01 July 2003
Wireless technologies working in concert add up to reliability.
By Raymond J. Wadsworth
The word wireless today has received a great deal of attention and has absorbed our interests, defined more economical methods of data transfer, and lent itself to a new definition of supervisory control and data acquisition (SCADA).
Traditionally, wireless pertained either to narrowband, line-of-sight telemetry that required Federal Communications Commission (FCC) licensing or to low frequencies in the 902 to 928 bands that were limited in range and number of available points and were highly susceptible to signal interference, even with line-of-sight capability.
In ever emerging technologies, with advancements in signal conditioning, signal transmission, and reception, a wireless industrial environment is not only available on the market. With certain technologies, it also remains sound and true, in terms of data transmission and acquisition; multiplatformed in terms of applications; and expandable, in terms of future advancements or user needs.
This new generation of telemetry has embraced such technologies as spread spectrum (frequency hopping), omnidirectional transmission (360° transmissions), and data packaging (bundling multiples of the same data bits for transmission). All have their unique characteristics that enhance the transmission of any radio signal. When married together, they offer a powerful data highway that is cost effective and reliable, with a return on investment that far outweighs that of hard wire and conduit in many, if not most, instances.
Inherent to the adaptability of these technologies is the manner in which the signal is conditioned or readied for transmission. Such conditioning includes, but is not limited to data packaging, signal encryption, and unique identification (ID).
When all these technologies work in concert they afford the end user the ability to transmit data reliably and uniformly. Then, and only then, you can reliably address the issue of data acquisition and control.
The system architecture, in terms of defining the initial "footprint" of the system's integration, is dependent on the end user's needs now and in the future. The instrumentation that is or will be employed within the facility, the communication highway over which all data is to interface in a seamless manner, and the degree of control functionality that is to be incorporated, are "critical" components for the "backbone" of any wireless communication system.
When considering a number of multiple points within the facility that are closely bundled, there are available, wireless remote terminal units (RTUs) that have the capability to act as aggregators for data points or discretes via either a Modbus or analog/digital interface. The RTUs then send all the data back to the supervisory control systems via a wireless communication link between the RTUs and the receiver to the host. Each addressable data point does not have to be stationary. In many instances, points such as temperature may be mobile points that are still tracked wirelessly, because the architecture of the wireless system has developed a footprint for that specific facility.
Initially, wireless telemetry experienced a great deal of data collision due to many transmissions over a narrow band of frequencies. To avoid this problem, technologies advanced so that wireless data could transmit over numerous bands, which is also called frequency hopping.
At the hop
Spread spectrum (See related story on page 20.) provides secure and noise-free radio transmission and is ideally suited to data communications. Spread spectrum operates by spreading the transmitted signal over a wide range of frequencies, instead of concentrating it around a center frequency. Spread spectrum is a modulation technique, along with other familiar types such as amplitude modulation (AM) and frequency modulation (FM). Conventional radio transmission is narrowband transmission.
Spread spectrum, however, spreads power over a wide band of frequencies or bandwidth. There are currently three spread spectrum bandwidths authorized by the FCC: 902–928 megahertz (MHz), 2400–2483.5 MHz, and 5725–5850 MHz.
The major advantage of spread spectrum technology is the ability to spread the signal over a wide band at very low power. Consequently, the signal has a low probability of interference or interception. Interference during radio transmission happens when signals take multiple paths or outside forces such as weather, magnetic fields, and electrical fields affect the process. Spread spectrum technologies provide redundant signals, which greatly reduce the risk of losing a signal to interference or interruption. It also minimizes the effects of walls, structures, and terrain on the signal. This technology allows a user to encrypt data or information on the sending and receiving ends of the process.
These encryptions may layer, providing means to perform multiple tasks or data functions. Each signal transmitted has multiple packets of information or commands. Only one of the many packets transmitted needs to reach the receiver to complete the communication. To ensure the receiver only accepts the transmitted signals it expects, the transmitter encrypts the beginning and end of each transmitted data package. This enables the receiver to ignore all other transmitted signals by other radio signals on the same frequency bands, which ensures the integrity of the data.
This procedure maximizes the reliability of the transmission. Users effectively employ it in industrial, chemical, refining, pharmaceutical, oil and gas, food and beverage, and other applications.
Before the development of this technology, transmitted signals required line-of-sight transmission. This was not conducive to plant environments where you often find data points buried within a facility.
