Selecting the right safety logic system

Understanding the complete spectrum of safety logic systems can help ensure you choose the right system for your application.

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

When designing safety into your machinery, you need to ensure compliance with today’s more stringent standards, but you also must factor in how safety will interplay with productivity to keep downtime to a minimum. You also likely need to consider how flexible and scalable your safety system will be to keep up with your operations as they expand or evolve.

This article will cover the wide array of safety logic systems that are available today, discuss the pros and cons of each, and provide guidance to help you determine which system is right for your application.

Pivotal changes

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

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 you to assess and document the reliability of a safety system by adding quantitative calculations to your design. This includes 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 for 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 (ROI). This is all helping manufacturers improve the bottom line without losing sight of safety.

Your challenge is to select the best, most cost-effective safety system that ensures compliance while also maintaining optimal production capability and flexibility.

Out with the old

Compare a legacy safety system to the more advanced safety systems brought to the market today — 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. Additionally, legacy systems lack diagnostics. As a result, troubleshooting takes more time during downtime events because technicians need to manually locate the problem, identify the root cause and then fix the issue. Meanwhile, production remains at a standstill while this takes place.

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 as your control and motion systems. Such integrated safety systems can help you 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, which communicate to programmable safety systems via a single network cable, to reduce your wiring costs and improve installation times.
• Improved productivity: Flexible programming allows engineers to create maintenance modes of operation, such as safe speed or partial shutdown, to minimise machinery downtime.
• More advanced diagnostics: Detailed information can easily be made available to operators and maintenance technicians so they can immediately identify the location and root cause of a safety event.
• Greater flexibility: Uptime-enhancing strategies, such as zone control — in which an area that is being serviced either stops or comes to a safe speed while unaffected production areas continue to operate as normal — are easier to both implement and expand.

The spectrum of safety logic systems

Safety logic systems are scaled from simple single-input relays to more comprehensive integrated safety systems. Choosing the right system can be difficult, as there are a number of considerations that you need to factor in:

1. Category or Performance Level (PL) requirements
2. Functional requirements
3. Control requirements
4. System size and footprint
5. System complexity and logic requirements
6. Process complexity
7. Zoning requirements
8. Safety monitoring, diagnostics and information
9. Documentation, validation and reporting
10. Cost

The following overview of safety logic systems is intended to help guide you through the decision-making process.

Safety relays

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

• Pros: Cost-effective solutions for your simplest safety functions.
• Cons: Less flexible, less cost-effective and more physically burdensome for larger systems with several zones and safety inputs.

Safety relays are available in three basic types: single-and dual-input standalone relays; modular safety relay systems; and configurable safety relays.

Single-input relays are designed for relatively small safety applications and simple machines needing single zone control. These devices are limited to providing local diagnostics using LED indicators. Nowadays, gateway devices are also available to transfer diagnostic information to higher level devices and HMIs.

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

Sample applications for single- and dual-input relays would be package wrapping, form filling, cutting and slicing.

Modular safety relay systems are expandable single-relay systems that can provide safety control for larger, more complex manufacturing equipment. They allow you to combine multiple input and output modules per base unit 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 or control the information between individual relays, and they also offer diagnostic and communication functionality, and can provide error statuses to an HMI on a fieldbus network.

Sample applications for modular safety relays would include package palletising and carton filling.

Configurable safety relays are more flexible and easy to use, and are suitable for applications that require multiple safety circuits and control of several zones. These relays allow engineers to create, control and monitor the safety system in the same software environment as the standard controller, which reduces your programming time and can help increase productivity.

They also offer more advanced connectivity than other relays, 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. Diagnostic data can be communicated to controllers or graphic terminals, and local diagnostics are often available using LEDs or simple displays.

Sample applications for configurable safety relays would also include package palletising and carton filling.

Programmable safety controllers

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

It also can be a better fit 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.

• Pros: Cost-effective ‘middle’ solution for safety applications that land between a safety relay and an integrated safety system; ideal when there is an existing, standard machine controller and you want to add safety.
• Cons: Lack of advanced HMI diagnostics is cumbersome for large systems.

Sample applications for programmable safety controllers include loading/unloading bays and sealing and converting machines.

Integrated safety systems

Integrated safety systems are the best solutions for safety applications that require advanced logic. They are ideal when a large physical space needs to be safeguarded or when you need a modular and scalable system. These controllers are designed for systems that have more than three zones of control, multiple axes of motion control and high I/O counts, including up to 250 dual-channel inputs and 100 outputs.

Integrated safety. For a larger image click here.

An integrated safety system uses dual processors to run all of your standard control functions and your safety control functions simultaneously, from a single safety PLC platform. Safety memory should be able to be locked and protected so it can’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 you to display safety status 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.

• 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 this increased cost is often offset by reduced wiring efforts/costs and reduced panel space, as well as improved diagnostics, flexibility and productivity.

Sample applications for integrated systems include integrated packaging and bottling lines, flexible automotive assembly lines, metal forming and coating lines, printing presses.

The integrated future

The full range of safety logic systems will continue to provide effective and affordable safety functionality for the foreseeable future, but manufacturers and industrial operators are moving towards an integrated safety approach because of the overall machinery performance benefits that it can provide compared to more conventional architectures.

Integrated safety systems can do more than optimise safety in your plant. They can also help improve machinery uptime and serve as productivity enhancers. Integrated safety technologies can help reduce the amount of time it takes to design, program and start up a system. They can simplify your wiring demands and network integration, and they can better accommodate future safety changes compared to hardwired systems.

According to a recent Aberdeen report, best-in-class manufacturers are 48% more likely than their competitors to integrate their safety systems with their plant-floor automation systems. With the help of these investments, best-in-class 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. On the other hand, laggard performers (the bottom 30%) achieved a 76% OEE rate, a 10% repeat accident rate and a 14% unscheduled asset downtime rate.

Conclusion

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 continue to become more seamlessly integrated into control system designs, implementing safety should no longer be a separate discipline, but rather a concurrent and more fundamental part of the design process.

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

*Brian Taylor is Safety Components Business Director, while Tim Roback has the title of Safety Marketing Manager at Rockwell Automation.

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