Audio Tachometer for 2 stroke engine

[nospam]

Ross Herbert <rherber1SPAMEX@bigpond.net.au> wrote:

It likely doesn't need to filter out much at all. The rapid opening of a
two stroke exhaust port to release the results of a recent explosion
produces a huge pressure impulse. I would not be surprised to find it 10's
of db larger than any other source although I am assuming these high
performance models do not have silencers.

While this is true the fact remains that any spurious and random noise
source would interfere with the wanted sound. It is not simply a
matter of detecting the amplitude of the desired signal. Since an
electret is extremely sensitive to both sound and physical vibration,
and is physically mounted in the boat which is speeding (hopefully)
over the water, it will pick up every sound and vibration, including
harmonics and sub-harmonics, wanted or not. Filtering would not be as
easy as hoped for, I imagine.

If someone fires a gun next to you head you are telling me you might not be
able to count how many times because of someone rattling plates or having a
conversation on the next table?

If the impulse is 10 times larger than any other signal you don't need
fancy circuitry to detect it and ignore everything else.

My biggest worries would be the risk of hearing reflections of itself in a
resonant exhaust system, and the dynamic range of the microphone, even
mechanical damage to the microphone.

Aha!, you are tending to agree with the gist of my criticism.

Not really just speculating that if you put a microphone somewhere near the
end of the exhaust the impulse would be so loud that it would exceed the
dynamic range of a normal microphone and possibly damage it. If I were
doing the job one of the things I would try is using an element from a
piezo sounder as a pickup.

The OP should record a sample from a running engine and see what it looks
like. He might also consider using a directional microphone from the shore.

Assuming that one could design a system as proposed, the final test of
accuracy and reliability has to be realised. After extensive bench
testing it would need to be tried in practice on the water. If all
goes well, the tacho will record the rpm as intended, but, how will we
know if the readout is accurate?

Because one can't see any mechanism in the system to give a consistently
inaccurate readout and you already know approximately what the readout
should be?

We would need to have some known
accurate system to compare the readings against, wouldn't we? What
this means is that you would also have to install a reference system,
one which is known to be free of all of the negative aspects I have
referred to,

and how would you know that system is free from those aspects, and free
from other negative aspects you didn't refer to?

and is tried and proven to accurately measure rpm of the
motor or drive shaft.

And how did you prove this other system accurately measures the motor rpm?

With another system you haven't proven either?

At some point you have to say if it quacks like a duck it is a duck and in
this case duck quacks are not that hard to recognise.

This implies that a known reliable non-contact method such as magnetic
hall-effect (pulse), or opto reflective method must be used as the
signal source because these methods are well established and widely
used to perform the same function in industry already. Having said
that, doesn't this suggest that if there is already a reliable system
of deriving the required rpm data with which to compare that obtained
by the new "audio based" system, then why do we need the new system in
the first place?

Because the system the OP proposes can be stuck on (and removed from) any
boat with no modifications to the boat other that adding a bit of sticky
back velcro. That is why I suggested he considers listening from the shore
which is even more convenient.

 

[Rich Grise]

On Fri, 22 Apr 2005 19:07:35 +0000, Mook Johnson wrote:

I have a crazy idea to use a microphone near the exhaust of a 2 stroke
engine to measure the instantaneous RPM of the engine.

Yes, as has been noted, it's a crazy idea.

Just paint half of the driveshaft flat black, and the other half gloss
white, and use something like this:
http://zuff.info/Doc/TCRT1000.pdf

That showed up on a Google search for "reflective optical sensor".

If you want a better reflective surface, find some aluminum tape and
stick a strip on the driveshaft.

Check also for "optical tachometer" - I haven't bothered yet, because
I feel like I'm already barking up a tree.

Good Luck!
Rich

 

[Ross Herbert]

On Wed, 27 Apr 2005 01:30:00 +0100, nospam <nospam@nospam.invalid>
wrote:

Ross Herbert <rherber1SPAMEX@bigpond.net.au> wrote:

It likely doesn't need to filter out much at all. The rapid opening of a
two stroke exhaust port to release the results of a recent explosion
produces a huge pressure impulse. I would not be surprised to find it 10's
of db larger than any other source although I am assuming these high
performance models do not have silencers.

