trajectory in real time animated

Hi, i am working on a programm that shows on a graphic the trajectory of a spherical robot. The robot only can move straight or making circles, and the variables that i give to the programm are the velocity(vel), and the curvature radius(cr). So here my questions:

-how can i introduce the variables in real time? so each time i introduce the new ones the trayectory changes

-how can i generate the trayectory and show it? the main problem here is the function that describes the movement of the ball: linear movement while cr is zero, making a circle and knowing the center of that circle when cr is not zero, and back to the linear movement when cr is zero again with that new direction.

thanks