rigidBody
Create a rigid body
Description
The rigidBody object represents a rigid body. A rigid body is the
building block for any tree-structured robot manipulator. Each
rigidBody has a rigidBodyJoint object attached to it that defines how the rigid body can
move. Rigid bodies are assembled into a tree-structured robot model using rigidBodyTree.
Set a joint object to the Joint property before calling addBody to add the rigid body to the robot model. When a rigid body is in
a rigid body tree, you cannot directly modify its properties because it corrupts the
relationships between bodies. Use replaceJoint to modify the entire tree structure.
Creation
Syntax
Description
Input Arguments
Properties
Name — Name of rigid body
string scalar | character vector
Name of the rigid body, specified as a string scalar or character vector.
This name must be unique to the body so that it can be found in a
rigidBodyTree object.
Data Types: char | string
Joint — rigidBodyJoint object
handle
rigidBodyJoint object,
specified as a handle. By default, the joint is 'fixed'
type.
Mass — Mass of rigid body
1 kg (default) | numeric scalar
Mass of rigid body, specified as a numeric scalar in kilograms.
CenterOfMass — Center of mass position of rigid body
[0 0 0] m (default) | [x y z] vector
Center of mass position of rigid body, specified as an [x y
z] vector. The vector describes the location of the center of
mass relative to the body frame in meters.
Inertia — Inertia of rigid body
[1 1 1 0 0 0]
kg•m2 (default) | [Ixx Iyy Izz Iyz Ixz Ixy] vector
Inertia of rigid body, specified as a [Ixx Iyy Izz Iyz Ixz
Ixy] vector relative to the body frame in kilogram square
meters. The first three elements of the vector are the diagonal elements of
the inertia tensor. The last three elements are the
off-diagonal elements of the inertia tensor. The inertia tensor is a
positive semi-definite symmetric matrix:
Parent — Rigid body parent
rigidBody object handle
Rigid body parent, specified as a rigidBody object
handle. The rigid body joint defines how this body can move relative to the
parent. This property is empty until the rigid body is added to a
rigidBodyTree robot model.
Children — Rigid body children
cell array of rigidBody object handles
Rigid body children, specified as a cell array of
rigidBody object handles. These rigid body children
are all attached to this rigid body object. This property is empty until the
rigid body is added to a rigidBodyTree robot model, and
at least one other body is added to the tree with this body as its
parent.
Visuals — Visual geometries
cell array of string scalars | cell array of character vectors
Visual geometries, specified as a cell array of string scalars or
character vectors. Each character vector describes a type and source of a
visual geometry. For example, if a mesh file, link_0.stl,
is attached to the rigid body, the visual would be
Mesh:link_0.stl. Visual geometries are added to the
rigid body using addVisual.
Object Functions
copy | Create a deep copy of rigid body |
addCollision | Add collision geometry to rigid body |
addVisual | Add visual geometry data to rigid body |
clearCollision | Clear all attached collision geometries |
clearVisual | Clear all visual geometries |
Examples
Attach Rigid Body and Joint to Rigid Body Tree
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) --------------------
Build Manipulator Robot Using Denavit-Hartenberg Parameters
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. The Puma robot is a serial chain manipulator. The DH parameters are relative to the previous line 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:
Create a
rigidBodyobject and give it a unique name.Create a
rigidBodyJointobject and give it a unique name.Use
setFixedTransformto 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.Call
addBodyto 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
Compatibility Considerations
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.