Introduction to Fieldbus
This prompted the need to integrate the various types of digital instrumentation into field networks to optimize system performance. While the “if it works then use it” mentality progressed, it became obvious that a Fieldbus standard was required to formalize the control of smart instruments.
What is Fieldbus ?
to replace the existing 4 – 20mA analogue signal. The network is a digital, bi-directional, multidrop, serial-bus, communications network used to link isolated field devices, such as controllers, transducers, actuators and sensors. Each field device has low cost computing power installed in it, making each device a ‘smart’ device.
functions as well as providing bi-directional communication capabilities. With these devices not only will the engineer be able to access the field devices, but they are also able to communicate with other field devices.
Why Fieldbus ?
existing intelligent transmitters can do the same thing. Fieldbus is a complete system, with the control function distributed to equipment in the field, while still allowing operation and tuning from the control room using the digital
communication. It replaces the traditional 4-20 mA and the classic DCS where the control function was centralized to one or more ‘control cards.’
the advantages of 4-20 mA standards (the main advantage of 4-20mA is tight closed loop control).By providing various options for communication speed and device powering, the requirements for both intrinsic safety and minimum communication delay can be met.
Fieldbus devices must therefore be able to operate by themselves with a simple user interface in order to be economical in small systems. The cost of a host computer with dedicated software. and certainly a DCS. cannot be justified for a small system, even though costs are going down. There would also be a logistics problem for both users and manufacturers if they were forced to keep using analog technology in small systems.The possible complexity of a system where so many devices can be connected together (and where each device can perform the function of several conventional devices) requires a friendly user interface. The user must be freed from manual address assignment, as seen in smart transmitter protocols, and the painstaking job of tracking bits, bytes, words and memory addressees, as it is done in PLCs.
Direct Digital Control System
Limited communications from intelligent field devices
2-Way Digital Multi-drop communications
Eventually, the control system architecture and technology will move toward the following directions:
Digital Communication Advantages
looks as if that will not be the way.Some advantages of bi-directional digital communications, over 4-20 mA, and other good features are known from existing smart transmitter protocols:
a controller may talk directly, rather than going through D/A and A/D conversions, of which there may be many in a closed loop. Status is passed along with measurement and control data. It is therefore possible to 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.
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 of old transmitters.Some problems and disadvantages of existing protocols, in comparison to 4-20 mA technology, have also been seen:
the use of fieldbus technology due to the simplification of the collection of information from field devices. Measurement and device values will be available to all field and control devices in engineering units.
System performance is enhanced due to the ability to communicate directly between two field devices rather than via the control system. This also enables several related field devices to be combined into one device.
As well as the cost advantages that fieldbus technology embodies, there are many other miscellaneous advantages
that are included in the fieldbus package. Although it is a major challenge trying to develop a single worldwide protocol for process control, there are currently only two real protocols for fieldbus, being ISP and WorldFIP and while there is still two
protocols rather than a world standard, it is better than a possible many.
Work is being done to merge the two protocols into a standard which will be a major advantage. The fact that eventually all fieldbus equipment will be standardised will mean that expansion of a
system or addition of field devices will be extremely simple, requiring no interfaces or converters.
for the user to concern themselves with the Physical and User layers.
Benefits of Fieldbus
occur at all levels of production management – from real time field operations to the production management level. Some of the main benefits, which can be achieved by utilising Foundation Fieldbus technology, are:
to be performed and noise and errors are virtually eliminated. Digital sensors can be found today in our EJA pressure transmitter family as well as in our YEWFLO vortex flow meter family. Additional digital sensors are being developed
that maximise accuracy and enhance process plant performance.
Multi-variable and multi-functional devices including test and measurement devices, power measurement meters and our vast R&D resources develop specific IC’s. Combining technologies results in fewer devices to install and
maintain. For instance temperature, flow, static pressure and density can all be measured by only one device. Due to Foundation Fieldbus technology, all this information can be made available.
not feel confident to buy something if one does not understand how it works.The simplicity of analog technology makes it easy to be understood. That is the main reason people feel so comfortable with it. 4-20 mA devices may be operated using only a screwdriver and tested with the most basic current meter.
Almost anybody could configure and troubleshoot such devices.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 even 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. Though 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.
Control Loop Connections
Conventional Control Loop
Fieldbus Control Loop
Interoperability in FF
performance, quality and delivery time. They may mix and match the best of each type, just as they could with the 4-20 mA. They do not have to choose a device manufactured by a certain company just to match other devices of the same brand already installed (without Fieldbus, users would have been forced to develop special communication drivers in this case).