Design of ultrasonic cleaning machine based on STC single chip microcomputer

　　With the continuous development of ultrasonic technology, ultrasound is widely used in detection, cleaning, welding, medical and other fields, and even in the fields of textiles and aviation. At present, the research and application of ultrasound can be divided into two major fields: power ultrasound and detection ultrasound. Ultrasonic cleaning is one of the most widely used applications of power ultrasound. It converts the sound energy of power ultrasound into mechanical vibration through a transducer, and at the same time, strong ultrasound will produce a "cavitation effect" when it propagates in liquid. The shock wave emitted when the cavitation bubble suddenly closes can generate thousands of atmospheric pressures around it, and the direct and repeated impact on the dirt layer will, on the one hand, destroy the adsorption of dirt and the surface of the cleaning part, and on the other hand, it will cause the destruction of the dirt layer and detach from the surface of the cleaning part and disperse them into the cleaning liquid to remove impurities, dirt or grease on the surface of the object. Compared with other cleaning methods, ultrasonic cleaning has the characteristics of high efficiency, low energy consumption, clean and environmentally friendly, especially when cleaning complex parts, blind holes, and objects with many slits, its advantages are more prominent.

　　1 Overall design of ultrasonic cleaning machine

　　The ultrasonic cleaning machine designed in this paper is based on the STC single-chip microcomputer as the control core, including rectification and filtering, inversion, IGBT drive, PWM generation and control, frequency scanning display, power regulation, tuning and matching and impedance matching modules as well as related protection modules.

　　Figure 1 Schematic diagram of ultrasonic cleaning machine

　　In the ultrasonic cleaning machine, the 220 V 50 Hz mains power input is divided into two paths, one for generating high-power ultrasonic waves, and the other for power supply for detection, control and display, as shown in Figure 1. The power of the cleaning machine can be controlled by a bidirectional thyristor. The inverter module is a half-bridge inverter, which converts the DC voltage into a high-frequency AC voltage, and then through the transformer boost and inductor matching of the tuning matching and impedance matching module, it can be transmitted to the piezoelectric transducer with high efficiency and maximum power. Finally, the piezoelectric transducer converts the electrical energy output by the ultrasonic power supply into high-frequency mechanical vibration.

　　2 Design principles of each module of ultrasonic cleaning machine

　　2.1 Rectification, filtering and power regulation module

　　220 V 50 Hz AC is rectified by rectifier bridge B1 and filtered by electrolytic capacitor C12 to generate DC output voltage. Bidirectional thyristor TR1 is used for power regulation, C11 is a safety capacitor, and R11 and C11 are mainly used to eliminate high-frequency interference. U1 is an optocoupler, and the model can be MOC3021, with pins 1 and 3 connected to the power regulation module. Optocoupler U1 plays the role of isolating strong and weak electricity, enhancing the reliability and safety of the circuit.

　　Figure 2 Rectification and filtering module

　　During the operation of the ultrasonic power supply system, the rectifier filter module and the inverter module will generate heat. The two modules can be installed on an aluminum radiator for air cooling. In this way, the system can work more safely and reliably.

　　2.2 Inverter and pulse drive module

　　Since the half-bridge inverter circuit uses fewer power devices, has low cost, and is relatively simple to control, the ultrasonic cleaning machine designed in this article uses a half-bridge inverter circuit.

　　Figure 3 Half-bridge inverter module

　　In the half-bridge inverter circuit, the two fully controlled switch devices are IGBTs, namely Q1 and Q2 and diodes D11 and D12 form a half-bridge inverter. Complementary signals are applied to Q1 and Q2, and the two IGBTs O1 and Q2 are triggered in turn, that is, they are turned on alternately. At the same time, the capacitors C1 and C2 connected to the DC input terminal should be large enough, and C1=C2, and the capacitance can be selected to be above 2μF. Similarly, resistors R14 and R15 should also be large enough, and R14=R15, and the resistance can be selected to be above 100 kΩ. Fuses F11 and F12 are used to protect switch tubes Q1 and Q2 to prevent excessive current.

　　Transformer T1 and resistors R16, R17, R18, and R19 form a pulse drive module to provide complementary trigger signals for Q1 and Q2. Since the driving voltage of the IGBT should be less than 20 V, and the input voltage between T12 and T14 is about 12 V, the transformer T1 ratio is designed to be 1:1:1. R18 and R19 are used for current limiting, and resistors of about 20 Ω can be selected. In this ultrasonic cleaning machine, the upper and lower IGBT devices have a certain dead time to prevent both from being turned on at the same time.

　　2.3 Voltage Transformation and Linear Voltage Regulation

　　The 220 V 50 Hz AC power is stepped down to 12 V by transformer T4, and then rectified by rectifier bridge B4, filtered by C41, and linearly stabilized by U1 (L7812), and then outputs 12 V DC voltage to power the PWM generation and control module. At the same time, the DC 12 V is then regulated by U2 (L7805) to 5 V, providing power for the processor IAP15F2K61S2. LED1 is a light-emitting diode that serves as a power indicator. In order to reduce the voltage ripple coefficient, capacitors C43 and C44 are added for multiple filtering.

