This type control is actually a combination of two previously discussed control modes, proportional and integral. Combining the two modes results in gaining the advantages and compensating for the disadvantages of the two individual modes. The main advantage of the proportional control mode is that an immediate proportional output is produced as soon as an error signal exists at the controller as shown in Figure 22. The proportional controller is considered a fast-acting device. This immediate output change enables the proportional controller to reposition the final control element within a relatively short period of time in response to the error.
The main disadvantage of the integral mode is that the controller output does not immediately direct the final control element to a new position in response to an error signal. The controller output changes at a defined rate of change, and time is needed for the final control element to be repositioned.
The combination of the two control modes is called the proportional plus reset (PI) control mode. It combines the immediate output characteristics of a proportional control mode with the zero residual offset characteristics of the integral mode.