A Four-in-One Compact Design of Monostable Quasi-Static-Toggling Mechanical Energy Harvester

Quasi-static-toggling (QST) mechanical energy harvester (MEH) can slowly accumulate a certain amount of mechanical potential energy. After the triggering position, it releases and converts the stored energy into useful electricity in an instant. It was designed into microgenerators for powering some ultra-low-powered Internet of Things (IoT) devices. This article introduces a 4-in-1 QST-MEH design. It uses a compact and deformable iron cantilevered beam to inclusively embody four key elements, i.e., an iron core, an energy-buffering spring, a rebounding spring, and a mechanical lever, which form a monostable QST electromagnetic generator. Compared with the existing designs, the proposed design reduces the component number to the most extreme. It largely helps reduce manufacturing and assembly procedures toward low-cost implementation and reliable operation. Experimental result shows that about 0.25 mJ of energy can be harvested from each round of press-release action with the experimental prototype. Such an amount of energy is sufficient to power an iBeacon bluetooth low-energy transmitter for motion-powered IoT applications.


I. INTRODUCTION
Vibration energy harvesting technology has been extensively studied during the last two decades.By scavenging energy from ambient vibration and turning it into useful electricity, we can prolong the chemical batteries' lifetime or even replace batteries in some lowpower standalone Internet of Things (IoT) devices toward sustainable and ubiquitous intelligence [1], [2], [3], [4], [5], [6], [7].Due to the broadband and low-frequency features of many ambient motions, e.g., human motions, such as finger tap, arm swing, or footstep, many studies proposed different broadband [8], [9] and frequency up-conversion [10], [11], [12], [13] solutions.Recent studies show that some specific designs called quasi-static-toggling (QST) mechanical energy harvesters (MEH) make an even better performance than most existing broadband and frequency up-conversion designs under extremely low or almost zero frequency [14].The QST-MEH can be driven in a very slow motion because it accumulates a specific amount of mechanical potential energy before triggering a release. 1 In other words, the quasi-static energy accumulation process even needs not to involve a significant amount of kinetic energy before triggering.Such QST-MEH designs can be realized by using either of the prevailing electromechanical transduction mechanisms, such as electromagnetic (EM) [14], piezoelectric [15], or triboelectric [16] transducers.Besides possessing excellent performance under quasi-static operation, the EM QST harvester can easily generate an easy-to-process voltage output (usually more than 10 V).As we know, generating a relatively large voltage under very slow motions with an EM transducer, in particular, within a small-size constraint, is an important, difficult, and extensively investigated research topic in the field of EM energy harvesting [17], [18].The QST solution offers new insight into overcoming the lowvoltage-output problem for EM MEH devices.
The monostable QST-MEH was empirically engineered into commercial products of battery-free wireless switches for more than a decade [19], [20].Their energy and dynamics in operation were thoroughly studied in one of our recent papers [14].Two unique mechanisms, namely, potential energy precharging and instantaneous magnetic poles swapping, enable the realization of QST operation.The first feature is enabled by using an energy-buffering spring, which accumulates sufficient force to move the iron core against the magnetic adhesion.The second feature is realized by an magnetic iron core moving abruptly between the north and south magnetic poles.In addition, a rebounding spring is usually added to make a bistable MEH into a monostable one, which is more natural to the users' experience in pressing a mechanical press button.To better match the comfortable zone of pressing motion or a desirable stroke, a mechanical lever is also found in many existing designs.The breakdown and assembly pictures of a commercial monostable QST harvester developed by Chlorop Company, Ltd. in China are shown in Fig. 1(a) and (b).
Given the aforementioned four essential components toward the realization of a monostable QST-MEH device, large effort and cost were caused in the assembly procedures of these small QST generators.In this article, we demonstrate that such miscellaneous components and labor-intensive assembly problems can be eased by sophisticatedly adopting a compact deformable structural design.The breakdown and assembly pictures of the proposed design are shown in Fig. 1(c) and (d).In the new design, we use a single deformable iron cantilevered beam (component #12) to carry out the functions of four essential components, i.e., the rebounding spring (#2), energy-buffering spring (#3), iron core (#4), and lever (#5).A China Patent was filed in March 2023 based on the proposed 4-in-1 design [21].

