Stepper motor mount for Losmandy GM-8


John Scherer
 

I decided to upload the design to Thingiverse.  Its just easier that way ;-)


Howard Dutton
 

That should be a great solution John.


Howard Dutton
 
Edited

On Sun, Dec 22, 2019 at 08:51 AM, Khalid Baheyeldin wrote:
An advantage of those 5-phase motors is that they could be connected with just 1:1 pulleys for EQ4 (144 teeth at 1/64 microstep).
That can be helpful in cramped situations where a bigger pulley is obstructed (like my mount).
I tested a 1000 step 5-phase stepper motor yesterday.  It was smooth and accurate in its motion but shaft angular displacement was still there and every bit as significant as any of the 400 step NEMA17 bipolar motors I tested.  The MKS SERVO42B on the other hand should in theory do fairly well on a GM8 (provided torque is sufficient,) but I would guess not as well as adding a 4:1 reduction to a standard 400 step motor/driver.

Test results for the 5-phase and MKS SERVO42B are near the bottom of the page:
https://onstep.groups.io/g/main/wiki/Stepper-Motor-Accuracy


John Scherer
 

That 5 phase system is very nice, and the price it good I think, but from a price/performance position, the SERVI42B's is tough to beat.  The SERVO42 mounted on that 116 oz/in PKP246MD15A2 motor would have plenty of torque but I haven't looked closely enough at the SERVO42 to know if it could handle the current.  The other issue I have with my GM8 Stepper motor mount is the spacing between the motor and the mount is about 1mm with the PKP244MD08Bs I currently use, so I'll need to add spacing between the stepper bracket and the mount if I want to use motors longer than the PKP244MD08.  


Howard Dutton
 

On Fri, Jan 3, 2020 at 11:40 AM, John Scherer wrote:
That 5 phase system is very nice, and the price it good I think, but from a price/performance position, the SERVI42B's is tough to beat.  The SERVO42 mounted on that 116 oz/in PKP246MD15A2 motor would have plenty of torque but I haven't looked closely enough at the SERVO42 to know if it could handle the current.
I think the SERVO42B wouldn't need a bigger motor, others use NEMA17 400 step to directly drive the GM8... and the SERVO42 won't miss steps so you'd think it'd be ok.  Also little to no benefit in 400 step so a much wider range of (low cost) motors if one does want to swap.  Perhaps a pair of these if the stock motors aren't quite up to it.  Staying at a < 1A setting is a good idea from what the manual says.

  The other issue I have with my GM8 Stepper motor mount is the spacing between the motor and the mount is about 1mm with the PKP244MD08Bs I currently use, so I'll need to add spacing between the stepper bracket and the mount if I want to use motors longer than the PKP244MD08.
My G11 with 40 or 50 lbs onboard slews around just fine with a PKP244MD08 running at only 0.4A.  You are at twice the overall reduction with a smaller lighter mount so torque load will be rather modest and tracking accuracy very good.  My PKP244MD08 also tested best with the TMC2130 at about this setting (0.48A)  At 0.85A the TMC2130 driver (stealthChop) didn't control the shaft angle as well through the micro-step sequence.


Howard Dutton
 

On Fri, Jan 3, 2020 at 11:40 AM, John Scherer wrote:
That 5 phase system is very nice, and the price it good I think, but from a price/performance position, the SERVI42B's is tough to beat.
The SERVO42B is one noisy beast though.  Not like a super annoying pwm whine but more like pepper mill kind of thing.


Alexander Varakin
 

Howard,
Is it always noisy or only during slews?

John,
How are you planning to couple the timing belt gear to the worm?


Howard Dutton
 

On Fri, Jan 3, 2020 at 12:31 PM, Alexander Varakin wrote:
Is it always noisy or only during slews?
Always noisy, more so during slews and doesn't sound like a normal stepper motor due to the rapid constant corrections in position I guess.


Howard Dutton
 

On Fri, Jan 3, 2020 at 11:40 AM, John Scherer wrote:
That 5 phase system is very nice, and the price it good I think, but from a price/performance position, the SERVI42B's is tough to beat.
On paper the 5-phase motors should be incredible you just need extra reduction to get the tracking resolution up high enough.

By way of comparison, for example, the PKP244MD08 pull-out torque (stall) drops to 30 oz-in by 200 RPM.  The PKP546MN pull-out torque drops to 30 oz-in at 1000 RPM.  For my G11 that's 3.3 deg/s for the PKP244MD08 which is about my fastest reliable slew speed.  So perhaps the PKP546MN would be 5 x faster at 15 deg/s?  Or double or triple the reduction and still do insanely fast slews and have fantastic tracking.  But really I don't care much about slewing at 15 deg/s so seems like a lot of work.  And this doesn't factor in the low current I run the PKP244MD08 in to keep mid-band resonance down which 5-phase don't really suffer from.


Howard Dutton
 

I'm still excited to hear about how this works.  Would it fit on the G11?


Jean-Paul Nombret
 

I confirm 5 phases system is very nice.
On my home made equatorial mount I use an Oriental motor Vexta 5 phases with its proper driver of 250 microsteps (125000 micro steps)
I use a 40:1 friction drive between a rectified steel 5mm axe and e "dremel" rectified 200mm disk.
Result is 13970 steps/deg and a 36 minutes period.
It's completely silent  during tracking, just a very little noise during slews at 4.5 deg/s (the max with Atmega 2560)


John Scherer
 

I've made what I'm hoping will be the final design changes to my Losmandy GM8 belt drive stepper mount.  You can find all the info on the Thingiverse page.  I've redesigned the plate to make it rectangular rather than that bent-rectangle design I had before.  The benefit of this is that the same cover design fits both the RA and DEC stepper mount.  I've also added a bearing mount in the cover so if you wish, you could extend the shaft on the 60T pulley so that it would recess into the bearing for added stability.  A very quick video is here.  I'm waiting on couplers from R+W, they take a while but I think they are made to order.  The plan after that is to have them machined from aluminum.



Klaus
 

Nice Work