8 Control Sensors&1 INTRODUCTIONIn feedback control system - plant（设备）response is measured and compared with a reference input and the error is automatically employed in controlling the plant. It follows that a measurement system is an essential component in any feedback control system and forms a vital link between the plant and the controller. Measurements are needed in many engineering applications. The measurement process has to be automated, however, in control systems applications.A typical measurement system consists of one or more sensor-transducer units and associated signal-conditioning （信号调理）（and modification 改善）devices （see Fig.8.1）. Filtering to remove unwanted noise and amplification to strengthen a needed signal are considered signal conditioning. Analog-to-digital conversion （ADC）, digital-to-analog conversion （DAC）, modulation （调制），and demodulation （角军调）are signal modification （信号土曾强、信号改善）methods. Note that signal conditioning can be considered under the general heading of signal modification. Even though data recording is an integral function in a typical data acquisition （数据采集）system, it is not a crucial function in a feedback control system. For this reason, we shall not go into details of data recording devices in this book. In a multiple measurement environment, a multiplexer （多路转换器）could be employed prior to or following the signal-conditioning process, in order to pick one measured signal at a time from a bank of data channels for subsequent processing. In this manner, one unit of expensive processing hardware can be time-shared （时分复用）between several signals. Sensor-transducer （传感器-变送器）devices are predominantly （主要的）analog components that generate analog signals, even though direct digital transducers are becoming increasingly popular in digital control applications. When analog transducers are employed, analog-to-digital converters （ADCs）have to be used to convert analog signals into digital data for digital control. This signal modification process requires sampling of analog signals at discrete time points. Once a value is sampled, it is encoded into a digital representation such as straight binary code, a gray code, binary-code decimal （BCD） code or American Standard Code for Information Interchange （ASCII）. The changes in an analogsignal due to its transient （瞬态的）nature should not affect this process of ADC. To guarantee this, a sample-and-hold （取样保持）operation is required during each sampling period. For example, the value of an analog signals detected （sampled） in the beginning of each sampling period and is assumed constant （held） throughout the entire sampling period. This is, in fact, the zero-order hold （零阶保持器）operation. The operations of multiplexing （多路选择），sampling, and digitizing have to be properly synchronized under the control of an accurate timing device （a clock）for proper operation of the control system. This procedure is shown schematically in Fig.8.2.All devices that assist in the measurement procedure can be interpreted （角军释）as components of the measurement system. Selection of available components for a particular application or design of new components should rely heavily on performance （?性能）specification （规范）for these components. A great majority of instrument ratings provided by manufacturers are in the form of static parameters. In control applications, however, dynamic performance specifications are also very important.When two or more components are interconnected, the behavior of individual components in the overall system can deviate （偏离）significantly from their behavior when each component operates independently. Matching of components in a multicomponent system, particularly with respect to their impedance characteristics, should be done carefully in order to improve system performance and accuracy.8・ 2 SENSORS AND TRANSDUCERSThe output variable （or response）that is being measured is termed （）the measurand （被测量）・Examples are acceleration and velocity if a vehicle, temperature and pressure if a process plant （冶炼厂），and current through an electric circuit. A measuring device passes through two stages while measuring a signal. First, the measured is sensed. Then, the measured signal is transuded （or converted） into a form that is particularly suitable for transmitting, signal conditioning, processing, or driving a controller or actuato匚For this reason, output of the transducer stage is often an electrical signal. The measured is usually an analog signal, because it represents the output of a dynamic system