Wireless problems have straightforward solutions

Applications can be technically complex, but wireless technologies are maturing and the parameters for successful installations are well defined

By Jim Montague, executive editor, Control

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Wireless can be a huge help in process applications, saving cable and bringing in signals and data that could never be gathered before. However, there are so many different wireless devices, coverage topologies, antenna configurations, frequencies, communication protocols and other requirements that it can be daunting to pick the right solutions for individual sites and applications.

"Wireless is a major part of our everyday lives at home and work, but many industries have been slower to adopt it," said Patrick Harris, global sales manager for industrial wireless at Eaton. "So, there's still a lot of mystery, black box magic and confusion about wireless, and questions about how to apply it to get the biggest bang for your buck."

To give users some timely wireless know-how, Harris presented "Industrial Wireless—Be Careful What You Ask For!" on Sept. 1 at the NovaTech Futureproof Automation User Conference 2015 in New Orleans. Eaton's Elpro wireless products include remote I/O, PLC gateways, Ethernet components, serial devices, and network management software. "In short, we make boxes that replace cables, and we can talk to any device in the field or I/O in the cabinet," said Harris. "And, wireless has endless application possibilities, including hybrid applications, cellular modem backbone options, and device-to-device tunneling for security."

Some of the most popular wireless applications are:

  • Vibration and motor performance monitoring,
  • 3G cellular communications,
  • Eyewash and safety shower deployment notifications,
  • Long-haul Ethernet and serial data transfers,
  • Tank farm level and custody transfer monitoring,
  • Emissions monitoring and regulatory compliance reporting,
  • Gateway interconnection of legacy PLCs,
  • Video surveillance for mobile equipment and personnel tracking,
  • Gas leak detection and remote cathodic prediction monitoring, and
  • Remote field device monitoring and control.

"Two-way communications between remote pumps and control rooms using digital and analog signals can monitor pump flows and pressures, and allow control rooms to remotely start and stop pumps," said Harris. "Wireless can also transmit video from IP-based Ethernet cameras positioned around the perimeter of a facility, and deliver it to the security office. We recently helped with a wireless project for Union Gas, which had hundreds of gas leak monitors over 20 miles, and decided to use a mesh wireless network. We also worked with Pemex on an Ethernet backhaul for gathered data from its wells and pipelines covering hundreds of kilometers."

To design, procure, implement and maintain wireless solutions for process applications, Harris explained that the basic environmental considerations include:

  • Frequency and the fact that available distance decreases proportionally as frequency increases;
  • Receiver sensitivity that's measured by antenna gain and cable loss;
  • Background noise or radio frequency (RF) interference;
  • Transmitter power that's also determined by antenna gain and cable loss;
  • Attenuation of radio signal, which is affected by the height of antennas and obstructions; and
  • Other factors, including atmospherics and ground mineralization.

Along with tradeoffs between distance and data rates, and picking a suitable frequency, Harris says users should also pick the most appropriate of the two main spread-spectrum types. The first is direct-sequence spread spectrum (DSSS), which spreads data packets over a wide band, effectively transmitting each bit on many channels. DSSS allows higher data rates (>1 Mbps), but is vulnerable to interference. The second is frequency-hopping spread spectrum (FHSS), which changes frequency after each data packet. FHSS has slower data rates (115.2 Kbd), but it's robust and less vulnerable to interference.

"It all comes down to physics and wavelengths, and determining what's the best frequency for my application based on what type of data I want to send, at what speed, over what distance, and what obstacles and interferences need to be overcome," said Harris. “Also, lower frequencies translate to fatter Fresnel zones. These are football-shaped fields between the transmitter and receiver, and so lower frequencies and wider radius means a signal might be able to get over a taller object."

Harris added there are two main antenna-type choices—omnidirectional and Yagi directional. Omnidirectional antennas are mounted vertically; radiate energy at 360° in a mostly horizontal plane; use multiple transmitters and receivers in different directions; and are good for industrial plants. Yagi directional antennas radiate energy in a specific direction; must be aimed towards their transmitter or receiver; and require long range, but offer higher gain.

"Because of all these and other considerations, it's important to do a path study of the site where wireless will be used," said Harris. "This will show the Fresnel zones, indicate how high the antennas will need to be."

Harris says wireless application questions that need to be asked are:

  • In which country will these radios operate?
  • Does the end user have a licensed frequency they want to use, or is the license-free ISM band OK?
  • Is this a new wireless installation/network, or is the customer adding more radios to an existing network?
  • How many radios will be added at this time? Will there be more later that must be accommodated?
  • What other radio networks are in the area that might cause interference?
  • What are the implications of radio latency to this process? For example, what happens if it takes 20 ms for the signal to arrive?
  • What type of data is being transferred?
  • What power is available at each radio site?
  • How far apart will the radios be?
  • What physical features/obstructions exist between the two radio sites?
  • Is there a likelihood of lightning striking the antenna such that a lightning surge diverter should be specified?
  • Will the radios be installed in an area where they will need to be placed in an enclosure to protect them, or will they be installed in an existing cabinet/enclosure?
  • Who will be responsible for the pre-sales and post-sales support of the end user?