Self‐Powered Smart Proximity‐Detection System Based on a Hybrid Magneto‐Mechano‐Electric Generator

A hybrid magneto‐mechano‐electric (H‐MME) generator that combines a Mn‐doped Pb(Mg1/3Nb2/3)O3‐Pb(Zr,Ti)O3 piezoelectric single crystal and an electromagnetic induction coil is demonstrated to convert stray magnetic noise of 60 Hz into useful electrical energy. The authors optimize the design of the piezoelectric cantilever, the weight of the magnetic mass, and the distance between the magnet and coil through theoretical simulation and experiments to maximize each contribution for the H‐MME generator. The total root mean square output power of the H‐MME generator is 28.35 mW with a power density of 0.042 mW cm−3 Oe2 in a 5 Oe magnetic field of 60 Hz, which is sufficient for driving multiple functional Internet of Things sensors that detect temperature, humidity, light, UV radiation, air pressure, and sound. In addition, a self‐powered smart proximity‐detection system (including a microcontroller unit, an ultrasonic sensor module, a piezo speaker, and a Bluetooth module) is demonstrated by rectifying the output of the H‐MME generator and controlling it using a power management circuit. The superior output power of the H‐MME generator exceeds simple sensing functions and presents the possibility of achieving battery‐free self‐powered smart devices and systems.


Introduction
[3] In particular, the demand for IoT devices, which are key elements of a hyperconnected society, has expanded since the fourth industrial revolution.[6][7][8] Among the various types of energy sources, such as solar, [4] vibration, [5][6][7] and heat, [8] stray magnetic fields that generate around 50/60 Hz alternating current (AC) power lines can be easily accessed. [9,10][18][19][20][21] Among them, the combination of piezoelectric harvesters and electromagnetic coils has been reported to be efficient in generating a greater output power in tens of milliwatts. [21,22]Previously, Kwak et al. [22] demonstrated a hybrid energy harvester integrated with a piezoelectric cantilever in the second harmonic bending mode and an electromagnetic induction coil.However, the hybrid harvester had a high resonance frequency of 119 Hz; therefore, it is difficult to operate at the 50/60 Hz frequency of the stray magnetic noise surrounding us.Also, their long cantilever used to exploit the second bending mode causes the entire IoT sensor system to be too voluminous and thus limits practical application.Additionally, the output power optimization of the hybrid harvester considering the interaction of each piezoelectric and electromagnetic induction harvester should be further investigated.
In the utilization of the MME generators, the initial MME generators have limited uses, such as powering LEDs, due to the microwatt-level output. [23,24]As the level of output power is gradually increased, the MME generators have been applied to self-powered IoT sensor systems that detect the temperature and humidity in real time. [25,26]Recently, the multifunctional IoT sensor systems for monitoring the pressure, illuminance, sound, UV, motion, magnetic field, and even gas have been powered by MME generators. [22,27]Further, to ultimately realize the hyperconnected society, the development of smart devices/systems to facilitate not only passive environmental sensing but also informed preemptive action is essential.However, due to significant power consumption, implementing self-powered smart systems powered by MME generators is challenging and has not been reported so far.
Here, we demonstrate a hybrid MME (H-MME) generator that combines the piezoelectric effect and electromagnetic induction to produce a root mean square (RMS) output power of 28.35 mW under a H ac of 5 Oe at 60 Hz (corresponding to a power density of 0.042 mW cm À3 Oe 2 ).This was achieved by optimizing the design of the piezoelectric cantilever structure and the weight of the magnetic proof mass.In addition, the distance between the magnetic mass of the piezoelectric energy harvester (PEH) and the electromagnetic induction coil was optimized using theoretical simulations and experiments to maximize the output power.The optimal design of the H-MME generator can produce sufficient output power to drive multiple functional IoT sensors.Furthermore, the H-MME generator successfully powered a microcontroller unit (MCU)-based smart proximity-detection system (SPDS) that can detect distance using an ultrasonic (US) transducer and generate a warning sound with a piezo speaker; it can also communicate with a smartphone via a Bluetooth (BT) module, which effectively notifies of potential collisions by emitting a loud buzzer, aiding in accident prevention.

Performance and Structural Optimization of H-MME Generator
As shown schematically in Figure 1a, we developed an H-MME generator that produces electricity by combining piezoelectric action with electromagnetic induction from the H ac surrounding power transmission lines carrying a 60 Hz AC current.Installing the vibrating magnetic proof mass vertically facilitated better resonance frequency tuning at %60 Hz and reduced harvester damage from the influence of gravity. [26]The output voltage from the high-power H-MME generator (Figure 1a-i) is rectified and controlled by the power management circuit (Figure 1a-ii)) and is then used to drive the SPDS (Figure 1a-iii).A photograph of the H-MME generator, which combines a PEH and an electromagnetic energy harvester (EMH), is shown in Figure 1b; this was achieved by positioning a solenoid coil near the ends of permanent magnets.The piezoelectric source in the H-MME generator was fabricated using a Mn-doped Pb(Mg 1/3 Nb 2/3 )O 3 -Pb(Zr,Ti)O 3 (PMN-PZT) single crystal macrofiber composite (SFC), which is depicted in the exploded view in Figure 1c.The operating mechanism of the H-MME generator, which utilizes the up-and-down bending motions of the cantilever vibration to generate electrical power through piezoelectric and electromagnetic effects, is depicted in Figure 1d.In the initial horizontal state (i), when an AC magnetic field induces a magneto-mechanical torque to the cantilever to bend it upward, it produces compressive stress on the PMN-PZT to generate a piezoelectric potential (ii).After release (iii), the downward bending motion of the cantilever results in tensile stress in the piezoelectric layer, thus producing an alternating polarity piezo-potential with every cycle of up-and-down motion (iv).When the piezoelectric cantilever vibrates, the magnetic mass at the end of the cantilever results in a change in the magnetic flux (Φ), generating an electromagnetically induced voltage (ε) through Faraday's law as follows where N is the number of turns.Therefore, when the periodic up-and-down oscillation of the magnetic mass changes Φ (Figure 1d(ii,iv)), the electromagnetic induction coil generates electrical energy.Theoretical simulations of the PEH structure were performed using COMSOL Multiphysics to maximize the output power of the H-MME generator.In a previous study, we showed that a PEH with a wide and short cantilever is more effective at generating a high output power at the resonance frequency (i.e., 60 Hz) of the first bending mode than a typical narrow and long cantilever. [27]To adopt a wide and short structure, we simulated the stress distributions of the SFC active layer with three different dimensions: 26 mm Â 18 mm, 28 mm Â 14 mm, and 30 mm Â 10 mm using COMSOL Multiphysics (Figure S1, Supporting Information).The maximum normalized stress was obtained for the 30 mm Â 10 mm distribution and was thus used as the piezoelectric layer in the MME generator.The details of the boundary load conditions and simulated stress distributions are listed in Table S1, Supporting Information.In addition, we simulated the stress distribution applied to the piezoelectric SFC active layer (30 mm Â 10 mm) by varying the tip-end-to-base ratio (ζ) of the Ti elastic layer (Figure 2a).The cantilever with a ζ of 0.6 exhibited the maximum value of the normalized stress with an equivalent tip width of %21 mm (Figure 2b(i)).To further fine-tune the cantilever structure, we simulated the stress distribution of the SFC by varying the width of tip-end from 18 to 22 mm in short increments of 1 mm.The cantilever with a tip-end width of 20 mm exhibited the highest normalized stress across the SFC and was used as an optimized trapezoidal cantilever to harness the piezoelectric energy (Figure 2b(ii)).Details of the boundary conditions and simulated values are provided in Table S2, S3 and Figure S2, Supporting Information.In case of the Cu coil for the EMH, the induced output voltage (ε) increased as the number of turns (N) increased; thus, a higher number of turns tended to improve the output power of the EMH, as shown in Table S4 and Figure S3a, Supporting Information.Therefore, considering the total volume of the H-MME, a Cu coil with a diameter of 0.2 mm with 2200 turns was adopted for the EMH on an acrylic structure with an internal diameter of 37 mm (Figure S3b, Supporting Information).
After the structure of the cantilever and coil was set, the weight of the magnetic mass was adjusted to maximize the output power of the coupled H-MME generator (Figure 3a).We simulated the output power of the EMH as a function of the weight (wt) of the magnetic mass from 30 to 39 Â Â g with a fixed magnet-to-Cu coil distance of 1 cm (Figure 3b).The simulated output powers of the EMH exhibited an increasing trend with the magnet weight (Figure 3c).We also experimentally measured the output power from both the PEH and EMH by increasing the weight of the magnetic mass from 30 to 39 g, with the highest power at 39 g for both the EMH and PEH at 5 Oe (Figure 3d,e, respectively).The graph of the total RMS power of each harvester increased as the magnet weight increased (Figure 3f ).The details of the sum of the output of each harvester tune to the resonance frequency of 60 Hz and the corresponding power density are listed in Table 1.The measured output parameters (i.e., open-circuit voltage, short-circuit current, and RMS output voltage) for the PEH and EMH with different magnetic mass weights are shown in Figure S4, Supporting Information.We fixed the magnet weight of the H-MME generator at 39 g without a further  increase because the additional magnets caused a structural challenge in tuning the cantilever's resonance frequency to 60 Hz for first harmonic bending mode.
In addition to the magnetic mass, a critical factor affecting the output of the H-MME generator by the interaction of PEH and EMH is the distance (d) between each center of the magnets and the electromagnetic coil, as shown in Figure 4a.The theoretical model depicting the change in the magnetic flux lines with variation of d is shown in Figure 4b.We simulated the normalized output power from the EMH by varying the d between the magnets and the coil from À5, 0, 5, 10, 15, and 20 mm (Figure 4c).The normalized EMH output increased as the magnets was closer to the inner side of the coil and reached a maximum at the point where the magnets were positioned at the center of the coil (d = 0).This implied that the oscillation of the magnets at the center of the Cu coil was the most effective alternating magnetic flux (Φ).However, when the magnets moved further down the center of the coil (d = À5 mm), the output power was decreased because of the reduced variation in Φ.Based on this simulation, we experimentally evaluated the output characteristics of the H-MME generator, with a focus on varying d between the magnet and coil.Figure 4d shows the measured RMS output power of the EMH at d ranging from À5 to 20 mm under a 5 Oe input.The maximum output power of the EMH was measured at 20.4 mW RMS at d = 0, which was consistent with the simulation.In addition, we measured the output power of the PEH as a function of d at 5 Oe (Figure 4e).Interestingly, as the magnetic proof mass of the PEH cantilever gradually moved inside the coil, the RMS output power decreased.In other words, the PEH was measured with a maximum RMS power of 23.4 mW when it was located far from the coil (d = 20 mm).A decrease in the PEH power near the coil was anticipated because the vibration of the magnetic mass was damped by the electromagnetic induction. [21]The results of each output power of the PEH and EMH according to the distance between the magnet and coil are listed in Table 2.In addition, the measured output parameters (i.e., open-circuit voltage, shortcircuit current, and RMS output voltage) for the PEH and EMH with different distances between the magnet and coil are shown in Figure S5, Supporting Information.The total output was optimized for 40.7 mW RMS by summing the power of each harvester at d = 5 mm, considering the interaction of PEH and EMH (Figure 4f ).
Figure 5a,b shows the output power of the PEH and EMH according to the 60 Hz input H ac ranging from 1 to 6 Oe under the optimized conditions (i.e., wt. of magnets = 39 g and d = 5 mm), respectively.Each harvester was tuned to a resonance frequency of 60 Hz and its frequency spectra are provided in Figure S6, Supporting Information.For the H-MME generator to be utilized practically in IoT devices/systems, the output characteristics must be evaluated by simultaneously connecting the load resistors to each harvester (Figure 5c).Therefore, we measured the RMS output power of the PEH and EMH as a function of each load resistance under a 60 Hz H ac of 5 Oe (Figure 5(d-i  and d-ii, respectively)).To present the maximum output power performance of the H-MME generator, each output power measured in Figure 5(d-i and d-ii) was summed to obtain the total output power of the H-MME generator at each load resistance, as shown in Figure 5e.The maximum output power of H-MME generator was 28.35 mW RMS at the optimum load resistance (PEH: 40 kΩ and EMH: 2.2 kΩ).The corresponding power density is 0.042 mW cm À3 Oe 2 , considering that the volumes of PEH and EMH are 5.625 and 21.55 cm 3 , respectively.Figure 5f shows a comparison of the maximum RMS output power at the optimum load resistance under the 5 Oe H ac according to the harvester configuration.The output powers of independent PEH and EMH were 21.43 and 19.30 mW RMS at 70 and 1.1 kΩ, respectively.In hybrid mode, the maximum output power of PEH was decreased by the interference with the coil to 20.04 mW RMS, and the maximum output of EMH was increased to 24.47 mW RMS.The ratio of the output of each harvester to the maximum total power (28.35 mW RMS) of the H-MME generator at optimum load resistance was 47% (13.37 mW) for the PEH and 53% (14.98 mW) for the EMH.

Self-Powered Environment Monitoring System Based on the H-MME Generator
The high output power of the H-MME generator can be utilized as a power source for self-powered IoT sensor systems, as shown schematically in Figure 6a.The AC output voltage of each harvester was efficiently rectified to DC voltage using a commercially available integrated circuit (IC) (LTC-3588, Linear Technology, USA) to charge a supercapacitor (0.2 F) in parallel.The supercapacitor was connected to a multifunctional IoT sensor (SLTB004A, Silicon Labs Co. Ltd., USA) to monitor environmental factors, including temperature, humidity, sound, UV, ambient light, air pressure, and magnetic field, via BT communication with a smart device.A high power consumption of approximately 5.72 mW is required to drive various sensors in multifunctional IoT systems, as described in Figure S7a, Supporting Information.We successfully demonstrated that the H-MME generator can continuously operate a self-powered IoT environmental system by applying an input H ac of 5 Oe in a Helmholtz coil (Figure 6b).A real-time captured snapshot is enlarged in the inset of Figure 6b.A demonstration is provided in Video S1, Supporting Information.The supercapacitor (0.2 F) used in the IoT sensor system was charged by the PEH, EMH, and H-MME generators for 155, 55, and 38 s at a magnetic field of 5 Oe (Figure 6c).In addition, during the continuous operation of the IoT sensor system, the charged state of the supercapacitor was maintained, as shown in the charge-discharge curve in Figure 6d.

SPDS Based on the H-MME Generator
To address the limitations of conventional energy harvesters, which struggle to supply power to complex and advanced functional IoT devices and systems, we propose a solution using an H-MME generator to power an SPDS that can be operated using a MCU. Figure 7a shows a block diagram of each component of the SPDS, including the H-MME generator, power management circuit, MCU (Microchip Tech., USA), US sensor module (HC-SR04, OSEPP Electronics, USA), active piezospeaker (HYDZ, Guangzhou Yueneng Technology Co. Ltd, China), BT module (HC-06, OSEPP Electronics, USA), and smartphone (Galaxy Note 20, Samsung Electronics Co. Ltd., Korea).The operating power of the SPDS was 104 mW, as shown in Figure S7b, Supporting Information, which was significantly higher than the output power of the H-MME generator.Therefore, we applied a power management circuit to automatically cut off the load (i.e., MCU) when the voltage amplitude of the supercapacitor was less than 2.4 V and connected it to the load when it was recharged to 3.6 V, enabling the SPDS to be periodically operated using the stored energy in the supercapacitor.In addition, the US distance sensor module was connected to the MCU and configured to detect the distance to an object and send an alarm to the loudspeaker to generate a warning when the distance was less than 7 cm.Additionally, the detected distance could be transmitted to a smart device using a BT module.To create a compact and practical self-powered system, we assembled all components of the SPDS (Figure 7b) by housing an acrylic structure whose dimensions are provided in Figure S8, Supporting Information.We demonstrated a self-powered SPDS based on the H-MME generator by applying a H ac of 5 Oe induced by the Helmholtz coil, as shown in Figure 7c, which shows an enlarged integrated SPDS module in the inset.
Figure 7d shows the charge-discharge curves of the supercapacitor during SPDS operation.The initial charge time of the 0.4 F supercapacitor to 3.6 V using the H-MME generator was %300 s.
When tuning on the SPDS to detect the distance with an alarm, the voltage of the supercapacitor decreases below 2.4 V. Subsequently, the H-MME generator recharges the capacitor back up to 3.6 V for 180 s.Owing to the periodic on/off switching of the power management circuit, the H-MME generator ensures continuous operation of the SPDS, enabling real-time distance measurements of nearby objects.This standalone SPDS, which eliminates the need for conventional power sources such as batteries, can be used for an extended period to measure the distance between target objects or materials without physical contact.This nondestructive and efficient measurement method helps prevent accidents by alerting of objects or stray cattle approaching in dangerous proximity with a loud alarm. [28,29]or a demonstration of the self-powered SPDS in real-time operation, refer to Video S2, Supporting Information.

Conclusion
We developed a H-MME generator by combining a PMN-PZT single-crystal macrofiber composite-based PEH and an EMH.As a result of optimizing the critical factor to the output power (i.e., cantilever structure, weight of the magnetic mass, and distance between the Cu coil and magnets) through theoretical simulations and experiments, the total output power of H-MME generator was 28.35 mW (power density of 0.042 mW cm À3 Oe 2 ) at the optimal load resistance under a magnetic field of 5 Oe at 60 Hz.The impressive output power of the H-MME generator can sufficiently supply the continuous driving power of a multifunctional IoT sensor system, including temperature and humidity sensors, light and UV detectors, air pressure sensors, and microphones.In addition, we demonstrated a selfpowered SPDS using the output power of the H-MME generator and a power management circuit that automatically turns the connection between the capacitor and load on and off.The MCU-based SPDS not only serves as an independent device for noncontact distance measurement in remote and harsh environments but also prevents potential collisions by alerting of object proximity with a loud buzzer.The H-MME generator has significant potential to revolutionize the energy supply for IoT sensor systems and various other applications.

Experimental Section
Fabrication of the H-MME Generator: The H-MME generator was fabricated by combining a piezoelectric energy harvester with an electromagnetic induction coil.The primary material used for the piezoelectric effect was 1 mol% Mn-doped PMN-PZT SFC (Ceracomp Co. Ltd, Korea) in the d 32 -mode (Table S5, Supporting Information), which was used in our previous studies. [22,26]Oxygen vacancies were generated by doping acceptor defects, such as Mn, into the PMN-PZT matrix.These vacancies contribute to domain hardening, [30,31] effectively reducing electrical losses and enhancing the mechanical quality factor.A fiber-shaped PMN-PZT piezoelectric single crystal [10 (l) Â 0.5 (w) Â 0.2(t) mm 3 ] and epoxy [10 (l) Â 0.05 (w) Â 0.2(t) mm 3 ] were arranged with a width of 30 mm to serve as the piezoelectric layer matrix.Au electrodes were applied to the top and bottom surfaces.Electrical signals were transmitted through Cu wires connected to the Au electrodes, and the piezoelectric layer and electrodes were protected by a polyimide film to form the SFC.A titanium-aluminum alloy cantilever [36 (l) Â 36 (w) Â 0.3 (t) mm 3 ] (Grade 5 titanium alloy) was used as the elastic layer for generating mechanical stress.To increase the stress applied to the SFC, we designed the cantilever as trapezoidal shape (Figure 2).The SFC was attached to the cantilever region using an epoxy adhesive (DP-460, 3M, USA).A NdFeB alloy (N35 grade) magnet was placed at the end of the cantilever to induce vibration as a magnetic proof mass.The properties of the elements used in the MME generator are listed in Table S6, Supporting Information.The output power of the PEH was measured using wires connected to the electrode pads of the SFC.The EMH employed a coil structure created from transparent acrylic.A copper wire with a diameter of 0.2 mm was wound 2200 times and positioned near the vibrating proof mass to utilize the most of the magnetic flux change, thereby forming the H-MME generator.
Simulations: The structures of the SFC and cantilever in the PEH system were optimized using a finite element analysis in COMSOL Multiphysics.By comparing the various lengths and widths of the active layer of the SFC at a resonance frequency of 60 Hz, we observed that a larger width resulted in a higher stress distribution.The cantilever was simulated as a trapezoidal shape to apply higher stress to the active layer, and the stress distribution was analyzed by varying the ratio of the tip end-to-base of the cantilever.The parameters used in the simulation of the PEH, such as the boundary load, frequency, stress, and displacement, are provided in Table S1 and S3, Supporting Information.To optimize the output power of the EMH, we arranged a coil and evaluated the output power with varying magnetic masses ranging from 30 to 39 g.Furthermore, using the magnet with the highest output power (39 g), the output variation with respect to the distance between the magnet and the coil was simulated to optimize the H-MME generator.
Characterization: An AC magnetic field was generated by placing the H-MME generator vertically at the center of a Helmholtz coil (IM SYSTEM Co. Ltd., Korea) powered by function generators (WF1948, NF Corp., Japan) and a high-speed power amplifier (HSA 4051, NF Corp., Japan).The amplitude of the magnetic field was measured using a Gauss meter.The RMS voltage produced by the H-MME was measured using an oscilloscope (Wave Surfer 510, Lecroy Corp., USA) and the short-circuit current was measured using a source meter (2611, Keithley, USA).

