Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 122
THEORY OF CONTROL LOOPS
Original Instructions
12.2.4 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.
Figure 12-3 Proportional control with integrator action
If the setpoint increases by a large amount, the integrator output ramps up for all the
time that the temperature error is negative. When the sample temperature reaches the
proportional band, the integrator output may be at its maximum value. The integrator
then takes significant time to reduce its output to the required value, resulting in a large
temperature overshoot. To prevent this overshoot, the integrator output held at zero until
the sample temperature is within the proportional band.
A PID controller can be made to operate in this mode by setting the proportional band
(P) parameter to a positive temperature value, and the integral (I) parameter to a non-
zero action time.
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