VICI ITC - Manual (Page 9)

If zone heater power is simply applied or interrupted according to the comparator’s
verdict (correction required/not required) the result is wholly unacceptable. To
illustrate:
Assume that a zone is to be heated from room ambient to 75
o
C. If 100%
power is applied until the temperature reaches 75
o
, and then interrupted, the
temperature will overshoot. As the temperature settles back to 75
o
, 100%
power will again be applied in an effort to prevent the temperature from falling
below the setpoint. As a result, the temperature will overshoot again. In this
manner, the temperature will continue to oscillate about the setpoint.
As can be seen, on/off, stop/go, etc. are corrective measures that would make the
ITC unacceptable for all but the crudest applications.
The key to obtaining acceptable stability lies in applying heater power relative to
the need. Using the above example, but employing proportional control, more
reasonable results are obtained:
While the temperature rises from ambient toward 75
o
C, power is applied
continuously, just as before. However, at a point just below the setpoint, say
70
o
, the power is reduced to 95%. As the temperature continues to rise,
power is linearly reduced such that power will be applied 50% of the time
when the temperature reaches the setpoint, and only 5% when it reaches
80
o
. When the temperature begins to settle, the process is reversed. Heater
power is gradually increased as the temperature declines toward the setpoint.
As a result, the temperature will tend to stabilize at a point where the power
application is sufficient to maintain equilibrium.
In conclusion, the system’s tendency to oscillate is greatly reduced if some form of
proportional control is employed.
Commonly, two methods can be used to electronically control AC power: phase
proportioning, and time proportioning. With phase proportioning, some percentage
of each AC cycle is applied to the load. While this method is just fine for power
drills, it is not acceptable for instrument use. This is due to the fact that the power
is not switched at zero-crossings. Therefore, large amounts of RFI/EMI can be
generated. (If such electrical "noise" is generated, it may upset the operation of
other instruments in the vicinity.) With time proportioning, the average power is
controlled by dividing time into specified periods. During each period, the percent-
age of power ON versus OFF time is proportional to the difference between the
setpoint and the controlled temperature. Power is switched only at AC voltage
zero-crossings, avoiding RFI/EMI generation.
Figure 4 is a graphic representation of time proportioning as it is implemented in
the ITC. The heart of the process is the proportioning waveform. This sawtooth-
shaped waveform defines three important parameters of operation: lower tempera-
ture boundary, setpoint, and upper temperature boundary. The setpoint will always
be situated in the exact center of the waveform. The lower temperature bound-
ary represents the point below which 100% power will be applied. The upper
temperature boundary represents the point above which no power will be ap-
plied. And, as stated earlier, the setpoint denotes the point at which power will be
applied 50% of the time. Observe, also, that between the two boundary tempera-
tures, the average applied power is linearly proportional to the difference between
the setpoint and the actual temperature within the heated zone.
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