Microscope Objective Focusing Module
This microscope objective and focusing actuator came from an unknown optical block I found on eBay. Markings didn’t turn up any useful datasheets, so I decided to take it apart. I expected to find a simple piezo or voice coil actuator (similar to those you find in optical drives)… turns out it’s far more complicated than that.
Dismantling the module does reveal a voice coil style actuator, but there’s a lot more going on that a simple open loop voice coil:
The BGA IC is a iC-TW8 SIN/COS Interpolator. Typically this would take the input from a rotary encoder on a motor and use this to provide feedback creating a servo motor.
A couple of other parts indicate how this feedback mechanism is being used here. Firstly there’s what looks like a photointerrupter module, containing a LED and a photodiode:
Which when assembled aligns with this odd looking shiny surface next to the voice coil:
Under magnification we can get a better look at this. It seems to be a small piece of silicon with patterned metal surface. The surface is marked with 44 lines over ~10mm, roughly 200 micron spacing. The photointerrupter, being fixed would be able to measure the reflectivity of each of metal lines as the voice coil moves. In doing so providing feedback on the relative motion of the objective.
This seems like an expensive way to make an optical feedback mechanism, but is no doubt quite accurate.
The focusing module is therefore using optical feedback to allow precise relative positioning. This is somewhat surprising to me. In general, being able to very accurately control the relative focus position isn’t very useful. You usually have some other focus quality metric (either a via a focusing laser, or just from the image itself) which you can use as feedback to focus the image.
So I’m curious as to why such precise distance measurement was required here. Perhaps different layers of a substrate were being imaged at fixed distances? Given that this module was completely undocumented, and the seller didn’t know it’s origin we may never know. But I'll probably dig into some other parts from this optical block in a future post.
Whatever the reason, this was a surprisingly well engineered focusing module for what seems like a relatively low cost optical system.
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