DESIGN OF A SIX DEGREES OF FREEDOM HAPTIC DEVICE

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Degrees_of_Freedom

Degrees of freedomDr. Alex Yu"Degrees of freedom" have nothing to do with your life after you get married. Actually, "Degrees offreedom" (df) is an abstract and difficult statistical concept. Many elementary statistics textbook introduces this concept in terms of the number that are "free to vary" (Howell, 1992; Jaccard & Becker, 1990).Some statistics textbooks just give the df of various distributions (e.g. Moore & McCabe, 1989; Agresti & Finlay, 1986). Johnson (1992) simply said that degrees of freedom is the "index number" foridentifying which distribution is used.Probably the preceding explanations cannot clearly show the purpose of df. Even advanced statisticstextbooks do not discuss degrees of freedom in detail (e.g. Hays, 1981; Maxwell and Delany, 1986;Winner, 1985). It is not uncommon that many advanced statistics students and experienced researchers havea vague idea of degrees of freedom. In this write-up I would like to introduce this concept from two angles:working definitions and mathematical definitions.Working DefinitionsToothaker (1986), my statistic professor at the University of Oklahoma, explain df as the number ofindependent components minus the number of parameters estimated. This approach is based upon the definition provided by Walker (1940): the number of observation minus the number of necessary relations among these observations.Although Good (1973) criticized that Walker's approach is not obvious in the meaning of necessaryrelations, I do consider the above working definition the clearest explanation of df I ever heard. If it does not sound clear to you, I would like to use an illustration introduced by Dr. Robert Schulle, my SAS mentor atthe University of Oklahoma, :In a scatterplot when there is only one data point, you cannot do any estimation of the regression line. The line can go in any direction as shown in the following graph.Here you have no degree of freedom (n - 1 = 0 where n = 1) for estimation. In order to plot a regession line, you must have at least two data points as indicated in the following scattergram.In this case, you have one degree of freedom for estimation (n - 1 = 1 where n = 2). In other words, the degree of freedom tells you the number of useful data for estimation. However, when you have two data points only, you can always join them to be a straight regression line and get a perfect correlation ( r = 1.00). Thus, the lower the degree of freedom is, the poorer the estimation is.Mathematical DefinitionsThe following are indepth definitions of df:Good (1973) looked at degrees of freedom as the difference of the dimensionalities of theparameter spaces. Almost every test of a hypothesis is a test of a hypothesis H within a broaderhypothesis K. Degrees of freedom, in this sense, is d(K) - d(H), where "d" stands for dimensalityin parameter space.Galfo (1985) viewed degrees of freedom as the representation of the quality in the givenstatistic which is computed using the sample X values. Since in the computation of m,the X values can take on any of the values present in the population, the number of X values, n,selected for the given sample is the df for m. The n for the computation of m also expresses the"rung of the ladder" of quality of the m computed; i.e. if n = 1, the df, or restriction, placed on thecomputation is at the lowest quality level.Chen Xi (1994) asserted that the best way to describe the concept of the degree of freedom is incontrol theory: the degrees of freedom is a number indicating constraints. With the samenumber of the more constraints, the whole system is determined. For example, a particle moving ina three dimensional space has 9 degrees of freedom, 3 for positions, 3 for velocities, 3 foraccelerations. If it is a free falling and 4 degrees of the freedom is removed, there are 2 velocitiesand 2 accelerations in x-y plane. There are infinite ways to add constraints, but each of theconstraints will limit the moving in a certain way. The order of the state equation for a controllableand observable system is in fact the degree of the freedom.Cramer (1946) defined degrees of freedom as the rank of a quadratic form. Muirhead (1994)also adopted a geometrical approach to explain this concept. Degrees of freedom typically referto chi-square distributions (and to F distributions, but they're just ratios of chi-squares). Chi-squaredistributed random variables are sums of squares (or quadratic forms), and can be represented as thesquared lengths of vectors. The dimension of the subspace in which the vector is free toroam is exactly the degrees of freedom. For examples,Let X_1, ... , X_n be independent N(0,1) variables, and let X be the column vector whoseith element is X_i. Then X can roam all over Euclidean n-space. Its squared length, X'X =X_1^2 + ... + X_n^2, has _a chi-square distribution with n degrees of freedom.Same setup as in (1), but now let Y be the vector whose ith element is X_i-{X-bar}, whereX-bar is the sample mean. Since the sum of the elements of Y must always be zero, Ycannot roam all over n-dimensional Euclidean space, but is restricted to taking values in then-1 dimensional subspace of all n x 1 vectors whose elements sum to zero. Its squaredlength, Y'Y has a chi^2 distribution with n-1 degrees of freedom.All commonly occurring situations involving chi-sqaure distributions are similar. The most commonof these are in analysis of variance (or regression) settings. F-ratios here are ratios of independentchi-square random variables, and inherit their degrees of freedom from the subspaces in which thecorresponding vectors must lie.Rawlings (1988) associated degrees of freedom with each sum of squares (in multipleregression) as the number of dimensions in which that vector is "free to move." Yis free to fall anywhere in n-dimensional space and, hence, has n degrees of freedom. Y-hat, on theother hand, must fall in the X-space and , hence, has degrees of freedom equal to the dimension ofthe X-space -- [p', or the number of independent variables's in the model]. The residual vector e canfall anywhere in the subspace of the n-dimensional space that is orthogonal to the X-space. Thissubspace has dimensionality (n-p') and hence, e has (n-p') degrees of freedom.Selig (1994) stated that degrees of freedom are lost for each parameter in a model that isestimated in the process of estimating another parameter. For example, one degree offreedom is lost when we estimate the population mean using the sample mean; two degrees offreedom are lost when we estimate the standard error of estimate (in regression) using Y-hat (onedegree of freedom for the Y-intercept and one degree of freedom for the slope of the regression line).Lambert (1994) regarded degrees of freedom as the number of measurements exceeding the amount absolutely necessary to measure the "object" in question. For example, to measure the diameter of a steel rod would require a minimum of one measurement. If ten measurements are taken instead, the set of ten measurements has nine degrees of freedom. In Lambert's view, once the concept is explained in this way, it is not difficult to extend it to explain applications to statistical estimators. i.e. if n measurements are made on m unknown auantities then the degrees of freedom are n-m.NavigationIndexSimplified NavigationTable of ContentsSearch EngineContact。

Chapter 5 Two Degrees of Freedom

m1 0 m 0 m2
c1 c2 c c2
c2 c2 c3
mt cxt k xt F t x
k1 k 2 k k2
k2 k 2 k3
Consider a torsional system consisting of two discs mounted on a shaft as shown below. Parameters; k, J0 and Mt

J1 kt11 kt 2 2 1 M t1 1 J k k M
2 2 1/ 2
Frequencies ratios,
X 21 m112 k1 k2 k2 r1 1 X1 k2 m212 k2 k3
2 X 22 m12 k1 k2 k2 r2 2 2 X1 k2 m22 k2 k3
Free Vibrations
X 1 X 1 cos 2 X 1 sin 2
2
2
2 源自2 2 1/ 2 1/ 2
r1 x1 0 x2 0 1 2 r1 x1 0 x2 0 2 r2 r1 2
2
X 12 sin 2 r1 x1 0 x2 0 1 1 2 tan 2 tan X 1 cos 2 2 r1 x1 0 x2 0
x1 t x t x2 t
F1 t F t t F2
mt cxt k xt F t x
The solution involves four constant of integration. From the initial conditions;

A New Six Degree-of-Freedom Haptic Device based on the Orthoglide and the Agile Eye

