Ultrasonic cleaning is the main use of ultrasonic cavitation. Under the sound field, the micro bubbles in the liquid will produce high frequency vibration. When the sound pressure reaches a certain value, the bubbles rapidly increase and suddenly close, and the liquid collides with each other to produce a powerful shock wave. Although the displacement and velocity are very small, the acceleration is very large and the local pressure can reach thousands of atmospheric pressure. The destruction, spalling, separation and emulsification of the fouling film on the surface of the cleaning material can achieve a quick and efficient cleaning effect. At present, ultrasonic cleaning is recognized by the world as the highest efficiency and the best way of cleaning. The cleaning efficiency has reached over 98%, and the cleanliness of the cleaning has reached the highest level.
The ultrasonic cleaning equipment is made up of ultrasonic generator and ultrasonic transducer. The ultrasonic generator is an important part of the ultrasonic cleaning equipment, which is responsible for providing energy to the ultrasonic transducer during the working process. In order to achieve a high efficiency of transducers, in addition to providing enough power, the frequency of the generator is also consistent with the resonant frequency of the transducer. But the resonant frequency of the transducer usually changes due to heat and aging in the working process, and the ultrasonic generator in the market usually tracks the resonant frequency of the ultrasonic transducer by manually adjusting the output signal frequency. In order to solve this problem, a kind of DDS (direct number) is developed in this paper. The frequency synthesizer of the word type frequency synthesizer is used to automatically track the frequency of the generator, so that the frequency of the output signal can automatically track the resonant frequency of the transducer, and the transducer is always kept in the high efficiency state during the working process.
2 system design principle
In order to make the output frequency of the ultrasonic generator track the resonant frequency of the transducer in real time, an automatic frequency tracking circuit based on DDS is designed. The system general block diagram is shown in Figure 1. The system is composed of DDS signal generation circuit, power amplification circuit, matching circuit, current feedback circuit, single chip microcomputer control and some peripheral circuits. The system block diagram is shown in Figure 1.
Diagram 1 system block diagram
The working principle of the whole system is as follows: a single chip microcomputer controlled DDS chip AD9832 produces a sine wave signal with a certain frequency. This sine wave signal is added to the two ends of the primary coil of an isolated transformer after shaping and primary amplifier, and the two square wave signals from the opposite side of the transformer side sense are further driven by two. A half bridge push-pull complementary power amplifier consisting of a N channel MOS tube is used to drive the piezoelectric transducer through a matching circuit composed of an output transformer and an adjustable inductor. During the transducer working, the current sensor is used to feed back the working current of the transducer, and the MCU controls the corresponding frequency signal of DDS chip output according to the feedback information, so that the transducer works at the resonant point all the time and effectively improves its working efficiency.
The hardware structure of the 3 system
3.1 signal generating circuit
The frequency signal is produced by the DDS chip. The working principle of DDS is to generate sine wave with frequency and phase controllable by means of numerically controlled oscillator. Internal circuits generally include reference clock, frequency accumulator, phase accumulator, amplitude / phase conversion circuit, D/A converter and low pass filter (LPF). This design uses AD9832 DDS chip, AD9832 is a phase accumulating NC oscillator, built-in sine look-up table and 10 bit D/A converter CMOS chip. The maximum clock 25MHz can be connected, and the maximum frequency output is 10MHz. The signal generation circuit is shown in Figure 2.
Figure 2 signal generating circuit
The ATMEGA16 MCU is used as the main controller in the design. ATMEGA16 controls the three pins of AD9832's SCLK, XDATA and FXYNC according to the time sequence diagram, so that AD9832 outputs a sinusoidal current signal of frequency, which becomes a sinusoidal voltage signal of the corresponding frequency after a resistance is grounded. In order to meet the needs of subsequent circuits, the signal generates a square wave signal with positive and negative 12 volts through LM393 zero crossing comparison.
3.2 power amplifier circuit
The square wave signal generated by the signal generation circuit is a small power square wave signal, which can not directly drive the transducer, and must pass through the power amplifying circuit. There are two ways to achieve power amplification: one is to use a low voltage and large current circuit; the other is to use a high voltage and low current circuit. The design uses high voltage and low current circuit to achieve power amplification. In the design, a two stage power amplifier circuit is used to complete the drive, which can be divided into low voltage driving circuit and high voltage driving circuit. The low voltage drive circuit is shown in Figure 3.
Figure 3 low voltage drive circuit
The type of power pipe used in the low voltage drive circuit is IRF540, because the square wave signal produced by the signal generating circuit can not drive the power tube IRF540 directly, so the driving circuit of the first stage NPN triode is added. As can be seen from Figure 3, the square wave signal of positive and negative 12 volts is converted to a square wave signal with a certain driving power through a NPN tube. The square wave signal further drives the MOS tube IRF540, so that the 2 side coils that connect the transformer between the MOS tube source and the VCC can produce two square waves with the same phase and the same amplitude. The waveform is shown in Figure 4. The size of the VCC can be adjusted to change the magnitude of the side coil of the isolating transformer by adjusting its size to change the output power of the whole ultrasonic generator.
