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rigidBodyTree

Create tree-structured robot

Description

The rigidBodyTree is a representation of the connectivity of rigid bodies with joints. Use this class to build robot manipulator models in MATLAB®. If you have a robot model specified using the Unified Robot Description Format (URDF), use importrobot to import your robot model.

A rigid body tree model is made up of rigid bodies as rigidBody objects. Each rigid body has a rigidBodyJoint object associated with it that defines how it can move relative to its parent body. Use setFixedTransform to define the fixed transformation between the frame of a joint and the frame of one of the adjacent bodies. You can add, replace, or remove rigid bodies from the model using the methods of the RigidBodyTree class.

Robot dynamics calculations are also possible. Specify the Mass, CenterOfMass, and Inertia properties for each rigidBody in the robot model. You can calculate forward and inverse dynamics with or without external forces and compute dynamics quantities given robot joint motions and joint inputs. To use the dynamics-related functions, set the DataFormat property to "row" or "column".

For a given rigid body tree model, you can also use the robot model to calculate joint angles for desired end-effector positions using the robotics inverse kinematics algorithms. Specify your rigid body tree model when using inverseKinematics or generalizedInverseKinematics.

The show method supports visualization of body meshes. Meshes are specified as .stl files and can be added to individual rigid bodies using addVisual. Also, by default, the importrobot function loads all the accessible .stl files specified in your URDF robot model.

Creation

Description

robot = rigidBodyTree creates a tree-structured robot object. Add rigid bodies to it using addBody.

example

robot = rigidBodyTree("MaxNumBodies",N,"DataFormat",dataFormat) specifies an upper bound on the number of bodies allowed in the robot when generating code. You must also specify the DataFormat property as a name-value pair.

Properties

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This property is read-only.

Number of bodies in the robot model (not including the base), returned as an integer.

This property is read-only.

List of rigid bodies in the robot model, returned as a cell array of handles. Use this list to access specific RigidBody objects in the model. You can also call getBody to get a body by its name.

This property is read-only.

Names of rigid bodies, returned as a cell array of character vectors.

Name of robot base, returned as a string scalar or character vector.

Gravitational acceleration experienced by robot, specified as an [x y z] vector in meters per second squared. Each element corresponds to the acceleration of the base robot frame in that direction.

Input/output data format for kinematics and dynamics functions, specified as "struct", "row", or "column". To use dynamics functions, you must use either "row" or "column".

Object Functions

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centerOfMassCenter of mass position and Jacobian
externalForceCompose external force matrix relative to base
forwardDynamicsJoint accelerations given joint torques and states
geometricJacobianGeometric Jacobian for robot configuration
gravityTorqueJoint torques that compensate gravity
inverseDynamicsRequired joint torques for given motion
massMatrixJoint-space mass matrix
velocityProductJoint torques that cancel velocity-induced forces
getTransformGet transform between body frames
homeConfigurationGet home configuration of robot
randomConfigurationGenerate random configuration of robot
checkCollisionCheck if robot is in collision
addBodyAdd body to robot
addSubtreeAdd subtree to robot
getBodyGet robot body handle by name
removeBodyRemove body from robot
replaceBodyReplace body on robot
replaceJointReplace joint on body
subtreeCreate subtree from robot model
copyCopy robot model
showShow robot model in figure
showdetailsShow details of robot model
writeAsFunctionCreate rigidBodyTree code generating function

Examples

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Add a rigid body and corresponding joint to a rigid body tree. Each rigidBody object contains a rigidBodyJoint object and must be added to the rigidBodyTree using addBody.

