CODEWITHSUNDEEP
javaandroidaccelerometer
Vivian
Vivian

Reputation: 1061

How to use the accelerometer to detect virbration above 1Gs?

I'm doing android application which is something like a car blackbox which records the traveling process of the car.

But now I'm face with the problem of how am i going to integrate an accelerometer which is capable of detecting slight movement (Probably > 1Gs) when an accident occur it could trigger the video recording to stop and saving it to the Archive file, thus not losing the file as a result to the accident.. Anyone knows how to do the above mention task to monitor any forms of vibration?

I'm new to android/java could someone help guide me along? Thanks in advance...

This is part of the section of the video recording but now how am i going to incorporate accelerometer for "Auto-Archiving" purposes?

Upvotes: 3

Views: 2811

Answers (1)

Aleadam
Aleadam

Reputation: 40391

You need to use onSensorChanged(SensorEvent event) callback of the interface SensorEventListener

Get the details of the SensorEvent class here: http://developer.android.com/reference/android/hardware/SensorEvent.html

There's an example on the IBM's developerWorks page: http://www.ibm.com/developerworks/opensource/library/os-android-sensor/index.html

From the android reference pages:

public final float[]

values Since: API Level 3

The length and contents of the values array depends on which sensor type is being monitored (see also SensorEvent for a definition of the coordinate system used). Sensor.TYPE_ACCELEROMETER: All values are in SI units (m/s^2)

values[0]: Acceleration minus Gx on the x-axis

values[1]: Acceleration minus Gy on the y-axis

values[2]: Acceleration minus Gz on the z-axis

A sensor of this type measures the acceleration applied to the device (Ad). Conceptually, it does so by measuring forces applied to the sensor itself (Fs) using the relation: Ad = - ∑Fs / mass

In particular, the force of gravity is always influencing the measured acceleration: Ad = -g - ∑F / mass

For this reason, when the device is sitting on a table (and obviously not accelerating), the accelerometer reads a magnitude of g = 9.81 m/s^2

Similarly, when the device is in free-fall and therefore dangerously accelerating towards to ground at 9.81 m/s^2, its accelerometer reads a magnitude of 0 m/s^2.

It should be apparent that in order to measure the real acceleration of the device, the contribution of the force of gravity must be eliminated. This can be achieved by applying a high-pass filter. Conversely, a low-pass filter can be used to isolate the force of gravity.

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

      final float alpha = 0.8;

      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];

      linear_acceleration[0] = event.values[0] - gravity[0];
      linear_acceleration[1] = event.values[1] - gravity[1];
      linear_acceleration[2] = event.values[2] - gravity[2];
 }

Upvotes: 1

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