Energy Harvesting in Tire: State-of-the-Art and Challenges.
Energy harvesting is a widely used technology in various fields, such as the aerospace, oceanography, building, and automotive industries. Kinetic, solar, and thermal energy are the typical forms of energy that are scavenged, but technological advancements now enable power harvesting from vibrations and deformations, which are converted to energy that can be used to supply power to target applications. Energy harvesting techniques are particularly beneficial to the automotive sector, but innovations involving in-tire energy harvesting have been rare. Given the importance of tire parameters in vehicle dynamic control, sensors are embedded in tires to acquire real-time measurements and send data to a vehicle's electrical control unit via radio frequency transmission. The in-tire sensors used to measure key tire parameters are traditionally powered by embedded cell batteries. The problem is that the limited energy storage of such batteries necessitates frequent replacement. The tire pressure monitoring system (TPMS) is the most frequently adopted system in vehicles. TPMS companies state that the lifespan of a TPMS battery is 5-10 years, but such a service life is possible only for automobiles whose TPMS is mostly in idle mode or when a given battery is large enough to provide hundreds of milliamp hour capacity . Satisfying real-time control demands necessitate the collection and transmission of voluminous data, which diminish battery life and reduce intervals between battery replacements. To eliminate the need for battery replacement and ensure real-time data transmission, introducing energy harvesting systems in tires is necessary.
In-tire energy harvesting has elicited considerable attention in the automotive field. An in-tire energy harvester can convert a number of energy types to electricity. Harvested energy can be used to directly charge tire sensors or stored in a rechargeable cell, depending on the sensors' power requirements and the amount of harvested energy. The development of intelligent tire technology has motivated the installation of numerous sensors in tires, thus dramatically increasing electrical demands. Bowen and Arafa  summarized various harvesting systems aimed at supplying power to TPMS sensors. Current in-tire energy harvesters are designed primarily for TPMSs. The rapid development of sensor technology and the growing demand for vehicle safety are expected to increase the number of developed in-tire sensors. Aside from TPMS sensors, the sensors used to measure or estimate the lateral, longitudinal, and vertical force of tires and other significant tire parameters are worth studying. Jo et al.  adopted a configuration that uses measured strain value to calculate tire force and accordingly guarantee improved control over vehicle behavior. Singh et al.  summarized the types of information obtained from intelligent tire sensors to show how such data affect control systems. One of the challenges encountered in in-tire energy harvesting is satisfying mounting power demands. Previous study of Kubba et al.  reviewed different tire pressure measurement approaches as well as battery-less TPMSs developed by individual researchers or by world's leading tire manufacturers. However, state-of-the-art research still need to be included in and summarized in a view that can present the overall research process and future development of different in-tire energy harvesting methods.
A tire is the only source of contact between a vehicle and the road; thus, collecting data on tire performance is essential to enhancing overall vehicle safety and performance. The effective functioning of vehicle control systems, such as antilock braking systems, electric stability control, adaptive cruise control, and TPMSs, requires accurate and real-time data, including contact force/torque, wheel speed, slip angle, and tire pressure. Various control systems equipped with different sensors need varying amounts of power supply; these variances determine the design principle, size, integrated approach, and output power of an energy harvester.
Theoretically, tires present sufficiently large energy potential, but the amount of harvested energy depends on the manner by which tires are used and optimized. The energy generated by tires originates from vibrations, rotations, deformations, thermal power, and air flow, among other sources. However, energy is released from such sources only for as long as a vehicle is running. In-tire energy harvesters enable the extraction of energy from every vehicle cycle, converting source energy to the electricity needed to power sensors or charge cell batteries. Advanced research has introduced several mechanisms, such as piezoelectric, electromagnetic, and electrostatic generators, whose harvested energy ranges in the microwatt to the megawatt level. Despite these promising advancements, in-tire energy harvesting design continues to be confronted with various challenges, including robustness, size, dynamic balance, compatibility with micro-electromechanical systems (MEMS), efficiency, and output power density. In addition, more and more researchers concern about the performance under uncertain factors and also potential problem caused by implementing the energy harvesting systems in tire. Kim  studied the algorithms to prevent the cross-connection errors between wrongly matched control modem (Tx) and energy harvesting charging device (Rx), which will occur frequently if the vehicle is located in close proximity to another vehicle or surrounded in the environment with dense wireless communicating devices.
