Audio Tachometer for 2 stroke engine

[Mook Johnson]

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.

The idea is non-contact RPM measurment in the presence is wind and road
noise. (lets say a motor bike running at high speed)

For this application I'm interested in 10,000 to 40,000 RPM which is 166 -
666 Hz. If I acquire this signal and BP filter it then run it through a
FFT, do you guys think I can distinguish the (pop pop pop) frequency of the
exhaust stroke from the noise?

The high level requirements are

a. non-contact RPM measurement
b. sensor must be waterproofable (is that a word? )
c. must output a signal that can be acquired by a small microprocesor or DSP
with A2D or Digital.
b. must be small relative to a 1.5" X 2.5" x .5" overall package
e. must be vibration tolerant (survivability and measurment accuracy)
f. At least several hour run time on a 2.8V, 800maH battery
g. Supply voltage is flexable but desired to be below 12V dc.

 

[martin griffith]

On Fri, 22 Apr 2005 19:07:35 GMT, in sci.electronics.design "Mook
Johnson" <mook@mook.net> 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.
snip
questions

1) why?
2) instantaneous revolutions per minute?


martin

After the first death, there is no other.
(Dylan Thomas)

 

[dalai lamah]

Un bel giorno Mook Johnson digitň:

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.

Just out of curiosity: if you can put a microphone near the exhaust (and
therefore you need to mount the sensor somewhere on the vehicle), why can't
you put a knock sensor on the cylinder or a capacitive pickup on the spark
plug cable?

Anyway, I think you could have better chances by receiving the EM spikes
generated by the sparks. If you put an AM radio near the engine, you will
hear a very clear RPM signal, and I guess it's even better with a 2-stroke,
one-cylinder engine.

--
asd

 

[Mook Johnson]

AM radio is a very interesting option for one type of this application.

The other type is a glow plug engine that doesn't have an EM signiture once
it is running.

The actual use for this device would be to monitor the 2 stroke engine on
high performance R/C boats. The plan is to have the unit velcroed to the
boat and it just listens to his engines whine and logs the RPM. Thats why
it needs to float since the 38" boat may be running at 80+ MPH and often
flip over and fly off the water spectacturlary. If the unit was seperated
from the boat I want it to float so it can be retrieved.

The attempt is to make the unit easily transportable fom boat to boat if
there are several boats to performance monitor and tune all that needs to be
done is to remove the unit forn the velcro and stick it on the other boat.
No external wires or plumbing.






"dalai lamah" <antonio12358@hotmail.com> wrote in message
news:do57h44lmzw$.1n7so9qprvvx2$.dlg@40tude.net...

Un bel giorno Mook Johnson digitň:

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.

Just out of curiosity: if you can put a microphone near the exhaust (and
therefore you need to mount the sensor somewhere on the vehicle), why
can't
you put a knock sensor on the cylinder or a capacitive pickup on the spark
plug cable?

Anyway, I think you could have better chances by receiving the EM spikes
generated by the sparks. If you put an AM radio near the engine, you will
hear a very clear RPM signal, and I guess it's even better with a
2-stroke,
one-cylinder engine.

--
asd

 

[Mook Johnson]

See other reply explanation of why. You're right instantaneous RPM is not a
real term. essentially I'd like an RPM update every last 100mS which is the
output of an averaging filter over last 1 second of acquired RPM values.




"martin griffith" <martingriffith@XXyahoo.co.uk> wrote in message
news:49qi611bftu11t2e3jj62f2sqdg40ku1j9@4ax.com...

On Fri, 22 Apr 2005 19:07:35 GMT, in sci.electronics.design "Mook
Johnson" <mook@mook.net> 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.
snip
questions
1) why?
2) instantaneous revolutions per minute?


martin

After the first death, there is no other.
(Dylan Thomas)

 

[Rich Grise]

On Sat, 23 Apr 2005 11:28:04 +0000, Mook Johnson top-posted:

AM radio is a very interesting option for one type of this application.

The other type is a glow plug engine that doesn't have an EM signiture
once it is running.

The actual use for this device would be to monitor the 2 stroke engine on
high performance R/C boats. The plan is to have the unit velcroed to the
boat and it just listens to his engines whine and logs the RPM. Thats why
it needs to float since the 38" boat may be running at 80+ MPH and often
flip over and fly off the water spectacturlary. If the unit was seperated
from the boat I want it to float so it can be retrieved.

