自动化专业英语PartⅤ-Ⅵ 课文原文内容

Part ⅤSensors and TransmittersIn a feedback control system, the elements of a process-control systemare defined interms of separate functional parts of the system . The four basic components of controlsystems are thesensors, transmitter , controller , and final control elements . Thesecomponents per form the three basic operations of every control system: measurementdecision, and action.Sensors and transmitters perform the measurements operation of control system. Thesensor produces a phenomenon, mechanical, or the like related to the process variable that itmeasures. The function of transmitter in turn is to convert the signal from sensor to the formrequired by the final control device. The signal, therefor e, is related to the process variable.Two analog standards are in common u se as a means of representing the range ofvariables in control systems. For electrical systems we use a range of electric current carriedin wires , and for pneumatic systems we use a range of gas pressure carried in pipes . Thesesignals are used primarily to transmitvariable information over some distance, such as to andfrom the control room and the plant .Fig .5 . 9 shows a diagram of a process- controlinstallation where current is used to transmit measurement data about the controlled variableto the control room, and gas pressure in pipes is used to transmit a feedback signal to a valveto change flow as the controlling variable .Fig .5 .9 Electrical current and pneumatic pressures are the most common means of information transmitter in the industrial environmentCurrent signal The most common current transmission signal is 4 to 20 mA . Thu s , inthe preceding temperature example, 20℃might be represented by 4 mA, and 120℃by 20 mA, with all temperatures in between represented by a proportional current . The gain is略That is , we can say that the gain of sensor/ transmitter is ratio of the span of the output tothe span of input .Current is used instead of voltage because the system is then les s dependent on load . Voltage is not used for transmission because of its susceptibility to changes of resistance inthe line .Pneumatic signals The most common standard for pneumatic signal transmitter is 3 to15 psi . In this case, when a sensor measures some variable in a range it is converted into a proportionalpressure of gas in a pipe . The gas is usually dry air .The pipe may be many hundreds of meters long , but as long as there is no leak in the system the pressure will be propagated down the pipe . This English system standard is still widely used in the U .S ., despite the move to the SI system of units . The equivalent SI range that will eventually be adopted is 20 to 100 kPa.The two cases presented show that the gain of the sensor/ transmitter is constant over its completeoperating range . For most sensor/ transmitter this is the case; however , there are some in stances , such as a differential pressure sensor used to measure flow, when this is not the case . A differential pressure sensor measures the differential pressure ,h, across anorifice . This differential pressure is related to the square of the volumetric flow rate F . Thatis F2 ah .The equation that describes the output signal form an electronicdifferential pressure transmitter when used to measure volumetric flow with a range of 0～F maxgpm isM F = 4 + 16 F2/ ( F max )2Where MF = output signal in mAF = Volumetric flowFrom this equation the gain of the transmitter is obtained as follows:K′r = d MF/ d F = 216 F/ ( F max )2with a nominal gainK′T = 16/ F maxThis expression shows that the gain is not constant but rather a function of flow . T he greater the flow is , the greater the gain . So the actual gain varies from zero to twice the nominal gain .This fact results in a nonlinearly in flow control system . Nowadays most manufacturesoffer differential pressure transmitters with built-in square root extractor s yielding a line r transmitter .The dynamic response of most sensor/ transmitter s is much faster than the process . Consequently , their time constants and dead time can often beconsidered negligible andthus , their transfer function is given by a pure gain . However , when the dynamics must be considered , it is usual practice to represent the transfer function of the instrument by a first-orderor second-order system:G( s) = K / ( T s + 1)or G( s) = K / ( T2 s + 2 Tξs + 1)WORDS AND TERMS3 .1 Numerical ControlNumerical control is a system that uses predetermined instructions to control a sequenceof manufacturing operations. The instructions are coded numerical values stored on sometype of input medium, such as punched paper tape , magnetic tape, or a common memory for program storage . The instructions specify such things as position ,direction , velocity , and cutting speed . A partprogram contains all the instructions required to produce a desiredpart . A machine program contains all the instruction s required to accomplish a desired process . Numerical control machines per form operations such as boring , drilling , grinding , milling , punching , routing , sawing , turning , winding ( wire ) , flame cutting , knitting( garments ) , riveting , bending , welding , and wire processing .Numerical control ( NC) has been refer red to as flexible automation because of therelative ease of changing the program compared with changing cams , jigs , and templates .The same machine may be used to produce any number of different parts by using different programs . The numerical control process is most justified when a number of different partsare to be produced on a particular machine: it is seldom used to produce a