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rigidBodyJoint

Create a joint

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Description

The rigidBodyJoint objects defines how a rigid body moves relative to an attachment point. In a tree-structured robot, a joint always belongs to a specific rigid body, and each rigid body has one joint.

The rigidBodyJoint object can describe joints of various types. When building a rigid body tree structure with rigidBodyTree, you must assign the Joint object to a rigid body using the rigidBody class.

The different joint types supported are:

  • fixed — Fixed joint that prevents relative motion between two bodies.

  • revolute — Single degree of freedom (DOF) joint that rotates around a given axis. Also called a pin or hinge joint.

  • prismatic — Single DOF joint that slides along a given axis. Also called a sliding joint.

Each joint type has different properties with different dimensions, depending on its defined geometry.

Creation

Syntax

jointObj = rigidBodyJoint(jname)
jointObj = rigidBodyJoint(jname,jtype)

Description

jointObj = rigidBodyJoint(jname) creates a fixed joint with the specified name.

example

jointObj = rigidBodyJoint(jname,jtype) creates a joint of the specified type with the specified name.

Input Arguments

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Joint name, specified as a string scalar or character vector. The joint name must be unique to access it off the rigid body tree.

Example: "elbow_right"

Data Types: char | string

Joint type, specified as a string scalar or character vector. The joint type predefines certain properties when creating the joint.

The different joint types supported are:

  • fixed — Fixed joint that prevents relative motion between two bodies.

  • revolute — Single degree of freedom (DOF) joint that rotates around a given axis. Also called a pin or hinge joint.

  • prismatic — Single DOF joint that slides along a given axis. Also called a sliding joint.

Example: "prismatic"

Data Types: char | string

Properties

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This property is read-only.

Joint type, returned as a string scalar or character vector. The joint type predefines certain properties when creating the joint.

The different joint types supported are:

  • fixed — Fixed joint that prevents relative motion between two bodies.

  • revolute — Single degree of freedom (DOF) joint that rotates around a given axis. Also called a pin or hinge joint.

  • prismatic — Single DOF joint that slides along a given axis. Also called a sliding joint.

If the rigid body that contains this joint is added to a robot model, the joint type must be changed by replacing the joint using replaceJoint.

Example: "prismatic"

Data Types: char | string

Joint name, returned as a string scalar or character vector. The joint name must be unique to access it off the rigid body tree. If the rigid body that contains this joint is added to a robot model, the joint name must be changed by replacing the joint using replaceJoint.

Example: "elbow_right"

Data Types: char | string

Position limits of the joint, specified as a vector of [min max] values. Depending on the type of joint, these values have different definitions.

  • fixed — [NaN NaN] (default). A fixed joint has no joint limits. Bodies remain fixed between each other.

  • revolute — [-pi pi] (default). The limits define the angle of rotation around the axis in radians.

  • prismatic — [-0.5 0.5] (default). The limits define the linear motion along the axis in meters.

Example: [-pi/2, pi/2]

Home position of joint, specified as a scalar that depends on your joint type. The home position must fall in the range set by PositionLimits. This property is used by homeConfiguration to generate the predefined home configuration for an entire rigid body tree.

Depending on the joint type, the home position has a different definition.

  • fixed — 0 (default). A fixed joint has no relevant home position.

  • revolute — 0 (default). A revolute joint has a home position defined by the angle of rotation around the joint axis in radians.

  • prismatic — 0 (default). A prismatic joint has a home position defined by the linear motion along the joint axis in meters.

Example: pi/2 radians for a revolute joint

Axis of motion for joint, specified as a three-element unit vector. The vector can be any direction in 3-D space in local coordinates.

Depending on the joint type, the joint axis has a different definition.

  • fixed — A fixed joint has no relevant axis of motion.

  • revolute — A revolute joint rotates the body in the plane perpendicular to the joint axis.

  • prismatic — A prismatic joint moves the body in a linear motion along the joint axis direction.

Example: [1 0 0] for motion around the x-axis for a revolute joint

This property is read-only.

Fixed transform from joint to parent frame, returned as a 4-by-4 homogeneous transform matrix. The transform converts the coordinates of points in the joint predecessor frame to the parent body frame.

