Analog phase lock control system for mechanical resonance

Abstract

The invention relates to a simulation phase-locked control system for mechanical resonance. The simulation phase-locked control system comprises an excitation element, a harmonic oscillator and a vibration pick-up element which are connected in sequence, the vibration pick-up element is connected with the control unit through the first analog switch, the closed-loop self-excitation loop, the second analog switch, the first alternating current distributor and the waveform conversion circuit in sequence; the vibration pick-up element is connected with the control unit through a third analog switch, a phase-locked loop, a fourth analog switch, a second alternating current distributor and a waveform conversion circuit in sequence; the first alternating-current distributor is connected with the excitation element, and the second alternating-current distributor is connected with the phase-locked loop and the excitation element; the control unit is further connected with the first analog switch, the second analog switch, the third analog switch, the fourth analog switch and the phase-locked loop to control the same. The method perfectly avoids the defects of inaccurate positioning of the real resonant frequency of the harmonic oscillator under a single closed-loop self-excitation system, difficulty in initial frequency positioning under a single phase-locked loop system, difficulty in oscillation starting of the system and the like, and retains the advantages of the two.

Description

Technical Field

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

[0002] At present, there are a lot of liquid density measurement needs in the fields of petrochemical, aviation, papermaking, brewing, etc. For example, in the trade transfer of oil products, since my country uses mass as the final settlement unit, the measured liquid volume needs to be multiplied by the real-time density of the oil products to get the final value used for trading. For example, the aviation field needs to measure fuel density accurately in real time to obtain fuel quality and then evaluate the flight time of the aircraft. Most density measurement scenarios have high requirements for the real-time, accuracy, and stability of density measurement.

[0003] At present, there are two main density measurement methods: the density meter based on the Archimedes principle and the density meter based on the resonance principle. The density meters based on the Archimedes principle mainly include density bottles, glass floats, etc. Their main characteristics are poor real-time performance, large liquid volume, poor safety, low automation, and need to be sampled on site and then sent to the laboratory for measurement. Therefore, it is difficult to meet the density measurement needs of the above application scenarios. Density meters based on the resonance principle include resonant tube density meters, short cylinder density meters, tuning fork density meters, etc. Their characteristics are that they can be measured online, so real-time performance can be well guaranteed. In addition, they also have the advantages of good safety and easy to realize automated measurement. Therefore, in recent years, resonant density meters have gradually replaced density bottles, glass floats, etc. and become the mainstream density measurement equipment.

[0004] Although resonant density meters are classified in various ways due to the different shapes of the resonators, in fact, the principles of all resonant density meters are the same. Resonant density meters mainly consist of four parts: a resonator, an excitation element, a vibration pickup element, and a control system. The resonator is the main frequency element, which is used to generate a natural 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 tubes, tuning forks, etc.; the excitation element is mainly used to provide excitation force to excite the resonator to produce forced vibrations. Common ones include piezoelectric ceramics and electromagnetic coils; the vibration 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 vibrometers, etc.; the main function of the control system is to process the phase relationship between the excitation signal and the vibration 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, the resonant frequency needs to be converted into the corresponding liquid density value. According to the principle of mechanical vibration, it can be inferred that the density of the measured liquid has a quadratic function curve characteristic about the natural period of the resonator, and the coefficient of this function can be obtained by calibrating 2 to 3 liquids with known densities. The common control system is mainly closed-loop self-excited oscillation, and 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 that the resonator automatically maintains the resonant state to achieve density measurement.

[0005] However, the resonant density meters currently installed in various large oil storage bases, petrochemical plants, aircraft, 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, which is very easy to cause the resonator's natural frequency drift. Therefore, the density meter has low accuracy and poor stability.

