CN112229502A - System and method for measuring natural frequency of simply supported beam - Google Patents
System and method for measuring natural frequency of simply supported beam Download PDFInfo
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Abstract
The invention discloses a system and a method for measuring the natural frequency of a simply supported beam, wherein the system comprises the following components: the device comprises a piezoelectric acceleration sensor, a charge amplifier, a data acquisition card, a DAQmx module, a data preprocessing module, a signal analysis module, an excitation signal unit for generating an excitation signal source, a power amplifier, an electromagnetic vibration exciter and a display recording module for displaying measured data; the electromagnetic vibration exciter converts electric energy into mechanical energy to provide an exciting force for the simply supported beam, the piezoelectric sensor detects the vibration acceleration of the simply supported beam, and after the vibration acceleration is processed by the charge amplifier, a vibration signal is transmitted to the input end of the data acquisition card; the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware, the data preprocessing module completes filtering and windowing interference elimination processing of signals, the signal analysis module performs FFT operation on input signals and output signals to obtain amplitude spectrum functions of the input signals and the output signals, and the ratio of the input signals to the output signals is taken to obtain a response characteristic curve. The invention has high measurement precision and high reliability.
Description
Technical Field
The invention relates to the technical field of simply supported beam measurement, in particular to a system and a method for measuring the natural frequency of a simply supported beam.
Background
The study of the natural frequency is of great importance in engineering, where all working parts in mechanical engineering inevitably vibrate. Both high-frequency vibration and low-frequency vibration can cause damage to components, so that the service life of the components is reduced along with the time if the vibration is light, and the components are directly damaged if the vibration is heavy; therefore, the research on the natural frequency realizes active vibration avoidance, reduces the loss of parts in work and becomes very significant. However, the natural frequency is not all harmful to engineering, and in the measurement operation, the natural frequency can be used as a low-pass filter, because the signal higher than the natural frequency is very weak and can also be used to determine the size of the sampling frequency.
In the prior art, a plurality of platforms and tools for testing vibration exist, but hardware equipment is expensive, and precision instruments have service life limitation, so that precision is reduced. The traditional methods are difficult to complete effective analysis of the engineering practical problem, the classical theory of elastic mechanics can only solve the problem of simpler structural shape and bearing load due to the difficulty of solving the edge value problem of partial differential equation, and the problem of complex geometric shape, irregular boundary, crack or thickness mutation, geometric nonlinearity, material nonlinearity and the like is very difficult to try to obtain an analytic solution according to the classical elastic mechanics method.
Disclosure of Invention
In order to overcome the defects and shortcomings in the prior art, the invention provides a system and a method for measuring the natural frequency of a simply supported beam. The invention adopts virtual instrument technology, utilizes the labVIEW software platform to carry out inherent frequency measurement, only needs to measure relevant input and output signals and select a proper signal analysis function to obtain an inherent frequency measurement result, solves the function limitation of the traditional electronic equipment, can add other functions on the labVIEW software platform according to the measurement requirement, and integrates the functions into a computer CPU for calculation.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides a system for measuring natural frequency of a simply supported beam, which comprises: the device comprises a piezoelectric acceleration sensor, a charge amplifier, a data acquisition card, a DAQmx module, a data preprocessing module, a signal analysis module, a vibration excitation signal unit, a power amplifier, an electromagnetic vibration exciter and a display recording module;
the piezoelectric acceleration sensor is connected with a charge amplifier, the charge amplifier is connected with a data acquisition card, the data acquisition card is connected with a DAQmx module, after being processed by the charge amplifier, a vibration signal of the simply-supported beam is transmitted to the input end of the data acquisition card, and the data acquisition card collects an analog signal;
the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware, the data preprocessing module is used for finishing filtering and windowing interference elimination processing of signals, the signal analysis module is used for carrying out FFT (fast Fourier transform) operation on input signals and output signals to obtain amplitude spectrum functions of the input signals and the output signals, the ratio of the amplitude spectrum functions to the output signals is taken to obtain a response characteristic curve, the excitation signal unit is used for generating an excitation signal source, the electromagnetic type exciter is used for converting electric energy into mechanical energy and providing an excitation force for a simply supported beam, and the display recording module is used for displaying measured data.
As a preferable technical scheme, the piezoelectric sensor adopts an YD-12 type piezoelectric acceleration sensor, the data acquisition card adopts a PCI-6221 acquisition card, the charge amplifier adopts a BZ2101 single-channel charge amplifier, the power amplifier adopts a GF-20 power amplifier, and the electromagnetic vibration exciter adopts a JZ series electric vibration exciter.
