消除加速度计读数中的重力影响

Question

I am developing an application, in which I have used Accelerometer for sensing linear accelerations , but after some time I get to know that Accelerometer does not show linear accelerations ( also have gravity effect on it ).

So for removing gravity I have tried some method and reached the conclusion that I must have to design a filter.

And for designing filter I have tried the following code.

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];
 }

which is taken from Android Developers .

Now I have some problem in the code above:

1. What are alpha, t and dt?
2. How to get the value of t and dt and how to calculate them?
3. Also let me know that the code i am using for getting linear accelerations will work or not?

Answer 1

This method is 'damping' the variation of the sensor values. It's called "exponential smoothing" . In a fast-changing variable, you can find an estimation of the average by taking the latest value, and letting it count for a fraction (alpha).

This algorithm uses this filter to find the constant, which 'must' be gravity. Total acceleration = gravity + instantaneous acceleration, so the instantaneous acceleration can be calculated by event.values - gravity .

This works unless you're eg constantly accelerating (rocket launch, speeding up on highway, ...). The factor alpha tells you how fast the 'average' can change, and needs to be bigger when you want to measure faster-changing accelerations. This is reflected by the term dt : when smaller (higher event rate) with respect to the filter time constant, alpha becomes bigger. The time-constant tells you how fast the effect of a ripple is decaying in the average.