Piezoelectric accelerometer signal conditioning method and circuit for low frequency vibration measurements

Abstract

The application belongs to the technical field of electric digital data processing, and discloses a piezoelectric accelerometer signal conditioning method and circuit for low-frequency vibration measurement.The method comprises the following steps: selecting multiple IC piezoelectric accelerometers to be reversely docked and installed, forming two branches, and being used in pairs so that the sensed alternating acceleration signals are opposite to each other; using matched transistors to form a multi-channel constant current source for power supply; using multiple instrument amplifier chips to form a differential amplification circuit, adopting a direct current coupling connection form, introducing the signals of multiple IC piezoelectric accelerometers into the differential amplification circuit, doubling the sensitivity of the alternating acceleration signals while offsetting the direct current bias voltage; the similar direct current bias voltages are subtracted to be zero; and using the high common mode rejection ratio characteristic of the instrument amplifier chip, the common mode noise is further suppressed through two-stage differential amplification.The application further suppresses the common mode noise through two-stage differential amplification, and improves the signal-to-noise ratio of a low-frequency broadband test signal.

Description

Technical Field

[0001] This invention belongs to the field of electrical digital data processing technology, and particularly relates to a signal conditioning method and circuit for an IC piezoelectric accelerometer used for low-frequency vibration measurement. Background Technology

[0002] This built-in IC-type piezoelectric accelerometer integrates the traditional piezoelectric acceleration sensing element and preamplifier circuit into a single package. It features anti-interference, low noise, and long-distance signal transmission. It has now become a standardized product system. Its output interface uses a standard coaxial cable connector, and the coaxial output interface is a two-wire system: the shield wire serves as the ground wire, and the core wire combines the power supply and signal output wires. In use, this core wire is connected to a constant current source to power the internal IC circuit; simultaneously, it is connected to a DC blocking capacitor to isolate the DC bias voltage while outputting the measured alternating acceleration signal.

[0003] Piezoelectric accelerometers are used for low-frequency vibration measurement. To address the high output impedance issue, piezoelectric accelerometers with built-in ICs utilize an integrated preamplifier circuit to achieve impedance matching and reduce output impedance. However, the preamplifier circuit within these IC-based piezoelectric accelerometers uses a field-effect transistor (FET) amplifier, requiring an external constant current source. Since the power supply line also serves as the signal output line, an external DC blocking capacitor is needed to isolate the DC bias voltage of the FET amplifier before coupling the AC signal out. Therefore, the selection of the external DC blocking capacitor is crucial; ideally, it should effectively isolate the DC voltage while minimizing AC signal loss. However, as is known from basic electronic circuit principles, capacitors always exhibit capacitive reactance when alternating signals pass through them. Furthermore, the value of this capacitive reactance is related to the operating frequency; the lower the frequency, the greater the capacitive reactance, and the greater its impact on low-frequency signals. The piezoelectric ceramic sensing element of the piezoelectric accelerometer used for low-frequency micro-vibration measurement can operate at frequencies as low as 0.05Hz. The capacitive reactance of the DC blocking capacitor (the lower the frequency, the larger the required DC blocking capacitor value) is also as high as hundreds of kΩ, which places high demands on impedance matching. If the capacitive reactance of the DC blocking capacitor is considered as part of the internal resistance of the accelerometer with a built-in IC, it can be seen that in actual use, the external DC blocking capacitor of the traditional signal conditioning circuit affects the low-frequency response of the accelerometer.

[0004] Based on the above analysis, the problems and defects of the existing technology are as follows: When using piezoelectric accelerometers for low-frequency vibration measurement, both the low-frequency response of the sensor and the signal-to-noise ratio of the measurement are usually required. However, the existing IC-type piezoelectric accelerometers have poor anti-interference and noise reduction performance and cannot simultaneously avoid using capacitive devices to isolate DC voltage, resulting in poor signal-to-noise ratio in the test and affecting the test sensitivity and accuracy. Summary of the Invention

[0005] To overcome the problems existing in related technologies, the present invention discloses a signal conditioning method and circuit for a piezoelectric accelerometer used for low-frequency vibration measurement. Specifically, it relates to a signal conditioning method and circuit for an IC piezoelectric accelerometer used for low-frequency vibration measurement.

[0006] The technical solution is as follows: A piezoelectric accelerometer signal conditioning method and circuit for low-frequency vibration measurement includes:

[0007] S1. Select two IC piezoelectric accelerometers and install them in opposite directions on the adapter mounting base to form two branches. The two are used in pairs with opposite vibration pickup directions, so that the sensed alternating acceleration signals are out of phase with each other, while the internal IC power supply and operation mode are the same.

[0008] S2 uses matching transistor pairs to form two constant current sources to power the internal IC circuits of the two paired IC piezoelectric accelerometers, making the DC bias voltages of the two IC circuits approximately equal.

[0009] S3 uses multiple instrumentation amplifier chips to form a composite differential amplifier circuit. It adopts a DC-coupled connection to introduce the signals of two IC piezoelectric accelerometers into the differential amplifier circuit. This cancels the DC bias voltage and doubles the sensitivity of the alternating acceleration signal. The difference between similar DC bias voltages is zero.