As a preferred technical solution, the excitation signal unit adopts pulse excitation, and includes: the DAQ assistant VI, the splitting signal VI, the oscillogram VI, the next integral multiple millisecond VI waiting function, the frequency response function VI and the spectrum measurement function VI;
the DAQ assistant VI is used for setting a task of collecting data or outputting signals, the splitting signal VI is used for splitting the collected signals into pulse signals and simply supported beam vibration signals, the oscillogram VI is used for displaying nodes of waveforms, waiting for the next integral multiple millisecond VI is used for controlling the time interval between every two sampling data, the frequency response function VI is used for performing frequency response calculation on the two collected signals, outputting an amplitude spectrum and a phase spectrum, and the frequency spectrum measurement function VI is used for observing the frequency spectrum characteristics of the input pulse signals.
As a preferred technical solution, the parameters waiting for the next integer multiple of millisecond VI include a millisecond multiple and a millisecond timer, the millisecond multiple is used for specifying a time interval of VI running, and the millisecond timer is used for returning to the waiting time.
As a preferred technical solution, the parameters of the frequency response function VI include: time signal X representing time waveform data X, time signal Y representing time waveform data Y, a window representing a window function, amplitude representing the amplitude and frequency range of the returned average frequency response, and phase representing the phase and frequency range of the returned average frequency response.
As a preferable technical solution, the excitation signal unit uses a sine wave signal as an excitation signal, and includes: reading VI by a Chirp signal VI and DAQmx, creating a virtual channel VI, writing VI by a sampling clock VI and DAQmx, and starting a task VI;
the method comprises the steps that a Chirp signal VI is used for generating a Chirp signal serving as an excitation signal source, DAQmx reading VI is used for reading sampling data in a task or a virtual channel specified by a user, the virtual channel VI is created for selecting AI voltage and representing an output voltage signal, a sampling clock VI is used for configuring the number of samples to be acquired or generated and creating a required buffer area, the DAQmx writing VI is used for writing data in the task or the virtual channel specified by the user, and the starting task VI is used for enabling a DAQ task to be in a running state.
The invention also provides a measuring method of the natural frequency measuring system of the simply supported beam, which comprises the following steps:
generating an excitation signal source, converting electric energy into mechanical energy by an electromagnetic vibration exciter, providing an excitation force for the simply-supported beam, detecting the vibration acceleration of the simply-supported beam by a piezoelectric sensor, processing the vibration acceleration by a charge amplifier, transmitting a vibration signal of the simply-supported beam to the input end of a data acquisition card, and collecting an analog signal by the data acquisition card;
the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware;
filtering and windowing the signal to eliminate interference;
and performing FFT operation on the input signal and the output signal to obtain an amplitude spectrum function of the input signal and the output signal, obtaining a response characteristic curve by taking the ratio of the input signal and the output signal, and displaying the measured data.
As a preferred technical scheme, the excitation signal source adopts pulse excitation, a plurality of vibration pickup points are equally divided on the simply supported beam, an acceleration sensor is sequentially placed on the vibration pickup points, the simply supported beam is hammered by a pulse hammer each time, frequency characteristic curves measured by the plurality of vibration pickup points are recorded, pulse excitation signals and response signals are converted into voltage signals through a charge amplifier to be used for a data acquisition card to acquire data, and finally the voltage signals are input into a DAQ assistant VI, a splitting signal VI, a waveform diagram VI, a frequency response function VI and a spectrum measurement function VI to be processed and displayed.
Preferably, the number of the vibration pickup points is 9, and an amplitude-frequency diagram obtained by measuring the position of the second vibration pickup point is selected for data processing.
As a preferred technical scheme, the excitation signal source adopts a Chirp signal, ports of collected signals and output signals on a data collection card and a wiring board are set by creating a virtual channel VI, then, a sampling rate and a sampling number are set by a timing VI, a clock source for analog output is set on the timing VI of signal collection, excitation data is written into a DAQ write VI cache, and the DAQ starts VI execution of signal output and input;
the excitation and response signals are converted into an array signal VI through dynamic signals and transmitted to an index array, then two signals, namely a Chirp excitation signal and a response signal of the device, are respectively taken out, FFT processing is carried out, amplitude values of the two signals are taken out to be divided, then the frequency of the response signal is taken as an abscissa, and an amplitude-frequency response oscillogram is created and displayed.
