Analog Phase-Locked Control System for Mechanical Resonance

Abstract

The present invention relates to an analog phase-locked control system for mechanical resonance, comprising: an exciting element, a resonator and a pickup element connected in sequence; the pickup element is connected to a control unit via a first analog switch, a closed-loop self-excitation loop, a second analog switch, a first AC distributor and a waveform conversion circuit in sequence; the pickup element is connected to the control unit via a third analog switch, a phase-locked loop, a fourth analog switch, a second AC distributor and a waveform conversion circuit in sequence; the first AC distributor is connected to the exciting element, and the second AC distributor is connected to the phase-locked loop and the exciting element respectively; the control unit is further connected to the first analog switch, the second analog switch, the third analog switch, the fourth analog switch and the phase-locked loop respectively to control them. The present invention perfectly avoids the defects that the true resonance frequency of the resonator cannot be accurately located in a single closed-loop self-excitation system and the initial frequency is difficult to locate and the system is difficult to start up in a single phase-locked loop system, and retains the advantages of both.

Description

Technical Field

[0001] The present invention relates to the technical field of density measurement based on the resonance principle, and particularly to an analog phase-locked control system for mechanical resonance. Background Art

[0002] At present, there is a large demand for liquid density measurement in the fields of petrochemical industry, aviation, papermaking, brewing, etc. For example, in the trade settlement of oil products, since China uses mass as the final settlement unit, the measured liquid volume needs to be multiplied by the real-time density of the oil product to obtain the final value for trading. Another example is that in the aviation field, it is necessary to measure the fuel density accurately in real time to obtain the fuel mass and then evaluate the endurance time of the aircraft. Most density measurement scenarios have high requirements for the real-time performance, accuracy, and stability of density measurement.

[0003] At present, the density measurement methods mainly include two categories: density meters based on Archimedes' principle and density meters based on the resonance principle. Density meters based on Archimedes' principle mainly include density bottles, glass hydrometers, etc. Their main characteristics are poor real-time performance, large liquid consumption, poor safety, and low automation level. They need to sample on-site and then send the samples to the laboratory for measurement. Therefore, it is difficult to meet the requirements of the above application scenarios for density measurement. Density meters based on the resonance principle include resonance tube density meters, short tube density meters, tuning fork density meters, etc. Their characteristics are that they can perform on-line measurement, so the real-time performance can be well guaranteed. In addition, they also have advantages such as good safety and easy realization of automatic measurement. Therefore, in recent years, resonance density meters have gradually replaced density bottles, glass hydrometers, etc. and become the mainstream density measurement devices.

[0004] Although resonant density meters have various classifications due to different shapes of the resonator, in fact, the principles of all resonant density meters are interrelated. A resonant density meter mainly consists of four parts: a resonator, an exciting element, a pickup element, and a control system. Among them, the resonator is the main frequency element, which is used to generate an inherent frequency value that can reflect the density value of the contained liquid. According to different shapes, it can be divided into long straight tubes, short cylinders, tuning forks, etc.; the exciting element is mainly used to provide an exciting force to stimulate the resonator to generate forced vibration. Common ones include piezoelectric ceramics, electromagnetic coils, etc.; the pickup element is mainly used to pick up the vibration signal of the resonator, which can be vibration displacement, velocity, and acceleration. Common ones include piezoelectric ceramics, electromagnetic coils, laser vibration meters, etc.; the main function of the control system is to process the phase relationship between the excitation signal and the pickup signal. According to the frequency characteristics of the resonator itself, the phase relationship is maintained at the resonant frequency point to keep the resonator in a resonant state. In addition, it is necessary to convert the resonant frequency into the corresponding liquid density value. According to the principle of mechanical vibration, it can be deduced that the measured liquid density value has a quadratic function curve characteristic with respect to the inherent period of the resonator. The coefficients of this function can be obtained by calibrating with 2 to 3 liquids with known densities. The common control system is mainly closed-loop self-excited oscillation. Its principle is mainly to adjust the loop gain product to be greater than 1 and the loop phase sum to be equal to 0 at the resonant frequency point, so as to make the resonator automatically maintain the resonant state and realize density measurement.

