Safety PLCs Connected to Systems Using PROFINET Provide High levels of Protection
Safety has become an important aspect of equipment design over the past several years, but it’s still a challenging issue for those who design and integrate industrial automation technology. It’s important to understand the differences between safety PLCs and PACs as well as the network’s safety protocols.
All industrial processes have some level of risk for injuring people, damaging investments or destroying nature. In some processes it may be easy to avoid risk without imposing special requirements on automation systems. However, there are typical applications associated with high risk, such as presses, saws, tooling machines, robots, conveying and packing systems, chemical processes, off-shore technology and fire and gas sensing. These applications need special functionality and technology.
In years past, safety automation relied on relays. Relay-based system were hard-wired, leading to increased costs due to additional wiring and engineering. They were also less flexible with lower availability. Stop positions of machines were undefined, so resuming operations required tedious, time-consuming efforts.
In the U.S., this situation changed dramatically with the enactment of NFPA 79 2002, which permitted the use of safety PLCs. That was an important step for product designers, since safety relies on trusted technology or material and tested safety products.
Controllers and software that have been proven in use in industrial applications are now fulfilling machine safety standards such as the IEC 61508, ISO 13849-1, NFPA 79 and others. As these controllers take over more safety requirements, there’s an increasing focus on the networks that tie them together.
Examining one of the safety protocols, PROFIsafe, shows how this aspect of safety is implemented. PROFIsafe is an integrated safety protocol that has an additional layer on top of the existing Industrial Ethernet (PROFINET) protocol. PROFIsafe secures the whole path from the location where a safety signal originates (such as an F-Module in a remote I/O device) to the location where it is processed (F-Host) and vice versa. It reduces the error probability of the data transmission between an F-Host (safety controller) and an F-Device to the level required by or better than the relevant machine safety standards. It is possible to transmit safety messages on the existing standard bus cables in coexistence with the standard messages.
Using this approach also allows the use of standard PLCs with integrated but logically separated safety processing (diverse operation).The safety controller using PROFIsafe has additional in-built safety measures which include:
· The consecutive numbering of the PROFIsafe messages (“sign-of-life”)
· A time expectation with acknowledgement (“watch-dog”)
· A codename between sender and receiver (“F-Address”)
· Data integrity checks (CRC = cyclic redundancy check)
Using PROFIsafe’s Consecutive Number lets a receiver can see whether or not it received the messages completely and within the correct sequence. In safety technology, it not only matters that a message transfers the correct process signals or values. The updated actual values must arrive within a fault tolerance time, letting the respective F-Controller/Device automatically initiate any necessary safety reactions such as a stoppage of movement.
For this purpose, the F-Devices utilize a watchdog timer. This is not the case in standard controllers. Different codenames between unique sender receivers is not a part of the standard controller. A cyclic redundancy check (CRC) plays a key role in detecting corrupted data bits during transmission. Each safety controller has the safety certification mark on the controller itself.
As in standard controllers, PROFIsafe can be used on any of the transmission channels be it copper wires, fiber optics, wireless, or it can be used on open Industrial Ethernet Backbones. It covers the need for high availability (not available in a standard controller) and low power consumption in process automation as well as the demand for short reaction times within milliseconds.
One of the major advantages is the possibility for devices to report diagnosis information to the operator in exceptional situations such as failures or errors, some of which not covered by a standard controller. Good diagnostic information helps reduce down times and related costs.
Modern F-Devices such as drives with integrated safety now can be designed in, and other fail-safe devices that work together with or as the safety controller to make a fully integrated safety controller solution.
The Safety PLC’s and related fail-safe I/O devices using the PROFIsafe protocol can be used for safety applications up to SIL3 according to IEC 61508 /IEC 62061,or PL “e” according to ISO 13849-1.
To conclude, here are some questions that should simplify the task of selecting safety controllers.
What questions do I need to ask while selecting a safety controller?
· Is the safety controller, certified for use in machine safety applications by the AHJ – Authority Having Jurisdiction?
· Has it been proven in use and how many devices are installed and for the OEM’s-worldwide installation & support?
· Can I get a completely integrated safety solution, i.e. Safety PLC’s, Safety drives, Safety Motor-starters, safety sensors etc?
· Does safety have wired and wireless capability and can it be used on a PC-based system, if required?
· Availability of safety diagnostics, is it available upto channel level?
· Is it easy to upgrade, 5 years down the line, without a big cost-impact or redoing the complete application?
·What about machine safety lifecycle support?
What technical questions do I need to ask about a safety controller?
· Do I need to protect my F-Controller/Device against very high voltages coming across the network cable from an unknown/other source?
· Is it safe to use the same 24V power supply that I use for the standard devices in my network?
· How do I test my F-Controller/Devices for the “increased immunity” that is required by IEC 61508?
· What are the installation rules?
· What are the security requirements?
See www.profisafe.net for more detailed information about PROFIsafe.Have an Inquiry for Siemens about this article? Click Here >>