Challenge 2.2: Rotate Motor Forward 5 Times

Category: Single Motor Position Control

Robot Base

• Required Robot: Any

Overview

This challenge builds upon basic motor rotation by introducing multi-revolution position control. Students will learn to calculate and control precise motor rotations using encoder feedback, which is essential for mechanisms like elevators, arms, and conveyor systems that need to move specific distances or complete multiple rotations.

Challenge Description

Create a subsystem that uses a button press to rotate a motor forward exactly 5 complete revolutions, then automatically stop. The motor should rotate smoothly and stop precisely at the target position using encoder feedback for position control.

Required Hardware

Competition Robot

• Motor: NEO with SparkMax controller (any available motor)
  • Elevator Primary Motor: SparkMax (CAN ID: 52)
  • Intake Arm Motor: SparkMax (CAN ID: 60)
• Controller: Xbox Controller (Port: 0)

Testbed Robot

• Motor: Any TalonSRX with encoder
  • Front Left Motor: TalonSRX (CAN ID: 20) with DIO encoder (ports 0, 1)
  • Front Right Motor: TalonSRX (CAN ID: 22) with DIO encoder (ports 4, 5)
• Controller: Xbox Controller (Port: 0)

Implementation Steps

1. Create a new Subsystem class for position-controlled motor
2. Add motor controller and encoder instances with proper initialization
3. Create a method to convert revolutions to encoder units
4. Implement a command that sets the target position to current position + 5 revolutions
5. Use position control (PID) to drive the motor to the target position
6. Add button binding in RobotContainer to trigger the 5-revolution command
7. Add telemetry to display current position, target position, and motor output

Success Criteria

• Motor rotates exactly 5 complete revolutions when button is pressed
• Motor stops precisely at the target position without oscillation
• Command completes automatically when target is reached
• Multiple button presses work correctly (each press adds 5 more revolutions)
• Telemetry shows accurate position feedback throughout the motion

Hints and Troubleshooting

• For Step 1: Create a subsystem class like public class PositionMotorSubsystem extends SubsystemBase. Consider naming it based on the mechanism you're controlling (e.g., ElevatorSubsystem or ArmSubsystem).
• For Step 2: For Competition robot using NEO, initialize with motor = new CANSparkMax(CAN_ID, MotorType.kBrushless); and get the built-in encoder with encoder = motor.getEncoder();. For Testbed robot, use motor = new WPI_TalonSRX(CAN_ID); and create a separate encoder instance.
• For Step 3: NEO encoders report in rotations by default, so 5 revolutions = 5.0 encoder units. For quadrature encoders on Testbed, calculate: targetCounts = revolutions * pulsesPerRevolution * 4 * gearRatio. Use the encoder specs from the hardware documentation.
• For Step 4: Create an InstantCommand that calculates the new target position: () -> setTargetPosition(getCurrentPosition() + 5.0). Store the target position in an instance variable.
• For Step 5: For NEO motors, use the SparkMax PID controller: pidController = motor.getPIDController(); pidController.setReference(targetPosition, ControlType.kPosition);. For TalonSRX, use motor.set(ControlMode.Position, targetCounts);. Set appropriate PID constants in the constructor.
• For Step 6: In RobotContainer, bind to a button like: driverController.a().onTrue(subsystem.rotateForwardCommand());. Make sure the command requires the subsystem.
• For Step 7: In the subsystem's periodic() method, add: SmartDashboard.putNumber("Current Position", getCurrentPosition()); SmartDashboard.putNumber("Target Position", targetPosition);
• Common Issue: If the motor doesn't stop at the target, check your PID constants. Start with P=0.1, I=0, D=0 and tune from there.
• Common Issue: If positions are incorrect, verify your encoder conversion calculations and make sure the encoder is properly connected and configured.
• Safety Tip: Add reasonable position limits to prevent the motor from rotating indefinitely if there's a sensor failure.

Challenge Extensions

• Bidirectional Control: Add a second button that rotates the motor backward 5 revolutions.
• Variable Revolutions: Use different buttons for 1, 3, 5, and 10 revolution movements.
• Speed Control: Add a parameter to control how fast the motor rotates to the target position.
• Soft Limits: Implement software limits that prevent the motor from exceeding safe position ranges.
• Homing Sequence: Add a limit switch and create a command that homes the motor to a known zero position before executing rotations.
• Continuous Operation: Create a command that continuously rotates the motor 5 revolutions, waits 2 seconds, then repeats until cancelled.

Learning Resources

• WPILib Documentation: PID Control
• WPILib Documentation: Encoders
• REV SparkMax PID Control
• CTRE TalonSRX Position Control
• WPILib Example: Encoder Bot
• FRC Team 3476 - Position Control Examples
• Chief Delphi: Motor Position Control Discussion
• REV Robotics: Understanding Encoder Feedback