Resonant frequency tracking control system and control method for ultrasonic transducer

Abstract

The invention relates to the technical field of metering detection, in particular to an ultrasonic transducer resonant frequency tracking control system and method, and the system comprises a programmable DDS module, an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, and an ultrasonic frequency control module. The programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit; the phase detection circuit is used for obtaining phase difference; the ultrasonic signal processing unit is used for converting the measured time difference into a pressure parameter; the programmable DDS module is used for determining the frequency quantity needing to be changed according to the phase difference; and the ultrasonic frequency control module is used for determining the tracking frequency value corresponding to the maximum current value as a resonant frequency value, adding the resonant frequency value with the frequency quantity needing to be changed, and determining and controlling the output frequency of the ultrasonic signal generator. Through the control system and the control method, the anti-interference capability and the stability of the circuit are higher.

Description

Technical Field

[0001] The present invention relates to the technical field of measurement and detection, and in particular to an ultrasonic transducer resonant frequency tracking control system and a control method. Background Art

[0002] Ultrasonic transducers are devices that convert electrical energy into sound energy and are commonly used in the fields of medicine, industry, and testing. Ultrasonic waves have many advantages in measuring the flow and pressure of fluids. Compared with traditional mechanical and electromagnetic instruments, they have strong adaptability to pipe diameters, are non-contact with fluids, are easy to use, and are easy to manage digitally. Since the ultrasonic transducer itself has an inherent resonant frequency, it can only operate most stably and efficiently at this inherent resonant frequency. However, the resonant frequency of ultrasonic transducers will drift due to factors such as temperature, environment, and aging of components. Therefore, choosing a suitable transducer is crucial to whether the entire system can work as expected.

[0003] The main resonant frequency tracking methods currently on the market usually adopt the following methods: 1. Use phase-locked tracking. This method achieves frequency matching between the ultrasonic power supply and the ultrasonic transducer and its tool head by connecting a series or parallel reactance capacitor, and then detects the output voltage and current of the entire power supply, and achieves frequency tracking by voltage and current phase locking. Since the PLL tracking circuit has high requirements for the feedback signal obtained, once the feedback signal is interfered by the outside world, the PLL may not be able to lock the resonant frequency. In addition, the PLL tracking frequency range is determined by external capacitors and resistors, and different capacitor and resistor values need to be changed for different frequency bands. This control matching has a very small adaptation range. Changes in the parameters of the transducer and its tool head will cause its frequency to shift, which will lead to phase tracking failure and can only be limited to a certain frequency range.

[0004] 2. Maximum current feedback method. The maximum current feedback method can achieve the maximum current in the circuit when the external voltage remains unchanged. The software is used to determine the current size on the line for each sampling and search for the frequency value under the maximum current. Frequency tracking and matching are achieved through optimization and intelligent editing of the control algorithm. In order to solve the problems of narrow tracking range of PLL, different frequency bands, different filter network parameters, and the maximum current feedback method involves many components, complex line parameters, easy zero drift, relatively high failure rate, and poor flexibility. In addition, there are shortcomings such as narrow control area and complex implementation. Due to the multi-peak problem in the ultrasonic vibration system, the maximum current feedback method is difficult to track in real time if the phase-locked loop hardware circuit is used alone. It may track to a non-resonant point and is prone to lock or loss of lock.

[0005] III. Adopt current perturbation tracking. This method requires perturbing the output frequency at all times and is difficult to adapt when the load changes rapidly. The tracking speed is slow and it is easy to cause oscillation of the power supply itself.

[0006] For the existing frequency automatic tracking methods, most of them linearly adjust the output and have a slow frequency tracking response. The analog phase-locked frequency tracking is vulnerable to signal interference and deadlock phenomena occur. Moreover, the frequency offset process caused by external factors is non-linear and it is difficult to explain with an accurate mathematical model. To sum up, the above methods have deficiencies such as transducer detuning, difficult parameter debugging, and slow tracking speed, and are not the best solutions. Summary of the Invention

[0007] To solve the above technical problems, the present invention proposes a resonant frequency tracking control system and control method for an ultrasonic transducer, which is easy to implement in hardware, has higher circuit anti-interference ability and stability, and can quickly, accurately find and track the resonant frequency of the ultrasonic transducer in real-time, improving the efficiency and stability of the ultrasonic transducer.

[0008] The present invention is implemented by adopting the following technical solutions: A resonant frequency tracking control system for an ultrasonic transducer includes a programmable DDS module, an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, and an ultrasonic frequency control module connected in sequence; the programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit at the same time; the phase detection circuit is used to input an excitation signal into the ultrasonic transducer, control the time delay of the excitation or reception pulse of each element in the array of the ultrasonic transducer, collect the operation state data of the ultrasonic transducer in real-time, test the phase difference between the voltage and current on the load, and obtain the phase difference; the ultrasonic signal processing unit is used to measure the time difference between the ultrasonic transmission signal and the reception signal and convert the measured time difference into the measured pressure parameter; the programmable DDS module is used to determine the frequency amount that needs to be changed according to the phase difference; the ultrasonic frequency control module is used to track the resonant frequency and current of the ultrasonic transducer, determine the resonant frequency value as the tracking frequency value corresponding to the maximum current value, and add the resonant frequency value to the frequency amount that needs to be changed confirmed by the programmable DDS module to determine and control the output frequency of the ultrasonic signal generator.

[0009] It also includes an adaptive filter and a low-pass filter, and the ultrasonic signal generator, the adaptive filter, and the low-pass filter are connected in series at the output end of the ultrasonic frequency control module in sequence.

