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Foundation Fieldbus Automation Systems

Foundation Fieldbus is a digital communications system used in automation along with Profibus, Modbus or HART. The technology appeared a little later than its competitors: the first edition of the standard is dated 1996 and currently includes two information exchange protocols 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 kbit / s. In this case, the power supply of field devices can be arranged from the data bus. The HSE network is based on high-speed Ethernet networks (100/1000 Mbit / s) and is used to build an automated process control network at the level of controllers and enterprise management systems.

The technology is applicable in the construction of industrial control systems of any industrial facilities, but it is most widely used at enterprises in the oil and gas industry and the chemical industry.

Technology features


Foundation Fieldbus was developed as an alternative to the traditional model of automated control systems based on analog sensors and gained a number of advantages over both the traditional model and digital systems based on Profibus or HART.

One of the main advantages lies in the high degree of reliability and fault tolerance of Foundation Fieldbus H1 systems , 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 a 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 enables the system to continue to operate even if the controller fails. This requires that the field devices be configured appropriately and that a reliable field bus power supply system is provided.

Additional benefits from the digitalization of the control system and the use of smart sensors are the ability to receive more data, in addition to measurement, from each field device, which ultimately expands the process monitoring area, which in traditional analog systems is limited by the signal input-output system .

The use of bus topology in the H1 network allows reducing the length of cable lines, the amount of installation work, and eliminating the use of additional equipment in control systems: input-output modules, power supplies, and in explosion hazard zones - spark protection barriers.

Foundation Fieldbus H1 allows the use of 4-20 mA sensor communication cables, which can be used to upgrade older control systems. Thanks to the use of intrinsic safety principles, the technology is actively used in explosive atmospheres. Standardization itself guarantees the interchangeability and compatibility of equipment from different manufacturers, and thanks to gateway devices it is possible to interconnect the network of field devices and the control systems of enterprises built on Ethernet.

The most similar Foundation Fieldbus H1 has with Profibus PA systems. Both technologies are based on the same physical layer standard, therefore the 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) are the same for these systems. Also in both systems it is possible to use up to 4 repeaters, due to which the segment length can 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 the Foundation Fieldbus H1 network are:

  • decentralized control system controller (DCS);
  • fieldbus power supplies;
  • block or modular interface devices;
  • bus terminators;
  • intelligent field devices.

Also in the system may be present 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 departing from it (Spur), to which field devices are connected. The trunk cable starts at the bus power source and usually ends at the last interface device. Four types of topology are allowed for communication between the controller and field devices: point-to-point, loop, bus, and tree. Each segment can be built using a separate topology, or using their combinations.



When using a point-to-point topology, each field device is connected directly to the controller. In addition, each connected field device forms its own network segment. Such a topology is inconvenient, because it deprives the system of almost all the advantages that are inherent in Foundation Fieldbus. Too many interfaces are involved on the controller, and for supplying field devices from the data bus, each communication line must have its own field bus power supply. The length of communication lines is 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 loop topology implies the serial connection of field devices to each other. Here, all field devices are combined into one segment, which allows less resources to be used. However, this topology also has drawbacks - first of all, it is necessary to provide methods in which failure of one of the intermediate sensors does not lead to a break in communication with the others. Another drawback is due to the lack of protection against short circuits in the communication line, in which information exchange in the segment will be impossible.

Two other network topologies, bus and tree topologies, have the greatest reliability and practicality, which are most widely used in practice when building H1 networks. The meaning of these topologies is to use interface devices to connect field devices to the trunk. Interfacing devices allow each field device to be connected to its own interface.

Network settings


Important issues when building an H1 network are its physical parameters - how many field devices can be used in a segment, how long can a segment be, how long can branches be. The answer to these questions depends on the type of power supply and energy consumption of field devices, and for explosive facilities, ways to ensure intrinsic safety.

The maximum number of field devices in segment (32) can be achieved only if they are supplied from local sources locally and in the absence of intrinsically safe means. When supplying sensors and actuators from the data bus, the maximum number of devices can be only 12 or less, depending on the methods for ensuring intrinsic safety.


Dependence of the number of field devices on the supply method and intrinsic safety methods.

The length of the network segment is determined by the type of cable used. The maximum length of 1900 m is achieved using a type A cable (twisted pair with a shield). When using a cable of type D (not a twisted multicore cable with a common screen) - only 200 m. By segment length is understood the sum of the lengths of the main cable and all branches from it.


Dependence of segment length on cable type.

The length of the branches depends on the number of devices in the network segment. So, with the number of devices up to 12, this is the maximum 120 m. When using 32 devices in the segment, the maximum branch length will be only 1 m. When connecting field devices with a loop, each additional device reduces the branch length by 30 m.


Dependence of the length of the branches on the trunk 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, they use special software packages such as DesignMate from the FieldComm Group or Fieldbus Network Planner from Phoenix Contact. The programs allow the calculation of the physical and electrical parameters of the H1 network, taking into account all possible limitations.

Purpose of system components



Controller

The controller's task 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 participants with scheduled (scheduled) or unplanned messages, carries out diagnostics and synchronization of all devices.

In addition, the controller is responsible for the 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 another communication protocol. For the upper level of the system, the controller provides monitoring and control functions on the part of the operator, as well as the functions of remote configuration of field devices.

There can be several Active Link Planners on the network, guaranteeing a backup 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 supply system in the H1 network plays a key role, because for the very possibility of information exchange, a voltage in the range from 9 to 32 V DC must be supported in the data cable. Regardless of whether the field devices are powered by the data bus or locally by power sources, bus power sources are required on the network.