[0010] S4 utilizes the high common-mode rejection ratio (CMRR) of the composite differential amplifier in the integrated instrumentation amplifier chip. After two stages of differential amplification, the first stage adopts a symmetrical cross-input method to reduce the input characteristic differences introduced by the potential difference at the input terminals and improve the balance at the input terminals, thereby improving the CMRR of the differential circuit. The second stage further suppresses common-mode noise through differential operations based on the differential output of the first stage.

[0011] In one embodiment, in step S1, multiple IC piezoelectric accelerometers have the same model number.

[0012] In one embodiment, in step S2, the circuit structure of each constant current source of the multiple constant current sources is identical.

[0013] In one embodiment, in step S3, the composite differential amplifier circuit is divided into two stages of differential amplification: the positive and negative input terminals of the input stage are cross-input to reduce the input characteristic difference introduced by the reference potential imbalance. After the two signals are subtracted in the two first-stage instrumentation amplifier chips at the input terminals, the common-mode signal is reduced and the differential-mode signal is strengthened before entering the second-stage instrumentation amplifier chip for differential amplification. There is no DC blocking capacitor in the entire composite differential amplifier circuit channel. When the IC piezoelectric accelerometer signal is introduced into the differential amplifier circuit, after the DC bias voltage is subtracted, the DC bias voltage output by the two IC piezoelectric accelerometers is equal and becomes zero after subtraction, while the alternating acceleration signal is 180 degrees out of phase and doubles after subtraction.

[0014] In one embodiment, in step S4, the composite differential amplifier is divided into two stages of differential amplification. The input stage has cross inputs at the positive and negative input terminals. The two signals are cross-subtracted in the two first instrumentation amplifier chips at the input terminals and then enter the second-stage instrumentation amplifier chip for subtraction and differential amplification.

[0015] In one embodiment, after step S4, the signal output enters the integration circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier, forming a servo feedback loop to reduce zero-point drift.

[0016] Another object of the present invention is to provide a piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement, comprising:

[0017] The IC piezoelectric accelerometer employs a preamplifier circuit composed of an internal field-effect transistor follower to sense the inversion of alternating acceleration signals.

[0018] Matched transistor pairs form a relatively consistent dual constant current source to power the preamplifier circuit; a composite differential amplifier circuit composed of multiple instrumentation amplifier chips is used to introduce multiple IC piezoelectric accelerometer signals into the differential amplifier circuit in a DC-coupled connection form, which cancels the DC bias voltage and doubles the sensitivity of the alternating acceleration signal; the difference between similar DC bias voltages is zero; and the high common-mode rejection ratio of the instrumentation amplifier chip is utilized to further suppress common-mode noise after two-stage differential amplification.

[0019] In one embodiment, the preamplifier circuit includes:

[0020] IC piezoelectric accelerometers consist of multiple low-frequency IC piezoelectric accelerometers connected in reverse to form two branches. They are used in pairs to make the sensed alternating acceleration signals out of phase with each other.

[0021] A field-effect transistor follower uses a drain-follower configuration to convert the charge signal of a piezoelectric element into a voltage signal output.

[0022] In one embodiment, the matching transistor pair includes: multiple constant current sources simultaneously powering multiple corresponding IC piezoelectric accelerometers, generating consistent DC bias voltages.

[0023] In one embodiment, the composite differential amplifier is divided into two stages of differential amplification: the positive and negative input terminals of the input stage are cross-input to reduce the input characteristic difference introduced by the reference potential imbalance. After the two signals are subtracted in the two first-stage instrumentation amplifier chips at the input terminals, the common-mode signal is naturally reduced and the differential-mode signal is strengthened before entering the second-stage instrumentation amplifier chip for differential amplification.

[0024] After the signal is output, it enters the integrating circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier, forming a servo feedback loop to suppress zero-point drift of the low-frequency circuit.

[0025] Combining all the above technical solutions, the advantages and positive effects of this invention are as follows:

[0026] First, addressing the technical problems existing in the prior art and the difficulty in solving them, this paper closely analyzes, in conjunction with the technical solution to be protected by this invention and the results and data obtained during the research and development process, how the technical solution of this invention solves the technical problems, and the inventive technical effects brought about by solving these problems. The specific description is as follows:

[0027] To meet the signal-to-noise ratio requirements for low-frequency broadband vibration measurements and to accommodate the universal coaxial cable output interface of the built-in IC piezoelectric accelerometer, this invention provides a signal conditioning method and hardware circuit for an internally mounted IC piezoelectric accelerometer used in low-frequency broadband vibration measurements: two accelerometers of the same model with good consistency are selected and installed in reverse for pairing; a matching transistor pair is used to form a relatively consistent dual constant current source to power the internal IC of the accelerometer, thereby improving the consistency of the two low-frequency measurement branches;

[0028] A differential amplifier circuit is constructed using multiple instrumentation amplifier chips and a DC-coupled connection is adopted to introduce the piezoelectric accelerometer signal into the differential amplifier circuit. This cancels the DC bias voltage and doubles the sensitivity of the alternating acceleration signal. It avoids the signal loss caused by the capacitive reactance of the DC blocking capacitor in the circuit when a single piezoelectric accelerometer is used, as well as the change in the static operating point introduced by the large-value DC blocking capacitor. This improves the signal-to-noise ratio and makes the static operating point relatively stable during low-frequency measurements.

