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Author Topic: Motion controllers - Take me to school  (Read 7243 times)

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Offline sn96

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Re: Motion controllers - Take me to school
« Reply #30 on: September 06, 2017, 09:03:59 PM »


Quote
Hmm......, spend your money so you feel good, let us know how all goes!  ;D RICH

LOL you so silly. I get it... I get it. You hate the idea of servos put on a small machine where steppers would work just fine. Still doesn't change my fascination about the crazy speed and performance of AC servos.
« Last Edit: September 06, 2017, 09:05:46 PM by sn96 »
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Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #31 on: September 06, 2017, 09:14:25 PM »
Hi Mike,
I notice the servo you are looking at has rated torque of 58 oz.in.  Seems very wimpy to me...don't under do it...you'll regret it each time the servo
faults. Craig

I saw the max torque was 200+ oz.in. but I see that it's short burst acceleration, and that the 50+ oz.in is continuous.  There are other models that have the max/continuous torqe in a more usable range.
________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #32 on: September 06, 2017, 09:32:57 PM »
There are not many options in the nema 23 form factor with a high torque much more than 90oz in in a 1/4" shaft configuration. There are two ratings: one is peak torque that  the motor can deliver for a short period of time, say like overcoming sticky ways as an example, and the continuous rating is how much torque it can provide continuously if you try to grip it with your hands. Steppers would just go into a "ZZZzzzzZZ" after max stall was applied, were as the ac servo would constantly try to turn the shaft with 90oz in of force even if the motor is fully stalled. This is how I understand it anyway.
The largest nema 23 servo in a 1/4" shaft  with the largest torque is this one:

https://www.teknic.com/model-info/CPM-SDSK-2321S-RQN/

Also what is neat about these servos are the built in driver boards. All you need is a pc with a BOB USB or Ethernet, and wire step and direction outputs directly to the servos.
« Last Edit: September 06, 2017, 09:44:06 PM by sn96 »
________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP

Offline ger21

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Re: Motion controllers - Take me to school
« Reply #33 on: September 06, 2017, 10:06:12 PM »
Quote
were as the ac servo would constantly try to turn the shaft with 90oz in of force even if the motor is fully stalled.

No, the drive will fault once it gets a certain amount out of position.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: Motion controllers - Take me to school
« Reply #34 on: September 06, 2017, 11:29:58 PM »
Hi Mike,
the most powerful 23 size from Clearpath is in the HP series CPM-SDHP-2341S-ELN, 165 oz.in but to get it they
had to drop the rpm back to 1680 rpm and up the price to $631.00

At slow speeds steppers produce a lot of torque, better, considerably better than a servo of the same size. At high speeds
the servo comes into its own but that won't matter a damn if the servo can't accelerate fast enuf to stay within its
following window and it fault out.

If you're really keen on servos you will probably need to go up to 34 size units.

The cheaper alternative is low inductance, high torque steppers with a decent high voltage drive, 68V or better.

On the basis of my previous post I would recommend a continuous torque of at least half my steppers/gearbox,ie
350 oz.in

Keep looking, keep talking, keep comparing, all of these are free, its too bloody late when you've just dropped a grand on the
wrong thing!

Craig
My wife left with my best friend...
     and I miss him!
Re: Motion controllers - Take me to school
« Reply #35 on: September 07, 2017, 03:27:19 AM »
Hi Mike,
ger21 is right. My servo is set with a following error of 20 encoder counts. It is an 8000 ppr encoder so 20 counts equates to 0.9 degrees, a half-step
of a two phase stepper.

I can set the following error window to be smaller, 4 counts I think is minimum, but then I have to slow everything down otherwise the drive faults out
all the time. I can likewise increase the following error but then there could be a noticeable inaccuracy between where I wanted the part cut and where it
actually gets cut, on the other hand you can push the speed up without running foul of following errors.

Craig
My wife left with my best friend...
     and I miss him!

Offline RICH

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Re: Motion controllers - Take me to school
« Reply #36 on: September 07, 2017, 06:51:53 AM »
Quote
I get it. You hate the idea of servos put on a small machine where steppers would work just fine

Not realy....do / buy whatever you want.
Our thinking is different.
Your learning and many replies are servo  / stepper application.
So continue learning and take into consideration posted replies.

I would suggest that you just take a moment and define what you want out of the Sherline.

1. Max axis velocity for rapids
2. Max feed rate which will be required to accomplish the machining for whatever tooling and
    material you will use.
3. Required acceleration
4. Accuracy and repeatability of the axes.
5. Then review the machine and answer this question:
     Is the current machine ( basic and all components) acceptable to do 1. to 4. above as is, with
      with steppers, with servos?

Have fun in school,

RICH

Offline sn96

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Re: Motion controllers - Take me to school
« Reply #37 on: September 07, 2017, 02:32:16 PM »
I appreciate all shared information from all of you. I am learning there is definetly more than what meets the eye. Ill reply more later. Im at work.


________________
Mike

Sherline 5400 with Sherline steppers
Visual Mill 6.0 Standard
Mach3
Windows XP
Re: Motion controllers - Take me to school
« Reply #38 on: September 07, 2017, 02:57:02 PM »
Hi Mike,
just been scanning around some of the other servo manufacturers and commonly a 400W servo has a rated torque of 1.27Nm which is equivalent to
160 oz.in. A 400W servo seems ridiculously large on Sherline mill.

My mill having gearboxes and so on is unusual and so my recommendation may likewise be way off as well. Do you have a specification for the torque of
the existing Sherline steppers? You know they work so you might say just double the torque and say 'now it'll work better'.

Craig
My wife left with my best friend...
     and I miss him!

Offline ger21

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Re: Motion controllers - Take me to school
« Reply #39 on: September 07, 2017, 04:12:44 PM »
I have a new 400w servo at home. It's slightly larger than a 570oz nema 23 motor. And probably 10x more powerful, if used properly. The servo has 160oz continuos torque at up to 3000 rpm, and  still about 100oz in at over 4000 rpm.
The 570oz stepper is down to about 160oz of torque at about 800-1000 rpm. If you use a 4:1 gear reduction, the servo has 4x the torque of the stepper. But the servo's peak torque rating is almost 3x higher, so when acceleration up to speed, you then have over 10x the torque of the stepper, in a similar size package.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html