【正文】 necessary for modern industrial control systems are in ,however,sampling operation,or discretization may be entirely fictitious and introduced only to simplify the analysis of control system which actually contains only continuous inputs and/or outputs are in ,however,sampling operation,or discretization,may be entirely fictitious and introduced only to simplify the analysis of control system which actually contain only continuous elements. In this lesson,we shall be concerned with discretetime systems which the signal representing the control efforts is piecewise constant and changes only st discrete points in there are several different types of sampling operation of practical importance,we shall list them as follows: (1)Periodic(conventional) sampling:In this case,the sampling instants are equally spaced,or tk=kt(k=1,2,3....) Multipleorder sampling:The pattern of the tk is repeated periodically,or tk+r tk=constant for all k. Multipleorderrate sampling:In this case,two concurrent sampling operations occur at tk=pT1 and qT2,where T1,T2 are contants and p ,q are integers. Random sampling:In this case,the sampling instants are random,or tk is a random variable. Here we shall treat only the case which the samplng is periodic. inclusion of digital puter in an otherwise analog system produces in digital form(usually as binary numbers) in part of the system then takes the form of a mixed digitalanalog introduction of a digital puter in a control system requires the use of digitaltodigital conversion of an analog signal to the corresponding digital signal(binary number)is an approximation because the analog signal can take an infinite number of values,whereas the variety of different numbers which can be formed by a finite number of values,whereas the variety of different numbers which can be formed by a finite set of digits is approximation process is called quantization. The process of quantizing (converting a signal in analog form to digital form)may be fulfilled by means of some specific range of input magnitudes is divided into a finite number of disjoint intervals hi which are not necessarily magnitudes fallinjg within each interval are equated to a single value within the single value is the digital approximation to the magnitudes of the analog input ,if x si the analog input,the digital output is given by y=Q(x ) Where Q is the quantizing function. The function x(t) is a discretetime operation of digital control systems involves quantization both in amplitude and in s hall next present the definitions of several terms. transducer is a device which converts an input signal into an output signal of another form.(The output general,depends on the past history of the input). Analog analog transducer is a device which converts an input signal into an ouput signals occur only at discrete instants of time (usually periodic),but the magnitudes of these signals may be any value within the physical limitations of the system. Sampleddata is a transducer in which the input and output signals occur only at discrete instants of time(usually periodic),but the magnitudes of the signal,as in the case of the analog transducer,are unquantized. 無(wú)錫職業(yè)技術(shù)學(xué)院 外文翻譯 5 Digital digital transducer is one in which the input signal is a continuous function of time and the output signal is a quantized signal which can assume only certain discrete levels. Analogtodigital digitaltoanalog transducer is one in which the input signal is a quantized signal and the output signal is a smoothed continuous function of time. Analog controllers and digital considering the types of controllers which are used in industrial control system,we may divide them into the following three categories: Analog controllers or puters:Analog controllers or puter represent the variables in the equations by continuous physical controllers can be designed which will satisfactorily serve as nondecision making controllers. Digital controllers or puters:These operate only on is an important function in digital controllers,and they are currently being used for the solution of problems Involving the optimal overall operation of industrial plants. Analogdigital controllers or puters:These are often called hybrid are binations of amalog controllers and digital of high performance controllers are of this type. Advantages of digital controllers over analog of the advantages of digital controllers over amalog controllers may be summarized as follows: (1)Digital controllers are capable of performing plex putations with constant accuracy at high puters can have almost any desired accuracy in putations at relatively little increase in the other hand,the cost of analog puters increases rapidly as the plexity of the putations increase if constant accuracy is to be maintained. (2)Digital controllers are extremely versatile. By merely issuing a new program,one can pletely change the operations being feature is particularly important if the control system is to receive operating information or instructions from some puting center,where economic analysis and optimization studies are being made. Because of the inability of conventional techniques to adequately handle