[0005] However, at present, the resonant density meters equipped on various large oil storage bases, petrochemical factories, airplanes, and ships have problems such as poor performance parameters of the resonator itself, low quality factor and density measurement sensitivity, and the measurement circuit only uses a simple closed-loop self-excited oscillation system, with poor anti-disturbance ability and extremely easy to generate the phenomenon of drift of the inherent frequency of the resonator. Therefore, the density meter has low accuracy and poor stability.

[0006] The core components of a closed-loop self-excited oscillation circuit include an amplifier, an analog filter, a phase shifter, and a variable gain amplifier. When the pickup element picks up the vibration signal, which is usually a very weak small signal, it needs to be amplified by the amplifier, and the amplification factor ranges from 10 2 ~10 9 vary; then it is necessary to filter out the interference of noise signals by the filter. The center frequency and bandwidth of the filter usually need to be set according to the density measurement range. Specifically, if the density measurement range of the density meter is 0 to 3000 kg / m 3, the corresponding natural frequency of the harmonic oscillator is 5000 - 3000 Hz. Then the center frequency of the filter should be set to 4000 Hz, and the bandwidth should be set to 2000 Hz. Since the filter will generate a phase lag, a phase shifter is needed to compensate for the lagged phase. Usually, an all-pass filter is used, which can generate a phase shift of 0 - 180°. The specific phase compensation value needs to be determined according to the phase lag amount of the filter at the resonance frequency point of the harmonic oscillator. For example, if the filter has a 30° phase lag at the resonance frequency point of the harmonic oscillator, the phase shifter needs to be set with a 30° phase lead for compensation. The main function of the variable gain amplifier is gain adjustment, which realizes reducing the amplification factor when the loop gain product is greater than 1 and increasing the amplification factor when the loop gain product is less than 1. Usually, it can be implemented by a variable gain amplifier circuit built around an operational amplifier or a dedicated voltage-controlled gain chip such as LMH6503.

[0007] Since the harmonic oscillator, analog filter, and phase shifter all have phase-frequency characteristics, the principle of closed-loop self-excited oscillation determines that the harmonic frequency finally excited by this system depends on the frequency point where the loop phase sum is 0. The harmonic oscillator does not work at a fixed resonance frequency point, but within a frequency range, which depends on the liquid density range of the actual usage scenario of the densitometer. If it is desired that the densitometer can measure liquids within the range of 0 - 3000 kg / m 3 and the corresponding natural frequency of the harmonic oscillator is 5000 - 3000 Hz, then if the closed-loop self-excited oscillation system is to accurately excite the true natural frequency of the harmonic oscillator under different liquids, it is required that the different phase lag amounts generated by the analog filter at each frequency point within this frequency range be accurately compensated by the phase shifter. That is, it is required that the phase-frequency characteristic of the phase shifter in the 3000 - 5000 Hz frequency range is exactly complementary to that of the analog filter, which is extremely difficult to achieve, at least impossible for the all-pass filters commonly used currently. Therefore, a resonant densitometer using a closed-loop self-excited oscillation control system cannot ensure that its harmonic oscillator always works at the true resonance frequency point, and it can even be said that in most cases, it works at a non-resonance frequency point. When the resonant densitometer works at a non-resonance frequency point, this frequency is not only related to the liquid density but also related to various parameters such as the elastic modulus and damping of the harmonic oscillator, which will inevitably lead to a decrease in the accuracy of the calculated liquid density value. Summary of the Invention

[0008] To solve the above technical problems, the present invention provides an analog phase-locked control system for mechanical resonance, comprising: an excitation element, a resonator, and a pickup element connected in sequence; the pickup element is connected to a control unit via a first analog switch, a closed-loop self-excitation loop, a second analog switch, a first AC distributor, and a waveform conversion circuit in sequence; the pickup element is connected to the control unit via a third analog switch, a phase-locked loop, a fourth analog switch, a second AC distributor, and the waveform conversion circuit in sequence; the first AC distributor is connected to the excitation element, and the second AC distributor is connected to the phase-locked loop and the excitation element respectively; the control unit is further connected to the first analog switch, the second analog switch, the third analog switch, the fourth analog switch, and the phase-locked loop respectively to control them.

