Design Of Control System Of Embedded Pulse Neutron Generator Based On FreeRTOS

The neutron generator is a small acceleration device capable of generating neutrons.It has the advantages of high yield,good monochromaticity,safe controllability,easy portability and convenient maintenance.It is widely used in petroleum logging,coal quality analysis and explosive detection.In the actual test,it was found that the real-time performance of the neutron generator console lower-level computer system developed by the research group in the past was not enough,the AD sampling accuracy on the MCU was low,the linearity of the driving sampling circuit was poor,and the wire connection of the hardware system led to increased signal interference 3.The long-distance communication has a large bit error rate,and the system as a whole will have zero drift.This will affect the experimental staff's grasp of the real-time state of the neutron tube,which will directly or indirectly affect the neutron yield and its stability.In response to the above problems encountered in actual testing,on the basis of inheriting the excellent experience of the predecessors,a pulse neutron generator control system based on the FreeRTOS real-time operating system was designed.By means of targeted design,it was well solved.The problems encountered in the above experiments have improved the stability of the neutron yield and greatly reduced the period and cost of later development and maintenance.The design of the control system is mainly composed of two parts: hardware system and software system.Hardware: mainly includes the minimum system circuit of the core board of the lower computer,driving sampling circuit,storage power supply,ion source power supply,accelerating pole power supply and He3 detector.The main control chip of the core board adopts an ultra-low power consumption embedded microcontroller STM32f407ZGT6 with ARM-CortexM4 as the core for industrial control field,which realizes six-channel AD data acquisition,generates three-channel PWM signal to control three-way power supply,two Circuit pulse provides ion source trigger pulse signal and delay synchronization signal and serial communication RS-232,RS-485 and other circuits.The design of the driver board abandons the separate design of the previous generation of the control board and the core board and the wire-type data transmission method,and integrates the two to form the overall design of the core board + the driver board.While improving hardware integration,the plug-in connection greatly enhances the stability and reliability of signal transmission.In the design of the driving sampling circuit,a low-pass filter is designed using a precision op amp to convert the PWM signal generated by the main control chip into a continuously adjustable analog signal to realize the control of the three-way power supply;at the same time,the AD acquisition circuit is designed to enter the main control Reasonable layout and planning before the chip,to achieve real-time collection of power information of each channel.The power supply control circuit includes an isolation circuit and an active filter circuit;the power supply voltage and current collection is formed by a voltage follower circuit and a signal amplification circuit.It was found in the experiment that when the accelerating pole works at a higher voltage,sudden discharge is easy to occur,which will cause different degrees of interference to the line and the entire control system,and even damage electronic devices.In order to better enhance the stability of the control system,a safe,shock-resistant RS-485 isolated communication module is added to the communication.And each part of the system integrates effective device protection,such as transient suppression diodes,fuses,etc.,to ensure the effective and stable operation of the control system.Software: mainly includes the neutron generator lower computer program based on the FreeRTOS real-time operating system of the core control board,and the LabVIEW control interface program of the upper computer.In the programming of the lower computer,it mainly includes the transplantation of FreeRTOS on STM32f407ZGT6.At the same time,on-chip hardware resources are used to complete AD acquisition,sampling filtering,PWM signal output,and asynchronous communication with the upper computer.According to the actual working state of the generator,different tasks are created and given corresponding priorities.The operating system performs task scheduling,which guarantees the real-time performance of the system.In the aspect of code writing,a task-based programming method is used,which greatly improves the readability and later maintenance of the code;in terms of communication,the ModBus industrial communication protocol is transplanted on the main control chip.Reading and writing realizes effective data transmission between upper and lower computers.Aiming at the problem that the on-chip AD acquisition accuracy is not high,oversampling technology is used to improve the acquisition accuracy through software,from the original 12 bits to 16 bits.In the program design of the host computer,in response to the zero drift of the hardware system,the imaginary value of the zero point is adjusted in the host computer,which improves the friendliness of the interaction between the console and the user.In response to the problem of large current impact caused by the switch of the host computer,a PID control algorithm is added to each control section to extend the adjustment time to reduce the impact of large current on the neutron tube,and at the same time can achieve precise control;for the entire sampling circuit The temperature drift and time drift appearing in the method use the least square method to correct the nonlinear error in the system,so that the nonlinear error of the system sampling is reduced to 0.2%.In this paper,the actual test of the designed neutron generator control system is carried out: the data acquisition accuracy test and the linearity test of the entire sampling circuit are carried out for the oversampling technology and the least squares linear fitting,and the storage power supply and ion source are also tested.Three-way power supply such as power supply and accelerating pole power supply were tested for step-voltage linearity.The experiment proves that the parameters of the three-way power supply can be adjusted linearly with the step size of the host computer and the adjustment accuracy has also been increased.Among them,the storage power supply voltage adjustment accuracy is 0.8mV,the adjustment range is 0-5V;the ion source power supply voltage adjustment accuracy is 1.3V,the adjustment range is 0-3000V;the accelerating electrode voltage adjustment accuracy is 0.017 kV,the adjustment range is 0-120kV;The performance of the DT neutron generator designed in this paper meets the design requirements.The neutron yield can reach 1.0 × 108 n / s,the life span can reach 8000 hours,and the neutron yield stability is within 0.28%.