Programmable Safety Controller: A New Paradigm for Safety

by Michael Paradiso, Application Engineer, OEM/Global Accounts Solutions Engineer, Fremont, CA

Beyond compliance with new international safety standards, programmable safety controllers offer designers flexibility and simplified configuration.

In motion control, safety has always been an important factor. Over the years, improving the safety in motion control processes has been a key focus of system designers. So the advent of safety programmable controllers (or safety PLCs) has changed the framework within which today’s manufacturers and system integrators approach these processes.

The new paradigm for flexible motion control eliminates hard relays and integrates the safety application into a programmable safety controller. This shift is fairly recent. And within the last five years, momentum has gained significant traction.

Technology typically precedes standards, which has been the case with programmable safety controllers. The drivers of programmable safety controller adoption include a host of international standards: EN ISO 13849-1/2 (safety of machinery—safety related parts of control systems), IEC/EN 62061 (safety of machinery—functional safety of electrical, electronic and programmable electronic control systems), and IEC 61131 (programmable logic controllers). As these standards have been instituted, industry has moved faster to incorporate the new safety technology.

The Evolution of the Paradigm
Many of today’s legacy safety applications are characterized by a “black box” approach; devices, systems, or objects that can only be viewed in terms of input, output, and transfer characteristics without any knowledge of their internal workings. This outmoded technology is frequently found in safety solutions that are separate from the automation systems, often implemented after the design, construction, and implementation of the machine or system.

This practice leads to a reactive approach to safety, requiring operators or maintenance personnel to bring a machine to a full stop, or putting it in a safe state, before working on the machine to correct the problem. Results include frustrating shutdowns, increased downtime, and lower productivity. In response, plant floor personnel often develop bypasses to disable the safety systems, increasing the risk of serious injury and not really improving productivity levels. Typically these safety systems involve hard wiring and electromechanical components such as relays. These systems are inevitably being replaced by redundant standard PLCs and programmable safety controllers.

By combining risk and time in the definition of safety functionality, the international standards offer a better approach to safety system design. The standards provide for better integration, more flexible configuration, and easier reporting of information to upper-level controls. Programmable safety controllers are bringing safety elements to the table that were previously unavailable. There’s no longer a need for hardwiring, but there is a growing need for systems to be monitored in a more advanced way.

Standard PLCs used in safety configurations are typically arranged in pairs. The second, redundant controller is used to achieve a safe and orderly shutdown if the primary controller fails. These PLCs also have additional I/O inputs for monitoring safety system outputs, and other outputs to test the safety system’s input modules. Further, safety applications using standard PLCs often require custom software to manage the safety system.

Programmable safety controllers are different. Standard PLCs typically have a single microprocessor to execute a program, Flash memory for storage, RAM for calculations, communications ports and I/O to detect and control the machine. Programmable safety controllers have dual microprocessors and circuits for synchronous detection and constant monitoring of Flash and RAM. Additionally, standard PLC inputs have no means of testing the functionality of their input circuits, while programmable safety controllers do.

Furthermore, while standard PLCs have a single output switching device, programmable safety controllers contain test points both behind and downstream of their output drivers. In a system with two safety switches, if a failure is detected at either switch (due to switch or microprocessor failure), or at the downstream test point, the operating system of a programmable safety controller automatically acknowledges system failure. Then it defaults to a known state on its own and triggers an effective equipment shutdown. The bottom line is that programmable safety controllers provide better monitoring and self-diagnostics than standard PLCs, and are an even further improvement over traditional hardwired electromechanical systems.

Benefits of Programmable Safety Controllers
There are quite a few benefits of programmable safety controllers including compliance with evolving safety standards, the ease of integration, a lower cost of ownership, an improvement in configurability and flexibility, and the leveraging of safety function.

Historically, manufacturers kept safety technology discrete from their automation systems. Companies that continue to do this will have to employ staff whose main function is to maintain, monitor, and control safety systems, all of which can be a significant cost, compounded by the greater complexity of the systems, which presents engineering, integration, and maintenance issues. Investing in programmable safety controllers can help companies manage risk better while facilitating compliance with international standards. Especially in today’s increasingly global marketplace, this can be a significant advantage.

Configuring a safety system with programmable safety controllers makes integration much easier. With the old method, monitoring one axis of movement requires one relay, one device. What if two or three or more axes of motion are needed? In the old paradigm more relays and contactors would have to be added and hardwired together into the safety system, which adds cost and complexity. With the new paradigm, a sensor can simply be added and programmed.

Another benefit is that the total cost of ownership goes down in multiple ways. First, the number of hardware components is reduced significantly. Second, there is less time and labor involved in building and implementing the safety system. Traditionally there might be 100 wires in a safety system. Now the same functionality can be achieved with only five wires, vastly simplifying the engineering effort. Moreover, fewer maintenance and dedicated resources are needed to operate safety systems designed with programmable safety controllers.

A safety light curtain illustrates how the new paradigm can improve productivity. In the past, when the operator was at the tool and had to reach in to remove or reposition a part, the entire process was stopped by the curtain. With a safety system designed to the new paradigm, the machine knows when a hazard isn’t present, allowing the action to take place without shutting down the entire process.

Taking Action
The advent of the programmable safety controller has empowered the development of active safety systems, not reactive ones. The old method slowed production, put operators looking to improve production at risk, and was more costly and less efficient over time. Now operators can keep processes going and use one component instead of five or six. It’s not a complicated thing, as the programmable safety controller is designed to be a drop-in replacement.

The implementation and broad acceptance of international standards will inevitably move the new paradigm forward. There will be safer, less expensive and more productive safety systems because of this technology. People used to think of safety in terms of cost; increasingly they’re seeing it for what it has become: a competitive value in today’s tough economic environment.

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