Is Safety I/O's Fault Detection and Diagnostics Worth the Price?

Mike Garrick, product marketing lead specialist,
Phoenix Contact Interface (www.phoenixcontact.com)

Open to Interpretation

Let's explain the technology at the heart of your question. In the past, seamless communication was nearly impossible, because no single network was able to integrate safety and standard control systems, while also enabling the seamless transport of data across multiple plant floor physical networks. That changed with emergence of standards such as the common industrial protocol (CIP). CIP is an application protocol for industrial networking that is independent of the physical network. The CIP provides a set of common services for control, configuration, collection and sharing across all of the CIP networks.

Given these integrated networking capabilities, the answer to your question depends on how it is interpreted.

Interpretation 1: You have a safety PLC with your safety devices wired to it, and each safety component has a third contact wired into a standard I/O point for machine diagnostics and annunciation. You also might have several components wired in series and connected to a single pair of safety PLC inputs, which means you probably do not have enough safety I/O to go around. If this scenario is accurate, then you are missing the boat. Rather than receiving information about which door is open or which light curtain is interrupted via additional standard PLC inputs, it would be much more cost-effective to receive this information across a communications network between the safety and standard PLCs. Using an integrated communication network, like CIP, would reduce labor time and costs associated with purchasing additional I/O and wiring the networks. Also, if you are wiring safety devices in series and running them into a single pair of safety PLC inputs, you are losing the ability to diagnose which of those devices are tripped. Use additional safety I/O, one pair of each device, and you will be able to tell exactly which device is causing your machine issues and get the machine back into production sooner. If you are using standard I/O with your safety PLC as part of the safety system, then you are not achieving SIL 3.

Interpretation 2: You are using an integrated controller that handles safety and standard control functions for your machine. This control architecture delivers value by its ability to perform safety and standard control with a single controller in a single application environment and often with a single network that supports the safety and standard data and communication. The value that the safety portion of the control system delivers is to help ensure the controller will respond to a demand with a lower probability of dangerous failure than a standard implementation. The higher the SIL or performance-level capability of the controller, the lower the probability the system will fail to danger. While a safety system including safety I/O does have higher diagnostics coverage and fault detection than standard I/O to achieve a SIL or performance level, these capabilities are used internally to detect faults and shut down the device, rather than provide additional status information to the operator. Using safety I/O where it's not required will add unnecessary cost and not deliver the incremental capabilities you mentioned. If this interpretation of your question is accurate, your system already should provide good diagnostics to identify device faults on the standard side, depending on the system you specified. In this case, take full advantage of your integrated safety controller to implement safety where it's needed and standard control where it's not, and then optimize your design.

Tim Roback, marketing manager, safety systems,
Jeff Gellendin, product manager, safety PLCs,
Rockwell Automation (www.rockwellautomation.com)

It's All About the AS-Interface

Since you are designing to SIL 3, you already have identified the added benefit of getting some diagnostics information. The unfortunate fact is that you are paying for this by having to run many individual wires, resulting in a high level of complexity during the design-and-build phase, not to mention the increase in possible failures at all those connection points.

Safety networks can do much more than provide better diagnostics as can easily be seen in the case of AS-Interface Safety at Work. This technology has been designed to reduce the overall cost of ownership by addressing all aspects of the installation.

For instance, with AS-Interface Safety at Work, users do not need a safe-rated PLC, and yet they still get SIL 3. Additionally, if an installation is designed to be controlled by PLC A and later needs to be switched to PLC B, the only piece of hardware that is swapped out is a gateway. The safety function remains untouched and works exactly the same way as before; no changes to the safety configuration are needed.

Diagnostics is another strong aspect of this technology. By connecting the safe devices to the AS-Interface network, the PLC will receive data concerning the state of the individual contacts; working with aux-contacts is finally a thing of the past. Armed with this information, a programmer can finally create intelligent HMI screens that not only show which safety door has been opened, but also point out if a magnetic switch is welded or an e-stop has a sticky contact.

The cost of switching to AS-Interface Safety at Work technology is surprisingly low.

Helge Hornis, Ph.D, manager, intelligent systems,
Pepperl+Fuchs (www.pepperl-fuchs.com)