Steppers V Servos from an Automation guy

[David Dustin]

My History:
I am a product of the Industrial Automation industry, having started as a maintenance mechanic and advanced to heading a top-notch Automation team, and then becoming an Engineering Manager. I have designed, programmed, installed and debugged systems with Steppers and Servos.
There are many applications where steppers are perfectly acceptable and there are others where a servo is required.
Even if we had to use steppers because of some spec, we could still easily make them "closed loop" by adding an encoder (either linear or rotary).

We had a machine that used a stepper motor with a linear encoder that sharpened round razor blades (yeah I know you will all figure it out pretty quickly). It would consistanly place the blade to within .0002" (two ten thousanth of an inch).

Micro-stepping produces very smooth motion. Cogging, or very course motion in a stepper is usually because it is not micro stepping but moving in 1.8 degree increments. 1.8 degree steppers are very cheap.

Servos (at least the ones I used) had a resolver for commutation and positioning, they provided absolute (vs incremental) rotary positioning within 1 revolution. Of course most machine tools use linear encoders to control positioning.

When the steppers we used were equiped with encoders they were almost always the type that produced a "quadrature" pulse trail and also had a Z pulse for homing.

Bottom line is I am perfectly compfortable using a machine equiped with steppers. the only time I have ever seen them "lose" pulses is when they either accel too aggressively (and that has to be VERY agressively) or are interfered with.

Just my 2 cents on the subject........

David

[Rodne Gold]

I have the converse experience , I have 7 Isel engravers , all using steppers , no encoders and they lose steps quite often.
The real problem is lack of utility with open looped steppers. I can stop my laser at any time , manually move the head away , inspect the work and then resume where I left off , not really possible with any closed loop system. There are other advantages that relate to positioning before starting the job etc.

Here's a good source for an analysis of each type of system
http://www.techno-isel.com/Tic/H834/HTML/H834P041.html

[George M. Perzel]

Rodne;
"I can stop my laser at any time , manually move the head away , inspect the work and then resume where I left off , not really possible with any closed loop system."
I know you use LaserPro lasers-how do you do what you described above? Can you do this with a Mercury?Thanks
Best regards;
George
LaserArts

[Scott Shepherd]

I've said it before, the stepper versus servo for lasers is hype in marketing, not anything based in fact.

Closed loop servos can certainly be used in lasers, as well as steppers. Neither is better. Period. Full stop.

On my stepper system, I can hit the pause button, open the door, make any adjustment I needed and hit the pause button again and it picks up where it left off.

One thing you don't have with steppers is an encoder strip that gets dirty that you have to constantly clean or suffer quality issues. I too have been around servo and stepper motors for over 20 years. I worked for the company that invented the FMS (Flexible Manufacturing System). I've worked with stepper systems (which happen to be the motor that was on most of the CNC machines that put a man on the moon, rockets into space, and the space shuttle into flight). Closed loop servos took it to the next level. Most people never benefited from the difference between the two.

I've made 1000's and 1000's of parts for aerospace and other high speed machinery on stepper systems. Kearney & Trecker and Cincinatti were two HUGE cnc manufacturing companies who's machines used steppers and their equipment produced just about every high tech product in the world for a long time. How'd they do it with such inferior technology?

Simple, it's not inferior, it's just different.

Servo's are great when there is a lot of torque involved, but the light head and lack of mass doesn't require a servo. You can certainly use it, and that's acceptable, but it's hardly superior in performance to a micro stepper.

[David Dustin]

"I too have been around servo and stepper motors for over 20 years. I worked for the company that invented the FMS (Flexible Manufacturing System). I've worked with stepper systems (which happen to be the motor that was on most of the CNC machines that put a man on the moon, rockets into space, and the space shuttle into flight)."

Ahhh a fellow Enginerd.........

[Rodne Gold]

George , press pause , move the head away ,. inspect , press resume at it starts where it leaves off.
There have been many discussion on various forums on steppers vs closed loop servos and the argument goes on forever , chev vs Ford.

I find it odd that anyone would defend an open loop stepper system against a close loop servo type motor. Closed loop is patently better under almost any circumstance and thats what we are talking about re lasers of the type we mostly use , closed loop servo vs open stepper....

Its kinda like circumnavigating the globe with a GPS vs counting how manyknots east and then west and then ne etc.
I have been doing CnC work for almost 30 years , and have also used many machines , written post processors and so on.
So in as much as you guys might have made parts for aerospace , I have just about as much experience under my belt and would infinitely prefer to work with a closed loop servo system ....so we differ.....Ford vs Chev

[Scott Shepherd]

Rodney, there is no argument that closed loop servo systems are better than steppers for CNC machines. Large, heavy tables, high torque, etc.

The argument is in reference to lasers. Let's be clear, closed loop servos gain you accuracy. Accuracy into the .00005" range. Last time I checked, I didn't need .00005" or even .0005" accuracy on positioning on my laser.

The fact remains that I can do everything on my stepper system that you can on your servo. You can't produce anything that I can't. So if there is no manufacturing advantage, then what's the point? So I can have something more to maintain?

