Controllers can be adjusted to function correctly in many different applications.
Each controller usually has three adjustment modes:
• Proportional. Controller output changes by an amount related to the size of the error
• Integral. Controller output changes by an amount related to the size and duration of the error
• Derivative. Controller output changes by an amount related to the rate of measurement change
With pneumatic controllers and early electronic controllers, each mode added to a controller made it more expensive. Most electronic controllers available today are equipped with all three modes at no additional cost. The unneeded modes can be turned off.
Most control applications use proportional-plus-integral control. Proportional-plusintegral-plus-derivative is sometimes used for temperature control with delays (deadtime) of several minutes. Proportional-only control is sometimes used in noncritical services such as draining vessels.
Note that the proportional and integral actions depend on the error (defined as setpoint measurement), but the derivative action only depends on the measurement. Controllers are constructed this way so there will be no large change in controller output when the operator enters a new setpoint for the controller.
