Close Loop Stepper vs BLDC servo vs AC Servo

[z05]

Hi Guys, I am relatively new to servo area. I have purchased close loop stepper system for my mill cnc conversion project, and it looks like, while being twice cheaper, I will be missing out on a lot of features. Looks like encoder on CL stepper system will not actually help me with positioning at all but can only be used as a failsafe, which was not my goal at all.

So, I went to alibaba to see what I can get instead of this system which cost me $80*3 + $80 for DC supply = $320 total, that is for 3Nm motor system.

Now back to alibaba offers, I could get a servo there with actually more torque, such as 220V AC servo (my mill is 220V always) for $120ish, which is just little more compared to my current buy, and dont need a 48V power supply on it too.

However I dont really know what exactly the difference would be between AC and DC servo system. If anyone could shed some light for me, would appreciate. I did googling, but also wanted to hear "insiders" opinions from home CNC machine perspective.

Thanks!

Similar Threads:

• Lathe Spindle Drive Options - AC Servo vs BLDC vs Stepper vs DC Brushed
• hybrid Closed Loop Stepper vs dmm servo
• EASY SERVO (close loop step servo) - JMC electromechanics
• Newbie- Closed loop servo vs real close loop in motion controller

[Al_The_Man]

By DC servo system, I assume you mean BLDC?
The performance between the two is roughly the same, with the BLDC having slightly more torque over the AC.
DC brushed is hardly used anymore, although for many years, this was the norm in most industrial CNC machines.
For my DIY projects I mainly used BLDC with no problems.

[z05]

Yes, BLDC is what I meant. The reason I am asking, first of all, by design AC servo would have less inertia due to lighter rotor, it would not require and additional DC power supply for it, it would not create electromagnetic noise. By all this, I can say it is a superior choice and allows for a cleaner wiring too. However, I am trying to make sure there is no limiting factor in its design, as I am not talking about spindle here, but about using that for motion control for XYZ. I need to have a positioning feature on such motor, so I dont have to use limit switches on my machine etc. I have seen BLDC servos with such function, but not AC. Which is why I am seeking some advice here.

[Al_The_Man]

Where did you get that AC servo have lighter rotor?
They are virtually identical, only the commutation is different.
How do you get around no limit switches??
Post the info you have to back this up!

[z05]

Well, even my 3D printer has no limit switches with stall detection, same thing is even easier with close loop stepper, as encoder can sense that with higher accuracy beyond one whole step.
And I saw a build with servos on youtube a while ago with alsolute positioning controlled by servos. Won't recall that now, as I watched a lot of related videos.

AFA your claim about motors goes, ...thats weird to hear. Thats kind of "motors 101" thing, they are nothing but identical let alone "virtually". 23k posts on this forum, I'd expect you to know such basics. I don't think I need to link you a documentation for that if you need to see it, just google. I am sure there is a bunch.

[Jim Dawson]

I guess that if the motors are small enough and the frame is robust enough then no travel safety limits would be required because you could let the axis slam into a hard stop in case of a runaway or programming error and thus the encoder error would spike past its limit and shut down the system.

On a mill, you really don't need home limits if you always 0 off of the part. The one exception to this is if you have a tool changer where you need to bring the Z axis to a fixed position to effect a tool change.

If you do use multi-turn absolute encoders, then you could have an encoder based home position and have software limits, and have a fixed Z position in the case of a tool changer. I think most major manufacturers of AC servos have multi-turn absolute encoders available as an option. Or as an alternative absolute linear scales could be used.

In the case of my machines, I do use safety travel limit switches because my servos are large enough to actually do some mechanical damage before the encoder errors out. So tripping the limit switch at the very least will reduce the energy of a potential collision in the case of some kind of a oops.

Almost all AC servos are BLDC motors today, they have permanent magnet rotors, almost all are 3 phase, and most are commutated by encoder or resolver feedback to the drive. I have seen some that are commutated by Hall effect sensors in the windings for feedback to the drive, but this is not common in larger modern motors.

As far as electrical noise, AC servos are just as bad, if not worse than, any other high energy device, you just need to observe proper grounding and cable shielding techniques to prevent problems.

Brushed DC servos are not in common use any longer, but I think are still available. They are 50 or so year old technology. There would be no advantage to using them in a new installation.

[Al_The_Man]

AFA your claim about motors goes, ...thats weird to hear. Thats kind of "motors 101" thing, they are nothing but identical let alone "virtually". 23k posts on this forum, I'd expect you to know such basics. I don't think I need to link you a documentation for that if you need to see it, just google. I am sure there is a bunch.

This is not my claim or assertion but from reputable manufacturers, as the the design of the motors, they are identical, 3ph AC P.M. and BLDC, the BLDC is described as such as they are effectively a brushed DC motor turned inside out.
I have taken AC servo's Fanuc etc and converted to BLDC. Simply by applying the right commutation.

[z05]

So one human is identical to another one that is turned inside out? lol. You just called the opposite identical. But hey, you answered your own question. I think your judgement of "identical" is way off. Inside out in this case means putting coils on stator together with most of the weight, which is why AC motor will suffer from less inertia obviously. Simple physics. I have quite an experience developing high performance high speed robots (just not with servos), so that for me is 101. I look at that always.

