A method and system for monitoring the full cycle of bolt loosening

Abstract

The application provides a bolt loosening whole-cycle monitoring method and system, comprising the following steps: a piezoelectric sensor is attached on a bolt connection structure; an excitation signal is applied to the piezoelectric sensor, and a voltage signal and a current signal of the piezoelectric sensor are detected; the excitation signal comprises a chirp sweep signal and a simple harmonic sine signal which are linearly superimposed; an impedance time-domain signal of the piezoelectric sensor is calculated according to the voltage signal and the current signal, and a frequency-domain transformation is performed to obtain an impedance frequency-domain signal; a linear loosening factor and a nonlinear loosening factor are respectively calculated according to the impedance frequency-domain signal; initial loosening of the bolt is detected according to the nonlinear loosening factor, and mid-late loosening of the bolt until complete loosening is detected according to the linear loosening factor. The method combines the advantages of linear electromechanical impedance method and nonlinear ultrasonic method by using nonlinear electromechanical impedance method, and can simultaneously utilize the impedance spectrum and nonlinear characteristics of the bolt structure to track and monitor the whole process of bolt loosening.

Description

Technical Field

[0001] This invention relates to the fields of infrastructure, aerospace, and machinery, and specifically to a method and system for full-cycle monitoring of bolt loosening. Background Technology

[0002] Bolts are used extensively in most industrial equipment and structures. During long-term service, bolted structures are subjected to various external loads, leading to loosening. As loosening intensifies, the bolt's tightening effect disappears, causing structural failure, resulting in significant economic losses and even serious safety accidents. Ultrasonic bolt loosening detection methods can be broadly categorized into two types: linear and nonlinear. Linear methods, including energy methods and traditional mechanical impedance methods, primarily utilize the linear characteristics of ultrasound, such as amplitude attenuation, energy transfer rate, time of flight, and resonance, to detect bolt loosening. However, for the initial stages of bolt loosening, the changes in these linear characteristics are often very subtle and cannot be effectively detected. Nonlinear methods utilize the nonlinear characteristics of ultrasound, such as high-frequency response, mixing response, and subharmonics. These nonlinear characteristics are highly sensitive to the contact nonlinearity generated between the connected structures in the early stages of bolt loosening. However, when bolt loosening reaches a certain level, the contact nonlinearity on the connection surface disappears. Therefore, nonlinear methods are only suitable for monitoring the initial stages of bolt loosening and cannot cover the entire loosening process.

[0003] Existing bolt loosening detection and monitoring technologies cannot monitor the entire process of bolt loosening from its initial stage to its final stage using a single piezoelectric sensor distribution. Current linear ultrasonic methods can only detect bolt loosening to a certain extent and are not sensitive to initial loosening. Existing nonlinear methods can only detect initial bolt loosening. When bolt loosening becomes more severe, nonlinear methods cannot detect it. Summary of the Invention

[0004] This invention provides a method and system for full-cycle monitoring of bolt loosening. It utilizes the mechanical impedance spectral response and nonlinear acoustic characteristics of the structure to provide a technical method for full-cycle monitoring of bolt loosening through nonlinear impedance method.

[0005] To achieve the above objectives, the present invention provides a method for full-cycle monitoring of bolt loosening, comprising the following steps:

[0006] A piezoelectric sensor is attached to the bolted connection structure;

[0007] An excitation signal is applied to the piezoelectric sensor, and the voltage and current signals of the piezoelectric sensor are detected; the excitation signal includes a linearly superimposed chirp sweep signal and a simple harmonic sine signal;

[0008] The impedance time-domain signal of the piezoelectric sensor is calculated based on the voltage signal and the current signal, and the impedance time-domain signal is transformed in the frequency domain to obtain the impedance frequency-domain signal.

[0009] The linear loosening factor and the nonlinear loosening factor are calculated based on the impedance frequency domain signal. The initial loosening of the bolt is detected based on the nonlinear loosening factor, and the middle and late loosening of the bolt is continuously monitored based on the linear loosening factor until it is completely loosened.

[0010] A further improvement of the present invention is that:

[0011] The piezoelectric sensor is connected in series with a sampling resistor; the excitation signal is generated by a function generator and amplified by a power amplifier before being applied to the two ends of the circuit formed by the piezoelectric sensor and the sampling resistor connected in series; the two ends of the sampling resistor are connected to a voltage detection system for detecting the voltage signal of the sampling resistor.