The rest of the article is organized as follows: The first section describes the energy harvesting target, which is the guideline of the harvester design. The second section presents three main mechanisms that are used in the in-tire energy harvesting, in which various examples have been given to explain the technical development. The third section introduces the methodologies for modeling, simulation, and experimentation, giving reference for further research. The last section concludes the sense of in-tire energy harvesting, compares the main methods, and summarizes the development and challenges confronted by the energy harvesting.
This article concerned more about the process of in-tire energy harvesting research, including the first step estimation of the power needed, mechanism choices, structure design, mathematical model simulation, optimization methods, prototype manufacture, installation, and experiment conduction. An overall review of the principles, performance, weight, and size is compared among the current mechanisms. In depth, conclusion is derived based on the great deal of researches. Last but not least, challenges are analyzed, including efficiency, cost, size, weight, dynamic balance, reliability, maintenance, manufacturing, and so on. The conclusion and challenges analysis will give insight into the future research and work.
Energy Harvesting Target for In-Tire Sensors_______
A general understanding of the power consumption of in-tire sensors is essential to clearly establish the output power goals of harvesters. TPMS power consumption, which is enabled by Freescale technology , is 7 mA at 434 MHz at 5 d Bm (d BmW) and 900 nA in idle mode; the idle mode consumption is equivalent to a maximum supply current of 0.14 [mu]A under 3 V. Kubba et al.  indicated that a TPMS s power requirement in the transmission frequency range is around 450 [mu]W under 60 times per minute. This power consumption serves as a good reference standard for establishing the objectives of energy harvesting design.
A sensor system typically comprises a conditioning circuit, a microcontroller, and a communication module. The functional operations of the microcontroller are classified into active and sleep modes, which require different power supply levels. The communication module implements data receipt and transmission. Xin Xue et al.  estimated the power requirements of wireless sensor nodes to be 3.015 mW for the microcontroller, 73 mW for the communication module, and at least 1.8 V as the minimum supply voltage for a Tmote Sky sensor node. As can be seen, the communication module accounts for a large proportion of energy consumption. Slow vehicle speeds eliminate the need for frequent data transmission, thereby reducing power demands. The power generation in most proposed energy harvesters is strongly related to vehicle speed. High speed is generally accompanied with large power harvesting potential, indicating that the power target of a harvester should be consistent with a vehicle's working condition. Low-power sensor systems are being gradually developed by sensor suppliers , a development that facilitates the satisfaction of power requirements through the use of in-tire energy harvesters.
Potential Sources and Volumes of Energy from Tires
The mechanisms currently used in energy harvesting systems are grouped in piezoelectric, electromagnetic, and electrostatic principles. The most popular harvester resources are piezoelectric materials. Piezoelectric generators are characterized by a simple structure, relatively compact size, easy installation or integration in tires, and compatibility with MEMS. Correspondingly, researchers tend to design energy harvesters on the basis of a piezoelectric structure. Nevertheless, the structure presents certain disadvantages, for example, piezoelectric material can only bear finite number of times of deformation, which seriously limited the lifespan of piezoelectric devices. The working principle of all electromagnetic generators is based on electromagnetic induction, but researchers have thus far presented distinct devices. The harvesting effect of an electromagnetic mechanism depends largely on relative motion amplitude and the speed of a magnet and coil activated by tires. Normally, electromagnetic configurations can be elaborately designed to ensure a sufficiently large generated energy density and extended operational life. However, most designs are relatively large scale, complex, and difficult to integrate with MEMS. As for electrostatic generators that function similar to a capacitor, changes in the distance or area between electrical conductors are used. Less studies have been devoted to electrostatic generation than the first two methods mostly because the former requires external voltage; scholars are eager to address the inconvenience that this prerequisite presents to effective tire functioning.
Mechanisms other than the three mainstream ones mentioned above are scare and different from each other. Take the research of Guo et al.  as an example; they have designed and fabricated compressible hexagonal-structured triboelectric nanogenerators (CH-TENGs) to harvest tire rotation energy. The CH-TENGs are obtained from Cu-coated fluorinated ethylene propylene (FEP) film and polyimide layer and can be stacked in parallel connections as shown in Figures 1 and 2. The maximum instantaneous power is demonstrated to be 1.9 mW with the CH-TENG in 8 units, a weight load of 10 N, and a speed of 2.51 m/s, which shows a potential of 1.2 W with 500 units and a running speed of 100 km/h.
The principle that underlies a piezoelectric generator is the direct piezoelectric effect; that is, electric charge accumulates on the surface of piezoelectric materials as a response to mechanical stress. Piezoelectric materials, such as crystals and ceramics, exhibit polarity and deformation under external force, thereby generating electricity due to polarization. The power harvesting effect of the piezoelectric generator depends strongly on design, material, and coupling coefficient.
Piezoelectric Materials Different piezoelectric materials have various properties that substantially influence harvester performance. The most extensively employed material is PZT (lead zirconate titanates), which is made of lead zirconate titanate, a kind of piezoelectric ceramic with high voltage and dielectric constants. Bulk and thin film PZTs easily break under high frequency loads , thereby constraining their suitability for vehicle tires. The piezoelectric polymer, polyvinylidene fluoride, is resistant to high strain but sensitive to temperature. Beeby et al.  compared the piezoelectric constants of different piezoelectric materials and found that the electromechanical coupling coefficient is essential to the efficiency with which mechanical energy is converted to electrical energy. The piezoelectric strain constant, planar coupling factor, permittivity, and aging rate of a material also considerably affect the performance of generators. To achieve high energy density, piezoelectric ceramic composition is synthesized, and the material is processed under special conditions . The properties of piezoelectric composites depend on the distribution of processing steps, as well as the types of connectives, polymer matrices, processing methods [151, and materials used to construct a harvester. A material can be structured into fibrils, plates, layers, grains, and powder form. The two-step sintering technique  for connecting materials effectively reduces grain size and improves energy density. Singh et al.  fabricated a piezoelectric bimorph transducer by using a high energy density composition that corresponds to 0.9 Pb ([Zr.sub.0.56][Ti.sub.0.44])[O.sub.3]-0.1 Pb [([Zn.sub.0.8/3][Ni.sub.0.2/3])[Nb.sub.2/3]][O.sub.3] and Mn[O.sub.2] (PZTZNN) to optimize the piezoelectric constant.
Research on materials can improve the performance of energy generators. The ultimate goal for these technologies is for them to be as compact as possible yet still achieve relatively high output power.
Installation of Piezoelectric Harvesters Piezoelectric harvesters are typically installed either on the rim or in the inner liner of a tire; each installation option is suited to a particular structure. For harvesters that scavenge energy from the deformation and stress that occur upon tire-ground contact, mounting on the inner liner of a tire is a more effective approach because this installation type adapts to the strain-driven mode, thus providing the electricity generated by the direct deformation of a piezoelectric material. For harvesters that obtain energy from vibration, both installation methods are practical, and choice is determined by the specific design of the piezoelectric material. Basing choice on design is appropriate for cases wherein changes in centrifugal and gravity force occur. Under such conditions, inertial energy harvesters are typically used. The rotation of a wheel can serve as an energy source, as enabled by a spring-mass system normally mounted on the wheel's rim. The stress status of the harvester changes as the wheel rotates, thereby activating piezoelectric materials, which then generate electricity.
Structure Development The basic structure of a piezoelectric generator is a cantilever made of piezoelectric materials. Usually, one end of the cantilever is fixed either to the rim or to the inner liner of a tire; the other end is freestanding and subjected to stress that originates from tire deformation, rotation, or vibration. Because this basic structure leaves much room for optimization, researchers have put forward a number of methods for improving structure performance.