The attempt is to make the unit easily transportable fom boat to boat if
there are several boats to performance monitor and tune all that needs to
be done is to remove the unit forn the velcro and stick it on the other
boat. No external wires or plumbing.

If this description of your scenario is really accurate, then the actual
RPM value really doesn't make any difference. No one who gets off on
spectacular crashes is going to be concerned with _actual_ values - just
make something up. What you _really_ need is a secret magic box that does
nothing, (except survive the crash, of course) and a display on your box
that shows impressive numbers. "Yeah, I was cranking out THIRTEEN GRAND
when it crashed. Gonna hafta work on that next year!" and show them the
display on your control box that's showing, oh, I don't know - 13052?
17449.5?

Who really cares?

Break a Leg!
Rich

"dalai lamah" <antonio12358@hotmail.com> wrote in message
news:do57h44lmzw$.1n7so9qprvvx2$.dlg@40tude.net...
Un bel giorno Mook Johnson digitň:

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.

Just out of curiosity: if you can put a microphone near the exhaust (and
therefore you need to mount the sensor somewhere on the vehicle), why
can't
you put a knock sensor on the cylinder or a capacitive pickup on the
spark plug cable?

Anyway, I think you could have better chances by receiving the EM spikes
generated by the sparks. If you put an AM radio near the engine, you
will hear a very clear RPM signal, and I guess it's even better with a
2-stroke,
one-cylinder engine.

--
asd

 

[Mook Johnson]

I think you have real boats confused with R/C boats. These boats are radio
controlled and 3' to 4' long. Key to these is speed with some over 100MPH
actual speed not scale. Racers play with props, strut angle and depth, boat
ride attributes, engine modifications, exhaust tuning, nitromethant % of
fuel mix, and on and on and on to get the speed. All kinds of dials for
them to play with in the search for the ultimate setup.

A boater will typically flip his boat 5- 10 times during a practice session.
There is a jon boat on site to go pick it up, clear the water and out she
goes again. The problem with the curent technology for RPM measurement is
that if the boat hits the water upsidedown at 60+mph it commonly rips that
unit and its associated wires off and sinks to the bottom. So for that
reason, boaters will be a litter conservative on how they run the boats so
the don't flip it.. Kind of defeats the purpose.






"Rich Grise" <richgrise@example.net> wrote in message
newsan.2005.04.23.15.04.10.839402@example.net...

On Sat, 23 Apr 2005 11:28:04 +0000, Mook Johnson top-posted:

AM radio is a very interesting option for one type of this application.

The other type is a glow plug engine that doesn't have an EM signiture
once it is running.

The actual use for this device would be to monitor the 2 stroke engine on
high performance R/C boats. The plan is to have the unit velcroed to the
boat and it just listens to his engines whine and logs the RPM. Thats
why
it needs to float since the 38" boat may be running at 80+ MPH and often
flip over and fly off the water spectacturlary. If the unit was
seperated
from the boat I want it to float so it can be retrieved.

The attempt is to make the unit easily transportable fom boat to boat if
there are several boats to performance monitor and tune all that needs to
be done is to remove the unit forn the velcro and stick it on the other
boat. No external wires or plumbing.


If this description of your scenario is really accurate, then the actual
RPM value really doesn't make any difference. No one who gets off on
spectacular crashes is going to be concerned with _actual_ values - just
make something up. What you _really_ need is a secret magic box that does
nothing, (except survive the crash, of course) and a display on your box
that shows impressive numbers. "Yeah, I was cranking out THIRTEEN GRAND
when it crashed. Gonna hafta work on that next year!" and show them the
display on your control box that's showing, oh, I don't know - 13052?
17449.5?

Who really cares?

Break a Leg!
Rich








"dalai lamah" <antonio12358@hotmail.com> wrote in message
news:do57h44lmzw$.1n7so9qprvvx2$.dlg@40tude.net...
Un bel giorno Mook Johnson digitň:

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.

Just out of curiosity: if you can put a microphone near the exhaust (and
therefore you need to mount the sensor somewhere on the vehicle), why
can't
you put a knock sensor on the cylinder or a capacitive pickup on the
spark plug cable?

