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Any good DC stepper Motors to recommend

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23 replies to this topic

#1 azure1961p

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Posted 09 February 2019 - 08:55 AM

It'd need to turn an azimuth base , 30" diameter, 1 time in 24 hours. The loading on the azimuth is about 60 pounds factoring rocker box and OTA.

 

Thanks.

 

Pete



#2 gregj888

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Posted 09 February 2019 - 11:04 AM

Pete,

 

The only way to really do this is to measure the force needed to rotate the base.  String and weights, fishing scale, torque wrench … any way you have to measure it.  You want the force perpendicular to the edge.  Then double or triple the measured number for headroom.

 

I'll point you at the motors with attached planetary gear heads.  These add some backlash but drastically cut the power needed and increase the number of steps which is usually a good thing.  

 

If the force needed is large, you may need to change bearings in the support, but one thing at a time.  You can always find a big enough motor, but may not want to haul the battery pack it would need.



#3 TxStars

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Posted 09 February 2019 - 04:35 PM

Here ya go, this will turn it...  lol

 

https://www.ebay.com...03530315&chn=ps

 

As Greg points out you may not want to haul the power pack needed to run this..

If you have gear reduction you can use a smaller motor..

 

There are many options that will drive your scope when done correctly.

 

https://www.ebay.com...5.c100534.m4207


Edited by TxStars, 09 February 2019 - 04:41 PM.

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#4 sixtysomething

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Posted 10 February 2019 - 04:16 AM

The thing about stepper motors is they "snap" from one step to the next and that snap will show up in the eyepiece or on the camera unless the stepping rate is fairly high, steppers are inherently not smooth by the nature of the way they work. The implication is that you need a large numerical gear ratio between the stepper and the azimuth base.

 

Steppers can be tamed to some extent, microstepping drivers is the way you accomplish that, for telescope drives I'd say the driver is almost more important than the particular stepper you use. The cheapest microstepping driver board I've used without some sort of issue is this one. 

 

https://www.amazon.c...e?ie=UTF8&psc=1

 

The great majority of easily available steppers are 200 steps per revolution, there are some which are 400 steps per revolution and all else being equal a 400 step motor would be superior for your purpose to a 200 step one since the steps are only half the size which means the snap from one step to the next is only about half.

 

With the driver board I listed and sufficient gearing to keep the stepping smooth at the focal point this motor would be more than adequate for the system you described.

 

https://www.amazon.c...olution stepper   

 

If you plan on slewing your azimuth base at high speeds from time to time then there are other issues with how fast you can get a stepper to turn, they run out of torque fairly quickly at higher stepping rates so your overall gear ratio becomes a major design point balancing smoothness with ability to slew rapidly, more of one gives less of the other.



#5 gregj888

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Posted 10 February 2019 - 01:57 PM

To add to Sixty's comments, micro-stepping has very little torque, almost non from micro-step to micro-step.  In general, micro-steps are non-deterministic so can't be used for position without an additional encoder or guider.  Micro-stepping is used to smooth motion.  

 

There are a couple of issues tied into slewing.  Steppers slew fastest in full step mode and track best when micro-stepping.  It's really nice to have a controller that changes modes as it accelerates to slew speed.  The OnStep controller can do that.  

 

I'm not sure it the driver listed has those inputs (would be where the selector switches are).  If it can, then it will work in this mode.  If not, one of the little single board drivers may be worth a look.

 

The motor listed will work, probably with some gearing, but note it will draw close to 5 amps...  No right answer, it's a balancing act.

 

BTW, this is the reason the higher end systems use servo motors.  They have a much higher dynamic range and less cogging (a term for the step to step snap).  



#6 laixiaolue

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Posted 10 February 2019 - 06:23 PM

For stepper used in telescope, I would suggest a close loop stepper, like this one https://www.ebay.com...60/113422102890

 

Normal steppers consumes current even though they won't move. If you run it a whole night, you cannot run it on battery, and the motors get really hot. A close loop stepper consumes no current when it is idle, and it is reliable because it won't lose steps.

 

Just search "close loop stepper" from eBay or aliexpress, there are many with different sizes, nema23, nema17, nema34, etc, pick the right one for your design. Close loop nema 34 is actually very powerful, you can use it to drive a spindle to mill metal.

 

If you go to China, don't forget to buy some;) they are really cheap in China.



#7 TxStars

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Posted 10 February 2019 - 08:47 PM

It might take a little searching, but a 0.9° step closed loop would be great.



#8 gregj888

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Posted 10 February 2019 - 10:21 PM

 laixiaolue, what software do you use to control the closed loop stepper?  ASCOM and/or INDI drivers?  

 

It's actually better to put the encoders on the telescope axis's to do away with PEC and other gear issues while allowing for clutched movement.  

