Introduction to robot and link classes

These notes will eventually migrate to the Sphinx documentation for each class, but here they are easier to work on.

WORK IN PROGRESS

Robot class

A Robot instance is essentially a container of links with metadata and some methods that apply (almost) all subclasses.

Abstract class

The Robot class is abstract. The current subclasses are

• ERobot the most general robot that can be branched
• DHRobot classical Denavit-Hartenberg model

The abstract methods are:

Method	Purpose
fkine	forward kinematics
jacob0	Jacobian in world frame
jacobe	Jacobian in end-effector frame
hessian0	Hessian in world frame
rne	inverse dynamics: recursive Newton Euler
isspherical	robot has spherical wrist
ets	ETS representation

Links

A Robot class acts like an iterator over the list of links comprising the robot.

It can also be used to slice the robot, eg. robot[1] is link 1 and robot[:3] is the first three links.

Links can also be found by name, eg. robot['link1'].

Base and tool

The forward kinematics is computed with respect to the base. By default this is the world frame (identity SE(3)), but can be set to an arbitrary value, eg. robot.base = X.

The .base property always returns an SE3 instance, but if the attribute ._base is None this means there is no base set, and .base will return SE3().

NOTE: For symbolic operations this is useful, since premultiplying the link transforms by an identity matrix will lead to unncessary 1.0 * is symbolic expressions.

Metadata and properties

Property	Purpose
n	number of joints
nlinks	number of links
nbranches	number of branches
hasdynamics	has dynamic parameters
hasgeometry	has geometry model
hascollision	has collision model
structure	robot structure as string
revolutes	joints which are revolute (bool array)
prismatics	joints which are prismatic (bool array)

Concrete methods

Kinematics

Method	Purpose
IK_xx	inverse kinematics

NOTE: inverse kinematics uses the fkine and jacob0 methods
of the subclass

Dynamics

Method	Purpose
inertia	rigid-body mass tensor
coriolis	rigid-body Coriolis and centripetal term
gravload	rigid-body gravity load
accel	rigid-body forward dynamics
inertia_x	operational-space mass tensor
coriolis_x	operational-space Coriolis and centripetal term
gravload_x	operational-space gravity load
accel_x	operational-space forward dynamics
dynamics	dynamic parameters as a table
dynamics_list	dynamic parameters as a list

NOTE: many of these methods require

Graphics

Method	Purpose
plot	plot/animate the robot
teach	interactively "teach" the robot

NOTE: plot and teach invoke the backend which calls back to
robot methods

Backend support:

Link class

This is an abstract base class for all links. It contains metadata about the link and a number of methods.

Metadata and properties

Property	Purpose
name	number of joints
isrevolute	true if link's parent is via a revolute joint
isprismatic	true if link's parent is via a prismatic joint
hasdynamics	has dynamic parameters
hasgeometry	has geometry model
hascollision	has collision model
m	link mass (float)
r	link CoM relative to link frame ndarray(3)
I	link inertia tensor about CoM ndarray(3,3)

Abstract method A

Denavit-Hartenberg defined robots

Is based on concrete subclasses

DHRobot

DHLink

Property	Purpose
theta	DH rotation about z-axis (float)
d	DH translation along z-axis (float)
a	DH translation along x-axis (float)
alpha	DH rotation about x-axis (float)
mdh	uses modified DH convention (bool)

A number of subclasses are useful in the creation of robot models using DH conventions

Method	Purpose
RevoluteDH	revolute joint using standard DH convention
PrismaticDH	prismatic joint using standard DH convention
RevoluteMDH	revolute joint using modified DH convention
PrismaticMDH	prismatic joint using modified DH convention