Networking Safety - Having Your Cake and Eating It Too

Two more components need to be mentioned. The gateway enables communication on AS-Interface and exchanges data upwards with the PLC. Any of the commonly used upper level networks, from DeviceNet to PROFIBUS, PROFINET to EtherNet/IP, is supported. In a simplistic sense, the gateway is a translator between the upper-level network going to the PLC and AS-Interface. The other component is the safety monitor. This is where the safe contacts reside and it can be thought of as a “networked, fully configurable safety relay.”

When the e-stop is in its released state, it constantly sends the dynamic safety code, 4-bits at a time. The safety monitor reads this data, compares it to the expected data and, as it finds a match, concludes that the e-stop is in the released state. If, on the other hand, the door is opened, the data from the door interlock switch contains only 0000 packets, indicating to the safety monitor that the door has been opened. From the user point of view and for the purpose of diagnostics, the details of the safety code sequence are not important. What is important for the user is that the PLC receives the same information as the safety monitor and it is simple to add an additional rung of PLC logic (see Figure 4) that evaluates as true as soon as the guard door is opened or the e-stop is pushed. This logic replaced the need for the auxiliary contacts used on standard hardwired solutions.

But what about the two failure cases (welded contact and intermittent contact) we talked about earlier? To address those cases, we need to discuss another quality of the 4-bit data packets. Since each safety device still contains two redundant safe contacts internally, the inventors of SaW designed the system in such a way that the state of each contact is reflected in 2 of the 4 bits. The following table summarizes the possible states of a safety device and how those states influence the data transmitted over the network. And because this data is also transmitted to the PLC, a user application can evaluate it and identify previously undetectable states.

Thus far we talked only about why and how a networked safety solution simplifies and enhances safety diagnostics at the PLC level. As mentioned earlier, it is the safety monitor that makes safety happen. Quickly looking at the steps involved in setting up the safety monitor shows another reason why networking safety in general — and AS-Interface Safety at Work in particular — simplifies and enhances installations.

Once all components are connected to the two-conductor network cable, power is applied, bringing everything to life. The next step is assigning addresses to the safe devices. Pushing a couple of buttons on the gateway is all it takes in terms of configuration.

Lastly, a drag-and-drop software tool is used to define the logic that ties the states of safe devices to OSSDs. It is here where reset and simultaneity conditions are configured, timing behavior is specified and logic relations between various safe and non-safe inputs are set. This logic is then downloaded to the safety monitor and activated. Besides the fact that numerous types of safety relays are replaced by just one type of safety monitor, any future changes to the safe logic are performed though software. Once the network is up and running, wires are never touched again, clearly an advantage for any application.

APPROVALS

Before applying a new safety technology, check if the necessary local and international approvals have been obtained by device manufacturers. The good news is that networked safety can be used in applications up to CAT 4 (according to EN 954), SIL3 (according to IEC61508) and in some cases even PL e (according to ISO 13849). In North America, OSHA, by means of NFPA 79, has approved using networked safety since its 2002 maintenance cycle.

With automation equipment being built on either side of the Atlantic and then shipped across the ocean, it makes a lot of sense to utilize safety hardware that follows global standards and regulations. Safety networks, while not entirely new, still are new enough to embrace the idea of global market penetration and approvals.

Today's networked safety solutions are powerful and flexible. Still, engineers are working on additional solutions and devices, so it is a good idea to ask the supplier of any available solution for forward and backward compatibility. Another important issue is configuration software and tools. Will the tool that's available today still work in 5 years? And if not, what is the cost of keeping up with software developments?

State of safety device	Data transmitted on AS-Interface	Note
Normal operation, both safe contacts are closed	Dynamic code sequence	Usually this state is not directly evaluated by the PLC
Normal operation, both safe contacts are opened indicating that the safety device has been used to indicate a safety function (e.g., machine shutdown)	Static 0000 bit pattern	This input bit pattern is evaluated by PLC ladder logic using four “examine OFF” statements in series (see Figure 4)
One of the safe contacts in the safe device is welded	00xx or xx00, where xx indicates bit toggling while the 00 part is constant for as long as the safe device is activated	The PLC can evaluate this situation and automatically inform maintenance the very instant the error occurred. PLC programmers even have the ability to annunciate the required replacement part to further reduce downtime.
Intermittent contacts	Multiple occurrences of 00xx or xx00 followed by toggling 4-bit packets indicates an intermittent contact	The PLC can evaluate this situation and automatically inform maintenance the very instant the error occurred. PLC programmers can program the system to suggest the proper maintenance steps depending on the safety device exhibiting the fault.

Helge Hornis is responsible for Intelligent Systems at Pepperl+Fuchs Inc. He has worked with a large number of users on RFID and sensor bus solutions, has managed integration projects at key customers and was directly involved in the development of a unique 3D position measuring system. He also holds a seat on the board of AS-Interface USA, the North American arm of AS-International.