in feed backcontrol applications. Transducer output is discrete in direct digital transducers. This facilitates the direct interface of a transducer with a digital processor.The sensor and transducer stages of a typical measuring device are represented schematically (图解地) in Fig.8.3 (a). As an example, consider the operation of a piezoelectric (压电的)accelerometer (see Fig.8.3 (b)). In this case, acceleration is the measured. It is first converted into an ineTtiaO 惯“性的)force through a mass element and is exerted on a piezoelectric crystal within which a strain (张力) (stress 应力)is generated. This is considered the sensing stage. The stress generates a charge inside the crystal, which appears as an electric signal at the output of the accelerometer. This stress-to-charge conversion or stress-to-voltage conversion can be interpreted as the transducer stage.Measuring deviceMeasured(Typically-^•TransmittableAnalogy A complex measuring device can have more than one sensing stage. More often, the measured goes though several transducer stages before it is available for control and actuating purposes. Sensor and transducer stages are functional stages, and sometimes it is not easy or even feasible to identify physical elements associated with them. Furthermore, this separation is not very important in using existing devices. Proper separation of sensor and transducer stages (physically as well as functionally) can be crucial, however, when designing new measuring instruments.In some books, signal-conditioning devices such as electronic amplifiers ate also classified as transducers. Since we are treating signal-conditioning and modification devices separately from measuring devices, this unified classification is avoided whenever possible in this book. Instead, the term transducer is used primarily in relation to measuring instruments. Following the common practice, however, the terms sensor and transducer will be used interchangeably to denote measuring instruments.8.3 ANALOG SENSORS FOR MOTION MEASUREMENTsignal)8.3.1IntroductionMeasurement of plant outputs is essential for feedback control. Output measurements are also useful in performance evaluation （性能评价）of a process. Furthermore, in learning systems （e.g., teach-repeat operation （示教操作）of robotic manipulators 机器人操作臂），measurements are made and stored in the computer for subsequent use in operating the system. Input measurements are needed in feed forward control. It is evident 明显的，therefore, that the measurement subsystem is an important part of a control system.The measurement subsystem in a control system contains sensors and transducers that detect misbrand and convert them into acceptable signals一-typically, voltages. These voltage signals are then appropriately modified using signal-conditioning hardware such as filters, amplifiers, demodulators, and analog-to-digital converters. Impedance matching （阻抗匹酉己）might be necessary to connect sensors and transducers to signal-conditioning hardware.Accuracy of sensors, transducers, and associated signal-conditioning devices is important in control system applications for two main reasons. The measurement system in a feedback control system is situated in the feedback path of the control system. Even though measurements are used to compensate for the poor performance in the open-loop system, any errors in measurements themselves will enter directly into the system and cannot be corrected if they are unknown. Furthermore, it can be shown that sensitivity of a control system to parameter changes in the measurement system is direct. This sensitivity cannot be reduced by increasing the loop gain, unlike in the case of sensitivity to the open-loop components. Accordingly, the case of sensitivity to the open-loop components. Accordingly, the design strategy （策略）for closed-loop （feedback）control is to make the measurements very accurate and to employ a suitable controller to reduce other types of errors.Most sensor-transducer devices used in feedback control applications ate analog components that generate analog output signals. This is the case even in real-time direct digital control systems. When analog transducers are used in digital control applications, however, some type of analog-to-digital conversion （ADC） is needed to obtain a digital representation of the measured signal. The resulting digital signal is subsequently conditioned and processed using digital means.In the sensor stage, the signal being measured is felt as the "response of the sensorelement.This is