single par t continually on the same machine . Numerical control is ideal when a part or process is defined mathematically . With the increasing u se of computer- aided design (CAD) , more and more processes and products are being defined mathematically . Drawings as we k now them have become unnecessary ―a part that is completely defined mathematically can be manufacturedby computer-controlled machines . A closed-loop numerical control machine is shown in Fig .5 .13 .The NC process begins with a specification ( engineering drawing or mathematicaldefinition ) that completely defines the desired par t or process . A programmer uses the specification to determine the sequence of operations necessary to produce the par t or carry out the process . The programmer also specifies the tools to be used , the cutting speeds , and the feed rates . The programmer uses a special programming language to prepare a symbolic program . APT ( Automatically Programmed Tools ) is one language used for this purpose . A computer converts the symbolic program into the part program or the machine program . In the past , the pa r t or machine program was stored on paper or magnetic t ape . The numerical control machine operator fed the tape into the machine and monitored the operation . If a change was necessary , a new tape had to be made . Now, it is possible to store the programin a common database with provision for on-demand distribution to the numerical control machine . Graphicterminals at the matching center allow operators to review programs and make changes if necessary .The x position controller moves the work piece horizontally in the direction indicated bythe + x a r row . The position controller moves the milling machine head horizontally in the direction indicated by the + yarrow . The z position controller moves the cutting tool vertically as indicated by the + z arrow . The following actions are involved in changing the x-axis position .( 1 ) The control unit reads an instruction in the program that specifies a+ 0 .004-inch ( in .) change in the x position . ( 2 ) The control unit send s a pulse to the machine actuator . (3 ) The machine actuator rotates the lead screw and advances the x- axis position + 0 .001 in . (4 ) the position sensor measures the + 0 .001-in . measured motion and sends another pulse . Steps ( 1 ) through ( 5 ) are repeated until the measured motion equals the desired + 0 .004 in .Computerized numerical control ( CNC ) was developed to utilize the storage andprocessing capabilities of a digital computer . CNC uses a dedicated computer to accept the input of instructions and to perform the control functions required to produce the part . However , CNC was not designed to provide the information exchange demanded by the recent t r end toward computer-integrated manufacturing (CIM) . The idea of CIM is to“get the right information- to the right person- at the right time- to make the right decision .”“I t link s all aspects of the business-f rom quotation and order entry through engineering , process planning , financial reporting , manufacturing , and shipping-in an efficient chain of production .”Direct numerical control ( DNC ) was developed to facilitate computer-integrated manufacturing . DNC is a system in which a n umber of numerical control machines are connected to a central computer for real- time access to a common database of part programs and machine programs . General Electric used a central computer connected to DNC machines through a communication s network in the automation of its steam turbine-generator operation s (STGO) .“A typical turbine-genera tor consists of more than 100 , 000 parts , someof which are manufactured in thousands of different configurations to meet the specific needs of each custom-designed unit . Through the CIM system, customers can specify a needed par t and receive replacement components that suit the original configuration of more than 4 , 000 operating STGO installations . In some cases , the small-parts shop can now manufacture and ship some emergency parts the same day the order is received .”4 .1 Relay ControllersAn industrial control system typically involves electric motors , solenoids , heaters orcooler , and other equipment that is operated from the ac power line . Thus , when a control system specifies that a“conveyor motor be turned on ,”it may mean starting a 50-HP motor . This is not done by a simple toggle switch .Instead , one would logically assume that a small switch may be used to energize a relay with contact ratings that can handle the heavy load , such as that shown in Fig .5 .16 . In this way , the relay became the primary control elementof discrete-state control systems .When an entire control system is implemented u sing relays , the system is called a relay sequencer . A relay sequencer consists of a combination of many relays , including specialtime-delay types , wired up to implement the specified sequence of events . Inputs are switches and push buttons that energize relay , and outputs are closed contacts that can turn lights on or off , start motor s , energize solenoids , and so on .The wiring of a relay control system can be described by traditional schematic diagrams ,such as those shown in Fig .5 .16 . Such diagrams are cumbersome , however , when many relays , each with many contacts , are used in a system . Simplified diagrams have been gradually adopted by the industry over the years . As an example of such simplification , a relay’s contacts need not be placed directly over the coil symbol , but can go anywhere in the circuit diagram with a number to associate them with a particular