[0010] The phase detection circuit includes a dual voltage comparator and a dual D flip-flop.

[0011] The programmable DDS module includes a phase accumulator, a phase register, a digital-to-analog converter, a waveform memory, a digital-to-analog converter, and a phase-amplitude conversion module, and is used to output the frequency of the sine wave and the amplitude signal of the sine wave.

[0012] The frequency of the output sine wave specifically refers to: using an external reference clock Tc and an N-bit phase register to increment by a step M, and the phase register returns to the initial position after every 2 N / M external reference clocks, and a sine wave frequency is output at the output end of the digital-to-analog converter.

[0013] The amplitude signal of the output sine wave specifically refers to: the phase accumulator accumulates the frequency control word K under the control of the reference clock, uses the accumulated value as an address code to query the amplitude value stored in the waveform memory, adds the output of the phase register to the phase control word and then inputs it to the sine lookup table address, maps the input address phase information into a digital quantity signal of the sine wave amplitude, drives the digital-to-analog converter to perform D / A conversion, and outputs an analog quantity.

[0014] The ultrasonic signal processing unit includes a microprocessor, an ADC digital-to-analog converter, and a TDC time-to-digital converter.

[0015] Measuring the time difference between the ultrasonic transmit signal and the receive signal specifically refers to: when the ultrasonic transducer collects the ultrasonic excitation signal, starting the ADC digital-to-analog converter to collect the measurement signal; after the data collection is completed, performing FIR digital filtering on the data, performing a convolution operation, and performing a fast Fourier transform on the output result to obtain the frequency response, changing the relative proportion of the frequency components contained in the input signal, outputting the input at each moment multiplied by the corresponding weight coefficient, and then performing superposition to obtain the weighting coefficient; performing an autocorrelation algorithm operation on the filtered data to determine the statistic of the time series data correlation, calculating the ultrasonic signal propagation time, measuring the correlation of the random process, finding the period value of the periodic signal sequence submerged by noise through autocorrelation operation, then converting the time of the pulse to be measured into the corresponding phase, and finally obtaining the time value of the pulse to be measured through calculation.

[0016] When the ADC digital-to-analog converter collects the measurement signal, it collects the waveform, and at the same time enables direct memory access (DMA) data transmission. At the rising edge of the clock, A / D conversion starts, and at the falling edge of the clock, the voltage conversion data is read. The A / D sampling frequency is controlled by controlling the frequency of the clock signal. At the same time, the TDC time-to-digital converter is driven to record the transmit signal time node. The result of the ADC sampling is directly stored in the RAM area of the specified waveform memory by DMA. The output of the waveform memory becomes a staircase wave after digital-to-analog conversion.

[0017] Converting the measured time difference into the pressure parameter to be measured specifically means that after reading the calibration data and after multiple measurements of the time interval and temperature measurement are realized, median filtering is respectively performed on the two groups of measured time values, and median filtering is performed on the measured temperature value; the two time values measured are fused through the Kalman fusion algorithm to obtain the final ultrasonic propagation time, and through bilinear interpolation calculation, the measured time value is converted into the pressure value of the liquid in the pipeline to be measured.

[0018] A method for tracking and controlling the resonant frequency of an ultrasonic transducer, in which the ultrasonic signal generated by the ultrasonic signal generator is transmitted to the ultrasonic transducer; the phase difference between the voltage and current on the load is tested to obtain the phase difference; the phase difference is compared with the given phase difference, and the frequency amount to be changed is output through fuzzy PI control; the resonant frequency and current of the ultrasonic transducer are tracked, the tracking frequency value corresponding to the maximum current value is determined as the resonant frequency value, and the resonant frequency value is added to the frequency amount to be changed to determine and control the output frequency of the ultrasonic signal generator, so that the output frequency of the ultrasonic signal generator can track the resonant frequency point of the ultrasonic transducer within the corresponding range.

[0019] Compared with the prior art, the beneficial effects of the present invention are manifested in: 1. In the present invention, through the phase detection circuit, a programmable DDS module is connected in parallel, and a ultrasonic signal processing unit connected to the output end of the ultrasonic transducer, and a ultrasonic signal generator connected in series in sequence through the ultrasonic frequency control module, the design is reasonable and the hardware implementation is easy.

[0020] In the present invention, the programmable DDS module combines hardware and software tracking algorithms through the phase detection circuit to perform frequency tracking of the ultrasonic transducer. For the real-time working condition of the ultrasonic transducer, it can quickly, accurately and stably find and track the new resonant frequency of the ultrasonic transducer, improve the efficiency, stability and digital frequency automatic tracking of the ultrasonic transducer, and has the advantages of small calculation amount, fast tracking speed and high accuracy.

[0021] Aiming at the problems of phase detection error and non-ideal problems, the present invention controls the frequency of the frequency tracking signal provided by the ultrasonic signal generator to track the resonant frequency point of the transducer within a certain range. Compared with the traditional control, it has stable and reliable operation, continuously adjustable ultrasonic power, and can maximize the potential of the ultrasonic transducer. The working frequency is automatically tracked, so that the output matching is better, the power is stronger, the efficiency is higher, and it has more advantages than the phase locking and static feedback methods.

[0022] 2. In the present invention, through the low-pass filter designed by matching, a large number of high-order harmonics can be filtered out, the voltage waveform of the ultrasonic signal generator can be improved, and a good sine wave voltage and current can be obtained at both ends of the ultrasonic transducer.