While this is true the fact remains that any spurious and random noise
source would interfere with the wanted sound. It is not simply a
matter of detecting the amplitude of the desired signal. Since an
electret is extremely sensitive to both sound and physical vibration,
and is physically mounted in the boat which is speeding (hopefully)
over the water, it will pick up every sound and vibration, including
harmonics and sub-harmonics, wanted or not. Filtering would not be as
easy as hoped for, I imagine.

If someone fires a gun next to you head you are telling me you might not be
able to count how many times because of someone rattling plates or having a
conversation on the next table?

This analogy is in no way comparable to the task the OP wants to
achieve.

If the impulse is 10 times larger than any other signal you don't need
fancy circuitry to detect it and ignore everything else.

Exactly. And a simple electret mic with op-amp filtering plus a PIC to
handle the data collection and readout is simply not good enough to
achieve this in practice.

My biggest worries would be the risk of hearing reflections of itself in a
resonant exhaust system, and the dynamic range of the microphone, even
mechanical damage to the microphone.

Aha!, you are tending to agree with the gist of my criticism.

Not really just speculating that if you put a microphone somewhere near the
end of the exhaust the impulse would be so loud that it would exceed the
dynamic range of a normal microphone and possibly damage it. If I were
doing the job one of the things I would try is using an element from a
piezo sounder as a pickup.

An electret mic and a piezo transducer ain't a lot different in this
environment.

The OP should record a sample from a running engine and see what it looks
like. He might also consider using a directional microphone from the shore.

Assuming that one could design a system as proposed, the final test of
accuracy and reliability has to be realised. After extensive bench
testing it would need to be tried in practice on the water. If all
goes well, the tacho will record the rpm as intended, but, how will we
know if the readout is accurate?

Because one can't see any mechanism in the system to give a consistently
inaccurate readout and you already know approximately what the readout
should be?

Oh, it's that simple eh, "one can't see any mechanism", is that it?
You don't seem to understand that this is a totally unscientific
method of establishing facts and is nothing more than guessing.

We would need to have some known
accurate system to compare the readings against, wouldn't we? What
this means is that you would also have to install a reference system,
one which is known to be free of all of the negative aspects I have
referred to,

and how would you know that system is free from those aspects, and free
from other negative aspects you didn't refer to?

Simply because when any method of measurement is established it has to
be compared against a previous accepted standard and can be shown to
be as good or better in performance.

and is tried and proven to accurately measure rpm of the
motor or drive shaft.

And how did you prove this other system accurately measures the motor rpm?

See above answer. To spell it out specifically... In the beginning
when tachometers were first designed they were totally mechanical with
a physical connection to the rotating part being counted, and using
proven mathematical relationships and engineering techniques.

With another system you haven't proven either?

If they aren't proven to work as intended then how do you suppose they
get to be used in industry and commerce to support the correct
maintenance and operation of mechanical plant using rotating parts
such as motors etc. In order for them to be accepted they first had to
be compared with what existed before.

At some point you have to say if it quacks like a duck it is a duck and in
this case duck quacks are not that hard to recognise.

Again, a totally unscientific way of establishing fact from fiction.

This implies that a known reliable non-contact method such as magnetic
hall-effect (pulse), or opto reflective method must be used as the
signal source because these methods are well established and widely
used to perform the same function in industry already. Having said
that, doesn't this suggest that if there is already a reliable system
of deriving the required rpm data with which to compare that obtained
by the new "audio based" system, then why do we need the new system in
the first place?

Because the system the OP proposes can be stuck on (and removed from) any
boat with no modifications to the boat other that adding a bit of sticky
back velcro. That is why I suggested he considers listening from the shore
which is even more convenient.

If you look around you will readily find non-contact optical
tachometers which can also simply be "stuck on (and removed from any
boat" and which will do exactly what the OP wants. I don't intend to
do that for you but I have noted several items which would do the job.
Readily available small products capable of measuring to 100,000 rpm
sell for around AUD300+ depending upon capabilities required. On
systems such as high speed pcb drilling machines the tacho's can
measure to 300,000 rpm using optical means. Optical sensing eliminates
99.999% of the possible causes of measurement error, which just isn't
the case for audible sensing.

