JPH0450628A - Shaking method for mechanical structure for measuring frequency response function - Google Patents

Abstract

PURPOSE:To prevent a truncation error substantially and to enhance the accuracy in measurement of a frequency response function by feeding back the free vibration data of a mechanical structure for a specified time. CONSTITUTION:The output from an irregular-wave generator 3a is intermittently connected and disconnected with a switch 9a. The output becomes the burst irregular wave. The wave is inputted into a shaker 2 through an adder 10. The acceleration response signal of an object 1 which is a mechanical structure and which is measured with an accelerometer 5 is converted into the acceleration signal with an amplifying and integrating device 7a. Thereafter the signal is inverted and inputted into the adder 10 through a switch 9b. The switch 9a is opened after the specified time. At the same time when the excitation of the object 1 is finished, the switch 9b is closed. The speed signal from the amplifying and integrating device 7a acts as a feedback signal. Therefore, the truncation error in the FET operation in a frequency analyzer 8 is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a vibration excitation method in experimental mode analysis for identifying vibration characteristics of mechanical structures.

[Prior Art] In experimental modal analysis for identifying the vibration characteristics of a mechanical structure, the first step is to perform an excitation experiment on the object and measure its frequency response function.

Since the measurement accuracy of this frequency response function has a great influence on the subsequent mode analysis, it is extremely important to minimize the errors that occur at this stage.

FIG. 2 is a configuration diagram of frequency response function measurement using the conventional vibration method disclosed in Japanese Patent Application Laid-Open No. 60-227136. In the figure, (1) is an object that is a mechanical structure, (
2) is a vibrator for forcibly vibrating the object (1);
(3) is a broadband wave generator for driving the vibrator (2); (4) is a force detector that measures the excitation force of the vibrator (2);
(5) is an accelerometer that measures the vibration response of the object (1);
(13) and (7) are amplifiers that amplify the signals from the force detector (4) and the accelerometer (5), respectively, and (8) is where the signals from these amplifiers (8) and (7) are input. This is a frequency analyzer.

Next, the operation will be explained. First, a broadband wave generator (
In step 3), the vibrator (2) is driven to forcibly vibrate the object (1). The excitation force at this time is measured by a force detector (4), and the vibration response of the object (1) is measured by an accelerometer (5). These measurement signals are then analyzed by a frequency analyzer (8).
and calculate the frequency response function using FFT operation.

By the way, in FFT operation, which is a finite Fourier transform, only a part of the waveform is cut out by applying a window function on the time axis to the target waveform, and the operation is performed on this part of the waveform, so the original time waveform is It is assumed that the length of the function is the period. In other words, since we are assuming an infinitely long waveform in which the cut waveforms are repeatedly arranged, the amplitude at the end of the time window and the amplitude at the beginning must be continuous. if,
If this condition is not met, a large error called truncation error will occur.

Incidentally, a pure irregular wave is basically suitable as the broadband wave used in the above embodiment. This is because, due to the statistical nature of pure irregular waves, the influence of nonlinearity of mechanical structures can be reduced and only the linear components necessary for modal analysis can be obtained. This purely irregular wave is not periodic in nature, so no matter what length of time window you use,
The length of the time window is not defined as a period. That is, the last value and the first value of the time window cannot be continuous, and therefore, when performing FFT calculation, a truncation error always occurs.

For this reason, conventionally, the pure irregular wave was not used as it was, but the third
A burst irregular wave was used that generated a purely irregular wave only partially within the time window as shown in the figure. Since the burst irregular wave has an amplitude of 0 at the beginning and end of the time window, it can be regarded as having a period with this time window length, and therefore, truncation errors can be prevented.

However, when performing frequency response function measurements, even if the burst irregular wave itself is not accompanied by a truncation error, the problem of truncation error still occurs. Figure 4 shows the waveforms at each point in Figure 2 when a burst irregular wave is used to drive the vibrator (2), and (a) is the waveform that drives the vibrator (2). , (b) is the waveform of the actual excitation signal measured by the force detector (4), and (c) is the waveform of the response signal measured by the accelerometer (5). According to the figure, since the mechanical structure has a limited damping capacity, even after the amplitude of the burst irregular wave given to the excitation am(2> becomes O, the mechanical structure remains free to oscillate. Therefore, even if the initial amplitude of the time window is 0, the final amplitude of the response signal waveform is not 0, and therefore a truncation error occurs.

Conventionally, as a countermeasure to this problem, a method has been used to shorten the generation time of pure irregular waves and lengthen the time for free vibration to decay. FIG. 5 shows this, in which (a) is a burst irregular wave whose generation time is shortened, and (b) is a waveform diagram of a response signal of a mechanical structure due to the burst irregular wave of (a).

[Problems to be Solved by the Invention] However, since the free vibrations of mechanical structures are attenuated exponentially, the above method does not provide an essential solution, and it is not possible to completely eliminate the truncation error ( In addition, shortening the excitation time as shown in Figure 5 reduces the excitation energy applied to the target mechanical structure, which in turn reduces the power of the excitation signal and response signal to be measured. Therefore, it becomes relatively susceptible to the influence of external dark vibrations and electrical noise from sensors, leading to a decrease in measurement accuracy.

