Hello CLin,
1. We need to take the sensor off every time if we need to calibrate it. Because we need to rotate the sensor while calibrating. And the offsets would change after some time for MEMS sensor. That's what we called Bias Repeatability. So we need to calibrate them periodically. For your application, it is impossible to calibrate only once and use it permanently.
2. When you detect slight movements of bumper in car, do you have any requirements for the resolution of the detection?
