Detection of print marks with unsteady quality

Using the example of hologram marks

The detection stability of contrast sensors is highly dependent on the teach-in. If the teach-in was done faulty or under bad conditions, wrong switching events of the sensor can easily occur.

In most applications a contrast sensor (KT) meets pretty stable conditions. For example the print quality of packaging material is monitored within the packaging machine far before the KT has to read the marks. Thus mostly it is sufficient to teach a KT once at the beginning of new material.

However if the contrast is changing from mark to mark it often leads to problems.

Example application: Hologram marks which are fabricated by laser exposure

By a look on the diagram it is obvious that the holograms in this example have a very high contrast using the green sending light. On the first view this is no problem at all for a KT from SICK – but by analyzing the diagram in detail you’ll see the risk:

The indicated thresholds in the diagram assume a teach-in of the first mark. If the sensor would place the threshold in the middle between background and mark (50% / Default setting) lots of marks wouldn’t be detected because of the varying exposure process of the hologram marks. Even if the teach-in was configured to place the threshold at 25%, some marks on this material wouldn’t be detected (see red circles).

In this example the background signal is very stable. Because of that the threshold could be placed closer to the background signal without creating any problems. Using this setting a stable detection of marks with unsteady quality is possible.

To be able to manually place the threshold there are different options:

KTM: Threshold configurable via IO-Link
KTS / KTX: Threshold configurable via IO-Link and display

Older sensors:
KT5: Variants with potentiometer
KT8CAN: Threshold configurable via CAN
KT10: Adjustment of threshold possible via +/- buttons