Question about anti-backlash gears
Question about anti-backlash gears
(OP)
Hello all,
I've recently attempted to field an antenna rotator system and have been stalled by what appears to be a backlash problem. I currently have a closed-loop feedback in place using an optical encoder. The encoder is driven via a 60-tooth, 24 DP, anti-backlash gear mated with a normal. 60-tooth, 24 DP spur gear directly coupled to the load. Having done some accuracy testing, it looks like we're seeing as much as 0.5 degrees of over-travel in this system whenever it reverses direction. This would be consistant with 0.5 degrees of lost motion at the encoder (thus the servo drive "thinks" it has to travel 0.5 degrees further to meet the position). Since this is my first time using anti-backlash gears, I have to ask if any more experienced people out here have any suggestions?
I've recently attempted to field an antenna rotator system and have been stalled by what appears to be a backlash problem. I currently have a closed-loop feedback in place using an optical encoder. The encoder is driven via a 60-tooth, 24 DP, anti-backlash gear mated with a normal. 60-tooth, 24 DP spur gear directly coupled to the load. Having done some accuracy testing, it looks like we're seeing as much as 0.5 degrees of over-travel in this system whenever it reverses direction. This would be consistant with 0.5 degrees of lost motion at the encoder (thus the servo drive "thinks" it has to travel 0.5 degrees further to meet the position). Since this is my first time using anti-backlash gears, I have to ask if any more experienced people out here have any suggestions?





RE: Question about anti-backlash gears
Have you taken a look at the spring(s) to see if it is broken, missing, or disconnected? Also, the two plates of the anti-backlash gear should rotate freely against each other. If they stick, you may need to clean and/or lubricate them.
Don
Kansas City
RE: Question about anti-backlash gears
Mike Halloran
Pembroke Pines, FL, USA
RE: Question about anti-backlash gears
What is your gear ratio?
I was a design lead for a drive system for waffer handling within a PVD (Physical Vapor Deposition) equipment. This turned out to be a high precision gearhead 3 arc-seconds. This was driven by the robotic end-effectors needing a near absolute position for waffer pickup.Todays, motion control electronics can then be programmed to correct error in position.....ie backlash can be compensated for electronically by using an external absolute encoder for comparison to the shaft encoder.
it appears that something else is wrong with your system. I would consult with a motion control expert.
Best Regards,
Heckler
Sr. Mechanical Engineer
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RE: Question about anti-backlash gears
The encoder is using SSI interface and is currently giving us 32770 steps per revolution. Our motion control board uses a non-floating point processor, so we sometimes lose a few counts doing the math, but a few counts is well within our error tolerance and is nowhere near enough to cause this kind of error.
The gear springs are in good position, the halves do slide across each other easily. Prior to engaging them, I tried to vary the amount of "pre-load" and it seemed like 1-tooth offset felt like too little and I wasn't able to easily do more than 2 teeth of offset. So, the anti-backlash gear is preloaded with a two-tooth offset (not sure if this is the correct terminology - but I mean that the two halves the anti-backlash gear are phased two teeth apart to preload the springs).
The ratio of the gears driving the encoder is 1:1 (60 teeth:60 teeth).
Our previous rotator did not have closed-loop feedback and used a homing-sequence and limit switches to find "home" and then used the servo resolver to keep track of all motion from that point. We had to implement a backlash compensation routine in the code to compensate for the planetary reducer's backlash and that worked reasonably well. Since we are not using limit switches or a homing sequence this time, we aren't able to use the backlash compensation because every time we cycle power, we have a 50/50 chance of doubling the error instead of removing it - depending on which direction the rotator last traveled.
RE: Question about anti-backlash gears
Mike Halloran
Pembroke Pines, FL, USA
RE: Question about anti-backlash gears
RE: Question about anti-backlash gears
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Question about anti-backlash gears
RE: Question about anti-backlash gears
RE: Question about anti-backlash gears
somewhere. The error seems not possible
without something else wrong other than
the anti backlash pinion.
RE: Question about anti-backlash gears
If it is consistent then it's probably electrical.
I have a lot of expertise with gearing, but I'm not your best servo-mechanism person -- though probably up to par for a mechanical engineer.
We have an application where there weren't enough encoder positions (not accurate enough)to get consistent results, but that's already been covered. Actually there was more involved than just the positions, it's seems there was response speed and compensation involved as well. The processor couldn't keep up with it. my 2 cents.
http ://www.ang elfire.com /planet/ga dgetforum/ index.html
RE: Question about anti-backlash gears
The problem is VERY repeatable, and we haven't ruled out an electrical problem. Our servo/software guru has been "playing" with the drive tuning, but that doesn't seem to make much difference except in how much time it takes to settle down after a move. One thought that HAS occurred is that we originally ordered the encoders with a 13 bit signal (8192 counts), and decided that a 15 bit would work better for our needs (Greg is correct - it is actually 32768 counts now). The encoder vendor did change one of them for us but our software expert discovered a way to change it externally by changing the signal being sent to the encoder. We're going to do some testing tomorrow like what zekeman is suggesting.
Thanks again
RE: Question about anti-backlash gears
That was only a general rule of thumb I gave you. In the end you will probably find 2 to 3 problems layered under the onion skin. Probably more electrical but possibly a mechanical interaction as well -- my 2 cents as a product assurance test engineer (in a previous life) for monochrome laser printers.
http ://www.ang elfire.com /planet/ga dgetforum/ index.html
RE: Question about anti-backlash gears
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