Compared with the prior art, the invention has the following advantages and beneficial effects:
(1) the invention adopts virtual instrument technology, utilizes the labVIEW software platform to carry out natural frequency measurement, only needs to measure relevant input and output signals and select a proper signal analysis function to obtain a natural frequency measurement result, solves the function limitation of the traditional electronic equipment, can add other functions on the labVIEW software platform according to the measurement requirement, and integrates the functions into a computer CPU for calculation.
(2) Because the excitation always precedes the response, and the abscissa can cause errors because of the frequency value dislocation of the signal when outputting the oscillogram, the invention solves the problem of data synchronization of input and output through the synchronization function of the DAQmx module, analyzes the data of the excitation and response signals, and then extracts each floating point data from the waveform to the array through various functions VI, thereby improving the measurement precision and having high reliability.
(3) The invention adopts software to generate sine scanning excitation signals, simultaneously compares the excitation signals with the simply supported beam vibration signals through a synchronous clock technology, measures the output-to-input amplitude ratio under any same frequency, and provides theoretical guarantee for accurately measuring the natural frequency.
Drawings
Fig. 1 is a schematic structural diagram of a system for measuring natural frequency of a simply supported beam according to the present embodiment;
FIG. 2 is a schematic structural diagram of a signal acquisition subsystem according to this embodiment;
FIG. 3 is a schematic structural diagram of an excitation signal output subsystem according to this embodiment;
FIG. 4 is a schematic view of a hammering measurement structure of the pulse hammer according to the present embodiment;
fig. 5 is a schematic structural diagram of the signal splitting VI in this embodiment;
FIG. 6 is a schematic diagram illustrating the structure of the present embodiment waiting for the next integer multiple of millisecond VI;
FIG. 7 is a schematic structural diagram of a frequency response function (amplitude-phase) VI according to the present embodiment;
FIG. 8 is a time domain diagram of a pulse signal according to the present embodiment;
FIG. 9 is a frequency domain diagram of the pulse signal of the present embodiment;
FIG. 10(a) is a first amplitude-frequency diagram of the vibration pickup point of the present embodiment;
FIG. 10(b) is a second amplitude-frequency diagram of the vibration pickup point of the present embodiment;
FIG. 10(c) is a third amplitude-frequency diagram of the vibration pickup point of the present embodiment;
FIG. 10(d) is a fourth amplitude-frequency diagram of the vibration pickup point of the present embodiment;
FIG. 10(e) is a magnitude-frequency diagram of a fifth vibration pickup point of the present embodiment;
FIG. 10(f) is a magnitude frequency diagram of a sixth vibration pickup point of the present embodiment;
FIG. 10(g) is a seventh amplitude-frequency diagram of the vibration pickup point of the present embodiment;
FIG. 10(h) is an amplitude-frequency diagram of the eighth vibration pickup point of the present embodiment;
FIG. 10(i) is a magnitude-frequency diagram of a ninth vibration pickup point in the present embodiment;
FIG. 11 is a schematic diagram of the front three-order vibration modes of the simply supported beam according to the present embodiment;
fig. 12 is a schematic diagram of a sine frequency sweeping structure according to the present embodiment;
fig. 13 is a schematic structural diagram of the Chirp Pattern VI in this embodiment;
fig. 14 is a schematic structural diagram of creating a virtual channel VI according to this embodiment;
fig. 15 is a schematic structural diagram of the sampling clock VI according to this embodiment;
FIG. 16 is a structural diagram of the DAQmx write VI of the present embodiment;
FIG. 17 is a schematic structural diagram of a start task VI according to the present embodiment;
fig. 18 is a schematic structural diagram of reading VI by DAQmx in this embodiment.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Examples
As shown in fig. 1, the present embodiment provides a system for measuring natural frequency of a simply supported beam, including: the system comprises a signal acquisition subsystem and an excitation signal output subsystem;
as shown in fig. 2, the signal acquisition subsystem includes: the device comprises a piezoelectric acceleration sensor, a charge amplifier, a data acquisition card, a DAQmx module, a data preprocessing module, a signal analysis module and a display recording module;
the piezoelectric acceleration sensor is connected with a charge amplifier, the charge amplifier is connected with a data acquisition card, the data acquisition card is connected with a DAQmx module, after being processed by the charge amplifier, a vibration signal of the simply-supported beam is transmitted to the input end of the data acquisition card, and the data acquisition card collects an analog signal;
the signal analysis module mainly only carries out FFT operation on the input signal and the output signal to obtain an amplitude spectrum function of the input signal and the output signal, and obtains a response characteristic curve by taking the ratio of the input signal to the output signal;
the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware, the data acquisition card adopts a PCI-6221 acquisition card, the signal acquisition subsystem and an excitation signal output subsystem are the same data acquisition card, signal acquisition refers to the acquisition of vibration measurement signals, the signal acquisition is used as an analog input end of the acquisition card, and the excitation signals need an analog output end of the acquisition card to generate excitation waveforms.
