Stepper Motor's help?

[Dave1]

Hi guys,

I need help in understanding stepper motors? How would I go about making sure I choose the right stepper motor? What part of the specification do I need to pay attention to? 

I need help in selecting the correct stepper motors for my Fullerscopes mount. I know the original Fullerscopes synchronous motors were 8 Lb/in. I don't know the actual torque value of the AWR motors themselves. But I do know that combined with a 3:1 or 2:1 pulley system they put out between 1.8-3 Nm ( 15.93-26.55 Lb/in ). Here's AWR page http://www.awrtech.co.uk/motors.htm . 

What I also don't understand is the gear reduction ? Or max slew speed deg/s?

I have also read about rpm of the stepper motors being important? And the steps?

 

David

[markse68]

For a starting point 8lb inch is about 0.9 Nm so if you aimed for 1-2Nm output torque you should be good. For gearing bear in mind steppers have limited max speed of about 600rpm before their torque falls right off. So you need to work back from the final slew speed you’d like or if you only want sidereal motion multiplied by the worm ratio and then chose an intermediate gear ratio to multiply to give 600rpm on the input. Then use that intermediate  gear  ratio to divide the 1-2Nm to get your motor torque and maybe add a bit for losses. You could use micro stepping drives to get smoother quieter motion but they use power to hold between step position. But full step you'd need a high enough gear ratio to not see the steps at the ep which would limit your max slew speed . I think I got that about right but will reread just in case

[Dave1]

@markse68

 The Beacon Hill 6 1/2 " gears provide 360:1, is that worm ratio? Sorry for the very basic question, but I am completely new to this, and keen to learn. Most of what you've just said has gone straight over my head.

Do I multiply 360 by 2 ( if the pully used from the motor to the worm is a 2:1 ratio ) and does that then tell me final gear reduction? 360*2=720?

And if I used a 360 worm gear with a 3:1 system from the motor, 360*3=1080 ?

How do I choose an intimidate gear? Or is that the 2:1 or 3:1 from the stepper motor?

How do I choose what final slew speed I'd like?

The motors will probably use the On Step system http://instein.eu/onstep.htm. Which includes the micro stepping drive/box.

David

[markse68]

Hi Dave, yes you have 360:1 worm ratio and if you use the belt multiply that by the 2 or 3:1.  It looks like there’s a planetary or spur gear stage on the motor itself? That will have a ratio too which you multiply also. That will give you the final ratio which will divide the input motor speed to give you an output speed (speed the scope moves) and multiplies the motor torque minus losses (friction in the gears).  The trouble with choosing the max slew speed is you need to base it on the stepper max speed of 600rpm but you need to keep enough gear ratio to not have issues seeing the steps at the eyepiece. The higher the final gear ratio the slower the max slew rate but the smaller the step and smaller the motor. If you want faster slew rate you need lower final gear ratio, stronger motors and need to think about output step size. Not sure what would be an acceptable output step size though. Are there or could you add clutches so you could slew  the scope by hand then let the motors take over?

[Dave1]

@markse68

Looking at AWR photo's, I don't see any form of gearbox in use. Though I am no expert.

So 360*3=1080 

1080/600=1.8 ( degree ) ?

I'm not sure about the whole clutch idea at this stage. I'm still not sure if I understand all this. I'm still no closer really yet to understanding or selecting a stepper motor. Put a piece of machinery in front of me, I can fix it, measure, do checks, even do basic specification for manufacturer. But when it turns to this type of stuff I really struggle.

 

Edited May 8, 2020 by Dave1

[markse68]

Hi Dave, this looks like a gear head but it needs the extra gearing because it’s a higher speed lower torque motor. In your case looking at the numbers I don’t think you need any extra gearing than the worm at all.

If you’re going to use microstepping drives then you probably don’t need to worry about steps- you can work out what one micro step will be in the output once you’ve decided on motors and gearing

suppose you want 10 deg  per second slewing, that would be 1.66 rpm. The worm ratio of 360:1 would mean you’d need to turn the worm at 600rpm (handy) with a torque of 1-2 Nm. So you could buy stepper motors with that sort of torque- Nema  23 perhaps and drive the worm 1:1. With 256 microsteps that would give you 256 steps per degree of output motion. Sounds ok to me but I don’t know. You couldn’t go any faster than that unless you really over rated the motor torque to allow for torque fall off or geared up a more powerful motor but at loss of resolution. You could go slower than that and get higher resolution- 5deg per second would double the resolution to 1/512 degree by adding the 2:1 belt drive. You may not want to run the motor at full speed for noise reasons.

[markse68]

Sorry Dave, I completely misread your original post and didn’t click the link to AWR motors. I thought you were talking about your original motors 🤦‍♂️. 

[markse68]

And I think I made a mistake about the max rpm- the trinamic boards can drive the motor up to about 300rpm and AWR site seems to spec for 120rpm so slower slew  rates. Sorry for the confusion

[markse68]

So what was your question again 🤪

Are you going to buy a system from AWR or looking to replicate it?

[Dave1]

Sorry Mark for the confusion. No I'm not using the original motors and gearbox's that come with the mount. As they are 12v DC motors. Wrong type of motors.

I want to try and replicate the AWR set up or to use the AWR set up as a basis for understanding. As they are the only company bar maybe Beacon Hill, that offer a GoTo kit for the Fullerscopes MKIV mount. 

That's why I've been quoting 3:1 or 2:1. AWR in another area of its website states it needs between 100-200 Ncm for the Fullerscopes to work http://www.awrtech.co.uk/retrofit.htm.

[markse68]

I think you could drive the worm direct with a motor like this: https://www.omc-stepperonline.com/nema-23-stepper-motor-8mm-single-shaft-for-face-mask-machine-and-respiratory-ventilator.html

or chose one half that torque and gear it 2:1 with belt but that would halve the slew speed. It requires 2.8 amps for full torque though so better check the drive can handle that. 

What will you be powering it with?

[Dave1]

Please see this thread for an update on this project https://stargazerslounge.com/topic/353841-fullerscope-mkiv-tracking-or-goto-help/?tab=comments#comment-3852630

 

 

[Starwatcher2001]

Sorry for replying to an old thread, but I'll add this is case it's useful to someone.  I've been away from ATM for a long time, so forgive me if this is common knowledge.

When motorising my dob, I found that adding a weight to the tail end of the stepper, like a flywheel, helped increase slewing speed significantly. It does require that you have the ability to ramp the slew rate up from an initial slow rate to a faster one over a period of a few seconds, and then ramp down at the end. This helps overcome the tendency to stall the motor. The compromise is the bigger the flywheel, the faster the eventual slew, but the more likely it is to stall. In my case I experimented with circular lead weights to find a good value.