Kinematic wearable energy comparison

The growing consumer demand for sustainable energy has prompted the exploration of alternative energy systems. One technique that has recently been exploited is the harnessing of mechanical vibrations to produce clean, stable, and portable energy. The present research analyses two different methods that harness kinetic energy and convert it to electrical power: piezoelectric and inductive charging. Comparative analysis was conducted through experimentation with two representative designs, wherein the prototypes were attached to a volunteer and the power output was measured during ambulatory movement. It was hypothesized that energy harvesting through inductance would have a higher efficiency and power density than piezoelectrics. The final results exhibited that the piezoelectric converters were more efficient per weight in harvesting energy, refuting the hypothesis and claims from previous studies. The overall power output per weight results demonstrated that piezoelectric strips were the most efficient prototype during participant running with respect to power density, which was measured at 0.553 W/kg. In contrast, the inductance-based prototype had a measured power density of 0.0091W/kg. Piezoelectrics are a more marketable and effective mechanism, due to their greater portability and flexibility in configuration. Electromagnetic inductance suffers by comparison due to design and manufacturing complexity. As the cost associated with piezoelectrics declines, this technology has the potential to build a new market in both attire and charging solutions for handheld devices. 1. Background Embedded wearable technology is an exciting and relatively new application of smart materials, seeking to harvest the energy of human movement to increase battery life of electronics. The two leading technologies currently researched today for kinetic energy harvesting through wearable technology are piezoelectrics and electromagnetic inductors [1]. Piezoelectric generators create electricity when they are subjected to mechanical stress, creating a charge during strain resulting in an electric field. Piezoelectric research shows that energy can be harvested from ambient vibrations to generate sustainable power. Several studies and experiments have been conducted to investigate the efficiency of harvesting energy via piezoelectric generators. Experiments Journal of Undergraduate Research & Scholarly Work conducted by Swallow et al. leveraged an innovative, custom-built piezoelectric generator, which was tested by mounting onto a vibrating beam structure. Results showed that piezoelectrics produce sufficient voltage output to power wearable microsystems. The team proposed future work seeking to integrate their design into an energy harvesting glove [2]. In 2013, Zu and Su published an investigation studying piezoelectric energy harvesting from oscillating beams [6]. The research showed that a piezoelectric generator mounted on a simply supported beam can produce electrical power when subjected to compressive dynamic loading [6]. Currently, the most commonly employed kinetic energy harvesters are based on electromagnetic inductance. Electromagnetic generators leverage induction, which is voltage production from the relative motion of the magnetic flux gradient caused by a conductor’s movement. This induced voltage is proportional to the temporal rate of change in the magnetic flux [5]. A study published by Janky et al. in 2004 investigated harnessing electrical energy produced by the ambient vibrational motion of moving vehicles through conversion into electromagnetic power [4]. These types of ambient vibration generators have resulted in numerous issued patents to the aforementioned research team and may offer the ability to substantially extend battery life for many electrically powered devices, particularly in wearable or handheld applications. In another example, Steve Vetorino of Applied Innovative Technologies developed the first magnetic-force flashlight. Vetorino’s straightforward approach induces current and stores it in a capacitor through simple manual oscillation of the flashlight. The capacitor then delivers the power to an LED, yielding 20 minutes of light for 30 seconds of manual perturbation [8]. These are just a few examples of inductor-based energy harvesting, which is becoming increasingly more feasible and correspondingly increasing in market circulation. Although both piezoelectric and inductor-based energy harvesting have been extensively studied, few researchers have compared the two energy production methods directly. Specifically, this investigation seeks to ascertain which generator can yield a higher power output efficiency, in terms of power output per weight (controlled for cost), when used to harvest energy from human motion. Drawing from prior studies, the study hypothesis is that dynamic induction will harvest and convert electrical energy from the kinetic motion of an ambulatory subject more efficiently than a piezoelectric generator [3]. This hypothesis was assessed through attaching inductance and piezoelectric prototypes to a volunteer during semi-controlled ambulatory movement. 