[0023] 3. The present invention extracts a feedback voltage proportional to the oscillating current of the ultrasonic transducer manipulator through a phase detection circuit, enabling the closed-loop system to oscillate freely at the mechanical resonance frequency of the transducer. As a result, the ultrasonic transducer is excited to vibrate, and its vibration frequency is the natural frequency of the ultrasonic transducer itself. Since the system self-oscillates at the mechanical resonance frequency of the ultrasonic transducer, the circuit system can promptly track even the slightest change in the resonance frequency of the ultrasonic transducer, ensuring that the operation always remains in the optimal state. Consequently, good frequency automatic tracking performance is obtained, guaranteeing that the feedback voltage is related only to the mechanical oscillating current within a wide frequency band, with reliable tracking and small offset. Furthermore, the transducer efficiency and signal-to-noise ratio of the ultrasonic transducer (PMUT) can be improved, purer-mode ultrasonic waves can be excited and received, interference from other ultrasonic signals can be reduced, and the anti-interference ability and stability of the circuit are higher.

[0024] 4. The phase detection circuit includes a dual voltage comparator and a dual D flip-flop. With this configuration, it can be directly connected to a computer interface or connected to a digital voltmeter through a low-pass filter to form a phase detector. It has the advantages of simple circuit structure, high precision, and strong anti-interference ability. The detection accuracy is better than 0.1° in the range of 2 MHz to 5 MHz, reducing the phase shift changes caused by input signal and external condition variations.

[0025] 5. The present invention precisely controls the phase of the phased beam in the transmit / receive state by controlling the time delay of the excitation or reception pulses of each element in the ultrasonic transducer array to obtain optimal beam characteristics. By changing the phase relationship of the ultrasonic waves transmitted / received by each element when reaching or coming from a certain point within an object, the changes in the focal point and beam orientation are achieved. The ultrasonic transducer uses many small piezoelectric wafers, which form a wafer group. The total energy radiated by the wafer group forms an ultrasonic beam, generating a controllable beam angle and focusing depth, and realizing sound field control through the combination of local wafer units. By driving each array unit with the same pulsed voltage and precisely controlling the phase of the phased beam in the transmit / receive state, optimal beam characteristics can be obtained.

[0026] 6. The present invention establishes a closed-loop for automatic tracking of the resonance frequency. Based on the effectiveness of the resonance frequency automatic tracking strategy using the PI control algorithm, it ensures that the impedance angle of the system is stabilized in a certain compact set and can guarantee the tracking control of the system to the natural resonance frequency, making the operation more reliable. Moreover, it can not only ensure the tracking of the system to the natural resonance frequency but also guarantee the dynamic performance of its impedance angle, enabling the system to maintain a good working state and better stabilize near the natural resonance frequency, thereby achieving an ideal control effect.

[0027] 7. The present invention adopts an ultrasonic frequency control module to track the resonant frequency and current of the ultrasonic piezoelectric transducer, and determines the tracking frequency value corresponding to the maximum current value as the resonant frequency value. It can not only automatically search for the resonant frequency of the transducer before the ultrasonic transducer starts to work, so that the designed power supply can automatically adapt to ultrasonic transducers with different resonant frequencies, but also accurately track the resonant frequency drift that occurs during the operation of the ultrasonic transducer. Compared with analog PI control, the parameter adjustment is more convenient.

[0028] 8. The present invention uses a phase detection circuit to compare two sinusoidal signals with a phase difference having a time interval or pulse width with the positive or negative zero-crossing moment. The voltage U0 and current signal I0 with a phase difference △t of φ are respectively converted into square waves by corresponding zero-crossing comparators, and square wave signals are output. The square wave output at a certain frequency contains a large number of harmonic components.

[0029] 9. Through this tracking control method, the present invention can ensure that the output power of the ultrasonic transducer is relatively stable and will not change significantly due to changes in the load. In this way, accurate results can be obtained through the ultrasonic transducer resonant frequency technology, and the pressure of the liquid and other medium characteristics can be accurately measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will be further described in detail below in conjunction with the specification drawings and specific embodiments, where: Figure 1 is a schematic structural diagram of the present invention; Figure 2 is a schematic diagram of related method time measurement in the present invention; Figure 3 is a schematic diagram of time-pressure conversion in the present invention. SPECIFIC EMBODIMENTS

[0031] Embodiment 1 As a basic embodiment of the present invention, the present invention includes an ultrasonic transducer resonant frequency tracking control system, which includes a programmable DDS module, and an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, and an ultrasonic frequency control module that are connected in sequence. The programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit at the same time.

[0032] The phase detection circuit is used to input an excitation signal into the ultrasonic transducer, control the time delay of the excitation or reception pulse of each element in the array of the ultrasonic transducer, collect the operation state data of the ultrasonic transducer in real time, measure the phase difference between the voltage and current on the load, and obtain the phase difference. The ultrasonic signal processing unit is used to measure the time difference between the ultrasonic transmission signal and the reception signal, and convert the measured time difference into the pressure parameter to be measured. The programmable DDS module is used to determine the frequency amount to be changed according to the phase difference. The ultrasonic frequency control module is used to track the resonant frequency and current of the ultrasonic transducer, determine the resonant frequency value corresponding to the maximum current value, and add the resonant frequency value to the frequency amount to be changed confirmed by the programmable DDS module to determine and control the output frequency of the ultrasonic signal generator.

[0033] Embodiment 2 As a preferred embodiment of the present invention, the present invention includes a resonant frequency tracking control system for an ultrasonic transducer, which includes a programmable DDS module and an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, an ultrasonic frequency control module, an adaptive filter, and a low-pass filter that are connected in sequence to form a closed loop. The programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit at the same time.

