Driving Sensorless Brushless Motors

One can now find an abundance of sensorless BLDC motors, mainly from China. These motor are primarily addressing the Drone and Hobby market but, thanks to their excellent characteristics and low cost, they find their ways in an increasing number of robotics and industrial applications.

sensorless BLDC thrusters with roboteq motor controller

Roboteq FBL2360 controller with 2 T200 sensorless BLDC thrusters from Bluerobotics

Roboteq controllers now support sensorless BLDC control. Compared to typical BLDC Electronic Speed Controllers (ESCs), the Roboteq controller brings advanced, professional features such as forward/reverse operation, monitoring of all operating parameters (current, voltage, temperature, speed), closed loop operations, USB/Serial computer connectivity, CANbus networking, MicroBasic scripting and many others.

A short video shows Roboteq's FBL2360 dual channel brushless controller driving an outrunner BLDC motor. The motor starts up smoothly and can operate a lower speeds than ordinary BLDC speed controllers. The oscilloscope capture shows the perfect commutation timing, noticeable by the totally symetrical BEMF rising and falling ramps on the floating phase.

Brushless DC (BLDC) motors is usually operated with a rotor-position sensors (typically Hall sensors), since the electrical excitation must be synchronous to the rotor position. For reasons of cost, reliability, mechanical packaging and especially if the rotor runs immersed in fluid, it is desirable to run the motor without position sensors, which is known as sensorless operation.

Sensorles control is a technical challenge. The motor windings of a brushless motor are energized using six-phases trapezoidal commutation. Each phase is equivalent to 60 electrical degrees. Six phases make up one electrical revolution.

For every phase, two windings are energized and one winding is not energized. The fact that one of the windings is not energized during each phase is an important characteristic of six-phase control that allows for the use of a sensorless control algorithm. When a BLDC motor rotates, each winding generates BEMF. This BEMF, and therefore the rotor position, can be sensed by monitoring the floating winding.

brushless commutation on roboteq controllers

The figure above shows the state of the transistor switches during one of the six commutations phases. The current flows from the battery through the U winding, then through the V winding to ground. The W winding is floating. Since no current is flowing through W, the W terminal has the voltage potential of the BEMF that is induced in the W winding, added to the voltage at the motor's center point. The center point motor is exactly half the V+ voltage, assuming that all windings have the same resistance.


The figure above shows the voltage at each of the windings during the 6 commutation phases. Notice the voltage of the floating phase ramping between 0 and V+, and between V+ and 0, as the rotor moves 60 degrees. The point in the ramp that is exactly at half the supply voltage can easily be detected using comparators. That point is called Zero Crossing and is used to synchronize a commutation timer inside the controller. The BEMF waveform of the motor varies as both a function of the rotor’s position and its speed. Detection of rotor position using the BEMF at zero and very low speeds is not possible. At startup, the motor is operated by applying a fixed commutation sequence and runs as a stepper motor until sufficient speed is gathered to detect Zero Crossings. Sensorless can therefore NOT be used if no torque is required a stall or very slow speed. Nevertheless, there are many applications (e.g., fans, pumps, watercraft thrusters) that do not require speed or positioning control at low speeds.

Sensorless control is supported in the FBL2360 controller. It is also available on the high power RGBL18xx family. It will be available on the MBL1xxx family in Q3 2016.

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