[0009] The analog phase-locked control system for mechanical resonance of the present invention can be used to build a closed-loop control system for a resonant density meter and other resonant sensors. It solves the technical problems existing in the commonly used closed-loop self-excitation oscillation system of the current resonant density meter, such as poor anti-disturbance ability, easy frequency drift, and difficulty in working at the true resonant frequency point of the resonator, as well as the technical problems existing in the commonly used phase-locked loop system, such as difficult initial frequency point positioning and difficult startup. That is to say, the present invention perfectly avoids the defects of inaccurate positioning of the true resonant frequency of the resonator under the single closed-loop self-excitation system and difficult initial frequency positioning and difficult startup of the system under the single phase-locked loop system, and retains the advantages of both.

[0010] Preferably, the pickup element is further connected to the first analog switch and the third analog switch respectively via a primary conditioning circuit. Thus, through this primary conditioning circuit, the signal output by the pre-stage pickup element can be optimized.

[0011] Preferably, the primary conditioning circuit includes an impedance matching network and a multi-stage amplifier circuit connected in sequence, and the multi-stage amplifier circuit is connected to the first analog switch and the third analog switch respectively. According to the present invention, the impedance matching network can be used to match the output impedance of the pre-stage pickup element and the input impedance of the post-stage multi-stage amplifier circuit, ensuring that the weak small signal collected by the pickup element can be effectively amplified. In particular, if the pickup element is a piezoelectric ceramic, the impedance matching network can be replaced by a charge amplifier. The multi-stage amplifier circuit can be used to preliminarily amplify the weak small signal collected by the pickup element.

[0012] Preferably, the closed-loop self-excitation loop includes a first analog filter circuit, a phase shift circuit, and a variable gain amplifier circuit connected in sequence between the first analog switch and the second analog switch.

[0013] Preferably, the phase-locked loop includes a second analog filter circuit, a phase detector, a low-pass filter, a PI controller, and a voltage-controlled oscillator that are sequentially connected between the third analog switch and the fourth analog switch, and the second AC distributor is also connected to the phase detector via the third analog filter circuit.

[0014] Preferably, the second analog filter circuit and the third analog filter circuit are symmetric analog filter circuits.

[0015] Preferably, the waveform conversion circuit includes a Schmitt trigger, a level conversion network, and a voltage follower that are sequentially connected. The first AC distributor and the second AC distributor are respectively connected to the Schmitt trigger, and the voltage follower is connected to the control unit.

[0016] Preferably, the control unit is connected to the voltage-controlled oscillator to control it.

[0017] Preferably, the parameter settings of the second analog filter circuit and the third analog filter circuit are exactly the same.

[0018] Preferably, the resonator is a mechanical frequency element for generating a resonant frequency that can reflect the liquid density value, the excitation element is an element for generating an excitation force to make the resonator vibrate under forced vibration, and the pickup element is an element for picking up the vibration signal of the resonator. Description of the Drawings

[0019] Figure 1 is a schematic principle block diagram of an analog phase-locked control system for mechanical resonance according to an embodiment of the present invention.

[0020] Figure 2 is Figure 1 the specific signal flow diagram of the illustrated analog phase-locked control system.

[0021] Figure 3 is Figure 2 the algorithm flow chart of the control unit of the illustrated analog phase-locked control system. Detailed Embodiments

[0022] The present invention will be further described below in conjunction with the accompanying drawings and the following embodiments. It should be understood that the accompanying drawings and the following embodiments are only used to illustrate the present invention, rather than limiting the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. The specific process parameters and the like in the following examples are also only an example within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, rather than being limited to the specific values in the following examples.

