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Posted on July 31, 2017

Energy-harvesting bracelet could power wearable electronics

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Researchers have developed an energy harvesting bracelet that harvests biomechanical energy from the wearer's wrist movements.
 
The energy harvesting bracelet (EHB) is based on two mutually exclusive circular motion permanent magnetic movers which capture energy through the natural motions of the wearer's wrist. The EHB can transform the translational motion in any orientation except the axial into the rotational motion of the movers, which passes through four coil transducers and induces significantly large electro-motive forces across the coils. A prototype EHB is shown to produce power that can charge a capacitor with 470 μF 25 V up to more than 0.81 V during at most 132 ms from any single excitations.
 
With the rapid increase in wearable electronics, more efforts have been focused on harvesting human motion kinetic energy to power these devices. Many different approaches have been taken for scavenging waste biomechanical energy, such as electromagnetic, piezoelectric and triboelectric effects. The dominant approach is the electromagnetic effect which usually can provide sufficient output power. The translational motion levitating magnet is the most common structure of the electromagnetic energy harvesters.
 
The EHB works due to electromagnetic induction, in which the interaction between a moving magnetic field and an electrical conductor generates a voltage. Electrically conductive copper coils inside the EHB wind around an inner shell where there are two moving magnets that rotate around the bracelet in response to the wearer's wrist movements. As the magnets move through the copper coils, they generate a voltage due to electromagnetic induction. The faster the motion, the more power is generated.
 
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Figure 1 above shows a broken-out sectional view of the energy harvesting bracelet (EHB). Two movers (60° circle arc, 29.5 mm inner radius, 40.5 mm outer radius, circular cross section) that repel each other are located in a tube (57 mm inner diameter, 83 mm outer diameter, and 0.5 mm wall thickness) winded by four coils (∼800 turns, ∼85 Ω, 0.15 mm diameter copper wire, 45° circle arc, and ∼0.6 mm thickness). In order to place the leads of coils, two grooves are fabricated in the tube. All of those are packaged in a shell (52 mm inner diameter, 88 mm outer diameter, and 1.45 mm wall thickness). The mover consists of two magnets (NdFeB, N42, disk, 10 mm diameter, and 4 mm height) that attract each other, a magnetic core (permalloy, 45° circle arc, 31.55 mm inner radius, 38.45 mm outer radius, square cross section, and 7 mm side length), and a mover's shell. Two steps with 6.8 mm width and 0.3 mm height are placed in the two ends of the mover's shell to reduce the contact area of the tube. Then, when the movers rotate in the tube, the friction between them can be decreased significantly, which can enhance the performance of EHB. The shell, tube, and the mover's shell are made of stereolithography material (DSM Somos Imagine 8000) by 3D printing.
 
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When the EHB is triggered by movement, two movers will do circular motion in the tube and induce significantly large electro-motive forces across four coils. According to the Faraday's Law, the coil can produce an output voltage proportional to the turns, thus enabling the waste biomechanical energy harvesting. There is no requirement of the initial position relationship between the movers and the coils. So, the EHB can be worn without any considerations and requirements. Due to the structural features of the two movers, which proceeds in a circular motion in the tube, and the suspension characteristics, the EHB is completely capable of harvesting the rotation and vibration energy.
 
Source and images: Applied Physics Letters
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