8.2 Back-EMF Motor Control Mode
(Submit Feedback)The closed loop of a BLDC motor control is a Phase-Locked Loop (PLL) based on the rotor position. Note that this inner loop does not attempt to modify the position of the rotor; instead, it modifies the commutation times to match whatever position the rotor has. An outer speed loop changes the rotor velocity, and the commutation loop locks to the rotor position to commutate the phases at the correct trigger times.
The Back-EMF sampler monitors the motor phase voltages. The neutral point Virtual Null simulator calculates the neutral point of the motor based on the following equation:
The three motor phase voltages are compared with the neutral point. The outputs of the three comparators are available synchronously for estimating the zero-crossing time events accordingly. In ‘Star wound ’ motor configuration, no extra wire connection is required. For ‘Delta wound ’ motors, there is no physical neutral point available, so the reference point must be estimated.
As the microcontroller knows which motor phase is floating and which two phases are driven to low and high, it is always capable of monitoring the comparator output of the floating phase.
When the Back-EMF signal crosses the neutral point or Virtual Null (VN), the Zero-Crossing Detector (ZCD) will switch the ZC_X signal. The MCU may use this signal as a ‘30 degrees before crossing ’ reference point. The MCU must commutate the system after 30 degrees of electrical rotation have occurred. See Figure 8-2.
The Zero-Crossing Detection (ZCD) method is typically used in six-step per electrical revolution commutation strategies. The motor is driven by energizing two windings at a time. One winding stays unenergized at all times, and the voltage (Back-EMF) on that unenergized winding can be monitored to determine the rotor position.
For more accuracy in rotor angle estimation, it is recommended to use complementary PWMs for the H/L side MOSFET bridge. The comparative Back-EMF waveforms are shown below.
When the motor being driven is at rest, the Back-EMF voltage is equal to zero. The motor rotor needs to be initially rotated for the Back-EMF sensor to lock onto the rotor position and commutate the motor. It is necessary to bring the rotor from rest up to a speed fast enough to generate sufficient Back-EMF voltage (enough to allow the Back-EMF sampler to send it).
Depending on the motor construction and the rotor speed, filtering algorithms are required for stable Zero-Crossing Detection (ZCD).