Anyway, I think you could have better chances by receiving the EM spikes
generated by the sparks. If you put an AM radio near the engine, you
will hear a very clear RPM signal, and I guess it's even better with a
2-stroke,
one-cylinder engine.

--
asd

 

[Rich Grise]

On Sat, 23 Apr 2005 15:13:39 +0000, Mook Johnson top-posted:

I think you have real boats confused with R/C boats. These boats are
radio controlled and 3' to 4' long. Key to these is speed with some over
100MPH actual speed not scale. Racers play with props, strut angle and
depth, boat ride attributes, engine modifications, exhaust tuning,
nitromethant % of fuel mix, and on and on and on to get the speed. All
kinds of dials for them to play with in the search for the ultimate setup.

A boater will typically flip his boat 5- 10 times during a practice
session. There is a jon boat on site to go pick it up, clear the water and
out she goes again. The problem with the curent technology for RPM
measurement is that if the boat hits the water upsidedown at 60+mph it
commonly rips that unit and its associated wires off and sinks to the
bottom. So for that reason, boaters will be a litter conservative on how
they run the boats so the don't flip it.. Kind of defeats the purpose.

So get a plastic bag, put your precious instrumentation package in it,
dump in some styrofoam peanuts, and shrink-wrap it, as was suggested about
a week ago.

Or, use expanding urethane wall foam.

Cheers!
Rich

"Rich Grise" <richgrise@example.net> wrote in message
newsan.2005.04.23.15.04.10.839402@example.net...
On Sat, 23 Apr 2005 11:28:04 +0000, Mook Johnson top-posted:

AM radio is a very interesting option for one type of this application.

The other type is a glow plug engine that doesn't have an EM signiture
once it is running.

The actual use for this device would be to monitor the 2 stroke engine
on high performance R/C boats. The plan is to have the unit velcroed
to the boat and it just listens to his engines whine and logs the RPM.
Thats why
it needs to float since the 38" boat may be running at 80+ MPH and
often flip over and fly off the water spectacturlary. If the unit was
seperated
from the boat I want it to float so it can be retrieved.

The attempt is to make the unit easily transportable fom boat to boat
if there are several boats to performance monitor and tune all that
needs to be done is to remove the unit forn the velcro and stick it on
the other boat. No external wires or plumbing.


If this description of your scenario is really accurate, then the actual
RPM value really doesn't make any difference. No one who gets off on
spectacular crashes is going to be concerned with _actual_ values - just
make something up. What you _really_ need is a secret magic box that
does nothing, (except survive the crash, of course) and a display on
your box that shows impressive numbers. "Yeah, I was cranking out
THIRTEEN GRAND when it crashed. Gonna hafta work on that next year!" and
show them the display on your control box that's showing, oh, I don't
know - 13052? 17449.5?

Who really cares?

Break a Leg!
Rich








"dalai lamah" <antonio12358@hotmail.com> wrote in message
news:do57h44lmzw$.1n7so9qprvvx2$.dlg@40tude.net...
Un bel giorno Mook Johnson digitň:

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.

Just out of curiosity: if you can put a microphone near the exhaust
(and therefore you need to mount the sensor somewhere on the vehicle),
why can't
you put a knock sensor on the cylinder or a capacitive pickup on the
spark plug cable?

Anyway, I think you could have better chances by receiving the EM
spikes generated by the sparks. If you put an AM radio near the
engine, you will hear a very clear RPM signal, and I guess it's even
better with a 2-stroke,
one-cylinder engine.

--
asd

 

[Joerg]

Hello Mook,

The actual use for this device would be to monitor the 2 stroke engine on
high performance R/C boats. The plan is to have the unit velcroed to the
boat and it just listens to his engines whine and logs the RPM. Thats why
it needs to float since the 38" boat may be running at 80+ MPH and often
flip over and fly off the water spectacturlary. If the unit was seperated
from the boat I want it to float so it can be retrieved.

You might want to think about taping a simple transmitter to it and have
it send the audio over to a receiver on shore, which in turn is
connected to the sound card of a laptop. That can do the FFT, logging
etc. This way not much is lost in case the boat suffers a hard crash.
Well, except for the boat, of course.

This reminds me of my wild days back in school. A friend built these
mean machines and I was the electronics guy. Usually it was a souped up
moped engine sitting in a rather small hull. These things really
screamed across the lake, being airborne most of the time. Until the law
showed up, that is.

