What is a Resolver?
A resolver is a type of encoder or a sensor for measuring rotation angle.
Among devices that detect the angle of rotation, the resolver reads changes in the magnetic field by electromagnetic induction to detect the angle of rotation, applied to the rotating equipment in which the resolver is mounted.
Uses of Resolvers
Resolvers are mainly used in industrial equipment. You will not see them in white goods or commercial facilities. Servo motors are the main target for measuring rotation angles.
For servo motors to operate accurately, it is necessary to detect and provide feedback on the motor’s speed and other parameters.
In recent years, resolvers are sometimes used in electric vehicles.
Principle of Resolvers
The basic principle of resolvers is almost the same as that of a transformer.
Rotation changes the relative position of the primary and secondary cores, causing a misalignment between the primary and secondary. The principle of the resolvers is to detect the angle of rotation by measuring the AC value. For example, when the rotation angle is 0 degrees, the relative positions of the cores do not change, so no input-output voltage shift occurs.
In resolvers, the part that rotates in conjunction with the object is called the rotor, and the part with a built-in coil is called the stator. On the other hand, an oval shape provides twice the output, and a triangle shape provides three times the output.
Thus, the output signal magnification can be controlled by the rotor shape. Resolvers thus have the advantage of being able to withstand use in adverse environments because of their simple structure consisting of a core and coil.
Other Information on Resolvers
1. How to Use Resolvers
Signal output from resolvers is converted to rotational angle and angular velocity by RDC, which stands for Resolvers Digital Converter, which converts them into digital signals that a CPU can process.
RDC can also compensate for manufacturing variations of resolvers when digitizing rotational angle signals. The arithmetic processing performed on servomotors and motors used in automobiles is generally based on PID control. The amount of energy to be given to the motor is determined by comparing the target rotational speed with the rotational angle and angular speed detected by the resolvers.
To improve positioning and control accuracy, the time difference between rotation angle detection and energy amount determination timing must be minimized, but this depends on the upper limit of CPU operating frequency.
2. Future of Resolvers
Resolvers have a simple basic configuration but are expensive. In addition to the cost of components, it is expensive to continue to manufacture resolvers that guarantee high accuracy.
Resolvers have multiple stator windings, and it is important to wind all copper wires uniformly. Any variation in winding will affect the output signal and lead to a decrease in the accuracy of position detection.
In recent years, magnetic sensors have been adopted as an alternative to resolvers. There are various types of magnetic sensors, but most are MR sensors that use the magneto-resistive effect. The magneto-resistive effect is a phenomenon in which electrical resistance changes depending on the strength and direction of an external magnetic field. Magnetic sensors are selected from the following according to the application.
- AMR (Anisotropic magnetoresistance effect) element
- GMR (Giant magnetoresistance effect) devices
- TMR (Tunnel magnetoresistance effect) devices
The advantages of magnetic sensors are that they can be integrated with bearing rings and do not require signal processing circuits, such as RDC.