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Vibration Testing
I've come across a marvelously cheap vibration sensor that I want to convert
into an engine vibration instrument for a Kitplanes article. The electronics for me is relatively trivial...the mechanics of vibration are a little harder to fathom. From a mechanical engineering point of view on a horizontally opposed engine, there are (as with most things) three axes of freedom -- fore and aft, side to side, and up and down (longitudinal, lateral, vertical). The sensor I have reads two axes, and my hit is that fore-aft is the least interesting vibration mode of the engine. The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. Comments and thoughts from the technonerds amongst us appreciated. (It has nothing, repeat NOTHING to do with the fact that such a meter might have detected a crack in that cylinder WAY BEFORE it departed the engine on the way home from Oshkosh {;-) ) Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#2
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Thirty years ago I worked for a company by the name of IRD Mechanalysis.
Their business was vibration detection, monitoring and analysis for preventive maintenance of heavy machinery. At that time they were a subsidiary of H H Robinson Company, a construction company based in Pittsburgh PA. We built our own crystal-transducers to work with the monitoring equipment. I never worked in the analysis end of the business, so I cannot tell you where the frequencies indicating "good" and "bad" lie. Each device is different. A problem with each component was indicated by a different frequency. The only aviation use I am aware of was to dynamically balance props. Jim Weir wrote: I've come across a marvelously cheap vibration sensor that I want to convert into an engine vibration instrument for a Kitplanes article. The electronics for me is relatively trivial...the mechanics of vibration are a little harder to fathom. From a mechanical engineering point of view on a horizontally opposed engine, there are (as with most things) three axes of freedom -- fore and aft, side to side, and up and down (longitudinal, lateral, vertical). The sensor I have reads two axes, and my hit is that fore-aft is the least interesting vibration mode of the engine. The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. Comments and thoughts from the technonerds amongst us appreciated. (It has nothing, repeat NOTHING to do with the fact that such a meter might have detected a crack in that cylinder WAY BEFORE it departed the engine on the way home from Oshkosh {;-) ) Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#3
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"Jim Weir" wrote in message ... Comments and thoughts from the technonerds amongst us appreciated. I suppose it depends what you're going to do with the data. If it's just an idiot sensor that something is going wrong with the engine, probably a sum of the total vibration would be best. If you're trying to test something, two meters would be better than one (either summed or switched). Frankly I kindof detest switched meters. I replaced that kludge in the navion with individual fuel gauges. Seems like the hifi apps should have some stereo vu-meters available, I know you used to be able to get a stereo bar graph led display at RadioSnack a few years back. |
#4
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In article ,
Jim Weir wrote: I've come across a marvelously cheap vibration sensor that I want to convert into an engine vibration instrument for a Kitplanes article. If you really wanted to try to diagnose problems with it you should try to couple it to RPM. Then you could measure vibration in terms of the order relative to the moving parts. If you could include the phase of the crankshaft you could probably spit out enough information to do a dynamic prop balance on a serial port. Alternatively you might be able to infer RPM by doing an FFT on the raw data. That would be a neat party trick. As far as mounting it seems like getting it as far forward as possible (where you should see the largest magnitudes) would be good. And if you only get 2 axes then I'd go with your idea and ignore push/pull and keep side/side and up/down. -- Ben Jackson http://www.ben.com/ |
#5
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I don't think serial ports need dynamic prop balancing.
You guys sure know how to complex up a bonehead simple idea, don'cha? {;-) Jim (Ben Jackson) shared these priceless pearls of wisdom: If you could include the phase of the -crankshaft you could probably spit out enough information to do a dynamic -prop balance on a serial port. Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
#6
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Jim Weir wrote: The sensor I have reads two axes, and my hit is that fore-aft is the least interesting vibration mode of the engine. The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. My take is that summing the two axes would be perfectly acceptable. I agree that fore-and-aft is unlikely to produce any interesting results. In fact, I'd bet that any mechanical problem would show up as an increase in lateral vibration. Vertical vibration would tend to occur only just before things come apart, IMO. George Patterson If a man gets into a fight 3,000 miles away from home, he *had* to have been looking for it. |
#7
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Jim if the sensors are that cheap why not just use two mounted 90 degrees
apart. If its analog output you could just sum the two together to get ~total vibration or convert to digital feed a cheap microprocessor chip and display all three. Logging all three channels would let you post process the data all you want and would not be all that difficult. John Jim Weir wrote: I've come across a marvelously cheap vibration sensor that I want to convert |
#8
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On 29-Sep-2004, Jim Weir wrote: The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. Hmm... Interesting bit of instrumentation there. My take is that since summing the two axes will involve some trivial electronics, why not add a bit more trivial electronics and have a means to (at least momentarily) individually look at either one axis or the other. I envision a 3-position toggle switch which is spring-loaded to the center ("mixed") position. Toggle to either "side" position will select one or the other axis for an individual reading. -- -Elliott Drucker |
#9
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Jim Weir wrote...
The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. You gotta decide what you're going to do with it and what it's going to tell you in the cockpit. Are your procedures going to be different if one axis reads high and the other reads normal ("Holy cow, lateral vibes are off scale! Good thing it's not vertical!")? Personally I'd think that total vibration (RSS) would be sufficient to indicate a problem that warranted more detailed troubleshooting on the ground. Too much information in the cockpit can be just as bad as not enough. Dave 'standard gauge' Hyde |
#10
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Well if you can produce a Power Density Spectrum, it might be useful for
engine condition monitoring. On my helicopter, I have a two channel device with a photo sensor to give me some idea the crank angle (if I'm checking engine balance) or rotor position if I'm doing a rotor balance. However, I put a scope probe on the out put of the accelerometer and can't find an intelligent signal. I'm told that my processor does a tracking filter based on the RPM derived from the photo sensor output. The sensors that I'm using are the piezoelectric type and the processor integrates their output to derive an "inches per second" or ips reading. I find the vibration analyzer both mandatory and frustrating. I will surely follow any KitPlanes article you produce with your sensors. Good luck. "Jim Weir" wrote in message ... I've come across a marvelously cheap vibration sensor that I want to convert into an engine vibration instrument for a Kitplanes article. The electronics for me is relatively trivial...the mechanics of vibration are a little harder to fathom. From a mechanical engineering point of view on a horizontally opposed engine, there are (as with most things) three axes of freedom -- fore and aft, side to side, and up and down (longitudinal, lateral, vertical). The sensor I have reads two axes, and my hit is that fore-aft is the least interesting vibration mode of the engine. The question is whether to have a two-channel meter (which complexes up both the design AND the panel space), a single meter switchable between lateral and vertical) or a single meter with the two axes summed together. Comments and thoughts from the technonerds amongst us appreciated. (It has nothing, repeat NOTHING to do with the fact that such a meter might have detected a crack in that cylinder WAY BEFORE it departed the engine on the way home from Oshkosh {;-) ) Jim Jim Weir (A&P/IA, CFI, & other good alphabet soup) VP Eng RST Pres. Cyberchapter EAA Tech. Counselor http://www.rst-engr.com |
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