Redundant motion sensors on Android?


I'm developing a tool which receives motion sensor data and sends it to a machine learning algorithm, which ultimately will deduce different types of movement.

I read the Motion sensor guide and it seems like there is some redundancy in the data you can get from the sensors. For example: the accelrometer data contains gravity data and the linear acceleration data shows acceleration without acceleration due to gravity.

So my question is: do i really need all the sensors to get all forms of motion or can I give up some of them?

EDIT: (clarifying the question)

I want to collect the minimal data that will allow me to deduce the same things. What I'm looking for is user behavior: the angle which the user holds his phone, the way the user moves while using his phone, etc..

The answer I'm looking for should include the sets of sensors that have high correlation within them, such that only some of the sensors in this set are required to deduce the same type of motion\movement\rotation\acceleration\etc..

The term "Motion" in the question have no precise meaning. So I answer more generally.

"The way one holds his phone" is nothing but the orientation of the phone.There are three sensors which individually tells the orientation of the phone.

  1. Accelerometer sensor
  2. Orientation sensor
  3. Rotation Vector sensor

Among them only the accelerometer is physical sensor and other two are virtual sensors (they don't have special piece of hardware, they use accelerometer data and report the orientation in different formats).

  1. The orientation sensor is deprecated so you can't use it.
  2. Rotation vector sensor tells the orientation encoded in a quaternion. If your code is based on quaternions then normalize the sensor output using SensorManager.getQuaternionFromVector() and continue. If your code is based on rotation matrix then obtain rotation matrix by calling SensorManager.getRotationMatrixFromVector() passing sensor output and continue. If you want the orientation alone get it by calling SensorManager.getOrientation() passing rotation matrix obtained previously.
  3. Using accelerometer sensor we can find the orientation, but the recommended approach is to combine it with magnetic field sensor output. Call SensorManager.getRotationMatrix() by passing the output of accelerometer output and magnetic field sensor output and get the rotation matrix. If your code is based on rotation matrix, just continue. If you want the orientation alone get it by calling SensorManager.getOrientation() passing rotation matrix obtained in previously. If your code is based on quaternion call SensorManager.getQuaternionFromVector() by passing rotation vector (orientation) obtained previously.

"The way one moves his phone" - Here I consider four motions.

  1. Change of position (Simple translation) and rate change of position (velocity) - No sensor to detect them.
  2. Rate of change of velocity (Simple acceleration) - Accelerometer detects it. But it also contains the gravity component. Normally we need acceleration without gravity component. This could be calculated simply as explained here. However there is another virtual sensor called Linear Acceleration which does the job for us.
  3. Change of orientation (Rotation) - Whenever the orientation changes the accelerometer, orientation and rotation vector sensors report us (gyroscope also reports, but is explained in next point). How to use this sensor to get the current orientation is explained in first part of the answer.
  4. Rate of change of orientation (Angular velocity) - Whenever the orientation changes the gyroscope sensor reports. The output is three numbers representing angular acceleration along x, y and z axes. The unit is radians per second.

Output of the gyroscope sensors is not accurate in long term and the output of accelerometer is not accurate in short term, so combine them to get steady output. For details see this question.

Now it is clear that the gyroscope and accelerometer is required in minimum. However using wide range of sensors minimizes our work.