In this paper, a stopper-engaged two-degrees-of-freedom nonlinear piezoelectric energy harvester is proposed. The proposed design consists of a cutout outer main beam and an auxiliary inner beam to achieve two close resonant frequencies. Impact-based nonlinearity is introduced to the system by adding a stopper to the end of each beam to broaden the harvesting bandwidth. By making the first two resonant frequencies close enough and introducing the nonlinearity, a broad bandwidth which connects the first two resonant frequencies can be achieved. Consequently, it is more adaptive to frequency variant environment than a conventional design. The governing equations of the electromechanical coupling system are derived based on the Euler-Bernoulli beam theory and piezoelectric constitutive equations. The mechanical stopper is modeled as a piecewise linear system. Finally, the voltage responses of the stopper-engaged piezoelectric energy harvester is numerically examined and validated with experimental results. Several settings of the gaps of the stoppers are discussed to see their impact on the performance. Overall, the simulated experimental results indicate that the broadband effect can be realized by adding two stoppers. The nonlinear effect is sufficient to improve the voltage output at the anti-resonant frequency, which is between the first two resonant frequencies, and completely connect two modes to achieve a wide bandwidth.
Musculoskeletal disorders may affect labor efficiency, cause disability, impair one’s work ability, and lower one’s quality of life. This consequently leads to a larger expenditure of medical resources. We aimed to design easy-to-open assistive devices for pneumatic tube systems to improve ergonomics and reduce musculoskeletal complaints of workers. We followed a design control process, including designs of motors, gears, sensors, and V-shaped connecting rods. Efficacy was evaluated by examining risks based on job strain index, user satisfaction, and musculoskeletal complaints of operators before and after the system’s implementation on a Nordic musculoskeletal questionnaire. We designed three assistive devices: two semiautomatic and one automatic. Each semiautomatic device costs about 300 US dollars and required space of 10 × 18 × 38 c m 3 . The automatic device costs about 3000 US dollars and required space of 28 × 38 × 50 c m 3 . The job strain index score decreased from 36 (very high risk) to 3 (low risk) with the semiautomatic devices and to 0 with the automatic device. Musculoskeletal complaints in the neck and upper limbs were reduced, with a significantly higher satisfaction rate for female operators. Our novel design of an automatic cap opening device for a pneumatic tube system was effective in improving ergonomics and reducing musculoskeletal complaints.
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