The structure and working principle of EHPS brushless DC motor For the brushless DC motor used in this system, the appearance structure of the rotor and stator is shown. The structure of the motor is visible. The rotor of the motor is external. In addition, the total number of stator slots of the motor is 12. The rotor contains 7 permanent magnets, ie 7 pairs of poles. According to the formula: electrical angle = number of pairs of poles × 360° total slot Number, or electrical angle = number of pairs of poles × mechanical angle, can calculate the electrical angle of the brushless DC motor is 210 °, from the electrical angle can be drawn out of the star vector vector diagram of the motor windings, as shown. According to the principles of symmetry of the four-phase windings and the maximum of the resultant torque, which slot conductors are included in each winding. Assume that the groove conductor numbers are 1, 8, and 3 are Phase A, 10, 5, and 12 are Phase B, 7, 2, and 9 are Phase C, and 4, 11, and 6 are Phase D.
Brushless DC motor driver circuit diagram When the motor rotor position sensor signal is input to the DSP, the DSP determines that one of the four phases of A, B, C, and D is the current required conduction phase according to the commutation control word and passes the output. The PWM chopper signal controls a power switch. When a phase winding is turned on, the current flows through the winding through the positive terminal of the 12V power supply, and finally flows through the sense resistor to the power ground. The current direction is shown by the solid line in the figure.
The four-phase winding turn-on order of this motor is determined as follows. First, according to the experimental observation, the four-phase winding back electromotive force waveform diagram of the motor and the corresponding commutation control word are obtained. Secondly, according to the back EMF waveform, the commutation control word at the moment of the peak value of the back EMF of each phase winding is recorded. Finally, the PWM signal is output by program instructions to turn on the phase. The correspondence between the commutation control word and the commutation, as shown.
Table 1 The relationship between the commutation control word and the commutation Because the structure of the motor winding is double-winding, in fact, the relationship between the A-phase winding and the B-phase winding is the main winding and the auxiliary winding. The A-phase winding and the B-phase winding are double-stranded and closely coupled together. After the A-phase main winding is energized, due to magnetic field storage, the B-phase auxiliary winding is turned on. Because the A-phase main winding current is attenuated, the winding generates a back EMF and the same magnitude of back EMF is induced at the same name of the B-phase winding. When the back electromotive force is greater than the motor voltage, the protection diode of the power tube of the B-phase winding is turned on to complete a freewheeling operation. The winding current direction is as shown when the motor is freewheeling. This freewheeling principle also applies to C, D phase windings.
BLDCM Control Strategy for EHPS In the actual EHPS system, the rotational speed of the motor needs to receive the external vehicle speed signal and the steering wheel angular speed signal through the central control unit, and obtain relevant processing and look-up table.
The angular velocity signal of the steering wheel used in the POLO vehicle comes from an inductive position sensor developed by Hella. The sensor can realize non-contact measurement of line displacement and angular displacement in a simple and compact space condition. The output frequency of the sensor is generally not Change the pulse signal whose pulse width changes with the position of the steering wheel. The signal is characterized by an increase in the steering wheel angle and an increase in the signal duty cycle. Each time the steering wheel rotates through 60°, the signal waveform changes repeatedly. Due to factors such as sensor design, the minimum and maximum values ​​of the duty cycle of the signal cannot reach 0% and 100%, and there is a certain signal rise time. The signal is introduced into the I/O port of the DSP for steering angle speed calculation processing.
In order to ensure that the vehicle can get more power when it is stationary or at low speed and when the steering wheel is operated quickly, and the vehicle gets a little boost or even no help when the vehicle is running at a high speed and slowly operating the steering wheel, the control strategy for this motor is formulated in this paper. A set of curves is fitted through MATLAB's internp1 function, as shown. This set of curves is the primary basis for formulating control strategies. The eleven curves in the figure represent the motor speed characteristics of eleven vehicle speed ranges. The main design idea is that when the steering wheel steering angular velocity sensor has no input signal or the input signal value is very small (stop mode), the motor is maintained at a constant speed according to the vehicle speed signal. When the speed sensor input signal exceeds a certain value (assistance mode), the motor speed is increased according to the current speed of the vehicle in different proportions until it increases to the maximum value of the motor speed. It can be seen from the figure that in the online phase, the higher the vehicle speed, the smaller the slope of the curve, and the smaller the vehicle speed, the larger the slope of the curve. The purpose of this design is to be able to ensure that the driver can get a good steering feel during the driving of the vehicle. When the vehicle speed is low, it can provide greater assistance, and when the vehicle speed is high, the assistance is reduced or even no assistance is provided. Secondly, for this control strategy, the transition from stop mode to assist mode is slow, and the steering feel is still deteriorating. Therefore, corresponding countermeasures need to be taken. The document proposes that the problem of slow response can be effectively solved by setting a higher stopping speed of the motor. Finally, because each curve represents a range of vehicle speeds rather than a fixed speed, it is necessary to add a hysteresis link in the software program. Otherwise, the controller may experience a number of booster characteristics due to small changes in the vehicle speed signal. Repeatedly switch between.
The interrupt handling of the motor speed characteristics designed in the control strategy can be described as follows: Immediately after the interruption, the scene is immediately protected and the A/D conversion is started. Second, the program needs to determine the current vehicle speed according to the vehicle speed sensor signal, and make relevant calculations to determine the rotation angle speed according to the steering wheel angle speed sensor signal. The target speed of the current motor is determined by the vehicle speed and the steering wheel angle speed.
When the motor target is given, the program passes the motor's PI speed adjustment and current PI adjustment to control the motor to reach the target speed, complete the motor's commutation work, and finally interrupt the exit and resume the site. Interrupt handler flow, as shown.
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