Field of the invention

[0001] The present invention relates to a method and an apparatus for determining Angle of Attack (AoA) and wind speed. AoA may be the relative angle between the course of the aircraft body and the direction of the wind and the wind may be the airflow relative to the aircraft, i.e., the wind speed may be a relative airflow speed relative to the aircraft. More particularly, the present invention relates to a wind energy harvesting, piezoelectric based, and wireless sensor for reliable and simultaneous measurement of AoA and wind speed, which can increase safety and decrease system redundancy in an aircraft. The present invention further relates to a method and a system thereof.

Background

[0002] The last decade has witnessed countless aerospace innovations involving sensors and systems, which indeed solved many challenges and overcame many obstacles. Yet, the airline industry has witnessed several dreadful accidents resulting in tremendous loss of lives. Apart from pilot errors, many such accidents are attributed to sensor malfunctions, inaccuracies, and lack of redundancy (multiple sensors) in the system. This calls for innovation in sensing systems that can reduce fatalities even further and make flying the safest mode of transportation. Further, preferably, the sensing systems should not require additional wiring.

[0003] Investigations reveal that issues in the measurement of wind speed and angle of attack (AoA) may be the causes of some accidents. For example, in some accidents one of the primary causes was autopilot failure which resulted from erroneous AoA and wind speed measurements.

[0004] Furthermore, traditional AoA and wind speed sensors require wires and/ batteries for power supplies, which increase the cost and complexity of AoA and wind speed sensing/measuring. Wired communications between the sensors and the servers may further worsen these issues.

[0005] Therefore, there is a need for an easy to use and accurate AoA and/or wind speed sensing method, apparatus and system thereof.

Summary of the invention

[0006] The present invention relates to a method and apparatus for sensing angle of attack and wind speed. More particularly, the present invention relates to a wind energy harvesting, piezoelectric based, and wireless sensor for reliable and simultaneous measurement of AoA) and wind speed, which can increase safety and decrease system redundancy in an aircraft. The present invention further relates to a method and a system thereof.

[0007] According to an embodiment of the present invention, an apparatus comprises, at least one piezoelectric film configured to harvest wind energy, at least one processor, which is configured to determine at least one of an angle of attack and an airflow speed relative to the apparatus based on energy information of the harvested wind energy by the at least one piezoelectric film. [0008] According to an embodiment of the present invention, a method for determining at least one of an angle of attack and an airflow speed relative to an aircraft, comprises, collecting experimental data comprising data of at least one of the angle of attack and the airflow speed , and corresponding data of energy information of harvested wind energy by at least one piezoelectric film; harvesting wind energy with the at least one piezoelectric film; and determining at least one of the angle of attack and the airflow speed based on energy information of the harvested wind energy by the at least one piezoelectric film according to the experimental data.

Brief description of the drawings

[0009] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[0010] Fig. 1 shows a conceptual view of the AoA on an airborne body according to the present invention.

[0011] Fig. 2 shows an apparatus according to the present invention.

[0012] Fig. 3 shows a schematic of an inverted flag according to the present invention.

[0013] Fig. 4 shows a schematic of an inverted flag according to the present invention.

[0014] Fig. 5 shows dynamical modes of an inverted flag according to the present invention.

[0015] Fig. 6 shows an apparatus according to the present invention.

[0016] Fig. 7 shows a method to determine AoA and wind speed according to the present invention.

[0017] Fig. 8 shows examples of surface polynomials according to the present invention.

Description of embodiments

[0018] Embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. However, the embodiments of the present disclosure are not limited to the specific embodiments and should be construed as including all modifications, changes, equivalent devices and methods, and/or alternative embodiments of the present disclosure.

[0019] The terms “have,” “may have,” “include,” and “may include” as used herein indicate the presence of corresponding features (for example, elements such as numerical values, functions, operations, or parts), and do not preclude the presence of additional features.

[0020] The terms “A or B,” “at least one of A or/and B,” or “one or more of A or/and B” as used herein include all possible combinations of items enumerated with them. For example, “A or B,”

“at least one of A and B,” or “at least one of A or B” means (1) including at least one A, (2) including at least one B, or (3) including both at least one A and at least one B.

[0021] The terms such as “first” and “second” as used herein may modify various elements regardless of an order and/or importance of the corresponding elements, and do not limit the corresponding elements. These terms may be used for the purpose of distinguishing one element from another element. For example, a first printing form and a second printing form may indicate different printing forms regardless of the order or importance. For example, a first element may be referred to as a second element without departing from the scope the present invention, and similarly, a second element may be referred to as a first element. [0022] It will be understood that, when an element (for example, a first element) is “(operatively or communicatively) coupled with/to” or “connected to” another element (for example, a second element), the element may be directly coupled with/to another element, and there may be an intervening element (for example, a third element) between the element and another element. To the contrary, it will be understood that, when an element (for example, a first element) is “directly coupled with/to” or “directly connected to” another element (for example, a second element), there is no intervening element (for example, a third element) between the element and another element.

