In the process of vibration, the parameters to be measured include amplitude, frequency and acceleration. Figure 1 shows a part of the test method. In figure 1(a), the weight 2 is suspended on the spring 1, and the light emitted by the light source 3 is reflected by the diaphragm 4 and the mirror 5, and then projected onto the photoelectric device 6. When the weight 2 vibrates, the mirror 5 vibrates with it, thus causing the change of luminous flux in the photoelectric device 6. In fig. 1(b), the photoelectric sensor has a prism 5 that vibrates together with the weight 2. If the luminous flux on the photoelectric device 6 increases, the luminous flux on the other photoelectric device 6' decreases, and vice versa. Two photoelectric devices are connected to form a bridge circuit with good sensitivity and stability. The photoelectric sensor in figure 1(c) has a pair of gratings 5 and 5'. The grating 5 is fixed, and the grating 5' vibrates up and down with the weight 2, so the luminous flux projected on the photoelectric device 6 also changes accordingly. Its advantage is high sensitivity and it is suitable for small displacement measurement.

Schematic diagram of vibration measurement by photoelectric sensor

Fig. 2(a) shows the principle of measuring the vibration of a rotating body or a rotating shaft. Next to the shaft or rotating body 2, a fixed shading plate 3 is installed, and its height can be moved up and down by screws. When the rotating body 2 is stationary, a gap with a width of 0 is formed between its bus bar and the light shielding plate 3. If the rotating body is asymmetric or eccentric, the width of the gap will change when it rotates, so the luminous flux and photocurrent generated by the light source 1 will also change. After amplification, it will be recorded by a recorder or displayed by an oscilloscope, so that the frequency and amplitude of vibration of the rotating body can be tested.

Measurement of vibration of rotating body by gap size change

In order to improve the sensitivity of the instrument, two photoelectric devices can be used. When the rotating body vibrates, the current on one photoelectric device increases and the current on the other photoelectric device decreases. The principle is as follows: when the rotating body 4 does not vibrate, the light emitted by the light source 1 is reflected by the mirrors 2 and 3, and the luminous fluxes on the photoelectric devices 5 and 6 are equal. Because the bridge is now balanced, there is no input signal in the amplifier 7. When the rotor vibrates in the vertical direction, the gap above the shaft increases and the gap below it decreases (and vice versa), so the luminous flux on the photoelectric devices 5 and 6 is different, and the bridge is out of balance, and there is voltage output, which can be recorded by the meter 8 after being amplified by the amplifier 7.

Schematic diagram of laser measuring vibration of high-rise building

The picture shows the schematic diagram of measuring wind-induced vibration displacement and frequency of high-rise buildings with laser beams. The whole device consists of transmitting and receiving parts. The laser emitting part 1 is placed on the static foundation beside the building, and the power is 0.4~ 1 MW. The single-mode He-Ne laser emits a beam with an emission angle of radian, which is reflected to the receiving target 4 at the top of the building through the internal focusing transmitting telescope 2 and the right-angle prism 3. The receiving part is installed at the top of the building, and consists of receiving target 4, amplifier and arithmetic unit 5, recorder and oscilloscope 6. The receiving target is between four or two photovoltaic cells, and the differential amplification has no output. When the building vibrates due to wind and other reasons, the position of the light spot in the target changes, and its waveform can be displayed by recorder or oscilloscope after amplification and operation.

Secondly, the tiny vibration is measured by grating and oscilloscope.

Measurement of micro-vibration with grating and oscilloscope

The picture shows the principle of measuring micro-vibration by grating and oscilloscope. The light emitted by the light source 1 is directed to the gratings 3 and 4 in parallel through the lens 2, and there are 100 lines on the gratings 3 and 4 every millimeter. The grating 3 is fixed, and the moving grating 4 vibrates with the measured vibrating body. During installation, the reticle of the grating 3 is inclined at a slight angle relative to the reticle of the grating 4, and they form moire fringes under the irradiation of parallel light. When the moving grating 4 moves in the opposite direction perpendicular to the reticle relative to the fixed grating 3, the moire fringe will move in the direction parallel to the reticle. When the grating moves by one grating pitch, the moire fringe moves by one fringe width.

