IMU Overview

Accelerometer

The accelerometer detects the instantaneous acceleration of the camera. The data provided by the accelerometer determines whether the camera is getting faster or slower, in any directions, with a precise value in meter per second squared (m/s²).

When an accelerometer is static, it is still measuring an acceleration of 9.8m/s² which corresponds to the force applied by the earth gravity. This force has always the same direction, from the camera to the center of the earth. The gravity allows to compute the camera’s absolute inclination and detect events like free falls.

Gyroscope

A gyroscope measures the angular velocity of the camera in degrees per second (deg/s). When combined with the accelerometer, both sensors can estimate the orientation of the camera at a high frequency.

Output Data

The following information is accessible from the camera sensor stack:

Output Data Description Units
Accelerometer
linear_acceleration Acceleration force applied on all three physical axes (x, y, and z), including the force of gravity. Values are corrected from bias, scale and misalignment. m/s2
linear_acceleration_uncalibrated Acceleration force applied on all three physical axes (x, y, and z), including the force of gravity. Values are uncalibrated. m/s2
linear_acceleration_covariance Measurement noise of the uncalibrated linear acceleration of the accelerometer. Provided as a 3x3 covariance matrix.
Gyroscope
angular_velocity Rate of rotation around each of the three physical axes (x, y, and z). Values are corrected from bias, scale and misalignment. deg/s
angular_velocity_uncalibrated Rate of rotation around each of the three physical axes (x, y, and z). Values are uncalibrated. deg/s
angular_velocity_covariance Measurement noise of the uncalibrated angular velocity of the gyroscope. Provided as a 3x3 covariance matrix.
Orientation
pose_covariance Measurement noise of the pose orientation. Provided as a 3x3 covariance matrix.
camera_imu_transform Transform between IMU and Left Camera frames.

Using the API

Accessing the IMU data can be done through the SensorData class. Data is stored in the class SensorData::IMUData which amongst others contains the accelerometer and gyroscope values.

You can retrieve IMU data with the following code:

SensorsData sensors_data;
SensorsData::IMUData imu_data;

// Grab new frames and retrieve sensors data
while(zed.grab() == SUCCESS){
  zed.getSensorsData(sensors_data, TIME_REFERENCE::IMAGE); // Retrieve only frame synchronized data

  // Extract IMU data
  imu_data = sensors_data.imu;

  // Retrieve linear acceleration and angular velocity
  float3 linear_acceleration = imu_data.linear_acceleration;
  float3 angular_velocity = imu_data.angular_velocity;
}
sensors_data = sl.SensorsData()
imu_data = sl.SensorsData.IMUData()

# Grab new frames and retrieve sensors data
while zed.grab() == sl.ERROR_CODE.SUCCESS :
  zed.get_sensors_data(sensors_data, sl.TIME_REFERENCE.IMAGE) # Retrieve only frame synchronized data

  # Extract IMU data
  imu_data = sensors_data.imu

  # Retrieve linear acceleration and angular velocity
  linear_acceleration = imu_data.linear_acceleration
  angular_velocity = imu_data.angular_velocity
SensorsData sensors_data = new SensorsData();
IMUData imu_data = new IMUData();

RuntimeParameters runtimeParameters = new RuntimeParameters();
// Grab new frames and retrieve sensors data
while(zed.Grab(ref runtimeParameters) == ERROR_CODE.SUCCESS){
  zed.GetInternalSensorsData(ref sensors_data, TIME_REFERENCE.IMAGE); // Retrieve only frame synchronized data

  // Extract IMU data
  imu_data = sensors_data.imu;

  // Retrieve linear acceleration and angular velocity
  float3 linear_acceleration = imu_data.linearAcceleration;
  float3 angular_velocity = imu_data.angularVelocity;
}

Pose

One the key reason of having an accelerometer and gyroscope in the camera it that their data can be fused to estimate camera orientation. The accelerometer provides gravity orientation, while the gyroscope estimates the rotation applied to the camera. Fused at high frequency, the combination of both sensors provides a robust orientation estimation.

IMU pose can be retrieved in imu_data.pose.

Code Example

For code example, check out the Getting Sensor Data tutorial.