Building Robots at School

September 22, 2008

Wheel Diameters, Bumps and Traction

Filed under: teaching,Tech Ed,VEX — dtengineering @ 6:04 pm

The students are making good headway on their first class challenge.  All five teams have built something that actually moves and have managed to set up their CPU and controller so that they can actually control it.  But they are noticing that some robots climb the “bump” from the cement to the foam better than others.  They have also noticed that some robots are really good at pushing the other robots around.  So I figured it was time to talk a little bit about the importance of wheel diameter and tire traction when it comes to designing robots.

on the right, a standard two-motor, four wheel drive robot.  In the back, a four-motor, four wheel drive robot.  Since there is nothing linking the front and rear wheels together, when one wheel loses contact with the ground, the motor attached to it is no longer helping to push.  On the left a robot with two different diameter wheels... but with the same gearing leading to each wheel.  This is a common "rookie mistake" as the two wheels will have the same rotational velocities, but different circumferences, meaning each wheel will want to travel at a different speed.  Note the front "omni-wheels", which allow for easier turning... a factor that becomes more important as the robots get bigger and heavier.

Three Robots: on the right, a standard two-motor, four wheel drive robot. In the back, a four-motor, four wheel drive robot. Since there is nothing linking the front and rear wheels together, when one wheel loses contact with the ground, the motor attached to it is no longer helping to push. On the left a robot with two different diameter wheels... but with the same gearing leading to each wheel, a typical "rookie mistake".

The article on torque briefly discusses the importance of wheel diameter in determining the pushing power and speed of the robot.  (Larger wheel = faster, but less pushing power.)  But a larger wheel also makes it easier to climb over obstacles.  This is because as the wheel gets bigger, the angle between level ground and the wheel’s tangent at the contact point gets smaller.  This means that a smaller wheel needs to rapidly accellerate upwards to get over an obstacle, while a larger wheel can “keep on rolling”.  Both axles will move upwards the same amount (since the obstacle is the same size for each wheel), but the smaller wheel will have to do it much more rapidly, “wasting” energy by rapidly converting forward velocity into upward velocity.  At the extreme end, when the wheel radius is equal to or less than the height of the obstacle, the robot must come to a complete halt in the horizontal direction.  This is one reason why “monster trucks” have such big wheels… it allows them to climb over big obstacles.

The second reason that big wheels help is that they raise the axles up off the ground, providing greater ground clearance.  This helps prevent getting “high centred” on an obstacle. This happens when the weight of the robot comes to rest on the chassis (where it touches the obstacle), taking weight off of the drive wheels (typically leaving one set spinning in the air) and thus reducing the amount of traction that can be used to push the robot up and over the obstacle.

A Bored of Notes on the Importance of Wheel Diameter for Clearing Obstacles

A Bored of Notes on the Importance of Wheel Diameter for Clearing Obstacles

So it sounds like big wheels are the best choice, but as with everything, there is a trade-off.  If you have size contraints on your robot, larger wheels mean that the axles, and thus the points where the wheels contact the ground, are also closer together.  This means that the robot is more likely to tip forward or backward under heavy accelleration or braking.  This is often compounded by the fact that the higher ground clearance provided by higher wheels often also results in a higher centre of gravity for the robot, making it more likely to tip in any direction. 

Of course this discussion has been based on four-wheel drive robots.  There are many other solutions to climbing obstacles and maintaining a low centre of gravity.  Six and eight wheel drive systems, using smaller wheels… typically with at least one set of wheels raised up a little higher than the others are quite commonly used for climbing over obstacles.  Tank Treads are also often used.  Other, more exotic drive systems such as snakes, walkers and other creative solutions are used for climbing obstacles as well.

Tips on How to Maximize Pushing Power

Tips on How to Maximize Pushing Power

Other than the importance of going over an obstacle, the other thing that students are discovering is that some robots ‘push’ better than others.  There are a number of factors that can affect this, as shown on the board of notes, to the right.  Maximizing pushing power is a balance between torque and traction.  Torque comes from the motors and gears, and is converted into a pushing force by the interaction of the wheel and the ground.  To maximize pushing power, drive the robot against an immovable barrier.  If the wheels spin, then you have more torque than you do traction.  To make your robot push harder, you need to imcrease the traction, either by increasing the co-efficient of friction between your tire and the ground, or by increasing the amount of force pushing downwards on your tires (ie: make the robot heavier, if you can.)  Alternatively, you may wish to increase your gearing or wheel diameter and trade off some of the excess torque for an increase in speed.  If the motors stall, however, and the wheels stay still, then you have more traction than torque.  You can increase your pushing power by reducing your gearing or wheel diameter (at the cost of reducing your speed)… or by adding more motors to your drivetrain.  A more complex solution, should you need both high torque AND high speed is to build a two-speed gearbox that can “shift on the fly”.

One final note, shown by the left hand robot in the photo at the start of this post, is that if you are going to put different sized wheels on a drive train, then it makes sense to link them with the appropriate gear ratio.  If both wheels spin at the same RPM, and one has a bigger circumference than the other, it is going to want to go faster than the smaller wheel.  The only time these two wheels won’t be “fighting” with each other is when the robot is stopped or the motors are stalled.

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