Super Systems 3 Series - Manual (Page 48)

Operations Manual Series 3
48
11.2.1 Proportional Band ‘PB’
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). 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 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 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 until finally becoming unstable.
The proportional band may be set in engineering units or as a percentage of the controller range.
11.2.2 Integral Term ‘TI’
In a proportional only controller, an error between setpoint and PV must exist for the controller to deliver power. Integral is used
to achieve zero steady state control error.
The integral term slowly shifts the output level as a result of an error between setpoint and measured value. If the measured
value is below setpoint the integral action gradually increases the output in an attempt to correct the error. If it is above setpoint
integral action gradually decreases the output or increases the cooling power to correct the error.
The diagram below shows the result of introducing integral action.
The units for the integral term are measured in time (1 to 9999 seconds in Series 3 controllers). The longer the integral time
constant, the more slowly the output is shifted and results in a sluggish response. Too small an integral time will cause the
process to overshoot and even oscillate. The integral action may be disabled by setting its value to Off.
Temperature
Time
Setpoint
Increasingly narrower
proportional band
Proportional
only control
Temperature
Time
Setpoint
Proportional + Integral
control
Proportional band
Temperature
Setpoint
wide
Output
100%
0%
50%
narrow