Center of Mass

The center of mass significantly impacts the robot's maneuverability. Make sure to consider its effect on each subsystem during the design stage, rather than addressing it after the robot is built.

The center of mass (COM) or center of gravity (COG) of a robot is the mean location of all the mass of a robot.

Robot in 3D Cartesian Coordinate System

Measuring COM Position

• Model the robot in CAD, and use a "measure"-type tool.]

• Lift the robot by the chassis using 2 fingers. When the robot balances on your fingers, the XY location of the COM will lie in the line formed by your 2 fingers.

• Hang the robot, and observe how the robot tilts to settle.

Vertical COM Position (Z)

When a robot brakes, the friction between the wheels and the floor rapidly decelerates the robot (velocity is in the opposite direction of acceleration in this case). This creates a torque around the center of mass of the robot, jerking the back of the robot up. In many cases, the rear wheels lose grip with floor as well.

Example from 6105C Blackout Robotics Tipping Point

To mitigate jerking ...

• lower the height of the COM off the ground

  • use lighter materials in the upper sections of the robot (ex. 1x1 L channels on arms, plastic screws / nuts)

  • use heavier materials in the lower sections of the robot

• decrease the max acceleration available to the driver

• implement motion profiling and other motor control techniques

The magnitude of the torque is proportional to the height and magnitude of the braking/slowing force.

Horizontal COM Position (XY)

Turning

The turning center of the robot is less predictable if the COM (in the horizontal plane) is far from the midpoint of all the wheels. Keeping the COM centered side-to-side and front-to-back ensures more stable and predictable turning.

Hanging

Robot Hanging	Simplified Diagram
RI3D for Over Under by JHAWK

The end effector used during a hang is typically implemented with a joint. If the horizontal COM is offset from the point where the robot hangs, the robot will tilt until the COM aligns directly beneath the hanging point.

Control

To control the position of the COM in the XY plane ...

• place towers optimally after building the chassis

• add/remove weight selectively around the robot