Mark Snodgrass
Member
I had a problem on a hydraulic servo controlled cylinder, which I believe was caused by the PID loop and the cylinder reaching a hard stop at one end. I want to ask the PID experts out there (PETER)if my thinking on what I saw is correct.
Here is the setup: I have a cylinder that extends to a home position, when a board on a lug chain is indexed to the right position the cylinder retracts to pull the board onto another chain (there are 4 identical cylinders, only two are used depending on length of the board). The control (1771-QB) uses a .250" PID band, and outside of the band uses feed forward to drive the servo.
What would happen is if a few lugs were empty or the line had to stop for a few minutes, on the next one or two retract strokes it would not get into position, but once it was continuously running it acted right. I also noticed it would never get to the target home position, but since it was within the in position band of .200" it would still run. I came in over the weekend to look at it closely, and found that at the home position was physically hitting the hard stop of the pin slide (the slide shaft was replaced recently) while the other cylinders had about of inch clearance. After a few measurements I offset the zero move the home position away from the hard stop by about .200" and the problem went away.
I think what was happening is when it was at home position, the integral term would wind up causing the control to be biased away from the retract position, and it not getting into position. The operator would turn off then turn on the hydraulics (which sends a reset to the QB) which resets the integral term and it runs normally until the next pause. Does this sound plausible?
I am working to determine why the slide is not correct, either not install correctly or it was the wrong length. We were trying everything we have ever seen cause similar problems (cylinder, servo valve, hyd pump, accumulator, temposonic) without any results. This one was stumping me, but if my conclusion is right, I have another nugget to store away in my brain.
Thanks guys.
Here is the setup: I have a cylinder that extends to a home position, when a board on a lug chain is indexed to the right position the cylinder retracts to pull the board onto another chain (there are 4 identical cylinders, only two are used depending on length of the board). The control (1771-QB) uses a .250" PID band, and outside of the band uses feed forward to drive the servo.
What would happen is if a few lugs were empty or the line had to stop for a few minutes, on the next one or two retract strokes it would not get into position, but once it was continuously running it acted right. I also noticed it would never get to the target home position, but since it was within the in position band of .200" it would still run. I came in over the weekend to look at it closely, and found that at the home position was physically hitting the hard stop of the pin slide (the slide shaft was replaced recently) while the other cylinders had about of inch clearance. After a few measurements I offset the zero move the home position away from the hard stop by about .200" and the problem went away.
I think what was happening is when it was at home position, the integral term would wind up causing the control to be biased away from the retract position, and it not getting into position. The operator would turn off then turn on the hydraulics (which sends a reset to the QB) which resets the integral term and it runs normally until the next pause. Does this sound plausible?
I am working to determine why the slide is not correct, either not install correctly or it was the wrong length. We were trying everything we have ever seen cause similar problems (cylinder, servo valve, hyd pump, accumulator, temposonic) without any results. This one was stumping me, but if my conclusion is right, I have another nugget to store away in my brain.
Thanks guys.