Nonlinear ultrasonic-based method for imaging internal micro-foreign objects in soft-pack batteries

Abstract

The application discloses a kind of based on nonlinear ultrasonic soft package battery internal small foreign matter imaging method, belong to power battery manufacturing field, the method includes the following steps: using nonlinear ultrasonic measuring system to stimulate and receive nonlinear ultrasonic on soft package battery surface, extract nonlinear ultrasonic time domain signal carrying foreign matter information;Soft package battery is scanned using three degrees of freedom motion platform C, based on the position of focusing ultrasonic transducer and the nonlinear ultrasonic time domain signal and spectrum, obtain acoustic nonlinear parameter and the spatial distribution of acoustic nonlinear parameter;Acoustic nonlinear parameter is normalized, and the functional relationship of normalized acoustic nonlinear parameter and different foreign matter is constructed;Based on the functional relationship of normalized acoustic nonlinear parameter and different foreign matter, the spatial distribution of acoustic nonlinear parameter, the size and position of foreign matter are inverted.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power battery manufacturing, and particularly relates to a soft package battery internal micro-foreign matter imaging method based on nonlinear ultrasonic. BACKGROUND

[0002] The preparation of a lithium ion battery generally includes three stages (i.e. pole piece preparation, cell assembly and packaging formation). For the laminated sheet in the cell assembly, the micro-foreign matter formed in the die cutting may enter the battery interior, leading to the performance degradation of the lithium ion battery, internal short circuit, and even heat runaway in the charging and discharging process. Therefore, the detection of the micro-foreign matter is particularly important, and the nondestructive testing can detect defects without damaging the internal structure of the lithium ion battery, which is expected to solve the above problems.

[0003] For the detection of the internal foreign matter of the soft package battery, the nondestructive testing methods based on ultrasonic, ray, neutron diffraction (Neutron radiography) and electrochemical impedance spectroscopy (Electrochemical impedance spectroscopy) are widely used. The electrochemical impedance spectroscopy (Electrochemical impedance spectroscopy) mainly studies the charge transfer process in the electrode reaction and the electrolyte conductivity behavior, and focuses on analyzing the impedance characteristics of the electrode and electrolyte interface, but cannot determine the position of the internal foreign matter of the soft package battery. The ray is extremely harmful to the human body, and requires relatively complex equipment and high cost. In addition, the inconvenience and high cost limit the popularity of the neutron diffraction method.

[0004] Compared with the above nondestructive testing technologies, the ultrasonic testing can accurately detect the internal defects of the lithium ion battery and has no harm to the human body. The traditional C-scan utilizes the transmission, refraction and reflection of ultrasonic waves and the linear parameters such as the amplitude and arrival time of the ultrasonic waves, and detects macroscopic defects (usually larger than 1mm in size) according to the spatial position of the ultrasonic transducer, but cannot determine the size and position of the micro-defects with a size much smaller than the wavelength. Therefore, the application provides a soft package battery internal micro-foreign matter imaging method based on nonlinear ultrasonic. SUMMARY

[0005] To solve the above technical problems, the application provides a soft package battery internal micro-foreign matter imaging method based on nonlinear ultrasonic to solve the problems in the prior art.

[0006] To achieve the above purpose, the application provides a soft package battery internal micro-foreign matter imaging method based on nonlinear ultrasonic, which comprises:

[0007] A nonlinear ultrasonic measurement system is used to excite and receive nonlinear ultrasonic on the surface of the soft package battery, and the nonlinear ultrasonic time domain signal carrying the foreign matter information is extracted.

[0008] acquiring the acoustic nonlinear parameter and the spatial distribution of the acoustic nonlinear parameter based on the position of the focused ultrasonic transducer and the nonlinear ultrasonic time domain signal and the spectrum;

[0009] normalizing the acoustic nonlinear parameter to construct a function relationship between the normalized acoustic nonlinear parameter and different foreign matters;

[0010] inverting the size and position of the foreign matter based on the function relationship between the normalized acoustic nonlinear parameter and different foreign matters and the spatial distribution of the acoustic nonlinear parameter.

[0011] Optionally, the nonlinear ultrasonic measurement system comprises an arbitrary function generator, a pulse amplifier, a focused ultrasonic transducer and an oscilloscope.

[0012] The process of acquiring the nonlinear ultrasonic time domain signal carrying the foreign matter information by the nonlinear ultrasonic measurement system comprises:

[0013] generating an excitation signal by the arbitrary function generator, and amplifying the excitation signal by the pulse amplifier to drive the focused ultrasonic transducer.

