Building LEGO Structures
(This page was copied from MIT)

Also see the downloading firmware and using Robolab in the ECC documents

Most people consider LEGO to be a childhood toy, but the LEGO Technic and Mindstorms systems make excellent construction materials for building robots. The best way to learn how to build with them is to just sit down and play with them. Since they can be taken apart as easily as they are put together, no design must ever be final. You can experiment with building, redesigning, and rebuilding until you are satisfied with the results.

The curse of building with LEGO, however, is that the pieces fall apart easily. If you have ever assembled a LEGO set and then dropped it on the floor, it probably shattered into a bunch of little pieces. This makes building a robot tough, but by employing a technique known as bracing, you will be able to build strong robots that can be handled and even dropped without falling apart.

LEGO Dimensions

lego dimensions
figure 1: LEGO dimensions

Before you begin construction, it is important to understand the building blocks you will be using. One of the first things you will notice is that structural LEGO pieces come in two different heights. The taller ones, bricks and beams, are 3/8" tall, while the shorter ones, flats and plates, are 1/8" tall. These are convenient measurements, since three flats can be stacked to equal the height of a brick. The dimensions of these basic LEGO pieces are shown in figure 1.

Unfortunately, neither of these heights are the same as the standard LEGO width. Instead, this distance, the Fundamental LEGO Unit (FLU) is 5/16", thus the height and width of a beam have a ratio of 6:5. All is not lost, though, because with a little creative stacking, it is possible to create vertical spacings which are integral multiples of horizontal spacings. The simplest such stack is two flats and one beam which yields a height of 2 FLU, though other stacks are possible. As you will see below, this property will be essential to building your robot.

Beams and Connectors

Another important aspect of the Technic and Mindstorms systems are the beams and connectors. The beams are the long structural pieces with holes through their sides. Besides their obvious use as structural components, they can be used in conjunction with the little gray and black connectors to build elaborate structures.

The connectors fit into the holes in the side of the beams and allow them to be joined side to side. This frees you from only being able to stack pieces on top of one another, thus opening up the ability to build significantly more complicated structures. Since the connections created in this manner can be rotated to any angle, you can even introduce diagnol constructs to your robots or create moving joints.

Note that the two types of connectors are functionally different. The black ones fit more snugly into the holes and resist rotation. The gray ones, on the other hand, rotate freely inside the holes for use in moving parts. It is okay to use the gray connectors in place of the black ones, but using a black connector in a moving joint will damage the connector and hole.


lego bracing
figure 2: A simple braced structure

In order to build a strong robot, you will have to master the technique of bracing. Structures built simply by connecting pieces together with their nubs will not be able to handle the stresses imposed on them by the operation of the robot. Instead, you must find a way to augment the structure with braces to make it stronger.

The basic idea of bracing is to create a stack of pieces between two beams, such that the holes in the top and bottom beams are separated by an integral FLU spacing (actually, only even numbers are possible). Then, using the connectors, you attach a beam vertically alongside the stack so that it holds the stack together. The concept is simple, but it is very powerful. One such structure is shown in figure 2.

Since bracing imposes constraints on how a structure can be built, it will be necessary to consider how your robot's structure will be braced from very early on in the design process. With experience, you will be able to build robots which can carry heavy loads and resist falling apart even when dropped on the floor. You will also be able to determine where braces are needed (and also of importance, where they are not). Figure 3 shows a brace inside a real LEGO robot.

bracing picture
figure 3: A brace inside a real robot