Kinetix pick and place "shuddering"

we have a pick and place setup that consists of a Y-axis mast that moves back and forth on a horizontal carriage x-axis.

The mast hangs on, and is moved up and down, and back and forth by means of a cogged belt that is routed on the pullies of two servo motors controlled by Kinetix drives and a control Logix PLC

The motors both turning in the same direction will move it in the horizontal, and the same two motors turning in opposite directions will move it up and down.

This whole setup has been operating for probably 15 years but lately when the mast is moved to near the end of the x-axis ,and then down the y- axis ( a place ) it "shudders" as if something mechanical is causing it...

After rebuilding and adjusting this thing about three times over the past week, I am completely convinced there is nothing mechanical beyond the motors' gearboxes causing this. And it only happens intermittently during the "place" move.

We get no errors on the Kinetix drives and the motors have to turn pretty precisely to move the thing straight up and down. I can't help but thinking that a problem in one motor's gearbox would cause the mast to move at least partially sideways as well as straight up and down.

It seems to me that the shuddering should in itself be enough to cause some sort of error on the Kinetix, and I can't imagine the PLC giving the Kinetix some command that would cause it to run correctly one time, and shudder the next.
I hope someone has an idea... thanks

I wanted to add, that there are brakes on these motors, but I do not think they are employed during any of the "moves", and the shuddering occurs when the motors would have to exert some sort of force on the mast to keep it from falling by gravity. there is a pick move on the other end of the carriage that also would fall by gravity, but it doesn't go as low.

It is hard for me to understand how a motor would hold a weight from falling by gravity while allowing it to descend at a controlled speed. It probably takes about 20 - 25 pounds to hold this thing up.

I am convinced there is nothing mechanical beyond the motors' gearboxes causing this .

My first thought when I saw that a PnP is shuddering is that one of the belts is slipping. That would cause the shuddering motion, but not fault the Kinetix. The reason for that is the servo is moving the correct number of counts in the proper amount of time, so as far as the Kinetix is concerned, there is no problem. You said the machice was rebuilt, but could you be more specific please? Were the belts and all drive pulleys replaced, or just inspected? Have you checked the tempeture of the gear boxes when the machine is running, and do you have a similar machine to compare it to? It's possible that you have a loose set screw or worn shaft where the motor and gearbox connect. The only way to know for sure is to pull them apart and have a look. In my experience a truly bad gear box will cause the drive to fault with either a velocity, or over current error, but that could depend on how much error the drive is programed to accept. When the shuddering takes place, are the pick heads ending up at the right pick and place locations? It may take many cycles for them to be off a noticable amount, but if you have belt slippage, you will eventually notice that the heads are out of position for the pick and place. On our PnP machines, we have 13 of them in this plant, The brake is only applied when the machine is at rest. I really think you are looking at a drive belt problem either in the belt itself, or one of the drive pulleys.

Hey! thank you very much for your thoughtful and knowledgeable reply.

Not to be argumentative, but to ask, or try to discuss... because you obviously have an understanding of this and because this kind of work kinda requires that you make deductions from what you can observe...

The belt is cogged and is well tensioned, although I will certainly look into it.
Could be the pullies on the shaft, or the gearbox on the motor , as you said...
but the pick head does end up in the correct place... which makes me wonder about ONE motor or gearbox . Seems that should make it move sideways some. and throw off every other position too.

The y axis (the mast) is aluminum and about 3-1/2" square and 5' long; it runs up and down on "plastic" rollers that are on bearings. The belt has it's own pullies on the same shaft but they are free to turn independently of the mast rollers.

The mast rides back and forth on a carriage that runs on a rail assembly that has a motor / gearbox on each end. The motors are stationary.

Every bearing and roller and the mast, carriage and rails itself has been replaced and adjusted with eccentrics etc. by me. It works freely but doesn't wobble. I don't know how it could be any better. Releasing the brakes allows the mast to fall - not quite as fast as it would without the belts and motors - but quite freely. The belt and drive pulleys were not replaced, but inspected. Maybe I will inspect them a little closer or at least replace the belt.

The only time this thing does this is on the "place" move. The belt has been off three times. I imagine the pullies and gearboxes are not in the same position as they have been originally but they might be.

I never looked into it but I imagine it would take four or five turns of the pullies to complete a place move.

I will certainly look into the pullies.


Is there nothing in the drives / PLC that could cause this without throwing an error of some kind?

My initial thought is backlash.

From your description it sounds like the axis is decelerating when the shuddering occurs. Over time some part of the drivetrain may have worn to where there is now some backlash and the drive is having a hard time dealing with it.