Scholars initially attempted to improve the length, thickness, and number of layers of the basic structure to augment its efficiency. In this regard, the types and combination of piezoelectric materials are other critical issues for investigation [12, 17, 18, 19]. A given material and the structure must correspond with each other to enable maximum output power. The superposition of several identical units is a simple but effective way of amplifying output power . Some researchers added a mass proof to the free end of the cantilever  to increase the vibration amplitude of a piezoelectric material and correspondingly improve the amount of harvested energy. In experiments, researchers realized that vibration frequency significantly affects harvest results. Common linear generators have a narrow frequency range. Given that a tire has a wide vibration spectrum, generators that use resonance at a certain frequency are minimally effective. Studies on methods of expanding the bandwidth of energy harvesting systems are currently underway [16, 20, 21], and the goal of bandwidth expansion has resulted in the creation of nonlinear designs. Tuned frequency generators or a combination of different frequency oscillators are also being developed. Researchers have recently created designs that are based on both piezoelectric and electromagnetic principles. These designs present promising potential. Furthermore, Xie et al.  developed a mathematical model for piezoelectric ring energy harvesting technology, of which the structure is quite different from cantilevers.
Hu et al. [1Z] presented a nanogenerator for energy harvesting that capitalizes on tire rotation. A material combined with five layers, including polyester substrate, ZnO nanowire, and electrodes, was used to construct the nanogenerator. The combination enables flexibility and a maximum output power density of 70 [mu]W [cm.sup.-3]. The experiment first involved the application of the initial nanogenerator to a bicycle, after which a parallel-connected nanogenerator was constructed. The updated design is applicable to wider vehicle tires. Apart from the novel design, additional insights were provided by Hu Y's work. For example, the results indicate that the amount of harvested energy can serve as a good indicator of tire pressure because deformation magnitude is strongly related to tire pressure. The authors also used a free cantilever beam structure, whose optimization method is parallel connection. The maximum output power of the structure is 120 [mu]W [cm.sup.-3]. The structure and mounting method are depicted in Figure 3.
Singh et al.  conducted experiments on the basis of data acquired by triaxial accelerometers to identify fundamental natural frequencies at different speeds. The authors also analyzed the vibration spectrum of a tire. The outdoor tests and the analysis of mean power curves combined with the spectrum show that the optimum output power produced by the energy harvester is 60-80 Hz, which is suitable for a wide range of tire speeds. This structure is illustrated in Figure 4.
To overcome the limited operating bandwidth of most energy harvesters, Roundy and Tola  developed an energy harvester for rotational environments. The harvester (Figure 5) uses the dynamics of an offset pendulum and nonlinear bistable oscillating motion. In the structure, a proof mass is connected to a piezoelectric beam through a small ball. The motion of the proof mass is similar to that of a pendulum swinging back and forth, and the movement generates electricity by causing beam deformation. The experimental results indicate that this harvester can support more than one transmission per minute when applied to a TPMS. Nevertheless, the design still needs further improvement.
Sadeqi et al.  put forward a novel piezoelectric-based energy harvester that uses a mass-spring system to broaden frequency. The specific components of the harvester are a coupled mass-spring system attached to a PZT beam. It generates 75-125 [mu]W of power at a vehicular speed of 70-80 km/h, and its resonance frequency is tuned by changing the spring stiffness and tip mass offline. The harvester is schematically shown in Figure 6.
Xie et al.  propose the dual-mass piezoelectric ring tire harvester shown in Figure 7. The design consists of a series of discrete PZT4 patches circumferentially embedded in a polymer ring, placing between two steel belts pasted in the inner liner of the tire. The simulated results show that the harvested power is up to 42.08 W with central angle 9 equals 30[degrees], radius of the ring equals 0.25 m, width equals 0.01 m, thickness equals 0.01 m, and number of patches equals 3. The theoretical model indicates great potential and efficiency; however, the practical energy harvesting ability still needs to be verified by further experimental test.
