stackshot auto-dist behaviour

[johan]

I've just started using this mode and I'm wondering if this is normal behaviour?

Using auto-dist mode in high precision and setting 4µm, I set start, then end, then press up to let it do its thing. The machine then goes to where I set the start, takes a shot, but then on the second shot seems to move moves miles forward away from that point and starts doing 4µm increments. Is this normal behaviour?
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[rjlittlefield]

Yes, this is normal behavior.

What's happening is that when you push UP to start shooting, the rail reverses direction from what it did while setting start-->end. It runs back to the start position, takes one picture, then reverses direction again and has to take up backlash before moving to take the second picture. After that, movement continues in one direction, so the backlash is already taken up and the motor just has to turn enough to do the 4 micron movements.

In theory, if you have the backlash value set exactly correct in the Config menu, the actual step between first and second images will be exactly 4 microns despite the big turn of the screw. In practice, it's very difficult to set the backlash value accurately enough to work that way, and even if you do there's likely to be a small framing difference between the first and second shots because the carriage will twist ever so slightly when it reverses direction.

You can avoid all these issues simply by pressing FWD or BACK as needed to move the rail beyond the start position, before pressing UP to start the automation. That way movement will be in the same direction even for the first frame, and all of the steps between frames will be short.

In the Zerene Stacker controller, this process can be automated by setting the "Prerun distance".

Backlash is different for every rail when you're working down at the micron scale. Procedures for setting it accurately are described HERE, but it's both more accurate and far easier to just use the prerun approach so that all the movements you care about are made in the same direction.

--Rik

[johan]

Fantastic, thank you Rik. What a great explanation, makes it so obvious and so simple. Thank you again.
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[stevekale]

Hi Rik, I was wondering about the same thing. From the link you provided:

"Rock the fine focus knob a small amount back and forth"

Where is this knob?

[rjlittlefield]

[stevekale]

That would explain why I couldn't find it. Just out of interest, with something like the "auto distance" mode for the Stackshot, does Zerene show data like the total distance between start and end and the step distance (given a prescribed number of steps)? One thing I have found in my playing around so far is that it is a bit unfortunate that the Stackshot doesn't provide more information for a run.

[rjlittlefield]