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What Might Make an Accelerometer More Robust?

Last month STMicroelectronics announced a new accelerometer “engineered to withstand stresses of modern mobile life.” They see those stresses arising from increasingly thinner phones and the mechanical and thermal challenges they cause. They called out board bending as a particular challenge to the mechanical integrity of the works inside the accelerometer package.

So how do you improve the mechanical structure of the accelerometer to do this? First, it helps to realize that there are two structures in ST’s accelerometers. One operates in-plane and provides both x and y acceleration information. A separate structure is used for the out-of-plane z axis acceleration. On older models, these two structures were set side by side.

To illustrate how things might be improved, they made reference to stability in an airplane, even though the comparison can’t be taken too literally. If you want the smoothest ride in the plane, you sit in the middle, between the wings. Especially to the extent that the middle has the least stress and that stresses radiate out from that, there’s more disturbance (bumpiness) at the extremes – the wingtips and nose and tail – than in the middle.

It turns out that the z-axis accelerometer is the most sensitive, so improving it was a goal. So they moved it to the middle of the die layout rather than having it off on one side. And where would the x/y structure go if the z structure is hogging the middle? Symmetry is achieved by splitting the x/y structure and putting one half on either side of the center z structure. The two halves become the “wings.”

The other improvement was to double the number of anchoring points on the z structure from 2 to 4. This reduced the stresses on those points, making them less subject to failure.

You can find more details on the performance of this acceleromete

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