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Week 10 of 12 · Mechanical Reliability

Feeder Hopper & Agitator Singulation

Solve the classic mechatronic challenge: particulate bulk flow, hopper bridging physics, and oscillating agitator geometry to feed exactly one Skittle at a time.

~5 hrs CAD + Mechanical Iteration Singulation

Concept Particulate Bulk Mechanics & Bridging

Singulation (converting a jumbled pile of candy into a single-file queue) is one of the most deceptively difficult problems in robotics. In software, queues are arrays. In physical machines, solids jam due to friction and force vectors.

Hopper Bridging / Arching: When bulk Skittles pour into a funnel, inter-particle friction causes the candies to form an interlocking structural arch over the exit hole. If the hopper funnel angle is too shallow (<50° from horizontal), the compressive force from candies above locks the arch in place, stopping all flow.

The Two Engineering Solutions:
1. Steep Funnel Wall Angle (≥65°): Forces normal gravity forces to exceed static wall friction.
2. Active Agitation (agitator.stl): A small rotating or oscillating paddle at the base of the funnel physically disrupts forming arches and aligns candies into a single-file flat orientation before dropping into the wheel nests.

Kit What you need this week

  • 3D printer + PLA filament
  • A full bag of fresh Skittles (at least 50 candies)
  • Digital calipers & notebook for jam logging
  • Week 6 inspector wheel assembly
  • Reference parts: 3D Files/feeder_body.stl, cone.stl, agitator.stl, the_middle_maker.stl

Build Step by step

Step 1

CAD: Model the steep hopper cone (`cone.stl`)

In Fusion 360:

  • Create a revolved cone with an upper opening of ∅ 70mm tapering down to an exit bore of ∅ 18mm over a vertical drop of 55mm (cone wall angle = 68° from horizontal).
  • Model a 1.5mm wall thickness with a reinforced upper rim.
  • Add a top lid interface flange (top_closure.stl) so candy cannot spill over when agitated.
Step 2

CAD: Design the singulation throat & agitator

Below the funnel sits the singulation throat (the_middle_maker.stl):

  • Design the drop aperture as a calibrated rectangle: 15.0 mm × 10.0 mm. A single Skittle (∅ 13.4mm × 6.5mm) passes easily when oriented flat, but two Skittles side-by-side (26.8mm) or on edge are physically blocked.
  • Model the oscillating paddle (agitator.stl): an eccentric sweeping arm that sweeps across the throat floor with 1.5mm clearance, clearing candy jams without crushing their sugar shells.
Step 3

Print & bench fit

Print the feeder housing in 0.2mm layer height with 3 perimeters. Assemble the hopper directly above the wheel pocket loading station (Position 4).

Step 4

The 50-candy feed trial & failure logging

Load 50 Skittles into the hopper and run 50 continuous indexing cycles. Log each cycle outcome into a 3-column table:

  • Clean Feed (1 Skittle): Dropped cleanly into the nest pocket.
  • Double Feed (>1 Skittle): Second candy fell into or bounced off the wheel. (Fix: reduce throat aperture length by 0.5mm).
  • Bridge Jam (0 Skittles): Candies locked above the throat. (Fix: increase agitator sweep angle or increase funnel steepness).
Step 5

Iterate CAD geometry

Based on your log, adjust the specific throat dimension in Fusion 360, reprint the throat insert, and retest until you achieve 10 consecutive clean feeds.

68° Steep Funnel (Anti-Bridge) Singulation Throat 15mm × 10mm Aperture (Blocks Side-by-Side Double Feed) Agitator Paddle (agitator.stl)
Figure 10.1: Hopper funnel wall angle, singulation throat geometry, and anti-jam agitator paddle.
Make it yours
  • Try designing a gravity-fed zig-zag slide feeder instead of a vertical hopper cone and compare jam rates.
  • Add a soft silicone or TPU wipe blade to the agitator tip to see if flexible contact reduces candy chipping.

If it breaks Troubleshooting

Candies bridge and stop fallingThe funnel walls are too shallow. Increase wall angle to >65° or increase the oscillating amplitude of the agitator paddle.
Candies getting crushed / chippedThe agitator paddle is too close to the throat edge. Increase clearance between paddle sweep and throat wall to 2.0 mm.
This week's checkpointThe feeder hopper and agitator are mounted above the inspector wheel. Ten consecutive Skittles feed cleanly into indexing nests without a jam or double-feed.
Next week: Full integration — unifying the feeder, inspector wheel, AS7341 spectral classifier, sorting slide, and LCD menu into one robust state machine.
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