[0029] The differential amplifier circuit adopts a composite instrumentation amplifier circuit structure. Through input stage cross-coupling, it reduces the fading of common-mode rejection ratio (CMRR) in the higher frequency band caused by input level differences. The reference potential input terminal of the subsequent instrumentation amplifier is connected to an integral servo circuit to suppress zero-point drift of the low-frequency circuit.

[0030] Second, considering the technical solution as a whole or from the perspective of the product, the technical effects and advantages of the technical solution to be protected by this invention are specifically described as follows:

[0031] This invention provides a signal conditioning method and hardware circuit for an internally mounted IC piezoelectric accelerometer used for low-frequency broadband vibration measurement: Two low-frequency IC piezoelectric accelerometers of the same model with good consistency are selected and installed in reverse for pairing; a matching transistor pair is used to form a relatively consistent dual-channel constant current source to power the internal IC of the accelerometer; a differential amplifier circuit is constructed using multiple instrumentation amplifier chips, and a DC-coupled connection is used to introduce the piezoelectric accelerometer signal into the differential amplifier circuit, which cancels the DC bias voltage and doubles the sensitivity of the alternating acceleration signal; the differential amplifier circuit is a composite instrumentation amplifier composed of multiple instrumentation amplifier chips, which compensates for the fading of the common-mode rejection ratio (CMRR) in the relatively high-frequency range caused by the difference in input terminal level through input stage cross-coupling, ensuring the measurement bandwidth of the accelerometer; utilizing the high CMRR characteristics of the instrumentation amplifier chips, after two-stage differential amplification, common-mode noise is further suppressed, and the signal-to-noise ratio of the low-frequency broadband test signal is improved.

[0032] Compared with the prior art, the advantages of the present invention further include:

[0033] This invention employs a DC-coupled output signal and common-mode voltage cancellation method to design a signal conditioning circuit: Accelerometers of the same model are paired with the accelerometer to be used. The two piezoelectric accelerometers with built-in ICs are connected in reverse to form two branches, making the sensed alternating acceleration signals out of phase. Matched transistor pairs are used to form a dual constant current source with good consistency, supplying power to the internal IC circuits of the two accelerometers respectively, so that the DC bias voltages on the two branches are approximately the same. The accelerometer signal from the IC circuit is output to a differential circuit composed of an instrumentation amplifier. At this time, the DC bias voltage, as a common-mode signal, is canceled, while the alternating acceleration signal... Since the signals are inverted differential signals, the difference is retained after differential subtraction. According to the basic principles of signal processing, subtracting two inverted signals doubles the amplitude, thus doubling the measurement sensitivity. The difference between the approximately equal DC bias voltages is approximately zero, eliminating the need for a DC blocking capacitor to isolate the DC voltage. This avoids the impact of using a DC blocking capacitor in AC coupling circuits on low-frequency acceleration measurements (according to Thevenin's theorem, the capacitive reactance of the DC blocking capacitor acts as a voltage divider in the circuit, causing acceleration signal loss). The useful acceleration signal is doubled, while the useless DC common-mode signal is approximately zero after subtraction, naturally improving the signal-to-noise ratio of the test.

[0034] Third, as supplementary evidence of the inventive step of the claims of this invention, it is also reflected in the following: the use of two paired IC piezoelectric accelerometers connected in reverse, and a DC-coupled signal conditioning circuit, avoids the inherent low-frequency response limitation of AC coupling circuits using DC blocking capacitors, extending the effective operating frequency band of the piezoelectric accelerometers to lower frequencies. Its lower frequency limit is no longer limited by the performance of the matching circuit, better matching the operating frequency characteristics of the piezoelectric element. This avoids the use of expensive, high-capacitance, low-loss capacitors, improving cost-effectiveness. Attached Figure Description

[0035] The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure.

[0036] Figure 1 This is a flowchart of the IC piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement provided in an embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of a conventional IC-type piezoelectric accelerometer and its external circuit provided in an embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the reverse docking installation of paired piezoelectric accelerometers provided in an embodiment of the present invention;

[0039] Figure 4 This is a schematic diagram of the connection relationship between the accelerometer and the signal conditioning circuit using DC coupling provided in an embodiment of the present invention;

[0040] Figure 5 This is a schematic diagram of the dual-channel matched constant current source circuit provided in an embodiment of the present invention.

