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
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.
Un bel giorno Mook Johnson digitò:
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
questions
1) why? 2) instantaneous revolutions per minute?martin
After the first death, there is no other. (Dylan Thomas)
On Sat, 23 Apr 2005 11:28:04 +0000, Mook Johnson top-posted:
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
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.
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.
On Sat, 23 Apr 2005 15:13:39 +0000, Mook Johnson top-posted:
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
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.
...
----- 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
Hello Mook,
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
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.
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
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.
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.
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.
I can see what you mean when you say these guys are serious....
I used electrets in a fan-failure application. We made a vacuum-formed fan tray with little notches for the electrets to be near the fan blades; these were bare-motor, unenclosed fans, so we got close. I found that the electrets saturated easily with this much pressure blast. That was OK with me, because I bandpass filtered (MF-10s) the signals to determine fan failure. But for a wide-range measurement, I'd suggest a low-sensitivity dynamic mic or something less likely to saturate. Saturation will lower ambient rejection, so you might wind up measuring the RPMs of the Harly that's passing you.
Agree that there should be plenty of signal, and processing should be simple; no FFTs or anything like that.
Sounds like more experimenting, and less talking, might be in order here.
John
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.
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