There's been quietly growing interest in wireless networking solutions for sensing and control applications, but machine builders for the most part have found it impractical.
Wireless networks have been seen as a problem in rugged factory environments because of the limits of traditional wireless network architectures that left networks vulnerable to physical obstructions, electrical noise and single points of failure, among other problems.
What may change the factory floor perspective of wireless is mesh network architecture. "High return on investment, low replacement costs and the ever-increasing pressure to cut costs, makes wireless sensor networking especially suited for industrial networking," says Charlie Chi, senior analyst with networks researchers ON World (www.onworld.com). "Mesh networking in particular provides many advantages such as large-scale networks, real-time monitoring and new applications that were not previously possible."
Venkat Bahl, vice president of marketing at Ember (www.ember.com) says mesh networking architectures can bypass the interference and physical limitations that have restricted extensive networking in industrial facilities. "Mesh is based on new wireless protocols that favor reliability and versatility over speed and combines hard wiring's reliability with wireless networking's versatility," says Bahl.
Mesh networks use low-cost battery-powered sensor modules and embedded networking intelligence. The modules house microprocessors, radio frequency (RF) chips, and sensors for temperature, pressure, acidity, etc., depending on the requirement. The microprocessors contain embedded networking intelligence based on international wireless networking standards.
Star (top) and Mesh Topologies
Each module in a mesh network acts as a sender, receiver, and a router. "The modules create a spider web of data paths with no central point of failure," says Bahl. "Mesh networks are self-healing and self-organizing, requiring no human intervention to re-route traffic. They are virtually immune to interference. If a transmission from a node hits a roadblock on one data path, the embedded routing capabilities relay the information to a node on an alternate path."
Another mesh network developer, Millenial Net similiarly describes wireless mesh networking systems as made up of three fundamental modules. "The endpoint--a.k.a. reduced-function device or RFD--is a radio-based device that provides the direct connection to analog and digital sensors," says Mark Pacelle, vice president of marketing for Millenial Net. "Routers, also called full-function devices or FFDs, are used to extend the range of a network, route around obstacles and provide back-up routes in case of network congestion or device failure. Finally, gateways interface the sensor network to a host PC or directly to a controller."
The most critical part of a wireless sensor network is the network system software or protocol, says Pacelle. "This lets endpoints, routers and gateways self-organize at power-up and quickly re-configure as devices join, leave, or move around the network," he says. "They also adapt to changes in the network traffic and propagation conditions. The capabilities enable mobility of individual devices or the entire network, and minimize installation and operating costs."
It should be noted that few expect wireless mesh networks to displace existing networks. Companies that deploy mesh networks might find they can extract more value from their current networks for relatively little cost. The ON World analysts, though, believe that "initially, wireless sensor networks will be targeted at monitoring applications displacing wired systems such as DeviceNet, Modbus and Fieldbus."
The benefits of mesh architecture include simplified deployment. "In a star network, if a node is out of range from its access point, there is no recourse other than to move the node or the access point, which is often not an option," advises Bahl. "In a mesh network, extra nodes can be quickly deployed to serve as repeaters to fill in any holes in the network."
In addition, since nodes only need to communicate with their nearest neighbors rather than reach an access point in a single hop, transmitter power can be reduced.
Reliability increases too. "In real-world environments, environmental factors can cause frequent interruptions to communication between any given pair of nodes," says Bahl. "A mesh architecture offers multiple links among nodes, so messages automatically travel along any available link, greatly enhancing the reliability of the overall network."