The change of moire fringes is received by photoelectric devices, and their respective fields of view are projected onto photoelectric devices 6, 7, 8 and 9 through the combination of prism and lens 5. The arrangement of photoelectric devices makes their signals differ by 90 degrees, and these signals are amplified by amplifiers 10, 1 1, 12 and 13 respectively. Assuming that the signals of photoelectric devices 6 and 8 are in opposite phases, they are amplified and applied to the horizontal deflection plate; The signals of photoelectric devices 7 and 9 have opposite phases and are amplified and applied to the vertical deflection plate. When the movable grating 4 vibrates with the measured object, the bright and dark areas of moire fringes change constantly, and the illuminance obtained by photoelectric devices also changes constantly. According to the principle of Lissajous diagram of oscilloscope, after amplification, an arc appears on the screen of electron ray tube 14, and the arc length of the arc is proportional to the amplitude of vibration. When the circular arc is closed to form a circle, the amplitude of vibration is equal to the pitch of the grating. Therefore, if the arc angle is marked on the screen of the X-ray tube in advance, the vibration amplitude can be measured from the arc length.

Three, interference vibration measurement method

Michelson interferometer can be used as an interference device for measuring mechanical vibration. The light intensity of the interference fringe can be obtained by combining the sum of the light intensities of the reference beam and the measuring beam (excluding reflection and other losses):

In the formula:

ψ —— phase difference between reference beam and measuring beam;

λ-monochromatic light wavelength;

-fixing the distance between the reference mirror and the beam splitting prism;

-The distance between the movable measuring mirror and the beam splitter prism;

χ —— the distance that the measured object moves with the vibration of the measured object.

If the measured object resonates, i.e.

Substitution, and then the interference fringe intensity change part obtained by substitution is.

If the photoelectric device works in the linear region, the output AC photoelectric signal is:

The relationship between x and u waveforms

The waveforms of X and U are shown in the above figure, and the photoelectric signal U is a periodic modulated wave. This is because the frequency of photoelectric signal is equal to the frequency of light and dark changes of interference fringes, that is, it is proportional to the speed of vibration displacement. At the upper limit position, … and the lower limit position, … of vibration displacement, the moving speed is the smallest, the interference fringe moves the slowest, the instantaneous frequency is the smallest, and the photoelectric signal density is the smallest. At the equilibrium position of vibration displacement, ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

If the moving measuring mirror moves, the light beam changes back and forth and the interference light changes once, a pulse can be obtained. The time between and is a vibration period, and one period of vibration is equivalent to change, so the number of counting pulses corresponding to one period of vibration can be obtained by the following formula, or the amplitude of vibration can be obtained by the number of pulses counted by the counter:

Because of the good monochromaticity, coherence and wavelength stability of laser, laser interferometry has been used as a higher benchmark for vibration measurement in various countries.

Measuring vibration by laser can be ignored because its wavelength error is very small (

When the vibration frequency is 0, the fringe frequency recorded by the counter is:

The ratio of design frequency to measured vibration frequency is "frequency ratio":

Therefore, we can get the relationship between the measured amplitude and,

In actual measurement, when the frequency of fringe change is recorded by a counter, it is recorded by a counter gate circuit controlled by the measured vibration frequency, so the number displayed by the counter is the frequency ratio, which can be obtained by knowing the wavelength of the light source.

Fig. (a) is a schematic diagram of a vibration meter, and fig. (b) is a block diagram of a counter of the vibration meter.

The control logic of the counter is to convert the measured simple harmonic vibration into an electrical signal through the vibration pickup, amplify and shape it through the shaper (2) to form a vibration pulse, and send it to the gate (2) controlled by the trigger (2). When the sampling pulse generator sends out the sampling pulse, flip the trigger (2), and the gate (2) is divided into two ways: one way is to turn the trigger (1) and open the gate (1), so that the photoelectric signal of the interference fringe passes through the amplifier and shaper (1) and enters through the gate (1). The other path leads to the frequency divider, which works according to the required average number of vibration cycles. For example, if 8 vibration cycles are used, 9 frequency divisions will be used. When the first vibration pulse opens the door (1), the frequency divider will not output a pulse until the ninth vibration pulse enters the frequency divider. This frequency-division pulse acts on the flip-flops (1) and (2) respectively, causing them to turn over, and the doors (1) and (2) are closed at the same time, so that the continuous vibration pulse can no longer open the doors (1), which plays a self-locking role. At this time, the digital tube displays the RF reading. When the sampling pulse generator outputs the next pulse, on the one hand, the counter and frequency divider are reset to zero, and at the same time, the door (2) is opened to repeat the above measurement process.

I copied it, ashamed.