Proceedings of Virtual Concept 2005Biarritz, France, November 8th – November 10th, 2005 A New Six Degree-of-Freedom Haptic Devicebased on the Orthoglide and the Agile EyeChablat D., Wenger P.Institut de Recherche en Communications et Cybernétique de Nantes1, rue de la Noë, 44321 Nantes, FrancePhone : 024******* Fax : 024*******E-mail : {Damien.Chablat, Philippe.Wenger }@irccyn.ec-nantes.frAbstract:The aim of this paper is to present a new six degree-of-freedom (dof) haptic device using two parallel mechanisms. The first one, called orthoglide, provides the translation motions and the second one produces the rotational motions. These two motions are decoupled to simplify the direct and inverse kinematics, as it is needed for real-times control. To reduce the inertial load, the motors are fixed on the base and a transmission with two universal joints is used to transmit the rotational motions from the base to the end-effector. The main feature of the orthoglide and of the agile eye mechanism is the existence of an isotropic configuration. The length of the legs and the range limits of the orthoglide are optimized to have homogeneous performance throughout the Cartesian workspace, which has a nearly cubic workspace. These properties permit to have a high stiffness throughout the workspace and workspace limits that are easily understandable by the user.Key words: Haptic device, interface, parallel mechanism, orthoglide, agile eye.1-IntroductionThe aim of this paper is to introduce a new haptic device that can be used as a new interface in a Virtual Reality system. Virtual Reality, characterized by real-time simulation, allows a human to interact with a Virtual 3D world and to understand its behavior by sensory feedback [1]. Associated with this definition, four key elements are usually considered: (i) virtual world, (ii) immersion (of human), (iii) interactivity, and (iv) sensory feedback. Most often, Virtual Reality systems provide only visual sensory feedback to the user (mono or stereo vision). However, aural or haptic interfaces exist that allow the user to hear or to feel the virtual environment. We want our device to be able to feel the virtual environment.The haptic feedback is associated with the sense of touch. For a user, the existence of an object in a virtual world is verified by coming into physical contact or touch. Usually, a haptic device enables input and output communication between the computer and the user.Haptic devices play important roles in the recognition of virtual objects. With these devices, the user can feel the rigidity or weight of virtual objects. Most robotic manipulators have large-scale and high-cost hardware, which inhibits their application to human-computer interaction.Our device was specifically developed for desktop use. It provides haptic feedback, which strongly enhances human capabilities in the major application areas of virtual reality, such as scientific visualization and 3D shape modeling.The six-dof Orthoglide features two parallel mechanisms mounted serially with a suitable system, which makes it possible to know and control the orientation of a stylus, even though all the motors are fixed. These features contribute to a significant reduction of the inertia when compared to serial architectures commonly used in haptic devices [2]. Thus, the inertia of the manipulator's moving parts is so small that compensation is not needed. This hardware design is compact, and it has the ability to carry a relatively large payload. Furthermore, the parallel kinematic architecture increases drastically the stiffness of the structure.2-State of the art of haptic devicesBasically, haptic devices that enable the user to touch an object can be classified into two main types: the first one provides only one single point of contact and the second one provides a single point of contact with a torque or multiple points of contact. In the next subsection, we will characterize their properties and introduce several examples.2.1-Single point of contactMany haptic devices have only three degrees of freedom, such as the Phantom Desktop [2] depicted in Fig. 1. With such a device, the user drives a stylus in space and can feel a single point of contact with an object. The force display provides stimuli to fingertip but no torque is provided.Figure 1: The Phantom Desktop provides one single point ofcontact to the user To provide high stiffness and a low inertia, parallelmechanisms can be also experimented as depicted in Fig. 2.This design has been optimized to avoid the singularconfiguration inside the workspace [3].Figure 2: The “syntaxeur” from ONERA/CERT [3] 2.2- Single point of contact with torque or multiple points of contact With a three-dof haptic device, the user can only manage the motion of one point. However, in virtual environment, he may also want to handle objects. For example, when one assemblesa screw in a hole, it is necessary to produce a feedback byforces and torques to feel the contact. This feature is related tothe problem of grasping.Only a six-dof devices can provide forces and torques in thesame times and there are three types of mechanisms:-Serial: The main problem of the serial type is the lack ofstiffness. Therefore, the position and the rotation are coupled.An example of the first type, depicted in Fig. 3(a), wasoriginally designed at McGill University [4] and is nowcommercialized by MPB [5]. The Phantom Premium six-dofdevices is also of the serial type but with a closed loop in theserial chain [2].-Parallel: An example of this type, depicted in Fig. 3(b), was designed at Tsukuba University [6]. The main advantage ofthis device is that all the actuators are fixed on the base. However, its rotation motions are limited and are function ofthe position. The computation of the inverse and directkinematic models is not simple, which is not compatible with real time applications.-Hybrid: An example of this type, depicted in Fig. 3(c), was designed at École Fédérale de Lausanne and is now commercialized by [7]. A parallel architecture provides high stiffness but the shape of its Cartesian workspace is generallynot simple and few of them have an isotropic configuration.(a) (b)(c)Figure 3: Three haptic devices with six-dof with (a) serialarchitecture, (b) parallel architecture and (c) hybridarchitectureThere is also a haptic device [8] made of a Delta [9] a two-dof agile eye [10] mounted serially with a revolute joint. Such a design enables to decouple the translation motionsand the rotation motions. The kinematics models are simple but its main drawbacks are the actuated joints of the wrist, which increase the inertia in motion and its volume. The aimof our paper is to propose a design with the same advantagebut without these drawbacks.Figure 4: A six-dof haptic device from Tohoku University 3- Constraints to design a new haptic interface The main desirable characteristics are quick motion and a regular workspace shape. In addition, compactness is also desirable, since the haptic interface should be treated in the same way as a conventional computer mouse. Quick motions could be achieved with a parallel mechanism since the massof the end-effecter can be reduced [11].It is possible to built a compact haptic device with a six-dof parallel mechanism, as shown by Long [12]. But in general, fully six-dof parallel mechanisms (e.g. Stewart Platform [13], HEXA [14]) have a restricted orientation workspace. To cope with this problem, it is possible to mount a spherical wrist on a translating mechanism.Without a force/torque sensor in the haptic interface, a fully isotropic mechanism would be needed to ensure uniform force controllability in each direction. Unfortunately, six-dof isotropic mechanisms do not exist because isotropy cannot be defined due to the non-homogeneity of the jacobian matrix [16]. Conversely, using a force/torque sensor would increase the inertia and the volume of the device.In general, the size of the workspace of haptic devices is relatively small compared to the size of the simulated virtual environment. Thus, there are several problems relating to the correspondence between displacements in these two spaces as scale factor, reachable space and directions of the translations or senses of rotations. To simplify these problems, the shape of the workspace of haptic devices must be close to a cube because its projection of the plan of view is the screen of the computer.To summarize these constraints, we can define an ideal haptic interface with the following constraints:•To be able to display motions in 6-dof,•To be able to realize quick motion (low inertia),•To have a regular dextrous workspace like a cube•To be compactOther design constraints, taking into account the control and the software can be found in Hayward [17].3.1-Fully isotropic mechanismsA three-dof fully isotropic parallel mechanism was defined in [18, 19, 20]. This device was firstly designed for milling applications. The linear joints can be actuated by means of linear motors or by conventional rotary motors with ball screws. The output body is connected to the linear joints through a set of three RRR identical chains where R stands Revolute joints.Figure 5 depicts an example of such a mechanism where the three prismatic joints c are orthogonal, and the three joint axes d, e and f, are parallel. Its workspace is defined by the intersection of three cylinders and no singular configurations exist inside.We could use this mechanism as a haptic device with three-dof because full isotropy is clearly an outstanding property. Unfortunately, bulky legs are required to assure stiffness because these legs are subject to bending. To increase the stiffness means to increase the inertia. Moreover, it would very difficult to mount a wrist on this mechanism and to actuate if from fixed motors. Another solution would be to add one or two legs [21, 23] but mechanical interferences would reduce the size of the workspace.The Orthoglide is part of the Delta linear family. It was optimized to have an isotropic configuration. Thus, its leg lengths as well as its range limits were also defined to have homogeneous performances throughout its Cartesian workspace [23]. The criterion used was related the Jacobian matrix and made it possible to characterize the velocity factor amplifications. The resulting machine, the Orthoglide, features three fixed parallel linear joints, which are mounted orthogonally and a mobile platform, which moves in the Cartesian x-y-z space with fixed orientation. The interesting features of the Orthoglide are a regular Cartesian workspace shape and good compactness.AA2Figure 6: OrthoglideThe three legs are PRPaR identical chains, where P, R and Pa stands for Prismatic, Revolute and Parallelogram joint, respectively. The arrangement of the joints in the PRPaR chains was defined to eliminate any special singularity [24]. Each base point A i is fixed on the i th linear axis such that A1A2 = A1A3 = A2A3. The points B i and C i are located on the i th parallelogram as shown in Fig. 7.iFigure 7: Legs kinematics of the Orthoglide3.3-Wrist mechanismThe architecture of the wrist can be either serial or parallel. Insome haptic devices, the actuators are fixed to the base and themotion is transmitted to the end-effector by using cable. Inother cases, the motors are embedded.To have high stiffness, the parallel architectures are suitablebut it suffers from a limited workspace. An exhaustive study ofsuch mechanisms can be found in [25]. Some of them are ableto have a fixed center point in the same loci of the center of theuser hand. Moreover, particular designs can provide unlimitedrotations about this central point, allowing very largeworkspace, limited only by the low-scaled dexterity andmechanical interferences. An example of such a device hasbeen tested at Laval University [26].Figure 8: The ShaDe deviceA parallel wrist with actuation redundancy can be employed toavoid the singularities [15]. An example of such a device isdepicted in Fig. 9. But it would be very difficult to actuate thisdevice with four motors fixed on the base of a translatingmechanism.Two coincidentmotorsTwo coincidentmotorsFigure 9: Redundant spherical wristAn hybrid architecture derived from the two-dof agile eye isdepicted in Fig. 10. It is the same one that is used in [8]. Itsmain property is that there are unlimited motions about its lastjoint. Unfortunately, there is no way to place the last motors onthe base.Figure 10: The hybrid agile eye with two-dof in serial with arevolute actuated joint eThe kinematic behavior of this mechanism is good becausethere is a set of orientations where the Jacobian matrix isisotropic as shown in Fig. 10. In the home posture, the jointaxis e is orthogonal to the joint axes c and d.3.4-Design of the legsThe aim of our design is to find a mechanism that allows usto avoid to carry the motors of the wrist and to reduce itsvolume.We have chosen to use the agile eye with two or three dofsbecause with such a mechanism we can easily achieve theorientation range needed for haptic devices. We can alsonotice that this design can be simplified to have a six-dofmechanism with only three-dof haptic feedback.The kinematics of the new leg is depicted in Fig. 11. On theneutral fiber of a leg, a mechanism with two universal jointsis used to transmit the rotary motion from the base of the legto the spherical wrist. From the kinematics of the Orthoglidedepicted in Fig. 7, we have added a revolute actuated joint cfixed on the base followed by a prismatic joint and a revolutejoint, called d and e, respectively. This former is carried bythe prismatic actuated joints. Two universal joints f, locatedon B i and C i, are placed on the neutral fiber of theparallelogram and finally, a revolute joint g allows us tomount the agile eye on the previous end-effector of theOrthoglide.iFigure 11: New design for the legs of the OrthoglideProceedings of the 18th International Symposium of Robotic Manipulators, IFR Publication, pp. 91-100, 1988.[10] Gosselin C., Hamel J.F., “The agile eye: a high performance three-degree-of-freedom camera-orienting device,” IEEE Int. conference on Robotics and Automation, pp.781-787. San Diego, 8-13 Mai, 1994.[11] Birglen, L., “Haptic Devices Based on Parallel Mechanisms. State of the Art”, in /Reviews/Review003.html[12] Long, G. L. and Collins, C. L., “A Pantograph Linkage Parallel Platform Master Hand Controller for Force-, IEEE 18, No. 2,pp. 166-175, 2002.[27] Chablat D. and Wenger P., “Dispositif de déplacement et d'orientation d'un objet dans l'espace et utilisation en usinage rapide”, Brevet français : FR2850599, demande PCT : WO2004071705, Déposant: Centre National de la Recherche Scientifique CNRS/Ecole Centrale de Nantes, 2004.。

