MercuryiTC
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Page 145
• There will be a delay between heat being applied to the heater and a change of the
temperature reading of the sensor.
• The system must respond as quickly as possible to a change of setpoint.
When a steady state is established, the heat provided by the controller exactly balances the
heat loss to the surroundings.
The criteria for good control are:
• Control accuracy
The temperature of the sample must be as close as possible to the setpoint.
• Control stability
Fluctuations above and below the setpoint temperature must be as small as possible.
• Control response
The sample temperature must follow changes of setpoint as quickly as possible.
14.2.1 Open loop operation
In an open-loop system, a fixed heater power is applied and the system is allowed to reach
equilibrium. Any changes in the heat loss from the system produce corresponding changes in
the sample temperature. The system also takes a long time to reach equilibrium. Open loop
control is obtained by setting the Heat parameter box on the Control Loop Configuration page
to Manual mode (see section 4.3).
An example of open-loop control is a hot-plate on a domestic oven.
14.2.2 On-Off control
In an on-off control system, the heater power is either maximum (if the measured temperature is
below the setpoint) or zero (if the measured temperature is above the setpoint). The control
accuracy and control response can be very good with this control system. The system also
responds to changes in heat loss from the sample.
Control stability is poor, since the sample temperature oscillates above and below the setpoint.
The magnitude of this oscillation depends on the thermal properties of the system. This control
system is an appropriate choice if the temperature oscillation it produces can be tolerated.
An example of on-off control is a thermostat controlling the temperature of a room.
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