Electronic “noise” generated by motors and other devices can interfere with data signals, either corrupting them enough to cause problems, or drowning them entirely in a sea of static. Physical barriers, such as equipment and structures between the transmitter and receiver, also can block or interfere with data signals.
Fortunately, a number of techniques exist for solving these problems. The first is to situate the antennae so that a clear, uninterrupted line of sight exists between them. Although signals can pass through some materials, they bounce off others. “Using the right antennae for the application will help as well,” notes Gabe Sierra, industry marketing manager at Emerson Process Management (www.emer sonprocess.com). He adds that mounting the antennae so that they are higher than the obstructions also will aid signal transmission.
Another emerging option is self-organizing networks, also known as mesh networks. In these networks, each device is not only a sensor measuring some process parameter, but also a wireless data router that broadcasts information. Besides collecting data locally and sending it to a computer or controller somewhere for processing, these devices also receive and pass along data from other devices on the network. The data travels from device to device until it reaches its destination.
Work around obstructions
The network of transmitters works around a variety of obstructions. If, for example, a sensor on one side of a steel tank wants to send information to a device on the other side, the signal can travel around the tank by way of another measurement device, perhaps a temperature transmitter on a nearby pipe. Even if an obstruction is temporary, such as a forklift truck passing by, the network finds a way around it automatically and delivers the data to its destination by a different path. Best practice is to ensure that these temporary interruptions occur as infrequently as possible by locating the antennae high and keeping the distances between devices short, typically within 100 feet.
Most manufacturers of these wireless networks of sensors, transmitters and receivers use their own proprietary schemes to make the creation of paths as efficient as possible. Because of the growing interest in the technology, the Institute of Electrical and Electronics Engineers, New York, last year issued its IEEE 802.15.4 standard for wireless communication between sensors.
No matter which vendor’s communications scheme that you select for your plant, Emerson’s Sierra recommends using devices rated for the area classification (for example, electronics used near chemicals that can burn or explode when ignited by sparks and electrical current). Doing so will simplify installation of the network. Without properly rated devices, you would have to design a way to make the measurement and send it by wire to a safe area for broadcasting, which just adds costs and defeats the benefits.
In situations where noise from motors and other sources is a problem, Sierra suggests deploying devices that transmit using the frequency-hopping spread spectrum (FHSS) or direct-sequencing spread spectrum (DSSS) method, both of which were developed for the military to avoid enemy jamming. DSSS spreads a signal across a range of frequencies and transmits each separately, and the receiver recombines the signals.
Rather than splitting the signal, FHSS works much like the old wireless telephones that would let you change channels when interference began corrupting your signal. FHSS differs in that it changes frequencies automatically. With this method, any obstruction at one frequency would last for only a fraction of a second. Sierra reports that FHSS seems to mitigate signal interference better in industrial settings than DSSS. Consequently, data transmitters that use it and can fit into self-organizing networks are a technology that promises some clarity in data transmission.
James Koelsch, [email protected], is an Automation World Contributing Editor.
