Building Robots at School

June 23, 2009

VEX Workshop: Centre of Gravity

Filed under: Uncategorized — dtengineering @ 11:36 pm
The centre of gravity of a robot is very important for a number of reasons… not least of which is how it affects traction.  The video, in the post on Drive Systems below, of the “dancing” bobcat excavator is a great example of how the centre of gravity can shift, but here is a simplified case:  by adjusting where you put “heavy” components, such as the microcontroller and battery, you can adjust the location of the centre of gravity, and also adjust how much weight is supported by each wheel.  In this short clip the centre of gravity moves backwards on the robot, shifting more weight to the rear wheels (green arrows) and reducing the weight on the front.

A similar effect happens when an arm lifts a load over the robot.  Here the robot starts out fairly well balanced, as the weight of the glowing purple person at the front of the robot along with the arm and gripper counteracts the weight of the heavy block of robot stuff at the back.  As the arm raises, however, the weight of the arm and load shifts backwards on the robot, eventually leading to the point where there is no longer enough weight on the front wheels to keep them on the ground.  At this point the front wheels raise off the ground and the robot tips over backwards, smashing the poor glowing guy into the ground.

To help keep the robot as stable as possible it is important to distribute loads evenly fore and aft on the robot, and to keep in mind that loads will shift while the robot is moving, particularly when you use a simple one degree of freedom arm as shown here.  You can also help stabilize the robot by putting the point where your wheels contact the ground as far to the front and as far to the back as possible.  One of the down sides of using large wheels is that they limit where that contact point can be located.  In the image below you can see that the smaller red wheels can be located further to the front and rear of the robot than the larger gray wheels.  This helps make the robot more stable.

 Small wheels allow the axles to be placed closer to the front and rear, increasing stability.

 

 

 
 
 
 
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