Wireless takes control for industrial applications
Wireless sensing is being used worldwide in tens of thousands of process industry applications, and manufacturers are reporting that wireless networks are in high demand and a growing segment within the industry. For connecting instrumentation to control and monitoring systems in the process industries WirelessHART from the FieldComm Group and ISA100 from The Instrumentation, Systems, and Automation Society (ISA) are widely used.
While wireless is primarily used for monitoring equipment and diagnosing problems, current trends indicate that wider use of wireless control is just around the corner, especially with new tools to make networks easier to set up and use.
Wireless equipment benefits
A primary reason for using wireless is low cost. In comparison, a wireless transmitter can be installed and connected to the control system at a fraction of the cost of using a wired transmitter. For about one fifth to one third of the cost of a wired transmitter, a wireless transmitter can be installed and connected to the control system. ("Information from wireless transmitters saves energy," Control Engineering, July 2016, p.23.)
Another major reason for using wireless is due to easy installation, particularly for battery-powered wireless transmitters. Wireless transmitters don’t need signal wiring back to a corresponding wireless gateway, which is in turn hardwired to one or more control and monitoring systems. Battery-powered units don’t require power wiring, so they can be installed virtually anywhere to monitor just about anything.
This means that equipment such as steam traps (see Figure 1), heat exchangers, pumps, compressors, and pressure relief valves easily can be monitored wirelessly. In many cases, these types of equipment are too expensive to monitor with wired instrumentation, often because they are in a location where a wired infrastructure is unavailable.
Wireless works in truly remote locations, such as offshore platforms, wellheads, lift stations, pumping stations, pipelines, tank farms, and oil terminals. In these situations, the wireless instruments are connected to a gateway, which can be hardwired to a local control and monitoring system. The gateway also can be hardwired to a long-range wireless transmission system.
Using its attributes of low cost and simple installation, wireless helps fulfill the needs of plants to optimize operations, save energy, increase efficiency and reduce maintenance costs.
For example, wireless sensors and specialized software (see Figure 2) can detect steam trap problems. These systems diagnose the problem, determine the probable cause, and notify maintenance that a device is failing or operating erratically. When the maintenance person goes into the plant to troubleshoot, he or she will "know before they go." Similarly, because WirelessHART provides access to all of the diagnostic and status conditions available in HART instrumentation, plant engineers and technicians can detect pending problems before the problems shut down a process.
To date, wireless has almost exclusively been used for monitoring and maintenance purposes, primarily because of update rates. To preserve battery life, most plants set the update time of a wireless transmitter to a relatively slow rate compared to wired transmitters. With an 8-second update rate, a battery can last from 5 to 7 years. Unfortunately, an 8-second update rate is not suitable to control fast processes, such as liquid or gas flow.
Despite the update rate, wireless control has already started for fast processes. For example, wireless control is being used on remote tanks to prevent overflows. When wireless level transmitters detect a potential overfill condition, the control system sends a signal to a valve actuator fitted with a wireless adapter to shut off flow. In Mexico, work is underway to shut down remote pumps wirelessly.
Recent trends and developments are changing, making wireless more suitable for real-time control in certain applications.
New developments in wireless controls
Various advances and developments are coming together to make wireless control more feasible.
Energy harvesting converts energy found in a process plant to electrical power that can be used by wireless devices. One energy harvesting device designed to power wireless instrumentation is the recently released a thermal harvester (see Figure 3). Thermal harvesters generate electricity from temperature differences between industrial heat sources and ambient air.
Heat sources are found throughout industrial facilities on rotating equipment-such as motors, fans, compressors, and pumps—as well as pipes carrying steam and other heated process fluids. Configurations are available for a wide range of potential heat sources including flat or cylindrical surfaces at temperatures up to 450°C (845°F). Because thermal energy harvesting solutions work with a battery, they have the additional security of redundant power; that is, the battery takes over in case there’s a disruption of heat to the energy harvester.
Table 1 shows the battery life (in years) of a wireless pressure transmitter with energy harvesting. With as little as a 40°C (72°F) heat differential, an energy harvester enables operation at a 1-second update rate without using back-up battery power for over 10 years.
