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from the Drive tug of war department...
Need help driving a chain conveyor w/ 2 motors
Motion control, servos, steppers, etc. topic
Posted by Jeff Groves on 9 July, 2001 - 1:16 am
Hi all,

I have a problem here that I could use a little help with.... I have a overhead monorail chain conveyor project that I am working on. I am attempting to retrofit the 3 HP eddy current drives on this conveyor with 3 phase motors and AB 1305 VFDs. I just got done re-installing the eddy current drives after a failed attempt to make the motor/VFD combination work. The conveyor has two identical cat drive units, with appproximately 600/1 reduction from the motor to the output shaft of the drive.

The trouble that I am having is I can't seem to balance the two drives out. One gets ahead of the other and I end up with too much tension on one side of the chain loop or the other. There is a chain take up on one side of the loop which provides a little forgiveness, but the other side of the loop is a direct chain connection between the two drives. I have tried a number of different configurations in the VFDs, but after the conveyor runs for a while it gets out of whack and needs human intervention to get balanced out again. The conveyor layout is relatively flat, so the parts won't present a large load.

I am thinking about finding some kind of a load cell to allow me to monitor the tension on the side of the loop without the take up and vary the speed of one drive based on the chain tension. That way I can balance the chain tension evenly between the two drives. I was hoping to be able to simulate this by monitoring the drive current and varying the speed of the drive to keep the tension constant, but the gear reduction is so high that the motor doesn't even seem to notice that a conveyor is connected to it! I did go a little wild on the motor size, 5 HP that we had on the shelf from a scrapped machine, but I essentially derated them to about 2-3 HP by limiting the current with the VFDs.

If anyone out there has experience with a problem like this I would sure appreciate your comments. At the start of this project this didn't look like it was going to be a big deal, but I spent all July 4th weekend fighting with it and I couldn't figure out a way to make it work.

Thanks in advance for the help.... I'm not an engineer so go easy on me with the PID loops and all that stuff... If anyone knows of a device that will measure the tension on the chain and get me 4-20 ma back to the PLC I would appreciate that also!

Jeff


Posted by Kirk S. Hegwood on 9 July, 2001 - 4:39 pm
On a number of projects that we have done, we did what your trying to do and
with no real luck. We now run up to five motors on a monorail system off of
one drive. The gear reductions and the motors are all the same size and
this works well. We also have done small two motor systems the same and
with equal luck. There are some spring take-ups and proximity switches that
monitor jams, but this rarely happens. Also, this monorail system is
driving some machinery at the same time.

Good luck,


Kirk S. Hegwood
President
Signing for Hegwood Electric Service, Inc.
Kirk.Hegwood@HegwoodElectric.com


Posted by Jason Halligan on 10 July, 2001 - 1:53 am
Hi Jeff,

We run non mecanhicly interlocked chains using synchroniser cards, we work through our local manufacturer at www.pdl.co.nz.

These synchronisers measure motor or gearbox rev's via rotary encoders and match rpm for rpm which compensates for minor actual load factors.

We also use torque limit arms on the main drive chain, just a spring loaded arm that hits a standard limit-switch just in case a non electrical condition is breched, ie a piece of steel is jammed in chain for an early smoko.


Best of luck,

Jason Halligan
JAS.KIM@xtra.co.nz


Posted by Scott Whitlock on 10 July, 2001 - 10:17 am
I've seen load sharing a few times on similar conveyor systems, and it always uses a *single* VFD to control two or more motors. You need to impedance match the motor leads, but it works well because the AC voltage to each motor will be at exactly the same frequency, so they always operate at exactly the same speed.

Of course, this means you need the same gear ratio on each of the CAT chains, but I don't think that's a problem.


Posted by Bill Sturm on 10 July, 2001 - 10:46 am
You need a "load sharing" setup. There are several techniques for doing
this. I have done this by sending the actual torque out of one drive as an
analog value and using it as the direct torque setpoint into the second
drive. Have you asked AB tech support for any suggestions? I am not sure
if the 1305 is even capable of such an application. This is actually a very
tricky application. Have you tried turning the velocity loop gains way
down on the two follower drives, so that they don't fight as much.

Also try turning the velocity integral term to zero on the slave
drives. This might help a lot. Try this first, actually. Then adjust the
velocity proportional gains to balance out the system.

The load cell idea may work, as long as the drive has an internal PID
loop. Another idea would be to add encoders to the motors and purchase
closed loop drive controllers from an outfit such as Contrex or Drive
Control Systems. They could position lock the salve drives with the
master. They could also explain exactly what you need to do.

Good luck,

Bill Sturm

If you need more help, email me directly or through the list. Directly is
faster.


Posted by Bob Desrochers on 10 July, 2001 - 1:39 pm
Hello Jeff,

If those VFDs were attached to a small plc-5 (via RIO) the problem should
be easily solved. These drives can be configured to report output power
back to a plc in either percent drive capacity or KW as a "word" of
information with good resolution. (defined range = 0 to 32767) The scale
can be trimmed for your application to further improve resolution. (less
than motor nameplate) I believe that it could reasonably show you the
difference in torque between the two running units. This would allow you to
"trim" the speed of one (as a follower) so the work would be "shared"
equally between the two thus eliminating a binding chain. The benefit of
this strategy is there would be no sensor to fail/maintain.
With 600:1 boxes I would try to "learn" the drag of the individual boxes
(without the chain) so the proper torque balance/offset would arrive sooner
;)

Bob


Posted by RogerD on 10 July, 2001 - 5:08 pm
Hi,

You will never get 2 1305's to do that, they throw around way to much current and regulate poorly.

The best way is to run the master as and the slave off the same digital reference(pulse train)or better yet close the loop and follow$$$$.

The cheap and dirty way is to size the same motors right on and put them on the same drive of at least the total HP(some vendors higher).


Posted by Tom Gianni on 11 July, 2001 - 11:51 am
You should contact AB tech support. This is a typical VFD application, and
the supplier should know best if and how your VFD model can do it; as well
as offer alternate solutions.

Some ideas: First make sure the 2 independent VFDs have identical
calibration for the min/max speed range and speed/command volts ratio. I am
assuming they both get the same command signal. Of course the motors should
be identical models as well. Note this is basically the same as running both
motors off one drive. If this isn't the cause, using the VFDs in open-loop
control is a likely suspect.

In open-loop control, each motor can have slightly different speed depending
on slip, which depends on the instantaneous load each motor sees. Your load
application may require closed-loop speed control or even angular
synchronization. The fact that old eddy-current drives did this application
tells me high performance angular synch should not be required. Another
possible solution is master-slave configuration where the slave can either
be operated as a speed slave (possibly driven off a master feedback
pulse-train)or simply a torque helper. Torque helper may not work since you
don't have a rigidly coupled load. If bi-directional, I think torque helper
absolutely won't work.

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