And your proposed method of "listening from the shore" is just as
hair-brained as the electret mic on the boat.

 

[Mook Johnson]

While it is completely understood that hall or opto sensing would provide
dead accurate RPM measurement and is in use now by other similar products,
the downfall is that it requires wires to connect the external hall sensor
to the package. the pacakage cannot be placed close enough to the output
shaft or flywheel to directly make the measurement. Wires are something I'm
trying to avoid.

Don't assume I'm limited to a PIC processor. I'm mor than happy to throw a
DSP at it and compute FFTs and FIR filter the input signals to get the
answer.

The basis behind the original question is that I suspected that I could
sample the signal and filter it and FFT it and the tallest peek would be the
loudedt point whish I would assume to be the engine noise. LOUD. But Ross
is correct, the boat goes through hell while skimming across the water at
70+ MPH. Vibration, water spray, etc.

The groundrules for using this device is that a single boat on the water is
run then brought in.

The exhaust of these engines is tuned (expansion chamber) to there are echos
and reverbs all over.

Would be a interesting DSP problem, the information I'm after is in there
but how do I get it out.


"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
news:26qt61hqlvqu35ce2an5gne48dk84jq1up@4ax.com...

On Wed, 27 Apr 2005 01:30:00 +0100, nospam <nospam@nospam.invalid
wrote:

Ross Herbert <rherber1SPAMEX@bigpond.net.au> wrote:

It likely doesn't need to filter out much at all. The rapid opening of a
two stroke exhaust port to release the results of a recent explosion
produces a huge pressure impulse. I would not be surprised to find it
10's
of db larger than any other source although I am assuming these high
performance models do not have silencers.

While this is true the fact remains that any spurious and random noise
source would interfere with the wanted sound. It is not simply a
matter of detecting the amplitude of the desired signal. Since an
electret is extremely sensitive to both sound and physical vibration,
and is physically mounted in the boat which is speeding (hopefully)
over the water, it will pick up every sound and vibration, including
harmonics and sub-harmonics, wanted or not. Filtering would not be as
easy as hoped for, I imagine.

If someone fires a gun next to you head you are telling me you might not
be
able to count how many times because of someone rattling plates or having
a
conversation on the next table?

This analogy is in no way comparable to the task the OP wants to
achieve.


If the impulse is 10 times larger than any other signal you don't need
fancy circuitry to detect it and ignore everything else.

Exactly. And a simple electret mic with op-amp filtering plus a PIC to
handle the data collection and readout is simply not good enough to
achieve this in practice.


My biggest worries would be the risk of hearing reflections of itself in
a
resonant exhaust system, and the dynamic range of the microphone, even
mechanical damage to the microphone.

Aha!, you are tending to agree with the gist of my criticism.

Not really just speculating that if you put a microphone somewhere near
the
end of the exhaust the impulse would be so loud that it would exceed the
dynamic range of a normal microphone and possibly damage it. If I were
doing the job one of the things I would try is using an element from a
piezo sounder as a pickup.

An electret mic and a piezo transducer ain't a lot different in this
environment.


The OP should record a sample from a running engine and see what it
looks
like. He might also consider using a directional microphone from the
shore.

Assuming that one could design a system as proposed, the final test of
accuracy and reliability has to be realised. After extensive bench
testing it would need to be tried in practice on the water. If all
goes well, the tacho will record the rpm as intended, but, how will we
know if the readout is accurate?

Because one can't see any mechanism in the system to give a consistently
inaccurate readout and you already know approximately what the readout
should be?

Oh, it's that simple eh, "one can't see any mechanism", is that it?
You don't seem to understand that this is a totally unscientific
method of establishing facts and is nothing more than guessing.


We would need to have some known
accurate system to compare the readings against, wouldn't we? What
this means is that you would also have to install a reference system,
one which is known to be free of all of the negative aspects I have
referred to,

and how would you know that system is free from those aspects, and free
from other negative aspects you didn't refer to?

Simply because when any method of measurement is established it has to
be compared against a previous accepted standard and can be shown to
be as good or better in performance.


and is tried and proven to accurately measure rpm of the
motor or drive shaft.

