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Pose frame semantics: legacy behavior

Pose Frame Semantics: Legacy Behavior

In version 1.4 and earlier of the SDF spec, the <pose> element represents a relative coordinate transformation between a frame and its parent. Link frames were always defined relative to a model frame and joint frames relative to their child link frames.

In version 1.5 of the SDF spec, the frame attribute string was added to <pose> elements to allow poses to be defined relative to a named frame instead of the default frames from version 1.4. For example, this would allow an SDF model to define its kinematics like a URDF, with joint frames defined relative to the parent link frame and a joint's child link frames relative to the joint frame.

The 1.5 specification also adds <frame> elements which can define named coordinate frames in addition to the existing link and joint frames in the model.

This document is a work in progress to define the semantics of the pose frame attribute.

Legacy behavior

Element naming rules in sdf 1.4

In sdf 1.4, sibling elements of the same type must have unique names. For example, the following models are invalid because links, joints, and collisions with the same parent do not have unique names.

<sdf version="1.4">
  <model name="model">
    <link name="link"/>
    <link name="link"/> <!-- INVALID: Same name as sibling "link"! -->
  </model>
</sdf>

<sdf version="1.4">
  <model name="model">
    <link name="link1"/>
    <link name="link2"/>
    <link name="link3"/>
    <joint name="joint" type="fixed">
      <parent>link1</parent>
      <child>link2</child>
    </joint>
    <joint name="joint" type="fixed"> <!-- INVALID: Same name as sibling "joint"! -->
      <parent>link2</parent>
      <child>link3</child>
    </joint>
  </model>
</sdf>

<sdf version="1.4">
  <model name="model">
    <link name="link">
      <collision name="collision">
        ...
      </collision>
      <collision name="collision"> <!-- INVALID: Same name as sibling "collision"! -->
        ...
      </collision>
    </link>
  </model>
</sdf>

The following model contains collision elements with the same name, but the models are valid because the elements are not siblings, but rather children of different links.

<sdf version="1.4">
  <model name="model">
    <link name="link1">
      <collision name="collision">
        ...
      </collision>
    </link>
    <link name="link2">
      <collision name="collision"> <!-- VALID -->
        ...
      </collision>
    </link>
  </model>
</sdf>

Sibling elements of different types are not mandated to have unique names, so the following is valid, though it is confusing and not recommended.

<sdf version="1.4">
  <model name="model">
    <link name="base"/>
    <link name="attachment"/>
    <joint name="attachment" type="fixed"> <!-- VALID, but RECOMMEND AGAINST -->
      <parent>base</parent>
      <child>attachment</child>
    </joint>
  </model>
</sdf>

It is allowed to create a link named world, but there is special treatment for a joint with world specified in the <parent> or <child> tags, so it is not recommended to do so.

<sdf version="1.4">
  <model name="model">
    <link name="world"/> <!-- VALID, but RECOMMEND AGAINST -->
    <link name="world_link"/> <!-- VALID, better -->
  </model>
</sdf>

Parent frames in sdf 1.4

With the exception of joint frames, all <pose> tags in sdf 1.4 define a relative transform between the current element's frame and the frame of its parent xml element.

For example, in the following model, the link frame is defined relative to the model frame, and the inertial, collision, visual, and sensor frames are all defined relative to the link frame.

<sdf version="1.4">
  <model name="model">
    <pose>...</pose>
    <link name="link">
      <pose>...</pose>
      <inertial>
        <pose>...</pose>
      </inertial>
      <collision name="collision">
        <pose>...</pose>
      </collision>
      <visual name="visual">
        <pose>...</pose>
      </visual>
      <sensor name="sensor">
        <pose>...</pose>
      </sensor>
    </link>
  </model>
</sdf>

The one exception is the joint frame, which is defined relative to the child link frame. In the following example, the joint frame is defined relative to the link frame of link2.

