Quantcast

Down-Home Vibrometer

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

sming

Well-Known Member
Joined
Apr 10, 2019
Messages
105
Not sure if you really need "AI" and a neural network to do frequency analysis of your engine sound.
But an arduino is probably not the good plateform for that anyhow. Good idea though!
 

BrianW

Well-Known Member
Joined
Jul 2, 2018
Messages
140
Location
Altus SW Oklahoma
Not sure if you really need "AI" and a neural network to do frequency analysis of your engine sound.
But an arduino is probably not the good plateform for that anyhow. Good idea though!
There you go: probably don't need a smart sensor, but if you do, probably shouldn't use an Arduino. Ah, if only I had specified the use of an nRF52840 from Nordic Semiconductors, a 32-bit ARM® Cortex™-M4 CPU running at 64 MHz. with 1MB of program memory.. .
[forgive my making fun - this is a description of an Arduino Nano 33 BLE for $20) <g> ]
 

FinnFlyer

Active Member
Joined
Nov 19, 2019
Messages
29
Location
Bell, FL
OK, so I got the ADXL335 and hooked up with a photo sensor, mounted it to the neck of RWS redrive in my RV-3B and hooked it up to a 4-channel scope. Channel 1 for triggering and channel 2 for the z-axis from the ADXL335. The scope has a number of math functions, like Integral, Differential, Average, etc.
Running the prop up to about 1,400 RPM I simply could not see any pattern, even when removing the 25g weight the builder had added when balancing the prop with a Dynavibe. Other than problems with noise on the trigger signal (must be a bad ground because I filtered it to about 100 Hz), why can I not see a consistent pattern between the two trigger pulses? I mean, even if I did a single-short trigger, I should still see a definite acceleration during the 360 degree interval.

What am I missing?

I guess I'll try it on the RV-4 where I know the prop is out of static balance.

Finn
 

rv7charlie

Well-Known Member
Joined
Nov 17, 2014
Messages
1,145
Location
Jackson
Hey Finn,

Have you seen the comments from RV12 drivers that they've had problems balancing Rotax 912 props due to multiple vibration freqs from the engine itself, the gear box, and the prop? Since drives typically use 'hunting tooth' ratios, the freqs don't look related to each other.

Charlie
 

trimtab

Well-Known Member
Joined
Apr 30, 2014
Messages
120
Location
rocky mountains, rocky, usa
1. Collect data with a dummy channel indexed to an odd multiple greater than one of the rotating mass.

2. FFT the results.

3. Extract the magnitude and phase data, and relate it to the position to mass compensate.

This requires very little effort, and coarse data at some sampling rate greater than twice the rotational speed will eventually capture the vibrations similar in frequency to the engine speed. Go for 10x the engine speed to capture higher frequency components.

It is largely how tire spin balancers work, with almost no other tricks, without strain gauges.
 

FinnFlyer

Active Member
Joined
Nov 19, 2019
Messages
29
Location
Bell, FL
Hey Finn,

Have you seen the comments from RV12 drivers that they've had problems balancing Rotax 912 props due to multiple vibration freqs from the engine itself, the gear box, and the prop? Since drives typically use 'hunting tooth' ratios, the freqs don't look related to each other.

Charlie
No, I haven't, but that sort of makes sense, except the Rotary 13B is supposed to be smooth.
So the trick appears to be to extract only the prop vibrations from the signal. Probably what trimtab is talking about (I still need to wrap my head around his post).

Finn
 

FinnFlyer

Active Member
Joined
Nov 19, 2019
Messages
29
Location
Bell, FL
What are you using as a reflector on the prop blade?
Reflective alum tape on one bolt shank end, the other bolt ends and nuts black with a sharpie. I am getting an excellent signal from the one bolt.
But overnight I realized why my filtering doesn't work. I have a 100K resistor from photo transistor to +5V. Then at the other end of the cable 47nF to ground. I need to add a 100K resistor just before the 47nF. Looks like high frequency noise picked up along the length of the cable.

Finn
 

FinnFlyer

Active Member
Joined
Nov 19, 2019
Messages
29
Location
Bell, FL
1. Collect data with a dummy channel indexed to an odd multiple greater than one of the rotating mass.

2. FFT the results.

3. Extract the magnitude and phase data, and relate it to the position to mass compensate.

This requires very little effort, and coarse data at some sampling rate greater than twice the rotational speed will eventually capture the vibrations similar in frequency to the engine speed. Go for 10x the engine speed to capture higher frequency components.

It is largely how tire spin balancers work, with almost no other tricks, without strain gauges.
OK, so not as simple as I thought it would be.

1. Three pulses per rotation or one pulse for every three rotations?
2. The o'scope has a FFT match function. but minimum resolution is 50Hz, so that probably won't work. I'm not familiar with the math behind FFT. A scope normally shows amplitude as function of time. I think a FFT of that plots amplitudes (magnitudes) against frequencies (spectrum)? Is there a simple way (or formula) to express or do that?
3. Don't know how to do that. Unless a second component of a FFT plot would be the phase data. Can you explain how or point me to a tutorial?

Actually conceptually I do get it: the FFT lets me extract only the frequency I'm interested in (rotation of prop). Then by looking at phase and magnitude of just that, I should see what I'm interested in. So basically using FFT as a filtering function. Wouldn't an analog notch filter do the same thing?

Finn
 
Last edited:

trimtab

Well-Known Member
Joined
Apr 30, 2014
Messages
120
Location
rocky mountains, rocky, usa
The FFT (or DFT, in the case of digital acquisition), yields a real and imaginary part.

The square root of the sum of the squares is the magnitude of each spectral line.

The phase is extracted from the arctangent of the imaginary divided by the real.
 

BrianW

Well-Known Member
Joined
Jul 2, 2018
Messages
140
Location
Altus SW Oklahoma
OK, so I got the ADXL335 and hooked up with a photo sensor, mounted it to the neck of RWS redrive in my RV-3B and hooked it up to a 4-channel scope. Channel 1 for triggering and channel 2 for the z-axis from the ADXL335. The scope has a number of math functions, like Integral, Differential, Average, etc.
Running the prop up to about 1,400 RPM I simply could not see any pattern, even when removing the 25g weight the builder had added when balancing the prop with a Dynavibe.
/snip/

Finn
So the scope shows a straight line?
low level random noise?
high-level noise?
A scope shot would be worth a thousand words!

Anyway - the accelerometer signal needs a low pass filter: if you run the prop at 1400 rpm, you need to low pass the frequencies below 24Hz only - you only have one mass to balance with, per prop revolution. The peak to peak signal then represents the imbalance. The location of a peak in relation to the time between triggers indicates the position. Signal preparation is the name of the game!

Good luck!
 
Last edited:
Top