Help sizing and choosing servo motors

[Magnus B]

Hello

I'm new to this forum and pretty new to CNC also actully.

I have a Sajo NC45 milling machine. It's a Swedish 70's NC machine that weighs about 5000 lbs I would guess. I'm going to take all the old DC servos of it and putting AC servos on it. Actually 2 of the 4 are already gone. But for know I'm only going to replace the x, y and z servos leaving the knee non-motorized.

I have measured what torque makes the different axis move. When I apply about 2-2.5 Nm I get a good acceleration on the table, but the spindle is a bit harder to move. I have taken it apart to lube it up, but I still think it needs a bit more torque to work properly.

The measuring were done with the machine all turned off and not during operation. The servos on it is marked 6.7 Nm cont and 21 Nm stall, so they are fairly big.

So no for the questions. How big servos should I choose based on this information? Anyone have a recommendation what to look for? I have been searching Ebay but not all have the specs. We have 240 volts 1ph and 400 3ph over here in Sweden so I guess I need to ger 240 v servos. Or do I? How thoes that work? I have seen servos with all kinds of operational voltages. Do they need to be run with a transformer or does the controllers somehow change the voltage? I have found servos with a great spec but the operational voltage on them is 200vac. Can I make that work?

Next step would be the controllers of course. If you have a good recommendation there please let me know. I have been looking at the Allen Bradley ultra 3000 series, since they seem to be quite easy to find and are able to run 240v.

Sorry for all the newbie questions, but I need to know to make this thing work for me.

Thanks,
Magnus

[serriadh]

The voltage rating of the servos is the maximum voltage they expect from the servo drives that are controlling them. The servos themselves won't ever be connected directly to mains power. Many commercial servo drives plug straight into 1ph or 3ph mains AC and have an internal transformer.

The servo drives are inevitably going to be the most expensive bit, and you'll need to make sure your motors are matched to your drives and your drives are matched to your power supply capabilities.

[Al_The_Man]

Why not go by the same torque value for the motors as the old ones?
I assume they got the design right in the first place.
You can also get drives which operate direct off the 1 phase or 3 phase supply, so if you get the suitable rated drives and motors you can connect direct to the incoming mains supply, the low voltage control is going to be isolated from the supply.
Al.

[Karl_T]

Just some thoughts for you.

I like to buy used stuff for a fraction of new. DC servos are much cheaper as they are no longer used by industry. You need to change brushes every few thousand hours of operation. Not an issue for the hobbyist. Same story for the DC drives. My favorite for value is AMC.

Mach can't use most servo drives, but EMC can. So, I'd suggest EMC as your control software.

[serriadh]

Originally Posted by Karl_T Mach can't use most servo drives, but EMC can. So, I'd suggest EMC as your control software.

I don't think this is entirely correct. Any servo drive that closes the feedback loop (ie, does its own PID processing) and takes a step/direction signal from a host will work just fine with Mach, and that covers a very large number of devices indeed.

[H.O]

Hi Magnus,
Welcome to CNCZone! I tend to agree with Al, why not try to get in the neighbourhood of what was there from the beginning. Don't forget to factor in the reduction ratio (if there is one) between the motor and screw. Newer motors may have a higher speed-rating than the older which would let you get away with less torque from the motor by adjusting the reduction ration. (If you don't want higher speed as well, of course)

As for Mach3 and servo systems it's being done all the time. In EMC2's case the position loop is closed in EMC2 itself which does offer some benefits. However, there are several hardware solutions available for running analog servo amplifiers with Mach3 and most modern digital servo drives offer step- and direction interface.

You can have a look at my web-site for an overview of some of the ways Mach3 can run servos.

/Henrik.

[Magnus B]

Hi all.

Thanks for your replies. Getting servos that matches the ones that's already there seems like a logical way and that was my original thought. But I still wonder if it's the continous or stall torque I should match. Seems to differ a lot between different servos. On the other hand if I can go with DC servos it seems like it's a bit easier to find servos that delivers the required torque.

I did not look at DC servos at all since I thought they weren't controllable with modern equipment. Just replacing the resolvers with encoders and matching with the right controller seems like a good choice for me. I have not made any efforts to try the servos I have, but they should work. I still need som kind of drive unit I guess.

But this gives some new possible solutions.... Too many solutions maybe. It feels like I'm back on square 1. Well with some new knowledge of course. The question is still: What should I do? But I must say DC servos seems like a good option.

Thanks,
Magnus

[serriadh]

Originally Posted by Magnus B I did not look at DC servos at all since I thought they weren't controllable with modern equipment. Just replacing the resolvers with encoders and matching with the right controller seems like a good choice for me.

You may not even have to do that... there are bits of hardware available that can take a resolver signal and output a quadrature signal as if normal incremental encoders were being used. Pico Systems offer such a board (see here) for $150, for example. It may also be quite possible to find some servo drives which accept a resolver input.

The question is still: What should I do?

I'd start with finding out the parameters of your existing servos, and having a quick peek on ebay to see what sort of drives might be easily available to you!

[Karl_T]

Originally Posted by Magnus B But this gives some new possible solutions.... Too many solutions maybe. It feels like I'm back on square 1. Well with some new knowledge of course. The question is still: What should I do? But I must say DC servos seems like a good option. Thanks, Magnus

Good decision for DC servos. While you're shopping for them, decide on your control software. Two main choices are EMC2 and Mach3. Make this selection before shopping for servo drives.

As I see the comparison, Mach has a strong following and it works well. Main drawback is its a step-direction only control with limited I/O. There are work arounds for both issues. EMC2 has a fairly good following, its a bit more geeky to install, has more capability. This is just my 2 cents on the controls - lots of strong opinions here.

Karl