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Computer-Controlled Systems

In many modern systems, the controller (or compensator) is a digital computer. The advantage of using a computer is that many loops can be controlled or compensated by the same computer through time sharing. Furthermore, any adjustments of the compensator parameters required to yield a desired response can be made by changes in software rather than hardware. The computer can also perform supervisory functions, such as scheduling many required applications. For example, the space shuttle main engine (SSME) controller, which contained two digital computers, alone controlled numerous engine functions. It monitored engine sensors that provided pressures, temperatures, flow rates, turbopump speed, valve positions, and engine servo valve actuator positions. The controller further provided closed-loop control of thrust and propellant mixture ratio, sensor excitation, valve actuators, spark igniters, as well as other functions (Rockwell International, 1984).

3. 
   Analysis and Design Objectives 9
   

4. Analysis and Design Objectives

In Section 1.1 we briefly alluded to some control system performance specifications, such as transient response and steady-state error. We now expand upon the topic of performance and place it in perspective as we define our analysis and design objectives.
Analysis is the process by which a system’s performance is determined. For example, we evaluate its transient response and steady-state error to determine if they meet the desired specifications. Design is the process by which a system’s performance is created or changed. For example, if a system’s transient response and steady-state error are analyzed and found not to meet the specifications, then we change parameters or add additional components to
meet the specifications.
A control system is dynamic: It responds to an input by undergoing a transient response before reaching a steady-state response that generally resembles the input. We have already identified these two responses and cited a position control system (an elevator) as an example. In this section, we discuss three major objectives of systems analysis and design: producing the desired transient response, reducing steady-state error, and achieving stability. We also address some other design concerns, such as cost and the sensitivity of system performance to changes in parameters.