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Re: How to analyze change in frequency?

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  • Tim S
    It sounds like Jeremy has a lot of experience with this type of stuff, but when reading it I immediately thought of using an FFT function on the scope to read
    Message 1 of 23 , Mar 3 5:22 AM
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      It sounds like Jeremy has a lot of experience with this type of stuff, but when reading it I immediately thought of using an FFT function on the scope to read out the frequency of the sensor signal directly. On my cheap $400 DSO, it has built in FFT functions. I would hook your sensor up to the scope, and then you could view both the fundamental and secondary frequencies directly on the scope. There may be some chips out there like certain DSP's that can do the FFT for you without the use of an expensive Digital Oscilloscope but I don't know any off hand.


      --- In Electronics_101@yahoogroups.com, "A6intruder@..." <A6intruder@...> wrote:
      >
      > I have a long term project which will seek to measure the degree of
      > torsional resonance that exists at the flywheel of my car engine. I have a
      > Hall effect sensor mounted to sense the flywheel gear teeth going by the
      > sensor. As the engine goes under load the power pulses actually cause
      > minute changes in the frequency of those gear teeth going past the sensor.
      > On a V-8 engine there are four power pulses per revolution of the flywheel.
      > As the engine loads up and the power pulses get larger this minute change in
      > frequency four times per revolution will grow in amplitude. If the
      > mechanical drive system (engine & transmission) hit a point of rotational
      > resonance those minute changes in frequency will become significantly
      > larger.
      >
      > So my question is basically how do I analyze this frequency?
      >
      > I have a regular oscilloscope. I could monitor the basic waveform but how
      > would I detect the minute change of frequency? Would the waveform shrink
      > and expand if I get the time base just so?
      >
      > If has been 30 years since college calc classes, am I seeking a derivative
      > or integral of the basic frequency? Can I achieve this with a certain mode
      > on the oscilloscope (I 'm a very basic scope dope)?
      >
      > Ideally once I learn the terms and analysis I am trying to achieve I would
      > want to collect this data during operation and record it on my laptop and
      > sync it to the engine parameter data that I am already able to
      > collect/record.
      >
      > What are the right terms/process I am looking for?
      >
      > The sensor is already mounted in the car.
      >
      > Data sheet:
      > http://media.digikey.com/pdf/Data%20Sheets/Hamlin%20PDFs/55505.pdf
      >
      > I thought John Popelish or someone else in the group gave me the right
      > process a few years ago but I couldn't find that post in the archives just
      > now.
      >
      > Thanks,
      >
      > Dan Nicoson
      >
    • Howard Hansen
      ... FFT will work when the engine is running at a steady speed. But FFT won t work when the engine is accelerating or there is a variable load on the engine.
      Message 2 of 23 , Mar 3 1:37 PM
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        On 3/3/2013 7:22 AM, Tim S wrote:
        >
        >
        > It sounds like Jeremy has a lot of experience with this type of stuff,
        > but when reading it I immediately thought of using an FFT function on
        > the scope to read out the frequency of the sensor signal directly. On
        > my cheap $400 DSO, it has built in FFT functions. I would hook your
        > sensor up to the scope, and then you could view both the fundamental
        > and secondary frequencies directly on the scope. There may be some
        > chips out there like certain DSP's that can do the FFT for you without
        > the use of an expensive Digital Oscilloscope but I don't know any off
        > hand.
        >

        FFT will work when the engine is running at a steady speed. But FFT
        won't work when the engine is accelerating or there is a variable load
        on the engine.