[0023] The expression “configured to (or set to)” as used herein may be used interchangeably with “suitable for,” “having the capacity to,” “designed to,” “ adapted to,” “made to,” or “capable of according to a context. The term “configured to (set to)” does not necessarily mean “specifically designed to” in a hardware level. Instead, the expression “apparatus configured to...” may mean that the apparatus is “capable of...” along with other devices or parts in a certain context.

[0024] The terms used in describing the various embodiments of the present disclosure are for the purpose of describing particular embodiments and are not intended to limit the present disclosure. As used herein, the singular forms are intended to include the plural forms as well, unless the context clearly indicates otherwise. All of the terms used herein including technical or scientific terms have the same meanings as those generally understood by an ordinary skilled person in the related art unless they are defined otherwise. The terms defined in a generally used dictionary should be interpreted as having the same or similar meanings as the contextual meanings of the relevant technology and should not be interpreted as having ideal or exaggerated meanings unless they are clearly defined herein. According to circumstances, even the terms defined in this disclosure should not be interpreted as excluding the embodiments of the present disclosure.

[0025] Wind in this document refers to an airflow , which may be relative to the aircraft. E.g., the wind speed is a speed of the airflow relative to the aircraft.

[0026] AoA is a fundamental aerodynamic parameter which may be defined as the angle between the reference line of an airborne body (e.g., the course line of the aircraft body, or the chord line of an airfoil, where the airfoil is the cross-sectional shape of a wing) and the relative wind velocity vector. The wind velocity vector may be a wind velocity relative to the aircraft, e.g., a vector indicate the relative airflow to the aircraft. Similar to wind speed, which may be a wind speed relative to the aircraft. Fig. 1 depicts a conceptual view of the AoA on an airborne body. In fig. 1 , AoA 103 is the angle a between the wind (or air flow, used interchangeably in this application) velocity vector 102 (e.g., wind direction) and the chord line 104 of the airborne body 100.

[0027] Accurate measurement of AoA along with the wind speed is crucial for maintaining appropriate lift and preventing stall regimes during take-off, landing, and cruise. The sensors for these have played instrumental roles in realizing the human desire to fly, starting from the advent of the first human aircraft in 1903. Although these sensors have been integral elements of aircraft and unmanned aerial vehicles (UAVs, or uncrewed aerial vehicles), for a century, even today they are prone to inaccuracies and faulty measurements. They are implemented in most of today’s aircraft and UAVs, especially for an autonomous flight or autopilot system.

[0028] An apparatus according to the present invention is shown in fig. 2. Fig. 2 is an example based on the present invention concept and does not limit the present invention to the exact configurations in fig. 2. The apparatus can accurately sense AoA and wind speed as well as harvest wind energy.

[0029] Fig. 2 shows an apparatus 200 to determine at least one of AoA and wind speed. The apparatus 200 may comprise a piezoelectric film 201 or more including 201 , 202, 203, 204 and 205, each of which may be mounted on a frame 207 via a clamp 206. The frame 207 and clamp 206 may be omitted. For example, the at least one piezoelectric film 201 to 205 may be directly mounted on a wing of the aircraft or on the tip of the head of the aircraft. And/or the film 201 to 205 may be fixed to the frame 207 and/or the aircraft via tapes, screws and/or any other alternatives.

[0030] Piezoelectric films may utilize flutter when subjected to wind. Flutter is one of the many aeroelastic phenomena that occur in dynamic fluid-structure interactions. Flutter has traditionally been deemed as a destructive phenomenon in the domain of aeroelasticity and is considered as a highly nonlinear problem due to the large strains and geometric deformations that it causes in addition to its transient behaviour.

[0031] Flutter may originate from the aeroelastic instability of a compliant structure, for instance, a flexible sheet (e.g., the at least one piezoelectric film 201) immersed in a flowing fluid. An elastic plate subjected to axial fluid flow begins to flutter above the critical velocity of the fluid, where the destabilizing forces overpower the stabilizing effect of the rigidity of the structure. The present invention uses the flutter phenomenon to harvest energy from the flow field (e.g., wind). For example, in the present invention, fluid kinetic energy using flutter may be translated into strain energy of the structure. This strain energy can then be harvested into usable electrical energy via the piezoelectric effect of the films 201 to 205.

[0032] The apparatus 200 according to the present invention may comprise at least one piezoelectric film (201 to 205, e.g., thin elastic piezoelectric film), which may be placed in the wind flow. The at least one film 201 to 205 may have two potential configurations: a regular flag and an inverted flag, either or both of which may be used for the purpose of the present invention.

