A resolver is a rotary transformer used to measure degrees of rotation. Resolvers can loosely be described as the analog equivalent to a rotary encoder, and provide position and speed feedback for a variety of industrial applications.
Resolvers consist of a rotary transformer, a rotor and a stator. When the transformer is excited by a current, it induces current in the rotor due to electromagnetic induction. As current flows through the fixed windings on both the rotor and stator, the feedback is used to determine the position of the rotor relative to the stator.
Because resolvers do not include any electronic parts, they are much more rugged than rotary encoder solutions and can provide reliable, accurate feedback in harsh environments that feature high temperatures, vibrations, radiation and contaminants.