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Basic Considerations

For a preliminary design of a beetleweight (3 lb) robot, you wihttps://wiki.robojackets.org/index.php?title=3lbers&action=editll need to consider the following:

  • Weapon type
  • Drive type (number of wheels, invertible, etc)
  • How it will be maneuvered (gyroscope stabilization, tank or mixed drive)
  • Armor vs weapon weight (how much are the wheels/back protected)
  • Materials used for armor, weapon, and frame (Various aluminum alloys, steel, plastic, titanium, etc)
  • How the frame is assembled (bolted - where do the bolts go and is it practical, welded - how to put it together properly)
  • How the weapon is made (can it be machined in the required shape, does it require CNC or can be done by hand)

Weapon Considerations

Common Weapons Types

Drum Spinner

Radii, Misti

This weapon is a spinning drum with teeth at the front of the robot. It is driven similarly to a vertical or horizontal spinner with a pulley system between the weapon and weapon motor.

Ring Spinner

Qti, Smitti Werbenjagermanjensen

This weapon is a spinning ring with teeth that surrounds the entire frame. Generally, the ring is supported by "rollers" which have bearings to reduce friction and support the ring with a male-female interface. The ring is typically driven directly by the weapon motor which has a casing of a high friction material such as rubber.

Shell Spinner

The weapon is a spinning "shell" that encompasses the robot. Similar to a ring spinner, the shell has teeth on it which serve as the weapon. These are driven either with a pulley system on a central support rod, or the same way as a ring spinner.

Vertical Spinner

Dat Boi

A robot with a vertically spinning weapon at the front of the robot. Typically this weapon is complemented by a surrounding wedge to lift opponents into the weapon. It is also driven similarly to a drum or horizontal spinner with a pulley system between the weapon and weapon motor.

Horizontal Spinner

Schwifti, Ki

The weapon is a flat spinning bar or disk at the front of the robot. It is also driven similarly to a drum or vertical spinner with a pulley system between the weapon and weapon motor.

Weapon Drive

Typical weapon drive employs a belt and pulley drive train. A rubber or urethane belt runs between a motor with a pulley, and a pulley on the weapon. When using a brushless out-runner motor, the outer can of the motor can be used as a pulley directly, but requires guides to prevent the belt from sliding up or down the motor. Direct drive weapons, where the motor shaft is directly connected to the weapon, is highly discouraged, as the shock from weapon hits can shatter the motor magnets, breaking the motor entirely.

Common motor specs at this scale:

  • Power: 500-800W
  • Voltage: 11.1 (3s) - 14.8 (4s)
  • KV: Depends on weapon type. Small spinners such as drums, disks, and bars, aim for weapon speeds greater than 8000RPM. Large spinners such as ring and shell spinners are in the 2000-5000RPM range

Weapon Material

Weapons are usually built from aluminum (6061) and mild steel. Large weapons such as drums and ring spinners employ an aluminum frame with steel teeth. Smaller weapons such as bars and disks can be made entirely from steel. It is critical that steel be used for the part of the weapon that comes in contact with other bots, as aluminum is too soft and deforms on direct impact. Toolsteel teeth may be used however it must be fully heat treated after machining to be effective. It is also quite expensive and usually not worth it.

It is important to run FEA on the weapon design, as it will highlight key points of failure. Typical forces a weapon experiences at this scale ranges between 300-600lbs.

Drive Train Considerations

Speed and Torque

A typical 3lber can move around 50-100 in/s. Usually gear ratios necessary to achieve these speeds can supply more than enough torque to allow the robot to push opponents around the arena.

Ground Clearance

A problem most 3lbers have is being too close to the ground. Debris and uneven arena floor can easily get underneith the robot, propping the wheels off the ground and preventing movement. While considered beneficial for preventing wedges from getting underneath the bot, the loss in control due to low clearance usually nullifies this benefit. It is far better to have a highly mobile bot with high clearance, greater than 0.375" from bottom plate to ground.


Here are some common calculations used to find things such as weapon speed, drive speed, pushing force, and power requirements.


ω = RPM *2π/60

v = ω*r

P = ω * τ

α = τ*J

F = τ*r

T = ω/α *0.63

t = T*3

ωoutput = ωinput * e

τoutput = τinput / e

P: Power (Watts)

RPM: Rotations per Minute

KV: RPM/Volt

V: Voltage

ω = Angular Velocity (rad/s)

α = Angular Acceleration (rad/s2)

τ = Torque (N-m)

J = Mass moment of inertia (N-m2)

F = Force (N)

T = Time Constant (s)

t = Spinup time to reach 98% max RPM (s)

r = Wheel/pulley radius (m)

v = Tangential velocity (m/s)

e = Gear Ratio (Drive gear/Driven gear)

NOTE: It is typically easiest to use si units for calculations then convert to imperial later.

How to Build a BattleBot Tutorials

Tutorial 1

Tutorial 2

Tutorial 3