Machine & Operator Safety
Pneumatics in machine safety and ISO 13489
February 22, 2016 by Erl Campbell Aventics
Feb. 16, 2016 – Machine builders are required to meet certain standards when developing machines for their customers. The goal is to make the workplace safer and increase overall productivity. ISO standard 13489 is one that is currently required for machines being delivered in Europe and is leading the way globally. The standard addresses safety and the reliability of machine control systems and their effects on the risk level to personnel.
It is important to note that the ISO standard covers the control aspect of the machine and not the actual moving components. For example, pneumatic actuators should be selected separately based on operating pressure, speeds and load or force required. The machine builder goes through a process to identify hazards and risks and determine levels of protection needed to mitigate the risk to a performance level based on the severity of the hazard, frequency of exposure, and potential to safeguard the hazard. The process results in a Category and Performance Level for each area of concern. The category determines the amount of logic required, and the Performance Level is based on the length of life of the machine. Typically the higher the category, the higher the performance level needs to be to satisfy the category. Suppliers of components have developed many devices for specific safety functions, however in most instances, these functions can be performed by standard components when implemented properly in a safety circuit.
Pneumatic circuits to perform safety functions have been developed. The circuits increase in complexity due to the extra channels and sensors that are needed to accomplish the required control. The components used would need to be tested to ensure they are reliable to a given level based on the level of safety required. Valves are measured with a B10 value. The B10 value is the number of cycles until 10 per cent of the components tested in an endurance test had failed. When it comes to electrical parts, like a sensor on a cylinder, we use MTTF which is “mean time to failure” and this number is measured in years. When these values are intended to reflect the case of a dangerous failure of the component, they are expressed with a subscript d. The standard B10 or MTTF number can typically be converted mathematically to a B10d or MTTFd to reflect the dangerous failure case. The dangerous failure number is typically higher by a factor of two since not all failures would be catastrophic and most components would only have diminished function once the B10 number was hit. This reliability data for standard pneumatic components can be used in a freely-available software program called SISTEMA to calculate a performance level for a given circuit design. The circuit used to perform a safety function should be reviewed by a third party like the IFA (“Institute for Occupational Safety and Health of the German Social Accident Insurance”). Some pneumatic manufacturers provide a library of the needed values for use in the software.
The machine builder’s need to meet these new guidelines has spurred development of new components to make implementation easier. Many pneumatic manufacturers now have products designed for safe exhaust, for example. Category 3 safe exhaust is the most common function required on machines containing pneumatic systems. The product can be as simple as using two solenoid valves with a sensor on each one to monitor the valve position, or can be a complete integrated device. Category 3 requires that the control system has redundancy and condition monitoring. The verification would need to be handled by a separate safety relay. The system can be upgraded by using increased monitoring through a safety PLC to achieve Category 4. The device could also be implemented in a modular package using only the functionality needed. The modular concept would allow the designer to choose a soft start valve or to integrate a pressure switch and can be combined with the normal preparation of compressed air 3(i.e. filters, regulators, etc.). Other functions that are commonly addressed are safe holding, prevention of unexpected movement, and safe pressurization. Again, depending on the level of protection required, these can be accomplished with standard components. Rod locks with monitoring, pressure operated check valves, soft start valves, the most common pressure switches, and directional function required on control valves are components that machines containing pneumatic systems can be used in circuits to develop the necessary control.
The future development of products for safety circuits could be integration into individual manifolds, allowing sections of the machine to be exhausted instead of the whole machine. Already, valves can be monitored for consistent shift times and that information can be used to alert the machine operator of possible problems before a failure. We may even see actuators with a complete control system integrated. The regulations have increased the need for further development of these types of solutions.
Machine safety guidelines help machine builders examine their machines for risk and possible areas where personnel can be harmed. The main concepts are to identify the severity of the risk, the length or frequency of exposure, and the potential to avoid the hazard. The parts selected for control of a specific function should have a measured expected service life (B10, MTTF). Using these concepts and devices to perform given functions will ensure reliable operation of the control circuit for a given machine movement, however the machine builder will need to make sure the machine is safe. The higher reliability of the machine control will also make the machine more efficient and productive over its useful life.
Erl Campbell serves as a key account manager for the food and beverage industry at Aventics Corporation, where he has worked since 1998. He has two technical engineering degrees from Greenville Technical College and has studied mechanical engineering at the University of North Carolina.
This column was originally published in the January/February 2016 issue of Manufacturing AUTOMATION.