Create a rigid body tree.

rbtree = rigidBodyTree;

Create a rigid body with a unique name.

body1 = rigidBody('b1');

Create a revolute joint. By default, the rigidBody object comes with a fixed joint. Replace the joint by assigning a new rigidBodyJoint object to the body1.Joint property.

jnt1 = rigidBodyJoint('jnt1','revolute');
body1.Joint = jnt1;

Add the rigid body to the tree. Specify the body name that you are attaching the rigid body to. Because this is the first body, use the base name of the tree.

basename = rbtree.BaseName;
addBody(rbtree,body1,basename)

Use showdetails on the tree to confirm the rigid body and joint were added properly.

showdetails(rbtree)
--------------------
Robot: (1 bodies)

 Idx    Body Name   Joint Name   Joint Type    Parent Name(Idx)   Children Name(s)
 ---    ---------   ----------   ----------    ----------------   ----------------
   1           b1         jnt1     revolute             base(0)   
--------------------

Use the Denavit-Hartenberg (DH) parameters of the Puma560® robot to build a robot. Each rigid body is added one at a time, with the child-to-parent transform specified by the joint object.

The DH parameters define the geometry of the robot with relation to how each rigid body is attached to its parent. For convenience, setup the parameters for the Puma560 robot in a matrix [1]. The Puma robot is a serial chain manipulator. The DH parameters are relative to the previous row in the matrix, corresponding to the previous joint attachment.

dhparams = [0   	pi/2	0   	0;
            0.4318	0       0       0
            0.0203	-pi/2	0.15005	0;
            0   	pi/2	0.4318	0;
            0       -pi/2	0   	0;
            0       0       0       0];

Create a rigid body tree object to build the robot.

robot = rigidBodyTree;

Create the first rigid body and add it to the robot. To add a rigid body:

  1. Create a rigidBody object and give it a unique name.

  2. Create a rigidBodyJoint object and give it a unique name.

  3. Use setFixedTransform to specify the body-to-body transformation using DH parameters. The last element of the DH parameters, theta, is ignored because the angle is dependent on the joint position.

  4. Call addBody to attach the first body joint to the base frame of the robot.

body1 = rigidBody('body1');
jnt1 = rigidBodyJoint('jnt1','revolute');

setFixedTransform(jnt1,dhparams(1,:),'dh');
body1.Joint = jnt1;

addBody(robot,body1,'base')

Create and add other rigid bodies to the robot. Specify the previous body name when calling addBody to attach it. Each fixed transform is relative to the previous joint coordinate frame.

body2 = rigidBody('body2');
jnt2 = rigidBodyJoint('jnt2','revolute');
body3 = rigidBody('body3');
jnt3 = rigidBodyJoint('jnt3','revolute');
body4 = rigidBody('body4');
jnt4 = rigidBodyJoint('jnt4','revolute');
body5 = rigidBody('body5');
jnt5 = rigidBodyJoint('jnt5','revolute');
body6 = rigidBody('body6');
jnt6 = rigidBodyJoint('jnt6','revolute');

setFixedTransform(jnt2,dhparams(2,:),'dh');
setFixedTransform(jnt3,dhparams(3,:),'dh');
setFixedTransform(jnt4,dhparams(4,:),'dh');
setFixedTransform(jnt5,dhparams(5,:),'dh');
setFixedTransform(jnt6,dhparams(6,:),'dh');

body2.Joint = jnt2;
body3.Joint = jnt3;
body4.Joint = jnt4;
body5.Joint = jnt5;
body6.Joint = jnt6;

addBody(robot,body2,'body1')
addBody(robot,body3,'body2')
addBody(robot,body4,'body3')
addBody(robot,body5,'body4')
addBody(robot,body6,'body5')

Verify that your robot was built properly by using the showdetails or show function. showdetails lists all the bodies in the MATLAB® command window. show displays the robot with a given configuration (home by default). Calls to axis modify the axis limits and hide the axis labels.

showdetails(robot)
--------------------
Robot: (6 bodies)

 Idx    Body Name   Joint Name   Joint Type    Parent Name(Idx)   Children Name(s)
 ---    ---------   ----------   ----------    ----------------   ----------------
   1        body1         jnt1     revolute             base(0)   body2(2)  
   2        body2         jnt2     revolute            body1(1)   body3(3)  
   3        body3         jnt3     revolute            body2(2)   body4(4)  
   4        body4         jnt4     revolute            body3(3)   body5(5)  
   5        body5         jnt5     revolute            body4(4)   body6(6)  
   6        body6         jnt6     revolute            body5(5)   
--------------------
show(robot);
axis([-0.5,0.5,-0.5,0.5,-0.5,0.5])
axis off

Figure contains an axes object. The hidden axes object with xlabel X, ylabel Y contains 13 objects of type patch, line. These objects represent base, body1, body2, body3, body4, body5, body6.

