Low cost vibration sensing for analysis

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Bill-Higdon

Well-Known Member
The discussions of vibration sensing and analysis reminded me of some tech I've seen for vibration sensing some of this is from the people who build their own turbine engines.
Here are 2 links from that and also a more recent article on low cost vibration sensing.
http://users.atw.hu/aerotarget/Balancing/Manual.pdf Uses speakers as sensors
Wuchtmaschine uses piezo discs as sensors

https://www.edn.com/low-cost-vibrat...63fac042321bf9b6ca34e12e79f5542#comment-29079 this is a more recnt article and uses custom make sensors

trimtab

Well-Known Member
The discussions of vibration sensing and analysis reminded me of some tech I've seen for vibration sensing some of this is from the people who build their own turbine engines.
Here are 2 links from that and also a more recent article on low cost vibration sensing.
http://users.atw.hu/aerotarget/Balancing/Manual.pdf Uses speakers as sensors
Wuchtmaschine uses piezo discs as sensors

https://www.edn.com/low-cost-vibrat...63fac042321bf9b6ca34e12e79f5542#comment-29079 this is a more recnt article and uses custom make sensors
This looks really interesting. Thanks.

Daleandee

Well-Known Member
Interesting stuff, although the first link brought up a Trojan Warning ...

crusty old aviator

Well-Known Member
Supporting Member
What is the price range for “low cost?” I have a $1700 Aces Sport dynamic prop balancer (+$200/yr for recal) that has a pretty good accelerometer.

Bill-Higdon

Well-Known Member
Interesting stuff, although the first link brought up a Trojan Warning ...
Didn't on my pc

DanH

Well-Known Member
Are talking about measuring X, Y, Z axis vibration, or torsional?

Sensing torsional vibration with an external accelerometer fails to consider the nature of the second mode. There may not be a block reaction. See the arrows on the sketch.

Then there is calibration. Even for the first mode, how do you translate an external acceleration to a shaft stress?

I am constantly amazed at the electronics and computer skills seen at the hobby level these days. Forget accelerometers. Cook up an inexpensive digital shaft transmitter, and a receiver. The industry standard stuff is pricy mostly because it is low volume.

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crusty old aviator

Well-Known Member
Supporting Member
Are talking about measuring X, Y, Z axis vibration, or torsional?

Sensing torsional vibration with an external accelerometer fails to consider the nature of the second mode. There may not be a block reaction. See the arrows on the sketch.

Then there is calibration. Even for the first mode, how do you translate an external acceleration to a shaft stress?

I am constantly amazed at the electronics and computer skills seen at the hobby level these days. Forget accelerometers. Cook up an inexpensive digital shaft transmitter, and a receiver. The industry standard stuff is pricy mostly because it is low volume.
This is new to me. Where would the shaft transmitter usually be located. I was involved in a project where we mounted strain gauges inside an IC engine and used transmitters. Is this similar?

GeeZee

Well-Known Member
I think it was a Brian Carpenter column in sport av magazine where he mentioned the existence of smart phone apps that use the accelerometers in the phone for vibration analysis.

GeeZee

Well-Known Member
This is probably already known by you torsional vibration “aficionados”. I thought it was really interesting. Especially the “Torsiograph“ they developed to measure WWII radial engine crankshaft twist.

DanH

Well-Known Member
This is new to me. Where would the shaft transmitter usually be located. I was involved in a project where we mounted strain gauges inside an IC engine and used transmitters. Is this similar?

Yes, exactly, but it doesn't necessarily need to go inside the engine. Strain gauge location is dependent on the physical configuration of the drive system, as it needs a shaft section suitable for gauge adhesion. The propshaft is a good spot when possible. This particular shaft was designed with an eye toward measurement. One idea considered at the time was a shaft extension spool with one of the common propeller bolt patterns, like the SAE 2 as seen here, with a permanently installed bridge and transmitter. That way anyone could bolt it into their drivetrain to see torsional results. Yes, the spool would affect the result to some small degree, but it should not be hard to develop a correction factor or method. Any such downside would be offset, in the practical sense, by the ease of plug-and-play.

