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Old October 20th, 2018, 11:52 AM   #1
mike3475
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chain conveyor speed control opinion

Hi,all:
we have over head chain conveyor drove by PF525,the total conveyor length is about 1350 foot, its speed jumping too much (not stable), there is encoder connected to this conveyor. can we add speed PID control to make the speed smoother?

thanks!

Last edited by mike3475; October 20th, 2018 at 11:54 AM.
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Old October 20th, 2018, 12:06 PM   #2
jdbrandt
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Quote:
Originally Posted by mike3475 View Post
Hi,all:
we have over head chain conveyor drove by PF525,the total conveyor length is about 1350 foot, its speed jumping too much (not stable), there is encoder connected to this conveyor. can we add speed PID control to make the speed smoother?

thanks!
You could do lots of things, including the encoder idea.

Have you static tuned the drive (reference Parameter 40)?

I would think that finding the root cause of the slowing down would be a higher priority versus the encoder idea. Are we taking a minor slowing, like 1 or 2 percent, or something more noticeable?
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Old October 20th, 2018, 12:14 PM   #3
shawn_75
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Is this possibly a slack problem in the chain? Like the slack is building in some spot and then quickly releasing causing the encoder to see this as a change in overall speed? 1350 feet of chain seems like a lot of chain to allow slack to build up. Perhaps add multiple encoders over the length of the conveyor and average the results. Or maybe your encoder needs moved to a location less likely to see the slack build and release.
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Old October 20th, 2018, 01:12 PM   #4
mike3475
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we have about 150 racks hanged up on the conveyor, we have different weight racks, rack weight is between 200 to 500 pounds, i just checked reference parameter#40 is 0. right now the speed setting is 3.6 inches/s. we checked the trend for actual speed of the encoder,i couldn't post the trend here (3MB), the max jumping: 4.8 inches/s , the min jumping : 2.5 inches/s, when using eye we can't see the speed jumping.
Thanks!
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Old October 20th, 2018, 01:56 PM   #5
thingstodo
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Quote:
Originally Posted by mike3475 View Post
we have about 150 racks hanged up on the conveyor, we have different weight racks, rack weight is between 200 to 500 pounds, i just checked reference parameter#40 is 0. right now the speed setting is 3.6 inches/s. we checked the trend for actual speed of the encoder,i couldn't post the trend here (3MB), the max jumping: 4.8 inches/s , the min jumping : 2.5 inches/s, when using eye we can't see the speed jumping.
Thanks!
You said there is an encoder on the conveyor. Is it on the drive sprocket, an idler sprocket, a tensioning sprocket?

The speed up/slow down is likely dependent on your loads and where they are. But that's just a guess based on experience with other systems.

As @jdbrant suggested - finding the root cause of the encoder reading jumping is the most important step. Everything else you do should be related to what that problem turns out to be
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Old October 20th, 2018, 02:13 PM   #6
GaryS
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You might want to consider installing a second, helper drive at about the half way point.
To share the load some people call them pony drives. I have seen many of them used on long conveyor systems. If you do, setup the slave as a torque follower not speed follower
speed followers are never stable on long conveyor systems the tend to fight each other causing instability.
Any number of things can cause the speed to very friction , bad bearings, uneven loading.
Check the drive first see if the speed command is stable , is the frequency output stable just a few thigs to check.
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Old October 20th, 2018, 03:32 PM   #7
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This sounds like a Stribeck friction problem.
I agree with GaryS about the second drive and what not to do.
Maybe more drives will help but start with two.
Both ( all ) drives need to follow the same virtual master that isn't affected by friction.
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Old October 20th, 2018, 03:34 PM   #8
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Itís usually never a good idea to try and program around a bad design or mechanical issue. Sometimes itís better to add hardware rather than software.
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Old October 20th, 2018, 04:42 PM   #9
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Does your encoder have enough resolution to account for the low speeds of the conveyor? Where is the encoder mounted, as in is it on a high RPM shaft or a low RPM shaft?
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Old October 20th, 2018, 05:27 PM   #10
mike3475
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we use EDON model:A-4480 line tracker, it has small chain which have a couple blocks ,this block will gear with the chain conveyor,from above many thoughts I think the encoder actual speed is not real the chain conveyor speed, since there are many chains ( the first is the main chain conveyor, the second is the motor drive chain, the third is the encoder chain), another thing is this conveyor has too much load, it will cause slack issue, we are unable to add another motor to drive this conveyor, so it is hard to control this conveyor speed from encoder feedback.
Thanks everyone!
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Old October 20th, 2018, 11:25 PM   #11
GaryS
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It sounds like you have identified one of your problems. Conveyor has too much load.

