How to know if Android device is Flat on table

Question

I'm using accelometer sensor to detect whether my device is flat on a table or not. The weird thing is even when I put my phone flat or rotate it on it's side the value is always between 90 and 100! This shouldn't be correct! am I missing something? Here is my code:

   float[] values = event.values;
    // Movement
    float x = values[0];
    float y = values[1];
    float z = values[2];
    float norm_Of_g =(float) Math.sqrt(x * x + y * y + z * z);

    // Normalize the accelerometer vector
    x = (x / norm_Of_g);
    y = (y / norm_Of_g);
    z = (z / norm_Of_g);
    int inclination = (int) Math.round(Math.toDegrees(Math.acos(y)));
    Log.i("tag","incline is:"+inclination);

    if (inclination < 25 || inclination > 155)
    {
        // device is flat
        Toast.makeText(this,"device flat - beep!",Toast.LENGTH_SHORT);
    }

Edit: I'm using this code : How to measure the tilt of the phone in XY plane using accelerometer in Android

you should use `log.i(,)` and not `System.out` which is for the terminal — Skaros Ilias, Jun 19 '15 at 22:27
Accelerometers are often more sensitive than humans. I had to disable my laptop's accelerometer because it was feeding unwanted input to many applications even when I just pressed keys. — o11c, Jun 19 '15 at 22:29
Also `System.out` is line-buffered by default, so output is only visible when a newline is sent. — o11c, Jun 19 '15 at 22:31
okay I used Log and when my device is flat it's always a value between 90 - 100. based on math that shouldnt be correct — Hirad Roshandel, Jun 19 '15 at 22:39
I never claimed to know anything about this, but in that case consider these values as a "flat" condition. maybe the table is not completely right, of course what @o11c said makes sense to me. — Skaros Ilias, Jun 19 '15 at 22:53
Could you be looking for orientation rather than accelerometer? — tachyonflux, Jun 19 '15 at 22:55
@HiradRoshandel how do you know the orientation of your axes? — o11c, Jun 19 '15 at 22:59
@o11c im using this http://stackoverflow.com/questions/11175599/how-to-measure-the-tilt-of-the-phone-in-xy-plane-using-accelerometer-in-android/15149421#15149421 — Hirad Roshandel, Jun 20 '15 at 00:39
Consider using https://developers.google.com/android/reference/com/google/android/gms/location/DetectedActivity#STILL instead. I'm guessing you don't really care about the orientation of the device (I often have phones sitting on my desk at an angle). — James Moore, Nov 19 '18 at 19:42

score 2 · Accepted Answer · edited May 23 '17 at 11:43

You're using the y-axis instead of the z-axis as used in the answer you linked.

The value of acos will be near-zero when the argument is near one (or near 180 degrees when near negative one), as seen in this picture:

Arccos(x)

As such, your inclination will be near zero (or 180) degrees only when the y axis is normalized to about one or negative one, eg when it is parallel to gravity, (thus, the device is "standing up").

If there's no other error, simply switching from:

int inclination = (int) Math.round(Math.toDegrees(Math.acos(y)));

to

int inclination = (int) Math.round(Math.toDegrees(Math.acos(z)));

should do it.

score 0 · Answer 2 · answered Jun 20 '15 at 05:51

I used the code on this page Motion Sensors

public void onSensorChanged(SensorEvent event){
  // In this example, alpha is calculated as t / (t + dT),
  // where t is the low-pass filter's time-constant and
  // dT is the event delivery rate.

  final float alpha = 0.8;

  // Isolate the force of gravity with the low-pass filter.
  gravity[0] = alpha * gravity[0] + (1 - alpha) * event.values[0];
  gravity[1] = alpha * gravity[1] + (1 - alpha) * event.values[1];
  gravity[2] = alpha * gravity[2] + (1 - alpha) * event.values[2];

  // Remove the gravity contribution with the high-pass filter.
  linear_acceleration[0] = event.values[0] - gravity[0];
  linear_acceleration[1] = event.values[1] - gravity[1];
  linear_acceleration[2] = event.values[2] - gravity[2];
}

The intention of this is to factor out the force of gravity (over repeated measurements) in the accelerometer values, leaving just the acceleration component in each direction.

The force of gravity is measured along each axis, so once you know which axis represents the device lying flat, you can just check if the majority of the force of gravity lays in just that axis. That would mean the device is laying flat on the table.

You can also look at the linear acceleration to make sure the device isn't moving.

thanks but how do you get the array gravity[] ? how is it initialized — Hirad Roshandel, Jun 20 '15 at 19:55
i just initialized it to 0. it stabilizes pretty quickly private float[] gravity = new float[]{0,00}; — dangVarmit, Jun 20 '15 at 21:51

How to know if Android device is Flat on table

2 Answers2