        T^he other Howard

        >
        > --- In Electronics_101@yahoogroups.com
        > <mailto:Electronics_101%40yahoogroups.com>, "A6intruder@..." wrote:
        > >
        > > I have a long term project which will seek to measure the degree of
        > > torsional resonance that exists at the flywheel of my car engine. I
        > have a
        > > Hall effect sensor mounted to sense the flywheel gear teeth going by the
        > > sensor. As the engine goes under load the power pulses actually cause
        > > minute changes in the frequency of those gear teeth going past the
        > sensor.
        > > On a V-8 engine there are four power pulses per revolution of the
        > flywheel.
        > > As the engine loads up and the power pulses get larger this minute
        > change in
        > > frequency four times per revolution will grow in amplitude. If the
        > > mechanical drive system (engine & transmission) hit a point of
        > rotational
        > > resonance those minute changes in frequency will become significantly
        > > larger.
        > >
        > > So my question is basically how do I analyze this frequency?
        > >
        > > I have a regular oscilloscope. I could monitor the basic waveform
        > but how
        > > would I detect the minute change of frequency? Would the waveform shrink
        > > and expand if I get the time base just so?
        > >
        > > If has been 30 years since college calc classes, am I seeking a
        > derivative
        > > or integral of the basic frequency? Can I achieve this with a
        > certain mode
        > > on the oscilloscope (I 'm a very basic scope dope)?
        > >
        > > Ideally once I learn the terms and analysis I am trying to achieve I
        > would
        > > want to collect this data during operation and record it on my
        > laptop and
        > > sync it to the engine parameter data that I am already able to
        > > collect/record.
        > >
        > > What are the right terms/process I am looking for?
        > >
        > > The sensor is already mounted in the car.
        > >
        > > Data sheet:
        > > http://media.digikey.com/pdf/Data%20Sheets/Hamlin%20PDFs/55505.pdf
        > >
        > > I thought John Popelish or someone else in the group gave me the right
        > > process a few years ago but I couldn't find that post in the
        > archives just
        > > now.
        > >
        > > Thanks,
        > >
        > > Dan Nicoson
        > >
        >
        >



        [Non-text portions of this message have been removed]
      • John Popelish
        ... I think an FFT could be made to work, but it would take a bit of up front computation. The sample period for the FFT would have to be exactly one or an
        Message 3 of 23 , Mar 3 3:29 PM
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          On 03/03/2013 04:37 PM, Howard Hansen wrote:
          >
          > FFT will work when the engine is running at a steady
          > speed. But FFT won't work when the engine is
          > accelerating or there is a variable load on the engine.

          I think an FFT could be made to work, but it would take a
          bit of up front computation.

          The sample period for the FFT would have to be exactly one
          or an integer multiple of rotations, and that would take
          tooth counting.

          The intervals tooth to tooth would have to be measured by
          some high frequency counter and then reciprocled to turn
          period into velocity samples, every tooth.

          Then that sequence of velocities of variable periods would
          have to be resampled, to make interpolated velocity samples,
          uniformly spaced in time, through one (or multiple rotations).

          At that point, a normal FFT could be calculated that would
          produce the magnitudes and phases of the harmonics of the
          rotational frequency.

          There may be a few mathematical short cuts possible, that
          would simply this process and still give useful results.
          I'm not a good enough FFT mathematician to see them, without
          a lot of digging.

          Fortunately, you don't need and really cannot use the FFT of
          every single rotation, so data could be taken for one
          rotation and then the FFT calculated during the next few
          rotations, with just tooth counting continuing, during that
          process, so the next FFT could start on the same tooth, to
          keep the phase information from jumping around. The running
          average of several of these results could produce a usably
          smooth display on some bar graph indicator.

          --
          Regards,

          John Popelish
        • jeremy youngs
          ... on this project though if you dont have a magnitude of impulse (accelerometer) or the frequency of vibration (transducer pick up) one will not be able to
          Message 4 of 23 , Mar 3 3:38 PM
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            >
            > john
            >


            > yes without going into the strategies and engine controller uses
            > quadrature encoding in the crank sensor to monitor instantaneous crankshaft
            > acceleration for misfire detection. which is exactly what you have outlined
            > here.
            >
            on this project though if you dont have a magnitude of impulse
            (accelerometer) or the frequency of vibration (transducer pick up) one
            will not be able to properly calculate the data for proper fourier analysis
            and will be lacking enough detail to have any substantial quantitative
            results , you can measure 0 drive train harmonics by traking rotation or
            acceleration ther is simply not enough information to calculate the other
            polynomial variables. Simply in order to do what the o.p is aking 3 sensors
            will be needed and the crank seneor would be very easy to just access at
            the engine controller by simple back probing. arduino shields are available
            with accelerometers and transducers availabe it is possible an electret
            with enough frequency spectrum would work as well.



