A rotary encoder using built-in self test ability with redundant and fault tolerant position sensing. Wherein the rotary encoder is used with a variety of rotary equipment including a valve actuator including the generation of speed data (1). The use for diagnosing valve actuator problem and other rotary equipment includes the frequency analysis performed upon speed or position or torque or thrust or vibration data including the speed or position data of the valve provided by the rotary encoder.