Voice Coil Rail

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ray_parkhurst
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Post by ray_parkhurst »

Finally found time to assemble the voice coil system as I had originally planned. My concept was to integrate a speaker mounted horizontally within a B&L B-stand base. This can then be added to any B&L A-stand to enable narrow-range but high precision stacking. Here are pics of the result with the 1.5" speaker I tested earlier:

Image

And here is a closeup showing a coin on a sub-stage above the voice coil stage:

Image

Peter's software works similarly for use with voice coil as with a stepper, and at my request he has enabled both to work simultaneously. This may help folks who would use a stepper system for coarse focusing, and the voice coil for the actual stack. I've tried this and it does work.

The only noticeable difference in the software is its range of max 4096 positions, which I assume means the DAC he used is 12-bit. Peter has included a software adjustment for the reference voltage, so this gives a sort of "gain control" to the system. I initially did not see this feature, and was limited to ~100um range from the speaker (which was marginal for coin stacking), then discovered the default reference voltage is 0.125. I upped it to 0.5 and now have plenty of range (about 500um) for coin stacks. The maximum reference voltage is 0.875V.

I have not tried calibrating my speaker, as I really don't see a need to do so. It's probably a good feature if you push the speaker into strong nonlinearity, but the linearity over the range I am interested seems good enough.

The version of hardware I have from Peter is unidirectional. I don't know status of his bidirectional efforts.

The system is a real pleasure to use and has much the same adjustment feel, and all the precision, of my analog power supply shown earlier in this thread.

ray_parkhurst
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Joined: Sat Nov 20, 2010 10:40 am
Location: Santa Clara, CA, USA
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Post by ray_parkhurst »

One thing I see that is very different with the voice coil rail vs stepper-based systems is the stable alignment. All the stepper-based rails I've tried move the stage side to side a little as it moves up/down. The voice coil rail does not do this. It's a bit odd to look through a stack of images taken with a telecentric lens using the voice coil rail. The images don't change in scale or position, just in focus from highest to lowest feature. If anyone is interested I can send or post some source pics from a stack to show this effect.

mawyatt
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Post by mawyatt »

Ray,

Nice work with Peter's controller and your setup! Get on Peter to supply you with a Current Mode Bidirectional Controller/Driver :D

The rails I have do exhibit some stage wobble as I call it (side to side movement) except the THK KR20 which seem free from any wobble. The "Off Axis Loading" concept significantly reduces this wobble if applied correctly (normal to the screw thread face). It certainly made an improvement in mine and others rail systems, and for a few rubber bands and bolt doesn't cost much either.

Looking forward to see your coin images from this and your best 1~2X lens.

Best,

Mike

ray_parkhurst
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Post by ray_parkhurst »

I have a KR26 that I just finished integrating (with Peter's conversion kit) but have not fully evaluated yet. Hopefully it has minimal wobble. I have a couple KR20's that will be integrated as well. I wonder if the KR20 has lower wobble given the 1mm pitch vs 2mm of the KR26. We'll see once I get something built.

lothman
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Post by lothman »

ray_parkhurst wrote:I wonder if the KR20 has lower wobble given the 1mm pitch vs 2mm of the KR26. We'll see once I get something built.
since the KR26 has a much higher load capacity than the KR20, I think it should wobble lesser. On the other hand friction (needed torque) for moving should be smaller on the KR20, but since all the THK rail move so smooth that should be no problem.

mawyatt
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Post by mawyatt »

My KR20 has no discernible wobble at 10X, I haven't tried beyond that though. I specifically got the KR20 because of the 1mm thread pitch, with 400 step motor gives a fundamental 2.5 micron step before engaging micro-stepping, however I suspect the KR26 will be just as good at 2mm pitch.

Mike

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