Automotive cruise control system is an excellent example of the type of electronic feedback control system. Recall that the components of a control system include the plant, or system being controlled, and a sensor for measuring the plant variable being regulated. It also includes an electronic control system that receives inputs in the form of the desired value of the regulated variable and the measured value of that variable from the sensor. The control system generates an error signal constituting the difference between the desired and actual values of this variable.
It then generates an output from this error signal that drives an electromechanical actuator. The actuator controls the input to the plant in such a way that the regulated plant variable is moved toward the desired value.
In the case of a cruise control, the variable being regulated is the vehicle speed. The driver manually sets the car speed at the desired value via the accelerator pedal. Upon reaching the desired speed the driver activates a momentary contact switch that sets that speed as the command input to the control system. From that point on, the cruise control system maintains the desired speed automatically by operating the throttle via a throttle actuator.
Under normal driving circumstances, the total drag forces acting on the vehicle are such that a net positive traction force (from the powertrain) is required to maintain a constant vehicle speed. However, when the car is on a downward sloping road of sufficient grade, constant vehicle speed requires a negative tractive force that the powertrain cannot deliver. In this case, the car will accelerate unless brakes are applied. For our initial discussion, we assume this latter condition does not occur and that no braking is required.
The plant being controlled consists of the powertrain (i.e., engine and drivetrain), which drives the vehicle through the drive axles and wheels. As described above, the load on this plant includes friction and aerodynamic drag as well as a portion of the vehicle weight when the car is going up and down hills.