PID Loop Operation
Maintenance
and Troubleshooting
8–14
PID Loop Operation
Determining a suitable sample rate (Addr+07):
1. Operate the process open-loop (the loop does not even need to be
configured yet). Place the CPU in run mode (and the loop in Manual mode,
if you have already configured it). Manually set the control output value so
the PV is stable and in the middle of a safe range.
2. Try to choose a time when the process will have negligible external
disturbances. Then induce a sudden 10% step change in the control value.
3. Record the rise or fall time of the PV (time between 10% to 90% points).
4. Divide the recorded rise or fall time by 10. This is the initial sample rate you
can use to begin tuning your loop.
Control
Output
PV
10% of full output range
Rise Time
10%
90%
Sample
Rate
In the figure above, suppose the measured rise time response of the PV was 25
seconds. The suggested sample rate from this measurement will be 2.5 seconds.
For illustration, the sample rate time line shows ten samples within the rise time
period. These show the frequency of PID calculations as the PV changes values. Of
course, the sample rate and PID calculations are continuous during operation.
NOTE: An excessively fast sample rate will diminish the available resolution in the
PV Rate-of-Change Alarm, because the alarm rate value is specified in terms of PV
change per sample period. For example, a 50 mS sample rate means the smallest
PV rate-of-change we can detect is 20 PV counts (least significant bit counts) per
second, or 1200 LSB counts per minute.
The Loop Parameter table for each loop has a data location for the sample rate.
Referring to the figure below, location V+07 contains a BCD number from 00.05 to
99.99 (with an implied decimal point). This represents 50 mS to 99.99 seconds. This
number may be programmed using DirectSOFT32’s PID Setup screen, or any other
method of writing to V-memory. It must be programmed before the loop will operate
properly.
Process Variable
S
Error Term
+
–
Control OutputSetpoint
X X X X
Sample Rate–V+07
Loop
Calculation
BCD
Sample Rate
00.05 to 99.99 sec
Programming the
Sample Rate
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