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Vibration Monitor (Hyde, Wanttaja?)
Necessity, as they say, is a mother.
I am in the process of reinventing a square wheel called a vibration monitor. The electronics is relatively trivial IF the input and output parameters are known. What we know is that the engine is going to have a fundamental frequency at cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a fundamental frequency of 2400/60 or 40 Hz. But wait, he said. There are going to be other (sub) harmonics of that frequency that will be of some interest. And, those harmonics will change as a function of the engine being a two or four stroke, four or six cylinder. So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are going to be of most interest to us and what is their mathematical relationship to the fundamental? As an extra bonus question, my sensor is going to be an old phonograph cartridge. Should I use the lightest weight "needle" that I can find? How about a tiny little ball of lead at the tip of that needle? Would that help the sensor? Or hinder it? Lastly, once I get this sucker up and running with you all's good ideas, is anybody game to bolt it onto their flying machine and report results? I can do it for the 182, but I'd really like some other real-world reports. Jim |
#2
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In article ,
RST Engineering wrote: Necessity, as they say, is a mother. I am in the process of reinventing a square wheel called a vibration monitor. The electronics is relatively trivial IF the input and output parameters are known. What we know is that the engine is going to have a fundamental frequency at cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a fundamental frequency of 2400/60 or 40 Hz. But wait, he said. There are going to be other (sub) harmonics of that frequency that will be of some interest. And, those harmonics will change as a function of the engine being a two or four stroke, four or six cylinder. *most* of the stuff 'of interest' is going to occur at the frequency of ignition in the #1 cylinder. You'll have "similar things" happening at the appropriate phase delay for each cylinder. *IF* everything is behaving exactly the same, that _should_ give you a composite signal at (no. of cylinders) * (cyl #1 ignition frequency). One also has to consider any drive-shaft powered 'accessories', that may be operating at a _different_ speed than the main engine. (gear ratio, and/or belt drive with different pulley sizes) One form of 'bad news' is something that is going on in one cylinder that is _different_ than what is happening in the rest of 'em. This may be merely 'different amplitude', or it may be 'different wave-form. Another form is something "recurring" at a frequency _other_ than what can be explained. e.g., a flat spot on a roller in a roller bearing, will have a characteristic frequency based on how many times the roller rotates, per shaft rotation. Which is likely to be some "weird" ratio. So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are going to be of most interest to us and what is their mathematical relationship to the fundamental? As an extra bonus question, my sensor is going to be an old phonograph cartridge. Should I use the lightest weight "needle" that I can find? How about a tiny little ball of lead at the tip of that needle? Would that help the sensor? Or hinder it? Oh Lordie! To get an _accurate_ measure of vibration, the 'system under test', and the 'testing system' must be *isolated* (mechanically, "vibrationally") from each other. Then you detect the vibration in the system under test, by measuring the instantaneous differences in position, relative to the testing system. When the 'testing system' is mounted _on_ the 'system under test', there is a complication of 'signal' being transferred *through* the mounting, which is then *not* detected by the pick-up, because _both_ components are affected. You can 'approximate' isolation with some sort of a 'suspension' system -- e.g. springs. This, however, ends up "complicating" things, because what it does is just introduce a 'delay' in the transfer through the suspension mechanism, *and* a probable, delayed, induced "negative" component restoring 'equilibrium'. Theoretical analysis can get *really* hairy real quick. To accurately track vibration, you have to have a sensor that will _move_ as far as the maximum 'excursion' of the system under test. It has to have enough structural strength that the sensor, itself doesn't "flex", yet inertia has to be low, to enable it to 'mimic' every motion of the system under test. You've got two *entirely* different kinds of 'sensor' possible: 1) something 'firmly attached' to the airframe, with a "Cats whisker" in contact with the engine, this measures engine movement relative to the airframe. 2) something 'firmly attached' to the engine, using a cat's whisker against a 'suspended' (as supported by a suspension) mass to compare against. This measures the 'movement' of the sprung mass, relative to the engine. Which is "more or less" equivalent (though opposite in sign, naturally to the movement of the engine relative to the "surround". In either case, the "cat's whisker" should be as light and rigid as possible In the latter case, "bigger is better" for the 'reference mass', subject to the suspension mechanism, and resonances, etc. in *it*. |
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RST Engineering wrote:
snip As an extra bonus question, my sensor is going to be an old phonograph cartridge. Should I use the lightest weight "needle" that I can find? How about a tiny little ball of lead at the tip of that needle? Would that help the sensor? Or hinder it? A lot depends on the range of frequencies, any sensor will have its own fundamental frequency- multiple sensors may be a good idea. Geophones seem to be good up to a few hundred Hz, cheap but difficult to get qty one. For the application a speaker may be the clue (as a second sensor). -- regards jc LEGAL - I don't believe what I wrote and neither should you. Sobriety and/or sanity of the author is not guaranteed EMAIL - and are not valid email addresses. news2x at perentie is valid for a while. |
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On Mon, 07 Mar 2005 13:07:25 +1100, jc wrote:
/// A lot depends on the range of frequencies, any sensor will have its own fundamental frequency- multiple sensors may be a good idea. Geophones seem to be good up to a few hundred Hz, cheap but difficult to get qty one. For the application a speaker may be the clue (as a second sensor). Building on this idea - it appears that piezo speakers with a loaded wafer have done well in this application. I'm guessing that loading the ceramic to a natural frequency well below 40Hz would be good. The output is a function of acceleration of the casing, I imagine. Brian Whatcott Altus OK |
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RST Engineering wrote:
Necessity, as they say, is a mother. [snip] As an extra bonus question, my sensor is going to be an old phonograph cartridge. And you *need* to use that phonograph needle? Analog Devices sells cheap MEMS-based accelerometers (e.g. ADXL202). You could even use two or 3 axes. The output a 5V square wave whose duty cycle is proportional to acceleration. Frank |
#6
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Hello Jim,
What we know is that the engine is going to have a fundamental frequency at cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a fundamental frequency of 2400/60 or 40 Hz. Just a thought: Usually the time between the onset of a vibration and some catastrophic failure is rather short. I believe what also needs to be detected is the higher frequency stuff that can happen well before this. Such as friction sounds from a bearing or cam not getting enough lubrication. Regards, Joerg http://www.analogconsultants.com |
#7
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RST Engineering wrote:
Lastly, once I get this sucker up and running with you all's good ideas, is anybody game to bolt it onto their flying machine and report results? I can do it for the 182, but I'd really like some other real-world reports. Jim I should be able to volunteer a rotary by the end of the summer. Since you won't know what frequencies should be there, could you make it a sort of historical tracking system, with maybe some sort of alarm that goes off if something starts going out of whack from historical data? |
#9
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Jim, I have a Memsic demo board, which displays LEDS on two axis with their tilt and motion sensor. I am done with it, would you like to play with it? George Graham RX-7 Powered Graham-EZ, N4449E Homepage http://bfn.org/~ca266 |
#10
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Love to. You got my address?
Jim "George A. Graham" wrote in message ... Jim, I have a Memsic demo board, which displays LEDS on two axis with their tilt and motion sensor. I am done with it, would you like to play with it? George Graham RX-7 Powered Graham-EZ, N4449E Homepage http://bfn.org/~ca266 |
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