VICI ITC - Manual (Page 16)

2.4 Proportioning Bandwidth
Given that the controlled temperature is reasonably accurate, stability becomes a
most important measure of system performance. Perfect stability is obtained by
applying the exact amount of power required to offset a system’s demand. In
addition, the power must be applied
instantaneously whenever a demand occurs.
Think about this. Theoretical notions like "exact" and "instantaneous" soon reveal
the meaning of the term, "optimum".
In attempting to achieve optimum stability, we assume that the user will experiment
with the proportioning bandwidth adjustment pot. (Refer to Section 2.1, item 7.) In
keeping with this assumption, we offer the following explanation of bandwidth
adjustment.
2.41 Bandwidth Defined
Rigorously defined, bandwidth is the peak to peak value of the proportioning wave-
form, expressed in degrees centigrade. The bandwidth pot controls the height of
the waveform. More importantly, the height determines the slope of the diagonal.
In Figure 9, two waveforms are shown: one with 3° bandwidth, and the other with
6° bandwidth. In each case, the controlled temperature is depicted 1 below the
peak height of the waveform. Notice that the resulting power applications are
different. In fact, power is applied twice as long in the 3° example as in the 6°
example. This is due to the slope of the diagonal, and, as we shall see, is a very
useful thing to remember.
The important thing to notice in Figure 9 is that as the controlled temperature
changes within the bandwidth, the resulting change in heater power is dependent
on the slope of the diagonal. More specifically, the
rate of change in applied
power is controlled by the slope of the diagonal.
If, in each case, the temperature falls 1° (1° excursion), the resulting changes in
applied power are dramatically different. In the 3° example, application changes
from 33- 1/3% to 66- 2/3%. The same 1 excursion in a 6° bandwidth causes
application to change from 16- 2/3% to 33- 1/3% per degree and 16- 2/3% per
degree, respectively. By observation, increasing the bandwidth decreases the
amount of change in average applied power for a given change in temperature.
How does all this relate to stability improvement? Well, assuming that a stability
problem exists, it may be attributable to excessive heater power. By this, we mean
that the heater is simply too powerful for the application. The situation usually
results from designing the system to heat quickly and operate over a broad
temperature range. The problem is characterized by the controlled temperature’s
tendency to spend most of the time above the bandwidth, occasionally falling into
its upper reaches. The temperature will not stay within the bandwidth because
Note: Current models of the ITC may have fixed valued resistors in the trim
pot location for the bandwidth calibration. If the ITC needs further calibration
they may be replaced with 10K trim pot and the following text will explain the
band width adjustments.
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