模拟与数字转换器前面我们已经提到，人们在模拟转换器、信号调节器和A/D转换器等的使用上已经积累了大量的经验。因此，目前大部分的系统自然都采用这些技术。然而，还有很大一部分测量方法实质是数字的，在个别的测量仪中使用这些方法时，需要用到一些积分电路，如频率计数和计时电路等来提供指示输出。另外，如果把这种转换器和电脑相连的话，就可以省去一些器材；因为很多有积分电路执行的工作可以由计算机程序代为执行。柯林斯把在控制和测量系统中处理的信号分为以下几类：（1）模拟式。尽管系统的被测数最初通过传感器得到的是模拟信号，然后通过设计或采用原有的方法将模拟形式的信号转换成电模拟信号。（2）数字码式。产生的信号是并行的数字信号，每一位的基数权重由预先编定的号码系统决定。在本书中这些仪器称作直接数字转换器。（3）数字式。其中的函数是指测量参数时用到的量度标准，如对重复信号取平均值。这些仪器在后来称为频域转换器。特别地，一些模拟转换器适合用一些特别的技术来把模拟量转换成数字输出。其中最通用的方法是同步法和相似仪器的方法，即产生载波频率的调制输出的方法。在用作普通的模拟量输出仪器时，输出量必须经过解调。解调后输出的是直流信号，支流信号的大小和方向描述了转换器运动元件的偏移。虽然使用传统的A/D转换技术可以用来产生数字信号，在提供高精度时采用这些新技术将同步输出直接变为数字输出，比用A/D转换方法更快。直接数字转换器实际上用得很少，因为在自然现象中很少有那种由温度变化、压力变化等因素作用而产生的可测量的离散的变化量。在普通的仪器系统中使用直接数字转换器有如下优点（即使在完成安装时不使用计算机）：（1）容易产生、处理和存储信号，如打控带、磁带等；（2）高精度和高分辨率的需要；（3）高介数字信号对外部噪声的抗干扰性；（4）在简化数据描述时的人机工程学优势（例如：数字读出器能避免读刻度或图表时的判度错误）。在直接数字转换器中最能起作用的发展是轴编码器。轴编码器在机***和飞行系统中被广泛应用。利用这些设备能达到很高的精度和分辨率，而且这些设备能进行激动连接，给出任何可测量物理偏移的直接数字输出。这类系统通常的缺点是仪器的惯性及编码器***了相应的速度，因而也***了操作频率。频域转换器在线系统（测量量较少时）有着特殊的地位。因为计算机能担当A/D转换系统的部分工作，能用它自己的寄存器和时钟来计算脉冲和测量脉冲宽度。在这种系统的设计中必须考虑到计算机存取和处理转换器输出所需的时间。自动控制的应用虽然自动控制应用范围实际上是无限的，但是我们的讨论仅限于现代工业中常见的几个例子。伺服机构虽然伺服机构本身并不是一种控制的应用，但是这种装置在自动控制中却是常用的。伺服机构，或简单称“伺服”，是一种闭环控制系统，其中的被控变量是机械位置或机械运动。该机构的设计使得输出能迅速而精确地响应输入信号的变化。因此，我们能把伺服机构现象为一种随动装置。另一种控制输出变化率或输出速度的伺服机构称为速率或速度伺服机构。过程控制过程控制是用来表示制造过程中多变量控制的一个术语。化工厂、炼油厂、食品加工厂、鼓风炉、轧钢机都是自动控制用于生产过程的里子。过程控制就是把有关诸如温度、压力、流量、液位、黏度、密度、成分等这样一些过程变量控制为预期值。发电电力工业首先关系到能量的转换与分配。发电量可能超过几千万千瓦的现代化大型电厂需要复杂的控制系统来表明许多变量的相互关系，并提供最佳的发电量。发电厂的控制一般认为是一种过程控制的应用，而且通常有多打100个操纵变量受计算机控制。自动控制已广泛地用于电力分配。电力系统通常由几个发电厂组成。当负载波动时，电力的生产和传输要受到控制，使该系统达到运行的最低要求。此外，大多数的大型电力系统都是相互联系的，而且两系统之间的电力流动也受到控制。数字控制有许多加工工序，如镗***、钻***、铣削和***都必须以很高的精度重复进行。数字控制是一个系统，该系统使用的是称为程序的预定指令来控制一系列运行。完成这些预期工序的指令被编程代码，并且存储在如穿孔纸带、磁带或穿孔卡片等某个介质上。这些指令通常以数字形式存储，故称为数字控制。指令辨认要用工具、加工方法（如切削速度）及工具运动的轨迹（位置、方向、速度等）等参数。运输为了向现代化城市的各个地区提供大量的运输系统，需要大型、复杂的控制系统。目前正在进行的几条自动运输系统中有每隔几分钟的高速火车。要保持稳定的火车流量及提供舒适的加速和停站时的制动，就需要自动控制。飞机的飞行控制是在运输领域的另一项重要应用。由于系统参数的范围的广泛以及控制之间的相互影响，飞行控制已被证明为最复杂的控制应用之一。飞机控制系统实质上常常是自适应的-，即其操纵本身要适应于周围环境。例如一架飞机的性能在低空和高空可能是根本不同的，所以控制系统就必须作为飞行高度的函数进行修正。船舶转向和颠簸稳定控制与飞行控制相似，但是一般需要更大的功率和较低的响应速度。张琦，杨承先现代机电专业英语M北京：清华大学出版社，20051/Analog and Digital TransducersAs mentioned previously/ considerable experience has been accumulated with analog transducers signal conditioning/ signal/ conditioning/ A/D converters etc// and it is natural that the majority of current systems tend to use these techniques/ However/ there are a number of measuring techniques that are essentially digit in nature/ and which when used as separate measuring instruments require some integral digital circuitry/ such as frequency counters and timing circuits/ to provide an indicator output/ This type of transducer/ if coupled to a computer does not necessarily require the same amount of equipment since much of the processing done by the integral circuitry could be programmed and performed and performed by the computer/Collins classifies the signals handled in control and instrumentation systems as follows/(1) Analog/ in which the parameter of the system to be measured although initially derived in an analog form by a sensor is converted to an electrical analog/ either by design or inherent in the methods adopted/(2) Coded digital/ in which a parallel digital signal is generated/ each bit radix weighted according to some predetermined code/ These are referred to in this book as direct digital transducers/(3) Digital/ in which a function/ such as mean rate of a repetitive signal/ is a measure of the parameter being measured/ These are subsequently referred to as frequency domain transducers/Some analog transducers are particularly suited to conversion to digital outputs using special techniques/ The most popular of these are synchros/ and similar devices/ which produce a modulated output of a carrier frequency/ For ordinary analog use/ this output has to be demodulated to provide a signal whose magnitude and sign represent any conventional A/D technique to produce a digital output/ there are techniques by which the synchro output can be converted directly to a digital output while providing a high accuracy and resolution/ and at a faster rate than is possible in the A/D converter method/(1) The ease of generating/ manipulating and storing digital signals/ as punched tape/ magnetic tape etc//(2) The need for high measurement accuracy and discrimination/(3) The relative immunity of a high level digital signal to external