how to measure compressor vibration on a vehicle
September 7, 2011 7:28 PM Subscribe
the vibration of a vehicle compressor is a response to the input of the engine, and the resonant charcteristics of the comrpessor, and the stiffness of the system it is mounted to. how does one measure the influence of the bracket on the response of the system?
i think the simple answer is that it is transmission path analysis, but this is difficult to do in practice. i want to measure tri axial vibration and demonstrate if the compressor bracket is stiff enough.
is it scientifically correct to test the components on their own to evaluate stiffness and response (isolated from the vehicle)
or can this only be tesed properly in te vehicle?
and is there a simplified way to describe the steps that should be taken? please msg for any further info
i think the simple answer is that it is transmission path analysis, but this is difficult to do in practice. i want to measure tri axial vibration and demonstrate if the compressor bracket is stiff enough.
is it scientifically correct to test the components on their own to evaluate stiffness and response (isolated from the vehicle)
or can this only be tesed properly in te vehicle?
and is there a simplified way to describe the steps that should be taken? please msg for any further info
If you have enough information you can simulate this. The problem with such a situation is it's awfully difficult to get enough information, and the closer the bracket's resonance(s) is(are) to any of the principal excitations the more difficult it will get.
Unless you're working for Pratt and Whitney, getting a fast answer involves making an overkill bracket, testing that, and then comparing the results to the performance of the candidate bracket. You will not get -an- answer that way. You will very likely get information that leads you to the answer.
posted by jet_silver at 7:39 PM on September 7, 2011
Unless you're working for Pratt and Whitney, getting a fast answer involves making an overkill bracket, testing that, and then comparing the results to the performance of the candidate bracket. You will not get -an- answer that way. You will very likely get information that leads you to the answer.
posted by jet_silver at 7:39 PM on September 7, 2011
I'm not an engineer, but it seems to me that the way to get to this is to figure out a way to measure what the compressor wants to do if it was completely unrestrained. You might not be able to measure this directly, but by looking at the construction of it, you can see what the internal motion of the parts is, and extrapolate from there.
Very simple example. Look at a guitar string. It has the fundamental frequency when it is tethered at two ends. It wants to vibrate at that frequency. If you were going to somehow mount that guitar string to a car and reduce the transmitted vibration, you might add another bracket in the middle of the string, at the node of its resonant frequency. This kills the fundamental without transmitting too much vibration through that mount, because the energy goes out via the vibrations in the two octaves. But if you put that mount out of tune, say an inch away from the node, you are causing it to vibrate all over the place because you are "grabbing" it in a "loud" part of the string. This will transmit more vibration through the mount, but it will also kill some of the resonance of the string. If you can damp that mount such that it kills the transmission through the mount, you've done a good job of silencing the system.
posted by gjc at 7:15 AM on September 8, 2011
Very simple example. Look at a guitar string. It has the fundamental frequency when it is tethered at two ends. It wants to vibrate at that frequency. If you were going to somehow mount that guitar string to a car and reduce the transmitted vibration, you might add another bracket in the middle of the string, at the node of its resonant frequency. This kills the fundamental without transmitting too much vibration through that mount, because the energy goes out via the vibrations in the two octaves. But if you put that mount out of tune, say an inch away from the node, you are causing it to vibrate all over the place because you are "grabbing" it in a "loud" part of the string. This will transmit more vibration through the mount, but it will also kill some of the resonance of the string. If you can damp that mount such that it kills the transmission through the mount, you've done a good job of silencing the system.
posted by gjc at 7:15 AM on September 8, 2011
Best answer: Mount a three axis accelerometer and monitor it. Do FFTs on the time domain data and you can see the resonance peaks.
In the old days, Tektronix made Fourier analyzer which were used for this type of thing.
Nowadays, you can use sound cards and spectral analysis software, like spectrum lab.
You can also use digitizing oscilloscopes with FFT capability, of which there are many, many.
If you are in an industrial environment, rent Tek 3000 series DSOs. You might be able to monitor the accelerometer outputs directly with it, though I haven't done this.
I did this exact experiment on mounting pads for a control box on a land mine removal tractor to see what the effect on resonance was from changes to the pads. It does work.
Ask more questions if this isn't enough to get you started. Memail, too, if you wanna, or email to address in profile.
posted by FauxScot at 7:32 AM on September 8, 2011
In the old days, Tektronix made Fourier analyzer which were used for this type of thing.
Nowadays, you can use sound cards and spectral analysis software, like spectrum lab.
You can also use digitizing oscilloscopes with FFT capability, of which there are many, many.
If you are in an industrial environment, rent Tek 3000 series DSOs. You might be able to monitor the accelerometer outputs directly with it, though I haven't done this.
I did this exact experiment on mounting pads for a control box on a land mine removal tractor to see what the effect on resonance was from changes to the pads. It does work.
Ask more questions if this isn't enough to get you started. Memail, too, if you wanna, or email to address in profile.
posted by FauxScot at 7:32 AM on September 8, 2011
This thread is closed to new comments.
and the compliance and the dampening of the various mounts included in the system described, and the additive/subtractive/canceling properties of the total structure, at various frequencies, and and the reflected impact torques/vibration to the prime mover of the various elements in the total system.
Element analysis is fine, but systemic analysis is nearly impossible. You build, and you test, and you measure, and thereafter, you pay close attention to your failure reports, and redesign accordingly.
posted by paulsc at 7:38 PM on September 7, 2011