Context & users

Many manual wheelchair users could benefit from intermittent power assist, but most kits are expensive, heavy, or require permanent frame changes. Goal: a low-cost, removable, motorized friction-assist that keeps manual control intuitive.

Requirements / constraints

• No frame modification; compact add-on

• Predictable, safe behavior; safety interlock (e-stop), clear states, no creep

• Performance: works on flat and mild inclines; roadmap for weather/debris tolerance

• Build envelope: complete within an eight-week semester and a limited budget

Solution architecture

• Drive: motorized friction roller on the rear tire with 6.25:1 gearbox and NEO brushless motor (sized from load cases)

• Structure: modular, serviceable brackets (printed + machined) sized for stiffness and repeatability

• Engagement: linear-rail mechanism with momentary push control to apply/remove assist consistently

• Electronics: enclosed controller and wiring harness designed for field access and quick swaps

Engineering highlights

• Sizing & calcs: torque/speed trade-offs (target ≈ 137 W, 28 N·m stall); contact pressure for traction without tire wear

• FEA: bracket stiffness and housing stress; geometry tuned to avoid slip/chatter under load

• Controls: simple state diagram (idle → assist → e-stop); current limit and safe default behaviors

Build & test

• Bench: printed fixtures for rapid iteration; roller load, current draw, and basic thermal rise tests

• Field: measured speed on flat/mild inclines; logged current and temperatures; tuned roller compound and preload

• Tuning goal: preserve fine manual control while providing noticeable powered assist

Results

• Assist: walking-speed assist on flat; usable assist on mild inclines

• Handover: smooth handover between powered and manual modes

• Safety: integrated e-stop; no failures during multi-session, multi-km tests

What I’d ship next

• Weather sealing and debris guards on the roller/mechanism

• Manufacturable enclosure and harness; standardized hardware

• Endurance and wear testing; long-duration thermal and duty-cycle characterization

• Control refinements (engagement feel, current ramp, battery management)