[0023] Specifically, Figure 1It is a schematic structural diagram of an analog phase-locked control system for mechanical resonance according to an embodiment of the present invention. As Figure 1 shown, the analog phase-locked control system for mechanical resonance in this embodiment includes: an exciting element, a resonator, and a pickup element connected in sequence. The pickup element is connected to the control unit via a first analog switch A1, a closed-loop self-excitation loop, a second analog switch A2, a first AC distributor, and a waveform conversion circuit in sequence. The pickup element is also connected to the control unit via a third analog switch B1, a phase-locked loop, a fourth analog switch B2, a second AC distributor, and the aforementioned waveform conversion circuit in sequence. Additionally, the aforementioned first AC distributor is also connected to the aforementioned exciting element, and the aforementioned second AC distributor is also connected to the phase-locked loop and the exciting element respectively. Furthermore, the control unit is also connected to the aforementioned first analog switch, second analog switch, third analog switch, fourth analog switch, and phase-locked loop respectively to control them.

[0024] Specifically, in this embodiment, the aforementioned control unit may be a micro-control unit MCU. The aforementioned resonator is a mechanical frequency element for generating a resonance frequency that can reflect the liquid density value, and can usually be a long straight pipe, a short cylinder, a tuning fork, etc. The aforementioned exciting element is an element for generating an exciting force to make the resonator vibrate under forced vibration, and can usually be an electromagnetic coil, a piezoelectric ceramic, etc. The aforementioned pickup element is an element for picking up the vibration signal of the resonator, and the vibration signal can be vibration displacement, velocity, acceleration, etc. Common pickup elements include piezoelectric ceramics, electromagnetic coils, laser vibrometers, etc.

[0025] Also as Figure 1 shown, in this analog phase-locked control system, the pickup element is also connected to the first analog switch A1 and the third analog switch B1 via a primary conditioning circuit respectively. Through this primary conditioning circuit, the signal output by the previous-stage pickup element can be optimized.

[0026] Figure 2 is Figure 1 the specific signal flow diagram of the analog phase-locked control system shown, and Figure 2 further shows Figure 1 the more specific structure of the analog phase-locked control system shown. As Figure 2 shown, the aforementioned primary conditioning circuit includes an impedance matching network and a multi-stage amplification circuit connected in sequence, and the multi-stage amplification circuit is connected to the first analog switch A1 and the third analog switch B1 respectively. Thus, the impedance matching network is used to match the output impedance of the previous-stage pickup element and the input impedance of the subsequent multi-stage amplification circuit to ensure that the weak small signal collected by the pickup element can be effectively amplified. Particularly, if the pickup element is a piezoelectric ceramic, the impedance matching network can be replaced by a charge amplifier. And the multi-stage amplification circuit is used to preliminarily amplify the weak small signal collected by the pickup element.

[0027] Also asFigure 2 As shown, the above-mentioned closed-loop self-excitation loop includes a first analog filter circuit, a phase-shifting circuit, and a variable-gain amplifier circuit that are sequentially connected between the first analog switch A1 and the second analog switch A2.

[0028] Regarding the above-mentioned closed-loop self-excitation loop, the details are further described as follows.

[0029] The first analog switch A1 can be used to control the conduction and cutoff of the pickup signal from the pickup element in the closed-loop self-excitation loop. The first analog filter circuit is used to filter out the noise existing in the pickup signal amplified by the multi-stage amplifier circuit. The center frequency and bandwidth of this first analog filter circuit need to be set according to the density measurement range. Specifically, if the density measurement range of the densitometer is 0~3000 kg / m 3 , and the natural frequency of the resonator corresponding to it is 5000~3000 Hz, then the center frequency of the filter can be set to 4000 Hz, and the bandwidth can be set to 2000 Hz.

[0030] The phase-shifting circuit can be used to compensate for the phase lag generated after the above-mentioned pickup signal passes through the first analog filter circuit. The specific phase compensation value needs to be determined according to the phase lag amount of the filter at the resonant frequency point of the resonator. For example, if the filter has a 30° phase lag at the resonant frequency point of the resonator, then the phase shifter needs to be set with a 30° phase lead for compensation.

