Technical Report: Vibration and its Sensors 1

Vibrations can be broadly classified into the following three types based on their characteristics.

• Linear vibration

• Bending vibration

• Torsional vibration

Furthermore, in order to quantitatively capture these vibrations, the following three physical quantities are generally used.

As shown in the diagram above, each physical quantity can be converted to the other by differentiating and integrating. When velocity is constant with respect to frequency, the output characteristics of displacement and acceleration with respect to frequency are generally represented as shown in the following table.

From this table, we can deduce the following:

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

Therefore, generally speaking, measuring displacement is more sensitive at low frequencies, while measuring acceleration is more sensitive at high frequencies.

In equipment diagnostics, displacement and velocity are measured up to several hundred Hz, while acceleration is measured at higher frequencies.

To select the appropriate sensor for accurately detecting vibrations, the following points should be considered:

What is the physical quantity being targeted?
• Displacement
·speed
·acceleration

・ 測定対象物の大きさ

Sensors come in two types: contact and non-contact. When using a contact sensor, the mass effect (explained later) must be considered. Furthermore, regardless of whether it's a contact or non-contact sensor, the required measurement area S of the sensor and the area S' of the object being measured must be considered. (Accurate measurement is impossible unless S'/S > 1.)

- Magnitude and frequency range of vibration of the object

It is necessary to determine the approximate magnitude and frequency range of the vibration being measured. Incorrect estimations at this stage could potentially damage the sensor.

• Measurement Environment: Check the temperature, humidity, and presence of dust, oil, and water in the measurement target and surrounding environment.

Each measurement method has its strengths and weaknesses.

After checking the above, you will proceed to select the most suitable sensor for your measurement target. Please refer to the table below for a summary of our product compatibility.

The natural frequency of an object changes with its mass. Therefore, when a sensor is attached, the mass of the sensor is added to the object, reducing its natural frequency. Consequently, if the mass of the sensor is not sufficiently small compared to the mass of the object being measured, it will change the natural frequency, resulting in measurement errors. As shown in the figure above, if the mass of the object being measured is M, the mass of the sensor is m, and the natural frequency of the measurement system is fe, then from the equation in the figure, the natural frequency decreases by Δfe. A good guideline for the sensor mass is 1/50 of the mass of the object being measured. When the mass m is 1/50 of M, the rate of change of frequency Δfe/fe is 0.01. Note that the mass referred to here is not the mass of the entire object being measured, but the mass of the structure to which the sensor is attached, and it may be surprisingly light, so caution is necessary.

▼Non-contact type