Fieldbus foundation specifies “Fieldbus is a digital two-way, multidrop communication link among intelligent measurement and control devices”. It is one of local area networks dedicated for industrial automation. Modern industries could not survive without information technologies and networks in the 21st century. From production line to enterprise level, digital communication supports all economical and social activities by its latest and powerful technologies. Fieldbus is part of it and cannot be separated from others.
FIELDBUS BENEFITS :
Fieldbus is expected to reduce the life-cycle cost of production line and then total cost of ownership (TCO) of the plant.
Planning Phase :
Fieldbus allows us to integrate our plant assets on a single plant automation system on the digital communication networks. We can connect devices from multiple suppliers without custom software and these network-based systems allow for smaller control rooms, smaller cabinet rooms and more information productivity.
Installation phase :
Fieldbus offers reduced installation and material cost by replacing the traditional one to one wiring scheme with networking or multi-drop configuration while intelligent field instruments make commissioning and plant startup much faster and less expensive.
Operation Phase :
Fieldbus integrates various installations of control function into one system to effectively optimize control of our plant. In addition, a unified human machine interface (HMI) is provided for operation.
Maintenance Phase :
Fieldbus allows for the reporting of self diagnostics, calibration, and environmental conditions of field instruments without disturbing the plant control. It will also allows to our stock for spare parts.
Renovation Phase :
Enhanced functionality of field instruments is endless. Fieldbus devices are becoming standard off the shelf instruments, which make it very cost-effective and easy for we to extend the life of our plant.
FOUNDATION FIELDBUS :
1. Fieldbus is not a product but a technology to make above benefits available to end – users. Following two condition are necessary to make them come true :
• Many vendors provide Fieldbus instruments.
• Those devices are interoperability.
2. Fieldbus foundation was established in 1994 to achieve these goals. Its major actives are :
• To promote a single international Fieldbus to both users and vendors.
• To deliver foundation Fieldbus specification.
• To Provide technologies for Fieldbus implementation including education.
• To install an infrastructure to achieve interoperability.
COMMUNICATION TECHNOLOGIES :
• Communication specifications are often explained with reference to the open system interconnect (OSI) layered model.
• Fieldbus is specified according to the simplified OSI model, consisting of there (3) layers : Physical layer (PHL), Data link layer (DLL), & Application layer (APL).
Physical layer :
• Physical layer is a mechanism to transmit and receive electric or optic signals to / from medium so that a data consisting of ones and zeros is transmitted from one made to the others.
• Physical layer interests are wires, signals, waveform, vtg, and others all related to electricity and optics.
• IEC / ISA standards specify minimum amplitude and worst waveform of received signal at a device at the any place of the Fieldbus with.
Physical layer receiver cricket must be able to receive this signal.
Data Link Layer :
• Data link layer is mechanism to transfer data from node to the othersthat need the others that need the data. It also manager the priority and order of such transfer requests.
• Data link layer interests are data, address, priority, medium control, and other all related to message transfer.
Application layers :
Application layers consist of two sub layers :
• Fieldbus access sub layer (FAS) manager data transfer while.
• Fieldbus message specification (EMS) encodes and decodes user data.
Fieldbus access sub layer :
• Fieldbus access sub layer (FAS) is a part of secure communication. Since Fieldbus doesn’t have layers between DLL and APL, FAS directly maps APL requests to DLL services.
Fieldbus message specification :
Fieldbus message specification (FMS) is a service interface for user applications to use Fieldbus services. When service is requested, it encodes request to transfer it to the other applications. Receiving FMS decodes the request to notify it to the application.
ADVANTAGES OF FIELDBUS :
Some advantages of bi-directional digital communications, over 4-20 mA, and other good features are known from existing smart transmitter protocols :
• Higher accuracy and data reliability.
• Multi-variable access
• Remote configuration and diagnostics
• Reduction of wiring
• Use existing ‘analog wiring’
Higher accuracy can be attributed to digital communication, since the microprocessors in, e.g., a transmitter and a controller may talk directly, rather than going through D/A and A/D conversion, of which there may be many in a closed loop. Status is passed along with measurement and control data. It is therefore possible t determine if the information is reliable or not. All data are checked and guaranteed free from distortion due to noise or an impedance mismatch that may affect analog undetected signals. Multi-variable access, meaning, e.g., that a pressure transmitter is not limited to a single output for pressure, but also informs process temperature. Another example is access to set point and manipulated variable of a controller in the same device, or multiple channel inputs for a temperature transmitter.
