Displacement

[Sarah Kefayati]

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Hello all,

 

Sorry that my question is very low-level but is causing me trouble:

 

when I scan through the Z axis with my starting step set as zero,when I come back through the steps,I  notice that I have displacement and I am not at zero again.I am trying beads embedded agarose gel.I assumed maybe it is because of the gel dehydration and I repeated it with a short period of time with very low number of steps but I still have displacement.

 

Could it be of a stage holder or lens-moving motor?

 

Thank you and sorry again 

Sarah

[Craig Brideau]

Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal Either your sample is moving, or you have mechanical backlash in your stage or lens motor.  When you move in Z, does the lens move, or does the stage move?  Any mechanical system with gears experiences what is called 'backlash'.  What it means is that when you go X in one direction, then tell the system to reverse X again you don't end up quite back at your starting point.  Backlash is caused by the slight play in the teeth of the gearing which is necessary for them to rotate.  There's not much you can do about it except use a linear position scale, or try overshooting.  To do overshooting, follow this example:  Start at some negative value, then move up to '0' and stop.  Then go up 100 units, then go down 110, then go up 10.  This seems wacky, but what you are doing is always approaching your start point from an upward-moving direction.  This ensures that the slop in your gearing is under the same condition you started from.

Craig

On Fri, May 9, 2008 at 11:21 AM, Sarah Kefayati <[hidden email]> wrote:

Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal

Hello all,

 

Sorry that my question is very low-level but is causing me trouble:

 

when I scan through the Z axis with my starting step set as zero,when I come back through the steps,I  notice that I have displacement and I am not at zero again.I am trying beads embedded agarose gel.I assumed maybe it is because of the gel dehydration and I repeated it with a short period of time with very low number of steps but I still have displacement.

 

Could it be of a stage holder or lens-moving motor?

 

Thank you and sorry again 

Sarah

[Kevin Ryan-2]

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http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal

This is gear backlash, or mechanical hysteresis. You will find this in any mechanical and many electrical systems - there is unavoidable mechanical distance (gear spacing) to take up when you change directions.
 
To avoid backlash, you need to always approach your sample from the same direction. What I try to do is to first isolate how much backlash there is. Approach a sample from a long distance away, take a picture. Go past the sample, come back the same distance according to the stage motor, and see how far away you are. Move in small steps until you are actually back at the sample. The extra distance you have to go to get back to the sample is the amount of backlash in that axis.
 
The solution to bacdklash is to always approach the sample from the same direction. If moving up in Z is the preferred direction (and it is, since moving down opens the possibility of gravity drifting the stage down over time), then when moving down moving past the sample position further than the backlash distance and then moving back up takes out the backlash.
 
In operation, removing backlash in this fashion involves moving directly to the desired location in one direction, and moving past the sample and then returning in the preferred direction. Very simple, and very effective.
 
Many image acquisition systems (including ours, Image-Pro) have the option of applying antibacklashing while acquiring.
 
Kevin Ryan
Media Cybernetics, Inc.

________________________________

From: Confocal Microscopy List on behalf of Sarah Kefayati
Sent: Fri 5/9/2008 13:21
To: [hidden email]
Subject: Displacement


Search the CONFOCAL archive at http://listserv.acsu.buffalo.edu/cgi-bin/wa?S1=confocal 
Hello all,
 
Sorry that my question is very low-level but is causing me trouble:
 
when I scan through the Z axis with my starting step set as zero,when I come back through the steps,I  notice that I have displacement and I am not at zero again.I am trying beads embedded agarose gel.I assumed maybe it is because of the gel dehydration and I repeated it with a short period of time with very low number of steps but I still have displacement.
 
Could it be of a stage holder or lens-moving motor?
 
Thank you and sorry again
Sarah