centerOfMass

Center of mass position and Jacobian

Syntax

com = centerOfMass(robot)

com = centerOfMass(robot,configuration)

[com,comJac] = centerOfMass(robot,configuration)

Description

com = centerOfMass(robot) computes the center of mass position of the robot model at its home configuration, relative to the base frame.

com = centerOfMass(robot,configuration) computes the center of mass position of the robot model at the specified joint configuration, relative to the base frame.

[com,comJac] = centerOfMass(robot,configuration) also returns the center of mass Jacobian, which relates the center of mass velocity to the joint velocities.

Examples

Calculate Center of Mass and Jacobian for Robot Configuration

Load a KUKA LBR iiwa robot model from the Robotics System Toolbox™ loadrobot. This is specified as a rigidBodyTree object.

lbr = loadrobot("kukaIiwa14")

lbr = 
  rigidBodyTree with properties:

     NumBodies: 10
        Bodies: {[1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]  [1×1 rigidBody]}
          Base: [1×1 rigidBody]
     BodyNames: {'iiwa_link_0'  'iiwa_link_1'  'iiwa_link_2'  'iiwa_link_3'  'iiwa_link_4'  'iiwa_link_5'  'iiwa_link_6'  'iiwa_link_7'  'iiwa_link_ee'  'iiwa_link_ee_kuka'}
      BaseName: 'world'
       Gravity: [0 0 0]
    DataFormat: 'struct'
    FrameNames: {'world'  'iiwa_link_0'  'iiwa_link_1'  'iiwa_link_2'  'iiwa_link_3'  'iiwa_link_4'  'iiwa_link_5'  'iiwa_link_6'  'iiwa_link_7'  'iiwa_link_ee'  'iiwa_link_ee_kuka'}

Set the data format to "row". For all dynamics calculations, the data format must be either "row" or "column".

lbr.DataFormat = "row";

Compute the center of mass position and Jacobian at the home configuration of the robot.

[comLocation,comJac] = centerOfMass(lbr);
show(lbr);
hold on
plot3(comLocation(1),comLocation(2),comLocation(3),Marker="x",MarkerSize=30,LineWidth=5);
hold off

Input Arguments

robot — Robot model

rigidBodyTree object

Robot model, specified as a rigidBodyTree object. To use the centerOfMass function, set the DataFormat property to either 'row' or 'column'.

configuration — Robot configuration

vector

Robot configuration, specified as a vector with positions for all nonfixed joints in the robot model. You can generate a configuration using homeConfiguration(robot), randomConfiguration(robot), or by specifying your own joint positions. To use the vector form of configuration, set the DataFormat property for the robot to either 'row' or 'column'.

Output Arguments

com — Center of mass location

[x y z] vector

Center of mass location, returned as an [x y z] vector. The vector describes the location of the center of mass for the specified configuration relative to the body frame, in meters.

comJac — Center of mass Jacobian

3-by-n matrix

Center of mass Jacobian, returned as a 3-by-n matrix, where n is the robot velocity degrees of freedom.

References

[1] Featherstone, Roy. Rigid Body Dynamics Algorithms. Springer US, 2008. DOI.org (Crossref), doi:10.1007/978-1-4899-7560-7.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

When creating the rigidBodyTree object, use the syntax that specifies the MaxNumBodies as an upper bound for adding bodies to the robot model. You must also specify the DataFormat property as a name-value pair. For example:

robot = rigidBodyTree("MaxNumBodies",15,"DataFormat","row")

To minimize data usage, limit the upper bound to a number close to the expected number of bodies in the model. All data formats are supported for code generation. To use the dynamics functions, the data format must be set to "row" or "column".

The show and showdetails functions do not support code generation.

Version History

Introduced in R2017a

R2024a: Static memory allocation support

centerOfMass now supports code generation with disabled dynamic memory allocation. For more information about disabling dynamic memory allocation, see Set Dynamic Memory Allocation Threshold (MATLAB Coder).