Modern factories depend on vastly more automation and intercommunication than their predecessors did to operate efficiently. Assembly lines need just-in-time delivery of components at each phase of assembly, but as assembly lines become leaner and meaner, precious little storage room exists at each position to hold material for future assembly. This means that very rapid, accurate methods of locating, moving, placing and accounting for inventory are needed for production to proceed. A line that goes down because one station lacks the raw material it needs can cost companies thousands to hundreds of thousands of dollars per hour. Meanwhile, up above the assembly floor, lighting, HVAC and other environmental control and monitoring equipment not only contribute to the health and safety of the workers, but also to the quality of products rolling down the line.
The Zigbee Alliance is dedicated to creating reliable, cost-effective, low-power, wirelessly networked monitoring and control products based upon an open, global standard. The alliance, an industry association whose membership is open to all, consists of original equipment manufacturers, semiconductor manufacturers, end users and others that use the IEEE 802.15.4 wireless-communication standard and add value to it through flexible networking, enhanced security and interoperability and compliance testing.
Zigbee (www.zigbee.org) addresses wireless-sensor and control developers' concerns about radio technology, protocol and network creation. The alliance aims for reliable data delivery, rugged security, long battery life, adjustable data latency and system simplicity, all at a system cost that is much more attractive than current standards-based or proprietary solutions.
Zigbee builds on the powerful yet lightweight IEEE 802.15.4 standard, ratified in May 2003 after three years of standards work by individuals from companies such as Invensys, Honeywell, Philips, Mitsubishi, Motorola and many others. It is a simple packet data protocol with features specifically designed for flexible, long-lived, distributed sensor and control systems and networks. Devices can be either battery- or line-powered; the advantage of 15.4 over all other wireless protocols is that battery operation was highlighted from the very start. Battery life, depending on application and battery type, can range from many months to many years.
The protocol provides strong quality-of-service through packet acknowledgement capabilities, packet error checking, use of direct-sequence spread spectrum and the ability to change frequencies to avoid interferers, along with security levels that range from none to strong encryption with authentication. IEEE 802.15.4 specifies three different bands of frequency operation with a total of 27 RF channels, making it a truly global standard. The protocol was designed for, and works optimally in, applications requiring data and control reliability. It is also intended to quickly deploy networks that can either autonomously configure to move data and control or let a network management agent manage the network.
Zigbee networks can consist of a few devices or of thousands to millions. Network structures range from traditional star, to cluster trees and ultimately, mesh networks. Each network has its place in the industrial or commercial environment. An industrial facility with acres under one roof can be managed with regularly spaced nodes, all communicating wirelessly with one another via Zigbee and providing seamless connectivity. The protocol takes care of assembling the network and managing the traffic; various network tools will be available to manipulate and optimize the network for the facility manager's specific needs.
When equipment is line-powered, long battery life isn't important, but robustness and reliability are. When equipment depends on remote sensors or other control mechanisms, deploying Zigbee devices provides robustness and reliability. It can be used to stick an occupancy sensor, light-level sensor or control panel on a wall without having to drag wire and power, saving hundreds to thousands of dollars in installation costs while producing a unit that could last the shelf life of a primary battery. (The shelf life for a lithium battery ranges between 10 and 20 years). The savings rapidly add up when assembly or commercial areas undergo regular remodeling or reconfiguring to adapt to changing staff or factory needs. Instead of dragging more wire, Zigbee makes these kinds of changes more a matter of "peel and stick." By design, the changes are transparent to the centralized control.
In addition to enabling factory monitor and control functions, Zigbee provides the ability to track and locate raw materials or finished products as they move in, through and out of the facility. Raw and finished material transporters are looking at Zigbee as a replacement for older RFID technology. Intelligent, two-way Zigbee transceivers acting as tags on material not only provide the basis for locating the material in the factory, but also monitor and report on such qualities of the material as temperature, humidity, vibration and the like.
As a chassis moves down the line, an intelligent Zigbee tag acts as the traveler for that product. It collects information about time/date/assembler at each station, any discrepancies or detours to or from rework, and then logs it all as a part of the finished product's permanent history. That tag can remain on the product throughout the product's life, always serving as the first point of contact for any service, maintenance or warranty work. As the finished product moves from the line to the warehouse, the warehouse recognizes the product and adds it to its inventory count. Shipped to the distributor or end user, the tag always serves as a two-way communications portal for information about the device. When on-site service is required, the maintainer might carry a Zigbee-enabled cell phone or PDA to communicate with the device and perform troubleshooting.
Jon Adams is director of the Motorola Wireless and Mobile Systems Group (Tempe, Ariz.).
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