Mechanisms

Mechanisms move game pieces or change the robot's physical configuration. The job is to produce repeatable motion safely, even when the load and geometry are not exactly as expected.

Define the contract

Document what the mechanism can do, what must be true before it moves, how it detects completion, and what it does when a sensor or actuator gives an unexpected result.

Coordinate explicitly

When mechanisms can collide or compete for power, use named safe states and explicit transition rules. Avoid spreading interlock logic across unrelated buttons and commands.

Separate goals from control

The rest of the robot should request a physical goal. The mechanism decides how to reach it within its limits.

public enum ElevatorGoal {
  STOW(0.05),
  INTAKE(0.18),
  SCORE_HIGH(1.42);

  public final double heightMeters;

  ElevatorGoal(double heightMeters) {
    this.heightMeters = heightMeters;
  }
}

Named goals give controls, autonomous code, telemetry, and tests a shared vocabulary. Keep the physical value beside the goal or in configuration, not in button bindings.

Enforce limits in the subsystem

private double safeVoltage(double requestedVolts, Inputs inputs) {
  if (!inputs.encoderConnected()) return 0.0;
  if (inputs.upperLimit() && requestedVolts > 0.0) return 0.0;
  if (inputs.lowerLimit() && requestedVolts < 0.0) return 0.0;
  return MathUtil.clamp(requestedVolts, -maxVolts, maxVolts);
}

Operator controls may add another layer of protection, but the subsystem remains responsible for its own physical limits.

Know the mechanism state

Track whether the mechanism is uninitialized, homing, ready, moving, or faulted. Position alone does not tell the full story.

public enum MechanismState {
  UNINITIALIZED,
  HOMING,
  READY,
  MOVING,
  FAULTED
}

Define what transitions are allowed and what event causes each one. Homing must have a timeout and a response for a switch that never changes.

Test the edges

• Approach both travel limits slowly.
• Interrupt motion in both directions.
• Disconnect or invalidate each position sensor.
• Start from an unknown position.
• Apply a heavier and lighter load.
• Request conflicting mechanism states.
• Brown out or disable during motion, then re-enable safely.

For mechanism programmers

Mechanism code commonly involves gearing, current limits, motion profiles, limit detection, homing, feedforward, closed-loop control, command requirements, and coordination with the rest of the superstructure.