Advanced Robot Control

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Advanced Robot Control

• Programming for Robustness with RoboLab

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Positioning

• Absolute
• Uses features or landmarks of the course
• Relative
• Robot keeps track of its moves
• Relies on Odometry

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Positioning Problems

• Absolute
• May have difficult time finding small landmarks
• Some landmarks robots are easily damaged
• Relative
• Error accumulates with every move
• If too many errors, robot maybe too far off
  course to find landmark later

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Common Sources of Error

• Rotation Sensor Resolution
• Gear Backlash
• Program Execution Speed
• Wheel Spin/Skidding
• Inside Turn Wheel

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Rotation Sensor Resolution

• Robot only knows position with plus or minus one
  count (at best)
• Gear backlash increases error beyond one count
• Use finer resolution to reduce error (Minimize
  Distance per Count)
• Rotation sensor should be at same speed as motor
  (or up to 1-1/2 times higher)

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Program Execution Speed

• Rotation sensor not read continuously
• RCX may not see a target
• RCX will not react instantly

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Wheel Spin at Startup

• Caused by sudden application of motor torque, not
  enough weight on drive wheels
• Wheels and rotation sensor turn before robot
  starts
• Skip or changes direction due to jump from
  start

8
Skidding

• Caused by rapid application of motor braking and
  not enough weight on drive wheels
• Robot told to stop but continues to move
• Rotation sensor doesnt see move
• Sends robot off position, affecting next move by
  robot

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Turns

• Errors are magnified in turns
• Any slight direction error can cause larger
  side-to-side error
• Braking of inside wheel
• Any movement of the inside wheel lessens the
  overall turn true angle is shorter than with a
  locked wheel
• Turns made with two counter-rotating wheels
  doubles rotation sensor resolution errors
• Additional errors if wheels dont turn at same
  speeds

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Non-Programming Solutions

• Set a reasonable speed-Try gearing robot for 10
  to 15 inches per second
• Allows one wheel to be locked in turn
• Gear rotation sensor for 1/8 of travel per count
  or less
• Measures position as precisely as practical
• Minimize backlash by avoiding multi-stage gearing

• Avoid loose gear meshes
• Keep weight on driving wheels
• Gain traction
• Minimize slipping and skidding
• Weight shifts with accel/decel
• Match motors use two motors with same output
  speeds
• Use motor test jig

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Motor Test Jig

• Build a motor test jig using
• Load motor with worm geartrain
• Test and record motor data
• Run for turn, record counts
• Forward and reverse
• Different power levels

Picture of Motor Test Jig
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Programming Solutions

• Creeping
• Precise Forward/Reverse/Turns
• Square Up to Lines
• Line Following using shades of gray
• Experimentation

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Creeping

• Moves Robot Slowly by providing a series of taps
  to the robot
• Overcome static friction
• Provides braking and speed control
• Offers these Advantages
• Go slowly to minimize wheel slippage
• Minimize distance error due to polling error
• Better chance of sensing narrow lines
• Bump up against landmark with much less force

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Why Not Use Low Power Levels?

• Often dont provide enough power to overcome
  static friction
• Robot still rolls easily enough that speed is
  still too high

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How to Creep

• Create a loop to wait for rotation (or time,
  light level or button press)
• Start motors at medium power level
• Wait for a very small time (1/100 sec)
• Stop the motors
• Wait for a very small time (1/100 sec)
• End loop

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Creep Example
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Precise Turns/Forward/Reverse

• Power applied gradually
• Reduce power before target
• Creep forward/backward until reach target

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Precise Startup

• Uses subroutine (to save memory)
• Position target passed from main task via
  container
• Sets up intial target
• Try using 10 to 20 counts short of actual target
• Loops until initial rotation target
• Branches to different power levels based on timer
  to provide smoother acceleration
• Avoids wheel slip at startup

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At Initial Target

• Coast or Creep
• If coasting, coast until time
• Could possibly coast past target
• Creeping applies pulsed braking
• No skidding
• Self correcting using closed loop positioning
• Moves forward or reverse to final target count
• Too far creeps in reverse
• Too short creeps forward

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Routine Details

• One subroutine can be used for left turns and
  forward
• Container 7 is set to 0 or 1 to choose left or
  forward
• Reverse or right turns are done similarly

• Stored as subroutines to save memory
• Target counts are passed using blue container
• Set container for forward/reverse or left/right

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Square Up

• Line up robot to edge of line
• Uses two Light Sensors
• Moves robot so each sensor seeks dark/light edge
• Know exact spot when parked
• Accuracy in direction
• Accuracy in position (1 axis)

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Square Up Setup

• Square up to dark line
• Each sensor is different
• Needs to be set before running
• Separate sub-vi that calibrates light levels
• Grabs light values
• Calculates and stores threshold values

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How it works

• For each sensor
• If sensor sees
• Light Creep one pulse forward, Reset Container
  to 0
• Dark Add 1 to container
• Do until container is set to 2 which means both
  sensors made it to the dark line
• Robot waddles to the line
• Repeat process with motors set for reverse and
  looking for light instead of dark
• Repeat loop two times to assure exact placement

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Line Following

• Follow line edge using light sensor
• Reads average value of light within a circle
• Seeking halfway between light and dark
• Based upon Light level sensed Motor Behavior
  will set motors to creep to steer robot toward
  line edge
• Can be separated into the 7 zones (shades of
  grey)
• Can go straight or turn depending on value
• Go faster and straighter near middle zones
• Go slower and turn sharper in zones away from
  middle

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Program Example

• Create an outer loop
• rotation sensor target
• Create a decision tree within the loop
• Made with container forks for branching for
  different response to each light level range
• Use Creeping within each branch
• Each of the 7 conditions can be setup and tested
  individually

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Experimentation is Key

• Alter creep speed and turn radius
• Watch robot to see how it behaves
• Adjusting height of light sensor
• Changes size of circle being read
• Changes sensitivity
• Adjust location of light sensor
• Change weight distribution

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Memory Management

• Use Subroutines(Subs) for routines called
  repeatedly
• Pass parameters to Subs using Containers
• Use Containers as flags (for program forks) to
  get multiple functions per Sub.
• Use utility programs to show memory usage and
  clear out slots.
• Get to know memory usage of program elements

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Show Memory vi example
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Erase Slot vi example
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One Final Thought

• The lesson is in the struggle and not in the
  victory