what is resolver?
Dear colleagues, hello
A resolver is an electromechanical sensor that converts rotary motion into angular position information. It is commonly used in motor control systems, industrial automation, and robotic applications. Resolvers measure the absolute angular position of a rotating shaft using alternating current (AC) signals and are less affected by electromagnetic noise or signal degradation.
Resolvers typically consist of the following components:
- Rotary transformer: Includes a rotating rotor and a stationary stator. The rotor follows the motor shaft, while the stator provides a fixed reference frame.
- Primary windings: Located on the stator; they provide the excitation AC signal that induces voltages on the rotor side.
- Secondary windings: Located on the rotor; they detect the induced voltages depending on the shaft angle and convert them into sine and cosine output signals.
The resolver’s output voltages vary with the rotor’s angular position. These analog signals are then processed by a converter circuit to provide precise position feedback to the control system.
Main advantages of resolvers:
- High temperature tolerance: Operates reliably even in extreme thermal environments.
- Strong electromagnetic immunity: Highly resistant to electrical noise and magnetic disturbances.
- Long lifespan: With no optical parts or fragile components, resolvers are mechanically durable and suitable for long-term applications.
Resolvers provide a reliable and robust feedback solution for harsh environments. However, they typically offer lower resolution than encoders and require complex signal processing to interpret the analog sine and cosine outputs. Depending on the project requirements, either resolvers or encoders can be selected accordingly.
Field experience and practical insights:
From years of experience in industrial systems, I’ve observed that resolvers outperform encoders in environments exposed to heat, vibration, or electromagnetic fields. While encoders may fail due to dust or heat buildup, resolvers continue operating without signal drift — especially in motor housings running above 100°C.
In servo drive repairs, I’ve seen that resolver cables can carry stable feedback even near high-current inverter outputs where interference is intense. The analog nature of their signal makes them easier to filter and more stable over time compared to digital square-wave outputs.
A key lesson from the field: resolver-to-digital converters (RDCs) play a critical role. Even a perfect resolver will give inaccurate results if the converter isn’t properly matched. Always ensure your drive or control card supports the same excitation voltage and frequency.
Resolvers are not about “maximum precision” — they are about maximum reliability. They are the choice when you need feedback that never quits, not even in the toughest industrial environments. That’s why they’re still found in aircraft actuators, heavy-duty CNC machines, and military-grade servos today.
In short, a resolver may look simple, but it’s a survivor — a sensor designed by engineers for engineers, where endurance matters more than elegance.
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