 

If I read the data sheet correctly the encoder is only 1000 ticks.  You have to assume +/-1  encoder ticks, 200 motor steps per revolution x say 8 micro-steps, you don't have enough resolution.  If you want to do 16 or 32 micro-steps, you really need a minimum of 12,000 discernable ticks.  

 

Please correct me if I've misread anything here.

 

So with what I see right now, I'm not sure I see the benefit.  I'd go with the DC servo systems and get the extra dynamic range.



#9 laixiaolue

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Posted 10 February 2019 - 11:26 PM

Greg, we are talking about stepper motor here, servo is another story. It depends on your design target. 1000 pulse per revolution applied to a worm gear is good enough for visual purpose. If you want higher accuracy, you have to pay more money for servo or closed loop stepper with better encoder.

 

The major advantage of closed loop stepper is that it is compatible with normal stepper. If you already have a stepper system, just replace the steppers and controllers with closed loop steppers. you can finish the upgrade in minutes. Another advantage is that closed loop stepper runs much faster, it can easily goes to 1000 RPM with losing torque.  It is still slower than servo motor, but much faster than normal stepper which can only do about 500RPM.



#10 gregj888

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Posted 11 February 2019 - 12:15 AM

 laixiaolue,  I don't see how a closed loop stepper has any higher RPM than an open loop stepper assuming the inductance and voltages are the same.  This shouldn't be contingent on open or closed loop control.

 

The mating motor has a 1000 line encoder so I assume 4000 ticks per rev.  That's enough to allow some micro-stepping for smoothing, but no change in rate so lower dynamic range.  You can use it for position so that a benefit by a factor of about 10.  This is nice, but there are tradeoffs.  You can lower the gear ratio, but then you need a bigger motor with more holding torque.   

 

The closed loop won't hurt anything, I just don't see a real benefit for the price but would depend on what I was trying to do I suppose. 



#11 TxStars

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Posted 11 February 2019 - 03:36 AM

The encoder on the stepper is not for positioning of the telescope, it is so you know exactly how far the stepper turned.

At high speed it runs more like a servo motor, the encoder lets you know how much it rotated.



#12 azure1961p

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Posted 11 February 2019 - 07:30 AM

Thanks so much guys. Really, I didn't anticipate the great info here. If I could just throw in another query ...

 

The Stepper would turn on the side of the azimuth base.  Think of a flat gear without teeth.   I would have an engager for the stepper to contact the side of the azimuth base with enough pressure to turn the rocker box.  Question is this...

 

With a stepper turning around a  30" base once in 24 hours I am left with the following needs then:

 

1.  A wheel that will engage the side of the base without slipping.  The base is birch plywood but I figure maybe a strip of Emery to provide some tooth to grip.

 

2.  The wheel on the stepper is a big player.  It can be steel and I just use a section of rubber shrink tube to assure good grip on the base.  But diameter is key.

 

Any suggestions guys?

 

Thanks in advance.

 

Pete



#13 gregj888

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Posted 11 February 2019 - 11:55 AM

Pete,

 

Google "motorize Dob telescope" there are a lot of options.  The timing belt versions seem to have done pretty well but I'd only used Eq drives with worms.  The Sidereal Tech has some info and I think the OnStep forum does too.

 

TxStars, yes, I understand.  With the steps also being deterministic I would prefer a planetary or worm drive on the stepper and the encoder on the telescope axis.  I'm not against the closed loop stepper, just think there's a marginal benefit in this case.  When you add the cost of the 2 closed loop steppers and controllers, you're better to halfway to a proven servo based system, the basis for that comment.



#14 TxStars

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Posted 11 February 2019 - 01:40 PM

Yea there are some stepper systems on the market as well as the servo ones.

 

https://www.365astro...ope-Mounts.html

 

http://astroproservice.com/

 

http://www.eqdrive.com.ua/en/



#15 laixiaolue

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Posted 11 February 2019 - 01:49 PM

Believe it or not, a closed loop stepper runs much faster at the same torque level.

 

 laixiaolue,  I don't see how a closed loop stepper has any higher RPM than an open loop stepper assuming the inductance and voltages are the same.  This shouldn't be contingent on open or closed loop control.

 

The mating motor has a 1000 line encoder so I assume 4000 ticks per rev.  That's enough to allow some micro-stepping for smoothing, but no change in rate so lower dynamic range.  You can use it for position so that a benefit by a factor of about 10.  This is nice, but there are tradeoffs.  You can lower the gear ratio, but then you need a bigger motor with more holding torque.   

 

The closed loop won't hurt anything, I just don't see a real benefit for the price but would depend on what I was trying to do I suppose. 