[0014] The focused ultrasonic transducer converts the amplified excitation signal into ultrasonic waves, and emits the ultrasonic waves to the surface of the soft package battery.

[0015] The ultrasonic waves interact with the foreign matter in the soft package battery after passing through the soft package battery, generate a nonlinear effect, and obtain nonlinear ultrasonic waves.

[0016] The focused ultrasonic transducer receives the nonlinear ultrasonic waves and converts them into electrical signals.

[0017] The oscilloscope records the electrical signals as nonlinear ultrasonic time domain signals.

[0018] Optionally, the three-degree-of-freedom motion platform comprises an oil tank, a sliding table, an upper computer, a servo motor, a ball screw, a clamping device and a motion controller.

[0019] The process of acquiring the nonlinear ultrasonic time domain signal carrying the foreign matter information by the nonlinear ultrasonic measurement system comprises:

[0020] The upper computer establishes communication with the motion controller through an Ethernet interface, and the upper computer establishes communication with the oscilloscope through a USB interface.

[0021] The upper computer transmits the size and resolution requirements of the scanning area to the motion controller through the Ethernet according to the communication protocol.

[0022] The motion controller controls the servo motor to drive the slide table to move along a preset trajectory according to the requirement, so as to realize C scanning of the soft package battery.

[0023] Optionally, the motion controller controls the servo motor to drive the slide table to move along a preset trajectory according to the requirement, so as to realize C scanning of the soft package battery, and the process comprises:

[0024] Step one, the motion controller reads the stroke and step length of the focused ultrasonic transducer in the y-axis and z-axis according to the scanning area size and resolution requirement transmitted by the upper computer, sends a pulse to the servo driver of the z-axis to make the focused ultrasonic transducer move in the positive direction of the z-axis;

[0025] Step two, after the focused ultrasonic transducer moves one step in the z-axis direction, update the displacement amount of the focused ultrasonic transducer in the z-axis in the motion controller; if the displacement amount of the z-axis does not exceed the stroke of the z-axis, the focused ultrasonic transducer continues to move in the positive direction of the z-axis; if the displacement amount of the z-axis reaches the stroke of the z-axis, send an instruction to the y-axis servo driver to make the focused ultrasonic transducer move one step along the y-axis, and then move in the reverse direction of the z-axis;

[0026] Step three, during the movement of the focused ultrasonic transducer in the reverse direction of the z-axis, when the displacement amount of the z-axis reaches the stroke of the z-axis again, the focused ultrasonic transducer moves one step along the y-axis, and then continues to move in the positive direction of the z-axis;

[0027] Step four, steps one to three are executed in a loop until the displacement amounts of the focused ultrasonic transducer in the z-axis and y-axis exceed the respective strokes, forming an S-shaped trajectory and completing the scanning.

[0028] Optionally, the process of constructing the functional relationship between the normalized acoustic nonlinear parameter and different foreign matters comprises:

[0029] Performing fast Fourier transform on the nonlinear ultrasonic time domain signal to extract the amplitude of the fundamental wave and the amplitude of the second harmonic wave;

[0030] Calculating an acoustic nonlinear parameter based on the amplitude of the fundamental wave and the amplitude of the second harmonic wave;

[0031] Performing normalization processing on the acoustic nonlinear parameter to obtain a normalized acoustic nonlinear parameter;

[0032] Analyzing the corresponding relationship between the normalized acoustic nonlinear parameter and different foreign matters to construct a functional relationship between the normalized acoustic nonlinear parameter and different foreign matters.

[0033] Optionally, the calculation expression of the acoustic nonlinear parameter is:

[0034]

[0035] In the formula, β is an acoustic nonlinear parameter, E1 is a Young's modulus, E2 is a high-order elastic constant, A1 is an amplitude of a second harmonic wave, A0 is an amplitude of a fundamental wave, k0 is a wave number of the fundamental wave, and x is a propagation distance of the nonlinear ultrasound.

[0036] Optionally, the process of determining the size and position of the foreign matter comprises:

[0037] According to the normalized acoustic nonlinear parameter and the position of the focused ultrasonic transducer, a spatial distribution of the acoustic nonlinear parameter is obtained.

