...ranging from emergency stop switches to guard switches, light curtains and safety relays or controllers, must be designed to be fail-safe in order to perform their proper function. However, machine builders need to take care when specifying safety components to avoid jeopardising machine availability.
For example, consider a machine where a guard needs to be opened frequently and an electrosensitive protection device would be inappropriate. Depending on the design of the control system, each time the guard is opened, the guard switch is activated and the safety relay is tripped. If the guard switch is of the tongue-operated type, mechanical wear is inevitable. Similarly, traditional electromechanical safety relays will also wear, though the life expectancy is typically greater than that of the guard switch.
Eventually one of these components will wear so that it ceases to function, though its final act should usually be to render the machine inoperable but safe.
One way to extend the time before this occurs or to avoid it altogether is to specify non-contact gate switches, such as those from the Pilz PSEN range, and a solid-state safety controller that is not subject to electromechanical wear in the same way as conventional safety relays. The Pilz PNOZmulti software-configurable modular safety controller uses dual redundant transistors to perform the switching function, with an electromechanical relay monitoring the transistor output and only operating in the event of one or both of the transistors not switching as they should. Electromechanical wear of these backup relays is therefore virtually non-existent due to the small number of times they are required to operate, and solid-state transistors do not wear at all, so the safety controller can be considered to be non-wearing.
Because the PSEN guard switches are magnetically operated, they are effectively wear-free as well. Furthermore, the IP67 housings are designed to withstand wet or dusty environments, and the PBT (polybutylene terephthalate) casing material has low water absorption, is resistant to temperature extremes, and has extremely stable electrical properties. If conditions are particularly harsh, the switching part of the sensor (with its M12 electrical connector) can be protected by polycarbonate or another non-magnetic material, leaving only the actuator part of the switch exposed. Should the switch be damaged, perhaps through accidental impact, it can be quickly changed thanks to the use of an M12 Brad Harrison connector - with no need to disturb the wiring unless that has also been damaged.
Pilz offers the PSEN sensors in two compact styles: rectangular units for use where space is at a premium, and cylindrical versions with threaded bodies. The switching range and tolerated misalignment depend on the model selected, but the maximum switching range is 8mm. They are also certified as evasion-proof in accordance with VDE 0660.
A further advantage of the PSEN switches is that a single device can be used on a guard to meet the requirements of BS EN 954-1 Category 4. If traditional switches are used, two would be required to provide the necessary redundancy; the PSEN switches have dual internal contacts that meet this need. And for BS EN 954-1 Category 4 applications, it should be noted that the combination of a PNOZmulti safety controller and a PSEN switch has BG approval in accordance with EN 6047-5-3.
For Category 2 or Category 3 applications, users have a further option of using a Pilz interface unit to link multiple PSEN switches in series, thereby reducing the number of inputs used on the PNOZmulti controller.
Almost any application requiring more than a handful of safety relays can benefit from a PNOZmulti safety controller, particularly if the safety control system is likely to need to be upgraded or if series machines are being built. Recent additions to the range of PNOZmulti modules include communications interfaces for linking to Profibus DP, DeviceNet, Interbus, CANopen - and now CC-Link as well. But most importantly, when a PNOZmulti is used in combination with PSEN non-contact switches, the result is a safety control system that is highly unlikely to ever impair the machine's availability.