Design Innovation Paper: Design Innovation Papers

Design and Treadmill Test of a Broadband Energy Harvesting Backpack With a Mechanical Motion Rectifier

[+] Author and Article Information
Yue Yuan, Mingyi Liu, Wei-Che Tai

Department of Mechanical Engineering,
Virginia Tech,
Blacksburg, VA 24061

Lei Zuo

Department of Mechanical Engineering,
Virginia Tech,
Blacksburg, VA 24061
e-mail: leizuo@vt.edu

Contributed by the Design Innovation and Devices of ASME for publication in the JOURNAL OF MECHANICAL DESIGN. Manuscript received December 4, 2017; final manuscript received April 16, 2018; published online May 23, 2018. Assoc. Editor: Ettore Pennestri.

J. Mech. Des 140(8), 085001 (May 23, 2018) (8 pages) Paper No: MD-17-1804; doi: 10.1115/1.4040172 History: Received December 04, 2017; Revised April 16, 2018

The energy harvesting backpack that converts the kinetic energy produced by the vertical oscillatory motion of suspended loads to electricity during normal walking is a promising solution to fulfill the ever-rising need of electrical power for the use of electronic devices in civilians and military. An energy harvesting backpack that is based on mechanical motion rectification (MMR) is developed in this paper. Unlike the conventional rack-pinion mechanism used in the conventional energy harvesting backpacks, the rack-pinion mechanism used in the MMR backpack has two pinions that are mounted on a generator shaft via two one-way bearings in a way that the bidirectional oscillatory motion of the suspended load is converted into unidirectional rotation of the generator. Due to engagement and disengagement between the pinions and the generator shaft, the MMR backpack has broader bandwidths than the conventional energy harvesting backpacks; thus, the electrical power generated is less sensitive to change in walking speed. Two male subjects were recruited to test the MMR backpack and its non-MMR counterpart at three different walking speeds. For both subjects, the MMR backpack for most of the time generated more power than the non-MMR counterpart. When compared with literature, the MMR backpack had nearly sixfold improvement in bandwidth. Finally, the MMR backpack generated nearly 3.3 W of electrical power with a 13.6 kg load and showed nearly two- to tenfold increases in specific power when compared with a conventional energy harvesting backpack.

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Grahic Jump Location
Fig. 1

MMR-based PTO unit: (a) CAD design drawing and (b) working principle of MMR (the mounting bearings are not shown)

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Fig. 2

Design of a suspended-load backpack frame: (a) front view and (b) side view

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Fig. 7

Measured average power of two subjects at different electrical resistance. Walking speed: 4.83 km/h. Suspended load: 13.6 kg. The power was averaged over the last 30 s in each test trial. Three test trials were averaged to obtain the final average power and error bars.

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Fig. 8

Instant power of MMR and non-MMR backpack for eachsubject: (a) subject A and (b) subject B. Walking speed: 4.83 km/h. The optimal electrical resistance of each subject (cf.Fig. 7) was used.

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Fig. 6

Treadmill test setup: (a) Backpack experimental rig. The PTO is installed in the cutout of the moving board while the racks are fixed to the fixed board. A 13.6 kg dead load is also attached to the moving board. A laser displacement sensor is used to measure the relative displacement of the suspended load. (b) Illustration of treadmill testing.

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Fig. 5

Simulated frequency response functions of MMR and non-MMR backpacks show that the former has a broader bandwidth. Note that both backpacks are subjected to sinusoidal excitations of varied excitation frequencies and a constant amplitude of 25 mm. Also note that the measured mechanical damping coefficients cmen=125, cmde=9, and cg = 31 Ns/m and simulated optimal stiffness k = 3172 (MMR) and k = 5345 N/m (non-MMR) are used (cf. Fig. 4). Finally, cm,100%en=125 and cm,50%en=62.5 Ns/m.

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Fig. 4

Average power versus spring constant and electrical resistance: (a) MMR backpack and (b) Non-MMR backpack. Excitation conditions: base excitation with 25 mm amplitude and 1.96 Hz excitation frequency. Note that the measured mechanical damping coefficients cmen=125, cmde=9 and cg = 31 Ns/m are used.

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Fig. 3

Mathematical model of MMR-based energy harvesting backpack. Note that the system consists of two states: engagement and disengagement states. During engagement state, the generator engages the backpack. During disengagement state, they are separate.

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Fig. 9

Average power of two test subjects at (a) different walking frequencies and (b) different walking speeds: 4.02, 4.83 and 5.63 km/h. Note that the leg length of subject A is shorted than subject B (cf. Table 2); thus, the fundamental frequencies of walking are higher; see Table 3. Three test trials were averaged to obtain the final average power and error bars at each walking speed.

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Fig. 10

Comparison of MMR energy harvesting backpack with literature [57]. Note that the dash lines are regeneration of Fig.4 of Ref. [5], where I, II and III represent backpack loads of 20 kg, 29 kg, and 38 kg, receptively. Dash-dotted line is of Fig. 4 of Ref. [5]. Dotted lines are regeneration of Ref. [7], where I and II represent backpack loads of 7.94 kg (17.5 lbs.) and 15.9 kg (35lbs.), respectively.



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