Functional safety: Understand the fundamentals

No matter where they are located in the world, industrial engineers responsible for designing and developing systems for industrial and factory applications need to be aware of the new functional safety design practices that have been demonstrated to successfully reduce the risk of system failures that can result in injury, costly damages or plant shutdowns. In addition, by recognizing and integrating functional safety processes, manufacturers can expand their markets and customer base worldwide.

Even though these safety standards are unified under an international umbrella, adoption rates have been slow as manufacturers and system designers evaluate and consider which set of standards they need to follow, while at the same time providing the greatest competitive advantage.

Where is functional safety critical?

There are many different systems within a warehouse, industrial or manufacturing plant that can benefit from applying functional safety processes and standards.

For example, in a bottling plant, implementing functional safety in the system design could enable the speed of the bottling line or the torque to be adjusted to a ‘safe’ level while brief inspections or repairs carried out, all without production having to completely stop. Similarly, on a printing press, implementing functional safety could allow the rollers to be cleaned with little or no real interruption to production and, crucially, little or no risk to the operator.

To prevent injury in a conveyor application, a sensor could be used to detect when someone is within eight feet of the machine and signal to the controller to reduce the speed of the belt. Instead of completely shutting the conveyor down production could be maintained with a slower speed and reduced risk to safety.

Other applications include the timber industry, where functional safety designs can be critical to the safe operation of tree harvesting and de-barking machinery as the system monitors the positioning of raw lumber that needs to be sawn and shaped into planks.

Steel mills also require strong functional safety operations to ensure the safe, accurate pouring of molten steel and the shaping and rolling of metal ingots and steel plates.

In escalators and moving walkways, speed sensors are vital, as well as in elevators where position control of the cab is essential. In recent applications, specifically the emergence of co-operative robots (or ‘cobots’ as they are sometimes known), the ability for a robot to co-operate effectively with a human counterpart is entirely dependent on safety—notably the ability to register contact and/or reduce the amount of force being applied.

Industrial storage and supply facilities, warehouses, equipment yards, etc., can all be made safer and more efficient by implementing functional safety policies. For example, many facilities use automated guided vehicles (AGVs) for quickly moving products in and around a warehouse or to different parts of a production line. These AGVs often rely on functional safety-rated encoders to measure the speed and direction of the vehicle and help ensure safe operation.

In all of the industries and applications highlighted above, as well as many others, designing in and implementing appropriate levels of functional safety can help reduce downtime and prevent serious damage or injury.

While functional safety is mandatory in systems design in the UK and Europe, driven by European Directives EN ISO 13849-1 and EN 62061, in the US however, a different set of safety rules apply. To understand more, it is helpful to first explore the European Standards that came into force in 2012.

Designing for industrial safety

The European Commission machinery directive says that industrial systems and machinery should operate as safely as possible, with minimum risk of injury to humans. However, as well all know, in the real world, there is no such thing as “zero risk”. Instead, the directive lays out a path to achieve a level of “acceptable risk” for specific industrial environments.

Within those environments and the machinery operating inside, if safety is dependent on control systems (encoders, sensors etc.), then these sub systems must be designed so to ensure that the probability of functional faults is sufficiently low. And if it turns out this is not possible, any faults that actually do occur should not lead to the loss of the safety function.

Until recently, the safety-related parts of a machine control were designed in accordance with EN 954-1 based on the calculated risk. However, with the emergence of new and more advanced hardware and software components, the standards of measuring and monitoring safety were upgraded. Today, the core functional safety standard is IEC/EN61508 and it includes several detailed standards relating to specific areas of manufacturing and design, most notably EN ISO 13849 and IEC/EN 62061.

EN ISO 13849-1 (developed with specific reference to machinery safety.)

This standard can be applied to safety-related parts of control systems and all types of machinery, regardless of the type of technology and energy used. These parts may include but are not restricted to relays, valves, position switches, PLCs, motor control units, pressure sensors etc. The performance of a safety function is specified by the term “Performance Level” (PL), with a safety rating categorized between the low “a” and highest rating “e”.

IEC/EN 62061 (written with specific reference to electrical/electronic components.)

This standard defines the requirements and provides the recommendations for the design, integration and certification of safety-related electrical, electronic and programmable electronic control systems for machinery. The performance of a safety function is described by the term “Safety Integrity Level” (SIL), categorized between 1 and 4, where ‘4’ is for the most complex, plant-level systems in the highest risk environments. (For the purposes of this article, we shall satisfy ourselves with Levels 1 – 3, as applied to industrial machinery.)

Designing for industrial and plant safety

Designing safety into industrial applications combines the procedures taken by the engineer during design and development, as well as those implemented by the user once the system is installed and operational.

Measures taken during the initial design phase are always preferable and are usually more effective than those taken just by the machine operator.

Design considerations

Whether the measures are taken before the system is designed, or after it has been installed, the design has to take into account the following factors:

• Evaluating the risk and deciding on the need for risk reduction.
• Identifying the hazards and any associated hazardous situations.
• Estimating the risk for each identified hazard and hazardous situation.
• Establishing the limits and the intended use of the machinery.

Defining the machine’s safety functions is a critical aspect of reducing risk. This includes the safety functions of the control system, for example, to prevent the machine from starting unexpectedly, over-speeding, running too slow, etc.

It is similarly important to recognize that a machine has different operating states (e.g., automatic and setup modes) and that the protective measures in these different modes may be completely different. Indeed, it might be that to achieve the levels of safety required, one or more safety-relevant control parts and several different safety functions are included, based on the operating mode.

Functional safety for industrial plants

Bringing system design into line with global safety norms makes sense for equipment manufacturers will enable them to better market their machines and compete on a worldwide scale.

The advantages of adopting functional safety encompass protecting people, the equipment and the environment in which they operate. However, functional safety design also improves productivity, enabling systems to continue to operate while minor maintenance or repairs are undertaken.

Of course, changes to an existing engineering and design process can add expenses as well as time to implement, but with the new generation of sensors, encoders and controllers that are now available, engineers have the building blocks to design safer systems with comparative ease and only minimal cost.

Scott Orlosky is a product manager for position sensors at Sensata Technologies; Jean-Marc Hubsch is an engineering manager focusing on encoders at Sensata Technologies, where he also is the technical expert managing the functional safety and certified hazardous area encoder product lines. 

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