Joints and Links (Thu Nov 14, lect 22) | previous | next | slides |

Robots can have their own body parts that have to be controlled

Logistics

  1. dynamaze
  2. cargo
  3. dashboard
  4. sort
  5. typist
  6. race
  7. convoy
  8. autopilot
  9. bowling
  10. guard

Final Projects

(T) Final Submissions

What is URDF?

  • Uniform Robot Definition Format
  • Defines a collection of links held together by joints
  • Joints
    1. Are not “visible” - all they do is to show how two links move relative to each other
    2. See urdf documentation for all the details
  • Links
    1. Are “visible”
    2. Can have many other properties

URDF Example

Joint Types

  1. revolute - a hinge joint that rotates along the axis and has a limited range specified by the upper and lower limits.
  2. continuous - a continuous hinge joint that rotates around the axis and has no upper and lower limits.
  3. prismatic - a sliding joint that slides along the axis, and has a limited range specified by the upper and lower limits.
  4. fixed - This is not really a joint because it cannot move. All degrees of freedom are locked. This type of joint does not require the axis, calibration, dynamics, limits or safety_controller.
  5. floating - This joint allows motion for all 6 degrees of freedom.
  6. planar - This joint allows motion in a plane perpendicular to the axis.

.launch file concerns

URDF Roles

  • Describes geometry of a robot
  • Names various links
  • There are others

robot_state_publisher

  • Looks at the urdf and publishes all the tfs which are statically tied to the base_link
  • Using urdf, publishes static_tf

joint_state_publisher

  • Looks at dynamic links (those that can move) and updates the tf as they move
  • What would cause them to move?

Manipulators

Kinematic Chains

  • A set of links and joints together form a kinematic chain
  • If one end is attached to something fixed (like the floor) it is “grounded”
  • The other end will usually have some kind of gripper, sensor, or effector
  • This system can be complex and can be analyzed mathematically

Coordinate Frames

  • Ground based mobile robots have two important coordinate frames: the map and robot frames.
  • Robot arms have many more coordinate systems.
  • The links are fixed and of known length
  • The joints are all in a known position represented by one or more angles
  • Joint space and task space
  • In ROS, names in URDF are used to name the frames

Kinematics

  • Forward Kinematics
    1. If you set all the joint angles a certain way, where will the end be in space?
    2. This has a single solution and is tractable
  • Reverse Kinematics
    1. If you want the end to be in a certain location, what should the joint angles be?
    2. This might have zero (out of reach), or more solutions

Charlie Arm

Thank you. Questions?  (random Image from picsum.photos)