Starting Control Of Sensorless Brushless DC Motor

Brushless DC motor (BLDCM) has many advantages such as simple structure, good reliability and speed performance, so it is widely used in various fields of national production. Traditional BLDCM has position sensor inside, which directly affects the size of the motor, the cost and operational reliability. So it is important to study sensorless control strategy for BLDCM. Among the available methods so far, back-emf method is the simplest and most applicable. But there is a starting problem when using back-emf method to realize the sensorless control.Research on BLDCM starting problem has been done in this paper. First, several sensorless start-up strategies are analyzed. Then, by analyzing the dynamic equation of BLDCM, the relations between the commutating time and the source voltage, and between the commutating time and the load torque are found. These relations can be described by function fitting. Cubic spline interpolation function has continuous second derivative, which avoids great calculation, acute oscillation, bad numeric stability and insufficient smoothness in section interpolation. So cubic spline interpolation has the capability to fit the sample data perfectly. According to these advantages, an interpolation start-up strategy is proposed.In this method, an initial commutating signal is given first to conduct two set phases in order to fix the rotor at a certain position. Then, with the changing of the source voltage and the load torque, the corresponding commutating time samples are collected. Then, for the first step in no-load condition, cubic spline interpolation functions are fitted off line using source voltages as the inputs and commutating times as the outputs. The simulation results show that during open loop starting, these functions can provide accurate start-up commutating times under any given source voltage; during closed-loop starting, with the changing of the effective value of the source voltage controlled by PI regulator, interpolation functions can adjust the commutating times on line to make sure they are accurate. When start-up procedure is over, the system is switched to back-emf running successfully.For the second step, the effect of load torque is taken into consideration, then bicubic spline interpolation functions are fitted off line using source voltages and load torques as the inputs and commutating times as the outputs. The simulation results show that the performance of changing load starting and rated load starting is improved significantly. So bicubic spline interpolation starting is more adaptable for the situations like greater rated load starting and larger load range starting.At last, the BLDCM control system is designed based on DSP TMS320F2812, which is produced by TI corp. Experiment is designed to testify the cubic spline interpolation starting strategy proposed in this paper. Experiment result shows that this method can give accurate start-up commutating signals, start the motor successfully and switch to back-emf running smoothly.