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Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 126
THEORY OF CONTROL LOOPS
Original Instructions
The control function is driven by two error signals:

Temperature Error
the difference between the temperature measured by the sensor and the
temperature set-point.

Heater Voltage Error
the difference between the actual heater voltage delivered by the heater control loop
and the target heater voltage.
Both error signals are scaled by the error sensitivities (see Section 3.2.10 ), and both are
also compensated for non-linearity of the valve (the error sensitivity decreases with
increasing temperature and decreasing flows).
The sensitivity to a temperature error is such that the valve responds to large
temperature errors, but is relatively unaffected by small errors. This means that fine
control of temperature near the setpoint is dominated by the heater.
When the system is in control at the setpoint, the temperature error is, by definition,
zero. Control of the valve position is thus dominated by the heater voltage error, and the
cooling is adjusted until the heater voltage reaches its target value. The system is at
equilibrium when both the temperature error and the heater voltage error are zero.
12.3.4 Heater target tables
The target heater voltage is contained in the Heater Target Table (HTT). This table
contains a set of target heater voltage values, each corresponding to a specified
temperature range. The target voltage value in the table is scaled to the maximum
heater output voltage (see
Section 3.2.14 ), and is multiplied by the gas flow scaling
factor (see Section 3.2.10 ), to obtain the final target heater voltage. The user can select
which Heater Target Table to use, but can not edit a Heater Target Table. Contact OINS
to obtain a Heater Target Table to suit your application.