对自由的看法作文英语初中

When discussing the concept of freedom,it is essential to consider its various dimensions and implications in our lives.Freedom can be viewed from different perspectives,such as political,personal,and social.Here are some key points that could be included in an essay on the topic of freedom for middle school students:1.Definition of Freedom:Begin by defining what freedom means.It can be the absence of coercion or restraint in ones life,or the power to act or speak or think without hindrance.2.Importance of Freedom:Explain why freedom is crucial.It allows individuals to make choices,express opinions,and pursue their dreams without undue interference.3.Types of Freedom:Discuss different types of freedom,such as freedom of speech, freedom of religion,and freedom of movement.Each type has its own significance and implications.4.Historical Context:Provide a brief historical context where freedom has been fought for and won.This could include examples from the American Revolution,the Civil Rights Movement,or the fall of the Berlin Wall.5.Freedom and Responsibility:Highlight the relationship between freedom and responsibility.With freedom comes the duty to use it wisely and not to harm others or infringe upon their rights.6.Challenges to Freedom:Discuss the challenges that can threaten freedom,such as authoritarian regimes,censorship,and surveillance.Its important to recognize these threats to maintain and protect freedom.7.Personal Freedom:Reflect on how personal freedom affects an individuals growth and development.It can lead to creativity,innovation,and the pursuit of happiness.8.Social Implications:Explore the social implications of freedom.A society that values freedom is likely to be more open,tolerant,and progressive.9.The Role of Education:Emphasize the role of education in fostering a sense of cation can teach critical thinking,which is essential for making informed decisions and standing up for ones rights.10.The Future of Freedom:Conclude by considering the future of freedom.Discuss how technology,globalization,and changing social norms might affect the concept of freedomin the years to come.Remember to structure your essay with an introduction,body paragraphs for each point, and a conclusion that summarizes your main ideas and leaves a lasting impression on the e clear and concise language,and provide examples to support your arguments.。

【托福阅读】精选阅读真题长难句50句

【托福阅读】精选阅读真题长难句50句1、 Later experiments in which researchers played recordings of songs to youngbirds showed just how precise this influence was, many of them would learnthe exact pattern of the recording they had heard.2、 The crude song of a bird reared in isolation gives some clues as to what thisrough idea may be the length, the frequency range and the breaking up intonotes are all aspects of chaffinch song shared between normal birds andthose reared in isolation.3、 Whatever the nature of the learning rules in a particular species, there is nodoubt that they are effective, it is very unusual to hear a wild bird singing asong which is not typical of its own species despite the many different songswhich often occur in a small patch of woodland.4、 Chemical analysis of bones enables archaeologists to determine theproportion of meat to vegetable foods in the diet by measuring the proportionof calcium to strontium in ancient bone because strontium in place of calciumin bones comes primarily from ingested plants.5、 Each dwelling had a different arrangement of the giant bones, which camefrom the skeletons of long-dead animals retrieved from the surrounding areaby occupants of the site, not from animals they had recently hunted.6、 In a precedent-setting decision, the Federal Energy Regulatory Commissionordered the dam removed after concluding that the environmental andeconomic benefits of a free-flowing river outweighed the electricity generatedby the dam.7、 Built nearly a century ago to provide power to lumber and paper mills in thetown of Port Angeles, these dams blocked access to upstream spawning bedsfor six species of salmon on what once was one of the most productivesalmon rivers in the world.8、 The Hetch Hetchy Dam in Yosemite National Park might be taken down toreveal what John Muir, the founder of the prestigious environmentalorganization Sierra Club, called a valley “just as beautiful and worthy ofpreservation as the maj estic Yosemite.”9、 A chance collision between two comets, or the gravitational influence of oneof the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—mayoccasionally alter the orbit of a comet in these regions enough to send it tothe inner solar system and into our view.10、 These comets appear to be distributed in all directions from the Sun, forminga spherical shell around the solar system, called the Oort cloud, after theDutch astronomer Jan Oort.11、 After the arrival of hunter-gatherers in the southwestern region of NorthAmerica, several alternative types of agriculture emerged, all involvingdifferent solutions to the Southwest’s fundamental problem: how to obtainenough water to grow crops in an environment in which rainfall is so low andunpredictable that little or no farming is practiced there today.12、 That fate actually befell the Mimbres, who started by farming the floodplainand then began to farm adjacent land above the floodplain as their population ca me to exceed the floodplain’s capacity to support it.13、 However, when drought conditions returned, that gamble left them with apopulation double what the floodplain could support, and Mimbres societycollapsed suddenly under the stress.14、 One more strategy was to plant crops at many sites even though rainfall waslocally unpredictable and then to harvest crops at whichever sites did getenough rain to produce a good harvest and to redistribute some of theharvest to the people still living at all the sites that did not happen to receiveenough rain that year.15、 Sanitation problems caused by larger, more sedentary populations wouldhave helped transmit diseases in human waste, as would the use of animaldung for fertilizer.16、 The increase in many of these came not only from the fact that fewer peoplewere dying from infectious disease and were living longer but also from theresults of modern lifestyles in developed countries and among the upperclasses of developing countries – a more sedentary life leading to lessphysical activity, more stress; environmental pollution, andhigh-fat diets.17、 This evolution may have been encouraged by what some authorities considerour overuse of antibiotics, giving microorganisms a greater chance to evolveresistance by exposing them to a constant barrage of selective challenges.18、 Therefore, on many reefs it is the fast-growing, branching corals thatultimately dominate at the upper, shallower portion of the reef, whereasmore massive forms dominate in deeper areas.19、 The fact that almost all small invertebrates on reefs are so well hidden orhighly camouflaged is another indicator of how prevalent predation is onreefs and its importance in determining reef structure.20、 Finally, they need reliable methods of storage because, where plant foodscannot provide a dietary safety net, planning has to be precise and detailedto ensure that there is enough to tide them over in periods of shortage.21、 Coins also provide a valuable source of written records: they can revealinformation about the location where they are found, which can provideevidence about trade practices there, and their inscriptions can beinformative about the issuing authority, whether they werecity-states (as inancient Greece) or sole rulers (as in Imperial Rome or in the kingdoms ofmedieval Europe).22、 The great risk with historical records is that they can impose their ownperspective so that they begin not only to supply the answers to ourquestions but subtly to determine the nature of those questions and even ourconcepts and terminology.23、 Not all botanists agree with an African-South American center for theevolution and dispersal of the angiosperms, pointing out that many of themost primitive forms of flowering plants are found in the South Pacific,including portions of Fiji, New Caledonia, New Guinea, eastern Australia, andthe Malay Archipelago.24、 To elaborate, before the eighth century, the elite marriage practice, whichwas an important instrument of political alliance making, had encouragedrulers to maintain multiple palaces: that of their own family and those of theirspouses, who commonly remained at or near their native family headquarters,at least for some years after marriage.25、 Nearly five billion years ago, some external influence, such asa shock wavetraveling from a catastrophic explosion (supernova), may have triggered thecollapse of this huge cloud of gases and minute grains of heavier elements,causing the cloud to begin to slowly contract due to the gravitationalinteractions among its particles.26、 Nearly five billion years ago, some external influence, such asa shock wavetraveling from a catastrophic explosion (supernova), may have triggered thecollapse of this huge cloud of gases and minute grains of heavier elements,causing the cloud to begin to slowly contract due to the gravitationalinteractions among its particles.27、 Steady overseas demand for colonial products created a prosperity thatenabled colonists to consume ever-larger amounts not only of clothing but ofdishware, home furnishings, tea, and a range of other items both produced inBritain and imported by British and colonial merchants from elsewhere.28、 Such materials as iron and nickel and the elements of which the rock-formingminerals are composed—silicon, calcium, sodium, and so forth—formedmetallic and rocky clumps that orbited the Sun.29、 This act was intended less to raise revenue than to serve as a protective tariff(tax) that would benefit British West Indian sugar producers at the expenseof their French rivals.30、 Parliament used British tax money to pay modest incentives to Americansproducing such items as silk, iron, dyes, hemp, and lumber, which Britainwould otherwise have had to import from other countries, and it raised theprice of commercial rivals’ imports by imposing protective tariffs on them.31、 The concept of chromatic adaptation was proposed in 1883; and thehypothesis was accepted for about 100 years, until it was realized that suchzonation did not necessarily occur and that the distribution of seaweedsdepended more on herbivory (the consumption of plant material),competition, varying concentration of the specialized pigments, and the abilityof seaweeds to alter their forms of growth.32、First, demand for news increased as Europe’s commercial and politicalinterests spread around the globe—merchants in London, Liverpool, orGlasgow, for example, came to depend on early news of Caribbean harvestsand gains and losses in colonial wars.33、 Industries with high concentrations of employment in urban areas, where aworker’s change of employer does not necessarily require investing in achange of residence, appear to have higher rates of job turnover thanindustries concentrated in nonmetropolitan areas do.34、 Some researchers, for example, have argued that a particular kind of pottery,called Ramey incised (which is incised with figures of eyes, fish, arrows, andabstract objects and was used by the people in the area of present-dayMissouri and Illinois at about A.D 900), was primarily used to distribute foodbut was also used to communicate the idea that the society’s elite, for whomthe pots were made, were mediators of cosmic forces.35、 There are several limiting factors, but results from a recent experimentsuggest that in areas of the ocean where other nutrients are plentiful, ironmay be one of the most important and, until recently, unrecognized variablescontrolling phytoplankton production.36、 In 1894, C. Lloyd Morgan, an early comparative behaviorist, insisted thatanimal behavior be explained as simply as possible without reference toemotions or motivations since these could not be observed or measured.37、 Assuming that in the early 1770s at least half of the demand for grain fromfarmers with surpluses was satisfied through long-distance channels, theproportion of grain produced for consumption beyond the local marketprobably accounted for about a quarter of total grain production consumedby humans.38、Sea turtles’ eggs are laid at night to minimize the likelihood of their discoveryby predators, and the offspring, when ready to emerge from their eggshellsand dig their way out of the sand, hatch at night for the same reason.39、 In short, therefore, the site Memphis offered the rulers of the Early DynasticPeriod an ideal location for controlling internal trade within their realm, anessential requirement for a state-directed economy that depended on themovement of goods.40、 In particular, research has focused on determining why such an apparentlyinhospitable place as Chaco, which today is extremely arid and has very shortgrowing seasons, should have favored the concentration of labor that musthave been required for such massive construction projects over brief periodsof time.41、 In one case, seeds of the arctic lupine, a member of the pea family recoveredfrom ancient lemming burrows in the Arctic, germinated in three days eventhough they were carbon-dated at more than 10,000years old!42、 In fact, unpredictability is probably a greater problem than is the severity ofthe unfavorable period.43、 Similar reasoning suggested that one could estimate total elapsed geologictime by dividing the average thickness of sediment transported annually tothe oceans into the total thickness of sedimentary rock that had ever beendeposited in the past.44、 The process that marine creatures use to create light is like that of thecommon firefly and similar to that which creates the luminous green colorseen in plastic glow sticks, often used as children’s toys or for illuminationduring nighttime events.45、 But its appearance under a microscope is even more spectacular, the livingcopepod appears as if constructed of delicately handcrafted, multicoloredpieces of stained glass.46、 Cleared lands would more likely have been worked by hand tilling, with littledirect help from animals, and the vast forests natural to Northern Europeremained either untouched, or perhaps cleared in small sections by fire, andthe land probably used only so long as the ash-enriched soil yielded goodcrops and then abandoned for some other similarly cleared field.47、 Because nests at the edges of breeding colonies are more vulnerable topredators than those in the centers, the preference for advantageous centralsites promotes dense centralized packing of nests.48、 In discussing the growth of cities in the United States in the nineteenthcentury, one cannot really use the term “urban planning,” as it suggestsmodern concerns for spatial and service organization which, in most instances,did not exist before the planning revolution called the City BeautifulMovement that began in the 1890s.49、 There the limits that topography imposed on production have been tightenedby climate, with the result that agricultural output has been more modest andless reliable, making the risk of crop failure and hardship commensuratelygreater.50、 Because liquid water was present, self-replicating molecules of carbon,hydrogen, and oxygen developed life early in Earth’s history and haveradically modified its surface, blanketing huge parts of the continents withgreenery.。

