Enclosureless I/O systems have been available for years, but new installations often still use traditional enclosure-based strategies. This happens despite the fact that a centralized enclosure approach has several downsides. What are some of the trade-offs between enclosure-based systems and enclosureless-based I/O systems, and what applications could each technology potentially best suit?
According to Matt Behrns, marketing manager at Pentair, the approach a designer selects depends more on the automation philosophy of the end user rather than specific applications. "One advantage an enclosure-based I/O strategy provides is flexibility," says Behrns. "Often, I/O requirements change rapidly based on end-user needs. Using an enclosure-based I/O strategy enables integrators, panel shops, OEMs or electrical contractors and installers to add or modify I/O quickly. Additionally, users can select many different types of I/O to meet their needs for a given application and are not 'locked in' to specific equipment vendors."
Another advantage is environmental protection. "I/O systems are often placed in harsh or outdoor environments, or in controlled indoor environments" says Behrns. "An enclosure-based design allows engineers to use the appropriate level of protection and not over-design systems. Widely available Type 12, 4 or 4X enclosures can be used to provide the appropriate level of ingress protection and corrosion resistance. Exposed connectors can often be weak leak and failure points in an enclosureless I/O system."\
Lastly, an enclosure-based I/O strategy leads to a reduced need for specialized connectors. "While connector-based wiring systems are improving and there are more available solutions especially for harsh environments requiring IP66 or Type 4/4X protection, the solutions often force the user into a specific set of solutions," says Behrns. "Using a traditional wiring method makes maintenance easier and reduces specialty cable requirements."
On the other hand, advantages to using an enclosureless I/O design include that the designs can be directly mounted on a machine, reducing wiring costs. "In addition, the approach makes it simple to plug and play different sensors into the design," says Behrns. "However, one disadvantage is sometimes designers mix IP ratings within the I/O system, which can cause failures in the field. The designer of enclosureless systems must be clear on the ingress protection requirements and make sure the products selected meet those needs. Otherwise, failures occur, causing downtime."
Behrns says cost differences between the two approaches are difficult to evaluate. "For example, the perception is that enclosure-based designs are expensive but many users select metallic enclosures for their designs when a cost-effective nonmetallic fiberglass or polycarbonate enclosure provides the same level of protection at a lower price point."
Using enclosed I/O products that communicate with a host over a standard interface like USB or Ethernet has gained widespread acceptance, says Earle Foster, senior vice president of marketing at Sealevel Systems. "This configuration allows the I/O circuit to be mounted close to the signals being monitored, eliminating the cost, inconvenience and possible error sources associated with long wire runs," he says. "Enclosed I/O products are also easy to maintain or replace, and the enclosure can protect the electronics from damage in the industrial environment."
In contrast, using an I/O circuit without an enclosure is attractive to OEMs and system builders that plan to install the I/O as part of a larger system, says Foster. "This approach conserves space, reduces cost and allows the user flexibility in mounting. In certain cases, both enclosed and board-only versions of products using the same I/O PCB are available. This allows the user to decide which method works best for their particular application."
Recall that a traditional enclosure-based I/O includes a main enclosure, with every device hardwired or connected with a disconnect at the device, and the cable for each device runs back to the controlled enclosure, says Bruce Bator, director of engineering, Phoenix Mecano. "An enclosureless system also has a main control panel and a central cable and includes many multi-conductor cables that travel to a distribution point or distribution enclosure. The systems involve much shorter cable runs that are much more resistant to noise."
In some environments, applications involving traditional enclosure-based systems work well, says Bator. "A centralized approach can be an advantage because it provides a lot of protection to system elements,” he says. “However, applications involving larger, more complex equipment can make the enclosure-based strategy cumbersome because it involves a big control unit versus smaller junction boxes located in other areas. Also, a downside is the large number of terminations can lead to loss in the system, which negatively affects the signal. And, because the many terminations are manual, the system can be difficult to implement. In addition, users must perform a lot more troubleshooting, and the repair is typically more difficult because of the need to replace extensive cabling."
An attractive alternative can come from enclosure-less based I/O systems, says Bator. "For example, a conveyor system in our plant employs an enclosureless I/O design because it includes units in a remote warehouse where each center must have its own run," he says. "The enclosureless approach lets us wire sensors to a single distribution point and then run a single cable, which eliminates cabling and makes maintenance easier. Another benefit is the design time is a lot quicker. Enclosureless designs are less complex because they let designers divide a system into smaller segments."