Figure 1 .
Figure 1.a) Schematic of a functioning H-MME generator, which is a combination of a PEH and an electromagnetic energy harvester (EMH).(i) Schematic of the output power generated by the H-MME generator in a stray magnetic field environment, (ii) rectifying circuit with load connection control for power management of the capacitor, and (iii) and self-powered proximity-detection system.b) Photograph of H-MME generator showing the integration of PEH and EMH in a Helmholtz coil.c) Exploded view of the piezoelectric SFC for PEH.d) Schematic of the operating mechanism of the H-MME generator.

Figure 2 .
Figure 2. a) Schematic of the optimization of the output performance of the PEH by adjusting the tip-end-to-base ratio (ζ) of the cantilever structure.b-(i) Finite element analysis to compare the stress distribution for varying ζ from 0.2 to 1.0 and (ii) additionally varying the width of tip-end from 18 to 22 mm for fine-tuning.c) Comparison of normalized maximum stress according to tip-end width ranging from 18 to 22 mm obtained from (b).

Figure 3 .
Figure 3. a) Schematic of the H-MME generator to optimize output power as a function of the weight of the magnetic proof mass.b) Theoretical simulation model for electromagnetic induction in the copper coil as a result of the variation in the magnetic flux with the magnet weight.c) Simulated normalization of the EMH output power, d) measured RMS output power of the EMH, and e) measured RMS output power of the PEH with varying weights of the magnetic mass ranging from 30 to 39 g. f ) Graph showing the tendency of the output power of the H-MME generator (i.e., the sum of the output power of PEH and EMH) according to the increase in the weight of the magnetic mass.

Figure 4 .
Figure 4. a) Schematic of the H-MME generator to optimize the output power according to distance from the center of each magnet and electromagnetic coil.b) Theoretical simulation model for electromagnetic induction in the copper coil as a result of a variation in the magnetic flux with a change in magnet distance.c) Simulated normalization of EMH output power, d) measured RMS output power of the EMH, and e) measured RMS output power of the PEH with varying distances of the magnet from the Cu coil ranging from À5 to 20 mm.f ) Graph showing the tendency of the output power of the H-MME generator (i.e., sum of the output power of PEH and EMH) according to the distance of magnet from the Cu coil.

Figure 5 .
Figure 5. a) RMS output power of the PEH and b) the EMH as a function of load resistances measured according to the input magnetic field intensities ranging from 1 to 6 Oe.c) Schematics for configuration to characterize the output power of the H-MME generator in combination with PEH and EMH with load resistance connection.d) The measured RMS output power of (i) PEH and (ii) EMH as a function of each load resistance at 5 Oe.e) Total RMS power of the H-MME generator obtained by summing (d-i) and (d-ii).f ) Comparison of the maximum output power of the independently measured PEH, EMH, and H-MME generator at the optimum load resistance.

Figure 6 .
Figure 6.a) Schematic of the H-MME generator, rectifying circuit, multifunctional IoT sensor, and smart device for a self-powered IoT sensor system.b) Photograph of the experimental setup with the H-MME generator inside a Helmholtz coil for application to power a multifunctional IoT sensor connected with a smart device (Inset shows a captured photograph for real-time measurement of environmental parameters using the multifunctional IoT sensor).c) Graph for charging a supercapacitor (0.2 F) from 0 to 3.6 V using the PEH, EMH, and H-MME generators at an input magnetic field of 5 Oe.d) Charging and discharging curve of the supercapacitor (0.2 F) during the real-time wireless communication of the H-MME generator-based self-powered multifunctional IoT sensor system.

Figure 7 .
Figure 7. a) Block diagram for the SPDS including the H-MME generator, power storage management circuit, US transducer, speaker, and smart device.b) Schematic of the self-powered SPDS with all the components assembled in a compact housing.c) Captured snapshot demonstrating the self-powered SPDS detecting distance and transmitting it to the smart device (The inset shows the photograph of the self-powered SPDS).d) Charging and discharging of a supercapacitor (0.4 F) according to the periodic operation of the SPDS by automatically turning ON/OFF the connection with the supercapacitor.

Table 1 .
Comparison of RMS output power and power density of the H-MME generator with the varying weight of the magnetic mass.

Table 2 .
Comparison of output power density with distance of the proof magnet from the Cu coil, where 0 mm is defined as the position where the coil center coincides with the proof mass.