[0034] The phase detection circuit includes a dual voltage comparator and a dual D flip-flop, and is used to input an excitation signal into the ultrasonic transducer, control the time delay of the excitation or reception pulse of each element in the array of the ultrasonic transducer, collect the operation state data of the ultrasonic transducer in real time, measure the phase difference between the voltage and current on the load, and obtain the phase difference.

[0035] The ultrasonic signal processing unit includes a microprocessor, an ADC digital-to-analog converter, and a TDC time-to-digital converter, and is used to measure the time difference between the ultrasonic transmission signal and the reception signal, and convert the measured time difference into the pressure parameter to be measured. The programmable DDS module includes a phase accumulator, a phase register, a digital-to-analog converter, a waveform memory, a digital-to-analog converter, and a phase-amplitude conversion module, and is used to determine the frequency amount to be changed according to the phase difference, output the frequency of the sine wave, and output the amplitude signal of the sine wave.

[0036] The ultrasonic frequency control module is used to track the resonant frequency and current of the ultrasonic transducer, determine the resonant frequency value corresponding to the maximum current value, and add the resonant frequency value to the frequency amount to be changed confirmed by the programmable DDS module to determine and control the output frequency of the ultrasonic signal generator.

[0037] Embodiment 3 As another preferred embodiment of the present invention, the present invention includes a method for tracking and controlling the resonant frequency of an ultrasonic transducer. The ultrasonic signal generated by the ultrasonic signal generator is transmitted to the ultrasonic transducer. The phase difference between the voltage and current on the load is measured to obtain the phase difference. The phase difference is compared with a given phase difference, and the amount of frequency change required is output through fuzzy PI control. The resonant frequency and current of the ultrasonic transducer are tracked, and the tracking frequency value corresponding to the maximum current value is determined as the resonant frequency value. The resonant frequency value is added to the amount of frequency change required to determine and control the output frequency of the ultrasonic signal generator, so that the output frequency of the ultrasonic signal generator can track the resonant frequency point of the ultrasonic transducer within a corresponding range.

[0038] Embodiment 4 As another preferred embodiment of the present invention, the present invention includes a resonant frequency tracking control system for an ultrasonic transducer. Referring to the accompanying Figure 1 drawings, it includes a programmable DDS module, an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, an ultrasonic frequency control module, an adaptive filter, and a low-pass filter. Among them, the programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit at the same time. The phase detection circuit is arranged on the connection line at the input end of the ultrasonic transducer, and the ultrasonic signal processing unit and the ultrasonic frequency control module are arranged in sequence at the output end of the ultrasonic transducer. The output end of the ultrasonic frequency control module is connected in series with the ultrasonic signal generator, the adaptive filter, and the low-pass filter, thereby forming a resonant frequency tracking control loop controlled by the DDS module.

[0039] The ultrasonic signal generator is usually called an ultrasonic generating source. Its function is to generate ultrasonic signals, and there should also be a feedback link, mainly providing two aspects of feedback signals: the first is to provide an output power signal. By adjusting the power amplifier according to the power feedback signal, the power amplification can be stabilized; the second is to provide a frequency tracking signal. When the ultrasonic transducer works at the resonant frequency point, its efficiency is the highest and it works most stably. The frequency tracking signal can control the ultrasonic signal generator to make the frequency of the ultrasonic signal generator track the resonant frequency point of the ultrasonic transducer within a certain range, so that the ultrasonic signal generator works in the best state.

[0040] The ultrasonic signal generator includes a high-frequency inverter module, a tuning and matching module, a rectifying and filtering module, and a sampling and control module. Among them, the high-frequency inverter module uses an intelligent power module (IPM) to perform half-bridge high-frequency inversion on the DC voltage. The center frequency nominal for the transducer assembly is selected through an LC matching circuit. Based on the tuning and matching module and the equivalent parameters of the transducer, the resonance point of the intersection of the two sets of parameters of the ultrasonic transducer is matched, and the pulse width modulation (PWM) frequency and output power of the ultrasonic transducer are adjusted. The PWM output frequency is reduced through adjustment, and then a DC voltage with adjustable amplitude is output through a BUCK chopper circuit. Finally, the output of the regulator adjusts the pulse frequency of the PWM to control the output frequency of the ultrasonic signal generator, and the output adjustment amount adjusts the PWM frequency of the inverter bridge to achieve the change of the output frequency of the ultrasonic signal generator. While the tuning and matching module reaches the matching resonance point, the rectifying and filtering module filters the output of the high-frequency inverter module, expresses the resonance frequency of the ultrasonic transducer as a composite variable, and then uses the feedback signal to adjust this composite variable, thereby realizing the tracking of the resonance frequency of the ultrasonic transducer. In this embodiment, the ultrasonic signal generator can consider adopting the circuit form of a switching power supply in terms of conversion efficiency. Its advantage is that it does not require strict circuit matching and allows the operating frequency to change continuously and rapidly.

[0041] The ultrasonic signal generator samples the voltage and current signals at the resonance matching end, judges the working state of the ultrasonic vibration system, changes the control word of the DDS waveform generator and the output of the PWM wave, thereby realizing the automatic frequency tracking and power adjustment of the ultrasonic transducer, and ensuring the efficient and stable operation of the ultrasonic vibration system.

