Del. Task Identifier D6.3T6.2 Work Package Haptic Illusion Based Systems

Author(s)Michael Fritschi (TUB)Davide Dente (UNIPI)Date30th September 2003Del. /Task Identifier D6.3/T6.2Work Package Haptic Illusion Based SystemsPartner(s)TUB, UNIPIWork Package Leader LSCConfidentiality Level PublicAbstract:This deliverable investigates technologies for pin actuating particularly with regard to the design of a combined pin actuator.Also a design for a multi actuated pin is presented.Thereby two actuators are divided in different function modules.These modules will be serial linked to an combined pin actuator.With this method we want to pursue a new basic approach to enhance the pin actuator technology.Contents1Design of tactile pin actuators(Part of TUB)11.1Introduction (1)1.2Mechanical design of the multiple actuated pin (1)1.2.1Low frequency module (2)1.2.2High frequency module (4)1.3Summary (4)References4 2Investigation and evaluation of technologies for actuating the pin(Part of UNIPI)6NOTE:The information in this document is provided as is and no guaranteeor warranty is given that the information isfit for any particular purpose.The user thereof uses the information at his/her sole risk liability.1Design of tactile pin actuators(Part of TUB)1.1IntroductionThe configuration most existing tactile displays are based on is a matrix like arrangement of pinsactuated perpendicular to the skin.Mostly they are designed to stimulate the cutaneous sense of the humanfinger tip.To provide a realistic tactile exploration the choice of the actuator playsan important role.The range of possible actuators is basically constrained by requirement for small size high force.With the additional need for high bandwidth the power density comes in a region of W cm[1].This value is far from the current available actuator technology. The overview of current devices categorized by actuators given in the deliverable6.1report[2] shows that in fact good positioning accuracy and high forces at the pins can be achieved but the overall performance is mostly not satisfied.At this reasons we decided to pursue a new basic approach to enhance the pin actuator technology.Our concept is to use two actuators in combination to actuate one pin.In this way we presume to be able to satisfy the requirement for excursion and bandwidth in a very large range of the human cutaneous perception.The sections below describe the mechanical design of the multiple actuated pin.1.2Mechanical design of the multiple actuated pinNearly every known tactile display uses one actuator per pin to generate a tactile stimulus to the human skin.The purpose of the multi actuated pin is to evaluate the possibility of serial connected actuators to enhance the performance of tactile displays.Before the design of these two individual actuators the design guidelines for the entire multiple actuated pin have to be defined.Thereby the specifications should be derived from thresholds of human cutaneous perception.Based on the investigations given in the deliverable6.1report[2]the main important facts are summarized in Tab.1.With these data essential target specification of the multiple actuated pin in terms of excur-sion,frequency,and force can be made:Excursion:The overall excursion is physically limited by the compression of the tissue between the surface and the bone of the indexfinger.The upper limit is approximately3 mm.Frequency:The frequency bandwidth of the cutaneous perception is about Hz.The sensitivity of indentation increases until a maximum at around230Hz[5].Force:In the static case the maximum force is required at an indentation of mm.With an impedance of N mm a force of N is needed.To satisfy these demands with a combined actuator concept different operative ranges are de-vised to the individual actuators.A reasonable division of the task domain can be made by the required actuation frequency.Investigations based on psychophysical observations showed that there is a threshold in detecting vibrations[6].The lower limit for the fuse of pulses into oneTable1:Tactile sensation and mechanical impedance of the humanfinger.Adapted from[3][4]Indentation Threshold(mN/mm)StaticHzHzm(lat.)mNTemporal SolutionSuccessive tap timeBandwidth senseReaction timemmmmMechanical Impedance(static)Range offinger pat indentationInitial contact mmFurther indentation mmFurther indentation mmindentation mmLateral indentationexcursion(%relative to the size in actuated direction).However,due to the high force they can provide the excursion can be raised by a mechanical transmission as schematically illustrated in