[0006] The core components of the closed-loop self-excited oscillation circuit include amplifiers, analog filters, phase shifters, and variable gain amplifiers. When the vibration pickup element picks up the vibration signal, it is usually a very weak signal and needs to be amplified by an amplifier with a magnification of 10. 2 ~10 9 The filter is then required to filter out the interference of noise signals. 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 densitometer is 0~3000kg / m 3, the corresponding natural frequency of the resonator is 5000~3000Hz, then the center frequency of the filter should be set to 4000Hz, and the bandwidth should be set to 2000Hz; since the filter will produce phase lag, a phase shifter is required to compensate for the lagging phase. Usually, an all-pass filter is used, which can produce a 0~180° phase shift. The specific phase compensation value needs to be determined according to the phase lag of the filter at the resonant frequency point of the resonator. For example, if the filter lags 30° at the resonant frequency point of the resonator, the phase shifter needs to be set to 30° phase advance for compensation; the main function of the variable gain amplifier is gain adjustment, which reduces the amplification factor when the loop gain product is greater than 1, and increases the amplification factor when the loop gain product is less than 1. It can usually be achieved by using a variable gain amplifier circuit built around an operational amplifier or a dedicated voltage-controlled gain chip such as LMH6503.

[0007] Since the resonator, analog filter and phase shifter all have phase-frequency characteristics, the principle of closed-loop self-oscillation determines that the harmonic frequency generated by the system depends on the frequency point that makes the loop phase sum equal to 0. The resonator does not work at a fixed resonant frequency point, but works within a frequency range. The range depends on the liquid density range of the actual use scenario of the density meter. If the density meter is expected to be able to measure 0~3000kg / m 3 For liquids in the range of 5000~3000Hz, the corresponding natural frequency of the resonator is 5000~3000Hz. If the closed-loop self-excited oscillation system is to accurately excite the real natural frequency of the resonator under different liquids, the different phase lags generated by the analog filter at each frequency point in the frequency range must be accurately compensated by the phase shifter, that is, the phase-frequency characteristics of the phase shifter in the frequency range of 3000~5000Hz are required to be complementary to the analog filter, which is extremely difficult to achieve, at least it is impossible to achieve with the all-pass filter commonly used at present. Therefore, the resonant density meter using a closed-loop self-excited oscillation control system cannot guarantee that its resonator always works at the real resonant frequency point, and it can even be said that in most cases it works at a non-resonant frequency point. When the resonant density meter works at a non-resonant frequency point, the frequency is not only related to the liquid density, but also to various parameters such as the elastic modulus and damping of the resonator, which will inevitably lead to a decrease in the accuracy of the converted liquid density value. Summary of the invention

[0008] In order 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 vibration pickup element connected in sequence; the vibration 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 vibration 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 respectively connected to the phase-locked loop and the excitation element; 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.

[0009] The analog phase-locked control system for mechanical resonance of the present invention can be used for the construction of closed-loop control systems for resonant densitometers and other resonant sensors. It solves the technical problems of poor anti-disturbance capability, easy frequency drift, and difficulty in working at the true resonant frequency point of the resonator in the closed-loop self-excited oscillation system commonly used in current resonant densitometers, as well as the technical problems of difficulty in locating the initial frequency point and difficulty in starting the oscillation in the commonly used phase-locked loop system. In other words, the present invention perfectly avoids the defects of inaccurate positioning of the true resonant frequency of the resonator in a single closed-loop self-excited system and difficulty in locating the initial frequency in a single phase-locked loop system, and difficulty in starting the system, while retaining the advantages of both.

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

[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 front-stage vibration pickup element with the input impedance of the rear-stage multi-stage amplifier circuit, ensuring that the weak signal collected by the vibration pickup element can be effectively amplified. In particular, if the vibration 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 signal collected by the vibration pickup element.

[0012] Preferably, the closed-loop self-excited 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.

[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 which 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 symmetrical analog filter circuits.

[0015] Preferably, 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.

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

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

[0018] Preferably, the resonator is a mechanical frequency element for generating a resonant frequency that can reflect the liquid density value, the exciting element is an element for generating an exciting force to force the resonator to vibrate, and the vibration pickup element is an element for picking up the vibration signal of the resonator. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It 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 yes Figure 1 The specific signal flow diagram of the analog phase-locked control system is shown.

