Robot Dynamics Library

This page shows a number of examples that hopefully will bootstrap the learning process for users wanting to use RDL. If there is a specific example or use case that you would like to see that is not already shown here, please open an issue so we can get it added.

2 Link Pendulum

In this example, we will construct a robot model of a 2 link pendulum robot.

// For this example, we will create a shared pointer to a model, but it's also perfectly valid to
// construct an instance directly. Many RDL functions take pointers to a model, this is really the
// only motivation for using a pointer instead of an instance directly.
// Upon construction, a body for world and a world frame are created, but no more bodies/frames exist until we add them.
// Create body 1
RobotDynamics::Body body1(1.0, Math::Vector3d(0.,0.,-0.1), Math::Vector3d(0.1,0.1,0.1);
// Create body 2
RobotDynamics::Body body2(2.0, Math::Vector3d(0.,0.,-0.2), Math::Vector3d(0.1,0.1,0.1);
// Now add the two bodies of the pendulum. We will use Model::appendBody here, but Model::addBody would work
// as well. The 'addBody' function requires as an argument the parent body you are adding to, but 'appendBody'
// adds the body to the previously added body.
unsigned int body1_id = model->appendBody(Math::SpatialTransform(), JointTypeRevoluteX,body1,"body1");
unsigned int body2_id = model->appendBody(Math::SpatialTransform(RobotDynamics::Math::Xtrans(RobotDynamics::Math::Vector3d(0.,0.,-1.0))),
// Each time a body is added, a reference frame is created for the body. These frames can be accessed by,
RobotDynamics::ReferenceFramePtr body1_frame = model->bodyFrames[body1_id];
RobotDynamics::ReferenceFramePtr body2_frame = model->bodyFrames[body2_id];
// Frames are alse created at the body's center of mass. These frames can be accessed by,
RobotDynamics::ReferenceFramePtr body1_com_frame = model->bodyCenteredFrames[body1_id];
RobotDynamics::ReferenceFramePtr body2_com_frame = model->bodyCenteredFrames[body2_id];
// Now update kinematics(see the available functions from the kinematics module or look in Kinematics.h)
void updateKinematics(*model, Q, QDot, QDDot) // Q,QDot, and QDDot here are vectors of joint positions, velocities, and accelerations
// Until kinematics have been updated, the transforms stored in each reference frame will not necessarily be correct.
// Also, you should be sure to update the kinematics each control tick.
// Now some examples of some simple things you can do utilizing the reference frames
// Create a point at the origin of body2_frame
FramePoint body2_origin(body2_frame, 0.,0.,0.);
// Now lets express that point in body1_frame
body2_origin.changeFrame(body1_frame) // body2_origin will now be expressed in the reference frame attached to body1
// To express the frame point(or any geometric quantity that has a frame) in world frame, this con be done by,
body2_origin.changeFrame(model->worldFrame); // Now it's in world frame
Definition: rdl_eigenmath.hpp:54
void updateKinematics(Model &model, const VectorNd &Q, const VectorNd &QDot, const VectorNd &QDDot)
Updates and computes velocities and accelerations of the bodies.
Definition: Kinematics.cpp:19
std::shared_ptr< ReferenceFrame > ReferenceFramePtr
Definition: ReferenceFrame.hpp:68
SpatialTransform Xtrans(const Vector3d &r)
Get pure translation transform.
Definition: rdl_eigenmath.hpp:1097
@ JointTypeRevoluteX
Definition: Joint.hpp:186
std::shared_ptr< Model > ModelPtr
Definition: Model.hpp:750
Describes all properties of a single body.
Definition: Body.hpp:88
Contains all information about the rigid body model.
Definition: Model.hpp:112