[0042] The ultrasonic signal generator converts the mains power (voltage 220V or 380V, frequency 50Hz or 60Hz) into a high-frequency alternating current signal that matches the ultrasonic transducer. When the phase feedback condition is met, it forms an oscillator with power amplification and resonates at the mechanical resonance frequency of the transducer. After power amplification, it drives the ultrasonic transducer to work. The ultrasonic signal generator generates a signal with a specific frequency, forming a signal with the same frequency as the ultrasonic transducer. This signal can be a sine signal or a pulse signal. This specific frequency is the working frequency of the ultrasonic transducer. After being smoothed by a low-pass filter, the signal becomes a waveform with a continuously changing required frequency. The adaptive filter tracks the time-varying characteristics of the input signal, quickly locks the resonance frequency at the millisecond level, and tracks the dynamic changes of the load, thereby realizing the tracking and control of the resonance frequency of the ultrasonic transducer.

[0043] The low-pass filter can adopt a finite impulse response (FIR) digital low-pass filter. A passive filter can be composed of resistors, inductors, and capacitors, or an active filter can be formed using operational amplifiers. Compared with passive filters, active filters have better effects, steeper characteristic curves, easier input and output impedance matching, and convenient frequency characteristic adjustment. The low-pass filter designed through matching can filter out a large number of high-order harmonics, improve the voltage waveform of the ultrasonic signal generator, and enable a good sine wave voltage and current to be obtained at both ends of the ultrasonic transducer. Therefore, an active filter can be adopted in this embodiment.

[0044] In an alternative embodiment, the phase detection circuit can be composed of a dual voltage comparator and a dual D flip-flop. It can be directly connected to the computer interface or connected to a digital voltmeter through a low-pass filter to form a phase detector. This circuit has the advantages of simple structure, high precision, and strong anti-interference ability, and its detection accuracy is better than 0.1° in the range of 2 MHz to 5 MHz.

[0045] The phase detection circuit collects the operating state data of the ultrasonic transducer in real time, tests the phase difference and frequency difference between the voltage and current on the load to obtain the phase difference, and at the same time participates in tracking the natural resonance frequency of the ultrasonic transducer used in the test as an important part of the control system. During the tracking process, based on the propagation of ultrasonic waves at the resonance frequency of the ultrasonic transducer, the phase detection circuit inputs an excitation signal into the ultrasonic transducer, controls the time delay of the excitation or reception pulses of each element in the array of the ultrasonic transducer, and generates a reflected excitation signal through the transmission of electromagnetic frequency to reach the surface material.

[0046] To improve the resolution of the ultrasonic transducer, which is beneficial to increasing the signal-to-noise ratio and extracting the required signals, the ultrasonic transducer can adopt piezoelectric transducers, magnetostrictive transducers, electromagnetic transducers, and laser transducers with a wafer diameter of 10 mm to 12 mm. Considering the use environment and design difficulty, a working frequency of 2 MHz to 5 MHz is adopted, and a sine signal is used in the experiment. The characteristic parameters of the ultrasonic transducer include resonance frequency, bandwidth, electromechanical coupling coefficient, electroacoustic efficiency, mechanical quality factor, impedance characteristic, frequency characteristic, directivity, transmission and reception sensitivity, etc. Ultrasonic transducers for different purposes have different requirements for performance parameters. For transmitting transducers, large output power and high energy conversion efficiency are required; for receiving transducers, a wide frequency band, high sensitivity, and high resolution are required. Therefore, in the specific design process of the ultrasonic transducer, relevant parameters of the ultrasonic transducer must be reasonably designed according to specific applications.

[0047] In an alternative embodiment, the resonant frequency of the parallel branches of the piezoelectric vibrator of the ultrasonic transducer, the mechanical quality factor are matched with the power supply operating point. The influence of the matching inductance parameter on the current magnitude in the series-parallel resonant frequency region and the phase difference between the current and voltage of the secondary of the high-frequency transformer is then tuned by the series matching inductance Lp. When the matching inductance is adjusted to the optimal value, after being stepped up by the high-frequency transformer, it drives the ultrasonic transducer. After passing through the inductance-capacitance matching circuit, it is applied to the transducer. The ultrasonic transducer resonates to generate mechanical energy to do work on the external load. In this way, the entire matching impedance is inductive, the output current gradually decreases, the current of the output ultrasonic transducer is the largest, and the phase difference between the current and voltage of the secondary of the high-frequency transformer is zero. Regardless of how the inductance value changes, the current and voltage waveforms of the secondary of the high-frequency transformer remain unchanged. This can not only reduce the calculation time but also improve the accuracy. In an actual circuit, the goal of adjusting the matching inductance is to make the phase difference between the current and voltage of the high-frequency transformer zero and without obvious distortion. At this time, the value of the matching inductance is optimal, that is, the ultrasonic transducer is in a resonant state.

[0048] The programmable DDS module is composed of a waveform memory, a phase accumulator, a phase register, a phase-amplitude conversion module, and a digital-to-analog converter DAC. After the waveform memory finishes the acquisition, the DDS module detects the phase difference between the voltage signal and the current signal at both ends of the ultrasonic transducer fed back through the phase detector circuit. It compares and calculates the sampled current and voltage with a sine wave of the same frequency, and at the same time calculates the initial phase of the current and the initial phase of the voltage. Subtracting these two phases gives the phase difference between the current and voltage, and calculates the real-time current and voltage phase difference values, which can adjust the output frequency of the pulse width modulation PWM signal in real time to achieve automatic frequency tracking.

[0049] The phase accumulator accumulates the frequency control word K under the control of the reference clock, uses the accumulated value as the address code to query the amplitude value stored in the waveform memory, adds the output of the phase register to the phase control word and then inputs it to the address of the sine lookup table, maps the input address phase information into a digital quantity signal of the sine wave amplitude, and drives the DAC to perform D / A conversion to output an analog quantity.