This invention has been made to solve the above problems, and can essentially prevent truncation errors, increase the generation time of the excitation signal, and improve the accuracy of frequency response function measurement. The purpose of this study is to obtain a vibration excitation method for mechanical structures.

[Means for Solving the Problems] A method of exciting a mechanical structure for measuring a frequency response function according to the present invention includes exciting a mechanical structure by applying an excitation signal to a vibrator for a predetermined period of time. Immediately after this excitation ends, information on the free vibration of the mechanical structure is fed back to the exciter.

[Function] In this invention, immediately after the end of excitation for a predetermined time, information on the free vibration of the mechanical structure is fed back to the exciter, and this acts to apparently increase the damping coefficient of the mechanical structure. The free vibration of the mechanical structure is damped in a short time after the excitation is finished.

[Example] FIG. 1 is a configuration diagram of frequency response function measurement using an excitation method according to an example of the present invention.

In the figure, the same reference numerals as in FIG. 2 indicate the same or corresponding parts, and the explanation will be omitted. (3a) is an irregular wave generator, (7a) is an amplification and integrator that amplifies the signal from the accelerometer (5) and converts it into a speed signal. (9a) is an irregular wave generator. The first switch (9b) is a first switch that connects and disconnects the output from the generator (3a), and the second switch (9b) connects and disconnects the output from the amplification and integrator (7a).

This first switch (9a) and second switch (9b)
are synchronous, when the first switch (9a) is closed, the second switch (9b) is open, and when the first switch (9a) is open, the second switch (9b)
is configured to be closed. (10) is an adder whose input side is connected to the first switch (9a) and the second switch (9b), and whose output side is connected to the vibrator (2).

Next, the operation will be explained. The output from the irregular wave generator (3a) is interrupted by the first switch (9a) to become a burst irregular wave, which is then sent to the exciter (10) via the adder (10).
2). At this time, the acceleration response signal of the object (1), which is a mechanical structure, measured by the accelerometer (5) is
After being converted into a speed signal by the amplification and integrator (7a), the signal is inverted and input to the adder (10) via the second switch (9b). This speed signal is a feedback signal to the exciter (2), but while the first switch (9a) is closed and the irregular wave is being excited, the second switch (9b) is closed. Since it is open, there is no feedback.

However, after a predetermined period of time has elapsed, the first switch (9a)
opens, and at the same time the excitation to the object (1) ends, the second
switch (9b) is closed and the amplifier and integrator (7a)
The speed signal from the motor acts as a feedback signal. This speed feedback has the effect of apparently increasing the damping coefficient of the mechanical structure, so that the free vibration of the mechanical structure is damped in a short period of time after the excitation ends.

Therefore, truncation errors in the FFT calculation in the frequency analyzer (8) are prevented.

In addition, although irregular waves were used to drive the vibrator in this embodiment, other broadband waves may be used. Further, although mechanical switches are used as the first switch and the second switch, equivalent means such as electrical switching elements may be used.

Furthermore, when inputting the burst irregular wave directly to the adder without going through the first switch, the second switch that controls the feedback signal may be synchronized with the burst irregular wave signal.

In addition, in this example, the speed signal of the mechanical structure was fed back, but if there is something that acts to limit the free vibration of the mechanical structure, that signal may be fed back, and the free vibration for feeding back is also possible. In order to obtain vibration information, a sensor such as an accelerometer may not be used, and the back electromotive force from the vibrator may be used.

[Effects of the Invention] As described above, according to the present invention, immediately after exciting the mechanical structure for a predetermined period of time, information on the free vibration of the mechanical structure is fed back to the exciter. The free vibrations are damped in a short time, essentially preventing truncation errors, and increasing the generation time of the excitation signal.
The accuracy of frequency response function measurement can be improved.

[Brief explanation of drawings]

Fig. 1 is a block diagram of frequency response function measurement using an excitation method according to an embodiment of the present invention, Fig. 2 is a block diagram of frequency response function measurement using a conventional excitation method, and Fig. 3 is a block diagram of frequency response function measurement using an excitation method according to an embodiment of the present invention. Figure 4 is a waveform diagram showing a regular wave. Figure 4 is a waveform diagram showing the signal at each point when a burst irregular wave is used to drive the vibrator in Figure 2. , is a waveform diagram showing a burst irregular wave whose generation time is shortened and a response signal of a mechanical structure in that case. In the figure, (1) is an object that is a mechanical structure, (2)
is an exciter, (3a) is an irregular wave generator, (4) is a force detector, (5) is an accelerometer, (6) is an amplifier, (7a) is an amplification and integrator, (8) is a frequency In the analyzer, (9a) is a first switch, (9b) is a second switch, and (10) is an adder. In addition, in each figure, is shown. Same symbols indicate the same or equivalent parts

Claims (1)

[Claims] A mechanical structure is excited by applying an excitation signal to a vibrator for a predetermined period of time, and immediately after the excitation ends, information on free vibration of the mechanical structure is fed back to the vibrator. , a method of excitation of mechanical structures for frequency response function measurement.