[0031] The variable-gain amplifier circuit, for example, can use a variable-gain amplifier, which can be used for gain adjustment to achieve reducing the amplification factor when the loop gain product is greater than 1, and increasing the amplification factor when the loop gain product is less than 1. Usually, it can be implemented by a variable-gain amplifier circuit built around an operational amplifier or a dedicated voltage-controlled gain chip such as LMH6503.

[0032] The second analog switch A2 can be used to control the conduction and cutoff of the excitation source of the excitation element in the closed-loop self-excitation loop.

[0033] The first AC distributor can be used to distribute the AC signal generated by the variable-gain amplifier circuit. One path is distributed to the excitation element to generate an excitation force, and the other path is distributed to the waveform conversion circuit for subsequent input to the MCU to calculate the signal frequency.

[0034] Also as Figure 2 shown, the above-mentioned phase-locked loop includes a second analog filter circuit, a phase detector, a low-pass filter, a PI controller, and a voltage-controlled oscillator that are sequentially connected between the third analog switch B1 and the fourth analog switch B2. Moreover, the above-mentioned second AC distributor is also connected to the phase detector via the third analog filter circuit. In addition, the control unit is also connected to the voltage-controlled oscillator to control it. Specifically, the above-mentioned second analog filter circuit and the third analog filter circuit are symmetric analog filter circuits. Preferably, the parameter settings of the second analog filter circuit and the third analog filter circuit are exactly the same.

[0035] Regarding the above-mentioned phase-locked loop, it is described in further detail as follows.

[0036] The third analog switch B1 can be used to control the conduction and cut-off of the pickup signal from the pickup element in the phase-locked loop.

[0037] The second analog filter circuit can be used to filter out the noise existing in the pickup signal amplified by the multi-stage amplifier circuit. The center frequency and bandwidth of the second analog filter circuit need to be set according to the density measurement range.

[0038] The third analog filter circuit can be used to match the phase lag generated by the second analog filter circuit. The two are symmetric analog filter circuits, and their parameter settings should be exactly the same, so that the pickup signal and the excitation signal generate the same phase lag amount, thereby ensuring that the relative phase difference between the two signals will not change due to the existence of the phase-frequency characteristic of the filter circuit.

[0039] The phase detector can be used to compare the phase difference between the pickup signal after multi-stage amplification and filtering and the excitation signal after filtering. If the phase difference is not 0, a DC voltage control signal is output. The amplitude of the DC voltage control signal is related to the magnitude of the phase difference between the two. The larger the phase difference, the larger the DC voltage output by the phase detector, and vice versa.

[0040] The low-pass filter can be used to filter out the high-frequency components existing in the DC voltage control signal output by the phase detector.

[0041] The PI controller can be used to adjust the DC voltage control signal output by the phase detector. If the phase detector continuously outputs a voltage control signal, the PI controller will produce an integral effect, increasing the frequency adjustment strength of the subsequent voltage-controlled oscillator and quickly reducing the phase deviation.

[0042] The voltage-controlled oscillator can be used to generate the excitation signal required by the excitation element. Its initial output frequency is set by the MCU, and the subsequent output frequency is adjusted by the phase detector and the PI controller. When the phase difference between the pickup signal after multi-stage amplification and filtering and the excitation signal after filtering is 0, the output of the phase detector is 0, and the PI controller no longer adjusts, and the output frequency of the voltage-controlled oscillator remains unchanged.

[0043] The fourth analog switch B2 can be used to control the conduction and cut-off of the excitation source of the excitation element in the phase-locked loop.

[0044] The second AC distributor can be used to distribute the AC signal generated by the voltage-controlled oscillator. One path is distributed to the excitation element to generate an excitation force, another path is distributed to the waveform conversion circuit for subsequent input to the MCU to calculate the signal frequency, and another path is distributed to the third analog filter circuit for phase comparison with the pickup signal after multi-stage amplification and filtering.

[0045] Also as Figure 2 shown, the above waveform conversion circuit includes a Schmitt trigger, a level conversion network, and a voltage follower connected in sequence. The above first AC distributor and second AC distributor are respectively connected to the Schmitt trigger, and the voltage follower is connected to the control unit.

[0046] Regarding the above waveform conversion circuit, it is further described in detail as follows.

