A low-noise capacitive MEMS acceleration sensor and a noise reduction method

Abstract

The present invention relates to a low-noise capacitive MEMS acceleration sensor, which includes a three-axis signal conditioning unit and a sensor group composed of a plurality of sensor units connected in parallel. The three-axis signal output ends of the sensor group are correspondingly connected to the three-axis signal conditioning unit. The present invention also provides a noise reduction method based on the capacitive MEMS acceleration sensor, which includes taking a plurality of sensor units and connecting them in parallel to form a sensor group; taking a three-axis signal conditioning unit and correspondingly connecting the three-axis signal conditioning unit to the three-axis signal output ends of the sensor group. The technical solution of the present invention can, to a certain extent, achieve the noise reduction level of the MEMS acceleration sensor required by users, and the product price is significantly reduced compared with directly selecting a MEMS acceleration sensor with a comparable noise level.

Description

Technical Field

[0001] The present invention relates to the field of sensors in the field of microelectromechanical system (MEMS) technology, and particularly to a low-noise capacitive MEMS accelerometer and a noise reduction method. Background Art

[0002] With the development of MEMS technology, inertial sensor devices have become one of the most successful and widely used microelectromechanical system devices in the past few years, and the microaccelerometer is an outstanding representative of inertial sensor devices. The theoretical basis of the microaccelerometer is based on Newton's second law. Inside a system, velocity cannot be measured, but its acceleration can be measured. If the initial velocity is known, the linear velocity can be calculated by integration, and then the linear displacement can be calculated. As the most maturely applied inertial sensor, the current MEMS accelerometer has a very high integration level, that is, the sensing system and the interface circuit are integrated on one chip.

[0003] Most of the existing MEMS acceleration sensors adopt a capacitance transducer-based architecture, and charge feedback is used to achieve closed-loop feedback control to realize the measurement of acceleration. The capacitive MEMS accelerometer has excellent zero-frequency response. At extremely low frequencies, it has high sensitivity and excellent temperature stability, but there are significant differences in the product noise level. From the perspective of the noise level, MEMS accelerometers can be classified into civilian and consumer-grade MEMS accelerometers and dedicated industrial-grade MEMS accelerometers according to their manufacturing processes. Since the MEMS sensor system is easily affected by switching noise, Brownian motion thermodynamic noise, etc., the noise level of civilian and consumer-grade MEMS accelerometers is much higher than that of dedicated industrial-grade MEMS accelerometers. The noise level of civilian and consumer-grade MEMS accelerometers is generally between 20 and 150 μg / √Hz, and the noise level of dedicated industrial-grade MEMS accelerometers is generally between 0.5 and 10 μg / √Hz.

[0004] In application fields such as seismic exploration, earthquake early warning, structural health monitoring, debris flow and landslide monitoring, high-precision and low-noise MEMS acceleration sensors are required. Generally, MEMS acceleration sensors with a noise level lower than 10 μg / √Hz are preferably selected. However, at present, the noise levels of civilian and consumer-grade products are generally greater than 20 μg / √Hz. Only the noise levels of some special MEMS acceleration sensors can reach 1 μg / √Hz, which can meet the noise level requirements of the above fields. However, most professional MEMS acceleration sensors have single-axis output, and the price of each chip is very expensive, making it difficult to be used in batches in practical applications. Therefore, how to reduce the noise index of MEMS acceleration sensors within the range of controllable costs based on existing civilian and consumer-grade MEMS accelerometers to meet the actual needs of application fields has become an urgent problem to be solved. Summary of the Invention

[0005] In view of the above technical problems, the technical solution of the present invention provides a low-noise capacitive MEMS acceleration sensor, which includes a three-axis signal conditioning unit and a sensor group composed of a plurality of sensor units connected in parallel. The three-axis signal output terminals of the sensor group are correspondingly connected to the three-axis signal conditioning unit.

