URDFParser

A simple parser libaray for URDF Files. That returns a robot object which can be used to access links, joints, transformation matrices, etc.

Usage:

parser = URDFParser()
robot = parser.parse(urdf_filepath, floating_base = False, alpha_tie_breaker = False)

Where the tie breaker is used to order joints with the same parent link.

alpha_tie_breaker=False # URDF ordering used
alpha_tie_breaker=True # Joint name ordering used

Instalation Instructions:

There are 4 required packages beautifulsoup4, lxml, numpy, sympy which can be automatically installed by running:

pip3 install -r requirements.txt

Robot API:

The main API is as follows where XXX can be replaced by:

• joint: a joint object (see API below)
• link: a link object (see API below)
• Xmat: a sympy transformation matrix with one free variable as defined by its joint (also 4x4 homogenous version)
• Xmat_Func: a function that returns a numpy matrix when passed a value for the free variable (also 4x4 homogenous version)
• Imat: a numpy 6x6 inertia matrix
• S: a numpy 6x1 motion subspace matrix

# A single object by its ID or by its name as defined in the URDF
get_XXX_by_id(lid) # jid for joints 
get_XXX_by_name(name)
# A list of the objects that occur in the given bfs level
get_XXX_by_bfs_level(name)
# A list of the object ordered by their IDs or by their names as defined in the URDF
# Note: The base link/inertia exists at index -1 and so will appear at the beginning of the list
get_XXXs_ordered_by_id(reverse = False)
get_XXXs_ordered_by_name(reverse = False)
# A dictionary of objects by their ID or by their name as defined in the URDF
# Note: The base link/inertia exists at index -1
get_XXXs_dict_by_id()
get_XXXs_dict_by_name()

The API also includes the following functions:

# get the robot name
get_name()
# get the robot type (if applicable)
is_serial_chain()
# get the number of positions and velocities in the robot state as well as numbers of links and joints
# note: links should be joints + 1 when including the base, num_joints = num_pos
#       num_vel = num_pos for fixed base (and is one larger with quaternion)
get_num_pos()
get_num_vel()
get_num_bodies() # assumes fixed world base frame included for fixed base robots
get_num_joints()
get_num_links()
get_num_links_effective() # num_links - 1 (base link is not used in many RBD algorithms when fixed)
# get the max bfs_level
get_max_bfs_level()
# get the IDs at a given bfs level and the bfs level for a given id
get_ids_by_bfs_level(level)
get_bfs_level_by_id(jid)
# get the ID of the parent(s) of a given link(s) by id
get_parent_id(lid)
get_parent_ids(lids)
get_unique_parent_ids(lids) # remove duplicates
# get the full list of parents ordered by id
get_parent_id_array()
# test if there is a repeated parent by ids
has_repeated_parents(jids)
# get the subtree IDs for a given id and total count and test if in a subtree
get_subtree_by_id(jid)
get_total_subtree_count()
get_is_in_subtree_of(jid,jid_of)
# get the ancestor IDs for a given id and total count and test if an ancestor
get_ancestors_by_id(jid)
get_total_ancestor_count()
get_is_ancestor_of(jid,jid_of)
# get all joints that have parent link name as the parent or child link name as the child
get_joints_by_parent_name(parent_name)
get_joints_by_child_name(child_name)
# get the joint that has parent link name as the parent and child link name as the child
get_joint_by_parent_child_name(parent_name,child_name)
# see if the following joints have the same S (useful for codegen)
are_Ss_identical(jids)

Joint API:

# get the name, id, and bfs of the joint
get_name()
get_id()
get_bfs_id()
get_bfs_level()
# get the parent and child link name
get_parent()
get_child()
# get the Xmat or Xmat_Func for this joint as defined above
# also get the 4x4 homogenous transformation matrix
get_transformation_matrix()
get_transformation_matrix_function()
get_transformation_matrix_hom()
get_transformation_matrix_hom_function()
# get the S for this joint as defined above
get_joint_subspace()
# get the velocity damping coefficent for this joint
get_damping()

Link API:

# get the name, id, and bfs of the link
get_name()
get_id()
get_bfs_id()
get_bfs_level()
# get the link's spatial inertia matrix
get_spatial_inertia()