The addition of an integral element to a proportional controller modifies the response (Figure 7.23) removing the offset and giving a steady-state value the same as the input set value. This elimination of offset is because the integral mode gives a controlling response which is proportional to the area under the error-time graph up to the current point and so can give a controller output signal even when the error has become zero.. There are many situations where we require a controller to continue giving an output signal even when the error is zero. For example, with the water level control system of Figure 7.20, if the outflow changes to a new rate then the controller has to receive an error signal to maintain the water level constant and still give the new flow rate. With a PI controller, we can still have a controller output with zero error and so there is no
need for offset. PI control tends to be used with systems where load
disturbances occur frequently.