[0047] The Schmitt trigger can be used to convert the sinusoidal AC signals input by the first and second AC distributors into square-wave AC signals, and the upper and lower threshold voltages can be adjusted according to the actual situation.

[0048] The level conversion network can be used to convert the square-wave AC signal output by the Schmitt trigger into the level range supported by the IO port of the MCU.

[0049] The voltage follower can be used to match the output impedance of the level conversion network and the input impedance of the IO port of the MCU.

[0050] In addition, regarding the MCU, its functions can include: one is to control the conduction and cutoff of the first to fourth analog switches A1, A2, B1, B2; the second is to capture the square-wave signal output by the voltage follower and calculate the signal frequency; the third is to convert the calculated average signal frequency into a liquid density value in combination with known calibration coefficients; the fourth is to output the measurement result to a screen such as an LCD for display.

[0051] Further referring to Figure 3 , Figure 3 shows Figure 2 the MCU algorithm flowchart of the analog phase-locked control system shown. As Figure 3 shown, first in step S1, the system is powered on; subsequently, in step S2, the MCU is initialized: next, in step S3, the MCU controls the first analog switch A1 and the second analog switch A2 to conduct, and the third analog switch B1 and the fourth analog switch B2 to cutoff; then, in step S4, the closed-loop self-excited loop conducts, the phase-locked loop cuts off, and the resonator starts to oscillate; subsequently, in step S5, the MCU captures the PWM output signal of the voltage follower, that is, the MCU captures the PWM output signal from the closed-loop self-excited loop; then, in step S6, the signal frequency f1 is calculated; next, in step S7, it is judged whether the calculated frequency f1 is a valid resonance frequency. If the frequency f1 is a valid resonance frequency, that is, judged as "Y" in step S7, it is considered that the resonator has started to oscillate, and then enters step S8. If the frequency f1 is not a valid resonance frequency, that is, judged as "N" in step S7, then enter step 9. In step S9, the MCU reports an error, that is, the MCU displays "ERROR".

[0052] Specifically, the step of determining whether the above-mentioned judgment frequency f1 is the effective resonance frequency can be used to prevent the closed-loop self-excitation loop from generating chaotic frequency values, resulting in incorrect initial frequency setting of the phase-locked loop. Usually, a frequency setting value can be artificially set and compared with the calculated frequency f1. If the frequency f1 is greater than the frequency setting value, then the frequency f1 is determined to be the effective resonance frequency.

[0053] As described above, after the signal frequency f1 is determined to be the effective resonance frequency, it enters step S8, where the MCU sets the initial excitation frequency of the phase-locked loop to f1; subsequently, in step S10, the MCU controls the first analog switch A1 and the second analog switch A2 to turn off, and the third analog switch B1 and the fourth analog switch B2 to turn on; furthermore, in step S11, the closed-loop self-excitation loop is turned off and the phase-locked loop is turned on, that is, it switches to the phase-locked loop; subsequently, in step S12, the MCU captures the PWM output signal of the voltage follower, that is, the MCU captures the PWM output signal from the phase-locked loop; then, in step S13, the signal frequency f2 is calculated; subsequently, in step S14, according to the calculated frequency f2, the density value 𝜌 is converted; finally, in step S15, the result is displayed on the MCU. In addition, as Figure 3 shown, in this embodiment, the above steps S12 to S15 are repeatedly looped to perform multiple calculations. For example, in a specific embodiment, it can be repeated up to 10,000 times, and then the liquid density value is obtained by converting according to the average value of multiple signal frequencies in combination with the known calibration coefficient.

[0054] The key points of the present invention are:

[0055] (1) First, use the closed-loop self-excitation loop to quickly start the resonator and locate the resonance frequency point of the resonator under the current liquid medium filling. Then, the MCU collects the current resonance signal through the waveform conversion circuit and calculates the frequency. If it is determined to be a valid frequency value, it is considered that the resonator has started to oscillate. Then, the MCU controls the first analog switch A1 and the second analog switch A2 to turn off, and the third analog switch B1 and the fourth analog switch B2 to turn on, switches to the phase-locked loop, and then the phase-locked loop finely tunes the resonance frequency.

