Design of Ultrasonic Therapeutic Instrument Based on AT89C2051 Single Chip Microcomputer

Abstract: This paper presents a design scheme of an ultrasonic physiotherapy instrument based on AT89C2051 single-chip microcomputer control. This scheme adopts a dual-channel input circuit of high-frequency and low-frequency signals, which can generate specific ultrasonic energy with low waveform peak and strong penetration, so it can act more deeply on muscles and bones, and accelerate healing. Practice has proved that this method is economical and simple to implement.
Keywords: AT89C2051; MAX038; ultrasonic wave; physiotherapy instrument

0 Introduction
At present, most ultrasonic therapy instruments (physiotherapy type) at home and abroad generally work in the form of continuous sine wave (1-3 MHz) or pulsed sine wave (low-frequency modulation of about 100 kHz), and their output sound intensity is ≤3.0 W/cm2. The problem is that ultrasonic waves mainly act on skin, muscle and connective tissue, and the emission waveform has high peak and poor penetration, so the therapeutic effect on deeper lesions is not obvious. Therefore, it takes a long time to treat and promote muscle recovery and bone healing. The physiotherapy device designed in this paper can generate specific ultrasonic energy with low waveform peak and strong penetration, which can act more deeply on muscles and bones, and accelerate healing. This design uses the design idea of ​​circuit modularization, and combines the actual circuit to design a simple and applicable multi-parameter ultrasonic therapy device. At present, there are no similar product reports on this design at home and abroad. The product design principle involves many cutting-edge research results and reports on the treatment of bone injuries with ultrasound.

1 System composition principle
The system is mainly divided into 5 main modules. Among them, the single-chip control module is the core of the system, which controls the intensity of the ultrasonic wave. Others (such as MAX038) mainly generate high-frequency waves, and NE555 constitutes a multivibrator. The ultrasonic wave generated after mixing is amplified and output, which can act on the injured part of the human body. Its basic system composition is shown in Figure 1.

The basic working principle of this system is that the integrated chip MAX038 outputs a high-frequency signal first, and the multivibrator composed of NE555 obtains a low-frequency signal through the two-frequency division circuit composed of 7474 and the low-pass filter composed of MAX029 and peripheral circuits, and then modulates the high-frequency signal with the low-frequency signal to obtain ultrasonic waves of appropriate frequency. Since the signal is relatively weak, a power amplifier is needed to amplify the signal. The power amplifier circuit here adopts a three-stage amplifier circuit, and finally outputs it through the transducer, and the output intensity of the ultrasonic wave is controlled by the single-chip microcomputer.

2 Generation of high-frequency signals
The MAX038 high-frequency waveform generator chip of Maxim can be used to generate high-frequency signals. Various waveform curves of this method can be expressed by trigonometric function equations. It can generate function signals of various waveforms. Function signal generators can also be made of general devices such as transistors and op amp ICs, and Maxim's function signal generator MAX038 has high frequency and good precision, so it is called a high-frequency precision function signal generator IC. The high-frequency signal generation circuit composed of this chip is shown in Figure 2.

In Figure 2, pins 3 and 4 (i.e., pins A0 and A1) are the waveforms that control the output, and the frequency of the output waveform is determined by the external capacitor C1 and pins 8 and 10.

3 Low-frequency signal generation circuit
The low-frequency signal generation module circuit can use NE555 to form a multivibrator. When the oscillator outputs a waveform signal, it is connected to a 7474 latch as the main component to form a two-frequency division circuit, and finally a MAX298 is used to form a low-pass filter for filtering, and the required low-frequency signal wave can be obtained. The MAX29X series 8th-order low-pass switched capacitor filter produced by MAXIM has been widely used in ADC anti-aliasing filtering, noise analysis, power supply noise suppression and other fields due to its easy use, simple design and small size (8-pin DIP package). The reason why MAX03 8 is not used here to generate low-frequency waves is mainly because when the DC levels of the two waveforms are different, the effect after mixing is not obvious. Using MAX298 for filtering can achieve good filtering effect within the frequency range of 5 kHz to 10 kHz. The specific low-frequency signal generation circuit is shown in Figure 3. The oscillation period is determined by R1, R2 and C1 in NE555. The specific formula is:

4 Design of Mixing Circuit
Mixing can be realized by multiplication circuit, and the circuit can be designed by the following relationship:

Because the multiplication circuit can easily realize the multiplication of two analog signals, and with the multiplication circuit as the basic unit, it can also be easily composed of division, exponentiation and square root operation circuits. Therefore, in the field of radio communication, the multiplication circuit can also be used to form modulation and demodulation circuits. At present, there are integrated circuits that can realize multiplication operations on the market, which are called analog multipliers.
The expression of analog multiplier is:

In the formula, k is the proportional coefficient, and the proportional coefficients of various circuits are generally different.
In the design of this circuit, the multiplier is mainly used to realize the multiplication of two signals to convert the low-frequency signal to the intermediate frequency. This design requires two frequency signals, one is the carrier (i.e., high-frequency signal) and the other is the modulating wave (i.e., low-frequency signal). In order to realize AM modulation, a reasonable multiplier circuit is required. This design selects the MC1496 integrated double-balanced analog multiplier from Motorola. MC1496 is an excellent analog multiplier produced by Motorola. It can be used as a broadband, suppressed carrier double-sideband balanced modulator. It does not require a coupling transformer or a tuning circuit. It can also be used as a high-performance SSB multiplication detector, AM demodulator/demodulator, FM demodulator, mixer, frequency multiplier, phase detector, etc.
MC1496 can be powered by a single power supply or a dual power supply, and the effects are the same. When powered by a single power supply, its DC bias is achieved by external components. This design uses a single power supply, and the output of a single power supply is a ground signal. In the dual power supply, the output is a differential signal of dual output. The specific circuit is shown in Figure 4.

