The software and hardware design of ultrasonic tooth cleaning machine based on single chip microcomputer

This paper discusses the hardware and software design of ultrasonic tooth cleaning machine with single chip microcomputer as the core. The tooth cleaning machine uses current sampling feedback to automatically scan and search for the resonance point. The resonance frequency and oscillation intensity are digitally locked, and the resonance point drift is extremely small, which solves some inherent defects of ultrasonic tooth cleaning machine designed with analog oscillation circuit.

1. Hardware Design

The hardware circuit block diagram is shown in Figure 1. The basic working process of the dental cleaning machine is as follows: the TL494 core oscillator circuit generates a push-pull pulse output with controllable duty cycle under the control of the MPU, and the MPU sends data serially to the oscillation frequency control circuit to control the oscillation frequency of the oscillation generating circuit, so that the duty cycle and frequency of the oscillation signal generated by the oscillation circuit are controlled by the MPU. The oscillation signal is amplified by the power amplifier circuit, and after being boosted by the high-frequency transformer, it drives the piezoelectric ceramic piece to convert the ultrasonic oscillation electrical signal into an ultrasonic mechanical vibration signal. The mechanical vibration can effectively remove dental plaque and calculus, thereby achieving the effect of beautiful teeth.

1. Power supply design

The power of the ultrasonic tooth cleaning machine is 10-20 W in normal operation, and it is required to work in a wide voltage range of 180-250 V. In order to meet the requirements and reduce the heat generation of the power supply part, the power supply part of this circuit adopts a switching power supply. The circuit schematic diagram of the whole machine is shown in Figure 2.

This switching power supply uses Motorola's DC-DC control chip MC34063. The circuit has the characteristics of simple circuit, low cost, high efficiency and low temperature rise. The core component MC34063 is a monolithic bipolar linear integrated circuit, which contains a temperature compensated bandgap reference source, a duty cycle control oscillator driver and a large current output switch. The output voltage U = (1 + R2 / RI) · 1.25 V, the current limiting resistor is 1 Ω, so the input current is limited to 0.3 V / 1 Ω = 0.3 A.

2. Oscillation circuit

There are many ways to generate oscillation signals. The simplest method is to generate PWM output directly by PIC16F73. This method is simple and convenient, but it has two defects: First, it cannot generate push-pull oscillation signals. Therefore, the power amplifier circuit can only work in the positive half cycle, with low efficiency and serious heat generation, which is not conducive to the stable operation of the circuit. Second, the resonance point of the piezoelectric ceramic is (30±5)kHz, and the resonance frequency bandwidth is ≤80 Hz. The PWM output of PIC16F73 is at a frequency of 25 to 35 kHz, and the step frequency is ≥100 Hz. Therefore, the PWM output of PICl6F73 may not find the best resonance point of the piezoelectric ceramic. The oscillation circuit designed by the author satisfactorily solves the above problems.

The oscillation circuit control chip uses TLt94, the internal block diagram of the chip is shown in Figure 3, and the specific circuit is shown in Figure 2. The push-pull oscillation signal is output from pins 9 and 10 of TL494. The frequency of the signal is determined by the capacitor Ct and resistor Rt connected to pins 5 and 6 of TL494. Rt and Ct should be low-temperature drift resistors and capacitors. The signal oscillation frequency calculation formula is: fosc = 1.1/2Rt·Ct; the duty cycle of the signal is determined by the external signal voltage of pins 1 and 2 of TL494.

3. Frequency control

To meet the requirements of 25-35 kHz oscillation frequency of piezoelectric ceramics and ≤80 Hz step frequency, Rw in the circuit of Figure 2 is a coarse adjustment potentiometer with a resistance of 20 kΩ, and the digital potentiometer IC4 is a fine adjustment potentiometer under the control of PICl6F73. After calculation, the coarse adjustment of Rw (with 1C4 as 5 kΩ) makes the fosc range of 24.5-35.7kHz, which meets the requirements. The fine adjustment digital potentiometer IC4 uses MCP41010 with a total resistance of 10 kΩ and 256 adjustable levels. The communication between MCP41010 and PICl6F73 adopts the convenient and fast SPI method, and the step resistance is 39.0625 Ω. The step frequency of the oscillator is:

The step frequency is 30.4 Hz when the oscillation frequency is 35 kHz, and 15.6 Hz when the oscillation frequency is 25 kHz. From the above data, it can be seen that the use of digital potentiometer to control the working mode of TL494 can meet the requirements of the resonance bandwidth of piezoelectric ceramics.

