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    标题：9.8 Android运动传感器开发

    目前，Android 平台支持的运动传感器包括以下 5 种：
    • TYPE_ACCELEROMETER。
    • TYPE_GRAVITY。
    • TYPE_GYROSCOPE。
    • TYPE_LINEAR_ACCELERATION。
    • TYPE_ROTATION_VECTOR。

    本节教程将对这几种传感器的用法做简单介绍。

    加速度传感器

    获取加速度传感器实例的代码如下： 
    1. private SensorManager mSensorManager;
    2. private Sensor mSensor;
    3. ...
    4. mSensorManager=(SensorManager)getSystemService(Context.SENSOR_SERVICE);
    5. mSensor=mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    从传感器获取数据并计算三个方向的加速度的代码如下： 
    1. public void onSensorChanged(SensorEvent event) {
    2.     // In this example, alpha is calculated as t / (t+dT) ,
    3.     // where t is the low-pass filter's time-constant and
    4.     // dT is the event delivery rate.
    5.  
    6.     final float alpha = 0.8;
    7.  
    8.     // Isolate the force of gravity with the low-pass filter.
    9.     gravity[0] = alpha * gravity[0] + (1 - alpha) * event.values[0];
    10.     gravity[1] = alpha * gravity[l] + (1 - alpha) * event.values[1];
    11.     gravity[2] = alpha * gravity[2] + (1 - alpha) * event.values[2];
    12.  
    13.     // Remove the gravity contribution with the high-pass filter.
    14.     linear_acceleration[0] = event.values[0] - gravity[0];
    15.     linear_acceleration[1] = event.values[1] - gravity[l];
    16.     linear_acceleration[2] = event.values[2] - gravity[2];
    17. }
    该计算方法仅是举例使用，实际计算方法要针对应用而确定。

    重力传感器

    重力传感器是加速度传感器的一种，其数据处理方式也相似。此处不再重复重力传感器的数据计算方法。获取重力传感器的代码如下： 
    1. private SensorManager mSensorManager;
    2. private Sensor mSensor;
    3. ...
    4. mSensorManager=(SensorManager)getSystemService(Context.SENSOR_SERVICE);
    5. mSensor=mSensorManager.getDefaultSensor(Sensor.TYPE_GRAVITY);

    陀螺仪

    陀螺仪可以在三个纬度上测量设备的旋转情况。获取陀螺仪传感器的代码如下： 
    1. private SensorManager mSensorManager;
    2. private Sensor mSensor;
    3. ...
    4. mSensorManager=(SensorManager)getSystemService(Context.SENSOR_SERVICE);
    5. mSensor=mSensorManager.getDefaultSensor(Sensor.TYPE_GYROSCOPE);
    从陀螺仪数据计算三个纬度旋转情况的代码如下： 
    1. // Create a constant to convert nanoseconds to seconds.
    2. private static final float NS2S = 1.0f / 1000000000.0f;
    3. private final float[] deltaRotationVector = new float[4]();
    4. private float timestamp;
    5.  
    6. public void onSensorChanged(SensorEvent event) {
    7.     // This timestep's delta rotation to be multiplied by the current rotation
    8.     // after computing it from the gyro sample data.
    9.     if (timestamp != 0) {
    10.         final float dT = (event.timestamp - timestamp) * NS2S;
    11.         // Axis of the rotation sample, not normalized yet.
    12.         float axisX = event.values[0];
    13.         float axisY = event.values[1];
    14.         float axisZ = event.values[2];
    15.  
    16.         // Calculate the angular speed of the sample
    17.         float omegaMagnitude = sqrt(axisX * axisX + axisY * axisY + axisZ * axisZ);
    18.  
    19.         // Normalize the rotation vector if it's big enough to get the axis
    20.         // (that is, EPSILON should represent your maximum allowable margin of error)
    21.         if (omegaMagnitude > EPSILON) {
    22.             axisX /= omegaMagnitude;
    23.             axisY /= omegaMagnitude;
    24.             axisZ /= omegaMagnitude;
    25.         }
    26.         // Integrate around this axis with the angular speed by the timestep
    27.         // in order to get a delta rotation from this sample over the timestep
    28.         // We will convert this axis-angle representation of the delta rotation
    29.         // into a quaternion before turning it into the rotation matrix.
    30.         float thetaOverTwo = omegaMagnitude * dT / 2. Of;
    31.         float sinThetaOverTwo = sin(thetaOverTwo);
    32.         float cosThetaOverTwo = cos(thetaOverTwo);
    33.         deltaRotationVector[0] = sinThetaOverTwo * axisX;
    34.         deltaRotationVector[1] = sinThetaOverTwo * axisY;
    35.         deltaRotationVector[2] = sinThetaOverTwo * axisZ;
    36.         deltaRotationVector[3] = cosThetaOverTwo;
    37.     }
    38.     timestamp = event.timestamp;
    39.     float[] deltaRotationMatrix = new float[9];
    40.     SensorManager.getRotationMatrixFromVector(deltaRotationMatrix, deltaRotationVector);
    41.     // User code should concatenate the delta rotation we computed with the current rotation
    42.     // in order to get the updated rotation.
    43.     // rotationCurrent=rotationCurrent * deltaRotationMatrix;
    44. }

    线性加速度传感器

    线性加速度传感器是传感器的一种。其获取实例的代码如下： 
    1. private SensorManager mSensorManager;
    2. private Sensor mSensor;
    3. ...
    4. mSensorManager=(SensorManager)getSystemService(Context.SENSOR_SERVICE);
    5. mSensor=mSensorManager.getDefaultSensor(Sensor.TYPE_LINEAR_ACCELERATION);

    旋转向量传感器

    旋转向量传感器能反映出当前设备的状态，其返回值是旋转角度与旋转轴的集合。获取旋转向量传感器实例的相关代码如下： 
    1. private SensorManager mSensorManager;
    2. private Sensor mSensor;
    3. ....
    4. mSensorManager=(SensorManager)getSystemService(Context.SENSOR_SERVICE);
    5. mSensor=mSensorManager.getDefaultSensor(Sensor.TYPE_ROTATION_VECTOR);

    目录

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