Preface With the rapid development of computer network technology, the replacement of digital and analog decentralized control systems by all-digital field control systems has become an inevitable trend in the development of industrial control systems. The fieldbus-based all-digital control system is a hot issue of concern to all countries in the world today.
Due to the promising prospect of fieldbus, in the early 1990s, in order to occupy the world market rapidly and driven by common economic interests, several major industrial automation instrument companies in the world began to take the road of strong alliances, and several major organizations have been established. The incompatibility of fieldbus standards has made the formulation of fieldbus international standards more complex. On the one hand, in order to gain the recognition of users, on the one hand, for the common interest, several major associations and foundations are formed to participate in the formulation of international standards. The most representative organizations are ODVA Supplier Association, FIELDBUS Foundation (PP) Foundation, and P-NET.
The FOUNDATION fieldbus held a technical conference for fieldbuses in September 2002. Here we will introduce the (FF) fieldbus as an example.
1. Introduction Field bus is not only a communication protocol, but also a complete control system. It is often called a field control system (FCS).
1.1 What is field control bus?
Fieldbus is an all-digital, serial, and bi-directional communication system that connects sensors, actuators, controllers, and other "field" devices to form a system with control functions.
1.2 The composition of the fieldbus consists of an automation communication network, an open fieldbus, and a standard user layer.
1.3 Fieldbus features decentralized control, interoperability, and economy.
The DCS system has successfully replaced the DDC system because it is more decentralized and digitized than the DDC. The FCS further extends this concept and is more decentralized and digital than the DCS.
1.4 Fieldbus Advantages Fieldbus technology, and in particular the structure of the FCS control system, has brought great benefits to the plant. The use of fieldbus can reduce hardware investment. Simplified system costs for the corresponding wiring and installation of design drawings are also greatly reduced. Overall, overall system costs can be reduced by 30%.
2. Comparison of Fieldbus Control System and Other Control Systems From Fig. 1, it is known that DDC, DCS, and FCS are different in structure. The direct digital control system (DDC) is to directly input the on-site analog signal to the computer, which is directly performed by the computer. The operation processing, and the computer output control signal to control the field device, due to the limited computing power of the computer, this type of control is generally not very large, is the control of the concentration, the danger is large:
Distributed Control System (DCS), on-site signal input and output equipment point-to-point input DCS system processing card on the operation processing, control signals through a special output card output, to the field equipment for control. The computer (operating station) acts as a man-machine interface and does not directly participate in the control. The size of the system depends on how much the card is handled. Its control is decentralized, and the information is concentrated on the operation station through the network. The system can vary in size. Diversification of dangers, safety gains.
The FCS system uses the controller and microprocessor built into the field instrument to calculate the process parameters, and then controls another instrument actuator on the site through the bus to achieve the purpose of controlling the device. It can be seen from Figure 1. Its control is all digital signals, and more dispersed, and the size of the system can be determined with the size of the branch bus.
The development of fieldbus began in the mid-1980s. At that time, as a substitute for 4-20mA, it was born out of DCS and retained the essence of DCS's design philosophy. Therefore, there is no essential difference between fieldbus and DCS. DCS is well-received in the control field and has been recognized by many users. Because of this, FCS has not completely started but has inherited many successes of DCS, such as digital technology and decentralized control, and further developed them to achieve more functions. Whether it is FCS or DCS, or even DDC, their basic points are the same. The operation control of the plant is accomplished through the central control room's operation station and field devices.
3. Advantages of fieldbus control system compared with other control systems FCS control system has the following advantages in addition to the above-mentioned savings:
3.1 Operationality Mainstream fieldbuses have now become international standards, and products from different manufacturers around the world can be connected and work together. In the near future, a wide variety of fieldbus devices coming to market will cover all possible measurement and control requirements, satisfy all possible applications, and seamless fieldbus systems can be easily integrated without using special drivers or icons. Using analog signals to bridge the gap in proprietary systems, users can choose the best equipment from different vendors without worrying about compatibility. In this way, due to the wide range of choices, users can get the best price-performance products without being tied to a single vendor.
3.2 Better Data Validity and Integrity Fieldbus uses digital signals, either "0" or "1", and has better robustness than analog signals that tend to attenuate and distort. Digital signal transmission uses error correction mechanisms to ensure accuracy. By referring to the status information corresponding to measurement and manipulation parameters, the operator can make better and more confident decisions. Digital communication enables multi-parameter transmission, and more field device information can be obtained from the operator station, including equipment station number, equipment identification, range, structural material, and calibration information. See Figure 2.
3.3 Improve product quality Because FCS has no A/D, D/A conversion, the signal integrity and accuracy can be improved, the accuracy of the entire system can be improved correspondingly, and the product quality can also be changed less. The calibration information of the equipment, including who , when, where, how to calibrate the equipment, etc., are stored in the equipment according to the ISO9000 standard. Keeping track of this information helps keep your equipment running at its best. Remote diagnostics and operational assessments can find out if the equipment is operating under the required conditions.
Through these methods, it is possible to clarify the cause of the quality change and make the equipment perform well.
3.4 Reduced expansion and variable costs The field bus has a flexible scale, which can include a loop, and can also include thousands or even more loops. Based on the original field bus system, new loops can be added with little or no hardware. Or modify the original circuit.
3.5 Increase Security Field bus diagnostic information is not only reported to the operator, but the equipment itself uses this information to take safety measures to ensure system safety when the equipment fails. Due to the prompt and accurate prompt of the fault, the operator can more quickly locate the fault and formulate corresponding measures.
4. Fault tolerance of the fieldbus The fieldbus has at least the same fault tolerance as the DCS, but both use different fault tolerance capabilities. The DCS uses bus redundancy and the fieldbus uses fault isolation. Like DCS, FCS uses redundant operating stations (including corresponding interfaces) and DCS controller network redundancy (data highways). Similarly, FCS with controller layer network is also redundant (some FCS systems do not exist Layer network). As with DCS, the FCS power system is also redundant. DCS requires controller redundancy. In FCS, the control is performed by field devices. There is no controller and therefore there is no redundancy problem.
4.1 Point-to-Point Installation The point-to-point cable connection method is used for the field device and the I/O card in the DCS. If the cable fails, it will only affect the circuit associated with the attached device. However, if the I/O card fails, it will affect all related loops, usually 8 or more loops. Therefore, in the DCS, a worst-case failure will result in the loss of 8 loops.
In FCS, a circuit requires a minimum of two devices, a transmitter and an actuator. According to the conservative statement in the fieldbus installation specification, a bus can be connected to 12 bus-powered devices (the number of non-bus-powered devices can be up to 32). In most cases, up to 16 devices can be connected to one bus, and up to eight loops can be provided to 16 devices. If the cable fails in the FCS, the number of failed devices and the number of loops depends on the fault location of the cable. If you are at the end of the cable, only one loop may be affected, and in the worst case if the cable fault occurs at the control room, it may affect all 8 loops.
In the above situation, the fault tolerance capabilities of DCS and FCS are the same, that is, one fault loses eight loops.
4.2 Critical loops In the DCS, critical loops sometimes use redundant I/O cards. A small fault in the worst case can only lead to the loss of one loop, that is to say, the loop has a higher integrity. The FCS can achieve the same integrity, placing only the critical loops on one bus, so that a cable failure will only result in the loss of one loop.
4.3 Full redundancy In the petrochemical industry, equipment investment is not a problem, and the entire system I/O cards are redundant to achieve higher loop integrity. In this regard, FCS can do the same.
5. Will FCS threaten DCS?
Because users need a simpler and fully operational solution to reduce the cost of system maintenance, expansion and upgrade, FCS is exactly what happened. It uses digital communications to make I/O cards and cables greatly reduced, and field devices With its own microprocessor, standard modules are used to completely distribute the control to the field, eliminating the need for controllers. In order to achieve predictive maintenance, data status indication, fault diagnosis and fault indication are designed, all of which make the field bus not only It is a communication protocol, but it is a complete replacement for DCS, or forcing DCS to turn to FCS.
6. Conclusions DCS is no longer the natural choice for process control systems. FCS can not only achieve basic control and interlocking but also advanced control, ESDC (emergency shutdown) and ERP (enterprise resource management). The advanced fieldbus-based FCS system can completely replace the traditional DCS, and through the enhancement of equipment management, while improving the product quality and output, the total cost of ownership (TCO) of the system is reduced, and the production efficiency is greatly improved. This is something that other systems cannot do today.
