Let me get this straight. You want to replace a long twisted-pair cable with a wireless link? Are you nuts?
That's what I said to my brother several years ago when he wanted my input on his project for a new control-system design in a Texas process control plant. He needed to install eight resistive temperature devices (RTDs) to measure the temperature on pipes to control their temperature with heat trace, a type of wire heating element.
The project was pretty straightforward, but the runs of cable from the RTDs to the controllers were very long, from a couple hundred feet up to a thousand feet. And installation of even simple twisted pair was very expensive.
In a harsh industrial environment, most cable runs have to be in conduit, and redundant runs were required in critical operations. On top of that, all wiring had to be installed by a licensed plant electrician. That cost was blowing the budget by several hundred thousand dollars. He correctly reasoned that you could actually design and install wireless links on those sensors and have money left over. So that's what he did.
That same scenario is playing out all over the world as engineers discover that it's less expensive to put wireless links in place of wiring in most new industrial systems. It's even simpler and cheaper now than a few years ago when the above project was implemented. Today, engineers have multiple wireless options to monitor sensors remotely or to perform remote control on a pump, lights, HVAC system, or whatever.
Knowing that the average cost of industrial wiring is about $200/foot, you have to wonder how many industrial networking projects or systems are delayed or never implemented because of the unbelievable cost of laying cable (think backhoe or conduit) coupled with the related complexity. Wireless has existed for many years in industry, but the new technologies, lower cost and power consumption, improved reliability, and security now makes it a truly viable option.
THE INDUSTRIAL ARENA
Industrial networks are used in factories, process control plants, warehouses, oil and gas pipelines, building controls, hospitals, trucks and automobiles, public utilities, and city facilities like lighting and traffic control. The older network and protocols (e.g., DeviceNet, FieldBus, Profibus) are slowly being replaced by none other than the ubiquitous Ethernet. Because it's so widely adopted, the cost per node is very low. Moreover, equipment from multiple vendors lets you do just about anything.
A key rationale for Ethernet in the factory is the ability to tie the plant into the office as provided for in many of the newer automated Computer Integrated Manufacturing (CIM) systems. Many companies have developed hardened Ethernet equipment to withstand the harsher industrial environment. And, the Ethernet/Industrial Protocol (IP) standard has emerged to deal with some problems of using an office local-area-network (LAN) technology in a real-time industrial setting. This protocol uses standard off-the-shelf Ethernet products but enhances them with software that ensures deterministic messaging. Thus, monitored time-critical data isn't lost, and control signals don't arrive too late, as might occur with standard Ethernet messaging.
Furthermore, many older protocols are being repackaged in TCP/IP and transmitted via Ethernet. Wired Ethernet industrial networks have seen phenomenal growth, and that pace continues unabated. With the wide availability of wireless Ethernet (Wi-Fi), suddenly wireless becomes a viable option in the plant. In addition, a slew of newer wireless technologies has emerged over the past few years, many that are fully adaptable to industry and some designed particularly for industrial networking.
WIRELESS ALTERNATIVES
Wireless networks operate in four basic topologies: point-to-point (also known as peer-to-peer), point-to-multipoint, multipoint-to-point, and mesh (Fig. 1). Point-to-point is found in the simpler systems, where only a single sensor is used or only a simple control function is required. More complex systems employ the multipoint configuration. The most common is where multiple sensors must be read and multiplexed into a central control point. The point-to-multipoint approach is less common but serves a broadcast function to multiple nodes when needed. The mesh topology isn't yet widely used, but wider adoption is expected as the price drops for wireless technologies.
The mesh network uses many closely spaced nodes (Fig. 1d). Each node can communicate with its nearby neighbors that are within range. The nodes can exchange data between themselves or store and forward data meant for another more distant node that's out of range. One of the nodes also serves as a connection to a wired node.
The neat thing about the mesh is that it allows nodes to transmit over a longer range than possible with a normal line-of-sight (LOS) link. The mesh is also more reliable because if one node fails due to a power loss or other defect, communications are still maintained. Data is simply routed through another nearby node. If LOS obstacles occur, the link can be revised to get the data through via another path.
Aside from its reliability, the mesh offers the benefit of requiring only very low power because the distances between nodes is usually short. Low power means lower power consumption and longer battery life in nodes that are independent of standard power.
Mesh networks are beginning to appear in industry for monitoring large networks of sensors. Such networks are sometimes referred to as micro-meshes. The military has already started adopting mesh networks in battlefield systems, and homeland security is an ideal application for this growing technology. As the cost, size, and power consumption of the nodes (sometimes referred to as motes for remotes) improve, many organizations are studying and researching mesh networks in an attempt to define how hundreds or even thousands of sensors can be used.
When it comes to designing and implementing a wireless industrial network, there are more options than you may think. The choice of a technology depends greatly on the application and whether the wireless connection is embedded or added on.
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