The next trend is the increasing availability of wireless adapters that allow legacy instrumentation and plant equipment to connect to a wireless network. Several instrument vendors offer adapters that convert a standard 4-20mA transmitter to WirelessHART or ISA100.
Hundreds of thousands of 4-20 mA instruments enabled with HART communications are installed in process plants worldwide, but many plants have yet to exploit all of HART capabilities for each instrument. Many plants use handheld HART interfaces to detect the instrument’s status and diagnostic information manually, but this information is unavailable to the control room.
Interfacing a legacy instrument into a control system often requires installation of a HART modem, power-wiring to the modem, and software in the control system to extract the data. Some legacy control systems can’t accommodate HART data without extensive custom coding. WirelessHART protocol adapters address these issues by converting the HART protocol to Modbus RTU or one of the Ethernet protocols.
For example, the Monroe County waste treatment plant in Rochester, N.Y., relied on operator rounds to take flow meter readings for three lines. The flowmeters were located in a separate building in the mile-long plant, and were too far away from the main control system for hardwiring. No conduit was available, and the existing input/output (I/O) rack of the local control system was full. So operators would travel to the building once an hour, write down the instantaneous flow readings, and manually calculate the total of the flows.
The plant purchased three wireless adapters for the three magnetic flow meters that needed to be brought online and one wireless gateway that provided an infrastructure for the wireless network. Now the plant has instantaneous flow information that operators can monitor from the control console.
Wireless adapters also can be installed on control devices. On control valves, for example, a wireless adapter can be installed on a digital valve positioner for a wireless closed-loop control. A similar setup could be feasible for control of a variable-speed motor drive.
Another interesting development is a modified proportional-integral-derivative algorithm that can be used to control processes with a response time of 30 seconds or less, such as a distillation column. The modified programming modifies the PID algorithm to automatically account for slower update times from wireless sensors. Research at the University of Texas shows that real-time control of a dividing wall distillation column using the advanced PID programming with wireless transmitters is comparable to control achieved using PID control with wired transmitters.
Wireless control is coming
All of the pieces are in place to apply wireless control for many applications. Figure 4 shows a typical wireless system with wireless sensors, access points, and a mesh network that connects them to the control system.
Wireless transmitters can operate with an update rate of 1 to 2 seconds, which is fast enough for most process control applications. The wireless system’s network manager can schedule whatever measurement update rates are needed from the wireless sensors, so critical sensor data can get to the control system quickly.
If the control system uses fieldbus or 4-20mA connections to control devices, there’s no problem. However, if control signals are being transmitted via wireless, the problem is that network managers cannot schedule wireless output transmit times the same way wireless inputs are scheduled. It can take anywhere from 10 seconds to several minutes for a wireless control signal to reach a device.
There are developments in the works for a network manager that allows the scheduling of wireless output signals to complete the last piece of the wireless control puzzle.
The wireless advantages of low-cost and simple installation help plants increase efficiency, optimize operations, reduce maintenance costs, and find small problems to assist in preventing or minimizing downtime.
Until now, because of slow response times and battery life issues, wireless transmitters have been used only for monitoring, not control. Recent developments—including energy harvesting, wireless adapters, and an advanced control algorithm—mean that wireless control is right around the corner for virtually any application.
Melissa Stiegler is the director of wireless product management at Emerson Automation Solutions. Edited by Emily Guenther, associate content manager, Control Engineering, CFE Media, email@example.com.
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About the Author
Melissa Stiegler is the director of wireless product management for Emerson Automation Solutions, responsible for the management for Emerson’s pervasive sensing and wireless portfolio. Stiegler received a Master’s in business administration from the University of Minnesota-Carlson School of Management and a Bachelor of Science degree in Electrical Engineering from the University of Minnesota-Twin Cities. Stiegler is an active participant in the Society of Women Engineers (SWE), as well as numerous other initiatives that promote science, technology, engineering and math (STEM) education to young people. She also is involved in Women in Engineering to enhance the diversity of ideas and approaches which drive business growth and financial results.
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