II. CONFIGURATION AND OPERATION
Fig. 2 shows the configuration of the proposed single-beam QST-MEH device.In the iron cantilevered beam OAB, different segments fulfill the functions of one or two key components of a monostable QST-MEH device.The AB segment directly touches one of the iron plates, which are magnetized by either the north or south magnetic poles, respectively; therefore, this segment acts as the iron core guiding the magnetic flux.When it deforms, the same segment also buffers an amount of potential (strain) energy, which is later released and converted into regulated electricity in the affiliated energy conversion circuit.The OA segment acts as a rebounding spring, which drags the AB segment back to the default monostable position after each round of press-release action.Before the tip of the beam breaks the magnetic adhesion, the pressing force at point A and the drag force at point B (magnetic attracting force minus contact force) obey the levering relation, i.e., |F A /F B | = OB/OA.
A cycle of press-release action goes through three key phases, as shown in Fig. 2. The tip of the iron beam is initially biased to the top iron plate (Position #1 in Fig. 2).In this position, as the tip is attracted by the north pole of the magnet, the magnetic flux in the iron beam flows from point B to point A. In this case, the magnetic flux through the coil is Φ 0 .The deforming phase starts when an external pressing force is applied downward at the middle point A of the iron beam.The beam undergoes sagging bending (Position #2 of Fig. 2).As long as F B exceeds the magnetic attractive force, the contact force to the top iron plate drops to zero; the tip of the beam starts to disengage from the top iron plate and hit the bottom iron plate (Position #3 in Fig. 2).The magnetic flux reversely flows from point A to point B. In this case, the magnetic flux through the coil is approximately −Φ 0 .Accelerating by the sum of the pressing force and the attraction force of the south pole, which are in the same downward directions, the tip of the beam leaves the upper iron plate and arrives at the lower one in just 1 ms [14].Thus, the triggering phase finishes almost in an instant.During this triggering phase, the magnetic flux through the coil changes by ΔΦ = −2Φ 0 .After the triggering phase, the external pressing force is gradually withdrawn.Owing to the rebounding segment OA, the beam resumes from Position #3 to Position #1.The magnetic flux reverts to the original B to A direction.As a result, the magnetic flux through the coil changes from −Φ 0 to Φ 0 , the magnetic variation ΔΦ = 2Φ 0 in the resuming phase.
According to Faraday's law of EM induction, the voltage generated in the coil is formulated as follows: where N is the number of turns in the coil, Φ is magnetic flux, ΔΦ = 2Φ 0 or −2Φ 0 is the change of magnetic flux at a triggering instant, and 2 d is the gap between two iron plates, as shown in Fig. 2. Assuming a uniform magnetic flux variation across the plates' gap, the generated voltage is approximately proportional to the moving velocity dx/dt.Given N and ΔΦ/2 d are both pretty large numbers, a rapid beam tip movement produces a short voltage pulse, whose magnitude is up to tens of volts.Such a voltage level is easy to be converted into a regulated digital voltage output (usually 1.8 V or 3.3 V for CMOS ICs) than those in many linear EM designs [17].

III. MODEL
The theoretical lumped model of a QST harvester was comprehensively studied in our previous paper [14].It is compatible with this new design.Therefore, here we only sketch the key steps for mathematically modeling the behavior of this monostable QST harvester.
Fig. 3 shows the equivalent model summarizing the mechanics of the proposed design before a quasi-static triggering.The displacements of the pressing point and tip are denoted as x A and x B , respectively.The OA and AB segments of the beam are modeled with two lumped stiffness K OA and K AB , respectively.The magnetic force effect at the beam tip is modeled as a nonlinear negative stiffness where K g3 and K g1 are two constants approximately fitting the practical nonlinear magnetic force according to the Duffing-type model.The stoppers formed by the upper and lower iron plates are modeled by a piecewise stiffness K s (x).The functional relations of the aforementioned four stiffness components are qualitatively illustrated in Fig. 3(b)-(e), respectively.The equivalent mass only affects the dynamics after triggering; therefore, it is not shown in this equivalent quasi-static model, in which the combination of potential energy enables the desired QST operation.When the beam's tip adheres to the top magnetic guiding plate, the total potential energy of the QST system is expressed as follows: where H(•) is the Heaviside unit step function.As x A continues to proceed in the downward (negative) direction during a press action, after attaining the critical point, where F AB + F g = 0, the beam's tip B breaks the magnetic adhesion, rapidly moves toward the bottom plate and triggers the instantaneous EM induction.The release action goes through a symmetric triggering process from the bottom plate to the top plate.

