JKS
Member
Good morning, all. I suppose a little bio is in order, to start: I recently began this new job as the plant's PLC Controls Technician (I know... fancy title...). Honestly, this is the first time that I have ever been paid for this type of work, although I have been utilizing PLC's for most of my career. I am a Master Electrician by trade and schooling, and have mainly self-taught the PLC world to myself. Yeah, I know that can be pretty scary, but I don't try to do anything I'm not comfortable with, and we have an outside contractor that we bring in for any high-level stuff. Or I search here for great answers and explanations. The plan is to get me up to speed over the next couple of years while weaning off utilizing the outside guys. But I digress... Here's my current problem:
Our process water system is supplied by two 90 gpm pumps driven by 7-1/2 hp motors controlled by AB PowerFlex 70's. Generally, these pumps take turns supplying the water to the system, unless our pressure drops to below 40 psi, in which case they both kick in to bring us back up. Under normal conditions, however, the analog signal is sent to the drive from a PID. Everything seems to be set up Ok and working, although I think that the system could use a good tuning. Here are the PID's parameters (sorry I don't have screen shots at the moment):
Control Block: N20:53; Process Variable: N20:14; and Control Variable: N20:20.
Kc= 2.0
Ti = 0.1
Td= 0.00
Loop Update = 0.10
Control Mode = E+SP-PV
PID Control = Auto
Time Mode = Timed
Limit Output CV = 0
Deadband = 0
Setpoint = 4915
Smax = 16383
Smin = 0
PV = 4915
CV% = 64
Output Max = 100
Output Min = 50
SE = 0
The only flags that are enabled are: Timed Mode (TM), Output Limiting (OL), and PID Enable (EN).
The 4-20 mA pressure transducer that is currently reading the output from the pumps is about 15 feet away. It will generally hold our Set Point of 60 psi, although you can watch and see a definite drop in the pressure at times under load. The system will eventually be a looped system, but for right now, it is ended approximately 350 feet from the feed.
We have a machine that, it is theorized, is running low on water. It is located in the opposite back corner of the plant from the feed, approximately 250+ feet of pipe away. I was instructed to add a new transducer to the line near where the mid-point will be, ~300+/- feet away. The engineers are theorizing that controlling the pressure from that point might be a more accurate spot in relation to what the system is doing on the back end.
Things that are known:
- If the system pressure gets to high (+95 psi), the pressure relief will blow off.
- As one would imagine, simply plugging the new transducer in for the PID's CV doesn't work: it causes the system to oscillate from ~45 psi to ~75 psi.
- Because of how the system is reading the new transducer, (over Ethernet through a RIO), it seems to have a better response time in the processor than the old, which is read directly off an analog input card on the SLC. (There actually appears to be points where that card "locks up" and freezes the PV, but I am not sure as to why. I believe this is most of our problem with the system, as it allows a drop in pressure. When it does start reading correctly again, it is usually bordering on kicking in the second pump.)
- The PLC enclosure looks like a family of large gerbils sleeps there. Too many hands have been in there over the years, and really the entire thing needs to be re-done. It is in the plan, but not for a long while, and we still need to run.
- I will not be able to run any step-tests on the system for a few weeks, as we will be running production.
- All of the scaling and set-up in the program seems to be ok, from what I have read on here and in the AB manual.
Things I do not know:
- What the hell I'm doing. Actually, where to start on trying to get things tuned in. I know that blindly stabbing at settings is not the correct way to do things, but it is all I have to go on for the moment.
- Whether this is the correct road to be traveling down or not. Where is the best spot to be controlling a system such as this?
- Whether all of the settings are where they should be. Are things set up correctly? Is there a better way in which to control it all? I'm just not sure at this point.
- Shouldn't there be some Differential setting? I realize that it is not always needed, but with a system such as this and the rapid changes that it sees, I would think that it would be a necessity... But what do I know?
