Mini-Project 4: A Pivoting Roller Intake

The goal#

A pivoting intake that deploys over the bumper, spins compliant rollers to grab a game piece, and retracts. Two motions: a pivot (position) and a roller (raw speed).

Step 1 — The pivot#

The pivot only needs to fight the intake's own weight (light, ~3-4 lb at a short moment arm), so a NEO 550 (REV-21-1651) or a single NEO through a MAXPlanetary 9:1 + a small chain reduction is plenty. Because the arm is light, you can often skip a full ProfiledPIDController and use a simple position loop with a kG term, but profiling still gives gentler deploys that don't bounce game pieces out.

Mount a hard stop at the deployed position so the mechanism rests on metal, not on motor torque. Use an absolute Through Bore Encoder V2 (REV-11-3174) on the pivot axle for reliable startup angle.

Step 2 — The rollers#

Rollers want speed and grip, not precision. Run 2 in compliant wheels on a 1/2 in hex shaft, driven by a NEO 550 or a brushed motor through a light reduction (e.g. 3:1 to 4:1) so surface speed stays high. Set a modest current limit (20-30 A) so a jammed game piece doesn't cook the motor.

Step 3 — Command-based sequencing#

The magic is the sequence: deploy, then spin, then sense, then retract. In WPILib command-based:

public Command intakeSequence() {
  return Commands.sequence(
      pivot.setGoalCommand(DEPLOYED_RAD),
      Commands.waitUntil(pivot::atGoal),
      rollers.runCommand(0.8) // duty cycle
          .until(this::hasGamePiece),
      Commands.parallel(
          rollers.stopCommand(),
          pivot.setGoalCommand(STOWED_RAD)));
}

Step 4 — Sensing the catch#

A cheap, reliable game-piece detector is a beam-break sensor or a current spike on the roller motor (current rises when a piece loads the rollers). Either feeds hasGamePiece(). Avoid relying solely on driver timing; an automatic stop prevents intake jams and double-loads.

Step 5 — Mechanical robustness (lessons from real teams)#

• Use compliant/squishy wheels so misaligned game pieces still funnel in; rigid wheels reject anything off-center.
• Polycarbonate side guides funnel the piece into the rollers.
• Route the roller motor power and the encoder cable through the pivot with a service loop so repeated deploy/retract cycles don't fatigue the wire. Pivoting intakes are a top source of broken wires mid-event.