Any talented electron benders out there? Want to change the landscape? Adapt WifI gear to beam the signal off the shaft, and display on a phone.

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wsimpso1

Super Moderator
Staff member
Guys, there are reasons these low tech low buck schemes are not widely used.
• The fundamental principle in instrumentation of any type is to figure out what you need to know the details on, and then sense that or something very closely related. If we are concerned with torsional vibe, you measure torsional accelerations or displacements, not linear accels of a housing that may or many not have anything to do with torsionals;
• Sensors used must be at least four times faster than the fastest vibe response you may have, with ten times faster being better still. Why? If you can not see the waveform of the signal, you are getting huge noise and maybe no signal at all. You can even draw a "wrong" conclusion about what is happening, which is worse than not being able to understand what is happening;
• Power the sensor adequately to make a usable and clean signal;
• Get the signal out of the sensor and to your recording equipment;
• Make sense of the signals.
The predominant equipment on this stuff grew out of fundamentals and real needs.

Rotec and its competitors use either optical or metallic tooth targets sensed with laser tachs, mag pickups, or reluctance pickups. They measure the rotating speed of each wheel with lots of teeth and way fast sensors. Data taking equipment is really fast - 10GHz to 100Ghz clock on the time stamp for every tooth passage. No linear pickups of housings are normally done.

Then we get to processing, because we are not concerned with wave form shape (sinusoidal is usually assumed) we want to know what is vibrating with what and what mode is big at what frequencies. Fast Fourier Transforms of each signal, FFT on one signal vs another signal, and so on. Now maybe there is software out there that does all this once you have stored the data, but if it gets cumbersome, we are unlikely to get much from it.

Is the data useable? Sure our 4x rotation (2x firing) on a 6000 rpm 4 banger is only 400 Hz, but if you want to know that you have a stiff system, you had better KNOW that your lowest mode freq is well above 400 Hz. You have to be able to sense vibrations to at least 800 Hz. Go with a 7000 rpm V8 and and now you need more like a 1kHz. OK, maybe that is not hard with sensors designed and marketed for this purpose, but what is the frequency response of your stereo voice coil when mechanically driven? What is its signal rejection in other axes? You do not know, and so you will have to characterize your sensors in some way, and select ones that are fast enough.

Now if you have the ability to get data and process it, can you analyze for A vibrating relative to B, A to C, A to D, B to C, B to D, and C to D? Frequently we find that we need to sense further afield than we started out, or worse, we find that some internal assembly has all sorts of vibe content and need details from within.

This drives the need for expensive sophisticated systems... If your repurposed speakers and strain gages and oscilloscopes can extract usable results, fabulous. Somehow I do not see most of them rotating with the parts, and the strain gages end up needing telemetry to get the signals out. the exercise ends up being about getting the telemetry to work, not about gathering info on what is happening in the machine. Somehow pointing a laser tach at a paper target seems way more reliable and useful.

I shall remain skeptical until shown differently. Please, prove me wrong. I would love to know there is a system out there that is cheap, easy to use, and covers the bases...

Billski

wsimpso1

Super Moderator
Staff member
This is new to me. Where would the shaft transmitter usually be located. I was involved in a project where we mounted strain gauges inside an IC engine and used transmitters. Is this similar?
Maybe. If you have rotating wheels, you really need to sense accel/decel on each one to figure out what is going on. Yes, you could mount strain gages on the shafts between wheels, then use telemetry to get the strains out. If the shaft in between the wheels has a straight section to instrument. Many systems are pretty compact... Use hollow shafts and put the strain gages on the ID? Ugh.

The smart way is we put a strip of paper printed with alternating dark and white bars on the wheel with a little rubber cement. Point a laser tach at this thing. I know, the strip will have a discontinuity. Rotec and its competition actually have a tool for correcting for missing teeth, out of place teeth, discontinuities, and so on. We can correct for these common errors.