PROPORTIONAL PLUS RESET CONTROL SYSTEMS
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
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.
Example of Proportional Plus Reset Control
Lets once more refer to our heat exchanger example (see Figure 23). This time we will apply
a proportional plus reset controller to the process system.
Figure 23 Heat Exchanger Process System
The response curves shown in Figure 24 illustrate only the demand and the measured variable
which represents the hot water outlet temperature.
Assume the process undergoes a demand disturbance which reduces the flow of the hot water out
of the heat exchanger. The temperature and flow rate of the steam into the heat exchanger
remain the same. As a result, the temperature of the hot water out will begin to rise.
The proportional action of the proportional plus reset controller, if acting alone, would respond
to the disturbance and reposition the control valve to a position that would return the hot water
out to a new control point, as illustrated by the response curves. Youll note that a residual error
would still exist.