[0006] As an improvement, each of the sensor units includes a capacitive MEMS acceleration sensor, a power supply filtering circuit, and a bandwidth conditioning circuit.

[0007] As a further improvement, the power supply filtering circuit includes an inductor connected to the power supply terminal, and the other end of the inductor is connected to the voltage input terminal of the acceleration sensor; the power supply filtering circuit further includes a capacitor connected in series with the inductor, and the other end of the capacitor is grounded and connected in parallel with the acceleration sensor.

[0008] As a further improvement, the bandwidth conditioning circuit includes a three-axis buffer and a frequency selection capacitor bank.

[0009] As a further improvement, the three-axis signal output terminals of each acceleration sensor are respectively connected to one end of a corresponding frequency selection capacitor and are correspondingly connected in series with the input terminals of the three-axis buffer; the signals at the ends of all the frequency selection capacitors are grounded; the output terminals of all the three-axis buffers are correspondingly connected to the three-axis signal output terminals of the array sensor.

[0010] As a further improvement, the three-axis signal conditioning unit includes a three-axis operational amplifier, and the three-axis signal output terminals of the sensor array are respectively correspondingly connected to the input terminals of the three-axis operational amplifier.

[0011] The present invention also provides a noise reduction method based on a capacitive MEMS acceleration sensor, which includes the following steps:

[0012] Take a number of sensor units and connect them in parallel to form a sensor group;

[0013] Take a three-axis signal conditioning unit and connect the three-axis signal output ends of the sensor group to the three-axis signal conditioning unit correspondingly.

[0014] Preferably, the obtaining method of the sensor unit includes the following steps:

[0015] Take a number of capacitive MEMS acceleration sensors;

[0016] Take a number of power supply filtering circuits;

[0017] Take a number of bandwidth conditioning circuits;

[0018] Connect the power supply filtering circuit to the power supply end of the capacitive MEMS acceleration sensor;

[0019] Connect the bandwidth conditioning circuit to the output end of the capacitive MEMS acceleration sensor.

[0020] Preferably, the obtaining method of the power supply filtering circuit includes the following steps:

[0021] Take a number of inductors and a number of first capacitors;

[0022] Connect the input end of each inductor to the power supply circuit, and connect a first capacitor in series to the other end;

[0023] Connect the end of the first capacitor to the ground;

[0024] Connect the first capacitor in parallel with the capacitive MEMS acceleration sensor.

[0025] Preferably, the obtaining method of the bandwidth conditioning circuit includes the following steps:

[0026] Take a number of three-axis buffers and a number of frequency-selective capacitors;

[0027] Connect each frequency-selective capacitor to the three-axis signal output end of each capacitive MEMS acceleration sensor;

[0028] Connect the input end of each three-axis buffer to the three-axis signal output end of each capacitive MEMS acceleration sensor correspondingly.

[0029] Based on the existing analog-output civilian MEMS accelerometers (belonging to the first type of capacitive MEMS accelerometers), the technical solution of the present invention forms a MEMS accelerometer array through the cascaded array noise reduction technology of multiple MEMS accelerometers, achieving the effect of reducing the noise level of the MEMS accelerometer by √N times while increasing the cost by about N times.

[0030] To a certain extent, the technical solution of the present invention reaches the noise reduction level of the MEMS accelerometer required by users, and the product price is significantly reduced compared with directly selecting a MEMS accelerometer with a comparable noise level. Brief Description of the Drawings

[0031] Figure 1 Schematic structural diagram of a low-noise capacitive MEMS accelerometer;

[0032] Figure 2 Power supply filter circuit diagram;

[0033] Figure 3 Circuit diagram of a low-noise capacitive MEMS accelerometer;

[0034] Figure 4 Noise detection diagram of an 8-chip parallel array of MEMS accelerometers;

[0035] Figure 5 Noise detection diagram of a 16-chip parallel array of MEMS accelerometers. Detailed Embodiments

[0036] The technical solution of the present invention will be specifically explained below in conjunction with the drawings and embodiments.

