Why Not Use Steppers

[Steve Etter]

Hi all,

My company builds a pretty large CNC machine and the new owner has begun to question whether we need to use servo motors rather than stepper motors. I have done quite a bit of searching on the web but can't seem to find any in-depth comparisons of servos vs. steppers with respect to application.

About the most relevant thing I have found so far comes from NetMotion.com's Application / Advisor where they wrote, regarding Positioning Applications, "Use stepper motor if torque is lower than 500 oz-in, less 2000 rpm, low to medium acceleration rates" and "Servo can handle effectively when load is mostly inertia instead of friction. The ability to overdrive servo motor in intermittent duty allows a smaller motor to be used. If positioning is critical in micron level use servo."

Well that is good information, but it is not enough for the boss. Does anybody have a link to more documentation I can use to explain when to use a servo and when to use a stepper? The more technical the better.

Thanks
Steve

[HuFlungDung]

Let him buy some steppers and try them out

The worst thing about steppers is people expecting them to keep track of position by the number of steps they were commanded to move. If you have a controller that will keep cranking out pulses until the thing is where it is supposed to be, then there should not be a whole lot wrong with them, I suppose.

But, then you have to buy an encoder, which is usually part of a servo system and can account for $300 to $500 worth of the cost, per axis.

[Steve Etter]

HuFlungDung wrote:

Let him buy some steppers and try them out

[FRUSTRATION VENT]

That, unfortunately, is just what he wants to do. The bad part about this I will be the one who has to install them on the machine, re-configure it, try everything to make them successful, and then, when I can't make them do the job, hear how it still must be possible because his theory says so. To top all that off, all this work will have to be done on site (a half-day drive from home) at a customer's facility while trying to make production.

[/FRUSTRATION VENT]

So, while I have attempted to explain the open-loop nature of the stepper motor, its inability to handle varying loads, and the fact that they are a poor choice for higher motor speeds, he says "They work on my plotter. We should be able to make them work here". My verbal explanations are simply not good enough. Nor, I am sure, will considered opinions from this forum.

That is why I am looking for published documentation.

Steve

[balsaman]

Is the machine presently working as designed? If so, if it ain't broke, don't fix it.

Eric

[HuFlungDung]

Steve, if your boss is paying you, then he gets to call the shots. Do your best, that's all you can do. The performance of the machine will be the ultimate judge anyways.

What kind of machine is this? Is it something that just "moves around" like a plotter, or does it meet some resistance, like a heavy table etc?

[Steve Etter]

The machine is a really large router. As to whether it is or is not working as designed is a matter of some debate. The customer is constantly asking it do a little more than he was the day before and he is reporting back that it is not always working as he would like. My company's owner (and my direct boss) had no affiliation with the company when the machine was originally designed so he is coming in from the back-end. Unfortunately the previous owners flew by the seat of the pants on many of their designs and there are few if any documents saying what the hard design criteria really were. I designed the controls and, in my opinion, the controls are more than capable. The mechanical component and design choices are where any real deficiencies exist. Of course, its my baby (controls wise) so I might not be the most unbiased person out there. The boss is a mechanical PE and his view is EXTREMELY biased in the opposite direction of mine. He also has vertually no controls design experience.

Oh, I agree that I will end up having to do what he directs, but I have no intent of doing so blindly or quietly if it is against my better judgement. That's why I am looking for publications on the issue. If I can show him that ardent professionals in the field agree that steppers are wrong for applications similar to ours, I am confident he will listen and back down. He definitely does not want to head down the wrong road. He just wants more credible information than he believes I can supply on my own.

Steve

[HuFlungDung]

http://www.aveox.com/indprods.htm

http://www.asiimaging.com/precise.html

http://www.motionautomation.com/technica.htm
(scroll way down)

And if that's not enough, tell him HuFlungDung says servo motors are best