Human Power Harvesting: “The Pendulator”

1. Human Power Harvesting:“The Pendulator” Jack Ingram Kristin Sensmeier ECE 445

2. The Idea Picture this: A Marine is 200 miles away from the closest electrical outlet and he needs to power his radio, but his battery pack is dead. What can he do? Solution: “The Pendulator!”

3. Objective Generate power at a rate of 1W from human movement

4. Existing Related Research • Shoe-mount generator • Sole of shoe (downward force) • Side of shoe (horizontal force) • Playground Equipment • Merry-go-round, swings, see-saw • Harvest energy using compressed air system

5. Motivation • New twist on the same concept of power harvesting • Add a small amount of work (mass) to a person’s body and be able to recover electrical energy from their movement (pendulum  motor system)

6. Block Diagram Note: Design Option #1 was chosen as the final design.

7. Pendulum Trip to Farm and Fleet • Chose the 5/32” gear wrench to serve as the ratcheting mechanism • Allows for turns of only 6 degrees per ratchet • Ratcheting gear itself is near lossless • Also looked at possible materials for pendulum itself

8. Ac Gear Motor Single phase motors too large and expensive Torque required to turn the shaft of the motor was too high for this system Dc Gear Motor Affordable and compact design Available for the voltage levels and gear ratios that were required for our project Motor Selection Winner: Dc Gear Motor!

9. Motor Specifications • DC PM Gear Motor platform (1.61.065.404-6V) by Buehler Motor Group • 56:1 gear ratio • Compact dimensions of 29 x 40 mm • Low initial torque of the shaft due to the lower input voltage of 6V

10. Pendulum Assembly: Fabrication Machine Shop • Fly wheel added to pendulum design • Pendulum effectively 7.5 inches long and weighs 9.6 oz. • Decided to use 3 inch diameter wheel of brass weighing 1 lb. 5 oz. for the fly wheel • Fly wheel allows for additional energy to be added to the system • Motor integrated into pendulum design to form complete assembly

11. Pendulum Assembly

12. Dc-dc Converter Design • Determined boost converter was needed (from ~1-2V to 4.5V) • Voltage level makes design from scratch difficult (problem: powering logic, switching) • Decided to go in search of a chip to meet our project’s needs

13. Converter Chip: Selection • Found chip with appropriate specifications from Texas Instruments • Synchronous Boost Converter, TPS61120PW • Minimum input voltage of 1.8V and variable output voltage option (max. 5.5V) • Surface mount chip package

14. Converter Chip: Implementation • Required to surface mount chip • Used Easytrax software to design a PCB board • Soldered pins into PCB board in order to interface with protoboard   

15. Converter Chip Components • Design of Inductor (20 µH) • Powdered iron core, T184-52 Green/Blue from Micrometals • AL ≈ 135 nH/turn • 13 turns, 20 AWG wire • Design of Capacitor (100 µF) • Calculated using needed output voltage vs. input voltage, also average operating frequency of chip at 500kHz and output ripple voltage of 10mV • Additional resistors used for voltage division for output level and load

16. Converter Chip: Schematic

17. What changed from original design? • Chose dc over ac system • Added fly wheel instead of external gear ratio • Did not attach battery charger

18. Pendulum Assembly: Testing Procedure • Mimicked swing of pendulum by walking to produce an output from motor • Varied the pendulum’s range of motion (22, 45, 67 and 90 degrees) • Connected outputs of motor to oscilloscope to determine output voltage

19. Pendulum Assembly: Testing Output Voltage and Current Waveforms at 90 degrees

20. Pendulum Assembly: Testing Output Power • P = VI • Vmax = 3.46 V, Imax = 75 mA • Pmax = 295 mW over a period of 400 ms

21. Pendulum Assembly: Testing Issue • Due to constant starting and stopping, the original waveforms from the motor were extremely rough and inconsistent Solution: • Added a 2200 µF capacitor across output of motor

