Most machine automation systems have a few points of safety-related I/O mixed in with many more points of standard I/O. There are many ways to handle a small amount of safety I/O, but most of these approaches have drawbacks.
Safety I/O can be controlled by a hardwired relay system, while standard I/O is controlled by a modern digital control system via a high-speed digital network. This solution is tried and true, but hardwired systems have many drawbacks, including a lack of diagnostic information.
A simple but expensive solution is to control the entire machine with a safety controller, safety I/O, and a safety network. Yet another expensive solution is to provide two automation systems, one safety-rated and one not.
A somewhat more elegant solution for controlling safety I/O now is provided by Beckhoff Automations new TwinSAFE bus terminals. This new product line consists of a safety controller, a safety input terminal, and a safety output terminal. The product line recently received all the required certifications for use in North America.
TwinSAFE bus terminals from
As alluded to by Harris, machine automation systems with TwinSAFE terminals can be implemented with standard controllers and standard digital networks, saving the expense of safety-rated components.
There are three products in the new system. KL6904 TwinSAFE bus terminal contains a safety-rated controller, four safety-rated outputs, and interfaces with Profibus and EtherCat digital networks.
KL1904 safety bus terminal has four digital inputs with potential-free contacts rated at 24 VDC. KL2904 safety bus terminal has four 24 VDC digital outputs rated at 0.5 amps.
A typical Beckhoff Automation machine control system would have a standard, non-safety rated controller communicating to standard I/O and to one or more KL6904 safety-rated controllers.
In effect, KL6904 is a distributed safety controller. It has four safety-rated outputs but no inputs, so it must be used with a KL1904 input terminal. Communication between the controller and the inputs is via the main host controller. KL1904 lets machine builders implement safety in small I/O increments and in a distributed fashion.
The result, says Beckhoff, is a low-cost control system with safety-rated components applied only as needed. For small configurations, the tasks of a failsafe PLC can be handled within the KL6904, relates Rob Rawlyk, Beckhoffs applications manager. For increased functionality, multiple KL6904s can work together. This is a far more streamlined and cost-effective alternative than a traditional safety PLC.
Rawlyk adds that TwinSAFE bus terminals extend the tried and tested Beckhoff I/O system with safety functions. Safe signals can be mixed with standard signals and are easily distinguished by terminal color because safe terminals have yellow plastic housings while standard terminals have white plastic housings, he says. Maintenance is simplified significantly through faster diagnostics and by easy replacement of components.
The TwinSAFE bus terminal system is just now being introduced in North America, but it has been in use for some time in Europe.
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