Servo - February 2009

switch) and light. You could buy additional sensors such as the motor rotation types mentioned above, and the simplicity of the RCX allowed you to readily make your own. The NXT's sensors include the touch and light switches as with the RCX, but also have a microphone for detecting sound and ultrasonic, for measurement, and determining nearby movement.

Various third parties sell enhanced sensors for the NXT. For example, Hi Technic Products (www.hitechnic. com) offers an electro optical proximity detector similar in function to the popular Sharp IR units commonly used in homebrew robotics. They also provide color sensors, compasses, accelerometer and tilt sensors, gyroscopes, and a multiplexer that lets you connect up to four NXT touch sensor switches to one input port.

Construction Parts

Both the RCX- and NXT-based robotics kits come with hundreds of LEGO Technic parts. The exact assortment varies between the two packages and, of course, you are free to combine parts from other standard LEGO and LEGO Technic kits, such as the low-cost Bionicle line. You can also purchase specific parts sets from retailers, including LEGO's own online store. The basic building block is the LEGO brick. Bricks are composed of one or more raised "studs" or "nubs" on the top and corresponding sockets on the bottom. The studs and sockets mate, allowing you to stack bricks one on top of another. Plates are like bricks, but are a third the height. Like bricks, plates have one or more studs on the top and corresponding sockets on the bottom, and are made to be stacked together. And finally, beams are specific to the Technic brand of LEGO parts, included with the Mindstorms sets. Beams are bricks with one row of studs and have holes down their sides. Beam variations include L- and hooked-shaped versions which can be used for things such as building robotic grippers.

Assembly parts hold things

together such as attaching one beam to another or a wheel to a beam. Connector pegs are typically used with the LEGO Technic line to attach beams together. Connectors pegs are either 1/2, 3/4, or full -- the difference lies in the length of the connector on one side of its middle collar. Some connector pegs are half connector and half axle (see below for more information on axles). Available in a wide variety of sizes and shapes, friction connectors are used with both LEGO Technic and standard parts for such things as connecting beams and wheels.

Other assembly parts include the bushing and the cross axle. For turning parts, there are wheel hubs provided in a number of different sizes; pulleys which are used with string, rubber bands, or O-rings to create a kind of belt; and gears which are used to construct more sophisticated motion transfer mechanisms. Rounding out the parts list are toggle joints, sloped beams, catches, cross blocks, axle connectors, cams, and a raft of others. Check out the LEGO website for more information on LEGO parts. For some good books on LEGO, go to the SERVO webstore at

( http://store.servomagazine.com).

All these parts are made to snap together. When properly constructed, the snap-together system provides for relatively strong joints. In fact, most "purists" frown on building LEGO mechanisms in any other way, including using fasteners or glues. Still, depending on the type and size of robot you're constructing, you might consider cementing non-movable pieces using ordinary household cement (such as Duco), epoxy, contact, or CA (cyanoacrylate) glues. If you want a very permanent bond, use clear ABS plastic solvent (LEGO parts are made of ABS plastic). ABS solvent cement is available at most home improvement stores, as well as many hobby stores. Another approach is to use hot melt glue. You can use either the standard or low-temperature variety.

Temporary constructions can be taped together using black electrical tape or even strapping tape used for box packaging. Do note that most tapes leave a sticky residue. Acetone can be used to remove this.