converted by the transducer into the transmitted (or measured) quantity. In this respect, the output of a measuring device can be interpreted as the "response of the transducer." In control system applications, this output is typically (and preferably) an electrical signal. Note that it is somewhat redundant (重复的) to consider electrical-to-electrical sensors-transducers as measuring devices, particularly in control system studies, because electrical signals need conditioning only before they are fed into a controller or to a drive system. In this sense, electrical-to-electrical transduction should be considered a "conditioning" task rather than a "measuring” function.8.3.2Motion TransducersBy motion, we mean the four kinematics variables:•Displacement (including position, distance, proximity 接近，and size 大小or gage位移/厚度)•Velocity•Acceleration•Jerk振动/冲击/加加速度Note that each variable is the time derivative of the preceding one. Motion measurement is extremely useful in controlling mechanical responses and interactions in dynamic systems. Numerous examples can be cited of situations in which motion measurements are used for control purposes. The rotating speed of a work piece and the feed rate of a tool are measured in controlling machining operations. Displacements and speeds (both angular and translatory) at joints (revolute and prismatic) of robotic manipulators or kinematics linkages are used in controlling manipulator trajectory. Angular speed is a crucial measurement that is used in the control of rotating machinery, such as turbines, pumps, compressors, motors, and generators in power-generating plants. Proximity sensors(to measure displacement) and accelerometers(to measure acceleration) are the two most common types of measuring devices used in machine protection systems for condition monitoring, fault detection （故障定位），diagnosis, and on-line （often real-time ）control of large and complex machinery. The accelerometer is often the only measuring device used in controlling dynamic test rigs （设备）.Displacement measurements are used for valve （阀）control in process applications. Plate thickness（or gage） is continuously monitored by the automatic gage control （AGC 自动厚度控制）system in steel rolling mills （车L钢厂）・以上716完成A one-to-one relationship may not always exist between a measuring device and ameasured variable. For example, although strain gages （张力应力计,应变计）are devices that measure strains（and, hence, stresses and forces）, they can be adapted to measure displacements by using a suitable front-end auxiliary sensor element, such as a cantilever （悬臂）（or spring）・Furthermore, the same measuring device may be used to measure different variables through appropriate data interpretation （信号解释）techniques. For example, piezoelectric accelerometers with built-in microelectronic integrated circuitry are marketed as piezoelectric velocity transducers. Revolver signals that provide angular displacements are differentiated to get angular velocities. Pulse-generating （or digital）transducers, such as optical encoders and digital tachometers （转速计），can serve as both displacement transducers and velocity transducers, depending on whether the absolute number of pulses generated is counted or the pulse rate is measured. Note that pulse rate can be measured either by counting the number of pulses during a unit interval of time or by gating a high-frequency clock signal through the pulse width. Furthermore, in principle, any force sensor can be used as an acceleration sensor, velocity sensor, or displacement sensor, depending on whether an inertia element （converting acceleration into force）, a damping （阻尼）element （converting velocity into force）, or a spring element （converting displacement into force）, respectively, is used as the front-end （前端）auxiliary senso 匚We might question the need for separate transducers to measure the four kinematics variables—displacement, velocity, acceleration, and jerk■一because any one variable is related to any other through simple integration or differentiation. It should be possible, in theory, to measure only one of these four variables and use either analog processing (through analog circuit hardware) or digital processing (through a dedicated processor) to obtain any of the remaining motion variables. The feasibility (可彳亍'性) of this approach is highly limited, however, and it depends crucially on several factors, including the following:(1)The nature of the measured signal (e.g., steady, highly transient, periodic, narrow-band 窄带,broad-band 宽带).(2)The required frequency content of the processed signal (or the frequency range of interest).(3)The signal・to・noise ratio (SNR) of the measurement.(4)Available processing capabilities (e.g., analog or digital processing, limitations of the digital processor, and interface, such as the speed of processing, sampling rate, and buffer size).(5)Controller requirements and the nature of the plant (e.g., time constants, delays, hardware limitations).(6)Required accuracy in the end objective (on which processing requirements and hardware costs will depend).For instance, differentiation of a signal (in the time domain) is often unacceptable for noisy and high-frequency narrow-band signals. In any event, costly signal-conditioning hardware might be needed for preprocessing prior to differentiating a signal. As a rue of thumb (木艮据经验)，in low-frequency applications (on the order of 1 Hz), displacement measurements generally provide good accuracies. In intermediate-frequency (中频的) applications(less than 1 kHz), velocity measurement is usually favored 有利的.In measuring high-frequency motions with high noise levels, acceleration measurement is preferred. Jerk is particularly useful in ground transit (ride quality 适用于乘坐的品质)，manufacturing (forging 锻造，rolling 车L制，and similar impact-type 冲击型operations), and shock isolation (delicate 精密and sensitive 敏感equipment) applications.Our discussion of motion transducers will be limited mainly to the following types of devices:Potentiometers (电位计)(resistively coupled (电耦) devices)Variable-inductance 变电感transducers (electromagnetically coupled devices)Eddy current transducersVariable-capacitance 变容transducersPiezoelectric transducers8.4 DIGITAL TRANSDUCERS8.4.1 IntroductionAny transducer that presents information as discrete samples and that does not introduce a quantization error when the reading is represented in the digital form may be classified as a digital transducer. A digital processor plays the role of controller in a digital control system. This facilitates (使利于，促进) complex processing of measured signals and other known quantities in order to obtain control signals for the actuators (执行器、激励器)that drive the plant (机械设备)of the control system. If the measured signals are in analog form, an analog-to-digital conversion (ADC) stage is necessary prior to digital processing. There are several other shortcomings of analog signals in comparison to digital measuring devices for digital control systems.Digital measuring devices (or digital transducers, as they are commonly known) generate discrete output signals such as pulse trains (脉冲序歹ll) or encoded data that can be directly read by a control processor. Nevertheless, the sensor stage of digital measuring devices is usually quite similar to that of their analog counterparts. There are digital measuring devices that incorporate microprocessors to perform numerical manipulations and conditioning locally and provide output signals in either digital or analog form. These measuring systems are particularly useful when the required variable is not directly measurable but could be computed using one or more measured outputs (e.g., power =force*speed). Although a microprocessor is an integral part of the measuring device in this case, it performs not a measuring task but, rather, a condition task. For our purposes, we shall consider the two tasks separately.The objective of this section is to study the operation and utilization of several types of direct digital transducers. Our discussion will be limited to motion transducer. Note, however, that by using a suitable auxiliary front-end sensor, other measured■一such as force, torque, and pressure 一-may be converted into a motion and subsequently measured using a motion transducer. For example, altitude (高度)(OT pressure) measurements in aircraft and aerospace applications are made using a pressure sensing front end, such as a bellows (膜盒)or diaphragm (膜片)device, in conjunction with an optical encoder to measure the resulting displacement. Motion, as manifested in physical systems, is typically continuous in time. Therefore, we cannot speak of digital motion sensors in general. Actually, it is the transducerstage that generates the discrete output signal in a digital motion measuring device. Commercially available direct digital transducers are not as numerous as analog sensors, but what is available has found extensive application.When the output of a digital transducer is a pulse signal, a counter is used either to count the pulses or to count clock cycles