Servo vs stepper resolution

[BigDon]

Servos like Clearpath and so on are gaining popularity quick. Normally most people with a stepper driven 4th axis would use a 50 or 100:1 ratio. Would a servo allow you to bring that down to a lot lower ratio and still have good resolution? I have a choice on motors to finish off my cnc. Is servos worth the extra money?

Thanks for any help!

 

[RBC]

Servos like Clearpath and so on are gaining popularity quick. Normally most people with a stepper driven 4th axis would use a 50 or 100:1 ratio. Would a servo allow you to bring that down to a lot lower ratio and still have good resolution? I have a choice on motors to finish off my cnc. Is servos worth the extra money?

Thanks for any help!

Don

Clearpath motors are definitely the direction machines like we use are, and should be, taking.

As for resolution, it's a little tricky.

Stepper motors are typically 200 steps per revolution. Now, these are "full" steps and most drives do use micro stepping that raises the steps per revolution to 2000. There are some drives that go far beyond that, but due to the overlap of tolerances, anything over 2000 just doesn't really do anything. Micro stepping is accomplished by the drive actually balancing the motor between positions by applying partial power to both sides of the full step and using the balance to achieve the position. Micro stepping is real, and it does work, but it's accuracy is really dependent on the linearity of the motor, which can vary greatly. Even on the best motors, I don't depend on micro stepping for really tight resolution. Personally, on my stepper systems, I don't go higher than .200" pitch. Pitch is the distance traveled with one revolution of the screw. With .200" pitch, I get .001" resolution at full step, and .0001" at micro step. Of course, you can go with more pitch for less critical applications like the long axis on a turning center.

Clearpath motors come standard with 800 count encoders. Basically, it's about the same as steps per revolution. The Enhanced version motors come with 6400 count encoders. I should mention that they both can be programmed with much higher resolution, but this is really "gearing". Meaning that if I set the standard motor with an 800 count encoder to a resolution of 2400, then the motor will only move after it receives 4 step pulses from the controller. My preference for screws used with clearpath motors is split between 5mm or around .200" pitch for motors with 800 count encoders, and 10mm or just under .400" pitch with the enhanced 6400 pulse encoders. The ultimate in resolution would be to use the enhanced, 6400 encoder motors with 5mm pitch screws. You won't have the higher rapids as you would with the bigger screws, but the resolution is .00003".

Now, for the index that you're asking about. The simple answer is yes, you could use something with a lower reduction ratio. Especially if you use the 6400 count enhanced motor. But, the question really is, why? There are perfectly suitable harmonic drives on Ebay for around $400. They are 50 to 1 ratio. For me, that's really the easiest way to go. Just make sure your clearpath motor uses the same shaft size as the harmonic drive so it's easy to put the 2 together. The harmonic drive will not only give great accuracy, which is very important with the index axis, but it's also going to be very "stiff". You really don't want your index to have any "give" at all if you're doing accurate inlay work.


I know that's long, but I felt it important to give some of the foundation on the differences.


Royce

 

[BigDon]

Thanks Royce! That helps a lot actually. I was trying to do my own research but could not find the numbers for Clearpath.

Thanks again!

 

[RBC]

No worries Don.

For anyone who is considering a clearpath motor setup, I strongly recommend using an external motion control board like the Ethernet SmoothStepper. (ESS)

The ESS is a drastic improvement over the PC's parallel port. It connects through an ethernet connection which is fast and very reliable. It increases Mach3's kernel output speed up to 4Mhz so you can take advantage of the clearpath motors additional speed and resolution capabilities. It has 3 emulated parallel port so you have plenty of room to hook up all kinds of extras. Basically, it becomes the PC's parallel port, but it also adds 2 more of them for more inputs and outputs.

I've built one machine with the ESS and clearpath motors already. I'm collecting the parts for the second right now, and hope to start converting some of my current machines soon.

I will have some Gecko G540 controls and motors available for those who still want to use stepper motors. If you're interested email me at rbunnell@obcues.com


Royce

 

[BigDon]

To use the ESS, you still need a BOB to connect the Clearpath to, right? What board are you using?

 

[RBC]

To use the ESS, you still need a BOB to connect the Clearpath to, right? What board are you using?

Don

It's really more of a terminal board than a breakout board. It adds some filtering and it brings power and ground to convenient places to make wiring a little easier.

Here's the one I use:

http://cnc4pc.com/product_info.php/...board-p-187?osCsid=23vb4tvn120pik712jcs4n82h2

You do still have to supply a good 5 volt source to power the ESS, but this terminal board will get all it needs from the ESS.


Royce

 

[conetip]

Royce,Which 800 step ,and 6400 step motors do you recommend?
I am looking at making a piston trim device for my model engines,can't afford a submicron Hardinge.
If I go about it the right way, should be able to make something to do the job and have better than 0.001mm diameter control.
Thanks, Neil

 

[RBC]

Royce,Which 800 step ,and 6400 step motors do you recommend?
I am looking at making a piston trim device for my model engines,can't afford a submicron Hardinge.
If I go about it the right way, should be able to make something to do the job and have better than 0.001mm diameter control.
Thanks, Neil

Neil

Well, I'm talking about the Clearpath motors by Teknic. Here's their motor listing page. In case you're not familiar with them, they are small servo motors with a quality drive built in. All you do is bring in DC power, and the enable, step, and direction signals straight to the motor and you're off and running. I wire them straight to the ESS terminal board.

https://www.teknic.com/products/clearpath-brushless-dc-servo-motors/wizard/allresults/

Clearpath's come in 4 different types of motors. Those that begin with an M are their motion control motors. they're meant more for point to point or similar type of control, not cnc. The 2 types of motors that are good for CNC are the SDSK, better known as stepper killers, and the SDHP which is the high performance version.