The piezoelectric sensor adopts an YD-12 type piezoelectric acceleration sensor, and the acceleration variable borne by the shell is converted into an electric signal to be output. A mass is present in the sensor body, which, due to its inertia during vibration, produces an acceleration relative to the housing which is converted into a force which in turn acts on the piezoelectric element, on the two end faces of which charge of opposite polarity is generated.
In this embodiment, the charge amplifier adopts a BZ2101 single-channel charge amplifier, which is mainly characterized in that:
(1) the attenuation of the equivalent capacitance of the input cable to the signal is eliminated, so that the cable with a certain length can be used in a measuring system, and the measured signal is not obviously attenuated;
(2) with a dial switch indicating input charge sensitivity, piezoelectric accelerometers with different charge sensitivities can be used to normalize the output of the instrument;
(3) the power supply system has two power supply modes of alternating current and direct current, and is suitable for various application environments.
In this embodiment, the BZ2101 single-channel charge amplifier includes a charge conversion unit, an adaptive amplification unit, a low-pass filtering unit, an output amplification unit, and an overload indication circuit;
a charge conversion unit: the function is to convert the input charge quantity into voltage quantity output;
an adaptive amplification unit: the sensitivity of the sensor can be adjusted from 0 to 20db by different gains, so that output normalization is achieved;
a low-pass filtering unit: the active filter is composed of an operational amplifier and a resistance-capacitance element, and divides into five steps of 1kHz, 3kHz, 10kHz, 30kHz and 100kHz, and the frequency drops respectively.
An output amplification unit: the gain of the high-pass filter is 0dB and 20 dB.
An overload indicating circuit: when the output signal is larger than the peak value of 10V, the overload indicator lamp is on, and the output signal enters a saturation region.
In this embodiment, after being processed by the charge amplifier, the vibration signal of the simply supported beam is transmitted to the input end of the data acquisition card, and the data acquisition card not only needs to collect the analog signal, but also needs to be linked with the hardware driver of the computer. Besides collecting signal data, the present embodiment also designs an excitation signal output subsystem, which needs to output an excitation signal to the electromagnetic vibration exciter to drive the electromagnetic vibration exciter to excite the simply supported beam to vibrate.
As shown in fig. 3, the excitation signal output subsystem includes: the system comprises an excitation signal unit, a DAQmx module, a data acquisition card, a power amplifier and an electromagnetic vibration exciter;
in this embodiment, the excitation signal unit generates an excitation signal source, and also uses DAQmx as a hardware drive to output an analog signal to the data acquisition card, and outputs the analog signal to the power amplifier through an AO (analog output) port in the shielded I/O junction box, and finally outputs the analog signal to the electromagnetic vibration exciter, wherein the electromagnetic vibration exciter converts electric energy into mechanical energy, and provides an excitation force for the simply supported beam;
in the present embodiment, the power amplifier is a GF-20 power amplifier, which is mainly used for driving loads such as a small-sized vibration exciter, a vibration table, a loudspeaker and the like, and has a usable frequency of from DC to 100KHz and an output power of 20VA (5 Ω). The amplifier can adopt current negative feedback and voltage negative feedback to become a voltage generator with low output impedance or a current generator with high output impedance.
In this embodiment, the electromagnetic vibration exciter is a JZ series electric vibration exciter, the model is JZ-1, and the electromagnetic vibration exciter is a converter capable of converting electric energy into mechanical energy and providing an exciting force to the simply supported beam.
In this embodiment, the data acquisition card, model PCI-6221 by data acquisition card NI, is installed on a computer motherboard, has both a/D and D/a conversion functions, and can simultaneously input and output data, and the connection interface with the computer uses a PCI bus to display the acquired data on the computer, which completes data analysis and calculation.