[0021] Figure 3 yes Figure 2 The algorithm flow chart of the control unit of the simulated phase-locked control system is shown. DETAILED DESCRIPTION

[0022] The present invention is 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, but not to limit 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 belong to the protection scope of the present invention. The specific process parameters and the like in the following examples are also only examples within a suitable range, that is, those skilled in the art can make a selection within a suitable range through the description of this article, and are not limited to the specific values ​​​​exemplified below.

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

[0024] Specifically, in this embodiment, the control unit may be a microcontroller unit MCU. The resonator is a mechanical frequency element, which is used to generate a resonant frequency that can reflect the liquid density value, and may usually be a long straight tube, a short tube, a tuning fork, etc. The exciting element is an element used to generate an exciting force to force the resonator to vibrate, and may usually be an electromagnetic coil, a piezoelectric ceramic, etc. The vibration pickup element is an element used to pick up the vibration signal of the resonator, and the vibration signal may be a vibration displacement, velocity, acceleration, etc. Common vibration pickup elements include piezoelectric ceramics, electromagnetic coils, laser vibrometers, etc.

[0025] Also like Figure 1 As shown, in the analog phase-locked control system, the vibration pickup element is also connected to the first analog switch A1 and the third analog switch B1 via the primary conditioning circuit. The primary conditioning circuit can optimize the signal output by the front stage vibration pickup element.

[0026] Figure 2 yes Figure 1 The specific signal flow diagram of the simulated phase-locked control system is shown, and Figure 2 It further shows Figure 1 The more specific structure of the analog phase-locked control system is shown in FIG. Figure 2 As shown, 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 A1 and the third analog switch B1 respectively. Therefore, the impedance matching network is used to match the output impedance of the front-stage pickup element with the input impedance of the rear-stage multi-stage amplifier circuit to ensure that the weak 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 is used to preliminarily amplify the weak signal collected by the pickup element.

[0027] Also like Figure 2 As shown, the closed-loop 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 A1 and the second analog switch A2.

[0028] The above-mentioned closed-loop self-excited loop is further described in detail as follows.

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

[0030] The phase shifting circuit can be used to compensate for the phase lag generated by the above-mentioned vibration pickup signal after passing through the first analog filtering circuit. The specific phase compensation value needs to be determined based on the phase lag of the filter at the resonant frequency point of the resonator. For example, if the filter has a phase lag of 30° at the resonant frequency point of the resonator, the phase shifter needs to be set to a 30° phase advance for compensation.

[0031] A variable gain amplifier circuit, for example, a variable gain amplifier can be used for gain adjustment to reduce the amplification factor when the loop gain product is greater than 1, and to increase the amplification factor when the loop gain product is less than 1. This can usually be achieved using 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 on and off 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 way is distributed to the excitation element to generate the excitation force, and the other way is distributed to the waveform conversion circuit for subsequent input into the MCU to calculate the signal frequency.

[0034] Also like Figure 2 As shown, 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 B1 and the fourth analog switch B2, and the 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 second analog filter circuit and the third analog filter circuit are symmetrical analog filter circuits. Preferably, the parameters of the second analog filter circuit and the third analog filter circuit are exactly the same.

[0035] The above phase-locked loop is further described as follows.

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

[0037] The second analog filter circuit can be used to filter out the noise in the vibration pickup signal after being 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 produced by the second analog filter circuit. The two are symmetrical analog filter circuits, and their parameter settings should be exactly the same, so that the pickup signal and the excitation signal produce the same phase lag, thereby ensuring that the relative phase difference between the two signals will not change due to the existence of the phase-frequency characteristics 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 the same 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 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 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 the voltage control signal, the PI controller will produce an integral effect, increase the frequency adjustment strength of the subsequent voltage-controlled oscillator, and quickly reduce 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 PI controller. When the phase difference between the vibration pickup signal after multi-stage amplification and filtering and the excitation signal after the same filtering is 0, the phase detector output is 0, the PI controller is no longer adjusted, and the output frequency of the voltage-controlled oscillator remains unchanged.

