Design of control system for high voltage pulse source of electro-biological effect

In recent years, developed countries have shown increasing interest in nonlethal weapons with soft lethality. Among them, the study of biological incapacitation effects of high-voltage pulses is an important topic in the development of such weapons. The electro-biological effect high-voltage pulse source is a key device. In order to study the effects of different pulse parameters on organisms, the amplitude, width and frequency of the pulse are required to be adjustable within a certain range. Therefore, the high-voltage pulse source is realized based on the all-solid-state rigid tube modulation technology and consists of three parts: a DC high-voltage power supply, a series IGBT modulation switch and a control system. The high-voltage pulse source control system based on the 8051 microcontroller has the advantages of simple design and reliable circuit. However, its program design is generally a front-end and back-end system with poor real-time performance, so that the stability of the output pulse waveform cannot meet the experimental requirements. This paper designs the program based on the embedded real-time operating system Small RTOS51, and optimizes the sampling signal filtering algorithm to make the output voltage accuracy of the high-voltage pulse source stable to 0.1%. Therefore, it can be reliably used in biological effect experiments.

1 Hardware Design

The electro-biological effect high-voltage pulse source is required to generate a pulse high-voltage output in a wide range of 500 to 5 000 V. The control system manages the work of all components of the high-voltage pulse source and is the core of achieving high reliability. The main functions of the control system are: continuously scanning the keyboard to obtain the power supply working parameters/commands, controlling the high-voltage pulse voltage amplitude according to the set parameters and the continuously sampled load voltage value, so that the power supply outputs at a predetermined constant voltage; sampling the primary switch power supply state to calculate the power supply output power in real time to protect the implementation object; and displaying the working status of the power supply on a dot matrix color graphic LCD screen in real time. The principle of the high-voltage pulse source is shown in Figure 1.

The hardware of the control system mainly consists of the control core, pulse width modulation module, voltage control and sampling module, keyboard and display module, and power-off data storage module. The control system structure block diagram is shown in Figure 2.

1.1 Voltage output control and sampling module

When the high-voltage pulse is output, the control system continuously samples the output voltage amplitude and compares it with the set value, and adjusts the output of the primary high-voltage switching power supply according to the comparison result. The accuracy of output voltage sampling and the output control accuracy of the primary high-voltage switching power supply are the key to achieving accurate and stable output of high-voltage pulses. Based on MAX526 and MAX197, 12-bit resolution output control and sampling can be achieved. In order to reduce the impact of high-frequency interference caused by the primary switching power supply of the high-voltage pulse source on the voltage control and sampling circuit, in addition to reasonable wiring of the ground wire, a 5 Ω resistor is connected in series to the digital voltage source of MAX526 and MAX197 for power supply filtering. The peripheral circuits of MAX526 and MAX197 are shown in Figure 3.

1.2 Pulse Width Modulation Module

The electro-biological effect source requires a continuously adjustable PWM signal with a frequency of 1 to 500 Hz and a pulse width of 1 to 200 μs to control the high-voltage pulse output. The PWM signal can be realized by using the built-in counter of the 8051 microcontroller, but considering that the program design is based on Small RTOS51, like most embedded operating systems, Small RTOS51 will frequently turn off all interrupts to protect the execution of key code in the system, which will cause the PWM signal generated by the software to have low accuracy. Therefore, this design uses the 8254 cascade method to realize the PWM signal.

8254 has three independent 16-bit subtraction counting channels, using a single -5 V power supply, and a maximum counting frequency of 10 MHz. The cascade method allows counters 0 and 2 to use the same 1 MHz external crystal oscillator. Counter 2 works in mode 3, and the count value is fixed to 20 to generate a 50 kHz square wave. The square wave signal is used as the clock of counter 1, and the count value of counter 1 is the frequency value, generating a 1-500 Hz frequency adjustable square wave. Counter 0 works in mode 1, and the count value is the pulse width value. The 1-500 Hz signal output by counter 1 is used as the gate control to achieve a continuous output of 1-500 Hz and 1-200μs. The 8254 peripheral circuit is shown in Figure 4.

1.3 Keyboard, display and power-off data storage module
The operation interface of the electro-biological effect source consists of a 1×5 keyboard and a 65536 true color graphic intelligent LCD display device. The 5 keys are directly scanned by the IO port of the single-chip microcomputer to obtain the key value. The LCD device has a built-in 32-bit ARM processor and RS232 serial port, which can be easily connected to any MPU with serial port function, suitable for rapid product development.
In addition to saving user parameters, the power-off data storage chip is also used to store intermediate variables of the program. EEPROM chip is a commonly used low-cost power-off data storage chip, but its access speed is too slow, which will take up too much time of the MPU and reduce the real-time performance of RTOS. Therefore, the system uses NVRAM chip DS1220 with built-in lithium battery and battery management function, and its data access time is 50 μs each time.

2 Software Design
Small RTOS51 is a preemptive embedded kernel developed specifically for 51 single-chip microcomputers, providing basic services such as message queues, semaphores, and interrupt management, and its functions meet the application of most engineering projects. Program design based on RTOS can ensure the stability of the output of the electro-biological effect high-voltage pulse source.
2.1 Program architecture design
The electro-biological effect high-voltage pulse source control system program is divided into 4 tasks. The priorities from high to low are: keyboard processing task, primary switch power supply status monitoring and display task, communication task with LCD display device, system operation and system status display task. The relationship between the task and the RTOS kernel and interrupt is shown in Figure 5.

2.2 Software Filtering Algorithm
The output sampling signal of the high-voltage pulse power supply with a high-frequency switching power supply as the primary energy source will inevitably have various high-frequency random interference signals. Even after being processed by a low-pass filter circuit, the sampling signal still has many burrs, as shown in Figure 6.

Therefore, software filtering must be used to obtain accurate sampling signals. In the case of severe high-frequency random interference, the median filter algorithm is often used, but the disadvantage of this algorithm is poor sensitivity. Therefore, this paper proposes an improved median average filter algorithm. Its basic principle is: sample N data in each group, sort the N data, remove the largest two values ​​and the smallest two values, and average the remaining (N-4) values, and use the average value as the final sampling result. In this paper, N=10. According to the algorithm, the signal in the above figure is sampled, the maximum sampling value is 4.01 V, the minimum is 3.99 V, the jitter range is ±0.01 V, and the third least squares method is used for numerical simulation, as shown in Figure 7. It can be seen that the algorithm can effectively filter out high-frequency random interference signals and has high sensitivity.

3 Conclusions
The output accuracy and stability of most high-voltage pulse sources based on high-frequency switching power supplies as primary energy sources do not meet the requirements of biological effect experiments. The core of achieving high accuracy and stability lies in the design of the control system. The sampling algorithm with strong anti-interference ability and high sensitivity is the key to solving the problem. This paper designs the program based on RTOS and proposes an improved sampling signal filtering algorithm. Compared with the traditional median filtering algorithm, the output pulse accuracy of the power supply and the reliability of the system are improved. Actual measurements show that the output accuracy remains at 0.1% during long-term continuous operation, which can be reliably used in biological effect experiments.