With omnidirectional transmissions, the signal broadcasts over a 360° spherical profile. In many instances, signal deflection by structures such as process vessels, pipe racks, or other metallic objects will enhance the signal's ability to transmit back to the receiver.
Signal transmission is between 3/4 mile and 11/2 miles depending on the transmitter applied. These distances can increase to a range of seven miles with solar-powered repeaters with battery backup. With less than one watt of power output, FCC site licensing is not required.
Applications within the architecture of a wireless telemetry system include temperature, pressure, flow, vibration, levels, analytical, and discretes. All have an addressable identity within the overall configuration of the system.
Secondary functionality such as motor start/stop, valve open/close, alarm, and enunciation can work within the system as well via commands from the supervisory control system. These signals transmit back to the field by utilizing transceivers that have the capability not only to receive data, but also to transmit commands back to instruments to initiate specific functions.
These applications can be addressed wirelessly by single-point sensors that are self-powered, self-transmitting devices that do not require line of sight, do not require FCC licensing, and do not have limitations in terms of addressable points. All single-point transmitters are intrinsically safe and nonincendive.
After addressing all of these wireless issues, the next question is: How can I incorporate wireless into my existing facilities' SCADA?
Wireless telemetry and data acquisition systems can either be stand alone or work in concert with existing SCADA packages, usually via a Modbus RTU or Modbus ASCII format. The interface to the existing SCADA system occurs via a serial port connection over RS485 or RS232 links. Hardwired connections for either analog or digital signals can also connect to either the programmable logic controller or distributed control system. Most systems are configurable to meet the specific requirements of the facility.
If additional data points are requested of an existing hardwired system, it is merely a plug-and-play environment once the host receiver is in place.
Each transmitting data point will establish high and low thresholds, addressable IDs, and specific clock settings, which will allow the transmitter to send packaged data at specific intervals of time. This enhances the collection of data, because you are no longer polling plant instruments, a time consuming process when considering numerous points within the facility. The receiver merely acts as an aggregator of transmitted packaged transmissions. Each receiver is capable of accepting up to 300 individual registers and can expand to 1,500 to 2,000 depending on transmitter clock settings.
Wireless RTUs offer additional functionality via an encapsulated programmable logic controller, which can initiate functional commands should a limit threshold be violated. This would include a limited number of I/Os and would, for instance, turn off a pump, close an associated valve, and turn on an enunciator if a high- or low-level limit is violated.
The same can happen for any limit violation, be it temperature, pressure, level, flow, vibration, critical emissions, or any analytical measurement for quality assurance.
You can incorporate all of this in a single, multiplatformed seamless configuration, allowing operations to address any data point or bring a discrete command into the overall system configuration. Plant operations rely on this configuration to achieve efficient and cost effective production schemes that would otherwise be cost prohibitive or impact online production levels.
Wireless telemetry for data acquisition, motor control, and system integration has been an ongoing evolution and will continue to expand its capabilities as fast as technology dictates. Automation projects no longer are restricted by budget constraints and extended project planning. Wireless telemetry systems can bring in hundreds of points in a matter of weeks—not months. Implementation costs for materials and labor are minimal when compared to that of most any hardwired system. Project schedules are dramatically compressed, allowing for much more rapid accomplishments of project milestones, system integration, and start-up dates. Adding new points as systems expand or reconfigure is quick, efficient, and usually costs less than $900 dollars a point. W
Behind the byline
Raymond J. Wadsworth is a sales manager at FEDD Systems, Inc., a Houston-based manufacturer and systems integrator in the wireless market.
Up-front works will save time, money
In the wireless world today, the "buy and try" approach is probably the most common method, and is the basis for the success of 802.11b in the wireless local area network (LAN) market. It works because companies do not usually have a problem getting the system to work in the end. The problem is the system may cost more than originally planned. Typically, you can solve most problems with more hardware. In the case of a wireless LAN, installing more access points for a given area usually solves the problem. In the wireless telemetry case, adjusting antenna position, adding a radio frequency amplifier, or installing a repeater usually fixes things.
There are very few cases where things won't work, but most installations take some tailoring to be completely effective. A user can do this tailoring up front with a site survey or after the fact with the buy and try method.
When using the buy and try method, do a little up-front work such as:
In this very tight economy, doing these items may save some time and even more money.
Editor's note: This dispatch was an excerpt from the IMS 02 paper entitled, "The Wireless Network Installation Conundrum: A Telemetry Perspective" written by H. Philip White, chief technology officer and vice president of Internet Services at Xsilogy Inc. in San Diego.
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