DMM AC servo questions gearing/encoders/motor sizing.. please help

I'm in the process of gathering materials for my next system build and these dmm ac servos seem to be a really sweet package. I'd like to get 500ipm rapids but that is looking like a huge pipe dream after running the #s.

I'm currently using linuxcnc on my small ~4'x4' router. I'd like to stick with it if at all possible. I'm getting decent ~150ipm rapids because I've only got something like a 2k encoder on my Dc brushed servos...

I can get something like 40k pulses/second out of my PC. As I understand it there is no smooth stepper in the linux cnc world so this is going to be the limiting factor. Even with a 10mm pitch screw, I'm still only at 57IPM. an 1:8 gear results in 461IPM, but then I'm only able to move 5 thou minimum and I've probably hobbled the servo motor at that point.

Some quick numbers.. maybe I've screwed up my math... please tell me I am off :)

pulses/sec mm pitch pulse/rev rev/sec inch/min geared 1:2 motor:screw geared 1:4 motor:screw geared 1:8 resolution geared 1:2 res geared 1:4 res geared 1:8 res 25000 5 16384 1.525879 18.02219181 36.04438361 72.08876722 144.1775344 0.00030518 0.000610352 0.001220703 0.002441406 35000 5 16384 2.13623 25.23106853 50.46213706 100.9242741 201.8485482 0.00030518 0.000610352 0.001220703 0.002441406 40000 5 16384 2.441406 28.83550689 57.67101378 115.3420276 230.6840551 0.00030518 0.000610352 0.001220703 0.002441406 25000 10 16384 1.525879 36.04438361 72.08876722 144.1775344 288.3550689 0.00061035 0.001220703 0.002441406 0.004882813 35000 10 16384 2.13623 50.46213706 100.9242741 201.8485482 403.6970965 0.00061035 0.001220703 0.002441406 0.004882813 40000 10 16384 2.441406 57.67101378 115.3420276 230.6840551 461.3681102 0.00061035 0.001220703 0.002441406 0.004882813

There is no SmoothStepper for LinuxCNC but the boards from Mesa Electronics are well supported and some offer step/dir generators as well as giving you tons of I/O options.

This post mentions one combination: http://www.cnczone.com/forums/linuxc...ml#post1191576

bob

Quote:

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Originally Posted by rowbare

There is no SmoothStepper for LinuxCNC but the boards from Mesa Electronics are well supported and some offer step/dir generators as well as giving you tons of I/O options.

This post mentions one combination: http://www.cnczone.com/forums/linuxc...ml#post1191576

bob

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Rowbare,

Thanks. So your thought is that I can use the mesa board as a step multiplier?

It turns out there is a "Gear_Num" parameter in the drive that seems to give some control of the coarseness off the encoder. I sent an email to DMM to get more details.

The mesa board has a step generator so you aren't limited to 40k pulses per second. The pulse rate is in in the MHz so you can use a high encoder count and still get the rated speed from the motor. That should give you all the speed you want. It would also allow you to gear down and increase torque at lower speeds. Or if you haven't bought the screws yet, you could select a finer pitched screw. However you have to be sure to look up the critical speed for the screw you are planning to use to make sure you don't overspeed it.

While looking for a link about critical speed I came upon this: http://www.nskamericas.com/cps/rde/x...w_Tutorial.pdf. You might find it interesting.

bob

It turns out that I can actually configure the servo drive/motor to act look it has 2000 count encoder. This should get me close to the speeds I want.
Even with the 2000 count encoder with a 5mm or 10mm pitch screw I'm still under a thou with my minimum resolution. Probably good enough for a home build.

I'm looking at the mesa cards now.

I have various ballscrews on hand. I have a set of 4' and a set of 5.5' heavy duty thk rails so this is the building block for the router. I also have some rather large diameter Star/rexroth ballscrews that I got at an industrial auction this past summer. I may use the 32mm diameter 40" screw for my y axis and a large 50mm diameter 65" long screw for my X axis. I think I'm going to have to do a rotating nut for that though...

That's a good overview of ballscrews thanks.

blau_schuh

The Encoder is still 16384 counts per rev, the step pules is all that you need to be concerned with,for working out your gearing/IPM, is 2000 pules per rev, with the large diameter Ballscrews you have ,you will need a larger motor size than the 400W you have,Gearing will help, but 750W would be more in line for those Ballscrews if you are going to use them