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Design of lower limb exoskeleton


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Design of lower limb exoskeleton for paraplegic patients

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Design of lower limb exoskeleton

  1. 1. Design of Exoskeleton for Paraplegic Patients Alok Bharadwaj | Aditya S. N | Anirudh V Kaushik R V College of Engineering Bangalore
  2. 2. Overview • Kinematics • Dynamics • Design • Analysis • Controlling • Design Features – Comfort level, – Manufacturability, – Compactness and structural stability.
  3. 3. Kinematics of Mechanism 1200 1200
  4. 4. Kinematics of Mechanism • Hip (Passive joint)
  5. 5. Kinematics of Mechanism • Hip and Knee joint • Use of mechanical stoppers to limit the angular movement of limbs.
  6. 6. Kinematics of Mechanism • Foot joint (Ball and Socket)
  7. 7. Dynamics of Mechanism Assumptions Mass of all links = 4kg Mass of all motors = 4kg Mass of battery pack = 2 kg Weight of human considered = 1000N Leg portion Length Mass Weight L1 450mm 5kg 50N L2 500mm 3kg 30N
  8. 8. Dynamics of Mechanism • Case 1 - Standing up Figure A Figure B
  9. 9. Dynamics of Mechanism • Standing up Torque required by motor at M2 = (1000 N) X (450 mm) = 450000 Nmm However, as the motion begins, the position of upper body keeps changing. Thus, due to change of Center of gravity, required torque is much lower progressively. Also, the crutches take up most of the body weight as the person can easily bend upper body to stand up. This value is rounded off to 50000 Nmm shared by each leg. Thus, torque at each motor M2 is 25 Nm.
  10. 10. Dynamics of Mechanism • Case 2 - Climbing stairs Torque required by knee motor = (1100 N) X (335 mm) =368500 Nmm After considering the load taken up by using crutches and bending forward in order to climb stairs, this value is considerably lesser. (Assumed to be 50 Nm for each knee motor in each leg) Thus, the maximum torque at knee considering previous case too becomes 50 Nm.
  11. 11. Dynamics of Mechanism • Case 3 – Raising leg (maximum extent) Torque required by motor at hip = (110 N) X (450 mm) =49500 Nmm Thus, the maximum torque required at the hip joint is around 50 Nm at each motor of the hip joint.
  12. 12. Design • Frame
  13. 13. Design • Thigh Link
  14. 14. Design • Shin Link
  15. 15. Design • Leg
  16. 16. Design • Complete Assembly – Ez Walk
  17. 17. Design • Draft
  18. 18. Design • Choosing the Battery: – For given maximum load @ 20rpm, the torque required is 50Nm. – Required power = 100W per motor at 24V DC – Total max power input = 400W for motors – Overall power output = 500W – Battery assumed to last for two hours – Capacity needed 40000mAh – Weight of LiPo batteries ~ 3.5Kg
  19. 19. Analysis • Material Used: Al 6061 – Ultimate strength: 310Mpa – Yield strength: 276Mpa – Shear strength: 207Mpa – FOS considered: 3 • Failure mechanism: Von Mises
  20. 20. Analysis • Critical Link: Thigh link
  21. 21. Analysis • Critical section
  22. 22. Controlling the mechanism
  23. 23. Initiating walking • Angle measurement using strain gauge measures the angle of upper body with respect to ground. • Length of step proportional to step length.
  24. 24. Use of Ultrasonic Sensors • 2 sensors are used in each leg – one at hip strap and one at foot • Hip sensor – senses variation in the terrain • Foot sensor – senses the vertical distance to the ground
  25. 25. Different cases requiring control Walking
  26. 26. Different cases requiring control Climbing Stairs UP
  27. 27. Different cases requiring control Climbing Stairs DOWN
  28. 28. Safety Feature • Crutches are provided to give proper balance when the person bends forward • While climbing down, if the person leans forward (variation in strain gauge), the mechanism is turned off preventing his fall.
  29. 29. Advantages of Design Comfort • Use of straps for even distribution of load • Weight evenly distributed on both sides • Weight of mechanism not being transferred to the body.
  30. 30. Advantages of Design Manufacturability • Links used and Aluminum 6061 are readily available in the market. • Assembly is simple. Most assembly is just through press fit. Fasteners are required only at a few places. • No requirement of a CNC for manufacturing of any component • Ball socket joints can be readily bought off the shelf
  31. 31. Advantages of Design Weight • Aluminum has a very low density making the mechanism very light • Evenly distributed loads at all locations. No point loads. • Weight evenly distributed b/w both legs resulting in no lateral CG offset. This makes the mechanism easy to use • Low weight motors
  32. 32. Advantages of Design Special features • Adjustable links • Portability – ease of assembly. Links can be press fitted into each other. Very less use of fasteners.