Engineering vibration value of physical parameters commonly used by means of displacement, velocity and acceleration. Due to the frequency range of vibration displacement amplitude is usually the amount is very small, and the displacement, velocity and acceleration between can convert each other, so in the actual use ZhongZhen momentum with acceleration value to measure the size of the general. Unit is commonly used for: 2 meters per second (m/s2), or the gravitational acceleration (g).
To describe the vibration signal is another important parameter signal frequency. The vast majority of the engineering vibration signal can be decomposed into a series of specific frequency and amplitude of the sine signals, therefore, for a vibration signal measurement, is actually made up the vibration signal of the sinusoidal frequency components of measurement. Of sensor examination of main performance index is also according to the regulations of sensors in the frequency range of the measurement accuracy of amplitude level to assess.
Charge the accelerometer output type is not suitable for low frequency measurement
Due to the low frequency vibration acceleration signal is very small, but the small charge high impedance signal are very susceptible to interference; When measuring the volume of the object, the greater the measuring frequency is lower, the SNR of the signal problem is more outstanding. Therefore in the present with built-in circuit should choose as far as possible under the condition of acceleration sensor has become increasingly common electrical noise is small, low frequency characteristics of low impedance voltage output type piezoelectric acceleration sensor.
The low-frequency cutoff frequency of the transducer
The high-frequency cutoff frequency of similar with sensor, low-frequency cutoff frequency refers to a specified sensor frequency response amplitude error (plus or minus 5%, plus or minus 5% or + / - 3 db) sensor can measure the minimum frequency signal. The greater the error value the low-frequency cutoff frequency is relatively lower. So different low-frequency cutoff frequency index of the sensor must be compared under the condition of the same error.
Low impedance voltage output type sensor is sensitive to low frequency characteristic is by sensors core body and a built-in integrated electrical parameters of the circuit. Simulated circuit of the frequency response characteristics can be used to describe the first-order high-pass filter properties, so the low frequency response of the sensor and the cutoff frequency can be completely with the first order to determine the time constant of the system. From a practical point of view, as a result of the VLF frequency response of the sensor calibration is difficult, and through the sensor to step signal of time domain response can be measured the time constant of the sensor; So low frequency response of the sensors and the characteristics of the first-order high-pass filter characteristics, almost the same by calculation can be conveniently to obtain the low frequency response of the sensor and the corresponding frequency of frequency.
The sensitivity of the sensor, dynamic and response characteristics of the low frequency noise
General requirements for low frequency measurement sensor has higher sensitivity to meet low frequency small signal measurement. But the increase in sensitivity is often limited. Although the acceleration sensor sensitivity is can reach 10 v/g or higher, but high sensitivity often bring other negative effects, such as the stability of the sensor, resistance to overload ability, as well as the interference of the sensitivity of the surrounding environment. Therefore the pursuit of high sensitivity is not necessarily can solve the measurement of small signal, instead of high resolution and low noise sensor tend to be more easy to solve practical problems in engineering application. So choose with low electrical noise sensor in the low frequency measurement is particularly important.
In order to show that the sensor can measure the minimum signal most commercial accelerometer are also provide resolution or electrical noise. The vast majority of domestic broadband electrical noise index of the sensor is generally labeled 20 mu V, and BW - broadband electrical noise index of the sensor has been reduced to 10 mu V. For low frequency small signal measurement, however, only provide broadband electrical noise and can not fully reflect the resolution of the acceleration sensor in the low frequency range measurement; This is because caused by the internal circuit is directly proportional to the reciprocal of the size and frequency of the low frequency noise, the so-called 1 / f noise and electrical noise when measuring low frequency sensor output exponentially increases. So, the value of low frequency noise sensor and broadband electrical noise index is completely different and lower frequency, the differences are more obvious. So common in the very low frequency measuring sensor its resolution of sensor output electrical noise power spectral density. The practical significance of this indicator is the noise of the sensor under specific frequency size, the unit is the Hz or mu mu V /) is commonly used in Hz/g). BW - sensor built-in electric circuit noise power spectrum density of typical values for 3 Hz @ mu V /) 10 Hz.
