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In addition to the pneumatic actuation system,
I improved Boadicea's strength to weight ratio is by building
the robot with lightweight materials. Boadicea's structural components
are mostly injection molded plastic and
carbon fiber composites. Additionally,
the composite and plastic construction of Boadicea allows rapid
prototyping and testing of new designs.
Injection molding is normally used in high volume projects because of its economic advantages. However, there are several reasons why it is appropriate for producing small robot parts, even in limited quantities.
A primary advantage of injection molding is that it allows more complex part geometry. For example, thin walled parts are much easier to make with a molding process because the part does not need to be clamped in a vise. Another advantage of using plastic parts is the wide variety of plastic materials which are available. The number of polymers, fillers, and reinforcing options available is dizzying; plastic materials offer a far greater range of properties than metals [Dexter 79]. Boadicea has parts made from several different plastic compounds.
I could not have built Boadicea without insert molding. This technique involves placing bushings, shafts, and structural elements in the molds, and then casting plastic around the inserts. Insert molding eliminates most post-machining operations and simplifies assembly. A more significant benefit, however, is that this process guarantees proper alignment and positioning of critical features. Insert molding is also the best way to fasten parts to the composite tubes used in Boadicea's body.
On a walking robot, the primary metric for selecting structural elements is the stiffness to weight ratio, which is extraordinarily high for carbon fiber composites. Boadicea uses composite tubes for structural parts in its body. The uniaxial tubing on Boadicea has nearly four times the stiffness to weight ratio of an equal size aluminum tube. High modulus graphite composites can have a specific stiffness nearly seven times that of aluminum, and about five times the tensile strength [Kalpakjian 92].
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