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A 'yaw rate sensor' is a gyroscopic device that measures a vehicle’s rotation around its vertical axis. The output is usually in degrees per second or radians per second and signal is a signed number. The angle between the vehicle's heading and vehicle actual movement direction is called yaw angle.
There are basically two forms of Yaw rate sensors: Piezoelectric type and Micromechanical type.
The measurement is based on the coriolis effect.
In the piezoelectric type, the sensor is a "tuning fork" shaped structure with four piezo elements(two on top and two below). During straight ahead driving, the upper ones produce no voltage as no Coriolis force acts. But in cornering, the rotational movement causes the upper part of the tuning fork to leave the oscillatory plane creating an AC voltage which is proportional to the yaw rate and oscillatory speed. Its sign depends on the direction(left or right)
In the Micromechanical type, the Coriolis acceleration is measured by a micro mechanical capacitive acceleration sensor placed on an oscillating element. This acceleration is proportional to the product of yaw rate and the oscillatory velocity which is maintained electronically at a constant value.