Question regarding servo drives and ratings. How much speed would I lose?

[radioactive]

Hi everyone, getting ready to bring an old CNC veritcal milling machine up to current technology and have a question about servo drives. The Supermax YCM-40 currently has servo drives model SEM 30M4-59, 2400 rpm, 142 volts, 30 pulse amps. I've read on this forum that these use around 10A continuous.

In my research I've found that some Viper 200 servo drives would work with this setup (rated at 170V, 20A) however carry a heavy price tag of $240 each.

Gecko 320's are rated at 80V, 20A and are only $119 each. I think I understand that the Gecko's would work however my max speed would be reduced. Is this the only downside?

I've noticed in a lot of posts that when people switch to a smooth stepper board they could get very quick rapid speeds however backed them down for safety concerns. So it seems that quite a few people don't use their max speed anyway?

I'm just curious how slow the machine would be setup like this or if it would only be noticed in a production environment where every second counts?

Thanks for any feedback...

[Torchhead]

First take your current setup with any belt reduction and leadscrew pitch and calculate the speed you can get now. Once you have that number in IPM of travel then multiply it by 2 (using 70 VDC as a rating for the Gecko) That will then tell you the max you can expect. You won't lose torque (which comes from current) but you will have basically half the upper speed. Is it enough? depends on if you are using more than 1/2 the speed currently....

Another approach would be to replace the motors with ones that have higher RPM rating at the 70 VDC target. The 850 oz-in we use in our Complete Packages are rated at over 3000 RPM at 72 volts DC. Put in the same place as the older motors and using a new power supply (which you HAVE to do with the Gecko's) you will actually get more speed.

In the end, dollar wise it might be better to just go with the higher voltage drives. and everything remains the same (except the control software)

TOM CAUDLE
www.CandCNC.com
Totally Modular CNC Electronics

[Torchhead]

then multiply it by 2...

Woops. Sorry I meant DIVIDE by 2. Less voltage means less RPM. RPM is proportional to the voltage in a linear relationship

TOM CAUDLE
www.CandCNC.com

[radioactive]

I should be able to get some of that information tomorrow when I bring the machine home. Not sure what it is capable of currently.

So the Gecko's would require a new power supply (additional cost - not sure what these cost?), however I would save $360 on drive costs.

Wasn't planning on replacing the servo's but I'm curious how much new ones cost and if they come with encoders already mounted? I still need to find encoders to install as well. Current servo's are rated at 3 NM which I think is around 426 ozin?

[Al_The_Man]

DC servo's of that size and quality can be very costly, don't the existing servo's have encoders?
Al.

[radioactive]

Originally Posted by Al_The_Man DC servo's of that size and quality can be very costly, don't the existing servo's have encoders? Al.

I believe they are tachs?

[Al_The_Man]

There would also have to be a way of position feedback.
On this vintage of a machine, the tach would have gone back to the drive for velocity control, and position feedback would either be an encoder or a resolver back to the controller.
Al.

[Torchhead]

Originally Posted by radioactive I should be able to get some of that information tomorrow when I bring the machine home. Not sure what it is capable of currently. So the Gecko's would require a new power supply (additional cost - not sure what these cost?), however I would save $360 on drive costs. Wasn't planning on replacing the servo's but I'm curious how much new ones cost and if they come with encoders already mounted? I still need to find encoders to install as well. Current servo's are rated at 3 NM which I think is around 426 ozin?

850 (peak) oz-in 34 frame DC servos with 500 line encoders and cables are $235.00 ea. You will spend 150.00 or more for a 65-72VDC 20A supply. The Gecko's will set you back $342. Shipping on motors and power supply is another $100.00. Then you still need to figure out how/where to mount the new power supply and wire it and the Gecko's up to some form of interface (Breakout Card?) to the PC (through in another $100.00)

New encoders to fit your current motors are in the $50.00 ea range

The $360 on drives + new encoders is starting to look better and better.....

TOM CAUDLE
www.CandCNC.com

[radioactive]

I got the machine home today and in the garage (had to remove the spindle motor to get it in the door). I believe the existing servo's had tachs for velocity feedback as Al mentioned earlier. The machine also has glass scales which I assume it used for position feedback on the X & Y axis.

The price listed above for new drives with encoders was not as bad as I had thought, however I would like to try and reuse the existing drives for now. I will keep that route as an option though. Where can I find encoders for that price? I checked US digital and the cheapest I could find were $90 each...

Thanks...

[Al_The_Man]

Renco still have some on ebay for $20.00 they are dumping old stock.
1/4" shaft size though.
Al.

[radioactive]

Existing servo drives are reduced with a belt 2:1, ballscrews are 5 TPI.

Would 500 ppr encoders be fine? If I calculated it correctly, it would be 500x2x5=5000 counts per inch. Or one step would equal 0.0002". Did I read that Mach automatically divides the counts by four?

I'm looking at the Renco's on ebay and figure I can thread the existing hole in the end of the servo shafts and make an adapter to attach with a 1/4" shaft to attach to the encoders.

Do I remove the tach assemblies (has a four brush setup and four wires coming off of it)? or just leave them in place and cut the wiring?

[Torchhead]

Your math is off:

500 lines = 2000 pulses per rev
2:1 belt X 5:1 ballscrew = times 10 (10 turns = 1")
2000 X 10 = 20,000 pulses = 1"

Mach does not divide the encoders. It does not use the encoders for position control.