Determine whether user is moving or not.

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Hi All,

I am developing an android app that starts a timer when the user is moving. The timer is starting ass planned, using the accelerometer. The problem, however is to get the timer to stop when the user stops. This should be when the user is at a complete standstill. I am having difficulty with it because if you move too slow the timer stops, or sometimes it stops and I just don't even know why.

Here is my code, which is not working as planned:

Java

@Override
    public void onSensorChanged(SensorEvent event) {
        // get the change of the x,y,z values of the accelerometer
        deltaX = Math.abs(lastX - event.values[0]);
        deltaY = Math.abs(lastY - event.values[1]);
        deltaZ = Math.abs(lastZ - event.values[2]);

        //Zero out gravity on the accelerometer.
        gDeltaX = (float)((0.9 * gDeltaX) + (0.1 * deltaX));
        gDeltaY = (float)((0.9 * gDeltaY) + (0.1 * deltaY));
        gDeltaZ = (float)((0.9 * gDeltaZ) + (0.1 * deltaZ));

        deltaX = deltaX - gDeltaX;
        deltaY = deltaY - gDeltaY;
        deltaZ = deltaZ - gDeltaZ;

        //Make sure process starts only once
        if (flag == 0) {
            if ((deltaX > 10) || (deltaY > 10) || (deltaZ > 10)) {
                //Start the timer
                startTime = SystemClock.uptimeMillis();
                timeHandler.postDelayed(updateTimerMethod, 0);
                flag = 1;
            }
        } else if (flag == 1) {
            if ((deltaX < 1) && (deltaY < 1) && (deltaZ < 1)) {
                //Stop the timer.
                timeSwap += timeInMillies;
                timeHandler.removeCallbacks(updateTimerMethod);
                flag = 2;
            }
        }
    }

If there is any alternate way of stopping the timer, rather than using the accelerometer when it has no movement, I am all open to suggestions. I was thinking the the accelerometer can be the initiallizer, but what to use when stopping??

Posted 27-Jul-15 9:34am

Christopher Smit

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Sergey Alexandrovich Kryukov · Answer 1 · 2015-07-27T10:09:00

When you ask about "moving", the question would be: moving relative to what? :-)

Apparently, it is impossible to tell the speed by an accelerometer; it is prohibited by the fundamental physical principles: all phenomena in different reference system moving at uniform speeds go in the same way, so you cannot detect any difference between them from inside the system. In other words, you would detect the same acceleration in systems at different coordinates and speeds, it is principally impossible to tell one from another.

I understand that you are doing a different thing: really detecting acceleration, not speed. This would be enough to detect the moment of time when some motion has started, if the previous state was pretty much complete rest. But what happens when one stops? Generally, it's no different from starting to move: the device just experience different acceleration. This is not enough? Let's consider that your user never change the orientation of the device (a questionable assumption itself), and initial acceleration is positive, then, to come to complete stop, you would need equivalent amount of acceleration with negative sign. But what is "equivalent amount"? Quite apparently, not identical acceleration profile, but some acceleration with the same first integral of motion, speed. Note that you would have to integrate the motion during all the motion, without any stops. It cannot be done precisely, by many reasons. In all cases, integrated values may accumulate the errors.

Instead of trying to find out the speed, you may do something very different. For example, get a statistic of the average acceleration level during some period of time. In other words, detect the vibrations at the frequencies and amplitudes detectable by an accelerometer. Assume some low level of such vibrations as "no motion".

—SA