3.1.2 Sensorless Control

Another method is to use sensorless control, which only requires a few passive components on the control board.

This method works by sensing the voltage on a floating phase induced by the movement of the magnets in the rotor, called Back Electromotive Force (BEMF). BEMF gives the microcontroller data about the current position of the rotor.

As the voltage induced in one coil is proportional to the speed of the rotor, at very low speeds, the amplitude of the BEMF is very small, noisy and difficult to filter. As such, there is an open-loop starting procedure in which the rotor follows the magnetic field generated by the microcontroller asynchronously. The speed grows continuously until the BEMF has a detectable amplitude.

The upside of sensorless control is that any sensored motor can be driven in Sensorless mode and can be used as a fail-safe control method, where it is possible. It is also more reliable as the system does not depend on external inputs, other than the inherent BEMF of the motor.

Unlike sensored control, where the hall effect sensors provide a logical signal, without any noise, sensorless control needs signal filtering and conditioning due to motor construction, power supply noise and other factors.

Because of the BEMF filtering, abrupt changes in motor speed cannot be detected precisely and may get the system out of lock. The sensored control does not suffer from this problem, as the Hall sensors provide the signal regardless of the motor operating conditions.