Abstract

Crack! is a 30 lb combat robot that I led the development and manufacturing of as the captain of the Irobotics Sophomore Combat Robotics Team. It features a 3lb independent battle hardened drivetrain, an electronically actuated ar500 hammer capable of delivering over 5000lbf, a welded Ti-6Al-4V wedge, and a machined 2024-T6 aluminum chassis. The ambitious design was constructed on time and under budget. The robot is in its second year of iteration and has competed in two of the largest combat robotics tournaments in North America.

V1

CRACK! has gone through several itterations during its development. The pictures above are from the first version, constructed for RoboBrawl 2023. This design was our team's first robot and gave us valuable experience that we applied to the second version.

The Chassis of Crack is entirely composed of 2024-T6 aluminum. Every panel is designed to interlock to increase rigidity and strength. The panels were manufactured by water jetting 2D profiles on our University's waterjet. All components were designed to minimize the cutting time and cost. Careful Tolerances were required due to the accuracy of the waterjet and the effects of combat. After cutting, all additional features were machined on a Bridgeport Mill. As a manufacturing  and maintenance consideration, every fastener was standardized to 1/4-20. 

Many 30 lb. combat robots feature powerful weapons which can carry Kjs of kinetic energy. Our solution deflecting the weapons is a large armored skirt. The wedge is made from 0.125 in. Ti-6Al-4V which is welded at the seams and reinforced with internal gussets. To source the material for the wedge, I negotiated a sponsorship with TMS Titanium, a California based titanium outlet. The components were cut on a waterjet and TIG welded by me with grade 2 Ti welding rods. 

The drivetrain is arguably the most important component of a combat robot. To control your opponent and position yourself to use your weapon, you must have a reliable drive. Crack's drivetrain features four brushless outrunners that output a combined total of 3.4 hp and are capable of rotating at 50K rpm. The motors attach to planetary gearboxes which are modified to have a hex output. The robot can be run with mecanum wheels or standard wheels with cleats. Mecanum wheels allow the robot to move in any direction because of their angled rollers and independent drive. Standard wheels and cleats can be added when more traction is required. To further increase traction, magnets are attached for an additional 80 lbs of downforce. 

In comparison to weapons that store energy through rotating a disk, hammers are far less damaging and require careful design to make them effective. Gear reduction must be carefully calculated as increasing the torque output directly increases kinetic energy, but limits the maximum angular velocity. The reduction of the hammer was matched to the MOI of the hammer so the motor hits its maximum rpm at the end of the 180-degree swing. A tension spring is run under the weapon to store potential energy on the backswing. The bottom profile of the hammer is an Archimedes cam which reduces the radius as spring tension increases, producing a constant torque on the output. All parameters were calculated on a custom Excel calculator, which takes motor parameters and hammer MOI as inputs, and outputs the ideal gear reduction, spring stiffness, and spacing. A slip clutch on the output shaft reduces the shock on the delicate gearbox and motor.