Build Your Own Combat Robot

Chapter 10: Weapons Systems for Your Robot 219
ground from the impact. The spinning, low-mobility attack of the thwack bot
makes it impossible for it to choose its angle of attack, letting its opponent line up
its attack strategy as it sees fit. A secondary attack mode of ramming and pushing
can help in those cases.
Overhead Thwack Bots
This type of bot was first used in the Spirit of Frank (Robot Wars, 1995). Examples
include Toe Crusher, Over Kill, and Mjollnir. The overhead thwack bot features a
wide, two-wheeled base, with the main body being built entirely between the two
wheels and fitting into their radius, and a long weapon–tipped boom such that the
body flips over and brings the weapon down on the opponent whenever the robot reverses
direction rapidly.
Thwack Mechanism Design
Like the thwack bot, the overhead thwack bot uses its motor torque to power an
impact weapon. Unlike the conventional thwack bot, the overhead thwack bot
attacks by reversing its drive power rapidly, the reaction torque from the drive
motors swinging the entire body end over end and bringing the tail end down in
front of it violently.
The challenge comes in getting enough inertia into the body of the robot, with
significant force and accuracy to hit the target. The same rapid reversal of drive
power that brings the weapon over will also drive the robot away from the target.
Attacking with an overhead thwack bot is accomplished by charging at a target
and then slamming itself into reverse just before impact. The entire robot has to be
balanced just right, such that the robot flips over quickly before it starts to back up
significantly. Insufficient or uneven wheel traction can cause the robot to veer to
one side while flipping, causing the weapon to miss its intended target. Widely set
wheels will help with accuracy.
Figure 10-7 shows an overhead thwack bot.
While a conventional thwack bot can take several revolutions to get up to
speed, an overhead thwack bot must produce all its weapon power in less than one
half of a full revolution of its drive wheels. The electrical and mechanical drive
power components have to be optimized for a high rate of energy delivery—high
current rate batteries, thick wiring, high-horsepower motors, and very rugged
drive gearing are a must. All the main components must fit between the drive
wheels for the robot to flip freely. Usually, these bots have large-diameter wheels
set wide apart to allow sufficient room between them for the main body. Of
course, large-diameter wheels usually means a high gear reduction to get the right
speed and torque, and large wheels and a high gear reduction will make the wheels
respond more slowly to rapid motor power reversal.
Optimizing an overhead thwack bot for maximum damage is difficult. The
best tactic is to increase drive motor power as much as possible. Increasing
the length of the tail and the weight of the mass at the end of the tail will increase