Towards Self-Powered Wearables: An Improved Approach For Wrist-Worn Energy Harvesting

Nowadays, the wearable tech scene is booming all over the world. These have been very well received in the sports, healthcare, and entertainment industries (see Fig. 1). People are interested in such a product (wearable) that is always on, requires no attention, and never needs to be charged; simply wear and forget. The human body is an abundant source of energy.

Potential sources include body heat, motion, exhalation and even blood pressure. Harnessed properly, this energy can be used to directly power wearables: everything from small sensors to power-hungry communication devices and displays. The possibilities are endless.

Fig. 1: Wearable technology infographics (Courtesy: WittySparks via https://www.wittysparks.com).

Harvesting energy from basic human activities (e.g., walking, running, jogging, performing office task, etc.) is a hot topic in recent days. Vibration energy harvesters convert kinetic energy generated by human-body motion into electrical energy by employing compatible electromechanical transduction systems. Generally, an inertial mechanism is used for electromechanical coupling. A proof-mass, mounted in a reference frame attached to the vibrating body, couples the kinetic energy (while the body is in motion) to a transducer (piezoelectric, electromagnetic, or other) that generates electrical power.

Among different locations on human-body, the most common and easiest one is the wrist since most wearable gadgets available in the market are wrist-worn (e.g., smartwatches, activity trackers, fitness gadgets etc.). Investigation says human-body-induced-motion exhibits low-frequency, large-amplitude, and random characteristics. Capturing energy from such low-frequency, large-amplitude random motion of the human-body requires clever design approaches.

Credit: Pexels

Depending on the source of vibration, some harvesters have been developed as resonant, others as wideband which utilize linear or nonlinear inertial-mass motion. However, the power output of such linear energy harvesters is limited by the internal travel range of its inertial-mass motion, especially for low-frequency excitations (e.g., human-body motion). To overcome this limitation of linear motion based harvesters, devices with rotational inertial mass have been adopted by researchers that utilize an eccentric rotor structure to couple the kinetic energy into the transducer element. However, the proof-mass rotational amplitude of such structures is quite small for human-arm motion during daily activities; a larger rotational amplitude results in higher power output. Effective design of the rotational unit enhances the rotational amplitude, which increases the output power.

Recently, researchers at the University of Utah, USA reports a new approach to wrist-worn energy harvesting by utilizing an improved eccentric rotational structure which is capable of generating over six times higher power than its conventional counterpart. Published in Applied Energy, it uses a sprung eccentric rotor structure for mechanical coupling and an electromagnetic transducer incorporated within the rotational structure for energy conversion (see Fig. 2).

The electromechanical behavior of the system was investigated via numerical and finite element analysis, and the performance of a fabricated prototype of ∼3.5 cm3 functional volume (the volume without crappy housing) was verified by a series of pseudo-walking signal (single frequency sinusoidal signal derived from motion of a driven pendulum that approximates the swing of a human-arm during walking). The authors say that verifying its performance on human subjects is ambiguous since the same result may not be reproduced (on the same subject) due to variation in the motion from one run to another. Therefore, the pseudo-walking test was initially done for a robust validation of the theoretical investigation. And they are looking forward to testing on humans to prove its potential in the real world.

Fig. 2: Schematics of a sprung eccentric rotor (left) and the proposed electromagnetic energy harvester using sprung eccentric rotor (right).

Theoretical simulations show that under certain pseudo-walking input excitations, the dynamic response of the eccentric rotor is greatly influenced by the stiffness of the torsional spring which, in turn, affects the power and voltage generation of the system. It was further justified by benchtop tests (see Fig. 3). The performance of the sprung device (with optimal or near-optimal spring stiffness) is very promising compared to its unsprung counterpart. The power output of the sprung device, with optimum spring stiffness, 1 Hz frequency, and ±25° rotational amplitude, is about 6 times higher than the power generated by the unsprung one under the same excitation conditions.

Results indicate that a sprung rotational electromechanical transducer effectively couples the extremely low-frequency motion (generated during the human-like-arm swing) and improves the energy harvesting performance significantly. This important finding is one step ahead towards self-powered wearables for human-body-induced motion.

Fig. 3: Average power versus torsional spring stiffness at 0.91 Hz (left) and measured power under various pseudo-walking excitation profiles (right).

These findings are described in the article entitled An electromagnetic rotational energy harvester using sprung eccentric rotor, driven by pseudo-walking motion, recently published in the journal Applied Energy. This work was led by Miah Abdul Halim, Robert Rantz, and Shad Roundy from University of Utah, in collaboration with their industrial partners Qian Zhang, Lei Gu, and Ken Yang from Analog Devices Inc.

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