Multi-Pixel Technology also uses the principle of optical triangulation that is utilized with SPT sensors. However, MPT sensors make use of not a single threshold between receiver elements, but many more, even up to 128 thresholds. While SPT sensing detects up to a certain distance but not beyond that, MPT sensing can also define sensing windows within which objects are sensed. MPT sensors are flexible enough that multiple distances can correspond to multiple outputs of the same sensor. For example, an SPT device could detect pill bottles or medical syringes closer than 65mm. But an MPT device could also detect those bottles at a distance between 48mm and 65mm, or it could trigger one discrete output when something is closer than 48mm and another output when something is closer than 65mm.
Pulse Ranging Technology is a method of distance measurement. PRT offers continuous information on the distance from the sensor to the observed object. It utilizes very high-intensity bursts of light that leave the sensor’s emitter, are reflected by the sensed object, and are detected by the sensor’s receiver. Based on the amount of time for the light to be emitted and to be picked up by the sensor’s receiver and using the speed of light in air as a constant, the distance to the object can be accurately calculated by the sensor. PRT is used not only to measure distance along a line of sight, but also to calculate dimensions of an object. Or it can act in the same capacity as an SRT or PRT sensor and detect objects within a certain distance or range.
An alternative to PRT is one based on phase-correlation, in which the emitter sends light continuously and measures the distance to an object based on the phase angle of the reflected light. Phase correlation sensors have weaker optical signals, and are thus more prone to interference from ambient lighting. They can detect different colored objects at difference distances and can also detect “phantom objects” when a background object is out of phase by 360 degrees.
When an application calls for simple presence detection, a conventional presence sensor is ideal. But when some objects must be seen and others ignored, or an object must be seen only at certain locations, or when the object’s presence and its position must both be known, SPT, MPT, or PRT distance-based sensors are the best options.
Jeff Allison is product marketing manager – photoelectric sensors for Pepperl+Fuchs.