According to Xinhua News Agency report, from 0:00 on March 31, the 500-meter spherical radio telescope (FAST) officially opened to the world for applications. Since the start of operation through national acceptance in early 2020, China SkyEye’s FAST facility has been operating stably and reliably. The number of pulsars discovered has exceeded 300, and major breakthroughs have been made in research fields such as fast radio bursts.
The successful operation of this "Giant Sky Viewing Eye" is also inseparable from the support of automation technology. Today I will share the design of the on-site bus optical fiber network of its reflective surface control system. Please read it~
Overview
FAST (Five-hundred-meter Aperture Spherical radio Telescope) is the world's largest single-aperture radio telescope. As a multidisciplinary basic research platform, FAST has the ability to extend the observation of neutral hydrogen to the edge of the universe, observe dark matter and dark energy, and explore the origin and evolution of the universe; it can take a year to discover thousands of pulsars and study extreme conditions The matter structure and physical laws of the world; hope to discover the material of strange stars and quark stars; discover the neutron star-black hole binary star, without relying on the model to accurately determine the mass of the black hole; detect the gravitational wave by accurately measuring the arrival time of the pulsar; join as the largest station The International Very Long Baseline Network, imaging the hyperfine structure of celestial bodies; it is also possible to discover the giant maser galaxy with high redshift, realizing the first observational breakthrough of methanol super maser outside the Milky Way; it is used to search and identify possible interstellar communication signals and find Extraterrestrial civilization and so on.
FAST can also extend my country's space measurement and control capabilities from the geosynchronous orbit to the outer edge of the solar system, and increase the downlink data rate of deep space communications by 100 times. The measurement accuracy of pulsar arrival time has been increased from the current 120 nanoseconds to 30 nanoseconds, making it the most accurate pulsar timing array in the world.Make a pulsar clock for the prospective research of autonomous navigation. Carry out high-resolution microwave patrols, diagnose and identify weak space signals with a resolution of 1 Hz, and serve as a passive strategic radar for national security. As the incoherent scattering radar receiving system of the "Meridian Project", it provides high-resolution and high-efficiency ground observations; tracks and detects coronal mass ejections, serving space weather forecasts, etc. The effect is shown in Figure 1.
Figure 1 FAST renderings
FAST basic structure
One of the main basic structures of FAST is to use natural karst landforms to build a ring beam 6725 with a diameter of 500 meters. A steel cable is woven into a spherical cable net, as shown in Figure 2, a triangular aluminum panel with a side length of about 12 meters on 4600 meshes is used to reflect the electromagnetic wave. The 2225 cable net nodes are pulled down by an electro-hydraulic actuator "actuator" with a certain tension, so that the cable net is stretched into a reference sphere. When performing astronomical observations, some nodes were further tightened according to calculations while others were loosened to form a 300-meter-diameter paraboloid with a focal ratio of 0.47 on a 500-meter-caliber spherical surface. The focal point of the paraboloid was just suspended on steel cables. At the feed source (receiving device) in the sky, to focus the received cosmic radio signal. The focus of the reflecting surface and the feed device are moved synchronously with time through the control system in order to track or actively observe a certain celestial body. From a large scale, the cable net steel cable is equivalent to a spring, which deforms under the pulling force of the actuator, forming a parabola whose focus points to a controllable movement, so as to achieve the function of focusing weak cosmic electromagnetic signals. The working frequency of FAST is in 70MHz~3GHz (wavelength 0.1~4.3 meters).
Figure 2 Schematic diagram of cable net structure
FAST control system
According to FAST’s requirements for reflector controlThe industrial mature PLC controller is used to build the control system as shown in Figure 3. The FAST master control system compiles and issues observation commands to each subsystem according to the requirements of astronomers’ observations. The reflector control system parses this observation command and converts it into position control commands for 2225 node actuators.
