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Precise backlash measurement

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If you look at the values, both show a clear trend for decreasing values.
My suspicion would be that there is slip somewhere in your focuser train:  Either it is a Crayford-type that works just by friction between the axis and the tube, or you have connected the focus motor to the fine drive knob that usually also has some intrinsic slip.

But if your focuser has slip, backlash correction is quite futile anyhow. :(
 
2 years 11 months ago #70422

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Hi Peter,

These readings were from the WO Z61 with a 2" Rack/Pinion focuser, and I'm connected directly to the course knob (not the fine). Any ideas what else can be wrong?

 
2 years 11 months ago #70431

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In that case it could be temperature variation.  If you put out the telescope from a warm room into the cold, it will change focus until it's temperature stabilized.  For a test like the above, I'd wait at least an hour after having brought the telescope out (My 140mm needs almost 2 hours...)
Or the coupler between motor and focus axis isn't solidly fixed.  Apart from that, I'm out of ideas....
 
2 years 11 months ago #70439

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Replied by Gene N on topic Precise backlash measurement

Hi Mohamed,

This is on your WO Z61 with focuser motor under comtrol of the MyFocuserPro2?

How fast are you driving (steps/microsteps per second)? What type of stepper motor.

Drive too fast and miss steps, not enough motor torque and miss steps.
Not all clamped down on the shafts and slippage as Peter mentions.

What was the orientation of the scope, high alt, low alt ?

Gene
PS: For my video, sorry, it was meant to show a low repeatable backlash stepper motor gearhead.
2 years 11 months ago #70446

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Hi Gene,

Please find my answers below...

>>This is on your WO Z61 with focuser motor under comtrol of the MyFocuserPro2?
Correct. It is MyFocuserPro2

>>How fast are you driving (steps/microsteps per second)? What type of stepper motor.I use STEPPERONLINE 27:1 Planetary Gearbox Nema 17 , and I configured it to 1/4 step


>>Drive too fast and miss steps, not enough motor torque and miss steps. Not all clamped down on the shafts and slippage as Peter mentions.>>What was the orientation of the scope, high alt, low alt ?I did this focusing test close to the meridian & equator (South, ALT about 40-50degrees)


 
Last edit: 2 years 11 months ago by Mohamed.
2 years 11 months ago #70535

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Replied by Gene N on topic Precise backlash measurement

Hi Mohamed,

Thanks!

For the 'steps/second' part of the question, when you did the testing, on the Main tab of the driver, what value did you have for 'Speed' Focus Speed?

Source code allows values of 0, 1 or 2

Which looks to directly correspond to Slow or Medium or Fast
as defined in the documents on sourceforge myFP2-307-3.pdf

Had you done any tuning of the motor speeds as given in the document section
FINE-TUNING THE STEPPER MOTOR SPEED
?

Did you happen to try the backlash experiment at different speeds or different stepping, which you mentioned was set to 1/4 step?

This is one area that can cause strange results, how fast driving based on chosen step mode, just looking to rule it out.

Another dealing with stepping is power supply to the phase coils of the motor drooping.

ALT: With your angle of 40-50 degrees, this would seem to present leverage on the drawtube when changing directions, giving it a chance to tilt.

Gene
2 years 11 months ago #70589

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Hi Gene

Thank you for your response. The speed was always set to "2". I shall perform the test again today and share the results
2 years 11 months ago #70664

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Hi Gene,

After setting the speed to zero, I started to get more consistent results, and I make an assumption that my backlash in this configuration is 250 steps...

Please advise your thoughts...

 
2 years 11 months ago #70679
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Replied by Gene N on topic Precise backlash measurement

Hi Mohamed,

Your iterations show decreasing position numbers for both forward and reverse, the delta's.
Is each set of measurements made with a different filter or just different focus runs?
Looking at the numbers
8910 - 6328 = 2582/4 = 645.5 *degrees/step(0.067) = 43.2485 degrees output shaft movement
8651 - 6119 = 2532/4 = 633 *degrees/step(0.067) = 42.411

Where the /4 is your 1/4 stepping and 0.067 from your 27:1 motor/gearbox reduction

Gene
2 years 11 months ago #70696

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Hi Gene

This set of numbers taken using one filter and mostly same star. I was pointing to NCP with angle 33d

From your math, is the backlash 42 steps?
2 years 11 months ago #70703

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Hi Mohamed,

If all the focus runs were done without changing filters and same star, looks really strange.

All reverse/no reversed should be the roughly the same values for -every- focus run and -not- be different by 2500+ steps difference between runs 1 and run 6

The '43' was actually the 2500 steps difference translated to stepper motor output shaft rotation difference in degrees betweens runs 1 and run 6.

Gene
2 years 11 months ago #70720

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