            --
            jeremy youngs


            [Non-text portions of this message have been removed]
          • Jan Kok
            Regarding using a Hall effect sensor to measure rotational resonance: it may work if the resonant frequency is substantially lower than the frequency of the
            Message 5 of 23 , Mar 3 6:26 PM
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              Regarding using a Hall effect sensor to measure rotational resonance: it
              may work if the resonant frequency is substantially lower than the
              frequency of the square wave that you get from the Hall effect sensor. If
              the resonant frequency you're looking for is close to or higher than the
              frequency of the sensor square wave (and actually, if there are strong
              frequency components above the frequency of the square wave), then the Hall
              effect sensor won't give you reliable info about the frequency variations,
              i.e. small, high-frequency changes in the rotation rate.

              It has to do with sampling theory. Suppose you use an A/D converter to
              sample a 900 Hz sine wave at a 1000 samples per second rate. In 10 ms the
              sine wave goes through 9 cycles, and you take 10 samples. The result is
              that the samples, if plotted, look like a single cycle of a sine wave at
              100 Hz. So 900 Hz looks like 100 Hz after sampling. The signal you are
              sampling must not have frequency components greater than 1/2 the sampling
              frequency, in order to avoid seeing spurious frequencies in the sampled
              data.

              With the Hall effect sensor, you can sample on the rising and falling edges
              of the sensor square wave, and what you are sampling is the width of the
              pulses. If you get 500 Hz from the sensor, then you can sample 1000 times
              per second and the maximum resonant frequency you can see accurately is
              below half that rate, i.e. less than 500 Hz.



              On Sun, Mar 3, 2013 at 4:38 PM, jeremy youngs <jcyoungs76@...> wrote:

              > >
              > > john
              > >
              >
              >
              > > yes without going into the strategies and engine controller uses
              > > quadrature encoding in the crank sensor to monitor instantaneous
              > crankshaft
              > > acceleration for misfire detection. which is exactly what you have
              > outlined
              > > here.
              > >
              > on this project though if you dont have a magnitude of impulse
              > (accelerometer) or the frequency of vibration (transducer pick up) one
              > will not be able to properly calculate the data for proper fourier analysis
              > and will be lacking enough detail to have any substantial quantitative
              > results , you can measure 0 drive train harmonics by traking rotation or
              > acceleration ther is simply not enough information to calculate the other
              > polynomial variables. Simply in order to do what the o.p is aking 3 sensors
              > will be needed and the crank seneor would be very easy to just access at
              > the engine controller by simple back probing. arduino shields are available
              > with accelerometers and transducers availabe it is possible an electret
              > with enough frequency spectrum would work as well.
              >
              >
              >
              > --
              > jeremy youngs
              >
              >


              [Non-text portions of this message have been removed]
            • AlienRelics
              I believe he said the resonance he is looking for would be at 1/4 the rotational frequency. Why not measure the period between gear teeth, and compare how it
              Message 6 of 23 , Mar 3 9:27 PM
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                I believe he said the resonance he is looking for would be at 1/4 the rotational frequency.

                Why not measure the period between gear teeth, and compare how it changes? It seems to me that what you are looking for is FM impressed on the pulses from the gear teeth.

                Steve Greenfield AE7HD

                --- In Electronics_101@yahoogroups.com, Jan Kok <jan.kok.5y@...> wrote:
                >
                > Regarding using a Hall effect sensor to measure rotational resonance: it
                > may work if the resonant frequency is substantially lower than the
                > frequency of the square wave that you get from the Hall effect sensor. If
                > the resonant frequency you're looking for is close to or higher than the
                > frequency of the sensor square wave (and actually, if there are strong
                > frequency components above the frequency of the square wave), then the Hall
                > effect sensor won't give you reliable info about the frequency variations,
                > i.e. small, high-frequency changes in the rotation rate.
                >
                > It has to do with sampling theory. Suppose you use an A/D converter to
                > sample a 900 Hz sine wave at a 1000 samples per second rate. In 10 ms the
                > sine wave goes through 9 cycles, and you take 10 samples. The result is
                > that the samples, if plotted, look like a single cycle of a sine wave at
                > 100 Hz. So 900 Hz looks like 100 Hz after sampling. The signal you are
                > sampling must not have frequency components greater than 1/2 the sampling
                > frequency, in order to avoid seeing spurious frequencies in the sampled
                > data.
                >
                > With the Hall effect sensor, you can sample on the rising and falling edges
                > of the sensor square wave, and what you are sampling is the width of the
                > pulses. If you get 500 Hz from the sensor, then you can sample 1000 times
                > per second and the maximum resonant frequency you can see accurately is
                > below half that rate, i.e. less than 500 Hz.
                >
                >
              • jeremy youngs
                I believe he said the resonance he is looking for would be at 1/4 the rotational frequency. Why not measure the period between gear teeth, and compare how it
                Message 7 of 23 , Mar 3 10:18 PM
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                  I believe he said the resonance he is looking for would be at 1/4 the
                  rotational frequency.

