Building the Inspector Wheel & Optical Shroud

Measure your Skittles with calipers

Measure 5 random Skittles across diameter and thickness. You will typically find ∅ 13.0–13.4mm diameter and 6.0–6.8mm thickness. Define your pocket parameters:

pocket_diam = 14.2 mm    # 13.4mm max + 0.8mm clearance
pocket_depth = 7.0 mm   # Full candy thickness
draft_angle = 7.0 deg   # Draft to prevent gravity sticking

Model the 4-pocket inspector wheel

In Fusion 360:

• Sketch a circle ∅ 60mm on the XY plane and extrude 8.0mm.
• Create a circular pattern of 4 nest pockets at r = 20.0 mm radius. Extrude as Cut with a 7° taper angle.
• Add a 1.2mm high retaining rim around the outer edge to retain candy under centrifugal motion.

Model the D-shaft collar & eccentric homing cam (`cam.stl`)

The inspector wheel must trip the limit switch once per revolution to establish an absolute physical datum ($0.0^\circ$) without using a hard stop that would block continuous 360° forward rotation. On the underside of the wheel (or as the dedicated interlocking part cam.stl):

• Central Hub Boss: Extrude a central cylinder (∅ 24.0mm base circle, height 12.0mm).
• D-Shaft Positive Bore: Cut a 5.15 mm D-shaft bore with a 4.5 mm flat edge offset.
• Transverse M3 Grub Screw Clamp: Add a transverse M3 heat-set insert pilot hole (∅ 3.2mm × 4.5mm depth) perpendicular to the D-flat. Tightening an M3 grub screw against the flat guarantees zero rotational slipping over thousands of cycles.
• Eccentric Lobe Profile: Model a smooth tangential ramp that rises +3.5mm radially over a 25° angular sweep with $R=4.0\text{mm}$ blend fillets. As the wheel spins, this lobe gently depresses the roller microswitch lever on plate_stepper.stl without mechanical shock or contact bounce.

Model the optical darkroom shroud (`inspection.stl`)

The AS7341 sensor board mounts directly above the pocket wheel:

• Model a rectangular hood that encloses the sensor board with M2.5 mounting standoffs.
• Position the sensor's optical aperture at a calibrated 8.0 mm focal height above the Skittle's top crown.
• Add internal light-baffling lips that overlap the wheel without touching (0.8mm air gap), creating a labyrinth seal against ambient photons.

Print, assemble & calibrate indexing

Print the parts (black PLA is ideal for the optical shroud to absorb stray light). Install the M3 heat-set insert, slide the hub onto the motor shaft, and tighten the grub screw against the D-flat. Bolt the optical shroud over the assembly.

Code the 3-phase homing & 90° indexing routine

Write an automated 3-phase homing routine in Python that eliminates mechanical switch hysteresis and indexes 90° between pockets:

import RPi.GPIO as GPIO
import time
from adafruit_motorkit import MotorKit
from adafruit_motor import stepper

SWITCH_PIN = 20
STEPS_PER_REV = 200  # 1.8° NEMA-17 stepper (200 steps/rev)
OFFSET_TO_POCKET_1 = 14  # Calibrated step offset to center Pocket #1 under sensor

GPIO.setmode(GPIO.BCM)
GPIO.setup(SWITCH_PIN, GPIO.IN, pull_up_down=GPIO.PUD_UP)
kit = MotorKit(0x60)

def precision_home_wheel():
    print("Homing inspector wheel...")
    # Phase 1: Fast Seek forward until cam trips switch (active LOW)
    steps = 0
    while GPIO.input(SWITCH_PIN) == GPIO.HIGH:
        kit.stepper1.onestep(direction=stepper.FORWARD, style=stepper.SINGLE)
        steps += 1
        time.sleep(0.008)
        if steps > 250:
            kit.stepper1.release()
            raise RuntimeError("Homing timeout: Cam switch not found!")
    
    # Phase 2: Slow reverse release to eliminate mechanical hysteresis
    time.sleep(0.05)
    while GPIO.input(SWITCH_PIN) == GPIO.LOW:
        kit.stepper1.onestep(direction=stepper.BACKWARD, style=stepper.SINGLE)
        time.sleep(0.03)  # Slow precision speed

    # Phase 3: Step forward by fixed offset to center Pocket #1 under optical sensor
    for _ in range(OFFSET_TO_POCKET_1):
        kit.stepper1.onestep(direction=stepper.FORWARD, style=stepper.SINGLE)
        time.sleep(0.01)

    kit.stepper1.release()
    print("✓ Wheel homed and Pocket #1 centered under optical shroud.")

def index_pocket(steps_forward=50):
    """Index exactly 90 degrees (50 steps on 200-step motor)"""
    for _ in range(steps_forward):
        kit.stepper1.onestep(direction=stepper.FORWARD, style=stepper.SINGLE)
        time.sleep(0.008)
    kit.stepper1.release()