coil . These simplifications resulted in the ladder diagram in use today .Ladder DiagramsThe ladder diagram is a symbolic and schematic way of representing both the system hardware and the process controller . It is called a ladder diagram because the various circuit devices connected in parallel across the ac line form something that looks like a ladder , with each parallel connection a“rung”on the ladderIn the construction of a ladder diagram , it is understood that each rung of the ladder is composed of a number of conditions or input states and a single command output . The nature of the input states determines whether the output is to be energized or not energized . T he following example illustrates many features of a ladder diagram construction and its application to control problems .Example: The elevator shown in Fig .5 .17 employs a platform to move objects up anddown . The global objective is that when the UP button is pushed , the platform carriessomething to the up position , and when the DOWN but ton is pushed , the plat form carries something to the down position .The following hardware specifications defined the equipment used in the elevator :Output elements:M1 = Motor to drive the platform upM2 = Motor to drive the platform downInput elements:LS1 = NC limit switch to indicate UP positionLS2 = NC limit switch to indicate DOWN positionST ART = NO push button for STARTSTOP = NO push button for STOPUP = NO push button for UP commandDOWN = No pus h but ton for DOWN commandThe following narrative description indicates the required sequence of events for theelevator system .1 .When the START but ton is pushed , the platform is driven to the down position .2 .When the STOP button is pushed , the platform is halted at whatever position itoccupies at that time .3 .When the UP button is pushed , the platform, if it is not in downward motion , isdriven to the up position .4 .When the DOWN but ton is pushed , the platform, if it is not in upward motion , isdriven to the down position .Prepare a ladderdiagram to implement this control function .SolutionLet us prepare a solution by breaking the requirements into individual tasks . Forexample, the firsttask is to move the platform to the down position when the STARTbut ton is pushed .This task can be done by using the START but ton to latch a relay , whose contacts also energize M2 ( the down motor ) . The relay is released , stopping M2 , when the LS2 limitswitch open s . Pushing START energizes CR1 if L S2 is not open ( platform not down ) .CR1is latched by the contacts across the START button . Another set of CR1 contacts starts M2to drive the platform down . When L S2 opens , indicating the full down position has been reached ,CR1 is released , unlatched , and M2 stops . These two rungs will operate only whenthe START button is pushed .For the STOP sequence , let us assume a relay CR3 is the master control for the rest ofthe system . Because STOP is a NO switch , we cannot use it to release CR3 . Instead , we use STOP to energize another relay ,CR2 , and use the NC contacts of that relay to release CR3 .This is shown in Fig .5 .18 . You can see that when START is pushed ,CR3 in rung 4 isenergized by the latching of the CR1 contact and the NC contact of CR2 . When STOP ispushed ,CR2 in rung 3 is energized , which causes the NC contact in rung 4 to open andrelease CR3 .Finally , we come to the sequences for up and down motion of the platform . In eachcase, a relay is latched to energize a motor if CR3 is energized , the appropriate button hasbeen pushed , the limit has not been reached , and the other direction is not energized . T he entire ladder diagram is shown in Fig .5 .17 . A NC relay connection is used to ensure that theup motor is not turned on if the down motor is on , and vice versa . Also , it was necessary to add a contact to rung 2 to be sure M2 could not star t if there was up motion and some joke r pushed the START button .Part Ⅵ1.1Transmission of Electrical EnergyElectrical energy is carried by conductors such as overhead transmission lines and underground cable . Although these conductors appear very ordinary , they possessimportant electrical proper ties that greatly affect the transmission of electrical energy . In this section , we study these proper ties for several types of transmission lines: high volt age , low-voltage, high-power , aerial lines , and underground lines .Principal components of a power distribution systemIn order to provide electrical energy to consumers in usable form, a transmission and distribution system must satisfy some basic requirements . Thu s the system must1 .Provide, at all times , the power that consumers need2 .Maintain a stable, nominal voltage that does not vary by more than±10%3 .Maintain a stable frequency that does not vary by more than±0 .1 Hz4 .Supply energy at an acceptable price5 .Meet standards of safety6 .Respect environmental standards .Fig .6 .1 shows an elementary diagram of a transmission and distribution system .I tconsists of two genera ting station s G1 and G2 , a few substations , an inter connecting substation and several commercial , residential , and industrial loads . The energy is carriedover lines designated extra-high voltage ( EH V ) , high volt age ( H V ) , medium voltage(MV) , and low voltage ( LV ) . This voltage classification is made according to a scale of standardized voltage .Transmission substations ( Fig .6 .1 ) serve to change the line voltage by mean s of step-upand step-down transformers and to regulator it by means of static vary compensators , synchronous condensers , or transformers with variable taps .Distribution