IV. PROTOTYPE AND IOT SYSTEM
To validate the operation of the proposed compact QST-MEH design, a mechanical prototype is manufactured, as shown in Figs.1(c) and (d) and 4(b).The mechanical component values of the prototyped QST harvester are listed in Table I.Some of the equivalent lumped parameter values are obtained according to the measurement results.By connecting the coil's output of the prototyped micropower generator to a bluetooth low-energy (BLE) iBeacon transmitter, which has an energyharvesting specific power management unit as introduced in [14], we successfully make an efficient motion-powered BLE iBeacon device,  as shown in Fig. 4(a) and (b).In a round of press-release action, the QST device can reliably generate sufficient energy to power and drive the iBeacon transmitter to send out one beacon packet.Together with a mobile app shown in Fig. 4(c), which was customized in one of our previous studies, as the receiver-side display [22], we successfully built and integrated all mechanical, electrical, and information modules of a motion-powered IoT system from sketch.Different from many previous studies on kinetic energy harvesting, which puts more emphasis on power output, in this system, energy is the linkage ensuring the successful integration of all parts from generation to conversion and consumption.

V. PERFORMANCE EVALUATION
To further evaluate the performance of this compact QST-MEH design, we measure the input mechanical characteristics of the prototyped energy harvester with a manual micropositioning stage (for applying a stroke in an extremely slow way) and a static force gauge.The output electrical characteristics are measured using an oscilloscope.The experimental setup is the same as that introduced in [14]; therefore, the description is not repeated here.Fig. 5 shows the relation between the stroke and applied force at the pressing point A during one round of press-release action in the experiment.Starting from the x A = 0 position, it first goes through compression with a linear stiffness along the press trajectory.The force magnitude suddenly drops at the stroke around x A = −0.28mm.A second steep but smaller force drop is found around x A = −0.33 mm.The two separated force drops, rather than just one drop, might be due to the effect of a stiff energy-buffering spring [14].When being released, the force steeply changes when x A = −0.21mm and −0.17 mm.The area enclosed by the press and release trajectories corresponds to a mechanical input energy of about 4.6 mJ, according to the work cycle analysis [23].Due to the levering effect of the proposed design, this first prototype is suitable to operate under relatively small stroke and large force scenarios.Fig. 6 shows the electrical output characteristics.Fig. 6(a)-(c) shows the open-circuit output voltage from the coil.Fig. 6(d) illustrates the harvested storable dc energy obtained by connecting the QST energy harvesting to a bridge rectifier and a buffer (filter) capacitor.The Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.press and release actions trigger one positive and one negative pulse, respectively, as overviewed in Fig. 6(a).As further illustrated in the enlarged views shown in Fig. 6(b) and (c), the short pulses at the open-circuit condition last for only 1.5-2.5 ms with a magnitude of more than 10 V.It tells the rapidness and strength of the instantaneous energy release.By connecting the pulsed output voltage to a bridge rectifier and a filter capacitor, we can obtain a stable dc output voltage, whose corresponding stored energy can be formulated according to the energy equation of a capacitor, i.e., E h = CV 2 dc /2, where C is the capacitance and V dc is the rectified dc voltage across the filter capacitor.Fig. 6(d) shows that, by changing the capacitance value of the buffer capacitor, the harvested energy differs.It attains a peak harvested energy of about 0.25 mJ when the buffer capacitance is 22 µF after one round of press-release action.As it was proven in our previous studies [15] that the affiliated BLE iBeacon transmitter consumes about E BLE = 0.2 mJ energy for sending one information packet, the prototyped QST-MEH-based system can provide sufficient energy to send out a BLE packet after only one press-release action.Therefore, the cyber-electro-mechanical codesign introduced in Fig. 4 successfully functions.