[0050] In this design, an external button group is required to control the parameter characteristics of the DDS output signal, including adjusting the type and frequency of the output signal, etc. The button interface module has three inputs: clock, reset, and buttons, and four outputs: digital display, row buttons, number of buttons, and new sampling data. Among them, the new sampling data is used as the input of stepclk in the DDS module. By changing the frequency control word input to the phase accumulator through the external button module, the output result of the accumulation operation is changed, and thus the frequency of the output signal is changed.

[0051] The programmable DDS module is based on a phase accumulator that recursively sums a digital input tuning word at the sampling clock rate, generating a time series of digital words at the output of the phase accumulator. These digital words increase linearly until the phase accumulator reaches its maximum value of 2 N and then flips, with the output having a fixed modulus of 2 N .

[0052] For a DDS based on a phase accumulator, the frequency of the sine wave output at the DAC output: f o = (M × f C ) / 2 N Where: f o is the synthesized frequency, 2 N is the modulus of the phase accumulator, M is the frequency tuning word, N is the width of the phase accumulator, f C is the sampling frequency.

[0053] The phase register returns to its initial state after every 2 N / M clock cycles of fc. Correspondingly, the sine lookup table goes through a cycle and returns to its initial position. The period T0 of the sine wave output at the phase output of the direct digital frequency synthesizer DDS module is Tc2 N / M, that is, the output sine wave frequency f out = M × fc / 2 N . By controlling the phase accumulation with the frequency control word until overflow, the high 8 bits are taken as the address to query the sin table, and through the way of triggering the module integration and converting with the sin and cos modules into a sine signal, thus controlling the output waveform. Then, through the zero-crossing comparison of the triggering module, it is converted into a triggered square wave pulse signal to obtain the triggering pulse at the corresponding frequency. Among them, Tc and f c are respectively the period and frequency of the external reference clock. The triggering module plays an important role in frequency tracking by changing the input frequency, and can change in real time the frequency required by the ultrasonic transducer to obtain triggering pulses at different frequencies.

[0054] The DDS module, according to the adaptive resonant frequency tracking control algorithm and the phase accumulator, uses the external reference clock Tc and the N-bit phase register to increment by step M. The phase register returns to its initial position after every 2 N / M external reference clocks, and outputs a sine wave at the DAC output.

[0055] Subsequently, at the input of the angle amplitude converter of the phase amplitude conversion module following the phase accumulator, according to the obtained number of bits P, the range of the time series digital frequency control word generated by the phase accumulator is from zero to 2p -1. Linearly map the binary values in the range of 0 to 2 to the radian angles in the range of 0 to 2π. The angle-amplitude converter converts a P-bit word into a D-bit amplitude value. At the same time, extend the angle-amplitude conversion function to perform sine and cosine conversions simultaneously. Immediately following the angle-amplitude converter, a D-bit DAC converts the D-bit digital amplitude value generated by the angle-to-amplitude converter into an analog signal, resulting in a sine wave at the output of the DAC, whose frequency is determined by the average switching rate of the phase accumulator.

[0056] The DDS module introduces a digital PI control algorithm in resonance frequency tracking. The PI control strategy is to set the proportional coefficient Kp and integral coefficient Ki of the PI controller. According to the PI control principle and the resonance conditions of the transducer, establish a PI control structure for automatic frequency tracking. The control deviation is formed by the given value x(t) and the actual value y(t). The deviation forms a control quantity for controlling the controlled object through the linear combination of proportional and integral, and discretize the analog PI. Compare the output feedback quantity with the set value, and perform PI calculation on the obtained difference and then output the control quantity. The digital PI control algorithm is more convenient for adjusting control parameters compared to the analog PI.

[0057] The DDS module introduces the PI algorithm, obtains the frequency that needs to be changed through PI control and adds it to the current frequency to get the required resonance frequency, automatically tracks the resonance frequency through software algorithms. At the same time, based on the resonance frequency tracking signal fed back by the ultrasonic frequency control module, use the feedback signal to determine the change in the resonance frequency of the ultrasonic transducer, and determine the output frequency of the ultrasonic signal generator according to the magnitude of the phase difference △t.

[0058] The ultrasonic frequency control module compares two sine signals with a phase difference of time interval or pulse width with the positive or negative zero-crossing moments, sends them to the zero-crossing comparison circuit after filtering, converts them into square waves through the corresponding zero-crossing comparators, and converts them into square waves of the same frequency. Then, perform an exclusive OR operation on the obtained square wave signals. The pulse width of the square wave after the exclusive OR operation can reflect the magnitude of the phase difference between the current signal and voltage signal of the ultrasonic transducer, and obtain a high-level square wave proportional to the phase through phase / time conversion. The ultrasonic frequency control module is based on the direct phase / time conversion method and phase difference measurement method of the ultrasonic signal processing unit, tracks the resonance frequency and current of the ultrasonic transducer, determines the resonance frequency value as the frequency value corresponding to the maximum current, and drives the ultrasonic signal generator to output the adjusted ultrasonic signal.

[0059] The control system can obtain an accurate voltage output signal according to requirements through the conversion of time signals. This method measures by converting the time interval into a voltage signal, and can realize functions such as time synchronization and timing of the circuit. By adjusting the parameters in the circuit, output signals with different measurement time delays can be obtained, improving the stability and reliability of the entire circuit.

[0060] The DDS module communicates with the TDC time-to-digital converter in the ultrasonic signal processing unit through the SPI interface to complete the initialization and clock calibration of the time measurement TDC; the TDC time-to-digital converter takes the time interval as the delay time of the control signal and the drive signal. After the system is powered on, the TDC chip is software reset by sending the instruction 0x50, and then the chip is initialized.

