Fwd: eddy current brake

[tom cloud]

>With a 4-point crank wheel and one sensor for that wheel, you would have 4
pulses, each 90=B0 apart.  A sensor wheel could be constructed so that it
gives a rising edge and a falling edge four times per revolution, with the
falling edge 45=B0 out of phase with the rising edge.  This provides the 8
points necessary to have a resolution of one event  every 45=B0.
>
>I'm responding to this post because it took me a couple seconds to
understand what was intended.  I first thought a wheel with four small (a
couple degrees wide) teeth was being described.  I then realized that Tom
was describing a wheel with teeth that spanned 45=B0 each.  However, I'm=
 still
puzzled when he states "2 revs =3D 360".  One revolution equals 360=B0.  Two
crankshaft revolutions, however, does equal 360=B0 of camshaft revolution=
 (oh,
now I get it.  Never mind.)
>
>When you start talking about PLL, however, I'm lost.  What exactly does a
PLL do

A PLL (phase locked loop) works like an op-amp, except its two inputs
are pulse waveforms (i.e. frequency) rather than analog voltages.
Basically, it outputs a DC voltage corresponding to the diff in the
phase or freq of the two inputs.

You can feed a freq into one input (say the pulses from your crank)
and let the DC output control a VCO (voltage controlled oscillator).
Take the output of the VCO and feed it into a divide by 'n' counter
whose MSB goes back into the other input of the VCO.  That forces the
VCO freq to be 'n' times the "reference" (our crank signal).

   |crank pulses| ----> |       |
                        |  PLL  | --> | VCO | ------> {output 'n' times}
                  ----> |       |                 |   {crank PRR       }
                  |                               |
                  ----- [ divider ] <--------------

"PRR"  =3D=3D=3D>> pulse repetition rate

Actually, the above is a little misleading.  The "PLL" above is
actually the phase comparator.  A PLL consists of both the
phase comparator and the VCO.

Two major problems with a PLL.  "Capture" -- ability to 'lock on'
to an input.  It will 'hunt' while seeking capture and the output
will be erroneous / unrelated to the phase relationship of the inputs.

"Lock Range" is the range of frequencies it can effectively output.
The capture range is smaller than the lock range.  The problem, I
believe, in the auto app here is -- will the PLL capture quickly
enough for you to start your car? and will it be able to track the
entire rpm range you want without going out of lock (which would be
bad).


Tom Cloud <cloud at peaches.ph.utexas.edu>