Ready for real-time wireless control?

Staff -- Control Engineering, 3/1/2006

Natick, MA—A large percentage of those using wireless technology in industrial applications are applying it to control functions, or both control and monitoring, according to a recent study by Venture Development Corp. (VDC). Further analysis indicated that control applications are largely in setup/maintenance; and reliability concerns are inhibiting adoption for operational real-time usage.

Input of study respondents on the question of "industrial monitoring" vs. "control applications" found:

• 76% said both;
• 16% indicated monitoring only; and
• 8% leaned to control only.

What applications use which wireless products? Data communication, controller programming, and controller maintenance were the three most identified, both currently and planned. So wireless use in machine- and process-setup and troubleshooting is far more common than for operational real-time control.

Users also identified wireless control of cranes/hoists—over 25 years a well-established industrial market for wireless operational real-time control. While it's "operational real-time" control, it's not fully automatic—the operator is an integral part of the control loop.

Two big problems inhibit operational real-time control.True real-time automated control poses a much greater concern to implementers than monitoring control applications. Two most identified user problems encountered in RF/microwave wireless industrial monitoring and control are:

• Signal reception drop-outs/blockage; and
• RF interference.

Consequently, users remain doubtful about wireless technology for operational real-time control, where even a slight delay in, or corruption of, data communication could have serious impacts. Currently, industrial wireless operational real-time control is most commonly found in applications with slowly changing variables, such as temperature and flow control of slowly fluctuating processes, or for operator-based control, rather than in automated systems where high-speed machinery is being controlled.

Controlling slow processes—with lesser data rates—requires less bandwidth. Therefore many existing, industrial, operational real-time wireless-control applications function in the 400-, 800-, and 900-MHz bands that provide adequate channel bandwidth. Higher-speed applications can require more bandwidth, necessitating higher frequency bands, such as 2.4 GHz or 5 GHz. Generally, moving to higher frequency bands makes signal drop-outs/blockage worse. Interference varies with the local environment.

A key justification of wireless is wiring cost reduction; savings tend to be greater for large distributed systems (often slow process-type applications, rather than high-speed machinery). Thus, there may be little savings in wiring for many discrete manufacturing operations, and use of wireless technology for operational real-time control may not be attractive.

Furthermore, higher RF/microwave frequency band operation may be needed to ensure adequate bandwidth. Usually, as the frequency goes up, so does RF/microwave component cost. However, wireless technology may still be attractive for use with portable operator interface terminals for machine setup/troubleshooting.

It appears that the greatest near-term potential for operational real-time usage will be for slow-moving, process industry applications, where data transmission faults don't pose a serious risk. For these, VDC analyst Jake Millette points out, "Wireless operational real-time control holds many potential advantages over wired solutions, but even here small steps must be taken as mistakes can be expensive."

It's expected RF/microwave wireless technology in operational real-time control will eventually extend into other applications in discrete and process manufacturing.