Select The Right Safety Logic System

Understanding the complete spectrum of safety logic systems can help users specify the correct ones for particular applications.

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

When designing safety into machinery, engineers need to ensure compliance with today’s more-stringent standards, while also factoring in how safety will interplay with productivity to keep downtime to a minimum. They also should consider how flexible and scalable a safety system will be with expanding or evolving operations. Maintenance personnel—often armed with experience working on legacy equipment—need to help ensure that a new or upgraded safety system is designed to properly mitigate safety risks.

Working together, engineering and maintenance departments face the challenge of determining which safety logic system is right for each application. Their goal is to select the best, most cost-effective option that can ensure compliance, while maintaining optimal production capability and flexibility.

Pivotal changes

A combination of contemporary safety standards and advances in technologies has brought machinery safety to the connected, information-enabled state in which it now exists.

The implementation of ISO 13849 and IEC 62061, and the withdrawal of EN 954-1 in 2011, ushered in a new era of safety standards. EN 954-1 specified safety-function characteristics and performance categories, but it didn’t require risk to be measured using quantitative calculation. Today’s more rigorous standards require engineers to assess and document the reliability of a safety system by adding quantitative calculations to the design. This includes factors such as proving component reliability (mean time to dangerous failure) and common-cause failure fractions (design, wiring, and assembly issues that could cause system failure).

As a result, these new standards allow a more methodical risk-assessment process. When combined with the latest programmable safety technologies, machinery can achieve more predictable performance, greater reliability, and better return on investment. This is helping manufacturers improve the bottom line without losing sight of safety.

Out with the old

Compare a legacy safety system to the more advanced safety systems entering the market. The differences are night and day.

Legacy safety systems consist of standard programmable logic controllers (PLC), with each input, logic, and output safety device hardwired. The significant amount of wiring involved in these systems makes installation more complex, resulting in longer start-up times and more difficult system upgrades. Legacy systems also lack diagnostics. Consequently, troubleshooting takes more time during downtime events, as technicians need to manually locate the problem, identify the root cause, and then fix the issue. Meanwhile, production remains at a standstill.

The contemporary electronic safety systems that are replacing these dated systems deliver a streamlined architecture, meaning that safety applications can be programmed using the same software that is employed for the control and motion systems.

Such integrated safety systems can help optimize safety, enhance productivity, and reduce costs in multiple ways:

Simplified wiring. I/O (input/output) devices can be directly wired to the safety I/O modules that communicate with programmable safety systems, using a single network cable 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 identify the location and the root cause of any
safety event.

Greater flexibility. Uptime-enhancing strategies, such as zone control—wherein an area being serviced either stops or comes to a safe speed, while unaffected production areas continue to operate as normal—are easier to implement and expand.

Safety logic systems are scaled from simple single-input relays to more comprehensive integrated systems. Choosing the most appropriate system for an application often can be difficult, given the number of factors to consider, including:

• Category or performance-level (PL) requirements
• Functional requirements
• Control requirements
• System size and footprint
• System complexity, logic requirements
• Process complexity
• Zoning requirements
• Safety monitoring, diagnostics, and information
• Documentation, validation, and reporting
• Cost.

The following overview of available systems can help in the decision-making process. Note that safety-design tools are available to help incorporate them into your systems.

Contemporary electronic safety systems deliver a streamlined architecture, meaning that safety applications can be programmed using the same software as control and motion systems.

Safety relays

Pros: Cost-effective solutions for the simplest safety functions

Cons: Less flexible, less cost-effective, and more physically burdensome for larger systems that use several zones and safety inputs

Safety relays are appropriate for minimal zone control with local hardwired I/O. They use simple safety logic—with little-to-no motion-control capabilities. Various safety-relay options are available, from basic single function and single-input designs, to more advanced configurable devices, for a range of safety functions.

Single-function relays. These types of devices are designed for relatively small safety applications and simple machines needing single-zone control. In the past, such relays have generally been limited to providing local diagnostics using LED indicators. Now, gateways allow them to send diagnostic data to a controller or human-machine interface (HMI).

Dual-input relays combine the functionality of two safety relays into one device. They are best suited for small standalone machines. Any logic that is used with these relays is usually configured by switches on the relay—and is very limited, usually to simple Boolean or time-based functions. Dual-channel relays also usually provide only LED-based local diagnostics.

