Servo motors

Electronics related to CNC

Servo motors

Postby dark_helmet » Wed Feb 08, 2012 11:30 pm

Researching building a gantry mounted laser with 48-60" X and maybe 36" Y. I happen to have 3 brushed servos on the shelf, and was thinking I could maybe use them, only problem is they are a bit large and would most likely be overkill. They are 87 oz/in per amp with a max 21 amp @ 65 volts. So it looks like the gecko G320X would drive them just fine. Thinking linear encoders, but haven't gotten that figured all out yet either.

I could see that they would probably stay cool running light loads of a laser with small amp, but just wondering what the drawbacks might be to using too large a motor. Would probably be able to push some nice acc/decel on the X axis and get nice speeds for engraving, but with a gantry mounted tube would have to limit the Y axis down a bit to be gentle on tube. Would most likley require some gearing down to the belt drives on laser but that shouldn't be a problem I don't think.

I could have access to a scope to tune them as well, though the knee mill I did a retrofit on years ago I can't recall using a scope to tune them so would have to look up how to do that again. Been awhile since I fiddled with this stuff.

Just looking for thoughts/ feedback to see if I'm nuts and should just by some smaller steppers and be done or if this would work and could possibly be better or at least use stuff I already have.
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Re: Servo motors

Postby macona » Thu Feb 09, 2012 10:19 am

Ii have done lots of stuff with servos. In fact, I almost never use steppers if I can get away with it.

Skip the geckos, I have used them and they suck. I would look at the Whale3 drives. One thing is you tune via USB so no scope needed. A good review here:

http://www.thecubestudio.com/ServoDriveReview.htm

Linear encoders may not work very well. The problem is you need a very, very stiff system to use them. This means preloaded ball bearing linear guides and probably ball screws or a very tight belt drive. Any lost motion in the system can cause tuning issues and lack of performance. My cnc mill was originally set up like this and it severely limited the performance of the machine because of backlash in the ball screws. I am using linear encoders on mine but I have gone though and am going though a lot of effort to stiffen everything.

Putting encoders on the motors themselves is usually the best bet. Don't worry about having too big of a motor, well, unless these things are really massive. Disadvantages of a too big motor is an inertial mismatch between the motor and the load which can make it more difficult to tune. It can also lower the accel and decel times since the mass of the armature is just that much more and you will be moving one of the motors on it's axis.
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Re: Servo motors

Postby dark_helmet » Thu Feb 09, 2012 3:49 pm

Thanks for the info. Those drives look good.

The linear encoder problem sounds reasonable and I guess I could just stick with the rotary encoders and go with a tad larger reinforced belt to keep and belt stretch overshoot problems to a minimum. The motors themselves that I have are just under 11 lbs each, but I think it should be fine as I know they are powerful and responsive. I'd only have to worry about moving the weight of one of them but would be the Y axis which would need to be kept a bit gentle with the laser tube gantry mounted as well.

Starting to get the general stuff figured out and will probably have to start drawing this all up soon and price it out. Fun times ahead.
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Re: Servo motors

Postby dark_helmet » Thu Feb 09, 2012 3:52 pm

One more quick question. Will those whale drives work with a DSP?
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Re: Servo motors

Postby macona » Thu Feb 09, 2012 10:24 pm

Belts really don't stretch. If they do you have broken the internal strands and the belt is not long for this world.

It seems they have done a revision of the product line. Whale3 has been replace with the DG2S-08020

When you do get your encoders you want differential output. If not you will need a converter board: http://www.cncdrive.com/Difflinedriver.html

The drives have a common ground input so you will need to wire it like a gecko G251 or G540. I think there is info on how to do that out there.
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Re: Servo motors

Postby KyleAdam » Thu May 24, 2012 11:52 am

There are several different options available in the CNC servo motors department and the motors come in a wide range of sizes.Every servo motor repair Industrial Servo & Controls Repair Inc. is carried out with specialized equipment and Perfect and correct alignment of feedback devices is very crucial.
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Re: Servo motors

Postby gavztheouch » Thu May 24, 2012 1:08 pm

I have have had some experience using www.granitedevices.fi for servo drives. I am really happy with them, they are very easy to set up as they have a built in scope and there are a few tutorials on the website on how to go about doing the tuning.

However if I was going to use servos again I would look out for a second-hand set of Panasonic or Yaskawa AC servo motors and drives on ebay. For $300 or less you can pickup a 200w or 400w system with all the cables, and there is no power supply to buy, plus they are easy to setup and run.
Timothy hay for rabbits delivered over the UK http://www.timothyhay.co.uk
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