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.

Industrial wireless instrumentation is rapidly becoming the technology of choice for many industrial applications. It is defined as the merger of wireless sensor network (WSN) technologies with industrial field instrumentation and it has become increasingly popular in the process industries. In wireless Instrumentation, an autonomous battery powered sensor system that integrates wireless technology to enables remote sensor values to be reported to a central hub. The central hub can be a Field Gateway that interfaces to a PLC or RTU systems;  or directly to Cloud SCADA host.

Wireless Instrumentation networks are used for monitoring of process or environment parameters when cabling is not feasible to connect the sensors due to cost or time constraints. Wireless instruments typically monitor a single process parameter or multiple process variables with a multi-variable transmitter.

COMPONENTS

Wireless Instrumentation is comprised of the Wireless Instrumentation Electronics and the Sensor. The Wireless Sensor Electronics Package contains the wireless communications equipment, battery system, and sensor interface housed in a weather proof package.

• Power source: In Wireless Instrumentation, devices are battery powered to provide rapid deployment with minimum installation costs.  These battery powered systems are designed to operate from 1 to 5 years between battery changes.
• Wireless communication technology: Unlicensed Radio (900 MHz, 2.4 GHz), Cellular, or Satellite technology is used in Wireless Instrumentation. The ISA100.11a standard for industrial wireless systems is mainly followed. Another standard used is Wireless HART which uses the same 2.4 GHz frequency band, but is more limited in use. 
  • Unlicensed radio is typically used to provide communications from local instruments to a field gateway for use by a PLC/RTU.
  • Cellular and Satellite technologies will direct communicate to a Cloud Host.
  • Licences Free Wireless Networks allow connectivity up to 1/4 miles from the sensor to field gateway. Range of the sensor can depend on the TX power of the wireless sensor, the frequency, and RF obstructions.
  • Cellular technologies are ideal for municipal water applications since there would be reliable cellular coverage. Satellite technologies will be deployed in remote industrial applications like oil and gas, pipelines, or mining where there may be no reliable terrestrial coverage.

• Instrumentation Signal Interfaces: The Wireless Instrumentation can be integrated to devices including sensor element or Wireless Sensor nodes that interface to standard  analog ( 4-20 ma, 1-5 VDC), digital (contact closures) instrumentation signal levels or data protocol (RS485 Modbus, Hart).
• Sensors: Wireless Instruments may be supplied complete with the sensing element. This is common for wireless instrumentation for pressure, temperature, and level measurements. The function of the wireless sensor is typically to report the process variable to the PLC/RTU or SCADA and not perform control logic.

Applications

Industrial wireless instrumentation is being applied to a wide variety of applications today, which includes:

• Monitoring applications that track the status of equipment or a process state, such as temperature or vibration.
• Alerts and alarms that track the status of a process state, such as temperature, or a safety state, such as hydrocarbon gas level. Exceptions are reported to an operator for appropriate action.
• Control applications such as controlling valve movements, motor operations etc.

Above shown is a control loop where a process variable (PV) is send from a wireless instrument to the controller and the controller sends back manipulated variable (MV) to the final control element.

Above mentioned are the most common areas of application of wireless instrumentation. But it doesn’t mean that this technology can’t be used in other areas of application such as automated safety instrumented function. Even though above mentioned applications the ones being currently practiced and this technology hasn’t made big inroads in safety applications. 

Advantages:

• Lower cost, especially when large numbers of instruments are installed.
• Manageability. When wired connections fail, they are typically complete failures that occur without notice. Wireless failures are usually transient, and those transient problems can mostly be avoided by preventative maintenance linked to wireless diagnostics.
• Flexibility. After a wireless system is installed, it is easy to add new wireless instruments and also to report more data from existing instruments using wireless adapters.
• Security. Wireless security extends to the field instrument and does not rely on physical security of the transmission medium. (Some fieldbus technologies assume that field wiring is secure and therefore have no cryptography on the field instrument.)
• Redundancy within a wireless network. Typically, a wired instrumentation relies on a single wire to each instrument, with various opportunities for failure. A well-designed wireless system has redundancy built in at all steps in the transmission chain without any failure-prone connectors. Field experience is demonstrating that a redundant wireless channel can be every bit as reliable as a non-redundant wired channel, particularly when wires are long and/or subjected to challenging conditions.
• Redundancy at the plant level. A wireless system can be used to add redundancy to wired reporting, with the same data reported through wired and wireless channels.

Disadvantages:

• Battery maintenance. Battery maintenance of wireless devices constitutes a factor that somewhat offsets wireless cost savings.
• Limited wireless instrumentation. Today, there is a limited range of available wireless instrumentation.
• Continuous sampling is required. In some cases, it is not technically feasible to sample and report process data continuously under battery power.
• Limited reporting rates. Wireless instruments and systems can be configured to support reporting as frequently as every second with a transmission latency of a fraction of a second. Sub-second reporting rates are not typically used for battery-powered instruments at this time.
• Spectrum management. Wireless instrumentation shares the radio spectrum with other systems and applications. Spectrum management generally needs to be considered when each new wireless system is installed, and should also be continuously monitored.

Future Scope

Industrial wireless instrumentation is widely becoming a norm nowadays. But there are still hesitancy towards this new technology platform. Through extensive theoretical analysis, laboratory experiments and pilot installations, it has become apparent that wireless instrumentation is ready for adoption at scale in non-critical monitoring applications. For control and safety systems, however, the currently available solutions have limitations which must be addressed through research and innovation before they are able to fulfil the more stringent requirements found in these types of applications.