Is it possible to run it through a cycle without actually picking up a part to see if it works any better when not carrying a part? Better performance with reduced load lends credence to the diagnosis of backlash. You could also try reducing the deceleration rate to see if that makes it better. Try jogging to the place where it shudders and stop there with the servos enabled and holding position. Then try to push the mast out of position and observe how well it resists your efforts. With the servos disabled and the brake off, move the mast to the problem area and try moving it forward and back to see if you can feel any looseness compared to other areas of travel.

Realolman,
You're absolutely right in your assumptions about the belts and gearboxes not being the problem. Belt slippage would certainly cause shuddering, we see it all the time here, but it would of course also throw the system progressively further out of alignment.

On our PnP systems, we have a prox that we passively home the system to. It defines the left and right hand motor positions to known values. After each cycle the PnP is sent to these coordinates, and if the prox is not made, then belt slippage has occurred.

Our PnPs our much larger than yours, approximately 10 feet of x-axis and 5 feet of y-axis. We occasionally see whipping of the belts before they slip as they wear. They have to be tensioned a few times as they age before being replaced.

As you know, another issue is the roller bearings. You've checked those, so they aren't the issue, but I've seen a bad one cause excessive heat and shuddering at end of travel like you describe.

It's rare, but I've also seen the motor brake cause similar problems, but usually it will cause overtemp or position error faults. And if that were the problem, the pick head wouldn't drop freely when you release the brake.

How sure are you that no one has modified the code, or changed move speed, accel, decel, or jerk settings? If your velocity or decel is too high, you're going to notice it the most exactly where you're seeing the shudder now. The pick head and mast are furthest extended as you are putting the most torque on the system, from a full-on vertical move to a quick stop at the place position. On our machines, the operators can bypass our mickey mouse FactoryTalk security on the HMI and jack around with the speed settings. Seems to happen every time we get a new young guy operator who wants to beat the other shifts haha.

This is where I would be looking, especially once you've gone through all the mechanical. If you want to post or pm me the code, I'd be happy to take a look at it for you. Probably not much anybody can do to diagnose this without being on-site, but you never know.

Cheers,
Dustin

thank you both very much... replies like yours are what makes this site A+.

I have to leave for a commitment, so I can't digest what you've said, but

I wanted to thank you all . I appreciate it very much

As you said earlier, I would replace the belt. I don't think it is slipping. I think it's spring rate changed; it got spongier. So your system is now on the ragged edge of oscillating at a natural frequency that can be excited by something in the control system. When the belt was new and stiffer the natural frequency was far enough away from the the excitation that it wasn't a problem. You might be able to confirm this by tightening or loosening the belt some and seeing what happens with the shudder.

Keith

Is this correct?

• The shuddering only occurs when carrying load.
• You are not loosing position.
• the machine is existing

The above all imply that you have a load issue - usually the mechanical system has changed - someone added weight or something mechanical is not optimal
given that you have totally verified the mechanical system after the gearbox output I would be looking at the gear box, brake, servo motor and feedback.


Most likely is
brake - not releasing correctly due to electrical or mechanical issues


Tests that may help
disconnect the PnP belts - can you turn by hand? - if so replace the servo motor as the brake is not applying correctly.
Manually release the brake on the motor (no PnP belts) - can you turn by hand ?


Electronic tests
set the servo up to show torque feedback - http://www.plctalk.net/qanda/showpost.php?p=427942&postcount=2
trend the torque on the disconnected system - at constant speed the torque should be very stable and low
Trend the torque on a full system - looking for mechanical tightness as you travel

thank you all very much for your replies

... looks like I'm gonna be busy tomorrow

Not being able to see your machine, it's a bit of a guess. I know on ours it takes less wear than you would think to get the belt to slip. Usually we notice it when the axis it either in the accel or decel. Belt strech is not something we have had an issue with on our machines, but then again, it's not one of our machines, so I wouldn't rule it out. We have in the past had issues with the set screw coming loose in the gearbox, or the shaft simply getting worn, but usually the problem get very bad, very fast when that happens. The same is true if one of the cogged pulleys is coming loose. If you have time could you post the manufacture and model number, as well as how many cycles a minute the machine runs.


"On our PnP systems, we have a prox that we passively home the system to. It defines the left and right hand motor positions to known values. After each cycle the PnP is sent to these coordinates, and if the prox is not made, then belt slippage has occurred."

On our machines the axis are only homed during the start up routine, so if something is slipping you usually don't know until you see it out of position, or the carriage crashes into something it is not supposed to.



"Most likely is
brake - not releasing correctly due to electrical or mechanical issues"

Again, this shows just how many different machine setups there are...
the brake on our machine is only used if the machine is between cycles, or stopped. The brake does not engage at any point during a pick or place motion.
It's is nice to see different points of view, and different aproaches to trouble shooting an issue like this. It will certainly help me keep a more open mind to different possible causes the next time we have a problem like this. Thank you all.

Will.