On the ”degrees of freedom” of the lasso

p p
ˆ = arg min y − β
β
x j βj
j =1
2
subject to
j =1
|βj | ≤ t.
(1)
Or equivalently ˆ = arg min y − β
β
p
p
x j βj
j =1
2
+λ
j =1
|β j | ,
(2)
where λ is a non-negative regularization parameter. Without loss of generality we assume that the data are centered, so the intercept is not included in the above model. There is a one-one correspondence (generally depending on the data) between t and λ making the optimization problems in (1) and (2) equivalent. The second term in (2) is called the 1norm penalty and λ is called as the lasso regularization parameter. Since the Loss + P enalty formulation is common in the statistical community, we use the representation (2) throughout this paper. Figure 1 displays the Lasso estimates as a function of λ using the diabetes data (Efron et al. 2004). As can be seen from Figure 1 (the left plot), the Lasso continuously shrinks the coeﬃcients toward zero as λ increases; and some coeﬃcients are shrunk to exact zero if λ is suﬃciently large. In addition, the shrinkage often improves the prediction accuracy due to the bias-variance trade-oﬀ. Thus the Lasso simultaneously achieves accuracy and sparsity. Generally speaking, the purpose of regularization is to control the complexity of the ﬁtted model (Hastie et al. 2001). The least regularized Lasso (λ = 0) corresponds to Ordinary Least Squares (OLS); while the most regularized Lasso uses λ = ∞, yielding a constant ﬁt. So the model complexity is reduced via shrinkage. However, the eﬀect of the Lasso shrinkage is not very clear except for these two extreme cases. An informative measurement of model complexity is the eﬀective degrees of freedom (Hastie & Tibshirani 1990). The proﬁle of degrees of freedom clearly shows that how the model complexity is controlled by shrinkage. The degrees of freedom also plays an important role in estimating the prediction accuracy of the ﬁtted model, which helps us pick an optimal model among all the possible candidates, e.g. the optimal choice of λ in the Lasso. Thus it is desirable to know what is the degrees of freedom of the lasso for a given regularization parameter λ, or df (λ). This is an interesting problem of both theoretical and practical importance. Degrees of freedom are well studied for linear procedures. For example, the degrees of freedom in multiple linear regression exactly equals the number of predictors. A generaliza2

心向自由无界的英语作文

The heart yearns for freedom,a concept as boundless as the universe itself.It is a fundamental human desire,a driving force that propels us to explore,to dream,and to create.In the realm of English composition,the theme of freedom can be approached from various angles,each offering a unique perspective on this timeless ideal.In the context of personal growth,freedom is the space to make choices and to learn from the consequences of those choices.It is the liberty to express oneself without fear of judgment or retribution.This freedom allows individuals to develop their own identities, to pursue their passions,and to contribute their unique talents to the world.Freedom of thought is another crucial aspect of this theme.It is the ability to question,to challenge,and to form ones own opinions.In an English composition,one might discuss the importance of critical thinking and the dangers of blindly accepting information.The essay could explore the historical figures who fought for intellectual freedom,such as Galileo Galilei,whose pursuit of truth led to his persecution by the Church.The freedom of movement is a topic that resonates with many,especially in todays globalized world.An English composition on this subject could delve into the benefits of travel,such as cultural exchange and personal enrichment.It could also address the challenges faced by refugees and immigrants,who seek freedom from persecution and poverty.Political freedom is a complex and often contentious issue.An English essay on this theme might examine the principles of democracy,human rights,and the rule of law.It could also critique authoritarian regimes and explore the struggles of those who fight for freedom in oppressive societies.In the realm of creativity,freedom is the lifeblood of artistic expression.A composition on this topic could celebrate the works of writers,musicians,and artists who have broken free from traditional constraints to create groundbreaking pieces.It could also discuss the role of censorship and how it stifles creativity.Finally,spiritual freedom is a deeply personal and introspective theme.An English composition on this subject might explore the idea of finding inner peace and liberation from societal expectations.It could delve into the teachings of various spiritual traditions and how they offer paths to freedom from suffering and desire.In conclusion,the theme of freedom is multifaceted and can be explored from numerous perspectives in an English composition.Whether it is the freedom to be oneself,to think independently,to move without barriers,to participate in a democratic society,to createwithout limits,or to seek spiritual enlightenment,the pursuit of freedom is a journey that enriches the human experience.。

自由之心飞扬的英语作文

The heart of freedom soars high in the sky,embodying the essence of human spirit that yearns for liberty and autonomy.It is a feeling that transcends physical boundaries and resonates with the core of our being.In the vast expanse of human history,the quest for freedom has been a relentless pursuit. It is the driving force behind countless revolutions and the inspiration for many a poets verse.The heart of freedom is not confined to any one nation or culture it is a universal desire that unites people from all walks of life.The essence of a free heart lies in its ability to dream,to aspire,and to seek out new horizons.It is the courage to stand up against oppression,to question the status quo,and to fight for ones beliefs.It is the unwavering conviction that every individual has the right to live life on their own terms,free from the shackles of tyranny and injustice.In the realm of the arts,the heart of freedom finds expression through the brush strokes of a painter,the notes of a composer,and the words of a writer.It is the passion that fuels the creative process,allowing artists to explore the depths of their imagination and to share their visions with the world.In the political sphere,the heart of freedom is the foundation of democracy and human rights.It is the belief that every person has a voice and that their opinions should be heard and respected.It is the commitment to upholding the principles of justice,equality,and fairness in the governance of a nation.In the personal realm,the heart of freedom is the inner strength that enables us to break free from the constraints of our past and to embrace the possibilities of our future.It is the resilience that allows us to overcome adversity and to grow from our experiences.As we navigate the complexities of life,let us not forget the importance of nurturing the heart of freedom within us.It is a precious gift that enriches our lives and fuels our journey towards a brighter tomorrow.By embracing the spirit of freedom,we can rise above the challenges that confront us and soar to new heights of personal and collective achievement.。

自由之心飞翔的英语作文

The heart of freedom soars high,transcending the confines of society and the limitations of the mind.It is the essence of human spirit,the yearning for liberty that propels us to explore the unknown and to challenge the status quo.In the realm of thought,freedom is the cornerstone of creativity.It allows us to think beyond the boundaries of what is taught and to question the norms that govern our lives. It is the freedom of thought that has led to the greatest discoveries and innovations in history,from the scientific breakthroughs of the Renaissance to the technological advancements of the modern era.The heart of freedom also beats within the arts.It is the freedom to express oneself without fear of censorship or judgment that has given birth to the most profound works of literature,music,and visual art.It is the freedom to explore different cultures and perspectives that has enriched our understanding of the human experience.In the political sphere,the heart of freedom is the foundation of democracy.It is the right to vote,to speak ones mind,and to live without oppression that defines a free society.It is the freedom to choose ones leaders and to hold them accountable that ensures the protection of our rights and the betterment of our communities.However,the heart of freedom is not without its challenges.It requires courage to stand up for ones beliefs,even in the face of adversity.It demands resilience in the pursuit of justice and equality.And it necessitates a commitment to the principles of truth,fairness, and compassion.But the rewards of a free heart are immeasurable.It brings a sense of fulfillment and purpose to life,a joy in the pursuit of knowledge,and a passion for the betterment of humanity.It is the heart of freedom that drives us to strive for a world where all individuals can live in harmony,free from the shackles of prejudice,ignorance,and injustice.In conclusion,the heart of freedom is a powerful force that shapes our lives and our world.It is a force that must be nurtured and protected,for it is the key to unlocking our full potential as individuals and as a global society.Let us embrace the heart of freedom and let it soar to new heights,inspiring us to create a brighter and more just future for all.。

什么是自由的真谛英语作文

The essence of freedom is a profound and multifaceted concept that has been explored and debated throughout history. It is a fundamental human desire, deeply rooted in our nature as individuals who seek autonomy, selfexpression, and the ability to make choices that shape our lives. In this essay, we will delve into the various dimensions of freedom, examining its significance and the challenges that come with it.Firstly, freedom is often associated with the absence of external constraints or coercion. It means being able to act according to ones own will without being forced or manipulated by others. This form of freedom is crucial for personal growth and development, as it allows individuals to explore their interests, talents, and passions without limitations. It is the freedom to pursue ones dreams and aspirations, to make decisions that align with ones values and beliefs.However, true freedom goes beyond mere physical or legal constraints. It also encompasses the internal realm of the mind and spirit. Emotional freedom, for instance, involves being liberated from the shackles of fear, anger, and resentment. It is the ability to let go of past traumas and to live in the present moment with a sense of peace and contentment. This type of freedom is essential for mental wellbeing and overall happiness.Intellectual freedom is another critical aspect of the true essence of freedom. It refers to the capacity to think independently, to question established norms and beliefs, and to engage in openminded dialogue with others. This freedom is vital for the advancement of knowledge, as it fosters creativity, innovation, and the pursuit of truth.Moreover, freedom is closely linked to the concept of responsibility. With the power to make choices comes the obligation to consider the consequences of our actions. True freedom is not about acting recklessly or selfishly it is about making informed, ethical decisions that contribute to the greater good. It is about using our autonomy to promote justice, equality, and compassion in society.However, achieving and maintaining freedom is not without its challenges. There are various forces that can threaten our autonomy, such as oppressive governments, societal pressures, and even our own insecurities and biases. To preserve our freedom, we must be vigilant and proactive in defending our rights and liberties. This may involve engaging in political activism, advocating for social change, or simply cultivating a strong sense of selfawareness and selfrespect.In conclusion, the true essence of freedom is a complex and multidimensional concept that encompasses physical, emotional, and intellectual autonomy. It is about the ability to make choices that reflect our values and contribute to our wellbeing and the wellbeing ofothers. While freedom comes with its own set of responsibilities and challenges, it remains a fundamental aspect of the human experience, one that we must strive to protect and uphold.。