　　Figure 4 Buck and linear regulation

　　2.4 PWM generation and control module and drive module

　　In this ultrasonic cleaning machine, KA3525A is used as the PWM generation and control chip. As shown in Figure 5, the setting range of the oscillation frequency of KA3525A is 20~40 kHz. A resistor Rd is connected in series between pins 5 and 7 of the chip to adjust the dead time in a larger range. The oscillation frequency of KA3525A can be expressed as:

　　In the formula: CT and RT are the capacitor and resistor of the oscillator connected to pin 5 and pin 6 respectively; Rd is the discharge terminal resistor connected to pin 7. Here: Rd, CT, and RT are R52, C5, and (R51+Rp51) in the figure respectively. Among them, Rp51 is a precision adjustable resistor, that is, the PWM output frequency can be adjusted by R1 and R2. Pin 8 is connected to a capacitor C51 for soft start to reduce the startup impact of the power switch tube. Pins 11 and 14 output two complementary PWM waves, which are amplified by medium-power transistors Q1, Q2, Q3, and Q4, and then driven by pulse drive transformer T1 to drive two IGBTs, controlling the inverter module to achieve half-bridge inverter (as shown in Figure 3). The high-frequency transformer T1 plays the role of isolating strong electricity from weak electricity, enhancing the driving ability and reliability of the power supply. [page]

　　Figure 5 PWM generation and control module

　　2.5 Power Regulation Module

　　The implementation principle of power regulation: The voltage on the power regulating resistor is detected through an AD port of the IAP15F2K61S2 microcontroller, and then the AD value is obtained through analog-to-digital conversion. Then the zero-crossing delay trigger of the bidirectional thyristor TR1 is controlled according to this value, that is, the output power is controlled by controlling the phase of the trigger pulse. Among them, Figure 6 is a zero-crossing trigger principle diagram. The 12 V AC is rectified by diodes D31 and D32 and current and voltage are limited by R31, R32, and R33, and then the zero-crossing point is detected by transistor Q3. When the grid voltage passes through zero, P3.3 generates a negative pulse. In addition, the P3.3 port of the IAP15F2K61S2 microcontroller is an external interrupt port, and the zero-crossing point of the power frequency voltage is obtained by detecting the zero-crossing pulse.

　　Figure 6 Zero-crossing trigger schematic

　　2.6 Tuning and Impedance Matching Module

　　The matching between ultrasonic power supply and transducer mainly includes tuning matching and impedance matching. In tuning matching, in order to reduce the reactive loss caused by electrostatic resistance and make the piezoelectric transducer output the maximum power, it is necessary to make the transducer close to the pure resistance state through matching to improve the output efficiency of ultrasonic power supply. In addition, if the tuning matching is completed, that is, when the load is in the pure resistance state, in order to make the power supply output the maximum power, it is necessary to make the actual load and the optimal output impedance of the power supply equal, and the implementation method is: through a high-frequency transformer, the impedance of the transducer is transformed into the optimal output impedance of the ultrasonic power supply, so that the piezoelectric transducer outputs the maximum power.

　　Figure 7 is a tuning and matching and impedance matching module of an ultrasonic cleaning machine, wherein the dotted box is an equivalent circuit diagram of a piezoelectric transducer.

　　Figure 7 Tuning matching and impedance matching

　　Among them, Co is the static capacitance of the piezoelectric transducer, which is mainly the capacitance generated by clamping. It is a real electrical quantity; Ro is the dielectric loss resistance of the piezoelectric transducer. It is generally believed that Ro is infinite and usually negligible; Ld, Cd, and Rd are the dynamic inductance, dynamic capacitance, and dynamic resistance of the piezoelectric transducer, respectively. When Ld and Cd are in resonance, the series branch is a pure resistor. Under the action of series inductance tuning and matching, the entire load of the ultrasonic power supply presents a pure resistive property. When the output voltage of the power supply is stable, the power obtained on the resistive load is only related to the resistance value of the load. Therefore, a high-frequency transformer is required to perform impedance transformation so that the ultrasonic power supply can output at maximum power.

　　3 Conclusion

　　According to the actual needs, this paper takes a single-chip microcomputer of STC model IAP15F2K61S2 as the control core, and proposes the overall design scheme of the ultrasonic cleaning machine system. According to the design scheme, the software and hardware are designed and debugged to ensure that its operating frequency is continuously adjustable in the range of 20~50 kHz and the dead time is stable, so that it can generate high-power ultrasonic waves after matching with the ultrasonic power supply and piezoelectric transducer. Finally, an ultrasonic cleaning machine with power regulation, frequency modulation and timing functions is produced according to the design. Through field tests, this ultrasonic power supply system can work stably for a long time.