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Beyond Wireless: The Other New Features of HART 7.0

Author : Michael Babb, Control Engineering Europe

12 July 2010

With wireless technology its most prominent advance, a great deal of interest has been generated in the user community for the latest version of HART. There are, however, other new features that give increased functionality for wired application process automation. We present here a short list of the six most important new features of HART 7.0.

ProComSol’s DevCom2000 Rev 4.0 Communicator Software for HART adds support for HART 7.0. The package provides full DD access to HART instrumentation for configuration and monitoring, and now it can be used for both wired and wireless devices.
ProComSol’s DevCom2000 Rev 4.0 Communicator Software for HART adds support for HART 7.0. The package provides full DD access to HART instrumentation for configuration and monitoring, and now it can be used for both wired and wireless devices.

Providers of HART-based instruments see new opportunities for the users of their technology.

‘Initially customers considered HART an optional technology which was specified - albeit rarely used in the day to day practice,’ said Frank Hils – Director Projects and Solutions for Endress+Hauser.

‘This has considerably changed over the last years. HART functionality has become a true functional requirement. Customers recognised HART technology’s ease of use and compatibility with existing 4-20mA engineering practices. More and more application fields were opened up by HART.

‘Just recently we see another user group showing interest in HART integration,’ he said.

‘Maintenance management demands integration of predictive condition monitoring information in their computerised maintenance management systems (CMMS). Hence, Endress+Hauser launched an initiative with SAP to integrate devices into both, the plant asset management systems at the customer’s site, e.g. SAP PM (plant maintenance), and the SAP Customer Service application SMP at Endress+Hauser. This integration solution will help the maintenance crews in process plants reduce plant downtime and reduce operating costs.

‘Intelligent measuring instruments will become fully enabled as smart devices which are able to create error tickets, notifications or alerts and to transfer processing data to the Service Management Application for installed base configuration and management as well for service related processes.

MACTek’s WirelessHART adapter—otherwise known as the ‘Bullet’—connects to any wired HART device to allow it to transmit wirelessly, while maintaining the 4-20 mA communications loop. Up to eight instruments can be multi-dropped with a single Bullet.
MACTek’s WirelessHART adapter—otherwise known as the ‘Bullet’—connects to any wired HART device to allow it to transmit wirelessly, while maintaining the 4-20 mA communications loop. Up to eight instruments can be multi-dropped with a single Bullet.

And just the reverse will happen: the Service Management Application is able to access remotely the device for problem diagnosis and resolution or operator support and to perform software updates. This initiative will considerably reduce the instrumentation maintenance management bill in the process industry.’

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We list here the six most important new features of HART 7.0.
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1. Report by exception
HART has been traditionally a ‘command and response’ system; HART instruments don’t transmit any values unless they’re requested to do so by the host. However, it was recognised that in some important cases, it is imperative that the instrument report a critical condition even though it has not been prompted to do so. For example, a level instrument monitoring the liquid level in a tank may observe the tank is going into an overfill condition and needs to warn the host about the situation.

This also allows the HART 7 enabled system to never miss an important process change or diagnostic alert because of a change that may have occurred and subsequently returned to normal within the normal poll time of the system.  This intelligent data publishing allow both time and value criteria to trigger data publication.

2. Synchronised sampling
This feature was implemented especially for control applications, where it is necessary to determine the order of execution of different elements of the system. For example, a control program 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 may also benefit from synchronised sampling. For example, a Coriolis meter may first read the flow value, then the temperature, then the pressure, to provide a compensated flow measurement. Before, the host system would have to issue separate commands for each part of the sequence; now the HART-enabled instrument does it. The compensation is also more accurate since all of the readings can be made as the same time.

3. Time or conditioned based alert
As noted previously, HART is a ‘command and response’ system; HART instruments don’t transmit any values unless they’re specifically requested to do so by the host. With HART 7.0 the instruments can be programmed to send values at specific time intervals, and, depending on the scale of the value, change the frequency and send values at different time intervals. For example, a temperature transmitter can be programmed to ‘send values every 10 minutes, but if the temperature exceeds 120 degrees, send the value every minute.’

4. Time stamp data
This feature is especially useful in analysing problems that have occurred. In this case, it is helpful to know exactly when various readings were taken from instruments in the system. All HART 7.0-enabled instruments add a ‘time stamp’ to the reading it sends to the host system so that the operator can read the log and determine what happened. For the time stamps to be accurate, all instruments involved must be synchronised. For wired devices, the host must set the clocks on each instrument, one at a time; for wireless systems, the gateway can perform this task on all instruments it is connected to simultaneously.

Yokogawa EJX110A pressure transmitter and Moore Industries HIM (HART Interface Module)
Yokogawa EJX110A pressure transmitter and Moore Industries HIM (HART Interface Module)

5. PV trends
Instead of sending process variable values one at a time, the HART 7.0 instrument can be programmed to store and transmit, for example, the last 12 values all at once. It the values are not changing dramatically and it is not necessary to have frequent updates, this method can save on communications overhead. Many of the field devices also have internal scan rates that are much faster than a system would normally need the data. So the trends can be setup to store information at a faster rate but for a shorter duration.

6. WirelessHART
Many would agree this is the most important part of HART 7.0. It is the basis of a mesh networking standard that has extended the HART concept to wireless networking, and was recently given an extensive test by BASF in Germany. See our article, ‘NAMUR Report: WirelessHART.


SIDEBAR: HART Diagnostics in Safety Applications

Evaluation International was asked to evaluate Moore Industries’ HART Interface Module (HIM), a ‘smart’ signal processor for deriving discrete switch points from analogue or HART signals. The tests were performed with a Yokogawa EJX pressure transmitter and a STA trip amplifier.

The instruments were found to perform exactly as the manufacturers’ data sheets specified. The pressure transmitter output various diagnostic information over HART and the HIM was able to read and act upon those data.  The EJX, HIM and STA formed a very reliable monitoring and trip circuit. The use of HART diagnostics offers the opportunity for instrument loops to achieve very low Probability of Failure on Demand (PFD) and high Safe Failure Fractions (SFFs).

For more on this Evaluation International project, see our article, Using HART To Improve Safe Failure Fraction (SFF) in Protective Measures 


 


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