为什么要自由的英语作文

In the heart of every individual lies an innate desire for freedom. It is a fundamental human yearning that transcends cultural, social, and geographical boundaries. As a high school student, I have come to appreciate the importance of freedom in shaping our lives and enabling us to reach our full potential. This essay delves into the reasons why freedom is essential and how it impacts our daily existence.Freedom is the bedrock of personal growth. It allows us to explore our interests, make choices, and learn from our experiences. Without the freedom to pursue our passions, we would be confined to a life of monotony and stagnation. For instance, my love for music was nurtured by the freedom to join the school band and experiment with various instruments. This freedom not only honed my skills but also instilled a sense of discipline and teamwork.Moreover, freedom of expression is a cornerstone of a democratic society. It empowers individuals to voice their opinions, challenge norms, and contribute to the collective discourse. In my school, we were encouraged to participate in debates and discussions, which fostered critical thinking and respect for diverse viewpoints. This open environment helped me understand the importance of dialogue in resolving conflicts and promoting social harmony.Freedom also plays a vital role in fostering creativity and innovation. When we are free to think and experiment without fear of retribution, we can push the boundaries of knowledge and imagination. For example, the invention of the internet was a result of the freedom to explore newtechnologies and share information globally. This freedom has revolutionized the way we communicate, learn, and interact with the world.However, freedom is not an absolute concept. It comes with responsibilities and the need to respect the rights of others. As students, we must learn to balance our personal liberties with the wellbeing of our community. For instance, while we have the freedom to express ourselves, we must also be mindful of the impact our words can have on others.Another aspect of freedom is the ability to make choices and bear the consequences. This autonomy teaches us accountability and resilience. When I chose to participate in a challenging science project, I had to manage my time, overcome obstacles, and accept the outcome, whether it was successful or not. This experience taught me valuable lessons about perseverance and selfreliance.Freedom also enables us to forge our own paths and define our identities. It allows us to break free from societal expectations and pursue our unique dreams. My friend, who aspires to be a professional dancer, faces skepticism and discouragement from some quarters. Yet, the freedom to follow her passion gives her the strength to overcome these challenges and stay true to herself.In conclusion, freedom is an indispensable aspect of our lives. It is the catalyst for personal growth, creativity, and social progress. While it comes with responsibilities and challenges, the rewards of living a life guided by our own choices and values are immeasurable. As we navigate through ourhigh school years and beyond, let us cherish and uphold the freedom that enriches our lives and shapes our future.。

自由心海翱翔的英语作文

The concept of freedom is as vast as the ocean,and the human spirit,when unfettered,can soar as high as the sky.The freedom of the heart allows us to explore the depths of our emotions,to dream without boundaries,and to create without limitations. Here is an essay that encapsulates the essence of a free spirit soaring through lifes vast expanse.In the tranquility of the mind,where thoughts are unbound,the soul finds its truest expression.It is in this state of liberation that one can truly experience the joy of existence.The heart,when free,is like a bird that takes flight,soaring high above the mundane,touching the clouds with its wings.The freedom of the heart is not merely an absence of physical constraints,but a state of mental liberation where one is not confined by the expectations of society,the norms of culture,or the judgments of others.It is the ability to think independently,to question the status quo,and to seek truth without fear of retribution.This freedom is the birthplace of creativity.When the heart is free,it can give rise to art, music,literature,and innovation that can change the world.It is the source of the most profound scientific discoveries and the most touching artistic expressions.It is the wellspring of all human progress.The journey of a free heart is not always smooth.It is fraught with challenges and obstacles that test the strength of ones resolve.But it is in overcoming these challenges that the spirit truly grows.Each setback is an opportunity to learn,to adapt,and to become stronger.The freedom of the heart also comes with great responsibility.With the power to create, one also has the power to destroy.It is essential to use this freedom wisely,to consider the impact of ones actions on others,and to strive for harmony and balance.In conclusion,the freedom of the heart is a precious gift that should be cherished and protected.It is the key to a fulfilling life,a life filled with wonder,discovery,and the pursuit of excellence.Let us all strive to keep our hearts free,to soar to new heights,and to explore the boundless possibilities that life has to offer.。

自由定义英语作文模板

自由定义英语作文模板In the vast expanse of human experience, there exists a concept that has been the cornerstone of countless revolutions, the subject of innumerable philosophical debates, and the driving force behind the most significant societal shifts. This concept is none other than "freedom." Freedom,in its essence, is the ability to make choices without undue constraints imposed by others. It is the right to express oneself, to pursue one's dreams, and to live life on one'sown terms.Introduction:The pursuit of freedom has been a fundamental aspect of human nature since the dawn of civilization. It is an innate desire that fuels the human spirit, pushing individuals to breakfree from the shackles of oppression and to strive for a life of autonomy and self-determination.Body Paragraph 1: Historical PerspectiveThroughout history, freedom has been a powerful motivator for change. From the American Revolution to the fall of theBerlin Wall, the quest for freedom has led to monumentalshifts in the world order. It has been the rallying cry for those who have fought against tyranny and injustice, seekingto create a world where individuals can live without fear and with the ability to make their own choices.Body Paragraph 2: Personal FreedomOn a personal level, freedom is the cornerstone of individuality. It allows individuals to express their thoughts, beliefs, and aspirations without fear of persecution. It is the freedom to choose one's career, to love whom one wishes, and to live life according to one's values and principles.Body Paragraph 3: Freedom of SpeechOne of the most cherished aspects of freedom is the freedom of speech. It is the right to voice one's opinions, to challenge the status quo, and to engage in open dialogue. This freedom is essential for the growth of societies, as it fosters creativity, innovation, and the exchange of ideas.Body Paragraph 4: Limitations of FreedomWhile freedom is a fundamental right, it is not without its limitations. Absolute freedom can lead to chaos and disorder, which is why societies establish laws and regulations to maintain order and protect the rights of all citizens. The balance between individual freedom and societalresponsibility is a delicate one, requiring careful consideration and constant adjustment.Conclusion:Freedom is a complex and multifaceted concept that holds different meanings for different people. It is a gift that must be cherished and protected, yet it is also a responsibility that requires wisdom and restraint. As we continue to navigate the complexities of our modern world, it is the pursuit of freedom that will guide us towards a more just and equitable society.Closing Statement:In conclusion, freedom is not just a right; it is a privilege that comes with the duty to respect the freedoms of others. It is the bedrock upon which societies are built, and it is the beacon that guides us towards a brighter future. Let us honor the sacrifices made by those who fought for our freedoms and let us continue to strive for a world where all may enjoy the blessings of liberty.。

大学英语作文自由

大学英语作文自由Freedom is a precious gift that we often take for granted. It allows us to express ourselves, make choices, and pursue our dreams without fear of oppression or restriction. In a world where freedom is not guaranteed for everyone, we must cherish and protect this fundamental right.Freedom gives us the power to speak our minds and share our ideas with the world. It enables us to have open discussions, debate different viewpoints, and work towards finding common ground. Without freedom of speech, our society would be stifled, and progress would be hindered.With freedom comes the opportunity to explore new possibilities and push boundaries. It allows us to take risks, learn from our mistakes, and grow as individuals. When we are free to follow our passions and interests, we can discover our true potential and make a positive impact on the world around us.Freedom also means having the autonomy to make decisions that align with our values and beliefs. It empowers us to live authentically and pursue our own path, rather than conforming to societal expectations or norms. When we are free to be ourselves, we can live a more fulfilling and meaningful life.In conclusion, freedom is a fundamental human right that should be cherished and protected. It gives us the ability to speak our minds, explore new possibilities, and live authentically. Let us never take our freedom for granted and always strive to uphold it for ourselves and others.。

有代价的自由英语作文

In the pursuit of freedom,it is often said that there is a price to pay.This notion is deeply rooted in the human experience,as freedom is not merely an abstract concept but a tangible reality that can come with its own set of challenges and sacrifices.Here is an essay that explores the idea of the cost of freedom.Title:The Price of FreedomIntroductionFreedom is a fundamental human desire,a cornerstone of democracy,and a driving force behind many of the worlds greatest achievements.However,the path to freedom is not always smooth,and it often requires a significant investment of time,effort,and sometimes even personal sacrifice.The Struggle for Political FreedomThroughout history,the quest for political freedom has been fraught with conflict and bloodshed.From the American Revolution to the fall of the Berlin Wall,the fight for selfgovernance has demanded the ultimate price:the lives of those who stood up for their beliefs.The price of political freedom is often measured in the sacrifices made by individuals who have given their all for the cause of liberty.Economic Freedom and Its ConsequencesEconomic freedom,while less visibly violent,still comes with its own set of costs.The pursuit of free markets and individual wealth can lead to disparities in income and social inequality.The freedom to accumulate wealth without constraint can result in a society where the rich prosper while the poor struggle,highlighting the complex relationship between economic freedom and social justice.Cultural Freedom and Its ChallengesCultural freedom allows individuals to express themselves without fear of persecution. However,this freedom can also lead to clashes between different belief systems and values.The freedom to practice ones religion,for example,can sometimes conflict with the secular norms of a society,leading to tension and,in extreme cases,violence. Personal Freedom and Its SacrificesOn a personal level,the pursuit of freedom can require sacrifices in relationships,career,and even personal wellbeing.The freedom to choose ones path in life often means leaving behind the comfort of the familiar and stepping into the unknown.This journey can be lonely and difficult,testing the resolve of even the strongest individuals.The Value of the Price PaidDespite the costs associated with freedom,the value it brings to human life is immeasurable.Freedom allows for creativity,innovation,and the expression of individuality.It enables people to pursue their dreams and to live life on their own terms. The price of freedom,while sometimes steep,is a necessary investment in the human spirit and the progress of society.ConclusionIn conclusion,the price of freedom is multifaceted and can be heavy.It is a cost that is paid in various forms,from the personal sacrifices of those who seek to live authentically to the collective struggles of nations fighting for their rights.Yet,the rewards of freedomthe ability to shape ones destiny,to express oneself without fear,and to contribute to the betterment of societyare worth the price.As we continue to strive for greater freedom in all aspects of life,let us remember the sacrifices made by those who have come before us and honor their legacy by continuing to fight for a world where freedom is not just an ideal,but a lived reality.。