[0038] Based on the functional relationship between the normalized acoustic nonlinear parameter and different foreign matters, and in combination with the spatial distribution of the acoustic nonlinear parameter, the size and position of the foreign matter are inverted.

[0039] Compared with the prior art, the present application has the following advantages and technical effects:

[0040] The present application is directed to the production stage of soft-pack batteries, adopts nonlinear ultrasound and C scanning to image the soft-pack batteries, determine the position, quantity and size of metal particles, and effectively identify the batteries with micro metal particles mixed therein. The method can prevent the batteries with micro metal particles mixed therein from flowing into the market, avoid the internal short circuit caused thereby, and ensure the safe operation of lithium ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0041] The accompanying drawings, which form a part of the present application, are intended to provide further understanding of the present application, and are used to interpret the illustrative embodiments of the present application and the description, and do not constitute improper limitations to the present application. In the drawings:

[0042] Figure 1 A nonlinear ultrasonic measurement system and a three-degree-of-freedom motion platform for an embodiment of the present application;

[0043] Figure 2 A parameter spatial distribution diagram for an embodiment of the present application, wherein (a) is a spatial distribution diagram of an acoustic linear parameter, and (b) is a spatial distribution diagram of an acoustic nonlinear parameter. The copper particles with a size of 200-300 microns are in the dashed line box;

[0044] Figure 3 A time-domain diagram and a frequency spectrum for an embodiment of the present application; wherein (a) is a time-domain diagram of an ultrasonic wave not passing through copper particles, (b) is a frequency spectrum of the ultrasonic wave not passing through the copper particles, (c) is a time-domain diagram of an ultrasonic wave passing through the copper particles, and (d) is a frequency spectrum of the ultrasonic wave passing through the copper particles;

[0045] Figure 4 A flowchart of a method for imaging micro foreign matter inside a soft-pack battery based on nonlinear ultrasound for an embodiment of the present application. DETAILED DESCRIPTION

[0046] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other in the case of no conflict. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0047] It should be noted that the steps shown in the flowchart of the drawings can be executed in a computer system such as a group of computer executable instructions, and although the logical order is shown in the flowchart, in some cases, the steps shown or described herein can be executed in an order different from that herein.

[0048] Compared with conventional ultrasonic, nonlinear ultrasonic is more sensitive to the contact nonlinear and the change of third-order elastic constant caused by interface and foreign matter. Therefore, the present application uses a three-degree-of-freedom motion platform and nonlinear ultrasonic to complete the C-scan of the soft package battery according to the acoustic nonlinear parameter and the spatial position of the ultrasonic transducer, and realizes the imaging of foreign matter in the soft package battery.

[0049] As shown in Figure 4 The present embodiment provides a soft package battery micro-foreign matter imaging method based on nonlinear ultrasonic. First, the theory of nonlinear ultrasonic longitudinal wave in the soft package battery is provided.

[0050] For material nonlinearity, the one-dimensional physical equation can be written as:

[0051]

[0052] Wherein, sigma represents stress, and epsilon represents strain.

[0053] For geometric nonlinearity, the one-dimensional geometric equation can be written as:

[0054]

[0055] Wherein, u represents displacement, the longitudinal wave generated by the excited ultrasonic transducer along the x-axis direction, passes through the soft package battery, and reaches the receiving ultrasonic transducer.

[0056] The one-dimensional motion differential equation can be written as:

[0057]

[0058] Wherein, rho is density.

[0059] By combining formula (1), formula (2) and formula (3), the one-dimensional wave equation can be obtained:

[0060]

[0061] The second-order perturbation method is introduced:

[0062] u = u0 + u1 (5)

[0063] Substitute equation (5) into equation (4),

[0064]

[0065] Neglecting the higher order terms with u1, equation (6) is divided into a linear part and a nonlinear part, where the linear part is equal to:

[0066]

[0067] The nonlinear part is equal to:

[0068]

[0069] Let u0 = A0(x)exp[i(ω0t-k0x)], A0(x) is a real number, substitute equation (7). It can be known that A0(x) is a constant, and u0 is equal to where c 2 = E / ρ, k0 = ω0 / c, ω0 is the angular frequency, and k0 is the wave number.

[0070] Let u1 = A1(x)exp[i(ωt-kx)], A1(x) is a real number, substitute equation (8). According to the initial condition u1| x＝0 = 0, it is obtained that Therefore, u1 is equal to:

[0071]

[0072] The ratio of the amplitude of the second harmonic wave to the square of the amplitude of the fundamental wave is equal to:

[0073]

[0074] For a specific frequency of a longitudinal wave and a fixed position, k and x are constants. The acoustic nonlinear parameter β is constructed:

[0075]

[0076] Wherein, the acoustic nonlinear parameter is only related to the material parameters of the soft package battery.

