Advanced Robot Control
description
Transcript of Advanced Robot Control
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Advanced Robot Control
Programming for Robustness with RoboLab
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Positioning
• Absolute– Uses features or
‘landmarks’ of the course• Relative
– Robot keeps track of its moves
– Relies on Odometry
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Common Sources of Error
• Rotation Sensor Resolution• Gear Backlash• Program Execution Speed• Wheel Spin/Skidding• Inside Turn Wheel
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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)
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Program Execution Speed
• Rotation sensor not read continuously
• RCX may not ‘see’ a target
• RCX will not react instantly
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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 doesn’t ‘see’ move– Sends robot off position, affecting next move
by robot
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
don’t turn at same speeds
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Programming Solutions
• ‘Creeping’• ‘Precise’ Forward/Reverse/Turns• ‘Square Up’ to Lines• Line Following using ‘shades of gray’• Experimentation
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
‘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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Why Not Use Low Power Levels?
• Often don’t provide enough power to overcome static friction
• Robot still rolls easily enough that speed is still too high
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Creep Example
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Precise Turns/Forward/Reverse
• Power applied gradually
• Reduce power before target
• Creep forward/backward until reach target
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
‘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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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)
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
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
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Show Memory vi example
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
Erase Slot vi example
Martian Exchange Students FLL Team 16 Oak Creek, Wisconsin
“The lesson is in the struggle and not in the
victory”
One Final Thought: