This tutorial is designed to introduce the important factors you need to consider when designing the weapon for your beetleweight bot. Modified from the work of previous BattleBots leads and aspects of the BattleBots Bible (RioBotz Combot Tutorial 2.0).
Step 1: Getting Started
One of the requirements for the project is to create a moving weapon.
There are a lot of important factors to consider when designing spinning weapons. Of those factors, some of the most important are weapon shape, center of mass, moment of inertia, material, and bite.
Step 2: Weapon Shape/Bite
When designing the shape of the weapon it is important to consider the forces on the contacting surfaces and ensure that they are properly supported.
In addition, weapon bite plays an important role in determining the effectiveness of a weapon. The weapon bite is how much of the weapon comes into contact with the opponent robot.
The formula for weapon bite is weapon bite = (vx1 + vx2) ⋅ 2π / (n ⋅ ω).
- vx1 is your robot's velocity
- vx2 is the other robot's velocity
- ω is your weapon's velocity
- n is the number of teeth on your weapon
The goal is to maximize weapon bite while maintaining a high kinetic energy. Generally, having only a few teeth (1-3) is optimal.
Step 3: Center of Mass
Center of mass is an important part to consider when it comes to designing your weapon. The center of mass, by definition, is the average position of all mass located within an object. This point is important when it comes to making sure your weapon spins with stability. The closer your center of mass is located to the axis of rotation the less oscillation will occur with your weapon. This means your robot will have less vibration, which is important, especially for shell spinners.
Finding your center of mass is easy when using SolidWorks, as the program calculates it all for you.
- use Mass Properties to get a precise calculation
- to see where it is, Insert > Reference Geometry > Center of Mass
Just do your best to keep the center of mass located as close to the center of your weapon's shaft as possible.
Step 4: Moment of Inertia
Moment of inertia is another important piece of your weapon. It is a property that is directly related to how powerful your weapon is. The larger your moment of inertia is, the more power you have within your weapon.
There are a few ways to make your weapon have a larger moment of inertia:
One is to use heavier materials like steel. This is a surefire way to make sure your weapon will just have a larger inertia. However, this method is often inefficient in terms of weight and costly in terms of money.
The way that people usually end up increasing moment of inertia is to use a combination of materials. They often fill the inside with lighter materials like plastic or aluminum while having an outer radius of heavier steels. This increases the average radius at which mass is located, and thus increases the moment of inertia for the weapon. An increase in moment of inertia is an increase in the amount of energy within the weapon, making it inflict more damage.
One thing to consider with moments of inertia is that the larger the moment of inertia, the longer it will take the weapon to spin up.
Step 5: Weapon Blades/Inserts
Different parts of a weapon feel different forces. The tips of the weapon are impacted with high forces; these forces are weaker in the body of the weapon, as they are spread out through more material.
Additionally, the tip of the weapon often impacts another weapon or hard metal armor. Because weapons come into contact with hard metals, they are often made out of hard metals to prevent dulling/damage during impacts. However, since hard materials are more brittle, over-hardened weapons are subject to fracture.
Weapon inserts are designed to optimize the materials in both parts of the weapon. Also, if they break/get too dull you can replace them. The most difficult part of working with weapon inserts is attaching them into the weapon in a way that doesn't break them.
Step 6: Mounting
Another thing to consider is that forces have an equal and opposite reaction. This means that when your weapon hits another robot, all the force you impart on them goes back into your weapon mount. Thus, it's extremely important that your weapon mount can hold up to the forces.
This holds true if you mount your weapon directly to the weapon motor. If you mount it directly, all of the shock forces from the impact are transmitted directly to the motor. One way to minimize this is to use a belt between the motor and the weapon; during impact, the belt will slip and the shock force will not be transmitted to the motor.
Step 7: Next Steps
Once you have a better idea of what your weapon design should look like, start applying it to your sketches/CAD model!
If you have any other questions or want a more in-depth look at some of the equations behind weapon design, check in with us/your mentors.