[0077] As Figure 4 shown, specifically comprising the following steps:

[0078] The nonlinear ultrasonic measurement system is used to stimulate and receive nonlinear ultrasonic waves on the surface of the soft package battery, and obtain the nonlinear ultrasonic time domain signals carrying foreign matter information; the three-degree-of-freedom motion platform is used to perform C scanning on the soft package battery, and based on the position of the focused ultrasonic transducer and the nonlinear ultrasonic time domain signals and spectrum, the acoustic nonlinear parameters and the spatial distribution of the acoustic nonlinear parameters are obtained; the acoustic nonlinear parameters are normalized to construct the functional relationship between the normalized acoustic nonlinear parameters and different foreign matters; based on the functional relationship between the normalized acoustic nonlinear parameters and different foreign matters and the spatial distribution of the acoustic nonlinear parameters, an imaging algorithm is used to image the soft package battery to determine the size and position of the foreign matter.

[0079] Step S1, design of the nonlinear ultrasonic measurement system. As shown in Figure 1 , the nonlinear ultrasonic measurement system is designed and built by means of an arbitrary function generator, a pulse amplifier, an oscilloscope, a focused ultrasonic transducer, etc., to stimulate and receive nonlinear ultrasonic waves.

[0080] Step S11, the stimulation signal is formed by an arbitrary function generator, which is a sine tone burst with Hanning window, n cycles and frequency f. To drive the excitation transducer, the energy of the signal is enhanced by a pulse amplifier and enters the excitation ultrasonic transducer.

[0081] Step S12, according to the focal length of the focused ultrasonic transducer, the distance between the receiving ultrasonic transducer and the transmitting ultrasonic transducer is adjusted, the soft package battery to be measured, the excitation ultrasonic transducer and the receiving ultrasonic transducer are immersed in silicone oil, and the soft package battery to be measured is located between the excitation ultrasonic transducer and the receiving ultrasonic transducer.

[0082] Step S13, the excitation ultrasonic transducer converts the electrical signal into mechanical vibration, the ultrasonic energy at the focal point is the largest, the fundamental wave and the second harmonic wave pass through the soft package battery and interact with the foreign matter, the nonlinear ultrasonic waves carrying foreign matter information enter the receiving transducer and are converted into electrical signals, which are displayed on the oscilloscope.

[0083] Step S2, design of the three-degree-of-freedom motion platform. The three-degree-of-freedom motion platform is designed and built, the path is planned and the stroke is determined according to the size of the soft package battery, the fixed step length is selected, and the C scanning of the nonlinear ultrasonic waves is completed.

[0084] Further, the three-degree-of-freedom motion platform is composed of an oil tank, a sliding table, an upper computer, a servo motor, a ball screw, a clamping device and a motion controller, etc., and micron-level precision motion control is realized by means of closed-loop feedback, the focused ultrasonic transducer is carried, and the C scanning of the soft package battery is completed.

[0085] Step S21, the host computer transmits the requirement of the size and resolution of the scanning area to the motion controller through Ethernet according to Transmission Control Protocol / Internet Protocol (TCP / IP) protocol. The motion controller receives the instruction and completes protocol analysis, and then stores the above parameters into the register.

[0086] Step S22, the C-scan of the soft-pack battery is completed by the three-degree-of-freedom motion platform and the focused ultrasonic transducer. According to the data in the register in step S21, the motion controller performs the following operations:

[0087] (1) reading the trip of the z-axis of the focused ultrasonic transducer and the step of the z-axis, sending trip / step pulses to the servo driver of the z-axis to move in the positive direction of the z-axis, wherein the z-axis is parallel to the length of the soft-pack battery.

[0088] (2) moving the focused ultrasonic transducer by one step and updating the displacement of the z-axis of the focused ultrasonic transducer. When the displacement of the z-axis exceeds the trip of the z-axis, sending an instruction to the servo driver of the y-axis, wherein the y-axis is parallel to the width of the soft-pack battery.

