Introduction
With the continuous development of cement production equipment and technologies, almost all new dry-process cement production lines built since the 1990s have adopted computer technology, among which the central centralized control system and DCS distributed control system are dominant.
In the initial stage of constructing the 700 t/d new dry-process production line in 1993, our company conducted an in-depth analysis of domestic and foreign application status. Considering the capital constraints and the inability to afford imported complete system software, we decided to adopt a production guidance system to collect and monitor the process parameters of the rotary kiln in real time. Based on multimedia information processing and expert system inference algorithms, the system displays full-production-process information to operators, and delivers real-time operating conditions of key positions on the rotary kiln production line via voice, image, text and other forms. It also provides corresponding operation guidance to assist on-site production.
This application mode combines the computer’s production guidance function with operators’ initiative. It overcomes the shortcomings of conventional operation modes, such as observing kiln conditions manually and relying solely on experience for control. It standardizes operation methods, stabilizes thermal conditions, and ultimately achieves stable and high-yield production. Meanwhile, it avoids misoperations frequently encountered by most automatic control manufacturers caused by inaccurate detection elements, actuator faults, computer judgment errors and other factors.
The scheme adopted by our company is a distributed system composed of three-level computers: data acquisition – monitoring management – production guidance. The lower computers adopt STD industrial control computers and BITBUS network systems. The electrical interface complies with balanced transmission RS-485 standard, with twisted-pair wires as the transmission medium. The middle and upper computers adopt INTEL industrial PCs. The system has operated stably for three years since commissioning.
Supported by multimedia computers, the system processes images of material calcination inside the kiln captured by ordinary industrial cameras. Through image processing technology, it simulates manual kiln observation and obtains characteristic parameters such as temperature distribution in the burning zone that reflect internal kiln conditions. A real-time expert system is established by integrating conventional process parameters of cement rotary kilns, empirical data, operation experience of workers and experts, and real-time process signals. It displays kiln conditions through graphics, text and audio, and puts forward timely production suggestions.
The production process monitoring system composed of BITBUS network and IPC industrial control computers features excellent anti-interference performance and strong multi-level computer communication capability. It realizes the acquisition, processing, storage, transmission, display and alarm of massive production process data. As a low-investment, low-failure, user-friendly and highly practical computer system for data detection, processing and production guidance, it has basically achieved the expected goals.
According to the actual layout of monitoring points on the production line, three sets of data collectors are respectively arranged in the kiln tail, coal mill and kiln head control rooms where process parameter monitoring points are concentrated, forming three independent slave stations. Each slave station supports 32-channel data processing, with a total processing capacity of up to 128 channels.
Affected by traditional operation habits, production workers have long been accustomed to operating by observing instrument panels. Therefore, independent display instruments are configured for key parameters such as temperature, pressure, flow rate, rotating speed and load in the initial system design. Signals from sensors and transmitters first access digital display instruments, and then are connected to lower computers for data acquisition.
Instrument panels only display instantaneous values, while the computer screen supports regular refresh, automatic page turning and patrol display of process parameters. It also stores raw data for up to several months, facilitating production managers and operators to analyze kiln conditions at any time, track dynamic variation trends of various parameters, and provide a basis for continuously optimizing production guidance system software and verifying operation methods.
Since computers and display instruments share the same set of sensing and transmitting components, and the distances between on-site detection points and data acquisition lower computers as well as between middle and lower computers are relatively long, various on-site electromagnetic interferences easily threaten the operational reliability of the system.
2 Anti-Interference Measures
The reliability of a microcomputer system is determined by multiple factors, among which anti-interference performance is a core indicator. Interference refers to all unwanted internal and external signals that affect the measurement results of electrical testing systems or instruments. Mild interference degrades signal quality, while severe interference disrupts normal circuit functions, causes logical confusion and control failure.
Theory and practice prove that electromagnetic interference exists only when three conditions are met simultaneously: interference source, transmission path and interference-sensitive receiving circuit. Eliminating any one of the three can suppress interference. Accordingly, common anti-interference methods include: suppressing noise sources to eliminate the root cause of interference; cutting off transmission paths to decouple interference sources from affected equipment; enhancing the electromagnetic interference immunity of equipment to reduce its sensitivity to interference.
Improving equipment anti-interference capability must start from the design stage and run through manufacturing, commissioning and whole-life operation and maintenance. During system design and laboratory research, we repeatedly demonstrated and tested the configuration of the three-level computer system, BITBUS network connection and inter-device communication. In addition to developing universal software and hardware suitable for production process monitoring and data processing, we systematically studied on-site anti-interference measures and formulated a comprehensive optimization scheme.
2.1 Reasonable Selection of System Backplane to Reduce Internal Noise and Interference
In the design of STD industrial control application systems, standard modules are usually adopted in the mode of overall design – module selection – modular assembly – separate debugging – overall debugging – on-site operation to accelerate development, improve performance and enhance reliability.
The STD bus backplane (motherboard) interconnects data lines, address lines and control lines of all functional modules, and provides system power for each module through power planes on the backplane. Theoretically, the bus backplane impedance should be minimized to avoid distortion of high-frequency pulse signals. Any signal distortion on the backplane bus is regarded as noise interference.
The system adopts high-performance four-layer STD bus backplane. The middle two printed circuit layers serve as a large-area power plane and ground plane respectively. Signal lines on the backplane are isolated by ground wires to reduce crosstalk caused by distributed capacitance between lines, with multi-point connection to the inner ground plane. Bus sockets requiring power supply on the main board are connected to the power and ground planes nearby, shortening wiring length and reducing impedance and voltage drop. The characteristic impedance of signal lines matches the output impedance of bus drivers, providing sufficient signal driving power.