References

[1] Corke, P. I., and B. Armstrong-Helouvry. “A Search for Consensus among Model Parameters Reported for the PUMA 560 Robot.” Proceedings of the 1994 IEEE International Conference on Robotics and Automation, IEEE Computer. Soc. Press, 1994, pp. 1608–13. DOI.org (Crossref), doi:10.1109/ROBOT.1994.351360.

Make changes to an existing rigidBodyTree object. You can get replace joints, bodies and subtrees in the rigid body tree.

Load an ABB IRB-120T manipulator from the Robotics System Toolbox™ loadrobot. It is specified as a rigidBodyTree object.

manipulator = loadrobot("abbIrb120T");

View the robot with show and read the details of the robot using showdetails.

show(manipulator);
showdetails(manipulator)

Get a specific body to inspect the properties. The only child of the link_3 body is the link_4 body. You can copy a specific body as well.

body3 = getBody(manipulator,"link_3");
childBody = body3.Children{1}
body3Copy = copy(body3);

Replace the joint on the link_3 body. You must create a new Joint object and use replaceJoint to ensure the downstream body geometry is unaffected. Call setFixedTransform if necessary to define a transform between the bodies instead of with the default identity matrices.

newJoint = rigidBodyJoint("prismatic");
replaceJoint(manipulator,"link_3",newJoint);

showdetails(manipulator)

Remove an entire body and get the resulting subtree using removeBody. The removed body is included in the subtree.

subtree = removeBody(manipulator,"link_4")
show(subtree);

Remove the modified link_3 body. Add the original copied link_3 body to the link_2 body, followed by the returned subtree. The robot model remains the same. See a detailed comparison through showdetails.

removeBody(manipulator,"link_3");
addBody(manipulator,body3Copy,"link_2")
addSubtree(manipulator,"link_3",subtree)

showdetails(manipulator)

To use dynamics functions to calculate joint torques and accelerations, specify the dynamics properties for the rigidBodyTree object and rigidBody.

Create a rigid body tree model. Create two rigid bodies to attach to it.

robot = rigidBodyTree('DataFormat','row');
body1 = rigidBody('body1');
body2 = rigidBody('body2');

Specify joints to attach to the bodies. Set the fixed transformation of body2 to body1. This transform is 1m in the x-direction.

joint1 = rigidBodyJoint('joint1','revolute');
joint2 = rigidBodyJoint('joint2');
setFixedTransform(joint2,trvec2tform([1 0 0]))
body1.Joint = joint1;
body2.Joint = joint2;

Specify dynamics properties for the two bodies. Add the bodies to the robot model. For this example, basic values for a rod (body1) with an attached spherical mass (body2) are given.

body1.Mass = 2;
body1.CenterOfMass = [0.5 0 0];
body1.Inertia = [0.001 0.67 0.67 0 0 0];

body2.Mass = 1;
body2.CenterOfMass = [0 0 0];
body2.Inertia = 0.0001*[4 4 4 0 0 0];

addBody(robot,body1,'base');
addBody(robot,body2,'body1');

Compute the center of mass position of the whole robot. Plot the position on the robot. Move the view to the xy plane.

comPos = centerOfMass(robot);

show(robot);
hold on
plot(comPos(1),comPos(2),'or')
view(2)

Figure contains an axes object. The axes object with xlabel X, ylabel Y contains 6 objects of type patch, line. One or more of the lines displays its values using only markers These objects represent base, body1, body2.

