The control system of a brushless DC motor was studied and analyzed using Saber simulation software. The position sensor, electronic commutator, and three-phase inverter circuit in the control system were investigated and analyzed, and a simulation model was built, its functions verified, and its performance analyzed. Finally, the various functional modules were organically integrated. A comprehensive simulation experiment of the control system was completed. The simulation results demonstrate that the system design is reasonable, and the simulation results are consistent with the theoretical analysis.
Brushless DC motors were developed based on brushed DC motors. In 1955, D. Harrison et al. in the United States first applied for a patent to replace the mechanical brushes of a brushed motor with a transistor commutation circuit, marking the birth of the modern brushless DC motor. Compared to brushed motors, brushless DC motors use electronic commutation instead of mechanical commutation, resulting in higher speeds, greater output power, longer lifespan, better heat dissipation, no commutation sparks, lower noise, and the ability to operate in high-altitude, thin-film conditions. They are widely used in servo systems requiring high power-to-weight ratios, fast response speeds, and high reliability.
With the increasing functionality and speed of DSP digital control chips, control circuits based on digital signal processors and embedded control software will represent the future development direction of brushless DC motor control. Brushless DC motors must be used in conjunction with electronic commutators and position feedback devices for more flexible control, but this also increases the complexity of control hardware and algorithms. Utilizing mathematical simulation analysis in the design of brushless DC motor control systems allows for a better understanding of the system's dynamic characteristics, verification of circuit design correctness, and confirmation of the appropriate matching of components and control parameters, thus enabling more effective system design.
This paper uses Synopsys' power electronics simulation software Saber to establish a simulation analysis model of the control system of a brushless DC motor. The position sensor, electronic commutator, and three-phase inverter circuit in the control system are studied and analyzed. The simulation model is built, the function is verified and the performance is analyzed. Finally, the overall model is used to conduct a simulation test of the system.
1. Overall Motor Control System
The block diagram of the brushless DC control system is shown in Figure 1.
In a brushless DC motor control system, the controller generates motor speed regulation and direction control signals according to the control strategy. A position detector generates a position signal representing the motor rotor. The electronic commutator logically synthesizes the rotor position signal, motor speed regulation, and direction control signals to generate corresponding switching signals. These switching signals trigger the power switches in the inverter in a specific sequence, distributing power to the U, V, and W phase windings of the motor stator according to a certain logical relationship, thus enabling the motor to generate continuous torque. The design and modeling simulation of each part of the brushless motor control system will be described in detail below.
1.1 Modeling of Motor Position Sensor
The position detector detects the position of the rotor magnetic poles in a brushless DC motor, providing correct commutation information for the logic switching circuit. This involves converting the position signal of the rotor magnet poles into an electrical signal to control the commutation of the stator windings.
This paper uses Hall effect sensors to test the position of the motor rotor magnetic poles. Three Hall effect sensors are evenly distributed at 120° intervals on the stator, and the Hall effect rotor is the permanent magnet rotor of the motor. As the rotor rotates, the N, S poles of the permanent magnet rotor alternate. The three Hall effect position sensors sense the change in the rotor's magnetic field and output Hall signals HA, HB, and HC. Different combinations of these three signals represent different positions of the motor rotor.
Based on the physical installation location of the Hall sensors, the relationship between the three-phase Hall signals HA, HB, and HC and the rotor magnetic pole electrical angle θ is as follows:
Where -180°≤θ≤180°
A simulation analysis model of the motor Hall sensor was established, and then simulation analysis was performed. When the number of pole pairs of the motor is 2, the simulation results of the output Hall signals HA, HB, and HC corresponding to different motor rotor angles Angle are shown in Figure 2.
As shown in the diagram, within one electrical cycle, the 3-phase Hall position sensor has 6 possible combinations of coded states: 101, 100, 110, 010, 011, and 001. When the motor rotates forward, the HA, HB, and HC coded combinations are as follows: 011->001->101->100->110->010->011. When the motor rotates in reverse, the HA, HB, and HC coded combinations are as follows: 010->110->100->101->001->011->010.
1.2 Electronic commutator modeling
The main function of the electronic commutator is to generate control signals S1, S2, S3, S4, S5, and S6 to control the on/off state of six power transistors based on the Hall position signals HA, HB, and HC generated by the motor position sensor, the motor direction control signal DIR, and the motor speed regulation signal PWM. When the motor control signal DIR is "0", the motor rotates in the negative direction; when the DIR signal is "1", the motor rotates in the positive direction. The duty cycle of the PWM signal varies between 0 and 1.0. By controlling the duty cycle of the PWM signal, the motor speed is regulated; the larger the duty cycle, the higher the motor speed.
The output control logic of the electronic commutator is as follows: the PWM signal modulates the high-side transistor of the half-bridge to achieve the purpose of motor speed regulation.
In order to improve the reliability of the system, the commutation logic is implemented by using an integrated logic gate circuit of AND gate, XOR gate, and NOT gate to realize the logic commutation of the motor.
When the PWM duty cycle is set to 0.6, the simulation analysis results of the electronic commutator are shown in Figure 3, where S1 and S4 are the control signals of the high-side and low-side transistors of a half-bridge.
As can be seen from the simulation results above, the two transistors on the same half-bridge cannot be turned on simultaneously; the PWM modulation signal realizes the control of the high-side transistor of the half-bridge.
