Rotor Synchronization; Motor Driver Control; Current Mode Control - ST STM32 User Manual

UM2124
3.2

Rotor synchronization

The BLDC drive is a synchronous control drive: the maximum efficiency is achieved if the
commutation between two consecutive steps is performed only when the rotor is in the right
spatial position, which occurs when the BEMF signal and the phase current are
synchronized.
The following methods can be implemented to perform the rotor synchronization:
sensored drive, which uses position, Hall or encoder sensors to measure the rotor
•
position.
• sensorless drive, based on the BEMF detection. It analyzes the zero crossing of the
floating phase BEMF signal to establish the commutation point. The match between
the floating phase BEMF signal with respect to the ground point is used to generate
the commutation between two consecutive steps in order to achieve the rotor
synchronization.
The six-step firmware library v.1.0 or v.1.1 is only based on sensorless mode, no position
sensor is supported. The BEMF signal is analyzed for each step and a new commutation
point is calculated step by step. This method allows rotor position synchronization with the
drive control.
3.3

Motor driver control

The BLDC drive is performed modulating the phase voltage through a PWM sequence that
is generated by two different control methods:
• current mode (peak control): two control loops are needed, one inner and one outer.
The inner loop generates, by hardware (with analog comparator), the PWM sequence
for motor current regulation starting from an analog reference signal; this signal is
generated by the outer loop that contains a digital PI regulator that compares the
speed reference with the motor speed real feedback. It provides the torque control and
inherent current limitation in the windings.
• voltage mode: only one loop is needed, the PI regulator directly generates the PWM
regulating the duty cycle value.
3.3.1

Current mode control

Figure 1: "BLDC motor control sequence"
method. The inner loop contains all the components to acquire the motor current, amplify
and conditioning the signal to be compared with a reference value. This block output is
directly connected to the ETR timer function (MCU features) that regulate the duration of
the PWM signals. In this case, PWM Ton are managed by the ETR.
Implementation of six-step motor control
algorithm in STM32 Nucleo microcontroller
shows the current mode (peak current) driving
DocID029868 Rev 1 9/39