High and Low Frequency Measurement Piezoelectric Accelerometer Application Guide

high frequency:

High frequency cutoff frequency of the sensor

High frequency cutoff frequency refers to the highest frequency signal that can be measured within the specified sensor frequency response amplitude error (±5%, ±10% or ±3dB). The high frequency cutoff frequency is directly related to the error value. The larger the specified error range, the higher the corresponding high frequency cutoff frequency. Therefore, the high frequency cutoff frequency indicators of different sensors must be compared under the same error conditions.

The sensitive core of a sensor with a high high-frequency cutoff frequency must have a high natural frequency, so the sensitivity of the sensor is relatively low. When selecting a sensor for high-frequency measurement, in order to meet the high-frequency frequency response index of the sensor, it is necessary to appropriately reduce the sensitivity requirements. The high-frequency characteristics of a piezoelectric accelerometer depend on the first-order resonant frequency of the sensor's mechanical structure. In actual use, the first-order resonant frequency of the sensor is often its installation resonant frequency. The installation resonant frequency is determined by the natural frequency of the sensitive core inside the sensor, the overall mass of the sensor, and the installation coupling stiffness. The level of the installation resonant frequency will directly affect the high-frequency measurement range of the sensor, so under the premise of having a stable sensitive core resonant frequency, improving the installation coupling stiffness is an important condition to ensure high-frequency measurement.

Under the same installation conditions, the lighter the sensor, the higher its installation resonance frequency and high-frequency cutoff frequency. Of course, the most basic factor that determines the high-frequency response of the sensor is the natural frequency of the sensitive core inside the sensor. The internal sensitive core of BW-sensor adopts advanced memory metal from abroad. The sensitive core not only has a high natural frequency but also has very stable frequency response characteristics. The high-frequency response characteristics and consistency of BW-sensor are far superior to shear-type accelerometers designed and manufactured only by component tolerance matching or mounting screw tightening.

Sensor installation form, installation resonant frequency

The high-frequency cutoff frequencies provided by sensor manufacturers are obtained under ideal installation conditions. In actual use, the different installation forms and installation quality of the sensor will directly affect the installation coupling stiffness, and then change the high-frequency cutoff frequency of the sensor. The characteristics of different installation resonant frequencies corresponding to different installation methods (screws, bonding, magnetic base and handheld) have been described in many vibration measurement literature; but it is necessary to point out that when different forms of installation methods are combined (such as screw installation with magnetic base), the high-frequency response of the sensor will be restricted by the installation form with the lowest frequency response. The installation method of high-frequency measurement often adopts screw installation. In order to achieve the ideal effect, the surface of the measured object must meet the specified flatness and smoothness requirements and the specified torque when the sensor is installed, so as to increase the installation coupling stiffness as much as possible to ensure the high-frequency cutoff frequency of the sensor. The higher the high-frequency cutoff frequency of the sensor, the higher the installation requirements of the sensor. Therefore, users who use high-frequency measurement sensors must take the installation of the sensor seriously.

The influence of the sensor output connector and cable on the measurement signal

The signal output connector of the sensor is also an important factor that potentially affects high-frequency measurement. In practical applications, the connector and cable of the sensor are also components of the sensor. Various forms of connectors, the connection between the cable connector and the sensor, as well as the weight of the cable and the fixed form of the cable relative to the object being measured will directly affect the resonant frequency of the sensor. The lighter the weight of the sensor, the more significant the impact of the connector and cable on high-frequency measurement. Therefore, when the installation conditions permit, the connector form of the small high-frequency measurement sensor should first consider the conjoined cable. The conjoined cable has the characteristics of fewer moving parts and light weight, which is more suitable for high-frequency measurement.

Typical high frequency measurement sensors

Low impedance voltage output type

D111/D112 sensitivity 1mV/ms-2, frequency range 0.5Hz~10kHz, M5 top/side output

Weight: 12g, Size: 13mm (hexagonal) x 19mm (height), M5 screw mounting

D121/D122 sensitivity 2mV/ms-2, frequency range 0.5Hz~10kHz, M5 top/side output

Weight: 12g, Size: 13mm (hexagonal) x 19mm (height), M5 screw mounting

Charge output type

D21100 sensitivity 0.1pC/ms-2, frequency range 1Hz~12kHz, M5 top output

Weight: 7 grams, size: 10mm (hexagonal) x 19mm (height), one-piece M6 screw installation

D21103 sensitivity 0.1pC/ms-2, frequency range 1Hz~12kHz, M5 top output

Weight: 7 grams, size: 10mm (hexagonal) x 23mm (height), shell insulation, M6 screw installation

D221/D222 sensitivity 0.3pC/ms-2, frequency range 1Hz~12kHz, M5 top/one-piece cable side output

Weight: 2 grams, size: 7mm (hexagonal) x 12~16mm (height), M3 screw installation

Low Frequency:

Charge output accelerometers are not suitable for low frequency measurements

Since the acceleration signals of low-frequency vibrations are very small, and the high-impedance small charge signals are very susceptible to interference; the larger the volume of the measured object, the lower the measurement frequency, and the more prominent the signal-to-noise ratio problem. Therefore, in the current situation where acceleration sensors with built-in circuits are becoming more and more common, we should try to use low-impedance voltage output piezoelectric acceleration sensors with relatively small electrical noise and excellent low-frequency characteristics. [page]

Low frequency cutoff frequency of the sensor

Similar to the high frequency cutoff frequency of the sensor, the low frequency cutoff frequency refers to the lowest frequency signal that the sensor can measure within the specified sensor frequency response amplitude error (±5%, ±10% or ±3dB). The larger the error value, the lower the low frequency cutoff frequency. Therefore, the low frequency cutoff frequency indicators of different sensors must be compared under the same error conditions.