substations change the medium voltage to low voltage by means of step-down transformers , which may have automatic tap-changing capabilities to regulate the lowvoltage . The low voltage ranges from 120/ 240V single phase to 600V, 3-phase . I t serves to power private residences , commercial and institutional establishments , and small industry .Interconnecting substations serve to tie different power systems together , to enablepower exchanges between them, and to increase the stability of the overall network .These sub stations also contain circuit breakers , fuses , and lightning arresters , toprotect expensive apparatus , and to provide for quick isolation of faulted lines from the system . In addition , control apparatus , power measuring devices , disconnect switches , capacitors , inductors , and other devices may be part of a substation .Electrical power utilities divide their power distribution systems into two major categories:1 .Transmission systems in which the line voltage is roughly between 115kVand 800kV .2 .Distribution systems in which the voltagegenerally lies between 120V and 69kV .Distribution systems , in turn , are divided into medium-voltage distribution systems ( 2 .4kV to 69 kV ) and low-voltage distribution systems (120V to 600V) .Type of power linesThe design of a power line depends upon the following:1 .The amount of active power it has to transmit2 .The distance over which the power must be carried3 .The cost of the power line4. Esthetic considerations , urban congestion , ease of installation , and expectedload growthWe distinguish four types of power lines ,according to their voltage class:1 .Low- voltage ( L V) lines provide power to buildings , factories , and houses to drive motors , electric stoves , lamps , heater s , and air conditioners . The lines are insulated conductors , usually made of aluminum, often extending from a local pole-mounted distribution transformer to the service entrance of the consumer . The lines may be overhead or underground , and the transformer behaves like a miniature substation .2 .Medium- voltage ( MV) lines tie the load centers to one of the many substation s of the utility company . The voltage is usually between 2 .4kV and 69kV . Such medium-voltage radial distribution systems are preferred in the larger cities . In radial systems the transmission lines spread out like fingers from one or more substation s to feed power to various load centers , such as high- rise buildings , shopping centers , and colleges .3 .High-voltage ( H V) lines connect the main substations to the generating stations .T he lines are composed of aerial wire or underground cable operating at voltages below 230kV . In this category we also find lines that transmit energy between two power systems , to increase the stability of the network .4 .Extra-high- voltage ( E H V) lines are used when gene rating stations are very far fromthe load centers . We put these lines in a separate class because of their special electricalproperties . Such lines operate a t volt ages up to 800kV and may be as long as 1, 000 km .Components of a HV transmission lineA transmission line is composed of conductors , insulators , and supporting structures . Conductors . Conductors for high-volt age lines are always bare . Stranded copper conductors , or steel-reinforced aluminum cable ( ACSR ) are used . ACSR conductor s are usually prefer red because they result in a lighter and more economical line . Conductor s have to be spliced when a line is very long . Special care must be taken so that the joints have low resistance and great mechanical strength .Insulators . Insulators serve to support and anchor the conductors and to insulate themfrom ground . Insulators areusually made of porcelain , but glass and other synthetic insulating materials are also used .Supporting structures . The supporting structure must keep the conductors at a safeheight from the ground and at an adequate distance from each other . For voltages below70 kV, we can use single wooden poles equipped with cross-arms , but for higher voltages , twopoles are used to create an H-frame, the wood is treated with creosote or special metallic salts to prevent it from rot ting . For very high-voltage lines, we always use steel towers made of galvanized angle-iron pieces that are bolted together .The spacing between conductors must be sufficient to prevent arc-over under gusty wind conditions . The spacing has to be increased as the distance between towers and as the line voltages become higher .Construction of a lineOnce we know the conductor size, the height of the poles , and the distance between the poles ( span ) , we can direct our attention to stringing the conductor s . A wire supported between two points ( Fig . 6 .2 ) does not remain horizontal , but loops down at the middle . The vertical distance between the straight line joining the points of support and the lowest point of the conductor is called sag . The tighter the wire, the smaller the sag will be .Before under taking the actual construction of a line it is important to calculate the permissible sag and the corresponding mechanical pull . Among other things , the summer to winter temperature range must be taken into account because the length of the conductor varies with the temperature . Thus , if the line is strung in the winter , the sag must not betoo great , otherwise the wire will stretch even more during the summer heat , with the result that the clearance to ground may no longer be safe . On the other hand , if the line is installed in the summer , the sag must not be too small otherwise the wire , contracting in winter , may become so dangerously tight as to snap . Wind and sleet add even more to the tractive force, whichmay also cause the wire to break .2 .1 Grounding and Ground-Fault ProtectionT he importance of proper grounding for elect rical systems in buildings is oftenunder estimated . Unde r normal conditions , an elect rical system can continue to ope rate satisfactorily ( that is , deliver power to the utilization equipment ) even without prope r grounding . It is not until an abnormal condition has occur red , and after eithe r someone has been injured, equipment has been damaged , or a fir e has been star ted , that it is realized that imprope r or faulty grounding was the r eason . T her efore, a good understanding of the functions of grounding is essential for the proper design , in stallation , and maintenance of an electrical system . In most cases , the con nection is made by direct metallic contact withearth . The large mass of the ear th then serves as a zero potential r eferencepoint .T he study of grounding must begin by identifying the differ ent aspects of grounding:system grounding , equipment grounding lightning protection grounding , and staticelectricity grounding . Fig .6 .4 shows the basic difference between system grounding and equipmentgrounding .System grounding is the intentional elect rical connection to ground of one of the cur r ent- car rying conductors of the elect ricalsystem . Equipment grounding is the connection to ground of all the nonelectrical conductive materials that enclose or ar e adjacent to the energized conductor s . T he electrical code r equires that all equipment must be prope rly grounded , except in very ra re special cases . However , the application of system grounding is not so universal . Certain types of systems , such as the 120/ 240 volt , single-phase, threewire and the 208Y/ 120 volt , three-phase, four-wire syst ems used to supply lighting havealways been grounded . On the other hand , the 480 and 600 volt , three-phase syst ems usedto supply loads such as motor s have until recently us ually been operated ungrounded .①System grounding . The intentional connection to grounding of one of the current-carrying conductionsof the system .②Equipment grounding . The connection to grounding of all the nonelectrical conductive ma terials thatenclose or ar e adjacent to the energized conductors .T he primary purpose for grounding an electrical system is one of safety, that is , to limitthe potential to ground that otherwise could occur f rom accidental contact with highervoltage syst ems or f rom t ransientovervoltages . However ,ther e ar e other important benefits associa ted with grounded sy stems , as follows :1 .Se rvice r eliability is improved: The transient over voltage condition s tha t a re possiblewith ungrounded systems cannot occur . With the elimination of this overvolt age stressing of the ins ulation , fewer ground faults should occur ove r the operating life of grounded sy stems .2 .Much simpler to locate the first ground fault : With proper coordination , theovercur rent device ( circuit breake r or fuse ) near est to the fault operates to disconnect the faultedcircuit , thu s leaving the balance of the system operating .3 .Ground-fault protection can be easily added: Arcing ground faults can be difficult todetect and therefore require special attention .4 .Provides two voltage levels on the same system: Single-phase loads such as lighting can be connected across the line- to-neutral voltage (120 volts on a 208Y/ 120 volt system) . Three-phase loads such as motors can then be connected across the line- to-line voltages (208 volts) .As is the case with most arrangements ,ther e a re some drawbacks to the use ofgrounded systems , as follows :1 .T he fir st ground fault r esults in the immediate s hutdown of par t of the system .2 .T he re can be very high ground- fault cur rents on bolt ed-type faults . These largecur rents must flow over the equipment grounding circuit .Protective relayT he protective relay is defined as a device that causes an abrupt change in an elect ricalcontrol circuit when the measured quantity to which it responds changes in a prescribed manner .T he elect ricalcontrol circuit is usually the t rip circuit of a circuit breaker , and the measured quantity is the powe r circuit cur rent and/ or voltage as r epresented by the instrument t ransformer s .Protective r elays can be divided into two fundament al types elect romechanical relays and solid-state r elays . Elect romechanical r elays have been the standard for many year s and , in spite of the development of the newer solid-state units , ar e still widely used because of their proven r eliability . Solid- st ate units , since they have no moving par ts , have gr eater accuracy andfaste r reset times than elect romagnetic relays . However , solid-state relays have the drawback that they can initiat e false tripping of the circuit breaker s becau se they may imprope rly react to spurious t ransient voltage spikes . These transientvoltages , which may only last for a few microseconds , can be the r esult of disruption on the power system, suchas the switching of a powe r circuit . A solid-star e relay must have a filtering system thatblocks any chance of these transient conditions f rom t riggering any of its detection circuits . Electromagnetic relays , on the othe r hand , are inherently immune to t ransient disturbances . Solid-state r elays can offer the same operating cha racteristicsand , in fact , usually use the same type of hou sing and t erminalar r angements , so they ar e virtually inter changeable with electromagnetic units .。