[0012] During the detection process, the current signal of the piezoelectric sensor is calculated based on the voltage signal of the sampling resistor and the resistance value of the sampling resistor. The voltage signal of the piezoelectric sensor is obtained by subtracting the voltage signal of the sampling resistor from the excitation signal.

[0013] A further improvement of the present invention is that: the sweep frequency range of the chirp sweep signal is set to include at least two structural resonant frequencies of the bolted connection structure being detected; the simple harmonic sine signal is one structural resonant frequency of the bolted connection structure; and the frequency of the simple harmonic sine signal is lower than the lower limit of the sweep frequency range of the chirp sweep signal.

[0014] A further improvement of the present invention is that the frequency domain transformation is performed using a fast Fourier transform.

[0015] A further improvement of this invention is that the nonlinear loosening factor includes a third-order nonlinear loosening factor DI. 3rd and the fourth-order nonlinear loosening factor DI 4r The expressions for the two are as follows:

[0016]

[0017]

[0018] Among them: A base It is the amplitude of the impedance frequency domain signal at the fundamental frequency of the simple harmonic sinusoidal signal, A. 3rd It is the amplitude of the third harmonic of the impedance frequency domain signal under a simple harmonic sinusoidal signal, A. 4rd It is the amplitude of the impedance frequency domain signal under the fourth harmonic of a simple harmonic sinusoidal signal.

[0019] A further improvement of the present invention is that: when the third-order nonlinear loosening factor DI3rd Or fourth-order nonlinear loosening factor DI 4rd When the ratio of the value of the current test to the average value of several previous tests is greater than a first threshold, it indicates that the tested bolt has loosened prematurely; the first threshold is 1.5 to 3.

[0020] A further improvement of the present invention is that the expression for the linear loosening factor RMSD is:

[0021]

[0022] In the formula: f a f represents the lower limit of the sweep frequency range of the chirp sweep signal. b y represents the upper limit of the sweep frequency range of the chirp sweep signal. i This represents the spectral amplitude at the corresponding frequency point in the impedance frequency domain signal; This represents the spectral amplitude of the corresponding frequency point in the impedance frequency domain signal when the bolt is completely loosened.

[0023] A further improvement of the present invention is that: when early loosening of the bolt is detected, the initial value of the linear loosening factor RMSD is recorded; subsequently, the linear loosening factor RMSD is used to represent the degree of loosening of the bolt in each detection process; when the ratio of the linear loosening factor RMSD to the initial value is greater than a second threshold, it indicates that the detected bolt is completely loose; the second threshold is 1.8 to 3.4.

[0024] The present invention also includes a full-cycle monitoring system for bolt loosening, used to perform the above-described full-cycle monitoring method for bolt loosening, characterized in that it includes:

[0025] A function generator is used to generate excitation signals;

[0026] A power amplifier, connected to the output of the function generator, is used to amplify the excitation signal;

[0027] A piezoelectric sensor, the first end of which is connected to the first end of a sampling resistor; the second end of the piezoelectric sensor and the second end of the sampling resistor are respectively connected to the positive and negative terminals of the output of the power amplifier.

[0028] The voltage detection system has two channels that detect the excitation signal output by the power amplifier and the voltage signal across the sampling resistor, respectively.

[0029] The signal processing module is used to calculate the linear loosening factor and the nonlinear loosening factor based on the excitation signal detected by the voltage detection system and the voltage signal across the sampling resistor.

[0030] The beneficial effects of this invention are as follows: This invention utilizes a swept-frequency signal in the time domain to organically combine the frequency response and the time domain response, resulting in a novel nonlinear impedance spectrum. This nonlinear impedance spectrum contains the structural resonance information and covers the nonlinear frequency range, providing extremely rich structural response information, thus enabling a more complete real-time monitoring method and allowing for full-process tracking and monitoring of bolt loosening from its initial stage to its final stage. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the full-cycle monitoring system for bolt loosening in this invention;

[0032] Figure 2 This refers to the bolt connection structure in the embodiments of the present invention;

[0033] Figure 3 The waveforms of each voltage signal and the impedance frequency domain signal are shown.

[0034] Figure 4 The waveforms of the impedance frequency domain signal of the bolt under various tightening torques are shown.

[0035] Figure 5 These are candlestick charts obtained from multiple experiments. Detailed Implementation

[0036] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, unless otherwise specified, the following embodiments and features described therein can be combined with each other.

[0037] It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Therefore, the illustrations only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0038] Some exemplary embodiments of the invention have been described for illustrative purposes. It should be understood that the invention may be implemented in other ways not specifically shown in the accompanying drawings.