And how did you prove this other system accurately measures the motor rpm?

See above answer. To spell it out specifically... In the beginning
when tachometers were first designed they were totally mechanical with
a physical connection to the rotating part being counted, and using
proven mathematical relationships and engineering techniques.

With another system you haven't proven either?

If they aren't proven to work as intended then how do you suppose they
get to be used in industry and commerce to support the correct
maintenance and operation of mechanical plant using rotating parts
such as motors etc. In order for them to be accepted they first had to
be compared with what existed before.


At some point you have to say if it quacks like a duck it is a duck and in
this case duck quacks are not that hard to recognise.

Again, a totally unscientific way of establishing fact from fiction.


This implies that a known reliable non-contact method such as magnetic
hall-effect (pulse), or opto reflective method must be used as the
signal source because these methods are well established and widely
used to perform the same function in industry already. Having said
that, doesn't this suggest that if there is already a reliable system
of deriving the required rpm data with which to compare that obtained
by the new "audio based" system, then why do we need the new system in
the first place?

Because the system the OP proposes can be stuck on (and removed from) any
boat with no modifications to the boat other that adding a bit of sticky
back velcro. That is why I suggested he considers listening from the shore
which is even more convenient.

If you look around you will readily find non-contact optical
tachometers which can also simply be "stuck on (and removed from any
boat" and which will do exactly what the OP wants. I don't intend to
do that for you but I have noted several items which would do the job.
Readily available small products capable of measuring to 100,000 rpm
sell for around AUD300+ depending upon capabilities required. On
systems such as high speed pcb drilling machines the tacho's can
measure to 300,000 rpm using optical means. Optical sensing eliminates
99.999% of the possible causes of measurement error, which just isn't
the case for audible sensing.

And your proposed method of "listening from the shore" is just as
hair-brained as the electret mic on the boat.

 

[Mook Johnson]

Try this link

http://www.cmdracing.com/updated/speedfootage_gallery.htm

Tis is one of the fastest guys in the country. Everyone is chasing him.



"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:1HcppPPT4obCFws8@jmwa.demon.co.uk...

I read in sci.electronics.design that Ross Herbert
rherber1SPAMEX@bigpond.net.au> wrote (in
1lvr61dcs6nld22gcr5qis4plhogfcdeb0@4ax.com&gt about 'Audio Tachometer for 2
stroke engine', on Tue, 26 Apr 2005:

I can see what you mean when you say these guys are serious....

One for each foot and you could do autonomous water-ski.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[nospam]

Rich Grise <richgrise@example.net> wrote:

On Fri, 22 Apr 2005 19:07:35 +0000, Mook Johnson wrote:

I have a crazy idea to use a microphone near the exhaust of a 2 stroke
engine to measure the instantaneous RPM of the engine.

Yes, as has been noted, it's a crazy idea.

Just paint half of the driveshaft flat black, and the other half gloss
white, and use something like this:
http://zuff.info/Doc/TCRT1000.pdf

It isn't a crazy idea it is a likely feasible idea with vastly more utility
than your suggestion.

He wants to turn up at a boat meeting and be able to stick his tacho on any
boat with little more trouble than sticking on a bit of velcro. He doesn't
want to turn up at a meeting and issue boat owners with mechanical drawings
of a sensor assembly so they can go home and start painting shafts and
manufacturing mounting brackets so maybe next time they meet they can spend
ten minutes with a screwdriver fitting (and later removing) his tacho.

You can buy a handheld unit for measuring the blow rate of pile driving
hammers which operates by listening to the bangs. It works because even on
a noisy construction site in the vicinity of the hammer the blows are much
louder than anything else. A 'crazy' idea which works and has great
utility.

 

[Ross Herbert]

On Wed, 27 Apr 2005 02:36:42 GMT, "Mook Johnson" <mook@mook.net>
wrote:

While it is completely understood that hall or opto sensing would provide
dead accurate RPM measurement and is in use now by other similar products,
the downfall is that it requires wires to connect the external hall sensor
to the package. the pacakage cannot be placed close enough to the output
shaft or flywheel to directly make the measurement. Wires are something I'm
trying to avoid.

