FEH RObot 2014
Topics for today
- The team
- Early goals
- Chassis development
- Tools
- Sensors
- Code and Route
- Budget
- Results
- Future changes
The team
- Carter Hurd
- Chassis design and construction
- David Hayes
- Sensor setup and electronics
- Hopen Yang
- Documentation
- Ryan Niemocienski
- Route and programming
early goals
- Low number of servos and powered mechanisms
- Reliability without reliance on RPS
- Fast run time + efficient path
- Separate tools for tasks
- A robot that works even when positioned imprecisely
Omnidirectional platform
Pros:
-
Can
complete the course fast without spending time turning
-
Can
mount each tool to one side
-
Less
chance of losing orientation on course due to turns
Cons:
-
Omni
wheels can't go up ramps well
-
Expensive
to buy/manufacture omni
wheels
-
Can’t
effectively use encoders
Chassis Development
Version 1
- Used 3 gears to reduce cost (prototype had 4 gears)
- Had ability to put all wheels at 45 degree angles for turning in one spot
Version 2
- Simpler,
more reliable mechanism with no gears
- No ability to put wheels at 45 degree angles-
-
Not needed for our navigation plan
Version 3
- Bands held wheel housings tightly against chassis for driving straight
- Failed to change orientations smoothly
version 4
- Version 2 linkage setup with flexible hub
- Allows for smooth transitions hile still holding wheel housings against side of chassis
Other details
- 2 sheets of PVC for body enclosed delicate linkages and minimize flex
- Ball bearings to reduce friction when changing orientation
- Low, centered weight distribution
- Suspension to keep the 2 diagonally-oriented drive wheels on the ground when acceding ramp
- Enclosed wheel housings to support wheel axles
Tools
Powered tools
- Micro servo with short lever arm for button
- Mounted close to button for maximum leverage
- Mechanism can be lined up at start of match
-
Forklift to lift skid and flip switch
- Lifting skid allows for accurate navigation
- Bent forklift arms
- Servo mounted below chassis
- 2:3 ratio improved torque of forklift
Passive tools
- Wire clips secure pin
- Clips are hinged to rotate downward after getting pin
- Scoop dump
- Throws scoop when DROID runs into wall
Sensors
- Rotary encoders
- More consistent then optical sensor methods
- encoders embedded within drive wheels
- 3 CDS Cells
- Greatly increase effective light reading area
- Foil cones on 2 sensors to further increase reading area
Code + Route
key functions
- Move_clicks ()
- Drives the motor a certain amount of clicks, testing a given condition as the robot drives. Uses a Proportional Integral Derivative (PID) controller.
- Orient_X ()
- Orients robot to drive in X direction
- Orient_Y ()
- Orients robot to drive in Y direction
- Fix_heading ()
- Fixes the heading of the robot to a setpoint with a given tolerance according to RPS values
PID control
Our PID controller works on the errors of shaft encoding. Based on the previous, current, and predicted future error from the shaft encoders, the robot correctly sets the motor powers to drive straight.
- Proportional: The factor of the current error between expected and actual number of clicks
- Integral: Sum of all previous error
- Derivative: Error between previous error and current error
Sample PID code
Float proportion = error1 * KP;
Float derivative = (prev_error1 – error1) * KD;
Float integral = total_error1 * KI;
Float change = proportion + derivative + integral;
motor1power += change;
- Orient functions
- As servo changes direction of the wheels, the body rotates
- Motors rotate in opposite direction at the same time as orientation change, to keep heading constant
- Speed and timing must be exact
- Fix heading
- Moves towards desired heading until reached
- Desired heading based off of start heading
- Zero values discarded
cost saving measures
- $124 total spent
- Despite $20 in unused purchases/replacement parts
- Servos purchased for <$5 online
- Micro servo used where little force was required
- Body made entirely out of 2 sheets of PVC
- Shape-lock plastic used as alternative to 3D printing
- Prototype parts were mocked up and tested before added to the budget
Results
- Low number of servos and powered mechanisms
- No servos used to retrieve pin, flip switch, or deposit scoop
- Reliability without reliance on RPS
- Path used just 2 heading checks, and worked reliably
- Fast run time + efficient path
- Fastest perfect run during the competition
- Separate tools for tasks
- Omnidirectional platform allowed for this
- A robot that worked even when positioned imprecisely
- Bent forklift arms + 3 pin clips
Future Changes
- Make flexible center linkage hub out of rubber urethane
- Would stand more repeated orientation changes
- Modify chassis to accept VEX 393 motors
- RPS free heading checks? Remotely pressing charge zone?
the end
FEH 2014
By Carter Hurd
FEH 2014
- 1,585