over one pulse duration. The count is first represented as a digital word according to some code; then it is read by a data acquisition and control computer. If, on the in a coded form (e.g., binary, binary-coded decimal, ASCII), it can be directly read by a computer. In the latter case, the coded signal is normally generated by a parallel set of pulse signals; the word depends on the pattern of the generated pulses.8.4.2Shaft Encoders (轴角编码器)Any transducer that generates a coded reading of a measurement can be termed an encoder. Shaft encoders are digital transducers that are used for measuring angular displacements and angular velocities. Application of these devices include motion measurement in performance monitoring and control of robotic manipulators, machine tools, digital tape-transport mechanisms, servo plotters and printers, satellite mirror positioning systems, and rotating machinery such as motors, pumps, compressors, turbines, and generators. High resolution (depending on the word size of the encoder) , high accuracy (particularly due to noise immunity of digital signals and superior construction), and relative ease of adaptation in digital control system cost and improvement of system reliability, are some of the relative advantages of digital transducer over their analog counterparts.Shaft encoders can be classified into two categories, depending on the nature and themethod of interpretation (译码) of the transducer output: (1) incremental encoders(增量编码器)and ⑵ absolute encoders(绝对编码器).The output of an incrementalencoder is a pulse signal that is generated when the transducer disk rotates as a resultof the motion that is being measured. By counting the pulses or by timing the pulsewidth using a clock signal, both angular displacement and angular velocity can bedetermined. Displacement, however, is obtained with respect to 关于some referencepoint on the disk, as indicated by a reference pulse (index pulse) generated at thatlocation on the disk. The index pulse count determines the number of fullrevolutions.★An absolute encoder (or whole-word encoder 整字编码器)has many of pulse tracks 轨道on its transducer disk. When the disk of an absolute encoder rotates, severalpulse trains 脉7中串 -- equal in number to the tracks on the disk ------------- a re generated simultaneously. At a given instant, the magnitude of each pulse signal will have one of two signal levels (i.e., a binary state), as determined by a level detector. This signal level corresponds to a binary digit (0 or 1). Hence, the set of pulse trains gives an encoded binary number at any instant. The pulse windows (孑L ) on the tracks can be organized into some pattern (code) so that each of these binary numbers corresponds to the angular position of the encoder disk at the time when the particular binary number is detected. Furthermore, pulse voltage can be made compatible with some form of digital logic (e.g., transistor-to-transistor logic, or TTL). Consequently, the direct digital readout of an angular position is possible, thereby expediting 力口速，促进digital data acquisition and processing. Absolute encoders are commonly used to measure fractions of a revolution. However, complete revolution can be measured using an additional track that generates an index pulse, as in the case of incremental encoder.The same signal generation (and pick・off 拾取, 截取)mechanism may be used in both types of transducers. Four techniques of transducer signal generation can be identified:(1)Optical (photo sensor 光学传感器)method.(2)Sliding contact 滑动接触(electrical conducting 电导)method.(3)Magnetic saturation 饱和(reluctance 磁阻)method.(4)Proximity sensor method.For a given type of encoder (incremental or absolute), the method of signal interpretation is identical for all four types of signal generation.In many control applications, encoders are built into the plant itself, rather than being externally fitted onto a rotation shaft. For instance, in a robot arm, the encoder might be an integral part of the joint 关节motor and may be located within its housing 夕卜壳/夕卜罩.This reduces coupling errors (耦合误差)(e.g., errors due to backlash I'可隙，shaft flexibility 弹 '性/柔度，and resonances i皆振added by the transducer and fixtures 夹具)，installation errors (e.g., eccentricity 偏心、率)，and overall cost.8.4.3Digital Tachometers★Since shaft encoders are also used for measuring angular velocities, they can be considered tachometers. In classic terminology, a digital tachometer is a device that employs a toothed wheel ( ) to measure angular velocities. A schematic