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

Proportional Controllerwhere

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).

Proportional Controller

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