The SDSK motors come standard with the 800 count encoders, but can be ordered as "enhanced" which means it gets the 6400 count encoders.

The SDHP are already equipped with the 6400 count encoder, and they also have a much more aggressive torque curve.


I have used 2 different models, both the SDSK versions.

The 2311S motors are NEMA 23 and are rated at 290 ounce per inch peak torque with 58 ounces constant. The 2321S is also NEMA 23 or course and is rated at 492 peak and 98 constant. Both are rated much higher in speed than I'll ever run them. I use the 2311S for my Y axis on my turning center and the 2321S is for the X axis. They are setup like a mill, and not a lathe, so X is the long one, and Y cuts the diameter. I also use the smaller 2311S to turn my lathe spindles.

When I retrofit my inlay machines, I'll use the 2321S motors on both the X and the Y, and the smaller 2311S on the Z and A.

NEMA 23 and 34 are the only frame sizes they have.

I'm not sure what you're looking for, but look through the lists. They have very good information on their site. You can download the manuals as well, but keep in mind that much of the manual is for the motion control motors. The Step and Direction area is only a few pages, but it's all the information you need.


Royce

 

[conetip]

Thanks Royce, That is good info and I will look into the motor's you have mentioned.
Neil

 

[3raildraw]

Royce is right

 

[LGSM3]

Royce,Which 800 step ,and 6400 step motors do you recommend?
I am looking at making a piston trim device for my model engines,can't afford a submicron Hardinge.
If I go about it the right way, should be able to make something to do the job and have better than 0.001mm diameter control.
Thanks, Neil

Neil...your question is a very good one when considering resolution. Mostly because it likely involves a single axis move that will take place on a full step "or actual encoder count"

I think resolution is tossed around without much regard for linearity. This happens to be quite important depending on what your trying to achieve.

If you consider a stepper has 200 counts per rev, then it can only start or stop on a full step. Micro stepping is a bonus but only takes place interior of starting and stopping. For me it would be wise to design around the 200 steps per rev in lieu of 2000 when considering turning ultra precise od's

Servo motors are exactly the same. If the Clearpath has an 800 count encoder then thats what it is. You can electronically gear up or down all you want but it will start and stop on 1 of 800 place possible.

Anyway, tech is such these days that you can get a 16bit encoder on damn near any servo for short money and thats almost always gonna yield sub micron resolution without gearing "or microstepping".

Sorry, if i complicated matters. I would just hate to see someone design a machine around microstepping when it wont be microstepping

 

[conetip]

Thanks for the info about micro stepping. I am looking at designing it without micro stepping, then if micro stepping works will be a bonus. The X axis actually needs to be in the region of 0.0002 mm per step per side or in inches speak 10 millionths. To get top performance from model engines, sub micron machines yield the best results. I though the spindle would be the real sticking point, but if I make it an air bearing, then the problem is solved.
Thanks again for the positive fed back guys. I am sure others will find this threads information valuable as well.
Neil

 

[RBC]

Thanks for the info about micro stepping. I am looking at designing it without micro stepping, then if micro stepping works will be a bonus. The X axis actually needs to be in the region of 0.0002 mm per step per side or in inches speak 10 millionths. To get top performance from model engines, sub micron machines yield the best results. I though the spindle would be the real sticking point, but if I make it an air bearing, then the problem is solved.
Thanks again for the positive fed back guys. I am sure others will find this threads information valuable as well.
Neil

Neil

Given the kind of resolution you're looking for, I'd have to suggest going with regular servo's and with linear encoders.

The trouble with trying to achieve that kind of resolution is that it will be very hard and expensive to get screws with that tight of lead accuracy.

Renishaw makes both optical and magnetic linear encoders with resolutions down to 1 micron. they aren't that expensive, but you will have to use them with a more commercial servo drive and motor. Teknic, the maker of the Clearpath motors, also makes some very good drives that would suit your needs.


Good luck! That's some really tight tolerances to try to hold!


Royce

 

[RBC]

Just wanted to share some new information about the clearpath servo's.

I've known that the motors come either as the regular version with an "Input Resolution" of 800 counts per turn, or the enhanced version with an input resolution of 6400 counts per turn. The enhanced option does cost a little more, but the higher resolution allows the use of steeper pitch lead screws.

What I found out is that the motors actually have a higher count encoder than just what the "Input Resolution" indicates. If you look at the repeatability in the specs, it shows 0.03 degrees. If you work that out, it comes to 12800 actual encoder counts per revolution.

The higher count doesn't give any higher resolution, although I can't see a need for anything higher than 6400. What it does do is allow the controller some advantages so it can make the motor smoother, quieter, and most importantly have more stiffness. I've always been pleasantly surprised at how difficult it is to turn the motor shaft when stopped. Yes, you can turn it, but you can also tell that the motor works very quickly to attempt to keep the shaft position. It's very smooth and doesn't have much "rubbery" type of give to it.



Royce