In this embodiment, a single data conductor may be used to connect to a single patch panel, and this embodiment employs a shielded I/O junction box having multiple input and output ports, optionally an NI SCB-68A type junction box, for connecting I/O signals to a plug-in DAQ device equipped with 68-pin connection ports. When the shielded cable is connected, the SCB-68A can provide a firm and extremely low-noise signal terminal, the SCB-68A is compatible with single-connector, double-connector NI X series and M series DAQ equipment provided with 68-pin connectors, the connecting box is also compatible with most NI E series, B series, S series and R series DAQ equipment, the data acquisition card and the wiring board of the embodiment can integrate input and output into one module, the number of hardware equipment is reduced, and meanwhile, a hardware basis is provided for synchronous module-in and module-out measurement.
As shown in fig. 4, in this embodiment, the excitation mode may be pulse excitation:
according to the definition of the frequency response function, the frequency response is amplitude frequency response and phase frequency response. The amplitude-frequency response function, i.e. the function of the amplitude ratio of the output to the input with respect to the frequency of the input signal, is called the amplitude-frequency characteristic of the system, and is expressed as:
collecting a pulse signal hammered by a pulse hammer and an input signal of vibration of the simply supported beam, carrying out Fourier transform, and dividing the pulse signal hammered by the pulse hammer and the input signal to obtain a frequency response function, wherein the amplitude is taken as a vertical coordinate, the frequency is taken as a horizontal coordinate, and the frequency is displayed in a waveform chart.
In this embodiment, several nodes VI are used, including a DAQ helper VI, a splitting signal VI, a waveform diagram VI, a waiting time for the next integer multiple of milliseconds VI, a frequency response function (amplitude-phase) VI, and a spectrum measurement function VI, and an amplitude-frequency characteristic curve of the input and output, i.e., a peak value, that is, an approximate natural frequency, is obtained from the frequency response function VI.
The DAQ assistant VI is a multi-state VI for quickly setting a task of acquiring data or outputting signals, integrates the data acquisition and analog output functions of Measurement & Automation Explorer (hereinafter referred to as MAX platform for short), and comprises the steps of setting sampling rate and sampling number, selecting an input/output channel, selecting acquisition/output voltage, current, digital signals and the like, wherein two signals acquired by pulse excitation respectively correspond to two acquisition channels of a data acquisition card, one is an AI1 channel and is responsible for acquiring pulse signals, the other is an AI2 channel and is responsible for acquiring simply supported beam vibration signals, and the main parameters are shown in the following table 1:
TABLE 1 DAQ Assistant VI parameter Table
As shown in fig. 5, signal splitting VI: because a pulse signal and a simply supported beam vibration signal are acquired by using double channels, two signal data are included in a signal acquired by the DAQ assistant, the two signals need to be separated, and the separation principle is according to the sequence of AI ports, namely, a signal 1 is the pulse signal and a signal 2 is the simply supported beam vibration signal;
waveform diagram VI: a node for displaying a waveform;
as shown in fig. 6, wait for the next integer multiple of milliseconds VI: the VI is used to synchronize operations, is used within the loop structure, and controls the rate of loop execution. After the data acquisition is finished, the data are displayed on a waveform chart, but the waveform chart is quick in refresh rate, so that the VI is used for controlling the time interval between every two sampling data, and a user can see the images clearly without flickering the images; the parameters of the VI include: a millisecond multiplier and millisecond timer, wherein the millisecond multiplier specifies the time interval for VI operation, and the millisecond timer returns a wait time in milliseconds;
as shown in fig. 7, the frequency response function (amplitude-phase) VI: the frequency response calculation is carried out on the two collected signals, an amplitude spectrum and a phase spectrum are output, the function can simultaneously connect the pulse excitation signal (namely X (t)) and the response signal (namely Y (t)) of the device to the input end of the function, the calculated output is the amplitude frequency characteristic or the phase frequency characteristic of the vibration response signal, namely the ratio of Y (omega)/X (omega) is measured, as X (omega) is 1, Y (omega) is H (omega), namely the frequency response function, and the corresponding frequency of the peak value, namely the natural frequency of the device, is found from the curve of H (omega).
The VI main parameters are: time signal X (time waveform data X), time signal Y (time waveform data Y), a window (specifying a window function), an amplitude (amplitude and frequency range returning an average frequency response), and a phase (phase and frequency range returning an average frequency response);
spectral measurement function VI: the function has guiding significance for improving the excitation quality and replacing a proper measuring head;
the pulse excitation method is simple, has wide applicability, is not limited by the structure and the size of a measured object, can easily measure parts which cannot be installed and fixed and parts which are difficult to install a vibration exciter, and can quickly obtain the natural frequency of the parts.
