In our previous introductory column, we discussed "why we measure sound and vibration." This time, we will introduce the types and characteristics of detectors used to measure vibration.
The most common type of detector that immediately comes to mind for vibration measurement is the accelerometer, which is used in contact with the object being measured. This detector uses a piezoelectric element as its detection element and vibrates together with (as one with) the mounting surface of the object being measured, outputting a signal proportional to the vibration acceleration (m/s²). By inputting this signal into an analyzer, measurement becomes possible. Piezoelectric detectors do not respond to DC acceleration. The operating range of small accelerometers used for measuring mechanical vibration is generally from a few Hz. Measurement is not possible in a stationary or near-stationary state.
While many readers may assume that all contact-type accelerometers are the same, there are actually several types. Let's explore these different types.
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Accelerometer with built-in preamplifier
This sensor has a built-in preamplifier that outputs a voltage signal (in volts) proportional to the vibration acceleration. It is relatively resistant to noise and easy to use, making it commonly used for general mechanical vibration measurement. However, the built-in preamplifier requires an external constant current supply for operation. While many recent analyzers are equipped with this sensor power supply, if connecting to an analyzer without this function, a sensor amplifier must be provided between the sensor and the analyzer.
Charge output type accelerometer
The piezoelectric element of the detection element generates an electric charge proportional to the vibration acceleration. This type of detector outputs the electric charge directly as a signal. Because it does not have a built-in charge amplifier to convert it to voltage, it can be used at high temperatures and for measuring high acceleration. However, because it outputs an electric charge, it cannot be directly input into a voltage-input type analyzer. The equipment is configured with the detector, charge amplifier, and analyzer connected in that order. The electric charge output from the detector is converted into a voltage by the charge amplifier and input into the analyzer.
Servo-type acceleration detector
Although we do not handle them ourselves, there are servo-type acceleration detectors available. These detectors detect the movement of a pendulum caused by acceleration, and an electric current flows through an internal coil to return the pendulum to its original position. This current value is converted into a voltage proportional to the vibrational acceleration and output. A key feature is that they can detect stationary conditions (0Hz), such as gravitational acceleration. They are used in earthquake observation and micro-vibration measurement of civil engineering structures. However, due to their operating principle, the upper limit of the measurable frequency is often around 100Hz.
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We have described the types of mountable acceleration detectors so far. Next, we will introduce non-contact vibration detectors that do not require the detector to be directly attached to the object being measured.
When measuring minute components that cannot be fitted with mountable accelerometers (because they are too small to mount), or when measuring objects with a very high upper limit on the measurement frequency (such as in the ultrasonic range), the following non-contact vibration detectors are often used.
Laser Doppler Vibrometer
This device is a non-contact vibration detector that uses laser light. It outputs a voltage signal proportional to the vibration velocity (in m/s) based on the difference in frequency between the emitted and reflected laser light. The underlying principle is known as the Doppler effect; readers may have experienced how ambulance sirens sound higher when approaching and lower when passing by. This device applies that effect. Because it is non-contact and can measure frequencies up to MHz, which are unmeasurable by typical accelerometers, it can detect vibrations in minute components and ultrasonic vibrations. Points to note are that the detection signal unit is velocity (m/s) and the device is expensive.
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Next, we will introduce displacement detectors as vibration measurement sensors. For vibrations around 10 Hz, where vibration motion can be visually observed, and for measuring the runout of a rotating shaft, measurement in displacement (mm) is common, and displacement detectors are used.
Non-contact detectors are typically used for displacement measurements related to vibration. While various detector manufacturers sell displacement detectors based on different principles, what they all have in common is that they ultimately output a voltage signal proportional to the measured displacement (in mm or μm).
We have discussed the types of detectors used in vibration measurement, and finally, we will explain the relationship between the three parameters of vibration measurement: displacement, velocity, and acceleration.
Displacement, velocity, and acceleration are related by differential and integral calculus. For example, integrating acceleration once gives velocity, and integrating it twice gives displacement. Conversely, differentiating displacement once gives velocity, and differentiating it twice gives acceleration.
Therefore, if displacement, velocity, or acceleration can be measured, it can be converted to other units by performing differential or integral operations.
Our current lineup of vibration detectors is as follows:
If you're interested, please take a look.
NP Series Accelerometer with Built-in Preamplifier
NP Series Charge Output Type Accelerometer
LV-1800 Series Laser Doppler Vibrometer
VE Series Capacitive Displacement Detector
(Excerpt from the email newsletter issued on September 15, 2021)
