Implementing automated machinery safety

One of the most significant trends driving automation relates to machinery safety and how the integration of technologies is key to advances in this area. Here, Nigel Dawson, Festo GB’s Product Manager for Electric Drives, surveys interesting developments making life easier for design engineers.

While the subject of machinery safety is not new, it continues to play a key role in machine and plant construction. But, there are many different approaches and a great deal of uncertainty amongst machine builders in how to handle these issues and what degree of complexity and cost they need to go to, to adequately minimise risks; it isn’t surprising many machine builders and end users seek advice and support.

A fully integrated approach that monitors both the axis of a machine and allows safety-related clamping or braking is the best, safest solution!

A fully integrated approach that monitors both the axis of a machine and allows safety-related clamping or braking is the best, safest solution!

In applications that are not protected by physical safety guards, but where personnel can come into direct contact with plant components, the Machinery Directive 2006/42/EC indicates systems must provide adequate risk reduction through integrated safety functions.

In many cases an overall safety concept, requires the monitoring of moving axis, as well as safety-related clamping or braking, depending on the expected conditions. In higher risk applications two independent channels are required. In the past, machine builders would often design in their own safety solutions that took the safety switching device and wired in the STO, or Safe Torque Off, function. Frequently servo motor have been replaced by a motor and brake combination in vertical applications.

Now there are several problems with this approach; they do not take into account all the possible failure states – a coupling assembly breakage or slippage or a broken toothed belt in a parallel mounting kit could render the brake useless. These faults could still allow the carriage and load to fall, causing damage or injury.

A fully integrated approach that monitors both the axis of a machine and allows safety-related clamping or braking is the best, safest solution and this is exactly what Festo’s electric axis EGC unit does, it has an optional second channel displacement encoder and one or two channel clamping unit. The mechanical system can be monitored by both a motor encoder (first channel) and the linear displacement encoder (second channel) mounted on the axis providing two channel monitoring.

The axis can also be specified with single or dual-channel clamping units EGC-HPN which are suitable for holding a position, collision protection and, due to their emergency braking features, enhance safety in any vertical axes, for example, those which are typically used in lifting and stacking applications.

Of course legislative safety is not the only use of such features as additional encoders and mechanical braking systems. External encoders offer direct input into Festo servo controllers to allow unsurpassed positional repeatability on mechanical axis. For instance, it is now possible to achieve 10 micron positional repeatability on a standard belt drive when configured with a simple low cost encoder option, giving machine builders belt drive performance with ballscrew accuracy. Similarly vertical loads can be held safely for long periods without the need for high current usage on servo motors, by choosing a simple mechanical clamp option.

With today’s safety standards it is a more complicated task for designers to gather together all of the data from different manufacturers to calculate and document their own designed safety solutions. The Festo EGC axis provides a cost effective solution for compliance to the requirements of the Machinery Directive in a neat and self contained assembly and a single part number. Festo provides information on a wide range of electric and pneumatic safety functions through a Safety Guidelines manual that can be downloaded from their website and distributed through the Machinery Safety Alliance seminar program.

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