Center of Mass
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The center of mass (COM) or center of gravity (COG) of a robot is the mean location of all the mass of a robot.
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.
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
implement motion profiling and other motor control techniques
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.
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 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.
