Rotor Position Sensing
- Kevin Li
The commutation of a BLDC motor is controlled electronically. The stator windings must be energized in a particular sequence to enable rotation. It is therefore crucial to know the position of the rotor in order to determine which winding must be energized.
Hall Effect Sensing
Hall Effect Theory
If a current carrying conductor is kept in a magnetic field, the field exerts a transverse force on the moving charge carriers. These charge carriers will typically be pushed to one side of the conductor. The buildup of charge at the opposing sides of the conductor will balance with this magnetic influence, producing a measurable voltage drop. The presence of this measurable transverse voltage is called the Hall effect.
Hall Effect Sensing Implementation
Most BLDC motors have three Hall effect sensors embedded into the stator of the motor to sense rotor position. Hall effect sensors output a high or low signal, corresponding to a north or south pole respectively, whenever the magnetic poles of the rotor pass by them. Shown below is an example placement of Hall effect sensors on the stator (sensors a, b, and c):
Based on the outputs of the Hall effect sensors, the required commutation sequence can be determined and executed.
Hall sensors require a power supply. The input voltage required may range from 4V-24V. The input current required may range from 5mA-15mA. The output of the Hall sensor is typically an open-collector type and so a pull-up or pull-down resistor may be required.
Hall Effect Sensing Drawbacks
Embedding Hall effect sensors into the stator is complex because any misalignments with respect to the rotor magnets could result in inaccurate rotor position sensing. To simplify the process of mounting Hall effect sensors, some motors may have the Hall effect sensors and Hall effect sensor magnets placed on the rotor. The result is that whenever the rotor rotates, the Hall effect sensor magnets give the same effect as the main rotor magnets. The Hall effect sensors are normally mounted on a PCB and fixed to the enclosure cap on the non-driving end. This allows for the Hall effect sensors to be adjusted easily to align with the rotor magnets.
Back EMF Sensing
Back EMF Theory
When a BLDC motor rotates, each winding generates a voltage known as back electromotive force (BEMF) that opposes the main voltage supplied to the windings according to Lenz’s Law. The polarity of the BEMF is in the opposite direction of the energized voltage. The BEMF depends on three primary factors:
Angular velocity of the rotor
Magnetic field generated by the rotor magnets
The number of turns in the stator windings
We can relate the BEMF to key characteristics by the following expression:
Here, we define the parameters:
N: The number of winding turns per phase
l: The length of the rotor
r: The internal radius of the rotor
B: The magnetic field density of the rotor
ω: The angular velocity of the motor
Notice that for a completely designed motor, all parameters of the above expression are constant except for the motor’s angular velocity. It is trivial to see that as the angular velocity (or speed) of the rotor increases, the BEMF also increases.
The potential difference across a winding can be calculated by subtracting the BEMF value from the supply voltage.
Motors are designed with a BEMF in such a way that when the motor is running at its rated speed, the potential difference between the BEMF and the supply voltage (voltage drop across the windings) will be sufficient for the motor to draw the rated current and deliver the rated torque. If the motor is driven beyond its rated speed, the BEMF may increase substantially, thus decreasing the potential difference across the winding, and reducing the current drawn. The effect is a droop in achievable torque.
When the supply voltage becomes equal to the BEMF and motor losses, the drawn current and achieved torque become zero.
Back EMF Sensing Implementation
BLDC motors can be commutated by monitoring the BEMF signals instead of Hall effect sensors. The relationship between Hall effect sensors and BEMF, with respect to phase voltage, is shown in the figure below:
As we may recall, every commutation sequence has one positively energized, one negatively energized, and one open-circuited winding. As shown in the figure above, the Hall effect sensor signal changes state when the voltage polarity of the BEMF changes from positive to negative or from negative to positive. Thus, the BEMF zero-crossings provides data necessary for commutation.
With this method of commutation, Hall effect sensors and Hall effect sensor magnets can be eliminated in motor construct, simplifying both design and cost. This is advantageous if the motor is to operate in environments where occasional cleaning of Hall effect sensors is required.
The figure below shows a block diagram for BEMF sensing control of a BLDC motor:
Back EMF Sensing Drawbacks
Ideally, the Hall effect sensor signal changes state when the BEMF crosses zero. In practical cases, there exists a delay due to the winding characteristics of the stator. This delay should be compensated by the MCU.
Recall that BEMF is proportional to the speed of rotation. At very low speeds, the BEMF would be of very low amplitude and it would be difficult to accurately detect zero-crossing behavior. With BEMF sensing, the motor has to be started in open loop. When sufficient BEMF is generated to detect zero-crossing accurately, the control should be shifted back to BEMF sensing. The minimum speed at which BEMF can be sensed is calculated from the BEMF constant of the motor.