inductive vs optical

[dn]

> Andrew Dalgleish wrote:

> Car engines run over a relatively wide rpm range, so time-based detection   
> is out.

Why? As long as your timing resolution is high enough, and you don't rely
on the micro for doing the quadrature or handling a zillion interrupts,
a micro should be able to handle up to 7 or 8000 RPM easily.  8000 RPM is 
only about 1Khz.

> Generally, I'd say an inductive sensor is more reliable than an optical   
> sensor.  The sensor itself isn't affect by voltage spikes as much as a LED,   
> although the supporting analogue circuitry may be.  

An inductive sensor has the shortcoming of having much less resoulution.
It would be extremely difficult to build an inductive pickup with a couple 
of hundred teeth small enough to fit in a distributor, whereas it's no 
problem with an optical encoder...

> I can buy off-the-shelf optical sensors which provide a TTL signal I can   
> plug into a computer. I can print my own encoding wheel and reduce it   
> photographically.  Development time is the single most expensive part of 
> any design.  Hence three optical sensors (1 for index plus 2 for 
> quadrature angle), a printed wheel, an EPLD, about 2 hours of programming, 
> and I'm done.

Check out HP (Hewlett Packard).  They make an optical sensor setup that you
can buy different resolution disks for, or make your own.  They are the HEDS
5000, 6000, and 9000 series.  They even make one with 360 cpr, and are very 
small in size.  Coupled with an HCTL2000/2016 quadrature decoder chip, 
(X4 resolution, built in 12/16 bit counter w/direction, 8 bit parallel 
interface) you have an instant position sensing system with no programming 
or fabricating at all.

regards
dn

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