Accidental "beaching" of these machines usually resulted in a long
furrow with a smoldering and hissing pile at the end of it.

Regards, Joerg

http://www.analogconsultants.com

 

[Rich The Newsgroup Wacko]

On Sat, 23 Apr 2005 21:22:08 +0000, Joerg wrote:

The actual use for this device would be to monitor the 2 stroke engine
on high performance R/C boats. The plan is to have the unit velcroed to
the boat and it just listens to his engines whine and logs the RPM.
Thats why it needs to float since the 38" boat may be running at 80+ MPH
and often flip over and fly off the water spectacturlary. If the unit
was seperated from the boat I want it to float so it can be retrieved.
....
This reminds me of my wild days back in school. A friend built these
mean machines and I was the electronics guy. Usually it was a souped up
moped engine sitting in a rather small hull. These things really
screamed across the lake, being airborne most of the time. Until the law
showed up, that is.

Accidental "beaching" of these machines usually resulted in a long
furrow with a smoldering and hissing pile at the end of it.
----- Quote -----

So Richard and I decided to try to catch [the small shark]. With
a great deal of strategy and effort and shouting, we managed to maneuver
the shark, over the course of about a half-hour, to a sort of corner of the
lagoon, so that it had no way to escape other than to flop up onto the land
and evolve. Richard and I were inching toward it, sort of crouched over,
when all of a sudden it turned around and -- I can still remember the
sensation I felt at that moment, primarily in the armpit area -- headed
right straight toward us.
Many people would have panicked at this point. But Richard and I
were not "many people." We were experienced waders, and we kept our heads.
We did exactly what the textbook says you should do when you're unarmed and
a shark that is nearly two feet long turns on you in water up to your lower
calves: We sprinted I would say 600 yards in the opposite direction, using
a sprinting style such that the bottoms of our feet never once went below
the surface of the water. We ran all the way to the far shore, and if we
had been in a Warner Brothers cartoon we would have run right INTO the beach,
and you would have seen these two mounds of sand racing across the island
until they bonked into trees and coconuts fell onto their heads.
-- Dave Barry, "The Wonders of Sharks on TV"
----- End Quote -----
--
Thanks!
Rich

 

[Luhan Monat]

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.

The idea is non-contact RPM measurment in the presence is wind and road
noise. (lets say a motor bike running at high speed)

For this application I'm interested in 10,000 to 40,000 RPM which is 166 -
666 Hz. If I acquire this signal and BP filter it then run it through a
FFT, do you guys think I can distinguish the (pop pop pop) frequency of the
exhaust stroke from the noise?

The high level requirements are

a. non-contact RPM measurement
b. sensor must be waterproofable (is that a word? )
c. must output a signal that can be acquired by a small microprocesor or DSP
with A2D or Digital.
b. must be small relative to a 1.5" X 2.5" x .5" overall package
e. must be vibration tolerant (survivability and measurment accuracy)
f. At least several hour run time on a 2.8V, 800maH battery
g. Supply voltage is flexable but desired to be below 12V dc.

I would think background noise would not be a problem. You could
probably get a reading from several feet away. Those two-strokes make a
lot of noise.

The design should have an automatic gain control on the front end, and
then maybe just a zero-crossing detector feeding a PIC.


--
Luhan Monat (luhanis 'at' yahoo 'dot' com)
"The future is not what it used to be..."
http://members.cox.net/berniekm

 

[Robert Roland]

On Sat, 23 Apr 2005 11:28:04 GMT, "Mook Johnson" <mook@mook.net>
wrote:

The other type is a glow plug engine that doesn't have an EM signiture once
it is running.

Another idea that might be worth looking into, is to measure the
resistance of the glow plug. The temperature in the combustion chamber
will vary greatly during each revolution, and this temperature
variation might cause enough resistance variation to be detectable.

--
RoRo

 

[Ross Herbert]

On Fri, 22 Apr 2005 19:07:35 GMT, "Mook Johnson" <mook@mook.net>
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.

The idea is non-contact RPM measurment in the presence is wind and road
noise. (lets say a motor bike running at high speed)

For this application I'm interested in 10,000 to 40,000 RPM which is 166 -
666 Hz. If I acquire this signal and BP filter it then run it through a
FFT, do you guys think I can distinguish the (pop pop pop) frequency of the
exhaust stroke from the noise?

