Past safety standards allowed only for hard-wired safety components. New approvals such as IEC 615081, a standard of the International Electrotechnical Commission (www.iec.ch), and revised standards such as NFPA 792 (2002 edition, otherwise known as the National Electrical Code) promulgated by the National Fire Prevention Association (www.nfpa.org), are in place to accommodate safety networks and provide new standards for protection of life and limb. These new standards have led to a surge of interest in safety networks, not only for their improved safety functions, but also for the return on investment they promise when implemented with other networks.
Costly wiring
Despite the level of importance safety systems carry, there is an ever-present challenge for safety to be cost-effective while maintaining the high level of reliability and accuracy the systems require. Traditionally, safety systems are constructed using point-to-point hard-wired logic and safety relays that are connected to switching components and emergency stop buttons located in the field. Multiple wires from each device, fed through control cabinets, often result in large wire bundles running through the system. Due to the sheer volume of wires, installation time is considerable, and troubleshooting is complex.
Today, there are many different types of industrial networking or “fieldbus” technologies on the market. AS-i, DeviceNet, Ethernet and Profibus are just some of the systems gaining popularity, as engineers search for ways to streamline controls within their plants. Additional wiring, input cards and other compatible equipment are often needed to provide accurate diagnostics and process feedback. This increase in cost, as well as the increased level of data transmission, opened the door for these industrial networks.
As industrial controls make the progression to industrial networks from hard-wire-based controls, companies have begun to develop safety networking systems that function directly with these networks. One of these is Safety at Work. Developed by a consortium of experts on industrial controls, Safety at Work was designed to simplify wiring and overcome the challenges facing safety systems that operate with point-to-point relay logic.
The Safety at Work system is one of the first fully approved industrial safety networking systems. It carries approval from TüV, an international certification organization, for application up to control category 4 according to EN954-1 and SIL 3 according to IEC 61508. The system uses one two-wire flat cable that integrates directly with most standard PLCs, as well as most bus systems.
Safety at Work is comprised of three main components. A safety monitor resides in the main control cabinet and is responsible for the translation of safety messages throughout the network. An AS-i fieldbus network provides the communication infrastructure throughout the system. Safety input modules are used to interface directly with standard safety switching components and integrated Safety at Work emergency stop buttons.
In operation, each field safety module stores a particular data sequence. During installation and start up, the safety monitor learns these code sequences. While in operation, the safety monitor constantly compares the target sequence with the current sequence of the safety module. If a safety module provides a wrong code sequence, the safety monitor switches to the safe state.
A Control Platform Interface connects the network with most standard PLCs and higher-level networks. The system has the ability to handle multiple safety zones on one system.
Garrett Place, [email protected], is a product manager at ifm efector.
