I finished taking my control theory course, and I want to build an RC car that has cruise control. So I started off with the force analysis, and I got external forces that are constant. For example if the road is sloped, then the gravitational force is holding the car back. Another force is rolling resistance. I believe these are the only two considerable forces that will resist the car's movement; however, this does not provide a stable system.
Doing a little research online, I found that most models, if not all, say that the external forces are proportional to the velocity of the car. I understand how that can be related to the spring-damper model, but I don't see why the external forces are proportional to velocity when all the forces are constant. For instance, rolling resistance only depends on the weight/normal force of the car--not velocity. So how can I say they are variably dependent on velocity?
Also, let's assume that the forces are proportional to velocity, then how would I figure out that proportionally constant?
I am trying to create a model in simulink with some sort of controller, probably PID, then later transfer it into reality. And I am already stuck with the main system. Thanks for any help!