How about a reflective sensor tachometer such as the Advent A2103 or
A2103/LSR (check out the specifications link)
http://www.electricmotorsupplies.com/advent_hand_held_tachometer.htm

Good for reading up to 2M from the rotating shaft or flywheel. Could
either of these be mounted in a suitable position?

Don't assume I'm limited to a PIC processor. I'm mor than happy to throw a
DSP at it and compute FFTs and FIR filter the input signals to get the
answer.

If you could identify a specific and unique characteristic in the
engine sound which is not present, or very low in amplitude, in all of
the other noise generated within the boat, then I would say you would
have a very good chance of success using DSP. Not having done any
design work with DSP myself, I couldn't advise on how to go about it.
No doubt there are people on this group who will be able to do so.

The basis behind the original question is that I suspected that I could
sample the signal and filter it and FFT it and the tallest peek would be the
loudedt point whish I would assume to be the engine noise. LOUD. But Ross
is correct, the boat goes through hell while skimming across the water at
70+ MPH. Vibration, water spray, etc.

The groundrules for using this device is that a single boat on the water is
run then brought in.

The exhaust of these engines is tuned (expansion chamber) to there are echos
and reverbs all over.

Would be a interesting DSP problem, the information I'm after is in there
but how do I get it out.

That is the question indeed. It may well be possible but it would take
considerable time and effort to achieve with loads of testing and
field trialling which could take several months. If you can avoid the
problems (despite the fact that there would be satisafaction if you
could achieve the desired result), then you can save a lot of time and
money which would be better spent on other aspects of your
sport/hobby. If you can use one of the Advent models or similar then
the cost would be well worth it.

SNIP

 

[Mook Johnson]

This is precicely what I'm after and trying to avoid (wires and special
mounts)

I guy wants to know how fast his hot boat is running and what RPM range its
operating in, just velcro this puppy on and go for a spin bring it back.
You will have a log of the run. He can tweek from there if he's losing too
much RPM in the turns or it's slowing down at the end of the straights.





"nospam" <nospam@nospam.invalid> wrote in message
news:1kvt61997rc4bgarrvb48rquccjgo61ct7@4ax.com...

Rich Grise <richgrise@example.net> wrote:

On Fri, 22 Apr 2005 19:07:35 +0000, Mook Johnson wrote:

I have a crazy idea to use a microphone near the exhaust of a 2 stroke
engine to measure the instantaneous RPM of the engine.

Yes, as has been noted, it's a crazy idea.

Just paint half of the driveshaft flat black, and the other half gloss
white, and use something like this:
http://zuff.info/Doc/TCRT1000.pdf


It isn't a crazy idea it is a likely feasible idea with vastly more
utility
than your suggestion.

He wants to turn up at a boat meeting and be able to stick his tacho on
any
boat with little more trouble than sticking on a bit of velcro. He doesn't
want to turn up at a meeting and issue boat owners with mechanical
drawings
of a sensor assembly so they can go home and start painting shafts and
manufacturing mounting brackets so maybe next time they meet they can
spend
ten minutes with a screwdriver fitting (and later removing) his tacho.

You can buy a handheld unit for measuring the blow rate of pile driving
hammers which operates by listening to the bangs. It works because even
on
a noisy construction site in the vicinity of the hammer the blows are much
louder than anything else. A 'crazy' idea which works and has great
utility.

 

[YD]

On Wed, 27 Apr 2005 11:02:33 GMT, "Mook Johnson" <mook@mook.net>
wrote:

This is precicely what I'm after and trying to avoid (wires and special
mounts)

I guy wants to know how fast his hot boat is running and what RPM range its
operating in, just velcro this puppy on and go for a spin bring it back.
You will have a log of the run. He can tweek from there if he's losing too
much RPM in the turns or it's slowing down at the end of the straights.

After all this chewing around without reaching some consensus, why not
do some practical experimentation? Stick a mic somewhere close to an
engine, stationary of course, and scope it. My guess is you'll see a
rather well defined waveform from the exhaust with some higher
frequency low level crud superimposed.

- YD.

--
Remove HAT if replying by mail.