ABOUT CONTRAST SENSORS

Contrast sensors (designated by "KT" or "NT" as in the KT 10) canbe used in a variety of applications including positioning data processing forms, positioning labels, cans or tubes, and recognizing codes. The contrast sensor signals the machine control to cut, fold, or fuse material or to stop material feed.

Contrast sensors detect registration marks by monitoring changes in the light reflected by the background and the mark. The mark is the symbol on the object that the contrast sensormust detect; the background is the area around the registration mark and must be in contrast to the mark.

Contrast sensors recognize colors as gray scale values. Therefore, two colors such as red and greenthat have approximately the same gray scale value may be difficult for a sensor to distinguish. When a sensor lens is chosen to increase the operating range, the contrast between the mark and the background may need to be increased. An ideal contrast is black and white.

The receiver in the contrast sensordetects the reflected light and converts it into an analog voltage. This voltage is dependant on the color or reflectivity of the scanned material, thus allowing the registration mark to be detected. As part of the setup procedure, the voltage level for the lighter color is stored in memory. The switching threshold is then set in relation to the stored level.

Each time the analog voltage from the received light exceeds or drops below the switching threshold, an output from the sensor is triggered.

SENSING DISTANCE
The sensing distance is the distance from the front edge of the lens and the scanned material. This distance is a function of the selected optics. The sensitivity of the contrast sensor increases as the sensor-to-object distance decreases.

The sensing distance tolerance is approximately +/- 3 mm (+/- 0.12 in), depending on the contrast level. This is the range within the distance from sensor to material can vary and the sensor will still reliably detect marks.

LIGHT SPOT ORIENTATION
The light spot may be a vertical/parrallel to the body of the contrast sensor, or the light spot may be horizontal/perpendicular to it, depending on the model selected. For the most relaible detection, the long side of the light spot should be parallel to the long side of the mark.

LENS POSITION
The lens position of contrast sensors can be changed 90 degrees by moving the lensand the threaded cap. (Not available on KT 2, KT 3, or KT 10)

INSTALLATION
The best location for the KT/NT is where the scanned material experiences the least lateral and vertical movement (flutter). The sensor works best when positioned as close as possible to the designated focal point.

Lateral Movement can be compensated for by increasing the length of the mark. However, flutter must remain within the sensing distance tolerance, or the contrast and the operating reliability will be significantly reduced. An optimum arrangement would be to guide the material over a fixed roller with the KT/NT aimed at the surface of the roller.

Mounting brackets should be designed and installed with sufficient stability to prevent strong vibrations from affecting the KT/NT. The selected location should be shielded from extremely bright ambient light. The lens can be installed in either opening depending on the location of the scanning plane.

If the scanned material has a glossy (reflective) surface, the scanner should be mounted at an angle aproximately 15 degrees prependicular to the scanned material.


LINKS

Contrast Sensors Product Information Brochure (PDF)