Integrating HART data from smart devices
HART calling! Your field devices and actuators want to tell you more of what is happening in the plant, all you have to do is listen.
There is a resource present in your plant that can help facilitate asset management, but if you're like most companies, you aren't using it, or you aren't using it to the extent that you could. That resource is HART Communication, and many, if not most of the field instruments deployed in your process probably have it. This article will discuss how HART works, and the variety of ways you can integrate HART data from process sensors and actuators in your plant.
Most companies have hand-held communicators that use HART technology, and that becomes the main method for setting up new field devices or changing configuration settings in the plant. The underused aspect is the variety of ways that new and installed instruments can communicate with the automation system, delivering the main process variable, additional variables, and diagnostic information. In fact, HART can be the main technical enabler for a device management program.
What HART can do
The information available via HART can tell a great deal about individual field devices, how they're performing, and the networks serving them. The diagnostic information can warn you if there are individual device problems, loop current faults, or if a device needs maintenance. It can help ensure the accuracy of process variable data, that individual instruments are ranged correctly, and if there are deviations between the analog and digital communications.
But beyond diagnostics, HART Communication can also deliver the secondary process variables available on most smart devices already installed in your plant. Any instrument purchased over the last 10+ years is probably ready to send that information if you have the means to integrate it in your control and asset management systems.
There are two methods for transactions between the host and device. The most common is a master/slave approach where the host requests information from the slave device. The device sends information only when requested by a host such as a DCS, PLC, or asset management system.
HART-enabled devices can also be configured to publish process data in a burst mode. This only works for specific process data commands, but the host can still request other information as needed.
Choosing master-slave or burst will be driven by the needs of the process and criticality of a specific process variable. Each device can use either approach as the situation dictates.
Multi-drop: HART as a fieldbus
In normal applications the primary process variable is transmitted via the 4-20 mA analog signal, and the additional variables are carried within the superimposed digital information. This may be the traditional method, but it requires a cable for every device. HART can transmit the primary variable with the digital information if desired, making each field device entirely digital.
When using that method, multiple devices can be connected via a single cable wired in parallel, similar to a fieldbus, reducing the amount of cabling. Up to 15 devices can be connected on one segment, using a handheld communicator or PC-based configuration tool to assign the poll address for each. The 4-20 mA signal is fixed at a low value, typically 4 mA, so the loop can carry power to each device. Communication follows a master/slave pattern with the host polling each device such that it can send process variables and diagnostic information.
This approach does not provide the constant updating of the primary process variable as the normal 4-20 mA loop would, but if a small amount of latency can be tolerated, it can deliver a cost-effective and reliable stream of process data.
More than one variable
Most field instruments available today are actually multi-variable devices, even if this is not an obvious feature. Pressure sensors, flowmeters, and other instruments gather additional information to correct the primary variable or to monitor another aspect of performance. For example, many types of pressure sensors need temperature data to calculate the pressure value. This temperature or other secondary information can be sent to the host via HART Communication.
While these secondary variables (SV) might not be used for critical control, they are available and can help fill in gaps of information coverage without additional intrusions into the process or buying more hardware. A typical use case example is ensuring that a device is not frozen or overheated. Many host systems can be set up to access this data, with the best way being natively HART-enabled I/O at the host system.
This method allows delivery of information easily and as quickly as possible with effortless integration to control and maintenance platforms. All variables are available from every device using a minimum of cabling and hardware. Unfortunately, many systems running in process plants were installed before HART-enabled I/O cards were common, so finding smart devices deployed in conventional 4-20 mA I/O card situations is typical. Users and system vendors have created many work-around approaches to fill this gap.
A HART loop converter is an individual modem that can read the HART data lifted off an individual 4-20 mA loop. Loop converters are typically designed to access a secondary process variable and convert the digital signal representing this variable to an additional 4-20 mA signal.
Such a unit can send the signal to a larger automation system, and/or convert the data and display it in appropriate engineering units. Depending on the sophistication of the device, it can be programmed with relay outputs for alarms or other functions.
Individual converters are useful when a small group of devices need to be addressed, but when larger numbers are required, there are better ways to deal with the situation.
For an installation where a user wants to extract HART data from a large number of field devices but there are no native HART I/O cards installed in the process automation system, the typical approach is to use a HART multiplexer. These systems come in a variety of configurations and from a variety of manufacturers, but they have some basic characteristics that are common.
Multiplexers contain multiple HART modems that are ganged together such that they can extract and convert the digital data from a device while not interfering with the normal 4-20 mA loop signal. The I/O of the existing host device does not see a difference in the 4-20 mA signal, and it can continue to control and monitor the process just as it always has.
This approach is typically retrofitted to an existing control system and field wiring. The multiplexer takes the information from however many devices it handles, and typically sends the data to some sort of asset management system via an RS485 serial bus, Modbus TCP, Ethernet link, or the new HART-IP. Communication is bi-directional so the asset management system can both read information from a field device and remotely access and read its configuration.
The downside of working with multiplexers is that they can be complicated to install, since each individual field device needs to be connected. This effect can be minimized by making connections where the cables have already been brought together in one place like marshaling cabinets. Latency is also a factor since multiple inputs have to share one HART modem for given number of inputs. With careful planning and network management, multiplexers can still be a very effective and economical way to handle large deployments.
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