Safety Network, Is one enough?

By Control Engineering Staff September 19, 2007

Communication systems that cooperate will achieve a better overall level of safety, than single ‘universal systems’.

In safety networks, not only is it true that one size doesn’t fit all, but it’s also the case that design matters. A network intended for instrumentation at the field level, for example, could cause in unnecessary downtimes or worse, if misapplied.

Engineers would like to pick the right safety network for the job, just as they can for control aspects thanks to open networks. Now that choice of the best solution—even if it means using two networks—is possible because of 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 safety network solution wouldn’t have been available. Instead, engineers are faced with 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 held 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 and will continue to be a big challenge to those charged with finding and fixing the problem.

The other 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. 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 safety device.

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 and can be tapped off anywhere to start a new branch. Over such spans, an ASIsafe approach can collect the binary safety signals from hundreds of safety input devices, making it possible to put E-stops, interlock switches, light curtains etc, where needed. What’s more, because of its architecture, ASIsafe can achieve the highest safety category (CAT4/SIL3). All that’s needed to reach a category 4 is to bring two contact signals into a system, have the appropriate monitoring, which is done here, through ASIsafe.

Sometimes, ASIsafe doesn’t fulfill all the demands of complex applications.For example, an application requiring a large safety I/O count, a complete Tandem Line automotive application or one which requires greater safety programming flexibility may not have needs met by ASIsafe. Other examples where another network is needed are in higher level functions such as those 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 devices themselves.

So the best network solution for an application could 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 is no longer the case, due to the F-Link, which transfers Safe data from the ASIsafe components to the PROFIsafe system via standard data traffic between the two systems. The F-Link, having the appropriate safety certifications, allows for real automation integrated solutions where global e-stops are required, or where faster responses are needed to improve stop functions. Real cell interaction can be constructed using the standard topologies of ASi and PROFIBUS without any compromise to the overall control philosophy or design.

For the first time this allows safety data to be transferred between production cells to bring better coordinated functional safety across wider plant areas. Using innovative design this clears the path towards more sophisticated safety engineering systems covering multiple plant areas.

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.

There is another benefit to 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 when a failure occurred. This data is available even if the failure is only momentary. The advanced diagnostics capability also means that we can eliminate a second possible point of concern with single cable solutions, by the good diagnostics interaction between system and user. This also means that any fault in the cabling (open circuits, short circuits, short circuits to ground etc.) is immediately detected. On detection the system is switched off in the appropriate manner for the stop category. System restart is inhibited until the fault is cleared and the fault indicated either by means of LED’s on the front of the F-Link or via a HMI device.

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 device 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. Just about every possible safety application, including robotics, conveyor systems, handling systems, airport carousels and lots more, can benefit from this F-Link solution by providing a cost-optimum, easy to handle system with greater safety programming power and flexibility to achieve the safety demands of the application.

Making it safe to say that “Only networked safety networks can solve your safety design constraints”