[0037] Some embodiments include a low-noise capacitive MEMS accelerometer, the structure of which is as Figure 1 shown, including three-axis (X, Y, Z) signal conditioning units (BX, BY, BZ) and a sensor group composed of a plurality of sensor units (A 11 …A ij …A mn ) connected in parallel. The three-axis signal output terminals (Xo, Yo, Zo) of the sensor group are correspondingly connected to the three-axis signal conditioning units.

[0038] Among them, the voltage is usually noise-reduced and filtered by a parallel capacitor bank before being loaded onto the sensor array. A plurality of sensor units (A 11 …A ij …A mn ) are usually arranged in an array including shapes such as a rectangular array and a circular array, and also include a form of a multi-layer array combination.

[0039] The capacitive MEMS accelerometer is a capacitance sensor based on the principle of capacitance with a variable pole pitch, which includes a fixed electrode and a movable electrode (mass m). Under the action of an external force, the movable electrode undergoes displacement, causing a change in capacitance.

[0040] The tiny displacement of the mass m caused by acceleration can be converted into a change in differential capacitance, and the difference between the two capacitances is proportional to the displacement. The relationship between the input acceleration a and the change in differential capacitance C can be obtained as follows:

[0041]

[0042]

[0043] The main source of self-noise in MEMS accelerometers is the Brownian thermal noise of MEMS sensors. The thermodynamic noise equivalent formula of Brownian motion in MEMS accelerometers is

[0044]

[0045] where g is the noise, m is the equivalent mass of the capacitance, Q is the quality factor of the accelerometer, ω0 is the natural frequency of the accelerometer, and k is the Boltzmann constant.

[0046] The sensitivity from acceleration change to sensitive capacitance change is

[0047]

[0048] where △C is the capacitance change of the accelerometer, d0 is the differential capacitance gap in the static state, and the total capacitance change is C0.

[0049] When N MEMS accelerometers are connected in parallel to form an array, both the equivalent capacitance C0 and △C increase by N times. Therefore, the sensitivity from acceleration change to sensitive capacitance change remains unchanged. However, since the equivalent mass m of the capacitance plate increases by N times, the noise level is reduced by √N times.

[0050] Some embodiments include that each sensor unit includes a capacitive MEMS accelerometer, a power supply filtering circuit, and a bandwidth conditioning circuit.

[0051] Among them, the capacitive MEMS accelerometer mainly includes a civilian-grade and relatively low-cost MEMS accelerometer.

[0052] In some more specific embodiments, the power supply filtering circuit is as Figure 2As shown, it includes an inductor (FB is preferably a ferrite bead) connected to the power supply terminal, and the other end of the inductor is connected to the voltage input terminal of the acceleration sensor; the power supply filtering circuit further includes a capacitor C connected in series with the inductor FB, and the other end of the capacitor C is grounded and is in parallel with the MEMS acceleration sensor.

[0053] The system power supply VCC and GND are filtered through the ferrite bead FB and the capacitor C and then connected to the power supply terminals VCC and GND of each capacitive MEMS acceleration sensor to reduce the power supply crosstalk between the units in the sensor array.

[0054] In some more specific embodiments, the bandwidth conditioning circuit includes a three-axis buffer and a frequency-selective capacitor bank.

[0055] In some more specific embodiments, the circuit is as Figure 3 shown, the three-axis signal output terminals (Xo ij , Yo ij , Zo ij ) of each acceleration sensor are respectively connected to one end of a corresponding frequency-selective capacitor (CX ij , CY ij , CZ ij ) and are correspondingly connected in series to the input terminals of three-axis buffers (AX ij , AY ij , AZ ij ); the signals at the ends of all frequency-selective capacitors are grounded; the output terminals of all three-axis buffers are correspondingly connected to the three-axis signal output terminals (Xo, Yo, Zo) of the sensor group.