Change the mass of the second body. Notice the change in center of mass.

body2.Mass = 20;
replaceBody(robot,'body2',body2)

comPos2 = centerOfMass(robot);
plot(comPos2(1),comPos2(2),'*g')
hold off

Figure contains an axes object. The axes object with xlabel X, ylabel Y contains 7 objects of type patch, line. One or more of the lines displays its values using only markers These objects represent base, body1, body2.

Calculate the resultant joint accelerations for a given robot configuration with applied external forces and forces due to gravity. A wrench is applied to a specific body with the gravity being specified for the whole robot.

Load a KUKA iiwa 14 robot model from the Robotics System Toolbox™ loadrobot. The robot is specified as a rigidBodyTree object.

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

Set the gravity. By default, gravity is assumed to be zero.

kukaIIWA14 = loadrobot("kukaIiwa14",DataFormat="row",Gravity=[0 0 -9.81]);

Get the home configuration for the kukaIIWA14 robot.

q = homeConfiguration(kukaIIWA14);

Specify the wrench vector that represents the external forces experienced by the robot. Use the externalForce function to generate the external force matrix. Specify the robot model, the end effector that experiences the wrench, the wrench vector, and the current robot configuration. wrench is given relative to the "iiwa_link_ee_kuka" body frame, which requires you to specify the robot configuration, q.

wrench = [0 0 0.5 0 0 0.3];
fext = externalForce(kukaIIWA14,"iiwa_link_ee_kuka",wrench,q);

Compute the resultant joint accelerations due to gravity, with the external force applied to the end-effector "iiwa_link_ee_kuka" when kukaIIWA14 is at its home configuration. The joint velocities and joint torques are assumed to be zero (input as an empty vector []).

qddot = forwardDynamics(kukaIIWA14,q,[],[],fext)
qddot = 1×7

   -0.0023   -0.0112    0.0036   -0.0212    0.0067   -0.0075  499.9920

Use the inverseDynamics function to calculate the required joint torques to statically hold a specific robot configuration. You can also specify the joint velocities, joint accelerations, and external forces using other syntaxes.

Load an Omron eCobra-600 from the Robotics System Toolbox™ loadrobot, specified as a rigidBodyTree object. Set the gravity property and ensure the data format is set to "row". For all dynamics calculations, the data format must be either "row" or "column".

robot = loadrobot("omronEcobra600", DataFormat="row", Gravity=[0 0 -9.81]);

Generate a random configuration for robot.

q = randomConfiguration(robot);

Compute the required joint torques for robot to statically hold that configuration.

tau = inverseDynamics(robot,q)
tau = 1×4

    0.0000    0.0000  -19.6200         0

Use the externalForce function to generate force matrices to apply to a rigid body tree model. The force matrix is an m-by-6 vector that has a row for each joint on the robot to apply a six-element wrench. Use the externalForce function and specify the end effector to properly assign the wrench to the correct row of the matrix. You can add multiple force matrices together to apply multiple forces to one robot.

To calculate the joint torques that counter these external forces, use the inverseDynamics function.

Load a Universal Robots UR5e from the Robotics System Toolbox™ loadrobot, specified as a rigidBodyTree object. Update the gravity and set the data format to "row". For all dynamics calculations, the data format must be either "row" or "column"

manipulator = loadrobot("universalUR5e", DataFormat="row", Gravity=[0 0 -9.81]);
showdetails(manipulator)
--------------------
Robot: (10 bodies)

