The Feedback Control Loop: Controller Characteristics (1)

The design of the valve, process, and measurement should be made such as to minimize deadtime in the loop while providing a reliable, more linear response; then the controller can be tuned to provide the best performance, with an acceptable operating margin for robustness. The PID controller is the most widespread and applicable control algorithm, which can be tuned to provide near optimal responses to load disturbances. PID is an acronym for Proportional, Integral and Derivative modes of control.

Proportional mode establishes an algebraic relationship between input and output. The proportionality is set by a tunable gain parameter. This unitless parameter, controller gain (Kc), specifies percent change in output divided by percent change in input. On earlier versions of PID controllers, an alternate parameter, Proportional Band (PB), was defined as the percent change in input required to cause a 100 percent change in output. Thus by combining definitions, these two terms are related as follows: Kc = 100/PB.

The Integral mode is sometimes referred to as ‘‘reset’’ because it continues to take action over time until the error between measurement and set point is eliminated. The parameter to specify this action is Integral time, which can be thought of as the length of time for the controller to repeat the initial proportional response if the error remained constant. Note that as this parameter is made smaller, the reset increases as the control action is repeated in a shorter period of time. Some controllers use an alternate parameter, Reset, that is the reciprocal of Integral time and is referred to as repeats/unit time. This latter approach is perhaps more intuitive in that as the Reset parameter is increased, there is more reset action being applied.

The Derivative mode is sometimes referred to as ‘‘rate’’ because it applies control action proportional to the rate of change of its input. Most controllers use the process measurement, rather than the error, for this input in order to not have an exaggerated response to step changes in the set point. Also, noise in the process measurement is attenuated by an inherent filter on the Derivative term, which has a time constant 1/8 to 1/10 of the Derivative time. Even with these considerations, process noise is a major deterrent to the use of Derivative mode.