Development of Standing Wave Electron Linear Accelerator Test System for Industrial CT

Industrial computed tomography, referred to as Industrial CT (ICT), is a non-destructive testing technique for reconstructing images of internal structures of objects based on external projection data. Industrial CT is mainly used for non-destructive testing and flaw detection of industrial products, and X-rays of different energies are selected according to the material and size of the workpiece to be inspected. ICT technology can accurately and accurately reproduce the three-dimensional structure inside the object, and can quantitatively provide the physical and mechanical properties inside the object, such as the position and size of the defect, the change of the density and the level, the shape and the precise size of the shaped structure, and the inside of the object. Impurities and distribution. With the development of accelerator technology, its application fields have been extended to industrial flaw detection, industrial CT, container monitoring, irradiation, medical treatment, etc., so the requirements for the operating parameters and stability of the accelerator are also increasing.
According to the basic process of reconstructing CT images, the basic requirements for an industrial CT device are: the ability to measure the intensity of rays after X-rays penetrate the object being inspected, and to complete the scan between the X-ray machine-detector system and the detected object. Motion to obtain the complete data needed to reconstruct the CT image; the obtained data reconstructs the cross-sectional image of the object by a certain algorithm. From scanning to reconstructing images are controlled and implemented by a computer. Thus, an industrial CT system should include: a source of radiation, a detector of radiation and a collimator, a data acquisition system, an image reconstruction, and a control system [2].
The common sources of radiation for industrial CT are X-ray machines and linear accelerators, and the importance of linear accelerators in industrial CT is seen. To facilitate the testing, operation and maintenance of the operating personnel, the five components of the CT system are tested separately. After all the parts meet the indicators, the overall test is carried out to meet the test of user needs. This approach has now become a new way to test accelerators. This test system is designed to test various indicators of the accelerator and has certain practical value.
1 system overall plan
The test system can display the state information transmitted by the accelerator through the transmission medium correctly through the agreed manner, and at the same time, can transmit the control information to the upper device for display by modifying and setting the relevant state on the underlying device, thereby obtaining the accelerator operation. status. It is the bridge between operators to understand accelerator performance and remote testing.
Both the standing wave electron linear accelerator and the test system use an aviation plug, and the two ends of the connecting cable are electrically connected by using a pin-shaped aviation plug. The test system consists of two parts: the upper computer and the controller: the upper computer is composed of a computer (can be shared with the CT machine acquisition computer), and its main function is to provide a human-computer interaction interface to realize communication with the controller and display of the accelerator state; The controller is an embedded system consisting of different modules to complete communication with the host computer. At the same time, the operator can control the working state of the accelerator through the buttons on the controller, realize direct control of the action of the accelerator part, effectively reduce the loss caused by the accelerator failure, and facilitate the maintenance of the accelerator.
The controller directly controls the accelerator's "emergency stop", "power", "power on" and other related performances to control the launch and stop of the accelerator; the accelerator will also "bundle state" and "return test" The control signal value is returned to the controller, and the controller processes it and sends it to the host computer so that the operator can directly understand the performance of the accelerator. When the “synchronization signal” value on the host computer interface is 1, the host computer issues a command to the controller. After the controller processes, the controller controls the accelerator to emit a square wave with a frequency of 50 Hz to 250 Hz, which is in the square wave high level. In the microsecond time, when the accelerator is out of the beam, the operator can collect the data within a few microseconds of the accelerator launch, and then send it to the control system for image reconstruction to determine whether the detected object has cracks. In addition, the host computer also monitors the accelerator's vacuum, water flow, temperature control, high preheating, low preheating and other related performance, and simultaneously plots the dose value of the accelerator beam and redraws it afterwards. The controller can also separately control the control signals such as "enable control" and "breaking beam control" of the accelerator and display on the liquid crystal screen.
2 host computer
The human-computer interaction interface is the main component of the host computer, providing an intuitive and convenient operating environment for the operator, using the visual programming language Visual C++ as a development tool. The serial port of the upper computer communicates with the CT control computer, and sends information such as the working state, operating parameters and fault codes of the accelerator to the CT control computer, and is displayed by the main interface of the CT control computer. At the same time, the upper computer interface also completes the real-time rendering of the dose rate time curve and the function of post-repainting and monitoring and setting of the accelerator working mode.
The operation interface is mainly composed of a status display area, a parameter display area, a control signal area and a drawing area. The status display area has 18 display states. When the system is running, the working status of the system is displayed in the corresponding position of the status area. The parameter display area mainly reflects the trigger frequency value, dose value and beaming time of the accelerator. The control signal area gives the control signal value from the test system to the accelerator, wherein the "enable control" controls whether the accelerator is out of the beam; the "synchronization signal" controls whether the accelerator emits a square wave pulse; the "energy selection" performs the 6Mev and 9Mev for the accelerator. The choice of energy; "bundle state" and "return test" are the values ​​of the control signals that the test system controller reads back from the accelerator to reflect whether the accelerator is working properly. The plot area reflects the trend and trend of the dose rate during the bundle.
The upper computer can not directly apply the control signal to the accelerator. It is necessary to transfer the control signal value to the controller, and then perform corresponding processing through the controller, and then control the accelerator. This test system uses RS-232 for communication (long-distance communication can be changed to RS-422 protocol).
3 controller
Accelerator's emergency stop, power supply, power-on and other functions are directly connected to the controller through the aviation plug. The accelerator can be controlled by the relevant buttons on the controller, the high warm-up process, the high-heating process, whether the beam is discharged, and whether the accelerator is cut off. Power and other functions. In addition, the timing control of the "enable control" and "synchronization signal" of the accelerator needs to be controlled by the controller after the calculation process.
3.1 Controller hardware design
According to the design of the test system, the controller should design the interface, parameter mode, numerical input interface and display interface for communication with the host computer. Considering the need of anti-interference design, the controller hardware circuit is composed of the following five modules: communication module, display module, control module, optical isolation module and button processing module. The design of the control module and optical isolation module is described below.
(1) Control module: Based on the performance and price of the single-chip microcomputer and the consideration of the requirements of the control cabinet system for the single-chip microcomputer, the AT89C52 chip is selected. Design advantages: The price of the single chip is cheap, the required functions can be realized, and the cost performance is high. Design Disadvantages: The single-chip microcomputer is serially processed, which cannot satisfactorily meet the requirements for systems with high execution efficiency.
(2) Optical isolation module: The control side interface power supply uses an independent 5 V power supply, and the signal between the control cabinet and the accelerator is connected by a light-coupled differential device 6N136 with a speed of 1 Mb/s; the differential signal form is completed by a 5 V 26LS32 device. Input/output.
3.2 Controller software design
The C language is used to write the controller software program. It has good portability and can be quickly ported to the UNIX operating system when the system is expanded. Mainly composed of main program module, LCD display module, serial port module, button processing module and waveform generation module.
The main function of the main program control system, the functions of each part of the MCU are implemented in the corresponding subroutine. In order to ensure the real-time performance of data transmission and the operation of other parts of the controller, the serial port and waveform generation are implemented by interrupt mode.
3.3 Frequency value error analysis and correction This system uses timer 0 to realize the square wave timing time. Since the used microcontroller crystal oscillator is 11.059 2 MHz, the calculation formula of the timer 0 count parameter is: initial_value=t/MC. Where t is the desired timing, t=T/2=1/(2×freq); MC is the machine cycle of 89C52, ie MC=12/11.059 2, so initial_value=t/MC=460 800/freq. This gives the initial value of the timer 0:
TH0=(65536-initial_value)/256
TL0=(65536-initial_value)%256
Since the crystal oscillator of the single chip is 11.059 2 MHz, there is an error between the actual frequency value and the theoretical value.
Accelerators have higher requirements on frequency values. When the input frequency value is changed from 50 Hz to 250 Hz, the error range is within 0.5% when the frequency values ​​of 50 Hz, 100 Hz, 150 Hz, 200 Hz, and 250 Hz are required. For the remaining frequency values, the error range is within 2%. In order to meet the above requirements, when the frequency value is a multiple of 10, according to the frequency value read, the value of initial_value is directly given by continuous correction, instead of the initial_value value calculated by the formula. Values ​​in other ranges are multiplied by the frequency value of the initial_value value of the special frequency value in the vicinity to give a rough value of M. Among them, the value of M is less than the theoretical value of 460 800.
The frequency value calculated by this method has high precision, and can better meet the design requirements of the accelerator for high accuracy of frequency values.
The industrial standing CT electronic linear accelerator test system has the characteristics of modularization, and can be extended according to practical needs; the quality monitoring operation of the sub-system is realized. Thanks to the optical isolation module, the control cabinet can operate normally in an industrial environment, which enhances the stability of the system. In addition, the system compensates for the frequency error to meet the needs of the accelerator's normal operation. The system has been tested in the 9MeV standing wave electron linear accelerator industrial CT machine, and the operation is stable and reliable.