2.Selection of vibration sensors
Generally, vibration sensors can be broadly classified into contact type and non-contact type. Non-contact sensors measure relative vibrations from a so-called stationary point. While the advantage of non-contact is that it does not affect the vibrating object, it requires measures such as vibration isolation.
In contrast, the contact type does not require a fixed point, and can be installed and measured relatively easily.
Contact-type vibration sensor measurements mostly use a system called a seismometer, which detects the motion of a weight.
Using this principle, we measure the acceleration and displacement of objects. Below are the measurement principles of the size-scale system.
I will briefly explain the principle.
As shown in Figure 1, a system in which a single weight is supported by a resistor with a spring and damping element inside a housing is called a size-modulation system.
Here, if we let x be a coordinate fixed to the enclosure and y be a coordinate fixed to space, then x represents the relative displacement between the weight and the enclosure, and y represents the displacement of the vibrating body to be measured. Since the size-motion system in Figure 1 can be considered a 1-degree-of-freedom system, the equation of motion is:
(m is the mass of the weight, k is the spring constant, and c is the viscous damping coefficient)
This can be written as follows. Here, in the size-modulation system of Figure 1, fn is the undamped natural frequency.
ζ is the damping ratio.
The quantity we actually want to measure is the displacement y of the vibrating object, and we measure the displacement x of the weight from the sensor.
If we can do this and determine the relationship between x and y, we can measure the vibration of the vibrating body.
If the amplitude ratio of x and y is γ (relative displacement transmission coefficient), then
It will be. f This is the frequency at which the casing and vibrating element of a size-based system vibrate together, and is the natural frequency. f n and
When the ratio of γ is plotted on the horizontal axis, the graph looks like Figure 2.
Looking closely at the graph, we can see that the characteristics can be considered separately depending on whether f/fn is less than 1 or greater than 1.
You can.
① When f / fn ≫ 1
γ ≈ 1, and x ≈ y.
In other words, a quantity equivalent to the displacement of the vibrating body can be measured, and this system functions as a displacement sensor.
② When f / fn ≪ 1
γ becomes f²/fn², and it increases proportionally to f² (the shape of the graph shows that it increases with 12dB/octave), and since ω = 2πf, x is proportional to ω²y.
In other words, a quantity proportional to the acceleration of the vibrating body can be measured, and this system functions as an accelerometer.
In practice, piezoelectric Accelerometer, which utilize this principle, are commonly used as vibration pickups. When force is applied to a piezoelectric element, strain occurs, generating an electric charge, from which a voltage signal proportional to the relative displacement x can be obtained.
3. Principle and Features of Vibration Sensors
The frequency characteristics of a piezoelectric Accelerometer that detects acceleration signals based on the above principle are inherent.
Frequency f n Depending on the tolerance of the flat bandwidth, practically speaking, a bandwidth of 1/3 to 1/5 is possible.
It is being used.
(References) Japan Industrial Pollution Prevention Association (ed.), "Technologies and Laws for Pollution Prevention (Vibration Edition)"
(Excerpt from the email newsletter issuedonNovember21,2007)
