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 (firstname.lastname@example.org) 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.
Actuator: A part of the final control element that translates the control signal into action by the final control device in the process. Typical examples are motors, solenoids, and cylinders.
Checksum: An entry at the end of a block of data that corresponds to the binary sum of all information in the block. A checksum works in error-checking procedures to detect single bit errors and some multiple-bit errors.
FHSS: Frequency hopping spread spectrum is one of two types of spread spectrum radio, the other being direct-sequence spread spectrum. FHSS is a transmission technology used in wireless LAN transmissions where the data signal is modulated with a narrowband carrier signal that hops in a random but predictable sequence from frequency to frequency as a function of time over a wide band of frequencies.
Which wireless is right?
Instrumentation engineers are searching for the ideal wireless communications protocol, with an implicit premise that one wireless communications protocol will meet all of their needs. There is a fundamental flaw in the concept one-size meets all for wireless communication protocols.
Different wireless protocols serve different purposes. Unfortunately, various goals for a wireless protocol are diametrically opposed and self-conflicting. Here are two prominent conflicts.
Data rate: High data rate, in particular, shortens both range and battery life. The designers of common wireless protocols including Wi-Fi, 802.11, Bluetooth, and ZigBee are targeting for markets much larger than the market for wireless sensors in the process industries. These larger markets demand high data throughputs, with the result that the common protocols sacrifice battery life and range.
ZigBee and Wi-Fi protocols, in particular, arose by design to serve personal area networks, which sacrifice range and battery life for higher data rates. Industrial sensor networks need the higher range, longer battery life, and lower data rates from a protocol such as the WINA MVP.
Open network: Open and ad-hoc networks compromise security. Personal area networks, such as ZigBee, Wi-Fi, and 802.11, by design, work to encourage any node to automatically add in and be part of the network. The open, ad-hoc network concept compromises a fundamental tenant of security, which is to control access. If you let everybody in whenever they want, maintaining data security is a monumental task.
Controlled access networks, such as the WINA MVP, support multiple vendors but maintain network access control. Control of network access control places network security as a premier wireless network goal.
No running wires, no running water
These figures show typical applications of successful wireless sensors in tandem with control elements.
One shot shows a wireless sensor monitoring the state of an on-off valve. This sensor reports the actual position of the valve confirming it has gone to the position determined by the control system.
Another photo shows wireless monitoring of the condition of a safety shower. This element allows for the dispatching of a plant emergency response team to an appropriate safety shower or eye wash station, should that station suddenly go into use. It uses a magnetic proximity switch as an input and wirelessly sends the data back to the control room.
The final photograph shows a wireless acoustic sensor mounted on a pressure relief valve (PRV) for continuous monitoring of leaks or releases of that PRV.
Acoustic monitoring is noninvasive, so these devices can mount without downtime, and they easily relocate as needed to identify problem relief valves.
These are all examples of wireless installations in harsh industrial process environments, which deliver continuous data every day without the necessity of running wires from the control room to the sensor.
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