Accelerometer orientation detection

[bergo]

Hello Freescalers,

I'm using MMA8453q accelerometer in an embedded project,

I would like to know when the device is facing down (Z axis facing down),

After reading the relevant application note (AN4068) I'm still in the dark about this issue, and was hoping I could get some help here.

Just reading positive acceleration on Z axis is not enough (positive Z axis -> g force when device is upside down), as this can be happening when device is on it's side and in a car (for example).

Thanks a lot in advance,

[TomasVaverka · Accepted solution]

Bergo,

Accelerometers generally measure both gravity and linear acceleration. Advanced accelerometers usually use either a low-pass filter or a debounce counter to filter out fast transitions.

The MMA8453Q is a smart accelerometer that uses the debounce counter to eliminate the effects of high frequency motion for orientation detection. Chapter 3.6.2 in the AN4068 describes it in detail.

Note that the MMA8453Q has a fixed back/front trip angle (±75°). Only the MMA8451Q allows to choose between four different trip angles.

Regards,

Tomas