In this embodiment, the measuring method of pulse hammering is to equally divide 9 vibration pickup points on the simply supported beam, place an acceleration sensor on the vibration pickup points in sequence, hammer the simply supported beam with the pulse hammer each time, record the frequency characteristic curve measured at the 9 vibration pickup points, convert the pulse excitation signal and the response signal into a voltage signal through a charge amplifier for a data acquisition card to acquire data, and finally input the voltage signal into a DAQ assistant VI, a splitting signal VI, a waveform diagram VI, a frequency response function (amplitude-phase) VI, and a spectrum measurement function VI for processing and displaying.
After the lines are connected, setting parameters of the charge amplifier, wherein the parameters of the charge amplifier connected with the piezoelectric acceleration sensor are set as follows: the charge sensitivity is properly adjusted to 10.38pC/Unit, the measuring range is 1-11pC/Unit, the linearity is 1, and the low-pass filtering is 3 KHz. Similarly, the charge amplifier connected to the pulse hammer is set with the following parameters: the sensitivity was adjusted to 9.37pC/Unit, and the remaining parameters were identical to those described above.
Exciting the simply supported beam, observing data collected by a waveform diagram, and obtaining a pulse signal time domain waveform and a pulse signal frequency domain waveform as shown in fig. 8 and 9;
as shown in fig. 10(a) to 10(i), by measuring the amplitude-frequency diagram at 9 different points, it can be seen from the frequencies at the peak positions in the diagram that the three natural frequencies are about: 50Hz, 200Hz, 425 Hz;
the natural frequency of the third order measured by the third vibration pickup point is zero, and the natural frequency of the second order measured by the fifth vibration pickup point is zero;
as shown in fig. 11, the second order mode shape obtained from ANSYS is a sine function curve:
wherein L represents the length of the simply supported beam, x represents the relative coordinates on the simply supported beam, y represents the amplitude
The amplitude of the simply supported beam caused by the natural frequency of the second order is zero, and the amplitude of the simply supported beam caused by the natural frequency of the fifth order is zeroThe point vibration pick-up point is
Therefore, the amplitude of the second-order natural frequency on the amplitude-frequency diagram is zero. Similarly, the third order mode of oscillation is:
in that
The amplitude is zero, so the third-order natural frequency of the third vibration pickup point is also zero.
The pulse excitation experiment shows that the amplitude-frequency diagrams measured by placing the acceleration sensors at different points on the cross beam are different, the main reason is influenced by the vibration mode, but the natural frequency of the simply supported beam can be measured by verifying that the vibration is picked at different points. Therefore, in order to reduce the influence caused by different vibration pickup point positions and measurement errors caused by vibration modes, the acceleration sensor is placed at the position of the second vibration pickup point in the experiment, because each vibration mode has larger vibration amplitude at the position, and the amplitude-frequency diagram obtained by measurement is more obvious and accurate.
As shown in fig. 12, in this embodiment, a sine wave signal with a stably changed frequency may also be used as the excitation signal, which is similar to a sine sweep signal and is also a sweep function of the excitation signal, a fast sweep of a small range near the frequency is performed, then FFT transformation is performed on the excitation signal and the input signal, and a spectrogram is formed by using two amplitude ratios and the frequency.
In the embodiment, a Chirp signal VI, a DAQmx reading VI, a virtual channel VI creation, a sampling clock VI, a DAQmx writing VI, and a task VI start are set;
the sine frequency scanner signal generator is provided with a formula node, and the calculation of the frequency change is carried out according to a formula in the formula node, wherein the formula is as follows:
in the formula (f)1As the starting frequency, f2To end the frequency, fiIs the output frequency.
The embodiment adopts a Chirp signal as an excitation signal to design the frequency scanner, and the Chirp signal is also called as a linear frequency modulation signal and is characterized in that the frequency of the signal is linearly and continuously changed along with time, the amplitude value is kept basically stable in a certain frequency range, and the frequency range can be set by adjusting the parameters of the Chirp signal. The Chirp signal can set any starting frequency and ending frequency, can measure higher-order natural frequency values, and can continuously excite the simply supported beam to cause smaller errors.
