The coordinate system of the mobile robot platform (map,odom,base_link)

<span id="Label3"></p>The coordinate system of the mobile platform<p><p>Source:<span style="color:rgb(0,0,255)">HTTP://WWW.ROS.ORG/REPS/REP-0105.HTML#ID5</span></p></p>1 Summary (abstract)<p><p>This REP specifies naming conventions and semantic meaning for coordinate frames of mobile platforms used with ROS.</p></p><p><p>This rep specifies the naming conventions and semantics of the coordinate system of the mobile robot platform under Ros.</p></p><p><p></p></p><p><p>2 Purpose (motivation)</p></p><p>developers of drivers, models, and libraries need a share  Convention for coordinate frames in order to better integrate and  re-use software components. shared conventions for coordinate frames  provides a specification for developers creating drivers and  Models for mobile bases. similarly, developers creating libraries and  applications can more easily use their software with a  variety of mobile bases that are compatible with this  Specification. for example, this rep specifies the frames necessary  for writing a new localization component. it also specifies  frames that can be Used to refer to the mobile base of a robot. </p><p><p>For better integration and reuse of software modules, drivers, models, and library developers need a coordinate system for sharing conventions. The shared protocol of this coordinate system gives developers a specification for creating drivers and models for mobile DOCKS. similarly, developers using some mobile robot base software compatible with this specification can easily create the appropriate libraries and applications. For example, This rep specifies a coordinate system that must be written in a positioning module, and it also specifies the coordinate system associated with the mobile robot Base.</p></p><p><p>3 Description (specification)</p></p><p><p>3.1 Coordinate system frame (coordinate Frames)</p></p><p><p>3.1.1 World coordinates (map)</p></p><p><p>The coordinate frame called map is a world fixed frame with its z-axis pointing upwards. The pose of a mobile platform, relative to the map frame, should not significantly drift over Time. The map frame isn't continuous, meaning the pose of a mobile platform in the map frame can change in discrete jumps at an Y Time.</p></p><p><p>In a typical setup, a localization component constantly re-computes the robot pose in the map frame based on sensor observ ations, therefore eliminating drift, but causing discrete jumps when new sensor information Arrives.</p></p><p><p>The map frame is useful as a long-term global reference, but discrete jumps make it a poor reference frame for local Sensi Ng and Acting.</p></p><p><p>The map coordinate system is a world fixed coordinate system with its z-axis pointing upwards. The posture of the mobile platform relative to the map coordinate system should not move significantly over Time. Map coordinates are discontinuous, which means that the attitude of the moving platform in the map coordinate system can occur at any time with discrete jumps.</p></p><p><p>In a typical setup, the positioning module, based on sensor monitoring, constantly recalculates the posture of the robot in the world coordinates, thus eliminating deviations, but may jump when new sensor information Arrives.</p></p><p><p>The map coordinate system is useful as a long-term global reference, but jumping makes it a bad reference for local sensing and Actuators.</p></p><p><p></p></p><p><p>3.1.2 Odometer coordinate system (odom)</p></p><p><p>The coordinate frame called Odom is a world-fixed frame. The pose of a mobile platform in the Odom frame can drift over time and without any BOUNDS. This drift makes the Odom frame useless as a long-term global reference. however, the pose of a robot in the Odom frame are guaranteed to being continuous, meaning that the pose of a mobile platform In the Odom frame is always evolves in a smooth, without discrete jumps.</p></p><p><p>In a typical setup the Odom frame was computed based on an odometry source, such as wheel odometry, visual odometry or an I Nertia Measurement Unit.</p></p><p><p>The Odom frame is useful as an accurate, short-term local reference, but drift makes was a poor frame for long-term referen Ce.</p></p><p><p>The Odom coordinate system is a world fixed coordinate system. The posture of the moving platform in the Odom coordinate system can be moved arbitrarily without any bounds. This movement makes the Odom coordinate system not a long-term global reference. however, the attitude of the robot in the Odom coordinate system is guaranteed to be continuous, which means that the attitude of the mobile platform in the Odom coordinate system always changes smoothly without jumping.</p></p><p><p>In a typical setup, the Odom coordinate system is calculated based on a range source, such as a wheel odometer, a visual odometer, or an inertial measurement unit.</p></p><p><p>The Odom coordinate system is useful as a short-term local reference as a precision, but the offset makes it not a long-term reference</p></p><p><p></p></p><p><p>3.1.3 Base Mark (base_link)</p></p><p><p>The coordinate frame called base_link is rigidly attached to the mobile robot Base. The Base_link can attached to the base of any arbitrary position or orienation; For every hardware platform there would be a different place on the base that provides a obvious point of reference. Note that the REP 103 [1] specifies a preferred orientation for frames.</p></p><p><p>The base_link coordinates are rigidly connected to the mobile robot Base. The Base_link can be installed in any position in the base, and for each hardware platform, an obvious reference point is provided in different places on the Base. It is important to note that the preferred orientation of the coordinates specified by rep103[1].</p></p><p><p></p></p><p><p>Relationship between 3.2 coordinates (relationship between Frames)</p></p><p><p>We have chosen a tree representation to attach all coordinate frames in a robot system to each Other. Therefore Each coordinate frame has a parent coordinate frame, and any number of the child coordinate Frames. The frames described in this REP is attached as follows:</p></p><p><p>Map--odom-base_link</p></p><p><p>The map frame is the parent of odom, and Odom is the parent of BASE_LINK.  Although intuition would say that both map and Odom should being attached tobase_link, this isn't allowed because each frame Can only has one parent.</p></p><p><p></p></p><p><p>In a robotic system, we use a tree to correlate all coordinate systems, so each coordinate system has a parent and any sub-coordinate system, as Follows:</p></p><p><p></p></p><p><p>Map--odom-base_link</p></p><p><p></p></p><p><p>The world coordinate system is the parent of the Odom coordinate system, and the Odom coordinate system is the parent of BASE_LINK. While it is intuitive to say that map and Odom should be connected to base_link, this is not allowed because there can be only one parent class per coordinate system.</p></p><p><p>3.3 Coordinate system permissions (Frame authorities)</p></p><p><p>The transform from Odom to Base_link are computed and broadcast by one of the odometry Sources. The transform from maps to Base_link are computed by a localization component. however, The localization component does not broadcast the transform from map to Base_link.  Instead, It first receives the transform from Odom to base_link, and uses the Information-broadcast from Mapto Odom</p></p><p><p>The conversion from Odom to Base_link is calculated and published by the mileage meter Source. however, The positioning module does not publish a map-to-base_link conversion (transform). instead, The positioning module receives the transform from Odom to Base_link and uses this information to publish the map to Odom Transform.</p></p><p><p>The coordinate system of the mobile robot platform (map,odom,base_link)</p></p></span>