毕业设计 英文资料

Compensation of Control System

1．Multiple Constrains in Design

The performance of a feedback control system is of primary importance.We have found that a suitable control system should have some of the following properties.

1)It should be stable and present acceptable response to input

command,i.e,the controlled variable should follow the changes in the input at a suitable speed without unduly large oscillations or overshoots.

2)It should operate with as little error as possible.

3)It should be able to mitigate the effect of undesirable

disturbances.

A feedback control system that provided an optimum perfprmance without any necessary adjustments is rare ually it is necessary to compromise among the many conflicting and demanding specifications and to adjust the system parameters to provide a suitable and acceptable performance when it is not possible to obtain all the desired optimum specifications.

The preceding chapters have showen that it is often possible to adjust the system parameters in order to provide the desired system response.When the achievement of a simple performance requirement may be met by selecting a particular value of K, the process is called gain compensation.However,we often find that it is not sufficient to adjust a system parameter and thus obtain the desired performance.Rather we are required to reconsider the structure of the system and redesign the system in order to obtain a suitable one.That is ,we must examine scheme of the system and obtain a new design that results in a suitable system.Thus the design of a control system is concerned with the arrangement of the system

structure and the selection of suitable components and parameters.When we are not able to relax several perfprmance requirements,we must alter the system in some way.The alteration or adjustment of a control system in oreder to provider a suitable performance is called compensation.

In redesigning a control system to alter the system response,an additional component or device is inserted within the structure of the feedback system to compensate for the performance deficiency.The compensating device may be electric,mechanical,hydraulic,pneumatic,or some other type of devices or networks and is often called a monly an electric network serves as a compensator in many control systems.

Quite often,in practice,the best and simplest way to improve the performance of a control system is to alter,if possible,the plant itself.That is,if the system designer is able to specify and alter the design of the plant,then the performance of the system may be readily improved.For example,to improve the transient behavior of a servomechanism position controller,we often can choose a better motor for the system.Thus a control system designer should recognize that an alteration of the plant maybe result in an improved system.However,often the plant is unalterable or has been altered as much as possible and still result in unsatisfactory performance.Then the addition of compensator becomes useful for improving the performance of the system.

2．Types of Compensation

The compensator is placed in a suitable location within the system,and can be done in several ways .An additional component may be interested in the forward path.This is called the cascade or serial compensation.The transfer function of the compensator is designated as Gc(s),whereas that of the original plant(or process)is denoted by Gp(s).Alternatively,the compensator may be placed in the feedback