Selecting the right safety logic system

Choosing the correct system for an application can help optimise safety, enhance productivity and reduce costs. Brian Taylor and Tim Roback* explain.

Machine safety systems have evolved from strictly hardwired systems, where safety and standard logic were separate, to the multifaceted spectrum of newer programmable systems with varying levels of cost, complexity and wiring methods.

When designing safety into machinery, engineers must verify compliance with today’s more-stringent standards and determine how safety will interplay with productivity to keep downtime to a minimum. Engineers also should consider how flexible and scalable a safety system will be with evolving operations.

Pivotal changes

The implementation of ISO 13849 (AS/NZS 4024.1503:2014) and IEC 62061 (AS 62061), and the withdrawal of EN 954-1 in 2011, ushered in a new era of safety standards.

These more rigorous standards required engineers to assess and document a safety system’s reliability by adding quantitative calculations to the design. These standards allow a more methodical risk-assessment process and, when combined with the latest programmable safety technologies, can help machinery achieve more predictable performance, greater reliability and better return on investment.

Out with the old

Legacy safety systems consist of standard programmable logic controllers (PLCs), with each input, logic and output safety device hardwired. Significant wiring requirements make installation more complex, resulting in longer start-up times and more difficult system upgrades. A lack of diagnostics make troubleshooting more difficult and can lead to longer downtimes. Contemporary electronic safety systems deliver a streamlined architecture, meaning safety applications can be programmed with the same software used for the control and motion systems.

This can help optimise safety, enhance productivity and reduce costs in multiple ways:

• Simplified wiring: I/O devices can be directly wired to the safety I/O modules that communicate with programmable safety systems to reduce wiring costs and shorten installation time.

• Improved productivity: Flexible programming allows engineers to create maintenance modes of operation, such as safe speed or partial shutdown, to minimize machinery downtime issues.

• More advanced diagnostics: Information can easily be made available to operators and maintenance teams, allowing them to quickly troubleshoot safety events.

• Greater flexibility: Uptime-enhancing strategies, such as zone control, are easier to implement and expand.

Safety systems

Choosing the most appropriate system for an application often can be difficult, given the number of factors to consider.

The following overview of available systems can help ease the decision-making process. Safety relays are appropriate for minimal zone control with local hardwired I/O. Various options are available:

• Single-function relays are designed for relatively small safety applications and simple machines needing single-zone control. Gateways allow them to send diagnostic data to a controller or HMI.

• Designed for stand-alone machines, dual-input relays combine the functionality of two safety relays into one device. Any logic used with these relays is usually configured by switches on the relay and is very limited. Dual-channel relays usually provide only LED-based local diagnostics.

• Modular safety-relay systems provide safety control for larger, more complex manufacturing equipment. They allow engineers to combine multiple single-function relays to support multiple safety devices, and also offer diagnostic and communication functionality.

• Configurable safety relays are designed for applications requiring multiple safety circuits and control several zones. They enable a safety system to be created, controlled and monitored in the same software environment as the standard controller, which reduces programming time and can help increase productivity.

Advanced connectivity makes more information — including I/O values, logic status and diagnostics — available to users. A general-purpose programmable safety controller can provide more advanced functionality for safety applications that require some complex logic.

This could include systems that require multiple safety zones, distributed safety I/O or interlocking with other safety controllers.

Programmable controllers also are appropriate for applications where a safety PLC is excessive, such as when only a safety network is needed, or when simple and uncomplicated software is desired. Integrated safety systems are ideal for applications requiring advanced logic, situations where a large physical space needs to be safeguarded, or when a modular and scalable system is needed.

They are designed for use in systems that have more than three zones of control, multiple axes of motion control, and high I/O counts. An integrated safety system uses dual processors to run all standard control functions and safety-control functions simultaneously from a single safety PLC.

Safety memory can be locked and protected, while all standard functions work as they would on a regular controller. Standard logic and external devices can read safety memory within an integrated safety system, allowing the display of safety statuses on human-machine interfaces (HMIs), displays or marquees.

Multiple safety PLCs in an integrated safety system can share safety data for zone-to-zone interlocking, and a single safety PLC can use remote distributed safety I/O between different cells or areas.

Integrated future

While the full range of safety logic systems will continue to provide effective and affordable safety functionality for the foreseeable future, manufacturers are moving toward an integrated approach.

In addition to optimising safety, uptime and productivity, integrated technologies can help reduce design, programming and system start-up time.

They also simplify wiring and network integration, and can accommodate future safety changes better than hardwired systems.

*Brian Taylor is safety components business director with Rockwell Automation, and Tim Roback is safety marketing manager with Rockwell Automation. The article was first published in the Journal From Rockwell Automation and Our PartnerNetwork is published by Putman Media, Inc.

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