Fig1.Starting from a commercially available piezo crystal with a size ofFigure1:Transmission of piezo excursionmm the transmission ratio to reach the displacement of mm must be.The realization of this ratio with only one lever stage and a minimum arm length at the actuator size of mm results in a lever length of mm.A lever of this length disagrees with the demand of compact size.Thus,we designed the transmission mechanism can applying two lever stages. Fig.2shows the current mechanical design of the low frequency module with the two leverExcursionPiezo ActuatorFigure2:Mechanical design of the low frequency modulestages.With this mechanism a module size of mm can be achieved.The two lever arm bearings,located at their pivot points are realized by elastic bending of its thin paths and provide moreover the necessary restoring force for the piezo actuator.To prevent friction a complete cohesive system of transmission levers is aspired.The pin will be attached at the end of the second lever and implements excursion among the drawn direction.In further stepsfinite element analyses will be undertaken to prove the stability of the bearing and to optimize the module size.Due to the high demand on accuracy the module will be manufactured by wire-electro discharge technique.1.2.2High frequency moduleThe high frequency module must provide the stimuli above Hz.As a consequence of the higher sensitivity of the cutaneous perception of indentation the maximum excursion of the module can be chosen much lower compared to the excursion of the low frequency module. Experimental results by an air pressure actuated display[7]show that an amplitude eight times higher than the indentation threshold at Hz enables to stimulate the sensation of a hard brush. With an indentation threshold of m at Hz[6]the amplitude of m may be a good target specification for dimensioning the module.To realize these functionalities an off the shelf rod piezo stack actuator can be used.This can be integrated in the pin rod between the low frequency module and the pin nib attached to the skin of the humanfinger tip.1.3SummaryThis report describes the mechanical design for a combined actuated pin.Investigations have shown,that the use of piezo crystals for both modules,the high-and low frequency part,are the best solution in terms of performance.The design of the low frequency module is based on a two leaver transmission with bending bearings.As high frequency module an off the shelf rod piezo stack actuator will be used.A high risk part at the serial actuator chain is the dynamical interference of the compo-nents against each other.Especially the animation of the low frequency module at its resonant frequency by the high frequency module must be prevented.Ongoing work in items:Finite elements analyses of the low frequency moduleRealization of the low frequency moduleDynamic analyses and position control of low frequency moduleChoice of high frequency module and build up of the multiple actuated prototypeControl and data processing concept for the combined pin actuator References[1]G.Moy,“Human tactile spatial sensitivity for tactile feedback,”in Int.Conf.on Robotics&Automa-tion,pp.776–782,IEEE,April2000.[2]M.Fritschi,“Design of a tactile shear force prototype display,”March2003..[3]S.Biggs and M.Srinivasan,Handbook of Virtual Environments,wrence Earlbaum,Inc.,2002.[4]S.Biggs and M.Srinivasan,“Tangential versus normal displacements of skin:Relative effectivenessfor producing tactile sensations,”in HAPTICS’02,IEEE,2002.[5]K.Shimoga,“A survey of perceptual feedback issues in dexterous telemanipulation:Part II,”inVirtual Reality Annual International Symposium,pp.271–279,1993.[6]F.Reynier and V.Hayward,“Summary of the kinesthetic and tactile function of the human upperextramities,”tech.rep.,McGill Research Centre for Intelligent Machines,Montreal,1993.[7]N.Asamura and H.S.N.Yokoyama,“Selectively stimulating skin receptors for tactile display,”Computer Graphics and Applications,vol.18,pp.32–37,Nov/Dec1998.2Investigation and evaluation of technologies for actuating the pin(Part of UNIPI)D6.3 - Design of pin actuatorT6.2: design of pin actuator with integrated piezoelectric crystal.Development of o small sized piezoelectric actuator optimised for integration in a mechanical pin. Investigation and evaluation of technologies for actuating the pin.For a realistic haptic exploration in a virtual space, human touch sensation needs to be evoked in both tactile (cutaneous) and proprioceptive (kinaesthetic) senses. Although the state of the art of haptic kinaesthetic display technologies