Proportional-plus-integral control is the recommended control action for most applications. Often called PI control, it combines proportional action and integral action in one controller. The resulting control action has the fast response and stability of proportional action, but no offset. In eliminating offset, integral action serves as an automatic bias adjustment.
The output from a proportional-plus-integral controller may be expressed as follows:
Figure 300-12 shows the open loop response of proportional-plus-integral control. Proportional control immediately acts to reverse the error. Integral action then continues to change controller output until the error equals zero.
Figure 300-13 depicts proportional-plus-integral control for a closed loop. In response to a step change in load (top graph), the controlled variable (middle graph) falls below the setpoint. The integral action adjusts the bias from 50% initially to about 75% after the load change and shifts the position of proportional band (shaded area) on the scale. Notice that the percentage value of the proportional band is not changed. The lower graph shows the output of the controller.
