专业英语第8单元作业.doc

8 Control Sensors

8.1 INTRODUCTION
In 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（解调） are signal modification（信号增强、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 analog signal 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（解释） 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 TRANSDUCERS
The 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 actuator. 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 back control 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 inertia（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 device
Measured
Transducer
Signal sensor
(Typically → → →Transmittable
Analogy signal)

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 MEASUREMENT
8.3.1 Introduction
Measurement 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---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（阻抗匹配） 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 sensor element.” 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.
8.3.2 Motion Transducers
By 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（轧钢厂）.
以上716完成
A one-to-one relationship may not always exist between a measuring device and a measured 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 sensor.
We might question the need for separate transducers to measure the four kinematics variables---displacement, velocity, acceleration, and jerk---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（可行性） 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（根据经验）, 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 usef