Acceleration sensor selection, installation, troubleshooting (collection)

Acceleration sensor to meet the needs of the working environment and range, accuracy and other requirements, to make the right selection, let us know what technical requirements of the acceleration sensor.

Acceleration sensor selection requirements

Sensitivity

The sensitivity of the sensor is one of the most basic indicators of the sensor. The sensitivity of the sensor should be determined according to the measured vibration amount (acceleration value), but since the piezoelectric acceleration sensor measures the acceleration value of the vibration, and the acceleration value is proportional to the square of the frequency of the signal under the same displacement amplitude condition, The magnitudes of the acceleration signals in different frequency bands vary greatly.

The acceleration of the vibration of a large-scale structure may be quite small. For example, when the vibration displacement is 1 mm, the acceleration of the signal with a frequency of 1 Hz is only 0.04 m/s2 (0.004 g); however, the displacement of the high-frequency vibration is For a 0.1mm signal with a frequency of 10 kHz, the acceleration can reach 4 x 10 5m/s2 (40000g). Therefore, although the piezoelectric accelerometer has a large measurement range, for the vibration signal used to measure the high and low frequencies, the sensitivity of the acceleration sensor should be adequately estimated.

The most commonly used vibration measurement piezoelectric accelerometer sensitivity, voltage output type (IEPE type) is 50 ~ 100 mV / g, charge output type is 10 ~ 50 pC / g.

Measuring range

The measurement range of the acceleration sensor is the maximum measurement that the sensor can measure within a certain nonlinear error range. The nonlinear error of the general-purpose piezoelectric acceleration sensor is mostly 1%. As a general rule, the higher the sensitivity, the smaller the measurement range, and the smaller the sensitivity, the larger the measurement range.

The measurement range of the IEPE voltage output type piezoelectric accelerometer is determined by the maximum output signal voltage allowed within the linear error range. The charge output type measurement range is limited by the mechanical stiffness of the sensor. Under the same conditions, the maximum signal output of the sensing sensitive core subject to nonlinear nonlinearity of the mechanical elastic interval is much larger than that of the IEPE type sensor. Most of the values ​​are required. Determined by experiment.

In general, when the sensitivity of the sensor is high, the mass of the sensitive core is larger, and the range of the sensor is relatively small. At the same time, because the mass is larger, the resonant frequency is lower, which makes it easier to excite the resonant signal of the sensitive core of the sensor. As a result, the resonant wave is superimposed on the signal to be measured to cause signal distortion output. Therefore, when selecting the maximum measurement range, the frequency composition of the signal under test and the natural resonance frequency of the sensor itself should also be considered to avoid the resonance component of the sensor. At the same time, there should be enough safety space on the range to ensure that the signal does not produce distortion.

Measuring frequency range

The frequency measurement range of the sensor is the range of frequencies that the sensor can measure within the specified frequency response amplitude error (±5%, ±10%, ±3dB). The high and low limits of the frequency range are called high and low frequency cutoff frequencies. As the frequency is directly related to the error, the allowed error range is large and the frequency range is wide. As a general rule, the high frequency response of the sensor depends on the mechanical characteristics of the sensor, while the low frequency response is determined by the integrated electrical parameters of the sensor and subsequent circuits. A sensor with a high frequency cutoff frequency must be small in size and light in weight, whereas a high-sensitivity sensor used for low-frequency measurement is relatively bulky and heavy.

The frequency of the accelerometer should be higher than the vibration frequency of the measured object, and the accelerometer frequency response required by the multiplier analysis is higher. Civil engineering is low frequency, accelerometer can choose 0.2Hz ~ 1kHz or so, mechanical equipment is generally in the middle frequency band, according to equipment speed, equipment stiffness and other factors to comprehensively estimate the frequency, choose 0.5Hz ~ 5kHz accelerometer. Shock measurement is mostly high frequency.

Internal structure

The internal structure refers to the way of sensing the vibration of the crystal piece of sensitive material and the installation form. There are two types of compression and shearing. Commonly, there are central compression, plane shear, triangular shear and ring shear. The central compression frequency response is higher than the shear type, and the shear type environmental adaptability is better than the central compression type. If the integral type charge amplifier is used to measure the speed and displacement, it is better to use a shear type product, so that the obtained signal has small fluctuation and good stability.

Output type

Depends on the interface between the system and the acceleration sensor. Generally, the voltage and acceleration of the analog output are proportional. For example, 2.5V corresponds to an acceleration of 0g, and 2.6V corresponds to an acceleration of 0.5g. Digital output typically uses a pulse width modulated (PWM) signal.

If the microcontroller used only has digital inputs, such as BASIC Stamp, then only the digital output accelerometer can be selected, but an additional clock unit must be used to process the PWM signal, which is also a burden on the processor. .

If you are using a microcontroller with an analog input, such as PIC/AVR/OOPIC, you can use the analog interface's accelerometer very simply. All you need to do is add an instruction like "acceleraTIon=read_adc()" to the program. The speed of processing this instruction is only a few microseconds.

Built-in circuit

The built-in concept is to place a charge/voltage conversion amplifier circuit in the accelerometer to become a sensing element with a voltage output function. It can be divided into two power supplies (four wires) and a single power source (two wires with offset ICP). The built-in circuit below refers to the ICP type.

At present, the built-in circuit sensors are used in many places in China for mechanical failure and pile-based inspection. Many online monitoring projects also use such products.

The core of the ICP sensor is used to supply power and is also a signal output channel. Selection calculation of built-in circuit sensor sensitivity:

If you choose the most common 100mV/g, you can measure the vibration within 50g, because the dynamic range of the sensor is ±5Vp. If you measure 100g, use the 50mV/g accelerometer, and so on.

The advantage of the built-in circuit is low price, good anti-interference and long-term use, but its high temperature resistance and reliability are not as good as charge output products, and the dynamic range is also limited by the effect of output voltage and bias voltage.

Measuring the number of axes

For most projects, two-axis accelerometers are already available for most applications. For some special applications, such as UAV, ROV control, three-axis accelerometers may be suitable for a bit.

The three-axis acceleration sensor can realize the inclination of two-axis plus or minus 90 degrees or two-axis 0-360 degrees. The accuracy of the latter is higher than that of the two-axis acceleration sensor by 60 degrees.

The three-axis accelerometer has the characteristics of small size and light weight (gm), can measure space acceleration, and can fully and accurately reflect the motion properties of objects. It is widely used in aerospace, robotics, automotive and medical, consumer electronics and other fields.

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