[zero based BLDC series] brushless DC motor control principle based on the zero crossing detection method of back EMF

Please refer to the basic rotation principle of Brushless DC motor 《 be based on HALL Brushless DC motor control principle of Sensor 》, Basic principles and basic knowledge will not be repeated in this article .

The existence of position sensor limits the application of Brushless DC motor in some specific occasions , Such as ： Increase the volume of the motor system , More connecting wires , And the position sensor is at high temperature 、 The sensitivity becomes poor when operating under severe conditions such as high pressure and high humidity , Reduce the reliability of system operation .

According to the different detection principles , The sensorless control method of Brushless DC motor mainly includes ： Back EMF method 、 Flux linkage method 、 Inductance method and artificial intelligence method . among , Back EMF method is the most mature technology at present 、 One of the most widely used position detection methods .

The principle of back EMF zero crossing detection method

According to Faraday's law of electromagnetic induction and Lenz's law , When the rotor rotates, the induced potential will be generated in the stator winding , The direction of the induced potential is opposite to the voltage of the winding , The polarity is opposite to the excitation voltage , So it is generally called back EMF or back EMF .

When the rotor is 0 Angle ,A The opposite electromotive force crosses zero , Further delay 30 When the degree of ,HALL The sensor detects an edge signal , At this time, commutation is required , That is to say 30 When the degree of ,AB The winding is energized , Start detection C Phase winding back EMF .

When the rotor position 60 When the degree of ,C The opposite electromotive force crosses zero , Further delay 30 degree ,HALL The sensor detects C Phase edge signal , At this time, commutation is required . And so on , The detection of back EMF is always carried out on the winding of the third phase that is not energized , Then delay 30 Degree commutation .

Compare the three-phase back EMF with the three-phase winding conduction ,6 The zero crossing events of back EMF all occur on the winding without conducting the suspended phase , All commutation points lag the zero crossing of the corresponding back EMF 30° Electrical angle . The zero crossing signal of back EMF is delayed 30° Electrical angle , available 6 Discrete rotor position signals .

The zero crossing point of the back EMF of the brushless DC motor winding strictly reflects the position of the rotor pole , When BLDC When the back electromotive force of a phase winding crosses zero , The direct axis of the rotor coincides with the quadrature axis of the phase winding , therefore , As long as the zero crossing signal of the winding back EMF can be accurately detected , You can judge the key position of the rotor . Theoretically , Ideal commutation point hysteresis back EMF zero crossing 30° Electrical angle , In this way, the commutation operation of Brushless DC motor can be realized , Ensure that the motor rotates continuously in a fixed direction , Thus, the maximum output torque of the motor is guaranteed , Reduce torque ripple .

BLDC The magnitude of the back EMF depends on the following factors ： Rotor angular velocity 、 The magnetic field produced by the rotor magnet , Turns and air gap of stator winding . After the motor design is completed , Rotor magnetic field , The number of turns and air gap of stator winding are fixed , With the increase of rotor speed , The back EMF also increases .

BLDC At standstill or low speed, the back EMF is zero or very small , It cannot be used to judge the rotor position , Therefore, special starting technology is needed to start , Commonly used “ Three paragraph ” Self starting method .

Implementation method of zero crossing detection of back EMF

1.ADC Detection method to detect back EMF

AD Detection method is a method of calculating the zero crossing point of back EMF by software , Direct detection through voltage sampling circuit BLDC Three phase terminal voltage of , Then through the software method , Calculate the zero crossing point of the suspended opposite electromotive force .

In order to make the three-phase voltage signal MOTOR_U/ MOTOR_V/ MOTOR_W Fall in the MCU On film ADC Sampling range of the module , Conduct resistance partial voltage treatment , And then the partial pressure obtained AD_VOLTAGE_U/ AD_VOLTAGE_V/AD_VOLTAGE_W The signal is sent directly ADC The sampling pin of .

ADC The detection method needs to collect the bus voltage in each control cycle , To determine the theoretical neutral voltage , And compare it with the terminal voltage of the suspended phase , So as to obtain the zero crossing point of back EMF , After detecting the zero crossing of back EMF , Time delay 30° Electrical angle can be used for commutation .

2. Hardware comparator method

The method of obtaining zero crossing point by hardware comparator ： By suspending the opposite electromotive force （ PHASE_U/V/W ） With virtual neutral voltage （MITTLE） Compare to get zero crossing . This method does not need to consider at which time the zero crossing sampling is performed , Just check the hardware comparison in real time I/O state , You can judge the commutation time .

The virtual neutral point calculation circuit is shown in the following figure

In the figure ,MOTOR_U,MOTOR_V,MOTOR_W Separately connected to the motor A,B,C phase , After passing through a voltage dividing network PHASE_U,PHASE_V,PHASE_W Three signals and connect to ADC Channel pin .MITTLE It is the virtual estimated neutral point voltage , As long as AB Open during power on MOTOR_W and MITTLE Comparison ,AC Open during power on MOTOR_V and MITTLE Comparison ,BC Open during power on MOTOR_U and MITTLE Comparison , The zero crossing point of each phase can be successfully detected .