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P14007: Wheelchair Assist: Subsystem Design Review

P14007: Wheelchair Assist: Subsystem Design Review. Che -An Lee – Industrial and System Engineer Dan Schuster – Mechanical Engineer Phil Medalie – Mechanical Engineer Tom Elliot – Electrical Engineer. Agenda. Functional Decomposition System Architecture Movement Assist System

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P14007: Wheelchair Assist: Subsystem Design Review

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  1. P14007: Wheelchair Assist: Subsystem Design Review Che-An Lee – Industrial and System Engineer Dan Schuster – Mechanical Engineer Phil Medalie – Mechanical Engineer Tom Elliot – Electrical Engineer

  2. Agenda • Functional Decomposition • System Architecture • Movement Assist System • Critical Subsystems • Gradual Grade • Hill Holder • Charging System • Update CR and ER • Test Plan • Risk Assessment • Project Schedule

  3. Functional Decomposition

  4. System Architecture

  5. Movement Assist System • Concept Validation and Feasibility • Analysis of chosen design • Integration with Other Systems • Results and Conclusion

  6. Concept Validation and Feasibility 1 2 • 4 Main Concepts • Feasibility of Crank mechanism • Mechanical Advantage versus difficulty of integration • Chosen Concept 3 4

  7. Comparison and Analysis of Designs • Evaluation of straight line crank mechanism • Showed that the device can provide a mechanical advantage due to crank arm • Main concerns and issues are due to actual construction  binding and sticking in joints of links and moving the crank around two end points of arc • Risk of trying to construct this crank versus the benefit resulted in going with regular crank arm • Gear Sets • Large mechanical advantage • Risks in cost and disengagement • Couldn’t find standard made planetary gear sets with the required footprint  too thick/large • Regular Crank directly coupled to shaft provides lowest risk and best design due to ease of integration and has the benefits of a good mechanical advantage

  8. Analysis of Chosen Design • FBD of Model and EOM • Minimum Torque Required to move system uphill • Key Assumptions • Rolling without slip • Frictionless bearings • Lumped Masses • Torque input from user is constant through Φ • Required Torque to turn generator is lumped into 1 term, Tmotor, and is constant • Starts from Rest

  9. Results With 1.5m Crank

  10. Results with Hand on Wheel Rim

  11. Conclusion of Movement Assist • Crank Design is feasible by itself and should be able to be achieved • Difficulty is in the user activation/disengagement and integration with other systems • Freewheel • Shaft attachment • Integration with braking and ratchet • Based on CR (component of lowest priority) we are putting this system on hold to focus on more critical systems = Hill Holder, Gradual Grade, and Energy Recovery

  12. Entire System

  13. Gradual Grade

  14. Braking System • Avid BB7 Disc Brakes • Controlled by bicycle brake handles • 160mm discs • Offers more control and less wear than rim brakes • Easily adjustable braking force • Wear items easily and inexpensively replaced • Normal state is disengaged

  15. Braking System Mounts • Mounting disc • Threaded onto end of shaft • Used as lock nut to allow shaft to spin with disc • Caliper Mount • Bent sheet metal • Located with hole in frame • Caliper attached via bolts through spacers

  16. Hill Holder

  17. Ratchet System • W.M. Berg Ratchet and Pawl • R16S20-64 • 4” PD, 64 Teeth • Can handle ~600 lbf • Roll back distance of ~1 inch

  18. Pawl System • Sheet metal arm • Locating slot for vertical motion • Spring pin at top • Used for engaging locking feature and for handle • Pawl is press fit onto end of arm • System weight will provide force to lock pawl in place

  19. Shaft Design

  20. Axle to Wheel Attachment • Spline exists in wheel • Not a standard spline size • May end up cutting keyway into wheel

  21. Moving Axle Backwards • May need to move axle backwards • Provides more room for brakes • Larger bearings can be used • Should not impact ergonomics • Similar bracket already exists on chair

  22. Advantages to Design • Very little modification to frame needed • Should work for many chairs • Easily removed to return to regular chair • Uses purchased parts where possible • Shaft and pawl mount need to be made • Wear items easily replaced • Still folds up to small size

  23. Harnessing Power Electrical Stages Rectifiers Generators Voltage Regulators Log Amplifiers Voltage Divider Battery Inputs

  24. Selecting a Motor for Generator * Two Motors are needed at to avg a electrical rate of .5-1 Amp at 5V Constantly. * Therefore 1.25-2.5 Watts needed on each motor when is spinning at 1.2mph(avg Wheelchair speed). * Torque to move the wheelchair be increased to a point where an elderly person cannot move the chair.

  25. Selected Motor CRA101-ND MOTOR AC GEARED 2.7W 120V 12RPM Therefore 10 peak volts/rpm • Used To Calculate Current after Full Rectification

  26. Simple Full Rectifier

  27. Different Kinds of Lin-Log Amplifier

  28. Up The Hill

  29. Down The Hill

  30. Battery Battery 2 Venom 2000mAh 7.4V 2S 1P 20C 1/16 LiPo Pack Battery Type: Lithium Polymer (LiPO)Configuration: 2S1PWeight: 3.88 oz.Dimensions: 18 x 34 x 90mmCharge Rate: 1C (40A)Continuous Discharge: 20C (40A)Min Discharge Volts Per Pack: 5.5VMax Burst Rate: 30C (60A)Max Volts Per Cell: 4.2VMax Volts Per Pack: 8.4VWatt Hours: 14.8

  31. Updated Customer Requirement

  32. Updated Engineering Requirements

  33. Test Plan

  34. Risk Assessment

  35. Project Schedule

  36. Questions/ Comments?

  37. Holding Force Calculation S = 60200 psi F = 0.375 in Y = 0.426 for 64 teeth DP = 16 V  Disregard for low speeds W = 601 lbf of allowable tooth load

  38. Roll Back Distance Calculation • 64 Teeth • 22” wheel diameter • 69” circumference • 69”/64 Teeth = 1.08”

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