I've noticed a common problem among First robots. When commanded by their software to go straight, they tend to turn. I think I understand why. On this page I will explain the problem. I think I know a solution. But rather than provide a solution, I prefer to get you thinking about this. Before we proceed, we need some technical background.
DC permanent magnet motors can run either forward or backward. There is a natural assumption that they work well either way. That is not correct. In fact the commutator of the motor is not symmetric to the windings. The switching of the magnetic fields is started early because it takes time to actually switch. What this means is that a motor has a distinct forward and backward direction. And the motor is considerably faster, more powerful, and more efficient when running forward than in reverse. For the sake of this article I will assume that backward is 60% as good as forward.
Most robots at First use tank steering. There are several advantages to this.
One result of the decision to use tank steering is that the robot has two drive trains. And assuming there is a transmission, this means that there are two transmissions, one for each side.
Transmissions are complex systems. Once you get a design that works you want to stick with it. This means that the left and right side have the same transmission. In order to do that, typically the transmission is rotated 180 degrees between the left and right side. We need to do this because otherwise the motors stick out of the robot.
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But remember what I said about the direction of electric motors. As you can see in the picture, we now have the forward direction of the left and right sides pointing in opposite directions. And it is now obvious why the robot tends to turn. What is not obvious is what we should do about it.
What most teams do, at least initially, is to compensate in software. But when you do that you pay a high price. Software cannot turn 60% power into 100%. All it can do is to reduce power: 100% down to 60%. And since in each direction the robot wants to go there is always one drive train that is not capable of more than 60% That means that with a software solution the robot can never have more than 60% of what the motors are capable.
Clearly we need a different solution. And clearly that solution needs to be mechanical. It is the nature of that solution that I want you to think about.