Real World Instrumentation with Python

Control System Types

Up to this point we’ve seen only the basic outline of the domain of things called control systems. In this section we’ll take a look at some specific examples, and apply some of the concepts and terminology. We’ll start by examining examples of open-loop systems, then move on to closed-loop controls, sequential controls, and nonlinear controls. Ultimately we’ll end up at proportional-integral-derivative (PID) controls, the most common form of closed-loop linear control in use today.

Open-Loop Control

In Chapter 1, an automatic outdoor light was used as an example of a nonlinear open-loop system. Linear and nonlinear open-loop control systems operate on the basis of a specific relationship between a control input and the resulting output, and as we’ve already seen, an open-loop controller has no direct “knowledge” of what the actual system is doing in the form of feedback. The accuracy and repeatability of the input/output control relationship is solely dependent on the initial accuracy and calibration of the components in the system.

A gas stove is a familiar example of the input/output relationship in an open-loop control system. The amount of heat applied to a frying pan is determined by the gas control valve on the front of the range top (the control input), and the gas pressure at the valve might be limited by a pressure regulator somewhere in the line (usually at the gas meter outside the house). Once the burner is lit it will produce a flame with an intensity ...