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4. When the PID tuning values have been determined,
if there is a disturbance to the process variable as
control passes from one proportional band to the
other, set the Overlap/Deadband parameter to a
positive value to introduce some overlap. Adjust
this value by trial and error until satisfactory results
are obtained.
Method 2 – For Asymmetrical Processes
Use this method if the relative power available from
the primary and secondary actuators is not symmetri-
cal (e.g. if the maximum cooling power is less than the
maximum heating power)
1. Check that the scaled input limits and the setpoint
limits of the loop in question are set to safe and ap-
propriate levels for your process. Adjust if required.
2. Set the setpoint to the normal operating value for
the process (or to a lower value if overshoots be-
yond this value might cause damage).
3. Select On-Off control by setting the primary pro-
portional band to zero (the secondary proportional
band will automatically be set on-off control when
you do this).
4. Switch on the process. The process variable will os-
cillate about the setpoint. Record the peak-to-peak
variation (
V) of the oscillation (i.e. the difference be-
tween the on-going overshoot and undershoot), the
time period of the oscillation (
T) in minutes and the
maximum rate of rise (
dP) and fall (dS) as the oscil-
lation continues.
V = On-going Peak-to-Peak variation
T = Time period of oscillation (minutes)
dS = Maximum rate of rise
dP = Maximum rate of fall
R = Ratio dS/dP
Primary Proportional Band = Pb.P = V/R
Secondary proportional band = R x Pb.P
Integral Time = T minutes
Derivative Time = T/6
Pro
ce
ss
Va
riabl
e
Time
5. Calculate and enter the PID control parameters (pri-
mary proportional band, integral time and derivative
time) using the formulas shown, and observe the
process.
6. If symmetrical oscillation occurs, increase the pro-
portional bands together, maintaining the same ra-
tio. If the asymmetrical oscillation occurs, adjust the
ratio between the bands until it becomes symmetri-
cal, then increase the bands together, maintaining
the new ratio.
7. When the PID tuning values have been determined,
if there is a disturbance to the process variable as
control passes from one proportional band to the
other, set the Overlap/Deadband parameter to a
small positive value to introduce some overlap. Ad-
just this value by trial and error to find the minimum
value that gives satisfactory results.
Valve, Damper & Speed Controller Tuning
This tuning method is used when controlling devices
such as dampers, modulating valves or motor speed
controllers. It applies equally to modulating valves with
their own valve positioning circuitry, or in VMD mode
where the instrument directly controls the valve motor–
see Valve Motor Drive / 3-Point Stepping Control. It de-
termines values for the primary proportional band, and
integral time constant. The derivative time is normally
set to
OFF. This type of PI Control minimizes valve/mo-
tor wear whilst giving optimal process control.
In VMD modem the Motor Travel Time and Minimum
On Time must be correctly set to match the valve spec-
ifications before attempting to tune the controller.
This technique is suitable only for processes
that are not harmed by large fluctuations in the
process variable.
1. Set the setpoint to the normal operating process
value (or to a lower value if overshoot beyond this
value is likely to cause damage).
2. Set the Primary Proportional Band a value approxi-
mately equal to 0.5% of the input span for the loop
to be tuned. (Span is the difference between the
scaled input limits).
3. Set the Integral & Derivative time constants both to
OFF.
4. Switch on the process. The process variable should
oscillate about the setpoint.
5. Follow the instructions in the diagram below. At
each stage, allow sufficient settling time before
moving on to the next stage.
P.Pb is the Primary
Proportional Band, Int.T is the Integral Time Con-
stant.
Содержание Chromalox 4081
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