[0039] like Figure 1 , 2 As shown, the present invention provides a method for full-cycle monitoring of bolt loosening, which includes the following steps:

[0040] Step 1: Attach the piezoelectric sensor to the bolted connection structure. In this embodiment, the bolted connection structure consists of two aluminum plates (120×50×5mm). 3 (Al 6061). Two aluminum plates are connected by M12 bolts. One of the aluminum plates has a sheet piezoelectric sensor made of APC-850 material, with dimensions of 14.4 × 9.7 × 1.5 mm, attached to it. 3 .

[0041] Step 2: Apply an excitation signal to the piezoelectric sensor and detect the voltage and current signals of the piezoelectric sensor; the excitation signal includes a linearly superimposed chirp sweep signal and a simple harmonic sine signal.

[0042] The chirp sweep signal's sweep range is set to include at least two structural resonant frequencies of the bolted connection structure being tested. In this embodiment, the chirp sweep signal is the main resonant frequency range of the bolted connection structure being tested, which includes multiple structural resonant frequencies, allowing the chirp sweep signal to obtain a wider frequency response. In this embodiment, the simple harmonic sine signal is one structural resonant frequency of the bolted connection structure, and the frequency of the simple harmonic sine signal is lower than the lower limit of the chirp sweep signal's sweep range. In one specific embodiment, the chirp sweep signal is from 380kHz to 420kHz, with a duration of 10ms. The sine signal is 50kHz. The excitation signal is generated by a function generator and amplified to 150Vpp by a power amplifier.

[0043] In this embodiment, a piezoelectric sensor is connected in series with a sampling resistor. The excitation signal is generated by a function generator and amplified by a power amplifier before being applied to both ends of the circuit formed by the piezoelectric sensor and the sampling resistor connected in series. A voltage detection system for detecting the voltage signal of the sampling resistor is connected to both ends of the sampling resistor.

[0044] The voltage detection system is an oscilloscope with two channels. Since the oscilloscope cannot directly acquire the current across the piezoelectric sensor, in this embodiment, the oscilloscope detects the voltage signal across the sampling resistor. Dividing the voltage signal by the resistance of the sampling resistor yields the current of the piezoelectric sensor. The voltage signal of the piezoelectric sensor is obtained by subtracting the voltage signal from the sampling resistor from the excitation signal.

[0045] Step 3: Calculate the impedance time-domain signal of the piezoelectric sensor based on the voltage signal and the current signal, and perform frequency domain transformation on the impedance time-domain signal to obtain the impedance frequency-domain signal.

[0046] like Figure 1 , 2As shown in Figure 3, in this embodiment, an oscilloscope is used to acquire the voltage signals in the time domain of the two auxiliary channels (the amplified excitation signal and the voltage signals across the sampling resistors, respectively). Figure 3 As shown in (a), voltage 1 is the amplified excitation signal, and voltage 2 is the voltage across the sampling resistor. The voltage signal on the piezoelectric sensor is obtained by taking the difference. Figure 3 (b) The current in the piezoelectric sensor is the current across the sampling resistor, which can be obtained by dividing the voltage across the sampling resistor by the resistance value, such as... Figure 3 As shown in (c), dividing the voltage and current in the time domain yields the impedance signal of the piezoelectric sensor in the time domain. Then, by using a Fast Fourier Transform (FFT), the impedance time-domain signal is converted to the frequency domain, thus obtaining the nonlinear impedance (impedance frequency domain signal) proposed in this invention.

[0047] Step 4: Calculate the linear loosening factor and the nonlinear loosening factor based on the impedance frequency domain signal; detect the initial loosening of the bolt based on the nonlinear loosening factor, and detect the complete loosening of the bolt based on the linear loosening factor. The linear loosening factor RMSD quantifies the shift in the fundamental frequency spectrum, while the nonlinear loosening factor quantifies the amplitude ratio of higher-order harmonics relative to the excitation fundamental frequency harmonic.

[0048] Nonlinear loosening factors include third-order nonlinear loosening factor DI 3r and the fourth-order nonlinear loosening factor DI 4rd The expressions for the two are as follows:

[0049]

[0050]

[0051] Among them: A base It is the amplitude of the impedance frequency domain signal at the fundamental frequency (50kHz) of the simple harmonic sinusoidal signal, A 3rd It is the amplitude of the impedance frequency domain signal under the third harmonic (150kHz) of a simple harmonic sinusoidal signal, A 4rd It is the amplitude of the impedance frequency domain signal under the fourth harmonic (200kHz) of a simple harmonic sinusoidal signal.