                  Why not measure the period between gear teeth, and compare how it changes?
                  It seems to me that what you are looking for is FM impressed on the pulses
                  from the gear teeth.

                  Steve Greenfield AE7HD

                  steve i believe accel decel curve could be extrapolated via demodulation
                  as you suggest. however this would assume abunch of ommitted truths that
                  need reconciliation. just a few i wil miss them as i dont wish to write a
                  thesis here :)

                  1 this meaure is taken at the flywheel

                  2 what exactly are we looking for ? just the 8 impulses in 2 revolutions?

                  ( the op hasnt been as forward on what he is looking to do here as would be
                  required)

                  3 there will be MANY resonances as anything attached to the crankshaft will
                  have a natural resonant frequency

                  4 there will be 1st and second order harmonics dominating this analysis

                  some means of isolating the suspect resonance is needed ( also to note what
                  IS the suspect resonance? this has not been defined !!!!)

                  (without definition i cannot help lead to a resolution :)

                  5 what are the particulars of the test method i e under load ? going down
                  the road? idling in the driveway? these conditions will radically affect
                  the identifying of suspect parameters and will lead to different results

                  in short i think an arduino with accelerometer, transducer and ignition
                  pulse pick up is required. i believe in essence a 100 dollar eva could be
                  built to experiment.

                  if i were limited to only looking at one characteric the most info can be
                  derived with a transducer monted to the engine to get precise frequency
                  measurements to back calculate what component is causing the resonance.
                  further the second most important thing is being able to quantify the
                  magnitude of the imulse so you can identify which resonance is THE one that
                  you are looking for .

                  last is rotational tracking this is the least useful and the first
                  recommended as all other parameters can be calculated using the above two
                  methods, however being that crankshaft pulse is readily available to
                  backprobe at the ecm connector it would be very handy to compare these
                  three in a dso type arduino box and graph them in synchrony to paint the
                  whole picture and make the resonance readily identifiable.


                  What I really want to know is what exactly is the O.P. trying to do ie why
                  are we measuring resonances and comparing them to other vehicles?

                  As an auto tech i have never fixed a vehicle in my life :)

                  What i do is resolve concerns and i find this concern to not currently be
                  defined enough to resolve :)

                  ( oh p.s rate of resolved conerns 97 percent in 7 yrs withh chrysler :)


                  --
                  jeremy youngs


                  [Non-text portions of this message have been removed]
                • jeremy youngs
                  DOH !!!! :)) simple cheap effective had to remeber bak to the first nvh class here this is the down and dirty old school tool !!! ( maybe a little ot as its
                  Message 8 of 23 , Mar 3 10:22 PM
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                    DOH !!!! :))

                    simple cheap effective had to remeber bak to the first nvh class here
                    this is the down and dirty old school tool !!!
                    ( maybe a little ot as its not electronic !!)
                    http://www.treysit.com/16.html


                    did i just answer my own post doh enjoy

                    --
                    jeremy youngs
                  • A6intruder@myo-p.com
                    I am the OP and I have tried to be completely forward as to my goal, I stated this earlier: I am trying to figure out the basic
                    Message 9 of 23 , Mar 4 9:43 AM
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                      I am the OP and I have tried to be completely forward as to my goal, I
                      stated this earlier:

                      "I am trying to figure out the basic sensor/processing/display/record
                      process now on my car so that later I can use it to analyze another
                      combination of engine/gearbox/load."

                      The point is to be able to test a given combination of engine/gearbox/load
                      for destructive resonant frequencies in the full range of operation, engine
                      RPM varying from idle (700 RPM) to max RPM (5500 RPM). I figured my turbo
                      charged V-8 Mustang would be a good development test bed since I can load
                      the engine very heavily to see "normal" power pulses with this process
                      which aren't resonant in this parts combination. If there were a
                      destructive resonant frequency in the operational range it would surface as
                      a much larger magnitude than the regular power pulses from the engine, at
                      whatever frequency it exists.