Jim, yeah stall detection should be somewhat easy to program, well... "should be" haha. Ok, "multi-turn absolute encoders" is what I need then! Thank you for a keyword, I will surely ask that now from a supplier!

The reason why I don't want to have limit switches, is that they always get filthy and wiring is a PITA, I imagine the system without them could be much "cleaner" build.

Almost all AC servos are BLDC motors today, they have permanent magnet rotors, almost all are 3 phase, and most are commutated by encoder or resolver feedback to the drive. I have seen some that are commutated by Hall effect sensors in the windings for feedback to the drive, but this is not common in larger modern motors.

That's definitely something new and unexpected. Another thanks for a hint, will definitely ask that too!

And yeah, I was not really considering brushed at all. Just BLDC vs AC servo, or possible stay with my Close loop steppers. I don't want to show the mill I want to convert to not start a comedy club here, but its pretty basic and small. I figured since it's my first attempt to such conversion, I might as well start from some basic model to play with and then sell it after conversion and upgrade to the next step in size.

Oh well, whatever, I am showing it LOL: https://www.harborfreight.com/vertic...ine-40939.html , just promise to not disappoint me saying that I bought a turd. I am still waiting for it to arrive on Friday. Got super lucky that it was shipped next day after I purchased it. Website showed up to 3 months waiting time and Precision Matthews warehouse is quite deserted now, couldn't buy anything over there sadly.

I have taken AC servo's Fanuc etc and converted to BLDC.

Al, it is actually possible that we are talking about different things. Looks like there are converted DCtoAC models, which are not native AC motors imo. Sure they can be converted but true AC servo motor is made differently and the design differs significantly too.

[Jim Dawson]

Stall detection shouldn't be an issue, all servo drives have an fault output and will automatically shut down on a number of fault conditions. I think even your closed loop stepper drives have a fault output. The way I normally handle the fault signal in software is to have the fault from any drive execute code that is the functional equivalent of hitting the E-stop button, including de-energizing the E-stop relay.

DMM-Tech servos have a couple of multi-turn absolute encoder options. DMM | AC SERVO DRIVE | AC SERVO MOTOR | ROTARY ENCODER I normally use DMM servos in my projects. Delta may have those options also.

While the HF mill would not be my first choice, I'm not going to beat you up over it. Ya gotta start somewhere.

[Al_The_Man]

So one human is identical to another one that is turned inside out? lol. You just called the opposite identical. But hey, you answered your own question. I think your judgement of "identical" is way off. Inside out in this case means putting coils on stator together with most of the weight, which is why AC motor will suffer from less inertia obviously. Simple physics. I have quite an experience developing high performance high speed robots (just not with servos), so that for me is 101. I look at that always.

I am not talking about AC squirrel cage induction, but P.M. , both BLDC and P.M. AC, I defy you to take apart either and tell the difference between them.
The only give away is the Hall effect sensors on the BLDC, if it does not have the latest commutation tracks on the encoder.
Both have a P.M. rotor with 3ph coils on the stator, the fact that I have used AC servo's as BLDC supports it, just by adding commutation for BLDC.
BLDC only exercise two coils at any one time, the AC version is 3ph sinusoidal fed.

[z05]

I wansnt commenting on communation at all, I am not that familiar with that. I was strictly discussing hardware design actually. And I actually have plenty in my garage and unlike DC models all my AC have static coils, that's odd. I have to admit all my models are rather small. But I guess there are different types among AC motors then too. Smells like a learning curve lol!

[z05]

Jim, well I guess it is hard to beat a knee mill that cost me $1850 total delivered with a coupon. At least for the price. I do have realistic expectations for that. And I only need it for rough prototyping during my product development stage. And I dont want to shock you, but I was completing same tasks with 6040 CNC router before with POKEYS57CNC and MACH4, and was not too bad actually. Forget about tolerances of course. Eventually I will probably get a larger bench mill from PM for $4000ish and convert it. But as a training device HF model is less risk IMO. As I will probably mess up a lot of things on my first build. Once learning curve has passed, then I will be ready to get something better, and once I convert HF machine, it can be sold very fast and with profit, I am sure. Even my upgraded router was gone in one day on facebook marketplace, especially with current market shortage.

Current condition of my setup. Have not messed with LinuxCNC and mesa board yet, hopefully will be able to make motors move at least soon Got a nice 15in touchscreen too with arm, because why not Nobody said style isnt important haha. And since I work in 3D Printing industry I will be testing out an idea of 3D Printed motor mounts (you can see one on that picture), actually pretty sturdy to at least let me use it as a temporary mount and let me machine a permanent solution from metal. The worst thing about this HF mill, is that it has almost zero information online, and wasnt actually able to find any guides for CNC conversion, but I loved the idea to get something that rigid for so cheap, and a small size of bed doesnt scare me at all, as all products that I develop are super tiny anyways. Anyways, my mill is in texas already, will get reasy for Friday fun. Still have no bench for it, hustling on FB marketplace.

https://www.cnczone.com/forums/attac...d=460114&stc=1