What is a Fieldbus?
FieldBus is the general name for a number of competing standards such as Profibus (the German national standard); Interbus-S (originated from Phoenix Contactor), FIP (the French National standard) and Foundation FieldBus (the so-called international standard).
In old systems, the variables are sent to the instrument room and from there it is digitalized and sends to the control room, which is very expensive for cables and other mechanical expense.
Fieldbus approach to Typical Control System:
There has been a strong movement over the past five years towards the use of smart instruments. Essentially a smart instrument has the following features:
- Intelligent Digital microprocessor based sensor
- Digital data communications capability
Here digital sensors are used in plant. There are a number of intelligent digital sensors for most traditional applications. These include sensors for measuring temperature, pressure, level, flow, mass and density. A diagram of a typical Smart Instrument:
At a basic level, most smart instruments provide core functions such as:
- Control of range/zero/span of instruments.
- Diagnostics to verify functionality.
- Memory to store configuration and status information (such as Tag Numbers).
Accessing these functions allows major gains in the speed and efficiency of the installation and maintenance process. For example, the time consuming 4-20 mA loop-check phase can be achieved in minutes – a considerable time-saving.
There is a proliferation of standards on the market at present. Two common ones are HART (Highway Addressable Remote Transducer) and Profibus. HART is a hybrid analog-digital standard whilst Profibus is a pure digital standard. This section discusses the following two Protocols in more detail:
- HART Protocol
Benefits of using smart instruments are:
- Greatly reduced wiring costs
- Reduced installation and start-up time
- Improved on-line monitoring and diagnostics
- Easier change-out and expansion of devices
- Improved local intelligence in the devices
- Improved Interoperability between different (competing) manufacturers.