                      So if we can see the "standard" power pulses with this process on my car
                      then when I bolt a similar engine to different gearbox and load (say a
                      propeller) on a test stand then I can monitor to see if the new combination
                      has an resonant frequency within the normal range of operation.

                      I really doubt my car has a resonant frequency in its range of operation as
                      I have flogged that car on the street and at the drag strip throughout the
                      full range of operation and it hasn't self-destructed.

                      Everyone has been very helpful on this discussion, thank you. I have no
                      doubt the information I am seeking can be determined from the gear teeth
                      sensor, it is just a matter of processing that data stream. Jan has pointed
                      out some of the numerical limitations to that process.

                      Thanks,

                      Dan

                      -----Original Message-----
                      From: Electronics_101@yahoogroups.com
                      [mailto:Electronics_101@yahoogroups.com] On Behalf Of jeremy youngs
                      Sent: Monday, March 04, 2013 12:19 AM
                      To: Electronics_101@yahoogroups.com
                      Subject: Re: [Electronics_101] Re: How to analyze change in frequency?

                      I believe he said the resonance he is looking for would be at 1/4 the
                      rotational frequency.

                      Why not measure the period between gear teeth, and compare how it changes?
                      It seems to me that what you are looking for is FM impressed on the pulses
                      from the gear teeth.

                      Steve Greenfield AE7HD

                      steve i believe accel decel curve could be extrapolated via demodulation as
                      you suggest. however this would assume abunch of ommitted truths that need
                      reconciliation. just a few i wil miss them as i dont wish to write a thesis
                      here :)

                      1 this meaure is taken at the flywheel

                      2 what exactly are we looking for ? just the 8 impulses in 2 revolutions?

                      ( the op hasnt been as forward on what he is looking to do here as would be
                      required)

                      3 there will be MANY resonances as anything attached to the crankshaft will
                      have a natural resonant frequency

                      4 there will be 1st and second order harmonics dominating this analysis

                      some means of isolating the suspect resonance is needed ( also to note what
                      IS the suspect resonance? this has not been defined !!!!)

                      (without definition i cannot help lead to a resolution :)

                      5 what are the particulars of the test method i e under load ? going down
                      the road? idling in the driveway? these conditions will radically affect the
                      identifying of suspect parameters and will lead to different results

                      in short i think an arduino with accelerometer, transducer and ignition
                      pulse pick up is required. i believe in essence a 100 dollar eva could be
                      built to experiment.

                      if i were limited to only looking at one characteric the most info can be
                      derived with a transducer monted to the engine to get precise frequency
                      measurements to back calculate what component is causing the resonance.
                      further the second most important thing is being able to quantify the
                      magnitude of the imulse so you can identify which resonance is THE one that
                      you are looking for .

                      last is rotational tracking this is the least useful and the first
                      recommended as all other parameters can be calculated using the above two
                      methods, however being that crankshaft pulse is readily available to
                      backprobe at the ecm connector it would be very handy to compare these three
                      in a dso type arduino box and graph them in synchrony to paint the whole
                      picture and make the resonance readily identifiable.


                      What I really want to know is what exactly is the O.P. trying to do ie why
                      are we measuring resonances and comparing them to other vehicles?

                      As an auto tech i have never fixed a vehicle in my life :)

                      What i do is resolve concerns and i find this concern to not currently be
                      defined enough to resolve :)

                      ( oh p.s rate of resolved conerns 97 percent in 7 yrs withh chrysler :)


                      --
                      jeremy youngs


                      [Non-text portions of this message have been removed]



                      ------------------------------------

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                    • Howard Hansen
                      Hello Dan, A popular Frequency to Voltage Converter IC is the LM2907-N/LM2917-N IC. See: A recommended
                      Message 10 of 23 , Mar 6 5:13 PM
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                        Hello Dan,

                        A popular Frequency to Voltage Converter IC is the LM2907-N/LM2917-N
                        IC. See: <http://www.ti.com/lit/ds/snas555b/snas555b.pdf>
                        A recommended application circuit is Figure 16 on page 14. To be able
                        to detect variations in frequency you need a frequency to voltage
                        converter with a low output ripple. Hence this is why I recommended a
                        frequency to voltage converter with 2 pole Butterworth Filter to reduce
                        ripple.

