No Strings Attached

By Control Engineering Staff September 19, 2007

It isn’t only fictional puppets like Pinocchio that don’t want to be tied down by strings. The same is true for those involved in automation and related control applications. Going wireless on the factory floor can save money, reduce installation time, and offer greater flexibility when compared to the traditional wired approach. Wireless technology that meets the appropriate certifications can also keep operators and equipment safe, if the technology is implemented properly.

That implementation has to account for the factory floor itself. Unlike an office environment, plant floors are electromagnetically very noisy. A 2007 study done by the National Institute of Standards and Technology (NIST) looked at the wireless environment in an auto assembly, an engine, and a metal stamping plant. At the common wireless local area network frequency band of 2.4 and 2.5 gigahertz, the NIST investigators found heavy, constant traffic in the factories, a consequence of data transmitters, wireless scanners, and industrial equipment.

As might be expected, the plants were crowded with metal structures, some stationary and others mobile. Because of the metal, the plants had high levels of multipath interference, with wireless signals traveling in multiple complicated paths from transmitter to receiver. As a result, the signals arrived at slightly different times and with varying strength, phenomena that can cause errors and adversely impact the quality of communications.

While studies of the wireless environment in plants continue, the NIST researchers have determined certain preliminary steps that can be taken to improve the situation. Some involve people, such as restricting the personal electronics of those in high traffic areas. Others require changes to machinery, such as using equipment that emits little electromagnetic interference. The last category of suggestions includes the wireless technology itself, with the recommendation here being to use wireless systems with a high immunity to noise.

When it comes to safety, one of the most critical and basic elements is network connectivity, which the NIST recommendations will improve. It’s just as important, however, that wireless networks and devices meet the right certifications and have the proper features.

Consider, for example, some of the specifications of the Siemens Mobile Panel 277F with safety functionality. Like other industrial equipment, it meets housing standards with a protection rating of IP 65. Like other wireless equipment, it supports the important IEEE 802.11 a, b, and g standards.

It uses the standards-based safe network protocol PROFIsafe, which ensures safe communications and further enhances network connectivity. Behind the scenes, the protocol employs redundant packets, with an automatic resend of any packets that are scrambled. Both PROFIsafe and the PROFINET standard upon which it runs are network agnostic. Neither cares if the transport medium for the information is wired or wireless.

In the case of the Siemens wireless access point products used by the mobile panel, patented technology guarantees communication between devices. The Siemens Mobile Panel 277F with safety functionality also requires mandatory communication with a failsafe CPU compatible with PROFIsafe communication.

The device is certified according to Safety Integrity Level (SIL) 3. This is a safety related categorization arising from the EN 62061:2005 standard that applies to the functional safety of safety-related electrical, electronic and programmable electronic control systems. In addition to typical HMI or human machine interface functionality, the mobile panel also has two enabling buttons and an emergency stop button.

While not tied down by a cable, the mobile panel does retain one of the safety related benefits of a tethered device. With a cabled device, it’s easy to tell which piece of machinery a panel is communicating with. It’s the one that the panel is wired to. Thus, accidentally interacting with another machine is impossible, a situation that ensures that a command to stop or start is received and acted upon by the appropriate system. What’s more, if the cable is of the right length and the connection point correctly located, the cable itself helps guarantee that operators are in a safe place when machinery starts or stops.

The same isn’t necessarily true with wireless panels. The freedom to move about is also the freedom to be in the wrong place and to communicate with an unintended system. Standard wireless data technology is of little help in solving these problems. The range of the 802.11 standards can be hundreds of feet and determining the location of transmitter and receiver with respect to one another is difficult.

However, the use of another wireless technology, radio frequency ID or RFID, can overcome these issues. RFID has a range measured in a few tens of feet, is low power, and has its own set of governing standards. The Siemens Mobile Panel 277F with safety functionality uses this technology in a transponder that limits the effective ranges in which the panel can be identified.

In effect, this approach replaces a wired tether with a wireless one. In this way, the operator has failsafe control of the right machine. Furthermore, the assignment of operating screens and operator authorizations is clear anywhere in a plant. The transponder allows the definition of zones where certain functions are enabled, another safety feature.

As this mobile panel makes clear, the promise of wireless freedom can be realized, thereby enhancing productivity and safety. However, doing so safely requires devices that meet the right certifications and offer the right features. With that, it’s possible to cut the strings that have been holding automation applications—and operators—down.