[0061] The ultrasonic signal processing unit consists of a microprocessor, an ADC analog-to-digital converter, and a TDC time-to-digital converter. The main functions of the built-in pressure measurement software main program of the ultrasonic signal processing unit are mainly divided into: initialization module, correlation method time measurement module, time / pressure conversion module, system calibration module, and human-computer interaction module. After the software starts running, the main program completes the initialization of the detection system. The initialization mainly includes system clock initialization, I / O port pin initialization, ADC initialization, direct memory access DMA initialization, timer initialization, and initialization of the peripheral communication module, and switches different program functions according to user operations.

[0062] Refer to the attached instructions Figure 2 In the correlation method time measurement process, when the ultrasonic transducer collects the ultrasonic excitation signal, the ultrasonic signal processing unit starts the built-in ADC analog-to-digital converter, turns on the ADC to collect the measurement signal, collects the waveform, and at the same time turns on the direct memory access DMA data transmission. At the rising edge of the clock, the DAC starts A / D conversion. At the falling edge of the clock, the voltage conversion data is read. The A / D sampling frequency is controlled by controlling the frequency of the clock signal. At the same time, the time digital conversion TDC chip is driven to record the time node of the transmitted signal. The result of the ADC sampling is directly stored in the RAM area of the specified waveform memory by the DMA. The output of the waveform memory becomes a stepped wave after digital-to-analog conversion.

[0063] Wait for the ADC data acquisition to end. When the ultrasonic wave reaches the receiving end, the FIR digital low-pass filter performs FIR digital filtering on the data, performs convolution operations, and performs a fast Fourier transform (FFT) on the output result to obtain the frequency response, changes the relative proportion of the frequency components contained in the input signal, multiplies the input at each moment by the corresponding weight coefficient, and then performs superposition to obtain the weighting coefficient (tap coefficient).

[0064] The ultrasonic signal processing unit performs autocorrelation algorithm operations on the filtered data to determine the statistic of the correlation of the time series data, calculates the propagation time of the ultrasonic signal, measures the correlation of the random process. After using the autocorrelation operation to find the period value of the periodic signal sequence submerged by noise, it starts the TDC timing program and the time measurement program of the correlation method, converts the time of the pulse to be measured into the corresponding phase, and then can obtain the time value of the pulse to be measured through calculation. The ultrasonic signal processing unit measures two sine signals with a frequency of 1 kHz and a phase difference of 72° by using the phase / time conversion method. If the time scale pulse frequency is 500 kHz, it calculates the phase quantization error and the counter counting result, and then performs the calculation and display of time-pressure conversion.

[0065] Refer to the attached instructions Figure 3 The time-pressure conversion module reads and sets the system parameters and the flag bit of the ADC interrupt according to the parameters set by the user, reads the corresponding system calibration data table, uses the TDC time measurement and the correlation method time measurement method to achieve multiple measurements of the time interval and temperature measurement. The two groups of measured time values are respectively subjected to median filtering. At the same time, the RTD temperature value of the platinum resistance temperature sensor measured by using the TDC chip is also filtered by using median filtering. The final ultrasonic propagation time is obtained by fusing the two time values through the Kalman fusion algorithm. Through bilinear interpolation calculation, the measured time value is converted into the pressure value of the liquid in the pipeline to be measured.

[0066] Based on the above control system, this embodiment also includes a method for tracking and controlling the resonant frequency of an ultrasonic transducer. The ultrasonic signal emitted by the ultrasonic signal generator is filtered by an adaptive filter and a low-pass filter and then sent to the ultrasonic transducer. The phase difference between the sampling voltage and current of the ultrasonic transducer is used as the input of the phase measurement module built in the phase detection circuit. The phase measurement module calculates the magnitude of the phase difference between the current and voltage, compares the magnitude of the detected output phase with the given phase difference, and then uses the phase difference between the sampling voltage and current as the feedback quantity of the closed-loop system. Then, the error and the error change rate are used as the input quantities of the fuzzy PI control. After the fuzzy PI control, the frequency quantity to be changed is obtained. The changed frequency is added to the set resonant frequency, that is, the resonant frequency value determined by the ultrasonic frequency control module tracking the resonant frequency and current of the ultrasonic transducer, to obtain a new drive frequency control quantity and the required resonant frequency, achieving the tracking of the phase difference, and thus realizing the tracking of the resonant frequency.

[0067] After startup, the operator sets the parameters of the measurement site. The ultrasonic signal generator adjusts its built-in amplifier through the power feedback signal, outputs a frequency tracking signal to the phase detection circuit, and then to the ultrasonic transducer. The DDS module controls the frequency tracking signal and frequency provided by the ultrasonic signal generator, tracks the resonant frequency point of the transducer within a certain range, monitors the current in the output circuit, adjusts the driving signal frequency of the ultrasonic transducer in real time according to the results of online identification, and automatically calculates the frequency value to be adjusted through the corresponding resonant frequency tracking control algorithm.

[0068] In summary, after reading the present invention document, various other corresponding transformation schemes made by those of ordinary skill in the art without creative mental labor according to the technical solutions and technical concepts of the present invention all fall within the scope protected by the present invention.