[0056] (2) The phase-locked loop uses the second analog filter circuit and the third analog filter circuit as symmetric analog filter circuits to simultaneously filter the noise of the pickup signal and the excitation signal after multiple-stage amplification. The second analog filter circuit and the third analog filter circuit use exactly the same parameters.

[0057] (3) A PI controller is added between the phase discriminator and the voltage-controlled oscillator to perform PI adjustment on the deviation signal output by the phase discriminator.

[0058] Specifically, in a single closed-loop self-excited oscillation system, since the phase-frequency characteristics exist in all of the resonator, analog filter, and phase shifter, the principle of closed-loop self-excited oscillation determines that the harmonic frequency finally generated by the excitation of this system depends on the frequency point that makes the loop phase sum to 0. The resonator does not operate at a fixed resonance frequency point but within a frequency range, which depends on the liquid density range of the actual usage scenario of the densitometer. Since the phase shifter cannot achieve precise compensation at every frequency within the entire frequency range, the single closed-loop self-excited oscillation system will inevitably cause the resonant densitometer to operate at a non-resonant frequency point, thereby leading to a decrease in density measurement accuracy. The degree of decrease is determined by the quality factor of the resonator.

[0059] In a single phase-locked loop system, the startup of the system completely depends on the control voltage output by the phase detector. If the initial frequency of the voltage-controlled oscillator deviates too far from the true resonance frequency of the resonator, the vibration signal picked up by the pickup element will become extremely weak due to the high attenuation of the resonator itself far from the resonance point, and the control voltage output by the phase detector will be 0. Then the output frequency of the voltage-controlled oscillator will not change, and the system will ultimately not start up. Therefore, the single phase-locked loop system has high requirements for the setting of the initial frequency. Even if the initial frequency is accurately set near the true resonance frequency point of the resonator, the single phase-locked loop system cannot be applied to resonators with excellent performance of high quality factor and high density measurement sensitivity. Because high density measurement sensitivity means that the true resonance frequency values of the resonator under two media with relatively large density differences will also vary greatly. For example, a certain resonator has a quality factor of 2000 and a density measurement sensitivity of -3Hz / kg∙m 3 , when filled with air, the true resonance frequency is 7000Hz. Then when filled with water, the true resonance frequency is approximately 4006Hz. If a single phase-locked loop system is adopted and the initial frequency is set to 7000Hz, the system can start up in the air medium. However, when water medium is introduced, the true resonance frequency of the resonator immediately drifts to 4006Hz. At this time, the voltage-controlled oscillator still outputs an excitation signal of 7000Hz. Then, with a quality factor of 2000, the bandwidth of the resonator is only 2Hz, which means that the resonator will be greatly attenuated under the excitation of frequencies outside the range of [4005, 4007]Hz, and the pickup element will not be able to pick up the vibration signal. The output of the phase detector is 0, and the output frequency of the voltage-controlled oscillator will always remain at 7000Hz, and the system will not be able to start up.

[0060] In contrast, the present invention first enables a closed-loop self-excitation loop to quickly start the oscillation of the system and locate the resonant frequency point of the resonator under the current medium filling. Then, the MCU collects the current resonant signal through a waveform conversion circuit and calculates the frequency. If it is determined to be a valid frequency value, it is considered that the resonator has started to oscillate. Next, the MCU controls the first analog switch A1 and the second analog switch A2 to turn off, and the third analog switch B1 and the fourth analog switch B2 to turn on, switching to the phase-locked loop. The initial frequency of the voltage-controlled oscillator is accurately set by the MCU according to the signal frequency excited by the closed-loop self-excitation loop, and then the phase-locked loop finely tunes the resonant frequency. Through this design, the defects of inaccurate positioning of the true resonant frequency of the resonator in the single closed-loop self-excitation system and difficult initial frequency positioning and difficult system startup in the single phase-locked loop system are perfectly avoided, while retaining the advantages of both.