 Idx                Body Name                         Joint Name                         Joint Type                Parent Name(Idx)   Children Name(s)
 ---                ---------                         ----------                         ----------                ----------------   ----------------
   1                     base         base_link-base_fixed_joint                              fixed                    base_link(0)   
   2        base_link_inertia        base_link-base_link_inertia                              fixed                    base_link(0)   shoulder_link(3)  
   3            shoulder_link                 shoulder_pan_joint                           revolute            base_link_inertia(2)   upper_arm_link(4)  
   4           upper_arm_link                shoulder_lift_joint                           revolute                shoulder_link(3)   forearm_link(5)  
   5             forearm_link                        elbow_joint                           revolute               upper_arm_link(4)   wrist_1_link(6)  
   6             wrist_1_link                      wrist_1_joint                           revolute                 forearm_link(5)   wrist_2_link(7)  
   7             wrist_2_link                      wrist_2_joint                           revolute                 wrist_1_link(6)   wrist_3_link(8)  
   8             wrist_3_link                      wrist_3_joint                           revolute                 wrist_2_link(7)   flange(9)  
   9                   flange                     wrist_3-flange                              fixed                 wrist_3_link(8)   tool0(10)  
  10                    tool0                       flange-tool0                              fixed                       flange(9)   
--------------------

Get the home configuration for manipulator.

q = homeConfiguration(manipulator);

Set external force on shoulder_link. The input wrench vector is expressed in the base frame.

fext1 = externalForce(manipulator,"shoulder_link",[0 0 0.0 0.1 0 0]);

Set external force on the end effector, tool0. The input wrench vector is expressed in the tool0 frame.

fext2 = externalForce(manipulator,"tool0",[0 0 0.0 0.1 0 0],q);

Compute the joint torques required to balance the external forces. To combine the forces, add the force matrices together. Joint velocities and accelerations are assumed to be zero (input as []).

tau = inverseDynamics(manipulator,q,[],[],fext1+fext2)
tau = 1×6

   -0.0233  -52.4189  -14.4896   -0.0100    0.0100   -0.0000

You can import robots that have .stl files associated with the Unified Robot Description format (URDF) file to describe the visual geometries of the robot. Each rigid body has an individual visual geometry specified. The importrobot function parses the URDF file to get the robot model and visual geometries. The function assumes that visual geometry and collision geometry of the robot are the same and assigns the visual geometries as collision geometries of corresponding bodies.

Use the show function to display the visual and collision geometries of the robot model in a figure. You can then interact with the model by clicking components to inspect them and right-clicking to toggle visibility.

Import a robot model as a URDF file. The .stl file locations must be properly specified in this URDF. To add other .stl files to individual rigid bodies, see addVisual.

robot = importrobot('iiwa14.urdf');

Visualize the robot with the associated visual model. Click bodies or frames to inspect them. Right-click bodies to toggle visibility for each visual geometry.

show(robot,Visuals="on",Collisions="off");

Figure contains an axes object. The axes object with xlabel X, ylabel Y contains 29 objects of type patch, line. These objects represent 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, iiwa_link_0_mesh, iiwa_link_1_mesh, iiwa_link_2_mesh, iiwa_link_3_mesh, iiwa_link_4_mesh, iiwa_link_5_mesh, iiwa_link_6_mesh, iiwa_link_7_mesh.

Visualize the robot with the associated collision geometries. Click bodies or frames to inspect them. Right-click bodies to toggle visibility for each collision geometry.

show(robot,Visuals="off",Collisions="on"); 

Figure contains an axes object. The axes object with xlabel X, ylabel Y contains 29 objects of type patch, line. These objects represent 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, iiwa_link_0_mesh, iiwa_link_1_mesh, iiwa_link_2_mesh, iiwa_link_3_mesh, iiwa_link_4_mesh, iiwa_link_5_mesh, iiwa_link_6_mesh, iiwa_link_7_mesh, iiwa_link_0_coll_mesh, iiwa_link_1_coll_mesh, iiwa_link_2_coll_mesh, iiwa_link_3_coll_mesh, iiwa_link_4_coll_mesh, iiwa_link_5_coll_mesh, iiwa_link_6_coll_mesh, iiwa_link_7_coll_mesh.

More About

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References

[1] Craig, John J. Introduction to Robotics: Mechanics and Control. Reading, MA: Addison-Wesley, 1989.

[2] Siciliano, Bruno, Lorenzo Sciavicco, Luigi Villani, and Giuseppe Oriolo. Robotics: Modelling, Planning and Control. London: Springer, 2009.

Extended Capabilities

Version History

Introduced in R2016b

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