Many new projects and plant expansions are ideally suited to taking advantage of using a multidrop bus system. The major areas of savings are generally acknowledged to be in cabling and installation costs, commissioning time and ease of network expansion.
However, when considering the type of instruments to be used on a bus-based system it often becomes apparent that the choices are more restricted than with other methods of instrument connection. Taking Profibus as an example, this fieldbus has a large base of products available but is still not as large as with other more conventional technologies.
So the engineer is faced with a dilemma. He not only has to learn new technologies concerned with the instrument network but now he may also have to evaluate different methods of measurement and establish their suitability for his application.
Transmitters with Profibus output
Manufacturers need to consider offering their entire range of products in suitable form or risk losing business to other manufacturers.
Looking at level measurement as an example, it is relatively easy to obtain radar level transmitters with Profibus outputs. Vega offers both low and high frequency versions of its range with a variety of antennae and approvals to suit a wide range of applications. Whilst radar is very versatile and may be used in many applications, it is not always the solution that the customer requires. It is therefore important that other more established technologies are available with a suitable Profibus output.
Power consumption
It is also important that manufacturers design their product from the ground up with bus system capabilities considered. One of the main areas that can reduce the benefits of bus systems is power consumption. This is especially true for hazardous area applications where a limited amount of current is available for supplying the instruments. Looking at the FISCO (Fieldbus Intrinsically Safe Concept) as applied to Profibus this restricts the current to 110 mA on a bus segment and if a sensor requires a high fault current, then this must also be taken into consideration.
The Profibus standard effectively offers a minimum current of 10 mA per sensor and manufacturers need to aim at this level to enable as many sensors as possible to be used on a segment, as a higher consumption means more segments may be required, which costs more to install.
In some cases the power consumption of instruments can be so great that a separate power supply may be needed to the instruments, as sufficient power cannot be supplied via the bus network. This negates the advantages of using a bus system and indeed may make the engineer reconsider his options and revert to more conventional cabling systems.
One of the other advantages of the bus system is that configuration set-up and commissioning can be carried out from the supervisory level of the control system. However, this requires instrumentation that is designed to allow this. In many cases manufacturers are making their product Profibus compatible by only converting at the output point of their instrument to a Profibus protocol signal. This often means that set-up and calibration parameters are not available and only the measured value is transmitted. This also means that the instrument will require set-up in the field.
Vega's range of level instruments is designed to be fully compatible with Profibus communications and allow access to all set-up parameters and diagnostics, allowing the full capabilities of the system to be used. Vega is a member of PACTware, which means that all Vega instruments can be run from this fieldbus-independent software system. PACTware is an open platform in which individual manufacturers can integrate the operation of their field instruments. PACTware allows all instruments in a plant to be quickly and easily configured, set up and if necessary diagnosed and adjusted.
With any complex system, back-up and security of settings is important and this integration with the system software ensures this happens. Exchange of instruments following failure or damage becomes a simple process of uploading existing settings and therefore reducing plant downtime.
In the short term, engineers may be planning projects that have some instrumentation that may only be available currently with more conventional outputs such as 4 to 20 mA. In this case one solution may be to consider the Vegalog system. This allows connection of a variety of sensors to a bus system with a range of different output protocols available, including Profibus, Modbus and Ethernet TCP/IP. This will also allow set-up of scaling, linearisation and other functions within the Vegalog, removing much of the on-plant maintenance requirements.
In summary, it is important for manufacturers to offer instruments that are designed for bus systems from their conception and not just an add-on to an existing outdated product.
For more information contact Alan Wynn, Vega Instruments, 011 958 1901 alan.wynn@za.vega.com
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