Computer data acquisition system and anti-interfer

  • Detail

Introduction with the continuous development of cement production equipment and technology, almost all new dry process cement production lines since the 1990s have adopted computer technology. The central control system and DCS distributed control system are the main ones

at the initial stage of preparing to build a 700t/d new dry process production line in 1993, after a careful analysis of the application status at home and abroad, taking into account the actual situation that the company is limited by funds and unable to bear the introduction of rubber pull tester models into the complete set of system software, it is considered to adopt the production guidance system to collect and monitor the process parameters of the rotary kiln in real time. Based on the multimedia information processing means and the reasoning and judgment method of expert system, while displaying the information in the whole production process to the production personnel, the operator can obtain the real-time working conditions of the key parts of the rotary kiln production line in time in the form of voice, image, text and other expressions. And put forward the corresponding production operation methods to guide workers in production. This application mode, which combines the production guidance function of the computer with the subjective initiative of the operator, can overcome the shortcomings of manual observation of the working conditions in the kiln and the operator's operation control based on experience in the conventional operation, so as to achieve the purpose of unifying the operation method, stabilizing the thermal system, and then stabilizing and high production. At the same time, it can also avoid the misoperation often encountered by most automatic control plants due to the inaccuracy of detection components, actuator failure, computer judgment error and other factors

1 system introduction

the company's scheme is a distributed system composed of three-level computers "data collection - monitoring management - production guidance". STD industrial control computer and BITBUS network system are selected as the lower computer. Its electrical interface adopts RS-485 standard of balanced transmission, and the transmission medium is twisted pair. Intel industrial PC is selected as the middle computer and upper computer. After three years of operation, the system works normally. With the support of multi-media computer, it can simulate the manual observation and fire watching method through image processing technology for the information such as the burning reaction of materials in the kiln obtained by the ordinary industrial camera, and obtain the characteristic quantities such as the temperature distribution in the burning zone, which represent the working conditions in the kiln. A real-time expert system is established by integrating the conventional process parameters, experience data, operation experience of workers and experts and the process signals collected in real time. It can display the working conditions in the kiln through graphic, audio and other media, and put forward production guidance suggestions in time. The production process monitoring system composed of BITBUS network and IPC industrial computer has the advantages of good anti-interference performance and strong communication ability between multi-level computers. It also solves the problems of collecting, processing, storing, transmitting, displaying and alarming a large amount of information in the production process. It has become a set of computer data detection and processing and production guidance system with low investment, few faults, convenient use and strong practicability, and has basically achieved the expected goal

according to the actual distribution of the monitoring points of the production line, three sets of data collectors are respectively arranged in the kiln tail, coal mill and kiln head control rooms where the process parameter monitoring points are relatively concentrated to form three independent slave stations. The data processing capacity of each slave station is 32 channels, and the total data processing capacity can reach 128 channels

due to the influence of traditional operation methods, production workers have been accustomed to observing the instrument panel for a long time. Therefore, at the initial stage of system design, it is required that all important parameters of the production line, such as temperature, pressure, flow, speed, load, etc., should be monitored with separate instruments. Signals from sensing elements and transmitters must first enter the digital display instrument and then be connected to the lower computer for data acquisition. The instrument panel only displays the instantaneous display value. In addition to the functions of regular refresh, automatic page turning, patrol inspection and display of process parameters, the computer screen also stores the original data with a storage capacity of up to several months, which is convenient for the production managers and operators to analyze the kiln conditions at any time, find out the dynamic change trend of various parameters, and provide a basis for continuously revising the production guidance system software and inspection operation methods. In view of the fact that the computer and the display instrument share a set of sensing transmission elements, and the distance between each detection point and the data acquisition lower computer, the middle and lower computers is far, it is easy to cause various electromagnetic interferences on the production site to pose a threat to the operation reliability of the system

2 anti-interference measures

the reliability of microcomputer system is determined by many factors, and the anti-interference performance of the system is one of its important indicators. Traditionally, we call all kinds of external or internal useless signals that affect the measurement results of electrical measurement systems or instruments interference. The influence of interference may reduce the quality of the signal, or destroy the normal function of the circuit, resulting in confusion of logical relations and control failure. Theory and practice have proved that the generation of electromagnetic interference must meet the conditions of simultaneous existence of interference source, transmission path and interference sensitive receiving circuit. If any of the three conditions can not be met, the interference will be no longer the condition. Therefore, the measure to suppress or eliminate the interference is to try to destroy one or several of the basic conditions for the generation of interference. The usual method is to suppress the noise source and directly eliminate the cause of interference; Cut off the transmission path of electromagnetic interference and eliminate the interference coupling between the interference source and the disturbed equipment; Strengthen the ability of the disturbed equipment to resist electromagnetic interference and reduce its sensitivity to interference

improving the anti-interference ability of equipment must start from the design stage and run through the whole process of production, commissioning, operation and maintenance. In the stage of system design and laboratory research, we have repeatedly demonstrated and tested the composition of the three-level computer system, the connection of the BITBUS network and the communication between them. In addition to trying to develop a set of software and hardware that are universal and suitable for production process monitoring and data processing, the anti-interference measures of field equipment are also carefully studied and analyzed, and the most perfect scheme is formulated