游心向自由的英语作文

The pursuit of freedom is a fundamental human instinct,deeply rooted in our desire to explore the unknown and to express our individuality.Freedom,in its essence,is the ability to make choices without undue constraints,to follow ones heart,and to live life on ones own terms.In the realm of thought,freedom allows us to think creatively and critically.It is the freedom of thought that has led to the most significant advancements in science, technology,and the arts.Without the ability to question established norms and to imagine new possibilities,progress would be stifled,and society would remain stagnant.The freedom of speech is another crucial aspect of this pursuit.It empowers individuals to express their opinions,share their experiences,and engage in open dialogue.This freedom is the cornerstone of democracy,enabling citizens to hold their leaders accountable and to participate in the decisionmaking process that shapes their lives. Freedom also extends to the personal sphere,where it manifests in the choices we make about our careers,relationships,and lifestyles.The freedom to choose ones path in life is what gives us the autonomy to grow,to learn from our mistakes,and to find fulfillment in our endeavors.However,with freedom comes responsibility.The liberty to make choices also means the obligation to consider the consequences of those choices on ourselves and others.It is through this balance between freedom and responsibility that we can build a society that respects individual rights while also promoting the common good.In conclusion,the hearts yearning for freedom is a testament to the human spirits resilience and adaptability.It is this pursuit that drives us to break free from the shackles of oppression,to challenge the status quo,and to create a world where everyone can thrive.As we navigate the complexities of life,let us remember the importance of freedom and strive to uphold it in all aspects of our existence.。

不设限英文名句

不设限英文名句Freedom is a concept that has been explored and celebrated throughout history, and it continues to be a fundamental value in modern society. From the freedom of speech to the freedom of expression, individuals have long sought to break free from constraints and embrace their right to live and act as they choose. In this article, we will explore the theme of "Unlimited Freedom" and examine some famous quotes that encapsulate the spirit of this idea.1. "The only way to deal with unfree world is to become so absolutely free that your very existence is an act of rebellion." Albert Camus。

Camus, a renowned philosopher and writer, captures the essence of unlimited freedom in this powerful quote. He suggests that true freedom lies in the ability to defy the constraints of an unfree world and live in a way that challenges the status quo. This idea of rebellion as a form of freedom is thought-provoking and encourages individuals to embrace their autonomy and make choices that go against the norm.2. "Freedom is not worth having if it does not include the freedom to make mistakes." Mahatma Gandhi。

The Flight of Freedom

**The Flight of Freedom**In the vast expanse of human history and the depth of human consciousness, there soars an idea as powerful as a tempest and as liberating as a dawn – the flight of freedom. It is a concept that has kindled the hearts of countless souls and driven them towards the pursuit of a life unshackled.Freedom is not a mere word but a flame that burns within, guiding us through the labyrinth of existence. Take the story of Spartacus, the gladiator who led a rebellion against the might of the Roman Empire. Despite the overwhelming odds, his quest for freedom was unwavering. His fight was not just for his own liberation but for the freedom of all those oppressed, highlighting the indomitable spirit that freedom can inspire.Like a soaring eagle that breaks free from the chains of earth, freedom gives us the wings to reach for the skies. The American Revolution is a prime example. The colonists, tired of tyranny and injustice, rose up in pursuit of the ideals of freedom and self-governance. Their struggle led to the birth of a new nation, founded on the principles of liberty and equality, demonstrating the transformative power of the pursuit of freedom.However, the flight of freedom is not without its challenges. It often requires sacrifice and the willingness to confront adversity. The story of Mahatma Gandhi and his nonviolent resistance against British rule in India showcases the determination and perseverance needed. Through peaceful means, he fought for the freedom of his people, facing imprisonment and hardship but never wavering in his belief.In a world where constraints and limitations often seem to abound, the flight of freedom serves as a beacon of hope. It urges us to question the status quo, to stand up against injustice, and to strive for a world where every individual can truly be free. The life and works of Nelson Mandela, who spent years in prison yet emerged to lead South Africa towards a more inclusive and free society, is a testament to the enduring power of the pursuit of freedom.In conclusion, the flight of freedom is an eternal journey that knows no bounds. It is a force that shapes nations, empowers individuals, and brings about change. Let us embrace this flight with courage and determination, for it is in the pursuit of freedom that we find the true essence of our humanity and the promise of a brighter future.。

自由是有限度英语作文

自由是有限度英语作文英文回答：Freedom, an ethereal concept that has been the subject of countless philosophical debates and literary masterpieces, is not an absolute but a nuanced and evolving idea. The notion of freedom is inextricably intertwined with the limits that define it, for it is within these boundaries that the true nature of liberty is revealed.Throughout history, societies have grappled with the delicate balance between individual freedom and societal cohesion. The pursuit of unrestrained liberty can lead to chaos and the erosion of social order, while excessive restrictions stifle creativity, innovation, and personal growth. Therefore, it becomes imperative to establish parameters that safeguard both individual rights and the well-being of the collective.The limitations imposed on freedom can vary greatlydepending on the societal context and prevailing values. In democratic societies, for example, freedom of speech isoften curtailed to prevent the spread of harmful propaganda or hate speech that could incite violence or discrimination. Similarly, laws against theft and assault serve to protect individuals from being deprived of their property orphysical safety.It is also essential to consider the temporal dimension of freedom. While some restrictions may be necessary in the present to safeguard society, they may become outdated or counterproductive over time. The concept of freedom is constantly evolving, shaped by societal changes, technological advancements, and the evolving understandingof human rights.Moreover, the boundaries of freedom are not static but rather dynamic, subject to negotiation and redefinition. As societies progress, new challenges and opportunities emerge, requiring adjustments to the existing framework ofliberties and restrictions. The role of government iscrucial in this process, as it must balance the protectionof individual rights with the need to maintain a stable and just society.In conclusion, freedom is not a limitless concept but one that is circumscribed by the boundaries of societal norms, legal frameworks, and the evolving understanding of human rights. These limitations are not meant to stifle liberty but rather to ensure its preservation and the well-being of the collective. By embracing the dynamic nature of freedom and engaging in ongoing dialogue about its boundaries, we can strive to create a society where individual rights are respected while societal cohesion is maintained.中文回答：自由的边界。

