Accelerometer and/or Gyroscope for gesture sensing

Question

I want to sense 3D motion (gesture). I have 3 axis accelerometer. But I am confused if I need a gyroscope or not for orientation and rotation sensing.

1. Isn't it possible to measure them from accelerometer alone? Because when the sensor rotates along one axis (say z-axis), there will be change in y-axis and x-axis output in accelerometer.

2. Is gyroscope really necessary or is it just easier if I have one?

Answer 1

Consider an accelerometer experiencing a net acceleration of gx - gz.

There are an infinite possible ways to achieve this, here are few examples:

1. The sensor is oriented with z pointing towards the sky, and the user is moving it laterally relative to the ground with acceleration g.
2. The sensor is oriented x pointing towards the ground, and the user is moving it laterally relative to the ground with acceleration -g.
3. The sensor x/z plane is oriented at a 45 degree angle, and the user is moving it in a down-ward/left direction.
4. The sensor is rotating about some point, moving with a non-constant velocity such that the acceleration is gx - gz.

etc.

So a single accelerometer cannot determine its orientation if it's experiencing acceleration and gravity.