Automatic Cat Feeder

MEMS1049 — Spring 2026
Team 27 — James Puzon, Tom Kisiel, Sean Savidge

Project Demo

System Overview

Motor Analysis

The primary dispensing actuator is a stepper motor. It rotates in 90° increments to move a paddle wheel and dispense repeatable portions of food from the reservoir into the bowl.
For analysis, the interaction between the paddle and food is modeled as an equivalent drag/friction force on one paddle face.

Free-Body Diagram

Speed Derivation

Variable form

n = Nrev / tdispense

Where:

• n: required rotational speed (rev/s)
• Nrev: revolutions needed to dispense one full portion (rev)
• tdispense: desired dispensing time (s)

Numeric form

n = 2.5 rev / 5 s = 0.5 rev/s

0.5 rev/s × 60 s/min = 30 RPM

Torque Derivation

Step 1: Rotational Dynamics

Using Newton's second law for rotation:

ΣM = Iα

Summing moments about the shaft axis (CCW positive):

τMotor - FDrag · r = Iwheel · α

Step 2: Inertia Check

Estimated paddle-wheel inertia (plastic, ~20 g, R ≈ 0.02 m), approximated as a solid disk:

Iwheel ≈ (1/2) mR² ≈ (1/2)(0.02)(0.02)² ≈ 4×10-6 kg·m²

Worst-case angular acceleration (0 to 30 RPM in ~0.1 s):

αmax ≈ π rad/s / 0.1 s ≈ 31 rad/s²

Inertial torque:

Iα ≈ (4×10-6)(31) ≈ 1.3×10-4 N·m ≈ 0.013 N·cm

This is approximately 0.25% of the calculated resistance torque, so inertia is neglected (α ≈ 0):

τMotor = FDrag · r

Step 3: Drag Force Estimate

The paddle moves through granular food, so FDrag is modeled as Coulomb friction:

Wfood = mfood · g

FDrag = μfood · Wfood

Where μfood ≈ 0.5 (estimated kibble-plastic friction coefficient). This is an estimate; testing can refine it.

Step 4: Required Torque with Safety Factor

τMotor,req = SF · FDrag · r

Using SF = 2 to account for:

• Friction uncertainty
• Startup compaction/friction effects
• Secondary losses (e.g., wall rubbing, pellet jams)

Numeric form

Wfood = 0.496 kg · 9.81 m/s² = 4.87 N

FDrag = 0.5 · 4.87 = 2.44 N

r = 0.8 in = 0.0203 m

τMotor,req (no SF) = 2.44 · 0.0203 = 0.0495 N·m = 4.95 N·cm

τdesign = 2 · 4.95 = 9.9 N·cm

Torque-Speed Diagram

Motor Specifications

Motor: Pololu #1200 (SOYO SY42STH47-1206A)

• Rated voltage: 4 V/phase
• Rated current: 1.2 A/phase
• Coil resistance: 3.3 Ω/phase
• Holding torque: 3.2 kg·cm (31.4 N·cm)

Vrated = Irated · Rcoil = 1.2 × 3.3 = 3.96 V ≈ 4 V

Power Electronics Selection

Selected driver: Pololu A4988 Stepper Motor Driver Carrier

• Supported VMOT range: 8 V to 35 V
• Project supply: 12 V wall adapter (within range)

Circuit Design