Measurement of natural frequency, without touch
Funded by KaiQuan Pump group
Tradition impulse excitation method relys on hammer to exert impulse on measured system. There are two main drawbacks of this method. One is the limited input energy, which causes a low signal to noise rate (SNR) of response. The other drawback is that the hammer can only knock static surface, which indicates it is not suitable when machinery is rotating. However, natural frequency of machinery is related to its rotating speed because of gyroscopic effect. New device should be developed to measure natural frequency of rotating machinery.
A electromagnetic exciter can effectively solve these two drawbacks of impulse method, by giving system a series of non-touch periodic impulse. The principle is that the exciter can generate magnetic force when it was charged with current. Through controlling the charged current in exciter, desired periodical force can be achieved. Because the magnetic force doesn't need contact object, this exciter can realize non-touch exciting. The periodical force is a series of impulse. Exerting this periodical impulse on measured system, input energy can be increased significantly and drawback of limited energy of impulse method can be avoided. Experimental result shows excellent performance of this exciter in measuring natural frequency.
A electromagnetic exciter can effectively solve these two drawbacks of impulse method, by giving system a series of non-touch periodic impulse. The principle is that the exciter can generate magnetic force when it was charged with current. Through controlling the charged current in exciter, desired periodical force can be achieved. Because the magnetic force doesn't need contact object, this exciter can realize non-touch exciting. The periodical force is a series of impulse. Exerting this periodical impulse on measured system, input energy can be increased significantly and drawback of limited energy of impulse method can be avoided. Experimental result shows excellent performance of this exciter in measuring natural frequency.
Electromagnetism analysis of exciter
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Fig. 1 meshing figure of exciter
The exciter is composed of iron core and copper wire. The copper wire winds around two teeth of iron core. The iron core is maded up by piling up silicon steel sheet. There is a clearance between rotor and teeth. Usually, the clearance is 0.2~0.4mm. When charging wire with current, the iron core generates magnetic force, draging rotor down. Stoping providing exciter with current, the magnetic force disappears and the rotor springs back. This kind of draging and springback form a impulse to the system, and system's natural frequency can be gotten through measuring impulse response and conducting Fourier Frequency Transform (FFT).
The aim of electromagnetism analysis is to find the reasonable structure of exciter. That is, what kind of shape can generate force as large as possible when inputing same current. It is found the thickness of two teeth is a key factor. Broadening the thickness, the exciter can significantly increase its magnetic force. It is also found the force is almost propotional to the input current. While too large current will lead to saturation of magnetic circuit. Typically, the magnetic force can be around 800N when current is 2.4A (wire diameter is 0.5 mm), coiling numer of wire is 700.
The aim of electromagnetism analysis is to find the reasonable structure of exciter. That is, what kind of shape can generate force as large as possible when inputing same current. It is found the thickness of two teeth is a key factor. Broadening the thickness, the exciter can significantly increase its magnetic force. It is also found the force is almost propotional to the input current. While too large current will lead to saturation of magnetic circuit. Typically, the magnetic force can be around 800N when current is 2.4A (wire diameter is 0.5 mm), coiling numer of wire is 700.
Experiment of exciter
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Fig. 2 periodical impulse vs transient impulse
I have used this exciter to measure natural frequency of a multiple rotors test-rig. In this experiment, the rig is running around 2916rpm. Vibration signal is sampled by eddy current sensor and DAQ system. Two excitation methods are used to measure natural frequency of this system. One is traditional impulse method, that is, giving system single impulse. Another method is periodic impulse method realized by the electromagnetical exciter, giving system multi-impulse.
When giving system single impulse, and measuring its vibration response. It can be seen that the natural frequency is hard to find in frequency spectrogram after conducting FFT to the measured data. This is because the input impulse energy is too small to excit the whole system and natural frequency is overwhelmed by other signals. In contrast, providing the rotor system a periodical impulse (the frequency is two impulse per second), the FFT result of corresponding response shows clearly natural frequency in its spectrogram. The comparsion between periodic impulse and single impulse can be seen in Fig. 2. For traditional impulse method, the natural frequency is overwhelmed by noise and hard to be recognized. However, the periodic impulse method effectively avoid this drawback by providing system more impulse energy. It should be mentioned that sideband will appear in periodical impulse. This sideband means the impulse frequency can not be too high, otherwise the resolution of this method will become low.
Through experiment, it is found that natural frequency of one system can be more easily recognized by periodic impulse than by single impulse. The self-developed exciter can realize this periodical impulse method. Moreover, because of simple structure and small size of this kind exciter, it can have a wide application in field of measuring natural frequency for rotating machinery. Back to list >>
When giving system single impulse, and measuring its vibration response. It can be seen that the natural frequency is hard to find in frequency spectrogram after conducting FFT to the measured data. This is because the input impulse energy is too small to excit the whole system and natural frequency is overwhelmed by other signals. In contrast, providing the rotor system a periodical impulse (the frequency is two impulse per second), the FFT result of corresponding response shows clearly natural frequency in its spectrogram. The comparsion between periodic impulse and single impulse can be seen in Fig. 2. For traditional impulse method, the natural frequency is overwhelmed by noise and hard to be recognized. However, the periodic impulse method effectively avoid this drawback by providing system more impulse energy. It should be mentioned that sideband will appear in periodical impulse. This sideband means the impulse frequency can not be too high, otherwise the resolution of this method will become low.
Through experiment, it is found that natural frequency of one system can be more easily recognized by periodic impulse than by single impulse. The self-developed exciter can realize this periodical impulse method. Moreover, because of simple structure and small size of this kind exciter, it can have a wide application in field of measuring natural frequency for rotating machinery. Back to list >>