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Housing for accelerometer

IWe Ivan Govnov

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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?
 

KlausST

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HI,

1) Two questions in one: First answer: Yes, second answer: No.
The sensor location plays a big roles as you already noticed. But placing it at the center usually is the worst case. Often PCBs are mounted with 4 screws, one at each edge of the PCB. So placing the sensor at the center make the distance to the mounting point large. This means bad coupling. Placing the sensor close to one mounting point will improve performance. The rest of the PCB still will follow resonances. To reduce the effect of mechanical resonances maybe a milled slot around the sensor is a good idea.

2) all is a mass-spring problem. Every mass (of a component) will have effect. Mass and spring (stiffness of PCB) result in a resonance. But to reduce the effects of resonance you should dampen the resonance. Damping = dissipate oscillation energy into heat.

3) I´m sure that MEMS manufacturers provide design notees.

4) definitely yes. But the key is "damping". Maybe Epoxy, rubber, silicone... just add new mass, but low damping, then just the resonance frequency is reduced. But if you use gel with high internal energy dissipation you will get best results. (see 2)

Klaus
 

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