The low-frequency characteristics of low-impedance voltage output sensors are determined by the comprehensive electrical parameters of the sensor's sensitive core and built-in circuit. Its frequency response characteristics can be described by the first-order high-pass filter characteristics of the analog circuit, so the low-frequency response and cutoff frequency of the sensor can be completely determined by the time constant of the first-order system. From a practical point of view, since the calibration of the very low frequency response of the sensor is relatively difficult, the time constant of the sensor can be measured by the sensor's response to a step signal in the time domain; therefore, the low-frequency response of the sensor and the corresponding low-frequency cutoff frequency can be easily obtained by calculation by using the fact that the low-frequency response of the sensor is almost consistent with the characteristics of the first-order high-pass filter.

Sensor sensitivity, low-frequency noise characteristics and dynamic response range

Sensors used for low-frequency measurement are generally required to have relatively high sensitivity to meet the measurement of low-frequency small signals. However, the increase in sensitivity is often limited. Although the sensitivity of the acceleration sensor can reach 10V/g or higher, high sensitivity often brings other negative effects, such as sensor stability, overload resistance, and sensitivity to surrounding environmental interference. Therefore, the pursuit of excessive sensitivity does not necessarily solve the measurement of small signals. On the contrary, high-resolution and low-noise sensors are often easier to solve practical problems in engineering applications. Therefore, it is particularly important to choose sensors with low electrical noise in low-frequency measurements.

In order to indicate the minimum signal that the sensor can measure, most commercial accelerometers also provide resolution or electrical noise indicators. The broadband electrical noise indicators of most domestic sensors are generally marked as 20μV, while the broadband electrical noise indicators of BW-sensor have been reduced to 10μV. However, for low-frequency small signal measurement, only providing broadband electrical noise cannot fully reflect the resolution of the sensor's acceleration measurement in the low-frequency range; this is because the low-frequency noise caused by the built-in circuit is proportional to the inverse of the frequency, that is, the so-called 1/f noise. When the measurement frequency is very low, the electrical noise output of the sensor increases exponentially. Therefore, the value of the low-frequency electrical noise of the sensor is completely different from the broadband electrical noise indicator, and the lower the frequency, the more obvious this difference is. Therefore, the resolution of sensors used for very low frequency measurement is often expressed by the power spectral density of the sensor output electrical noise. The practical significance of this indicator is the noise level of the sensor at a specific frequency, and its unit is generally expressed in μV/√Hz or μg/√Hz. The typical value of the power spectral density of the built-in circuit electrical noise of BW-sensor is 3μV/√Hz@10Hz.

Effect of Transient Temperature Response of Sensors on Low Frequency Measurements

Due to the characteristics of piezoelectric ceramics, piezoelectric accelerometers will produce different degrees of charge output for sudden changes in temperature. The transient temperature response index of the sensor is a measure of the sensitivity of the sensor to temperature changes. This is especially important for low-frequency measurements. Since the signal of low-frequency measurement is very small, and the sensor is likely to produce an error equivalent to the low-frequency vibration signal due to changes in ambient temperature; these two signals are difficult to distinguish in the very low frequency range, so how to reduce the impact of ambient temperature changes on sensor output is very important in low-frequency measurements. The unit of the transient temperature response index of the sensor is g/oC, which means the acceleration output equivalent to each degree of transient temperature change. Its value is obtained by converting the voltage (charge) output and the sensor sensitivity.

The transient temperature response of the sensor is directly caused by the piezoelectric material, so the size of the charge output of the piezoelectric ceramic caused by the sudden change of temperature determines the quality of this indicator. BW-sensor uses piezoelectric ceramics with the best comprehensive performance indicators abroad and combines them with memory metal to make acceleration sensors for low-frequency measurement. After years of use in national defense weapons, aerospace and large structures, the sensors have been verified to have excellent low-frequency output stability and anti-interference performance. In actual very low frequency measurements, in order to reduce the impact of ambient temperature changes on the low-frequency signal output of the sensor, the sensor housing is protected by a heat-insulating protective cover as much as possible.

The influence of the sensor's mounting base and base strain on the measurement

Since low-frequency measurement sensors do not have high requirements for high-frequency response, any installation method of the sensor can generally meet the requirements. However, two issues need to be noted. First, the sensor should try to use an insulating base to avoid any noise caused by the ground loop affecting the measurement signal. Second, the influence of the measured structural strain at the sensor installation location on the sensor output should be considered, that is, the strain sensitivity of the sensor. Piezoelectric accelerometers in the form of shear structures have good base strain characteristics and can generally meet the usual low-frequency structural tests. If the structural strain is too large and affects the sensor's measurement signal, the influence of the structural strain on the sensor measurement can be reduced by reducing the contact area between the sensor and the measured structure.

Low impedance voltage output typical low frequency measurement sensor

D171/D172 sensitivity 100mV/ms-2, frequency range 0.04Hz~1.5kHz, broadband electrical noise 10μV

Low frequency resolution 1μg/√Hz@2Hz, M5 top/side output

D14105/D14205 sensitivity 10mV/ms-2, frequency range 0.1Hz~8kHz, broadband electrical noise 10μV

Low frequency resolution 1μg/√Hz@2Hz, M5 top/side output

Miniaturized design, weight 12 grams, size 16mm (hexagonal) x 21mm (height), M5 screw installation

D17110 sensitivity 100mV/ms-2, frequency range 0.04Hz~1.5kHz, broadband electrical noise 10μV

Low frequency resolution 1μg/√Hz@2Hz, insulated mounting base, top integrated cable output