inductive vs optical

[Paul E. Campbell]

> Missing/short/long pulse detection needs either time or frequency based
> detection. (Any other's I've missed?)

Phase. But really all three are from the same general concepts.

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

Incorrect. You need an adaptive system is all (measures changes in the
AVERAGE rate or average pulse length, not absolute sensing). The amount of
extra stuff to implement this is minimal (564 or a '4040 would probably be
overkill).

> Frequency based detection is usually done by a phase or frequency locked   
> loop, either analog or digital.
> 
> Either way, you still have a fairly complex filter to design to ensure
> under/over-shoot doesn't cause false (extra or missing) triggering.

Excuse me? You run a single active filter loop giving you a second order
response (practically infinite shaping possibilities for this case). Takes
an op amp and a pair of resistors and capacitors. And it's not even text
book..the PLL documentation I have is literally a case of "plug in the
numbers" design. For those who want all digital, one uses a lag/lead type
phase detector with a down/up input onto the register which stores the
modulus on a count to N (or divide by N) register.

> Multiple sensors (inductive or optical) remove the need for any loop   
> filter
> design. You do have to allow for sensor failure, and yes, you now have   
> more sensors which could fail.

Sensor sets cost about $3. PLL's cost about $2-$5 (depends on whether or not
it's all digital). Pretty much a flip of the coin.

> If you use two sensors for angle (eg quadrature encoding), you can test for
> either of these failing as well. Ok, so if the engine rocks backwards when
> you switch it off will confuse this, but you don't care at this point.
> I'd prefer to have the sensor checking than to know how far my engine   
> rocks back :-)

Another advantage of quadrature sensors is you are not entirely SOL if one
goes out because it just cuts your resolution in half and negates the
possibility of reverse direction sensing.

> 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.

Inductive sensors require more signal conditioning and they should be fed by
alternating current to prevent problems in residual fields.

> 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.

Not in the case of DIY because more development time is GOOD. Remember that
DIY's do not account for time so much as the thrill of designing and getting
it working. It's not a "get it out the door" problem.

I may get flamed with disagreement but "DIY" pretty much implies hobbyist.

BTW..how in the world do you get hold of cheap FPGA/EPLD type programmers
and the associated compilers? Unless it's a PAL or older GAL's, it seems that
the manufacturers want on the order of $0.5-10K for the tools.