Using gears

The mechanical design of a robot can often influence the performance or behaviour of a robot.

For example, it is possible to identify gear combinations (particularly compound gear combinations) which should result in a very fast moving robot, but which fails to move the robot at all through lack of what is commonly referred to as power, but is more strictly known as torque. This is because of a necessary trade-off between the speed of the output gear relative to the driver gear mounted on the motor, and the turning power, or torque, available at the output gear relative to the driver gear.

If you have written a timing based, open loop program to drive your robot over a particular distance, or get it to turn on the spot through a desired angle, the distance travelled will be dependent on the size of wheel and the gear ratio connecting the motor to the wheel.

If you are using the RCX robots, you can easily modify the gear ratio that connects each motor to its corresponding wheel shaft. Try modifiying the gears to see what effect it has on the distance the robot travels over a given time, the speed it travels at, and the rate at which it accelerates from a standing start.

Optional Exercises

As well as determining the load your robot can pull, the available torque also limits how steep an incline your robot can climb. Experiment with the steepest angle of slope up which your robot can climb with the different gear combinations.

A final experiment is to build a tray on skids, load it with weights and attach it to your robot (string works fine!) - what mass can your robot pull with different gear ratios?

You may have found that when you gear down the robot so that it travels slowly it can pull a greater load and climb a steeper slope.

You may also have noticed that your robot could pull a greater load when it is on a rough surface. This is because such a surface allows for greater traction.

You may additionally have found that by pushing down gently on the robot you could help it grip the surface better - that is, you increased the traction by essentially increasing the weight of the robot. Traction is a major consideration in robot design, and one that is often neglected in the early stages of design, though at the designers' peril as you can perhaps testify.

To see what the effect of different gear combinations is on the speed of rotation of each gear in a gear train, try out this gear simulator:

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