Auto Tach Pulsin?
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I am slowly putting together the pieces of an automotive project I am
working on. One of those pieces is a BS2-based tachnometer. I would
like to use as little processing time as possible to generate the
tachnomoter, and am willing to accept accuracy within 1 to 3%
overall. The car already has two possible signals to use. One is the
coil winding -- seems to me I could measure the time between each
coil discharge and calculate rpm from that. The other approach is to
take the crank angle reference sensor signal (like a hall effect
sensor--creates a sine wave with at least 2.5v amplitude) and measure
the cycle time on that with pulsin. Anyone have experience doing
either, or otherwise have tips on interfacing these dirty/spikey
signals to the BS2?
working on. One of those pieces is a BS2-based tachnometer. I would
like to use as little processing time as possible to generate the
tachnomoter, and am willing to accept accuracy within 1 to 3%
overall. The car already has two possible signals to use. One is the
coil winding -- seems to me I could measure the time between each
coil discharge and calculate rpm from that. The other approach is to
take the crank angle reference sensor signal (like a hall effect
sensor--creates a sine wave with at least 2.5v amplitude) and measure
the cycle time on that with pulsin. Anyone have experience doing
either, or otherwise have tips on interfacing these dirty/spikey
signals to the BS2?
Comments
to an opto-isolator with a transistor output that will always turn on at the
same angle (likewise off). Determining an exact angle at which to transition
your output takes a bit of work, but for a tach you would only need a once
per-rotation signal. Hence, precision with regard to the crank angle seems
to be irrelevant. You can detect the positive 1/2 of the sine wave by
setting the current to the emitter (LED) and you'll get a square pulse out
of the detector based on the overall current transfer ratio. If you can deal
with a short pulse, you can keep the current on the emitter side very low
and get a resulting pulse from the detector somewhat centered around the
peak of the sine wave. With the 2.5V AC signal, this should be pretty simple
to accomplish with very few components.
Chris
>
> I am slowly putting together the pieces of an automotive project I am
> working on. One of those pieces is a BS2-based tachnometer. I would
> like to use as little processing time as possible to generate the
> tachnomoter, and am willing to accept accuracy within 1 to 3%
> overall. The car already has two possible signals to use. One is the
> coil winding -- seems to me I could measure the time between each
> coil discharge and calculate rpm from that. The other approach is to
> take the crank angle reference sensor signal (like a hall effect
> sensor--creates a sine wave with at least 2.5v amplitude) and measure
> the cycle time on that with pulsin. Anyone have experience doing
> either, or otherwise have tips on interfacing these dirty/spikey
> signals to the BS2?
>
>
A simple y fool proof way is to use what is called a cross-over
detector.
Is simply a bipolar NPN like a 2N2222 and connect a clampping diode
base to emitter. Apply the AC signal to the base thru a resistor.
In the collector, a resistor to +5v.
Every time the sinewave goes thru zero, the collector saturates and
remains saturated thru the positive part of the cycle.
The diode clamps the junction to approximately 0.6 volts when the
sine wave goes negative.
There is no problems with resistor tolerances or changes in the
amplitude of the sine wave. The only changes are due to temperature
coefficient of the transistor Vbe which is approx. 2.2 mV/DegC
--- In basicstamps@y..., "Chris Loiacono (E-mail)" <chris01@t...>
wrote:
> Not exactly, but in a similar way, it's fairly common to input a
sine wave
> to an opto-isolator with a transistor output that will always turn
on at the
> same angle (likewise off). Determining an exact angle at which to
transition
> your output takes a bit of work, but for a tach you would only need
a once
> per-rotation signal. Hence, precision with regard to the crank
angle seems
> to be irrelevant. You can detect the positive 1/2 of the sine wave
by
> setting the current to the emitter (LED) and you'll get a square
pulse out
> of the detector based on the overall current transfer ratio. If you
can deal
> with a short pulse, you can keep the current on the emitter side
very low
> and get a resulting pulse from the detector somewhat centered
around the
> peak of the sine wave. With the 2.5V AC signal, this should be
pretty simple
> to accomplish with very few components.
>
> Chris
>
> >
> > I am slowly putting together the pieces of an automotive project
I am
> > working on. One of those pieces is a BS2-based tachnometer. I
would
> > like to use as little processing time as possible to generate the
> > tachnomoter, and am willing to accept accuracy within 1 to 3%
> > overall. The car already has two possible signals to use. One is
the
> > coil winding -- seems to me I could measure the time between each
> > coil discharge and calculate rpm from that. The other approach is
to
> > take the crank angle reference sensor signal (like a hall effect
> > sensor--creates a sine wave with at least 2.5v amplitude) and
measure
> > the cycle time on that with pulsin. Anyone have experience doing
> > either, or otherwise have tips on interfacing these dirty/spikey
> > signals to the BS2?
> >
> >