Based on CAN bus control system GaAs photocathode preparation
GaAs photocathode is a negative electron affinity photocathode, quantum efficiency, emission electron energy and angular distribution of the advantages of centralized, so the low light level image intensifier, semiconductor sensors, spin-polarized electron source and many other fields has been widely used [1], but the preparation process of GaAs photocathode is extremely complex, and conditions of preparation, there are stringent requirements. Preparation of GaAs photocathode now are mainly dependent on skilled operators for manual operation, this mode of operation not the waste of a lot of human and material resources, but also the quality and efficiency of the preparation can not be guaranteed. The process commonly used in the preparation of online spectral response tester [2], can only be used for cathode spectral response curve after preparation and testing, assessment of the quality of preparation of the cathode, the cathode of its preparation the amount of information in many of the other, such as vacuum degree, cesium source and oxygen source current, are not real-time collection capabilities, but also did not cesium (oxygen) source current preparation process computer control and automation, which greatly hampered China GaAs photocathode preparation of theoretical research, preparation quality and efficiency. In this paper, CAN bus, high reliability, low cost, configuration flexibility and speed and quick [3 ~ 5], designed based on CAN bus of the GaAs cathode Zhibei optical monitoring system can realize the above information in real time testing and cesium ( oxygen) source current of the control.
A control system composed of
Preparation of GaAs photocathode according to the requirements, we design shown in Figure 1 Preparation of GaAs photocathodes control system block diagram. The control system consists of three main components: ultra-high vacuum activated systems, monitoring equipment, computers and peripherals, including ultra-high vacuum system is activated for Preparation of GaAs photocathodes (Cs oxygen purification and heat activation), the preparation of a variety of the amount of information available through external monitoring device connected to the computer. Monitoring equipment in two parts, part of the complete photocurrent spectral response curve of the test, the other part of the completion of the test vacuum and cesium (oxygen) source current signal of the test and control.
The first part of the test is equivalent to a spectral response, spectral response curves of the test, first computer-controlled grating monochromator output of a certain wavelength of monochromatic light and light to the cathode surface, cathode ray produces a weak current, photocurrent amplified by the A / D is converted to digital signals, computer signals and the corresponding data processing power of monochromatic radiation on the spectral response curve by the cathode [2]. This part of the CAN bus is not used, this will not be introduced, focusing on the second part. In the second part, vacuum, cesium (oxygen) source current signal is achieved through the CAN bus monitoring, and measurement and control signals through the USB-CAN converter connected to the computer. This part of the design, combines the advantages of CAN and USB bus, so that it can achieve a more flexible and more powerful communication task signal Test & Control.
2 Control System Hardware Design
2.1 USB-CAN converter hardware design
USB-CAN converter to USB and CAN protocol conversion between the two buses, its block diagram shown in Figure 2. 89C52 microcontroller diagram for the converter control tasks and CAN communication task with the USB bus. SJA1000 CAN controller interface circuit and the 82C250, USB controller, USB interface circuit using generic device interface chip CH372. In the micro-controller, USB and CAN bus packets are used to receive interrupt, this approach can minimize the delay, real-time communication capabilities to improve the system.
More than 2.2 Hardware Design of the amount of information monitoring and control equipment
The original system design and simulation of the power vacuum and other equipment are working independently, and does not have the ability and computer communications. In order to achieve vacuum and cesium (oxygen) source current monitoring and control of information, but also to save costs, the program used in this paper is the original equipment, the amount of information to add more than one control module to make a digital measurement and control and communications functions. More than the amount of information the task of monitoring and control module is removed the device display and transmit the signal to the computer or the computer control commands sent by the transmission to the device, so in fact control module circuit consists of two parts, part of the complete vacuum, cesium (oxygen) source of current information on the test, the other part of the realization of cesium (oxygen) source current size, and on-off control.
In the vacuum, cesium (oxygen) source current information on testing, data collection and equipment in order to maintain the consistency of display data, the device used is displayed on the digital control information directly out. Vacuum gauge and the analog power display by the three digital control structure, so the two can use the same test circuit. Figure 3 shows the amount of information test module for multi-block diagram, chart or a power vacuum gauge of three digital tubes were selected with one pair of four electronic switches CD4052 a connected microcontroller address output from the latch strobe , followed by the output control CD4052 8 digital tube display information in paragraph 2, connected to the comparator LM358, upon comparison to a standard logic level output to the micro-controller, after the last micro-controller 3 digital control signals together, have the power vacuum gauge or display data. Comparator used here The main advantages of digital control can drive electric Butong flexible Diaozheng level compared Dianya, and Shu Chu logic level Wen Ding, Conger Shi Ju You better test the applicability of the circuit. To display collection information, can be under the control of micro controller through CAN bus transmission to the computer.