The high level requirements are

a. non-contact RPM measurement
b. sensor must be waterproofable (is that a word? )
c. must output a signal that can be acquired by a small microprocesor or DSP
with A2D or Digital.
b. must be small relative to a 1.5" X 2.5" x .5" overall package
e. must be vibration tolerant (survivability and measurment accuracy)
f. At least several hour run time on a 2.8V, 800maH battery
g. Supply voltage is flexable but desired to be below 12V dc.

I think the statement about "a crazy idea" in your first line sums up
your proposal.

For starters trying to accurately determine rpm at the update rate you
require (100mS)using the "sound" from a small motor would require some
serious signal processing which just couldn't be done with simple
analog electronics IMO. The fact that all sorts of other noises would
be present simultaneously would make the task akin to fishing an
intelligent signal out of a spacecraft out past Jupiter using a 10W
transmitter (not exactly but you get my drift).

Quite frankly I can't really see why you need to know how many revs
the motor on an R/C boat is doing anyway.You're still going to crank
it up as much as possible if you are trying to catch up in a race and
if it goes bang then it doesn't matter one hoot what the rpm's were at
the time.

 

[Mook Johnson]

The reason RPM is important is to make sure your setup the boat to run in
the RPM range where the engine has the most power. you can tell that from
the acceleration and jerk. Some guys get enginer builders to run their
engine on a dynomometer and get a HP vs RPM graph but never knwo what RPM
range their boat is running in.

The elite guys in this hobby are SERIOUS. They are trying to get every R of
performace out of the boats.


"Ross Herbert" <rherber1SPAMEX@bigpond.net.au> wrote in message
news:vptm611tfkuaud6p6jsgg7bsf3patld36f@4ax.com...

On Fri, 22 Apr 2005 19:07:35 GMT, "Mook Johnson" <mook@mook.net
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.

The idea is non-contact RPM measurment in the presence is wind and road
noise. (lets say a motor bike running at high speed)

For this application I'm interested in 10,000 to 40,000 RPM which is 166 -
666 Hz. If I acquire this signal and BP filter it then run it through a
FFT, do you guys think I can distinguish the (pop pop pop) frequency of
the
exhaust stroke from the noise?

The high level requirements are

a. non-contact RPM measurement
b. sensor must be waterproofable (is that a word? )
c. must output a signal that can be acquired by a small microprocesor or
DSP
with A2D or Digital.
b. must be small relative to a 1.5" X 2.5" x .5" overall package
e. must be vibration tolerant (survivability and measurment accuracy)
f. At least several hour run time on a 2.8V, 800maH battery
g. Supply voltage is flexable but desired to be below 12V dc.



I think the statement about "a crazy idea" in your first line sums up
your proposal.

For starters trying to accurately determine rpm at the update rate you
require (100mS)using the "sound" from a small motor would require some
serious signal processing which just couldn't be done with simple
analog electronics IMO. The fact that all sorts of other noises would
be present simultaneously would make the task akin to fishing an
intelligent signal out of a spacecraft out past Jupiter using a 10W
transmitter (not exactly but you get my drift).

Quite frankly I can't really see why you need to know how many revs
the motor on an R/C boat is doing anyway.You're still going to crank
it up as much as possible if you are trying to catch up in a race and
if it goes bang then it doesn't matter one hoot what the rpm's were at
the time.

 

[Ross Herbert]

On 24 Apr 2005 09:59:04 -0700, "BobG" <bobgardner@aol.com> wrote:

Oh you old guys are just full of crap. A 50 cent electret mic in a
plastic bag and an opamp preamp into an edge detect on a pic would work
just fine. Just measure the period of the zero crossings of the pops
and log the max values into eeprom every second or so. You near
Orlando? I'll help you with this project no prob.

Age has nothing to do with the practical realisation of measuring
physical phenomena using electronics. The experience gained over many
years of realising what is practical and what isn't, is lost on the
young would be's if they could be's as you obviously are.

