Servo - November 2015

Even with all their limitations, IR and/or ultrasonic ranging sensors can provide reasonably accurate (within 1/2 inch or so) and reliable information, especially in a controlled or known environment. This — plus the fact that rangers cost only slightly more than digital sensors — makes them an excellent choice for many hobbyists. Sometimes though, better accuracy, greater distances, and more reliability are needed.

Laser ranging sensors are significantly more expensive than IR or ultrasonic, but they solve many problems. Lasers generally have no trouble detecting dark or soft objects, and — perhaps even more important — they easily detect angled surfaces that can confuse IR and ultrasonic sensors. Furthermore, their detection range can be 50 times greater than IR and ultrasonic rangers.

The accuracy of a laser sensor (typically around 1 cm) is not significantly better than other types of rangers, but that’s not the entire story. The readings from IR and ultrasonic sensors often bounce or jitter around the actual value. The distance to a stationary object might, for example, be reported as 10 inches, then 9, then 11, then 10, etc. Such individual readings are obviously not truly accurate, but an accurate reading can often be obtained by averaging the readings over time. The readings from laser rangers — by comparison — are far more stable and repeatable than other types.

For many applications, a laser’s capabilities are worth the higher price. The extended range of a laser, for example, can greatly enhance a robot’s navigational behaviors because it can map its environment and more precisely determine its location by accurately measuring the distance to known objects and distant walls.

Lasers don’t just improve the standard mobile robotic behaviors we are all familiar with. They also make it simpler to deal with new technologies. For example, the extended range of a laser sensor can make it easy to determine the altitude of a drone, and the speed and accuracy of the measurements can enhance the ability to land autonomously.

Interfacing with many laser rangers can be difficult, but Parallax has made it easy with its new line of laser-based sensors. Figure 6 shows a typical model in their SF10 series which can measure distances up to 100 meters. Figure 7 shows the SF02 which has a maximum range of 50 meters. Both of these lasers were designed to be used as altimeters for small aircraft, but they also make excellent perimeter sensors for mobile robots.

No matter which model you choose, a major advantage of the Parallax lasers is their variety of interfaces which allows them to connect with almost any processor. The interfaces include USB, analog, CMOS serial, and I2C. Another advantage of these lasers is that they can be configured to meet your specific needs. Let’s look at an example to illustrate this point.