Machine Design

Sensor Sense: Multipixel-array sensors

Multipixel-array (MPA) sensing for target detection within specific distances combines the benefits of background suppression, sharply defined sensing, and insensitivity to object color with the distance-based detection of a distance-measurement sensor. This lets the user customize sensor-target detection based on the distance to an object, whereas a conventional photoelectric sensor simply detects an absence or presence of an object.

MPA-type sensors emit light that’s reflected by the target object back to the sensor’s light-sensitive receiver area. Unlike conventional photoelectric sensors that use only a single light-sensitive sensor, the MPA uses a pixel array consisting of many photosensitive sites. Earlier technologies used differential diode (two-segment) receivers. A target closer to or farther away from the sensor changes the angle of the light beam, splitting the amount of light that falls upon each light-sensitive diode. By measuring the difference in light striking each diode, a rough estimate of the target’s distance would determine if it was the valid target and not a false trigger.

In contrast, MPA sensors often use over 100 receiver elements per array for higher resolution. Triangulation calculations of the reflected light between the source and where it hits the pixel array determines the distance to the target object. In other words, MPA sensing is determined by the geometry of the reflected light. Closer objects reflect light at greater angles than farther objects. Where the greatest concentration of light centers on the array determines if the target is at the desired distance.

The high number of light-sensitive pixels in MPA sensors gives the ability to create multiple switching thresholds or to establish sensing windows that force the target to fall within a specific distance from the sensor before triggering the sensor output. This lets the sensor offer a higher level of control than standard background-suppression (BGS) models that have just one distance threshold.

And the fact that MPA sensors detect where the light is reflected rather than how much light is reflected makes these sensors insensitive to variations of an object’s reflectivity or color.

Sensing distances or spans are set using electronic rather than mechanical adjustment. The use of multiple sensing ranges lets a single sensor act as multiple single-point sensors.

Typical applications for MPA photoelectric sensing include presence checking, trigger or leading-edge sensing, stack-height detection, tension checking, stroke-height monitoring, fill-level monitoring, collision-avoidance monitoring, and web-break monitoring.

Pepperl+Fuchs supplied information for this column. | © 2012 Penton Media, Inc.

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