Pi Control Parameters; Proportional Band; Integral Time - Daikin MicroTech Operation And Maintenance

PI Control Parameters

Adjusting PI parameters can cause erratic unit operation,
and potentially damage the equipment .
PI control parameters should only be adjusted by trained personnel
having a complete understanding of how these parameters affect system
operation. Generally these parameters do not need to be adjusted from the
factory default settings.
Associated with each PI loop is a set of two adjustable
parameters: Proportional Band and Integral Time. When
the unit ventilator is properly sized for the space, the factory
settings for these parameters provides the best and most
robust control action (Figure
If field problems arise, first ensure these parameters are set
back to the factory default settings. If adjustment is required,
only make small adjustments to one parameter at a time.
After each adjustment, allow enough time for the system to
stabilize before making further adjustments. If you do not have
the means to graph the space performance, record the actual
measured value and setpoint for several minutes and then plot
the results using a spreadsheet to determine the correct action
to change the PI parameter.
Figure 18: 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
control oscillations that go fully above and below the setpoint.
A proportional band setting that is too large
an offset between the actual measured oscillation center and
the setpoint. A small offset is not necessarily a problem since
most systems have a small "natural" offset and the integral
function automatically works to eliminate or reduce this effect.
www.DaikinApplied.com
CAUTION
18).
(Figure 19) causes
(Figure 19) causes
Figure 19: Proportional Bands
Too Small
In general, it is best to start with a relatively large proportional
band setting (use of default setting is recommended) and
adjust to smaller values.
If you want the system to respond strongly to small changes in
the space, adjust the proportional band to a higher setting.
If you want the system to react weakly to small changes in the
space, adjust the proportional band to a lower setting.

Integral Time

The integral time, or integral action, causes the controlled
output to change in proportion to time difference between the
sensor value and setpoint. The difference over time between
the actual value and setpoint forms an "area under the curve"
(Figure 20). The integral action works to reduce this "area
under the curve" and to eliminate any natural system offset.
Figure 20: Integral Time
Area Under The Curve
The smaller the integral time, the faster the output ramps up or
down with small changes in the space. The smaller the integral
time, the quicker the system reacts to small changes in the
space. If the Integral Time is set too small, long oscillations
occur (Figure 20).
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 want the system respond strongly to small
changes in the space, lower the integral time. If you want the
system to react weakly to small changes in the space, adjust the
integral time to a higher setting.
33 OM 1280-2 • MICROTECH UNIT CONTROLLER
Description of Operation
Too Large
Too Small