By Scott Orlosky and Jean-Marc Hubsch, Sensata Technologies
Designing safety into industrial applications is typically a combination of the measures taken by the engineer during design and development, and those implemented by the user once the system is installed and operational.
Measures taken during the initial design phase are always preferable, and usually more effective, than those taken by the machine operator. But regardless of when the measures are taken, the design always has to take into account the following factors:
• Establishing the limits and the intended use of the machinery
• Identifying the hazards and any associated hazardous situations
• Estimating the risk for each identified hazard and hazardous situation
• Evaluating the risk and deciding on the need for risk reduction.
A key part of reducing risk requires defining the machine’s safety functions. This includes the safety functions of the control system, for example, to prevent the machine from starting unexpectedly, over-speeding, or even running too slowly.
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.
Consider, for example, the risk within a conveyor application, where the initial line of ‘protection’ could be a sensor that detects when a person is within eight feet of the machine. Rather than completely shutting the conveyor down, the controller first reduces the speed of the conveyor to reduce the risk. Production is therefore maintained, without compromising safety.
In a bottling plant, for example, designing in Functional Safety could enable the speed of the bottling line or the torque to be adjusted to a ‘safe’ level while a brief inspection can take place, or a repair carried out, without production being called to a halt. Similarly, on a printing press, implementing Functional Safety could enable the rollers to be cleaned with little or no real interruption to production and – crucially – little or no risk to the operator.
Within the timber trade, Functional Safety designs are critical to the operation of semi-automated tree harvesting and debarking systems and machinery, and in the speed and positioning of lumber to be sawn. The same is true within steel mills, for the safe and accurate pouring of molten steel and the shaping and rolling of ingots and steel plates.
In escalators and moving walkways, speed sensors are vital, and so too in elevators where position control of the cab and accurately determining weight and maximum loads is essential. In the most recent applications, and 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.
In all of the industries and applications highlighted, designing in appropriate levels of Functional Safety will help prevent serious injury or even death. But to be really certain, using components that are themselves certified to a specific Safety Integrity Level (SIL) by one of the recognized certification bodies enables users to achieve the system safety level requirements simply, effectively, and in the quickest possible time. They need only to feed the relevant data into the SISTEMA software (available free on the internet) for a final safety level to be calculated and recorded.
Standard products that are not individually safety rated can be used, of course, but may limit the designers to a lower level of safety rating for the entire system. Using certified products makes it easier for engineers to calculate and accurately claim a safety rating for a system overall, as well as providing important data such as a Mean Time to Failure. It also reduces the work (and cost) required of the OEM in designing Functional Safety as a machine upgrade
The advantages of adopting Functional Safety are not simply about protecting people, the equipment and the environment in which they operate; they are also about how Functional Safety design improves productivity, enabling systems to continue to operate while minor maintenance or repairs are undertaken as outlined in the examples above.
Of course, any change to an existing engineering and design process adds cost, but with the new generation of sensors, encoders and controllers now available, engineers have the building blocks to create a safer system with comparative ease and only minimal cost. Existing systems can also be easily upgraded to achieve a higher level of safety without having to design from scratch.