Not sure where to start.
Not sure where to start.
(OP)
So, we've installed this fancy new system with a dual motor driven chain, and the damned thing has had problems since day one. It slows down from time to time and interrupts production. We have considered a number of options to cure the problem, for example, we just changed the motors' encoders at the engineer's recommendation, and the problem is still occurring. We are not sure if the problem is one of controller programming, bad wiring, mechanical, or something else.
I have included some shots that may help to illustrate the issue further. There is an 8 hour time series of both of the motor's currents. When the chain is not loaded, you can see an oscillation with period of about 40 minutes, the time it takes for one complete chain cycle. When production starts, the current begins to rise, and peaks after about half an hour or so. Then the chain randomly jerks, slows down, and sometimes even fully stops then returns to it's normal operation.
I appreciate anyone who might be able to point me in the right direction. What other information should I provide to help mitigate the problem?
I have included some shots that may help to illustrate the issue further. There is an 8 hour time series of both of the motor's currents. When the chain is not loaded, you can see an oscillation with period of about 40 minutes, the time it takes for one complete chain cycle. When production starts, the current begins to rise, and peaks after about half an hour or so. Then the chain randomly jerks, slows down, and sometimes even fully stops then returns to it's normal operation.
I appreciate anyone who might be able to point me in the right direction. What other information should I provide to help mitigate the problem?





RE: Not sure where to start.
http://files.engineering.com/getfile.aspx?folder=4...
Here is a zoomed in version of one of the "spikes" on current:
http://files.engineering.com/getfile.aspx?folder=c...
And, here is a zoom in on one of the spikes affecting motor RPM:
http://files.engineering.com/getfile.aspx?folder=f...
RE: Not sure where to start.
RE: Not sure where to start.
RE: Not sure where to start.
making their output speed stiff.
One quick though without knowing much about the system is make one motor master of the correct speed of the chain and
the other motor is controlled to provide proportional torque only to aid the master.
RE: Not sure where to start.
An indexer just before the entrance to the cooler catches carcasses, and they ride the chain for 40 mins before exiting and being unloaded.
We changed the Encoders last night, bc we were only getting a 3.5V feedback signal, and 5V is the minimum. The number of "spikes" has halved today, but the problem is still significant.
RE: Not sure where to start.
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RE: Not sure where to start.
See if you can see the take-ups moving as the motors load up.
Check the chain for damp sections.
I have seen serious problems (shafts bending and teeth spitting out of exposed gears) in a blast freezer. This was a metal mesh conveyor which slid on plastic covered rails. As the blast room was cooled down, it went through a temperature zone where quantities of hoar frost formed on the sliders and was scraped off by the conveyor mesh. It turned out that during the period of hoar frost formation, the co-efficient of friction of the sliding conveyor increased many times normal. Although the open gear drives were over sized for the friction at normal operating temperatures, the gears would self destruct when the hoar frost formed due to the increased drag of scraping many feet of hoar frost off of the support rails.
If the problem is worse as the temperature drops below freezing and frost forms when the blast freezer is first started, it may be well to look for a hoar frost issue.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Not sure where to start.
It appears that you are running your chain backwards. Chain tensioners should always be at the lowest tension point in a chain loop. This is on the out-going side of a drive sprocket. I would guess that your problem is caused by the chain jumping a tooth on the drive sprockets. A chain as long as yours will have a lot of stretch in it. Going as slowly as it does the tooth jump is probably fairly slow and quiet.
RE: Not sure where to start.
I had a problem exhibiting similar symptoms from a different type of transducer a few years ago and the problem turned out to be power related: the average supply current was stated in the data sheet and we'd designed around that, but the actual current was a series of pulses and the cable resistance was too high to allow those pulses to be drawn without significant volt-drop. A decent-sized capacitor local to the transducer sorted that out, but the problem had us puzzled for a while.
RE: Not sure where to start.
RE: Not sure where to start.
It could be jamming. The red curve jumps right away when the chain catches because that drive is pulling a chain section with little give. The green current curve builds as that drive first pulls the slack out of the spring loaded take-up before it hard jambs.
RE: Not sure where to start.
RE: Not sure where to start.
My only other thought is that there is some sort of higher lever controller that is deciding which is the master and which is the slave, maybe by looking at both encoders to determine error as the chain stretches and expands in different temperature zones. If so, and the chain jumped a tooth, I can see that causing havoc in the control loop. The general slowdown might be the system changing back and forth between masters attempting to correct an error that was not accounted for in the design. Those blips of current dropping off looks to me as well to be evidence of that; as the chain slowly slips over the sprocket there is a momentary loss of load, which drops the motor current briefly.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Not sure where to start.
We are getting normal square waves on the A,A*, B, and B* lines on both drives, but the Z and Z* look strange. The documentation says that they should be square waves with frequency 1/3 of A and B, but we are seeing a strange form that looks a lot like noise.
RE: Not sure where to start.
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RE: Not sure where to start.
Most newer drives have a very good sensorless vector control algorithm. I can't see this application requiring closed loop vector control for these motors and I can't understand what else they would be doing.
You should be able to program a drive to output a 4-20mA speed signal. It might be telling to have both VFD speed and current on the same plot.
The applications I've seen basically have a master VFD. You tell the master VFD to run at a certain speed. A torque or motor load output is sent from the master to the slave VFD. The slave VFD is set up to run up to a speed where it is producing the same amount of torque or the same motor load.
Can you not involve the manufacturer or integrator?
RE: Not sure where to start.
http://literature.rockwellautomation.com/idc/group...
RE: Not sure where to start.
This may be pointing to a mechanical problem. However it may be a mechanical problem cause by a control issue.
Is this the output from the encoders? If so, have you bench tested the encoders? What type of wiring is used between the drives and the encoders? Length, wire gauge? Shielding? Possible voltage drop in the conductors?
Do the carcass supports run on rollers? are all the rollers turning or are some frozen?
Does the chain tend to bunch up on the output of the drive sprockets? Is there excess movement of the take-up rolls?
Consider: Is the chain speed actually slowing down or is a lost or faulty encoder signal indicating a stoppage or slowdown when the chain is actually running normally? The speed and current spikes may be indicative of an actual slow down, or they may be the drive's reaction to a faulty signal showing that the drive has lost speed abruptly.
Unforeseen occurrences; Is the portion of the chain subject to occasional wash down water which may be causing a problem as the wet chain transits the cold room? This is one example but the list of the unforeseen is endless.
If you have bunching or excess take-up roll movement, you may need more tension on the take-up rolls.
If the encoders give good signals on the bench it may be well to inspect the field wiring to the encoders.
A mechanical problem causing electrical upsets or an electrical problem causing mechanical upsets, that is the question.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Not sure where to start.
RE: Not sure where to start.
I still don't see where the encoders come into play but I expect the encoder is being used by the VFD for closed loop vector control. If this is true, you could try switching the VFD's to sensorless vector mode which eliminates using the encoders, assuming this speed-trim load sharing feature still works in sensorless vector mode.
It appears you have 2 encoders with 2 wire runs back to 2 different VFD's. In other words, 2 separate systems. Yet both seem to be exhibiting the same encoder issue. This would make me suspect a connection error. Both are wired wrong causing both to work incorrectly in the same manner.
RE: Not sure where to start.
RE: Not sure where to start.