In the precise world of motor control, there is an unassuming yet essential component: the resolver. It acts like an accurate position detector, continuously monitoring rotor angle and providing critical position feedback to the control system. This article explains how a resolver works.
What Is a Resolver?
Resolvers are available in several configurations, including variable-reluctance and wound-rotor designs. A resolver resembles a small motor and mainly consists of a stator and a rotor. A typical variable-reluctance resolver is used here as an example.
- Stator: Usually contains three windings: an excitation winding (EXC), a sine winding (SIN), and a cosine winding (COS).
- Rotor: A rotationally symmetric structure made from stacked silicon-steel laminations.
As the rotor turns, the sine and cosine windings induce voltage signals related to the rotor position. Processing these signals makes it possible to calculate the rotor angle with high precision.
How It Works: Electromagnetic Induction
The resolver relies on electromagnetic induction. A high-frequency AC signal, often several kilohertz, is applied to the excitation winding and creates an alternating magnetic field. This field couples into the sine and cosine windings.
Because the sine and cosine windings are physically orthogonal, the strength of their coupling varies periodically with rotor position.
- When the rotor field is parallel to a stator winding, that winding produces maximum voltage.
- When the rotor field is perpendicular to a stator winding, that winding produces zero voltage.
The sine and cosine outputs therefore vary as a sine wave and a cosine wave as the rotor rotates.
Signal Behavior: A Dance of Sine and Cosine
The animation below shows how the resolver stator, rotor position, excitation, sine, and cosine signals relate to one another.
The animation shows three key signals:
- Excitation signal (EXC): A high-frequency sine wave with constant amplitude that drives the resolver.
- Sine signal (SIN): A sine wave whose amplitude changes with rotor position.
- Cosine signal (COS): A cosine wave whose amplitude changes with rotor position and whose phase differs from the sine signal by 90 degrees.
Measuring both amplitudes makes it possible to uniquely determine rotor position over a full 0–360 degree range.
Decoding the Angle: From Analog Signals to Digital Data
The resolver outputs analog sine and cosine signals. A resolver-to-digital converter (RDC) converts them into data suitable for digital control systems.
An RDC generally performs three steps:
- Sample the sine and cosine signals at high speed.
- Calculate the current angle using digital signal-processing algorithms such as the arctangent function.
- Output the calculated angle to the control system as a digital signal.
Advantages and Applications
Resolvers are widely used in industry because of several advantages:
- High reliability: Robust construction, high-temperature and vibration tolerance, and strong immunity to interference.
- High precision: Accurate position feedback for high-performance motor control.
- Absolute position: The rotor position is available as soon as power is applied, without a reference point.
- Long service life: Contactless construction or low-wear slip rings provide a long operating life.
Common applications include:
- Electric vehicles: Traction motors and electric power-steering motors.
- Industrial automation: Servo motors and robot joints.
- Aerospace: Actuators and navigation systems.
- Defense: Radar and artillery control systems.
Conclusion
With its elegant electromagnetic sensing principle and strong performance, the resolver provides a dependable foundation for precise motion control in modern industry. It is used in applications ranging from electric vehicles and industrial robots to aerospace and defense systems.
References
- [1] Resolver Principles and Applications. Motor Control Technology.
- [2] Resolver-to-Digital Conversion Techniques. Analog Devices.