[0041] Figure 6 This is a schematic diagram of the composite differential amplifier circuit provided in an embodiment of the present invention;

[0042] Figure 7 This is a schematic diagram of the signal conditioning circuit provided in an embodiment of the present invention;

[0043] In the diagram: A, preamplifier circuit; A-1, piezoelectric accelerometer; A-2, field-effect transistor follower; B, matching transistor pair; C, composite differential amplifier circuit; C-1, first-stage instrumentation amplifier chip; C-2, second-stage instrumentation amplifier chip; C-3, final-stage instrumentation amplifier. Detailed Implementation

[0044] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0045] I. Explanation of the Implementation Example:

[0046] The IC piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement provided in this embodiment of the invention includes:

[0047] Two low-frequency IC piezoelectric accelerometers of the same model with good consistency are selected and installed in reverse to form two branches. They are used in pairs so that the sensed alternating acceleration signals are out of phase with each other.

[0048] A pair of matching transistors is used to form a relatively consistent dual constant current source to power the internal circuit of the IC piezoelectric accelerometer; in this way, the DC bias voltage on the two branches is approximately the same.

[0049] A differential amplifier circuit is constructed using multiple instrumentation amplifier chips, employing a DC-coupled connection. Signals from two IC piezoelectric accelerometers are introduced into the differential amplifier circuit, canceling the DC bias voltage while doubling the sensitivity of the alternating acceleration signal. The difference between similar DC bias voltages is approximately zero. The differential amplifier circuit is a composite instrumentation amplifier composed of multiple instrumentation amplifier chips. Through input stage cross-coupling, it compensates for the fading of the common-mode rejection ratio (CMRR) at relatively high frequencies caused by differences in input levels, ensuring the accelerometer's measurement bandwidth.

[0050] By utilizing the high common-mode rejection ratio of the instrumentation amplifier chip, and through two-stage differential amplification, common-mode noise is further suppressed, thereby improving the signal-to-noise ratio of the low-frequency broadband test signal.

[0051] Specifically, it includes:

[0052] The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement includes:

[0053] The first step is to select two IC piezoelectric accelerometers A-1 and install them in reverse on the adapter mounting base to form two branches. The two are used in pairs with opposite vibration pickup directions, so that the sensed alternating acceleration signals are out of phase with each other, while the internal IC power supply and operation mode are the same.

[0054] The second step is to use matching transistors to form two constant current sources for B, which will power the internal IC circuits of the two paired IC piezoelectric accelerometers A-1, so that the DC bias voltages of the two IC circuits are approximately equal.

[0055] The third step involves constructing a composite differential amplifier circuit 3 using multiple instrumentation amplifier chips. By adopting a DC-coupled connection, the signals from the two IC piezoelectric accelerometers A-1 are introduced into the differential amplifier circuit. This cancels out the DC bias voltage and doubles the sensitivity of the alternating acceleration signal. The difference between the approximately similar DC bias voltages is zero.

[0056] The fourth step utilizes the high common-mode rejection ratio (CMRR) of the instrumentation amplifier chip. After two stages of differential amplification, the first stage adopts a symmetrical cross-input method to reduce the input characteristic differences introduced by the potential difference at the input terminals and improve the balance at the input terminals, thereby improving the CMRR of the differential circuit. The second stage further suppresses common-mode noise based on the differential output of the first stage through differential operations.

[0057] Example 1

[0058] like Figure 1 As shown, the IC piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement provided in this embodiment of the invention includes the following steps:

[0059] S101: Based on the factory calibration certificate of IC piezoelectric accelerometer A-1, select two sensors with good sensitivity consistency (e.g., sensitivity difference not exceeding 0.5 dB) from the sensitivity response curve, and install the two sensors in opposite directions.

[0060] S102: Two constant current source circuits are constructed using matching transistors to transistor B. The circuit structure of each constant current source is the same, and high-precision resistors are selected to ensure that the circuit performance is approximately the same.

[0061] S103: Power the two IC piezoelectric accelerometers A-1 with a constant current source.

[0062] S104: A composite differential amplifier composed of multiple instrumentation amplifier chips. C: Divided into two stages of differential amplification. The input stage has cross inputs at the positive and negative input terminals. The two signals are cross-subtracted in the two first instrumentation amplifier chips C-1 at the input terminals, and then enter the second stage instrumentation amplifier chip C-2 for subtraction and differential amplification.

[0063] S105: After the signal is output, it enters the integrating circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier C-3, forming a servo feedback loop to reduce zero drift.

[0064] Example 2

[0065] like Figure 2 As shown, the working principle of the IC-type piezoelectric accelerometer 1-1 provided in this embodiment of the invention includes:

[0066] IC-type piezoelectric accelerometers with standard interfaces, such as Figure 2As shown: The piezoelectric ceramic element T1 is integrated with the preamplifier circuit A1 into a single package. Its output interface is a coaxial connector P1. There are only two external connections: one connects to the shielding layer of P1, and the other connects to the core wire of the coaxial connector, which also serves as a common line for power supply and signal output. The traditional connection method is: a constant current source supplies power forward, and a DC blocking capacitor C0 is used to isolate the DC bias voltage, coupling the alternating signal to the subsequent stage.

[0067] The DC blocking capacitor C0 has AC impedance, which is related to the frequency f.

