FTC Motor Encoders 20200304

Motor Encoder Demo

Why

• Where am I? is a key question a robot must know to get some things done.
• Odometry is the use of sensor data to estimate a robot's position: Odometry from Wikipedia.
• An encoder is an example sensor that can be used to answer that question: Encoder from Wikipedia.
• Dead reckoning is figuring out where you are based on where you were and your estimate on how far you moved in some direction: Dead Reckoning from Wiipedia.

Demo Code

• DriveForwardWithEncoderEpp.java

Exercise

• Exercise 1: Load the DriverForwardWithEncoderEpp Op Mode into Android Studio and execute it..

Motor Encoders

REV Motor Drive Characteristics

• MiniBot Hardware Kit REV-45-1171
• Core Hex Motor REV-41-1300
  • Free Speed 125 RPM
  • Gear Ratio: 72:1
  • Encoder Counts per Revolution at the motor: 4 counts/revolution REV-41-1336]
  • Encoder Counts per Revolution at the output (wheels): _________
• Gears - not relevant
  • 72 Tooth Plastic Gear REV-41-1336
  • 90 Tooth Plastic Gear REV-41-1337
  • Encoder Counts per Revolution at the wheels: _________
• Wheels
  • 90mm Traction Wheel REV-41-1354
  • Counts per mm: _________

Exercise

• Exercise 2: Verify the Counts per mm above with a physical test.

Encoder Diagram

FTC Motor Object Structure

References

• FTC API reference

Motor Class Specs

• DcMotorSimple extends HardwareDevice
  • getPower -- 64 bit double
  • setPower -- 64 bit double
  • ...

DcMotor extends DcMotorSimple

• • getMode
  • getPowerFloat -- 32 bit float
  • getTargetPosiition -- encoder value
  • isBusy
  • setMode
  • setPowerFloat -- 32 bit float
  • setTargetPosition -- encoder value
  • ...

• DcMotorEx extends DcMotor
  • getCurrent
  • getPIDCoefficients
  • getTargetPositionTolerance
  • getVelocity -- ticks per second and angle unitis
  • isOverCurrent
  • setPIDCoefficients
  • setVelocity -- ticks per second and angle units
• ...

Exercise

• Exercise 3: Change the DriveForwardWithEncoder to use Velocity control for both wheels instead of Power control. How does the robots move characteristics change? eg
  • Did the robot drive in a straighter line using the setVelocity than the setPower example?
  • Did the robot start out moving straight at the very beginning?
  • Did the robot stop gradually or abruptly when its target distance was reached?
  • How would you improve the robot's performance?

Spoiler Alert

• DriveForwardWithEncoderEppGio.java - The team's colaborative solution
  • Key changes
    • import DcMotorEx
    • leftMotor and rightMotor change to type DcMotorEx
    • type cast for the hardwareMap.dcMotor.get
    • change setPower to setVelocity
    • remove setting rightMotor to RUN_WITHOUT_ENCODER run mode

• DriveForwardWithEncoderSpeed.java - Ed's solution
  • Key changes - same as above pluss
    • Use this hardwareMap statement

        rightMotor = hardwareMap.get(DcMotorEx.class,"myRightMotor");

DcMotor and DcMotorEx differences

• DcMotor is designed for Modern Robotics / HiTechnic motor controllers
• DcMotor is designed for the Control Hub and Expansion Hub
• Reference: DcMotor vs. DcMotorEx

References

• REV Encoders
• REV Using Encoders - Motor Modes
• [https://docs.revrobotics.com/rev-control-system/programming/hello-robot-autonomous-robot/robot-nav-onbot-java/autonomous-navigation-onbot Encoder Navigation - OnBot
  • For Java code see: "Setting Velocity" subsection in the program "HelloWorld_EncoderAuton".
  • In is also described in a subsection "Turning the Divetrain Using RUN_TO_POSITION"
  • There is also an earlier sections called "Moving to a Target Distance."

Robot HW Architecture

• Proportional Integral Derivative from Wikipedia If you don't want to dig into the mathematics, look at the PID methods in the DcMotorEx class (getPIDCoefficients and setPIDCoefficients).

Optical Encoders

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