[0033] An inverted flag may be a mirror image of a regular flag, where the leading edge (oriented against the wind direction) is free to move, and the trailing edge is fixed. For example, the loose end of the film is directed against the wind direction, e.g., when harvesting wind energy and/or measuring AoA/wind speed. Figs. 3 and 4 show schematic view of the inverted flag geometry for an elastic sheet (e.g., the films 201 to 205). The sheet in a resting state (e.g., no wind pressure) is shown using a solid straight line 301 , and the curved dashed lines 302 indicate the curvature of the film (e.g., under wind pressure). A denotes the maximum displacement between the tips (loose end) in the y-axis, while L and W denote the length and width of the sheet, respectively.

The wind 303 has a speed of U.

[0034] When the films 201 to 205 are used for energy harvesting and/or AoA/wind speed determination (e.g., mounted on the aircraft), they may be mounted according to the x, y and z axis’s shown in fig. 3. For example, the x, y and z axes may be according to the aircraft in a parking state on land where x, y and z axes are perpendicular to each other. The x-axis may be parallel to the chord line 104 of the aircraft 100 and the x-axis is oriented in the same direction as from the head to the tail of the aircraft. The y-axis maybe perpendicular to the chord line 104 of the aircraft 100 and parallel to the horizontal plane. The z-axis may be perpendicular to the horizontal plane.

[0035] The regular flag is with the leading edge being fixed, and the trailing edge being free to move. For example, the fixed end of the film is against the wind, e.g., when harvesting wind energy and/or measuring AoA/wind speed.

[0036] The inverted flag is preferred in the present invention. The free front and fixed rear end may be, in general, more susceptible to instability when faced with external axial loading than a regular flag. To explain, a simple analogy of leaves fluttering in a breeze irrespective of their orientation to the wind, which justifies using this configuration as an alternative to the regular flag for flow-induced flapping.

[0037] There may be some important parameters which are used to characterize the dynamics of interaction between fluid flow and an inverted elastic film: mass ratio p, aspect ratio AR, and bending stiffness Kb of the elastic film. They may be calculated as shown in below Equations (1), (2) and (3),

[0038] Where p _s is the elastic sheet density, / is the fluid density, h is the thickness of the film, L is the length of the film (e.g., the longest edge of the film), l/l/is the width of the film (e.g., the second longest edge of the film, where the shortest edge may the thickness of the film), U is the free-stream velocity, E is the Young’s modulus, v is the Poisson’s ratio m represents the relative magnitude of structure to fluid inertial forces, while Kb characterizes the relative magnitude of bending forces to the fluid inertial forces and can be used to classify three dynamical regime or modes for such a configuration: a. Straight mode (e.g., Kb ³ 0.3): In this mode, the sheet remains straight. This regime exhibits behaviour analogous to fixed-point stability where after an initial disturbance, the sheet experiences positive damping and returns to a stretched-straight position. b. Flapping mode (e.g., 0.1 < Kb < 0.3): In this mode, the sheet will flap from side to side and the deflection will be periodic. The magnitude of oscillations is the maximum in this mode. c. Deflected mode (e.g., Kb £ 0.1): In this mode, the sheet bends in one direction and maintains a highly curved or deformed shape. [0039] The three modes are shown in Fig. 5. With different materials, the values of given in above examples may be different.

[0040] The advantages for using an inverted flag are the following: a. Higher harvested energy: The inverted flag may have a greater peak-to-peak amplitude of deflection than the regular flag. This means a greater magnitude of strain, which further implies a better potential for energy harvesting and sensing. The average strain energy with an inverted flag is approximately 100 times higher than a regular flag under the same flow characteristics and elastic properties. b. Reliability: The regular flag may exhibit chaotic flapping beyond a critical bending stiffness while the inverted flag only experiences limited cycle oscillations within a specific range of Kb. Thus, an inverted flag may exhibit higher stability and behaves more reliably than a regular one. c. Range: In order to sense AoA and wind speed, flutter must be induced over the typical operating range of wind conditions for any aircraft. An inverted flag can cause this. d. Robust: The accuracy of a regular flag is highly dependent on the wing/nose geometry, whereas the inverted flag may be robust to it.

[0041] During flutter, piezoelectric films translate the induced strain into electrical energy, which has been explained in JM McCarthy, S Watkins, A Deivasigamani, and SJ John. 2016, Fluttering energy harvesters in the wind: A review. Journal of Sound and Vibration 361 (2016), 355-377. This article is incorporated by reference into this application.

