Electrically Driven "Eyes": Resolver - CMVTE EV Conversion Kits

In the driving system of an electric vehicle, the motor is the "heart," and the motor controller (MCU) is the "brain." For the brain to accurately control the heartbeat of the motor, it must rely on "eyes" to sense the motor's speed and rotor position in real-time. These eyes are the Resolver.

Table of Contents

What is a Resolver?

A Resolver is an angle/velocity sensor based on the principle of electromagnetic induction. It is essentially a special type of rotary transformer that converts rotor angle information into electrical signals for the motor controller to process through electromagnetic coupling between the stator and rotor windings.

Key Function: The resolver doesn't output a direct angle value but rather two sine and cosine voltage signals that change with the angle. After processing by the resolver-to-digital converter (RDC) chip, accurate angle information can be obtained.

The Role of Resolver in the Drive Motor

In the control of permanent magnet synchronous motors (PMSM) or AC asynchronous motors, the key is vector control (Field-Oriented Control - FOC). FOC needs to know the exact position of the rotor magnetic poles to achieve precise decoupling of stator current and magnetic field, enabling maximum efficiency and torque.

The resolver's working principle is quite exquisite. It inputs a high-frequency sinusoidal signal (excitation signal) to the rotor coil and receives high-frequency induction signals induced in the coil. These signals can be transformed into corresponding sine and cosine information after processing. After software analyzes this information, we can obtain the absolute position of the stator and achieve precise motor control.

What the Resolver Does:

Rotating Speed Detection

Provides real-time rotating speed to ensure dynamic response of control systems.

Position Detection

Provides rotor angle information to support FOC vector control for precise motor operation.

Safety Redundancy

As a key sensor, functional safety must be guaranteed, commonly implemented through dual-channel redundancy design.

Therefore, without a resolver, there would be no precise control of high-performance electric drive systems.

Technical Features of Resolver

Compared to other common position sensors (e.g., Hall sensors, encoders), resolvers have the following advantages:

High Reliability

Simple structure with no optical components, resistant to dirt and dust. High temperature resistance (typically above 150°C), suitable for harsh motor compartment environments.

High Precision

Resolution can reach 16 bits or even 18 bits (implemented by the resolver chip). Angle error is less than ±0.1°, meeting precision control requirements.

Strong Anti-Interference Capability

Uses electromagnetic induction principle, insensitive to oil, dust, and vibration. Excellent EMC performance, suitable for complex automotive electromagnetic environments.

Of course, its shortcomings are also obvious:

Higher cost, requiring a special resolver-to-digital converter chip
Complex signal chain with high requirements for calibration and alignment

Resolver vs. Alternative Solutions

Sensor Type	Advantages	Disadvantages	Typical Applications
Resolver	High reliability, high precision, strong anti-interference, high temperature resistance	Higher cost, complex signal processing	High-performance EV drive motors, industrial motors
Hall Sensor	Low cost, simple implementation	Limited accuracy, temperature sensitivity	Low-end motors, simple position detection
Photoelectric Encoder	Very high precision, fast response	Poor anti-pollution ability, sensitive to environment	Clean industrial environments, robotics
Magnetic Encoder	Good balance of cost and performance, compact size	Magnetic interference sensitivity, limited temperature range	Mid-range EV motors, some industrial applications

Hall sensor: Low cost but limited accuracy, mostly used for low-end motors.

Photoelectric encoder: High precision but poor anti-pollution ability, not suitable for automotive environments.

Magnetic encoder: With rapid development in recent years, accuracy has significantly improved, and some applications have begun to replace resolvers. However, under high safety and reliability requirements, resolvers remain the first choice.

Therefore, the resolver is the mainstream solution for driving motors, though it faces challenges from magnetic encoders in some applications.

Conclusion

In the electric drive system, the resolver may seem insignificant, but it is the key link ensuring power performance and safety. It acts as the "eyes" of the motor, allowing the controller to see clearly and accurately, thereby achieving powerful performance and smooth driving control.

As the industry evolves, resolver technology will remain at the core for a long time, but we are also seeing the rise of new sensor solutions. In the future, whoever finds the best balance between reliability and cost will become the "new eyes" of electric drive.

Need Expert Advice on Motor Position Sensors?

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Frequently Asked Questions

Why is the resolver called the "eyes" of the electric drive system?

The resolver is called the "eyes" because it provides real-time feedback on the rotor's position and speed to the motor controller, much like how eyes provide visual feedback to the brain. Without this critical information, the controller cannot accurately control the motor's operation, similar to how movement would be impaired without vision.

What are the common symptoms of resolver failure in electric vehicles?

Common symptoms include: false reporting of motor speed when the vehicle is stationary; no motor output when accelerating despite normal high voltage; three-phase hardware overcurrent faults; IGBT faults; motor speed jitter; motor locked-rotor conditions; and unusual vibrations or sounds. Any of these symptoms may indicate resolver issues requiring attention.

How does resolver calibration work and why is it important?

Resolver calibration determines the angle deviation between the resolver's zero position and the motor's zero position. This is crucial because even a small misalignment can cause significant control errors. Calibration ensures measurement accuracy, which directly impacts motor efficiency, torque output, and overall system stability. Different terms like zero-angle calibration or motor angle calibration all refer to this essential process.

What's the difference between a resolver and an encoder?

Resolvers use electromagnetic induction and are analog devices that output sine/cosine signals, while encoders (optical or magnetic) are digital devices that output discrete position values. Resolvers are more robust in harsh environments (high temperature, dust, vibration) but require more complex signal processing. Encoders offer higher resolution in clean environments but are more sensitive to contamination and temperature extremes.

Are magnetic encoders replacing resolvers in electric vehicles?

Magnetic encoders are gaining ground in some applications due to their compact size, good accuracy, and lower cost. However, resolvers still dominate in high-reliability, safety-critical applications because of their proven performance in harsh environments, higher temperature tolerance, and stronger electromagnetic interference resistance. The choice depends on specific application requirements, cost considerations, and environmental factors.

What is resolver-to-digital conversion (RDC) and why is it necessary?

Resolver-to-digital conversion is the process of converting the analog sine and cosine signals from a resolver into digital angle and velocity information that the motor controller can use. This is necessary because resolvers output analog signals, while modern motor controllers process digital data. The RDC chip performs this conversion with high precision, enabling accurate motor control based on resolver feedback.

Can electric vehicles operate without a resolver?

While some simple motor control methods (like trapezoidal control) can operate without precise position feedback, high-performance electric vehicles require resolvers or similar sensors for Field-Oriented Control (FOC). FOC provides superior efficiency, torque control, and smooth operation but depends entirely on accurate rotor position information. Without this, vehicles would experience reduced efficiency, lower performance, and potentially unsafe operating conditions.

Electrically Driven “Eyes”: Resolver