Backlash Compensation howto

Hi there,

i´m new to SGP and also, have a new Focuser (Lacerta Motorfocus).
I think, the Motorunit has a Backlash, and i´d like to know how to handle this the best way. The description of the Motor offers two ways. 1. measuring the backlash and use the settings for compensation (requires a dial indicator, which i don´t have) or 2. using focusing in one direction only. Is this last option available in SGP?

How would you handle this to achieve best results?

Kind Regards
Frank

Watching with interest. I am also trying to fathom this one out. When I change focuser directions it takes about 7 steps to take out the backlash before the focuser starts to move in the opposite direction. What I don’t understand is that you can only seem to set backlash compensation either IN or OUT and not both. But as I said, I get backlash in both directions so I need the capability in both…
Or am I missing something [probably]?

I think there are two ways that backlash compensation can work.
In one the focuser moves to a position to one side of the focus position, then moves to the focus position. This means that the focus position is always approached from the same direction and, in theory, the gears are always loaded in the same way. The amount of correction doesn’t matter, as long as it’s greater than the mechanical backlash.
In the other the backlash is measured as the dead zone between movements in and out. One is chosen as the “true” position and the other is compensated for the backlash amount. In this case the position is reached in a single move, in either direction, but the amount of backlash is critical to getting good movement.

I think the first method is preferable because the focus position is always approached in the same way and the backlash isn’t critical but it will take a little longer.

With the second there may be separate in and out corrections but they should be equal because if they aren’t multiple movements in and out will lead to a drift in focus position.

Hi Chris,
So lets say I am in focus at position 1000 and I have, for example, 10 steps of backlash when changing direction. And say I need the focuser to move OUT to a new position for filter X, say 1100. Are you saying then that the focuser actually needs to move further out than 1100 - say 1120 - and then back again 20 to get onto 1100 to ensure that the backlash gets removed? so in that case the backlash compensation would be 20 and in the IN direction?

For the first option, yes. The mechanical backlash would need to be less than 20, which it is.

For the second option then if you want to move from 1000 to 1100 then you move to 1110. The extra 10 steps take up the backlash and, because the move was out the backlash is subtracted so although the motor is at 1110 it is reported as being at 1100.

If you then want to move from 1100 to 1000 you move to 1000. The motor moves from 1110 to 1000 and again the extra 10 steps remove the backlash. This time the motor position is reported.

In this case the backlash must be exactly 10. If it isn’t then the same reported position will be different depending on which way the position was approached.

This is complex but should be hidden from the user in the driver. You do need to know what sort you have because it affects how much care you need to take setting the backlash.

I’m struggling with this a bit as well. I generally image with two refractors (140mm f/7 or 98mm f/6.3) with MicroTouch focusers on each. As per Frank’s original post I also do not own a dial indicator or have any other way to accurately measure the backlash.

If I understand this correctly the backlash compensation amount does not have to be precise so long as it is in excess of the actual backlash. So is there an arbitrary number (ie a multiple of the step size or some other metric) that I can use and be relatively confident that I am inputting a usable number.

As well, if I am not sure of the direction of backlash compensation would it be a reasonable assumption to use IN because of the gravitational pull on the focuser

Thanks

I hope that this is largely an academic discussion and no one is actually suggesting to implement backlash both ways. Backlash removal is an ancient concept and at the core is the choice to make a measurement by approaching from the same direction every time so that the readings are consistent. Often there is a preferred mechanical direction to approach from that has less compliance, slop, and variability - and in the case of a focuser it is the direction that is pushing up against gravity. So you estimate how much slop there is - and as long as you make a move in the good direction that is greater than the slop amount, you know the reading will be good and repeatable - no matter where you started from.

Imagine having this bi-directional system set up where you have some presumed value for the slop and you actually use that value when moving from each direction. Then imagine you make 100 moves of one step in alternating directions. If you have a slight error in that estimate of slop, that error will accumulate with every small move - and you will have no idea where you are.

Backlash removal works very well from one direction - especially if it is set up in the optimal direction that has less slop. And it only kicks in when you need to go backwards - which during autofocus only happens at the start of the run, and at the end of the run when it returns.

The only thing needed in setting it up is getting the direction of the curve right - which is usually pushing up against gravity as the curve is taken right to left - and the backlash amount, which can be determined by feel or by watching the system focus after a change of direction. Too small a backlash is bad, but using a large value is probably fine as long as it isn’t too big - and you can always tell if things are working if the focus curve is repeatable.

Frank

Frank,

I’m still a bit confused on how to determine the adjustment amount. You outlined two methods “by feel” or by observing the amount after changing direction. I’m sure both of these are obvious but can you tell me what I am supposed to feel? And what amounts am I observing on focus change? Most importantly how do I translate this into a compensation amount?

Is there and arbitrary adjustment amount that I could use instead that would ensure I would be above the threshold.

Bach

Hi. Well if a focuser has a knob and it has backlash, then as you turn the knob a bit one way or another you will feel a gap and a certain amount of play where the knob is just moving freely and not actually moving the focuser. That is the main “slop” in a system that requires backlash compensation. It’s a completely dead region where turning it one way or the other has no effect at all.

But in addition there is a more subtle thing where even once you engage and things start moving, it doesn’t fully engage and it takes some time before it is moving at full speed. So there is a dead region where there is no motion, then a period where it starts to move - and then where it is going at a constant speed as you rotate.

For a good rack and pinion focuser this all may be very tiny - and only a small fraction of the turn of a knob. But for the primary focuser knob on an sct it is much more - and you can both feel it and see it as you focus in and out - and watch the star change with focus.

And if you can’t feel the focuser you can still watch what happens as you focus a bit and then change directions. Watch carefully as the focuser tube responds to the command to move and see if there is any delay. For a really good focuser it may be almost nothing - but you can still put in some backlash compensation because it won’t hurt - other than to slow things down a bit when you reverse direction.

Frank

Hi there,

sorry for my late reply. I´ve figured out how it works for me. I´ve buyed myself a dial gauge and simply measured the backlash, put the numbers in the Software, and boom it works.

CS Frank