Posted by arlene

Bandwidth and Range

The most fundamental parameters for selecting the proper sensor network are bandwidth (or throughput) in bits per second (bps, kbps, Mbps) and range (meters, kilometers, feet, or miles). The physics of RF communications are such that throughput goes down with increased range, assuming all other parameters are held constant. In fact, many 802.1 lb devices are rated for four different throughputs that increase with decreasing guaranteed range. The tank farm example typically requires a relatively long range for sensor networks (1 km or more) but with a relatively low data rate (<1 kbps). On the other hand, some experiments performing modal analysis involving arrays of accelerometers may require several Mbps over a range of only a few meters or tens of meters. As mentioned, the user must distinguish between component bandwidth and the throughput rate of useful data.

Number of Sensors per Network

Related to the network bandwidth is the number of sensors that would reasonably be attached to a single access point of the plant intranet. In some systems each sensor will have its own RF transmitter. In others, a sensor node may multiplex several sensors onto the communications bus. In both instances, the user must be aware of both the bus throughput maximum rates and the individual node/sensor maximum rates. Most busses can accommodate somewhere between 32 and 256 individual nodes. The user must decide which sensors need to be grouped onto a subnet to provide the necessary data coordination or control loops. Based on this the user must determine which subnet technologies support the requirements.

EMC

Electromagnetic compatibility (EMC) deals with compatibility in both directions. That is, it deals with limiting the emissions of the component being considered, as well as dealing with the potential susceptibility of the component to emissions from other devices. The Federal Communications Commission (FCC) rules focus mainly on limiting the emissions of electronic equipment such that they will not interfere with other devices. On the other hand, U.S. military standards (MIL-STDs) and European standards (e.g., International Electrotechnical Commission, or IEC) deal with both emissions and susceptibility.

First of all, wireless nodes must be selected within the constraints of the overall frequency planning of the facility. Typically, FCC guidelines are utilized as a general overview of which frequencies and power levels are acceptable within the planning of a region’s spectrum use. However, local spectrum management authority must make judgment calls as to where and when various types of radio equipment can be used. For instance, some facilities do not allow handheld radios in the control room. For similar reasons, many hospitals restrict the locations within which cell phones can even be turned on. That is because any cell phone that

Is turned on is periodically transmitting at least a pilot signal so that it can be associated with a particular cell tower.

The determination of the proper use of wireless equipment within a plant is not limited to the knowledge and control of carrier frequencies (spectrum management) but should also include the selection of radio equipment with compatible modulation types (waveforms). In some instances, conventional narrowband transmitters such as handheld radios have interfered with important plant processing equipment, while direct-sequence spread-spectrum radio equipment was utilized in the same area without upsetting the process. This is because direct- sequence spread-spectrum waveforms “spread” the energy over a wider bandwidth (typically by factors of 10:1 to 1,000:1) such that the effective volts/Hz energy levels are lower.

Another modulation type called frequency-hopping spread-spectrum is often used to good effect to overcome jamming signals. It has been used heavily by the military and in some commercial applications.Acurrently popular line of components called Bluetooth utilizes frequency-hopping modulation. In this instance, the dwell time of the carrier is under a few msec; however, during that interval the signal is similar to narrowband transmitters. Frequency-hopping devices have been known to interfere with direct-sequence devices such as wireless local area networks (LAN) utilizing IEEE 802.1 lb protocols. Because of this, some members of the IEEE 802.15 working groups have discussed separating IEEE 802.15.1 devices from direct-sequence devices by at least 6 feet.

Vendors should not only document FCC certification of their products, but they should also demonstrate that their units have worked in the presence of other wireless equipment. This wireless equipment should include handheld radios (which are typically narrowband licensed units), microwave ovens (some 2.45 GHz radio units are susceptible to the leakage fields from microwave ovens, even though their emanations are certified to be below permissible health limits), and cell phones. Also, the vendor should be able to show that their units do not upset existing plant equipment, especially other wireless links.

Some IEC documents call for the coordination of lightning protection experts, architects, and construction companies when building a plant to ensure that a thorough lightning protection system is in place. Analogously, an EMC expert should be consulted at plant startup and during major upgrades to ensure that all of the plant’s wireless communication devices, equipment such as RF heaters, and other electronics sensitive to EM fields will peacefully coexist.

When there were only a few types of wireless devices, EMC was less of an issue than it is now. However, the current proliferation of wireless communication devices will cause intersystem EMC to become a major consideration when selecting wireless equipment. Note that intrasystem EMC is under the purview of the designer of the wireless components and that adhering to FCC rules provides some measure of intersystem EMC. History has proven that good engineering practices and well-planned governmental controls are not enough, however. Some systems will invariably interfere with others; good spectrum management and spatial separation will minimize this interference. The use of directive antennas, direct-sequence spread spectrum, power control, and all other available technical means should be employed to reduce the possibility of EMC problems.

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