<sdf version="1.4">
  <model name="model">
    <pose>...</pose>
    <link name="link1">
      <pose>...</pose>
    </link>
    <link name="link2">
      <pose>...</pose>
    </link>
    <joint name="joint" type="fixed">
      <pose>...</pose>
      <parent>link1</parent>
      <child>link2</child>
    </joint>
  </model>
</sdf>

Given this longstanding behavior, this pattern should be followed for sdf 1.5+ when the pose frame attribute is empty. This would imply that the following models are equivalent to the previous models discussed in this section:

<sdf version="1.5">
  <model name="model">
    <pose frame=''>...</pose>
    <link name="link">
      <pose frame=''>...</pose>
      <inertial>
        <pose frame=''>...</pose>
      </inertial>
      <collision name="collision">
        <pose frame=''>...</pose>
      </collision>
      <visual name="visual">
        <pose frame=''>...</pose>
      </visual>
      <sensor name="sensor">
        <pose frame=''>...</pose>
      </sensor>
    </link>
  </model>
</sdf>

and

<sdf version="1.5">
  <model name="model">
    <pose frame=''>...</pose>
    <link name="link1">
      <pose frame=''>...</pose>
    </link>
    <link name="link2">
      <pose frame=''>...</pose>
    </link>
    <joint name="joint" type="fixed">
      <pose frame=''>...</pose>
      <parent>link1</parent>
      <child>link2</child>
    </joint>
  </model>
</sdf>

Parent frames in URDF

For comparison, the behavior of parent frames in Unified Robot Description Format (URDF) is given in this section. A URDF files contains links, joints, collisions, visuals, and inertials like SDFormat, but with several significant differences.

The first difference is that coordinate transformations are expressed using the attributes of the <origin/> tag instead of the value of <pose/>, which is a superficial difference as the numerical contents of each tag have a similar definition.

<pose>x y z roll pitch yaw</pose>

is equivalent to

<!-- URDF -->
<origin rpy='roll pitch yaw' xyz='x y z'/>

Similar to SDFormat, URDF inertial, collision, and visual elements are defined relative to their parent link frame with their own <origin/> tag, but links and robots (equivalent to models) do not have <origin/> tags:

<sdf version="1.4">
  <model name="model">
    <link name="link">
      <inertial>
        <pose>...</pose>
      </inertial>
      <collision name="collision">
        <pose>...</pose>
      </collision>
      <visual name="visual">
        <pose>...</pose>
      </visual>
    </link>
  </model>
</sdf>

is equivalent to

<!-- URDF -->
<robot name="model">
  <link name="link">
    <inertial>
      <origin rpy='...' xyz='...'/>
    </inertial>
    <collision name="collision">
      <origin rpy='...' xyz='...'/>
    </collision>
    <visual name="visual">
      <origin rpy='...' xyz='...'/>
    </visual>
  </link>
</robot>

The most significant difference between URDF and SDFormat coordinate frames is related to links and joints. While SDFormat allows kinematic loops with the topology of a directed graph, URDF kinematics must have the topology of a directed tree, with each link being the child of at most one joint. URDF coordinate frames are defined recursively based on this tree structure, with each joint's <origin/> tag defining the coordinate transformation from the parent link frame to the child link frame.

<sdf version="1.4">
  <model name="model">
    <link name="link1"/>
    <link name="link2"/>
    <joint name="joint" type="fixed">
      <pose>{xyz} {rpy}</pose>
      <parent>link1</parent>
      <child>link2</child>
    </joint>
  </model>
</sdf>

is decidedly not equivalent to

<!-- URDF -->
<robot name="model">
  <link name="link1"/>
  <link name="link2"/>
  <joint name="joint" type="fixed">
    <origin xyz='{xyz}' rpy='{rpy}'/>
    <parent link="link1"/>
    <child link="link2"/>
  </joint>
</robot>

The following image is used in URDF documentation to illustrate how coordinate frames are defined recursively from link1 -> joint1 -> link2 and link1 -> joint2 -> link3 -> joint3 -> link4:

urdf coordinate frames

This model in this image could be represented by the following URDF with model frame M, link1 frame L1, link2 frame L2, joint1 frame J1, etc. using the suffix notation for kinematic quantities described in the specify_pose tutorial.