As shown in fig. 13, the excitation signal is generated by a Chirp Pattern VI using a Chirp signal as an excitation signal source, and the main parameters are shown in the following table 2:
TABLE 2 Chirp Pattern VI parameter Table
As shown in fig. 14, a virtual channel VI is created: the AI voltage is selected in this created virtual channel VI function, representing the output voltage signal, with the main parameters as shown in table 3:
table 3 creating a virtual channel VI parameter table
As shown in fig. 15, the sampling clock VI: the sampling clock VI configures the number of samples to be taken or generated and creates the required buffers, the main parameters of which are shown in table 4 below:
TABLE 4 sampling clock VI parameter table
As shown in fig. 16, DAQmx writes VI: writing data in a user-specified task or virtual channel, the main parameters are as shown in table 5 below:
TABLE 5 DAQmx write VI parameter Table
As shown in fig. 17, start task VI: the VI functions to put the DAQ task into operation, starting to measure or generate, the main parameters are shown in table 6 below:
TABLE 6 Start task VI parameters Table
The above part is the analog output part of DAQmx, and the sweep excitation can be realized by matching with a sine sweep scanner, and the VI of analog input is described as follows:
as shown in fig. 18, DAQmx reads VI: reading the sampled data in the user-specified task or virtual channel, wherein the parameters are shown in the following table 7:
TABLE 7 DAQmx read VI parameter Table
In the signal acquisition module, the DAQ timing VI and the DAQ starting VI are the same as the analog output, the virtual channel VI is established to be a polymorphic VI, the mode needs to be selected as the AI voltage, and the other parameters are the same.
Firstly, setting ports of collected signals and output signals on a data collection card and a wiring board by establishing a virtual channel VI, then setting sampling rate and sampling number by a timing VI, and setting a clock source of analog output on the timing VI of signal collection in order to synchronize input and output, so that the operation of analog input and analog output can run according to the timing signal of analog output, excitation data is written into a DAQ write-in VI cache, the DAQ starts VI to execute signal output and input, a timing node is arranged in a While circulating structure and is set as the sampling rate, so that the analog output is synchronized with Chirp signal output, and finally the DAQ clears task VI and clears the cache.
The excitation and response signals are converted into an array signal VI through dynamic signals and transmitted to an index array, then two signals, namely a Chirp excitation signal and a response signal of the device, are respectively taken out, FFT processing is carried out, amplitude values of the two signals are taken out to be divided, then the frequency of the response signal is taken as an abscissa, and an amplitude-frequency response oscillogram is created and displayed.
In the embodiment, software is adopted to generate a sine scanning excitation signal, the excitation signal and the simply supported beam vibration signal are simultaneously compared through a synchronous clock technology, the output-to-input amplitude ratio under any same frequency is measured, and theoretical guarantee is provided for accurately measuring the natural frequency.
The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.
Claims (10)
1. A simply supported beam natural frequency measurement system, comprising: the device comprises a piezoelectric acceleration sensor, a charge amplifier, a data acquisition card, a DAQmx module, a data preprocessing module, a signal analysis module, a vibration excitation signal unit, a power amplifier, an electromagnetic vibration exciter and a display recording module;
the piezoelectric acceleration sensor is connected with a charge amplifier, the charge amplifier is connected with a data acquisition card, the data acquisition card is connected with a DAQmx module, after being processed by the charge amplifier, a vibration signal of the simply-supported beam is transmitted to the input end of the data acquisition card, and the data acquisition card collects an analog signal;
the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware, the data preprocessing module is used for finishing filtering and windowing interference elimination processing of signals, the signal analysis module is used for carrying out FFT (fast Fourier transform) operation on input signals and output signals to obtain amplitude spectrum functions of the input signals and the output signals, the ratio of the amplitude spectrum functions to the output signals is taken to obtain a response characteristic curve, the excitation signal unit is used for generating an excitation signal source, the electromagnetic type vibration exciter is used for converting electric energy into mechanical energy and providing an excitation force for the simply supported beam, and the display recording module is used for displaying measured data.
2. The system for measuring the natural frequency of the simply supported beam as claimed in claim 1, wherein the piezoelectric sensor is a YD-12 type piezoelectric acceleration sensor, the data acquisition card is a PCI-6221 acquisition card, the charge amplifier is a BZ2101 single-channel charge amplifier, the power amplifier is a GF-20 power amplifier, and the electromagnetic vibration exciter is a JZ series electric vibration exciter.
