How does a resolver work and what does a resolver do?
Resolvers are angular position sensors which can be used for position and velocity feedback in applications such as servo motors. Due to its design, the hollow shaft resolver boasts of a service life, which is well above average. Reliability as well as high precision and low space requirements, supplement its favorable characteristics. It remains fully operable even under extreme environmental conditions. Essentially, the resolver consists of a stationary stator and a movable rotor in terms of being a mechanical device.
Electrically, it consists of a transformer for supplying the rotor with power and a second transformer for determining angles. The first transformer has a concentric design and is functionally independent of angle values. The second, angle-dependent transformer is made of a stator winding and a rotor winding. The windings of these two transformer components are designed such that the number of windings in the grooves correspond to the values of a sine.
Negative values are realized by reversing the direction of the winding. The stator coils consist of two similar windings that are fitted in a relative position to each other rotated by 90°. If the rotor winding is energized, a sinusoidal magnetic flux is created that induces voltages in the stator coils as a function of the relative angular position of the rotor and stator. The amplitudes of the two voltages correspond to the sine or cosine. Thus, using a suitable evaluation circuit, it is possible to obtain the absolute angle data (Sine ϕ / Cosine ϕ = Tan ϕ, where ϕ = shaft angle). The term used for the basic version is a resolver with one pair of poles (1-speed-resolver).
The number of pairs of poles indicates how often the sine distribution of the rotor and stator windings is repeated during one revolution. The higher the number of pairs of poles, the higher the mechanical precision of the resolver. With multiple pairs of poles, the absolute angle data can be lost but a higher resolution is possible after digital conversion of the resolver signals.
What is the difference between resolver and encoder?
Both devices (resolvers and encoders) are angular position sensors, meaning by converting the mechanical movement into electrical signals, they measure the rotary position of a shaft. Resolvers are much more robust against extreme environmental conditions such as high temperatures as well as shock and vibration compared to encoders. Resolvers can serve as an alternative to the incremental and absolute encoders. Encoders provide a digital output signal whereas resolvers provide an analog output signal, thus requiring resolver-to-digital conversion. The decision between selecting a resolver or an encoder depends on the specific application, particularly on environmental conditions and on the control electronics. Resolvers are favorable for very harsh environments, but encoders can show high accuracy and are less complex to integrate into the control electronics.
