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However, once you go wireless, you’re in a different world. Interoperability of devices on the same network is still an issue, but it pales in comparison to the problem of [ital]coexistence[end ital] between wireless devices on the same or adjacent networks. Now you have to decide if adding a device or another network node will work, work part of the time, or simply bring down the entire network. Wireless communications are such that you may not even know the answer until you turn the new device on.
The SP100 committee has developed a taxonomy to describe the industrial wireless landscape. The committee has defined five different classes of wireless automation, ranging in complexity from Class 5 to Class 0.
Class 5: Monitoring without immediate operational consequences
The ISA SP100 Call for Proposal states, "This class includes items without strong timeliness requirements. Some, like sequence-of-events logs, require high reliability; others, like reports of slowly changing information of low economic value, need not be so reliable, since loss of a few consecutive samples may be unimportant."
Many experts, like Harris (Hesh) Kagan, director of technology for new business at Invensys Process Systems, and president of the Wireless Industrial Networking Alliance (WINA), believe this will be the largest category of wireless applications in the near future. "The biggest bang for the buck, the low hanging fruit," he says, "is probably going to be in the area of maintenance and condition monitoring, followed very quickly by an opportunity for incremental process measurements."
These applications include all of the condition-monitoring applications that are being used to add incrementally to asset management, as well as all of the process measurements Kagan is talking about.
In the old days, the only process variable that was measured was the one that was being used to control the loop: flow, temperature, level, pressure, density, pH, conductivity, and so forth.
The advent of huge relational databases optimized for process control, such as Invensys' Wonderware historian, Honeywell's PHD, or OSIsoft's PI, make it possible to actually use information from process sensors not directly involved in loop control. The problem is getting those sensors to connect to the historian's server. Wiring up a bunch of extra sensors just so you can see process optimization trends is simply not going to get past the funding gate. Wire is expensive for sensors not directly needed to make the process go. Wireless sensors, on the other hand, have a great deal to offer these applications.
Class 4: Monitoring with short-term operational consequences
"This class includes high-limit and low-limit alarms and other information that might instigate further checking or dispatch of a maintenance technician," reads Call for Proposals. "Timeliness for this class of information is typically low (slow), and measured in minutes or even hours."
This class of applications includes monitoring the inventory level of tanks in tank farms, as well as alarms that are designed only to actuate if the process goes significantly out of whack, but that don’t instantly require a response. Alarms, however, are a special case. They can be of any class (see “Alarms” sidebar below).
Class 3: Open-loop control
This class includes actions where an operator, rather than a machine, 'closes the loop' between input and output. Such actions could include taking a unit offline when conditions so indicate. Timeliness for this class of action is human scale, measured in seconds to minutes.
Class 2: Closed-loop, supervisory control
"This class of closed-loop control usually has long time constants, with timeliness of communications measured in seconds to minutes," adds Call for Proposals. "Examples are batch unit and equipment selection." The quintessential examples of closed-loop, supervisory control are oilfield pipeline and water/wastewater SCADA systems. These have been wireless for two decades, already.
Class 1: Closed-loop, regulatory control
"This class includes motor and axis control as well as primary flow and pressure control," says SP100. "It is a matter of semantics," adds Kagan.
"We get a lot of head-nodding," he continues, "when we ask end users if they want to use wireless for closed-loop control. However, when you peel the layers of the onion away, what we find out is that they’re really talking about using wireless for incremental process measurement. They’re not looking for closed-loop control. There’s no one I’m aware of, an end user, who’s interested in wireless, at this point in their lives, for closed-loop control."
Class 0: Emergency action
This class includes safety-related actions that are critical to both personnel and plant. Most safety functions are, and will be, performed through dedicated wired networks to limit both failure modes and susceptibility to external events or attack. Examples are safety-interlock, emergency-shutdown, and fire control. However, Angela Summers, Ph.D., president of SIS-TECH consultants and chair of ISA's TR84.02 and TR84.04 committee says, "Wireless sensors have not demonstrated the reliability necessary for SIS applications in the petrochemical and refining industry.
Interoperability versus Coexistence
It isn't enough, however, to limit your thinking to just these classes because they don't define all of the wireless applications you'll find in your plant. If you'll go back to Figure 1, you can see a lot of different standards, and each standard is designed for one or more distinct applications. For example, RFID is becoming more and more prevalent, even in process plants. RFID is being used today for worker-location systems, inventory management, and even batch-management applications. So, when you’re producing specifications for your process plant’s wireless systems, you need to take into account the effects of the RFID transmitters in your wireless control system, and vice versa. Perimeter security is another significant wireless application, and workers in your plant, probably you too, are already using walkie-talkies and cell phones, as well as wireless laptops, PDAs, and other wireless-enabled tools (see “Wireless While We Work” sidebar below). So, what will happen when somebody keys a microphone right next to your wireless loop controller?
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