The Modern Industrial Workhorse: PID Controllers
- Tuesday, 01 July 2014
Proportional-Integral-Derivative (PID) controllers are used in most automatic process control applications in industry today to regulate flow, temperature, pressure, level, and many other industrial process variables.
They date back to 1939, when the Taylor and Foxboro instrument companies introduced the first two PID controllers. All present-day controllers are based on those original proportional, integral, and derivative modes.
PID controllers are the workhorse of modern process control systems, as they automate regulation tasks that otherwise would have to be done manually. While the proportional control mode is the main driving force in a controller, each mode fulfills a unique function. Proportional and integral control modes are essential for most control loops, while the derivative mode is excellent for motion control. Temperature control is a typical application that uses all three control modes.
Without a PID controller, manual control of water temperature is a tedious process. For example, to keep a constant temperature of water discharged from an industrial gas-fired heater, an operator has to watch a temperature gauge and adjust a fuel gas valve accordingly (Figure 1). If the water temperature becomes too high, the operator has to close the gas valve just enough to bring the temperature back to the desired value. If the water gets too cold, he has to open the gas valve.
The control task done by the operator is called feedback control, because the operator changes the firing rate based on feedback from the process via the temperature gauge. The operator, valve, process, and temperature gauge form a control loop. Any change the operator makes to the gas valve affects the temperature, which is fed back to the operator, thereby closing the loop.
To automate temperature control with a PID controller, the following are required:
- Install an electronic temperature measurement device
- Automate the valve by adding an actuator (and perhaps a positioner) so it can be driven electronically
- Install a controller and connect it to the temperature measurement device and automated control valve
The operator sets the PID controller’s set point (SP) to the desired temperature, and the controller’s output (CO) sets the position of the control valve. The temperature measurement, called the process variable (PV), is then transmitted to the PID controller, which compares it to the set point and calculates the difference, or error (E), between the two signals. Based on the error and the controller’s tuning constants, the controller calculates the appropriate controller output to set the control valve at the correct position to keep the temperature at the set point (Figure 2). If the temperature rises above its set point, the controller will reduce the valve position and vice versa.
Each of the controller’s 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.
Proportional Control Mode
The proportional control mode changes the controller output in proportion to the error. If the error increases, the control action increases proportionally.
The adjustable setting for proportional control is called the Controller Gain (Kc). A higher controller gain increases the amount of proportional control action for a given error. If the controller gain is set too high, the control loop will begin oscillating and become unstable. If set too low, the control loop will not respond adequately to disturbances or set point changes.
For most controllers, adjusting the controller gain setting influences the amount of response in the integral and derivative control modes.
The Proportional-Only Controller
A PID controller can be configured to produce only a proportional action by turning off the integral and derivative modes. Proportional controllers are simple to understand and easy to tune: the controller output is simply the control error times the controller gain, plus a bias. The bias is needed so the controller can maintain a non-zero output while the error is zero (process variable at set point). The drawback is offset, which is a sustained error that cannot be eliminated by proportional control alone. Under proportional-only control, the offset will remain present until the operator manually changes the bias on the controller’s output to remove the offset. This is known as a manual reset of the controller.