Therefore, their main purpose is to maintain the required electrical parameters on the bus, as well as power devices connected to the network. Bus power sources differ from conventional power supplies in that they have the corresponding impedance of the output circuit at data transmission frequencies. If you directly use 12 or 24 V power supplies to power the H1 network, the signal will be lost, and information exchange on the bus is impossible.


FB-PS redundant fieldbus power supplies (4-segment assembly).

Given the importance of providing reliable bus power, power supplies for each network segment can be redundant. Phoenix Contact's FB-PS power supplies support Auto Current Balancing technology. DIA provides a symmetrical load between power sources, which favorably affects their temperature conditions and ultimately leads to an increase in their service life.

The power supply system of the H1 network is usually located in the controller cabinet.

Interfacing devices Interfacing

devices are designed to connect a group of field devices to the 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, the 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 eliminating the short circuit on the line, the previously blocked communication port starts working again.

Field barriers additionally provide galvanic isolation between non-intrinsically safe circuits of the main bus and intrinsically safe circuits of connected field devices (branches).

Physically, interface devices also come in two types - in block and modular designs. Block interface devices of the FB-12SP type with segment protection functionality allow the use of intrinsically safe IC circuits for connecting field devices in Zone 2, and the FB-12SP ISO field barriers allow connecting devices in Zone 1 and 0 with intrinsically safe IA circuits.


FB-12SP and FB-6SP interface devices from Phoenix Contact.

One of the advantages of modular devices is the ability to scale the system by selecting the number of channels necessary for connecting field devices. In addition, modular devices allow you to create flexible structures. In one control cabinet, segment protection modules and field barriers can be combined, that is, field devices located in different hazardous areas can be connected 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 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 to the control object as possible.

Surge Protection Devices

Field H1 networks can form very long segments, and communication lines can pass in places where the formation of surge surges is possible. By impulse overvoltage is meant the induced potential difference caused by lightning discharges or short circuits in nearby cable lines. Induced voltage, the magnitude of which is of the order of several kilovolts, causes the discharge currents to flow in kiloamps. All these phenomena occur within microseconds, but can lead to failure of the network components H1. To protect equipment from such phenomena, it is necessary to use an SPD. The use of SPDs instead of conventional bushings ensures reliable and safe operation of the system in adverse conditions.

The principle of its operation is based on the use of a quasi-short circuit in the nanosecond range for the flow of discharge currents in a circuit in which elements are used that can withstand the flow of currents of this magnitude.

There are a large number of varieties of SPDs: single-channel, two-channel, with interchangeable plugs, with various types of diagnostics - in the form of a blinker, dry contact. Modern diagnostic tools from Phoenix Contact allow monitoring of SPD using Ethernet-based digital services. The company's factory in Russia produces devices certified for use in explosive atmospheres, including Foundation Fieldbus systems.

Bus terminator

The terminator performs two functions in the network - it shunts the field bus current, which occurs due to the modulation of the signal and prevents the signal from reflecting from the ends of the trunk line, thus preventing the appearance of noise and jitter (phase jitter of the digital signal). Thus, the terminator avoids the appearance of inaccurate data on the network or data loss at all.

In each segment of the H1 network, two terminators are required, at each end of the segment. Bus power supplies and Phoenix Contact couplers are equipped with disconnectable terminators. The presence of unnecessary terminators in the network, for example, due to an error, will significantly reduce the signal level in the interface line.

Interchange between segments


The exchange of information between field devices is not limited to one segment, but is possible between different sections of the network, which can be connected through a controller or an enterprise network based on Ethernet. In this case, the Foundation Fieldbus HSE protocol or the more popular, for example, Modbus TCP, can be used.

When building an 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 size and the required convergence time of the network when communication channels are broken.

Integration of HSE-based systems with third-party systems is possible using OPC technology.

Explosion proof methods


To create an explosion-proof system, it is not enough to be guided only by the explosion safety characteristics of the equipment and the choice of its correct location at the facility. Within the system, each of the devices does not function by itself, but works within a single network. In Foundation Fieldbus H1 networks, information exchange between devices located in different hazardous areas is associated not only with data transmission, but also with electric energy transmission. The amount of energy that was permissible in one zone may not be acceptable in another. Therefore, to assess the explosion safety of field networks and select the optimal method for its provision, a systematic approach is used. Among such methods, the most widely used methods of ensuring intrinsic safety.

For 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 them is based on the use of spark protection barriers and implements a tried and tested concept, which was used in control systems based on analog 4-20 mA signals. This method is simple and reliable, however, it limits the power supply of field devices in Ex zones 0 and 1 80 mA. In this case, according to an 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 in fact leads to a point-to-point topology, when to connect a large number of field devices, the system must be divided into many segments. This method also significantly limits the length of the trunk cable and branches.

The FISCO concept was developed by the German National Metrology Institute and later became part of the IEC standards, and then GOST. To ensure 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 for output power, for field devices for power consumption and inductance, for cables for resistance, capacitance and inductance. Such limitations are associated with the fact that capacitive and inductive elements can accumulate energy in themselves, which in emergency mode, in the event 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 a large amount of 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 a segment. However, there are also restrictions on the length of the trunk cable and branches.

High Power Trunk technology is currently the most common intrinsic safety technology in Foundation Fieldbus networks because it lacks the disadvantages that exist in networks protected by barriers or built according to FISCO. With the use of HPT, it has become possible to achieve the limit value of field devices in a network segment.



The technology does not limit the electrical parameters of the network where this 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 the hazardous area, devices are used to interface with the functionality of field barriers, which limit the electrical parameters of the network to power the sensors and are located directly next to the control object. In this case, the explosion protection type Ex e (increased protection) is used throughout the segment.

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