Figure 4 isolation transformer output waveform
The half bridge push-pull complementary power amplifier consisting of two N channel MOS tubes is a high voltage drive circuit module of the whole system. The circuit diagram is shown in Figure 5.
Figure 5 high voltage drive circuit
The operating voltage of the circuit in Fig. 5 is obtained by directly rectifying and filtering the 220VAC power supply. Because of the great difference in the working voltage of the two stage power amplifier and the different way of taking power, it is necessary to add the isolation transformer between the low voltage drive circuit and the high voltage drive circuit. The design of the half bridge push-pull complementary power amplifier uses IRFP460 power transistor, which can withstand voltage 500V and allow the maximum current to be 20A. In order to increase the driving power of the generator, the 4 parallel half bridge push-pull complementary power amplifier circuit is adopted in the design. In Figure 5, only a half bridge push-pull complementary power amplifier circuit is painted in order to make the circuit diagram concise.
3.3 matching circuit design
In power ultrasonic equipment, the matching design of generator and transducer is very important. To a large extent, it determines whether the ultrasonic equipment can work normally and efficiently. The matching of ultrasonic generator and transducer includes two aspects: impedance matching and tuning matching. Impedance matching is to transform the transducer's impedance into the best load, that is, impedance transformation. Tuning matching is to make the input voltage and current at the two ends of the transducer in the same phase, so that the efficiency is the highest.
In the design, the output of the half bridge push-pull complementary power amplifier circuit is coupled to the transducer through the adjustable inductor to form a matching circuit after the transformer coupling. The circuit is shown as the line frame of Figure 5. The load of the ultrasonic generator is presented as a pure resistance state by the matching circuit, and the impedance transformation of the coupling transformer is used to transform the impedance of the transducer to the optimal load.
Design of 3.4 current feedback circuit
In order to solve the transducer frequency drift problem and obtain the best acoustic efficiency, the electrical signal output by the ultrasonic generator can track the resonant frequency of the ultrasonic transducer which is changed at work, that is called frequency automatic tracking. In this design, the output frequency of the ultrasonic generator is adjusted according to the current signal returned from the real-time feedback, and the output frequency of the generator is used to track the resonant frequency of the transducer in real time.
This module includes two parts: current sensor circuit and RMS conversion circuit. The model of the current sensor used in the module is CLSM-25. The current sensor is based on the Holzer effect and uses the magnetic balance method to make the output current proportional to the measured current. It is suitable for DC, AC or any other waveform, and can be used as a sampling element for measurement or feedback. In the design, the current signal output by the current sensor becomes a voltage signal through a suitable resistance to the ground, and the signal is amplified and sent to the AD637 for processing. AD637 is a true RMS chip, and its highest accuracy is better than 0.1%. The feedback signal is changed into a DC voltage signal through AD637, and the voltage signal is sent to the A/D pin of the ATMEGA16 microcontroller for processing. The ATMEGA16 microcontroller has 10 bit precision A/D, and the highest resolution rate reaches 15 kSPS, which fully meets the design requirements.
When the transducer operates at the resonant frequency point, the DC voltage feedback is the highest. In the design, the single-chip microcomputer collects the DC voltage signal of A/D port in real time, so that the transducer always works on the resonance point. Fig. 6 is the voltage signal of the transducer working at the resonance point.
Figure 6 transducer working voltage waveform
Figure 6 shows the voltage of the transducer when it is working normally. From the load waveform analysis, the sine characteristic of the transducer is very good, that is, the impedance matching and tuning matching are intact.
4 system software design
In this design, ATMEGA16 microcontroller is used as the main control chip. The internal integrated A/D of ATMEGA16 is used to sample the feedback voltage signals in real time. The software design flow chart of this frequency automatic tracking ultrasonic generator is shown in Figure 7.
Figure 7 flow chart of software design
In the design, the working frequency of the ultrasonic generator is high, so the feedback voltage can not be sampled directly to find the corresponding peak point in real time when the A/D sampling is sampled, but the voltage signal returned by the feedback is converted into a corresponding true value signal by using the AD637. The signal is a DC signal, which is conducive to the rapid measurement of the microcontroller and makes the generator adjust the frequency rapidly.
In the design, the generator always works at the frequency band of 30 Hertz around the resonant frequency point. Once the resonant frequency point is detected, the central frequency of the resonant frequency band is updated immediately. This method makes the ultrasonic generator work automatically at the resonance frequency point and effectively improves its working efficiency.
Based on the development of the DDS frequency automatic tracking ultrasonic generator, the resonant frequency of the transducer can be well tracked. The power amplifier circuit has high reliability, little heat, good matching effect and no high temperature phenomenon. In the case of long time working, the acoustical effect of the transducer is in a good state. At present, the equipment has been put into industrial application.
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