[0089] (3) after the movement of the z-axis is completed, keeping for a period of time, if the displacement of the z-axis does not reach the trip of the z-axis, continuing to move the z-axis in the movement direction; when the displacement of the z-axis exceeds the trip of the z-axis, moving the focused ultrasonic transducer by one step along the y-axis and then moving in the reverse direction of the z-axis.

[0090] (4) when the displacement of the z-axis of the focused ultrasonic transducer exceeds the trip of the z-axis, sending an instruction to the servo driver of the y-axis, moving the focused ultrasonic transducer by one step along the y-axis and then moving in the positive direction of the z-axis.

[0091] (5) repeating the above steps until the displacement of the z-axis and the displacement of the y-axis both exceed the trip of the z-axis and the trip of the y-axis, stopping the scanning, and obtaining the "S" shaped trajectory.

[0092] Step S3, the communication between the oscilloscope and the three-degree-of-freedom motion platform. The computer sends an instruction to receive the movement of the ultrasonic transducer to a specified position, the computer collects the waveform data on the oscilloscope by means of the data line, and the computer sends an instruction to receive the movement of the ultrasonic transducer to the next position.

[0093] The data synchronous acquisition system comprises an oscilloscope, an upper computer, a motion controller and a USB3.0 interface, and realizes time synchronization by means of a TCP / IP protocol. The focused ultrasonic transducer moves one step at a time, the motion controller sends an instruction to the upper computer, and the upper computer sends an instruction to the oscilloscope. The signal in the oscilloscope is returned to the upper computer at a rate of 480 Mbps through the USB3.0 interface, is analyzed by the upper computer, and is saved to a non-volatile memory.

[0094] Step S31: The focused ultrasonic transducer moves one step at a time along the z-axis or the y-axis, and the motion controller sends an instruction to the upper computer.

[0095] Step S32: The upper computer sends an instruction by means of the USB3.0 interface, acquires the excitation signal and the received signal in the oscilloscope, saves them in the non-volatile memory, and numbers them in the order of acquisition.

[0096] Step S4: A function relationship (such as a one-variable high-order polynomial) between the normalized acoustic nonlinear parameter and different foreign matters is constructed. Soft package batteries with different foreign matters are prepared, the different foreign matters are evaluated by means of nonlinear ultrasonic waves, the nonlinear ultrasonic time domain signals are received, the acoustic nonlinear parameter β is extracted, normalization is performed with respect to the maximum value, and a function relationship between the normalized acoustic nonlinear parameter β' and the different foreign matters is constructed.

[0097] The amplitude of the nonlinear ultrasonic time domain signal or the acoustic nonlinear parameter is extracted, normalization is performed with respect to the maximum value, and a function relationship between the normalized amplitude A0 or the normalized acoustic nonlinear parameter β' and the different foreign matters is constructed.

[0098] Further, the process of constructing the function relationship between the normalized acoustic nonlinear parameter and the different foreign matters comprises: performing fast Fourier transform on the nonlinear ultrasonic time domain signal, extracting the amplitude of the fundamental wave and the amplitude of the second harmonic wave, calculating the acoustic nonlinear parameter based on the amplitude of the fundamental wave and the amplitude of the second harmonic wave, performing normalization on the acoustic nonlinear parameter to obtain the normalized acoustic nonlinear parameter, analyzing the corresponding relationship between the normalized acoustic nonlinear parameter and the different foreign matters, and constructing the function relationship between the normalized acoustic nonlinear parameter and the different foreign matters.

[0099] Step S41: Soft package batteries with different foreign matters are prepared, and the different foreign matters are evaluated by means of nonlinear ultrasonic waves.

[0100] Step S42: The nonlinear ultrasonic time domain signal is received and is subjected to fast Fourier transform, the amplitude A0 of the fundamental wave and the amplitude A1 of the second harmonic wave are extracted, the acoustic nonlinear parameter β is calculated, and normalization is performed with respect to the maximum value.

[0101] Step S5: Imaging of the soft package battery. According to the function relationship between the normalized acoustic nonlinear parameter β' and the different foreign matters and the spatial distribution of β', the imaging of the soft package battery is realized.

[0102] The present application determines the size and location of the foreign matter in the soft package battery according to the distribution of the normalized amplitude or normalized acoustic nonlinear parameter.

[0103] In step S51, the spatial distribution of A'0 or β' is obtained according to the amplitude A'0 of the fundamental wave or the normalized acoustic nonlinear parameter β' and the position of the receiving ultrasonic transducer.