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DESIGN OF A SIX DEGREES OF FREEDOM HAPTIC DEVICEbyŞAKİR KABADAYISubmitted to the Graduate School of Engineering and Natural Sciencesin partial fulfillment ofthe requirements for the degree ofMaster of ScienceSABANCI UNIVERSITYSpring 2006© Şakir Kabadayı 2006 All Rights ReservedTo my brotherACKNOWLEDGMENTSFirst of all, I wish to express my deepest gratitude and appreciation to my advisor Assist. Prof. Dr. Kemalettin Erbatur for all his guidance and assistance not only throughout this thesis but all my graduate study. It has been a pleasure to work with him.I would like to thank Prof. Dr. Asif Şabanoviç who initiated this study and always be patient and tolerant with me.I would also like to acknowledge the members of my defense committee: Assoc. Prof. Dr. Mustafa Unel, Assoc. Prof. Dr. Mahmut Akşit and Assist. Prof. Dr. Erkay Savaş for finding time to serve as my jurors and for their comments on this work.I have to thank İlker Sevgen in addition to his friendship for his efforts in the completion of this thesis and Mehmet Güler for his “ultra-precise” manufacturing skills.Among my friends who contributed to the success of this thesis in any way, I am glad to mention the following names; Burak Yılmaz who is my part-time roommate and one of the smartest and passionate people I have ever known, Nusrettin Güleç who has been nothing but a real friend for almost seven years now, Özer Ulucay whom I can always count on, Arda Burnaz who supplied foods, drinks and his unconditional friendship, Esra Nur Şahinoğlu whose friendship was precious for me, Onur Özcan who is one of the most sincere person I have known, Deniz Güçlü who was always there for me, and all others Berk Çallı, Yeşim Hümay Esin, Merve Acer, Tolga Duran, Doğucan Bayraktar, Nevzat Ataklı, Furkan Pekmez…I would like to thank all Mechatronics graduate students for their friendship and creating a peaceful laboratory environment.I wish to convey my special thanks to Emre Güldoğan, for her invaluable support and love that helped me a lot not only throughout this thesis but also one of the darkest periods of my life. Whenever I search for a peace of mind and some happiness to get me through the day, she is at my side, she always was, and she always will be.I am indebted to my family for their trust, everlasting support and love that they have given me at all times. Especially I am grateful to my mother, Şükran and my sister, Makbule for their patience and faith in me, whatever I do.Special thanks go to my brother, Faruk, to whom I cannot express my gratitude with words.DESIGN OF A SIX DEGREES OF FREEDOM HAPTIC DEVICEŞakir KABADAYIMS Thesis, 2006Thesis Advisor: Assist. Prof. Dr. Kemalettin ErbaturKeywords: Haptic interface, Dynamics compensation, Design criteriaAbstractFrom the open-loop tele-operator systems of 1950’s to the modern kinesthetic training and surgery support setups, haptic systems took a long way of evolution. Application areas ranging from minimally invasive surgery to space training systems for astronauts, still there is a large room for improvements. The vast areas of emerging applications put a number of demands on haptic interfaces. Fidelity, large workspace and high force/torque capacity are among those demands.The thesis concentrates on the design of a haptic master arm. The mechanical system with an analysis of dynamics properties, electronic hardware, algorithms for forward and inverse kinematics and software for the integration of sensors and actuators are developed to create an infrastructure for haptic interaction. Though the major design criteria applied in this design are a large workspace and high force/torque capacity, dynamics compensation techniques are also discussed as part of the developed infrastructure. The main focus of the thesis is the design of this hardware and software base for haptic applications rather than the design of haptic control algorithms.A survey on haptic interfaces and master arm design criteria is presented firstly. A set of specifications for the master arm is determined for a general and multipurpose yet ergonomic use. Newton-Euler based simulation techniques are employed for the component selection. Sensors and controller hardware are selected according to the demands of the haptic control problem. Dynamics compensation techniques for the designed manipulator are considered and tested in simulation. Finally the designed master arm is assembled and electrically integrated.ALTI SERBESTLİK DERECELİ HAPTİK ARABİRİMİ TASARIMIŞakir KABADAYIYüksek Lisans Tezi, 2006Tez Danışmanı: Yrd. Doç. Dr. Kemalettin ErbaturAnahtar kelimeler: Haptik arabirim, Dinamik dengeleme, Tasarım kriterleriÖzetDokunma hisli (haptik) sistemlerin gelişimi, 1950’lilerdeki açık döngülü uzaktan komutalı sistemlerden bugünkü modern dokunsal öğretim ve cerrahi destek tertibatlarına kadar süren uzun bir yol katetmiştir. Uygulama alanları minimal invaziv cerrahiden astronotlar için uzay eğitim sistemlerine kadar uzanmakta olup, yine de gelişme için geniş bir alan bulunmaktadır. Çeşitli alanlarda gelişen uygulamalar dokunma hisli ara yüzler için birçok talep ortaya koymaktadır. Sadakat, geniş çalışma alanı ve yüksek kuvvet/tork kapasitesi bu talepler arasındadır.Bu tezde dokunma hisli ana kolun tasarımı üzerine yoğunlaşılmıştır. Sistemin dinamik özelliklerinin analizi ile oluşturulmuş mekanik sistem, elektronik donanım, ileri ve ters kinematik çözümleri için algoritmalar ve algılayıcı ve eyleyici uyumu için yazılım; dokunma hisli etkileşimin alt yapısını oluşturmak için geliştirilmiştir. Her ne kadar bu tasarımda ana tasarım kriterleri geniş iş alanı, yüksek kuvvet/tork kapasitesi olsa da geliştirilen altyapının bir parçası olarak sadakat kriterini geliştirmek amacıyla dinamik telafi teknikleri de tezde işlenmiştir. Tezin ana konusu dokunma hisli denetleme algoritmaların tasarımından çok dokunma hisli uygulamalar için yazılım ve donanım tasarımıdır.Tezde ilk olarak haptik arabirimler ve kol tasarım kriterleri üzerine yapılan literatür taraması sunulmuştur. Genel ve çok amaçlı, aynı zamanda ergonomik bir kol için tasarım belirtimleri belirlenmiştir. Newton-Euler tabanlı benzeşim teknikleri kullanılarak eyleyici ve transmisyon elemanları seçilmiştir. Haptik denetleme algoritmalarının gerektirdiği algılayıcı ve denetleyici donanımı seçilmiştir. Tasarlanan manipülatör için dinamik telafi teknikleri üzerinde durulmuş ve bu teknikler benzeşim ortamında denenmiştir. Son olarak tasarlanan kol monte edilmiş ve elektriksel bağlantıları yapılmıştır. Tez sonuçların sunulması ve tartışılması ile sonlandırılmıştır.TABLE OF CONTENTSACKNOWLEDGEMENTS (v)ABSTRACT (vii)ÖZET (viii)LIST OF TABLES (xi)LIST OF FIGURES (xii)LIST OF SYMBOLS (xv)ABBREVIATIONS (xvi)1 INTRODUCTION (1)2 LITERATURE SURVEY (4)3 DESIGN CRITERIA (10)3.1 Main Design Criteria (10)3.2 Ergonomics (11)3.3 Workspace (12)3.4 Force/Torque Capacity (12)3.5 Resolution (13)4 DESIGN OF THE KINEMATICS ARRANGEMENT (14)4.1 Parallel or Serial Arrangement? (14)4.2 Spherical Wrist (15)4.3 Kinematic Arrangement of the Main Axes (15)4.4 Placement of the Spherical Wrist (16)4.5 Link Sizes (19)4.6 Axis Assignment and Denavit - Hartenberg parameters (21)4.7 Forward Kinematics (23)4.8 Inverse Kinematics (24)5 COMPONENT SELECTION (26)5.1 Selection of Actuators (26)5.2 Stress Analysis (28)5.3 Design Verification with Newton-Euler Based Inverse Dynamics Simulation355.4 Selection of Sensors (52)5.4.1 Position Sensor Selection (52)5.4.2 Force Sensor Selection (54)5.5 Controller Hardware, Actuator and Sensor Integration (55)6 DYNAMICS COMPENSATION ALGORITHMS (59)7 ASSEMBLY (66)8 CONCLUSION (71)REFERENCES (72)APPENDIX A. HARMONIC DRIVE DATASHEET (76)LIST OF TABLESTable 4.1 D-H parameters for the designed arm (23)Table 5.1 Comparison of actuator options (27)Table 5.2 Maximum displacement of the links (32)Table 5.3 Max von-Mises stresses and safety factors (35)Table 5.4 Comparison of position sensor options (53)Table 5.5 Resolution of encoders (53)Table 5.6 Comparison of F/T sensor options (54)Table 6.1 Dominant dynamics effects at the joints (59)Table 6.2 Values ofω used for the joints (62)k nLIST OF FIGURESFigure 2.1. The first “force feedback” system developed by Goertz (4)Figure 2.2 The Pantograph linkage (5)Figure 2.3 SHaDe (6)Figure 2.4 SPIDAR-G (7)Figure 2.5 Phantom desktop (7)Figure 2.6 LHIfAM (8)Figure 2.7 ViSHARD6 (9)Figure 2.8 ViSHARD10 (9)Figure 4.1 The alternative revolute z axis (16)Figure 4.2 Main axis arrangement (16)Figure 4.3 Spherical wrist mounted to the bottom of z axis (17)Figure 4.4 Spherical wrist mounted on top of the z axis (17)Figure 4.5 Typical spherical wrist (18)Figure 4.6 Placement options for the spherical wrist (19)Figure 4.7 Comparison of link lengths and workspace (20)Figure 4.8 Workspace with a hole in the middle (21)Figure 4.9 Joint axes assignment (22)Figure 4.10 Complete axes assignment (22)Figure 5.1 Displacement distribution of link 1 (30)Figure 5.2 Displacement distribution of link 2 (30)Figure 5.3 Displacement distribution of link 3 (31)Figure 5.4 Displacement distribution of link 4 (31)Figure 5.5 Displacement distribution of link 5 (32)Figure 5.6 von-Mises stress distribution for link 1 (33)Figure 5.7 von-Mises stress distribution for link 2 (33)Figure 5.8 von-Mises stress distribution for link 3 (34)Figure 5.9 von-Mises stress distribution for link 4 (34)Figure 5.10 von-Mises stress distribution for link 5 (35)Figure 5.11 The inputs and outputs of the N-E algorithm (37)Figure 5.12 Total joint torque requirement and its components for shoulder joint, high joint speed, high end effector force/torque case (39)Figure 5.13 Total joint torque requirement and its components for shoulder joint, high joint speed, low end effector force/torque case (40)Figure 5.14 Total joint torque requirement and its components for shoulder joint, low joint speed, low end effector force/torque case (40)Figure 5.15 Total joint torque requirement and its components for shoulder joint, low joint speed, high end effector force/torque case (41)Figure 5.16 Total joint torque requirement and its components for elbow joint, high joint speed, high end effector force/torque case (41)Figure 5.17 Total joint torque requirement and its components for elbow joint, high joint speed, low end effector force/torque case (42)Figure 5.18 Total joint torque requirement and its components for elbow joint, low joint speed, low end effector force/torque case (42)Figure 5.19 Total joint torque requirement and its components for elbow joint, low joint speed, high end effector force/torque case (43)Figure 5.20 Total joint torque requirement and its components for vertical axis joint, high joint speed, high end effector force/torque case (43)Figure 5.21 Total joint torque requirement and its components for vertical axis joint, high joint speed, low end effector force/torque case (44)Figure 5.22 Total joint torque requirement and its components for vertical axis joint, low joint speed, low end effector force/torque case (44)Figure 5.23 Total joint torque requirement and its components for vertical axis joint, low joint speed, high end effector force/torque case (45)Figure 5.24 Total joint torque requirement and its components for roll 1 joint, high joint speed, high end effector force/torque case (45)Figure 5.25 Total joint torque requirement and its components for roll 1 joint, high joint speed, low end effector force/torque case (46)Figure 5.26 Total joint torque requirement and its components for roll 1 joint, low joint speed, low end effector force/torque case (46)Figure 5.27 Total joint torque requirement and its components for roll 1 joint, low joint speed, high end effector force/torque case (47)Figure 5.28 Total joint torque requirement and its components for pitch joint, high joint speed, high end effector force/torque case (47)Figure 5.29 Total joint torque requirement and its components for pitch joint, high joint speed, low end effector force/torque case (48)Figure 5.30 Total joint torque requirement and its components for pitch joint, low joint speed, low end effector force/torque case (48)Figure 5.31 Total joint torque requirement and its components for pitch joint, low joint speed, high end effector force/torque case (49)Figure 5.32 Total joint torque requirement and its components for roll 2 joint, high joint speed, high end effector force/torque case (49)Figure 5.33 Total joint torque requirement and its components for roll 2 joint, high joint speed, low end effector force/torque case (50)Figure 5.34 Total joint torque requirement and its components for roll 2 joint, low joint speed, low end effector force/torque case (50)Figure 5.35 Total joint torque requirement and its components for roll 2 joint, low joint speed, high end effector force/torque case (51)Figure 5.36 Animation window (52)Figure 5.37 Maxon motor and driver connections for current control mode (56)Figure 5.38 F/T sensor GUI (57)Figure 5.39 F/T sensor VB program GUI (58)Figure 6.1 PD control performance without fine tuning (63)Figure 6.2 PD control performance with fine tuning (64)Figure 6.3 PD control with gravity compensation (64)Figure 6.4 PD control with gravity and Coulomb friction compensation (65)Figure 7.1 Base and planar elbow manipulator assembly (66)Figure 7.2 Vertical axis mechanism (67)Figure 7.3 Spherical wrist mechanism (67)Figure 7.4 Spherical wrist mechanism (67)Figure 7.5 Full view