JPH0445098B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an irregular percussion excitation method used when measuring the frequency response function of an object to be measured such as a machine or a structure.

[Conventional technology]

Traditionally, in order to understand the dynamic characteristics of machines and structures, that is, the external forces applied to them and their dynamic responses such as vibrations, it is sufficient to measure their transfer functions (frequency response functions, etc.). It is said that Among them, there are various methods for measuring the frequency response function, as shown in Fig. 4 a, b, and c, but the simplest of these is the percussion excitation method shown in Fig. 4 c. This method uses a force detector 30a (load cell)
It is only necessary to hit the object to be measured such as the structure 10 with the attached impact hammer 30, and there is no need to install a vibrator 60 as in the other vibration methods shown in a and b in the same figure, so it is easy to use on site. can be executed. Instead,
It cannot be denied that this method is disadvantageous in accuracy in many respects compared to the method using the vibrator 60. Specifically, since the impact is performed manually, the excitation energy is relatively small.

Since the excitation waveform is impulse-like as shown in FIG. 5, the excitation energy is concentrated for a very short time after the impact, and thereafter becomes zero. Therefore, the crest factor becomes high, and it is susceptible to the influence of noise and nonlinearity of the structure 10 and the like.

There are drawbacks such as:

Therefore, especially in the case of a large structure, the excitation response becomes small and the measurement accuracy of the frequency response function tends to decrease.

One method to overcome this drawback is the irregular impact vibration method. This is as shown in Figure 6.
This is a method of performing multiple hits within the time window of the FFT device 20. This allows the crest factor of the excitation waveform to be lowered and the excitation energy to be increased. If the number of strikes is N, the excitation energy increases by about 10 logNdB.

[Problem that the invention seeks to solve]

In this way, in the irregular percussion excitation method, it is possible to increase the excitation force by the percussion excitation method, but
However, it has the following drawbacks.

○A Since one person swings the hammer 30, there is a limit to its excitation force.

○B Since one person swings the hammer 30, the period tends to be approximately periodic, and in this excitation method, which requires a constant time interval between strikes for measurement accuracy, the spectrum of excitation force is is discontinuous,
Or it becomes something close to it, and the accuracy of frequency response function measurement decreases.

The present invention was devised in view of the above-mentioned problems, and aims to improve the accuracy of frequency response function measurement by making the time intervals of impact irregular in the irregular impact excitation method. .

[Means for solving problems]

Therefore, as shown in FIG. 1, in the impact vibration method of the present invention, two or more impact hammers 3 simultaneously apply irregular impact vibration to an object 1 to be measured such as a structure. This method is hereinafter referred to as the multi-input irregular impact excitation method.

The number of vibrations to be applied may be any number as long as it can hit approximately the same point. Furthermore, in addition to hitting the same point from the same direction, it is also possible to hit from the front and back, for example, as shown in Figure 1. ).

As shown in FIG. 1, the excitation force signals from the force detectors 3a of the plurality of impact hammers 3 are added together by an electric adder 4 to become one excitation force signal, and after this, normal impact is performed. Similar to the vibration experiment, the vibration response signal from the accelerometer 5 and the like is input to the analysis device 2, and a frequency response function is calculated. Note that the signals from the hammers 3 striking in the same direction are sent directly to the adder 4.
However, the signal from the hammer 3 striking in the opposite direction is added to the adder 4 after having its phase inverted.

In this method, the excitation energy increases by about 10 logMdB compared to the conventional irregular impact excitation method (here, M is the number of impacts). Furthermore, the excitation waveform becomes more irregular as shown in FIG. 2, improving measurement accuracy.

〔Example〕

A low-speed diesel engine frame with a total weight of approximately 1000 tons,
The frequency response function was measured by applying the following four excitation methods.

(a) A hydraulic vibrator was installed on the No. 1 cylinder, and a sine wave vibration method was performed.

(b) A striking vibration method was performed in which the No. 1 cylinder was struck only once with the impact hammer.

(c) An irregular impact excitation method was implemented in which the No. 1 cylinder was hit as irregularly as possible by one person using an impact hammer.

(d) A multi-input irregular impact excitation method according to the present invention was carried out in which the No. 1 cylinder was hit irregularly by two people at the same time with an impact hammer.

The frequency response function measured by the above method is
It is shown in figures a to d (the figures a to d correspond to the above-mentioned vibration methods a to d, respectively). As can be seen from these figures, the sine wave excitation method (a) is naturally the most beautiful, but the second best is the multi-input irregular percussion excitation method (d) according to the present invention, and the natural frequency and This data is practically sufficient for the purpose of investigating mode shapes. In the percussion excitation method b and the irregular percussion excitation method c, the low-order modes in particular are disturbed, and it is difficult to say that the data is usable.

As described above, it can be seen that by using the new vibration method according to the present invention, the frequency response function of even a large structure such as a low-speed diesel engine can be measured relatively easily.

〔Effect of the invention〕

According to the multi-input irregular impact excitation method of the present invention as described above, the excitation energy can be increased compared to the case of excitation by one person, and the excitation waveform becomes more irregular, Since it is possible to prevent the excitation spectrum from becoming discontinuous, the frequency response function of large structures, etc., which could previously only be measured accurately using the sine wave excitation method, can now be measured practically using the percussion excitation method. It has become possible to measure with sufficient accuracy, and therefore it has become possible to significantly reduce the cost and time of vibration experiments.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an implementation method of the multi-input irregular impact excitation method according to the present invention, FIG. 2 is a graph diagram showing an example of an excitation waveform obtained by the excitation method of the present invention, and FIG. Figures a, b, c, and d are graphs showing the frequency response functions obtained when implementing the method of the present invention and the conventional method. Fig. 4(c) shows one using the irregular percussion excitation method, Fig. 4(d) shows one using the multi-input irregular percussion excitation method according to the present invention, and Fig. 4a, b, c is an explanatory diagram showing the conventional measurement method of the frequency response function, where a shows the sine wave excitation method, b shows the irregular vibration method, and c shows the method of measuring the frequency response function in the past.
shows the percussion excitation method, Fig. 5 is a graph showing the excitation waveform during excitation using the percussion excitation method, and Fig. 6 is a graph showing the excitation waveform during irregular percussion excitation. be. In the figure, 1 is an object to be measured, 10 is a structure, 2 and 20 are analysis devices, 20a is a frequency response function measuring device,
3 and 30 are impact hammers, 3a and 30a are force detectors, 4 is an adder, 5 is an accelerometer, and 60 is a vibrator.

Claims (1)

[Claims] 1 Hit the object to be measured with an impact hammer,
In the irregular impact excitation method, the excitation force of the impact hammer and the vibration response of the object to be measured are measured and a frequency response function of the object to be measured is determined from these signals. In this case, multiple impact hammers are used to strike irregularly at the same time, and if the respective excitation force signals are generated when they are struck from the same direction, they are added as is; on the other hand, when they are struck from the opposite direction, If the signal was generated when the object was hit by a vehicle, the phase is inverted and added, thereby creating a single excitation force signal. A multi-input irregular percussion excitation method characterized by determining the frequency response function of the object to be measured.