[0043] The fourth analog switch B2 can be used to control the on and 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 excitation force, another path is distributed to the waveform conversion circuit for subsequent input into 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 like Figure 2 As shown, 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.

[0046] The above-mentioned waveform conversion circuit 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 actual conditions.

[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 terminal 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 MCU's IO terminal.

[0050] In addition, regarding the MCU, its functions may include: first, by controlling the on and off of the first to fourth analog switches A1, A2, B1, and B2; second, capturing the square wave signal output by the voltage follower and calculating the signal frequency; third, converting the calculated signal frequency average value combined with the known calibration coefficient to obtain the liquid density value; fourth, outputting the measurement results to a screen such as an LCD for display.

[0051] Further references Figure 3 , Figure 3 Shows Figure 2 The MCU algorithm flow chart of the analog phase-locked control system is shown in Figure 1. Figure 3 As shown, first, in step S1, the system is powered on; then, 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 be turned on, and the third analog switch B1 and the fourth analog switch B2 to be turned off; then, in step S4, the closed-loop self-excitation loop is turned on, the phase-locked loop is turned off, and the resonator starts to oscillate; then, 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-excitation loop; then, in step S6, the signal frequency f1 is calculated; next, in step S7, it is determined whether the calculated frequency f1 is a valid resonant frequency. If the frequency f1 is a valid resonant frequency, that is, it is determined to be "Y" in step S7, it is deemed that the resonator has started to oscillate, and then the process proceeds to step S8. If the frequency f1 is not a valid resonant frequency, that is, it is determined to be "N" in step S7, the process proceeds to step 9. In step S9, the MCU reports an error, that is, the MCU displays "ERROR".

[0052] Specifically, the above-mentioned step of determining whether the frequency f1 is a valid resonant frequency can be used to prevent the closed-loop self-excitation loop from generating frequency chaos, resulting in an error in the initial frequency setting of the phase-locked loop. Usually, a frequency setting value can be manually set to compare with the calculated frequency f1. If the frequency f1 is greater than the frequency setting value, the frequency f1 is determined to be a valid resonant frequency.

[0053] As mentioned above, after the signal frequency f1 is judged as the effective resonant frequency, the process proceeds to step S8, and the MCU sets the initial excitation frequency of the phase-locked loop to f1; then, 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; further, 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; then, 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; then, in step S14, the density value 𝜌 is converted according to the calculated frequency f2; finally, in step S15, the result is displayed on the MCU. In addition, as Figure 3 As shown, in this embodiment, the above steps S12 to S15 are repeatedly cycled 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 converted according to the average value of the multiple signal frequencies combined with a known calibration coefficient.

[0054] The key points of the present invention are: (1) First, use the closed-loop self-excited loop to quickly start the resonator to locate the resonant frequency point of the resonator under the current liquid medium filling. Then, the MCU collects the current resonant signal through the waveform conversion circuit and calculates the frequency. If it is judged 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, switching to the phase-locked loop, and then the phase-locked loop fine-tunes the resonant frequency.

[0055] (2) The phase-locked loop uses a second analog filter circuit and a third analog filter circuit as symmetrical analog filter circuits to simultaneously filter out noise from the pickup signal and the excitation signal after multi-stage amplification. The second analog filter circuit and the third analog filter circuit use exactly the same parameters.

[0056] (3) A PI controller is added between the phase detector and the voltage-controlled oscillator to perform PI adjustment on the deviation signal output by the phase detector.

[0057] Specifically, in a single closed-loop self-excited oscillation system, since the resonator, analog filter and phase shifter all have phase-frequency characteristics, the principle of closed-loop self-oscillation determines that the harmonic frequency ultimately excited by the system depends on the frequency point that makes the loop phase sum equal to 0. The resonator does not operate at a fixed resonant frequency point, but rather operates within a frequency range that depends on the liquid density range of the actual use scenario of the densitometer. Since the phase shifter cannot achieve accurate compensation for every frequency in 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, which in turn leads to a decrease in density measurement accuracy. The degree of decrease is determined by the quality factor of the resonator.