Summary and Research Trend In summary, piezoelectric harvesters can be classified into different devices according to type of energy source (e.g., inertial, kinetic, rotational, vibrational). A basic piezoelectric generator consists of a beam and a proof mass. An end stop is used to limit the position of the beam and avoid the mechanical failure of the material. Proof mass, springs, and geometry designs are typically used to tune the natural frequency of a harvester; this application, in turn, enables adaptation to the wide vibration spectrum of tires. Potential power limits and efficiency refer to the equation given by Shafer et al. , which depend on parameters like mechanical damping ratio, acceleration magnitude, modal mass, system natural frequency, allowable stress in the piezoelectric material, etc. The expression indicates preference of beams with low length-to-width ratio and a low ratio of mass-to-cubed length. Given that integrating electrodes with piezoelectric materials is easily accomplished through deposition or etching, piezoelectric generators are compatible with MEMS. MEMS offers advantages in terms of size and therefore facilitates power generation at the micrometer scale.
Recent research is showing the trend to analyze the general and theoretical problem faced by most of the piezoelectric in-tire energy harvesters. Eshghi et al.  studied the uncertainty of a piezoelectric energy harvester, including manufacturing tolerances, environmental effects, and material properties. This article showed that reliability-based design optimization (RBDO) increases the reliability of piezoelectric type energy harvesters by 26% with sacrificing the objective function for 2.5% compared to traditional deterministic design optimization (DDO). Multiple design optimization studies are required to improve the reliability and performance of the harvester since there are already plenty of basic structure designs.
Electromagnetic generators operate under the electromagnetic induction arising from changes in magnetic flux. Relative motion in tires is constant because of vibration, rotation, and deformation, which can be transformed to electricity via electromagnetic induction. The basic structure of an electromagnetic generator is composed of a permanent magnet and coil sets. This structure can be modified in numerous ways, and the magnet, coils, and generator frame are all repairable. The guiding principle of electromagnetic generators is the maximization of the relative moving speed and amplitude of the magnet and coil sets.
Changes in Force Conditions in Tires Analyzing experimental data, Bonisoli et al.  confirmed that acceleration drops to zero upon contact between tires and the ground. The author proposed an electromagnetic harvester that is radially installed in tires. During a rotation cycle, the acceleration within the tires proceeds in four phases (Figure 8). In the first phase, the tires do not come into contact with the ground, and the harvester is subjected to centrifugal acceleration. In the succeeding phases, the harvester is subjected to peak acceleration, zero acceleration, peak acceleration, and centrifugal acceleration when the tires come into contact with the ground, are completely planted on the ground, are lifted from the ground, and return to no contact with the ground, respectively. Energy can be harvested from the aforementioned rotation because of the excitation caused by inertial force.
As tires rotate, changes in the gravity to which harvesters are subjected occur; no such alterations happen to acceleration. The design of Wang et al.  primarily applies gravity changes in tires. Wang et al.  comprehensively analyzed vibration, radial force, and gravity to develop a nonlinear suspended energy harvester based on electromagnetic induction.
Introduction of Typical Structures Using the Electromagnetic Mechanism Bonisoli et al.  presented an electromechanical energy scavenging device, which consists of a fixed permanent magnet, a floating permanent magnet, two reverse wound coils, and a shell (Figure 9). The entire device is mounted onto the inner layer of a tire so that it can employ sudden changes in acceleration as an energy source upon tire-ground contact. The floating magnet is subjected to the variable acceleration and magnetic force of the preload magnet. As the tire rotates, the floating magnet moves up and down along an external box, thereby altering the magnetic flux in the coils. The designed harvester can generate 2 mW of power at a vehicular speed of 40 km/h.
Wang et al.  proposed a novel weighted-rotor energy harvester that is mounted on the rim of a wheel (Figure 10). The tangential gravity of the rotator changes periodically during wheel rotation, thus creating relative motion between the rotator and the stator. Magnets are installed on the rotator, and coil sets are mounted on the stator to harvest kinetic energy from the rotating wheel. The use of a suitable weighted block allows the harvester to match the natural frequency of the wheel. This component allows for the production of as much output power as possible. When a 550-[ohm] resistor is connected in series, the average output power generated by the harvester is 399-535 mW at an experimental plate rotation speed of 300-500 rpm.