has witnessed a dramatic progress over the last decade, the need of an efficient cutaneous stimulator, regardless the particular technology is becoming pressing.A piezoelectric array actuator seems to offer a fair amount of advantages compared to few drawbacks.A piezoelectric actuator (PZT) can produce extremely fine position changes down to the subnanometer range. The smallest changes in operating voltage are converted into smooth movements. Motion is not influenced by friction or threshold voltages. PZTs can generate a force of several Newton, offering very fast response time. They do not produce magnetic fields nor are they affected by magnetic fields. They are especially well suited for applications where magnetic fields cannot be tolerated, as for instance in MRI studies. Moreover, displacement is based on solid state dynamics, without rotating shafts, and shows no appreciable wear.Two main types of piezo actuators are available: low voltage (multilayer) devices requiring about 100 volts for full motion, and high voltage devices requiring about 1000 volts for full extension.Modern piezo ceramics capable of greater motion include lead zirconatetitanate.Multilayer designs, constructed from 20 to 100 µm layers in low voltage piezo,allow achieving a relative expansion (strain) up to 0.2 %, if both the regularand inverse electric field is used. Longer travel ranges can be achieved bymechanical amplification techniques.A piezo actuator can reach its nominal displacement in approximately 1/3 of the period of the resonant frequency. Rise times on the order of microseconds and accelerations of more than 10,000 g's are possible. This feature permits rapid switching applications. Resonant frequencies of industrial reliabile piezo actuators range from a few tens of kHz (for actuators with total travel of a few microns) to a few kHz for actuators with more than 100 microns travel.Similar to electromagnetic devices, open loop piezo actuators exhibit hysteresis (they are also referred to as ferroelectric actuators). Hysteresis is based on crystalline polarization effects and molecular friction. The absolute displacement generated by an open loop Piezo depends on the applied electric field and the piezo gain which is related to the remaining polarization (10 to 15 % of the commanded motion). Hysteresis can be compensated by closed loop piezo actuators.PZs exhibit creep, an unwanted positive or negative increase in the displacement over time. Creep only occurs with open loop PZs. Like hysteresis, creep is related to the effect of the applied voltage on the remaining polarization of the piezo ceramics. Creep decreases logarithmically with time.An interesting piezo actuator technology is the bender type Actuator(Bimorph and Multimorph Design).A piezo bimorph operates similarly to a bimetallic strip in a thermostat.When the ceramic is energized the metal substrate is deflected with amotion proportional to the applied voltage. Bimorph actuators providing motion up to 1000 µm are available and greater travel range is possible. Apart from the classical strip form, bimorph disk actuators are available, where the center arches when a voltage is supplied. Instead of a Piezo/metal combination Piezo/Piezo combinations are possible where individual Piezo layers areoperated in opposite mode (contraction/expansion).An interesting application of this technology can be found inthe texture explorer by Yasushi Ikei and Masashi Shiratori[1]. They realized a 2x5 piezoelectric vibrotactile display.Project of a tactile pin array can be addressed to threestimulus form. Static shape normal stimulus addressing SA1 receptors (Merkel cells), dynamic shape normal stimulus (tactile flow) addressing SA1 and RA (Meissner) cells, and static shape shear stimulus addressing SA2 (Ruffini) corpuscles [2].Using miniaturized piezo actuators, we think that the best choice is to stimulate the RA corpuscles, due to their high sensitivity. Indeed, a very good stimulation can be achieved using a displacement of piezo actuators of just 40 microns in the frequency range of 40-60Hz, in which RA receptors are most sensitive.REFERENCES[1] Yasushi Ikei, Masashi Stiratori: TextureExplorer : A tactile and force display for virtual textures. Proceedings of the 10th Symp. On Haptic Interfaces For Virtual Envir. & Teleoperator Systs. (HAPTICS.02) 0-7695-1489-8/02 $17.00 © 2002 IEEE[2] J´erome Pasquero and Vincent Hayward, STReSS: A Practical Tactile Display System with One Millimeter Spatial Resolution and 700 Hz Refresh Rate. Proceedings of Eurohaptics 2003, Dublin, Ireland, July 2003.。