Proportional-integral-derivative (PID) control is the most common control algorithm used in industry today. The popularity of PID controllers can be attributed to their effectiveness in a wide range of operating conditions, their functional simplicity and how easily engineers can implement them using current computer technology. This article discusses PID control and practical implementations and provides a brief overview on how to tune PID controllers.
Why do engineers need control, and which types of control can they use? Engineers use control to either modify plant behavior to keep system output stable and improve response time (the time the plant takes to go from its current state to the state defined with the new input) or minimize the energy the plant uses to transition between states. To accomplish this, engineers can operate controllers themselves or use mechanical devices or PC or PC-related technology. Thanks to the development of computer-based systems, engineers can use PCs and appropriate hardware to interact with plants and read their output and input signals.
A plant is a physical process that must be controlled, meaning a PID controller needs to increase the stability or performance of the process. A tank with one input valve and a fixed output flow, where height level is regulated, is one example.
Other examples include a deposit where a chemical reaction takes place, and pH is maintained precisely, or a motor connected to a conveyor belt whose speed must be sustained.
All these systems share one common characteristic: they have one input that can be controlled to produce a desired change in the output signal.
Figure 1: All plant systems have one input that can be controlled to produce a desired change in the output signal.
A mathematical model can approximate how the plant behaves. A "black box" with one input and one output can represent this plant model. In the case of a DC motor connected to a conveyor belt, the input would be the voltage applied, and the output would be the speed at which it turns.
On a hydraulic system, the input would be how open the valve is, and the output would be the height on the tank. Finally, on a pH reactor plant, the input is the rate at which a chemical reactor is fed, and the output is the pH measured.
Figure 2. Open-loop systems such as this lack the information needed to change the value on the controller to correct error.
Open-loop systems lack the information needed to change the value on the controller to correct error. To do this, engineers must add feedback information, resulting in a closed-loop system.
Figure 3. A closed-loop system includes a feedback mechanism to correct error.