This class of vehicles differs from unicycle-type mobile platforms by the existence of one or several steering wheel(s) used to modify the vehicle’s orientation, alike the steering wheel of a conventional car. For these vehicles, the rear wheels are either free of actuation (front-wheel-drive cars), or actuated only for traction/propulsion purposes (rear-wheel-drive cars). This steering system causes a limitation of the vehicle’s turning capacities. For instance, pure rotation of the vehicle’s body is not possible for rear-wheel-drive cars, and is only possible by first rotating the steering wheel angle by 90 degrees for front-wheel-drive cars. In any case, pure rotation is not possible when the steering-wheel angle is not allowed to reach this latter value. Thus, the only way to modify the vehicle’s orientation is to produce a longitudinal motion, either forward or backward. Mathematically speaking, a car-like vehicle has only one degree of mobility, whereas unicycle-type vehicles have two. For this reason, their control is slightly more complicated. The extra complication shows also when writing the vehicle’s kinematic equations. The resulting model is 4-dimensional (3 variables for the position/orientation of the body, and one variable for the steering-wheel angle), whereas it is only 3-dimensional in the unicycle case. The number of control inputs is the same and equal to two. In the car case they are the body’s longitudinal velocity and the steering-wheel angular velocity. At the control design level, transverse functions for car-like vehicles depend on at least two extra variables, instead of only one in the unicyle case.

This video takes back the trajectory tracking task used to illustrate the control of a unicycle-type mobile robot.