22. Converter Chip: Testing • Needed to simulate potential input waveforms • Constant input to verify chip was functioning properly • Square wave input from function generator to simulate periodic input from pendulum (frequency was set to mimic pendulum’s frequency)

23. Converter Chip: Testing Constant Input Voltage and Resulting Output Voltage from Chip

24. Converter Chip: Testing Square Wave Input Voltage and Output Voltage from Chip

25. Converter Chip: Testing • Determined chip was functioning properly • Also, the output capacitor/resistor combination appeared to be holding voltage long after input went to zero • RC time constant (time for capacitor to discharge to 36.8% of initial voltage) • R3 + R5 = 1.8 MΩ + 200 kΩ = 2.0 MΩ • C = 100 µF + 2.2 µF = 102.2 µF • R*C = 204.4 s  3.41 minutes!

26. Overall System: Testing • Connected the pendulum assembly to the converter circuit • Able to produce enough voltage from the pendulum to power the logic and meet threshold requirements for the chip

27. Pendulum Assembly: Efficiency Input Energy • The energy injected into the motor via the pendulum and flywheel can be determined by the following equations of motion: • Pendulum: KE = m*g*l*sin(θ) where m = mass = 9.6oz = .272kg g = 9.81 m/s2 l = 7.5 in = .1905m θ = rotation of weight = 90 degrees • KE due to the pendulum = .508 J • Flywheel: KE = ½ * k * M * R2 * W2 where k = ½ (uniform circular disc) M = mass = 1lb. 5 oz. = .595kg R = radius = 7.5 in = .0762m W = rotational speed = 90 degree turn/400ms period = 37.5 RPM. • KE due to the fly wheel = 1.213 J • Total KE of the system = 1.71 J

28. Pendulum Assembly: Efficiency We can determine how efficiently the motor converted mechanical into electrical energy • η = [Pmax/(Total KEin/400ms)]*100 • η = 6.86 % (maximum efficiency)

29. Converter Chip: Efficiency • Maximum efficiency with square wave pulse input • Pin = 2.56V*0.220A = 0.563W • Pout = 4.00V*0.096A = 0.384W • η = 68.18% • Decreased efficiency due to inconsistent input, but still good

30. Overall System: Efficiency Complete Circuit Output Voltage, Current and Power Waveforms

31. Overall System: Efficiency Pmax = 350.9 mW Period = 400 ms η = [350.9mW/(1.71 J/400 ms)]*100 η = 8.2 %

32. Obstacles • Motor Issues • Shipping problems with Buehler Motor Group • Defective motor causes several weeks delay • Chip Issues • Incorrect dimensions from TI needed for surface mounting • Contacted Tech Support to obtain correct information, delay of several weeks • Functionality Issues • Not enough torque to turn shaft of motor, added fly wheel

33. Cost Analysis: Parts

34. Cost Analysis: Labor and Total Labor: ($35/hour)*2.5*(13 weeks)*(12hours/week)*2 engineers = $27,300 Total Cost: Labor + Parts = $27,363.98 (-6.28% change from initial estimate)

35. Final Results • Wanted 1W output power, were able to obtain 140mW maximum output power • Still a ways to go for this type of design, but progress

36. Scaling Factor • How much bigger would our system need to be to obtain 1W? • 140 mW max.  1W = about 7 times • Fly wheel: (21 oz.*7)/16 oz. = 9 lbs. • Pendulum: (9.6 oz.*7)/16 oz. = 4 lbs. • Motor: bigger motor to handle extra weight on the shaft • Chip would be able to handle increase until current increases to above 1.1A

37. Scaling Factor • Feasible to make the new system • However, not appropriate for the application we envisioned • Portable • Lightweight

38. Recommendations Placement of pendulum • Hip, knee, arm • Motor with higher gear ratio • Use both ranges of pendulum motion to create AC signal

39. Thanks to… • Prof. Chapman • Jonathan Kimball • Jing Tang • Chad Carlson • Machine Shop – Scott McDonald • Parts Shop – Jim Wehmer • Texas Instruments • Buehler Motor Group

40. Questions?