Computer Data Acquisition System and Anti-interference Measures

Cement Enterprise Computer Data Acquisition System and Anti-interference Measures

Excerpt from China Automation Network Special Recommendation


With the continuous development of cement production equipment and technology, almost all new dry cement production lines since the 1990s have adopted computer technology. Among them, the central centralized control system and the DCS distributed control system are the main ones.

The company established a 700t/d new dry process production line in 1993 at the initial stage. After earnestly analyzing the current domestic and international application status, taking into consideration the fact that the company is financially restricted and unable to withstand the actual situation of the introduction of complete set of system software, it considers using the production guidance system method for rotary kiln process parameters. Acquisition and real-time monitoring. Based on the multimedia information processing method and the reasoning and judgment method of the expert system, while displaying the information in the entire production process to the production personnel, various expressions such as voice, image, and text are used to enable the operator to timely acquire the key parts of the rotary kiln production line. Real-time operating conditions. And put forward the corresponding production and operation methods to guide the production of workers. This combination of the computer's production guidance function and the operator's subjective initiative can overcome the inadequacies of manual operation of observation of the kiln working conditions, and the operator's operation control based on experience, so as to achieve a unified operation method. Stabilize the thermal system and stabilize production and increase production. At the same time, it can avoid the misoperation caused by factors such as misalignment of detection elements, failure of actuators, and misjudgment by computers in most automatic control manufacturers.

1, system introduction

The company's plan is a distributed system consisting of three levels of data acquisition, monitoring, and production guidance. The lower computer chooses STD industrial computer and BITBUS network system. The electrical interface adopts the RS-485 standard for balanced transmission and the transmission medium is twisted pair. Middle machine and upper computer choose INTEL industrial PC. After three years of operation, the system is working properly. With the support of multimedia computers, information about the firing reaction of kiln materials obtained by ordinary industrial cameras can be simulated and observed through the image processing technology. The characteristics such as the temperature distribution of the firing zone can be used to characterize the kiln work. The characteristic amount of the situation. The real-time expert system was established by integrating conventional process parameters, empirical data, worker and expert operating experience, and real-time acquired process signals of the cement rotary kiln. It can display working conditions in the kiln through media such as graphic and audio, and promptly propose production guidance. The production process monitoring system composed of the BITBUS network and the IPC industrial control computer has the advantages of good anti-interference performance and strong multi-level computer communication capability. And solve the problem of the collection, processing, storage, transmission, display, and alarm of a large amount of information in the production process. Become a set of investment provinces, less faults, easy to use, practical computer data detection processing and production guidance system, basically achieved the desired goal.

According to the actual distribution of each monitoring point of the production line, we set up three sets of data collectors in the kiln tail, coal mill and kiln head control room where the process parameter monitoring points are relatively concentrated, and form three independent slave stations. Each slave station has a 32-channel data processing capacity and a total data throughput of 128 channels.

Due to the influence of traditional operating methods, production workers have long been accustomed to observing dashboards for operation. Therefore, in the initial stage of system design, all important parameters such as temperature, pressure, flow, speed, and load monitoring on the production line are required to have a separate meter display. The signals from the sensing element and the transmitter must first enter the digital display and then connect to the lower computer for data acquisition. The instrument panel only displays instantaneous indications. In addition to functions such as regular refreshing, automatic page turning, and inspection display of process parameters, the computer screen also stores raw data for storage for months, facilitating the production managers and operators to analyze the kiln at any time. In addition, the search for dynamic trends of various parameters and the basis for continuous revision of 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-level computer and the middle and lower-level computers is far, the electromagnetic noise at the production site may easily pose a threat to the system operation reliability. .

2, anti-jamming measures

The reliability of the microcomputer system is determined by a variety of factors, and the system's anti-jamming performance is one of its important indicators. It is customary to refer to various external or internal unwanted signals that affect the measurement results of an electrical measurement system or instrument as interference. The influence of interference will reduce the quality of the signal, but it will destroy the normal function of the circuit, causing confusion in the logic and failure of control. Theory and practice have proved that the generation of electromagnetic interference must meet the conditions for the simultaneous existence of interference sources, transmission paths, and interference-sensitive receiving circuits. If any one of the three conditions cannot be satisfied, the interference will no longer be a condition, so the measures to suppress or eliminate the interference are to try to destroy the basic conditions of one or more of the interferences. The method usually adopted is nothing more than suppressing the noise source and directly eliminating the causes of the interference; cutting off the transmission path of electromagnetic interference, eliminating the interference coupling between the interference source and the disturbed device; strengthening the ability of the disturbed device to resist electromagnetic interference and reducing its Sensitivity to interference.

Improving the anti-jamming capabilities of equipment must begin at the design stage and run through the entire process of production, commissioning, operation and maintenance. At the system design and laboratory research stage, we have repeatedly demonstrated and experimented with the composition of the three-level computer system, the connection of the BITBUS network, and the communication between each other. In addition to trying to develop a set of software and hardware that are versatile and suitable for production process monitoring and data processing, serious anti-jamming measures for field equipment have also been researched and analyzed, and the best possible solution has been formulated.

2.1 Reasonable selection of system backplane to reduce internal noise and interference

In the design of STD industrial computer application system, in order to speed up the research and development speed, improve the performance index and enhance the reliability, the standard template is often selected as the mode of “overall design-module selection-block assembly-distribution-total adjustment-field operation”. get on.

The function of the STD bus backplane (motherboard) is to connect the data lines, address lines, and control lines among the function templates in the system to each other, and provide the system power for each function template through the power lines (faces) on the backplane. It is theoretically required to reduce the impedance of the bus board as much as possible so that high-frequency pulse signals do not cause distortion. Any distortion of the signal on the backplane bus is considered noise interference.

The system uses a high-performance four-layer STD bus backplane. This new backplane uses the middle two printed circuit planes as large-area power planes and ground planes, respectively. At the same time, the signal lines on the backplane are also isolated by ground lines to reduce the crosstalk caused by the distributed capacitance between the lines. And the technology of connecting multiple points to the inner ground plane is adopted. At the same time, the bus sockets on the mainboard that require power can all be connected to the power plane and the ground plane nearby, thereby shortening the wiring length and reducing the impedance and voltage drop. The characteristic impedance value of the main board signal line is already close to the output impedance value 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 supply to eliminate interference from the power supply

The most harmful interference in the microcomputer system comes from the power supply. In order to prevent the introduction of interference from the power system, AC voltage regulators are usually used to improve the stability of the system; isolation transformers are used to eliminate the parasitic coupling interference between the primary and secondary and improve their resistance to common-mode rejection ratio; they are low at the power supply inlet termination. Pass filter or double T filter to eliminate power frequency interference and other measures.

We use industrial-grade high-reliability anti-jamming regulated power supply in the data collector. After the power frequency alternating current is transformed, rectified, filtered, and regulated, the voltage required by the system is obtained. And by increasing the number of filter stages, increasing the filter capacitor and other ways to eliminate the conductive noise from the grid and the ripple noise caused by poor filtering, with high voltage stability, anti-jamming performance and other characteristics. It also has high common-mode rejection ratio and series-mode suppression ratio, and can suppress interference in a wide frequency range.

2.3 Take optical isolation measures to eliminate electromagnetic interference from the process channel

The process channel includes the way for the forward channel, backward channel, and host or host to communicate with each other. The task of the system forward channel is to complete signal picking, adjustment and transformation. Because the forward channel is close to the measured object, and the sensor output is often a weak signal, the forward channel in the system is the main channel for interference and intrusion.

The most effective measure to suppress input and output interface interference is to cut off its coupling channel in isolation. We use the A/D conversion module with optocoupler in the forward channel. The optocoupler uses light as the medium for signal transmission so that it does not interfere with each other electrically. With the characteristics of one-way signal transmission, non-contact, large common-mode rejection ratio, and ease of cooperation with logic circuits, it is sufficient to suppress the interference of digital systems to analog signals, especially weak signals. Of course, this measure only solves the isolation between the signal and the earth. For a system with multiple analog signal inputs, the effects caused by high common-mode interference between the analog input channels also require the use of additional electromagnetic isolation amplifiers or linear amplifiers. Optocoupler isolation circuit to solve.

