Experts disagree about wireless sensor systems

A lively discussion of the current role of wireless in online condition monitoring.

By Paul Studebaker

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The experts we interviewed for our November, 2006 feature about online condition monitoring were quite calm and consistent until we started discussing the situation with wireless sensor systems. There, they break down into three camps:

  • Wireless sensors and data transfer systems are ready to revolutionize the cost-benefit equation for continuous monitoring – come and get them.
  • Wireless is a cost-effective alternative to wired networks for transmitting data or information, but wireless sensors are not yet ready for industrial applications – check back in a year or three.
  • Wireless is getting a lot of attention but due to serious battery life, radio frequency and data transmission load limitations, wireless sensors will remain suitable only for low-demand applications.

Why don’t all these smart people agree? One reason is because they have different criteria for what it takes to perform effective online condition monitoring. But, they also have very different assessments of the current state of the technology.

Wireless

The vision: A world unbound by wires

In a recent presentation on the future of predictive maintenance, Elsa Anzalone, account manager, Azima (www.azimainc.com) and Jack Nicholas, Jr., P.E., CMRP, senior technical advisor, Allied Reliability (www.alliedreliability.com) say wireless transmitters show promise of greatly reducing (by up to two-thirds) the cost to install and maintain predictive, condition monitoring systems. “To date, wireless sensors have been developed for vibration, speed, temperature, pressure, current, partial discharge activity and certain other key parameters,” say Nicholas and Anzalone. “Locally powered videocameras with wireless transmission capability may also be installed at remote stations for observation of critical gauges or areas of a facility.”

The majority opinion: Utopia is a ways off

Wireless sensors are available, “But today, there are serious limitations,” says Jack Dischner, president, Commtest (www.commtest.com). “They give you data when they’re programmed to wake up, not whenever you need it. Some have clipped frequency response, and some systems require repeaters sprinkled throughout the plant. The FCC limit of 0.3 Watts can mean slow data transmission.” 

While acknowledging that wireless can have a “dramatic effect” on bringing online systems to more “less critical” assets, “The current state-of-the-art for wireless sensors is perhaps not as close to utopia as some would have us believe,” says Steve Sabin, editor of GE's ORBIT magazine. Consider the following challenges that a wireless sensor supplier must address:

Data timing: Sensors are configured to collect and transmit data on a schedule, not based on what the machine is doing. Though this saves battery life, it creates a significant technical limitation of the devices themselves for applications involving variable-speed or variable-load machines. In these applications, the benefit of an online system is that it checks the machine state before testing to ensure repeatable test conditions for
trending, as well as identifying whether the operating state itself has changed. Without repeatable test conditions, monitoring is not always very useful.

Power: Transmitting the signal is only half the purpose of wires in a conventional sensor system. The other purpose is to supply power. Battery-powered sensors are an option, but if the number of battery-powered sensors in a plant proliferate, it isn’t hard to foresee thousands of batteries. Even with a long battery life of several years, that still amounts to thousands of batteries to change (and dispose of) each year. This takes the designer down even more challenging avenues such as self-powered technologies (solar, etc.).

Transmission integrity: Industrial plants are extremely noisy environments as far as the electromagnetic spectrum is concerned. Rarely is there a clear line-of-sight between sensors and the location of the host system (which might be in a control room). The reliability of transmission in such an environment creates very difficult challenges, particularly when it has to be balanced with cost-per-point, power consumption and other issues.

Licensing: Because wireless sensors use radio signals, there is the issue of licensing in multiple countries. There is no universal standard for radio frequency usage or licensing.

Transmission range. Peer-to-peer approaches can be used as part of a mesh architecture, where each device is capable of sending and receiving from every other device. Hence, each device doesn’t need a range of thousands of feet, it only needs to be able to communicate with its nearest neighbor, which will in turn relay the message to its nearest neighbor, etc. However, this requires sensors to be turned on more frequently, thus requiring more power.

Cost per point. Sensor costs are high and the user is limited to choosing
the sensor that has a transmitter, not necessarily the best sensor for his application. Embedding lots of signal processing and memory in each sensor raises the cost per point, but means the sensor can store information if communications are temporarily interrupted.

“Will we ever have a sensor that combines low total installed cost, small size, excellent transmission integrity, long battery life or battery-free operation, universal compliance with the RF licensing authorities globally -- in essence, all the benefits of hardwired sensors without the wires? It’s hard to say,” says Sabin. “What we can say with assurance is that it isn’t here today, and it isn’t likely to appear anytime soon.

“Are many people working on this? Yes, including ourselves. Is progress being made? Yes. Will we see wireless sensors in greater use in the not-to-distant future? Yes. Will these sensors possess all the ideal characteristics described above? No, we don’t think so. Like all technologies, we’ll see such sensors deployed where the benefits outweigh the costs, and there will be places where the cost of running wires eclipses the inconvenience of changing batteries, periodic signal interruption, the possible need for repeater antennas, etc.

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