Modular safety-relay systems. These expandable 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, including mats, light curtains, and switches, and to enable zone control.

Modular safety relay systems will usually have some type of backplane or bus and a master module to aggregate and/or control information between relays. They also offer diagnostic and communication functionality, and can provide detailed safety relay data to an HMI or controller on a fieldbus network.

Configurable safety relays. Flexible and easy-to-use configurable safety relays are ideal for applications that require multiple safety circuits and control several zones. These relays allow engineers and maintenance professionals to create, control, and monitor the safety system in the same software environment as the standard controller. This approach reduces programming time and can help increase productivity.

Configurable safety relays also offer more advanced connectivity than other devices, with embedded communication capabilities that enable users to easily perform partial or conditioned shutdowns. Significantly more information is available to the user, including I/O values, logic status, and diagnostics.

These data can be communicated to controllers or graphic terminals, and local diagnostics are available using LEDs or simple displays.

Programmable safety controllers

Pros: Cost-effective “middle” solution for safety applications that land between a relay and an integrated system; appropriate when there is an existing, standard machine controller and engineers want to add safety

Cons: Lack of advanced HMI diagnostics is cumbersome for large systems.

A general-purpose programmable safety controller can provide more advanced functionality for safety applications that require some complex logic—needs that a safety relay won’t quite meet. This could include systems that require multiple safety zones (three or more), distributed safety I/O, or interlocking with other safety controllers.

Programmable controllers can also be better fits for systems where a safety PLC would be excessive. This could include instances where a safety network is all that is needed or when simple and uncomplicated software is desired.

Integrated safety systems

Pros: Best suited for large, complex, and integrated systems; incorporates safety and standard control and I/O into one controller, providing more advanced and flexible safety functionality and greater connectivity; also offers the most advanced HMI diagnostics.

Cons: Most-expensive option, but increased cost is often offset by reduced wiring efforts/costs and reduced panel space, as well as improved diagnostics, flexibility, and productivity.

Integrated safety systems are the best solutions for applications that require advanced logic. They are well suited for situations where a large physical space needs to be safeguarded, or when a modular and scalable system is needed.

These controllers are designed for use in systems that have more than three zones of control, multiple axes of motion control, and high I/O counts, including as many as 250 dual-channel inputs and 100 outputs.

An integrated safety system uses dual processors to run all standard control functions and safety-control functions simultaneously from a single safety PLC platform. Safety memory can be locked and protected, so it won’t be modified, while all standard functions (motion, drive, sequential, and process) 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 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.

The 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. The overall machinery-performance benefits this approach provides are substantial, compared with more-conventional architectures.

In addition to optimizing plant safety, improving machinery uptime, and enhancing productivity, integrated technologies can help reduce design, programming, and system start-up time. Not only do they simplify wiring and network integration, these systems can accommodate future safety changes better than those that are hardwired.

According to a recent Aberdeen report, best-in-class manufacturers are 48% more likely than their competitors to integrate safety systems with their plant-floor automation systems. With these investments, such manufacturers (the top 20%) achieved a 90% overall equipment effectiveness (OEE) rate, a 0.2% repeat-accident rate, and a 2% unscheduled asset-downtime rate. Conversely, laggard performers (the bottom 30%) achieved a 76% OEE rate, a 10% repeat accident rate, and a 14% unscheduled asset-downtime rate.

Safety standards and technologies will continue to evolve, and the future points to more options and more flexibility to apply safety technology to meet specific needs. As safety and standard components become more seamlessly integrated into control-system designs, implementing safety should no longer be a separate discipline. Instead, it should be a concurrent and more fundamental part of the design process.

Regardless of the application, carefully evaluating risks and determining appropriate mitigation strategies in the early stages of machine design will help engineering and maintenance teams select the right safety solution. In turn, making safety a more natural part of the design process will help keep employees and machinery safer, while helping to improve the bottom line. 

Information in this article was provided by Brian Taylor, safety components business director, and Tim Roback, safety manager, at Rockwell Automation (rockwellautomation.com), Milwaukee.

To learn more about safety logic systems, visit:

isa.org

odva.org

dguv.de/ifa/Praxishilfen/Software/SISTEMA/index-2.jsp

rockwellautomation.com/global/solutions-services/capabilities/safety-solutions