[0042] Another aspect of the piezoelectric material for flutter is the effective stiffness of the sheet, which may be controlled to excite flutter in the required wind speed ranges. Polyvinylidene fluoride (PVDF) or polycarbonate film are examples for flutter harvesters (e.g., for films 201 to 205) which exhibit strong piezoelectric properties. They may be used in the present invention due to their low cost and easy availability. PVDF undergoes a poling process whereby the polymer is strained and placed within a strong electric or poling field. Moreover, a phenomenal feature of PVDF films is that they can be engineered to varying stiffness and toughness requirements. The PVDF sheet flapping under aeroelastic flutter can electrically be represented by an equivalent current source paired with a capacitance. If the terminal load is a pure resistor, then, the value of the optimal load resistance for maximum power transfer can be calculated. The circuit’s impedance can be calculated using Equation (4),

...(4) where j denotes the imaginary part, RL is the load resistance, and X _c is the source impedance of the PVDF sheet and can be calculated using Equation (5),

X _c — 2 p C ... (5) ' where C is the capacitance of the PVDF sheet, and / is the sheet’s flapping frequency.

[0043] The power harvested at the terminal load may be calculated by Equation (6), where / is the effective value of the generated source current. The maximum power ( P _max ) may be reached when RL = X _c .

[0044] Both of the AoA and wind speed affect the behaviour of an inverted flag under wind/axial flow. Experiments using stroboscopic imaging proved that there was a gradual growth of flapping amplitude when placed at an angle to a free flow stream. Additionally, the nature of oscillations depends heavily on AoA. Therefore, there is sufficient scope to leverage the energy information generated by the piezoelectric film to infer AoA and wind speed.

[0045] The energy information may comprise at least one voltage information, at least one current information and/or at least one power information. The at least one voltage information may comprise at least one of a peak voltage value, a zero-crossing voltage value, an average voltage value, a voltage phase value, and a frequency of fast Fourier transformation value of voltage signals. The at least one current information may comprise at least one of a peak current value and an average current value. The at least one power information may comprise at least one of a peak power value and an average power value. In the following examples, the voltage information is used as an example for the determination of the AoA and wind speed, however, the current and/or power information may be used based on the same principle.

[0046] For example, when using a polycarbonate film in a wind tunnel, the characteristic flow speeds, flutter amplitude and frequencies vary with AoA. Large amplitude oscillations could exist when AoA is less or equal to a certain angle, e.g., 26.8° of AoA in some experiment settings, which gives a large enough window for AoA sensing.

[0047] Thus, the present invention uses harvested energy information from at least one piezoelectric film, to determine AoA and wind speed. In the below example, voltage information is used as an example.

[0048] For example, information of the voltage generated by a piezoelectric film for sensing wind speed and AoA may include: (a) peak voltage ( V _P ) and/or (b) zero crossing frequency (f _z ) of the generated voltage. V _P may match with the peak amplitude of flapping, while f _z may relate to the flapping frequency, e.g., a frequency of the flag crossing the resting line (i.e., the position of the film when there is no wind, i.e., shown in the solid line 301 in fig. 3). These two features may represent the flutter characteristic in the electrical domain, e.g., different AoA a and wind speed U.

[0049] For example, V _P may be sampled once over a predefined period, e.g., a total of 100 samples and each sample is the highest voltage in a period of 500 ms; f _z may be sampled over time intervals with the same or another predefined period, e.g., the crossing frequency within in each of 500 ms intervals.

[0050] Each of the at least one films 201 may have optimally one of the flag orientation alternatives (i.e., flag orientations) when used to harvest wind energy and/or determining the AoA and wind speed. The flag orientations may include a parallel configuration and a perpendicular configuration. In the parallel configuration, the flutter is observed in the x-z plane, while in the perpendicular configuration (e.g., as shown in figs. 3 and 4), it is in the x-y plane. For example, the x, y and z axes may be according to the aircraft in a parking state on the land where x, y and z axis’s are perpendicular to each other. The x-axis may be parallel to the chord line of the aircraft and the x-axis has a direction from the head to the tail of the aircraft. The y-axis maybe perpendicular to the chord line of the aircraft and parallel to the horizontal plane. The z-axis may be perpendicular to the horizontal plane.

[0051] In the present invention, the perpendicular configuration is preferred since the perpendicular configuration may show a more uniform decaying trend of Vp with a.

[0052] Even if oscillations are not large when a film is in the deflected mode (as shown in fig. 5), low amplitude flutter oscillations may still create high frequency components in the flags. The variation in frequency for the film in perpendicular configuration shows its potential for carrying information regarding a and U in a certain AoA range, e.g., a e [0°, 10°]. Based on experimental results, it is evident that the film in the perpendicular configuration show that (a, U) can be obtained from the values of (V _P , f _z ).

[0053] A film may be most effective in determining the AoA within a certain range (e.g., a e [0°, 10°], [0°, 15°], [0°, 20°] etc.) and less effective when the AoA is outside the certain range. The range may be determined based on the physical property of the film. Therefore, a plurality of films may be comprised in the apparatus of the present invention for determining the AoA in a larger range, e.g., with five films 201 to 205 shown in fig. 2, each of which has a 10° AoA sensing range and with a total of 40° sensing range (e.g., only considering the range in between the films on both edges).

