estimateGravityRotation

Estimate gravity rotation using IMU measurements and factor graph optimization

Since R2023a

Syntax

[gRot,info] = estimateGravityRotation(poses,gyroscopeReadings,accelerometerReadings,Name=Value)

Description

The estimateGravityRotation function estimates the gravity rotation that helps transform input poses to the local navigation reference frame of IMU using IMU measurements and factor graph optimization. The gravity rotation transforms the gravity vector from the local navigation reference frame of IMU to the pose reference frame.

The accelerometer measurements contain constant gravity acceleration that does not contribute to motion. You must remove this from the measurements for accurate fusion with other sensor data. The input pose reference frame may not always match the local navigation reference frame of IMU, North-East-Down (NED) or East-North-Up (ENU) in which the gravity direction is known. So, it is necessary to transform the input poses to the local navigation frame to remove the known gravity effect. The estimated rotation helps align the input pose reference frame and local navigation reference frame of IMU.

[gRot,info] = estimateGravityRotation(poses,gyroscopeReadings,accelerometerReadings,Name=Value) estimates the rotation required to transform the gravity vector from the local navigation reference frame of IMU (NED or ENU) to the input pose reference frame.

Note

Input poses must be in the initial IMU reference frame unless you specify the SensorTransform name-value argument, then the poses can be in a different frame.

Examples

Estimate Gravity Rotation and Direction Using IMU Measurements and Factor Graph Optimization

Specify input poses in the first camera pose reference frame.

poses = [0.1 0 0 0.7071 0 0 0.7071; ...
         0.1 0.4755 -0.1545 0.7071 0 0 0.7071];

Specify 10 gyroscope and accelerometer readings between consecutive camera frames.

accelReadings = repmat([97.9887 -3.0315 -22.0285],10,1);
gyroReadings = zeros(10,3);

Specify IMU parameters.

params = factorIMUParameters(SampleRate=100, ...
                             ReferenceFrame="NED");

Specify a transformation consisting of 3-D translation and rotation to transform input poses from the initial camera pose reference frame to the initial IMU pose reference frame. Initial sensor reference frame has first sensor pose at it's origin.

sensorTransform = se3(eul2rotm([-pi/2 0 0]),[0 0.1 0]);

Specify factor graph solver options.

opts = factorGraphSolverOptions(MaxIterations=50);

Estimate gravity rotation using IMU measurements between camera pose estimates.

[gRot,solutionInfo] = estimateGravityRotation(poses, ...
                      {gyroReadings},{accelReadings}, ...
                      IMUParameters=params, ...
                      SensorTransform=sensorTransform, ...
                      SolverOptions=opts)

gRot = 3×3

    0.0058   -0.6775   -0.7355
    0.1023    0.7320   -0.6736
    0.9947   -0.0713    0.0736

solutionInfo = struct with fields:
             InitialCost: 2.3123e+03
               FinalCost: 19.0067
      NumSuccessfulSteps: 23
    NumUnsuccessfulSteps: 18
               TotalTime: 0.0021
         TerminationType: 0
        IsSolutionUsable: 1

Use gravity rotation to transform gravity vector from local navigation frame to initial camera pose reference frame.

% gravity direction in NED frame is along Z-Axis.
gravityDirectionNED = [0; 0; 1];
% gravity direction in pose reference frame.
gravityDirection = gRot*gravityDirectionNED

gravityDirection = 3×1

   -0.7355
   -0.6736
    0.0736

% gravity vector in pose reference frame.
gravityMagnitude = 9.81;
gravity = gravityDirection*gravityMagnitude

gravity = 3×1

   -7.2149
   -6.6076
    0.7221

Input Arguments

poses — Camera or lidar poses

N-by-7 matrix | array of se3 objects | table | array of rigidtform3d objects

Camera or lidar poses, with similar metric units as IMU measurements estimated by stereo-visual-inertial or lidar-inertial systems, respectively, specified as one of these pose types:

• N-by-7 matrix, where each row is of the form [x y z qw qx qy qz]. Each row defines the xyz-position, in meters, and quaternion orientation, [qw qx qy qz].

• Array of se3 objects.

