Sensors and feedback modes for BLDC/PMSM motors*

For a controller to properly drive a BLDC (Brushless DC) or PMSM (Permanent Magnet Synchronous Motor), it must constantly know the rotor position.

This information is essential for synchronising phase commutation, regulating the torque, speed, and in some cases, the absolute position.

Several feedback technologies exist, each suited to a different level of performance or precision.

Hall sensors: simplicity and robustness

The most common system relies on three Hall effect sensors, integrated directly into the motor.

These sensors detect the passage of the rotor's magnetic poles and provide digital information (0 or 1) to the controller.

• The three combined signals define six commutation sectors per electrical revolution.

• The controller switches the motor phases at each state change, which is sufficient to maintain a stable torque and smooth speed at moderate operation.

Advantages:

• Economical and highly reliable solution.

• Sufficient for most traction, actuator, or simple automation applications.

• Works even at very low speeds (unlike sensorless).

Limitations:

• Limited resolution (6 electrical positions per revolution).

• Slight torque ripple noticeable at low speed.

Hall sensors are therefore an excellent compromise of simplicity / robustness / cost for motors powered between 24 and 51.2 V in mobile or industrial applications.

Encoder (incremental or absolute): precision and advanced control

For applications requiring precise control of position, speed, or movement, motors are equipped with an encoder.

It can be:

• an incremental encoder (generating A/B pulses and a Z reference),

• or an absolute encoder, directly providing the rotor's angular position.

Some encoders deliver sin/cos signals, allowing for very fine interpolation of the position, with resolutions of several thousand points per revolution.

Advantages:

• Very precise control of torque, speed, and position.

• Essential for robotic, synchronised, or multi-axis systems.

• Allows for efficient and smooth vector control (FOC).

Limitations:

• Higher cost.

• Requires more rigorous wiring and configuration.

This type of feedback is preferred in high-precision electric actuators, machine axes, or agricultural robots, where movement reproducibility is essential.

Sensorless: sensorless control

It is also possible to drive a BLDC motor without physical sensors.

The controller then estimates the rotor position by measuring the back electromotive force (back-EMF) generated by the windings at the time of commutation.

Advantages:

• Reduction in cost and complexity of the motor (no sensors).

• Increased reliability in certain harsh environments (dust, vibrations).

Limitations:

• Ineffective at very low speeds or at start-up, as the back-EMF is too weak to be measured.

• Less robust during high torques or rapid load changes.

Sensorless solutions are suitable for applications where the motor mainly operates at a constant speed, and where assisted start (mechanical or software) is possible.

Synthetic comparison

In summary

The choice of sensor type depends on the expected performance level:

• For robust and economical control → Hall sensors.

• For fine and synchronised control → encoder.

• For minimal cost and stable speeds → sensorless.

Position feedback is an essential element of motor control, ensuring performance, safety, and durability of the system.

*: The technical information presented in this article is provided for informational purposes only. It does not replace the official manuals of manufacturers. Before any installation, handling, or use, please consult the product documentation and adhere to safety instructions. The Torque.works site cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.