[0054] For example, the apparatus 200 may comprise a plurality of piezoelectric films, e.g., an array of multiple piezoelectric films. The films may be single poled PVDF piezoelectric films (e.g., L = 30 mm, W = 15 mm and m = 10). All or some of the plurality of the piezoelectric films may be inverted flags, and/or in the perpendicular configuration (e.g., direction of flutter in x - y plane). Each of the films may function as both a sensing and wind harvesting element. Each of the films is mounted at an angle from the other (e.g., a certain degree difference on the x axis as shown in fig. 2). Each of the films may be held in place using specially designed rigid clamps that orient the flags at the required mount angle. Each film, based on the incoming wind direction (e.g., the wind direction relative to the aircraft), may exhibit prominent flapping in a certain range (e.g., [-10°,

10°], [-20°, -20°], or other range depending on the physical property of the film) from the central axis of the film (i . e . , the rest state as shown in 301 of fig. 3). Together, all films may form a wide sensing range for AoA and/or wind speed.

[0055] The apparatus 200 may be mounted at the tip of the airfoil or on a wing of the aircraft. Since the apparatus 200 faces the incoming wind without a bluff body, wake effects may be ignored if the apparatus 200 is mounted sufficiently far away from the fuselage. The mount may be manufactured using the requisite material based on wind speed and end application considerations, e.g., carbon-fiber. The volumetric size of the mount and apparatus 200 may be 27.5 cm3 and the weight of the films and sensor PCB without the mount may be about 60 g.

[0056] The examples of a method to determine the AoA and wind speed will be presented in fig. 7. The modules of the apparatus are presented first in fig. 6.

[0057] Fig. 6 shows modules that are included in the apparatus 600, where the apparatus 200 in fig. 2 is an example of the apparatus 600. The apparatus 600 according to the present invention may comprise a film unit 601 (e.g., comprising at least one piezoelectric film which may be the same as films 201 to 205) and at least one processor 602. The apparatus 600 may further comprise an energy conditioning unit 603 and/or a communication unit 604, which may be omitted as well. The apparatus 600 may further comprise a battery, not shown, to store the generated energy and/or to power some or all the electric components of the apparatus. The battery may be omitted.

[0058] The generated energy by the film unit 601 may be fed to one or more of the electric components of the apparatus 600 and provide electricity power.

[0059] The generated energy by the film unit 601 may be fed to the energy conditioning unit 604. The energy conditioning unit 604 may comprise at least one preamplifier 6011 . The voltage output of each film, V„ (t), where n = 1 , 2, ... , N, and is a number to indicate each film (e.g., film 1 , film 2, ... film N in the film unit 601), may be fed to the at least one preamplifier (e.g., each film may correspond to a different preamplifier), which may be a voltage amplifier functioning as a buffer between the piezoelectric films and the other electric components / circuit of the apparatus 600. The preamplifier 3011 may provide the required load resistance ROPJ as seen by the films. For example, a ROPJ for the films may be between 1 MW and 5MW for different flapping frequencies and a median value of 3.3MW may be used for the films.

[0060] The energy conditioning unit 603 may further comprise a conditioning circuit 6031 . The preamplifiers) voltage output may be fed to the conditioning circuit 6031 , before being sampled by an analog-to-digital converter (ADC) 6032. The voltage signal V„ (t) may be conditioned by the conditioning circuit to make it possible for the ADC 6032 to sample it. The conditioning circuit 6031 may be a resistive voltage divider circuit for scaling, which may map the generated voltage to a certain voltage range, e.g., between 0 to 2 V. High impedance resistors may be used to minimize the current. The negative swing of the voltage waveform may be inverted using an ultra low-power OpAmp, e.g., MAX44264, which may provide the inverted value to the conditioning circuit 6031 and is then fed to the at least one processor 602.

[0061] The ADC 6032 may be comprised in the energy conditioning unit 603, in the at least one processor 602, or as a separate component of the apparatus 600. The sampled voltage signal or the preamplifiers) voltage output may be fed to a power harvesting and rectifying circuit 605 for powering at least one electronic component of the apparatus 600. For example, LTC3588 may be used, which is a full-wave bridge rectifier integrated with a high-efficiency buck converter for the purpose of rectifying and stepping down the generated piezo voltage due to its extremely low- power consumption and small footprint. The generated voltage may be used to power the apparatus 600, e.g., powerthe at least one processor 602. For example, the output of the buck converter may be directly used to power the microcontroller.

[0062] The at least one processor 602, which may be configured to determine at least one of an angle of attack and a wind speed based on information of energy harvested by the at least one piezoelectric film comprised in the film unit 601 .

