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Lift Assist Arm Brace

Lift Assist Arm Brace. Group 44 Erin Gallagher, Sudheer Potru, and Ronson Yong Senior Design Spring 2006. Overview. Introduction Background Filtration and Amplification Microcontroller Motor and Mechanical Brace Testing Safety Successes and Challenges Future Work Conclusion.

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Lift Assist Arm Brace

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  1. Lift Assist Arm Brace Group 44 Erin Gallagher, Sudheer Potru, and Ronson Yong Senior Design Spring 2006

  2. Overview • Introduction • Background • Filtration and Amplification • Microcontroller • Motor and Mechanical Brace • Testing • Safety • Successes and Challenges • Future Work • Conclusion

  3. Introduction • Brace senses muscle contraction to assist weakened muscles with the lifting of light objects • Restores normal ability to perform everyday tasks

  4. Features • 60-lb rated lifting power • Lightweight • Easy to put on and wear • Padded for comfort • No external controls necessary • Battery power supply

  5. Biopotential • Nerve impulse reaches axon and releases acetylcholine (ACh) • ACh changes permeability causing Na+ to diffuse in • At threshold voltage, Na+ rushes in, creating action potential

  6. The Electromyelogram (EMG) • Picks up depolarization voltages of motor endplates • 1-5 mV signal • 20 – 450 Hz frequencies • Majority of signal from 20 – 150 Hz • Biphasic

  7. Original Block Diagram

  8. Final Block Diagram

  9. Filtration and Amplification Circuit (FAC) Schematic • Input from muscle electrodes • RC high-pass filter (8.2nF and 100MΩ) into differential amplifier • Instrumentation Amplifier (100X) • Bandpass filter (18 Hz to 145 Hz) • Additional 100X Amplification • Output to PIC

  10. Bandpass Filter Frequency Response Simulation (SPICE)

  11. FAC Design Considerations • Initial Design • Bandpass (20 to 450 Hz) • Bandstop (60 Hz) • Differential Amplification Gain = 1 • Used AD622 Instrumentation Amplifier • Final Design • Bandpass (18 to 145 Hz) Sallen-Key Topology • Bandstop unnecessary because differential amplification eliminates 60 Hz noise via common electrode • Differential Amplification Gain  100 to obtain strong EMG signal • Changed to AD623 which is single-supply

  12. Amplification, Rectification, and Envelope Detection • Rectification • Rectifies Biphasic EMG signal • Envelope Detection • Smoothes out signal • ~3 Hz RC High Pass

  13. Output of Instrumentation Amplifier

  14. Output of FAC (0-5V)

  15. Motor-Driving Circuitry (MDC) Schematic • ADC conversion • PIC determines difference between bicep and tricep contraction strengths • Difference used for PWM duty cycle • If either bicep or tricep significant, assign high/low outputs for which muscle is stronger • PWM feeds transistor and is fed into relays • Relays switch polarity of motor leads

  16. Software Flowchart

  17. PWM Output

  18. MDC Design Considerations • Initial Design • Two transistors w/ one having -12V • MOSFETs do not work with negative voltages • Second Design • 1 relay, 1 transistor • Motor does not run on -12V • Final Design • Transistor feeds 2 relays which determine polarity

  19. Original PCB Design Layout

  20. Final Circuit Board

  21. Brace Design and Build

  22. Extended and Contracted Brace

  23. Final Brace Prototype

  24. Effect of Electrode Placement on EMG Voltage

  25. Effect of Lifting Weight on EMG Voltage

  26. Bandpass Filter Frequency Response

  27. Amplifier Gain Verification

  28. PIC ADC vs. Voltage Input

  29. PIC Duty Cycle Vs. Bit Number

  30. Power Consumption • Instrumentation Amplifier, Op Amps, and Pic • 5V @ 29mA Average  0.145W • Motor • 12V @ 3A max  36W

  31. Temperature Safety • Motor • Max. 36W before circuit overheats • Transistor • Heat dissipation plane • Cloth covering • 49°C (120°F) requires two minutes for burn

  32. Other Safety Concerns • Quick release • Moving parts • Exposed screws • Loose wires

  33. Successes Clean EMG Signal output Good PIC control output Working mechanical linkage Challenges Motor Driving Circuit Switching delays Power limits and thermal performance issues Successes and Challenges

  34. Future Work • Quick release • Adjustable functioning in response to weight • Different circuitry to reduce switching delay • Lightweight material for brace • Smaller, lighter motor • Adjustable brace arms for all sizes • Tight-fit sleeve to wear under clothes • Built-in electrodes for easy application and tight skin contact • Expandability for other limbs

  35. Credits • Professor Raymond Fish • Professor Stephen Boppart • Professor Scott Carney • Ms. Hyesun Park • Mr. Alex Spektor • Mr. Kevin Lee • Machine Shop and Parts Shop Staff

  36. Thank You! • Questions? • Comments?

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