Ten years ago, we suffered through the multi-headed fieldbus monster, when eight different technologies were established as standards. Today, that effort seems like a rehearsal for what we're going through with multiple wireless technologies, many Ethernet protocols, several instrument buses and different device buses.
To some extent, this proliferation of networks and protocols is needed to meet application requirements, but the sheer multitude of options can be baffling to standards makers, OEMs and end users. This article will sort through available options, adding order to chaos.
The first distinction that needs to be made is among industrial network levels (Table 1 and Figure 1). At the top level, there are enterprise and control networks. These networks are generally Ethernet, and communicate among enterprise servers, HMIs, controllers and blocks of I/O. Although Ethernet is the de facto standard at this level, many different protocols compete (Figure 2).
One level down are instrument and sensor networks. Instrument networks typically are used to connect instruments and analyzers to HMIs and controllers, and sometimes directly to enterprise-level computing systems. These networks are optimized to transfer relatively large amounts of data at relatively slow speeds, making them a good fit for process control applications.
Sensor networks typically connect discrete sensors to controllers, and connect motor controllers to a main controller. These networks often carry power along with signals on the same wire. High speeds are possible because of the relatively small amounts of data transfer, making these networks a good fit for the discrete sensors and I/O often found in machine and robot builder applications.
Overlaying all three network levels is wireless, with myriad competing standards. Some of these standards are optimized for one level, such as WirelessHART for instrument-level networks. Others such as wireless Ethernet cut across multiple levels.
In the process industry, wireless gained a foothold primarily because of the cost savings over wired cable, says Richard McCormick, automation engineer at Mick Automation in Levis, Quebec, an industry consultant with extensive plant operations experience. "Wireless is definitively the biggest trend in automation these days, at least for monitoring," McCormick says. "The advantages are cost savings in wiring, and providing access to places so remote they were simply rejected every time a project looked at bringing those signals inside the DCS."
Rob Kearney, maintenance supervisor for North Star BlueScope Steel in Delta, Ohio, uses 32 wireless temperature transmitters to monitor furnace operation (Figure 3). "The new wireless solution eliminated almost 100% of the cable and conduit, while reducing maintenance costs by $200,000 annually," he says.
In the discrete manufacturing automation sector, wireless is used, again primarily for monitoring as opposed to real-time control applications. "Wireless makes it easier to communicate between devices when it's often not practical and/or feasible to run a hard-wired communications cable," says Sam Hoff, president of Patti Engineering, an automation systems integrator in Auburn Hills, Michigan.
But wireless has made little impact on many users, maybe because some are a little timid. "Wireless in the plant delivers a new way to provide proof-of-concept for large automation projects," says Tom Edwards, senior technical advisor at Opto 22. "Before investing in permanent wiring or building a full-blown wireless infrastructure throughout the factory or facility, people have begun dipping their toes in the water by using wireless components in one or two remote areas."
In other words, wireless is being used primarily for monitoring purposes in both process and discrete automation mainly because it costs less than hardwiring. Otherwise, users and OEMs seem reluctant. Maybe this is because automation engineers aren't comfortable with the technology or the multitude of standards.