No.2 Various measurement instrument : Sensors for vibration measurement
In the previous column, I introduced "Why we measure sound and vibration?" This time, I will introduce the types and features of detectors for vibration measurement.
When you hear “the general detector for vibration measurement”, what you come to mind immediately is an acceleration detector that is used in contact with the measurement target. The Ono Sokki’s accelerometers use piezoelectric elements for detection, and by vibrating together with the measurement target, they output signals proportional to the vibration acceleration (m/s2). Since the piezoelectric detector does not respond to DC acceleration, it cannot measure in a stationary state or a state near stationary. The frequency range for small accelerometers is typically from a few Hz.
There are some types of the contact type accelerometers. Let me introduce them.
Accelerometer with built-in preamplifier
The accelerometer with built-in preamplifier is a detector that outputs the voltage signals (unit: V) proportional to the vibration acceleration. It is relatively noise-resistant and easy to handle, thus it is often used for general mechanical vibration measurement. However, the built-in preamplifier needs constant current supply from the outside for driving. Recently, the number of analyzers equipped with CCLD for the sensor has increased, but when connecting to the analyzer without CCLD, it is necessary to prepare a sensor amplifier between a detector and an analyzer.
Charge output type Accelerometer
The piezoelectric elements generate electrical charge proportional to the vibration acceleration. It is suitable for use under high temperature and for measurement of great acceleration. A charge amplifier is required as it is a charge output. Connect it between an accelerometer and an analyzer so that ｔhe charge output from the accelerometer is converted into the voltage by the charge amplifier and input to the analyzer.
Servo type Accelerometer
There is a servo type accelerometer, which we don’t provide. The movement of the pendulum caused by acceleration is detected, the current flows to the internal coil, this current is converted into the voltage, and the voltage proportional to the acceleration is output. The feature is that it can detect a stationary state (0 Hz) such as gravitational acceleration. It is useful for seismic observation and tremor measurement of civil engineering structures. However, in principle, the upper limit of measurable frequency is around 100 Hz.
So far, I have introduced the contact type accelerometers. Next, I will introduce non-contact type vibration detectors that do not require to be directly attached to the measurement target.
A non-contact type vibration detector is used when measuring a small part which cannot be mounted on a detector, or when measuring an object with a very high upper limit of measurement frequency (ultrasonic range, etc.).
Laser Doppler Vibrometer
This detector is a non-contact detector that detects vibration with laser beam. A voltage signal proportional to the vibration speed (unit: m/s) is output ｂｙ the differences in frequency between the irradiation laser and the reflection laser. The original principle of this detector is the application of the Doppler effect, which the ambulance siren sounds higher in pitch when it is approaching than when it is receding. Since it is possible to measure the frequency up to MHz, which cannot be measured by a general accelerometer, vibration of minute parts and vibration in the ultrasonic region can be detected. Do take note that the detection signal unit is velocity (m/s) and it is relatively expensive.
Thirdly, I will introduce a displacement detector as a vibration measurement detector. In the case of vibration around 10 Hz where vibration itself can be visually observed and vibration measurement of the rotating shaft, measurement by displacement (mm) is common, and a displacement detector is used.
Non-contact detectors are normally used for displacement measurement related to vibration. Each manufacturer sells the displacement detectors with various principles, however, the common principle is that a voltage signal proportional to the measured displacement (mm or μm) is output.
Finally, I will simply explain the relationship between displacement, velocity, and acceleration, which are the three parameters of vibration measurement. Displacement, velocity, and acceleration are each related to differential/integration. For example, if acceleration is integrated to the first order, it becomes velocity, and if it is integrated to the second order, it becomes displacement. Conversely, if the displacement is first-order differentiated, it becomes velocity, and if it is second-order differentiated, it becomes acceleration.
Therefore, if you have one of the values for displacement, velocity, or acceleration, you can obtain another value by performing a derivative or integral operation.
The lineup of vibration detectors currently available at our company is as follows. Please take a look if you would like.
（KH）
【NP series Accelerometer built-in preamplifier】
https://www.onosokki.co.jp/English/hp_e/products/keisoku/s_v/np3000series.htm
【NP series Charge output type Accelerometer】
https://www.onosokki.co.jp/English/hp_e/products/keisoku/s_v/np2000series.htm
【LV-1800 series Laser Doppler Vibrometer】
https://www.onosokki.co.jp/English/hp_e/products/keisoku/s_v/lv1800.html
【VE series Electrostatic capacitance-type Gap Detector】
https://www.onosokki.co.jp/English/hp_e/products/keisoku/dimension/new_ve.html