Claims

1. An ultrasonic transducer resonance frequency tracking control system, characterized in that: It includes a programmable DDS module, an ultrasonic signal generator, a phase detection circuit, an ultrasonic transducer, an ultrasonic signal processing unit, and an ultrasonic frequency control module that are connected in sequence. The programmable DDS module is connected between the phase detection circuit and the ultrasonic frequency control module and communicates with the ultrasonic signal processing unit at the same time. The phase detection circuit is used to input an excitation signal into the ultrasonic transducer, control the time delay of the excitation or reception pulse of each element in the array of the ultrasonic transducer, collect the operating state data of the ultrasonic transducer in real time, test the phase difference between the voltage and current on the load, and obtain the phase difference. The ultrasonic signal processing unit is used to measure the time difference between the ultrasonic transmission signal and the reception signal and convert the measured time difference into the pressure parameter to be measured. The programmable DDS module is used to determine the frequency amount that needs to be changed according to the phase difference. The ultrasonic frequency control module is used to track the resonant frequency and current of the ultrasonic transducer, determine the resonant frequency value as the tracking frequency value corresponding to the maximum current value, and add the resonant frequency value to the frequency amount that needs to be changed confirmed by the programmable DDS module to determine and control the output frequency of the ultrasonic signal generator.

2. The resonant frequency tracking control system of an ultrasonic transducer according to claim 1, wherein: It further includes an adaptive filter and a low-pass filter. The ultrasonic signal generator, the adaptive filter, and the low-pass filter are connected in series at the output end of the ultrasonic frequency control module in sequence.

3. The resonance frequency tracking control system of an ultrasonic transducer according to claim 2, characterized in that: The phase detection circuit includes a dual voltage comparator and a dual D flip-flop.

4. The resonant frequency tracking control system of an ultrasonic transducer according to claim 2, characterized in that: The programmable DDS module includes a phase accumulator, a phase register, a digital-to-analog converter, a waveform memory, a digital-to-analog converter, and a phase-amplitude conversion module, and is used to output the frequency of the sine wave and the amplitude signal of the sine wave.

5. An ultrasonic transducer resonance frequency tracking control system according to claim 4, characterized in that: The frequency of the output sine wave specifically refers to: using an external reference clock Tc and an N-bit phase register to increment in steps of M, and the phase register returns to the initial position after every 2 N / M external reference clocks, and a sine wave is output at the output terminal of the digital-to-analog converter.

6. The resonance frequency tracking control system of an ultrasonic transducer according to claim 5, wherein: The amplitude signal of the output sine wave specifically refers to: the phase accumulator accumulates the frequency control word K under the control of the reference clock, uses the accumulated value as the address code to query the amplitude value stored in the waveform memory, adds the output of the phase register to the phase control word and inputs it to the sine lookup table address, maps the input address phase information into a digital quantity signal of the sine wave amplitude, drives the digital-to-analog converter to perform D / A conversion, and outputs an analog quantity.

7. An ultrasonic transducer resonance frequency tracking control system according to claim 2, characterized in that: The ultrasonic signal processing unit includes a microprocessor, an ADC digital-to-analog converter, and a TDC time-to-digital converter.

8. An ultrasonic transducer resonance frequency tracking control system according to claim 7, characterized in that: Measuring the time difference between the ultrasonic emission signal and the received signal specifically refers to: when the ultrasonic transducer collects the ultrasonic excitation signal, start the ADC analog-to-digital converter to collect the measurement signal; after the data collection is completed, perform FIR digital filtering on the data, perform convolution operations, and perform fast Fourier transform on the output result to obtain the frequency response, change the relative proportion of the frequency components contained in the input signal, output the input at each moment multiplied by the corresponding weight coefficient, and then perform superposition to obtain the weighting coefficient; perform autocorrelation algorithm operations on the filtered data, determine the statistical quantity of the correlation of the time series data, calculate the ultrasonic signal propagation time, measure the correlation of the random process, and use autocorrelation operations to find the period value of the periodic signal sequence submerged by noise. Then, convert the time of the pulse to be measured into the corresponding phase, and then obtain the time value of the pulse to be measured through operations.

9. An ultrasonic transducer resonance frequency tracking control system according to claim 8, characterized in that: When the ADC analog-to-digital converter collects the measurement signal, it collects the waveform and simultaneously enables direct memory access (DMA) data transfer. At the rising edge of the clock, start the A / D conversion. At the falling edge of the clock, read the voltage conversion data. Control the A / D sampling frequency by controlling the frequency of the clock signal. At the same time, drive the TDC time-to-digital converter to record the time node of the emission signal. The result of the ADC sampling is directly stored in the RAM area of the specified waveform memory by direct memory access (DMA). The output of the waveform memory becomes a staircase wave after digital-to-analog conversion.

10. An ultrasonic transducer resonance frequency tracking control system according to claim 9, characterized in that: Converting the measured time difference into the pressure parameter to be measured specifically refers to: after reading the calibration number and after multiple measurements of the time interval and temperature measurement, perform median filtering on the two groups of measured time values respectively, and perform median filtering on the measured temperature value; fuse the two time values through the Kalman fusion algorithm to obtain the final ultrasonic propagation time, and calculate through bilinear interpolation to convert the measured time value into the pressure value of the liquid in the pipeline to be measured.

11. A method for tracking and controlling the resonant frequency of an ultrasonic transducer, characterized in that: The ultrasonic signal generated by the ultrasonic signal generator is transmitted to the ultrasonic transducer; measure the phase difference between the voltage and current on the load to obtain the phase difference; compare the phase difference with the given phase difference, and output the frequency quantity that needs to be changed through fuzzy PI control; track the resonant frequency and current of the ultrasonic transducer, determine the resonant frequency value as the frequency value corresponding to the maximum current, add the resonant frequency value and the frequency quantity that needs to be changed, and determine and control the output frequency of the ultrasonic signal generator so that the output frequency of the ultrasonic signal generator can track the resonant frequency point of the ultrasonic transducer within the corresponding range.

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