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QUANTUM™ HD COMPRESSOR CONTROL PANEL
OPERATION
090.040-O (MAR 12)
Page 118
[Setpoint]
- A setpoint box is provided to the
value that you wish to control to.
[Proportional Band]
– This setpoint determines
the size of a region either above or below the
Control Setpoint. Within this region, the Pro-
portional component of the PID Output value is
the number between 0% and 100% that directly
corresponds to the difference between the Con-
trol Input (Actual) and the Control Setpoint (Set-
point). Outside of this region, the Proportional
component is either 100% or 0%. If the PI Action
is Forward, the Proportional Band extends above
the Control Setpoint. If the PI Action is Reverse,
the Proportional Band extends below the Control
Setpoint.
[Integration Time]
- This setpoint controls the
in
fl
uence that the Integral component exerts on
the PI Output value. The Integral component
works to push the Control Input toward the Con-
trol Setpoint by tracking the difference between
the Control Input and the Control Setpoint over
time.
[High Limit]
- The highest value that the output
can be.
[Low Limit]
- The lowest value that the output
can be.
[Off Value]
- If the PI Control is set to When
Running, this value sets the value of the PI out-
put when the compressor is off.
Setting up a PI Channel
NOTE:
The basic steps in setting up a device for PI
control is:
Setup Input channel
• First, decide which physical input channel
you will be using. Note: If you wish to use
an Auxiliary Analog channel, it will require
being completely setup as its own chan-
nel before attempting the remainder of this
procedure.
Setup PI parameters
•
From the Home screen, access the Menu.
Select
[Con
fi
guration]
> Proportional /Inte-
gral Control.
• On the Proportional / Integral Control
screen, select the Analog channel that has
already been set up, in this case, channel 1.
•
It is on this Proportional / Integral Control
screen that the remainder of the PI param-
eters are entered. For a detailed description
of these parameters, refer to the Overview
to Tuning a PI Controller.
Note:
Each device that is utilized for PI control must
also be properly calibrated.
Overview to Tuning a PI Controller
The purpose of this section is to give some basic
guidelines for tuning Proportional, Integral, and Dif-
ferential gains of a PI controller. To tune a PI control-
ler, it would be advantageous to hook the system up
to some test equipment to allow you to record the
appropriate variables. At the very least, the appropri-
ate data will have to be monitored and recorded by
hand from the system screen for subsequent evalua-
tion and possible spreadsheet graphing.
Proportional Band
The Proportional Band setpoint translates the
Proportional Gain into the units of the Control
Input channel. When the control Input is at the
Control Setpoint, the Proportional component
of the PI Output will be 0%. As the Control In-
put moves through the Proportional Band, and
away from the Control Setpoint, the Proportional
component will increase. If the Control Input is
in the middle of the Proportional Band, the Pro-
portional component value is 50%, and when the
Control Input reaches the outside of the Propor-
tional Band, the Proportional value will be 100%.
The value of the Proportional component is cal-
culated as the difference between the Control
Input and the Control Setpoint, divided by the
Proportional Band and then multiplied by 100.
Example:
Control Input: Discharge Temperature
Control Setpoint: 100° F
Dead Band: 0° F
Proportional Band: 10° F
Action: Forward
Control Input
Proportional Output
100° F
0%
102.5° F
25%
105° F
50%
108° F
80%
110° F
100%
112° F
100%
The value selected for the Proportional Band will
be determined by the stability of the system, the
accuracy required, and the average output value,
among other things. One way to pick a starting
value is to
fi
nd a range of allowable Control Input
values and then to select a Proportional Band
so that the Proportional value will reach 100%
before the Control Input moves outside of the
allowable range.
After the initial value is entered for the Pro-
portional Band, watch the system to see how it