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INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Precise backlash measurement

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If the focus routine always moves to final position in the same direction then oversizing the backlash steps when reversing basically makes it a do not care because the backlash cancel's out.

Think about it this way, currently at 500 with actual backlash = 25
Move back 200 physically moves it to 325 (175 = 200 - 25 backlash), counter though shows 300
Move forward 200 (physically moves 175 = 200 -25 backlash) up to 500 physical and counter shows 500
2 years 11 months ago #70105

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Thank you Ihoujin, will give this a try in the cloudy week...
2 years 11 months ago #70113

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The linear algorithm approaches the final position from one side in an effort to eliminate the consequences of backlash. Which is fine. But it's only true for that algorithm. Iterative and polynomial don't do that, so good backlash compensation is preferred and will more easily fit a better curve.

As a matter of opinion, I have come to prefer the speed and accuracy of polynomial with a reliable focuser.
2 years 11 months ago #70114

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Hi Gene,

The problem is as Ihoujin mentioned if you are approaching the final focus point from one direction as in linear it won't matter, but the problem is that (1) IN and OUT backlashes could not be equal, and (2) This confuses the algorithm as at the final position, the algorithm is not equally going IN and OUT, and that is the cause of the shift of backlash that causes the problem.
2 years 11 months ago #70116

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Hi Mohamed,

True backlash in opposite direction can vary but more than likely comes from drawtube tilting than gear train backlash, which is more repeatable -if- the gears are fixed to the shaft and do not ride on the shaft they are mounted to. Coming from drawtube tilt also implies amount may change depending on how far the drawtube is extended.

Here is a video I did of a stepper motor head used, gears only in the head. Repeatability is good considering any slip that may have occurred of the dial indicator on the moment arm and the 5 pound weight hanging from the 2 inch moment arm :-)

Stepper motor head video from 2012
2 years 11 months ago #70123

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One thing you can try:
Most (all?) focusers have an option to reverse the direction.  So do an auto-focus run with the linear algorithm (that works around a possible backlash) in the normal direction, and write down the value (also look at the curve to verify it is a reliable one).  Then activate the 'reverse' option in the focuser driver, and repeat the AF run.  This time, because of the reversal, it will always end moving outward, and you get the focus 'coming from the other side'.  The difference between the two values you get that way would be your backlash.
 
2 years 11 months ago #70210

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Hi Peter,

Thank you for your suggestion, but when I tried it, I got variable readings. Please find the below table...

 
2 years 11 months ago #70400
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Hi Gene,

Thank you for the video, but I couldn't grasp how it was related to the focuser backlash. From what I can tell your stepper motor worked well :)

 
2 years 11 months ago #70401

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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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