A PC as a tachometer

[BlueRidgeMark]

Now this is thinking outside the box...

I did some related work a while ago on a vaguely similar subject, except we were measuring the ion current across a spark plug immediately following a ignition event on 4 strokes.

Cool! Why? opcorn:

My suggestion would be to couple the spark signal directly to soundcard. This way you don't have as much background noise to deal with.

The super cheap way would be by making wraps around your ignition wire as close to the spark plug as you can. The more wraps the higher the gain will be...you will have to tune the preamp on your recording device to make sure you have enough amplification and aren't clipping.

Just off the top of my head, I would think that a spark plug wire is going to put out enough of an impulse to fry a computer audio pickup. They are looking for millivolts, not kilovolts. Even just wrapping a wire around the spark plug wire, I would expect you'd induce a few hundred volts on the pickup wire, if not more.

You could play with it to see how close you can get before it's too much, but I suspect your problem will be too much signal rather than too little.

 

[sperho]

sperho, I posted a video in the original thread before I saw this one. Can you tell what the saw is turning in the wood?

I measure that saw to be peaking at around 15,500 between cuts and for the 1st and 3rd cuts, it slowly drops to a low of about 10,300. In the 2nd cut, it only dropped down to about 11,400 and was held there pretty consistently. Did you have a small tach mounted during those cuts?

 

[Lakeside53]

If you use a wire around the plug lead idea, you need to clamp it with a pair of high speed diodes (like IN914) back to back, and a low pass filter -couple of resistors and a capacitor. This way the input is limited to +/- 0.6 volts and the high frequency noise is dumped. It wouldn't be hard to make a really good (and simple) filter so the pc can just worry about counting the fundemental frequency.

 

[sperho]

Here's the video that I've been using to try and replicate the tach (chaps video):

http://www.labonville.com/videos/v2.htm


I just can't figure out exactly what to do .... Anybody got any idea about Hz, like how to analyze??? I've done some half-assed attempts but don't know really know what the heck I'm doin... lol

I've been using S O N Y (it won't let you type that word for some reason .. weird) Vegas 6 for the video then Soundforge to analyze the audio ... still confused though.

In Soundforge, do a Help search on "spectrum analysis". It is capable (I only know this from Google, not because I own the program, or I'd give you more help...)

I skip the video editing bit and input the wave channel directly into CoolEdit and hit record at 44.1 kHz. That way I don't have to extract anything, I just digitally capture from my soundcard on the fly...

 

[dtnodya]

In Soundforge, do a Help search on "spectrum analysis". It is capable (I only know this from Google, not because I own the program, or I'd give you more help...)

I skip the video editing bit and input the wave channel directly into CoolEdit and hit record at 44.1 kHz. That way I don't have to extract anything, I just digitally capture from my soundcard on the fly...

Good idea for skipping the video editing aspect ... that'll save time and record better (to analyze).

OK, yeah I used the 'help' in Soundforge and found the spectrum analysis. The only thing I'm still confused on is the math to convert to (theoretical) RPM's. How are you getting RPM from Hz ... which frequency? Which peak? :help:

If you use a wire around the plug lead idea, you need to clamp it with a pair of high speed diodes (like IN914) back to back, and a low pass filter -couple of resistors and a capacitor. This way the input is limited to +/- 0.6 volts and the high frequency noise is dumped. It wouldn't be hard to make a really good (and simple) filter so the pc can just worry about counting the fundemental frequency.

That's a good idea also. Could mount it all in one of those small 'hobby boxes' ... know what I'm talkin' about?

 

[sperho]

The only thing I'm still confused on is the math to convert to (theoretical) RPM's. How are you getting RPM from Hz ... which frequency? Which peak? :help:

A power spectrum is audio power level plotted vs. frequency instead of power level plotted vs time (what you normally see as an audio track). This spectrum is generated by a mathematical manipulation called a Fourier transform. Assuming the chainsaw is generating the loudest sound in the recording, then the peak with the most power (highest on the dB scale) will be due to the saw. The lowest peak will be the fundamental frequency and it's frequency is directly proportional (by a factor of 60) to the RPM of saw. Remember the fuel/air mixture explodes once per revolution, so revs/min = 60 * revs/sec (in Hz). So if a the biggest amplitude and lowest frequency peak is 100 Hz, that is probably the fundamental (check the harmonics). So, RPM = 100*60 = 6000.

