fewayne wrote:

What you are describing is the result of excessive backlash. The linear algorithm assumes there is none and that is what the step calculations are based on.


Wait, what? Did you not previously write:

Linear focus is the way to go. Nails perfect focus every time and inherently compensates for backlash.


One of these things is not like the other :-) Hy's answer up the stream also seems to indicate that the algorithm is designed to minimize the effects of backlash.

I mean, "excessive" I might certainly buy, since a step size of 100 makes a visible focuser tube displacement but can be nearly eaten by backlash.

So, as nearly as I can tell, the backlash is actually in the motor; there's really nothing I can do about that without replacing it. Although I recognize that geartrain backlash in a commercial motor seems unlikely compared to my laughably crude setup displaying, shall we say, idiosyncrasies...still, when I reverse direction the thing gronks for a couple seconds and only then does the shaft start to turn. A subsequent move in the same axis makes the gronking and turning simultaneous.

When I ordered the timing pulley they sent me one with a bore about triple the size of what I'd asked for; coming up with a crude bushing for that left me with a setup that certainly might be susceptible to backlash, although as I noted that doesn't appear to be happening. I've ordered another, we'll see what arrives in a week or two on the slow boat from China.



Let me rephrase: The linear focuser inherently compensates for backlash, as it moves so far out, then back in, that it eats up all the backlash in the process.

HOWEVER: While establishing the V-curve, the Linear algorithm determines the calculated minimum on the assumption that there is no backlash. It moves out accordingly and then performs a calculated move towards that minimum. Any backlash will displace the second inward curve by that amount. In addition, if you excessive backlash, it is theoretically possible that the outward movement does not extend beyond the calculated minimum, which will send your focusing attempt off into Nirwana.

That's what I wanted to convey in fewer words and where I obviously failed miserably.

When you look at the curve I posted, it is obvious that there is essentially zero backlash, as the secondary inward curve nearly perfectly overlaps the primary V-curve.

If you have a geared stepper and a gearbox with a lot of play, that will be difficult to fix. I have opted against using a stepper with a gearbox and I am using direct drive microstepping instead. I was worried about steps being missed, but it seems to be working surprisingly well. No major problem with missed steps.

Jo

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