MercuryiTC
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Page 146
Figure 7 Response of an on-off control system
A PID controller can be made to operate in On-Off mode by setting the proportional band (P)
parameter to zero.
14.2.3 Proportional control
A proportional control system allows the heater power to be continuously varied. This
overcomes the oscillation problem produced by an on-off control system. The heater voltage at
any instant is proportional to the error between the measured temperature and the setpoint. If
the sample temperature drops, the heater voltage increases to compensate.
A proportional control system reaches equilibrium below the setpoint. This is because the
heater output at the setpoint (when the error is zero) is zero. Thus a negative error is required to
produce enough heater output to balance the heat-losses to the surroundings.
The temperature error can be reduced by making the proportional control apply to only a part of
the temperature range of the system. Below this proportional band the heater voltage is at its
maximum value. However reducing the proportional band too far produces temperature
oscillations (in the limit, a proportional band of zero degrees produces on-off control).
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