In a Portuguese manufacturing plant, an operator loads small parts by hand onto a resistance-welding machine and initiates the PLC-controlled welding cycle. After welding, the operator drops the completed assembly into a metal bin used to transport parts directly to the customer. From time to time, unwelded parts are placed in the bin, resulting in rejection of entire batches of assemblies by the customer. Manual inspection of each assembly is impractical; a sensor system must detect the presence of each item as it is dropped into the bin and check that the corresponding welding cycle has been completed. The sensing system must not increase the process cycle-time and must detect parts as they are dropped into the open bin, which is approximately 120cm (4ft) long and 90cm (3ft) wide. Parts are around 15cm (6in) long and a sensor with a rapid response time is necessary to detect fast-moving items.
The Contrinex range of Safetinex infrared light curtains, developed for use in high-reliability safety systems, is well-suited for this application. Comprising separate sender- and receiver-units and constructed with rugged one-piece aluminum-profile housings, these versatile sensor systems are ideal for the busy production environment. With a response time of less than 25 milliseconds and a resolution of 14mm, the Safetinex range satisfies the demanding customer specification for rapid response and reliable detection of the small parts. The compact design allows easy mounting at each end of the transport bin without impeding the operator or increasing the process cycle time. Safetinex light curtains require no direct connection between sender and receiver; The system provides two separate OSSD (Output Signal Switching Device) PNP outputs, and integral M12 connectors allow easy removal and replacement of individual units for maintenance. Connection to the customer’s PLC is via flexible PVC-sheathed cables. The system detects the presence of any item passing through the light curtain and immediately interrogates the PLC to confirm that a welding cycle has been completed since detection of the preceding item. If no welding cycle is registered, the system interrupts the process and sounds an alarm, alerting the operator.