[0056] The three-axis signal conditioning unit in some embodiments includes three-axis operational amplifiers, and the three-axis signal output terminals of the sensor group are respectively connected to the input terminals of the three-axis operational amplifiers.

[0057] In some more specific embodiments, a sensor group (array) is formed by paralleling 4 pieces of civilian-grade MEMS sensors (LIS344ALH, Mouser Electronics, Inc.) to achieve the goal of reducing the noise level by 2 times, with the cost increasing by about 4 times. Compared with directly selecting existing MEMS sensors with a noise level 2 times lower, the cost of achieving the same noise level through the noise reduction method of the array technology is only one-fifth.

[0058] A sensor group (array) is formed by paralleling 16 pieces of civilian-grade MEMS sensors (LIS344ALH, Mouser Electronics, Inc.) to achieve the goal of reducing the noise level by 4 times, with the cost increasing by about 16 times. Compared with directly selecting existing MEMS sensors with a noise level 4 times lower, the cost of achieving the same noise level through the noise reduction method of the array technology is only one-twentieth.

[0059] To further verify the noise reduction effect of the MEMS acceleration sensor group (array), 8 and 16 MEMS acceleration sensors (LIS344ALH, Mouser Electronics, Inc.) were respectively connected in parallel to form an array, which was connected to the same data collector for recording, and the noise spectral density of the MEMS acceleration sensor arrays (8 and 16) in three directions was respectively plotted. The dynamic range comparison is shown in Table 1:

[0060] Table 1. Relationship between the dynamic range of the MEMS sensor array and the number of arrays

[0061]

[0062] The noise of the 8-MEMS-acceleration-sensor array and the 16-MEMS-acceleration-sensor array were respectively detected, and the detection results reflect the noise reduction effect of the technical solution of the present invention. Among them, the noise level of the 8-MEMS-acceleration-sensor array is as Figure 4 shown; the noise level of the 16-MEMS-acceleration-sensor array is as Figure 5 shown. It can be seen from the results that the noise level of the 16-MEMS-acceleration-sensor array is reduced by √2 times compared with that of the 8-MEMS-acceleration-sensor array, and the dynamic range is increased by 3 dB, which is basically consistent with the theoretical calculation.

[0063] The present invention also provides a noise reduction method based on a capacitive MEMS acceleration sensor, and the method includes the following steps:

[0064] Take a number of sensor units and connect them in parallel to form a sensor group;

[0065] Take a three-axis signal conditioning unit and correspondingly connect the three-axis signal output ends of the sensor group to the three-axis signal conditioning unit.

[0066] Preferably, a method for obtaining a sensor unit includes the following steps:

[0067] Take a number of capacitive MEMS acceleration sensors;

[0068] Take a number of power supply filtering circuits;

[0069] Take a number of bandwidth conditioning circuits;

[0070] Connect the power supply filtering circuit to the power supply end of the capacitive MEMS acceleration sensor;

[0071] Connect the bandwidth conditioning circuit to the output end of the capacitive MEMS acceleration sensor.

[0072] Preferably, it also includes a method for obtaining a power supply filtering circuit:

[0073] Take a number of inductors and a number of first capacitors;

[0074] Connect the input end of each inductor to a power supply circuit, and connect a first capacitor in series to the other end;

[0075] Ground the end of the first capacitor;

[0076] Connect the first capacitor in parallel with the capacitive MEMS acceleration sensor.

[0077] Preferably, the method for obtaining the bandwidth conditioning circuit includes the following steps:

[0078] Take a number of three-axis buffers and a number of frequency-selective capacitors;

[0079] Connect each frequency-selective capacitor to the three-axis signal output end of each capacitive MEMS acceleration sensor;

[0080] Connect the input end of each three-axis buffer correspondingly to the three-axis signal output end of each capacitive MEMS acceleration sensor.

