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Harmonic drive EQ mount project

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4 replies to this topic

#1 lukasik

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Posted 16 October 2019 - 09:53 AM

Finally - I'm FREEEE.  Well sort of.  Last December I got two HD component sets for under $200.  They're used from an industrial robot, and look to be in excellent condition.  One set also had a serious crossed roller bearing on the output.  I bought another bearing to match for the second axis.  It took me 10 months to grind through the design as I picked away here and there when I could.  I want to do this project really well including finishes and all, so not to be hurrying.  I now have some 40+ sheets of detail drawings and a wee bit of machining to do - no CNC - just a manual lathe and mill.  The plan is to run it all with OnStep which I tried briefly in 2017 - those folks have really done impressive things with that platform!  I designed in axis release clamps outside of the HD drive trains to mimic having clutches like a typical worm drive might. The whole mess should weigh about 30 pounds (without tripod or counterweights).  Should easily carry 50 pounds or more depending on the placement of the load and acceleration rates - more if i use a counterweight.  HD's have pretty big periodic error so guiding will be needed for long focal length / duration imaging.

 

I think I'll super clean the shop before making chips - lots of chips.

 

Cheers,

 

Bob

Attached Thumbnails

  • Harmonic Drive Mount FINAL 4b.jpg
  • Harmonic Drive Mount FINAL 3b.jpg
  • Harmonic Drive Mount FINAL 2b.jpg

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#2 macdonjh

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Posted 16 October 2019 - 10:45 AM

I'll take one.

 

No, wait, I just bought a mount.  Oops.  Bad luck for me.



#3 theropod

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Posted 21 October 2019 - 09:16 AM

So, belt drives in the finals? My phone just won’t really let me see detail in the drawings. Yes, I’ll be watching this thread.



#4 Aleksandr Naumov

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Posted 21 October 2019 - 09:25 AM

Bob,

 

I would like to ask you:

 

- what is the reduction of HD ?

  (I see stepper motors on your drawings, think there is no internal gear drive)

 

- could you estimate backlash on these HD ?

 

- how do you estimate "pretty big periodic error" ?



#5 lukasik

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Posted 22 October 2019 - 09:40 AM

Hi Macdonjh - thanks.  Hopefully I'll end up with one!

 

Hi Theropod - the belt drives are on the input side of the harmonic drives:  90T and 17T for a 5.294: reduction from the stepper motors.

 

Hi Aleksandr - The HD sets are listed as 100:1.  Because of the mounting arrangement the RA axis may actually be 101:1 * 5.294 to give 534.7:1 overall.  The DEC axis will be 100:1 * 5.294 to give 529.4 overall.

 

The output backlash for the HD drives is zero.  However there is a small backlash on the input side due to the Oldham coupling used in the wave generator assembly. The units I have are spec'd at 9 arc-sec max backlash on the input.  So that equates to .09 arc-sec backlash between the motor and the output if you assume zero backlash in the belt drives.  HD drives are often described as "zero backlash", but there is that tiny bit that will depend on the ratio of the drive.
 

Regarding the periodic error:  I first became aware of this reading a thread in the mounts forum about the RainbowAstro RST-135.  Lots of good discussion in this thread.  My sets are either 30 arc-sec peak to peak if standard, or 15 arc-sec peak to peak if special.  I'd sure like to feel special, but don't really know what I have in hand.  This info all comes out of the HD component sets catalog.

 

I'll update as I successfully make some parts.

 

Best Regards,  Bob


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