Wang et al.  developed a nonlinear suspended energy harvester (NSEH) mounted on a wheel (Figure 11). The NSEH consists of a mass, a permanent magnet, springs, and coil sets. Kinetic equations are analyzed to derive the natural frequency of the NSEH, which exhibits nonlinear dynamic behavior. The parameters are optimized on the basis of the results of vibration analysis and magnetic flux simulation. The fabricated prototype generates 30-4200 [mu]W of power at wheel speeds of 200-900 rpm. These values are sufficient to cover the average power consumption of a TPMS.
Wang et al.  invented an innovative real-time wireless dynamic tire pressure sensor (DIPS) to detect the subsurface pavement profile using acoustic wave-based method. Considering the power supply requirement of the sensors, a strong magnetostatic coupling-based energy harvester has been designed along with the acoustic sensor. The energy harvester consists of a high permeability core solenoid that is fixed proximate a vehicle wheel, and a bias magnet array that is fixed in conjunction with a dust shield. The electromagnetic energy harvesting system that can provide power to the DIPS is shown in Figure 12.
Optimization of Output Power The basic elements of an electromagnetic energy harvester are magnets and coils, which can be combined by using springs, interaction forces, frames, or guides. The force condition of a magnet is usually complicated; its motional characteristics can be devised to provide more possibilities for maximizing output power. Parameters such as the electrical damping coefficient, coil turns, and wire gauge are the principal factors that influence the scale of output power.
An entire electromagnetic energy harvesting system comprises a mechanical subsystem and an electrical subsystem. The configuration of the electrical circuit can affect the damping coefficient of the magnet. Dynamic coupling means that optimization must consider the interaction between the two subsystems.
Soliman et al.  derived the maximum generated voltage equation of a harvester (i.e., Equation 1) on the basis of the analysis of the model schematic shown in Figure 13.
The maximum output generated at resonance is expressed as follows:
[mathematical expression not reproducible] Eq. (1)
where B is the magnetic field density, l denotes the effective coil length, [omega] presents the natural frequency, Y is the ground excitation amplitude, and [[zeta].sub.t] is the damping ratio determined by mechanical and electric dampers [291.
Ramadass et al.  optimized output efficiency by designing an interface circuit with a bias-flip rectifier and shared inductor. Optimization must simultaneously consider several parameters, so both the equation and simulation tool employed by the authors can be used to fine-tune configurations. Other than traditional electromagnetic principle, Liu et al.  take use of giant magnetostrictive material to convert the tire contacting force proportionally into output voltage.
An electrostatic harvester, a triboelectric generator, and electroactive polymers all operate under the principle of electrostatic generation, for which the relative movement among electrically isolated charged capacitor plates and the mechanical deformation of an electrode are used.
Yen et al.  presented and demonstrated a variable-capacitance vibration energy harvester that delivers 1.8 [mu]W of power to a resistive load. The principle is that vibration separates plates, thus changing capacitance. The system requires a startup voltage below 89 mV, which contributes to the considerable potential of the design. The harvester effectively converts vibration to electricity but requires enhancement in terms of application in tire systems and powering for TPMSs.
Renaud et al.  proposed a MEMS vibration energy harvester based on a [SiO.sub.2]-[Si.sub.3][N.sub.4] electret, whose footprint is only 1 [cm.sup.2]. When excited by a similar vibration in tires, the maximum output power generated by the harvester is 10-50 [mu]W. The schematic of the principle underlying the harvester is shown in Figure 14. Vibration causes the Si proof mass to move back and forth, thereby changing the electric charge between the electret and the electrodes.
The drawbacks of an electrostatic mechanism are that a startup voltage is required, and the generated power is relatively lower than that produced by piezoelectric and electromagnetic mechanisms. Its advantages are electromagnetic compatible connection to a load circuit and compact size.