[0068]

[0069] The lower the frequency, the greater the AC impedance. It is evident that the presence of capacitive components in the circuit is detrimental to low-frequency measurements. This not only increases the difficulty of impedance matching in subsequent circuits and reduces the signal integrity of low-frequency signal energy transmission to subsequent stages, affecting the low-frequency response of the measurement system, but also requires a higher DC blocking capacitor value as the operating frequency decreases. As is known from circuit design common sense, the stability of the reference potential deteriorates after an alternating signal passes through a large-capacity DC blocking capacitor, affecting the actual test results.

[0070] Using a DC-coupled circuit avoids the aforementioned problems. By removing capacitive components, low-frequency signals can be transmitted to subsequent circuits without loss. However, the DC bias voltage must be suppressed; otherwise, the circuit will fail to amplify, as the DC bias voltage will saturate the circuit during the first amplification stage. This invention employs two paired IC piezoelectric accelerometers, mounted in reverse configuration. The output signals are differentially subtracted to cancel out the DC bias voltage.

[0071] 2) such as Figure 3 The reverse docking installation diagram of the paired piezoelectric accelerometer 1-1 provided in this embodiment of the invention is shown.

[0072] After selection, two accelerometers with good consistency in sensitivity-frequency response test curves are paired up for use. In the vibration measurement direction, the two IC piezoelectric accelerometers A-1 are installed opposite each other on the mounting adapter, and the measured vibration alternating signal is out of phase.

[0073] 3) such as Figure 4 The connection diagram of the DC-coupled accelerometer and signal conditioning circuit is shown. The connection method of the signal conditioning circuit for the DC-coupled piezoelectric accelerometer is as follows: Figure 3As shown, constant current source 1 and constant current source 2 power two IC piezoelectric accelerometers A-1 respectively. The output signal lines of the two IC piezoelectric accelerometers A-1 are directly connected to the input terminal of the differential amplifier, i.e., the composite differential amplifier C, forming a DC-coupled connection. Constant current source 1 and constant current source 2 are constant current sources composed of matching transistor pair 2. Matching transistor pair 2 are highly consistent transistors produced by electronic device manufacturers, which can control the non-inconsistency error between constant current source 1 and constant current source 2 within a very small range.

[0074] Constant current source 1 and constant current source 2 supply power to two IC piezoelectric accelerometers A-1 respectively, and the DC bias voltages generated are approximately the same; however, the mechanical mounting positions of the two IC piezoelectric accelerometers A-1 are opposite; therefore, it can be considered that during vibration measurement, the DC bias voltage on the two branches is mainly a common-mode signal, while the vibration acceleration alternating signal is out of phase and is mainly a differential-mode signal.

[0075] After the common-mode signal and differential-mode signal enter the composite differential amplifier C, the common-mode signal is suppressed by subtraction; while the amplitude of the inverted differential-mode signal is doubled, that is, the test sensitivity is doubled.

[0076] 4) such as Figure 5 The above describes the circuit diagram of the matching transistor pair B, which is a schematic diagram of the principle of the dual-channel matching constant current source circuit. In order to ensure the consistency of the two constant current source branches, the matching transistor pair B uses transistor Q4 to form a dual-channel constant current source. Q4 contains two highly consistent transistors, which are devices manufactured by electronic device manufacturers specifically for matching consistency applications. For a single branch, it is a classic transistor constant current source circuit. Among them, diodes D1, D2, D3, and D4 are selected as Zener diodes with consistent forward voltage and volt-ampere characteristics. Resistors R3, R4, R5, and R6 are selected as metal foil resistors with high precision and temperature stability. Resistors RL1 and RL2 are load matching resistors.

[0077] Taking one of the branches (source 1) as an example:

[0078] Diodes D1, D2, and R5 are connected in series, forming a stable voltage, U, at the base of transistor 1 of the matching transistor pair Q4. D The base-emitter voltage of the transistor is U. BE If the current gain is β, then the output current of source 1 is:

[0079]

[0080] Similarly, the output current of source 2 is:

[0081]

[0082] 5) such as Figure 6The diagram shows a schematic of the composite differential amplifier C circuit, i.e., the circuit diagram of the composite differential amplifier circuit. Although the front-end uses a well-consistent IC piezoelectric accelerometer A-1, and the power supply uses a dual constant current source composed of matched transistor pairs, the DC bias voltages of the two branches are both in the range of several volts (about half of the power supply voltage), and there may still be slight differences between the two branches. For differential circuits (whether it is a differential amplifier with discrete transistor components or an integrated circuit structure), the difference in DC level between the positive and negative input terminals will cause changes in the operating characteristics of the internal components of the circuit input stage. This common-mode error of the differential circuit output introduced by the imbalance of the input DC level will cause a decrease in the common-mode rejection ratio (CMRR) of the differential circuit at higher frequencies: as the frequency increases, the common-mode rejection ratio (CMRR) drops rapidly—that is, at higher frequencies, the measured signal-to-noise ratio decreases. To address this situation, this invention uses multiple instrumentation amplifiers to form a composite differential amplifier to improve the common-mode rejection ratio and expand the operating bandwidth.

