‘Get connected’ to the benefits of HART 7
22 February 2011
HART Communication has come a long way since its introduction over 20 years ago. The latest version brings a further host of innovative features, which users of older versions of the communication protocol may not be aware of. Suzanne Gill reports.
The HART Communication Protocol was first conceived over 20 years ago, based on the same idea that brought Caller ID to analogue telephones. Over the years it has undergone continued development and the latest version of the protocol, which was introduced in 2007, is HART 7.
Users of the HART Communication Protocol have known for some time that it will allow them to gain quick, easy visibility to devices in the field. However, these same users may not be aware of many of the benefits offered by today’s HART technology. We hope that this Supplement will go some way to update your HART knowledge and encourage you to take a closer look at this multi-vendor communication protocol, which today, boasts more than 30 million installed devices worldwide.
The HART Protocol was originally created to offer a globally-accepted standard for sending and receiving digital information across analogue wires between smart devices and control or monitoring systems because many automation networks were, and indeed still are, based on traditional 4-20mA analogue wiring.
It is important to remember that HART 7 is not just all about wireless - although it must be said that it is this feature that usually gets top billing. Although it is a big feature of the offering, and of course we will comment on it, we should first reflect on some of the other features of HART 7.
Commenting on the benefits of HART 7, Charles Larson, director of technology for Moore Industries in the US, said: “The introduction of HART 7 has improved the availability of additional data and diagnostic information from devices. It has also increased the awareness of users to the wealth of information in HART devices that can be used to increase plant efficiency.”
Jim Cobb, Marketing Manager, Temperature, Emerson Process Management, said: “HART 7 brings along some of the advancements of HART 6 which were never widely adopted. This would include the long 32-character tag that is often needed to provide a seamless reference through the various applications that may deal with a device including data sheets, loop drawings, calibration records, and configuration data. This is now becoming a commonly asked for feature.
“HART 7 brings a number of new capabilities to the HART protocol. In most cases to realise the benefits, the customer will need to have both field devices and a system that support these new features. One important aspect is that with HART 7, devices now have complete multi-variable support, breaking the traditional concept of HART being for only a 4-20mA based, single-measurement device.. This is obvious with the WirelessHART implementation because it is all-digital by nature, however now all of the variables in a wired HART device can have its own status providing validation for each value whether it is a direct measurement or is calculated.”
Many industry commentators believe that the most significant addition to HART 7 is the ‘Report by Exception’ feature. Traditionally a ‘command and response’ system, HART instruments would only transmit values when requested to do so by the host. With the introduction of HART 7, however, this changed. Now, you no longer have to send regular commands from the host to the device. If there is a process or device status change, the device will automatically advise you. In addition to the obvious benefits that this can offer, think also of the bandwidth savings of this feature. If predictive maintenance is your goal, then the Report by Exception feature is a godsend too.
Jim Cobb continues: “The ‘smart’ updating capability, together with Events reporting provide a much more reliable way for system to obtain data from a HART field device. No longer are they required to poll for data but are proactively sent data only when it is required. This means that data is not lost because of measurement changes that happen in-between polls or Events that happen and return to their original state quickly. The Event reporting includes an acknowledgement mechanism that ensures the system or host receives the message.”
HART 7 instruments can also be programmed to store and transmit a selected number of values together. This promises even further bandwidth savings in applications where the values would not be expected to change drastically. Instruments can also be programmed to send values at specific time intervals that can be varied, dependent on a set of changing parameters that can be set by the user.
Another useful feature provides a great way to analyse and review problems that have occurred. All HART 7-enabled instruments now also add a ‘time stamp’ to any readings sent to the host system. This allows engineers to better track when and possibly why a problem may have occurred.
MACTek believes that many of the HART 7 features are important for both wired and wireless applications. Thomas Holmes, President, MACTek Corporation, said: “Many of the new features enable wireless technology to be more efficient and practical. Report by exception and time stamped data are examples of features that contribute to wireless implementation. Now, a device does not have to be addressed 24/7 by the host if the process measurement is not constantly changing - freeing up host resources. This feature allows either a time-based or measurement-based criteria trigger to initiate data publication. The report by exception feature allows the device to report when it sees a change in measurement value greater than a pre-selected value or if the device sees an internal problem. Time stamped data provides a valuable tool when evaluating the sequence of events or analyzing a process problem.”
A popular feature of HART 7 is ‘Synchronised Sampling’, particularly in control applications where it is necessary to determine the order of execution of different elements of the system. Scenarios could include a control programme that may need to first read the sensor’s value, then execute the PID algorithm, and then send a signal to the valve. Complex gas flow measurements also make good use of this feature, where traditionally, the host system would have to issue separate commands for each part of the sequence. A HART 7-enabled instrument is able to do this itself.
Many of the field devices that are now available also have internal scan rates that are much faster than a system would normally need the data. This allows the trends to be setup to store information at a faster rate but for a shorter duration.
Moore Industries anticipates that the new features of HART 7 will result in it being used more frequently in multi-drop applications in the future. Charles Larson of Moore Industries said: “Devices such as our TCM multichannel temperature concentrator system have multiple channels and can have multiple devices on a single line. The squawk and find device features will be very useful for configuring devices in multi-drop applications. The new sub-device feature will also be useful on our multichannel devices so that their internal data can be more logically organised. Exception-based reporting will improve the responsiveness of multi-drop applications by reducing the data that must be transferred.
Last, but by no means least, we could not put together a list of HART 7 features without mentioning WirelessHART. The approval in March 2010 of WirelessHART as an international standard (IEC 62591 ED.1.0) was the catalyst for product development and rapid deployment. In common with (and because it forms part of) HART 7, WirelessHART is also interoperable and backwards compatible with existing user investments in HART devices, tools and systems.
Commenting on the addition of wireless to the Standard, Gerrit Lohmann, Product Manager Product Interface at Pepperl+Fuchs said: “I feel that WirelessHART boosts traditional wired HART, allowing features that were available since HART 5 or HART 6 to move further into focus and become more prominent to end users.”
On the benefits that wireless brings to HART, Gareth Johnston, Wireless Product Manager for ABB in the UK, said: "Traditional 4-20mA field instruments have been successfully using the HART protocol to assist during commissioning and scheduled maintenance work for many years. However, some 20 years since HART 5 became available we are still not making the best use of the remote access that HART offers. The result is that instrument information remains locked away for the vast majority of users. Adding remote access via a WirelessHART upgrade adapter provides the key to unlock this information.
"Adding WirelessHART to existing HART instruments in the field is simple. Users now only have to connect an upgrade adapter to the instrument, either at a spare gland or at a junction box, then enter the network ID and password to the adapter which will then automatically join the existing WirelessHART mesh network. The original 4-20mA signal remains intact, but the user is now able to remotely access the instrument to monitor process, maintenance and configuration information."
Why use these new features of HART 7? Accessing and using device and process information from intelligent field devices can have a significant positive impact on plant operations. Users have found that they can lower operating costs, improve plant availability, enhance maintenance practices, improve safety, reduce downtime and maximize installed assets by getting connected with the power of HART Communication.
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