Light detection grids deliver poka-yoke automotive bracket assembly

When an automotive first-tier supplier needed to be able to guarantee that an assembly was welded correctly before it was despatched, and to implement a Poke Yoke control system, it turned to Contrinex.

In the manufacturing cell, 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.

There is a risk that from time to time, unwelded parts could be placed in the bin, resulting in the rejection of entire batches of assemblies by the customer. Manual inspection of each assembly is impractical and so a sensor system was sought to reliably detect each item as it is dropped into the bin and check that the corresponding welding cycle has been completed.

The control system needed to be designed to ensure that it did not extend the process cycle-time, and the sensor must be able to reliably detect fast-moving items as the assemblies are dropped or thrown into the despatch bin. The metal bin is approximately 120cm long and 90cm wide, whilst the parts are around 15cm long.

The Contrinex DGI range of infrared light detection grids was ideally suited to the application. Comprising separate sender- and receiver-units housed in rugged one-piece aluminium-profile housings, these versatile sensors are ideal for the busy factory production environment.

The system detects the presence of any item passing through the light grid and interrogates the PLC to confirm that a welding cycle has been completed since the preceding part was placed into the bin. If a correct welding cycle was not registered before the part entered the bin, the system suspends the process, disabling the welder and sounding an alarm to alert the operator.

With response times from 0.8 to 4.8 milliseconds – which is faster than safety light curtains – and a choice of resolutions from 0.9 to 25mm dependent upon the size of the light grid, the DGI light detection grids far exceeded the customer’s demanding specification for rapid response and reliable detection of the small parts.

The sensor’s compact design and ability to be spaced at up to 8m between sender and receiver enabled their easy mounting at each end of the transport bin without impeding the operator or increasing the process cycle time.

The DGI light detection grid sender only requires a power supply connection, whilst the receiver provides two Push-Pull (PNP and NPN), Light-ON and Dark-ON outputs. The 4-pin M12-pin connections simplify installation and reduce cost. The customer was especially keen to avoid complex solutions which required programming on a computer and were delighted to be offered a ‘normal’ sensor switched with only a potentiometer to adjust sensitivity if needed.