[0083] U1 and U2 are both integrated differential instrumentation amplifiers (C-1), serving as the first stage amplification. Their inputs are cross-coupled, suppressing common-mode signals and amplifying differential-mode signals, respectively. The inverted accelerometer signal acts as the differential-mode signal, doubling the sensitivity. The second-stage differential instrumentation amplifier (C-2), U3, amplifies the difference between the output signals of U1 and U2 while suppressing common-mode signals. Because the common-mode responses of U1 and U2 are correlated, the output common-mode errors caused by the common-mode error voltage are also similar. The common-mode error of U3 manifests as a common-mode signal, which is also suppressed in the second stage, while the differential signal from the previous stage is further amplified.

[0084] The output signal of U3 is fed back to the reference level input pin 5 of U3 through the integral servo circuit consisting of operational amplifier U4 (i.e., final stage instrumentation amplifier C-3), resistor R11, and capacitor C3. The DC voltage error of the previous stage input is also fed back to the previous stage instrumentation amplifier to cancel it out.

[0085] Example 2

[0086] like Figure 7 As shown, the IC piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement provided in this embodiment of the invention includes:

[0087] The preamplifier circuit 1, which uses an IC piezoelectric accelerometer with an internal field-effect transistor follower, includes:

[0088] IC piezoelectric accelerometer A-1, there are multiple IC piezoelectric accelerometers A-1. Select two low-frequency IC piezoelectric accelerometers of the same model with good consistency and install them in reverse to form two branches. They are used in pairs so that the sensed alternating acceleration signals are out of phase with each other.

[0089] The field-effect transistor follower A-2 is composed of field-effect transistors Q1 and Q2 and external resistors R1, Rs1 and R2, Rs2. Its main structure adopts the conventional and classic source-drain follower form in electronics, which converts the charge signal of piezoelectric elements T1 and T2 into a voltage signal output.

[0090] Matching transistor pair B forms a relatively consistent dual constant current source to power the internal circuit of the IC piezoelectric accelerometer.

[0091] The composite differential amplifier circuit C, composed of multiple instrumentation amplifier chips, is divided into two stages of differential amplification. The positive and negative input terminals of the input stage are cross-input to reduce the input characteristic differences introduced by the reference potential imbalance. After the two signals are subtracted from each other in the two first-stage instrumentation amplifier chips C-1 at the input terminals, the common-mode signal is naturally reduced and the differential-mode signal is strengthened before entering the second-stage instrumentation amplifier chip C-2 for differential amplification. There are no DC blocking capacitors in the entire composite differential amplifier circuit C channel, so it adopts a capacitor-free DC coupling connection. When the IC piezoelectric accelerometer A-1 signal is introduced into the differential amplifier circuit, after the DC bias voltage is subtracted, due to the previous selection of the consistency of IC piezoelectric accelerometer A-1, the DC bias voltage output of the two IC piezoelectric accelerometer A-1 is approximately equal, and the result after subtraction is approximately zero. However, the useful alternating acceleration signal is 180 degrees out of phase, and the result after subtraction is naturally doubled. Therefore, it can be said that while canceling the DC bias voltage, the sensitivity of the alternating acceleration signal is doubled.

[0092] In this embodiment of the invention, the composite differential amplifier circuit C, composed of multiple instrumentation amplifier chips, suppresses the fading of the common-mode rejection ratio (CMRR) caused by input level differences at relatively high frequencies through input stage cross-coupling, ensuring the measurement bandwidth of the accelerometer. After two-stage differential amplification, utilizing the high CMRR characteristics of the instrumentation amplifier chips (the CMRR of conventional instrumentation amplifiers reaches over 100 dB), the differential signal is amplified while common-mode interference is suppressed. The gain of amplifying the differential signal is more than 10,000 times that of suppressing the common-mode signal, further suppressing common-mode noise. The input stage cross-coupling reduces the impact of input level differences, slowing down the fading trend of the CMRR at high frequencies, which is equivalent to expanding the bandwidth, thus improving the signal-to-noise ratio of the broadband test signal.

[0093] In this embodiment of the invention, the signal output after differential amplification by the composite differential amplifier circuit C enters the integrating circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier C-3, forming a servo feedback loop to suppress the zero-point drift of the integrating circuit, i.e., the low-frequency circuit.

[0094] In a preferred embodiment, the selected low-frequency IC piezoelectric accelerometer A-1 is paired with a sensor whose sensitivity-frequency response test curves show good consistency; the paired IC piezoelectric accelerometers are mounted on the adapter mounting base in opposite directions.

[0095] A dual constant current source circuit with similar consistency is constructed using a matching transistor pair B and a high-precision resistor to power the internal circuits of the two paired IC piezoelectric accelerometers.

[0096] The power supply / signal combination line of the two paired IC piezoelectric accelerometers is directly connected to the positive and negative input stages of the differential amplifier circuit (without passing through the DC blocking capacitor of the conventional AC coupling circuit), forming a DC-coupled connection. The differential amplifier is a composite instrumentation amplifier circuit composed of multiple instrumentation amplifier chips. Through input stage cross-coupling and two-stage differential amplification, it amplifies the differential signal while suppressing the common-mode signal. Thus, after the constant current source is powered, the DC bias voltage on the power supply / signal of the two paired IC piezoelectric accelerometers is subtracted as the common-mode voltage. Since the two IC piezoelectric accelerometers are selected and paired for consistency, and the constant current source is composed of matched transistor pairs, consistency is also guaranteed. Therefore, the DC bias voltage output by the IC piezoelectric accelerometers is mostly suppressed, and the measured useful AC signal is an inverted differential signal (because the two accelerometers are installed in reverse), which is effectively amplified, and the sensitivity is doubled.

