Research on the design of secondary electron imaging system for electron beam welding Chang Hongbin and Xue Xing (Guilin University of Electronic Science and Technology, Guilin 541004, Guangxi) designed the secondary electronic imaging system. The principle is to synchronize the welding machine and the electron beam in the kinescope through the synchronous scanning system, and collect the secondary electrons bombarded from the workpiece with the acquisition board. This secondary electron signal is obtained by pre-amplification and video amplification, and finally obtained in the kinescope Weld image. And the system is there.
Jin A integration circuit A field oscillation!
The main requirements of the line apchH line oscillation line output hM voltage circuit synchronous scanning system for line and field scanning circuits are: the nonlinear distortion and geometric distortion of a grating should be small. Generally, the nonlinear distortion of line scanning is less than 12%. Since the human eye is sensitive to vertical distortion, the nonlinear distortion of the field scanning circuit should be less than 8%. The nonlinear distortion of the grating depends on the circuit design of the line and field scanning. The geometric distortion of the grating is generally required to be less than 1.5% ~ 3%. It is mainly determined by the winding mold and winding process of the deflection coil.
The synchronization performance of the line and field scanning circuits is better, the synchronization is stable and reliable, and the ability to suppress interference signals is strong. The field scanning circuit and interlace scanning have good performance, no parallel phenomenon, and high definition. The synchronization introduction range and holding range of the line scan circuit should be appropriate. On the one hand, it must ensure good synchronization when the temperature changes and power supply voltage fluctuations; on the other hand, it must ensure that the anti-interference ability is excellent, and there is no distortion at the top of the image.
The oscillation frequency is stable and less affected by changes in ambient temperature and power supply voltage.
The circuit has high efficiency and low loss. The efficiency of the line and field scanning circuits is mainly determined by the output stage of the line and field scanning circuits.
33 Design of the detection and amplification system 15 The function of the detection and amplification system is to receive and amplify various information generated by the interaction of incident electrons and substances, and convert it into a video signal (analog voltage or digital quantity). Its performance is important to the quality of secondary electron imaging images.
In the secondary electron detection, two different amplifiers can be used to amplify the secondary electron flow signal emitted from the sample. The first type of amplifier uses a signal current to flow through a high-value resistor to produce a voltage drop, which is then amplified by a general transistor or tube amplifier. The second type of amplifier uses some physical processes, that is, each secondary electron emitted on the sample generates an average of g new electrons, and g is greater than 1. This type of amplifier can be a secondary electron multiplier with a photomultiplier tube Scintillator or bombardment-induction conductivity multiplier. Because the first type is lower in cost and can meet the requirements, first consider using the first type of amplifier.
3.3.1 Pre-amplifier system The role of the pre-amplifier is to convert various imaging information generated by the interaction of incident electrons with the sample surface material into a voltage signal. It is required to have a low level of electrical noise and be able to freely select and distinguish various information. It can amplify the received information to a sufficiently large signal level, and the reception and amplification time is short enough. The LT1192 and LT1252 differential high-speed op amp chips are used here. The principle is as shown.
3.3.2 Video Amplification System The function of the video amplification system is to amplify the signal (1 ~ 1.2VH,) from the pre-amplifier circuit (up to 50 ~ 80V) through the main amplifier as the grid of the image signal modulation kinescope. You can adjust the brightness according to the image, change the size of the video amplification signal and the DC level of the amplifier, and realize the adjustment of the secondary electronic image contrast.
In order to meet the requirements of high voltage gain and low signal distortion, the common output amplifier is often used as the output stage of the amplifier. The ideal frame rate characteristic of the video output should be a straight line parallel to the horizontal axis in the range of 0 ~ 5.5MHz, but it is technically impossible to achieve. At present, only compensation methods can be used in the circuit to increase the gain of a certain frequency appropriately As shown. In the actual circuit, the emitter negative feedback compensation method is used, that is, a resistor is connected in series with the emitter of the viewing tube to generate current negative feedback, which reduces the gain, but expands the frequency bandwidth, and the frequency modulation end and low frequency end of the signal are obtained. Extension. In order to adjust the electrical contrast, we added an adjustable potentiometer to the emitter of the final stage discharge tube, so that the negative feedback formed is only for the AC component, so the size of the AC negative feedback can be changed by adjusting the potentiometer to adjust Contrast. The video output circuit is as shown.
3.4 The addition of line and field blanking signals produces images during the forward course of line and field scanning, and there is no video signal during the reverse course of scanning. Retraced bright lines will interfere with the image. In order to avoid the occurrence of retrace lines, line and field reversal pulses are used as blanking signals in the design of the output stage. If the cathode of the kinescope is applied with a positive voltage greater than a certain value, the electron beam is cut off. When the line and field reversal pulses arrive, the output tube of the video amplifier is cut off, the collector is at a high potential, and the cathode of the kinescope is cut off at a high potential, which eliminates the return line.
4. Conclusion The secondary electronic imaging system has a simple structure and a circuit module design, which avoids the requirement of precision optical instrument parts. It is a very effective means to completely solve the observation and alignment of the electron beam welding seam. The use of electron beam welding secondary electronic imaging can solve the defects of the current observation system, improve the quality of welding seam observation and alignment, and furthermore, the automatic tracking of the welding seam can be realized by extracting the welding seam information. Can adapt to the current and future development trend of electron beam welding. In the design of the secondary electronic imaging system, the research team consulted and studied a large amount of data at home and abroad, and has achieved the expected results at the experimental stage. But there is still a lot of work to be done. The next stage is mainly to improve the quality, stability and reliability of imaging and the digital conversion of analog signals, so as to realize the direct acquisition of welding seam information by the computer.
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