I was pretty sure that I was going to need help in getting this thing tuned in from the start (our guy was not available this week), so I have been reading and gathering information prior to posting here. Hopefully, I have enough so that some guidance can be provided, although I will certainly do my best to gather anything that might be asked of me. My plan at the moment is to try some different parameters in the PID and see what happens. Anyone with any thoughts, ideas, suggestions, whatever - please don't hesitate to post.
Thank you all for your time in advance (and sorry about the length).
- Jeff
Our process water system is supplied by two 90 gpm pumps driven by 7-1/2 hp motors controlled by AB PowerFlex 70's. Generally, these pumps take turns supplying the water to the system, unless our pressure drops to below 40 psi, in which case they both kick in to bring us back up. Under normal conditions, however, the analog signal is sent to the drive from a PID. Everything seems to be set up Ok and working, although I think that the system could use a good tuning. Here are the PID's parameters (sorry I don't have screen shots at the moment):
Control Block: N20:53; Process Variable: N20:14; and Control Variable: N20:20.
Kc= 2.0
Ti = 0.1
Td= 0.00
Loop Update = 0.10
Control Mode = E+SP-PV
PID Control = Auto
Time Mode = Timed
Limit Output CV = 0
Deadband = 0
Setpoint = 4915
Smax = 16383
Smin = 0
PV = 4915
CV% = 64
Output Max = 100
Output Min = 50
SE = 0
The only flags that are enabled are: Timed Mode (TM), Output Limiting (OL), and PID Enable (EN).
The 4-20 mA pressure transducer that is currently reading the output from the pumps is about 15 feet away. It will generally hold our Set Point of 60 psi, although you can watch and see a definite drop in the pressure at times under load. The system will eventually be a looped system, but for right now, it is ended approximately 350 feet from the feed.
We have a machine that, it is theorized, is running low on water. It is located in the opposite back corner of the plant from the feed, approximately 250+ feet of pipe away. I was instructed to add a new transducer to the line near where the mid-point will be, ~300+/- feet away. The engineers are theorizing that controlling the pressure from that point might be a more accurate spot in relation to what the system is doing on the back end.
Things that are known:
- If the system pressure gets to high (+95 psi), the pressure relief will blow off.
- As one would imagine, simply plugging the new transducer in for the PID's CV doesn't work: it causes the system to oscillate from ~45 psi to ~75 psi.
- Because of how the system is reading the new transducer, (over Ethernet through a RIO), it seems to have a better response time in the processor than the old, which is read directly off an analog input card on the SLC. (There actually appears to be points where that card "locks up" and freezes the PV, but I am not sure as to why. I believe this is most of our problem with the system, as it allows a drop in pressure. When it does start reading correctly again, it is usually bordering on kicking in the second pump.)
- The PLC enclosure looks like a family of large gerbils sleeps there. Too many hands have been in there over the years, and really the entire thing needs to be re-done. It is in the plan, but not for a long while, and we still need to run.
- I will not be able to run any step-tests on the system for a few weeks, as we will be running production.
- All of the scaling and set-up in the program seems to be ok, from what I have read on here and in the AB manual.
Things I do not know:
- What the hell I'm doing. Actually, where to start on trying to get things tuned in. I know that blindly stabbing at settings is not the correct way to do things, but it is all I have to go on for the moment.
- Whether this is the correct road to be traveling down or not. Where is the best spot to be controlling a system such as this?
- Whether all of the settings are where they should be. Are things set up correctly? Is there a better way in which to control it all? I'm just not sure at this point.
- Shouldn't there be some Differential setting? I realize that it is not always needed, but with a system such as this and the rapid changes that it sees, I would think that it would be a necessity... But what do I know?
I was pretty sure that I was going to need help in getting this thing tuned in from the start (our guy was not available this week), so I have been reading and gathering information prior to posting here. Hopefully, I have enough so that some guidance can be provided, although I will certainly do my best to gather anything that might be asked of me. My plan at the moment is to try some different parameters in the PID and see what happens. Anyone with any thoughts, ideas, suggestions, whatever - please don't hesitate to post.
Thank you all for your time in advance (and sorry about the length).
- Jeff
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