The digital communication allows the complete configuration to be changed remotely. Calibration can be done in operation without having to apply any input or measure the output. Similarly, the status of the self diagnostics may be interrogated. Reduction in wiring and simplification is achieved through connection of several devices on a single pair or wires – multidropping. Connection is a simple task, since everything is in parallel and terminal number matching is at a minimum. This means low cost and easy replacement f old transmitters.
DISADVANTAGES OF FIELDBUS :
Some problems and disadvantages of existing protocols, in comparison to 4- 20 mA technology, have also been seen :
• Communication speed too low for closed loop control.
• Poor or no interoperability between devices of different type and manufacturer.
• Devices must be polled for status.
• Not Interdevice.
The slowest option for Fieldbus is 25times faster, and far more efficient, than the most common smart transmitter protocol, ensuring tight closed loop control. The low speed version of Fieldbus was designed to used the same type of wiring as analog and smart transmitters, being able to easily replace them. However, it should be noted that in order to use the multidrop feature, transmitters must be connected in parallel.
TURNING TO FIELDBUS :
The impact on the user, when turning to Fieldbus will be great. The main points are :
• Even higher reliability operation
• Virtually unlimited flexibility
• Reduction in equipment cost
• Reduction in installation cost
• Mass of information.
The speed by which Fieldbus will capture the market depends largely on how fast the users are trained. One does not feel confident to buy something if one does not understand how it works. Field devices may report failures and problems immediately, enabling maintenance personnel to pinpoint errors instantly or even before they can cause any harm. The multivariable capability of Fieldbus allows control, totalization and other signal processing in the field. Therefore, a separate controller or other signal conditioning equipment is not necessary. The host may be a simple off-the shelf PC with MMI software. Multidropping of several devices on a single wire may drastically reduce the amount of needed cable. In many factories, a device can be a kilometer or eve further away from the control room. Since every loop needs at least two pairs of wires one for the transmitter, the other for the actuator a refinery can have several hundreds of such loops. In all, the cabling saving for a medium or large factory is immense. Transmitters and actuators are often located next to each other, but far from the operator console, an ideal situation for multidropping. Through prices of Fieldbus devices may initially be high, the reduction in number of devices and wiring, with associated cable trays and marshaling boxes, will yield a less expensive system manufacturers can no longer rely on proprietary technology to keep prices high. Fieldbus will bring open competition, which will eventually reduce prices.
Fieldbus has, on top of this, software function blocks which replace many functions today performed by hardware. This provides tremendous flexibility since the control strategy may be edited without having to rewire of change any hardware. Once physically connected, logical connections between function blocks may be changed, function blocks can be added and removed. More advanced devices may execute a virtually unlimited number of function blocks. If a system has to be expanded or improved, the need for additional hardware is minimized, just by letting existing devices execute more blocks.
FIELDBUS REQUIREMENTS :
The main requirements for Fieldbus were to overcome the problems of smart transmitter protocols while maintaining the advantages of 4-20 mA standards. By providing various options for communication speed and device powering, the requirements for both intrinsic safety and minimum communication delay
can be met. With 4-20 mA technology it is possible to build a control loop containing only a transmitter, a controller and an actuator. Fieldbus devices must also be capable of doing so, as well as acting in a larger control system. Fieldbus must be multipurpose and as versatile as 4-20 mA. Fieldbus devices must therefore be able to operate by themselves with a simple user interface in order to be economical in small systems.
Fieldbus specifies the basic functionality requirements, but must allow a manufacturer to add unique features to their device. These features benefit the user, and the manufacturer may uses them as marketing tools. The cost of shutting down a system can be very high in terms of production loss. To be able to configure the system while in operation is therefore a requirement met by Fieldbus.
CONCLUSION :
Fieldbus is already providing that it can have dramatic benefits for end users. Wiring cost savings of 80% to 90% over conventional installation are being realized. The myriad of configuration and diagnostic information available in Fieldbus device is greatly reducing device commissioning time. This additional diagnostic information often makes it possible to remotely diagnose a field device problem thus saving a costly trip to the field. Finally the benefits of moving control functions from the central control room out to the Fieldbus devices are resulting in better, more reliable control as well as a less complex centralized control system.
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