Speed ​​Control System of Two-phase Brushless DC Motor Based on DSP

0 Introduction

Rare earth permanent magnet brushless DC motors use high-performance rare earth permanent magnet materials and non-contact commutation technology. They are small in size, high in efficiency, spark-free, reliable in operation, and have speed regulation performance similar to that of ordinary DC motors. They are widely used in aerospace, precision instruments, industrial control automation, and other fields. Brushless DC motors use electronic commutation devices and no mechanical brushes; they use permanent magnet rotors and no excitation loss; the heat-generating armature windings are placed on the outer stator, which has good heat dissipation, high efficiency, strong overload capacity, and no commutation sparks. They are particularly suitable for high-speed fields and are a key development direction for high-speed motors.

At present, in some special fields, there are strict requirements on the size of the motor, the number of connections and reliability. In these occasions, the position sensorless brushless DC motor (BLDCM) has become an ideal choice. The project uses digital design technologies such as DSP and CPLD to build a small-volume, high-speed, high-reliability motor speed control system. The system uses a two-phase brushless DC motor, a position sensorless method, and uses the back electromotive force of the motor winding as a signal. The CPLD generates the motor commutation timing, and realizes the steady speed control of the brushless DC motor through hardware startup and phase-locked loop tracking. The system has the characteristics of simple structure and electronic control circuit, reliable operation, and easy maintenance.

1 Mathematical model of brushless DC motor

Taking two-phase conduction and three-phase star six-state as an example, the voltage balance equation of the three-phase winding of the brushless DC motor is:

In the formula: uA, uB, uC are the stator phase winding voltages; eA, eB, eC are the stator phase winding electromotive forces; iA, iB, iC are the stator phase winding currents; L is the self-inductance of each phase winding of the stator; M is the mutual inductance between every two phase windings of the stator; R is the three-phase stator resistance; p is the differential operator.

The equivalent circuit model of the permanent magnet brushless DC motor can be obtained from formula (1), as shown in Figure 1.

The electromagnetic torque generated by the stator winding is expressed as:

The rotor motion equation of a permanent magnet brushless DC motor is:

Where: Tn is the electromagnetic torque; TL is the load torque; B is the damping coefficient; ω is the motor mechanical speed; J is the moment of inertia.

2 Control strategy and hardware composition of the control system

2.1 Control strategy of control system

The system adopts speed and current double closed-loop control, and uses digital devices to form the speed control part. The current loop uses the traditional PI regulator. DSP, as the central controller, issues various instructions and forms the PI regulator of the speed loop, which can perform intelligent PID control; part of the CPLD forms a phase-locked loop to detect the error value between the input frequency and the feedback frequency, and the other part uses the armature back electromotive force to generate the inverter's commutation control signal. The control principle block diagram of the system is shown in Figure 2.

2.2 Hardware composition of the control system

The hardware connection block diagram of the brushless DC motor control system is shown in Figure 3, which mainly consists of a brushless DC motor, an inverter, a controller, and a power supply.

Since the motor has a high speed, reaching 19,500 r/min, a large-scale programmable logic device (CPLD) with a fast operation speed is used to process speed feedback and motor commutation signals, and TMS320F2812 (DSP) constitutes the speed controller. The speed feedback of the motor is processed by the built-in phase-locked loop 74LS297 of the CPLD to generate a deviation input to the DSP for speed loop correction, and then output to the current controller through the 4-channel, 12-bit resolution D/A converter DAC7724. After current correction, it enters the inverter. EPM7128S (CPLD) receives the back electromotive force of the motor winding to generate a conduction signal to control the switch of the four power amplifiers. The motor has two windings, A and B. The two windings are turned on in the forward and reverse directions in turn. If the stator winding is energized continuously in the order of ABAB, the rotor will rotate clockwise at a certain speed.

The hardware circuit connection diagram of the DSP core control part of the control system is shown in Figure 4.

3 Software Design

The system software consists of two parts: DSP program and CPLD program. When designing the software, we first conduct a system analysis and divide the entire program into submodules according to functional requirements. Considering the real-time requirements of the control system, interrupt programming is adopted. The entire DSP software system consists of a main program and several interrupt service programs.

The main function of the main program is to initialize the system, including initializing the DSP registers, interrupts, timers, GPIO, etc. Initialize the DSP to generate a working clock; initialize the internal modules of the DSP; disable global interrupts, initialize the interrupt vector table, and set the interrupts as needed; turn on global interrupts, enter the loop waiting main program, and wait for internal and external interrupt signals. The interrupt subroutine completes the correction control task of the speed loop. The flowchart of the DSP interrupt subroutine is shown in Figure 5.

The CPLD part completes the phase-change processing and speed feedback control functions of the motor, and uses a mixed design of graphical design and VHDL language to complete the programming of the timing part. The CPLD receives the back electromotive force of the motor winding, and forms the conduction control signal of the 4-way inverter through the on-chip logic circuit to control the power-on sequence of the motor winding; the phase-locked processing is performed through the built-in digital phase-locked loop 74LS297 of the CPLD to complete the constant control of the motor speed. The phase-locked loop and the phase-change processing circuit are packaged together using CPLD to form a complete speed feedback control module. The CPLD part of the program flow chart is shown in Figure 6.

4 Control system simulation and comparative analysis of its results

After VHDL description and compilation, EDA software can be used to perform timing function simulation. Functional simulation is performed in QuartusⅡ software development environment, and the timing simulation waveform is shown in Figure 7.

The speed controller is simulated using Matlab simulation software, and the obtained speed response curve of the simulation system is shown in Figure 8.

The continuous system is discretized using the bilinear transformation method, and the resulting digital system speed response curve is shown in Figure 9.

In the figure: n is the motor rotation speed; r is the motor revolutions.

The experimental results show that the speed response indicators of the improved digital system: rise time, adjustment time, overshoot, etc. have all been improved, and the system has good speed and stability.

5 Conclusion

Based on the analysis of the operating principle of brushless DC motor, a brushless DC motor control system solution based on TMS320F2812 is proposed, which makes full use of the powerful functions of DSP to enable the system to obtain higher control accuracy and dynamic and static characteristics. Phase-locked speed control is applied to the brushless DC motor system, and the simulation results of analog and digital systems are compared and analyzed, which proves that the system has strong robustness and adaptability, realizes the control of motor output torque and speed, and improves the speed regulation performance of the motor.