3. The system for measuring the natural frequency of the simply supported beam as claimed in claim 1, wherein the excitation signal unit is excited by pulses and comprises: the DAQ assistant VI splits the signal VI, the oscillogram VI, waits for the next integral multiple millisecond VI, the frequency response function VI and the spectrum measurement function VI;
the DAQ assistant VI is used for setting a task of collecting data or outputting signals, the splitting signal VI is used for splitting the collected signals into pulse signals and simply supported beam vibration signals, the oscillogram VI is used for displaying nodes of waveforms, waiting for the next integral multiple millisecond VI is used for controlling the time interval between every two sampling data, the frequency response function VI is used for carrying out frequency response calculation on the two collected signals and outputting an amplitude spectrum and a phase spectrum, and the frequency spectrum measurement function VI is used for observing the frequency spectrum characteristics of the input pulse signals.
4. The simple beam natural frequency measurement system of claim 3, wherein the parameters to wait for the next integer multiple of milliseconds VI include a multiple of milliseconds to specify a time interval for VI run and a millisecond timer to return the wait time.
5. The system of claim 3, wherein the parameters of the frequency response function VI include: time signal X representing time waveform data X, time signal Y representing time waveform data Y, a window representing a window function, the amplitude representing an amplitude and frequency range returning an average frequency response, the phase representing a phase and frequency range returning an average frequency response.
6. The system for measuring the natural frequency of the simply supported beam as claimed in claim 1, wherein the excitation signal unit uses a sine wave signal as the excitation signal, and comprises: reading VI by a Chirp signal VI and DAQmx, creating a virtual channel VI, writing VI by a sampling clock VI and DAQmx, and starting a task VI;
the method comprises the steps that a Chirp signal VI is used for generating a Chirp signal serving as an excitation signal source, DAQmx reading VI is used for reading sampling data in a task or a virtual channel specified by a user, the virtual channel VI is created for selecting AI voltage and representing an output voltage signal, a sampling clock VI is used for configuring the number of samples to be acquired or generated and creating a required buffer area, the DAQmx writing VI is used for writing data in the task or the virtual channel specified by the user, and the starting task VI is used for enabling a DAQ task to be in a running state.
7. The measurement method of the simple beam natural frequency measurement system according to any one of claims 1 to 6, characterized by comprising the steps of:
generating an excitation signal source, converting electric energy into mechanical energy by an electromagnetic vibration exciter, providing an excitation force for the simply supported beam, detecting the vibration acceleration of the simply supported beam by a piezoelectric sensor, processing the vibration acceleration by a charge amplifier, transmitting a vibration signal of the simply supported beam to the input end of a data acquisition card, and collecting an analog signal by the data acquisition card;
the DAQmx module is used as a hardware drive and interface of a signal acquisition program and acquisition hardware;
filtering and windowing the signal to eliminate interference;
and performing FFT operation on the input signal and the output signal to obtain an amplitude spectrum function of the input signal and the output signal, taking the ratio of the input signal and the output signal to obtain a response characteristic curve, and displaying the measured data.
8. The measurement method of the natural frequency measurement system of the simply supported beam according to any one of claims 1 to 5, wherein the excitation signal source employs pulse excitation, a plurality of vibration pickup points are equally divided on the simply supported beam, the acceleration sensor is sequentially placed on the vibration pickup points, and the simply supported beam is hammered by a pulse hammer each time, a frequency characteristic curve measured by the plurality of vibration pickup points is recorded, the pulse excitation signal and the response signal are converted into voltage signals by a charge amplifier for a data acquisition card to acquire data, and finally the voltage signals are input into a DAQ assistant VI, a splitting signal VI, a waveform diagram VI, a frequency response function VI and a spectrum measurement function VI for processing and displaying.
9. The measurement method of the system for measuring the natural frequency of the simply supported beam according to claim 8, wherein the number of the vibration pickup points is set to 9, and an amplitude-frequency pattern obtained by measuring the position of the second vibration pickup point is selected for data processing.
10. The measurement method of the natural frequency measurement system of the simply supported beam according to claim 1 or 6, characterized in that the excitation signal source adopts a Chirp signal, ports of collected signals and output signals on a data collection card and a wiring board are set by creating a virtual channel VI, then a sampling rate and a sampling number are set by a timing VI, a clock source of analog output is set on the timing VI of signal collection, excitation data is written into a DAQ write VI cache, and the DAQ starts VI execution signal output and input;
the method comprises the steps of converting excitation and response signals into array signals VI through dynamic signals, transmitting the array signals VI to an index array, respectively taking out a Chirp excitation signal and the response signals of the device, performing FFT (fast Fourier transform) processing, taking out amplitude values of the two signals, dividing the amplitude values, taking the frequency of the response signals as a horizontal coordinate, and creating an amplitude-frequency response oscillogram for display.
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