this screen, press the [Previous Screen]
key.
3. Setup PID parameters
• From the Operating Status screen, press
the [Menu] key. Next press the [Capacity
Control], then [More…]. Finally, press the
[PID Setup] key.
• On the PID Setup screen, press the key that
is adjacent to the Analog channel that has
already been set up, in this case, channel 1.
• It is on this Setup screen that the remainder
of the PID parameters are entered. For a
detailed description of these parameters,
refer to the Overview to Tuning a PID
Controller.
Note: After having performed the above setup procedure,
if any portion of step 1 or 2 must be changed, then repeat
the entire procedure to ensure that all parameters are
correct per the new settings. Also, be aware that each
device that is utilized for PID control must also be properly
calibrated.
Overview to Tuning a PID Controller
The purpose of this section is to give some basic
guidelines for tuning Proportional, I
ntegral, and Differential
gains of a PID controller. To tune a PID controller, 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 appropriate data will have
to be monitored and recorded by hand from the system
screen for subsequent evaluation and possible
spreadsheet graphing.
Differential Gain
Once ready, set all gains to zero (Proportional, Integral
and Derivative). If you suspect that you will not need
differential control, then skip down to the paragraph that
discusses the Proportional Gain. Otherwise, start by
adjusting the Differential Gain. You cannot use Differential
control alone the way that the software is coded, so set
the Proportional Gain to some small value (one or less).
Check to see how the system works. If it oscillates
Proportional Gain, you should be able to cure it with
Differential Gain. Start with about 100 times more
Differential Gain than Proportional Gain. Now, start
increasing the Differential Gain until you see oscillation,
excessive noise, or excessive (more than 50%) overshoot.
Note that the oscillation from too much Differential Gain is
much faster than the oscillation for not enough Differential
Gain. A good strategy is to push the Gain up until the
system is on the verge of oscillation, then back the Gain
off by a factor of two or four. At this point, your system will
probably be responding very sluggishly, so it is time to
tune the Proportional and Integral Gains.
Proportional Gain
If it isn't already, set the Proportional Gain to a starting
value between 1 and 100. Your system will probably either
show terribly slow performance, or it will oscillate. If you
see oscillation, drop the Proportional Gain by factors of
eight or ten until the oscillation stops. If you don't see
0scillation, increase the Proportional Gain by factors of
eight or ten until you start seeing oscillation or excessive
overshoot. As with the Differential Controller, a good
strategy is to tune right up to the point of too much
overshoot, then reduce the Gain by a factor of two or four.
Once you are close, fine-tune the Proportional Gain by
factors of two until you like what you see.
Integral Gain
Once you have the Proportional Gain set, start increasing
the Integral Gain. Your starting values will probably be
from 0.0001 to 0.01. Here again, you want to find the
range of Integral Gain that gives you reasonably fast
performance, without too much overshoot, and without
being too close to oscillation.
Proportional Only Control
There will be certain applications in which only
proportional control is required for good performance.
When this is the case, the Integral and Differential Gains
can simply be set to zero, and the Proportional Gain set to
a calculated value. An example will help clarify how the
Proportional Gain can be calculated to produce
predictable results:
Suppose you wish to control the Suction Pressure to
a setpoint of 50 PSIA, and you want the output to be
100% when the measured value us 35 PSIA (and
below), and you want the output to be 0% when the
measured value is 50 PSIA (and above).
Let the term PB represent the Proportional Band, and
PG represent the Proportional Gain that you wish to
calculate. Also, let the term R represent the Range of
the particular sensor. Then, if the input channel is set
for 1-5 Vdc use PG = (1.25 x R) / PB, if the input
channel is set for 0-5 Vdc, use PG = R/PG. In these
examples, the Proportional Band is 15 (50 - 35). The
Range of a Suction Pressure sensor is 0 - 200 PSIA.
Therefore, PG = (1.25 x 200) / 15 = 16.66. Since only
integer values are used by the Quantum™ control for
gain factors, 17 would be entered for the Proportional
Gain. Because of this roundoff, the output will reach
100%, close to but not exactly at 35 PSIA.
In this example, the Action of the control loop should
be set to Reverse (the further the measured value is
below the Setpoint, the higher the Output). If the
opposite is desired in a particular control loop (the
further the measured value is above the Setpoint, the
higher the Output), then the Action should be set to
Forward.
Where: R = Sensor Range
Temp 922.5
Suction 200
Discharge 500
PB = 1.25 R
PG
S90-010 O FRICK QUANTUM™ COMPRESSOR CONTROL PANEL
Page 46 OPERATION
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