Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001
· The system models we developed before allow us to predict how a system will behave. A separate, and important topic is computer control.
· With no controller we would set an input, and hope for an output. For example, push the gas pedal and hope for the right speed.
· A controller looks at the desired system condition, and the actual system condition, and then adjusts the input to bring the desired and actual closer. For example cruise control.
· The diagram below is a representation of a simple control system add to in the previous tank example,
· We have already dealt with deriving an equation for the process. In this case it was the valve tank combination discussed before. By itself the tank is an open loop system, we set the valve angle and hope for a liquid level.
·Next, we need to find an equation for the controller. This equation can be highly dependent upon the control method to be used. If we are to use a computer it is best to have a simple equation, as shown below, (NOTE: the form of the equation, and the values of the coefficients change the nature of the control problem).
· The controllers (equations) that follow will be put in the above form. These controllers can also be used individually, or combined to get more complex properties.
· Keep in mind that the typical objective of a control system is to minimize the error between the input and output. Another common goal is to do this as quickly, or efficiently as possible. One constraint we must observe is that the system should not become unstable.