Servo - July 2009

article on weapon shaft thicknesses.

PHOTO 2

Optimization

Combat robots are born overweight, and seem to get heavier after that. This may require a complete redesign or applying some optimization techniques to lose weight, improve strength, or (hopefully) both.

Optimizing the shape is one approach. Taking a solid disk and removing material without removing strength is a good example. One key factor in the Moment Of Inertia (MOI) of a disk is the mass at any given radius. This means that metal removed near the hub doesn’t count as much towards MOI as metal near the edge. Thin spokes aren’t as strong as thick ones, so a number of smaller holes may be better than a few big ones. Think of a honeycomb, web design, or a truss. Two good examples are shown in Photos 1 and 2.

There is a lot more help on this topic in the book (about 3-1/2 pages worth) and all of it is worth reading.

PHOTO 1

Drive!” Drive a lot. Hundreds of hours, not just a few. Good driving wins matches.

Structures

There are three main types of robot structure: trussed, integrated, and unibody. Trussed robots are built using bars, bolted or welded together, resulting in a very rigid and light structure. The armor is usually made out of plates attached to the bars. They are a fast type to build, and are easy to work with during events. The plates (and possibly truss members) can be removed and replaced with spares. The greatest disadvantage is the welded joints which can be weak points. Armor plates can also be ripped off during fights (Photo 3).

Integrated robots receive their name because the structure and armor are integrated into a single structure using screws or welds. The armor plates are also in the structure. Building this type of bot is harder, but they can be very compact and strong (Photo 4).

Unibody robots have their structure milled out of a solid block of material. Through milling, the sides, bottom, and pockets for

components are created. No welds or screws are needed except for installing the top. These are the lightest robots, and can be the most damage resistant. However, you can lose more than 80% of the material in the carving process, not to mention the amount of time spent to do this. Another downside is that a damaged structure can’t be replaced (Photo 5).

Another way to make unibody robots is through molding materials such as composites like carbon fiber or Kevlar. These are not very popular because of the expense, difficulty, and expertise needed.

Armor

Building and Testing

Full scale models are another very helpful building step. Components can be mocked up (before spending money) from PDF or CAD drawings. Design can take up to 60% of the total development time, so make it count. Once you’ve modeled the bot and are ready to cut metal, “measure twice, cut once” is the rule.

Testing is often short changed or overlooked. According to Carlo Bertocchini’s Law, “Finish your robot before you come to the competition.” Often bots are finished just before (or at!) a competition, leaving no time for testing. Many things can go wrong if the bot doesn’t have sufficient drive time before competing. This leads nicely to Show Master Dave Calkin’s main advice: LTFD or “Learn To Freakin’

Like structures, armor types come in three flavors: traditional, ablative, and reactive. Traditional armor plates are usually made out of tough, hard materials that try to absorb and transmit impact energy. Hardness causes damage to the opponent’s weapons, while toughness resists damage. This can be achieved using a composite type armor, as well as the typical metal plates. Depending on the materials,