PROPORTIONAL CONTROL SYSTEMS
At time t2, the measured variable decreases by 50oF, or 25%, of the measured variable span. The
25% controller input decrease causes a 50% controller output increase. This results in a
controller output increase from 3 psi to 9 psi, and the control valve goes from fully shut to 50%
The purpose of this system is to provide hot water at a setpoint of 150oF. The system must be
capable of handling demand disturbances that can result in the outlet temperature increasing or
decreasing from the setpoint. For that reason, the controller is set up such that the system
functions as shown in Figure 18.
Figure 18 Controller Characteristic Curve
If the measured variable drops below the setpoint, a positive error is developed, and the control
valve opens further. If the measured variable goes above the setpoint, a negative error is
developed, and the control valve throttles down (opening is reduced). The 50% proportional band
causes full stroke of the valve between a +50oF error and a -50oF error.
When the error equals zero, the controller provides a 50%, or 9 psi, signal to the control valve.
As the error goes above and below this point, the controller produces an output that is
proportional to the magnitude of the error, determined by the value of the proportional band. The
control valve is then capable of being positioned to compensate for the demand disturbances that
can cause the process to deviate from the setpoint in either direction.