Total knee replacement (TKR) surgeries have been increasing tremendously in the past few years particularly among active young people and elderly people suffering from knee pain. Hence, continuous monitoring of the load on the knee after the knee surgery is highly desirable for designing an efficient and more functional smart knee implant. In this paper, we demonstrate a smart knee implant system which consists of a triboelectric harvester and a front-end electronic system which continuously monitors the load on the knee by relying only on the power harvested by the triboelectric harvester. The TKR system consists of the femoral, tibial tray, and the ultra high mechanical polyethylene (UHMWPE) bearing parts. The designed triboelectric harvesters are placed between the tibial tray and the UHMWPE bearing for perfect load monitoring. The frontend electronic system is placed on the tibial tray to be powered by the harvester. The triboelectric harvester produces an AC signal which is processed using our proposed frontend electronic system to monitor the load on the knee. For instance, at a knee cyclic load of around 230 N, the harvester produces 6.5 μW power and 18 V RMS signal at a frequency of 1 Hz. The frontend electronic system consists of a LC filter to process the high voltages from the harvester, a rectifier to convert the AC signal into a DC signal, a regulator to convert this DC signal into a stabilized and ripple free DC signal to provide biasing to the final stage, Delta-Sigma ADC, which finally converts the analog signal into digital bits. The power consumption of the proposed design is approximately 5 uW. According to the proposed design, monitoring the load several times a day is feasible by relying only on the harvested power. The prototype of the proposed system has been fabricated on a printed circuit board (PCB) and tested with the designed harvester. The test results demonstrate that triboelectric energy harvesting is a promising technique for self-monitoring the load inside knee implants. Through this research, the knee implants could be improved and failures can be detected at an early stage.
One major challenge to the usability of implants in total knee replacement (TKR) surgery is the limited of the postoperative knee joint loading data; therefore, the ability to continuously monitor these loads is an attractive concept. Integrating an energy harvester to scavenge the energy from human motion enables this monitoring. Recently, Triboelectric Generators have gained attention for energy harvesting because of their flexibility and easy fabrication processes. We investigate a triboelectric energy harvester for load sensing of TKR under simulated gait loading. The performance of triboelectric harvester prototypes was measured under simulated gait loading using a VIVO joint motion simulator. During cyclical loading, triboelectric harvesters undergo a contact and separation mechanism, which led to a voltage potential being generated. The power output is related to the amount of compressive load and the frequency. Therefore, the output power can be used to estimate joint loading and can act as a load-sensing implant component. Aiming to include biocompatible materials, we evaluated the performance of titanium as the triboelectric layer and showed the output is higher compared to Aluminum.
KEYWORDS: Magnetism, Beam shaping, 3D modeling, Energy harvesting, Finite element methods, Energy conversion efficiency, Amplifiers, Transducers, Electromagnetism, Resonators
This paper describes the finite element modeling and experimental testing of a magnetic T-shaped piezoelectric energy harvester that activates the internal resonance phenomena to increase voltage output and frequency bandwidth. The harvester consists of two magnets and two coupled beams, a cantilever piezoelectric beam attached to a clamped-clamped beam. A finite-element model is used to obtain the global mode shapes and natural frequencies of the system. We controlled the distance between the two magnets to achieve a nonlinear phenomenon of internal resonance of the structure, where the 2:1 ratio is satisfied between the modal natural frequencies. The T-shaped structure is combined with the magnetic nonlinearity such that a large, distinct internal resonance can occur at much lower excitation levels compared to a T-shaped structure without magnetic nonlinearity. Presented experimental results validate the benefits of the T-shaped structure nonlinearity when combined with a magnetic nonlinearity to achieve higher bandwidth and large responses, which can improve the energy conversion efficiency of the vibration energy harvester.
Parametric excitation has been investigated for several years as an effective way to drive a structure parametrically into large distinctive responses. However, parametric resonance requires a minimum threshold of excitation to be triggered. To reduce the threshold, we propose a two-degree-of-freedom vibration system. This system consists of two perpendicular beams each with a tip magnet placed so the same poles face each other. The repulsive magnetic force couples the motion of the two beams. By decreasing the distance between the magnets, the threshold value for parametric excitation decreases. In addition, the repulsive magnetic force decreases the first resonance frequency of the vertical beam and thus its principal parametric resonance. Lowering the threshold excitation and parametric resonance frequency are two unique properties that make the device ideal for energy harvesting at low frequencies.
Converting ambient mechanical energy to electricity, vibration energy harvesting, enables powering of the low-power remote sensors. Nonlinear energy harvesters have the advantage of a wider frequency spectrum compared to linear resonators making them more efficient in scavenging the broadband frequency of ambient vibrations. To increase the output power of the nonlinear resonators, we propose an energy harvester composed of a cantilever piezoelectric beam carrying a movable magnet facing a fixed magnet at a distance. The movable magnet on the beam is attached to a spring at the base of the beam. The spring-magnet system on the cantilever beam creates the variable double well potential function. The spring attached to the magnet is in its compressed position when the beam is not deflected, as the beam oscillates, the spring energy gradually releases and further increases the amplitude of vibration. To describe the motion of the cantilever beam, we obtained two coupled partial differential equations by assuming the cantilever beam as Euler-Bernoulli beam considering the effect of the moving magnet. Method of multiple scales is used to solve the coupled equations. The cantilever beam with the two magnets is a bi-stable system. Making one magnet movable can create internal resonance that is explored as a mechanism to increase the frequency bandwidth. The effect of system parameters on the frequency bandwidth of the resonator is investigated through numerical solutions. This study benefits vibration energy harvesting to achieve a higher performance when excited by the wideband ambient vibrations.
KEYWORDS: Energy harvesting, Magnetism, Polymers, Resonators, Composites, Smart materials, Bistability, Magnetic sensors, Wind energy, Solar energy, Transducers, Complex systems
Ambient energy in the form of mechanical kinetic energy is mostly considered waste energy. The process of scavenging and storing such energy is known as energy harvesting. Energy harvesting from mechanical vibration is performed using resonant energy harvesters (EH) with two major goals: enhancing the power scavenged at low frequency sources of vibrations, and increasing the efficiency of scavenging energy by increasing the bandwidth near the resonant frequency. Toward such goals, we propose a piezoelectric EH of a composite cantilever beam with a tip magnet facing another magnet at a distance. The composite cantilever consists of a piezoelectric bimorph with an extended polymer material. With the effect of the nonlinearity of the magnetic force, higher amplitude can be achieved because of the generated bi-stability oscillations of the cantilever beam under harmonic excitation. The contribution of the this paper is to demonstrate lowering the achieved resonant frequency down to 17 Hz compared to 100 Hz for the piezoelectric bimorph beam without the extended polymer. Depending on the magnetic distance, the beam responses are divided to mono and bi-stable regions, for which we investigate static and dynamic behaviors. The dynamics of the system and the frequency and voltage responses of the beam are obtained using the shooting method.
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