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OM 750
PI CONTROL PARAMETERS
Associated with each PI loop is a set of two adjustable PI parameters.
These parameters are “Proportional Band” and “Integral Time”. When
the unit ventilator is properly sized for the space, the factory settings
for these parameters will provide the best and most robust control
action. See Figure 20.
If field problems do arise, first ensure these parameters are set back to
the factory default settings. If adjustment is then required, only make
small adjustments to one parameter at a time. After each adjustment,
enough time for the system to stabilize should be allowed before further
adjustments are made. You must have patience. If you do not have the
benefit of a means to graph the space performance, you can still record
the actual measured value and setpoint on paper for several minutes and
then plot the results using a spreadsheet to determine the correct action
in changing PI parameters.
Adjusting PI parameters can cause erratic unit operation and
equipment damage!
PI control parameters should only be adjusted by trained personnel
having a thorough understanding of how the parameters affect
system operation. Generally these parameters do not need to be
adjusted from the factory default settings.
CAUTION
!
Figure 20. Optimized PI Loop Control
PROPORTIONAL BAND
The Proportional Band, or proportional action, causes the controlled
output to change in proportion to the magnitude of the difference
between the sensor value and setpoint.
A Proportional Band setting that is too small (See Figure 21.) will cause
control oscillations that go fully above and below the setpoint.
Figure 21. Proportional Band too Small
A Proportional Band setting that is too large (See Figure 22.) will cause
an offset between the actual measured oscillation center and the
setpoint. A small offset is not necessarily a problem as most systems
have a small “natural” offset and the Integral function will automatically
work to eliminate or reduce this effect.
Figure 22. Proportional Band too Large
In general, it is best to start with relatively large Proportional Band
setting (the factory default setting is best) and adjust to smaller values.
If you wish the system to respond strongly to even small changes in the
space, you would lower the Proportional Band.
If you wish the system to react weakly to small changes in the space,
then you would adjust the Proportional Band to a higher setting.
INTEGRAL TIME
The Integral Time, or integral action, causes the controlled output to
change in proportion to time integral of the difference between the
sensor value and setpoint. The difference over time between the actual
value and setpoint forms an “area under the curve.” See Figure 23. The
integral action works to reduce this “area under the curve”, and to
eliminate any natural system offset.
Figure 23. Area Under the Curve
Figure 24. Integral Time too Small
The smaller the Integral Time, the faster the output will ramp up or
down with small changes in the space. The smaller the Integral Time,
the quicker the system can become “upset” due to small changes in the
space. If the Integral Time is set too small, long period oscillations will
occur. See Figure 24.
In general, it is best to start with a relatively large Integral Time setting
(the factory default setting is best) and adjust to smaller values.
If you wish the system to respond strongly to even small changes in the
space, you would lower the Integral Time.
If you wish the system to react weakly to small changes in the space,
then you would adjust the Integral Time to a higher setting.
