Technical Report on Vibration and Its Sensors 1

Vibrations can be broadly classified into three categories based on their characteristics.

• linear vibration

• bending vibration

• torsional vibration

In order to quantitatively capture such vibrations, three physical quantities are generally used.

Each physical quantity can be transformed into one another through differentiation and integration, as illustrated above. When the velocity is constant with respect to frequency, the characteristics of displacement and acceleration in relation to frequency can generally be represented in a table as shown below.

From this table, the following can be understood.

In the low frequency range, the sensitivity to displacement is high, and as the frequency increases, it shifts to velocity and then to acceleration.

From this, it can be concluded that at lower frequencies, it is generally more sensitive to measure displacement, while at higher frequencies, it is better to measure acceleration.

In equipment diagnostics, displacement and velocity are measured up to several hundred Hz, while acceleration is measured for frequencies above that.

To select the appropriate sensor for accurately detecting vibrations, it is necessary to consider the following points.

- What physical quantities are targeted
- Displacement
- Velocity
- Acceleration

・ 測定対象物の大きさ

There are two types of sensors: contact and non-contact. When using a contact sensor, it is necessary to consider the mass effect (described later), and regardless of whether the sensor is contact or non-contact, one must take into account the measurement area of the sensor S and the area S' of the object being measured. (Accurate measurement is impossible unless S'/S > 1.)

The magnitude of the vibrations of the target object and the frequency range.

It is necessary to determine the approximate size and frequency range of the vibrations to be measured. Misjudging these parameters here could potentially damage the sensor.

- Measurement environment: Check the temperature and humidity of the measurement subject and surrounding environment, as well as the presence or absence of dust, oil, and water.

There are strengths and weaknesses depending on the measurement method.

After checking the above, we will proceed to select the most suitable sensor for the measurement target. We have summarized our product offerings in the table below for your reference.

<Cautions for Contact and Non-Contact Selection>

Contact type

The mass effect refers to the phenomenon where the natural frequency of the object being measured is affected and changes due to the mass of the sensor attached for measurement.

The natural frequency of an object changes due to its mass, so when a sensor is attached, the mass of the sensor adds to the object, resulting in a decrease in the natural frequency. Therefore, if the mass of the sensor is not sufficiently small compared to the mass of the object being measured, it will alter the natural frequency and lead to measurement errors. In the diagram above, let the mass of the object being measured be M, the mass of the sensor be m, and the natural frequency of the measurement system be fe; then, according to the formula in the figure, the natural frequency decreases by Δfe. A guideline for the mass of the sensor is that it should be about 1/50 of the mass of the object being measured. When the mass m is 1/50 of M, the rate of change in frequency Δfe/fe is 0.01. It should be noted that the mass referred to here is not the total mass of the object being measured, but rather the mass of the structural part where the sensor is attached, which can sometimes be surprisingly light, so caution is necessary.

Non-contact

advantage point	does not affect the vibrating body. Effective for measuring parts that cannot be measured by contact methods, such as rotating bodies.
Points to note	The sensor needs to be vibration insulated.