LEGO Mindstorms NXT in the Classroom, Session Three: Sensing the World Around You

Building robots can be incredibly educational as well as fun, but it isn’t very interesting if your robots just stand still! To interact with their environment, robots need to have some means of sensing the world around them. LEGO gives you a number of out of the box choices, and other manufacturers have created aftermarket sensors that expand the initial range considerably.

Regardless of the sensors, students need to learn that robots don’t sense the world to the same degree as we do. As limited as some of our senses (sound or smell, for instance) may be in comparison to the rest of the animal kingdom, they are far superior to the sensors you’ll find in just about any robotics kit. Just as aspiring photographers have to learn to see the world through the lens of their camera, aspiring roboticists need to learn to see the world in the same way that their robot does. Without this crucial ability, it is far too easy to assign tasks that no robot can perform. Before we can do that, we need to take stock of our own senses.

Let the students identify the five senses, then encourage them to come up with specific instances in which we make use of them (for instance, the skin, an extremely sensitive touch sensor, is absolutely critical when picking up and grasping objects). Coming up with this list will make it easier to develop their own tasks and strategies for use in their own robots.

The LEGO Mindstorms NXT kit can “see” through the use of the light and ultrasonic sensors, though it needs two sensors for the price of our one (our binocular vision allows us to determine range to an object as well as orient ourselves relative to our surroundings - for that matter, our hearing does, as well). The light sensor sees in terms of light intensity, or how bright a light source is, as well as detecting colors on a greyscale; however, its field of vision is extremely limited, as is its resolution. Think of it as viewing the world through a peephole!

Though it is very limited, the sensor enables the NXT to perform such tasks as following a line or seeking a light source. The ultrasonic sensor performs only one function, using sound outside of the range of human hearing in the same fashion as a bat, relying on timing the reflection of that sound bouncing off objects to determine their range.

The NXT can “hear” after a fashion, but it is incapable of discerning patterns or frequencies in the noise it hears. It is restricted to being able to determine the amplitude or volume of that sound. (While many people have heard the term decibel, or dB, most don’t realize that it is measured on a logarithmic scale, just like a more commonly cited example, the Richter scale.) For those interested in discussing the science of robots with their kids, both the sound and light sensors measure the intensity of a given parameter at a specific distance from the source, and both are governed by an inverse square relationship (doubling the distance reduces the amount of energy received by a factor of four, and so on).

We rely extensively on our sense of touch when interacting with objects; our skins can convey presence, texture, and warmth. Our robot is not quite so sophisticated, since its touch sensor is a simple on-off affair. Still, this sensor can be a very useful way to trigger simple reactions.

An often overlooked sensor in the NXT is the rotation sensor included in each motor; it may not seem like a big deal, but it allows the motors to mimic your own ability to “feel” the position of your limbs (even with your eyes closed!) and move them at different speeds when necessary.

The easy part of working with motors is knowing what they do -- it’s a little more difficult to know exactly when to use a given motor and how. Take time to have students give their own examples of how we use their biological equivalents; this not only reinforces the similarities between humans and robots, but also establishes or reinforces the validity of using their own experiences as a source of building ideas.

Continue to the next article in the robotic series and learn about Giving Your Robots a Brain.