[0058] In a single phase-locked loop system, the system oscillation is completely dependent on the control voltage output by the phase detector. If the initial frequency of the voltage-controlled oscillator deviates too far from the true resonant frequency of the resonator, the vibration signal picked up by the vibration pickup element will become extremely weak due to the high attenuation of the resonator itself far away from the resonance point. The control voltage output by the phase detector will be 0, the output frequency of the voltage-controlled oscillator will not change, and the system will eventually not oscillate. Therefore, the single phase-locked loop system has very high requirements for the setting of the initial frequency. Even if the initial frequency is accurately set near the true resonant frequency point of the resonator, the single phase-locked loop system cannot be applied to resonators with excellent performance with high quality factor and high density measurement sensitivity, because high density measurement sensitivity means that the true resonant frequency value of the resonator when filled with two media with large density differences will also differ greatly. For example, a resonator has a quality factor of 2000 and a density measurement sensitivity of -3Hz / kg∙m 3 , when filled with air, the real resonant frequency is 7000Hz, then when filled with water, the real resonant frequency is about 4006Hz. If a single phase-locked loop system is used and the initial frequency is set to 7000Hz, the system can oscillate in the air medium. However, when the water medium is introduced, the real resonant frequency of the resonator immediately drifts to 4006Hz, and at this time the voltage-controlled oscillator still outputs an excitation signal of 7000Hz. Then, at a quality factor of 2000, the bandwidth of the resonator is only 2Hz, which means that the resonator will be greatly attenuated under frequency excitation outside the range of [4005, 4007]Hz, and the vibration pickup element will not pick up the vibration signal, the phase detector output is 0, the output frequency of the voltage-controlled oscillator will always remain at 7000Hz, and the system will not be able to oscillate.

[0059] In contrast, the present invention first enables the closed-loop self-excitation loop to enable the system to start oscillating quickly and locate the resonant frequency point of the resonator under the current medium filling. Then, the MCU collects the current resonant signal through the waveform conversion circuit and calculates the frequency. If it is judged to be a valid frequency value, it is considered that the resonator has started oscillating. 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, and switch 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 fine-tunes the resonant frequency. Through this design, the defects of inaccurate positioning of the true resonant frequency of the resonator under a single closed-loop self-excitation system and the difficulty in positioning the initial frequency and the difficulty in starting the system under a single phase-locked loop system are perfectly avoided, and the advantages of both are retained.

[0060] In addition, the phase-locked loop adopts a second analog filter circuit and a third analog filter circuit that are formed as a symmetrical analog filter circuit, and simultaneously performs noise filtering on the pickup signal and the excitation signal that have undergone multi-stage amplification, and 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 caused by filtering is also the same, thereby avoiding the problem of needing to perform phase compensation for a single analog filter circuit in a single closed-loop self-excitation system and a single phase-locked loop system. Under such a circuit structure, the filtering effect can be achieved without introducing a phase error, and the positioning accuracy of the true resonant frequency of the resonator can be greatly improved.

[0061] 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 of poor anti-disturbance capability, easy frequency drift, and difficulty in working at the real resonant frequency point of the resonator in the closed-loop self-excited oscillation system commonly used in the current resonant density meter, as well as the technical problems of difficulty in locating the initial frequency point and difficulty in starting oscillation in the commonly used phase-locked loop system.

[0062] Without departing from the purpose of the basic characteristics of the present invention, the present invention can be embodied in various forms. Therefore, the embodiments of the present invention are used for illustration rather than limitation. Since the scope of the present invention is limited by the claims rather than the specification, all changes within the scope defined by the claims or within the equivalent scope of the 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.

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 closed-loop 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.

5. The system according to claim 1, characterized in that 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 which 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.

6. The system according to claim 5, characterized in that The second analog filter circuit and the third analog filter circuit are symmetrical analog filter circuits.

7. 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.

8. The system according to claim 5, characterized in that The control unit is connected to the voltage controlled oscillator to control it.

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

10. The system according to any one of claims 1 to 9, 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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