<!-- URDF -->
<robot name="model">

  <link name="link1"/>

  <joint name="joint1" type="revolute">
    <origin xyz='{xyz_L1L2}' rpy='{rpy_L1L2}'/>
    <parent link="link1"/>
    <child link="link2"/>
  </joint>
  <link name="link2"/>

  <joint name="joint2" type="revolute">
    <origin xyz='{xyz_L1L3}' rpy='{rpy_L1L3}'/>
    <parent link="link1"/>
    <child link="link3"/>
  </joint>
  <link name="link3"/>

  <joint name="joint3" type="revolute">
    <origin xyz='{xyz_L3L4}' rpy='{rpy_L3L4}'/>
    <parent link="link3"/>
    <child link="link4"/>
  </joint>
  <link name="link4"/>

</robot>

For comparison, here is an SDFormat model with the same link names, joint names, frame names, and parent-child relationships.

<model name="model">
  <link name="link1">
    <pose>{xyz_ML1} {rpy_ML1}</pose>
  </link>
  <link name="link2">
    <pose>{xyz_ML2} {rpy_ML2}</pose>
  </link>
  <link name="link3">
    <pose>{xyz_ML3} {rpy_ML3}</pose>
  </link>
  <link name="link4">
    <pose>{xyz_ML4} {rpy_ML4}</pose>
  </link>
  <joint name="joint1" type="revolute">
    <pose>0 0 0 0 0 0</pose>
    <parent>link1</parent>
    <child>link2</child>
  </joint>
  <joint name="joint2" type="revolute">
    <pose>0 0 0 0 0 0</pose>
    <parent>link1</parent>
    <child>link3</child>
  </joint>
  <joint name="joint3" type="revolute">
    <pose>0 0 0 0 0 0</pose>
    <parent>link3</parent>
    <child>link4</child>
  </joint>
</model>

The definition of SDF coordinate frames is illustrated by the following image, in which all links are defined relative to the model, and each joint is defined relative to its child link. Since there is no offset allowed between the child link frame and the joint frame in URDF, the SDF joint poses are all zeros.

Once pose frame semantics are implemented in SDFormat, it will be possible to define an SDFormat model that behaves identically to a URDF model by specifying the parent reference frames according to the URDF convention.

Specifying parent and child link names for joints in sdf 1.4

Joints specify the parent and child links by name in the <parent> and <child> elements. The specified links must exist as siblings of the joint with one exception: if one but not both of the parent and child links is specified as world and a sibling link named world does not exist, the world link will be interpreted as a fixed inertial frame.

The following model contains joint with a valid specification of sibling links.

<sdf version="1.4">
  <model name="model">
    <link name="link1"/>
    <link name="link2"/>
    <joint name="joint" type="fixed">
      <parent>link1</parent>
      <child>link2</child>
    </joint>
  </model>
</sdf>

The following models contain joints with a valid specification of one sibling link connected to a fixed inertial frame as parent or child.

<sdf version="1.4">
  <model name="model">
    <link name="link"/>
    <joint name="joint" type="fixed">
      <parent>world</parent>
      <child>link</child>
    </joint>
  </model>
</sdf>

and

<sdf version="1.4">
  <model name="model">
    <link name="link"/>
    <joint name="joint" type="fixed">
      <parent>link</parent>
      <child>world</child>
    </joint>
  </model>
</sdf>

The following model contains a link named world, so the joint connects link to its sibling rather than connecting to a fixed inertial frame.

<sdf version="1.4">
  <model name="model">
    <link name="link"/>
    <link name="world"/> <!-- VALID, but RECOMMEND AGAINST -->
    <joint name="joint" type="fixed">
      <parent>world</parent>
      <child>link</child>
    </joint>
  </model>
</sdf>

The following model contains an invalid joint specification because the parent link does not exist.

<sdf version="1.4">
  <model name="model">
    <link name="link"/>
    <joint name="joint" type="fixed">
      <parent>fake_link</parent> <!-- INVALID: link with this name not found in this model -->
      <child>link</child>
    </joint>
  </model>
</sdf>

The following world also contains an invalid joint specification because, while link1 does exist in the world, it is not a sibling of the joint.

