Design News, February 2013

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s e n s o r s / m ac h i n e v i s i o n The Backbone of Industrial Wireless Networks The IEEE 802.15.4 standard fully defines a robust wireless personal area network (PAN) that specifically targets low-power, low-bandwidth networks commonly utilized in industrial monitoring and control. Here's how it works: an IEEE 802.15.4 transceiver integrated with an industrial switch or sensor communicates to a monitoring receiver that can handle multiple switches in a star-configuration network. Every network, and each wireless device in the network, has a unique identification number that allows the device and its associated monitor to encode their signals. This ensures that the communications link between them is private and virtually immune to crosstalk from other switches or networks. These 802.15.4 radios provide excellent results in large indoor open spaces and/or outdoor installations with a relatively open line-of-sight between device and monitor. A 35-dB link margin ensures that minor obstacles or even heavy precipitation will not compromise communications. In some installations, the signal can penetrate intervening walls depending on the materials. The allowable operating range of an IEEE 802.15.4 radio is more than 1,000 ft (304m). In extreme conditions such as heavy precipitation, rain, or snow, the signal could be reduced by approximately 75 ft (23m). Because 802.15.4 devices draw so little power they can be operated by industry-standard batteries, which ensures reliable operation and provides for almost limitless options for installation. It also eliminates the need for situation-dependent, unreliable, and expensive energy scavenging. A wireless sensor or switch may operate for several years without a battery replacement or recharging, depending on the design. Typically, the monitor/receiver unit in a wireless network can support either a single device or multiple devices. For example, the Honeywell Limitless WDRR receiver can support up to 14 different remote batterypowered wireless devices. In addition to continuously monitoring the sensor and switch status, the controller can also monitor the signal strength and battery levels for each individual device on its network. If a battery starts to die, or a switch gets blocked, the operator will know instantly and can take corrective measures. Wireless on/off controllers are also available. Powered by batteries, these wireless operator interface devices can be located for easy operational access without spending a lot of time to connect them to the control network or to a power source. The ease of configurability of wireless networks for new assembly lines makes wireless very attractive to the automotive industry, which often introduces new models every year. based on 802.15.4 point-to-point communications, Honeywell's Limitless switches can be configured with up to 14 devices to communicate with one receiver module. The wireless signal is received by either a panel-mount receiver, or an industrial DIN-rail module, which is then converted to an output. Outputs can be LeDs, buzzers, or standard electrical signals used by traditional controllers. No Interference Because 802.15.4 is a PAN standard it operates independently of other common wireless LAN technologies such as Bluetooth, WiFi, and cordless phones. This means there is no exchange of information or other interaction between a PAN and the LAN, eliminating the need for corporate IT department involvement. Secure Networks To ensure secure communications links, 802.15.4 radios can be set up to require registration with their network monitors. This registration or "pairing" process, which takes less than five seconds, is performed only once to establish the linkage. Once a device and its monitor have been paired, the two continually exchange information as long as both have power. IEEE 802.15.4 security techniques are part of the pairing process. When registration is activated, the monitor provides the switch with several pieces of information. The first is the monitor's individual 16-bit network identification number, which the monitor selects at power-up to avoid conflict with other networks in the vicinity. The second is a unique 16-bit address for the wireless devices, which the monitor creates. The third piece of data is a 128bit encryption key that the pair will use to encode future communications. T6 Trend waT c h : senso r s / m a c hi n e v i s i o n / a s u p p l e m enT To de s ign ne ws f eb r uary 2013 [www.designnews .com]

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