5 Power amplifier design
In addition to the general circuits of rectification, oscillation, amplification and protection in general amplifiers, the power amplifier circuit of the ultrasonic generator should also have some special circuits, such as automatic frequency tracking, matching, automatic power control, vibration system amplitude control and other circuits. These circuits are required by the special properties of the generator load (i.e., transducer). Usually, the generator has an optimal load value (sometimes also called output impedance). Only when the actual load is equal to this value can the generator work in the designed state and provide the rated output to the load. Otherwise, it is necessary to use the output transformer for impedance transformation. In addition, tuning must be performed because the transducer is a reactive load. Therefore, direct connection to the generator will produce considerable reactive losses, which will not only reduce efficiency, but also affect the safe use of the generator. Therefore, an element with opposite reactance is needed to "offset" the influence of the reactance component in the transducer, which is called tuning. In this power amplifier, the most important thing is matching. Since the piezoelectric transducer has static capacitance and the magnetostrictive transducer has static induction, when the transducer is in resonance, there is a phase angle φ between the voltage and current on the transducer, and its output power P=cosφ. Due to the existence of φ, the output power cannot reach the maximum value. Only when φ=0 can the output power reach the maximum value. Therefore, in order to make the voltage VRL on the transducer in phase with the current (φ=0), it is necessary to connect a counteracting impedance in parallel or in series with the transducer. For the piezoelectric transducer, it is sufficient to connect an inductor in parallel or in series, while the magnetostrictive transducer should be connected in parallel or in series with a capacitor.
For the power amplification part of the ultrasonic wave, this paper has made some measurements and calculations, and the actual circuit design is a three-stage amplification. Figure 5 shows its front-end circuit, in which a field effect tube is used as the first-stage front-end amplifier, and then a radio collector is used to amplify the current. Each amplifier has a radio collector output circuit added to the output end to reduce the output impedance, thereby achieving the effect of amplifying the current. The intermediate transistor amplifier circuit is shown in Figure 6. Its input and output are both filtered by capacitors, and an emitter follower is added at the output to reduce the output impedance. The final transistor amplifier circuit uses Toshiba's 2SC3281 power transistor, and its circuit is shown in Figure 7.

6 Single-chip microcomputer control
This design uses ATMEL's AT89C2051 single-chip microcomputer to form the control system. Since the system requirements are not very strict, that is, high computing speed and large memory are not required, but only certain shock resistance and low price are required to achieve the purpose of economical affordability. Therefore, the author chooses AT89C2051 CPU. The operating temperature range of this CPU is -40℃～125℃, with 20 pins. It is a simplified version of 8051 CPU and can fully meet the use requirements of this system. Since the main program mainly completes the module initialization and the call of its own program, its structure is clear and simple. Figure 8 shows its control software flow chart.

The second is the initialization module, which is used to restore the timer, interrupt, etc. to the most original state. For example, the timer works in working mode 1, and the interrupt priority of timer 1 is the highest. Its initialization flow chart is shown in Figure 9.
The third is power output adjustment. The output power of this physiotherapy instrument is divided into three levels (low, medium, and high). The single-chip microcomputer output is used to control the static bias voltage of the subsequent BJT circuit, thereby changing the static working point of the BJT, so that the output voltage amplitude at both ends of the piezoelectric crystal changes to adjust the output power. During the design, the interrupt service program can be called through the external key interrupt to achieve this function. The principle is to determine which interrupt to enter by the number of key presses, and call the corresponding subroutine. Since the power here is divided into three levels, the remainder obtained by dividing the number of key presses by 4 can be used to judge, that is:
if the number of key presses is 4N (N=0, 1, 2, 3...), then no power is output;
if the number of key presses is 4N+1 (N=0, 1, 2, 3...), then the low-power output subroutine is called;
and so on, three different power outputs can be achieved.
The following is part of the code:
INTOSER: ACALL DELAY: Delay program to remove key jitter
...
MOV B, #04H; Set the divisor to 4
MOV A, R3; Set the dividend
DIV AB
MOV A, B; Get the remainder
MOV R3, B
RL A
ADD A, R3
MOV R5, A; Set the offset to 3 bytes
MOV DPTR, #PMTB; Get the table header address
JMP @A+DPTR; Determine the number of key presses and jump to the corresponding output program

7 Conclusion
This design mainly gives a more detailed implementation plan from ultrasonic generation to control. Compared with other methods, this solution is simpler and has lower cost, and the devices used are relatively common. High-frequency and low-frequency signals are generated independently, which is convenient for adjusting the two signals. In addition, the use of a single-chip microcomputer as a control chip has high flexibility and can be modified according to different requirements to meet various practical needs.