VICI ITC - Manual (Page 8)

1.323 Differential Comparator
A differential comparator is used to compare the output of the D/A converter with
that of the input amplifier. Subsequently, the comparator’s output denotes whether
the zone temperature is higher or lower than the setpoint.
1.324 Heater Power Switch
In accordance with the comparator’s decision, the power circuitry will apply power
to the heater when the zone temperature is below the setpoint and interrupt power
when it is above the setpoint.
In strictly theoretical terms, the above four elements are all that would be required
to implement the controller’s function. However, practical application requires three
additional elements:
1.325 Thermocouple Break Detection
The most common physical malfunction in thermocouples is a break, or open
circuit. If a break occurs, the input amplifier can no longer report the zone
temperature, and usually will report the ITC’s temperature, instead. From the
preceding discussion, one can deduce that this is a potentially disastrous situation.
However, a separate circuit is employed specifically to detect a break condition. Its
output will cause the Heater Power Switch to be disabled, and a front panel
indicator to be illuminated whenever a break occurs.
1.326 Proportional Power Control
To this point, power application to the heater has been described as simply
"applied or interrupted". In reality, this would be analogous to trying to maintain a
dragster’s speed at 30 mph using full throttle applications only. Obviously, power
must be delivered according to the need. For this reason, the ITC employs pro-
portional power control, where net power is delivered to the heater according to
the difference between the setpoint and zone temperature. The proportioning
technique is discussed in Section 1.33.
1.327 Power Attenuation
Effective heater size can be tailored to the application by proper adjustment of the
power attenuation control. Excessive heater ratings are often the cause of system
instability at near ambient temperatures. Proportional control and power
attenuation work hand-in-hand to produce excellent temperature stability. (Refer to
Section 2.3.)
1.328 Zero Crossing Switch
This allows the heater to come on only if the AC waveform is at zero to suppress
noise on the powerline.
1.33 Proportional Power Control
Although the ITC’s input amplifier and D/A circuitry are accurate and predictable in
their temperature representations, they do not in themselves constitute a good
temperature controller. In a control system, the essential factor is stability. A
temperature controller is not doing its job if the zone temperature is allowed to
oscillate about the setpoint to a degree which upsets the process. In short, the
aim is to reduce a thermal system’s natural tendency to oscillate to a level where it
is not significant.
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