Servo - November 2015

Figure 3, which means you must use an ADC ( analog-to-digital converter) to obtain the reading. There are several things we can learn from this graph.

First, objects nearer than 5 cm can appear to be farther away because the voltage generated is the same as a more distant object. Also notice that the main part of the graph has an inverse relationship with the distance (the voltage decreases as the distance increases). Finally, observe the non-linearity of the voltage/distance relationship. This means calculations are necessary to extract the actual distance.

The Parallax PING))) sensor shown in Figure 4 utilizes sound waves to measure distances up to three meters. In order to obtain distance readings from ultrasonic sensors, you generally must write a program that triggers the unit to send out a burst of ultrasonic sound. The program then counts the time it takes for the wave to hit a remote object and reflect back to the receiver, and then uses that time to compute the distance to the object. This calculation is linear, so it is much easier than that required for the IR sensor of Figure 2.

The signal indicating the wave has been received is a digital (true/false) indication, but again, these sensors are often referred to as analog because the data they ultimately provide is a variable quantity.

These two examples demonstrate why it can be more difficult to obtain ranging information than to simply detect the presence of an object with a digital sensor. When distance data is not needed, the low cost and ease-of-use of digital sensors can make them attractive — especially for less sophisticated robots or beginning hobbyists. Even ranging sensors, however, have their own limitations.

IR sensors can only sense objects that reflect light back to them, so dark objects that absorb light or objects with surfaces that greatly disperse light can be difficult to detect. IR rangers are particularly susceptible to this problem because the amplitude of the IR wave is small to begin with and it decreases with the distance being measured.

Ultrasonic sensors generally require software routines to measure the time for sound waves to bounce off objects and return to them. The linearity of the time-distance relationship makes it easy to obtain fairly accurate readings, but ultrasonic sensors can have trouble reliably detecting soft objects (such as stuffed animals, curtains, etc.) that absorb sound waves.

Both IR and ultrasonic sensors can have trouble detecting surfaces that are not somewhat perpendicular to their beams because angled surfaces can reflect the waves off to the side instead of back toward the sensor. In addition, the IR and ultrasonic sensors generally used for robotics can only reliably measure distances of 3-10 feet.

Often, an object that is difficult to detect with ultrasonics is easily seen with an IR-based sensor (and vice versa). For that reason, there are advantages for giving your robot both types so it can spot objects detected by either sensor. Parallax provides a dual-mounting bracket for IR and ultrasonic sensors (Figure 5) to help facilitate such an arrangement.