Drivetrain

The drivetrain is the mechanical system that allows a robot to move. The drivetrain on the Security Tank is actually quite simple. It uses caterpillar tracks, also known as tank treads.

Tank treads consist of a belt or a chain that runs across at least two wheels or pulleys. They work in pairs, with one on each side of the vehicle. The treads essentially make two large, continuous surfaces that the robot drives on. This provides a large contact area with the ground and allows a tank to have maximum traction over all surfaces. Changing the power split between the left and right tracks allows the robot to steer. This is what the term tank steering refers to.

As you might have guessed, our Security Tank is equipped with two caterpillar tracks and has tank-style steering. One EV3 large motor drives each of the tracks; the left EV3 large motor is plugged into motor port B and drives the left caterpillar track, and the right EV3 large motor is plugged into port C and drives the right track.

There is no gearing between the EV3 motors and the tracks. Rather, the motors directly drive their respective tracks. This makes for a compact, robust, simple drive system. The downside is that the tank drives slowly. But this project is not about building a speed machine; that is something we will cover later in this book.

When you take a look at the underside of the robot, you can see both of the drive motors, each paired with a tank tread:

There are two more things you should notice: the first is that the EV3 brick is mounted on the bottom of the robot, between the two drive motors, facing downward towards the ground. The brick actually acts as a structural part of the chassis. Placing the EV3 brick here helps to make the tank very compact. However, the downside is that it is not conveniently located; the user will need to lift the robot to access the brick and switch the programs. This is an example of a trade-off you may face when engineering your own robots, and you will need to ask yourself which is more important for the specific robot you are building: accomplishing a mechanical goal, or maintaining ease of use (ergonomics) for the user. For this project, ease of use is sacrificed to make a neater, more compact robot.

The second thing that you may have noticed is that there is a small pulley attached to the right drive motor. This pulley drives a rubber band that runs around the spiked roller in the front of the tank. This system is set up so that when the tank drives, the spiked roller will turn as well. This is an example of a clever way in which you can accomplish more than one task with one motor!