Extra Distance Inductive Sensors Detect Presence of Metal Washers in Plastic Assemblies


A plastics manufacturer tests batches of control knobs for in-car audio systems before shipment to automotive assembly plants; each knob contains a small metal washer that occasionally becomes dislodged. A custom-built testing machine tests a tray of 70 knobs in a single cycle; long-distance inductive sensors, positioned directly below the knobs, confirm the presence of a washer in each assembly.


In operation, plastic control knobs for in-car audio systems are retained in position by small spring washers; during manufacture, a washer is fitted to each knob after molding. A Portuguese manufacturer transports batches of knobs to automotive assembly plants in transit trays, each containing 70 knobs.

Occasionally, washers are omitted or become dislodged, resulting in rejection of entire batches of knobs by the customer. Manual inspection of each knob is neither practical nor cost-effective; a sensor system must test each knob in-situ in the transit tray.

The size and position of the washers makes the task particularly demanding. Each 10mm-diameter washer is formed from 1mm-diameter steel wire; in a batch of knobs, located at 45mm centers in a transit tray, washers lie more than 10 millimeters above the sensors. The sensors must operate reliably in close proximity to one another, sensing the presence of the washers through the plastic tray.


Contrinex Extra Distance inductive sensors are ideal for this application; M18-diameter units from the Basic range operate reliably at sensing distances up to 12mm. Quasi-embeddable with chrome-plated brass bodies and PBTP sensing faces, these devices allow a minimum clearance between sensors of only 44 mm, comfortably accommodating the required spacing.

A bank of 70 sensors, mounted in a custom-built testing fixture, aligns exactly with the positions of the knobs in the transit tray. Locating lugs position the tray correctly and clamps secure it during the testing cycle; checking the presence of a washer in each of the 70 knobs is accomplished in a few seconds and in the event that a washer is missing, the position of the faulty knob is immediately identified.

This Contrinex inductive sensor has an industry-standard PNP normally-open interface and an integral M12 connector, allowing easy removal and replacement of individual sensors for maintenance. Connection to the customer’s test equipment is via a flexible PVC-sheathed cable.

With an unmatched combination of extended sensing distance and exceptional packing density, Contrinex Extra Distance inductive sensors deliver highly reliable test results. Previously, the customer had no reliable means of testing a complete tray of knobs in a single operation.

Customer Values

  • Reliable, repeatable testing of complete trays of knobs in a single cycle
  • Testing is completed in standard transit trays with no additional handling
  • Immediate identification of faulty assemblies
  • Elimination of batch rejection by the customer

Specific Product Advantages

  • Extended sensing distances up to 12mm
  • Exceptional packing density with 44mm clearance
  • Reliable sensing of small targets through plastic tray material