WirelessHART Does Control!
Reliable Control is Designed In, So Wired or Wireless – It’s the User’s Choice
Some still express doubts about taking WirelessHART beyond SCADA, condition monitoring, or diagnostics, and using it for process control. They fill pages, forums, and blogs with arguments and concerns about determinism, reliability, and security. Meanwhile, several noted experts have actually tested WirelessHART in laboratory trials, including direct comparisons to wired control.
The HART Communication Foundation technical note, Control with WirelessHART, explains that WirelessHART was designed specifically to support the wide range of process-industry use cases from simple monitoring to closed-loop control. The paper states, “Testing and field trials with wireless devices have demonstrated that the communication accuracy, stability, total performance, and reliability meets the demands of industrial process monitoring and control applications.
“Control application requirements for sampling intervals, jitter, and latency were specifically addressed and designed into the WirelessHART technology. In fact, control performance with WirelessHART can be comparable to that of a wired system using traditional field buses.”
Results on the commercial bioreactor were reported in May, 2009, by ISA Fellow Greg McMillan, a recognized authority on pH and advanced process control, in the Control Global article, “Is wireless process control ready for prime time?”.“I can envision a wireless world, but is wireless control ready for prime time in industrial processes?” McMillan asks. “Is wireless reliable, secure, and fast enough for process control? Will battery life be long enough? What is the motivation other than to be part of a neat new technology?”
For part of the answer, McMillan compared wired and WirelessHART for control of a single-use bioreactor (SUB) typically used for pilot or commercial production of high-value-added biopharmaceuticals. A 100-liter SUB skid with WirelessHART pH, temperature, and pressure transmitters, and a lab-optimized control system for research and development was used.
As a pH measurement expert, McMillan is impressed by the stability of wireless measurements. “The instrument suffering the most from ground problems for the past 50 years is the pH transmitter,” he writes. “I wish I had a dollar for every spike I have seen or users reported in a pH trend chart.”
The spike is usually caused by ground noise. In some cases, the spikes disappear when the electrode is removed and inserted in a beaker, indicating one side of the ground loop is through the process. In most cases, the ground noise is gone when the electrode is left in place and a lab meter is connected to its cable instead of the transmitter wiring. This indicates the other side of the ground loop is somehow established in the transmitter wiring.
Figure 1 is a trend chart of a WirelessHART transmitter and a wired conventional pH transmitter on a single-use bioreactor. A spike from ground noise appears in the wired pH, but not the wireless pH.
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Figure 1. A ground noise spike in a wired pH signal is not seen in the signal from a wireless pH transmitter.
“Most of the new measurement diagnostic research and development is going into wireless transmitters,” McMillan writes. “For example, wireless pH transmitters have improved glass impedance measurement, enabling better glass diagnostics, finer measurement resolution, and the setup for smart electrodes that have calibration history and electrode health embedded. If you want the latest advances in measurement technology, you need to consider wireless.”
In his article McMillan discusses security and reliability, including battery life concerns and how they relate to resolution, control band, and tuning requirements. Ultimately, the article concludes that with a comprehensive battery life management approach, exception reporting, and a secure, reliable self-organizing and optimizing network, wireless process control is ready for all but the fastest processes, provided the transmitter resolution settings are right for the application.
The above referenced HART technical note (Control with WirelessHART) thoroughly analyzes the technical details associated with sampling intervals, latency, and jitter: “Actual plant experience shows that average latency times are about 30 milliseconds. Using 30 milliseconds in our calculations reduces the loop execution time to less than 100 milliseconds and reduces the number of communications in the network. If it is important to reduce the delays introduced by multiple hops, additional network access points can be used.”
It also is possible to further reduce communication latency and address higher-speed control applications by using peer-to-peer communications between field devices. Running the control algorithm in a field device eliminates the need for wireless hops between that device and a gateway-resident algorithm. “Such an arrangement may also use less bandwidth, allowing for multiple control loops with minimal impact to overall bandwidth,” the technical note states.
An example in the technical note uses wireless network layouts that were “more complex than what experience has found in actual plant environments. The tech notes continues: “We also could have incorporated multiple access points to shorten communication paths, and allocated additional communication resources to further enhance the effectiveness of the WirelessHART network. And because all WirelessHART measurements include a timestamp, we could have used the timestamp in the control algorithm to further reduce the impact of any latency and jitter.”
The technical note concludes: “Even without utilizing any of these additional features, the example shows that the overall control performance of a typical WirelessHART network is comparable to that of traditional wired field buses. The WirelessHART protocol allows for secure, highly reliable, low-latency control with almost no impact on the bandwidth and absolutely no impact on process performance.”
In an interview for the February 2009 Automation World article, “Wireless Control in the Process Industries: Blasphemy or Common Sense?”, Ed Ladd, HART Communication Foundation Director of Technology Programs, acknowledges that while WirelessHART is designed for control applications, it is up to the user to decide whether or not wireless should be used for control (interview at www.automationworld.com/feature-6503).“My personal recommendation is to stick with noncritical control applications,” Ladd says. “If the process you are working with can kill someone or it is critical to your process, wire it, but not even that is infallible. The HART protocol does not eliminate one for the other. With HART, you can go wired, wireless, or both – users' choice.”