1.3 Modeling of Three-Phase Inverter Circuit
The function of the inverter circuit is to receive the control signal from the electronic commutator and convert it into the gate drive control signal of the six power transistors of the inverter circuit. By controlling the opening and closing of the power transistors, the motor power supply is converted into three-phase AC power U, V and W that can drive the brushless motor.
In a motor power drive circuit, a three-phase inverter bridge circuit has six power transistors. For a MOSFET power switch, the gate-source voltage Ugs must be greater than a certain threshold for it to conduct; this threshold varies for different power transistors.
Figure 4 shows the schematic diagram of a half-bridge circuit for a three-phase inverter.
For the low-end transistor Q4, since its source (s) is grounded, when controlling Q4 to turn on, it is only necessary to increase the voltage signal Ud at the gate of Q4 by more than the threshold. However, for the high-end transistor Q1, since its source potential U is floating, it is difficult to control Q1 to turn on by simply applying the voltage signal Up to the gate of Q1.
Based on the above analysis, power switching transistors generally employ two methods: direct drive and isolated drive. In the isolated drive mode, each of the six power switching devices uses an independent drive circuit, requiring an auxiliary power supply. Furthermore, the circuits are interconnected, increasing complexity and reducing reliability. In contrast, the bootstrap power bridge driver IC has independent low-side and high-side input channels, and the floating voltage is achieved using a built-in bootstrap circuit. It requires only a single DC power supply to output the drive pulses for the half-bridge power switching transistors.
The power drive integrated circuit for the three-phase inverter bridge in this paper uses the dedicated driver chip IR2110 manufactured by IR International Rectifier, Inc., and the power switching transistor is MOSFKTIRFP260N. The circuit of IR2110 driving a half-bridge is shown in Figure 5. In the figure, C1 and VD are the bootstrap capacitor and diode, respectively, and Rg is the gate series resistor.
The bootstrap capacitor C1 provides a floating power supply to the high-side IRFP260N. Before a half-bridge transistor turns on, it needs to charge the bootstrap capacitor C1. When the voltage across C1 exceeds the threshold voltage, the high-side transistor begins to conduct. The bootstrap capacitor must provide the gate charge required for the power transistor to turn on, and the voltage across it must remain essentially constant during the high-side transistor's conduction period. A bootstrap capacitor that is too small may result in significant ripple. The bootstrap capacitor value is typically 0.1–1 μF; here, a value of 1 μF is chosen.
When the high-side IRFP260N transistor is turned on, the bootstrap diode D1 must withstand the same voltage as the drain of the IRFP260N. Therefore, the reverse withstand voltage of the diode must be greater than the bus voltage, and it should be a fast recovery diode to reduce the feedback charge from the bootstrap capacitor to the power supply.
A simulation analysis model of the inverter circuit was established and simulation analysis was performed. The simulation analysis results of the control signals G1_C and G4_C of the high-side transistor Q1 and the low-side transistor Q4, the gate drive signal Q1_G of transistor Q1, the gate-source voltage Q1_GS, the midpoint potential U of Q1 and Q2, and the gate drive voltage Q4_G of transistor Q4 are shown in Figure 6.
In Figure 6, at time "1", the control signal Q4_C of the low-side power transistor Q4 is active. After passing through the driver IC IR2110, the gate drive signal Q2_G of Q2 is 11.988V. Its gate-source voltage is greater than the turn-on threshold of IRFP260, so Q2 turns on, and Q1 turns off. At time "2", the control signal Q1_C of the low-side power transistor Q1 is active. After passing through IR2110, the source potential U of Q1 is 90V. The gate potential Q1_C of Q1 is raised to 101.95V by the bootstrap capacitor. At this time, the gate-source voltage Q1_GS of Q1 is 11.95V, which is greater than the turn-on threshold of the power transistor, so Q1 turns on, and Q2 turns off. It can be seen that the design of the three-phase inverter circuit can reliably control the turn-on and turn-off of the power transistors.
2 System Functional Simulation
The brushless DC motor parameters are set as follows: 2 pole pairs, single-phase winding resistance of 1.65Ω, winding inductance of 1mH, back EMF coefficient ke=0.048, and rotor moment of inertia j=4.189x10-6kg*m2. The PWM duty cycle is set to 0.6, and the frequency is 10kHz. The entire motor control system is simulated. The simulation analysis results of the three-phase winding voltages U, V, and W, the motor speed Wrm, and the motor rotor mechanical angle Theta are shown in Figure 7.
As shown in the diagram above, due to the PWM duty cycle of 0.6, the motor is in an acceleration state regardless of whether it rotates in the positive or negative direction: when DIR is "0", the motor rotates in the negative direction; when DIR is "1", the motor rotates in the positive direction. The results show that the brushless DC motor control system is working normally.
3. Conclusion
This paper utilizes the simulation software Saber to model and analyze a brushless DC control system. System simulation experiments demonstrate that the control system operates normally, exhibits high simulation accuracy, and the simulation results are consistent with the theoretical analysis. Matlab/Simulink simulation software is primarily suitable for motor control system research, while PSpice simulation software is mainly suitable for power electronic circuit analysis. Saber software contains a rich library of power electronic components and motor models, offers high computational accuracy, and combines the advantages of both of these analysis tools. Therefore, simulation analysis of motor control systems based on Saber allows for the understanding of the system's dynamic characteristics, detailed design and analysis of circuit design, and verification of control strategies and algorithms. This provides a highly effective design tool for the application of motor control systems, enabling more efficient system and subsystem design.
