Design of an intelligent microwave therapeutic apparatus and its control system

Abstract: This paper mainly describes the design of the intelligent system of medical microwave therapeutic apparatus. The composition principle and function of microwave therapeutic apparatus are briefly introduced, and the software and hardware design of the intelligent automatic control system of the microwave therapeutic apparatus is described in detail. At the same time, some components of the microwave therapeutic apparatus are introduced accordingly. Through PID adjustment and segmented fitting, the intelligent control of the microwave therapeutic apparatus makes the operation of the microwave therapeutic apparatus more stable and reliable than that without this control scheme. The test results also show that the design is feasible. At the same time, the host computer software is developed to provide a platform for follow-up visits and statistical analysis of treatment effects.

1 Introduction

微波治疗是临床上一种新的治疗手段，其设备简单，治疗效果明显，使用安全，并发症少，对组织损伤小，因此得到医务界的肯定。微波治疗仪是利用微波在人体产生的热对患者的病变部位进行辐射，从而达到治疗目的，鉴于其治疗效果显著，是当今发展无损伤治疗的理想医疗设备。目前各医院使用的微波治疗仪进口的占多数，普遍存在体积大、价格昂贵、操作复杂等缺点，而国内生产的微波治疗仪技术还需更新、输出功率稳定性欠佳，致使治疗效果不明显，缺乏一些必要的保护措施，这使得整个系统的安全性不是很好，智能化程度不理想，使用操作起来显得不方便。所以有必要对其进行重新设计，尤其在安全性和控制系统方面要使用新的设计方法，以达到最佳效果。

The intelligent microwave therapy device designed in this paper has the following characteristics: (1) The control system uses overload protection in hardware and PID control plus segmented fitting method in software, which fundamentally changes the safety performance and output power of the therapy device; (2) An embedded operating system is used in the software design, which brings the operability, safety performance and stability of the entire microwave therapy system to a new level.

2 Microwave therapy mechanism

Microwaves are ultra-high frequency electromagnetic waves with a frequency of 300~300000MHz and a wavelength of 1mm~1m, and are non-ionizing radiation. Microwave
technology plays an important role in both medical diagnosis and treatment.

In the process of microwave therapy, thermal effect therapy and non-thermal effect therapy are distinguished according to the different microwave power densities used. At present, the mechanism of thermal effect therapy is relatively clear. Its therapeutic mechanism is: most of the human tissue is composed of polar molecules such as water and protein. Under the action of the microwave electric field torque, the polar molecules move in an orderly arrangement along the direction of the microwave electric field, and rotate back and forth with the alternation of the high-frequency electric field. During the rotation, they produce friction-like collisions with adjacent molecules to generate heat. The temperature of the body tissue in the irradiated area rises, and the blood vessels and microvessels expand, so as to achieve the purpose of accelerating the metabolic process, improving local nutrition, and enhancing tissue repair and regeneration. Unlike other thermal therapy methods, the heat source of microwave heating is not conducted from the outside, but generated by the biological tissue itself. This thermal effect has high efficiency and good balanced thermal stability [4-6].

3 System composition and principle block diagram

The treatment process is completed by placing the organism in a microwave radiation field using the microwave heating effect treatment principle. The instrument is mainly composed of a switching high-voltage source, a magnetron, a linear power supply, a waveguide, a microwave probe, a sensor, and a microcontroller. The pulse width modulated switching high-voltage power supply generates the high voltage required by the magnetron. The output power of the magnetron is stabilized by feedback control of the high-voltage DC current. The linear power supply provides a stable DC voltage to power the filament. The magnetron uses a medical magnetron, the waveguide (microwave cable) is used to transmit microwaves, and the microwave probe is inserted into the patient's affected tissue for treatment [5]. The sensor is used to monitor the output power of the magnetron. The microcontroller controls the normal operation of the entire system.

Through a full-bridge rectifier and a comparator, the sinusoidal alternating current is converted into a series of rectangular waves. At the falling edge of the rectangular wave, that is, the zero-crossing point of the sinusoidal alternating current, the system will receive an interrupt request, indicating that the thyristor and relay can be triggered. The appropriate trigger point is controlled by the system to achieve the required power. The power sensor collects the output of the magnetron to monitor the output power value and form a closed-loop control system [3]. At the same time, the output end of the sensor is also connected to the hardware protection circuit. If the control system fails and the output is not controlled, the output power may exceed the range required by this instrument. At this time, the hardware protection circuit will detect the output over-limit signal and force the output to shut down to enhance the reliability of the system. After the treatment is completed, the host computer software will output the relevant information of this treatment and save it to the database for follow-up use and statistical analysis of the treatment effect.

The system principle block diagram of the microwave therapeutic apparatus is shown in Figure 1.