[0061] In addition, the phase-locked loop adopts a second analog filter circuit and a third analog filter circuit formed as a symmetric analog filter circuit to filter out noise from the vibration pickup signal and the excitation signal after multiple-stage amplification at the same time. The second analog filter circuit and the third analog filter circuit adopt exactly the same parameters; since the parameters of the second analog filter circuit and the third analog filter circuit are exactly the same, the phase lag generated by filtering is also the same, avoiding the problem of phase compensation required in a single analog filter circuit in the single closed-loop self-excitation system and the single phase-locked loop system. Under such a circuit structure, not only can the filtering effect be achieved, but also phase error is not introduced, which can greatly improve the positioning accuracy of the true resonant frequency of the resonator.

[0062] In summary, the analog phase-locked control system for mechanical resonance of the present invention can be used to build a closed-loop control system for a resonant density meter and other resonant sensors. It solves the technical problems existing in the commonly used closed-loop self-excitation oscillation system of the current resonant density meter, such as poor anti-disturbance ability, easy frequency drift, and difficulty in operating at the true resonant frequency point of the resonator, as well as the technical problems existing in the commonly used phase-locked loop system, such as difficult initial frequency point positioning and difficult startup.

[0063] Without departing from the gist of the basic features of the present invention, the present invention can be embodied in various forms. Therefore, the embodiments in the present invention are for illustration rather than limitation. Since the scope of the present invention is defined by the claims rather than by the specification, and all changes falling within the scope defined by the claims, or within the equivalent scope defined by the claims, should be understood to be included in the claims.

Claims

1. An analog phase-locked control system for mechanical resonance, characterized in that: include: An exciting element, a resonator and a vibration pickup element connected in sequence; The vibration pickup element is connected to the control unit via the first analog switch, the closed-loop self-excitation loop, the second analog switch, the first AC distributor, and the waveform conversion circuit in sequence; The vibration pickup element is connected to the control unit via the third analog switch, the phase-locked loop, the fourth analog switch, the second AC distributor, and the waveform conversion circuit in sequence; The first AC distributor is connected to the excitation element, and the second AC distributor is connected to the phase-locked loop and the excitation element respectively; The control unit is also respectively connected to the first analog switch, the second analog switch, the third analog switch, the fourth analog switch and the phase-locked loop to control them; The closed self-excitation loop includes a first analog filter circuit, a phase shift circuit and a variable gain amplifier circuit which are sequentially connected between the first analog switch and the second analog switch; The phase-locked loop includes a second analog filter circuit, a phase detector, a low-pass filter, a PI controller and a voltage-controlled oscillator connected in sequence between the third analog switch and the fourth analog switch, and the second AC distributor is also connected to the phase detector via the third analog filter circuit; The control unit is connected to the voltage controlled oscillator to control it; The waveform conversion circuit includes a Schmitt trigger, which is used to convert the sinusoidal AC signal input by the first AC distributor and the second AC distributor into a square wave AC signal.

2. The system according to claim 1, characterized in that The vibration pickup element is also connected to the first analog switch and the third analog switch respectively via a primary conditioning circuit.

3. The system according to claim 2, characterized in that The primary conditioning circuit includes an impedance matching network and a multi-stage amplifying circuit connected in sequence, and the multi-stage amplifying circuit is respectively connected to the first analog switch and the third analog switch.

4. The system according to claim 1, characterized in that The second analog filter circuit and the third analog filter circuit are symmetrical analog filter circuits.

5. The system according to claim 1, characterized in that The waveform conversion circuit includes a Schmitt trigger, a level conversion network and a voltage follower connected in sequence, the first AC distributor and the second AC distributor are respectively connected to the Schmitt trigger, and the voltage follower is connected to the control unit.

6. The system according to claim 4, characterized in that The parameter settings of the second analog filter circuit and the third analog filter circuit are completely the same.

7. The system according to any one of claims 1 to 6, characterized in that The resonator is a mechanical frequency element used to generate a resonant frequency that can reflect the liquid density value, the exciting element is an element used to generate an exciting force to force the resonator to vibrate, and the vibration pickup element is an element used to pick up the vibration signal of the resonator.

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