of the haptic device (68)Figure 7.6 Close view of the vertical axis and spherical wrist (69)LIST OF SYMBOLSD Manipulator inertia matrixq Vector of joint variablesq & Joint velocity vectorq && Joint acceleration vectorJ Manipulator Jacobiana J Actuator inertiam J Motor side combined inertiag J Reduction system inertiag g Gravity constant)(q g Gravity effect vector),(q q C & Matrix for the computation of centripetal and Coriolis forcese F Force and torque vector exerted by the manipulator tool tip on theenvironment as expressed in the world framei α Denavit Hartenberg twist parameter for link ii d Denavit Hartenberg offset parameter for link ii θ Denavit Hartenberg angle parameter for link ii a Denavit Hartenberg length parameter for link if B Joint viscous friction constant matrixc b Joint Coulomb friction vectoru Generalized force/torque control inputτ Vector of joint force/torquek P K Proportional gain for joint kk D KDerivative gain for joint kABBREVIATIONSHartenbergD-H DenavitDOF Degree of FreedomLagrangeE-L EulerF/T Force / TorqueGUI Graphical User InterfaceLHIfAM Large Haptic Interface for Aeronautics Maintenance M/S Master / SlaveMIS Minimally Invasive SurgeryMS MicrosoftEulerN-E NewtonSCARA Selective Compliant Articulated Robot for Assembly SHaDe Spherical Haptic DeviceSPIDAR-G SPace Interface Device for Artificial Reality with Grip TCP/IP Transmission Control Protocol / Internet ProtocolBasicVB VisualCVC VisualViSHARD Virtual Scenario Haptic Rendering Device with 6 DOFRealityVR Virtual1INTRODUCTIONHaptic is a term that refers to sense of touch. Haptic manipulators or interfaces are force-reflecting devices which allow a user to touch, feel, manipulate, create and/or alter simulated objects in a virtual environment. The main purpose of using haptic devices is to enhance user experience when he/she is interaction with a simulated or remote environment. In order to realize tele-presence completely all related information should be at disposal of the user. Visual and aural feedback topics are well-covered in the literature in comparison to haptics. Haptic feedback conveys physical information about the environment such as inertia, friction, compliance, and roughness which can not be directly sensed by other sensory systems.A haptic interface together with computer hardware and software produces the sensation of touch and interaction with the environment. The environment in which the device is used can be either real physical surrounding or a simulated environment generated by software. Virtual environments might contain objects with masses and friction, springs and dampers and virtual walls. Examples of real environments might be remote locations or relatively nearby locations where user cannot access due to hazardous conditions.It has numerous application areas, such as robotic surgery, virtual reality (VR), tele-operation, entertainment industry etc. MIS (Minimally Invasive Surgery) is one of the application areas where haptic feedback is gravely desired. Compared to traditional surgery procedures, MIS employ small incisions through which the cameras and instruments are passed to carry out the operation. Haptic feedback is essential in such an application since the surgeon do not have direct visual or tactile feedback. It enables the surgeon to determine the rigidity of the tissue he/she is manipulating. ZEUS surgery robot from Computer Motion Inc. and DaVinci from Intuitive Surgical Inc. are two examples of widely-used surgery robots which do not have force feedback. Integrationof haptic feedback to these devices could not only improve the quality of operation but also increase the speed of the surgeon [1].Virtual Reality applications are used in various industrial areas such as medical operation, tele-operation and entertainment. With the incorporation of haptic feedback to VR applications, user’s perception of the environment becomes more realistic. Haptic devices are the successors of the historical hand controllers used in tele-operation and today find a wide usage in the VR applications. As a natural result of the improvements in the computer technology, rendering of virtual environments in real-time could be achieved. Development of virtual reality technology brought along the increasing demand for haptic devices. These robotic mechanisms form the kinesthetic counterpart of the VR environment as a complement to the visual, in some cases also aural, feedback. Desktop haptic devices are already finding usage as force “displays” for computer games, nano-manipulation applications and surgical simulators.The main objective of this technology is to create a realistic force-position interface between the user and the VR environment. The device commands motion and force to its slave counterpart, it receives feedback signals, depending on the control algorithm, and reflects the forces felt by the slave side to the user. The quality of this interface can be evaluated in terms of “impedance accuracy” and “impedance resolution” [2]. Impedance accuracy is the criteria for matching the impedance of the haptic device to the environments. Impedance accuracy plays an important role in the high-torque applications such as driving simulators and smart exercise machines. Impedance resolution or fidelity refers to the sensitivity of the device for discrimination of different impedances. Fidelity is more crucial for dexterous applications such as surgery where the impedance of the environment is changing.In order to achieve high impedance accuracy and resolution, dynamics of the haptic device should be optimized. Natural dynamics of the device diminish the realism of the haptic feedback since they are sensed by the user as a part of the simulated environment. Robotic researchers show great effort to reduce the natural dynamics of the manipulators by means of using more efficient drive trains and transmission mechanisms or higher strength-to-weight ratio materials.However, reduction of the natural dynamics cannot be achieved further by physical means for high force/torque output devices. For high output purposes, usage of large actuators, drive mechanisms and linkages lead to more inertia and friction thus resulting in high natural dynamics. The demand for haptic devices with highforce/torque capability is obvious since traditional industrial manipulators are used as haptic interfaces. As illustrated in [3], [4] industrial manipulators are used as driving and flight simulators. Motorized “smart” exercise machines [5] and Astronaut extravehicular training systems [6], [7] are other examples of application areas of manipulators as high output haptic devices.High fidelity criteria may also suggest that the dynamics of the device should be minimized relative to the impedance of task environment. Although the output of the haptic device is adequate, natural dynamics of the device might impair the impedance discrimination performance. During surgical procedures where the environmental impedance is changing, stiffness encountered during the penetration of the scalpel to a layer of tissue is a significant example of this phenomenon [8], [9].In addition to mechanical improvement to reduce dynamics, active control can be utilized to further cancel dynamical effects. Dynamics compensation can be employed as model feedforward or force feedback from an F/T sensor mounted on the haptic device itself. Gravity and friction feedforward are also used in dynamic compensation, however to employ inertial compensation force or acceleration feedback is required. Inclusion of feedback in the control loop leads to a more robust design as well, especially when the physical properties of the haptic device are changing.What this thesis aims at is the development of a harmonious collection of hardware and software components to form the infrastructure for general purpose haptic interaction. The primary specifications which are tried to be met are a large workspace and high force/torque capacity. Dynamic and kinematic properties, sensor and actuator specifications, strengths and weaknesses of the designed system as a haptic interface are presented in detail. What this thesis is not about is the development of haptic control algorithms. The focus is on kinematic arrangement, mechanical design and instrumentation.The next chapter presents a survey on haptic interfaces and haptic master arms. Chapter 3 develops the design criteria employed for the master arm designed in this thesis. The design of the kinematic arrangement to fulfill the workspace specifications follows in Chapter 4. Chapter 5 discusses the selection of actuators and transmission systems based on stress analysis and dynamics simulations. Sensor selection and control hardware integration is also presented in this chapter. A set of dynamics compensation tools are presented in Chapter 6. Chapter 7 discusses the production of the device, and evaluates the built mechanism. Finally, conclusions are presented.2LITERATURE SURVEYThe evolution of haptic devices dates back to the first “force feedback” system developed by Goertz for remote handling of radioactive materials. This system featured a master/slave (M/S) architecture where the master and slave arms were identical and were connected to each other via rigid mechanical link (Figure 2.1). After a while, these rigid links were replaced with servomotors which enabled the usage of the system over larger distances [10].Figure 2.1. The first “force feedback” system developed by GoertzBefore emergence of haptic devices, master/slave manipulators were used for tele-operation. In the beginning, passive replicas of the master manipulator were used at the remote site, which were commanded by the operator via the master manipulator. Later, master arms were motorized so that they could provide force feedback to the operator which was present on the slave side. Addition of force feedback to the M/S system increased task execution speed, especially in unpredictable or changing environments.Computational requirements were minimal since M/S systems employed joint-to-joint control. With the development of computer technology, complex kinematicscomputations could be carried out in real-time. This released the limitation of constructing master and slave arms identical. The JPL Force Reflecting Hand Controller was one of the first systems that employed different kinematics arrangement and it had better resolution than the devices built earlier [11].Later on, these M/S slave systems were begun to be used for simulation of virtual environments or reproducing forces sensed in a real physical environment. Usage of haptic devices brought new research areas with it. Transparency or low dynamics issues were scrutinized.There are various haptic devices in the literature that feature different kinematic structures, actuators, number of DOF, and have ranging workspace and force capabilities. All of these devices have their advantages and disadvantages depending on the application areas. Although it is still at an early stage, commercial haptic devices are also available.Pantograph is also a parallel mechanism device which was designed by [12] in McGill University. Pantograph was initially designed as 2 DOF, however there are different versions of that device that employ more DOF [13], [14]. It has a planar structure which the user commands with the fingertip thus enabling the device feedback forces. This process resembles exploring a surface. (Figure 2.2)Figure 2.2 The Pantograph linkageSHaDe is a haptic device designed recently by members of department of mechanical engineering, Laval University [15]. This haptic device differs from otherdevices in the literature with its spherical structure. It has three DOF and only gives rotational feedback around a central fixed point. The aim of the authors was to emulate the human wrist. It has some advantages compared to other devices such as pure rotation around a point located inside the user’s hand, large workspace and ergonomics. All actuators are fixed to the base of the device thus yielding a rigid structure. A force sensor is also used, placed between the end-effector and the joystick, in order to measure torques at the central point.Figure 2.3 SHaDeSPIDAR-G [16] (Space Interface Device for Artificial Reality with Grip) is a tension-based force feedback device that has seven DOF. It allows the user to interact with the virtual objects by manipulating two hemispherical grips located at a center point. Haptic sensation is achieved by controlling the tension of eight cables which are connected to the vertices of a cube. Its characteristics are smooth force feedback, no backlash, low inertia and safety. It is ideally suitable for engineering design applications.One of the most popular commercial haptic devices is SensAble’s Phantom. Several versions of this haptic device are available. Phantom Desktop [17], which is also available in Sabanci University Mechatronics laboratory, has six degree of freedom and three degrees of force feedback. It is a portable device and it can be connected to any computer having a parallel port. It provides the position of its stylus in x, y, and z axes and the rotation of its stylus as roll, pitch, yaw forming a total of six degree freedom. Force feedback is provided for only x, y, and z axes at a maximum force of 1.75 Newton. It has a purely serial kinematic structure and do not employ force or。