Methodologies for Modeling, Simulation, and Experimentation
Modeling methodologies chiefly encompass schematic, stress, and vibration analyses. Piezoelectric harvesters are usually simplified into mass-spring systems  to enable the setup and calculation of equations. These harvesters generally consist of mechanical, electromagnetic, and circuital subsystems, requiring different analysis methods. In order to facilitate calculations of the motion status of complex coupling subsystems, Simulink enables dynamics simulation [20, 25], magnetics finite element method enables magnetic field distribution analysis , Ansys allows for stress and strain calculation [35, 36], and finite element method is able to simulate deformation, stress condition, and strain distribution [37.38], Experiments can be conducted on actual car tires [21,39], but most researchers carry out experimentation on a designed test platform. Simulated vibration input is used as the excitation source [40, 41]. Wang et al.  designed a shaker fixed with a steel beam as the experimental embodiment of the vibration caused by a tire and mounted a vacuum-packaged MEMS harvester on the shaker. Renaud et al.  performed a shock test. Other researchers [20, 26, 43] fabricated a rotating plate to simulate tire rotation. Kulkarni et al.  carried out an experiment to validate the performance of pulse-based resonant harvesters. Steel and masses were dropped on the harvesters' surfaces to generate the required pulse.
Challenges for In-Tire Energy Harvesting Systems
Despite the progress made in in-tire energy harvesting technology, challenges continue to confront the field. To begin with, the types of energy harvesting innovations intended for tire systems differ across automobiles. With respect to physical conditions, the space within a tire is characterized by poor and restricted conditions. These present significant installation issues, specifically those related to weight and size. Under constrained physical environments, piezoelectric generators are a suitable alternative, but electromagnetic harvesters provide higher output power per unit volume. The problem common to the three mechanisms is a harvester's compatibility with the vibration frequency of tires. When road conditions and vehicle speed differ, the spectrum of tire vibrations also vary. Much work has been directed toward broadening bandwidth, considering that output power is usually maximized at resonance. Materials, structures, proof masses, springs, multiple structures, and nonlinear components can all be studied to determine how harvesters can correspond with a wide range of vibration frequencies. Another important matter for consideration is circuit management because a circuit's output is power. Load parameters should be optimized to reach high efficiency. The output power reported in existing research remains inadequate for satisfying the power demand of sensors.
Aside from examining the power and efficiency that can be obtained within tires, research should also be directed toward the variation of tire mass distribution, tire vibration, harvester masses, especially in relation to tire imbalance, which may result in ride disturbance. The harvester-introduced distribution of mass within a tire differs from the distribution observed during tire manufacture. Because of variations in the mass distribution within a harvester, imbalance markedly increases even when a tire is equipped with static and dynamic balancers.
The big challenge for the tire energy harvester also relates to the tire reliability performance as well as the cost effectiveness between the energy saved and cost associated with the maintenance and manufacture of the energy harvester. Since most of the energy harvesters are only prototypes tested on the test bench or on-road test, the cost of manufacture can be expensive and no information about maintenance is addressed. In this article, available information of material, size, and weight in the reviewed research have already been mentioned, and others are missed because of unavailable information. On the other hand, safety is critical so sometimes cost is subordinated to safety, especially in the research and development stage. In the future, cost would be reduced through volume production.
The increased attention elicited by tire technology and the rising demand for vehicular safety and steerability highlight the need to develop in-tire sensors that can keep pace with the advancement of automotive electronic control systems. Traditional batteries require periodic replacement, thereby causing maintenance problems related to mounting on tires. In-tire energy harvesting systems pose tremendous potential as a substitute for batteries with low lifespans. Many fields have begun taking advantage of the energy harvesting technology, and developers continue to gradually improve system efficiency. Numerous studies have applied energy harvesting devices to tires as a means of supplying power to sensors or charging batteries for essential systems such as TPMSs. Methods based on piezoelectric, electromagnetic, and electrostatic mechanisms are central to research on converting different types of energy to electricity. Possible energy sources include tire deformation, strain, rotation, and vibration. Table 1 summarizes the comparison of different mechanisms.