Figure 3 FAST reflector control system block diagram
From the control content, reflector control is not a high-speed control system (node scan cycle 0.5 seconds, node position accuracy RMS 20mm). But there are two characteristics that are not usually found in industrial control systems. First, the large number of nodes requires high reliability, and the actuator nodes are distributed in depressions with irregular terrain (there is no regular wiring bridge channel based on industrial installations), and the more severe is the extremely demanding electromagnetic compatibility requirements. . Because FAST is used to receive extremely weak cosmic radio signals, the electromagnetic radiation requirements of the International Telecommunication Union (ITU) for radio astronomy background are more than 10,000 times (80dB) stricter than the national military standard limit. The upper blue line in Figure 4 is the electromagnetic sensitivity requirement of GJB151A for military equipment, the lower solid red line is the limit minus 80dB, the red dashed line is the limit minus 94dB, and the lower blue line is the continuous and spectral line observation limit RA769 recommended by ITU. There are basically no such strict requirements in conventional industrial control systems. At the same time, Guizhou is a lightning-prone area, and large-scale lightning protection in the wild is an issue that must be considered. The surge protection device SPD can limit the induced lightning voltage below 1.5kV. If it is matched with an EMI filter, it will have a better suppression effect on the instantaneous overvoltage, which can usually be limited to a few hundred volts.
Figure 4 GJB151A standard and ITU-R RA769 limit requirements comparison
In order to meet the requirements of electromagnetic shielding, all power entering the shielded space must pass through the “wall” type power filter. Its incoming and outgoing wires are invisible to each other to prevent electromagnetic interference from inductive coupling between wires. The signal fiber enters and exits the shielded space through the waveguide .Electromagnetic sealing is different from water and gas medium sealing. Theoretically, the thickness of electromagnetic waves is about 0. If there is a non-conductive continuous long slit at the connection of the shielding conductor, although the slit is narrow enough to not leak water or air, the electromagnetic wave can still pass smoothly. And the wavelength passed is proportional to the longest continuous length of the gap. For deep holes, electromagnetic waves below the cut-off frequency will be greatly attenuated when passing through. Therefore, the deep hole electromagnetic waves through which water and gas media can easily pass, are difficult to pass. Note that when the optical cable passes through the waveguide, the possible metal reinforcing wire must be removed, because the metal wire may have the function of an antenna to transmit electromagnetic signals in the shielding room to the outside, thus destroying the shielding function, as shown in Figure 5.
Figure 5 Schematic diagram of fiber and power supply passing through the wall through the shielded space interface
The simple design method of the circular waveguide is as follows: _p7GHz span 7.6 ) Where D is the inner diameter of the waveguide, unit: cm.
FAST uses the frequency band 70MHz~3GHz, should choose fc﹥3GHz, for example fc=15GHz, it is approximately as follows:
insertion loss: A=32L in the waveguide type Length, unit: cm.
Based on these characteristics, the reflective surface control system adopts the following measures:
(1) Disperse PLC controller risks,Build 12 relay rooms with 120db shielding efficiency in the depressions, each relay room is equipped with a Siemens PCS7-300 controller and 4 Profibus-DP field bus interface cards, and each DP bus logic is connected with about 50 node drivers. In total, it is responsible for controlling approximately 200 nearby actuators. A total of 12×4=48 DP buses connect all 2225 actuators. In order to solve the difficulties of signal filtering and cable shielding and avoid the risk of lightning strikes, Profibus-DP fieldbus adopts optical fiber transmission. The 12 sets of PCS7-300 controllers are connected to the communication server of the observation building through the single-mode fiber double-ring network Profinet. For any single point failure of the main ring network, the self-healing function of the ring network immediately changes the information flow to ensure normal communication. But if two points fail at the same time or a switch fails, the system will automatically switch the communication from the main ring to the backup ring, which can also ensure normal communication. The distance from the reflecting surface to the main observation building is 1000 meters, and the distance between the relay rooms is about several hundred meters, and the distance from the relay room to the nearest actuator is in the range of 50 to 300 meters. This is not a long distance for single-mode fiber.
