Abstract: Aiming at the problem that the traditional ultrasonic power supply can not drive and lock the frequency of different resonant frequency segments, the problem of phase locking and frequency tracking in the wide frequency domain can not be realized. An FPGA-based automatic frequency search and tracking and dynamic matching are designed. Wide frequency domain ultrasonic power supply for resonant frequency transducers. According to the impedance characteristic curve of the transducer, a wide frequency domain frequency search method with dynamic step size is designed to quickly track the resonant frequency of the transducer, and adjust the output frequency in real time according to the phase difference of the voltage and current of the feedback circuit to lock the whole system. In the resonant state. The experimental results show that the designed wide frequency domain ultrasonic power frequency search is fast and the tracking is accurate, and the dynamic matching transducer has good adaptability.
0 PrefaceUltrasonic welding, cleaning and inspection technologies have broad application prospects [1]. However, the ultrasonic system resonance frequency drift, tracking lag and other issues have not been completely solved. Most of the ultrasonic devices currently developed are ultrasonic power supplies and their associated transducers. A single ultrasonic power supply cannot drive and lock the transducers with different resonant frequency segments, which can not achieve frequency tracking and phase locking in the wide frequency domain [2]. Therefore, the development of a broadband ultrasonic power supply with frequency automatic tracking and dynamic matching of different resonant frequency transducers has important application value [3].
In this paper, XILINX ZYNQ series FPGA (XC7Z0201CLG484I) is used as the main controller, and an ultrasonic power supply with automatic frequency search, tracking and dynamic matching transducer in wide frequency domain is designed. The output frequency is 20~40 kHz, which can drive different application type transducers in this frequency range, and has wide applicability in wide frequency domain.
The ultrasonic power supply is composed of a rectifier circuit, an inverter circuit, a matching circuit, a feedback circuit, and a main control circuit. The system structure block diagram is shown in Figure 1. The host controller FPGA integrates a dual-core ARM Cortex-A9 processor (Processing System, PS) and a traditional Field-Programmable Gate Array (FPGA) logic resource (Programmable Logic, PL). The PL unit of the FPGA mainly performs functions such as frequency search, frequency tracking, voltage and current phase discrimination, SPWM generation, and direct digital frequency synthesizer (DDS); the PS processor completes functions such as interface display and input parameter processing. The matching circuit consists of a high-frequency transformer and an improved T-matching network, which acts as a variable-resistance tuning, making the load purely resistive and improving the efficiency of the power supply.
DDS can be simplified to consist of a phase accumulator and a ROM sine wave storage table. Under the control of the system clock, the frequency control word is updated, and the frequency control word corresponds to the address of the ROM sine wave table, and the content stored by the address is the amplitude corresponding to a certain synthetic point of the sine wave, under the next system clock, The output of the phase accumulator adds a frequency control word again, corresponding to changing the address stored in the waveform, thereby generating the next amplitude point of the synthesized waveform until the phase accumulator overflows to form a complete sine wave [4-5]. The frequency of the sine wave changes with the change of the frequency control word, and its output frequency is:
Where N is the phase accumulator bit length, fclk is the system clock, and Fword is the frequency control word.
The DDS output sine wave signal is modulated with the high frequency triangular carrier signal generated inside the SPWM generation module, and the PWM wave whose pulse width changes according to the sine wave law is output, and the output frequency of the control inverter circuit is changed according to the sine wave frequency of the DDS output.
After the system is powered on, the frequency search is first performed according to the feedback current effective value I_in, the resonant frequency of the transducer is quickly searched in the frequency domain of 20 kHz~40 kHz, and the maximum current value I_max at the resonant frequency is recorded as the current threshold, and then the starting frequency is started. The tracking program, according to the voltage U of the feedback circuit and the phase difference of the current I, locks the output frequency at a frequency point where the phase of the voltage and current are consistent, and the PL also monitors the magnitude between the feedback current value and I_max in real time as a judgment of whether the system is in a resonant state. An auxiliary condition. Once the system is replaced by a transducer or the load is abrupt, the system will be detuned, and the feedback current value will be much smaller than I_max. At this time, the PL will select the frequency search program, re-search for the resonant frequency of the transducer, and then go to frequency tracking for phase locking.
2 frequency searchWhen the output frequency of the ultrasonic power source is equal to the resonant frequency of the transducer, the power supply has the highest working efficiency and the system works most stably [6]. Experiments show that when the transducer operates at the resonant frequency Fs, the current flowing through the load is the largest [7-8], so the resonant frequency search can be realized by detecting the current value of the transducer.
Figure 2 plots the impedance characteristics of a transducer with resonant frequencies of 22 kHz and 32.32 kHz in the 20 kHz to 40 kHz band. It can be seen from Fig. 2 that the impedance of the transducer changes greatly near the resonant frequency, that is, the current value changes significantly; and when it is far away from the resonant frequency, the impedance change is not obvious, so that the current change is weak. Based on this impedance characteristic of the transducer, a frequency search method based on current maximum is designed on the FPGA platform. The FPGA implementation of the frequency search is shown in Figure 3.
After the system is powered on, the frequency search module generates the frequency control word required for the 20 kHz output frequency. Under the system clock control, the current feedback current RMS value I_in is compared with the current value fed back from the previous cycle. If the difference between the two is less than the setting The threshold Ith indicates that the load current changes are not obvious at this time, and the system frequency is far away from the resonant frequency of the transducer. At this time, the frequency control word of the DDS is changed by the set large step step1; when the difference between the two is greater than Ith, this indicates The system frequency is close to the resonant frequency of the transducer, the frequency control word will be fine-tuned in the set small step step2 until the output frequency reaches 40 kHz, and the frequency search function is completed. At this time, the current maximum value I_max during the search is recorded. And output the frequency control word corresponding to the maximum current. The software flow chart of frequency search is shown in Figure 4. The comments for the variables in the figure are as follows:
(1) Freq: search frequency range (20 kHz~40 kHz)
(2) F_word_rem: resonant frequency control word
(3) I_in: input current RMS
(4) I_max: current maximum
(5) step, step 1, step 2: represents asynchronous
(6) Ith: set the threshold
(7) det_I: current difference between adjacent sampling periods.
After the system is powered on, the initialization frequency Freq is 20 kHz. Under each sampling period, the relationship between the input current I_in and I_max is compared. If the input current is greater than I_max, the input current value is assigned to I_max, and the frequency point at this time is assigned to F_word_rem, and the current value in the adjacent sampling period is compared to generate det_I, and then the relationship between det_I and Ith is compared. If det_I is greater than Ith, it means that the output frequency of the system is close to the resonant frequency point of the transducer, and the output frequency Freq will be changed in small step 2; otherwise, the output frequency is far away from the resonant frequency of the transducer, in order to speed up the search. The output frequency should be changed in the wide frequency domain with a large step 1 step. After the frequency domain search is completed, the output frequency word is the resonance frequency point F_word_rem corresponding to the current value maximum time, and the frequency search procedure is completed.
Figure 5 is a ModelSim simulation diagram of the frequency search program. The cursor position represents the current maximum point, which is the resonant frequency point of the transducer. Near the resonant frequency of the transducer, the output frequency word changes in small steps of step=10, and the resonant frequency point is searched step by step, and the resonant frequency is recorded as 20 260 and the current maximum value at this time is 3 471. Figure 5 shows that the variable step size search method based on the transducer impedance characteristic curve satisfies the design requirements.
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