Resolver.

Prologue

There are different types of electric motors that differ significantly from each other in terms of performance, size, operation. Just to name a few we distinguish the synchronous motors (SPM, IPM, reluctance), the asynchronous alternating current motors, the direct current motors and many others. 

Although some of these motors can also operate through a direct connection to the mains, the rotation, regulation and control of the motion of these motors almost always requires a more or less sophisticated control given by an inverter or by a servo-drive and a sensor position located on the motor rotor. 

There are several position sensors used in motors, the resolver is one of them. 

How does it work?

The resolver is an electromechanical inductive sensor for measuring angular displacements used in rotary applications and in motors. Technically similar to a transformer, it provides an absolute position on the single turn by providing a sinusoidal voltage signal. It is used to detect the speed, direction and position of the rotating element of the motor, the shaft. The digital transducer counterpart is called encoder.

The resolver consists of two sets of circular coils arranged on a ferrous core made of insulated laminations. A rotor coil (fixed on the motor shaft) and a stator coil fixed on the stator of the motor which works like a transformer that induces a voltage to the coil rotor through an air gap. Resolvers use a magnetic flux between the windings on the iron cores in a very similar way to the behavior of the transformers. By combining several coils properly out of phase with each other a voltage signal sine and cosine type is generated providing an analog voltage equivalent to the absolute angle between the rotor and the stator.

The analog signal supplied by the resolver is sent to the Servodrive which converts it into a digital signal with a resolution of 16Bit (65536ppr). The Servodrive can pass an incremental encoder type signal to a numerical control through a “Simulated Encoder” output. This output can be set to 4 types of resolutions: 256, 1024, 4096 and 16384 ppr.

Advantages

The resolver is comparable to a singleturn absolute encoder, but is normally considered alternative to the incremental encoder equipped with Halls sensors. The most immediate advantage is in the resolution on the revolution, since the interpolation of the signal is generally 12/14 bits which are equivalent to 4096 / 16384ppr. In the best case, as in HDT drives, this resolution rises to 16bit which equals 65536ppr. (internal resolution of the drive). An incremental encoder provides resolutions of 1024 or 2500ppr at most. A less apparent but clearly overwhelming advantage compared to the encoder consists in the fact that the resolver does not have an optical sensor or an on-board electronics and is therefore extremely robust and perfectly suitable for use on motors that must be placed in difficult environments with high temperature.

Disadvantages

The main disadvantage of the resolver is in the accuracy of the signal. The signal that is generated is created by windings and as such they do not guarantee a clean signal like that generated by an optical encoder. Therefore, although the resolution and positioning accuracy is higher than an incremental encoder, the signal cleaning is not the same and this generates noise during the motor control which in some applications may not be accepted.