PID Loop Operation
Maintenance
and Troubleshooting
8–62
PID Loop Operation
Troubleshooting Tips
Q. The loop will not go into Automatic Mode.
A. Check the following for possible causes:
S A PV alarm exists, or a PV alarm programming error exists.
S The loop is the major loop of a cascaded pair, and the minor loop is not
in Cascade Mode.
Q. The Control Output stays at zero constantly when the loop is in Automatic Mode.
A. Check the following for possible causes:
S The Control Output upper limit in loop table location V+31 is zero.
S The loop is driven into saturation, because the error never goes to zero
value and changes (algebraic) sign.
Q. The Control Output value is not zero, but it is incorrect.
A. Check the following for possible causes:
S The gain values are entered improperly. Remember, gains are entered
in the loop table in BCD, while the SP and PV are in binary. If you are
using DirectSOFT, it displays the SP, PV, Bias and Control output in
decimal (BCD), converting it to binary before updating the loop table.
Q. The Ramp/Soak Generator does not operate when I activate the Start bit.
A. Check the following for possible causes:
S The Ramp/Soak enable bit is off. Check the status of bit 11 of loop
parameter table location V+00. It must be set =1.
S The hold bit or other bits in the Ramp/Soak control are on.
S The beginning SP value and the first ramp ending SP value are the
same, so first ramp segment has no slope and consequently has no
duration. The ramp/soak generator moves quickly to the soak segment,
giving the illusion the first ramp is not working.
S The loop is in Cascade Mode, and is trying to get the SP remotely.
S The SP upper limit value in the loop table location V+27 is too low.
S Check your ladder program to verify it is not writing to the SP location
(V+02 in the loop table). A quick way to do this is to temporarily place an
end coil at the beginning of your program, then go to PLC Run Mode,
and manually start the ramp/soak generator.
Q. The PV value in the table is constant, even though the analog module receives the PV signal.
A. Your ladder program must read the analog value from the module successfully
and write it into the loop table V+03 location. Verify the analog module is generating
the value, and the ladder is working.
Q. The Derivative gain doesn’t seem to have any affect on the output.
A. The derivative limit is probably enabled (see section on derivative gain limiting).
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