diagram of one such device is shown in Fig.8.4. This is a magnetic induction tachometer of the variable-reluctance (变磁阻) type. The teeth on the wheel are made of ferromagnetic material. The two magnetic induction (and variable) proximity probes (探针) are placed facing the teeth radially 径向的，a quarter-pitch 四分之一齿距apart. When the toothed wheel rotates, the two probes generate output signals that are 90°out of 足巨离phase. One signal leads the other in one direction of rotation and lags 滞后the other in the opposite direction of rotation. In this manner, directional readings are obtained. The speed is the pulse width, as in the case of an incremental encoder以上718完成The advantages of these digital (pulse) tachometers over optical encoders include simplicity, robustness, and low cost. The disadvantages include poor resolution (determined by the number of teeth, the speed of rotation, and the word size used for data transmission), and mechanical errors due to loading, hysteriscs (迟滞)，and manufacturing irregularities.8.4. 4 Hall Effect SensorsConsider a semiconductor element subject to a DC voltage Uref. If a magnetic field is applied perpendicular to the direction of this voltage, a voltage Uo will be generated in the third orthogonal[^垂直的direction within the semiconductor element. This is known as the Hall Effect 霍尔效应(observed by E. H. Hall in 1897). A schematic representation of a Hall Effect sensor is shown in Fig. 8. 5.A Hall Effect sensor may be used for motion sensing in many ways-fbr example, as an analog proximity sensor, a digital limit switch, or a digital shaft encode匚Since the output voltage Uo increases as the distance from the magnetic source to the semiconductor element decreases, the output signal Uo can be used as a measure of proximity. Alternatively, a certain threshold level of output voltage Uo can be used to activate a digital switch or to create a digital output, hence forming a digital limit switch.A more practical arrangement would be to have the semiconductor fixed relative to oneanother in a single package. By moving a ferromagnetic member into the air gap between the magnetic source and the semiconductor element, the flux linkage can be altered. This changes Uo. This arrangement is suitable both as an analog proximity sensor and as a limit switch.Furthermore, if a toothed ferromagnetic wheel is used to change Uo, we have a shaft encoder ora digital tachometer (see Fig. 8. 6).The longitudinal 纵向的arrangement of a proximity sensor, in which the moving element approaches head-on toward the sensor, is not suitable when there is a danger or overshooting the target, since it will damage the sensor. A more desirable configuration is the lateral 狈面的arrangement, in which the moving member slides by the sensing face of the sensor. The sensitivity will be lower, however, with this lateral arrangement.以上女生完成The relationship between the output voltage Uo and the distance x of a Hall Effect sensor measured from the moving member is nonlinear. Linear Hall effect sensors use calibration 校正to liberalize 线性化their output.以上女生与719都要做Vocabulary1. absolute encoder 绝对编码器2. accelerometer [®k.selo'romital n. 加速度测量仪3. actuator「®ktjueito]n. 执行机构4. analog-to-digital Conversion, ADC 模数转换器5. angular ['®Qgjula] a. 角的6. auxiliary [□：g'ziljori] a. 辅助的7. as a rule of thumb 根据经验8. bellows 「belouz]n. 膜盒9. binary-coded decimal, BCDBCD码，二-十进制编码10. calibration [.loli'breifon]n. 标准；标定；刻度11. cantilever [!<®ntili：vo|n. 悬臂12. closed-loop n. 闭环13. crystal fkristl] a.&n.晶体的；水晶，晶体14. demodulation「di：t ID3djufleifon]n. 解调15. derivative [dl'rivativl n. 导数16. diaphragm ['daiafraem]n. 膜片17. differentiation [»dtf©»renJi'ei fon]n. 微分18. digital- to-analog conversion, DAC 数模转换器19. discrete [disl<ri：t] a. 离散的20. displacement [dis'pleismont]n・位移21. eddy「edi|n・涡流22. encoder [inloudo]n. 编码器23. error「er® n.误差，偏差24. expedite fekspidait)V.加速25. feedback ['fi：dbaBk]n. 反馈26. feedforward [ifizd'fo^ad]n. 前馈27. forging ['fo：d3ir)|n. 锻造2& hysteresis [ histo'rizsisl n. 磁滞29. immunity [i'mju：niti]n. 抗扰性30. impedance [im'pi：dans]n. 抗阻31. increment encoder 增量编码器32. inertia ja|n. 惯性33. integration [.inti'greijon]n. 积分34. interface rinte(：)Jeis|n. 接口35. jerk电狀】n. 振动36. kinematic [JcsinTnmtlc] a. 运动的，运动学的37. longitudinal [I3nd3rtju：dinl] a. 经度的；纵向的38. manipulations [Hiejlipjllleljen]n. 操作；控制；处理39. manipulator (me'nipjuleit©]n・机械手，操作器40. measurand「rnejorond]n. 被测量，被测对象41. modulation [.modju'leijenl n. 调制42. multiplexer rmAlti.plekse]n. 多路转换器43. offset fset]n. 偏心44. open-loop n. 开环45. orthogonal [ofOaganll a. 垂直的，正交的。
