The Bowmar Builders are an FLL team associated with Bowmar Elementary School. The purpose of this blog is to help other students interested in robotics to learn from what we have learned. As an FLL Robotics team we use the Lego EV3 robotics system and the Mindstorms graphical programming language.

Thursday, December 1, 2016

Problems with gripping, turning, and going over things

By Mateo B.

Today we discovered a problem with our robot. It was not turning very well at all. The wheels would keep slipping. So we tested it and found out that we needed to put more weight on the wheels. We changed our robot so that it was smaller and the center of gravity was just in front of the wheels.

We worked out the math that showed us what was wrong. The center of gravity was towards the front of the robot, which made the driving wheels have very little weight on them. Here is how you can figure this out for your robot.

The first thing to do is measure the distance between the wheels. We call this distance "L". Next, measure the distance from the back wheel to the center of gravity between the wheels. We call this distance "L_b". The ratio of these two numbers tells us a lot of information about how the robot will grip and roll over objects.

Once you have these two numbers you can use them in the fraction portion of the equation below.

As an example, say your "L" is 10 spaces and the center of gravity is roughly 2 spaces in front of the back wheels (L_b). The ratio for how much of the weight of the robot is on the back wheels is (1-2/10), or 0.8, which is 80%. The higher that percentage, the more grip your robot has. The problem with our last robot design was that we were at 30%. This left 70% on the front wheels, which made it very difficult to turn and to roll over things. We changed our robot design so that now it is at roughly 90% of the weight on the back wheels without an attachment, and 80% to 85% with an attachment on the front.

The equation shows us how there are four degrees of freedom for making the robot turn better. First, the "alpha" parameter is the coefficient of friction. (Coefficient means the number stuck on front of another number. It makes it smaller or bigger.) The coefficient of friction means how "sticky" or "slippery" the wheel is on the mat. The higher the coefficient of friction, the better. This number usually goes from 0 to 1 (for example, ice would be a small number, like 0.1. Concrete would be higher, like 0.8.) You can change the coefficient of friction by using tape on the wheels to get the dirt and dust off.

The second degree of freedom is the weight of the robot. Heavier = more sticky, but also more mass you have to move. So it pretty much equals out.

Third is the distance between the wheels, also called the wheel base. If you make the wheel base longer, without moving the center of gravity, then more weight is on the back. Shorter means less.

The last degree of freedom is where the center of gravity (also called center of mass) is located. Moving the center of gravity closer to the back wheels puts more of the weight on the back wheels, which is what you want.

If your drive wheels are on the front rather than the back, just flip the diagram around so that you are always measuring from your driving wheels.

For our robot we decided to both shorten the wheel base and move the center of gravity much further back. We moved the center of gravity by putting the heaviest part (the EV3 brick) just behind the wheels. This made the center of gravity just in front of the wheels. By making it shorter we made more it agile. The combination should let us go through the road walls and over the crossing barrier much more easily and with greater control.

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