Need For Plant Optimization

One of the top strategic initiatives of any organization is plant optimization. This need is driven by global pressure to increase product quality, reduce product costs, reduce plant downtime, improve plant safety, reduce time to market, and increase plant flexibility. To achieve plant optimization the number of different networks, gateways and systems in the plant hierarchy (figure 1) must be reduced while at the same time increasing information integration between automation systems and Management Information systems (MIS). To be costeffective the system must be built on standard, high volume, low cost networking technology.

Figure 1

This is accomplished by replacing proprietary automation systems and networks with a single, open, integrated fieldbus architecture. The open fieldbus architecture enables direct integration of sensors, devices, subsystems and data servers resulting in the lean hierarchy on the right in figure 1. The open specification allows a complete range of interoperable products from many suppliers.

FOUNDATION™ fieldbus - Open, Integrated Fieldbus Architecture

The Fieldbus™ Foundation is an international, not-for-profit, organization of over 180 manufacturers and users of automation equipment that was formed for the purpose of developing the open, integrated fieldbus architecture. The open specification developed by the foundation is FOUNDATION fieldbus™.

FOUNDATION fieldbus™ is based on the well-known the Open Systems Interconnection (OSI) Reference Model. The OSI model describes the network in terms of layers. The Physical Layer (layer 1) provides connection to the communication media (e.g. wire, fiber optics, wireless), while the Application Layer (layer 7) defines the services needed to send and receive messages.

The communication network is only part of the specification needed to achieve interoperability between devices and subsystems from different manufacturers. Networks only transport messages from point A to point B much like airplanes move passengers from country to country. To achieve true interoperability without translators (gateways), FOUNDATION fieldbus™ uses a common language above the application layer called the “User Layer.” The user layer defines the framework for performing control functions in the devices, and specifies how distributed control functions are linked across the network.

TheFOUNDATION fieldbus™ integrated architecture is shown in figure 2.

Figure 2

User Layer – Distributed Control

The advent of powerful, low-cost, microprocessors has made it cost-effective to distribute control functions into the field devices on a digital network (see figure 3). This reduces or eliminates the need for I/O converter subsystems and controller equipment which greatly reduces hardware and wiring costs, overall system power requirements, and cabinet space. The risk of system failure is greatly reduced because there is no single point of failure. If a device should fail, powerful diagnostics pinpoint the problem quickly, reducing the time to repair which improves system uptime and plant safety.

Figure 3

To achieve distributed control, functions such as an Analog Input (AI) in a flow transmitter or an Analog Output (AO) in a valve are encapsulated in function blocks. Functions such as Proportional Integral Derivative (PID) control can also be built into function blocks and run in a field device. In figure 3, the PID and AO blocks are running in the valve.

For basic control blocks the number and type of inputs and outputs are pre-defined by the FOUNDATION fieldbus™ specification. For more complex functions such as batch control, coordinated drive control, I/O gateways, and PLC sequencing, a special Flexible Function Block (FFB) can be used. The inputs and outputs of the FFB, and the FFB control algorithm are configurable by the end user.

Function Block Scheduling

After the location of the function blocks in the devices is determined, a function block schedule is generated and downloaded to each field device. When the device is put in service, function blocks in the device execute at offsets determined by the configuration system. See figure 4. In the example, the communication from the AI block to the PID block takes place over the fieldbus, but the communication between the PID and AO block is local to the valve and does not use the fieldbus. The period of time needed for execution of the function blocks is called the macrocycle.

Figure 4

Device Descriptions

Function blocks contain the information needed for the online control functions, but additional information is needed for configuration and display purposes. This additional information is provided by Device Description (DD) and Capability Files (CF). The DD/CF files provide the “drivers” that allow host systems to configure and operate the devices without custom programming (figure 5).

Figure 5

Device suppliers create the DD/CF files for their devices. The Fieldbus Foundation collects the DD/CF files and makes them openly available via the Fieldbus Foundation’s web site (www.fieldbus.org) and on CD-ROM.

Physical Layer

Referring to figure 2, OSI layer 1 defines the device connection to the physical media. The physical layer (PHY) ensures that devices can send and receive data bits correctly on the media.FOUNDATION fieldbus™ provides two profiles that can be selected based on application needs: H1 and HSE.

H1 runs at 31.25 kbit/s and is optimized for field device integration in process control and hybrid applications. HSE (High Speed Ethernet) runs at 100 Mbit/s and is designed as a high performance control backbone for integration of H1 and other subsystems, high density data generators such as PLCs and analyzers, and plant data servers. HSE uses standard industrial Ethernet networking equipment. The H1 physical layer is an approved IEC 61158 standard, and the HSE physical layer is an approved Ethernet/IEEE 802 standard.

H1 uses a single twisted pair wire that can be run up to 1900 Meters without a repeater. Up to 4 repeaters per segment can be used to extend the distance. Transmission is half-duplex which allows the same media to be shared by several devices. The topology can be a tree, bus or a combination.

H1 supports Intrinsic Safety (I.S.) applications with bus powered devices. To accomplish this, an I.S. barrier is placed between the power supply in the safe area and the device in the hazardous area. H1 supports the Fieldbus Intrinsically Safe Concept (FISCO) model. Fiber optic cable is an alternate media for H1 devices. Devices connect to the fieldbus through a star coupler. Distances from the device to the star coupler range up to 1660 meters depending on fiber size, wavelength and optical power budget.

