With the popularization of consumer electronics such as smart phones, devices are required to have higher functions and designability. Acceleration sensors are required to meet the needs of the working environment, range and accuracy. What are the technical requirements for the accelerometer?
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 a digital input, such as a 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.
External environment 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, etc. Many problems.
Temperature effect:
The change in temperature caused by the change in sensor output is caused by the characteristics of the piezoelectric material (sensitive core). There is no linear change between the sensor output and the temperature. Generally speaking, the output change at low temperature is larger than that at high temperature. In addition, because the temperature response of each sensor is difficult to maintain, the output of the sensor in actual use is rarely corrected by the temperature coefficient.
It is necessary to specifically point out that there are two kinds of steady state and transient state of temperature change. The change of sensor output sensitivity with temperature usually refers to the influence of steady state high and low temperature state on signal output. The effect of transient temperature changes on the sensor output is primarily reflected in low frequency measurements.
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 obtain the sensitivity index of the special high temperature from the manufacturer. The sensitivity index is the key to ensure the accuracy of the test. .
Location restrictions:
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 dustproof. Hand-held accelerometers (with long probes) are available for access.
Insulation, ground loop and magnetic field radiation:
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.
The ratio of the magnitude of the signal output caused by the action of the acoustic wave and the magnetic field to the sensor is called the acoustic sensitivity and magnetic sensitivity of the piezoelectric sensor. The most direct way to reduce the sensor's acoustic sensitivity is to increase the thickness of the sensor's casing. This is the most common measurement condition for most sensors. The most direct measure to reduce the magnetic sensitivity of the sensor is that the metal parts should be made of non-magnetic or weak magnetic materials as much as possible. In addition, the double-layer shielding shell structure can also reduce the magnetic sensitivity of the sensor.
Additional quality:
The mass of the accelerometer mounted on the vibrating structure is less than 1/10 of the mass of the structure itself, and is considered to have no significant influence on the signal to be measured.
Supporting instrument
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.
Acceleration sensor installation
Measurements with accelerometers, in order to make the data accurate and easy to use, can be installed in a variety of ways, there are several, for you to choose the application.
Screw mounting
Installed with screws, its frequency response can approximate the original calibrated frequency response and is called rigid mounting. The screw mounting is performed by tapping the hole along the axis of the vibration source on the object to be perforated.
2. Bonding installation
When the object to be tested is not allowed to drill, various adhesives such as "502", epoxy glue, double-sided adhesive tape, and plasticine can be used. It should be noted that the first two methods are used in a frequency close to the rigid mounting method, and the latter two are generally used in the low frequency field, and the measured frequency is greatly reduced. The bonding method is not suitable for impact measurement.
3. Magnetic seat
The advantage of the magnetic seat is that it does not damage the object to be measured and is easy to move. However, it should be considered that using the magnetic seat test will reduce the frequency response of the accelerometer (the magnetic seat should be removed from the road piece when it is used!), which may be less than one-third. When using, first install the magnetic base on the object to be tested, then screw on the sensor, or gently absorb the two on the object to be tested. The impact state causes the sensor to generate charge accumulation, which affects the accuracy of the test.
4. Mica sheet / PTFE film
Mica sheet installation has two functions, insulation and insulation. For high-temperature test specimens, the thickness of the 0.1 mm mica sheet can be used, and the accelerometer frequency response will be slightly reduced. Mica and PTFE are the best materials for the insulation between the test piece and the accelerometer.
5. Three-way sensor installation
Install the screws through the through holes and the side threads for testing or testing.
With the popularity of consumer electronics such as smartphones, devices are required to have higher functionality and designability. In this case, the demand for highly integrated components and miniaturization and low power consumption is strong. The combination of a small-package 3-axis accelerometer and a 3-axis gyroscope composite sensor has not only the features and functions of the above small package gyroscopes, but also industry-leading low power consumption of only 4mA. They have adapted to smartphones, tablets, game consoles, remote controls and other small smart devices, as well as the arrival of the era of artificial intelligence.
