Bridging Safety Networks – Why One Size Does Not Fit All

05/23/2007


In safety networks, as in shoes, not only is it true that one size doesn’t fit all but it’s also the case that design matters. A shoe that’s right for a formal affair is likely to turn an ankle on a sports court. Likewise, a network suited for instrumentation at the field level, for example, could make a plant floor stumble, or worse, if misapplied.

Engineers would like to pick the right safety network for the job, just as they can do for control aspects thanks to open networks. Now that choice of the best solution– even if it means using two networks – is possible due to Siemens F-Link and related devices. These devices link bit signals from binary safety sensors on the field level AS-i network to the byte level Profibus, bringing flexibility while cutting costs and boosting productivity.

In the past, such a dual network solution wouldn’t have been available. Instead, engineers faced two choices. In one, they could struggle with a hard-wired approach. The drawbacks here include a lack of flexibility during the design phase, high cost, and being hostage to intermittent connections. During operation, an intermittent connection can lead to regular machine shut downs because safety systems detect the fleeting interruption as a failure. The intermittent nature of the failure– and Murphy’s Law – almost guarantees that the connection will be fine when tested. Thus there’ll be seemingly nothing to fix, and the system will be started up only to go down again when the intermittent connection opens once more. Such a scenario proves Einstein’s Theory ofRelativity, as the time this yo-yoing goes on may seem like an eternity to those charged with finding and fixing the problem.

The other previous choice for engineers was to suffer through design trade-offs and performance drop-offs as they shoe-horned a less than optimum network into a plant floor. Safety is basically an on-off signal and safety switches should be placed in optimum locations, which means E-stops are typically separated by 10 or 20 feet from one another for switch after switch after switch. So with the block style I/O approach used in some networks, engineers confront two bad options. They can put in a drop every 20 feet, which is expensive, or they can opt to make the drops further apart and run expensive wiring from the drops out to each switch.

However, AS-i is well suited for this type of task, which is one reason why Safety as Work, embodied in Siemens ASIsafe, makes so much sense in such settings. The wiring is inexpensive and can run for long distances. Over such spans, an ASIsafe approach can collect the binary safety signals from hundreds of switches, making it possible to plant E-stops where needed. What’s more, because of its architecture, ASIsafe can achieve the highest SIL category, SIL4, with little additional cost. All that’s needed to reach a category 4 is to bring two contact signals into a system.

However, ASIsafe isn’t the right safety network for everything on the plant floor. Examples where another network is needed are in higher level functions such as is found in a PLC, an HMI panel for an operator, and a GUI for easy visualization and control. A network for the bit-level chatter of instruments and safety switches also isn’t the best one to handle integration with manufacturing information systems (MIS).

For such needs, a byte-oriented network like Profibus is the best answer. These networks sit above and abstracted from the field instrumentation level, just as the functions of a PLC, GUI, or MIS sit above and abstracted from the field instruments themselves.

So the best network solution for a given site might well be a mixture of AS-i and Profibus, but the challenge has been bridging the two, at least as far as safety signals are concerned. That's no longer the case, thanks to Siemens recently introduced F-Link. It brings the two networks together, acting as a gateway between ASIsafe and Profibus. Both safety and standard data signals move through it, with the link having either screw-type or spring-loaded terminals and all the appropriate safety certifications.

Being able to tie the networks together as needed brings a number of advantages. For one thing, the simplicity and ease of AS-i translates into easy design changes, even those that pop up in mid-stream after a project is already well underway.

Another characteristic benefits both original equipment manufacturers and those running machines on the plant floor. With F-Link, the system captures diagnostic information, allowing the trapping of a signal that can pinpoint exactly where a failure occurred. This data is available even if the failure is only momentary and fleeting.

For OEMs, this capability means that labor is reduced and machines are built faster, all because the diagnostics are more precise. That speed translates into more product out the door in a given amount of time and smaller lead times, both achieved without adding labor. For end users of the machines, better diagnostic information results in more uptime since the dreaded intermittent connection can no longer hold a production line hostage. When a fault appears, the switch responsible will be known and so troubleshooting will be faster.

Sometimes the right network for the job isn’t one– it’s two. With the ability to seamlessly bridge bit and byte level safety networks, F-Link allows engineers to pick the right one for the job, rather than settle for a less than optimum solution. That’s the way to put a plant floor on the right footing.

For more information concerning this or any other Siemens product or service, please stop by www.asksiemens.com/5





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