VI. DISCUSSION
Regarding the transduction mechanism used, the proposed MEH utilizes a classical EM transducer, which was wellstudied based on the Faraday's law of induction.Yet, the whole system differs from merely a linear EM harvester.Most linear EM harvesters output a rather small voltage, which is hard to process with a normal bridge rectifier.Nowadays, one of the standpoints endorsing the extensive investigations of triboelectric nanogenerator is that it outperforms the EM harvester at low-speed operations, such as body motion and ocean wave [28].Therefore, generating a relatively large and easy-to-process voltage under very slow motions, in particular, with a small-size design, is an important, difficult, and extensively investigated, rather than a trivial research topic for EM devices.It turns out that, over the last 20 years, since the first German Patent filed by EnOcean GmbH in 2003 [29], academic people have overlooked the effective QST solutions realized by engineers from industry.This article further carries on the study of QST EM harvesters and proposes a concise, effective, and novel design.It proves that the low-voltage output issue of an EM harvester at low-speed operation can be easily solved by introducing a supereasy structure.Table II lists some typical MEH in the literature for comparison.It highlights the advantages of the QST structures toward ultra-low-frequency operation and an easy implementation of complete IoT systems, particularly in response time, easy manufacturing with a small number of parts, etc.
According to the measured mechanical input energy of 4.6 mJ (see Fig. 5) and the optimal electrical output energy 0.25 mJ [see Fig. 6(d)], the conversion efficiency of this prototype is 5.4%.At this moment, we can only measure the input mechanical energy and the output electrical energy.The theoretical calculation of conversion efficiency is not ready yet because it involves a complex interactive process with a distributed parameter system, transient release behavior, electromechanical energy conversion, and mechanical collision, which is more complex than those linear or nonlinear systems discussed in the existing studies [30], [31].This article emphasizes the concise 4-in-1 design toward the functionality of a compact, self-contained, and self-powered IoT device.More comprehensive mechanics and electromechanical interaction will be analyzed in detail in our future work.

VII. CONCLUSION
In summary, this article has introduced a compact and effective QST EM energy harvester and its immediate application in a battery-free IoT system.The advance of the proposed design results from the utilization of a deformable iron cantilevered beam to inclusively replace four essential ingredients of a monostable motion-powered press button, i.e., an iron core, an energy-buffering spring, a rebounding spring, and a force-adapting lever.The first prototype of the 4-in-1 QST-MEH device records a mechanical input energy and an electrical output energy of 4.6 and 0.25 mJ, respectively, which results in a conversion efficiency of only 5.4% at this stage.Although the efficiency now is low, the harvested energy is sufficient for powering a BLE iBeacon device to send out one wireless packet for an immediate application.Such a compact and holistic design is superior to many complicated or one-sided designs in the related research field.It is promising to achieve better cost effectiveness compared with the existing commercial products of motion-powered wireless switches.In the future, more studies are to be carried out for an in-depth analysis, improvement, and the optimization of the proposed compact 4-in-1 QST-MEH device.

A
Four-in-One Compact Design of Monostable Quasi-Static-Toggling Mechanical Energy Harvester Shiyi Liu and Junrui Liang , Senior Member, IEEE Abstract-Quasi-static-toggling (QST) mechanical energy harvester (MEH) can slowly accumulate a certain amount of mechanical potential energy.After the triggering position, it releases and converts the stored energy into useful electricity in an instant.It was designed into microgenerators for powering some ultra-lowpowered Internet of Things (IoT) devices.This article introduces a 4-in-1 QST-MEH design.It uses a compact and deformable iron cantilevered beam to inclusively embody four key elements, i.e., an iron core, an energy-buffering spring, a rebounding spring, and a mechanical lever, which form a monostable QST electromagnetic generator.Compared with the existing designs, the proposed design reduces the component number to the most extreme.It largely helps reduce manufacturing and assembly procedures toward lowcost implementation and reliable operation.Experimental result shows that about 0.25 mJ of energy can be harvested from each round of press-release action with the experimental prototype.Such an amount of energy is sufficient to power an iBeacon bluetooth low-energy transmitter for motion-powered IoT applications.Index Terms-Electromagnetic (EM) generator, energy harvesting, Internet of Things (IoT), quasi-static-toggling (QST).

Fig. 2 .
Fig.2.Configuration of the proposed QST harvester and its operation in a press-release action.In the iron cantilevered beam OAB, the AB segment acts as the iron core and energy-buffering spring.The OA segment acts as the rebounding spring.The whole beam acts as a mechanical lever with a force ratio |F A /F B | = OB/OA.

Fig. 3 .Fig. 4 .
Fig. 3. Equivalent model of the QST-MEH.(a) Schematic diagram.Constitutive relations of (b) Rebounding spring K OA .(c) Energybuffering spring K AB .(d) Magnetic effect modeled as a nonlinear negative stiffness K g .(e) Stopper effect, which is modeled as a piecewise stiffness K s .

Fig. 5 .
Fig. 5. Experimental result of the mechanical input characteristics.

Fig. 6 .
Fig. 6.Experimental result of the electrical output characteristics.(a) Open-circuit voltage in a press-release cycle.(b) Enlarged view of the press instant (beam position from #2 to #3).(c) Enlarged view of the release instant (beam position from #3 to #1).(d) Harvested storable dc energy after one round of press-release action by using a bridge rectifier under different buffer (filter) capacitance values.