Example: eye(4)

This property is read-only.

Fixed transform from child body to joint frame, returned as a 4-by-4 homogeneous transform matrix. The transform converts the coordinates of points in the child body frame to the joint successor frame.

Example: eye(4)

Object Functions

copyCreate copy of joint
setFixedTransformSet fixed transform properties of joint

Examples

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Open Live Script

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)   
--------------------
Open Live Script

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

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.

Open Live Script

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

Load example robots as rigidBodyTree objects.

load exampleRobots.mat

View the details of the Puma robot using showdetails.

showdetails(puma1)
--------------------
Robot: (6 bodies)

 Idx    Body Name   Joint Name   Joint Type    Parent Name(Idx)   Children Name(s)
 ---    ---------   ----------   ----------    ----------------   ----------------
   1           L1         jnt1     revolute             base(0)   L2(2)  
   2           L2         jnt2     revolute               L1(1)   L3(3)  
   3           L3         jnt3     revolute               L2(2)   L4(4)  
   4           L4         jnt4     revolute               L3(3)   L5(5)  
   5           L5         jnt5     revolute               L4(4)   L6(6)  
   6           L6         jnt6     revolute               L5(5)   
--------------------

Get a specific body to inspect the properties. The only child of the L3 body is the L4 body. You can copy a specific body as well.

body3 = getBody(puma1,'L3');
childBody = body3.Children{1}
childBody = 
  rigidBody with properties:

            Name: 'L4'
           Joint: [1x1 rigidBodyJoint]
            Mass: 1
    CenterOfMass: [0 0 0]
         Inertia: [1 1 1 0 0 0]
          Parent: [1x1 rigidBody]
        Children: {[1x1 rigidBody]}
         Visuals: {}
      Collisions: {}


body3Copy = copy(body3);

Replace the joint on the L3 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(puma1,'L3',newJoint);

showdetails(puma1)
--------------------
Robot: (6 bodies)

 Idx    Body Name       Joint Name       Joint Type    Parent Name(Idx)   Children Name(s)
 ---    ---------       ----------       ----------    ----------------   ----------------
   1           L1             jnt1         revolute             base(0)   L2(2)  
   2           L2             jnt2         revolute               L1(1)   L3(3)  
   3           L3        prismatic            fixed               L2(2)   L4(4)  
   4           L4             jnt4         revolute               L3(3)   L5(5)  
   5           L5             jnt5         revolute               L4(4)   L6(6)  
   6           L6             jnt6         revolute               L5(5)   
--------------------

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

subtree = removeBody(puma1,'L4')
subtree = 
  rigidBodyTree with properties:

     NumBodies: 3
        Bodies: {[1x1 rigidBody]  [1x1 rigidBody]  [1x1 rigidBody]}
          Base: [1x1 rigidBody]
     BodyNames: {'L4'  'L5'  'L6'}
      BaseName: 'L3'
       Gravity: [0 0 0]
    DataFormat: 'struct'

Remove the modified L3 body. Add the original copied L3 body to the L2 body, followed by the returned subtree. The robot model remains the same. See a detailed comparison through showdetails.

removeBody(puma1,'L3');
addBody(puma1,body3Copy,'L2')
addSubtree(puma1,'L3',subtree)

showdetails(puma1)
--------------------
Robot: (6 bodies)

 Idx    Body Name   Joint Name   Joint Type    Parent Name(Idx)   Children Name(s)
 ---    ---------   ----------   ----------    ----------------   ----------------
   1           L1         jnt1     revolute             base(0)   L2(2)  
   2           L2         jnt2     revolute               L1(1)   L3(3)  
   3           L3         jnt3     revolute               L2(2)   L4(4)  
   4           L4         jnt4     revolute               L3(3)   L5(5)  
   5           L5         jnt5     revolute               L4(4)   L6(6)  
   6           L6         jnt6     revolute               L5(5)   
--------------------

References

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

[2] Siciliano, Bruno. Robotics: Modelling, Planning and Control. London: Springer, 2009.

Extended Capabilities

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

Version History

Introduced in R2016b

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