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Research Papers

Lower Limb-Driven Energy Harvester: Modeling, Design, and Performance Evaluation

[+] Author and Article Information
Jean-Paul Martin, Michael Shepertycky, Qingguo Li

Bio-Mechatronics and Robotics Laboratory,
Department of Mechanical and
Materials Engineering,
Queen's University,
Kingston, ON K7L 3N6, Canada

Yan-Fei Liu

Power Group,
Department of Electrical and
Computer Engineering,
Queen's University,
Kingston, ON K7L 3N6, Canada

1J.-P. Martin and M. Shepertycky contributed equally to this work.

Manuscript received June 4, 2015; final manuscript received February 5, 2016; published online August 24, 2016. Assoc. Editor: Carl Nelson.

J. Med. Devices 10(4), 041005 (Aug 24, 2016) (9 pages) Paper No: MED-15-1207; doi: 10.1115/1.4033014 History: Received June 04, 2015; Revised February 05, 2016

Biomechanical energy harvesters (BMEHs) have shown that useable amounts of electricity can be generated from daily movement. Where access to an electrical power grid is limited, BMEHs are a viable alternative to accommodate energy requirements for portable electronics. In this paper, we present the detailed design and dynamic model of a lower limb-driven energy harvester that predicts the device output and the load on the user. Comparing with existing harvester models, the novelty of the proposed model is that it incorporates the energy required for useful electricity generation, stored inertial energy, and both mechanical and electrical losses within the device. The model is validated with the lower limb-driven energy harvester in 12 unique configurations with a combination of four different motor and three different electrical resistance combinations (3.5 Ω, 7 Ω, and 12 Ω). A case study shows that the device can generate between 3.6 and 15.5 W with an efficiency between 39.8% and 72.5%. The model was able to predict the harvester output peak voltage within 5.6 ± 3.2% error and the peak force it exerts on the user within 9.9 ± 3.4% error over a range of parameter values. The model will help to identify configurations to achieve a high harvester efficiency and provide a better understanding of how parameters affect both the timing and magnitude of the load felt by the user.

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References

Figures

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Fig. 1

(a) Schematic of a gait cycle indicating time of cable retraction and cable extension of the right leg. (b) Both left and right cable length over a complete gait cycle. (c) Cable velocity of both left and right cable and combined positive velocity of both cables. Modified from Ref. [20].

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Fig. 2

The lower limb-driven energy harvesting device. (a) Schematic showing how the harvester is worn by the user. Modified from Ref. [20]. (b) Components of the retraction mechanism: a constant force spring exerting a force on the input pulley, while its other end freely coils about an idler pulley. Cable path: the cable passes over two sets of idler pulleys, redirecting its path to be coiled about the input pulley. Gear train: the input pulley transfers motion to an input shaft. Motion is then transferred to a drive gear via a one-way roller clutch (shown in removed section). The motion is amplified through a single stage gear ratio to the driven gear. The driven gear is mounted on the generator shaft (not shown in the figure).

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Fig. 3

Schematic showing internal components engaged in each state. (a) Coupled state. The input shaft is engaged with the remainder of the system. Engaged components include both left and right drive gears, driven gear, generator, input shaft, input pulley, and the retraction spring. (b) Decoupled state. The input shaft is not mechanically engaged with the remainder of the gear train due to the overrun of the roller clutch. Engaged components include input shaft, input pulley, and the retraction spring.

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Fig. 4

(a) Gear train assembly shows which components of the drive train are rotating at the same speed in each state. (b) Input and generator shaft angular velocity as a function of time. The generator shaft angular velocity is reduced by the overall gear ratio for direct comparison. Vertical dashed line indicates the time (tc) that decoupling begins, where the generator shaft overruns the input shaft. t0 indicates initiation of swing and t1 indicates the beginning of swing of the opposite limb.

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Fig. 5

Mechanical and electrical power produced in each test condition. Overall device efficiency (ηtotal) is the ratio of electrical power produced to the mechanical power required.

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Fig. 6

(a) The measured input angular velocity amplified by the overall gear ratio, N, and the model's predicted angular velocity of the generator, showing angular velocity after decoupling. Overlaid is the measured input angular acceleration. (b) Measured and predicted generated voltage for the M1(R3.5) condition. (c) Measured and predicted force exerted on the user for the M1(R3.5) condition. The coinciding phase of gait is shown as reference. Decoupling is indicated as a vertical hatched line.

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Fig. 7

(a) The angular velocity and angular acceleration of the M2(R3.5) condition. (b) and (c) show the electrical, inertial, and mechanical force contributions to total force, for conditions M2(R3.5) and M4(R12), respectively. Data are segmented to show a single step (50–100% of gait cycle), with vertical hatched lines indicating swing, beginning of cable extension, and the end of swing for the right leg.

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