Servo motors for plasma table

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20 May 2021 00:23 #209585 by txtrone
What are the pros/cons of servo motors on a plasma table? Are they better, or just different than steppers?

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20 May 2021 00:30 #209588 by thefabricator03
Generally speaking, Servos are superior to steppers. They dont loose steps and they dont have a exponentially decaying torque curve.

That comes at a price though as servos are more expensive than stepper motors. Most people who build their own machines generally try to do so on a budget so steppers tend to lend themselves to the cost conscious machine builder.

But from my reading. the China AC servos motors you can find all over the internet are suppose to be OK. I am thinking of buying a bunch of them for some up coming projects.
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20 May 2021 10:28 #209635 by tommylight
Also servos are much harder to protect from EMI and RF interference, so grounding and shielding is imperative on plasma cutters.

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20 May 2021 11:11 - 20 May 2021 11:14 #209637 by snowgoer540
I looked into servos pretty hard when I built my table, specifically Clearpaths as they are easy to work with, available, etc. In the end, I couldn't justify it for several reasons:

1. Plasma is an inherently inaccurate process. Per Hypertherm, one can only reasonably expect ±.030" with an automated process (although I know it's possible to dial it in and do better on a part for part basis) Hypertherm Article Source written by Jim Colt . At any rate, all the machines I have with steppers on them repeat very well, save for some sort of mechanical issue that binds them up, or crashing the machine. Which servos would perform no different.
2. As Tommy said, they are more susceptible to interference, and in our air plasma home shop environment, that makes them more difficult to be successful with.
3. I get 1200ipm rapids out of my 570 oz-in Nema 23 motors. Easily. I actually had it at 1500ipm for awhile, but it seemed like it was really beating the table around. I turned it down with longevity of the machine in mind.
4. I also have really good accelerations. I started with them set at 120in/s^2, but settled on 80, with longevity in mind.
5. I have yet to see a home-game level servo-driven plasma table without some sort of planetary gear reduction attached to them. Generally it seems 10:1, etc. Those planetary gear sets aren't cheap. Of course, there's other ways to do reductions, but the point is it seems to need way more reduction than stepper systems.
6. All that said, it's hard to justify the additional cost/aggravation.

If I was really worried about closed loop feedback, I would opt for closed loop steppers over servos I reckon. That said, I haven't seen the need for it thus far.

On the other end of things, the only real gains I could see were no stepper noise, and reduced harmonics. But with 3:1 reductions on a stepper system, apparently that eliminates (or greatly reduces) stepper harmonics.

tl;dr - The juice isn't worth the squeeze in a plasma setting. Just my $0.02, IMO, YMMV, etc.
Last edit: 20 May 2021 11:14 by snowgoer540.
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20 May 2021 11:52 - 20 May 2021 11:52 #209641 by bevins
I just finished a plasma Gantry machine with servo's and planetary 10:1 gear ratio. I have to say they are amazing. But the drawback is cost. The customer paid for them but he also asked to do a comparison with steppers. So I did.

Servos would not work without reduction. They choked and spit at me when I tried, I tried for two weeks trying to adjust them to no avail.

Steppers I got to work without reduction but they were right on the edge and I got no speed to talk about.

As soon as I put planetary reduction on the servo's they came alive. Really smooth. I was amazed. So the customer chose the servo's. I am running rack and pinion. It is a 5X11 foot Gantry table. The Z is using servo directly connected to ball screw no reduction.

That coupled with QTplasmaC, the customer is getting a really nice plasma table.

I would choose AC Servo's any day over steppers but it is the cost that makes the difference.

Just my 2 cents
Last edit: 20 May 2021 11:52 by bevins.
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20 May 2021 12:56 #209646 by txtrone
Thank you all for the informative responses. I felt compelled to perform some sort of due diligence into servos before I went any further with a motor, driver, and power supply upgrade.

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10 Jul 2021 01:23 - 10 Jul 2021 01:23 #214153 by BenDiesel
Thank you for this post. I'm looking at building a plasma table with dmm 750w 240v servos and a 3.2:1 reduction.
Last edit: 10 Jul 2021 01:23 by BenDiesel.
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