Contrinex DGI Light Detection Grids Provide Low-Cost Poka-Yoke of Automotive Bracket Assemblies


The automotive supply industries face high expectations and often severe penalties for supplying incorrect parts and therefore quality control systems like Poka-Yoke are common. Such error prevention systems typically interlock two operations together, for example, a part cannot be removed by an operator until all parts are present and a process has been completed.

In this application example, the correct assembly of a welded bracket at an automotive component supplier is ensured by Contrinex DGI infra-red light grids. The machine control system relies on the light grid’s ability to reliably detect each assembly as it is placed into a transportation bin, verifying that the bin only contains brackets that have been welded correctly.



An automotive first-tier supplier needed a Poka-Yoke system to guarantee that an assembly was welded correctly. 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 because of the high volume of parts being manufactured. Therefore 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 implementation of the control system could not extend the process cycle time so loading the assembly into a checking gauge was not practical. A sensing system was sought that would 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.


Customer Solution

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-profiles, 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 & NPN), Light-ON & Dark-ON outputs. The 4-pin M12-pin connections provide simplify, low-cost installation. The customer was especially keen to avoid complex solutions which required programming on a computer and was delighted to be offered a ‘normal’ sensor switched with only a potentiometer to adjust sensitivity if needed.


  • Well-proven, highly reliable detection
  • Ideal for detection and counting of small objects, or a wide variety of shapes and sizes
  • Choice of resolutions – 0.9 mm, 2 mm, 4 mm, 8 mm or 25 mm
  • Detection heights up to 2m and up to 8m distance
  • Fast response time – 0.8 ms … 4.8 ms
  • Robust aluminium extrusion housings and shatter-resistant PMMA optics
  • Fast & simple set-up – Adjust just one potentiometer – No programming, PLC or complexity
  • Low-cost M12 cable connections
  • -5°C… to +50°C – IP65 rating
  • Exceptional price/performance ratio

Click to download Product information, or click to download Contrinex’s Product Overview brochure to find out more.

Call PLUS Automation on 0121 58 222 58 or email to discuss how they can help you #MakeSenseofSensors using Inductive, Machine Safety, Photoelectric, Pressure, Process, RFID & Ultrasonic sensors from Contrinex, ReeR & Satron.

With over 2,000 sensors in UK stock, supported by stock held in Europe and short manufacturing lead times, they’d welcome the opportunity to talk about improving automation and machine performance, or how to reduce costs.


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