Motion Control Novice Requires Help with an unstable system

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Thread Starter

Mark Johnson

I have a servo controlled system using a Moog 504 type amplifier and Moog motor driven by a Trio Motion controller. The system is used to drive linear motion through a ballscrew/leadscrew at 0.6" pitch. When this system was installed it ran OK for three weeks and then problems began to occur again(the original ballscrew was 1.875" and showed similar problems until it was changed to 0.6").

1. The motion, which follows a smooth profile was said to be very jerky.

2. If left standing over the weekend with power on the axis will drift (not a massive movement approximately 50-100 thou).

3. It will also drift during the course of the day whilst running and has to be adjusted 10-20 thou daily.

4. It also shows signs of not being able to hold position or a lack of "stiffness". There is a load applied to the ballscrew whilst in a stationary position. When the system is running perfectly this can be apprximately 0-2 thou because of mechanical losses. It is now showing 6-10 thou which is causing problems because the position has to be held for a "pick-up" action to occcur.

The system is programmed with gains that are broad to cover a range of machines, but have worked in the past (although this machine is subtily different).

Thanks in advance for any advice or suggestions anyone can offer.

Mark Johnson
 
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Charles Makai

Mark,

I have had similar problems in the past caused by inertia mismatches in the drive train. It is not uncommon in ballscrew driven systems to drive the screw with a timing belt drive using a 1:1 pulley ratio. If this is true for your system, and the pulleys are steel, they can be a significant source of rotational inertia directly applied to the motor, which can de-stabilize your system.

Be sure also to check the pulleys are secure. If the pulleys are not keyed to the motor\leadscrew shaft they may be slipping.

Regards,
Charles Makai
Manufacturing Support Services
 
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Cameron Anderson

I have seen problems like this. One system, the coupler would slip from time to time. The other system that drove me nuts was that the motor had a bad feedback cable. The system would run fine for a week, then fail every 2 hours, then run for 8 hours then fail. What I mean by fail is that it was loosing position. We needed +/- 0.001 and we were gaining counts intermitently. It came down to that the encoder cable wore out because it is constantly being flexed. Put a new cable in and now the problem went away. As far as the spongey controls, you can bump up your gains. Also look in the controller and see if its position is off. You should be able to monitor the position. If it says it is not off, but you put an indicator on the actuator and it is off, then your gaining or losing encoder pulses or your mechanics is sliping. If the controller says its off, then either the mechanical system is changing and affecting the controls ability to position or again your gaining or losing pulses. I would suggest tuning the system again making it stiffer.

Cameron Anderson
Motion Control Specialist
 
I like to share some of my experiences on the similar positioning problem that you are facing.I agree with Mr. Cameron Anderson that gain tuning is very important in a system which requires very precise positioning. Tune it to get a stiffer system. Besides that, you can set the in-position windows parameter to make the specification for position error tighter. Another thing is the encoder cable might have some electrical noise interference. I don't know about your type of encoder cables. Some manufacturers sell those cables with motor in a complete set where this cable are shielded. But some machine maker will prefer buying motor with flying leads wires where there is only a bunch of short wires coming out from the motor and they will make an extension cable from this wires to their controller/drives. Of course this type of motor sold is cheaper. Again some machine builder will use the normal cable for the extension cable where there is no shield at all. We have experienced this where there is only a very tiny out of position throughout 8 hours operation. And the error is not accumulating. It simply occurs randomly. And finally found out that the shielding/grounding and the electrical layout system in the machine is easily affected by these interference. Hope these helps.
 
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Bob Talarico

Dear Mark,
I had similar problem on a machine a few years back. It turned out that the ball screw manufactured used a coupling between the motor and screw that was not torsionally stiff enough to han the load reflected back though system. The torsional wind-up was erratic and of course changed with the load. I finally found a company called "Diequa" who published the torsional stiffness and inertia of their metal bellows couplings. Also the .6 pitch may be to great allowing to much load to be reflected back to the coupling, you might want to look at that also.
Good Luck
Bob Talarico
 
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