(2) The actuator adopts the intelligent electro-hydraulic actuator with optical interface. In addition to the execution function, it has the functions of local closed loop, diagnosis, protection and health management. Therefore, it can only communicate through Profibus-DP fieldbus digital communication and The system realizes two-way transmission of multiple information.
system and 2225 actuators need 2225 bus optical interfaces. If the conventional photoelectric conversion mode is adopted, the number of equipment will be large, the cabinet will be large, the heat generation will be large, and the increase in complexity will lead to the system Reliability is reduced. Profibus-DP communication protocol is a single-master-slave protocol. The system uses broadcast communication to the actuators. At any time, only one actuator can respond to the master system. At the same time, the actuators of the slave station are connected to each other. No communication requirements. According to these technical characteristics, we have adopted the passive optical network technology PON (Passive Optical Network) that has been maturely used in Internet technology to greatly simplify the optical network structure.Reduce the number of equipment and the corresponding equipment occupation space and power consumption heat.
Due to the development of Ethernet and optical fiber technology and the pursuit of Internet transmission distance, speed and bandwidth, fiber to the home is bound to involve the performance and cost of optical network access. The price of the optical fiber interface of the Ethernet switch is unbearable for the network access of thousands of households. Therefore, the EPON technology of the Ethernet passive optical network was proposed in the 1990s and has been matured and widely used after 20 years of development. The typical PON system structure is shown as in Fig. 6.
Figure 6 Passive Optical Network (PON) system diagram
One of the key components in the PON system is the optical splitter (Optical splitter). It is a passive pure optical component, which has two manufacturing methods: fusion tapering and planar waveguide. It does not require a power source, does not generate heat, and is inexpensive. The main path light is allocated to each branch light path , on the contrary, the return light of each branch has a light wave "vector sum" operation function in the main light path. The technical characteristics of PON data transmission basically meet the requirements of the Profibus-DP communication protocol, but it also has the following two characteristics:
· The light emitted by the OLT is divided into multiple parts, and the energy of each beam Will be attenuated proportionally. Every time it doubles, the light attenuation is about -3dB (that is, 10log0.5). This loss needs to be compensated by luminous power or receiving sensitivity.
· All ONUs that are not in the communication state cannot be in the optical logic 1 state (that is, there is light).
The technical indicators of the photoelectric transceiver module we choose domestically produced are:
rate 0~2Mbps; luminous intensity ≥ -5dBm; wavelength 15dBm; 1310nm.
The formula for calculating the energy balance of the optical path is as follows:
luminous power-all loss-spare margin loss ≥ receiving sensitivity 2 span 2 ~ 0.15span 3span 2 ~ Fiber fusion splicing loss is 0.1dB; fiber distance loss is related to wavelength, roughly 0.25~0.4dB/km can be selected; spare loss energy margin is usually considered as 5~8dB. A calculation example is as follows:
-5dBm (luminous power) = -25dBm (receiving sensitivity) + 2dB (sum of connector loss) + 11dB (1:8 splitter loss) + 5dB (spare margin) +L×0.4dB/km (fiber distance loss)
can calculate the reliable transmission distance
L=(25-5-2-11-5)/0.4=2 /0.4=5km＞0.3km
It is interesting not to simply think that the input and output of the splitter are used in reverse to become the so-called "photosynthesizer".Even if the light from the two branches of the same light source is combined through a beam splitter, a stable combined light with doubled intensity cannot be obtained. Because the monochromatic laser has the factor of the frequency and phase vector sum, the peak value will be obtained. Although it is doubled, there is an unstable light signal with irregular full amplitude beating.
passive optical network can only be used in serial communication types without network conflicts. For example, mainstream communication networks such as Modbus and Profibus-DP conform to this regulation. However, communication protocols that survive network conflicts, such as CAN, cannot be used.
Due to the use of a 1-way 8-way splitter, the system saves 7/8=87.5% of the traditional photoelectric converters, which greatly reduces the power consumption, space and cost, and the Profibus-DP network The number of electrical nodes is also reduced in the same proportion, and repeaters can be used less or without, so this scheme has obvious advantages. The question is, wouldn’t it be better to use a 1-way 16-way splitter? Indeed it is, but the limiting factor is that the current receiving sensitivity of low-frequency optoelectronic transceiver modules is not enough to compensate for the higher splitting loss. The receiving sensitivity of the photoelectric transceiver module that transmits Ethernet signals without a DC component can reach -35dB, so even a 1-way 64-way splitter can be used. Figure 7 is an example of the optical path connection from the PLC cabinet in the relay room to the electronic board in the actuator.
