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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 |
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On Sun, 6 Mar 2005 09:47:58 -0800, "RST Engineering"
wrote: 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? Can't help you on the first issue, but adding the ball of lead to your needle is going to lower its OWN harmonic frequency. The harmonic of the needle may be brought within the normal range of the engine. Which means you'll get a false reading based on the characteristics of the sensor. I know it's contrary to the Weir philosophy, but commercial accelerometers are available for less than $100. Just because it's not in the Digi-Key catalog doesn't mean it doesn't exist. :-) http://www.vernier.com/probes/probes...plate=acc.html Since this is for an article, you might be able to sweet-talk Analog Devices out of some of their acceleration transducers...they're pretty cheap, though the company normally doesn't sell in lots of less than a thousand. http://www.analog.com/en/subCat/0,28...0%255F,00.html I got a buddy who works for a company that handles this stuff...let me email you his address. 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. Happy to stick it on my Fly Baby. A neat spot would be down on the axle next to the video camera.... Ron Wanttaja |
#3
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Two "whoops":
On Sun, 06 Mar 2005 18:17:58 GMT, I wrote: I got a buddy who works for a company that handles this stuff...let me email you his address. Whoops #1: I had his company name slightly wrong. They don't handle accelerometers. Happy to stick it on my Fly Baby. A neat spot would be down on the axle next to the video camera.... Whoops #2: I forgot this was an *engine* vibration monitor. Haven't pulled the G-level yet to put the engine down on the axle. Still willing to install the sensor, though. I posted my earlier adventures with a recording accelerometer about ten years ago. Here's the article: http://www.wanttaja.com/avlinks/accel.htm Ron Wanttaja |
#4
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Ron Wanttaja wrote:
On Sun, 6 Mar 2005 09:47:58 -0800, "RST Engineering" wrote: 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? Can't help you on the first issue, but adding the ball of lead to your needle is going to lower its OWN harmonic frequency. The harmonic of the needle may be brought within the normal range of the engine. Which means you'll get a false reading based on the characteristics of the sensor. I know it's contrary to the Weir philosophy, but commercial accelerometers are available for less than $100. Just because it's not in the Digi-Key catalog doesn't mean it doesn't exist. :-) http://www.vernier.com/probes/probes...plate=acc.html Since this is for an article, you might be able to sweet-talk Analog Devices out of some of their acceleration transducers...they're pretty cheap, though the company normally doesn't sell in lots of less than a thousand. http://www.analog.com/en/subCat/0,28...0%255F,00.html I got a buddy who works for a company that handles this stuff...let me email you his address. 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. Happy to stick it on my Fly Baby. A neat spot would be down on the axle next to the video camera.... Ron Wanttaja I see accelerometers like these on e-bay all the time. Dan, U.S. Air Force, retired |
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Ron Wanttaja wrote:
snip I posted my earlier adventures with a recording accelerometer about ten years ago. Here's the article: http://www.wanttaja.com/avlinks/accel.htm Ron Wanttaja On F-4s we had statistical accelerometers which had a 4 window display. Each window showed how many times a certain G had been reached. The display was electromechanical. It would be interesting to build a small digital equivilent to install in HB aircraft. It shouldn't be too difficult. Dan, U.S. Air Force, retired |
#6
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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. |
#8
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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 |
#9
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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 |
#10
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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? |
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