<sdf version="1.4">
  <world name="world_with_invalid_joint">
    <model name="model1">
      <link name="link1"/>
    </model>
    <model name="model2">
      <link name="link2"/>
      <joint name="joint" type="fixed">
        <parent>link1</parent> <!-- INVALID: link1 is not a sibling of joint -->
        <child>link2</child>
      </joint>
    </model>
  </world>
</sdf>

This section has discussed naming conventions for <joint> elements as children of a <model>. For completeness, it should be noted that the SDF specification allows for a <joint> to be a direct child of a <world> (see world.sdf:58), but the naming conventions for this case are not established, as this use case is not supported by Gazebo or any other known software.

Specifying parent and child link names for joints in sdf 1.5 with nested models

Support for nested models was added in sdf 1.5, which allows a <model> element to contain child <model>s. For example, the following model contains two links nested inside child models.

<sdf version="1.5">
  <model name="model">
    <model name="model1">
      <link name="link"/>
    </model>
    <model name="model2">
      <link name="link"/> <!-- VALID -->
    </model>
  </model>
</sdf>

The coordinate frame of each child model is defined relative to its parent element, following the convention from sdf 1.4.

A joint can specify the names of parent and child links from sibling models by specifying the sibling model name with the delimiter :: followed by the link name.

This model contains a valid joint specification with parent and child links from sibling models:

<sdf version="1.5">
  <model name="model">
    <model name="model1">
      <link name="link"/>
    </model>
    <model name="model2">
      <link name="link"/>
    </model>
    <joint name="joint" type="fixed">
      <parent>model1::link</parent>
      <child>model2::link</child>
    </joint>
  </model>
</sdf>

This model contains a valid joint specification with a child sibling link and the parent link from a sibling model.

<sdf version="1.5">
  <model name="model">
    <model name="nested_model">
      <link name="link"/>
    </model>
    <link name="link"/>
    <joint name="joint" type="fixed">
      <parent>nested_model::link</parent>
      <child>link</child>
    </joint>
  </model>
</sdf>

Please note: The future nesting behavior, the naming rules for child models (can they have same name as the parent model?), and the :: delimiter are under discussion and subject to change. Please see the proposal for the potential new behavior in SDF 2.0.

Phases of parsing kinematics of an sdf 1.4 model

This section describes phases for parsing the kinematics of an sdf 1.4 model. It does not discuss proper validation of collision and visual geometries, link inertia, and many other parameters. There are three phases for validating the kinematics data in a model:

  1. XML parsing and schema validation: Parse model file from XML into a tree data structure, ensuring that there are no XML syntax errors and that the XML data complies with the schema. Schema .xsd files are generated from the .sdf specification files when building libsdformat with the xmlschema.rb script.

  2. Name attribute checking: Check that name attributes are not an empty string "", and that siblings elements of the same type have unique names. This includes but is not limited to models, actors, links, joints, collisions, visuals, sensors, and lights. This step is distinct from validation with the schema because the schema only confirms the existence of name attributes, not their content. Note that libsdformat does not currently perform this check when loading an SDF using sdf::readFile or sdf::readString (see issue sdformat#216.

  3. Joint parent/child name checking: For each joint, check that the parent and child link names are different and that each match the name of a sibling link to the joint, with the following exception: if "world" is specified as a link name but there is no sibling link with that name, then the joint is attached to a fixed reference frame.

After these three phases have been completed, the model kinematics can be used in several ways:

  • Construct a kinematic graph with a node for each link and a directed edge for each joint from the parent link to the child. The graph may be a tree if there are no kinematic loops, in which case its kinematics are compible with URDF.

  • Recursively compute initial frame poses for links and joints using the rules for parent frames defined in the previous section.

Phases of parsing kinematics of an sdf 1.5 model

Parsing an sdf 1.5 model is similar to parsing an sdf 1.4 model, with the addition of nested models. Validation steps 1 and 2 are the same, but the procedure for checking parent and child names for joints must account for the :: separator, which allows links to be specified as the descendants of sibling models of a joint.