Figure 1 System principle block diagram of microwave therapeutic device

4 Control System

The control system is the core of the entire therapeutic instrument. All control instructions are issued by it, so the stability design of the control system is the key to the entire design.

In order to enhance the reliability and anti-interference of the instrument, a special hardware protection circuit is used. When the hardware fails and the microwave output is uncontrolled, when its power exceeds the set value, the hardware protection circuit will be triggered and the output will be automatically shut down, thus improving the safety of the entire microwave therapeutic instrument. In the software design, an embedded operating system is used, which greatly improves the stability and anti-interference of the entire system. In order to improve the reliability of control and the performance of rapid response, PID closed-loop control is used. At the same time, the output power of the microwave tube is affected by many factors such as the grid voltage, working temperature and working time. Its output has the characteristics of nonlinearity, lag and time variation. Therefore, the input of the sensor is segmented and fitted, and different correction coefficients are set in each segment to make the output more stable [1][2].

4.1 Control system hardware design

The hardware is mainly divided into the following parts. The power supply provides power for the control system; the keyboard and LCD display circuit complete the human-computer dialogue function; the control circuit of the magnetron achieves the treatment effect of different diseases of the patient according to the requirements of the program instructions; the hardware protection circuit provides reliable protection for faults in the control system; the power sensor and AD converter monitor the output of the microwave tube; the output drive is completed by the relay and the thyristor at the same time; the communication circuit completes the information exchange between the controller and the host computer; the buzzer drive circuit combines with the light-emitting diode to jointly alarm the end of preheating, the end of treatment and faults.

The hardware block diagram is shown in Figure 2:

Figure 2 Hardware block diagram

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4.1.1 Power supply and human-computer interaction circuit

In order to prevent interference from entering from the power supply, the power supply is composed of a switching power supply and a filter circuit, and has good anti-interference performance. For easy operation, a treatment and physiotherapy conversion key, a work pause key, a power adjustment key, a time adjustment key and a reset key are set; the display uses an LCD screen plus an LED indicator to display information such as current power, time and treatment status.

4.1.2 Magnetron control and drive circuit

The magnetron is a microwave emission source, which consists of an anode, a resonant cavity, a cathode and a magnetic field. When a 3.3V DC filament voltage is applied to the magnetron filament, the cathode is heated, and a DC high voltage of about 2000V is applied between the anode and the cathode. The electrons emitted by the cathode fly to the anode under the action of a strong magnetic field. There are multiple small resonant cavities on the anode. Before the electrons hit the anode, they oscillate in these resonant cavities, and the resonant frequency is about 2450MHz. After the treatment begins, the relay is closed, and the thyristor is triggered after the zero point to control the intensity of the microwave. At the same time, the power sensor starts working to monitor the microwave intensity. When the thyristor breaks down or other faults occur, the system cannot control the output of the magnetron, and the value collected by the power sensor is greater than the upper limit of the safe power. When disconnecting the thyristor is invalid, the hardware protection circuit is activated, the relay is closed, the high-voltage power supply is disconnected, and an alarm is sounded to remind the operator that the instrument has a fault.

4.1.3 Alarm and communication circuits

The alarm circuit is mainly composed of light-emitting diodes and buzzers. When the treatment is over or a fault occurs, the alarm circuit will be activated to notify the operator of the current status in an audible and visual manner. The communication circuit is mainly responsible for data communication between the instrument and the host computer. After the treatment is completed, the relevant information of the treatment will be uploaded to the host computer.

4.2 Control system software design

The system software consists of two parts: the host computer and the instrument software. The host computer software is written in VB, which mainly records the treatment information for follow-up visits and statistical analysis of the treatment effect. The microcontroller part uses the MicroC/OS-II embedded operating system, which greatly enhances the stability and anti-interference performance of the entire control system, while speeding up the design speed and making the code easy to maintain.

In the treatment state, the relay is closed first, and then the voltage zero crossing point is detected. After the zero crossing point, the thyristor is triggered with a delay. The delay time is controlled by the output drive circuit and the power sensor to form a PID closed-loop system. Since the output of the microwave tube does not change linearly, the input of the sensor is segmented and fitted with different correction coefficients before being used for PID adjustment. This improves the reliability and rapid response capability of the control system and makes the output more stable.

The program flow chart is shown in Figure 3.

Figure 3 Program flow chart

5 Conclusion

The author's innovation: The microwave power control scheme fundamentally solves the problem of microwave output power imbalance. The added protection circuit in the hardware has greatly improved the safety performance of the instrument. The control strategy adopts the method of segmented fitting plus PID control fusion, which has the characteristics of good linearization and the advantages of high PID control accuracy and good stability, overcoming the difficulties caused by nonlinearity, lag, time variation, etc. in microwave power control.

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