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Page 147
Figure 8 Proportional band control
A PID controller can be made to operate in proportional-band mode by setting the proportional
band (P) parameter to a positive temperature value.
14.2.4 Proportional control with integral action
Adding an integrator into the proportional band control system can reduce the inherent
temperature error to zero. The proportional band is set to a sufficiently large range to avoid
oscillation. The error signal is then used as the input of the integrator. The output of the
integrator is added to the output of the proportional control system.
If the sample temperature is below the setpoint, the negative error causes the output of the
integrator to increase. This increases the heater output, which causes the sample temperature
to rise. The output of the integrator stabilises at a positive value when the error is zero.
The integrator must be constrained to act more slowly than the response of the heater, to avoid
oscillations. The control response of this system may be slow, because of this. The response of
the integrator is characterised by the integrator action time. This is referred to as the I
parameter in the PID control loop.
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