[0063] The at least one processor 602 may be at least one microcontroller. For example, the microcontroller may be an ON Semiconductor’s RSL10 System-In-Package (RSL10 SIP), which contains an integrated on-board antenna, BLE radio SoC and all necessary passive components in a single package. It is ultra-low power and has a highly flexible multi-protocol 2.4GHz radio. Using this chip, the entire sensor design requires 360 mϋ to communicate at 6 dBm maximum transmit power. The ADC in RSL10 can record voltage between 0 to 2 V, and can sequentially sample 8 channels at a maximum sampling rate of 6.25 kHz with a resolution of 14 bits. The w(t) signal may be sampled after it is passed through the signal conditioning block, making it compatible with the SIP’s ADC.

[0064] The signal _n (t) may be recorded by the film unit 601 for each of the piezoelectric films at a predefined sampling rate for a predefined interval, e.g., 1 kHz sampling rate in every 500 ms. The sampled signal may be then fed to the at least one processor 602, e.g., the at least one processor 602 may be configured to implement a real-time algorithm for determination of AoA a and wind speed U.

[0065] Since the apparatus 600 may be fully self-powered, there may be no need for external power supply circuit. The apparatus 600 powers on with the incoming wind, which induces flutter of the piezoelectric films resulting in powering of the sensing circuit.

[0066] The apparatus 600 may comprise at least one communication unit 604, which may support different communication protocols, e.g., wired or wireless. The at least one communication unit 604 may be configured to transmit the determined results of the AoA and wind speed to an external device, e.g., a server, and/or fetch data from the external device, e.g., settings, firmware, etc. Wireless communication unit is preferred since less wire is used, which can be beneficial in the flying condition of the aircraft.

[0067] Fig. 7 shows a method for measuring/determining AoA and/or wind speed according to the present invention is presented below. Fig. 7 may be implemented by the apparatus 200 and/or 600 according to the present invention. In the below explanation of Fig. 7, generated voltage information is used as an example. However, other information of the harvested energy, e.g., the current information and/or the power information may be used to determine AoA and/or wind speed, which is based on the same principle.

[0068] The method for determining at least one of an angle of attack and/or a wind speed, may comprise step 701 of collecting experimental data comprising data of at least one of the angle of attack and the wind speed based and corresponding data of the information of the energy harvested by at least one piezoelectric film; step 702 of harvesting wind energy with the at least one piezoelectric film; and step 703 of determining at least one of the angle of attack and the wind speed according to the experimental data.

[0069] In the below example, the method can measure/determine AoA and wind speed by using generated voltage signal(s) from the at least one piezoelectric film from the incoming airflow.

[0070] In step 701 experimental data is collected, which may comprise data of at least one of the angle of attack and the wind speed based and corresponding data of the information of the energy harvested by at least one piezoelectric film. The experimental data may be pre-collected and stored in a memory for the purpose of the present invention. [0071] Step 701 may comprise a step to generate a look up table, or generate a regression model based on the experimental data of at least one of the angle of attack and the wind speed based and corresponding data of the information of the energy harvested by at least one piezoelectric film.

[0072] For example, a look up table may comprise at least one input entry and at least one output entry. The at least one input entry may include energy information, e.g., peak voltage and zero-crossing frequency. The at least one output entry may include the corresponding AoA and/or wind speed according to the energy information, which is collected during experiments.

[0073] With the lookup table, after harvesting energy in step 702 which generates new energy information, the AoA and/or wind speed may be determined according to the lookup table. If the exact energy information is not recorded in the table, a closest entry may be chosen. For example, if in the table, only a peak voltage of 2V and 2.5 V is recorded, a generated 2.1V peak voltage may chose the results based on the entry of 2V in the table, or calculating a proportional results based on two closest entries.

[0074] A regression model may be generated by using experimental data, which can be used to determine the AoA and/or wind speed. For example, a step of nonlinear surface generation may be performed based on the experimental data. In this example, the peak voltage V _pn and a zerocrossing frequency / _zn are used (n is the number of the film from 0 ro N-1 or 1 to N), however, other energy information may be used based on the same principle, e.g., an average voltage value, a voltage phase value, and a frequency of fast Fourier transformation value of voltage signals, and/or at least one current information comprises at least one of a peak current value and an average current value, wherein the at least one power information comprise at least one of a peak power value and an average power value.

[0075] In this example, from the generated voltage signal 7 _n (t) of each piezoelectric film, a peak voltage V _pn and a zero-crossing frequency / _zn may be measured (where n represents the number given to each film). For example, the peak voltage V _pn and the zero-crossing frequency / _zn may be measured by being averaged over certain intervals, e.g., in every 100ms or 500 ms. For example, time-series V _pn ( t _k ) and / _zn (£¾) may be sampled at 0.5 Hz (with interval of 500ms in interal tk).

[0076] An empirical nonlinear relation may be derived as V _pn = P a,U), / _zn = P ² {a,U) based on experimental data, where

Where i=1 is for V _pn and i=2 is for / _zn , J and K represent the total numbers (wind speed candiaate values U ^j from j=0 to J+1, and AoA candidate values a ^k from k=0 to K+1) of experimented/tested values of the wind speed U and AoA a. In this example, it is assumed J=K=2. However, other number may be used, e.g., with higher J and K for higher resolution of the experiments. P'j. _k is the element of the coefficiency.

