A couple months ago, I started to investigate building an auto-focus mechanism for cameras that could operate in high magnetic fields. This was for a work project (where I build MRI-compatible devices). Inside an MRI machine, the base magnetic field is usually 1.5 or 3 Tesla. At these fields, any motor or solenoid that relies on manipulating a local magnetic field will not work. Unfortunately, manipulating magnetic fields is one of the easiest ways to convert electricity into physical movement, so most auto-focus assemblies are built this way. There are some
Piezo "ultrasonic" ring motors. Many high-end DSLR (and SLR) lenses have silent ring motors that vibrate at ultrasonic frequencies to create rotation. Great! The only problem is that they are built from steel, and the steel cannot be used in the MRI environment. They are also too big -- I need a tiny lens about 1/4" in diameter, not 2" like a DSLR lens
Piezo wiggler motors. Looks interesting, but getting a sample is near impossible. The screw assembly is probably steel, and the system requires a fairly rigid and complex slide system
Liquid lenses. These lenses are built by sealing a small amount of water and oil inside a shirt button-shaped capsule. The walls of the capsule are metal with glass windows on the faces of the button. The water and oil will not mix, and the boundary between the fluids will bend light because the two liquids have different refractive indices. If the walls of the capsule are charged with electrical current, the shape of the water droplet will change because of electrostatic attraction. (check this out: http://www.youtube.com/watch?v=p1f6zLysilU ) The oil is made up of non-polar molecules, so it does not share this same attraction. By controlling the voltage on the capsule walls, the shape of the water can be controlled and thus the focal length of the lens formed by the water-oil interface. Neat!
The liquid lens is the metallic button-shaped thing on the left. I built a holder for the lens out of 1" squares of copper clad board.
There are only one or two manufacturers of liquid lenses: Varioptic, Philips, and maybe Siemens. Philips and Siemens would not discuss the liquid lens and would not sell any samples. Varioptic would send samples -- only $340 for each lens! What?! The evaluation kit was a mere $4000, while the full developer's kit was $12,750. I kid you not. What kind of company would charge a potential customer $12k for the privilege of testing their products? Insane!
Anyway, since I couldn't deal with any of the manufacturers directly, I set about finding an off-the-shelf product that contained a liquid lens. After many hours of searching, I found an actual product that had the Varioptic lens in it: Digitus DA-70817. There were many other prototypes and press releases for other liquid lens products, but it seems the Digitus camera is the only mass-produced device right now. Unfortunately, it is not available in the USA. It's not on eBay. Amazon.de has it, but will not ship to the USA. After many more hours of searching, I got in touch with a German distribution company called Assmann (no joke). I exchanged a few emails with a friendly person there, and she sent me five of these cameras (about 25 euro apiece with the usual outrageous international shipping charge).
I removed the lens from one of the cameras and mounted it between two pieces of copper clad board. To make the lens change focal length, I need to supply 0-70V AC to the lens. AC is required to prevent the liquid in the lens from polarizing over time and losing effectiveness. The voltage needs to be fairly high to deform the water drop sufficiently, and also needs to be changed rapidly for quick changes in focal length. Fortunately, there are two companies that make ICs specifically designed to directly drive liquid lenses. Unfortunately, one of the companies, Maxim, will not sell samples of their chip, and they will not even give out datasheets for it. Take a look: http://www.maxim-ic.com/req_full_ds.cfm?action=request&id=5949 How rude is that? WTF?! The other company that makes liquid lens drivers is Supertex. They will sell samples of their HV892 at a reasonable price, but the chip is only made in a 10-lead 4mm x 4mm DFN package with .65mm pitch. Apparently, this SMD footprint is so unusual, there are no DIP converters available from anywhere. Geeeez! I ended up designing my own PCB for the chip and having them made at http://www.expresspcb.com/. Here it is:
I interfaced the chip to an Arduino Duemilanove. The Arduino sends a value between 1 and 255 to the HV892 via the I2C bus. The value controls the HV892 output voltage, and hence the lens's focal length. I am not sure what the mathematical relationship is
I used a laser pointer to show the different focal lengths that the lens can achieve.