[0052] When the third-order nonlinear loosening factor DI 3rd Or fourth-order nonlinear loosening factor DI 4rd When the ratio of the value of the current test to the average value of several previous tests is greater than a first threshold, it indicates that the tested bolt has loosened prematurely; the first threshold is 1.5 to 3.

[0053] The expression for the linear loosening factor RMSD is:

[0054]

[0055] In the formula: f a f represents the lower limit of the sweep frequency range of the chirp sweep signal. b This represents the upper limit of the sweep range of the chirp sweep signal; the impedance frequency domain signal is a discrete frequency domain signal, y i For the spectral amplitude of the corresponding frequency point in the impedance frequency domain signal, in the above expression, the frequency points involved in the summation are located in the interval [f a ,f b ]between; This represents the spectral amplitude of the corresponding frequency point in the impedance frequency domain signal when the bolt is completely loosened. Obtained through actual measurement, for example, during the installation of bolted connection structures. sequence.

[0056] When early loosening of the bolt is detected, the initial value of the linear loosening factor RMSD is recorded; subsequently, the linear loosening factor RMSD is used to represent the degree of loosening of the bolt in each detection process; when the ratio of the linear loosening factor RMSD to the initial value is greater than a second threshold, it indicates that the detected bolt is completely loose; the second threshold is 1.8 to 3.4.

[0057] In this embodiment, the above method is verified through experiments. Figure 3 As observed in (d), the frequency spectrum of the obtained nonlinear impedance includes the fundamental frequency components of the harmonic sine wave and the chirp sweep signal, at 50kHz and 380kHz-420kHz, respectively. It also includes the higher-order harmonic spectrum of the nonlinear response (second order 100kHz, third order 150kHz, fourth order 200kHz), as well as the mixed response of the harmonic wave and the chirp sweep. Initial loosening of the bolt produces the aforementioned nonlinear response, while later loosening eliminates the nonlinear response, but a spectral shift in the chirp sweep remains.

[0058] In this embodiment, the bolt saturation torque is 60 Nm. Within the range of 0-60 Nm, the nonlinear impedance spectrum of the bolt structure is measured every 5 Nm. Figure 4 As shown. In this embodiment, 10 independent repeated experiments were conducted, and... Figure 5 The box plot shown illustrates the probabilistic relationship between the bolt loosening factor and the magnitude of the applied torque. Nonlinear loosening coefficients, third and fourth harmonics, are used to detect initial bolt loosening. Linear loosening coefficients can be used for subsequent monitoring.

[0059] When monitoring bolts throughout their lifespan, we first focus on the nonlinear loosening factor. For example... Figure 5When the torque is 35 Nm, the bolt has already begun to loosen prematurely. As the bolt loosens to a certain extent, the non-linear loosening factor decreases to a normal level, at which point the linear loosening factor needs to be continuously monitored. It increases monotonically with the severity of the loosening until the bolt is completely loosened.

[0060] like Figure 1 As shown, the present invention also includes a full-cycle monitoring system for bolt loosening, used to perform the above-described full-cycle monitoring method for bolt loosening, comprising:

[0061] A function generator is used to generate excitation signals;

[0062] A power amplifier, connected to the output of the function generator, is used to amplify the excitation signal;

[0063] A piezoelectric sensor, the first end of which is connected to the first end of a sampling resistor; the second end of the piezoelectric sensor and the second end of the sampling resistor are respectively connected to the positive and negative terminals of the output of the power amplifier.

[0064] The voltage detection system has two channels that detect the excitation signal output by the power amplifier and the voltage signal across the sampling resistor, respectively.

[0065] The signal processing module is used to calculate the linear slack factor and the nonlinear slack factor based on the excitation signal detected by the voltage detection system and the voltage signal across the sampling resistor. In this embodiment, the signal processing module is a computer connected to the voltage detection system. In large-scale applications, the function generator, power amplifier, voltage detection system, and signal processing module can be integrated on a single circuit board.