[0077] The coefficients ? ^■ _/,¾ , may be be determined using a robust linear least squares algorithm or any other suitable method. The coefficients may be determined with certain confidence, and an example is given in Table 1 (with 95% confidence), and the corresponding surface polynomials have been shown in Fig. 8.

Table 1 Coefficients of surface polynomials

[0078] For example, from the coefficients Pf _k , a rank may be determined with Equation (8). 10, 20 < U < 20. (8)

[0079] Because the rank is 2, based on the inverse function theorem, the relation ( V _pn , / _zn ) =(P ¹ (U, a), P ² ( U , a)) is invertible, allowing to determine a, U from V _pn , f _{z n} . Note that since the trend for 7 _pn and f _{z n} may be independent of the sign of AoA a, |a| may be obtained based on this regression model. However, the wind speed U can be directly obtained from the above inverted function.

[0080] In the present invention, a film 210 in the apparatus 200/600 may sense effectively in a certain range of the AoA and less effective when the AoA is outside the range. A plurality of films may be mounted in the device according to increase the total effective AoA sensing range, as shown in fig. 2. For example, if the AoA sensing range of a film is 10 degree, then multiple films (e.g., five in fig. 2) may be mounted with 10 degree from each other, in order to increase the sensing range.

[0081] In this example regression model, an additional step may be used to determine AoA range based on the film that received the strongest wind and/or generate the highest energy value (e.g., highest voltage, current, and/or power).

[0082] For example, with the structure shown in fig. 2, , if the AoA sensing range of a film is 10 degrees and five films are mounted with 10 degrees difference from each other, then the AoA e [-10°, 30°] may be effectively measured. For example, at least one of the five piezoelectric films is within an angular separation of 10° from the wind velocity vector U, which can be used to determine the AoA. Since the nonlinear surface regression may be applicable when a is within a certain effective range, it may be optional to determine the piezoelectric film which is within this range (i.e. , this film receives the strongest wind and/or generates the highest energy value), and which makes a minimal angle with U. Further, since the surface regression may give the absolute value of the angular separation, the sign of the angular separation may be determined. For example according to Algorithm 1 , where au is the angle between M-th film and the wind direction. In line 1 , the film (i.e., M-th film) with the strongest peak voltage is selected and then the sign of the angle au is determined. Algorithm 1 : AoA range dection algorithm

[0083] In Algorithm 1 if the films are in a perpendicular configuration, a film which makes a minimal angle with the wind velocity vector may also generate the largest voltage Vpn. If one of the piezo films (e.g., Mth) makes a minimal absolute angle au, then all other piezoelectric films make an absolute angle which is in the set {10i ± au, 1 £ i < N-1 as the number of the films}, and it can be observed that Vp ( au,U ) > Vp (10i ± au,U ), 1 < i < 4. The above numbers in the example are based on the assumption mentioned above. Thus, in other scenarios the numbers will adapt accordingly, e.g., according to the total number of films, the AoA effective sensing range, etc.

[0084] In the example regression mode, after the smallest angle <¾ is determined, the real AoA can be determined, e.g., by adding au and the mounting angle of the M-th film.

[0085] So far, in the regression model, the AoA and the wind speed are determined once, however, the above steps may be iterated to determine AoA in real time (e.g., once in every certain time period).

[0086] The present invention may have the below advantages:

[0087] Reliability. The small form factor of the apparatus 200 adds negligible weight or aerodynamic effects; thus, by deploying several redundant sensors/apparatuses, reliability can be increased. Currently used sensors rely on pressure tubes, called Pitot tubes, which need to be heated to prevent ice formation -the main reason for the Air France 447 crash some years ago.

On the contrary, due to its inherent flapping nature and the incoming wind, the present invention is less prone to ice formation. The present invention, using an unconventional physical phenomenon, ensures variability in sensing mechanisms that safeguard the system during times of catastrophe. Additionally, PVDF piezoelectric properties may be temperature dependent but it is linear, and the change in sensor readings can be corrected simply by adding a temperature sensor to the electronics at negligible cost. Thus the reliability of the present invention can be maintained over a wide range of operational conditions.

[0088] Enhancements to sensing range. The present invention is highly customizable to suit the sensing ranges required by the end applications. By translating the flutter characteristics of the piezoelectric flag into quantifiable features, the present invention estimates the wind speed and AoA while harvesting sufficient energy. The sensing range for AoA can easily be customized by choosing the appropriate number of piezoelectric films, as described in this application. The range of wind speed, over which the sensor can function, may require altering the flutter range of the flags. This is possible by manipulating the dimensions as well as the physical properties of the films used, namely Young’s modulus and Poisson’s ratio. Based on different film configurations, the present invention can be used to measure a wide range of wind speeds.

