Safety that sees the light
February 1st 2008

Handling and controlling the passage of large bulk items that need to pass through a safety light barrier into another area without causing total disruption of the work flow can be a challenge, explains Barry Goodwin of Leuze electronic

There are occasions when the momentary inhibition (or muting) of safety light curtains or multi-beam systems, in a controlled manner, allows for improved operational efficiency but at the same time affords staff maximum protection from the hazards of machinery This requirement can be applied to industrial production lines as well as warehousing environments, both pose the same problem, handling and controlling the passage of large bulk items that need to pass through a safety light barrier into another area without causing total disruption of the work flow. It is not difficult to imagine a long vehicle production line with highly automated precision welding robots ready to fuse the next body component as it progresses through a number of stages until a complete body shell emerges. Providing protection the entire length of the production track is relatively easy – perhaps fixed guarding areas with emergency access doors using interlock switches and safety relays. The main areas for concern are the entry and exit points, a position where the floor pan must enter or the car body emerge from within the 'wire cage'. There has to be a gap sufficiently large for the car body. At the instance of exit the car body blocks the opening, at other times the opening enables free access to people. To meet safety requirements, swing doors equipped with safety hinge switches or limit switches could be positioned across the exit point, opening automatically as the car body approaches; this would add a considerable time delay and cost to the operation as well as possibly creating a momentary loss of safety during the opening sequence. Placing standard safety multibeam systems or curtains across the exit point is a possible solution but without some form of inhibition control, every time a car body was to exit, it would break the beam and stop the line. The most practical solution is the application of safety light curtains or multi-beam systems with MUTING CONTROL, here sensors are used to provide a controlled 'mute' of the operation of the safety light guard when the car body approaches the exit point, and re-initiates its safety functions immediately it has moved clear of the light guard. [fig 1] Under conditions when the exit point is clear, the safety light guard provides an invisible barrier which, if broken will signal an alert and close down all hazardous movement. As a car body moves towards the exit zone point it activates sensors. The electrical output from the sensors is used to send an inhibit signal to the light guard outputs thus preventing a stop condition. Once the car body is clear of the sensors, the safety light guard returns to its normal protection field duties. A safe working environment is maintained throughout this muting operation since the car body physically blocks a person's access to the robot cell. In mechanical handling applications, fork lift trucks are often used to remove or place palletised loads onto conveying systems or other in-feeds to high-speed automated machines. As in the previous example, fixed guards may be in place either side of the conveyor-run, the problem area is the load/unload point where clear access is needed. The natural choice for protection at the man-machine interface are safety light guards, in this instance the operation of the light guard is inhibited by initiation of inductive loops in the floor, linked to a muting control [fig 2]. As the fork truck arrives at the drop-off or pickup points its mass is detected by the inductive loops, these mute the operation of the safety light guard allowing the infeed/discharge of pallets. During removal or placement of the palletised load, the access point onto the conveyor is blocked thus preventing human access. Any person within the inductive field will not be detected by the inductive loops and will thus not initiate a mute condition but by breaking the safety beam they trip the system to safe mode. The application of muting controls ensures the highest level of safety without hindering productivity. It is this ability to distinguish between products and persons that not only satisfys the safety requirements as laid down in the relevant European Standards, but helps the production manager optimise throughput.

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