[0097] In a preferred embodiment of the present invention, the internal circuit structure of IC piezoelectric accelerometer 1 is discussed using a common low-frequency IC piezoelectric accelerometer as an example: the main pre-amplifier circuit inside IC piezoelectric accelerometer A-1 is a field-effect transistor follower composed of matching resistor R1, current-limiting resistor Rs1, and junction field-effect transistor Q1, and T1 is a piezoelectric element; similarly, IC piezoelectric accelerometer A-1 includes: IC piezoelectric accelerometer 1 and IC piezoelectric accelerometer 2 are selected products of the same model and with good consistency.

[0098] IC piezoelectric accelerometer 1 and IC piezoelectric accelerometer 2 are powered by a dual-channel matched constant current source, forming two measurement branches. The dual-channel matched constant current source is a typical constant current source circuit consisting of a matching transistor pair Q4, Zener diodes D1, D2, D3, and D4, and high-precision resistors R3, R4, R5, and R6. After passing through load matching resistors RL1 and RL2, the current source powers IC piezoelectric accelerometer 1 and IC piezoelectric accelerometer 2 via a combined power supply / signal output cable. Simultaneously, the combined power supply / signal output cable for IC piezoelectric accelerometer 1 and IC piezoelectric accelerometer 2 is connected to the positive and negative input terminals of a differential amplifier circuit, forming DC coupling. Therefore, the signal input to the differential amplifier circuit includes not only the alternating acceleration signal but also a DC bias voltage.

[0099] In a preferred embodiment of the present invention, IC piezoelectric accelerometer 1 and IC piezoelectric accelerometer 2 are used as described above. Figure 3 If the installation methods are reversed, the two alternating acceleration signals are differential signals with opposite phases.

[0100] In a preferred embodiment of the present invention, the differential amplifier circuit is a composite amplifier mainly composed of instrumentation amplifiers U1, U2, and U3. U1 and U2 constitute the first-stage differential amplifier circuit, and their input terminals are cross-coupled to reduce the influence of common-mode rejection ratio (CMRR) changes caused by input terminal level differences. The DC bias voltage of the preceding circuit is subtracted, and the alternating acceleration signal is doubled before entering the second-stage differential amplifier U3, which further suppresses the common-mode signal, amplifies the differential-mode signal, and improves the common-mode rejection ratio (CMRR), thereby improving the signal-to-noise ratio during accelerometer measurement.

[0101] In a preferred embodiment of the present invention, operational amplifier U4, resistor R11, and capacitor C3 constitute an integrating circuit. Resistor R12 is an input balancing resistor that integrates the DC offset voltage at the output of the differential amplifier and the zero-point offset at extremely low frequencies, and feeds it back to the reference voltage input terminal (pin 5) of the second-stage differential amplifier U3 to form a servo loop. In this way, the low-frequency cutoff frequency of the circuit is determined by the RC time constant. For example, if resistor R11 is 10M ohms and C3 is 10 microfarads, the low-frequency cutoff frequency (-3dB) of the circuit is approximately 0.02Hz, which meets the requirements for low-frequency acceleration measurement.

[0102] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail or recorded in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0103] II. Application Examples:

[0104] By adopting the technical solution of the present invention, the lower limit of the measurement frequency of the piezoelectric accelerometer can be extended to the low-frequency end, and the function of the magnetoelectric vibration sensor commonly used in seismology can be replaced, which can be used in the fields of earthquake monitoring, mineral seismic exploration and other technical fields.

[0105] III. Evidence of the relevant effects of the embodiments:

[0106] The technical solution of this invention avoids the limitations of traditional IC-type accelerometers in low-frequency response characteristics caused by the matching AC-coupled signal conditioning circuit when used as a single unit. According to Thevenin's theorem, the lower frequency limit is no longer limited by the capacitive reactance of the DC blocking capacitor. A comparison of the inherent characteristics and natural properties of DC-coupled and AC-coupled circuits shows that the signal conditioning circuit of this invention can better match the operating frequency characteristics of the piezoelectric element. Furthermore, the reverse docking and differential signal processing techniques further suppress common-mode noise signals while doubling the amplitude of the useful differential-mode signal, thus improving sensitivity and enhancing the signal-to-noise ratio of the piezoelectric accelerometer during low-frequency measurements.