[0089] Customization for different applications. As mentioned above, the flag dimensions and material properties may influence the performance of the present invention under different flying conditions. However, it can easily be mounted on smaller aircraft which fly at lower altitudes. These aircraft are typically used for surveying or by hobbyists where an inexpensive, small-sized AoA sensor can add safety and aid the pilots in efficient flying.

[0090] Since there is little provision for wiring in these smaller aircrafts, a wireless, self-powered sensing is very lucrative based on the present invention.

[0091] The present invention minimizes power, cost, and size, which are highly valued features in these smaller aircrafts.

[0092] Further, the present invention is not limited to commercial aircraft. It can be easily mounted and used in windmills, where accurate measurement of AoA could lead to better control and thus higher energy harvesting.

[0093] The present invention relates to: an apparatus comprises, at least one piezoelectric film configured to harvest wind energy, at least one processor, which is configured to determine at least one of an angle of attack and an airflow speed relative to the apparatus based on energy information of the harvested wind energy by the at least one piezoelectric film.

[0094] The at least one piezoelectric film may be at least one polyvinylidene fluoride film.

[0095] Each of the at least one piezoelectric film may comprise a loose end and a fixed end.

[0096] Each of the at least one piezoelectric film may be mounted to be an inverted flag or a regular flag when harvesting wind energy, preferably, to be an inverted flag, wherein the inverted flag may be defined as having its loose end directed against airflow direction when harvesting wind energy, and the regular flag may be defined as having its fixed end against the airflow direction when harvesting wind energy.

[0097] Each of the at least one piezoelectric film may be in a perpendicular configuration wherein the at least one piezoelectric film may be configured to be perpendicular to a horizontal plane, wherein the horizontal plane may be defined in relation to the earth surface. [0098] The apparatus may further comprise a plurality of piezoelectric films, and/or wherein the plurality of piezoelectric films may be mounted evenly distributed on a frame with an angle to each other, wherein the frame may be perpendicular to the horizontal plane.

[0099] There may be more piezoelectric films above a zero angle of attack line than below the zero angle of attack line.

[00100] The energy information may comprise at least one voltage information, and/or at least one current information and/or at least one power information.

[00101] The at least one voltage information may comprise at least one of a peak voltage value, a zero-crossing voltage value, an average voltage value, a voltage phase value, and a frequency of fast Fourier transformation value of voltage signals.

[00102] The at least one current information may comprise at least one of a peak current value and an average current value.

[00103] The at least one power information may comprise at least one of a peak power value and an average power value.

[00104] At least one of the angle of attack and the airflow speed may be determined according to a lookup table and/or regression according to experimental data. The experimental data may include data of at least one of the angle of attack and the airflow speed and corresponding data of the energy information.

[00105] The controller may be configured to determine the angle of attack according to a piezoelectric film receiving the strongest airflow in the at least one piezoelectric film, wherein the piezoelectric film receiving the strongest airflow in the at least one piezoelectric film is a piezoelectric film which generates the highest voltage, and/or the highest current, and/or the highest power.

[00106] The apparatus may further comprise a communication unit, wherein the communication unit is configured to communicate the determined at least one of the angle of attack and the airflow speed to an external device.

[00107] The energy harvested by the at least one piezoelectric film may be configured to power at least one electronic component comprised in the apparatus.

[00108] An aircraft comprises the above apparatus.

[00109] The apparatus may be mounted at a tip of an airfoil or on a wing of the aircraft.

[00110] .A method for determining at least one of an angle of attack and an airflow speed relative to an aircraft, comprises, collecting experimental data comprising data of at least one of the angle of attack and the airflow speed , and corresponding data of energy information of harvested wind energy by at least one piezoelectric film; harvesting wind energy with the at least one piezoelectric film; and determining at least one of the angle of attack and the airflow speed based on energy information of the harvested wind energy by the at least one piezoelectric film according to the experimental data.

[00111] At least one of the angle of attack and the airflow speed may be determined according to a lookup table and/or regression based on the experimental data. [00112] The information of the energy may comprise at least one voltage information, at least one current information and/or at least one power information.

[00113] The at least one voltage information may comprise at least one of a peak voltage value, a zero-crossing voltage value, an average voltage value, a voltage phase value, and a frequency of fast Fourier transformation value of voltage signals, and/or wherein the at least one current information comprises at least one of a peak current value and an average current value, and/or wherein the at least one power information comprises at least one of a peak power value and an average power value.

[00114] The angle of attack may be determined according to a piezoelectric film receiving the strongest airflow in the at least one piezoelectric film, wherein the piezoelectric film receiving the strongest airflow in the at least one piezoelectric film may be a piezoelectric film which generates the highest voltage, and/or the highest current, and/or the highest power when harvesting wind energy.

[00115] . A storage medium configured to store instructions executed by at least one processor to perform the above method.