Hitherto, safety related control systems have been subject to the standard EN 954-1, which, by the end of this year finally gives way to EN ISO 13849-1 following a fairly lengthy transition period. Although it has governed the design of safety-related machine control systems since 1996, EN 954-1 has been criticised for its relatively simplistic approach; for example, it offers no adequate requirements for programmable electronic systems and does not take the probability of safety system component failure into account.
EN ISO 13849-1 introduces probabilistic techniques to cope with increasing electronic complexity. It also introduces the concept of ‘Performance Level’ (PL) – equivalent to the ‘Safety Integrity Level’ (SIL) classifications of the EN IEC 62061 standard, which oversees the functional safety of electrical, electronic and programmable electronic control systems.
There are five Performance Levels: ‘a’ through to ‘e’, where ‘a’ represents a low contribution to risk reduction and ‘e’ a high contribution to risk reduction. Generally, the five Performance Levels, PLa to PLe have equivalence to the four SIL levels 1 to 4, though there is no SIL equivalent to PLa. Defining PL or SIL depends upon the initial risk assessment carried out by the machine builder/installer and relates to the probability of a dangerous failure occurring per hour of operation.
Safe but productive
Rockwell Automation had the machine builder - and these imminent changes - very much in mind when it released the first product in its Allen-Bradley Powerflex 750 ac drive series towards the end of last year. As one might expect, the new 755 ac drive provides users with complete control of motor speed, torque and position, as well as offering ease of integration and application flexibility. However, the 755 offers several levels of safety, including an ‘enhanced’ safety feature that will maintain machine operation - and hence continuity of production - at a rate commensurate with operator safety.
So, in addition to Safe Torque Off – the PLd/SIL3 certified, basic level safety option that will prevent drive restarts following a trip - there is an additional ‘Speed Monitor Safety’ option. This combines the Safe Torque Off capability with Rockwell’s ‘Safe-Speed’ control core technology in a single piece of hardware embedded within the drive. Certified to PLe/SIL3, this option reduces both hardware and wiring costs and enables safety devices to be connected directly to the drive.
Where appropriate, with Speed Monitor Safety it may be possible to reduce and monitor the speed of an application to a point that allows the removal of an obstruction or the carrying out of certain maintenance work without bringing the machine to a complete standstill. This can have quite a positive effect on operations, improving productivity and limiting the incidence of downtime.
The embedded Speed Monitor Safety option provides a number of functions, including Safe Stop, Safe Limited Speed, Safe Maximum Speed, Safe Maximum Acceleration and Safe Direction. Safety programming wizards are provided for easy set-up and the CIP safety compliant drive comes with Ethernet IP and DeviceLogix control functionality embedded, so it can be scaled to implement a wide range of machine control tasks. In addition, the drive can be configured to provide a raft of operating data, such as cooling fan function, I/O relay cycles and motor run time hours to give advanced warning of potential faults.
The PowerFlex 755 features a slot-based mechanical architecture, which means that customers can ‘build’ a drive to meet their immediate needs - with as few or as many options for I/O, feedback, safety, communications, and auxiliary control power as their application requires.
Rockwell Automation has outlined the long-term financial benefits of machine safety integration in a white paper - ‘Proving the Value of Safety’ which can be viewed at: www.rockwellautomation.com/go/prsafety