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: