Future Design MCTB 4.3, User Manual

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MCTB
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9.4.1 Process Cascade Mode
This mode uses the low and high set points as the upper and lower range for the secondary loop set point. The
percentage of output of the primary loop is converted to calculate a linear set point between these two values.
The Process Low and Process High values are limited to the minimum and maximum values for the lower and
upper set point limits of the secondary loop as defined by the OEM in the MCTB configuration.
The process mode has the benefit of maximum change rates by allowing the secondary loop set point to go to
absolute limits. This allows the process to achieve set point as quickly as possible but does so at the cost of
stability. If the process has a large lag time, the control may oscillate uncontrollably once the process reaches
set point because of the large overshoot. In most cases, deviation is the recommended selection.
9.4.2 Deviation Cascade Mode
This mode uses the low and high set points as the upper and lower range to generate a set point offset for the
secondary loop set point relative to the primary loop set point. The secondary loop set point is calculated by
adding these values as offsets from the primary loop set point. As the primary loop set point changes so does
the window defined by the set points.
The deviation mode has the benefit of limiting the maximum and minimum overshoot that the secondary loop
can exhibit relative to the desired primary loop set point. This allows the system to be fine-tuned to the minimum
amount of overshoot required to obtain the desired set point.