CONTROL MODES AND CONTROLLERS
Basic forms of control action or “modes” used in most process control are: two-position or “on-off” control, proportional control, integral or “reset” control, and derivative or “rate” control. The latter three modes are often used in various combinations with each other.
Two-Position (on-off) Controllers
The simplest form of control action is “on-off” control, in which the controller output either energizes or de-energizes some two-state device such as a relay or an open-shut type valve. The two-position controller is used extensively in home heating and cooling systems, refrigerators, hot water tanks, air compressors, and other applications where the cost of more precise control is not justified. Most two-position controllers are reverse-acting, i.e., when the measured variable is above the set-point, the controller turns the manipulated variable OFF, and when the measured variable is below the setpoint, the controller turns the manipulated variable ON. A “deadband” or differential gap exists around the zero error condition to minimize cycling. This is often implemented as a pair of control points: one where the controller will “kick-on” and the other where the controller will “kick-off” as opposed to a single setpoint.
Proportional, Integral, and Derivative Control Modes
When the cycling nature of “on-off” control cannot be tolerated, a controller using some combination of the proportional, integral, and derivative modes is normally used. The normally used combinations of these modes are: (P) Proportional only; (PI) Proportional plus integral; (PD) Proportional plus derivative; and (PID) Proportional plus integral plus derivative. Integral mode is rarely used alone since it adds lag to the system without the benefits of proportional mode. Derivative mode is never used alone since its output is zero except when the error signal (PV – SP) is changing.
Direct and Reverse Acting Controllers
The error signal term (PV – SP) indicates “reverse” controller action (the controller output decreases when the measurement increases). Most controllers may be switched from one type of action to the other, and the choice depends upon the action of the final control element and the direction that the measured controlled variable responds to the manipulated variable.
1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas