Foundation Fieldbus is a digital communication system used in automation along with such as Profibus, Modbus or HART. The technology appeared somewhat later than its competitors: the first edition of the standard is dated 1996 and currently includes two protocols for information exchange between network participants - H1 and HSE (High Speed Ethernet).
The H1 protocol is used for information exchange at the level of sensors and controllers, and its network is based on the IEC 61158-2 physical layer standard, allowing a data transfer rate of 31,25 kbps. In this case, the field devices can be powered from the data bus. The HSE network is based on high-speed Ethernet networks (100/1000 Mbps) and is used to build an APCS network at the level of controllers and enterprise management systems.
The technology is applicable in the construction of automated process control systems for any industrial facilities, but is most widely used in the oil and gas industry and the chemical industry.
Technology Capabilities
Foundation Fieldbus was developed as an alternative to the traditional analog sensor-based automated control system model and received a number of advantages over both the traditional model and digital systems based on Profibus or HART.
One of the main advantages is the high degree of reliability and fault tolerance of systems H1, which is achieved due to two factors:
- use at the field level of intelligent devices (sensors and actuators);
- the ability to organize information exchange directly between field-level devices without the participation of the controller.
The intelligence of field devices lies in the ability to lay down control and information processing algorithms that are traditionally implemented in the controller. In practice, this allows the system to continue working even if the controller fails. This requires that the field devices are properly configured and that a reliable fieldbus power supply system is provided.
Additional benefits from digitalization of the control system and the use of smart sensors are the ability to obtain more data, beyond measurement, from each field device, which ultimately expands the field of view of the process, which in traditional analog systems is limited by the signal I / O system. .
The use of bus topology in the H1 network allows you to reduce the length of cable lines, the amount of installation work, to refuse the use of additional equipment in control systems: I / O modules, power supplies, and in hazardous areas - spark protection barriers.
H1 accepts 4-20 mA sensor communication cables, which can be used when retrofitting older control systems. Thanks to the use of the principles of intrinsic safety, the technology is actively used in explosive environments. The standardization itself guarantees the interchangeability and compatibility of equipment from different manufacturers, and thanks to gateway devices, it is possible to interface a network of field devices and APCS networks of enterprises built on Ethernet.
Foundation Fieldbus H1 is most similar to Profibus PA systems. Both technologies are based on the same physical layer standard, so these systems have the same data transfer rates, the use of Manchester coding, the electrical parameters of the communication line, the amount of possible transmitted power, the maximum allowable cable length in the network segment (1900 m). Also, in both systems, it is possible to use up to 4 repeaters, so that the segment length can already reach 9,5 km. Common are the possible network topologies in the control system, as well as the principles of intrinsic safety.
System Components
The main elements of a Foundation Fieldbus H1 network are:
- decentralized control system (DCS) controller;
- fieldbus power supplies;
- block or modular interface devices;
- bus terminators;
- intelligent field devices.
Also in the system there may be gateway devices (Linking Device), protocol converters, SPDs and repeaters.
Network topology
An important concept in the H1 network is the concept of a segment. It is a trunk communication line (Trunk), with branches branching off from it (Spur), to which field devices are connected. The trunk cable starts at the bus power supply and usually ends at the last coupler. Four types of topology are allowed for communication between the controller and field devices: point-to-point, stub, bus, and tree. Each segment can be built using a separate topology, or using their combinations.
With a point-to-point topology, each field device is connected directly to the controller. Each connected field device forms its own network segment. This topology is inconvenient because it deprives the system of almost all the benefits that are inherent in the Foundation Fieldbus. There are too many interfaces on the controller, and to power the field devices from the data bus, each communication line must have its own field bus power supply. The length of communication lines turns out to be too long, and information exchange between devices is carried out only through the controller, which does not allow using the principle of high fault tolerance of H1 systems.
The daisy chain topology implies a serial connection of field devices to each other. Here, all field devices are combined into one segment, which allows you to use fewer resources. However, such a topology also has drawbacks - first of all, it is necessary to provide methods in which the failure of one of the intermediate sensors will not lead to a break in communication with the rest. Another drawback is due to the lack of protection against a short circuit in the communication line, in which information exchange in the segment will be impossible.
Two other network topologies have the greatest reliability and practicality - bus and tree topologies, which are most widely used in practice when building H1 networks. The point of these topologies is to use couplers to connect field devices to the backbone. Interface devices allow each field device to be connected to its own interface.
Network settings
Important questions when building an H1 network are its physical parameters - how many field devices can be used in a segment, what is the maximum length of a segment, how long can branches be. The answer to these questions depends on the type of power supply and power consumption of the field devices, and for explosive sites, on the methods of ensuring intrinsic safety.
The maximum number of field devices per segment (32) can only be achieved if they are powered from local sources at the site and in the absence of intrinsic safety. When powering sensors and actuators from the data bus, the maximum number of devices can only be 12 or less, depending on intrinsic safety practices.
Dependence of the number of field devices on the method of supply and methods of ensuring intrinsic safety.
The length of the network segment is determined by the type of cable used. The maximum length of 1900 m is achieved using type A (shielded twisted pair) cable. When using cable type D (not twisted multi-core cable with a common screen) - only 200 m. The length of the segment is understood as the sum of the lengths of the main cable and all branches from it.
The dependence of the length of the segment on the type of cable.
The length of the spurs depends on the number of devices in the network segment. So, with up to 12 devices, this is a maximum of 120 m. When using 32 devices in a segment, the maximum branch length will be only 1 m. When connecting field devices with a loop, each additional device reduces the length of the branch by 30 m.
Dependence of the length of branches from the main cable on the number of field devices in the segment.
All these factors directly affect the structure and topology of the system. To speed up the network design process, special software packages are used, such as DesignMate from the FieldComm Group organization or Fieldbus Network Planner from Phoenix Contact. The programs allow you to calculate the physical and electrical parameters of the H1 network, taking into account all possible restrictions.
Purpose of system components
Controller
The task of the controller is to implement the functions of the Active Link Scheduler (LAS) - the main device that manages the network by sending service messages. LAS initiates information exchange between network members by scheduled (scheduled) or unscheduled messages, performs diagnostics and synchronization of all devices.
In addition, the controller is responsible for automatic addressing of field devices, acts as a gateway device, providing an Ethernet interface for communication with the upper level of the control system based on Foundation Fieldbus HSE or other communication protocol. For the upper level of the system, the controller provides operator monitoring and control functions, as well as remote configuration functions for field devices.
There can be several Active Link Schedulers in the network, guaranteeing the redundancy of their functions. In modern systems, LAS functions can be implemented in a gateway device that acts as a protocol converter for control systems built on a standard other than Foundation Fieldbus HSE.
Fieldbus power supplies
The power system in the H1 network plays a key role, because for the very possibility of information exchange, the voltage in the data cable must be maintained in the range from 9 to 32 VDC. Whether field devices are powered by the data bus or local power supplies, the network requires bus power supplies.
Therefore, their main purpose is to maintain the required electrical parameters on the bus, as well as powering devices connected to the network. Bus power supplies differ from conventional power supplies in that they have an appropriate output circuit impedance at data rates. If 1 or 12 V power supplies are used directly to power the H24 network, the signal will be lost, and information exchange on the bus is not possible.
Redundant fieldbus power supplies FB-PS (assembly for 4 segments).
Given the importance of providing reliable bus power, the power supplies for each network segment can be redundant. FB-PS power supplies from Phoenix Contact support Auto Current Balancing technology. DIA provides a symmetrical load between power supplies, which favorably affects their temperature regime and ultimately leads to an increase in their service life.
The H1 mains supply system is usually located in the controller cabinet.
Interface devices
Interface devices are designed to connect a group of field devices to the main data bus. According to their functions, they are divided into two types: segment protection modules (Segment Protectors) and field barriers (Field Barriers).
Regardless of the type, interface devices protect the network from short circuits and overcurrents in outgoing lines. When a short circuit occurs, the interface device blocks the interface port, preventing the short circuit from spreading through the system and thus guaranteeing information exchange between other network devices. After the short circuit on the line is eliminated, the previously blocked communication port starts working again.
Field barriers additionally provide galvanic isolation between non-intrinsically safe fieldbus circuits and intrinsically safe circuits of connected field devices (tap-offs).
Physically, interface devices are also of two types - in block and modular design. Block couplers type FB-12SP with segment protection functionality allow the use of intrinsically safe IC circuits for connecting field devices in Zone 2, and FB-12SP ISO field barriers allow connecting devices in Zone 1 and 0 with intrinsically safe IA circuits.
FB-12SP and FB-6SP couplers from Phoenix Contact.
One of the advantages of modular devices is the ability to scale the system by selecting the number of channels required to connect field devices. In addition, modular devices allow you to create flexible structures. In one distribution cabinet, you can combine segment protection modules and field barriers, that is, connect field devices located in different explosion hazard zones from one cabinet. In total, up to 12 two-channel FB-2SP modules or single-channel FB-ISO barrier modules can be installed on one bus, thus connecting from one cabinet up to 24 field devices in Zone 2 or up to 12 sensors in Zone 1 or 0.
Interface devices can be operated in a wide temperature range and are installed in explosion-proof enclosures Ex e, Ex d with a degree of dust and moisture protection of at least IP54, including as close as possible to the control object.
Surge protection devices
H1 field level networks can form very long segments, and communication lines can run in places where surge voltages are possible. Surge voltages are induced potential differences caused by lightning strikes or short circuits in nearby cable lines. The induced voltage, the value of which is on the order of several kilovolts, causes the flow of discharge currents of kiloamperes. All these phenomena occur within microseconds, but can lead to the failure of the components of the H1 network. To protect the equipment from such phenomena, it is necessary to use an SPD. The use of SPDs instead of conventional feed-through terminals guarantees reliable and safe operation of the system in adverse conditions.
Its principle of operation is based on the use of a quasi-short circuit in the nanosecond range for the flow of discharge currents in a circuit that uses elements that can withstand the flow of currents of this magnitude.
There are a large number of types of SPDs: single-channel, two-channel, with interchangeable plugs, with various types of diagnostics - in the form of a blinker, dry contact. State-of-the-art diagnostic tools from Phoenix Contact enable monitoring of SPDs using Ethernet-based digital services. The company's plant in Russia manufactures devices certified for use in explosive environments, including Foundation Fieldbus systems.
Bus terminator
The terminator performs two functions in the network - it shunts the field bus current that occurs due to signal modulation and prevents signal reflection from the ends of the main line, thus preventing the appearance of noise and jitter (digital signal phase jitter). Thus, the terminator avoids the appearance of inaccurate data on the network or the loss of data altogether.
Each segment of the H1 network must have two terminators, at each end of the segment. Phoenix Contact bus power supplies and couplers are equipped with switchable terminators. The presence of extra terminators in the network, for example, due to an error, will significantly reduce the signal level in the interface line.
Information exchange between segments
Information exchange between field devices is not limited to one segment, but is possible between different sections of the network, which can be connected via a controller or an Ethernet-based enterprise network. In this case, the Foundation Fieldbus HSE protocol or a more popular one, for example, Modbus TCP, can be used.
When building a HSE network, industrial switches are used. The protocol allows ring redundancy. In this case, it is worth remembering that in a ring topology, switches must use one of the redundancy protocols (RSTP, MRP, or Extended Ring Redundancy) depending on the magnitude and required network convergence time when communication links are interrupted.
Integration of HSE-based systems with third-party systems is possible using OPC technology.
Explosion protection methods
To create an explosion-proof system, it is not enough to be guided only by the explosion-proof characteristics of the equipment and the choice of its correct location on the site. Within the system, each of the devices does not function on its own, but operates within a single network. In Foundation Fieldbus H1 networks, information exchange between devices located in different explosive zones is associated not only with data transmission, but also with the transmission of electrical energy. The amount of energy that was acceptable in one zone may not be acceptable in another. Therefore, a systematic approach is used to assess the explosion safety of field networks and select the optimal method for ensuring it. Among these methods, the methods of ensuring intrinsic safety are most widely used.
With respect to fieldbuses, there are currently several ways to ensure intrinsic safety: the traditional method of intrinsic safety barriers, the FISCO concept, and High Power Trunk (HPT) technology.
The first of these is based on the use of IS barriers and implements a proven concept that has been used in control systems based on analog 4-20 mA signals. This method is simple and reliable, but limits the power to field devices in Ex Zones 0 and 1 to 80 mA. In this case, according to the optimistic forecast, it is possible to connect no more than 4 field devices per segment with a consumption of 20 mA, but in practice no more than 2. In this case, the system loses all the advantages that exist in Foundation Fieldbus, and actually leads to a point-to-point topology, when to connect a large number of field devices, the system must be divided into many segments. Also, this method significantly limits the length of the main cable and spurs.
The FISCO concept was developed by the "National Metrology Institute of Germany" and later included in IEC standards, and then in GOST. To guarantee the intrinsic safety of the field network, the concept involves the use of components that meet certain restrictions. Similar restrictions are formulated for power supplies in terms of output power, for field devices in terms of power consumption and inductance, for cables in terms of resistance, capacitance and inductance. Such limitations are due to the fact that capacitive and inductive elements can accumulate energy in themselves, which in emergency mode, in case of damage to any element of the system, can be released and cause a spark discharge. In addition, the concept prohibits the use of redundancy in the bus power system.
FISCO provides more current to power devices in the hazardous area compared to the field barrier method. 115 mA is available here, which can be used to power 4-5 devices in the segment. However, there are also restrictions on the length of the main cable and branches.
High Power Trunk technology is currently the most common intrinsic safety technology in Foundation Fieldbus networks because it avoids the disadvantages that exist in networks protected by barriers or built according to FISCO. With the use of HPT, it has become possible to reach the limit value of field devices in a network segment.
The technology does not limit the electrical parameters of the network where it is not necessary, for example, on the main communication line, where there is no need for maintenance and replacement of equipment. To connect field devices located in an explosive zone, interface devices with the functionality of field barriers are used, which limit the electrical parameters of the network to power sensors and are located directly next to the control object. In this case, the type of protection Ex e (increased protection) is used throughout the segment.
Source: habr.com