Learn Brushless Direct Current (BLDC) Motor Sensors
Introduction
Accurate rotor position sensing is essential for controlling Brushless DC (BLDC) motors, especially in applications requiring precise speed, torque, and direction control. There are several position-sensing topologies used in BLDC motor systems, each with unique characteristics, advantages, and disadvantages. This lesson introduces the three most common position sensing methods—resolver, optical encoder, and Hall effect sensor—explains how they work, and discusses their suitability for various applications.
Resolver
The resolver is the earliest and most robust position-sensing topology. It operates using electromagnetic induction to generate analog sine and cosine signals proportional to the rotor’s angle. These signals must be converted to a digital value using a resolver-to-digital converter. Microchip’s dsPIC® Digital Signal Controllers (DSCs) are well-suited for this task, as their high-speed Analog-to-Digital Converter (ADCs) and Digital Signal Processing (DSP) engines can efficiently process the resolver’s outputs and calculate absolute rotor position.
Resolvers provide continuous, absolute position feedback, making them ideal for servo applications in industrial control where high resolution and reliability are critical. Their robustness allows them to function reliably in harsh environments, including those with extreme temperatures, vibration, or contamination. However, resolvers are expensive and require specialized signal processing hardware, which can increase system cost and complexity.
Optical Encoder
Optical encoders are widely used for high-resolution position sensing. They consist of a disk with transparent and opaque sections, a light source, and photodetectors. As the shaft rotates, the encoder generates digital pulses corresponding to the disk’s movement. Optical encoders can be incremental, providing relative position information, or absolute, offering exact position data at any time.
Incremental optical encoders deliver high resolution—often hundreds of interrupts per revolution—but do not provide absolute position at power-on. Absolute encoders overcome this limitation but are more complex and costly. Optical encoders are suitable for servo applications requiring high accuracy, but they are sensitive to dust, dirt, and vibration, and their cost is moderate to high. Microchip’s microcontrollers and DSCs feature Quadrature Encoder Interface (QEI) modules, which simplify the processing of encoder signals and support precise position and speed control.
Hall Effect Sensor
Hall effect sensors are the standard position-sensing solution for low-cost BLDC motor applications. These sensors detect the magnetic field of the rotor and provide digital signals indicating rotor position, typically every 60 electrical degrees. One to three Hall effect sensors are commonly used, with three sensors providing commutation information for each 60-degree segment of the rotor’s rotation.
Hall effect sensors are simple, compact, and inexpensive, making them ideal for high-volume applications such as home appliances. They are easy to integrate using basic external interrupt inputs or channel change notification features available on Microchip microcontrollers. However, their resolution is low, as they only indicate commutation states rather than precise positions. For example, in a motor with five pole pairs, five electrical cycles from the Hall sensors correspond to one mechanical revolution.
Summary
| Feature | Resolver | Optical Encoder | Hall Effect Sensor |
|---|---|---|---|
| Output Type | Analog (sine/cosine) | Digital (pulses or code) | Digital (commutation signals) |
| Resolution | High (continuous) | High (up to thousands of counts per revolution) | Low (typically 6 states per electrical cycle) |
| Accuracy | High | High | Moderate |
| Robustness | Very high (resistant to noise, vibration, temperature) | Moderate (can be affected by dust, vibration) | Moderate (can be affected by EMI) |
| Cost | High | Moderate to high | Low |
| Size | Moderate to large | Small to moderate | Very small |
| Suitability for FOC | Yes | Yes | No |
| Absolute Position | Yes | Yes (absolute encoders) | No (only commutation state) |
| Installation Complexity | Moderate to high | Moderate | Low |
| Typical Applications | High-performance, harsh environments (EVs, industrial automation, aerospace) | Robotics, CNC, precision motion control | Basic BLDC control (fans, pumps, power tools) |
Choosing the right position sensing topology for a BLDC motor depends on the application’s requirements for resolution, reliability, and cost. Resolvers offer absolute position feedback and exceptional robustness, making them ideal for industrial servo systems. Optical encoders provide high resolution and accuracy, suitable for precise control, but are sensitive to environmental factors. Hall effect sensors are cost-effective and easy to implement, best for basic commutation in high-volume, low-cost applications. Microchip Technology supports all these sensing methods with dedicated hardware modules and software libraries, enabling engineers to design efficient and reliable BLDC motor control systems.
Learn More
- AN885 - Brushless DC (BLDC) Motor Fundamentals
- AN901 - Using the dsPIC30F for Sensorless BLDC Control
- AN1160 - Sensorless BLDC Control with Back-EMF Filtering Using a Majority Function
- AN2520 - Sensorless Field Oriented Control (FOC) for a Permanent Magnet Synchronous Motor (PMSM) Using a PLL Estimator and Equation-based Flux Weakening (FW)
- AN957 - Sensored BLDC Motor Control Using dsPIC Digital Signal Controllers (DSCs)
- Motor Control and Drive
- Microchip Supported Motor Types
- Motor Control Application Framework (MCAF)
- motorBench® Development Suite
- Proportional Integral Derivative (PID) Compensator
- Motor Control Terminology