2. Methodology 2.1 Study Design Journal of Undergraduate Research & Scholarly Work Generated power was measured during the ambulatory motion of a consistent volunteer. For these experiments, the electrical mechanisms were wired directly into a multimeter that simultaneously recorded the voltage, current, and power output during each trial. Specifically, the data recorded monitors output from a subject exercising on a treadmill. Exercise was conducted by consistent pacing on a treadmill (8-minute mile pace) for 1-minute intervals, a total of 8 runs were recorded. Measurement began once critical speed was reached and maintained constant. 2.2 Generator Hardware Selection To properly control for commercial applicability, each of the electromagnetic inductance or piezoelectric prototypes were constrained to a nearly identical cost. All the materials used in order to evaluate the power of the piezoelectric generators, two different forms of hardware were used. The first was a piezoceramic disk (GOE-3301, manufactured by Geodrum) with 35x0.58mm dimensions, weighing 9.9g. This prototype produced voltage derived from pressure applied from the subject’s heel. The second was a piezo film (605-000004, manufactured by Parallax,) with dimensions of 0.98x0. 52mm and a weight of 1.7g. The film was made from flexible, synthetic polymers and was attached to the subject’s hip to produce power from distortion generated during locomotion. Analyzing the behavior of the voltage produced by both normal (perpendicular) pressure and flexion allowed for variability in measurement when testing the piezoelectric energy generation method. Both of these mechanisms were selected due to their ability to harvest energy from weight transfer and vibrational movement while the test subject was running. 2.3 Generator Placement To provide power generation without disturbing gait, the piezoelectric ceramic disks were placed beneath the test subject’s right and left plantar fascia. Previous studies have demonstrated that the anatomy of the plantar fascia provides maximum pressure density directed to propulsion that is needed for the experimental running gait, which is likely to result in maximum energy transfer [6]. The piezo films were secured at the subject’s right hip and are activated by lateral flexion. These were positioned at different local maxima of torque while the human body is in motion to compare which location resulted in higher energy production: the midbicep or hip. As seen in figure 1A, the piezoelectric generator, denoted as “S”, was attached to the deltoid of the test subject. Kinetic energy is directly related to velocity and acceleration of the runner’s body mass. The hip was chosen as one of the focal points for measuring the energy produced by the piezoelectric strip due to the range of hip motion increasing as the gait lengthens, thus Journal of Undergraduate Research & Scholarly Work increasing energy expended. It should be noted, however, that at a steady pace, an increase in flexion results in a decrease in total leg extension. The piezoelectric generator attached on the hip of the test subject, denoted as “H”, is shown in figure 1B. The only physical modification made to both S and H generators was a pair of Magcraft Rare Earth 19.05x3. 18mm magnets placed on the ends of the piezoelectric films to provide additional momentum during movement without piercing the generator. As inductors generate energy most efficiently via linear translation, the ankle was selected as a stable point of maximum translation on which to affix the prototype, as shown in figure 1C. To properly secure the prototype to the subject under dynamic constraints, sweat resistant athletic scotch tape was used as additional support to minimize energy loss from reactionary vibration. The inductive generator was custom designed and built with common off the shelf (COTS) components in order to accommodate for the geometric and physical limitations of the design. The design was constrained to efficiently harness single axis movement. It was placed horizontally on the side of the foot (talus), parallel to the sole of the test subject. The inductive coil was housed in a cylindrical casing 3D printed using polylactic acid (PLA). The casing accommodated a 1-inch diameter copper coil spanning 4 inches in length made from a 14 American Wire Gage (AWG) copper wire. The electromagnetic inductor design is exhibited in Figure 2B. The composition of the design was intended for effective energy production while maintaining a feasible level of comfort for the test subject. 2.4 Data Collection In order to render each prototype wearable for the test subject, a 14 AWG copper wire was soldered to each generator tested and wired to a Morris Products Digital 600-voltmeter (Model #57030). T