"Harmonics" are integer multiples of the fundamental frequency (because of natural resonance phenomenon). Thus, if a fundamental frequency = 100 Hz, you would expect to see a series of peaks at 200, 300, 400, 500, etc. How high in frequency the harmonics go, depends on the initial sampling rate, bandwidth of the recording, the saw itself, and the environment. The harmonic peaks are useful because they can aid in an accurate determination of the fundamental frequency. So, my attached image above (post #20), you can see that the 3 highest power peaks are at 200, 400, and 600 Hz. The lowest is the fundamental and the others are harmonics. So, multiply 200 bangs (revs)/sec by 60 sec/min and you get 12,000 revs/min.

In my software, I can watch the power spectrum as the audio track is being played and can pretty easily look for highest and lowest RPMs acheived. A nice feature, but not really necessary. When you do this kind of analysis, you need to make sure that you select only the short time window (usually about 1 second) that you are interested in, so you get the cleanest power spectrum. If you create the power spectrum for the whole audio track at once, you will have a frequency mess and it'll be hard to discern what's going on. This is very important, especially because the RPM varies with time due to bogging, throttle blipping, etc. You must only analyze a time period where the RPMs sound pretty constant.

This is way more than most need/want to know, but since you asked... After I see a few more videos with tach information and keep seeing agreement, I'll probably look for some simple freeware that could allow others to easily do this, if they are so inclined.

 

[Goat]

Cool! Why? opcorn:

You can use the info w/ crankshaft angle to find when peak cylinder pressure occurs. Send that info back to the ecm and have it adjust the ignition timing on the fly to maintain peak torque output regardless. You can also use the info to detect combustion anomalies like a misfire or pre-ignition.

Just off the top of my head, I would think that a spark plug wire is going to put out enough of an impulse to fry a computer audio pickup. They are looking for millivolts, not kilovolts. Even just wrapping a wire around the spark plug wire, I would expect you'd induce a few hundred volts on the pickup wire, if not more.

You could play with it to see how close you can get before it's too much, but I suspect your problem will be too much signal rather than too little.

You'd only pickup millivolts. You are just trying to measure the magnetic field produced around the wire when the plug fires. So the voltage induced in the sense wire will be quite low. Lake's suggestion, to use a schottky diode to clamp the flyback is highly recommended!!!

I always keep my signal filtering as much in software as possible, gives you more flexibility, but that is just my preference.

 

[dtnodya]

Thanks for the excellent description!

I've been selecting short bits ... I'll do a screenshot and see what we got ...

 

[Lakeside53]

If you were realy good :jester: you'd be able to discriminate the sound of rough bearings, piston slap and rod issues... Hmmm... new ebay service - call, pay $10, run up the saw by the phone, and you'd get and instant diagnosis.

No doubt it would get outsourced to India within a year.:bang:

 

[blis]

hmmmm, just as side thought, this frequency stuff could "easily" be adapted to muffler making... Since if youre able to pick up the "right" frequencies and make the muffler vibrate at those frequencies you would be able to make tuned and compact pipe for chainsaw...

 

[dtnodya]

If you were realy good :jester: you'd be able to discriminate the sound of rough bearings, piston slap and rod issues... Hmmm... new ebay service - call, pay $10, run up the saw by the phone, and you'd get and instant diagnosis.

No doubt it would get outsourced to India within a year.:bang:

LMAO!!!

*off topic*
When I worked at a car dealership (service dept), I had a guy call one day ... He says, "my engine is making some weird sounds", I tell him to bring his car in and we'd check it out for him. He says, "here, listen" and holds the phone up to the running engine. All I heard was KKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKKkkSHHHHHHHHKKKKKKKKKKKKKKKk. What a moron!

 

[Stihl Crazy]

I measure that saw to be peaking at around 15,500 between cuts and for the 1st and 3rd cuts, it slowly drops to a low of about 10,300. In the 2nd cut, it only dropped down to about 11,400 and was held there pretty consistently. Did you have a small tach mounted during those cuts?

I don't run a tach when racing but I can put it on for testing. The saw in the video was set at 15,500. I have more videos if you want. This is an interesting topic. On the up cut I run 1000 less because of the extra pressure on the saw.

 

[sperho]

I don't run a tach when racing but I can put it on for testing. The saw in the video was set at 15,500. I have more videos if you want. This is an interesting topic. On the up cut I run 1000 less because of the extra pressure on the saw.