In the original system, the cesium (oxygen) source current size of the control is by manually adjusting the power of analog potentiometers to achieve and in order for the computer to the current size of the digital control, the most direct way is to use digital potentiometers replace the power supply analog potentiometers, click here to design the current control module block diagram shown in Figure 4. Power supply has a coarse potentiometer 10K and a 1K to fine-tune the potentiometer, respectively 10K and 1K digital potentiometer X9C103, X9C102 replaced the two figures in Figure 4 is in series with the potentiometer, and this access method can be make more precise current control, while the actual analog power control accuracy over the three show accuracy, which can meet the system requirements on the current control accuracy. Digital potentiometers or analog potentiometer output selection control signal by the microcontroller, after driving through the DPDT relay control to determine. Micro-controller can also output control signals to control the relay turn on or off analog power of the external 220V power supply, in order to achieve cesium (oxygen) source current off, and because control of the 220V AC power, the micro control between the device and driver must be optically isolated. The way the computer can be achieved through the preparation process of the cathode oxygen alternating the automatic cesium.
Based on CAN bus control system GaAs photocathode preparation
Figure 4 Cs (oxygen) source current control module block diagram
3 Control System Software Design
3.1 USB-CAN converter software design
USB and CAN bus converter software mainly for USB and CAN bus protocol conversion, monitoring devices in the communication between the computer and its software design includes CAN and USB interface chip, and CAN and USB initialization packet sent and receiver. USB and CAN bus controller chips were used to CH372 and SJA1000, CH372 has built-in firmware mode, easy to use. USB and CAN bus message receiver are used to implement interrupt mode, this method ensures the fastest response time and improve communication efficiency. Received USB packets can be sent directly to the CAN bus monitoring and control equipment, and received CAN messages sent through the two steps to the computer, the first interruption of transmission through the USB interrupt service routine triggered by the computer, after the bulk transfer will be through the CAN message sent to the computer, in this way can trigger the computer to collect real-time data processing, to improve the response speed of the computer.
More than 3.2 the amount of information monitoring and control software design
Points more than the amount of information monitoring and control software, test software and control software. Test software digital control function is displayed on the device data out, after decoding by the combination and measured the amount of information into. Testing software design is the key strobe control CD4052 turn all the output in multiple digital tube display information, then that information 组合 together, and then find the document type code table can get equipment, or display the current size of the vacuum. Control software is the function of the digital potentiometers and relays cesium (oxygen) source current size, and off the control, which control software is designed to focus the current size of the control. As the power supply output current and the potentiometer resistance is not necessarily linear, and different power supply situation is different, we should have some adaptive control algorithms. Reference manual adjustment of current, experience, control software is used in two-step adjustment algorithm, the first "coarse" after "fine-tune." Two-step adjustment method to compute the current set value first, and the current value of the difference, if the difference is greater, a "coarse" (regulation X9C103). If the difference had fallen to the X9C102 the adjustment range, then to "fine tune" (regulation X9C102), fine-tune the time by "split half" adjustment method (search algorithm with a similar dismantling and a half) to the current fast approaching and to set value. With the above two-step adjustment algorithm can adjust the output current to ensure a short time, high precision.
3.3 Computer software design
Some computer software comprises three modules, including more than the amount of information control module, the information display module and the performance parameters calculation module, which more than the amount of information control module including optical current A / D acquisition, vacuum collection, cesium (oxygen) source current of the acquisition and control. Photocurrent is collected regularly by a computer-controlled, vacuum and cesium (oxygen) source current collection is different, it is by the power vacuum gauge or display information changes its initiative sent to the computer, the computer receives data after the update the display or processing. In order to ensure that the computer in time to receive the information sent to the device, we used a pseudo-interrupt software and multi-threading. Pseudo-interrupt service routine is interrupted by the CH372 driver after the adoption of dynamic link library (DLL) calls in the application of simulation can be a timely response to USB interrupt transmission, multi-threading is to achieve parallel processing of multiple tasks, improve responsiveness and processing efficiency . Performance parameter calculation module of the test results were analyzed, and to obtain the desired preparation or cathode of various performance parameters.
4 Conclusion
CAN bus in the preparation of GaAs photocathodes the successful monitoring and control system for monitoring the amount of information to solve the system more reliable and real-time requirements on high difficulties, the success of measurement and control system would be useful to GaAs optical depth cathode Zhibeigongyi Jinxing of theoretical research, promote the advancement of the preparation level of GaAs photocathodes, the system has good prospects and promotion of value.