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Design of Control Systems

Performance Specifications

Control systems are designed to perform specific task. The requirements imposed on the control systems generally relate to accuracy, relative stability, and response characteristics. For trivial design problems the performance specifications may be given as precise numerical values. In other cases, the specifications may be given partially in terms of precise numerical values and partially in terms of qualitive statements. In the latter case the performanece spesifications may be modified in the process of design, because some specifications may never be satisfied due to conflicting requirements.

To state the performance specifications is the most important part of the dsign of control system. For example, if the accuracy at steady state is of prime importance, then there is no need to require rigid performance on the transient response, because this may lead to implementation of very expensive components.

System Compensation

Increasin the gain of the system will improve the steady state behaviour, but will result in poor stability or even may make the system unstable. This is the first step for adjusting the system for satisfactory performance, but adjusting the gain might not provide suffcient ferformance. In most of the cases it is necessary to redesign the system by addinig components or devices, so that the system will behave as desired. Such redesign is called compensation and the added device is called compensator. The compensator compensate the deficit performance of the original system.

Compensators

Widely employed compensators are the lead compensators, lag compensators, lag-lead compensators, PID compensators, velocity feedback compensators and many others. Most of these compensators may be electronic devices, or RLC networks and amplifiers. They may be realized by pneumatic or hydraulic devices, too. The selection of realization of the compensator depens mainly on the nature of the controlled plant. If it is dangerous to use electrically build compensator (e.g. the controlled plant involves flammable fluids) then pneumatic or hydraulic compensator should be choosen. If, however, no hazards exist then the cheaper electronic compensator is often used.

Design Procedures

Based on a priori knowledge about the physical characteristics of the plant to be controlled and, in many cases, relying on the trial-and-error approach, the designer sets up a mathematical model of the control system and adjusts the parameters of the compensator. The most time consuming part of the design is the checking the system perfoemance by analysis with each adjustement of the parameters.

The next step is to build a prototype of the system and test the performance of the closed loop system. Because, in most of the cases the model idealized and linearized, the actual system performance may vary compared to the design rezults. By trial and error the designer have to additionally adjust the parameters of the compensator until the system meets the specifications. The designer have to evaluate the final system whether it is reliable and economical, as well.