Piezoelectric materials are more applicable under small deformation with large stress. Various materials (PZT, PVDF, etc.) have different performance and need to be carefully chosen to make most use of their advantages. Generally, structure of stack, cantilever, spring, and strengthened mass end are applied with no requirement of complex geometries or add-ons. The output energy has the characteristics of high voltage and low current. However, the piezoelectric materials normally have limited number of deformation. The life of the piezoelectric energy harvester is greatly limited since the tire rotates at a high speed when driving, resulting in strict requirement in design of material and structure.
Electromagnetic mechanism has high energy density and high stability, requiring no independent voltage source. They are applicable to various vibration modes like rotation and reciprocating motion, especially vibration with large relative speed and amplitude. Gear mechanism can be applied to enlarge amplitude, but requires difficult integration with MEMS. Moreover, low efficiency and impact lead to reduced life expectancy.
Electrostatic energy harvesters are not as popular as methods mentioned above, requiring an initial polarization voltage or charge. Normally the output impedance is larger, which is not suitable for energy supply. Roddy et al.  demonstrated that electromagnetic and piezoelectric mechanisms have higher energy storage densities than electrostatic mechanisms.
The review is intended to give an overview of the history and current research of in-tire energy harvesting. Researchers can conduct their own innovative design of in-tire energy harvesting device on basis of this review. In the future, the most promising methods should be figured out for experiments which can afford high cost as well as low-cost volume production. Tire experiment sometimes requires to install expensive sensors and the experiment may last for a long time without interruption. In that case, in-tire energy harvesting device that actively generates electricity is necessary and to some extent regardless of the cost. Otherwise when considering the application on a mass-produced car, cost and reliability are the primary concerns. Under different conditions, the optimal designs of in-tire energy harvesting device can be quite different from each other. Last but not least, future work should be carried out to overcome the challenges, like efficiency, cost, size, weight, dynamic balance, reliability, maintenance, manufacturing, and so on.
Lab of ASE Tsinghua University
Liangyao Yu, Ph.D.
The authors would like to thank for support from the National Science Foundation of China (Grant No. 51775293, No. U1664263, and No. 11672148).
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Liangyao Yu, Xiaoxue Liu, Sheng Zheng, and Jinghu Chang, Tsinghua University, China
Received: 10 May 2018
Revised: 09 Jul 2018
Accepted: 09 Jul 2018
e-Available: 28 Dec 2018
TABLE 1 Comparison of different energy harvesting mechanisms. Mechanisms Piezoelectric Energy sources Inertia [16, 20], kinetic [16, 20, 21], rotation , vibration [16, 20], deformation [17 18, 19, 22] Basic elements Beam and proof mass [16, 21], end stop , composite materials [12, 17, 18, 12, 22], oscillators [16, 20, 21], springs [20, 21] Advantages and No need for external voltage, compatible characteristics with MEMS, relatively small size, high voltage output Disadvantages Limited lifespan and brittleness due to material, efficiency influenced by temperature Mechanisms Electromagnetic Energy sources Relative motion caused by vibration [25, 27, 29], rotation [25,26, 27], deformation  Basic elements Permanent magnet and coils [25, 26, 21], electric circuit [25, 26, 27, 30] Advantages and No need for external voltage, characteristics flexible structure design, coupled subsystems Disadvantages Difficult to integrate with MEMS, relatively large size Mechanisms Electrostatic Energy sources Vibration [32, 33], deformation , strain  Basic elements Capacitors [32, 33], electrodes [32, 33] Advantages and Compatible with MEMS, relatively characteristics compact Disadvantages Needs external voltage, relatively low output power
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|Author:||Yu, Liangyao; Liu, Xiaoxue; Zheng, Sheng; Chang, Jinghu; University, Tsinghua|
|Publication:||SAE International Journal of Passenger Cars - Electronic and Electrical Systems|
|Date:||Aug 1, 2018|
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