Sensor Sense: Time-of-flight sensing

July 12, 2007
Automated inventory storage-andretrieval systems rely on error-free position sensing of overhead cranes and vertical lifts.

Laser distance or displacement sensors use timeof-flight sensing to handle the positioning monitoring process.

Time of flight (TOF) measures the time needed for a burst of laser light to travel from the sensor to the target and back again. Because light travels at a constant speed, the time lapse between sending and receiving the light pulse represents the distance to the inventory.

TOF offers a longer sensing range with higher accuracy and resolution than the once widely used phase-correlation measuring principle. TOF is recognized today as the default technology for long-range laser-based distance measurement.

With TOF, diffuse-model photoelectric sensors generally can work at ranges from 5 to 10 m. Retroreflective models boost that range to handle distances from 30 to 200 m. Resolution is commonly less than 1 mm while accuracy can reach as high as a couple of mm. Output from the sensor is typically a 4-to-20-mA current loop that works over the minimum to maximum span of the sensor.

One clear advantage of laserdisplacement sensors with time of flight is the ability to measure distance without physical contact. In addition, installation is as simple as mounting a sensor and aligning it with the target. While a Class-1 infrared laser serves for taking measurements, alignment typically uses a Class-2 visible red laser. Installation for retroreflective mode is similar except a reflector mounts on the target.

Pepperl+Fuchs (am.pepperl-fuchs. com) provided information for this column.

A laser-displacement sensor bounces laser light off a reflective target back to its receiver. The light pulse takes approximately 6.67 nsec/m to make the round trip. The sensor uses the measured time to calculate the distance to the target with accuracy of a few mm over a distance of 200 m.

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