The signals that can be input to analysis devices such as FFT analyzers are voltage signals.
When measuring physical quantities such as acceleration, velocity, displacement, force, and sound pressure, detectors (sensors) that convert these physical quantities into voltages are used. A laser Doppler vibrometer measures the vibration velocity of an object non-contact using laser light and outputs a voltage proportional to the vibration velocity (m/s). There are also devices such as laser displacement meters that output a voltage proportional to the displacement (m, mm, μm, etc.).
This time, we will introduce how to set up the analysis equipment when measuring vibration velocity and displacement.
In the case of a laser Doppler vibrometer
A laser Doppler vibrometer measures the vibration velocity of an object by utilizing the Doppler effect of a laser beam. The VELOCITY OUT terminal of the laser Doppler vibrometer outputs a voltage signal proportional to the vibration velocity.
Figure 1 shows an example of connecting a laser Doppler vibrometer to an analysis device.
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Figure 1: Example of connection between a laser Doppler vibrometer and an analysis device.
The measurement range of the laser Doppler vibrometer is set to match the magnitude of the vibration velocity of the object being measured. Our LV-1800 laser Doppler vibrometer has three measurement ranges: 1 (m/s)/V, 0.1 (m/s)/V, and 0.01 (m/s)/V (four ranges when the LV-0800 option is added), each indicating the vibration velocity in m/s at which 1V is output. Figure 2 shows how to switch the measurement range of our LV-1800 laser Doppler vibrometer.
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Figure 2: Method for switching the measurement range of a laser Doppler vibrometer.
Figure 3 shows how to configure the DS-3000 series acoustic vibration analysis software. Set the EU type to "EU/V", leave the 0 dB reference value at 1 (normally), and leave the offset at 0 dB.
If the laser Doppler vibrometer's measurement range is 0.1 (m/s)/V, the unit name should be set to "m/s" and the EU value to "0.1", which is the value of the measurement range.
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Figure 3: Example of settings in DS-3000 series software.
In the case of a laser interferometer
A laser interferometer measures the displacement of an object by continuously counting the interference fringes that change with amplitude. The analog output of the displacement meter unit outputs a voltage signal proportional to the object's displacement. Similar measurements can be performed by connecting a digital displacement meter unit to our laser Doppler vibrometer.
Figure 4 shows an example of connecting a laser interferometer and an analysis device.
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Figure 4: Example of connection between laser interferometer and analysis device
Depending on the measurement purpose, switch the displacement sensor unit's coupling between AC and DC.
The measurement range of the displacement meter unit is set to match the displacement of the object being measured. Our laser interferometer LV-1200A and digital displacement meter unit LV-0121A have nine measurement ranges: 0.1 μm/V, 1 μm/V, 5 μm/V, 10 μm/V, 100 μm/V, 2 mm/V, 0.1 m/V, 0.5 m/V, and 1 m/V, each indicating the displacement in meters at which 1 V is output. Figure 5 shows how to switch the measurement range of our laser interferometer.
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Figure 5: Method for switching the measurement range of the laser interferometer.
Figure 6 shows how to configure the DS-3000 series acoustic vibration analysis software. Set the EU type to "EU/V". The 0 dB reference value is usually left at 1, and the offset is left at 0 dB.
If the measurement range of the laser interferometer is 100 μm/V, set the unit name to "m" and the EU value to "0.0001", which is the measurement range value converted to meters (m). You can also set the unit name to "mm" and the EU value to "0.1", or set the unit name to "um" (μm) and the EU value to "100".
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Figure 6: Example of settings in DS-3000 series software.
Other speedometers, displacement meters
In the previous sections, we introduced the setup methods for connecting laser Doppler vibrometers and laser interferometers, but the setup for analysis devices is the same even if you are using speedometers or displacement meters, due to other principles.
For a speedometer, you set how many m/s it takes for 1 V to be output from the speedometer. For a displacement sensor, you set how many meters (or mm, or μm) it takes for 1 V to be output from the displacement sensor.
summary
This time, we introduced how to set up speedometers and displacement meters, such as laser Doppler vibrometers and laser interferometers, when connecting them to analysis equipment such as FFT analyzers for analysis.
(Excerpt from the email newsletter issued on February 19, 2015)