#16 gregj888

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Posted 11 February 2019 - 03:25 PM

laixiaolue, I'll just say sure, but I don't understand why.  It sort of defies what I know about steppers.

 

TxStars, indeed...  I've retired one of Mel's old stepper board.

 

AFAIK, there's little difference between steppers (if micro-stepped) or DC servos for tracking and guiding.  The big difference is in slew speed/dynamic range.  The capabilities of DIY steppers and those factory built are generally price, but you have to look at specific units in both cases.

 

The OnStep, using the RAMPS 1.4 board gets you the electronics: arduino Mega, stepper drivers with extra channels (total of 5, Alt, Az, Rotation, focus and filterwheel (?) or just spares) for $20 or less...  a little more if you want the onboard display.   Stepper drivers are listed at 2a peak per phase but shouldn't be run that hard, 1A - 1.25 is probably as hard as I would push.  From there there are 4.5 amp board or you could use the modules listed above and wire them in. 

 

Pete, it's up to you which way to proceed.  There are a number of options listed here, all good and workable.  All the open loop stepper systems can be adjusted for current draw so you can optimize that some.  Geared steppers will give the needed power with a smaller motor (lower power) and smaller step size at the cost of slower slew speeds.  If you add on telescope axis encoders you can push to and the controller will pick up the control when released if this function is supported. 

 

The closed loop steppers will recover some resolution and slew speed from the gearing at the cost of needing a bigger motor and more current.  These can be geared down too, but not sure what you would gain.  I don't have a feel for current draw on a roller drive (I would expect the motor to be on more to keep the scope from moving off the set point compared to a worm drive) but laixiaolue may have some data.  Maybe a small worm reducer between the motor and telescope ???

 

No bad choices here...



#17 brave_ulysses

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Posted 11 February 2019 - 06:36 PM

hi greg,

 

i believe this is what laixiaolue is talking about:

https://www.robotics...r-control/10398

 

scroll down to

 

"Driving a Stepper as a 2-Phase Brushless Motor"



#18 gregj888

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Posted 11 February 2019 - 07:37 PM

Clay, I get it or think I do. 

 

When we drive a stepper using microsteps that's that we are doing the same thing electrically.  My understanding is that a stepper's speed is based on it's electrical and magnetic characteristics and those of the driver.  Inductance and drive voltages being the primary factors. 

 

You could change the voltage curve some under closed loop, but that shouldn't be any different than running full step by changing the mode of the driver.  In full step mode current is limited by the field buildup (related to the inductance) and how hard you push the current through the coils.  Servo control won't change that or at least I don't know how it can.

 

I'm not trying to challenge anyone here or be overly dense, but I don't see how adding feedback to the control changes the top speed. 

 

Closed loop will change the position reliability of the stepper between steps which is nice, if you can use it.  I can't see how to use it effectively when we are adding that precision to the motor shaft for most applications. 

 

Again, I'm not against using the closed loop steppers...



#19 azure1961p

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Posted 16 February 2019 - 08:53 AM

Hi Folks,

 

Thanks for a lot of great a d vice and as much, alternatives.  I've narrowed down my stepper needs so here goes...

 

30" Azimuth base will have a stepper turning a half inch wheel on the main spindle  2.5x per hour. 

 

So that the rub: 2.5 revolutions per hour.

 

I once bought steppers from Herbach and R ad that were 1/15 RPM.  What can I do to get it 2.5 revs per 60 minutes?   Is this possible?

 

Thanks,

 

Pete



#20 TxStars

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Posted 16 February 2019 - 11:49 AM

If you purchased a motor in the past that was 1/15 rpm it was likely a synchronous AC motor.

Stepper motors do not have a fixed rpm and will run any rpm you need them to run via the control board..

Most need 200 steps to make a full rotation so 2.5 rph would be "choppy"

As said earlier you would want to use gear reduction to smooth the movement out.


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#21 gregj888

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Posted 16 February 2019 - 06:44 PM

Without knowing how much force you need, maybe something like this?  This motor would need ~13 full steps per second so using microsteps should be fairly smooth.  You may want to go up a size or 3 on the motor, but it's a start.

 

 https://www.omc-step...684s-pg100.html



#22 brave_ulysses

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Posted 16 February 2019 - 07:54 PM

be sure you are aware of the shipping time if you buy from oms-stepperonline

 

my shipment has been on the slow boat for over a month...



#23 gregj888

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Posted 16 February 2019 - 09:58 PM

I just got a couple of small steppers from  oms-stepperonline in less than a week.  That's west coast US from their west coast US warehouse.   All I can say...


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#24 555aaa

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Posted 19 February 2019 - 05:24 PM

Can we ask what is going to control this thing? The azimuth axis doesn't go at constant speed if you are tracking the stars with a dob. 




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