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Article - July 2003
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Practical Considerations for Using Wireless I/O Technology

Guy Zebrick
Kele


The combination of today's wireless I/O devices and field survey tools, along with a modern facility management system, offers a practical way to control and monitor difficult to wire inputs and outputs with minimum risk and experience. 

In any commercial automated control or monitoring application, we use field devices to sense and control various mediums to ensure comfort, safety, and efficiency. The field devices, both inputs and outputs (I/O), are typically hard-wired from remote locations to building automation controllers. While hard-wired connections are common and practical in most situations, in others they are extraordinarily costly, wasting both time and money. For example, one faces significant challenges when hard-wiring in existing facilities where special wall and ceiling materials are present, such as marble, glass, and asbestos. In addition, projects that require exterior connections and involve trenching to remote buildings, exterior signs, or gates can also be difficult. 

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The list of difficult hard-wire applications is as long as an electrical contractor's change-order. Below are some examples in which replacing traditional hard-wired electrical runs with more practical wireless alternatives can be of great benefit.

  • Asbestos ceilings and insulation 

  • Paved parking lots

  • Marble wall

  • Solid sheet rock ceiling

  • Historical buildings

  • OSA temperature

  • Firewall penetration

  • Floor core penetration

  • Fast track change orders

  • Clean room alterations

  • Open office temperatures

  • Museums

  • Outside lighting

  • Day/Night control and night setback sensing

Are Wireless Alternatives Really Practical? Past vs Present Technology

Wireless data transfer is not new, but in the past the wireless technology available was either too unreliable or too expensive for use in commercial control applications. In addition, wireless radio devices were difficult to estimate, engineer, and install. Site licenses, complicated installations, and connectivity issues made dealing with these systems a frustrating task. Low cost wireless devices that operated with older technology relied on power-hungry, drift-prone, narrow band transmitters and receivers that were susceptible to radio interference. Only governments and the military could afford to use higher performance digital radio, which has been available for many years. Today, however, new license-free wireless I/O devices, like the Kele Frontier Wireless I/O System, which connect using the signal levels typically found in automation and control systems (0-10V and 4-20 mA analog, dry-contact digital), can be easier and more cost-effective to implement than traditional hard-wired devices. 

Control Solutions, Inc Some new wireless products operate with the same technology that the ultra-secure government and military wireless data systems utilize. This technology is called direct sequence spread spectrum (DSSS). DSSS is a form of radio transmission in which a digital data signal is spread over many frequencies all at once and then reassembled at the receiver. The receiver hears only the properly encoded digital signals for which it was programmed to listen, and it interprets the signal as a digital noise burst. Since the transmitted signal appears as nothing more than noise, it will not interfere with other systems. It also attenuates any narrow band interfering signals and provides received signal gain in the process of reassembly. The inherent digital coding ensures secure data transmission within the wireless network. DSSS is an excellent choice for wireless I/O data transfer because of the inherent receiver gain, security, fast signal acquisition, noise immunity, and low power consumption. 

A fast, low-power transmission is especially useful when battery-powered transmitters are desired. Wireless devices may be manufactured with built-in replaceable batteries, making them truly wireless; they may also have provisions for local external power, eliminating the need for periodic battery replacement. Regardless of the power used, wireless I/O transmitters typically send signals on a periodic basis, instead of the continuous signals that hard-wired connections provide. 

Receiver outputs will maintain the last received signal level and will update based upon the associated transmitter's configuration. Update rates of 30 to 60 seconds are typical. Shorter update times tend to crowd the airwaves, reducing data throughput and limiting the total number of transmitters within a system. Shorter update times will also reduce the life expectancy of battery-powered devices. Longer update times increase both battery life and the maximum number of usable transmitters within a system. Fortunately, 30- and 60-second update times are acceptable for many applications, such as space temperature control and point monitoring. At these update rates, it may be possible to have as many as 50 transmitters all operating within the same wireless network and still maintain data throughput of at least 95%. See Figure 1 below for an example of the relationship between data update times and transmitter quantities.

Figure 1 - Relationship Between Transmitter Interval and Quantity

Figure 1 - Relationship Between Transmitter Interval and Quantity 

Reliable wireless communication also requires an appropriate place on the radio dial. Ideally, the operating band should be one reserved for data transmission. The higher frequency bands at 2.5 Ghz and 5 Ghz are becoming crowded with the explosion of wireless Ethernet (Wi-Fi) and Bluetooth networks and may not be the best choice for wireless I/O devices. The operating band should be away from other popular frequencies and set aside for services like cellular phones, job site radios, or commercial radio and television. In addition, it should be away from the frequencies where natural noise (lightning crashes) or man-made electrical interference (microwave ovens) can cause interference. Fortunately, the United States Federal Communications Commission has reserved dedicated license-free bands precisely for low power digital data transmissions designated as the Industrial/Scientific/Medical (ISM) bands. One of the ISM bands, located near 915 Mhz on the radio dial, offers the added advantage that radio signals on this frequency band enjoy - good building penetration and propagation. 

Is Wireless the Way for You? Considering a Jobsite for Wireless Possibilities 

Now that you know reliable, low power, high performance wireless I/O transmitters and receivers that operate using DSSS technology on the 915 Mhz ISM band are available, how do you know whether they will work for your application? Fortunately, advanced wireless site survey tools that take advantage of DSSS technology and measure actual data throughput rates are available now, such as the Kele Frontier RST-K, as shown in Figure 2.

Figure 2 - Kele RST-K Survey Tool Kit

Figure 2 - Kele RST-K Survey Tool Kit 

This site survey device provides a very simple "go / no-go" indication of operating range in the field. No radio expertise is needed to perform a simple walking survey to verify actual operational distance needed between transmitters and receivers. Real world maximum operational ranges between transmitter and receiver are usually between 200' and 500' laterally and 100' vertically inside of a typical occupied commercial building. When used line-of-sight, the range may extend beyond mile. The range of the wireless signal varies greatly depending upon transmitter power, building construction materials, and layout. Figure 3 below illustrates typical operating range of a wireless DSSS signal.

Figure 3 - Typical Operating Range

Figure 3 - Typical Operating Range

If range or building structure prevents a good signal, wireless repeaters are available, such as the Frontier RR repeaters. Repeaters extend the range of a wireless signal, responding only to the transmitter for which they are programmed to hear. 

With today's technology and some site investigation, wireless I/O can be implemented with off-the-shelf products for a wide variety of applications. As more people discover the practical advantages of wireless technology and consider it a viable alternative to hard-wiring, the world of wireless solutions will become more well-rounded and no area will remain unexplored.

The Kele Company is a leading supplier of products, services, and solutions to the building automation and control industries. Located in Memphis, Tennessee, Kele manufactures and distributes the new Frontier Wireless I/O System. The Frontier line is a complete selection of wireless DSSS transmitters, receivers, repeaters, portable survey kits, and programming tools that are compatible with any building automation system. 

Visit the Kele website at http://www.kele.com/frontier/frontier.html for more information.

* Wireless I/O information based on extensive testing of the Kele Frontier Wireless I/O System


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