• Camera pose table returned by the poses (Computer Vision Toolbox) function of the imageviewset (Computer Vision Toolbox) object.

• Array of rigidtform3d (Image Processing Toolbox) objects.

gyroscopeReadings — Gyroscope readings between consecutive camera views or poses

cell array of N-by-3 matrices

Gyroscope readings between consecutive camera views or poses, specified as a cell array of N-by-3 matrices, in radians per second. N is the number of samples, and the three columns of gyroscopeReadings represent the [x y z] measurements.

Data Types: cell

accelerometerReadings — Accelerometer readings between consecutive camera views or poses

cell array of N-by-3 matrices

Accelerometer readings between consecutive camera views or poses, specified as a cell array of N-by-3 matrices, in meters per second squared. N is the number of samples, and the three columns of accelerometerReadings represent the [x y z] measurements.

Data Types: cell

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Example: estimateGravityRotation(poses,gyroscopeReadings,accelerometerReadings,IMUParameters=factorIMUParameters(SampleRate=100)) estimates the gravity rotation based on an IMU.

IMUParameters — IMU parameters

factorIMUParameters() (default) | factorIMUParameters object

IMU parameters, specified as a factorIMUParameters object.

Example: IMUParameters=factorIMUParameters(SampleRate=100)

SolverOptions — Solver options

factorGraphSolverOptions() (default) | factorGraphSolverOptions object

Solver options, specified as a factorGraphSolverOptions object.

Example: SolverOptions=factorGraphSolverOptions(MaxIterations=50)

SensorTransform — Transformation consisting of 3-D translation and rotation to transform pose or point in input pose reference frame to initial IMU frame

se3() (default) | se3 object

Transformation consisting of 3-D translation and rotation to transform a quantity like a pose or a point in the input pose reference frame to the initial IMU sensor reference frame, specified as a se3 object.

For example, if the input poses are camera poses in the initial camera sensor reference frame, then the sensor transform rotates and translates a pose or a point in the initial camera sensor reference frame to the initial IMU sensor reference frame. The initial sensor reference frame has the very first sensor pose at its origin.

Example: SensorTransform=se3(eul2rotm([-pi/2,0,0]),[0,0.1,0])

Output Arguments

gRot — Gravity rotation

3-by-3 matrix | se3 object | rigidtform3d object

Gravity rotation, returned as a 3-by-3 matrix, se3 object, or rigidtform3d (Image Processing Toolbox) object similar to input pose type. It contains the rotation required to transform the gravity vector from the local navigation reference frame of IMU (NED or ENU) to the input pose reference frame.

info — Factor graph optimization solution information

structure

Factor graph optimization solution information, returned as a structure. The fields of the structure are:

Field	Description
InitialCost	Initial cost of the non-linear least squares problem formulated by the factor graph before the optimization.
FinalCost	Final cost of the non-linear least squares problem formulated by the factor graph after the optimization. Note Cost is the sum of error terms, known as residuals, where each residual is a function of a subset of factor measurements.
NumSuccessfulSteps	Number of iterations in which the solver decreases the cost. This value includes the initialization iteration at 0 in addition to the minimizer iterations.
NumUnsuccessfulSteps	Number of iterations in which the iteration is numerically invalid or the solver does not decrease the cost.
TotalTime	Total solver optimization time in seconds.
TerminationType	Termination type as an integer in the range [0, 2]: 0 — Solver found a solution that meets convergence criterion and decreases in cost after optimization. 1 — Solver could not find a solution that meets convergence criterion after running for the maximum number of iterations. 2 — Solver terminated due to an error.
IsSolutionUsable	Solution is usable if 1 (true), not usable if 0 (false).

Use this information to check whether the optimization required for alignment has converged or not.

Data Types: struct

References

[1] Campos, Carlos, Richard Elvira, Juan J. Gomez Rodriguez, Jose M. M. Montiel, and Juan D. Tardos. “ORB-SLAM3: An Accurate Open-Source Library for Visual, Visual–Inertial, and Multimap SLAM.” IEEE Transactions on Robotics 37, no. 6 (December 2021): 1874–90. https://doi.org/10.1109/TRO.2021.3075644.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2023a