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Author Topic: Change Stepper to Servo  (Read 2853 times)

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Re: Change Stepper to Servo
« Reply #10 on: October 27, 2017, 05:18:11 AM »
Hi,
whats wrong with the open loop steppers?...are they losing steps or too slow? Many people seem to think that a closed loop is going to solve 'lost steps',
certainly that's the advertising hype,  if a motor is loaded to the extent its losing steps then feedback is going to try to overcome that but feedback
DOES NOT make a motor any more powerful. If it was losing steps because it lacked the torque to do the job, it will still lack the torque, lose steps and
then fault out 'following error' feedback or not.

Servos, either brushed DC or brushless AC maintain their torque at high speed and beat stepper motors hands down above 1000 rpm or so. Stepper motors
have lots and lots of torque, for their size, at low speed. In fact steppers beat servos hands down at low speeds, for torque anyway.

Steppers and their drivers are way cheaper than good servos and except for high speed are as good or better than servos of the same size. If you really really
need your machine to go fast then pay up....and for your high speed production machine you'd be stupid to buy cheap crappy servos, you need good ones which
will last.

I repair welding equipment for a living and I see cheap Chinese electronics all the time...don't go there....they can and do make good stuff but its not cheap!

Craig
My wife left with my best friend...
     and I miss him!
Re: Change Stepper to Servo
« Reply #11 on: October 27, 2017, 09:04:49 AM »
ggg
no i am ahobby user only
i mill now with my second milling machine the third bigger and massive one
for the new machine i need stronger motors
so if i take steppers then i need new driver

i can take the postep 60 but i then prefer i closed loop system
or i take the servos
a friend have them and have good experience with it
(https://www.youtube.com/watch?v=O_PR7S4j1og&t=94s)

which stepper cloes loop can you advice

thx
Re: Change Stepper to Servo
« Reply #12 on: October 27, 2017, 09:42:21 AM »
Hi,
34 size, that is 80mm square motors can be had in various lengths upto about 2000 oz.in (10 ft.lb or 15Nm). Low inductance windings promote
high speed torque in conjunction with a high voltage driver, ie minimize a steppers disadvantages.

American made Gecko drivers have a very good reputation, their 80V 7A models are around $150. Some experienced CNCers recommend Leadshine
AM 882 80V 8.2A drivers which are good value for money at $75.
My wife left with my best friend...
     and I miss him!
Re: Change Stepper to Servo
« Reply #13 on: October 27, 2017, 10:04:56 AM »
ok that are the drivers
but motors ?
and can i connect this leadshine to the pokeys
Re: Change Stepper to Servo
« Reply #14 on: October 27, 2017, 10:31:39 AM »
Hi,
there are lots to chose from, Leadshine 86HS85 are 8.5Nm units  for about $80, even bigger 86HS120 which are 12Nm are about $200.

Note these with the AM882 are open loop systems. Closed loop systems tend to cost more yet not really get any advantage, they are still stepper
motors, a closed loop driver done not make them any more powerful or faster. They will try to make up for lost steps if there are any but in most cases
if they lose steps they can't make them up because they're just plain overloaded so the drive faults 'following error' and shuts Mach down.

An 8.5Nm motor and drive would cost about $155 plus shipping. A 400W Delta servo is rated for only 1.27Nm and servo and drive cost about $380
and a 750W is still only 2.39Nm with servo and drive about $600. The servos will be much faster but also way more expensive. Used within their
limitations the open loop steppers will work well for a fraction of the price.
My wife left with my best friend...
     and I miss him!
Re: Change Stepper to Servo
« Reply #15 on: October 27, 2017, 11:20:51 AM »
thx for your detailed description
but thats the point
this servos with implemented encoder and driver cost 130€ + shipping
i need 4
x y z and a

gecko 150$ + Leadshine 86HS85 80$ are cost more
and i think servo are the future
Re: Change Stepper to Servo
« Reply #16 on: October 27, 2017, 11:48:20 AM »
Hi,
get some proper information about those servos. They sound too good to be true.....and therefore probably aren't true.

130 for proper servos....yeah right! Good luck.

Craig
My wife left with my best friend...
     and I miss him!
Re: Change Stepper to Servo
« Reply #17 on: October 27, 2017, 11:10:42 PM »
Hi,
found these on Ebay:
https://www.ebay.com/itm/200W-3000rpm-0-64N-m-NEMA24-Economical-AC-servo-motor-driver-kit-36VDC-1250-line/263174349390?hash=item3d4669e24e:g:vl0AAOSw7ZlZpTq8
which is pretty cheap, note that its only 0.64Nm.

https://www.ebay.com/itm/200w-60mm-0-64Nm-3000rpm-AC-Servo-Motor-drive-kit-absolute-20Bit-AC220V-JMC-CNC/263257457182?hash=item3d4b5e021e:g:jXYAAOSw6b9Z3bjT
This unit is made by the same company that you linked to but these are the real deal AC servo and note the price, $300 plus shipping. And like the
previous model I linked to still only 0.64Nm.

If you want Mitsubishi, Panasonic or Yasakawa double and triple and more those prices and still only 0.64Nm.

I think 130 EUR is highly optimistic or alternately very  very low powered 50 or 100W either way insufficient for a decent size router/mill.

Craig
My wife left with my best friend...
     and I miss him!
Re: Change Stepper to Servo
« Reply #18 on: October 28, 2017, 03:36:45 AM »
Hi
the holdingtorque of the stepper like leadshine is 8.5nm
but when you drive 500 revolution you only have about 3nm
at 1000 only 1nm

this servos have about 1.8kn at 300%
and when its right konfigured the have also much power at low feed rates
Re: Change Stepper to Servo
« Reply #19 on: October 28, 2017, 04:28:49 AM »
Hi,
you're right steppers lose torque at speed, low inductance units like these lose less than most but still only have a 1/4 of their torque at 1000 rpm, so
about 2Nm or 305 oz.in.

I've found a spec for the largest of the integrated servos from JMC, 180W and 0.57 Nm or 87 oz.in. The servos can temporaily do three to four times their
continuous rated torque but they'll fault out in 20-30 seconds. If you expect to run them in excess of their continuous rating you'll be disappointed.
Quite frankly I doubt 87 oz.in is enough for a substantial machine. If they lag behind because they don't have the torque to maintain the position/velocity
profile of your Gcode trajectory they will try to keep up and either overheat and fault 'over temp' or lag behind the position required and fault 'following error'
Closed loop feedback does not make servos immune from being overloaded and when overloaded they perform just as badly as any other overloaded motor.

Quote
and when its right konfigured the have also much power at low feed rates
wrong...they have 0.57Nm torque from 0 to 3000 rpm, they do not
have more torque when going slow.

You might be advised to get one and try it out before you buy the whole lot.

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