Logistics

• Discussion about PA: Teach a robot to dance!
• Review of Chapters up to 8: Teleop-bot

Projects

• Thursday’s Lab will be the Project Bazzaar
• More ideas can be found at ROS Robotics Projects
• Projects will be done in groups of 1 to 3 people
• The last ~6 weeks will be heavily focused on the projects

How to prepare for Thursday

• Meet with potential teammates to discuss your project
• Prepare a 5 minute pitch for your project for Thursday
• What will the demo look like?
• Whata are the big technical challenges?
• How hard or easy will it be?
• How interesting will it be to watch?
• What will you learn?
• (T) Team Project Proposal

Coordinate systems

• Where is a robot? How do we designate the location?
• If it’s a surface moving mobile robot, then an x,y coordinate would seem to be sufficient
• We need to determine the units: for us, usually Meters
• We need to designate the origin
• Anything else?

What about the “real world”?

• Where is that 0,0?
• Does it matter?

What part of the robot?

• What part of the robot are we talking about?
• The “middle”?
• Who says where the middle is?
• Does it matter how high off the ground it is?

Orientation

• What about where it is pointing?
• Is that part of the orientation?
• What does it mean to tell the robot to rotate right or left?
• Is that direction also part of the orientation in space?
• When would that matter?

Demo

What else?

• What other aspects of orientation in space?
• If the Robot has a camera mounted on it?
• If there is an arm?
• Robot pushing an elevator button

Actions

• Taking action based on orientation. Think about:
• Turn Right
• Turn North
• Point camera in the direction of the sound

“Pose”

• The combination of the robot’s position and orientation (direction it is pointing)
• Each one has 3 dimensions: x,y,z and r,p,y
• x,y,z are cartesian coordinates (position)
• r,p,y are Euler coordinates (orientation)

Multiple Coordinate systems

• Each component of the robot potentially has a coordinate system
• Many of them have a fixed relationship to each other
• Point 0,0 for the robot’s tf might be Point 10,0 for the camera’s tf
• There are coordinate systems attached to (each) robot
• But also to the rest of the “world” the robot knows about

Conversions

• Converting between coordinate systems
• Very common calculation is to convert a certain actual point
• From one coordinate system to another

ROS Units and data types

• In ROS, we use the following units:
  1. Distance: Meter
  2. Velocity: Meters/Second
  3. Angles: Radians (sometimes degrees)
  4. Orientation: Euler angles (roll pitch yaw) or Quaternions

Euler Angles

• Conventional way to represent orientation
• Pitch, Roll, Yaw (from airplanes)
• Can be radians or angles, but for ROS poses, it’s radians
• Reminder: Radians go from 0 (0 degrees) to 2 * PI (360 degrees == 0 degrees)

Quaternions

• Quaternion represents the same, but with 4 numbers
• There’s an exact conversion between Euler angles and Quaternions
• For subtle mathematical reasons, when combining a series of rotations, Quaternions work better
• Don’t try to interpret the x,y,z,r of a quaternion, it doesn’t have an intuitive mapping to x,y and z euler angles.

# type(pose) = geometry_msgs.msg.Pose
# convert from euler to quaternion

quaternion = tf.transformations.quaternion_from_euler(roll, pitch, yaw)
pose.orientation.x = quaternion[0]
pose.orientation.y = quaternion[1]
pose.orientation.z = quaternion[2]
pose.orientation.w = quaternion[3]

# type(pose) = geometry_msgs.msg.Pose
# convert from quaternion to euler

quaternion = (
    pose.orientation.x,
    pose.orientation.y,
    pose.orientation.z,
    pose.orientation.w)
euler = tf.transformations.euler_from_quaternion(quaternion)
roll = euler[0]
pitch = euler[1]
yaw = euler[2]

Datatypes in ROS

• ROS “messages”, also *.msg files
• Really amount to a C struct

Point

• Point (geometry_msgs/Point.msg)
• x,y,z: float64
• A point in 3D space
• Right hand rule
• positive x-forward, positive y-left, positive z-up

Vector3

• Vector3 (geometry_msgs/Vector3.msg)
• x, y, z: float64
• Simple, generic representation of 3 float64

Quaternion

• Orientation (geometry_msgs/Quaternion.msg)
• x, y, z, w: float64
• An orientation in space, in quaternion form
• Quaternions are another way to express orientation

Pose

• Pose (geometry_msgs/Pose.msg)
• position: Point
• orientation: Quaternion
• A combination of “where” and “facing what way”

Twist

• Velocity in free space, broken into linear and angular
• Twist (geometry_msgs/Twist.msg)
• linear, angular: Vector3

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