Voice Coil Rail

Have questions about the equipment used for macro- or micro- photography? Post those questions in this forum.

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mawyatt
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Post by mawyatt »

Peter,

The trick is to sense the voltage across R and force that current (V/R) thru the load which is your coil. So current doesn't depend on load, only V and R. I've used this concept to drive at YIG oscillator, and many other loads. You can also invert the concept to source current instead of sink current. Your op Amp will work fine, you don't need the RdC combo either.

Got to run to a meeting at UCSD.

Best,

Mike

ChrisR
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Post by ChrisR »

Linaerity of the movement may be important - or not very. You can afford to waste a third of your shots.


Central heating gas boilers (furnaces, in the US) have gas valves, which are modulated (turned down) by altering the pressure on a diaphragm using a solenoid. When they appeared, High/low pressure was as good as could be done, by hitting end stopps.
More recently, gradual control has been achieved. Not many bits accuracy, because they don't need it. They are a lot smaller than bass speaker coils though, I'll break one up to check the travel. Some are low voltage, some high..
Chris R

elf
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Post by elf »

mjkzz wrote: Not sure if you are addressing me, if so, my name is Peter.

In terms of patent, another intention of this post is to prevent people from patenting it as this post will serve as public domain knowledge. So at least for USA, since the "new" patent law (passed back in 2013?) will count this post as prior art, any patent issued (after this post) based on this idea will be invalidated.

Regards
Peter
Posting an idea in an Internet forum probably won't qualify it as prior art for blocking a patent application. Currently in the US it's first to file. However, I'm not a patent lawyer and haven't even played one on TV, so the information could be wrong :roll:

mjkzz
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Post by mjkzz »

Thanks Mike and Chris!

My thought is this, break the system apart -- a digital part and analog part, the digital part handles communication with computer, output precise voltage with unity gain impedance buffer (the op-amp), then the analog part which will convert that voltage into current with good performance.

mjkzz
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Post by mjkzz »

Hi elf,

The first to file rule applies to situation when there is NO public domain knowledge. If two parties working on solving the same problem, but none has disclosed their approaches, then whoever files first wins if two approaches are identical or similar

In 2013, a new patent law was passed in US that essentially says if anyone can provide reliable source of public domain info against a patent (before it was issued), that patent will be invalidated. This new law has made many patents worthless -- a published paper in a small academic circle can invalidate a patent worth hundreds of million dollars.

Of course, this might be US only, not sure about EU or other part of world.

And are there any professors in this forum? Maybe this is a good senior project for a BE.EE degree and if you are not in engineering discipline, it still can be a cross department project. So there, that ought to invalidate any attempts :-)

mawyatt
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Post by mawyatt »

mjkzz wrote:Thanks Mike and Chris!

My thought is this, break the system apart -- a digital part and analog part, the digital part handles communication with computer, output precise voltage with unity gain impedance buffer (the op-amp), then the analog part which will convert that voltage into current with good performance.
Peter,

If you use a DAC with buffer amp, so basic voltage mode, then you will want to place Rd in my schematic in series with the DAC/Buffer Amp output, keeping C shunt to ground on the Op Amp side of Rd. This will allow you to adjust the stepped DAC transitions into a more smooth result. For starters, select a time constant of about the DAC clock period. For ramp use this will create a more serrated ramp rather than a stepped ramp, thus reducing the harsh step transitions that could excite natural vibrations in your setup.

Best,

Mike

ray_parkhurst
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Post by ray_parkhurst »

I gave this concept a try using a 4" full-range speaker. The speaker has +/- 2-3mm of travel, depending on linearity expectations. This is more than I need for use with coin stacks under normal shooting conditions. I played around with the voltages and currents required for 100um of travel, which encompasses most of what I do, and found it takes ~50mA to move that much. I placed a 470ohm resistor in series with the voice coil, and drove the combo with a 25V power supply with coarse/fine voltage adjust (an HP 6215A). This system worked superbly, with "infinite" adjustability. It was really wonderful to be able to adjust focus using the voltage knob of an analog power supply! I saw no movements, no vibrations, and excellent repeatability. I highly recommend folks give it a try.

I shot a stack of a mintmark using a Nikon 20X MM objective (NA 0.4). 30 images, at ~2.5um per step. This is feasible with my stepper-based stacking setup, though the step size is not as uniform as I'd like, and this shows up as minor distortions when doing 3D renderings of the Depth Map stack. In the case of the voice coil stepper, the shots were perfectly spaced, and the rendering is the best I've been able to achieve.

The stack variable was voltage, given the high value of series resistance effectively making power supply into a current source. For this stack, the min Voltage was 3.0V, and max was 18.0V, and I snapped a shot every 0.5V, corresponding to 2.5um per shot. This system is clearly capable of finer steps, and in fact I would think it could do 10x better even with the supply I'm using. Range is the issue, since I'm linearizing the system by using current set by a large value resistor. A true current source would be needed to achieve the target ranges of the OP.

For some reason the forum is not allowing me to link to the pics I'd like to show.

mjkzz
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Post by mjkzz »

Wow, you beat me to it. Congratulation!!! I am bogged down with something else and I did not think of using something like 18V.

I will come back to it soon. Thanks for sharing.

mawyatt
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Post by mawyatt »

Ray,

The current source i sketched out for Peter should work fine for your voice coil. Just use a larger current sense resistor, like 50 ohms.

Since you only need 50ma, a smaller output transistor can be used. You could even parallel up a couple 2N2222s with a small 10 ohm emitter ballast resistors for the output.

You'll only need 5~10 volts with this current source, depending on how you set it up.

Thanks for posting, this is interesting indeed!!

Best

Mike

ray_parkhurst
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Post by ray_parkhurst »

Here is the result, first in 2D, then a perspective rendering.

Image

Image

mjkzz
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Post by mjkzz »

wow, I gotta get this thing going again... thanks for sharing.

mawyatt
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Post by mawyatt »

How do these Voice Coils behave regarding lateral and/or vertical movement when the coil is moving forward or reverse? Regular Focus rails have this effect to varying degrees.

I ask because I am wondering if you can stack images in Zerene with All Alignments OFF

Thanks,

Mike

ray_parkhurst
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Post by ray_parkhurst »

mawyatt wrote:How do these Voice Coils behave regarding lateral and/or vertical movement when the coil is moving forward or reverse? Regular Focus rails have this effect to varying degrees.

I ask because I am wondering if you can stack images in Zerene with All Alignments OFF

Thanks,

Mike
I saw virtually zero lateral movement when I did the stack referenced above. Of course the driver axis must be lined up with the optical axis for this to be true in general. This was one of the most interesting things about the method...as I adjusted the power supply voltage up/down, it acted like a continuously variable fine focus control with no vibration or movement. It is really a cool system.

dolmadis
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Post by dolmadis »

I know that this is a thread which in trepidation of my own ignorance of electronics that I should not venture but I would really appreciate seeing the rig on which these images have been achieved and what components were involved and how.

Thank you.



John

ray_parkhurst
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Post by ray_parkhurst »

dolmadis wrote:I know that this is a thread which in trepidation of my own ignorance of electronics that I should not venture but I would really appreciate seeing the rig on which these images have been achieved and what components were involved and how.

Thank you.



John
Here is a (blurry, no flash) annotated picture showing the system.

Image

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