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On this page
  • Measuring COM Position
  • Vertical COM Position (Z)
  • Horizontal COM Position (XY)
  • Turning
  • Hanging
  • Control

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  1. Hardware
  2. Design Fundamentals

Center of Mass

PreviousRPMNextIntroduction to VEX Parts

Last updated 8 months ago

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

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.

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.

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

Horizontal COM Position (XY)

Turning

Hanging

Robot Hanging
Simplified Diagram

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

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

implement motion profiling and other motor

The 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.

⚙️
control techniques
Hang
center of mass
RI3D for Over Under by JHAWK
Robot in 3D Cartesian Coordinate System
Example from
The magnitude of the is proportional to the height and magnitude of the braking/slowing force.
6105C Blackout Robotics Tipping Point
torque
turning center