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 a digital input, such as a 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.
External environment 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, etc. Many problems.
Temperature effect:
The change in temperature caused by the change in sensor output is caused by the characteristics of the piezoelectric material (sensitive core). There is no linear change between the sensor output and the temperature. Generally speaking, the output change at low temperature is larger than that at high temperature. In addition, because the temperature response of each sensor is difficult to maintain, the output of the sensor in actual use is rarely corrected by the temperature coefficient.
It is necessary to specifically point out that there are two kinds of steady state and transient state of temperature change. The change of sensor output sensitivity with temperature usually refers to the influence of steady state high and low temperature state on signal output. The effect of transient temperature changes on the sensor output is primarily reflected in low frequency measurements.
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 obtain the sensitivity index of the special high temperature from the manufacturer. The sensitivity index is the key to ensure the accuracy of the test. .
Location restrictions:
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 dustproof. Hand-held accelerometers (with long probes) are available for access.
Insulation, ground loop and magnetic field radiation:
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.
The ratio of the magnitude of the signal output caused by the action of the acoustic wave and the magnetic field to the sensor is called the acoustic sensitivity and magnetic sensitivity of the piezoelectric sensor. The most direct way to reduce the sensor's acoustic sensitivity is to increase the thickness of the sensor's casing. This is the most common measurement condition for most sensors. The most direct measure to reduce the magnetic sensitivity of the sensor is that the metal parts should be made of non-magnetic or weak magnetic materials as much as possible. In addition, the double-layer shielding shell structure can also reduce the magnetic sensitivity of the sensor.
Additional quality:
The mass of the accelerometer mounted on the vibrating structure is less than 1/10 of the mass of the structure itself, and is considered to have no significant influence on the signal to be measured.
Supporting instrument
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.
Acceleration sensor installation
Measurements with accelerometers, in order to make the data accurate and easy to use, can be installed in a variety of ways, there are several, for you to choose the application.
Screw mounting
Installed with screws, its frequency response can approximate the original calibrated frequency response and is called rigid mounting. The screw mounting is performed by tapping the hole along the axis of the vibration source on the object to be perforated.
2. Bonding installation
When the object to be tested is not allowed to drill, various adhesives such as "502", epoxy glue, double-sided adhesive tape, and plasticine can be used. It should be noted that the first two methods are used in a frequency close to the rigid mounting method, and the latter two are generally used in the low frequency field, and the measured frequency is greatly reduced. The bonding method is not suitable for impact measurement.
3. Magnetic seat
The advantage of the magnetic seat is that it does not damage the object to be measured and is easy to move. However, it should be considered that using the magnetic seat test will reduce the frequency response of the accelerometer (the magnetic seat should be removed from the road piece when it is used!), which may be less than one-third. When using, first install the magnetic base on the object to be tested, then screw on the sensor, or gently absorb the two on the object to be tested. The impact state causes the sensor to generate charge accumulation, which affects the accuracy of the test.
4. Mica sheet / PTFE film
Mica sheet installation has two functions, insulation and insulation. For high-temperature test specimens, the thickness of the 0.1 mm mica sheet can be used, and the accelerometer frequency response will be slightly reduced. Mica and PTFE are the best materials for the insulation between the test piece and the accelerometer.
5. Three-way sensor installation
Install the screws through the through holes and the side threads for testing or testing.
With the popularity of consumer electronics such as smartphones, devices are required to have higher functionality and designability. In this case, the demand for highly integrated components and miniaturization and low power consumption is strong. The combination of a small-package 3-axis accelerometer and a 3-axis gyroscope composite sensor has not only the features and functions of the above small package gyroscopes, but also industry-leading low power consumption of only 4mA. They have adapted to smartphones, tablets, game consoles, remote controls and other small smart devices, as well as the arrival of the era of artificial intelligence.
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