HSE uses standard Ethernet/IEEE 802 media options. HSE was developed and tested at 100 Mbit/s, but use of 1 Gbit/s speeds and higher can be added when needed. HSE uses a star topology. Standard twisted-pair Ethernet cables can run up to 100 meters between an Ethernet switch and the device. HSE full-duplex fiber optic cable can run up to 2000 meters between a switch and device. Switches are often interconnected with fiber optic cable to take advantage of the greater distances.

Communication Stack

Referring again to figure 2, OSI layers 2-7 are collectively called the communication “stack.” The stack controls access to the physical media, encodes and decodes user layer information into “packets,” and controls the delivery of the packets on the network. Devices must use the same stack or they will not interoperate on the network.

Access the physical media is defined by the data link layer (DLL). In H1 the DLL contains Link Master (LM) protocol that allows only one device at a time to access the media. The active LM is called Link Active Scheduler (LAS). H1 allows for redundancy of the LAS. If one LAS fails, one of the other LMs will become the LAS.

HSE does not use a LM, it uses standard Ethernet/IEEE 802 multiple access DLL protocol. In HSE networks the switch internally detects and resolves any collisions. HSE supports redundancy; both redundant interfaces and redundant devices.

H1 does not use OSI layers 3-6 because H1 does not need packet routing or packet splitting. HSE does need packet routing and splitting and uses standard Internet Protocol (IP) for the Network layer, and Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) for the Transport layer. H1 and HSE use a common layer 7 interface to the user layer. In H1 the services are defined by the Fieldbus Message Specification (FMS). In HSE the services are defined by the Field Device Access (FDA) Specification.

FMS and FDA provide the same Client/Server, Publisher/Subscriber, and Event Notification communication services. Client/Server is typically used for request/response communication between hosts and field devices. Publisher/Subscriber services are used for transfer of cyclic data between function blocks and for data acquisition by a host. Event Notification services are typically used by the devices for sending alarm and trend information to the hosts.

Standards summary: H1 PHY, DLL, and FMS are IEC 61158 standards. HSE PHY and DLL are Ethernet/IEEE 802 standards, HSE Transport and Network layers are Internet standards, and HSE FDA is an IEC 61158 standard.

Device Test and Registration

To verify that devices conform to specification, the Fieldbus Foundation maintains a rigorous test program. For control devices, DD/CF files, function blocks, the communication stack, and physical layer are verified and use of the Product Registration “Checkmark” is granted for devices that pass the tests (See figure 6). Because each host device is different, a special Host Interoperability Support Test (HIST) verifies the features claimed by the host suppliers.

Figure 6

End users should use equipment that has passed the Fieldbus Foundation test program. Devices passing device registration and host HIST tests are listed on the Fieldbus Foundation web site www.fieldbus.org. As of this writing, over 100 devices had passed registration testing.

FOUNDATION™ fieldbus Applications

The range of applications covered byFOUNDATION fieldbus™ is shown in figure 7. FOUNDATION fieldbus™ specifies twenty-one standard function blocks designed to address basic and advanced process control applications and more blocks are in development. The flexible function blocks (FFB) mentioned earlier, are application-specific and are designed to implement control strategies such as supervisory data acquisition, batch control, PLC sequencing, burner management, coordinated drives, and advanced I/O interfacing. FFBs are created using programming tools based on standards such as IEC 61131-3. FFBs are also used to map information from other networks such as sensor busses into the FOUNDATION fieldbus™ open architecture.

Figure 7

Integration and Interoperability

Interoperability is critical to the plant optimization strategy because it ensures that there are ample device suppliers to provide the range of devices needed. The Fieldbus Foundation’s open, integrated fieldbus architecture ensures integration and interoperability at the device level, subsystem level, and data server level in the plant hierarchy.

Field Device Integration

H1 is optimized for integration of field level devices from different suppliers as shown in figure 8. Because of Function block, DD/CF, and common stack technology, registered devices will “plug and play” on the H1 fieldbus without custom software programming.

Figure 8

Subsystem Integration

HSE integrates automation subsystems as shown in figure 9. As with field device integration, subsystem integration does not require custom software programming. HSE is a high performance control backbone built with low cost standard industrial Ethernet equipment. Linking Devices integrate H1 subsystems into the HSE backbone.

Remote plant sites are integrated with the enterprise backbone using a variety of standard wide area networking equipment and services, or by fiber optic cable. Standard firewall technology is used to implement plant security and safety policies.

Figure 9

Data Server Integration

Modern data servers are built on high performance PC platforms with Ethernet access. Data servers connect to the HSE backbone and provide a common platform for application software as shown in figure 10. Typical application software includes Human/Machine Interface (HMI), Enterprise Resource Planning (ERP), Configuration and Diagnostic tools, and Management Information System (MIS) databases.

Figure 10

Summary

FOUNDATION fieldbus™ provides the open, integrated fieldbus architecture needed for plant optimization. H1 provides an optimized solution for field device integration. HSE integrates H1 and other subsystems, high density data generators, and data servers without the need for custom software programming or gateways.

Figure 11

Data servers provide the platform for application packages and MIS integration. FOUNDATION fieldbus™ function blocks ensure uniform presentation of data at all levels of the hierarchy. Function blocks combined with IEC 61158 and Internet/Ethernet compliant networking enables devices from different manufacturers to interoperate on H1 and HSE. The foundation’s rigorous testing programs provide confidence that registered H1 and HSE devices and HIST-tested hosts will interoperate.

For more information please visit the Fieldbus Foundation web site at www.fieldbus.org, or call 512 794 8890.