disturbances(noise)/(4) Ergonomic advantages in simplified data presentation(e/g/ digital readout avoids interpretation errors in reading scales or graphs)/The most active development in direct digital transducer has been in shaft encoders/ which are used extensively in machine tools and aircraft systems/ High resolution and accuracies can be obtained/ and these devices may be mechanically coupled to provide a direct digital output of any parameter which gives rise to a measurable physical displacement/ The usual disadvantage of these systems is that the inertia of the instrument and encoder often limit the speed of response and therefore the operating frequencies/Frequency domain transducers have a special part to play in online systems with only few variables to be measured/ since the computer can act as part of an A/D conversion system and use its own registers and clock for counting pulses or measuring pulse width/ Access and process the transducer output/ In designing such systems/ consideration must be given to the computer time required to access and process the transducer output/Application of Automatic ControlAlthough the scope of automatic control application/ we will limit this discussion to examples which are commonplace in modern industry/ServomechanismsAlthough a servomechanism is not a control application/ this device is commonplace in automatic control/ A servomechanism/ or servo for short/ is a closed-loop control system in which the controlled variable is mechanical position or motion/ It is designed so that the output will quickly and precisely respond to a change in the input command/ Thus we may think of a servomechanism as a following device/Another form of servomechanism in which the rate of change or velocity of the output is controlled is known as a rate or velocity servomechanism/Process ControlProcess control is a term applied to the control of variables in a manufacturing process/ Chemical plants/ oil refineries/ food processing plants/ blast furnaces/ and steel mill are examples of production processes to which automatic control is applied/ Process control is concerned with maintaining at a desired value such process variables as temperature/ pressure/ flow rate/ liquid level/ viscosity/ density/ and composition/Much current work in process control involves extending the use of the digital computer to provide direct digital control (DDC) of the process variables/ In direct digital control the computer calculates the values of the manipulated variables/ The decisions of the computer are applied to digital actuators in the process/ Since the computer duplicates the analog controller action/ these conventional controllers are no longer needed/Power GenerationThe electric power industry is primarily concerned with energy conversion and distribution/ Large modern power plants which may exceed several hundred megawatts of generation require complex control systems to account for the interrelationship of the many variables and provide optimum power production/ Control of power generation may be as 100 manipulated variables under computer control/Automatic control has also been extensively applied to the distribution of electric power/ Power systems ate commonly made up of a number of generating plants/ As load requirements fluctuate/ the generation and transmission of power is controlled to achieve minimum cost of system operation/ In addition/ most large power systems are interconnected with each other/ and the flow of power between systems is controlled/Numerical ControlThere are rainy manufacturing operations such as boring/ drilling/ milling/ and welding which must be performed with high precision on a repetitive basis/ Numerical control (NC) is a system that uses predetermined instructions called a program to control a sequence of such operations/ The instructions to accomplish a desired operation ate coded and stored on some medium such as punched paper tape/ magnetic tape/ or punched cards/ These instructions ate usually stored in the form of numbers-0bence the name numerical control/ The instructions identify what tool is to be used/ in what way (e/g/ cutting speed)/ and the path of the tool movement (position/ direction/ velocity/ etc/)/TransportationTo provide mass transportation systems for modern urban areas/ large/ c
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