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Arm Mechanism

Rotary mechanisms enable the pivoting of mechanisms through rotational movement. These mechanisms come in many shapes and sizes. The arm mechanism is designed to control a pivot joint that rotates to a desired angle. It supports position control, velocity control, and duty cycle control while handling sensor conversions and simulation behind the scenes.

2910 pivot arm6036 pivot arm

Common Use Cases​

  • Intake pivots
  • Shooter or hood pivots
  • Climber arms

Typical Controls​

  • Position control for moving to a fixed angle.
  • Motion profile position control for smoother movements with controlled acceleration.
  • Velocity control for constant-speed motions.
  • Duty cycle for manual/teleop override.

Constructor Parameters​

The ArmMech constructor requires the following parameters:

  • logging_prefix - String prefix for logging (typically from getSubsystemKey())
  • mech_name - Optional name for multiple instances (pass null if only one arm)
  • motor_configs - List of MotorConfig objects, leader first
  • gear_ratio - Gear ratio as motor rotations / mechanism rotations
  • arm_length - Length of the arm in meters (used for simulation)
  • arm_mass - Mass of the arm in kg (used for simulation)
  • min_angle - Minimum angle in radians (used for simulation)
  • max_angle - Maximum angle in radians (used for simulation)
  • gravity_compensate - Optional boolean to enable/disable gravity compensation (default: true)

Example Construction​

// In your subsystem constructor
pivot_ = new ArmMech(
    getSubsystemKey(),           // Logging prefix
    "Pivot",                      // Mechanism name (optional)
    List.of(CONSTANTS.PIVOT_MOTOR_CONFIG),  // Motor configs
    CONSTANTS.PIVOT_GEAR_RATIO,  // Gear ratio
    CONSTANTS.PIVOT_LENGTH,       // Length (m)
    CONSTANTS.PIVOT_MASS,         // Mass (kg)
    CONSTANTS.PIVOT_MIN,          // Min angle (rad)
    CONSTANTS.PIVOT_MAX           // Max angle (rad)
);
// Set initial position (for zeroing)
pivot_.setCurrentPosition(CONSTANTS.PIVOT_HOME_POSITION);

Common Methods​

Control Methods​

  • setTargetPosition(angle_rad) - Standard position control
  • setTargetPositionMotionProfile(angle_rad) - Position control with motion profiling
  • setTargetPositionWithFF(angle_rad, feedforward) - Position control with arbitrary feedforward
  • setTargetPositionMotionProfileWithFF(angle_rad, feedforward) - Motion profile with feedforward
  • setTargetVelocity(velocity_rad_per_sec) - Velocity control
  • setTargetDutyCycle(duty_cycle) - Open-loop voltage control (-1.0 to 1.0)

Feedback Methods​

  • getCurrentPosition() - Returns current angle in radians
  • getCurrentVelocity() - Returns current velocity in radians per second
  • getLeaderCurrent() - Returns leader motor current draw in amps

Calibration​

  • setCurrentPosition(angle_rad) - Sets the current position (for zeroing/homing)

Simulation​

  • applyLoadTorque(torque_nm) - Applies external load torque at the arm shaft (simulation only)

Tips​

  • Use motion profile control for smoother movements with large position changes.
  • The mechanism automatically handles gravity compensation if enabled in the constructor.
  • Always set an initial position using setCurrentPosition() after construction to establish a known zero point.
  • Use feedforward control to improve tracking performance on demanding trajectories.
  • Keep units consistent (radians for angles, radians per second for velocities).
  • For multiple arms in one subsystem, provide a unique mech_name to distinguish them in logs.