[0081] Although this specification contains many specific implementation details, these should not be construed as limiting the scope of any invention or the scope of what can be claimed, but rather as illustrations of features that can make a specific implementation of a particular invention concrete. The specific features described in the context of separate implementations in this specification can also be implemented in combination with a single implementation. Conversely, the various features described in the context of a single implementation can also be implemented independently in multiple implementations, or in any suitable sub-combination. Additionally, although the features can be described above as acting in combination and even initially claimed as such, one or more features from the claimed combination can in some cases be removed from the combination, and the claimed combination can be directed to a sub-combination or a variation of a sub-combination.

[0082] Similarly, although the operations are described in a particular order in the figures, it should not be understood that such operations are required to be performed in the particular order shown or in sequential order to achieve the desired result, or that all of the illustrated operations are to be performed.

[0083] Particular implementations of the subject matter have been described. Other implementations are within the scope of the following claims. For example, the acts recited in the claims can be performed in a different order and still achieve the desired result. As an example, for the processing described in the figures to achieve the desired result, it is not necessary to require the particular order or sequential order shown. In certain implementations, multitasking and parallel processing can be advantageous.

Claims

1. A low-noise capacitive MEMS acceleration sensor, characterized in that, It includes a three-component signal conditioning unit and a sensor group composed of several sensor units connected in parallel. The three-component signal output ends of the sensor group are correspondingly connected to the three-component signal conditioning unit, and multiple sensor units are arranged in a rectangular array or a circular array; each of the sensor units includes a capacitive MEMS acceleration sensor, a power supply filtering circuit, and a bandwidth conditioning circuit. The power supply filtering circuit includes an inductor connected to a power supply terminal, and the other end of the inductor is connected to the voltage input end of the acceleration sensor; the power supply filtering circuit further includes a capacitor connected in series with the inductor, and the other end of the capacitor is grounded and is connected in parallel with the acceleration sensor. The bandwidth conditioning circuit includes a three-component buffer and a frequency selection capacitor group. The three-component signal output ends of each acceleration sensor are respectively connected to one end of each frequency selection capacitor and correspondingly connected to the input ends of the three-component buffers in series; the signals at the ends of all the frequency selection capacitors are grounded; the output ends of all the three-component buffers are correspondingly connected to the three-component signal output ends of the sensor group. The three-component signal conditioning unit includes a three-component operational amplifier, and the three-component signal output ends of the sensor group are respectively and correspondingly connected to the input ends of the three-component operational amplifier.

2. A noise reduction method for a capacitive MEMS acceleration sensor based on the capacitive MEMS acceleration sensor described in claim 1, characterized in that, It includes the following steps: Take several sensor units and connect them in parallel to form a sensor group; Take a three-component signal conditioning unit and connect the three-component signal conditioning unit correspondingly to the three-component signal output ends of the sensor group; The method for obtaining the sensor units includes the following steps: Take several capacitive MEMS acceleration sensors; Take several power supply filtering circuits; Take several bandwidth conditioning circuits; Connect the power supply filtering circuit to the power supply terminal of the capacitive MEMS acceleration sensor; Connect the bandwidth conditioning circuit to the output end of the capacitive MEMS acceleration sensor; The method for obtaining the power supply filtering circuit includes the following steps: Take several inductors and several first capacitors; Connect the input end of each inductor to a power supply circuit, and connect a first capacitor in series to the other end; Ground the end of the first capacitor; Connect the first capacitor in parallel with the capacitive MEMS acceleration sensor; The method for obtaining the bandwidth conditioning circuit includes the following steps: Take several three-component buffers and several frequency selection capacitors; Connect each frequency selection capacitor to the three-component signal output end of each capacitive MEMS acceleration sensor; Connect the input end of each three-component buffer correspondingly to the three-component signal output end of each capacitive MEMS acceleration sensor.

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