Voice Coil Rail

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mjkzz
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Voice Coil Rail

Post by mjkzz »

I am building a voice coil rail system for 50x magnification. Basically I modified a loud speaker and put subject on the center coil. I'd like to get some idea as how heavy most subjects are. Thanks

Beatsy
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Post by Beatsy »

An average housefly is about 12mg. FoV at 50x will be well under 1mm so I can't see the weight of any suitable subject (even a little bit of lead) being much of an issue at that magnification.

Interesting idea - are you using this method to achieve very fast stacking or something?

mjkzz
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Post by mjkzz »

Hi Beatsy,

Yes, the idea is to make travel distance of 2mm with 4096 steps, then use 4K video to shoot a stack -- at each step, the duration will be 3 times the duration for the frame rate so that at least one frame is captured without movement.

For example, a 25fps 4K video, it will not change position for 120ms (or to be conservative, make it 125ms), so we can get 8 samples per second. 1000 image stack will take about 125 seconds.

Detection of "good" image will be done with software, but initially, can be done manually.

I hope the device is small enough and has less magnetic interference with step motor to be placed on an X-Y table to achieve stitching as well.

mjkzz
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Post by mjkzz »

Actually 2 times the duration of frame rate is good enough . . . so we can actually make it faster.

lonepal
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Post by lonepal »

I am very curious about your system.
Can you share some pics and technical information?
Thanks.
Regards.
Omer

mjkzz
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Post by mjkzz »

lonepal, still at early stage, just finished circuit design. there are a lot to do, the finished device will not be like a 30W speaker :-)

zzffnn
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Post by zzffnn »

Very interesting! I see that travel distance and weight capacity limits the lowest possible magnification - so is 50x the lowest possible magnification?
Selling my Canon FD 200mm F/2.8 lens

mjkzz
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Post by mjkzz »

zzffnn: not sure what you meant by "lowest possible magnification", it should work for lower magnification except travel distance might be too short.

There are actually some existing, commercial voice coil actuator with LONG travel distance (a couple of inches) and high position accuracy. I am just trying to use a speaker to do the trick.

For a DIY'ed VCA, it is possible, and why not. But now, I just started the circuit.

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Post by rjlittlefield »

Rather than running in step-and-stop mode, you might consider using a smooth ramp, with the ramp speed adjusted to move say 1/2 of the diffraction-limited DOF per frame time. That way you wouldn't have to worry about either mechanical resonances or frame selection.

In the example at http://www.photomacrography.net/forum/v ... hp?t=11130, I was shooting with a 4X NA 0.1 objective, nominal DOF 0.055 mm, with a feed of 0.020 mm per frame at 20 fps, so 0.4 mm per second total. Final stack was 50 frames, 2.5 seconds.

--Rik

mjkzz
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Post by mjkzz »

Thanks Rik, I had similar experience with resonances, a couple of layer of tissue paper fixed it -- my rail was put on a hard wood table top and it was resonating with the surface and it was loud :-).

Thanks for the idea to take advantage of 1/2 diffraction limited DOF and do smooth ramping. I might try that since I am updating my driver to have 1/128 micro step (of course with larger motor and higher current to gain enough torque). Or maybe a servo motor (with rotary encoder for discrete steps).

But for higher magnification works, say an objective with 2um nominal DOF, at feed rate of 25fps means 0.025mm per second. It might be achievable THEORETICALLY with 1/128 micro stepping, 1mm pitch screw and a 400 step motor -- 0.025*400*128 = 1280 pulses per second. In practice, I am not sure, need to actually build it and experiment with it. Maybe a stepper with gear reduction box.

It seems at higher fps, we can gain some to our advantage, say at 50fps.

mjkzz
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Post by mjkzz »

Or make the coil move continuously at 1/2 nominal DOF rate . . .

mawyatt
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Post by mawyatt »

Peter,

Very interesting idea!!

As you know, voice coils are fundamentally magnetic in nature, thus the magnetic fields are proportional to current not voltage. Since you will be using a low rate of change applied current, the coil voltage will be small ( L*di/dt). Speaker coils have a modest inductance L, and can represent a modest impedance at audio frequencies (20~20kHz), but at your excitation equivalent frequency (probably around 1Hz or lower) the impedance will be very small. This makes voltage drive difficult and hard to control.

With this in mind the proper driving mode for the voice coil will be current based rather than voltage based, so the driver will provide a constant controlled current independent of load impedance, rather than the usual voltage mode. Basically this approach is somewhat counter intuitive since you seek to make the driver have a high output impedance rather than low impedance common to voltage mode drivers. The trick is to use an active device (Power Transistor or MOS) to do this so you don't need a high supply voltage.

If you are not familiar with these current mode techniques, having done this throughout my career, I can provide some assistance if you wish. It's not complicated and doesn't require lots of parts, nor is it expensive.

Keep us informed of your progress, this speaker based voice coil rail sounds very interesting indeed!!

Best,

Mike

Pau
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Post by Pau »

Not directly related, but...do you know about piezoelectric focuser rings and other piezo actuators?
For example: http://www.pi-usa.us/products/Microscop ... ge.php#PIF

High precision (and expensive!) stuff
Pau

mjkzz
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Post by mjkzz »

Mike, thanks! And here is what I think.

Lets put the word speaker aside, so our goal is to move something up and down (my initial goal is vertical setup) electromagnetically by 2mm max at very fine steps. So yes, as far as I know, this displacement (or rather force) will be proportional to current inside the voice coil if a permanent magnetic is used. But we are not making it oscillate, just moving it to displace some subject linearly, so this sounds like almost DC because it does not oscillate. Of course, since we are changing the current over time, there bound to have some oscillation if you analyze it in frequency domain, but I somehow think we can treat it as DC overall.

Since it is DC (or almost DC) operation, we really do not have to worry about frequency response characteristic of the setup and only consider the DC side of it. Having said that, I think since the resistance (rather than impedance) of the coil is the major factor here, we can turn a current mode device into a voltage mode one -- by adjusting the voltage across the coil, we can alter the current.

Base on the above, my circuit is basically a 12bit DAC and a voltage follower as a buffer to supply enough current with the coil as resistive load. I have not submitted the circuit for PCB fabrication yet, but will in a couple of days.

I am sure there will be something to be aware of from the frequency domain, such as oscillation when changing current (voltage) applied to the coil, or maybe even PCB design, and if they are causing enough problem, I think I will go for piezo route with simple voltage control (with less displacement)

mjkzz
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Post by mjkzz »

Hi Pau, thanks for the info. I am aware of piezo drive, but not that advanced one. My last resort is to use a piezo device and apply voltage on it to change position, though it will be limited to smaller displacement.

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