The applications of rich and successful wireless sensors and elements in industrial process environments are many. They prosper in the harsh industrial elements and deliver continuous data every day without the necessity of running wires from the control room to the sensor and the actuator.

Wireless technology is rapidly gaining converts in the industrial environment. It's now available in a wide range of different implementations including wireless phones, wireless local area networks (LANs), wireless keyboards, and wireless sensors.

Using the airwaves is allowing instrumentation engineers to gather much needed process information with unprecedented ease. The installation of a wireless sensor can be as simple as installing a gauge; but with measurement accuracies better than ±0.1%, features such as automatic self-calibration and direct communication to a plant's process control system, allow wireless sensors to gather the information needed to squeeze extra process performance and to examine parameters that are not presently monitored.

Success in the challenging industrial process environment puts special demands on wireless devices. Understanding these extra application demands and matching them to the right wireless products and technology is fundamental to a successful industrial wireless installation.

Guarded military secret

Wireless technology differs as much as wireless products differ. The first key to understanding is remembering the word wireless is an adjective that describes and modifies something else. A wireless phone, for instance, uses a different communications technology than a wireless LAN uses.

The term generally applies to those devices that communicate over the airways using a digitally based communications protocol. A key difference between wireless devices and the conventional radios, familiar to us all, is radios send their information in an analog signal.

Basic radio communications techniques developed in the beginning of the 1900's. These analog radio waves were at a fixed frequency. As fixed frequency analog signals are easily disrupted and intercepted, legislation and regulation were required to protect radio broadcasters and limit interference.

The military had a difficult time with interference and interception of basic analog radio communications and were searching for ways to make their communications secure. Toward the end of WWII, a famous patent went to Hedy Lamarr for her concept of frequency hopping radio transmission.

This patent was a closely guarded military secret for many years and became the backbone for secure military communications to the end of the 20th century.

With the release of the Lamarr patent, the Federal Communications Commission (FCC) established a set of radio frequencies that worked at low power without requiring a user license. With frequency hopping techniques, digital communications, and unlicensed radio transmission, the stage was set for the development of the many wireless devices that are available today.

Number of sinusoidal waves

Robust wireless communication inside the plant rests on several fundamental technology developments.

In digital communications, the information passes as a rapid succession of ones and zeros. In most protocols, a one digit is a given number of sinusoidal waves sent at one frequency, and a zero digit is a given number of sinusoidal waves sent at a slightly different frequency.

A disruption to a digital communication requires that no signal gets through or that a one comes through as a zero. These are major disruptions and very unlikely since ones and zeros are passed as different frequencies. Disruption in radio wave amplitude is fairly easy to do, but it is very difficult to alter the frequency of a radio wave. By putting together a succession of ones and zeros as a header, it is easy for the receiving radio to positively identify the source of the radio transmission and to verify its validity. By counting the number of ones and zeros and transmitting these counts in each message, the receiving radio can verify all the data transmitted properly.

Just like digital signal processing has improved computer computational accuracy and digital communication has improved digital television transmission compared its analog counterpart, digital wireless communication has made a marked improvement over analog signals that are more likely to be subject to interference.

Effects of background noise

The second foundation for robust plant wireless communications is the use of many frequencies. Virtually all robust plant wireless communications utilize multiple frequencies for communications. In North America, the FCC has set aside the radio spectrum from 902MHz to 928MHz for low-power, unlicensed radio communications.

Robust radio communications can take place by spreading the signal over this 26MHz spectrum. An effective technique for spreading the signal is to hop from one frequency to another. This is Frequency Hopping Spread-Spectrum (FHSS). The transmitting radio and the receiving radio simply hop from one frequency to another at exactly the same time, maintaining their own synchronized communication.