SERVO Rotary

[Sandra Collins]

Recently I bought a 30W fiber laser machine. It comes with a rotary and a Mark-While-Engraving rotary extension board. The board manual suggests for best performance such function should be combined with a SERVO Rotary. However, mine comes with a regular stepper rotary. The controller supports both SERVO and Stepper. I am relatively familiar with stepper motors/drives and the wiring but I know very little about SERVOs.

How difficult would be to replace the stepper motor and drive with a SERVO system? Does anyone have experience with such upgrades?

[Jerome Stanek]

You would need a new driver for a servo vs stepper

[Sandra Collins]

You would need a new driver for a servo vs stepper

Of course. Probably a new PSU as well. I imagine the hardest part would be to configure the encoder. The encoder software looks quite intimidating.

Could you recommend a motor/drive combo for this application?

[Rich Harman]

...The board manual suggests for best performance such function should be combined with a SERVO Rotary.

Have you used it and found the stepper deficient? I would think that a stepper would be fine unless you need that extra bit of acceleration that only a servo can provide.

[Kev Williams]

Adtually, acceleration of the rotary is the LAST thing you want, due to 'transition shock" (my term for lack of a better one!)

About the only benefit I can see in a servo vs stepper for a fiber rotary, is the servo will make a much smoother transition between steps, resulting in less 'bounce-back' shock of the heavy chuck. But engraving quality, or lack thereof, as these machines are infamous for the visible seams at every transition, a servo isn't going to help with this. As to the shock, you can eliminate this problem with steppers by increasing the "Pulses Per Round" settings. Factory default seem to be 6400 PPR for many machines, meaning of course each pulse = 1/6400th of a rotation. But here's what I've found with these machines: The speed of rotation between pulse points is 'static', not adjustable, meaning it takes X amount of time to rotate from one point to the next, regardless of how far away the next point is. Therefore, raising the pulses per round causes the chuck to rotate MUCH slower, the ratio being proportional to the 6400 default: 12,800 PPR will effectively cut the rotation speed in half, and 25,600 PPR will cut it in half again. I have mine set for 20,000, and it's smooth as glass when transitioning, meaning the chuck isn't rattling back & forth to a stop at the same time the laser is firing the next transition, the chuck moves so slowly the stop-shock almost non-existent.

But, while slowing the rotary between transitions makes for less engraving distortions (it's wonderful for the rotary-text function), it (a) doesn't change actual engraving speed, and (b) it doesn't help at all with visible seams. The seams are the result of minor inaccuracies in the part's actual diameter and actual rolling-radius at any given point. The only real fix, as many have found, is to have ridiculously small split sizes, such as .05 or .1mm- At these splits, good engraving can be had, but man, is it slow going!

And a servo stepper, IMO, will change none of the above, other than make the transitions maybe a little softer. Not sure that will be worth the time & money

[Sandra Collins]

Kev and Rich,

Thanks for the input. Maybe I wasn't clear in my original post - I don't need the SERVO for the standard EZCAD rotary functions. They use the so-called "handshaking method" - the graphics is engraved in segments. While the rotary stays still the galvo will engrave on X and Y. When the segment is complete the rotary will turn the part at a new angle then the galvo will engrave X and Y of the next segment and so on... I agree a regular stepper should be just fine for this method. Stitching the segments is usually the only challenge but as Kev suggested, adjustings the micro-steps helps.

However, I am interested in the SERVO upgrade because of the "special" MarkWhileRotating function of my machine. Please note that this method is very different than the standard rotary functions. As I mentioned in the first post the MarkWhileRotating function require a separate dedicated controller. 99.9% of the machines with normal LMC boards do not have this function. The main difference is that the MarkWhileRotating controller will turn OFF of the Y-axis of the galvo completely and replace it with the rotary axis. That way the X-mirror in the galvo and the rotary can work simultaneously. It is particularly useful for long continuous curves that have to be engraved around the entire perimeter of a cylindrical object without interruptions.

This is a demonstration of the function - Link

There are several problems related to the use of stepper motors with the MarkWhileRotating function:

1. Resolution - obviously the resolution (regardless of the number of magnetic poles and micro-steps) of the stepper (Y-axis) is much lower than this of the galvo (X-axis). The galvo uses SERVO motors. When I examine the engraved parts closely I can see that the curves are not smooth but rather wavy and distorted. Hatching is also problematic as the space between the hatch-lines is not even. In turn that affects the uniformity and the depth of the engraving.

2. Vibrations and resonance - all steppers "click" and vibrate. That causes resonance issues and it is affecting the quality of the line and hatch. It is significantly worse at higher speeds.

3. Acceleration and inertia - the stepper drives do not have a method of controlling and compensating acceleration and inertia. Even the closed-loop "smart" steppers offer only positional correction. There are special low-inertia SERVO motors used in these scenarios.

4. Speed - This is probably the main issue. To have a relatively stable operation the stepper has to run at very low speeds. It is fine for marking, but it doesn't work for deep engraving. When the laser beam moves slowly it melts the metal. To evaporate the metal it has to run at high speed. I have no idea how high it has to be. It would depend on the diameter of the part and the material. I imagine something in the 500-1000mm/s range.

I think these are the main reasons the MarkWhileRotating manual suggests using SERVO instead of stepper.

So to go back to the original question - does anyone know how to replace a 1-axis stepper system with 1-axis SERVO? Can you suggest a motor/drive combo? What would be the challenges?

[Kev Williams]

so in other words, you're talking vector engraving capabilities using the rotary. Very interesting...

[Jeff Watkins]

Sandra, I looked into adding the "Mark While Engrave Board" to my laser. I was led to believe that it would go between my current LMC board and rotary and have an additional connection to the board. They made it sound like it would work with the current stepper rotary.

I would contact Mrs. Sophy with BJJCZ as she is apparently the person that deals with customers outside of China. The may argue with you about not buying the machine from them, even though they don't sell machines, but be persistent and your should get an answer. sophy@bjjcz.com

Let us know what you find out.

[Sandra Collins]

I would contact Mrs. Sophy with BJJCZ as she is apparently the person that deals with customers outside of China.

Contacting BJJCZ was the first thing I did. I've been in communication with Sophy about other products as well. She is the only international sales rep. for the entire company and she's very busy. Typically she'll answer only sales related questions (basic info, availability, and pricing) with 1-2 sentences. Maximum 3 if they are short I don't think she is qualified to answer technical questions in-depth. She couldn't provide me with more information than what is already in the manual. BJJCZ suggests the use of SERVO rotary for this function but it doesn't offer one. In general, BJJCZ works with large clients. If I was a representative of a large factory and we were talking about several hundred controllers I am sure they would have assigned an engineer to consult me. But not in my case. So I am on my own.

I was led to believe that it would go between my current LMC board and rotary and have an additional connection to the board.

It is a bit more complicated. It also bridges the galvo signal and alters some of the LMC functions.

They made it sound like it would work with the current stepper rotary.

It does, but unless you intend to use it for, marking roughly machine parts, you may be disappointed with the quality. If you don't pair it with a SERVO axis the engraving quality will be significantly lower than what you would get from the standard rotary functions with a stepper.

I looked into adding the "Mark While Engrave Board" to my laser.

I would suggest to wait and see how this thread will develop. Hopefully, some of the SERVO experts would join the discussion.