2.1 reasonably select the system backplane to reduce the internal noise and interference

in the design of STD industrial computer application system, in order to speed up the research and development, improve the performance index and enhance the reliability, the standard template is often selected according to the mode of "overall design - module selection - building block assembly - separate commissioning - General commissioning - field operation"

std bus backplane (motherboard) is used to connect the data line, address line and control line among the function templates in the system, and provide system power for each function template through the power line (surface) on the backplane. Theoretically, it is required to reduce the bus backplane impedance as much as possible so that the high-frequency pulse signal will not be distorted when passing through. Any distortion of the signal on the backplane bus is considered noise interference

the system selects a high-performance four layer STD bus backplane. This new backplane uses the middle two layers of printed circuit surface as a large area power surface and ground surface respectively. At the same time, the signal lines on the backplane are isolated with ground wires to reduce the crosstalk caused by the distributed capacitance between lines. And the technology of connecting multiple points with the inner ground plane is adopted. At the same time, the bus sockets on the motherboard that need power can be connected to the power supply surface and ground surface nearby, so as to shorten the connection length and reduce the impedance and voltage drop. The characteristic impedance of the main board signal line is close to the output impedance of the bus driver, so that the latter can provide more abundant signal driving power to the signal line

2.2 correctly configure the internal and external power supplies to eliminate the interference from the power supply

the most harmful interference in the microcomputer system comes from the power supply. In order to prevent interference from the power system, AC voltage regulator is usually used to improve the stability of the system; The isolation transformer is used to eliminate the parasitic coupling interference between the primary and secondary, so as to improve its ability to resist common mode rejection ratio; Connect low-pass filter or double-T filter at the power inlet to eliminate power frequency interference and other measures

we select industrial high reliable anti-interference stabilized voltage power supply in the data collector. After the power frequency AC is transformed, rectified, filtered and stabilized, the voltage required by the system can be obtained. It also eliminates the conducted noise from electricity and ripple noise caused by poor filtering by increasing the filtering stages and filtering capacitors. It has the characteristics of high voltage stabilization accuracy and strong anti-interference performance. It has high common mode rejection ratio and series mode rejection ratio, and can suppress interference in a wide frequency range

2.3 photoelectric isolation measures shall be taken to eliminate the electromagnetic interference from the process channel

the process channel includes the forward channel, the backward channel and the host or the way of information transmission between the host. The task of the system forward channel is to pick up, adjust and transform signals. Because the forward channel is close to the measured object, and the sensor output is often weak signal, the forward channel in the system is the main channel of interference invasion

the most effective measure to suppress the channeling interference of input and output interfaces is to cut off their coupling channels by isolation. We select a/d conversion template with optoelectronic coupling device in the forward channel, because the optoelectronic coupler uses light as the medium for signal transmission, so that it does not interfere with each other electrically. It has the characteristics of one-way signal transmission, no contact, large common mode rejection ratio, easy to cooperate with logic circuit, and is sufficient to suppress the interference of digital system to analog signals, especially weak and small signals. Of course, this measure only solves the isolation between the signal and the earth. For a system with multi-channel analog signal input, the impact caused by high common mode interference between analog input channels needs to be solved by adding electromagnetic isolation amplifier or linear optocoupler isolation circuit

2.4 twisted pair communication is adopted to eliminate crosstalk caused by electromagnetic induction between lines.

process channels include forward channels, backward channels and host or host mutual information transmission channels. The task of the forward channel of the system is to offset the magnetic flux density generated by the interference source on the signal circuit, or it is also a common method to arrange the conductor close to the ground wire

in order to simulate the electromagnetic interference environment at the production site and test the anti-interference performance of long-distance signal transmission, we consciously increased the length of the signal line during laboratory commissioning, and also artificially arranged some interference sources indoors for electromagnetic noise interference, trying to accumulate data for on-site installation and commissioning

in the field wiring and static commissioning stage, the data acquisition system and computer data processing process are normal. However, when the rotary kiln system was put into operation and on-line unified commissioning, it was found that although enough attention had been paid to the anti-interference ability of the equipment in the system design stage, most of the possible interference had been eliminated. However, the high temperature and dusty environment at the cement production site, coupled with the fact that signal cables are mostly laid in parallel with power lines and control circuits along the kiln bridge or cable trench, high and low voltage cables, DC speed regulating devices along the way, high-power and high-voltage rectifying equipment with voltage up to 120kv, as well as a large number of electromagnetic rapping devices, frequency conversion equipment and other interference sources, still have a serious impact on the system

2.5 eliminate the interference of power supply and grounding system

after repeated debugging and comprehensive analysis and comparison, it is found that in addition to the strong current interference in the field environment, the interference from power supply and grounding system is more serious. Since the AC power supply of the computer network comes from the power panel of the nearest workshop, the internal backflow caused by the unbalanced current of the power system is ignored in the design and wiring, which will seriously interfere with the work of the computer network through the power lead-in wire. And their mutual coupling produces various internal interferenceAnalysis on preventive maintenance of injection molding machine
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