The procedure adopted when using the controller is to initially set the
mode as manual. The set point is then set to the required value and the
controller output manually adjusted until the deviation is zero and the
plant thus operating at the required set point. Figure 7.30 shows the
block diagram of the control system when it is being operated in manual
mode and the operator adjusting the controller output by adding in a
signal. The controller can then be switched to automatic control. When
this happens, the manual input signal is held constant at the value that
was set in manual mode.