Due to the promising prospect of fieldbus, in the early 1990s, in order to occupy the world market rapidly and driven by common economic interests, several major industrial automation instrument companies in the world began to take the road of strong alliances, and several major organizations have been established. The incompatibility of fieldbus standards has made the formulation of fieldbus international standards more complex. On the one hand, in order to gain the recognition of users, on the one hand, for the common interest, several major associations and foundations are formed to participate in the formulation of international standards. The most representative organizations are ODVA Supplier Association, FIELDBUS Foundation (PP) Foundation, and P-NET.
The FOUNDATION fieldbus held a technical conference for fieldbuses in September 2002. Here we will introduce the (FF) fieldbus as an example.
1. Introduction Field bus is not only a communication protocol, but also a complete control system. It is often called a field control system (FCS).
1.1 What is field control bus?
Fieldbus is an all-digital, serial, and bi-directional communication system that connects sensors, actuators, controllers, and other "field" devices to form a system with control functions.
1.2 The composition of the fieldbus consists of an automation communication network, an open fieldbus, and a standard user layer.
1.3 Fieldbus features decentralized control, interoperability, and economy.
The DCS system has successfully replaced the DDC system because it is more decentralized and digitized than the DDC. The FCS further extends this concept and is more decentralized and digital than the DCS.
1.4 Fieldbus Advantages Fieldbus technology, and in particular the structure of the FCS control system, has brought great benefits to the plant. The use of fieldbus can reduce hardware investment. Simplified system costs for the corresponding wiring and installation of design drawings are also greatly reduced. Overall, overall system costs can be reduced by 30%.
2. Comparison of Fieldbus Control System and Other Control Systems From Fig. 1, it is known that DDC, DCS, and FCS are different in structure. The direct digital control system (DDC) is to directly input the on-site analog signal to the computer, which is directly performed by the computer. The operation processing, and the computer output control signal to control the field device, due to the limited computing power of the computer, this type of control is generally not very large, is the control of the concentration, the danger is large:
Distributed Control System (DCS), on-site signal input and output equipment point-to-point input DCS system processing card on the operation processing, control signals through a special output card output, to the field equipment for control. The computer (operating station) acts as a man-machine interface and does not directly participate in the control. The size of the system depends on how much the card is handled. Its control is decentralized, and the information is concentrated on the operation station through the network. The system can vary in size. Diversification of dangers, safety gains.
The FCS system uses the controller and microprocessor built into the field instrument to calculate the process parameters, and then controls another instrument actuator on the site through the bus to achieve the purpose of controlling the device. It can be seen from Figure 1. Its control is all digital signals, and more dispersed, and the size of the system can be determined with the size of the branch bus.
The development of fieldbus began in the mid-1980s. At that time, as a substitute for 4-20mA, it was born out of DCS and retained the essence of DCS's design philosophy. Therefore, there is no essential difference between fieldbus and DCS. DCS is well-received in the control field and has been recognized by many users. Because of this, FCS has not completely started but has inherited many successes of DCS, such as digital technology and decentralized control, and further developed them to achieve more functions. Whether it is FCS or DCS, or even DDC, their basic points are the same. The operation control of the plant is accomplished through the central control room's operation station and field devices.
3. Advantages of fieldbus control system compared with other control systems FCS control system has the following advantages in addition to the above-mentioned savings:
3.1 Operationality Mainstream fieldbuses have now become international standards, and products from different manufacturers around the world can be connected and work together. In the near future, a wide variety of fieldbus devices coming to market will cover all possible measurement and control requirements, satisfy all possible applications, and seamless fieldbus systems can be easily integrated without using special drivers or icons. Using analog signals to bridge the gap in proprietary systems, users can choose the best equipment from different vendors without worrying about compatibility. In this way, due to the wide range of choices, users can get the best price-performance products without being tied to a single vendor.
3.2 Better Data Validity and Integrity Fieldbus uses digital signals, either "0" or "1", and has better robustness than analog signals that tend to attenuate and distort. Digital signal transmission uses error correction mechanisms to ensure accuracy. By referring to the status information corresponding to measurement and manipulation parameters, the operator can make better and more confident decisions. Digital communication enables multi-parameter transmission, and more field device information can be obtained from the operator station, including equipment station number, equipment identification, range, structural material, and calibration information. See Figure 2.
3.3 Improve product quality Because FCS has no A/D, D/A conversion, the signal integrity and accuracy can be improved, the accuracy of the entire system can be improved correspondingly, and the product quality can also be changed less. The calibration information of the equipment, including who , when, where, how to calibrate the equipment, etc., are stored in the equipment according to the ISO9000 standard. Keeping track of this information helps keep your equipment running at its best. Remote diagnostics and operational assessments can find out if the equipment is operating under the required conditions.
Through these methods, it is possible to clarify the cause of the quality change and make the equipment perform well.
3.4 Reduced expansion and variable costs The field bus has a flexible scale, which can include a loop, and can also include thousands or even more loops. Based on the original field bus system, new loops can be added with little or no hardware. Or modify the original circuit.
3.5 Increase Security Field bus diagnostic information is not only reported to the operator, but the equipment itself uses this information to take safety measures to ensure system safety when the equipment fails. Due to the prompt and accurate prompt of the fault, the operator can more quickly locate the fault and formulate corresponding measures.
4. Fault tolerance of the fieldbus The fieldbus has at least the same fault tolerance as the DCS, but both use different fault tolerance capabilities. The DCS uses bus redundancy and the fieldbus uses fault isolation. Like DCS, FCS uses redundant operating stations (including corresponding interfaces) and DCS controller network redundancy (data highways). Similarly, FCS with controller layer network is also redundant (some FCS systems do not exist Layer network). As with DCS, the FCS power system is also redundant. DCS requires controller redundancy. In FCS, the control is performed by field devices. There is no controller and therefore there is no redundancy problem.
4.1 Point-to-Point Installation The point-to-point cable connection method is used for the field device and the I/O card in the DCS. If the cable fails, it will only affect the circuit associated with the attached device. However, if the I/O card fails, it will affect all related loops, usually 8 or more loops. Therefore, in the DCS, a worst-case failure will result in the loss of 8 loops.
In FCS, a circuit requires a minimum of two devices, a transmitter and an actuator. According to the conservative statement in the fieldbus installation specification, a bus can be connected to 12 bus-powered devices (the number of non-bus-powered devices can be up to 32). In most cases, up to 16 devices can be connected to one bus, and up to eight loops can be provided to 16 devices. If the cable fails in the FCS, the number of failed devices and the number of loops depends on the fault location of the cable. If you are at the end of the cable, only one loop may be affected, and in the worst case if the cable fault occurs at the control room, it may affect all 8 loops.
In the above situation, the fault tolerance capabilities of DCS and FCS are the same, that is, one fault loses eight loops.
4.2 Critical loops In the DCS, critical loops sometimes use redundant I/O cards. A small fault in the worst case can only lead to the loss of one loop, that is to say, the loop has a higher integrity. The FCS can achieve the same integrity, placing only the critical loops on one bus, so that a cable failure will only result in the loss of one loop.
4.3 Full redundancy In the petrochemical industry, equipment investment is not a problem, and the entire system I/O cards are redundant to achieve higher loop integrity. In this regard, FCS can do the same.
5. Will FCS threaten DCS?
Because users need a simpler and fully operational solution to reduce the cost of system maintenance, expansion and upgrade, FCS is exactly what happened. It uses digital communications to make I/O cards and cables greatly reduced, and field devices With its own microprocessor, standard modules are used to completely distribute the control to the field, eliminating the need for controllers. In order to achieve predictive maintenance, data status indication, fault diagnosis and fault indication are designed, all of which make the field bus not only It is a communication protocol, but it is a complete replacement for DCS, or forcing DCS to turn to FCS.
6. Conclusions DCS is no longer the natural choice for process control systems. FCS can not only achieve basic control and interlocking but also advanced control, ESDC (emergency shutdown) and ERP (enterprise resource management). The advanced fieldbus-based FCS system can completely replace the traditional DCS, and through the enhancement of equipment management, while improving the product quality and output, the total cost of ownership (TCO) of the system is reduced, and the production efficiency is greatly improved. This is something that other systems cannot do today.