The majority of smart field devices installed worldwide today are HART enabled. It is installed in more than 30 million devices worldwide. HART (Highway Addressable Remote Transducer Protocol) is the global standard for sending and receiving digital information across analog wires between smart devices and control or monitoring system. More specifically, HART is a bi-directional communication protocol that provides data access between intelligent field instruments and host systems. A host can be any software application from technician’s hand-held device or laptop to a plant’s process control, asset management, safety or other system using any control platform. Most automation networks in operation today are based on traditional 4-20mA analog wiring, HART technology serves a critical role because the digital information is simultaneously communicated with the 4-20 ma signal. HART (Highway Addressable Remote Transducer) was developed by Fisher- Rosemount to retrofit 4-20mA current loop transducers with digital data communication. HART uses Bell 202 modem technology. HART modulates the 4-20mA current with a low-level frequency-shift-keyed (FSK) sine-wave signal, without affecting the average analog signal. HART uses low frequencies(1200Hz and 2200Hz) to superimpose the digital signal to the current loop.
A fieldbus trunk or segment—either FOUNDATION fieldbus H1 or PROFIBUS PA—is a single twisted pair wire carrying both a digital signal and DC power that connects up to 32 fieldbus devices(temperature, flow, level and pressure transmitters, smart valves, actuators, etc.) to a DCS or similar control system. Most devices are two-wire bus-powered units requiring 10 to 20mA, but it is also possible to have 4-wire fieldbus devices, typically where a device has a particularly high current draw. The fieldbus segment begins at an interface device at the control system. On a FOUNDATION fieldbus H1 (FF) system, the interface is called an H1 card; on a PROFIBUS PA system (PA), it is a PROFIBUS DP/PA segment coupler. In terms of signal wiring and power requirements for the segment, FF and PA are identical. Minimum device operating voltage of 9V.Maximum bus voltage of 32V.Maximum cable length of 1900m (shielded twisted pair).The DC power required by the bus is normally sourced through a fieldbus power supply or “power conditioner” which prevents the high frequency communications signal from being shorted out by the DC voltage regulators. Typical power conditioners make 350 to 500mA available on the bus
and usually incorporate isolation to prevent segment-to-segment cross talk. For PA, the “segment coupler” usually incorporates the power conditioning component. In FF segments, the power conditioners are separate from the H1 interface card and are often installed in redundant pairs to improve the overall reliability.
Integrating a HART device to a Fieldbus network using a gateway
There are a few products on the markets that convert HART to Fieldbus. The converter or gateway will work as a master for the HART signal from the field devices and as a slave to the Fieldbus network, sending process values and diagnostics to your control system. 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. Devices can also be configured to publish process data in 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 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 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 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, flow meters, 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 a temperature data to compensate the pressure value. This temperature or other secondary information can be sent to the host via HART. While these secondary variables 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. 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. Loop Converters a HART loop converter is an individual modem that can read the HART data lifted off an individual loop. Loop converters are typically designed to access a secondary process variable and convert the digital signal representing this variable to a 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. Multiplexers 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 process automation system, the typical approach is 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, and it can continue to regulate 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 or Ethernet link. Communication is bi-directional so the asset management system can both read information from a field device and set 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 networks have to share one HART modem. With careful planning and network management, multiplexers can still be a very effective and economical way to handle large deployments. Handhelds and Single Modems In many situations, a maintenance person or instrumentation engineer may need to configure or check the diagnostic information from an individual field device. There are many types of single HART modems and handheld communicators that can provide an interface easily and inexpensively. Such devices are not limited by manufacturer and can communicate with any instrument or actuator, thanks to the interoperability built into the HART protocol. Small single modems can be inserted into a loop whenever needed in the plant or maintenance shop. Such devices can communicate with a laptop or other host system using USB, RS232, or even Bluetooth wireless. Today’s handheld communicators and calibration devices are hugely versatile and can contain device descriptors for hundreds of devices. The maintenance technician can connect the unit to a field device and it can call up the relevant information from its memory. If necessary, the configuration can be changed or the diagnostic data retained for downloading back at the maintenance shop. While these individual communication devices can be very useful, they have the inescapable limitation that they require a technician to be involved and can only communicate with one device at a time. By contrast, a fully HART-enabled I/O system can perform all of the above functions on any field instrument or actuator at any time from the control room or maintenance shop. Work Practices: Commissioning and Maintenance When new field devices are going to be installed or brought in for maintenance in the shop, HART can be the means to get them configured properly and tested before returning to operation. The ability to check and verify performance before installation can save an enormous amount of time compared to installing and then removing sensors that aren’t ready for use. During the commission phase, HART information can be used to reduce the commissioning time. Once a new device has had its HART tag name assigned and ranged for that application in the process, it can be ready for the next round of checks where HART information can be provide ease during the checks: • Verification of the device’s location, physically and I/O; Calibration;Configuration and loop check; Alarm and interlock validation; and, Online operation validation. Similarly, field instruments and smart valve controllers can be checked in the shop before returning to operation. A well-developed maintenance program can prescribe a routine of tests tailored to an individual device or group via HART communication to check whatever attributes are most critical. Working through this procedure avoids problems and helps train new technicians. Once back in place, transmitters and valves can be given final verification before resuming the process. Or, tests can be performed in-situ rather than in the shop. HART can help test a variety of critical attributes, including: Verify proper tag and location; Verify wiring and power supply; Check signal integrity and grounding; Re-zero, check range and span calibration; Verify analog trim for DCS output; Send simulated process variable to verify DCS reading; Capture new valve signature for baseline; Set alarms and security configuration; and, Configure and calibrate additional process variables.