1. A Comparative Study of Wearable Kinematic Technologies Nicolas Ballesteros-Velasco, Stephanie Garcia, Gold Hood  Department of Mechanical Engineering, University of Texas at San Antonio One UTSA Circle, San Antonio, TX ,
Abstract
The present research analyses two different methods that harness kinetic energy, which is then converted to electrical power, namely the transduction mechanisms of piezoelectric and electromagnetics. Methodology included attaching a voltmeter to measure the output of both voltage (V) and currents (Amps), and for comparison measuring the stored charge as it discharged the capacitors on a breadboard. It was hypothesized that the results should suggest that inductive charging would have a higher efficiency and power density than the piezoelectric method. The final results yielded that the inductor is more effective than piezo electrics when converting kinetic energy to power, thus supporting the claimed hypothesis. However, due to size constraints and maintenance, electromagnetic power is not yet practical in wearable energy harvesting. Which leads to the conclusion that piezo electrics are more popular for systems that inherently involve kinetic energy harvesting. This method is most feasible to embed in consumer products and industrial standards of the future.
Introduction
Background
Electromagnetic generators use the induction that is propagated from relative motion of the magnetic flux gradient that the conductor undergoes. The electromagnetic force, voltage, induced in a circuit can be proportional to the time rate change of the magnetic flux [1]. Piezoelectric generators involve materials that create electricity when they are subjected to mechanical stress. These materials perform by generating a charge through strain which results in an electric field. The research regarding vibration based energy can be harvested from ambient vibrations to generate sustainable power. These types of ambient vibration generators may offer a superior alternative to batteries as the main power source for many electrical power devices, particularly in mobile applications. In 2013, Y Zhu J Zu and W Su from the department of Mechanical and Industrial Engineering at the University of Toronto published an investigation studying the design and analysis of piezoelectric energy. The study yielded positive results that affirmed that piezoelectric energy can be directed to increase the harvested voltage [2].
Hypothesis
Due to the wide array of movement and use of multiaxial movement, it is hypothesized that dynamic induction will harvest more electrical power converted from kinetic motion rather than that of a piezoelectric sensor in a experimental nature. Thus, implicating that the inductive method is more efficient than piezoelectric technology when converting body motion/multi axial movement into electrical power. However due, to the impracticality of weight and rigidness, the piezoelectric technology is more feasible to use on a consumer basis and design.
2. Methodology
The sensors or mechanisms were attached to the human test subject, in the location specified in Figure’s 1-3. While the subject was exercising on a treadmill tests were performed. Exercising pertained but was not limited to running on a treadmill (8-minute mile pace), and walking on a treadmill (20-minute mile pace). Each test consisted of walking and running within 2-minute spans, 1-minute span of walking 1 minute of running. The test was performed a total of 8 times. Thus, including break frames to control for stamina.
Method of Measurement
During the activity testing, the mechanisms were wired directly into a voltmeter that recorded the voltages, currents, and power outputs during the trial. The sensors were attached in location of most motion induced by subject during a run or walk. As seen in figure 1, the piezo electric “S” sensor was attached to the deltoid of the test subject. As for the piezo electric “H” sensor was attached on the hip of the test subject, refer to figure 2. For most tangential range of motion, the Inductor was placed on the ankle of subject, otherwise seen in figure 3. Due to vast amounts of constraints and sensors attached to the subject, regular sweat resistant athletic scotch tape was used.
3. Results
Figure 4 depicts the main results to the study, showing the amount of power produced by each component due to the subject’s motion during the testing. The overall power output results demonstrate that the Piezo electric ceramic disks were the least close competitor in respects to power output equating to 9.7% as much as the inductor while the piezo electric strips output were 27.9% and 47.9%. The overall efficiency is also dropped down for the piezo electrics as it was discovered that they were operating at a higher frequency than that of the inductor, therefore had the advantage to output more power. The Inductor peaked at output of 1.6 mW, while the piezo electric sensors peaked at 0.54 μW and 0.78 μW.
Conclusion
Although kinetic energy is not efficient, if it can be improved it can then progress to be manufactured in a more marketable way for public consumption on a consumerist level. The results deviate from expected values by more than 10%, thus making our study insignificant to compare with previous study. The study however was conducted with different hardware and in different terms which may explain the slight deviations. In conclusion, the research achieved the following goals: (1) provide more data supporting piezoelectric and electromagnetic energy harness to further enhance wearable technology, and (2) deducts that electromagnetic energy is more effective than piezo electrics, however due to constraining sizes and design, the piezo electrics are bound to be the better choice when making wearable energy harvesting technology.
Figure 1. Piezoelectric strip attached at deltoid.
Figure 2. Piezoelectric strip attached at hip.
Figure 3. Piezoelectric attached at ankle.
Figure 5. Current to Voltage comparison of each sensor.
Figure 4. Total and average power output comparison chart of the tested sensor.
Proceedings of the 2018 ASEE Gulf-Southwest Section Annual Conference The University of Texas at Austin
April 4-6, 2018