Figure 7 1/4 DP bus to the 16-channel optical connection diagram of the actuator
RS485 The transceiver has two usages, one is used under the control of the CPU, as The communication port of the "smart" device, its receiving and sending status is all under the planned control of the CPU. The second is to be used as a repeater. At this time, it does not know the origin of the signal and the baud rate . There are two types of methods to solve the direction and baud rate identification control.One type is to use CPU or FPGA type chips, which is generally considered to be more complicated and difficult to adapt to high-speed communication. The second is a hardware-based method. This method is relatively simple but requires the link layer of the communication protocol to comply with certain rules, that is, the first bit of each "byte" that constitutes an information frame must be a negative transition from 1 to 0 At the beginning, the idle state of all nodes is monitoring, and the bus stays at logic "1". For example, mainstream communication protocols such as Modbus and Profibus-DP conform to this requirement. This can design the RS485 transceiver like this:
idle state: monitoring state / RE=DE=0, the bus state is 1, the RO output is 1; ： _Span3span monitoring state/RE=DE=0, bus receives information, RO outputs according to the received information;
sending state: when DI=0 is sending/RE= strong when DI=0 , The bus state is 0, RO is high impedance, and it is pulled up to logic 1 by an external resistor, which is consistent with the idle state. This is a real send worthy of the name.
When DI=1/RE=DE=0, then the chip is nominally in the sending state, but it is actually receiving. The bus is high-impedance. Like the idle state, it is also pulled up to a logic 1 by an external resistor, and it just "likes" sending a 1.
So we can think that the so-called sending process actually only sends logic 0,The logic 1 is a natural result of bus bias. The repeater does not do any extra work on the data, thus achieving protocol independence, self-adaptive baud rate, and transparent data transmission. The RO pull-up resistor is to ensure that RO keeps logic 1 unchanged during such a "transmission" process. The receiving state /RE=DE=0 is for regular use and there is nothing special about it.
Profibus-DP communication physical layer adopts RS485 standard, the idle time of the master station stays in the logic "1" state, which has no effect on the reception from the slave station. But the idle time of the actuator slave station stays in the logic "1" state, which does not meet the PON requirements. For this reason, it is necessary to invert the signal sent from the slave station so that it is in the idle state at logic "0", that is, no light is emitted. After receiving by the photoelectric transceiver of the master station, it is reversed and restored to meet the original requirements of communication. A very simple signal reversal method for the master station is to reversely connect the bus terminals A and B of the 485 chip, and at the same time change the original pull-up resistor RO to ground, that is, pull-down. In this way, the transmission and reception are reversed at the same time, and similar changes are required on the slave actuator side to keep the two parties consistent. The master station RS485 transceiver configuration diagram is shown in Figure 8. Note that 4 of the bias matching resistors R1\2\3\RO are necessary and indispensable.
Figure 8 Master station protocol independent, baud rate adaptive, signal reverse RS485 transceiver
Model Research" (Project Number: 11273001)
About the author
Zhu Lichun (1964-), female, Chifeng, Inner Mongolia, researcher, Ph. Chief Engineer of FAST Measurement and Control System,In 1986, he graduated from Beijing University of Posts and Telecommunications Department of Telecommunications Engineering, graduated from the Graduate School of the Chinese Academy of Sciences, majoring in radio astronomy technology and methods, mainly engaged in the research of measurement control technology methods.
Zhang Weijie (1963-), a native of Beijing, senior engineer, project manager of Taiji Computer Co., Ltd., graduated from Beijing University of Technology in 1987 , mainly engaged in the design, construction, installation and management of industrial automation and other related industries Debugging and other related work.
Skeke (1945-), male, Dongyang, Zhejiang, professor-level senior engineer, deputy chief engineer of the Metallurgical Automation Research and Design Institute (retired), graduated from the Radio Department of Tsinghua University in 1970, engaged in industrial automation measurement and control technology methods And equipment research, product development and engineering contracting.
is an excerpt from "Automation Expo"
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