March/April 2012
Automation Basics

HART makes troubleshooting easy

From predictive maintenance to remote diagnostics, discover how easy it can be to make HART work for you

By Shannon Foos and Brian Root

Based on the same technology used to develop Caller ID, the HART Protocol was developed in the mid-1980s for use with field instrumentation. Just like those answering the phone sought a way to identify callers, manufacturers also sought identification information about the device connected on the other end of the wire. Plant engineers also wanted smart field instruments that allowed them to move away from hand-held devices and instead quickly identify problematic conditions from the control room.

HART history

The ability to remotely manage assets helped drive the initial adoption of smart instrumentation, but support slowed as the technology arrived at a time when knowledgeable plant personnel were retiring in waves. Many plant managers initially thought increased intelligence capabilities could help mitigate the brain drain. But they soon found installing new knowledge management systems became too big of a project when understaffed, and many opted to train new employees instead.

Now, today's new generation of data-savvy engineers want another chance at gaining manufacturing intelligence from field instrumentation. In addition, plant managers have cost-constraint pressures from global competition and need systems to become more efficient and improve productivity. These engineers are finding HART is one of the simplest ways to take advantage of the increased intelligence in process instrumentation. 

No longer is it necessary for maintenance engineers to travel to the field to access device information for first level troubleshooting and then make a second trip to the device with the necessary tools or replacement parts. When an instrument faults, operators can identify the source of failure immediately and easily distinguish between a failure that is process related (such as an empty pipe), installation related (such as a loose wire), or a failure that requires device replacement. This is made possible by HART's ability to collect instrument data into a host system. And in many cases, operators can utilize HART to proactively troubleshoot malfunctions before they bring an entire machine down.

How Hart Works  

How HART works

Highway Accessible Remote Transducer Protocol, or HART Protocol, is a global standard for communicating digital information across analog wires that connect a smart field device and a control or monitoring system. Communication occurs using standard instrumentation grade wire and using standard wiring and termination practices. HART provides two simultaneous communication channels through the 4-20mA signal that communicates the primary measured value and the digital signal that communicates more detailed information such as device status, diagnostics, and additional measured values.

HART data is organized into three different classifications-universal, common practice, and device-specific. All devices using HART Protocol must recognize and support the universal commands, as universal commands provide access to information useful in normal operations. Examples of universal commands include read primary variables and units, read manufacturer and device type, read current output and percent of range, and read sensor serial number and limits.

Common practice commands provide functions implemented by many, but not necessarily all, HART communication devices. The HART specifications recommend devices to support these commands when applicable. Examples of common practice commands include read a selection of up to four dynamic variables, write damping time constant, write transmitter range, and set fixed output current and perform self-test.

Device-specific commands represent functions unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on device-specific commands is available from device manufacturers. Examples of device-specific commands include read or write sensor type, start, stop or clear totalizer, and read or write alarm relay set point.

Whether out in the field or sitting in the control room, HART architecture allows engineers and operators to collect data using a variety of different host systems. From a hand-held device or laptop, to a plant's process control platform or asset management system, engineers and operators can quickly collect data in whichever software application is most appropriate for their needs. When collecting data from smart instruments, HART enables a variety of host applications to provide status information that not only indicates the measurement value, but the quality of that measurement as well. 

Because a large majority of today's automation networks are still based on traditional 4-20mA analog wiring, HART technology serves a crucial role by adding valuable digital information to the 4-20mA signal. Luckily, the majority of smart field devices installed today are already HART enabled, allowing users to easily "upgrade" to more information from the same device, once the automation system architecture is HART-enabled.

Troubleshooting with Hart  

Troubleshooting with HART

From increasing plant availability to improving regulatory compliance, HART's ability to provide engineers with quick and easy visibility into devices has a variety of benefits. One of the main troubleshooting advantages is HART's ability to decrease the time it takes to identify and resolve problems. This not only reduces maintenance costs, but it can improve safety for many industries where the most dangerous part of the job is venturing into the field to check on a device.

HART speeds troubleshooting in a variety of ways, starting by reducing the need for operators and engineers to cross tabulate information. Because HART bridges analog and digital data, it provides plant personnel with the ability to consistently check alarms. For example, an operator reacting to an alarm can confirm the fault's legitimacy by ensuring the analog and digital values match. This negates the need for the operator to look for a faulty transmitter or any other hidden causes that might trigger the same alarm. This consistency check is a simple way for plant personnel to confirm they are seeing the entire picture when analyzing a fault.

For issues related to the process and not the instrumentation, gaining a clear picture upfront saves time and allows the appropriate personnel to address the issue sooner. For example, plant operators notice an erratic flowmeter reading and notify the instrumentation team. Before the instrumentation team heads to the field, they look at the diagnostics and notice an "Empty Pipe" condition. Rather than going directly to the field, the instrumentation team engages the process engineers to assist in determining what is causing the lack of flow.

In the event that a maintenance engineer must travel into the field to address a fault, gaining a clearer picture upfront allows for proactive troubleshooting. Armed with information that accurately tells engineers what is wrong with an instrument, they can gather all necessary tools and spare parts before heading into the field. This ultimately helps save time and money and assists with safety for engineers as they are not entering potentially dangerous field settings more often than necessary.

In some situations, engineers only need to know whether to bring a tool belt or a replacement device into the field. The "sensor open circuit" diagnostic, for example, simply requires a screwdriver, ultimately saving the engineer time from gathering unnecessary tools and replacement devices.

To further aid troubleshooting, today's HART users leverage the technology to help bring meaningful data to the operator screen. Rather than a set of numbers popping up on the HMI when an alarm is triggered, advanced HART implementations from suppliers can show a detailed description of what is causing the problem. Therefore, there is no need for operators to cross reference an alarm code with a fault list in order to understand what the problem might be, which in turn saves time in critical situations where every second counts.

HART today and tomorrow

Plants around the world continue choosing HART communication because of its easy integration with smart instrumentation, feature-rich offerings, and cost-saving advantages. These benefits will only be enhanced as device integration and asset management technologies continue to flourish. Thanks to its flexibility and continuous updates that keep HART protocol in line with the latest instrumentation updates, it is safe to expect HART will remain a significant communications technology for years to come.

ABOUT THE AUTHORS

Shannon R. Foos, P.E. (srfoos@ra.rockwell.com), Segment Manager with the Information Software and Process Business at Rockwell Automation Inc., has more than 15 years of experience in process automation. Brian R. Root (brroot@ra.rockwell.com), Senior Application Engineer with the Information Software and Process Business at Rockwell Automation Inc., has more than 20 years of experience in the industrial and process controls industry.

Why smart instruments?

Instrumentation vendors are changing the playing field. All digital field devices provide:

  • Increased resolution and accuracy
  • Remote configuration
  • Better/remote diagnostics
  • Automatic and remote reconfiguration
  • More information about the device and the process

Smart instrumentation users receive the best of both worlds:

  • The analog signal provides compatibility and speed of the primary measurement variable.
  • The digital signal superimposed on the analog wave provides bi-directional communication of diagnostic and multivariable information transferred.