This will cause stability problems if that is the case then you defiantly need a second drive.
One way to tell is to look at the motor load a good reference point is a normal load should be 75% of full load. This will give you a buffer for those times when the load increases. If the load is above that then you need a second drive. I know you said that that is not possible I think you really mean that it is not in the budget but sometimes you have to find it in the budget. I don’t like to use those terms not possible, about the only thing that is not possible is me walking on the moon in my life time.
You said you have an encoder connected to the main line chain through as series of blocks or sprokets to monitor the line speed. I hope you are not using that for the speed feedback loop to the drive. If so that is the worst configuration you can have you will ever get it to be stable that way. If you using this arrangement just to monitor the line speed then you could actually be running very stable but the monitor encoder is just showing the instability in the chain and sprockets a better place to get the line speed would be from the motor or even just the use the signal from the drive.

For feed back to the drive the encoder should be mounted on the motor this will give you more pulses to work with, better resolution = better speed control.
As I have said before the best way to connect the second drive on a common chain conveyor is to connect the second or slave drives in torque follower arrangement this also has the advantage that the drives don’t have to be the same size (HP) you can add just what you need to handle the extra load. Each drive will supply the same proportional torque for the drive and speed will always match.
Example a 50HP and 20HP powering the same chain conveyor the master controls the speed. Sa the torque on the master goes up the torque from the slave will go up so at 50% torque both drive will be supplying 50% of their rated torque.
I hope that helps
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Old October 21st, 2018, 10:53 AM   #12
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Thanks GrayS!
we didn't add the encoder feedback to the chain conveyor speed control, that encoder is for our loader and unloader system line tracking (loader and unloader system is for loading and unloading the racks on the chain conveyor).
i agreed with your thought about using this encoder feedback to control the chain conveyor speed is bad idea and the best solution is directly get feedback from the chain conveyor motor or its drive.
To be honest I never monitored the chain conveyor motor torque current before, since we added many heavy racks at beginning of this year, then the chain conveyor started many issues , I believed added many heavy racks caused the conveyor issues.I will monitor the chain conveyor motor load next week, see if it is over 75% of the full load, if that is true, I will let my boss know and make decision to add the helper motor.
Thanks everyone to help us!
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Old October 21st, 2018, 12:02 PM   #13
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If the "Conveyor has too much load" then probably during the periods when the load is high, simply the motor does not have enough power to move it at the speed set

in addition if the system regulates the passage time and not the speed it can be that when the load decreases the speed increase to recover the lost distance.

This would explain the oscillations above and below the nominal speed.
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Old October 21st, 2018, 12:23 PM   #14
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1) The PF525 offers velocity vector control only, and although it has good torque control, it does not have an actual torque regulator in that tiny little package. So you can't do a second drive as a torque follower using that series of drive; there is no torque control mode in a 520 series drive.

2) The Autotune parameter, P040, ALWAYS shows the initialized status, whether or not the autotune function has been performed. If you are unsure of whether or not it was done, just do it again, it can't harm anything. If you can uncouple the load from the motor and perform a rotating autotune, that is better than the static tune, but if not, the static tune is the bare minimum that must be performed.

3) When you use Vector Control of any sort with a PF525, it IS a velocity PID loop control, you don't need to add another one. Double check that you are in fact using the Vector Control mode, either Sensorless Vector (SVC, P039 = 1, the factory default), which doesn't use the encoder, or Flux Vector that does (P039 = 4). Since you have the encoder anyway, I would use the Flux Vector control. If P039 is set to 0 because someone thought that was easier and they didn't do an Autotune, that may be your problem right there... this is DEFINITELY a Vector Control application.

If those checks and any changes result in no improvement, either your mechanical situation needs attention of your motor is just too small for the load. The idea of a slave drive is probably the best approach, but you will have to change to a PowerFlex 753 for both motors.
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Old October 22nd, 2018, 12:44 AM   #15
mike3475
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Good learning from you guys, our setting: P#39 is 1 (SVC); P#40 is 0 (ready/idle)
that encoder we used right now is not for the chain conveyor speed control feedback, it is for our other system, it is EtherNet based motion encoder (842E-CM-SIP2BA),I just found that it can't connect to PF525.
I will monitor the conveyor motor torque current, see if it is over the full load.
I appreciated everyone inputs.
thanks again!
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