Posted to News on 19th Feb 2013, 16:04
Electric axes with integrated safety features
To comply with the requirements of the Machinery Directive 2006/42/EC, machine builders and system integrators often create a safety concept that necessitates the monitoring of moving axes, as well as safety-related clamping or braking, depending on the expected conditions. In higher-risk applications two independent channels are required. Typically safety systems take an output from a safety switching device and connect it to the STO (Safe Torque Off) function of the drive. Often servo motors are replaced by a motor and brake combination in vertical applications.
There are several problems with this approach; it does not take into account all the possible failure states - for example, 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 safest approach, and this is what Festo's EGC electric axis 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 to provide two-channel monitoring.
The axis can also be specified with single- or dual-channel EGC-HPN clamping units that are suitable for holding a position, collision protection and, due to their emergency braking features, enhance safety in any vertical axes - for example, those used in lifting and stacking applications.
Positional repeatability
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 enable excellent positional repeatability to be achieved on mechanical axes. 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 ball screw 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 the safety systems they have designed. Festo provides information on a wide range of electric and pneumatic safety functions through a Safety Guidelines manual that can be downloaded from the company's website and distributed through the Machinery Safety Alliance seminar programme. The Festo EGC axis therefore provides a cost-effective option for compliance with the requirements of the Machinery Directive in a neat and self-contained assembly, and with a single part number.
Follow the links for more information about Festo's EGC electric axis and other linear drives and machinery safety products.
