accelerometer to measure less then 1mm movmenet

Thread Starter

DJ_AA

Joined Aug 6, 2021
152
Hi All

Is it possible to use the low-cost accelerometers to measure the movement of vibration that is less than 1mm?

Or do I need the very high end versions?
 

MrSalts

Joined Apr 2, 2020
1,762
Hi All

Is it possible to use the low-cost accelerometers to measure the movement of vibration that is less than 1mm?

Or do I need the very high end versions?
It really depends on how fast it goes fast. And exactly what "less than 1mm" means. What frequency is it? It is possible none of them work for you.
 

Jon Chandler

Joined Jun 12, 2008
192
It depends on the frequency of the vibration. As an example, consider the following. 1mm pk-pk equals:

14 g rms @ 100 HZ

3.6 g rms @ 50 Hz

0.89 g rms @ 25 Hz

0.14 g rms @ 10 Hz

0.036 g rms @ 5 Hz

Shameless plug: Check out VibCon in the Google Play Store to convert between commonly used vibration units.

SmartSelect_20220430-125102.jpg
 

Hymie

Joined Mar 30, 2018
1,081

Thread Starter

DJ_AA

Joined Aug 6, 2021
152
It really depends on how fast it goes fast. And exactly what "less than 1mm" means. What frequency is it? It is possible none of them work for you.
I am trying to measure vibrations , when a car is driving pass a structure or building.
 

panic mode

Joined Oct 10, 2011
2,134
accelerometers measure acceleration, not distance. without context 1mm is meaningless. what acceleration is acting on the sensor when passing building? of course there is vertical component due to planet gravity (about 9.8m/s^2), there is also side acceleration due to mass of objects in the surroundings such as pedestrians, trees, traffic lights and of course buildings. i hope you are not trying to sense them using cheap accelerometer.
 

MrSalts

Joined Apr 2, 2020
1,762
1mm is not huge, because it really depends how quickly am accelerometer can capture a value and update. 1mm at 1kHz requires a crazy quick read of you want 10 readings per displacement sweep (20k per left + right displacements at 10 readings per left AND right displacement at 1kHz. to look at peak acceleration across the node and zero-G as the vibrating object reaches its displacement limit.

I guess another key is how many datapoint can your bus transfer each second and at what resolution? If you're running i2C, good luck at 20k readings per second.
Another issue, what happens when the frequency is 0.0625 Hz at 2mm. How many bit resolution do you need for an accurate measurement. That ain't much movement beyond noise. The answer, it depends.

if he's measuring the deflection of a brick building caused by a passing car, the deflection will be way under 1mm. If he's looking at a house of cardboard, then, we'll, I dunno.
 
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panic mode

Joined Oct 10, 2011
2,134
a car strolling down the street will take couple of seconds to pass one building. if traveling on highway that may be about a second per building. so frequency of buildings passed by is 1Hz or lower. my question is what system can be used in a car to create 1mm displacement when car is passing some buildings?
 

Jon Chandler

Joined Jun 12, 2008
192
How do I say this? Bull excrement.

A 1mm pk-pk sine wave at 1kHz is a vibration level of 1423 g rms. That is beyond huge, and about 3× the range of a typical 10mV/g industrial accelerometer. But vibration at 1kHz is well within the range of accelerometer (industrial accelerometers typically have a 3dB point of 20kHz, but specialized accelerometers go even higher).

Reiterating again, a vibration of 1mm may be insignificant or huge depending on the frequency of the vibration.

Screenshot_20220430-162437_Edge.jpg
 

MrSalts

Joined Apr 2, 2020
1,762
a car strolling down the street will take couple of seconds to pass one building. if traveling on highway that may be about a second per building. so frequency of buildings passed by is 1Hz or lower. my question is what system can be used in a car to create 1mm displacement when car is passing some buildings?
The building won't vibrate at the frequency it takes the car to pass. It will vibrate at its resonant frequency. It really depends on the modulous of the materials used to build the building and the exact geometry of the building.
 

panic mode

Joined Oct 10, 2011
2,134
True but to detect that or measure such vibration, accelerometer would need to be attached to the building. How would one do such measurement if the sensor is in a car moving down the street?
 

Jon Chandler

Joined Jun 12, 2008
192
It all comes down to what the TS is attempting to measure. First it was a vague general question, then there's a "clarification" of when a car passes a building. It could be the vibration in a car or the vibration in a building or something else entirely.

Let's presume it's building vibration he's interested in. If truly low vibration levels are the concern, like for a sicilon wafer fab, an electronic microscope or other similarly sensitive actions, this is the area of very specialized instrumentation, starting with a seismic accelerometer like the Wilcoxon 731A. This nearly two pound mammoth has a sensitivity of 10g/V, a maximum input level of 0.5g and a frequency response of 0.05 – 450 Hz. The noise floor of this beast is in the micro-g range. Datasheet. Used models are available on ebay for around $300 but use caution going that route since dropping one of these on to a concrete floor will damage it.

Once you have a suitable transducer, you'll need some way to look at the data. The usual method is an FFT spectrum analyzer that works in the audio range.

This accelerometer has internal electronics that work from an ICP* constant current power supply. Power to the accelerometer is supplied as a constant current of 2 – 20mA (with a compliance voltage of around 20 -28 volts) on the signal conductor. The integral electronics of the accelerometer will bias this constant current to about 12 volts, and the AC output will be centered about this value. At the analyzer end, a DC blocking capacitor isolates the AC signal from the DC supply voltage.

This power supply is actually the easy part and can be easily DIY'd. Use an LM317 configured as a constant current source of about 3 or 4 mA to supply power to the signal line, and an electrolytic cap of 10uF (if memory serves) in line with the output to the analyzer.

*I am legally required to save that ICP is a registered trademark of PCB Piezotronics.
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
152
accelerometers measure acceleration, not distance. without context 1mm is meaningless. what acceleration is acting on the sensor when passing building? of course there is vertical component due to planet gravity (about 9.8m/s^2), there is also side acceleration due to mass of objects in the surroundings such as pedestrians, trees, traffic lights and of course buildings. i hope you are not trying to sense them using cheap accelerometer.
Well, the aim is to measure structural movement.I am thinking about using the following sensor:
ADXL313 Datasheet and Product Info | Analog Devices

Which goes down to 0.5G.

So aim I correct in understanding the output of the sensor will tell us how was accelerating in a certain direction, therefore, in theory, detect some type of movement or vibration. This sensor detects 0.5G at 10 bits, does that mean 4.9m/s^2? This implies that each bit reflects a movement of 0.47cm?
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
152
True but to detect that or measure such vibration, accelerometer would need to be attached to the building. How would one do such measurement if the sensor is in a car moving down the street?
The sensor will be fixed onto the wall of the building.
It's not only cars so it could be multiple elements effecting the structurebut would like to get a better understanding of vibration that could be affecting the structure..
 

panic mode

Joined Oct 10, 2011
2,134
Is the building surrounded by roads on each side? One or two way traffic? How far from the building?

i could see a lot of factors affecting reading, including wind, rain, A/C, activities of users in the building etc

is there a reason not to use conventional methods like vision, light or ultrasound beam etc
 
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