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Mount gear ratio calculator

Astrofriend

By Astrofriend
in DIY Astronomer

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Astrofriend

Astrofriend

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Have you ever had thoughts to build your own motor driven mount? Then you need to know how to get the gear ratios you need. You also need to know what angular resolution your motor works with, what does one step from your motor correspond to in arc seconds.

I have made a Mount Gear Ratios calculator on my homepage. It's new for today and there can be some mathematical erros in the calculation so double check everything before you buy any expensive parts!

 

Here it is:

http://www.astrofriend.eu/astronomy/astronomy-calculations/mount-gearbox-ratio/mount-gearbox-ratio.html

 

Let me know if you find anything wrong with it.

 

/Lars

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Astrofriend

Astrofriend

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Hi Gina,

Stepper motors makes it easier to find driver bord.

 

My EQ6 mount is fine but I want something little bit better. A true directdrive system had been nice, but too expensive today. Maybe cheaper in future because of the lack of mechanical gears.

 

/Lars

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Gina

Gina

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My plan is to use timing belt drive with two step reduction.  I shall probably use NEMA17 stepper motors with 0.9° step angle with 16x microstepping for tracking and full-step for slewing.  A4988 drivers controlled by a Raspberry Pi and INDI drivers.

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Astrofriend

Astrofriend

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Hi Gina,

I tested your figures in my gear ratio calculator. With a standard 180:1 worm drive and two 3:1 belt reducers (I have modified it to take two reducers), 400 step stepper motor and 16 bit micro step I get 0.125" / step. Good to not use 64 bit micro step, gives poor torque and not so good precision.

 

What kind of timing belt are you plan to use, glasfibre, steel inforced, kevlar etc ? I have a nylon belt with steel inforce in my belt reducer to my focuser. Maybe I can get i better with smaller teeth pitch and kevlar, but I don't have that experience today yet to judge. And I think my focuser is good and no need to improve it today.

A lot of people rebuilt their EQ6 mount to timing belt drive. But it looks to be very difficult to set the tension of the belt because of the incluser. The tension is important to get low backlash.

I have read at other threads you started, you build a lot of equipment your self I understand. Background from the industry, or?

/Lars

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Gina

Gina

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My DIY mount will be for widefield imaging.  A ZWO ASI 1600MM-Cool camera with wide angle vintage SLR lens.  No worm drive, just direct belt drive with MXL timing belts (or similar 2mm pitch).  A resolution of one second of arc is sufficient in this application.

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  • 3 weeks later...
  • 2 weeks later...
Astrofriend

Astrofriend

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I have done a lot of test of the focuser motor and the focuser. And I have also succeeded to lower the friction torque of the focuser shaft. From high 30 Ncm to 15 Ncm. I also found in my case, if the calculation give me that I need 40 Ncm the stepper motor and gearbox must deliver maybe 4 times more torque to be safe.

The stepper motor has most difficult to handle when just doing a few steps, when moving hundred steps or more it works fine. I noticed most problem when it doing temperature adjustment, in that case it move just a few step each time it correct the focus, the stepper motor very often stall in this case. If it stall it get out of sync and then the autofocus mode doesn't work at all.

 

The stepper motor also work better in halfstep mode than fullstep mode, anyone noticed this too?

 

This was the most important fix I did:

http://www.astrofriend.eu/astronomy/projects/project-focus-friction/page07-project-focus-friction.html

 

Old grease is not good!

 

/Lars

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Gina

Gina

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I see the problem.  I was comparing single step with just one set of coils energised with half step with two sets energised, forgetting the in-between half steps with only one set energised.  Sorry for the confusion.

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