                        To select component values for the Butterworth Filter one needs to know
                        the range of possible resonant frequencies. What is the range of
                        possible resonant frequencies? My guess is 43 to 367 Hz. This guess
                        comes from an engine RPM range of 650 to 5500 and 4 power pulses per
                        revolution.

                        The other Howard


                        On 3/2/2013 9:49 AM, A6intruder@... wrote:
                        >
                        > OK,
                        >
                        > Just answering Jan and Jeremy got me thinking.
                        >
                        > Without looking at any data sheets let's say we have an IC that converts
                        > frequency to voltage. The Hall effect sensor puts out a square wave signal
                        > with equal high and low durations. So each time the sensor waveform
                        > crosses
                        > a certain voltage (say 2 volts) it allows the IC to determine
                        > frequency and
                        > output the corresponding signal voltage.
                        >
                        > With a completely smooth turning gear wheel at a steady RPM we will get a
                        > very steady signal voltage. Any time the frequency starts to vary the
                        > signal voltage will go up or down. In the case of four power pulses per
                        > revolution the IC should see four areas of changing voltage in each
                        > rotation
                        > of the gear wheel. If you hooked an oscilloscope to the output of this IC
                        > you would see a steady horizontal line for a steady RPM with no power
                        > pulses. With power pulses you would start to see those four areas of
                        > changing voltage.
                        >
                        > The peak amplitude of the changing voltages would signify the magnitude of
                        > the resonance. Large resonance would be a faster change in frequency since
                        > the mechanical amplitude of the disturbance has to happen in the same
                        > amount
                        > of time...
                        >
                        > I'm thinking out loud here. I need to dig into some data sheets and
                        > see how
                        > fast these IC's really are...
                        >
                        > Thanks,
                        >
                        > Dan
                        >
                        > -----Original Message-----
                        > From: Electronics_101@yahoogroups.com
                        > <mailto:Electronics_101%40yahoogroups.com>
                        > [mailto:Electronics_101@yahoogroups.com
                        > <mailto:Electronics_101%40yahoogroups.com>] On Behalf Of Jan Kok
                        > Sent: Saturday, March 02, 2013 12:09 AM
                        > To: Electronics_101@yahoogroups.com
                        > <mailto:Electronics_101%40yahoogroups.com>
                        > Subject: Re: [Electronics_101] How to analyze change in frequency?
                        >
                        > A couple of questions: How many teeth are there on the gear that the
                        > sensor
                        > senses? And, is there some way to tell where the "0" position of the
                        > flywheel is?
                        >
                        > I would suggest buying or borrowing a digital storage oscilloscope with a
                        > large memory (say 1 million samples or so) that can store waveform
                        > data to a
                        > USB key.
                        >
                        > Capture a bunch of waveforms from the sensor, and analyze them at your
                        > leisure on your computer.
                        >
                        > You'll get a frequency modulated square wave from the sensor. The
                        > higher the
                        > RPMs, the higher the frequency and the shorter the period of the square
                        > wave.
                        >
                        > Write a program to measure the period of each pulse, and plot the
                        > frequencies of the pulses (inverse of periods). You can use the Excel
                        > graph
                        > wizard to plot your data.
                        >
                        > Does the graph of the frequencies look somewhat sinusoidal? That graph
                        > would
                        > show the frequency modulation of the sensor square wave, and the cycles in
                        > the sinusoid should correlate with the power strokes of the pistons. The
                        > sinusoid should have much larger amplitude when the engine is under load.
                        >
                        > The amplitude should also be larger at certain RPMs where there is
                        > rotational resonance, compared with RPMs where there is no resonance.
                        > However, my hunch is that it will be hard to see any resonance, for
                        > several
                        > reasons: the resonant frequency may be higher than the sensor squarewave
                        > frequency, friction in the engine and drivetrain will act to dampen any
                        > resonance, and the resonance may be buried in random noise.
                        >
                        > Sounds like a fun project. I'll be curious to hear how it turns out.
                        >
                        > [Non-text portions of this message have been removed]
                        >
                        > ------------------------------------
                        >
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                        >
                        >



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