The transient temperature response of the sensor for low frequency measurement
Due to the characteristics of piezoelectric ceramic, piezoelectric accelerometer to sudden changes in temperature will produce different degrees of charge output. The transient temperature response of the sensor is measuring sensor sensitivity to temperature changes. This was especially important for low frequency measurement. Due to the low frequency measurement signal is small, and sensor due to environment temperature change is likely to produce considerable error and low frequency vibration signal; The two signals at very low frequency range it is difficult to distinguish, so how to reduce the influence of environment temperature change on the sensor output in the low frequency measurement is very important. Index of the transient temperature response of the sensor unit is g/oC, transient temperature changes every once had quite the acceleration of output, its value through the voltage (charge) output and the conversion between the sensor sensitivity.
Transient temperature response of the sensor is the direct result of piezoelectric materials, so the piezoelectric ceramics to the charge output caused by temperature jump size determines the stand or fall of this metric. BW - sensor selection at present abroad the best comprehensive performance index of piezoelectric ceramic and made of memory metal used for low frequency measurement of the acceleration sensor for years by the national defense weapons, space and large structure by use of the sensor has the superior low frequency output stability and anti-jamming performance. Actually very low frequency measurement, in order to reduce the impact of environmental temperature change of sensor low frequency signal output, sensor shell as far as possible using the insulation protective sleeve.
Sensors installed base and the base of strain effect on the measurement
Due to the low frequency is not high to the requirement of high frequency response measurement sensor so sensor using any kind of installation method is generally can meet the requirements. But need to pay attention to two problems, one is the sensor should consider using insulating base as much as possible to avoid any influence caused by ground loop noise measurement signal. Secondly, should consider the sensor installed in the measured strain of sensor output, the influence of the size of strain sensor sensitivity. Shear structure in the form of a piezoelectric acceleration sensor has a good base strain characteristics, generally can meet the test usually low frequency structure. If too much strain have influence on the measuring signal of sensor structure, can be reduced to the contact area between the sensor and measured structure reduce the structural strain on the impact of the sensor.
With high frequency indexes corresponding to the sensor, the sensor of low frequency measurement is usually determined by the low-frequency cutoff frequency, low-frequency cutoff frequency must be same with the corresponding amplitude error. And high frequency characteristics, low frequency characteristic of the sensor have nothing to do with any mechanical parameters of the sensor, but only depends on the electrical characteristics parameters of the sensor. Of course sensor as a portion of the measurement system, the measuring signal of the low frequency characteristics will also be affected by and sensor with subsequent instrument electric parameters. According to the different forms of output signal, the following will be the charge output and low resistance voltage output acceleration sensor to discuss respectively.
Although type electric charge output acceleration sensor list low-frequency cutoff frequency, but generally give points out that the measured signal of low frequency characteristics determined by subsequent charge amplifier. In practice, as the core of charge type sensor body insulation impedance is greater than the charge amplifier input impedance at the input, the measurement system composed of sensors and a charge amplifier its low frequency of frequency should be determined by the low frequency characteristics of charge amplifier. But if the core insulation impedance of the sensor, the sensor may affect the low frequency characteristics of the whole measurement system. So ensure that the insulation resistance of the core body of charge low frequency output type accelerometer measurement is very important.
For IEPE sensors with constant current voltage source, it usually low frequency by the frequency of 0.1 Hz (5%). So usually low frequency characteristic of measurement system is decided by the low-frequency by the frequency of the sensor. Most of the low-frequency cutoff frequency general-purpose sensor for the 0.5 Hz ~ 1 Hz, specially used for low frequency measurement sensor low frequency of frequency can be extended to 0.1 Hz. Due to the low frequency calibration sensor is difficult, so manufacturers generally only provide test data of more than 10 hz. But in low frequency characteristics of the sensor and the first-order high-pass filter is very consistent, so the user can through the measured time constant to check the actual low frequency response of the sensor.
With IEPE type piezoelectric acceleration sensor measure VLF acceleration signal also need to pay attention to the problem are as follows:
When sensor voltage source and constant current exchange coupling fairly low frequency by frequency, low frequency characteristics of measuring system is made up of sensors and constant current each low frequency response of the voltage source, the measurement system of low-frequency cutoff frequency is higher than sensors or their low-frequency cutoff frequency constant current voltage source. Sensor should be with the ideal measuring system with dc balance constant current voltage source, the low frequency response of this system will depend entirely on the low frequency by the frequency of the transducer.
Can be measured accurately when sensors used to measure very low frequency is low frequency signal is not entirely decided with low frequency response characteristics of the system, the system of low frequency noise size will directly affect the measurement of low frequency signal. The transient temperature response of the sensor size also will directly affect the low frequency of the sensor measurement.
