Basics of industrial instrumentation signals 4

As we continue the subject of industrial instrumentation signals, lets do some HART talk. Although HART technology evolved in the industry 3 era, but it was the turning point in the instrumentation field as it generated the first digital signals along with the conventional 4-20 mA signal used for the process value indication and control. So it can be also counted as the start of industry 4 era where digital signals ruled.

This became possible by frequency shift keying, where a digital signal is super imposed on a conventional 4-20 mA current signal, enabling extra information sharing. This was not possible in simple transducers with 4-20 mA signals. Data like the transmitter's tags, status, diagnostics and poll address used can be transmitted over the analog signal to the control station.

This breakthrough was later developed to enhance the transducers potential to send more data over the conventional signal and ultimately replaced the analog part when fieldbus communication took over which was a full duplex (two way) communication between the field device and the central control system (PLC or DCS). HART communication on the other hand is half duplex, which means that only one device can send or request the data at a time. It uses 1200 Hz or 2200 Hz frequency for communication. HART communicators which are hand held devices are used to communicate externally to any HART enabled transmitter. Multi drop configuration with host device is also a key factor of this mode of communication. Here a unique poll address is assigned to each instrument in the field for identification and the host device can request data from each individual device using this poll address. Up to fifteen devices can be connected in a loop.

In some cases built-in displays can also be used to fetch/alter the information in a transmitter. The ranges, URV and LRV and other parameters can be changed without the need to remove the transmitter from the loop, though it is always advisable (CAUTION) to take necessary precautions, like if the loop is in automatic control, it should be first taken into manual control. Also for safety instrumented systems, loop overrides and other necessary precautions should be taken. The HART enabled instruments made it simple and also the other calculations and additional parameters from the transducers are sent over to the host device.

The last but not the least is the fieldbus communication. It is a totally digital duplex communication between various field devices and the host devices simultaneously. The network can be built in various topologies including ring, star, branched or daisy chain. Each one depends on the required engineering solutions for a specific process. Some times combination of these topologies can also be used.

Initially the manufacturers of field instrumentation started building their own communication protocols for the sovereignty of their systems. However two major protocols which are now more in use in industry are the Profibus and Fieldbus. It is very important to consider the communication protocol when designing a system as all field communications among field devices and the central processing systems like a DCS should be able to communicate effectively.

The fieldbus can be a simple sensor bus network, where many different sensors and switches communicate with a host through a single point hub. Then we have a higher level known as device bus network where many input and output devices communicate together including but not limited to transmitters, I/P converters, VFD drives or motor control centers etc. Then a more complex network used in factories or process plant is the control bus network which links PLC or a DCS with all devices to have simple communication but individual controls over a fast network. Most of the time in refineries, chemical and petrochemical plants or other installations this is a preferred network.

An enterprise bus network which is the most complex of all these networks is used where supervisory control, SCADA and information in real time is required. It interconnects different systems and shares high level information among various systems connected together for information gathering and display in real time for administrative level use also, in addition to doing process control and other assigned functions

A fieldbus system has many advantages like faster communications, reduces cabling in the field and in the control room, more reliable and multi faceted communication with detailed diagnostics of field devices, status related information, ease of installation and maintenance to name a few. Being a fast communication, it also provides better process control for any industry or plant. Being flexible in nature, addition of devices without complex revamps is also possible.

The fieldbus can also be configured to control loop locally in field where a master device which is configured for this purpose weather be a fieldbus transmitter or part of a final control element can take over control as a fallback strategy if the communication to host system is disrupted or lost. This is a very big advantage for loops in critical processes, and can handle any crisis of the main system smoothly

A single connection point allows hundred of devices to be connected reducing the need of cables run from field to a control station. There can also be redundant cables for additional safety. Here field devices also communicate among the same ring and with the central control, making it a better choice for field instrumentation. Multi variable transmitters in the field also allow one transmitter to monitor and send information related to flow, pressure and temperature to the main controller, instead of installing three individual transmitters for each variable. This can be used specially in compensated flows for gas to have a volumetric calculated value from the field itself.

Though another way of communication which is the wireless networks is also now developed, but still to date, fieldbus technology remains on the top when we discuss the different industrial signals for instrumentation. It is also a good choice for smart grids and electrical systems as well where SCADA system is used in power distribution monitoring. The topic is quite exhaustive and may need a book of voluminous size to cover all aspects of fieldbus communications, but since we are here just doing a basic introduction so we will not expand the topic further, though in detailed articles in future it will be covered.

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(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)