2.4 Using twisted pair communication to eliminate crosstalk caused by electromagnetic induction between wires

The process channel includes the way for the forward channel, backward channel, and host or host to communicate with each other. The task of the forward channel of the system is to offset the magnetic flux density generated by the interference source on the signal loop, or to arrange the conductor close to the grounding line is also a common method.

In order to simulate the electromagnetic interference environment at the production site and the anti-interference performance of the long-distance transmission of the test signal, we deliberately increased the length of the signal line during the debugging of the laboratory, and artificially arranged some interference sources for electromagnetic noise interference in the laboratory to try to install and debug the site. Accumulated information.

During field wiring and static debugging, the data acquisition system and computer data processing are normal. However, when the rotary kiln system was operated by feeding and on-line adjustment, it was found that although the anti-jamming capability of the equipment has been given sufficient attention in the system design stage, most of the interference that may occur has been eliminated. The high temperature and dusty environment in the cement production site, together with the signal cables, are mostly laid along the kiln bridge or cable trenches and power lines and control loops. The high and low voltage cables along the way, DC speed control devices, and voltages up to 120kV High-power high-voltage rectifier equipment, as well as a large number of various types of electromagnetic vibration device, frequency conversion equipment and other interference sources, the impact on the system is still very serious.

2.5 Elimination of Power Supply and Grounding System Interference

After repeated debugging and comprehensive analysis and comparison, it was found that in addition to strong interference in the field environment can not be ignored, the interference from the power supply and grounding system is more serious. Since the AC power of the computer network comes from the power screen of the nearest workshop, the internal reflow caused by the unbalanced current of the power system is neglected during the design and wiring, and the work of the computer network is seriously disturbed by the power lead-in. The mutual coupling between them generates various internal disturbances. We take the following measures to solve them one by one:

1) Add a dedicated computer power supply to eliminate the interference introduced by the AC power line.
We set up a 35mm2 three-phase four-wire special power line for the computer system from the plant substation, and place it directly on the kiln head position machine to control the site. Then from the upper computer separately to the three data acquisition stations phase output branch power line. Eliminates electromagnetic interference caused by backflow in the neutral line when voltage fluctuations or unbalanced three-phase currents occur in the power circuit.

2) Increase the cross section of the signal transmission line and the grounding line to reduce transmission interference.
Generally, the low-impedance connection of the common reference point of the device, circuit, and signal potential is called grounding. There are two main grounding purposes for electronic equipment, one for safety, that is, safety grounding; the other is for providing a reference potential to the system and providing a low-resistance path for high-frequency interference, ie, working grounding. Normal grounding is an important means of suppressing noise and preventing interference. Improper grounding can lead to increased interference and coupling. However, regardless of the grounding method, the common ground plane (or line) has a certain impedance. When the current flows through the ground, it will generate a voltage drop. In addition, the ground line may form a loop with other leads, thus becoming a source of interference. The interference introduced by the grounding system can be divided into ground resistance interference and ground loop interference according to its causes. The main measures to eliminate the grounding interference in the system are: increase the cross section of the grounding wire to reduce the impedance of the common ground wire; correctly select the grounding method and the grounding point to cut off the ground loop; and combine the grounding with the isolation and shielding to make it work Its anti-interference effect. When we doubled the section of the signal transmission line, we also received a significant reduction in noise.

3) Shield and ground the surrounding strong electromagnetic interference sources.
Shielding is the main measure to suppress the spatial field interference. It uses the shield to cut off or weaken the spatial coupling channel of the interference field to prevent its electromagnetic energy transmission. Shielding can be divided into three types: electric field shielding, magnetic field shielding and electromagnetic field shielding. Among them, the electromagnetic field shielding has both anti-electromagnetic interference and anti-radiation interference capability, that is, it also has the functions of magnetic field shielding and electric field shielding.

We use its own metal housing as a shield for strong electromagnetic interference around the network, and the shield is well grounded. When the electrical control panel is used as a shield, it is required that there are good electrical connections between the various parts of the housing, and any insulation between the housings will cause interference. The function of the shield is to cut off the electric field interference caused by the interference source through the distributed capacitive coupling channel. At the same time, the shield can be used to generate an eddy current magnetic field in the opposite direction under the influence of the high frequency magnetic field of the interference source to cancel and weaken its interference effect, and the influence of the high frequency electromagnetic field on the disturbed circuit is significantly reduced.


