Comment

1 April 2005

No copper wire needed

Hazardous, remote valves and actuators ideal customers for wireless connect.

By Clifford Lewis

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.

The probability of an interfering signal hopping in the same hopping pattern is virtually zero. This frequency hopping radio transmission formed the backbone of the military's secure communication strategies for many years. It is a well proven technique to eliminate the effects of background noise and give robust secure communications.

Security codes are a third fundamental building block of secure industrial wireless communications protocols. In every data transmission, the receiving device needs to match a security code embedded in the wireless message: No security code match, no communication.

Each message should also contain a checksum code. A checksum adds up the number of ones in a message and sends that number. The receiving device adds up the number of ones it receives and checks to see it is the same number of ones that transmitted.

Tight frequency filters

The fourth technology for modern wireless protocols is transceiver acknowledgement. Every industrial wireless device should be a receiver as well as a transmitter. After each message transmits, an acknowledgement from the receiving end should transmit back to the sender confirming receipt. To complete the acknowledgement cycle, the message needs to arrive at its destination and the security code and check sum code correctly received as well. When the sending unit gets its acknowledgement back, it knows the message went through properly. If the message acknowledgement does not arrive, the sending unit resends the message at another frequency to get the message through.

The fifth technology for modern wireless protocols is the implementation of tight frequency filters. These filters eliminate reception and transmission of radio frequency energy outside of the proper bands and make the communication links much less susceptible to interference.

Secure, robust signals with the five characteristics described above form the foundation for a solid industrial wireless communication network. Many wireless devices not designed for the difficult industrial environment contain some, but not all, of the above foundation blocks. The solid communication foundation, with all of these parts, is necessary for an industrial wireless sensor. With a firm foundation, wireless signals from wireless sensors designed for the industrial environment will get through any plant radio interference and they arrive at their destination intact.

The next steps rest on this secure foundation: 1) Get the signals where they need to be, and 2) translate the signals into a language easily understood in the industrial control community.

Line of sight not necessary

The actual communication from a wireless device travels through a Fresnel Zone.

Reflecting surfaces in the Fresnel Zone set up their own subset of Fresnel Zone communications. Provided there is not complete blockage of the Fresnel Zone to radio waves, the transmission wave will get through. A true line-of-sight is not necessary. The size of the Fresnel Zone is the combination of factors including transmitter power, receiver signal strength sensitivity, antenna height, and antenna spacing.

Industrial process sensors are frequently in hazardous areas, and units should come under the ratings for Class I, Div I, and Div II safety classifications.

Any wireless device needs to tie into existing control systems. Getting information into the myriad of existing control systems is not a small task. The 4-20mA signal and switch closures are universally translatable. Digital input allows more data flow at significantly lower cost, but generally adds a level of complexity to any system. Modbus and OPC servers offer degree of acceptance where large data flows are required.

Behind the byline

Clifford Lewis (clewis@mail.adaptiveinstruments.com) has a B.S. in chemical engineering and an MBA from Dartmouth. He is a vice president at Accutech. He also worked for Celanese and General Eastern during his career.