6: Robots and Simulators
Looking more widely now at the world of robotics

Building blocks

  • Robots are made up of many subsystems
  • Subsystems are ‘represented’ in the software
  • Actuation (e.g. wheels)
  • Sensing (e.g. cameras)
  • Computing (e.g. brains)

Actuation

  • Wheels - Differential Drive
    • Differential drive: Turtlebot3
    • Turning by different speed on two or more wheels
    • Sometimes there is a caster
    • Can only move forward and backward.
    • Not up and down or (directly) sideways
    • With more than 2 wheels, or a track - skid teering
    • Cars have “ackerman” steering (4 wheels, 2 that can steer)
  • Holonomic - non-Holonomic
    • Holonomic - can move in any direction
  • Manipulator Arm
    • Degrees of freedom
    • Coordiante transforms

Sensors

Lidar
  • Rotates and uses a laser to measure distance to nearest obstacle
  • Returns a vector of values corresponding to the directions
Limitation Operates in a plane, like a disk. Can't detect an obstacle above or below the plane!
Visual Cameras
  • Webcam sees a color picture
  • Matrix of dots, each dot has a color
  • Dot[x,y] = {r,g,b}
  • Data can be processed for example with opencv
  • A lot of data and a lot of processing
  • Bandwidth limits
Depth Cameras
  • Like a Microsoft Kinect
  • In addition to {r,g,b} also has distance
  • iPhoneX

Computation

  • Our Turtlebots have:
    1. Arduino to control the motor. Also with various sensors, lights, and i/o ports. Board is from Robotis and is called OpenCR
    2. Raspberry Pi which runs Ubuntu Mate and ROS
  • Distributed environment, typically:
    1. ROSCORE running on our roscore1.cs.brandeis.edu
    2. Some ROS nodes running on Raspberry Pi
    3. More Ros nodes running on a laptop or other computer
  • Workload is truly distributed across all of those
  • Eventually we may have to have a much beefier computer running all three
  • This would make the robot much more independent

Simulators

  • Software to allow work even without actual robots
  • A series of nodes which:
    • publish topics that a robot would publish (e.g. /odom)
    • Subscribe to topics that a robot would (e.g. /cmd_vel)
    • Use a “physics engine” to simulate what would happen in the real world
  • For example, simulator
    • Maintains an imagenary x,y where the robot is right now
    • Maintains a 3-d model of the robot
    • Has 3-d models of obstacles like walls
    • Changes all these states based on motion commands it receives
    • Of course, much more complex
  • We will not be using Stage
  • Gazebo
    • Really sophsticated 3d simulator plus visualization
    • Can do much more than Turtlebots
    • I have seen Gazebo models of a complete Amazon Warehouse Floor
    • With multiple Robots running around
  • RViz
    • While it looks similar, it is not a simulator
    • It “simply” visualizes topics that it subscribes
    • In a 3-d space
  • Stdr
    • Much simpler than Gazebo
    • We might use that.