[0066] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for full-cycle monitoring of bolt loosening, characterized in that, Includes the following steps: A piezoelectric sensor is attached to the bolted connection structure; An excitation signal is applied to the piezoelectric sensor, and the voltage and current signals of the piezoelectric sensor are detected; the excitation signal includes a linearly superimposed chirp sweep signal and a simple harmonic sine signal; The impedance time-domain signal of the piezoelectric sensor is calculated based on the voltage signal and the current signal, and the impedance time-domain signal is transformed in the frequency domain to obtain the impedance frequency-domain signal. The linear loosening factor and the nonlinear loosening factor are calculated based on the impedance frequency domain signal. The initial loosening of the bolt is detected based on the nonlinear loosening factor, and the middle and late loosening of the bolt is continuously monitored based on the linear loosening factor until it is completely loosened. The piezoelectric sensor is connected in series with a sampling resistor; the excitation signal is generated by a function generator and amplified by a power amplifier before being applied to the two ends of the circuit formed by the piezoelectric sensor and the sampling resistor connected in series; the two ends of the sampling resistor are connected to a voltage detection system for detecting the voltage signal of the sampling resistor. During the detection process, the current signal of the piezoelectric sensor is calculated based on the voltage signal of the sampling resistor and the resistance value of the sampling resistor. The voltage signal of the piezoelectric sensor is obtained by subtracting the voltage signal of the sampling resistor from the excitation signal. The sweep frequency range of the chirp sweep signal is set to include at least two structural resonant frequencies of the bolted connection structure being tested; the simple harmonic sine signal is one structural resonant frequency of the bolted connection structure; the frequency of the simple harmonic sine signal is lower than the lower limit of the sweep frequency range of the chirp sweep signal. Nonlinear loosening factors include third-order nonlinear loosening factors. and fourth-order nonlinear loosening factor The expressions for the two are as follows: ； ； in: It is the amplitude of the impedance frequency domain signal at the fundamental frequency of the simple harmonic sinusoidal signal. It is the amplitude of the third harmonic of the impedance frequency domain signal under a simple harmonic sinusoidal signal. It is the amplitude of the impedance frequency domain signal under the fourth harmonic of a simple harmonic sinusoidal signal; When the third-order nonlinear loosening factor Or fourth-order nonlinear loosening factor When the ratio of the current test value to the average of several previous tests is greater than a first threshold, it indicates that the tested bolt has loosened prematurely; the first threshold is 1.5 to 3. The initial loosening of the bolt is detected by nonlinear loosening factor, and the middle and late loosening of the bolt is continuously monitored by linear loosening factor until it is completely loosened. During the entire monitoring of the bolt, the nonlinear loosening factor is focused first. When the bolt loosening reaches a certain degree and the nonlinear loosening factor decreases to a normal state, the linear loosening factor is continuously monitored. The linear loosening factor shows a monotonically increasing law with the severity of the bolt loosening until the bolt is completely loosened.

2. The method for full-cycle monitoring of bolt loosening according to claim 1, characterized in that: The frequency domain transformation method is Fast Fourier Transform.

3. The method for full-cycle monitoring of bolt loosening according to claim 1, characterized in that: The expression for the linear loosening factor RMSD is: In the formula: This represents the lower limit of the sweep frequency range for the chirp sweep signal. This represents the upper limit of the sweep frequency range of the chirp sweep signal. This represents the spectral amplitude at the corresponding frequency point in the impedance frequency domain signal; This represents the spectral amplitude of the corresponding frequency point in the impedance frequency domain signal when the bolt is completely loosened.

4. The method for full-cycle monitoring of bolt loosening according to claim 3, characterized in that: When early loosening of the bolt is detected, the initial value of the linear loosening factor RMSD is recorded; subsequently, the linear loosening factor RMSD is used to represent the degree of loosening of the bolt in each detection process; when the ratio of the linear loosening factor RMSD to the initial value is greater than a second threshold, it indicates that the detected bolt is completely loose; the second threshold is 1.8~3.

4.

5. A full-cycle monitoring system for bolt loosening, used to execute the full-cycle monitoring method for bolt loosening as described in any one of claims 1 to 3, characterized in that... include: A function generator is used to generate excitation signals; A power amplifier, connected to the output of the function generator, is used to amplify the excitation signal; A piezoelectric sensor, the first end of which is connected to the first end of a sampling resistor; the second end of the piezoelectric sensor and the second end of the sampling resistor are respectively connected to the positive and negative terminals of the output of the power amplifier. The voltage detection system has two channels that detect the excitation signal output by the power amplifier and the voltage signal across the sampling resistor, respectively. The signal processing module is used to calculate the linear loosening factor and the nonlinear loosening factor based on the excitation signal detected by the voltage detection system and the voltage signal across the sampling resistor.

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