One of the Measure to Overcome the Forward Process Passage in Industrial Automation Control System

In the industrial field, when signals are transmitted between electrical equipments, meters, PLC control systems, and DCS systems that are far away from each other, interference often occurs, causing instability or even misoperation of the system. In addition to the influence of internal and external disturbances in the system, there is a very important reason for the grounding of various instruments and equipment. Under normal circumstances, the device housing needs to be connected to the ground, and the circuit system must also have a common reference ground. However, due to the potential difference between the reference points of the instrumentation devices, a ground loop is formed. Since the ground loop circulation will bring common mode and differential mode noise and interference, the system often cannot work properly. An ideal solution is to electrically isolate the equipment so that the originally interconnected ground networks become independent units and the interference between them will be greatly reduced.

So in the industrial automation control system, and instrumentation, sensor application, widely uses 4~20mA electric current to transmit the control, the examination signal. Because the 4~20mA current loop has strong anti-interference ability and the circuit is simple, it can be used to transmit tens or even hundreds of meters of analog signals. In general, if the transmission distance exceeds 10 meters, the current signal needs to be isolated.
MHM-05 is an analog signal isolator, also known as a safety barrier, also called a signal transmitter, and belongs to the category of signal conditioning. It mainly acts as an anti-interference. Because of its strong anti-interference ability, it is widely used in automatic control systems. Especially in industrial automation is getting higher and higher. As the number of primary sensor applications increases, the requirements for the accuracy and sensitivity of sensors increase. The control program is also becoming more and more complicated. The disturbances on the industrial site are intricate and complicated. It is complicated from the start of the control site such as the start of the motor, the switching of the high-current contactor, the phasing of the SCR, the electric pulse of the arc, the electromagnetic waves, and so on. In the interference group, it is necessary to fully consider the use of isolation measures to ensure the reliability of the system during system design and cut off all kinds of channels that may cause interference. The reliability of the entire system design is a very important part of the success or failure of the system. Therefore, the industrial standard remote analog signal DC 4-20mA is completely isolated from the system by a signal isolator, and there is no common point, which is generally referred to as an isolated isolation state. From the statistics of many automated engineering projects introduced, the proportion of 1:1 current signal isolators used in on-site analog signal acquisition is extremely high, which is really not unknown. In many domestic self-built projects, it is blocked during large-scale system integration. , and then resolving the isolation measures to solve the case is still less? Therefore, at the beginning of the system design, we must repeatedly emphasize the use of anti-jamming measures. This is a lesson from many real-life cases to our designers. This technical measure is indeed one of the most effective anti-jamming measures to overcome the forward process path in today's automated control systems.

The signal isolator is mainly used with industrial intelligent measurement and control systems such as DCS and PLC. It is used to improve and complement the system simulation I/O plug-in function and enhance the reliability of on-site and remote transmission signals. However, many signal isolators currently on the market still need to be connected with auxiliary power supplies, or they must be marketed with products with relatively low technical content, and inconvenience in use. From the perspective of system reliability, the addition of one more component not only increases the availability of one fault point, but also introduces new interference channels. With the development of new technologies and new processes, the performance of many devices continues to improve, and the power consumption of devices has dropped to a point where they can be ignored. Therefore, the newly designed MMH-05A signal isolator of Shanghai Yilun Electromechanical Co., Ltd. is used. An isolator designed without the need of an external auxiliary power source. It only has two inputs and two outputs, which bring convenience to system design and application. It also improves system reliability. The structure of the MHM-05A adopts a bayonet-type modular structure, which can be directly snapped into the standard DIN rail, so it is very convenient for installation, disassembly and maintenance. Can be mounted on the standard rails in multiple compact stacks to save or replace the control cabinet input and output terminals. The MHM-05C type is an electronic module structure that can be directly mounted on PCB printed circuit boards.

The MHM-05 series signal isolator is a passively isolated, single input, single isolated output. Enter 4-20 mA. Output 4-20 mA.