4. Intensity control

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This tooth cleaning machine is designed with a sensitive intensity control circuit. The RAl pin of PIC16F73 is connected to the potentiometer Rw1. When it is adjusted to different positions, the analog voltage input to RAl is different. After being converted into a digital signal by the internal A/D of PIC16F73, the signal determines the duty cycle of the PWM signal output by CCP1. After filtering, the PWM signal is sent to the 2nd pin of TI. 494 and compared with the reference voltage sent to the 1st pin, thereby determining the pulse width of the oscillation signal output by the 9th and 10th pins of TL494 to be between 0 and 48%. When the pin switch is disconnected, PIC16F73 determines that the RC3 input is at a high level, then the PWM output duty cycle of PIC16F73 is 0, and the oscillation signal output duty cycle of the 9th and 10th pins of TL494 is 0, thereby controlling the tooth cleaning machine to stop the mechanical oscillation output.

5. Push-pull power amplifier

In order to achieve a good tooth cleaning effect, the mechanical oscillation of ultrasonic mechanical oscillation must reach a certain intensity, that is, the oscillation signal output by TL494 sent to the piezoelectric ceramic must first be power amplified. Since the instantaneous current flowing through the power tube reaches 1.1 A, in order to reduce the heat generation of the power tube and reduce the heat sink, a field effect tube is used as the power driving tube. The field effect tube in this circuit is driven by a simple method. Practice has proved that the power amplifier circuit has stable performance, very little heat generation, and can effectively reduce the size of the circuit board. The signal after power amplification is boosted by a high-frequency transformer to a peak-to-peak value of 250 to 350 V and sent to the piezoelectric ceramic to be converted into ultrasonic mechanical oscillation.

6. Scanning search of resonance point

The automatic scanning and searching of the resonance point of the piezoelectric ceramic is a major feature and difficulty of this circuit. Since the resonance points of piezoelectric ceramics are different, in order to make the circuit adapt to various piezoelectric ceramics, the author designed an automatic scanning and searching circuit for the resonance point. When the PICl6F73 is just powered on and the pin switch is turned on, the PWM output pulse width of CCP1 is fixed to 80%, so that the output signal pulse width of the 9th and 10th pins of the TL494 remains unchanged. At the same time, the PICl6F73 periodically sends data to the digital potentiometer MCP41010, so that the resistance between the 6th and 5th pins of the MCP41010 steps from 0 to 10 kΩ, and the output frequency of the 9th and 10th pins of the TL494 changes in steps of 15.6 to 30.4 Hz. For a signal with a fixed duty cycle, when the oscillation signal frequency is consistent with the resonance frequency of the piezoelectric ceramic, the current flowing through the source and drain of the field effect tube is the largest. The current is converted into a voltage signal by the sampling resistor Ra, and is sent to RAO of PIC16F73 after amplification by the operational amplifier. PIC16F73 performs A/D conversion on the voltage to a numerical value Q, and memorizes the data P sent to the digital potentiometer when Q is the maximum value. When the digital potentiometer changes from 0 to 10 kΩ, the scanning search of the resonance point of the piezoelectric ceramic is completed. Sending data P to the digital potentiometer, TL494 outputs an oscillation signal of a fixed frequency, which is the resonance frequency of the piezoelectric ceramic. When selecting the operational amplifier, it is important to note that the bandwidth is greater than 2 MHz, because the peak voltage of the sampling resistor Ra changes very quickly during the scanning search. If the bandwidth of the operational amplifier is insufficient, the optimal resonance point of the piezoelectric ceramic may not be found. This circuit uses MCP602 with a bandwidth of 2.8 MHz.

2. Software Design

The hardware design of this tooth cleaning machine is slightly complicated, but the software design is relatively simple. The overall software flow chart is shown in Figure 4.

3. Anti-interference design

The tooth cleaning machine itself has solenoid valves, foot switches, high-frequency transformers and other devices that will generate strong interference. In addition, this machine is also used in a dental treatment table, which has several powerful motors working, which will cause serious electromagnetic interference to the tooth cleaning machine. When the interference signal comes, abnormal phenomena such as freezing, program flying, and damage to system parameters may occur, so some anti-interference measures are taken in both hardware and software.

1. Hardware anti-interference

Connect a power filter to the power input of the tooth cleaning machine to filter out high-order harmonics and pulse interference in the power grid. When selecting a single-chip microcomputer, choose a model with a hardware watchdog, or add an external watchdog circuit to effectively monitor whether the program is trapped in an infinite loop fault. Connect a 0.1 μF decoupling capacitor to the power input and common ground of each chip to optically isolate the signal sent by the foot switch. The above measures are all effective.

2. Software anti-interference

First, software redundancy. Any output signal and setting are constantly refreshed, and the cycle is set at 5 ms. The A/D conversion adopts the 8-time conversion averaging method to obtain the most accurate signal.

Second, software traps. Software trap technology is to forcibly introduce the captured flying program to the reset address 0000H through jump instructions, so that the program can be put on the right track. Setting software traps between control modules and unused program spaces can effectively suppress program flying and make program operation more reliable.