Servo - June 2009

MAN UFACTUR IN G:

RioBotz Comb t Tutorial

Summarized:

Design Fundamentals

● Original text by Professor Marco Antonio Meggiolaro; Summarized by Kevin M. Berry

Professor Meggiolaro of the Pontifical Catholic University of Rio De Janeiro, Brazil, recently translated his popular book, The RioBotz Combot Tutorial, into English. In the process, he greatly expanded it and solicited input from a wide range of robot builders. The updated version — 367 pages long with an incredible 895 figures, graphs, and photographs — is available free for download at www.riobotz.com.br/en/ tutorial.html. SERVO Magazine, as a service to the building community, is summarizing the tutorial in a series of articles beginning with part of Chapter 2, “Design Fundamentals.” Look for the rest of Chapter 2 and additional chapters in the future. All information here is provided courtesy of Professor Meggiolaro and RioBotz.

Scale Factor

One important thing to keep in mind during the design phase is the scale factor. If the physical dimensions (length, weight, height) of a bot are doubled, the volume cubes. However, the cross sectional area of a structural member just doubles, so its strength doesn’t keep up with the weight/volume. What does this have to do with combat robots? Everything.

If you want to take your 12 pound hobbyweight design and scale it up to a 120 pound middleweight, you’d need to multiply the weight by 10. You’d do this by multiplying all the dimensions by the cube root of 10, or 2.15. If you were going to double a 60 pound bot’s weight, you’d be building a 120 pound middleweight with a

scale factor of the cube root of two, or 1.26, so all dimensions should be increased by 26%.

Obviously, there is a lot more to this. Volume doesn’t automatically equal weight unless the density is the same, which means precise scaling of components. Using commercial parts with standard sizes, this won’t exactly happen. Also, stress and strength aren’t linear factors. The tutorial recommends raising the scaling factor to a power of 1.5 in critical components like weapon shafts to account for

TABLE 2 WEIGHT CLASS

• 60 pound Lightweight

• 120 pound Middleweight

• 220 pound Heavyweight

• 340 pound Super-Heavyweight

DIAMETER 0.5” 0.75” 1.0” 1.25”

Weight Classes

Combat robots range in size from the unnamed one ounce miniatures, through 50 or 75 gram fleaweights, up to 390 pound megaweights. Table 1 summarizes current weight classes.