Remove gravity from accelerometer data

Subtracts the rotated gravity vector from raw accelerometer data, leaving only dynamic (linear) acceleration. The gravity direction is determined by rotating the reference gravity vector (0, 0, -g) from the world frame into the sensor frame using the provided orientation quaternions.

Usage

removeGravity(accel, orientation, g = 9.81)

Arguments

accel: Numeric matrix (n x 3) of raw accelerometer readings in m/s^2.
orientation: A data.frame or matrix containing orientation quaternions. If a data.frame, must contain columns q_w, q_x, q_y, q_z (as returned by estimateOrientation()). If a matrix, must have 4 columns (w, x, y, z).
g: Numeric. Gravitational acceleration magnitude (default 9.81 m/s^2).

Value

Numeric matrix (n x 3) of dynamic (gravity-free) acceleration.

Details

For each time step, the gravity vector in the world frame (0, 0, -g) is rotated into the sensor frame using the conjugate of the orientation quaternion. This rotated gravity is then subtracted from the raw accelerometer reading.

References

Madgwick SOH, Harrison AJL, Vaidyanathan R (2011). "Estimation of IMU and MARG orientation using a gradient descent algorithm." IEEE International Conference on Rehabilitation Robotics.

Examples

n <- 100
accel <- matrix(c(rep(0, n), rep(0, n), rep(-9.81, n)), ncol = 3)
gyro <- matrix(0, nrow = n, ncol = 3)
ori <- estimateOrientation(accel, gyro, sampling_rate = 100)
dyn_accel <- removeGravity(accel, ori)