This is a very old thread, and I have not read all of it.
I had a white paper about someone who did make a digital indicator, and he had it hooked up to a personal computer.
I will try to find that article.
I always thought that a stepper motor would be ideal for actuating a D-valve (or other type steam valve), although introducing electronics to a steam engine is blasphemy to many.
I don't think I would ever use a stepper motor on a steam engine, but I am an EE and steam lover, so I am aware of the posibilities.
Some experimental internal combustion engines use solenoid actuated poppet valves; such valves could definitely be made to work on a steam engine. I wouldn't like to be far from shore relying on my own inexpertly potted electronics, but for an engine to play with in the shop - sure, could be fun, and would certainly make experimenting with different valve events easier. To tie back to the subject of this thread, a digital engine indicator could provide feedback back to the controlling computer to optimize valve events - more lead, overlap, etc.
A balanced slide valve could also be driven by a voice coil from one of those ghastly giant subwoofers often found in lowered cars; this would of course need position feedback but would make a pretty powerful and fast servo; the matching 12V audio amps could be pressed into service as the analog servo amplifier.
