7.1.3 Proportional Band Pb

 

This section describes the effect of the proportional term only, that is with the integral and derivative terms turned off. The proportional band, or gain, delivers an output which is proportional to the size of the error signal. It is the range over which the output power is continuously adjustable in a linear fashion from 0 to 100% (for a heat only controller) or +100% for a heat/cool controller. Below the proportional band the output is full on (100), above the proportional band the output is full off (0) as shown in the diagram below. The proportional band is measured in engineering units (e.g OC).

 

The width of the proportional band determines the magnitude of the response to the error. If it too narrow (high gain) the system oscillates by being over responsive. If it is too wide (low gain) the control is sluggish. The ideal situation is when the proportional band is as narrow as possible without causing oscillation.

 

 

The diagram also shows the effect of narrowing proportional band to the point of oscillation. A very wide proportional band results in straight line control but with an appreciable initial error between setpoint and actual temperature. As the band is narrowed the temperature gets closer to setpoint. If the proportional band is very narrow the loop becomes unstable resulting in an oscillatory response.

 

The proportional band is set as a percentage of the controller range.

 

In practice this ideal situation, as shown in the diagram above is not fully achievable since there is always a loss of temperature. This can be compensated for in two ways, either by adding an integral term as described in section 7.1.4, below or by manually adjusting the power output as described in section 7.1.9.