[0107] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention and within the spirit and principles of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A method for conditioning piezoelectric accelerometer signals for low-frequency vibration measurement, characterized in that, The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement includes: S1. Select two IC piezoelectric accelerometers (A-1) and install them in opposite directions on the adapter mounting base to form two branches. The two IC piezoelectric accelerometers (A-1) are used in pairs with opposite vibration pickup directions, so that the sensed alternating acceleration signals are out of phase with each other, while the internal IC power supply operation mode is the same. S2, using matching transistor pair (B) to form two constant current sources to power the internal IC circuits of the two paired IC piezoelectric accelerometers (A-1), so that the DC bias voltages of the two IC circuits are equal; S3, using multiple instrumentation amplifier chips to form a composite differential amplifier circuit (3), adopting a DC-coupled connection, introduces the signals of two IC piezoelectric accelerometers (A-1) into the differential amplifier circuit, canceling the DC bias voltage while doubling the sensitivity of the alternating acceleration signal, and the DC bias voltage is zero after subtraction. S4 utilizes the high common-mode rejection ratio (CMRR) of the composite differential amplifier (C) of the integrated instrumentation amplifier chip. After two stages of differential amplification, the first stage adopts a symmetrical cross-input method to reduce the input characteristic differences introduced by the potential difference at the input terminal and improve the balance at the input terminal, thereby improving the CMRR of the differential circuit. The second stage further suppresses common-mode noise through differential operation based on the differential output of the first stage.

2. The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to claim 1, characterized in that, In step S1, multiple IC piezoelectric accelerometers (A-1) have the same model number.

3. The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to claim 1, characterized in that, In step S2, the circuit structure of each constant current source in the multi-channel constant current source is identical.

4. The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to claim 1, characterized in that, In step S3, the composite differential amplifier circuit (C) is divided into two stages of differential amplification: the positive and negative input terminals of the input stage are cross-input to reduce the input characteristic difference introduced by the reference potential imbalance. After the two signals are subtracted in the two first-stage instrumentation amplifier chips (C-1) at the input terminals, the common-mode signal is reduced and the differential-mode signal is strengthened, and then enters the second-stage instrumentation amplifier chip (C-2) for differential amplification. There is no DC blocking capacitor in the entire composite differential amplifier circuit (C) channel. The IC piezoelectric accelerometer (A-1) signal is introduced into the differential amplifier circuit. After the DC bias voltage is subtracted, the DC bias voltages output by the two IC piezoelectric accelerometers (A-1) are equal and zero after subtraction. However, the alternating acceleration signal is 180 degrees out of phase and doubles after subtraction.

5. The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to claim 1, characterized in that, In step S4, the composite differential amplifier (C) is divided into two stages of differential amplification. The input stage has cross inputs at the positive and negative input terminals. The two signals are cross-subtracted in the two first instrumentation amplifier chips (C-1) at the input terminals and then enter the second stage instrumentation amplifier chip (C-2) for differential subtraction.

6. The piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to claim 1, characterized in that, After step S4, the signal output enters the integration circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier (C-3) to form a servo feedback loop and reduce zero drift.

7. A piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement, implementing the piezoelectric accelerometer signal conditioning method for low-frequency vibration measurement according to any one of claims 1-6, characterized in that, The piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement includes: The IC piezoelectric accelerometer uses a preamplifier circuit (A) composed of a field-effect transistor follower inside to sense the inversion of alternating acceleration signals. The matching transistor pair (B) forms a relatively consistent dual constant current source to power the preamplifier circuit (A); A composite differential amplifier circuit (C) composed of multiple instrumentation amplifier chips is used to introduce signals from multiple IC piezoelectric accelerometers (A-1) into the differential amplifier circuit in a DC-coupled connection manner. This cancels the DC bias voltage and doubles the sensitivity of the alternating acceleration signal. The difference between similar DC bias voltages is zero. Furthermore, by utilizing the high common-mode rejection ratio of the instrumentation amplifier chips, the common-mode noise is further suppressed after two-stage differential amplification.

8. The piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement according to claim 7, characterized in that, The preamplifier circuit (A) includes: IC piezoelectric accelerometer (A-1) There are multiple IC piezoelectric accelerometers (A-1). Multiple low-frequency IC piezoelectric accelerometers are installed in reverse to form two branches. They are used in pairs so that the sensed alternating acceleration signals are out of phase with each other. The field-effect transistor follower A-2 uses a drain-level follower configuration to convert the charge signal of the piezoelectric element into a voltage signal output.

9. The piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement according to claim 7, characterized in that, The matching transistor pair (B) includes multiple constant current sources that simultaneously power multiple corresponding IC piezoelectric accelerometers (A-1), generating consistent DC bias voltages.

10. The piezoelectric accelerometer signal conditioning circuit for low-frequency vibration measurement according to claim 7, characterized in that, The composite differential amplifier (C) is divided into two stages of differential amplification: the positive and negative input terminals of the input stage are cross-input to reduce the input characteristic difference introduced by the reference potential imbalance. After the two signals are subtracted in the two first-stage instrumentation amplifier chips (C-1) at the input terminals, the common-mode signal is naturally reduced and the differential-mode signal is strengthened. Then it enters the second-stage instrumentation amplifier chip (C-2) for differential amplification. After the signal is output, it enters the integrating circuit and is fed back to the reference potential input terminal of the final stage instrumentation amplifier (C-3) to form a servo feedback loop and suppress the zero-point drift of the low-frequency circuit.

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