[0104] In step S52, the size and location of the foreign matter are determined according to the functional relationship between A'0 or β' and different foreign matters and the spatial distribution of A'0 or β'.

[0105] As a specific embodiment of the present application, a nonlinear ultrasonic measurement system and a three-degree-of-freedom motion platform are designed, a soft package lithium ion battery with copper particles is prepared, and ultrasonic C scanning and acoustic nonlinear parameters are combined to realize the imaging of the soft package battery and identify the metal foreign matter with a size of 200-300 μm in the soft package battery.

[0106] The present application prepares a lithium ion battery with a thickness and capacity of 3.5 mm and 0.5 Ah (containing 3 positive electrode sheets and 4 negative electrode sheets), using a ternary positive electrode system of NCM523. In order to simulate the metal foreign matter that may be incorporated in the manufacturing process, the present application implants copper particles with a particle size of 200 μm-300 μm at the interface of the middle, the separator and the electrode sheet of the soft package battery.

[0107] The excitation signal is formed by an arbitrary function generator, which is a sine tone burst (amplitude ±500 mV) with Hanning window, 40 cycles and a frequency of 2.25 MHz. After passing through a pulse amplifier, the excitation center frequency and the focal length of the focused ultrasonic transducer are 2.25 MHz and 1 in. The generated ultrasonic waves carry the information of the internal foreign matter of the soft package battery, pass through the focused ultrasonic transducer with a center frequency and a focal length of 5 MHz and 1 in, are converted into electrical signals, and are recorded by an oscilloscope with a sampling rate of 250 M times / sec. At the same time, the electrical signals are uploaded to the host computer in real time at a rate of 480 Mbps through the USB 3.0 interface. With the help of the three-degree-of-freedom motion platform and the focused ultrasonic transducer, the trip of the z-axis of the focused ultrasonic transducer (trip=30 mm) and the step of the z-axis (step=0.5 mm) are read, the trip of the y-axis of the focused ultrasonic transducer (trip=50 mm) and the step of the y-axis (step=0.5 mm) are read, the area where the copper particles may exist is scanned, and an "S" shaped trajectory is obtained. The host computer synchronizes with the three-degree-of-freedom motion platform through the TCP / IP protocol, and sequentially stores the acoustic signals to the non-volatile memory.

[0108] First, the acoustic signal is subjected to a fast Fourier transform to extract the fundamental amplitude and the second harmonic amplitude A0 and A1; the maximum value of A0 is normalized to calculate the acoustic nonlinear parameter The maximum value of β is normalized to obtain the spatial distribution of A'0 or β' according to the fundamental amplitude A0 or the normalized acoustic nonlinear parameter β' and the position of the receiving ultrasonic transducer, as shown in Figure 2 The spatial distribution of copper particles is inverted.

[0109] Compared with the area without copper particles, the change rate of the normalized acoustic nonlinear parameter β' of the copper particle defect area is 39.8%, and the change rate of the normalized fundamental amplitude A'0 of the copper particle defect area is 6.3%, so the acoustic nonlinear parameter sensitivity is 6.3 times the linear parameter sensitivity, as shown in Figure 3 This proves the advantage of nonlinear ultrasonic in the detection of soft package battery micro-foreign matter.