2.2 Correct Configuration of Internal and External Power Supplies to Eliminate Power-Derived Interference
Power supply is the primary source of severe interference in microcomputer systems. To suppress interference introduced via power systems, multiple measures are adopted: using AC voltage regulators to improve system stability; installing isolation transformers to eliminate parasitic coupling interference between primary and secondary sides and enhance common-mode rejection ratio; connecting low-pass filters or double-T filters at power inlets to suppress power frequency interference.
Industrial-grade high-reliability anti-interference regulated power supplies are adopted inside data collectors. Industrial frequency AC power is processed through voltage transformation, rectification, filtering and voltage regulation to provide required voltage levels. Multi-stage filtering and enlarged filter capacitors are applied to suppress conducted noise from the power grid and ripple noise caused by insufficient filtering. The power supply features high regulation accuracy, strong anti-interference performance, high common-mode and series-mode rejection ratio, and wide-frequency interference suppression capability.
2.3 Optoelectronic Isolation to Eliminate Electromagnetic Interference via Process Channels
Process channels include forward channels, backward channels and information transmission paths between hosts. The forward channel is responsible for signal acquisition, conditioning and conversion. Close to measured objects and with weak output signals from sensors, forward channels are the main access path for interference.
Optoelectronic isolation is the most effective measure to suppress interference coupled into input and output interfaces. A/D conversion modules with optocouplers are adopted in forward channels. Optocouplers transmit signals via light with complete electrical isolation, featuring unidirectional signal transmission, contactless operation, high common-mode rejection ratio and easy compatibility with logic circuits. They can effectively suppress interference from digital systems on analog signals, especially weak signals. This measure realizes signal-to-ground isolation; for multi-channel analog input systems, high common-mode interference between analog channels is further eliminated by adding electromagnetic isolation amplifiers or linear optocoupler isolation circuits.
2.4 Twisted-Pair Communication to Eliminate Crosstalk from Inter-Wire Electromagnetic Induction
Process channels cover signal transmission paths of forward channels, backward channels and inter-host communication. A common anti-interference method is to counteract magnetic flux density generated by interference sources on signal loops or arrange wires close to ground wires.
To simulate on-site electromagnetic interference and verify long-distance signal transmission anti-interference performance, we intentionally extended signal lines and arranged artificial interference sources in laboratory commissioning to accumulate experience for on-site installation and debugging.
During on-site wiring and static debugging, the data acquisition system and computer data processing operated normally. However, during feeding and joint commissioning of the rotary kiln system, severe interference still occurred despite adequate early anti-interference design. The harsh high-temperature and dusty environment of cement plants, together with signal cables laid alongside power lines and control loops on kiln bridges and in cable trenches, are affected by various interference sources: high and low voltage cables, DC speed regulation devices, 120 kV high-power high-voltage rectifiers, electromagnetic vibrators and frequency conversion equipment.
2.5 Elimination of Interference from Power Supply and Grounding System
Repeated debugging and comprehensive analysis show that besides strong on-site electrical interference, interference from power supply and grounding systems is more prominent. The AC power supply of the computer network is taken from nearby workshop power panels. Neglecting internal circulating current caused by unbalanced current in power systems leads to severe interference to computer network operation via power lines, and mutual coupling generates internal interference. The following measures are adopted:
(1) Configure dedicated power supply for computer system to suppress interference from AC power lines
A 35mm² three-phase four-wire dedicated power cable is laid from the factory substation directly to the upper computer control room at the kiln head, with phase-split branch lines distributed to three data acquisition stations. This eliminates electromagnetic interference caused by neutral line circulating current due to voltage fluctuation and three-phase current imbalance in power loops.
(2) Increase cross-sectional area of signal lines and grounding wires to reduce transmission interference
Grounding refers to low-impedance connection of equipment and circuits to a common signal reference point. Electronic equipment grounding serves two purposes: safety grounding for personal safety, and working grounding to provide reference potential and low-resistance path for high-frequency interference. Proper grounding is critical for noise suppression; improper grounding aggravates interference coupling.
Any grounding plane or wire has inherent impedance, producing voltage drop when current flows through it and possibly forming loops with other wires, becoming a new interference source. Grounding interference is divided into ground resistance interference and ground loop interference. Key solutions include: increasing grounding wire cross-section to reduce public ground impedance; selecting proper grounding modes and points to cut off ground loops; combining grounding with isolation and shielding for optimized anti-interference effect. Doubling the cross-sectional area of signal lines achieves obvious noise reduction.
(3) Shield and ground surrounding strong electromagnetic interference sources
Shielding is the main measure to suppress spatial field interference, which cuts off or weakens spatial coupling paths of interference fields and blocks electromagnetic energy transmission. It is classified into electric field shielding, magnetic field shielding and electromagnetic field shielding; electromagnetic shielding integrates both electric and magnetic field suppression and anti-radiation capability.
Metal enclosures of surrounding strong interference sources are used as shielding covers with reliable grounding. When electrical control cabinets are used as shielding enclosures, all cabinet parts must maintain good electrical connection; insulation between cabinet components will cause interference leakage. Shielding covers suppress electric field interference via distributed capacitance coupling and generate reverse eddy current magnetic fields under high-frequency magnetic field excitation of interference sources, offsetting and weakening interference and significantly reducing the impact of high-frequency electromagnetic fields on sensitive circuits.