Well, hot tamales. If you don't mind, I'd like to look at 2 or 3 more videos where you know the RPMs before I consider this reproducible... If you'd slap on a tach and shoot a vid while testing, that'd be even better.

 

[Al Smith]

If you were realy good :jester: you'd be able to discriminate the sound of rough bearings, piston slap and rod issues... Hmmm... new ebay service - call, pay $10, run up the saw by the phone, and you'd get and instant diagnosis.

No doubt it would get outsourced to India within a year.:bang:

Actually,"Vibration anaylisis " is a science all of it's own.It was part of my job back in my navy days but I haven't kept up with it.The equipment they have today far surpasses the antiquated stuff we used way back when.However,even with the "blackpowder" stuff of old,we could determine bearing failures before they actually happened .That is to say before the faulty bearing led to a catastrophic equipment failure.

 

[sperho]

If you were realy good :jester: you'd be able to discriminate the sound of rough bearings, piston slap and rod issues...

Mail me some equipment for method development and we'll go into business together - you can be the store front and I'll provide diagnoses. Maybe a new 460 to start with. Yes, I believe that'll be a good starting point...

 

[Stihl Crazy]

Well, hot tamales. If you don't mind, I'd like to look at 2 or 3 more videos where you know the RPMs before I consider this reproducible... If you'd slap on a tach and shoot a vid while testing, that'd be even better.

The videos I have don't show the tach, but I can make some this weekend. Here is an 088. How much did it die in the third cut?

 

[sperho]

Here is an 088. How much did it die in the third cut?

With the fidelity of the recording* when you were running that beast, it was extremely difficult to see power spikes of that saw. I'll bet that thing sounds like Satan on fire in person. Were these videos recorded with a point and shoot camera? I wouldn't put too much money on this, but I measured around 7,200 RPM as the lowest RPM in the 3rd cut.


*And probably accoustics of the environment the recording was made, which was the same as the others, but this saw's sound waves were clearly interacting with the surroundings in a much different way... I would not say that "all stinger pipes are indiscernible", though. I've found some online that are readily discernible.

 

[precaud]

If you were realy good :jester: you'd be able to discriminate the sound of rough bearings, piston slap and rod issues...

This is done all the time with industrial machinery; I sell the equipment which does it. It's basically a series of FFT's, plotted as a 3-D "waterfall" display. The manufacturer would provide typical "signatures" that point to specific failures in progress.

 

[craneage2001]

Harmonic Analysis

I took what was said earlier and did some playing around...I used a 1 second snippet from that company's video about chaps, with the saw taching out at 12200.

Running the frequency analysis in Adobe Audition, I got a familiar graph...note the peaks, indicated by the arrows:

The peaks are at the following Hz, which calculate into RPMs as mentioned earlier, correcting for the various harmonic factors:

• 208 hZ = 1x > 12480 RPM
• 417 hZ = 2x > 12510 RPM (417*60/2)
• 624 hz = 3x > 12480 RPM
• 832 hZ = 4x > 12480 RPM
• 1246 hZ = 6x > 12460 RPM
• 3321 hZ = 16x > 12453 RPM
• 4978

= 24x > 12445 RPM

Using the harmonics helps verify the readings, and they all average out to 12471 RPM, which is pretty close to the tach reading. The slight discrepancy MIGHT be from the video compression causing a slight increase in frequency...either way, it's pretty close.

Thoughts?

 

[PES+]

What are you attemptig to do precisely?

You can find the rpm as you have done by analyzing the spectrum like you have but you have also found several of the problems involved with doing it.

Are you trying to offer dial up tach service?

In order to have enough data to find the fundamental frequency you need to have verification by the harmonics which you have plotted.

You have also found one of my main reasons for my leaving the recording industry years ago, an almost complete lack of reliable reference standards in the various digital compression and manipulation algorithms.

You have guessed quite correctly that the video codex treatment of the audio has skewed the results......the weird part which you can verify is pull the sample several more times from the site and the spectrum can and will shift in frequency. For the largest variation download the sample from a different computer using a different ISP and you will see.

The other thing you have found is the reflections involved in the recoding environment can make it difficult to near impossible to get an accurate read of the fundamental due to swamping of the fundamental and harmonics by standing waves and doppler pitch shifted reflections

A stick on transducer on the saw itself can eliminate those variables and would be quite accurate

As far as lakesides suggestions it is quite possible and can be done for rather low cost today.

Is there really a market for such a thing? Could be