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内蒙古工业大学本科设计说明书

Process-Control Systems

A process-control system is often a regulator．However，a complicated

process-control system may possibly contain several de vides which could be defined as servo-mechanisms．we shall consider systems with several inputs ,some know as controls because they may be manipulated and others called external disturbances ,which are quite unpredictable.

In such situations, one aspect of the control problem is to determine how the controls should be manipulated so as to counteract the effects of the external disturbances on the state of the system. one possible approach to the solution of this problem is to use a continuous measurement of the disturbances , and from this and the known system equations to determine what the control inputs should be as functions of time to give appropriate control of the system state.

An example of a typical process control system will now be considered．This system include：measured value (output ?o)，set value (input ?i)，deviation (minus error

????o??i)，transmitter (error detector )，control pneumatic motor control valve (output element)，process (load)．

This is quite different from industrial process control ，the other large area of applica- tion of automatic control systems，which includes such tasks as the control of flow rates in pipelines，temperatures in furnaces and chemical reactors，concentrations of reagents，and levels in tanks．These are almost invariably carried out with standard off-the-shelf control- lers，either electronic or pneumatic，which have provision for manual or remote entry of the set point yr，and often include a recorder which produces a continuous plot of y．In its simplest form，such a controller provides only proportional control action ：a power am- plifier in the controller produces a signal proportional to the error which can then be used to drive an actuator such as a flow regulating valve．More elaborate controllers include compensating networks which give proportional-plus-derivative (PD)，proportional-plus- integral (PI)，or proportional-plus-integral-plus-derivative (PID) control action．The lat- ter is also known as a three-term controller ，and is descried by

tdeu(t)?kpe(t)?kd?ki?e(?)d?

0dt 1

内蒙古工业大学本科设计说明书

where u is the controller output and e?yr?y is the error signal．thus the ideal transfer function of controller is

hc(s)?kp?kds?ki sthis，of course，cannot be achieved perfectly in practice；we have already commented on the difficulty of differentiating signals．In electronic controllers，the component proporti- onal to the derivative of the errors usually produced approximately by a lead network；in pneumatic devices，both integral and derivative components are produce by passive (lag and lead) networks ．A block diagram of a typical pneumatic PID controller is shown in Fig．1．

Fig.1 Block Diagram of a Pneumatic PID Controller

Fig．2 is a general representation of an open-loop control system．The input or con- trol u(t) is selected based on the goals for the system and all available a priori knowledge about the system．The input is in no way influenced by the output of the system，represen- nted by y(t)．If unexpected disturbances act upon an open-loop system，or if its behavior is not completely understood，then the output will not behave precisely as expected．

Fig．2 An Open-Loop Control System

Another general class of control systems is the closed-loop or feedback control system，as illustrated in Fig．3．In the close-loop system，the control u(t) is modified in some way by information about the behavior of the system output．A feedback system is

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内蒙古工业大学本科设计说明书

often better able to cope with unexpected disturbances and uncertainties about the system's dynamic behavior．However，it need not be true that close-loop control is always superior to open-loop control．When the measured output has errors which are sufficiently large，and when unexpected disturbances are relatively unimportant，close-loop control have a performance which is inferior to open-loop control．

Fig．3 An Closed-Loop Control System

In such situations ，one aspect of the control problem is to determine how the control should be manipulated so as to counteract the effects of the external disturbances on the state of the system．One possible approach to the solution of this problem is to use a continuous measurement of the disturbances，and from this and the known system equations to determine what the control inputs should be as functions of time to give appropriate control of the system state．

A different approach is to construct a feedback system，that is，rather than measure the disturbances directly and then compute their effects on the system from the model or system equations，we compare direct and continuous measurements of the accessible syst-

em states with signals representing their \，and use this signal to produce inputs to the system which will drive the error as close to zero as possible．Diagrams representing these two basic strategies of control are shown in Fig．4．

Consider a system which fails to meet a specification calling for zero steady-state error in response to a reference or disturbance input．There is then no choice but to insert the required number of integrators into the loop．

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内蒙古工业大学本科设计说明书

Fig．4 Schematic Representation

(a)open-loop；(b)closed-loop control strategies

There are thus two methods for improving the static accuracy of a sys tem when loop gain adjustment by itself will not work：inserting pure integrators into the loop or inserting a passive lag network．Of these，the former is more effective，at the cost of amore severe degradation of the transient response，so that a further step of compensation is usually nec- cessary to improve transients．There are several ways to implement time integration of a variable by analog (as distinct from digital) devices．Probably the most common integrati- ng device is the basic integrating circuit of the electronic analog computer，consisting of an operational amplifier with an input resistor and a feedback capacitor ．With the development of integrated circuits，such devices have become very compact，and quite cheap compared with systems employing mechanical displacements．In many industrial control systems signals are realized electrically because of the ease of transmitting， comparing，amplifying，and summing voltages，so that electronic integrators can be readily implemented．Nevertheless，all integrators are \． sometimes the performance specifications permit a nonzero steady-state error，so that additional integrators can be avoided，but the loop gain cannot be given a sufficiently high value without an unacceptable deterioration in transient performance．

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内蒙古工业大学本科设计说明书

过程控制系统

过程控制系统通常是一个调节器。然而一个复杂的过程控制系统可能包含好几个被称为伺服机构的装置。我们将讨论的系统有几个输入,其中某些输入称为控制量，,因为这些量是可以人为地控制的，而另外一些输入称为外部扰动,它们是非常难以预测.

在这些情况下、控制问题的一个方面在于确定如何处理控制量以便抵制外部干扰对系统状态的影响,解决这种问题的一个可能的办法是不断的测量扰动量，根据这个测量值和已知系统的方程确定的应有的控制输入量,以便对系统状态进行合适的控制。.

现在考虑一个典型过程控制系统的例子。该系统包括：测量值-输出θo 、设定值-输入θi、偏差-负误差(-θ=θo-θi)、变送器-误差检测器、控制器、气压电机控制阀-输出元件、过程-负载。

这些任务几乎总是用标准的和现成的控制器（调节器）来实现的，控制器可能是电子的或是气动的，它们有设定点的yr手动或遥控的输入装置，通常包括一个能连续绘制图形y的纪录仪。最简单的形式中，这样一个控制器只有比例控制作用：控制器中的功率放大器产生一个正比于误差的信号，用它来驱动例如流量调节阀门的执行元件。更精密的控制器包括能给出比例加微分(PD)，比例加积分(PI)，或者比例加积分加微分(PID)控制作用的校正网络。PID也叫做“三项”控制器，可用式描述：

tdeu(t)?kpe(t)?kd?ki?e(?)d?

0dt其中u是控制器的输出，e=yr-y是误差信号，其理想传函为

khc(s)?kp?kds?i

s当然，实际上这是不能完全得到的，我们已解释过对信号进行微分的困难性。在电子控制器中，正比于误差信号的微分项通常由超前网络近似产生；在气动装置中，积分和微分都是由无源的滞后与超前网络产生。图1是一个典型的气动PID控制器的方框图。

当然，实际上这是不能完全得到的，我们已解释过对信号进行微分的困难性。在电子控制器中，正比于误差信号的微分项通常由超前网络近似产生；在气动装置中，积分和微分都是由无源的滞后与超前网络产生。图1是一个典型的气动PID控制器的方框图。

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