Going the distance with pulse ranging technology

24 April 2009

Pepperl+Fuchs has redeveloped its pulse ranging technology (PRT) so that for the first time, it can be used in sensors of all sizes for a wide range of commercial applications. Unlike phase correlation, which is used in most laser distance measurement techniques, the time-of-flight is measured directly.

In the direct time-of-flight measurement process, a laser diode transmits short pulses of light—about 250,000 per second—which are reflected on the target object and then recaptured by a light-sensitive receiver element. By measuring the time difference between the original pulse and the reception of the “echo” the precise distance can be calculated. The instrument in fact makes thousands of distance measurements and takes a statistical average to get the actual distance.

One advantage over distance measurements that use phase shift is, obviously, when the phase shift is greater than 360 degrees, the measurement is no longer valid. Gerd Schwebel of P+F (photo) says the phase shift methods show a high dependency on ambient conditions such as extraneous light, changing measuring objectives, and interaction with other sensor systems.

Another advantage of pulse ranging technology is primarily due to the “chopper” or discontinuous effect of transmitting the red laser light. The short light pulses contain energy up to a thousand times more intense than sensors emitting permanent light beams. The process helps to deliver more accurate and reproducible results regardless of ambient conditions such as surface quality, colour or prevalent extraneous light.

The Pepperl+Fuchs portfolio includes several sensors with PRT: the first model is the cost-effective VDM28 distance sensor enclosed in a standard light barrier housing with a measurement range of 8 m.

The VDM100 offers a maximum range of 300 m, a repeat accuracy degree of <0.5 mm and is ideal for the rapid, precision positioning of stock feeders.

The VDM70 has a maximum range of 250 m and is suitable for the simple positioning of gantry cranes, lifters, elevator cars or for measurement tasks, while the VDM54 with a maximum range of 6 m is optimised for collision prevention applications on electronic suspended rails.