What are the types of wheel speed sensors used in vehicles?

Wheel speed sensors measure rotational speed for ABS, traction control, and stability systems. Types include: Variable reluctance (passive) sensors generating AC voltage from a toothed ring - simple but signal varies with speed; Active sensors (Hall effect or magnetoresistive) providing digital square wave output regardless of speed - more accurate at low speeds. Modern vehicles use active sensors for reliable operation from near-zero speed to maximum, enabling advanced control functions.