Optical Dist sensors

dn dn at dlogtech.cuc.ab.ca
Tue Mar 5 04:00:38 GMT 1996


 Andrew Dalgleish writes:

> Generating the pattern is pretty easy with a PC and printer, although the   
> resolution wouldn't be good enough. If you print it out at some huge   
> resolution then reduce it photographically it'd be ok. Any photographer   
> could do it for you. Or print it inverse then just use the film negative.

I actually did this one time to make my own encoder.  I generated the pattern
with a CAD program (Generic Cadd), output the file in Postscript, and printed
it on the laser printer on clear transparency film.  Worked great!  The 
pattern came out crystal clear, I then glued the transparent film onto an 
aluminum disk and used reflective opto sensors to read it.  I ended up using 
a commercial encoder, however, because the housing was too expensive to
machine in quantity.  I assume there would be a limit as to how small you 
could go using this method, but a decent laser printer could make the thing 
very small with very little loss in image quality.  Using a CADD program, 
you can scale the drawing to any size you want.

> You need at least two sensors anyway (one for cylinder #1 TDC and one for   
> angular resolution), so going to 3 or more isn't going to be much dearer.

All you really need is the 2 quadrature tracks, plus a single "index" marker
which generates a pulse once per revolution.  In this manner, you have a sync
pulse which tells you when you have gone one full turn, and simply reference
everything to the sync pulse...

> Makes me wonder why I piddled around using counter chips all these years.

Look into the Hewlett Packard HCTL2016 chip - it has a complete quadrature
decoder with digital filtering, and a 16 bit counter readable by any CPU.
Works like a damn, and is really easy to use...

> Hmm... time to investigate multiple sensor arrays.
> I recall seeing a 16-pin DIP with 8 photo-transistors in it years ago.

These were used in the ancient but venerable paper tape readers.  I doubt if
you could still find them, but I think TI used to make some.  They still
make linear CCD arrays, but these are kind of a pain to interface.  You could
always use discrete surface mount photo transistors, you can pack them really 
close together.  This is what most optical encoder manufacturers use.  
Difficult to work with SMD components, though. 

regards
dn


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 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
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