[0110] The above is only the preferred specific embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1.A method for imaging a small foreign object inside a soft-pack battery based on nonlinear ultrasonic, characterized in that, The method comprises the following steps: a nonlinear ultrasonic measurement system is used to stimulate and receive nonlinear ultrasonic waves on the surface of the soft package battery, and a nonlinear ultrasonic time domain signal carrying foreign matter information is extracted; the nonlinear ultrasonic measurement system comprises an arbitrary function generator, a pulse amplifier, a focused ultrasonic transducer and an oscilloscope; wherein the process of obtaining the nonlinear ultrasonic time domain signal carrying foreign matter information by the nonlinear ultrasonic measurement system comprises: generating an excitation signal by the arbitrary function generator, amplifying the excitation signal by the pulse amplifier to drive the focused ultrasonic transducer; the focused ultrasonic transducer converts the amplified excitation signal into ultrasonic waves, and transmits the ultrasonic waves to the surface of the soft package battery; after the ultrasonic waves pass through the soft package battery and interact with the foreign matter in the soft package battery, nonlinear effects are generated to obtain nonlinear ultrasonic waves; the focused ultrasonic transducer receives the nonlinear ultrasonic waves and converts them into electrical signals; the oscilloscope records the electrical signals as nonlinear ultrasonic time domain signals; a three-degree-of-freedom motion platform is used to perform C scanning on the soft package battery, and based on the position of the focused ultrasonic transducer and the nonlinear ultrasonic time domain signal and spectrum, acoustic nonlinear parameters and spatial distribution of the acoustic nonlinear parameters are obtained; the acoustic nonlinear parameters are normalized to construct a functional relationship between the normalized acoustic nonlinear parameters and different foreign matters, which comprises: performing fast Fourier transform on the nonlinear ultrasonic time domain signal to extract the amplitude of the fundamental wave and the amplitude of the second harmonic; calculating the acoustic nonlinear parameters based on the amplitude of the fundamental wave and the amplitude of the second harmonic; normalizing the acoustic nonlinear parameters to obtain normalized acoustic nonlinear parameters; analyzing the corresponding relationship between the normalized acoustic nonlinear parameters and different foreign matters to construct a functional relationship between the normalized acoustic nonlinear parameters and different foreign matters; based on the functional relationship between the normalized acoustic nonlinear parameters and different foreign matters and the spatial distribution of the acoustic nonlinear parameters, the size and position of the foreign matter are inverted. 2.The soft-pack battery internal micro-foreign object imaging method based on nonlinear ultrasound according to claim 1, characterized in that, The three-degree-of-freedom motion platform comprises an oil tank, a sliding table, an upper computer, a servo motor, a ball screw, a clamping device and a motion controller; wherein the process of using the three-degree-of-freedom motion platform to perform C scanning on the soft package battery comprises: the upper computer establishes communication with the motion controller through an Ethernet interface, and the upper computer establishes communication with the oscilloscope through a USB interface; the upper computer transmits the size and resolution requirements of the scanning area to the motion controller through Ethernet according to the communication protocol; the motion controller controls the servo motor to drive the sliding table to move along a preset trajectory according to the requirements, so as to realize C scanning on the soft package battery. 3.The soft-pack battery internal micro-foreign object imaging method based on nonlinear ultrasonic wave according to claim 2, characterized in that, the motion controller controls the servo motor to drive the sliding table to move along a preset trajectory according to the requirements, so as to realize C scanning on the soft package battery, which comprises: Step one, the motion controller reads the size of the scanning area and the resolution requirement transmitted by the host computer, reads the stroke and step length of the y axis and z axis, sends a pulse to the servo driver of the z axis, and moves the focusing ultrasonic transducer along the z axis in the positive direction; Step 2: When the focused ultrasound transducer is in z After moving one step in the axial direction, update its position in the motion controller. z The displacement of the shaft; if z The displacement of the shaft did not exceed z The travel of the axis then continues along the focused ultrasonic transducer. z If the axis moves in the positive direction; z The displacement of the shaft reaches z The travel of the shaft is directed towards... y The axis servo driver sends commands to cause the focused ultrasonic transducer to move along... y Move the axis one step, then along z The axis moves in the opposite direction; Step three, the focused ultrasound transducer moves one step along the z axis in the reverse direction while the z displacement of the axis again reaches the z stroke of the axis, the focused ultrasound transducer moves one step along the y axis, and then continues to move along the z axis in the forward direction; Step 4: Repeat steps 1 through 3 until the focused ultrasound transducer is in... z shaft and y The displacement of the axes simultaneously exceeds their respective strokes, forming an S-shaped trajectory and completing the scan. 4.The soft-pack battery internal micro-foreign matter imaging method based on nonlinear ultrasonic waves according to claim 1, characterized by, the calculation expression of the acoustic nonlinear parameters is: wherein β is the acoustic non-linear parameter, E 1 is the Young's modulus, E 2 is the higher order elastic constant, A 1 is the amplitude of the second harmonic, A 0 is the amplitude of the fundamental wave, k 0 is the wave number of the fundamental wave, x is the propagation distance of the non-linear ultrasound. 5.The soft-pack battery internal micro-foreign matter imaging method based on nonlinear ultrasonic waves according to claim 1, characterized by, the process of determining the size and position of the foreign matter comprises: the spatial distribution of the acoustic nonlinear parameters is obtained according to the normalized acoustic nonlinear parameters and the position of the focused ultrasonic transducer; Based on the function relation between the normalized acoustic nonlinear parameter and different foreign matters, combined with the spatial distribution of the acoustic nonlinear parameter, the size and position of the foreign matter are inversed.

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