PID Controllers Explained - Automatic Control

PID controllers are named after the Proportional, Integral and Derivative control modes they have. They are used in most automatic process control applications in industry.

PID controllers can be used to regulate flow, temperature, pressure, level, and many other industrial process variables. This blog reviews the design of PID controllers and explains the P, I and D control modes used in them.

Automatic Control

To relieve our operator from the tedious task of manual control, we should automate the control loop. This is done as follows:

Install an electronic temperature measurement device.

Automate the gas valve by adding an actuator (and perhaps a positioner) to it so that it can be driven electronically.

Install a controller (in this case a PID controller), and connect it to the electronic temperature measurement and the automated control valve.

A PID controller has a Set Point (SP) that the operator can set to the desired temperature. The Controller’s Output (CO) sets the position of the control valve. And the temperature measurement, called the Process Variable (PV) gives the controller its much-needed feedback. The process variable and controller output are commonly transmitted via 4 – 20mA signals, or via digital commands on a Fieldbus.

When everything is up and running, the PID controller compares the process variable to its set point and calculates the difference between the two signals, also called the Error (E).

Then, based on the Error and the PID controller’s tuning constants, the controller calculates an appropriate controller output that opens the control valve to the right position for keeping the temperature at the set point. If the temperature should rise above its set point, the controller will reduce the valve position and vice versa.

PID Control

PID controllers have three control modes:

Proportional Control

Integral Control

Derivative Control

Each of the three modes reacts differently to the error. The amount of response produced by each control mode is adjustable by changing the controller’s tuning settings.