Teach you to choose the correct acceleration sensor

Accelerometers are widely used due to their wide frequency response, large dynamic range, high reliability and ease of use. When users make general vibration and impact measurements, the main technical indicators concerned are: sensitivity, frequency range, internal structure, on-site environment and subsequent instrument configuration.

1, the choice of sensitivity

Yangzhou Jingming's product introduction gives the reference range, the purpose is to allow users to easily select the appropriate product in many different accelerometers. The minimum acceleration measurement is also called the minimum resolution, considering the post-amplifier circuit. Noise problems should be kept as far as possible from the minimum available value to ensure the best signal to noise ratio. The maximum measurement limit should consider the nonlinear influence of the accelerometer itself and the maximum output voltage of the subsequent instrument. The estimation method is: the maximum measured acceleration & TImes; the charge/voltage sensitivity of the sensor, whether the above value exceeds the maximum input charge/voltage value of the instrument, It is recommended that if the known acceleration range is selected, it can be selected in the “reference range range” of the sensor index (considering the frequency response and weight). At the same time, in the case of frequency response and weight, the sensitivity can be considered higher to improve the follow-up. The instrument inputs signals to improve the signal to noise ratio.

While taking into account the frequency response and weight, you can select the sensor sensitivity by referring to the following range: the vibration of civil engineering prototype and super large mechanical structure is about 0.1g~10g, and the accelerometer of 3000pC/g~300pC/g can be selected, mechanical equipment The vibration is about 10g to 100g, and an accelerometer of 20pC/g to 200pC/g can be selected, and an accelerometer with an impact of about 0.1pC/g to 20pC/g can be selected.

2, frequency selection

The frequency response curve given by the manufacturer is screw-mounted and generally divides the curve into two segments: the resonant frequency and the frequency of use. The value of the use frequency is given by the sensitivity deviation, which is ±10%, ±5%, ±3dB. The resonant frequency is generally avoided, but there are also special cases, such as bearing fault detection.

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.

The installation method of the accelerometer will also change the frequency response (the effect on the vibration value is small), the installation surface should be smooth and smooth, and the installation selection should be based on the principle of convenience and safety. Here is the frequency of use of the different KD1005 accelerometer installation: screw 5kHz, epoxy or "502" 4kHz, magnetic chuck 1.5kHz, double-sided adhesive 0.5kHz, it can be seen that the installation method has a great impact on the test frequency response Should pay attention to the choice.

Accelerometer's weight, sensitivity and frequency of use are inversely proportional, high sensitivity, heavy weight, low frequency of use, which is also the choice of skills.

3. 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.

4, 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.

5. Environmental impact

The environment at some test sites is relatively harsh, and many factors are considered, such as waterproofing, high temperature, installation location, strong magnetic field and ground loop, which will have a great impact on the measurement.

Waterproof: There are two concepts of waterproofing, shallow waterproofing and deep waterproofing, especially deep waterproofing. For example, the vibration monitoring of the permanent ship lock gate of the Three Gorges Project, the water depth is nearly 100 meters, it involves ground loop interference, high pressure water seepage, wire protection, long-term reliability. Sex and many other issues.

High temperature: The temperature range given by most manufacturers is the usable value, not the sensitivity of the high temperature condition. In fact, the sensitivity deviation is high at high temperature. Special users should request the special high-temperature sensitivity index from the manufacturer. The sensitivity index is to ensure accurate test. key.

Position limitation: Accelerometers are permanently installed on the site and will be subject to human collision. Industrial products should be selected. A protective cover should be installed outside the accelerometer. This can also act as insulation and dust. It should also be required to the manufacturer. For handheld parts, handheld accelerometers (with long probes) are available.

Insulation, ground loop and magnetic field radiation: Accelerometers and special conductors with special casing materials should be selected at the test site where radiation is strong. Such research is rare in China. For multi-point grounding, humidity, etc., to solve the test interference, a floating or insulated accelerometer can be used.

In order to overcome the loop current influence test generated by multi-point grounding, a floating or insulated sensor can be selected. Insulated accelerometers are available without special requirements and with little interference. For permanent monitoring or high interference conditions, the floating type should be used. The difference between these two names is that the outer casing of the insulated sensor is a signal ground, the base is insulated, and the outer shell of the floating type product is a shield layer, and a three-wire method is adopted.

Additional mass: The mass of the accelerometer mounted on the vibrating structure is less than 1/10 of the mass of the structure itself, and it is considered that there is no significant influence on the signal to be measured.

6, supporting equipment

Piezoelectric accelerometers, like charge output, can be matched with any kind of high-impedance input charge amplifier or collector with charge pre-function. There are many types of charge amplifiers, single, multiple, integral, quasi-static. It must be selected according to the measurement requirements.

There are also special cases, such as directly connecting the output signal of the piezoelectric sensor to a three-meter instrument (such as an oscilloscope) with a certain high resistance performance, and the signal can also be measured, but because the impedance matching is not enough, it can only qualitatively understand the dynamic condition.

The ICP type built-in accelerometer has a constant current adapter, and one instrument can supply constant current supply and signal output of multiple accelerometers. Some data acquisition instruments also come with a constant current function, which can be directly used with ICP sensors.

Ordinary charge output type sensors, such as those with a constant current output, can be used with the JM3861 constant current controller.

The dual-supply accelerometer can be powered by a dual supply or by a dual DC regulated supply.