Q
PID2 function seems to prevent Manipulated value (Mv) from lowering to allow Present value (Pv) to come up to Setpoint (SP). Wastewater outfall to ocean controlling Total Chlorine residual to 0.100 of 0-1 range (Wallace & Tiernan Micro 2000).
Using the pid2 function for dosage control of bisulfite to neutralize total chlorine in our wastewater treatment plant disinfected effluent to the ocean, we cannot control at the chosen setpoint of 0.100 out of a range from 0.000 to 1.000. The pid2 function is set at pb=80, reset=00.12 repeats/min, rate=00.02min or 00.00. output clamp low of 409 out of 4095. the engineering range is 0 to 1000.
what happens is the manipulated variable will not continue to go lower (to reduce the bisulfite addition) even thought the pv is lower than the setpoint desired. it is as if the equation for the pid2 locks up and our final amount of total chlorine residual stays at 0.000 instead of 0.100.
we have temporarily set up a method of reducing the bisulfite pump stroke by 1% (lowering the Mv by 40) if the Pv stays below the SP at 0.001 or below for more than 10 minutes. The control loop is slow, about 8 minutes, that is why the reset number is so small. the rate/derivative number was given a little to make the system respond slightly faster, but did not change the basic problem of the Mv not continuing to decrease even though the Pv is less than the SP.
The details of the equation and flow blocks in the ladder logic block library user guide do not give enough information to determine why the lock up occurs. We are hoping that someone at Modicon can figure out why we may be running into this problem. We understand that you may not know the details of our process but we can help on that end if needed for your diagnosis. We have tried many different settings for the gain, reset parameters without success. proworx 32 has a better help file, but just basically duplicates the user guide.
We are using the PID2 for chlorine addition to maintain a 5.5 ppm total chlorine residual out of a range of 0-20.0 (0-200units) with the same parameters (although "maintain" usually means slightly below the 5.5 mark).
Also, the pump stroke is controlled by the residual pid2 function and the motor speed is controlled by a direct ratio to changes in flow. It has been suggested to control speed by the residual number and the stroke by the flow changes as the motor speed is more linear than stroke changes. Perhaps a blend of the two?
Has anyone solved this sort of problem before and can make some suggestions? Thanks. Modicon does not seem to want to touch the problem as the person who wrote the function no longer exists.
Using the pid2 function for dosage control of bisulfite to neutralize total chlorine in our wastewater treatment plant disinfected effluent to the ocean, we cannot control at the chosen setpoint of 0.100 out of a range from 0.000 to 1.000. The pid2 function is set at pb=80, reset=00.12 repeats/min, rate=00.02min or 00.00. output clamp low of 409 out of 4095. the engineering range is 0 to 1000.
what happens is the manipulated variable will not continue to go lower (to reduce the bisulfite addition) even thought the pv is lower than the setpoint desired. it is as if the equation for the pid2 locks up and our final amount of total chlorine residual stays at 0.000 instead of 0.100.
we have temporarily set up a method of reducing the bisulfite pump stroke by 1% (lowering the Mv by 40) if the Pv stays below the SP at 0.001 or below for more than 10 minutes. The control loop is slow, about 8 minutes, that is why the reset number is so small. the rate/derivative number was given a little to make the system respond slightly faster, but did not change the basic problem of the Mv not continuing to decrease even though the Pv is less than the SP.
The details of the equation and flow blocks in the ladder logic block library user guide do not give enough information to determine why the lock up occurs. We are hoping that someone at Modicon can figure out why we may be running into this problem. We understand that you may not know the details of our process but we can help on that end if needed for your diagnosis. We have tried many different settings for the gain, reset parameters without success. proworx 32 has a better help file, but just basically duplicates the user guide.
We are using the PID2 for chlorine addition to maintain a 5.5 ppm total chlorine residual out of a range of 0-20.0 (0-200units) with the same parameters (although "maintain" usually means slightly below the 5.5 mark).
Also, the pump stroke is controlled by the residual pid2 function and the motor speed is controlled by a direct ratio to changes in flow. It has been suggested to control speed by the residual number and the stroke by the flow changes as the motor speed is more linear than stroke changes. Perhaps a blend of the two?
Has anyone solved this sort of problem before and can make some suggestions? Thanks. Modicon does not seem to want to touch the problem as the person who wrote the function no longer exists.