Q: What is Proportional Control?

Proportional controls are designed to eliminate the cycling associated with on-off control. A proportional controller decreases the average power used by the heater as the temperature approaches setpoint. This has the effect of reducing the heat that is added to the process so that the temperature approaches the setpoint without overshoot. This proportioning action can be accomplished by turning the output on and off for short periods of time.

This "time proportioning" varies the ratio of "on" time to "off" time to control the temperature. This is sometimes refered to as Pulse width modulation (PWM). The proportioning action occurs within a band either side of the set point. Outside this band (proportional band) , the controller functions as an on-off unit, with the output either fully on (below the band) or fully off (above the band). However, within the band, the output is turned on and off in the ratio of the measurement difference from the setpoint. At the setpoint (the midpoint of the proportional band), the on-time and off-time are equal. If the temperature drops below the set point, the on time becomes longer. If the temperature is above the set point, the off time will be longer. 

If should be noted, that if the proportional band is reduced toward 0, then the controller will again act as a switch, operating as ON/OFF control and will reintroduce cycling of the temperature which will lead to overshoot.

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