Proportional Controller

In many processes, the cycling that occurs with the two position ON—OFF control is undesirable. Steady process controlled in the absence of noises, is possible when the controlled variable is a continuous function of the error.

A most widely used form of continuous control mode is the proportional control action, in which a smooth, linear relation exists between the controller output and error. Hence, Proportional Controller can be represented as


P = controller output, including error

Po= controller output without error (%)

EP = error signal in proportional control action

KP = proportional controller gain or proportional constant between er­ror and controller output (%)

The above equation indicates one-to-one correspondence between Ep and P. The control action can also be expressed in terms of the Proportional Bandwidth (PB), defined as the error required to cause a 100% change in controller output and is usually expressed as percentage of the chart width.

Hence PB = 1/KP x 100%

From the above equations for proportional control action the following can be concluded

  1. If error is zero, the controller output is constant (=Po).
  2. In case error signal is present, the proportional controller output needs a correction of Kp% for every 1% error (EP).
  3. There is an error band about zero, having magnitude = PB, within which the output is not saturated at 0% or 100%.

The characteristics of a proportional control is as shown in Fig. 21.6 (a).

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