Do a search for "Epilog encoder strip" and see how many hits you get. Calculate that into time and material wasted to resolve the problem. Now do a search for stepper and see how many people have positioning errors with steppers.

Of course Trotec has a sealed system, so they don't have that issue, but Epilog certainly does.

I'm in business to make things. I'm not in business to maintain my machinery, and in my case, I'll take the open loop stepper that requires no maintenence over a closed loop servo that I have to take covers off to clean on a regular basis.

With any technological advances, there should be a clear performance difference, and I've yet to see a performance advantage clearly demonstrated by any laser manufacturer.

[Mike Null]

I have used both for lasers and cannot detect a performance diference in the work that I do. The one caveat, I think, is speed. The servo is faster.

[Rodne Gold]

Was this rehashing of an old topic made so the OP could feel better about his universal purchase , which seems to be a stepper motor laser?
So in other words , anyone that bought a closed loop servo driven machine wasted their money......Odd post indeed....

[David Dustin]

Was this rehashing of an old topic made so the OP could feel better about his universal purchase , which seems to be a stepper motor laser?
So in other words , anyone that bought a closed loop servo driven machine wasted their money......Odd post indeed....

Well I am a new member and a new laser owner.
I had seen the topic kicked around and thought I would throw my 2 (insert your lowest denomination of the currency of choice) into the mix.

It is not a justification of the purchase decision (at least I hope not in the deep recesses of my foggy brain).

Forgive me if these topics have been hashed over and over, they are all new to me...

David

[David Fairfield]

Welcome to the forum. An excellent way to join a conversation here is show what you're making, ask a how-to question, or answer a how-to question.

I love a good argument and I'm proud to say I've been thrown off at least a dozen forums. :cool: But I'm on my best behavior here because its too valuable a resource to risk pissing anyone off. If you like a debate, there are some really good web forums for arguing every topic under the sun.

As for steppers vs servos, I don't give a crap, I'm just happy my machine works as advertised. :)

Dave

[Doug Griffith]

This seems like such a touchy subject! Probably because each one of us already has a significant investment in one system or the other and take it personally.

In my opinion, David Dustin's original post was just his "2 cents" and not a debate. His last post was more or less an apology. Some of us like cake, some like pie. Eat as we wish.

Welcome to the forum.

Doug

[Scott Shepherd]

The one caveat, I think, is speed. The servo is faster.

Maybe on the Trotec, but it's not faster on other machines. I ran the exact same file on the Epilog and the ULS and the ULS time was over a minute faster on a 15 minute job that I run all the time. I believe it was 90 seconds faster, but I don't recall without looking for some old notes I took, so I know I can say 1 minute on the same job without having to look that up.

My only point in repeating this is that it doesn't matter to the degree the sales reps want you to believe it does. The bottom line for buying any laser is having it run real life examples of what you'll be doing with it rather than buying into the canned demo's that have been refined to perfection of years of doing it.

If the price/feature/performance of a servo machine satisfies your needs, go for it. If the stepper does all of that, then go for that. It's just not the issue that it's made out to be. There is enough difference in machines and their performance features that this should be the last thing considered and every time I have had people telling me about servos, it's been because they don't want to talk about something their machine doesn't do.

Want an education? Go to the manufactures and start asking questions like "What does your machine do that their machine won't do". You'll quickly start to get a picture of where the weak points of the systems are. Then, when you do shop, you can look specifically at the weak points and determine if those items are deal breakers. Most sales reps I have met have been in the business a long, long time. They know the ins and outs of the industry and they can tell you stories that are hidden from us. They can tell you the pivot point when a manufacturer went downhill, they can tell you who's outsourcing their driver coding, who abandoned a great platform for one full of bugs.

Ask any Xenetech rep who's been around about the point when that company changed directions. Ask them about Jay (the person who came up with it all), and what happened when he passed away. You'll start to get the real picture of who's doing what and who you should avoid.

Just my opinion.

[Mike Null]

Steve

Since we all seem bent on belaboring this issue for no good reason does this also refer to the fact that ULS will not post a raster speed.


There is enough difference in machines and their performance features that this should be the last thing considered and every time I have had people telling me about servos, it's been because they don't want to talk about something their machine doesn't do.

[Scott Shepherd]

I have no idea Mike, I've never looked for that information. I don't believe most of what sales people or brochures tell me. I like to base my opinion on personal experiences and from my own personal experiences with two machines (neither of them a Trotec), I can tell you my ULS finished the part faster than the Epilog did. Why? I have no idea. Do I care how? Nope. I just know it did. If that means someone's manual or brochure says different, then so be it. I can only speak from personal experience when I compared to machines that were both current model machines.

[Martin Reynolds]

I like steppers and servos. Steppers are similar to digital in nature: you can build a very precise positioning system with no feedback, as long as you don't miss steps.

Servos have a lot more torque for the size, but they have to have a closed-loop design.

Once you get to a closed-loop design, you can really start swinging the laser head around. Stepper or servo.

So the argument should really be about closed-loop or open-loop, and that will all be settled in the price of the system.