The difference in Background Suppression

A customer recently asked my advice selecting the right sensor for an application.

They needed to detect a plastic part approximately 3mm thick on a conveyor belt that is white with some black print on its surface. Looking from above at around 55mm range it sounded simple, but they had already tested with a background suppressed sensor from another supplier without success.

I tested with our equivalent sensor and it worked perfectly – so I had to explain the reason.

Background suppressed sensors are supposed to be colour blind and consider only the distance. The test procedure to measure how good they are at achieving that goal is explained below – in layman’s terms.

A sensor will be setup, so it detects test card number 1 (white) at a nominal range, for example 50mm. Test card number 2 (black) is then introduced, and the measurement is taken at the point it detects that card.     If it detected the second card at, for example, 45mm then it’s a 10% error.

Such tests are done at many different distances across the working range of the sensor to form a graph, tests are also done on different test cards which are Black/White, Grey/White and White/White to show the hysteresis.

When I showed the comparable graphs of my sensor to the competitor brand they had tested with it became clear why one worked and one didn’t. 

From the graph above you can see our sensor error at 50mm is circa 3% with our competitor error being circa 40% (on black/white shift). Our theoretical error can be calculated as 3% at 50mm = 1.5mm whilst our competitor 40% at 50mm = 20mm

So when you choose a sensor for an apparently ‘easy’ application my advice is to check a little deeper into the specifications or call the sensor company for them to clarify the sensor is the right model for your application.