I'll bet you that you would not be able to prove absolutely that the
proposed audio technique would accurately measure the real rpm of the
motor while the boat is actally racing, using an electret mic as the
transducer. How would you filter out ALL of the myriad noises of water
slapping the hull or other vibrations (ie. "NOISE") etc, AND retain
ONLY the engine combustion pops, ALL while the boat was actually under
racing conditions? Sure, you would get a figure, but would it be
accurate, and how would you know if it was or not?

I would venture to say that using sound as an input signal would
require some serious digital processing in order to extract the engine
pops and nothing else, and you would need to do a large amount of
real-time testing to design the correct algorithms.

Now a far more realistic signal source would be to have some opto
source on the driveshaft. That signal you could easily handle with
analog techniques.

 

[Ross Herbert]

Looks like BobG is offering to solve all your problems... You should
have this little project up and running in no time at all

On Sun, 24 Apr 2005 18:55:16 GMT, "Mook Johnson" <mook@mook.net>
wrote:

The reason RPM is important is to make sure your setup the boat to run in
the RPM range where the engine has the most power. you can tell that from
the acceleration and jerk. Some guys get enginer builders to run their
engine on a dynomometer and get a HP vs RPM graph but never knwo what RPM
range their boat is running in.

The elite guys in this hobby are SERIOUS. They are trying to get every R of
performace out of the boats.
SNIP

 

[Ross Herbert]

On Sun, 24 Apr 2005 18:55:16 GMT, "Mook Johnson" <mook@mook.net>
wrote:

The reason RPM is important is to make sure your setup the boat to run in
the RPM range where the engine has the most power. you can tell that from
the acceleration and jerk. Some guys get enginer builders to run their
engine on a dynomometer and get a HP vs RPM graph but never knwo what RPM
range their boat is running in.

The elite guys in this hobby are SERIOUS. They are trying to get every R of
performace out of the boats.

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

http://powerpete.20megsfree.com/shopping_page.html

 

[nospam]

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

On 24 Apr 2005 09:59:04 -0700, "BobG" <bobgardner@aol.com> wrote:

Oh you old guys are just full of crap. A 50 cent electret mic in a
plastic bag and an opamp preamp into an edge detect on a pic would work
just fine. Just measure the period of the zero crossings of the pops
and log the max values into eeprom every second or so. You near
Orlando? I'll help you with this project no prob.

Age has nothing to do with the practical realisation of measuring
physical phenomena using electronics. The experience gained over many
years of realising what is practical and what isn't, is lost on the
young would be's if they could be's as you obviously are.

I'll bet you that you would not be able to prove absolutely that the
proposed audio technique would accurately measure the real rpm of the
motor while the boat is actally racing, using an electret mic as the
transducer. How would you filter out ALL of the myriad noises of water
slapping the hull or other vibrations (ie. "NOISE") etc, AND retain
ONLY the engine combustion pops,

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.

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.

My guess is some quite crude analog filtering and perhaps ALC would produce
a reliable digital once per rev pulse for counting.

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.

 

[John Woodgate]

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

 

[Ross Herbert]

On Tue, 26 Apr 2005 12:55:24 +0100, nospam <nospam@nospam.invalid>
wrote:

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

On 24 Apr 2005 09:59:04 -0700, "BobG" <bobgardner@aol.com> wrote:

Oh you old guys are just full of crap. A 50 cent electret mic in a
plastic bag and an opamp preamp into an edge detect on a pic would work
just fine. Just measure the period of the zero crossings of the pops
and log the max values into eeprom every second or so. You near
Orlando? I'll help you with this project no prob.

Age has nothing to do with the practical realisation of measuring
physical phenomena using electronics. The experience gained over many
years of realising what is practical and what isn't, is lost on the
young would be's if they could be's as you obviously are.

I'll bet you that you would not be able to prove absolutely that the
proposed audio technique would accurately measure the real rpm of the
motor while the boat is actally racing, using an electret mic as the
transducer. How would you filter out ALL of the myriad noises of water
slapping the hull or other vibrations (ie. "NOISE") etc, AND retain
ONLY the engine combustion pops,

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.

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.

My guess is some quite crude analog filtering and perhaps ALC would produce
a reliable digital once per rev pulse for counting.

This runs counter to your previous statement.

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

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? If it were equally reliable (which I doubt is
possible), and cheaper to make, there may be an argument for
developing such a system, however, I very much doubt it.

Anyway, BobG is already on the trail and will have something knocked
up and running in a week or so no doubt....