I have an application that I am currently doing a bit of brainstorming on and thought you guys may have some good input.

I have already decided to use the LPC932 since I have the Keil emulator for it, but I'm willing to change if needed. So I'll have your standard A/D and interruptable I/O.

My application consists of a telescoping metal rod. The inner rod is threaded so rotational force (a motor in this case) can make the length of the rod longer and can reverse back to zero. The total length of possible movement is about 80mm. The step size is about half a milimeter. Each rotation of the threaded rod increases a known amount.

This particular metal rod will be in a wet environment (blood). Although if my method is small and stationary enough, I might be able to seal it off.

I'm kicking around ideas for precisely measuring the expanded distance at any given time. Whatever method I choose, I will need to make it accurate down to about a milimeter and it will need to know if the movement was forwards or backwards toward zero.

Cost is no option, but complexity is. I don't have a lot of PCB real estate to work with here.

Some ideas I have been pondering...

1) Rotary encoder
2) Makeshift linear potentiometer (not sure how it would handle blood all over the carbon trace).

How would you tackle this? I appreciate any ideas and suggestions!

Bruce