Housing for accelerometer

Thread Starter

Ivan Govnov

Joined Nov 27, 2020
10
The target of our educational project was creating a vibration sensor based on a MEMS-accelerometer. We found that housing itself of completed device and method of placing PCB in housing has a significant effect on instrument readings. This effect is observed in form of non-linear frequency response, for example, in the frequency range up to 300 Hz, the readings of our device and the control device practically coincided but in the higher-frequency range, there were areas of signal attenuation and subsequent areas of its amplification. This effect may be explained to resonance effects cased with housing properties and its pouring by compound.
So we have the following questions:
1) Can the accelerometer location in the rather compact PCB be significant, and is it correct that the better accuracy can be obtained by placing the accelerometer in the center of the PCB?
2) Could the placement of components other than the accelerometer in the PCB layout play an important role in sensor readings, given that there are no very bulky components and they are all relatively evenly distributed across the board?
3) Are there any recommendations for designing housings for devices with MEMS accelerometers?
4) Can pouring of the housing with compound significantly affect the readings of the accelerometer?
 

Yaakov

Joined Jan 27, 2019
3,737
If you've ever seen Chladni figures (
) then you will know the PCB will have various nodes and antinodes depending on the frequency, and they will change dynamically.

You will have to have the PCB be as small as practical, but there may be nulls at any given frequency once the wavelengths get smaller than the PCB.
 

KeithWalker

Joined Jul 10, 2017
2,034
If you are going to measure vibrations or linear acceleration and want the most accurate results, the accelerometer should be mounted firmly to the surface of the device you are measuring, with the minimum mass of anything else attached to it.The wiring from the accelerometer to the circuit board should be thin and flexible so that is does not add any significant distortion to the measured signal. Mounting it on a circuit board will introduce errors caused by the flexing and vibration of the PC board and the components on it.
 

drc_567

Joined Dec 29, 2008
1,156
... With reference to the video clip shown above, it may be possible to utilize a PCB with a relatively large surface area, and then try to locate a node point, at which any internal resonance frequencies would be nulled out.
... A similar principle occurs when a baseball bat intercepts a pitched ball at the so-called sweet spot, a few inches from the end of the bat, which is a node point, where flexural oscillations of the bat are minimal, resulting in more efficient power transfer to the pitched ball.
 
Last edited:

LowQCab

Joined Nov 6, 2012
1,308
The Printed-Circuit-Board that the Accelerometer is soldered to should only be
large enough to attach the device, plus a Bypass Capacitor, and a Ribbon-Cable.

Have 5 or 10 of them made.

Epoxy the tiny Board to the object you are measuring.

The Frequency-Response-Curve that you will notice will then be,
virtually 100%, caused by the materials and construction of the object.
.
.
.
 

KeithWalker

Joined Jul 10, 2017
2,034
The target of our educational project was creating a vibration sensor based on a MEMS-accelerometer. We found that housing itself of completed device and method of placing PCB in housing has a significant effect on instrument readings. This effect is observed in form of non-linear frequency response, for example, in the frequency range up to 300 Hz, the readings of our device and the control device practically coincided but in the higher-frequency range, there were areas of signal attenuation and subsequent areas of its amplification. This effect may be explained to resonance effects cased with housing properties and its pouring by compound.
So we have the following questions:
1) Can the accelerometer location in the rather compact PCB be significant, and is it correct that the better accuracy can be obtained by placing the accelerometer in the center of the PCB?
2) Could the placement of components other than the accelerometer in the PCB layout play an important role in sensor readings, given that there are no very bulky components and they are all relatively evenly distributed across the board?
3) Are there any recommendations for designing housings for devices with MEMS accelerometers?
4) Can pouring of the housing with compound significantly affect the readings of the accelerometer?
The answers to your questions are:
1) YES, NO
2) YES
3) NOT THAT I AM AWARE OF.
4) YES
 
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