一种新的无位置传感器BLDC控制方法
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A New Starting Method of BLDC Motors without Position Sensor
Wook-Jin Lee and Seung-Ki Sul
School of Electrical Engineering and Computer Science Seoul National University Seoul, Korea
wjlee@eepel.snu.ac.kr and sulsk@plaza.snu.ac.kr
Abstract— This paper presents a novel method to detect the rotor position of the BrushLess DC(BLDC) motor of a Hard Disk Drive(HDD) at standstill and a start-up method to accelerate the rotor up to a certain speed where the conventional position sensorless control methods based on the back EMF could work reasonably. The proposed initial rotor position estimation method is suitable to avoid the temporary reverse rotation or the starting failure of HDD. The principle of the estimation is based on the variation of the current response caused by the magnetic saturation of the stator core of BLDC motor when the current flows along the magnetic axis. This method can be implemented using only one current sensor at DC link of the inverter, that is prerequisite for HDD drive. It does not depend on the model of the motor, and it is robust to motor parameter variations. By the proposed method, it is demonstrated experimentally that a stable starting can be achieved even with severe mechanical disturbance.
Fig.1. Location of eight possible stationary voltage vectors for a VSI in the d-q plane according to the switching function (Sa, Sb, Sc)
rate of the change of current in the stator winding due to the change of inductance, it is possible to estimate the relative position between a rotor magnet and a stator winding. The inductance of the stator winding has its minimum value where the North pole of the rotor magnet lies in the corresponding stator winding axis in the case of the spindle motor of the HDD. Several directions of voltage vectors can be applied to the stator winding of the motor to identify the rotor position, and the direction that has minimum inductance can be decided as the d-axis direction [2]. Using a single current sensor in DC link of the inverter, with conventional method 60 degrees of resolution in the estimation of rotor position is possible at best, because the available voltage vectors with PWM inverter are restricted to the basic six nonzero voltage vectors. In Fig.1, the available voltage vectors with a 3-phase voltage source inverter are represented.
As the starting method from standstill to a certain low speed, various methods which inject extra voltage pulses periodically to measure the stator inductance varying with the rotor position were proposed. [4, 5] However, these may not be acceptable in HDD because of their complexity. As a simple and conventional method to rotate the motor at standstill, a predetermined voltage waveform had been applied. Then the rotor might rotate synchronously with the applied voltage [1, 6]. In this method, the electrical and mechanical parameters such as the torque constant, the friction coefficient, and the inertia of the rotor should be considered for the stable starting performance. Some unexpected disturbance or load torque during starting procedure may cause serious start failures [1].
(b) DC-link current waveform when a voltage vector applied
Fig.2. The inductance measurement method of the stator winding by applying one voltage vector during a sampling time(Ts)
Keywords-BLDC motor; HDD; sensorless; starting; saturation
I. INTRODUCTION
Recently sensorless operation of brushless dc(BLDC) motor using the back-EMF information, such as back-EMF zero crossing, is widely used and adequate for small or lowcost application such as Hard Disk Drive(HDD). But when the motor is at standstill or at low speed, it is impossible or very difficult to get the position information from back-EMF. Therefore, a special starting method is generally needed [1]. In HDD the power supply voltage is getting reduced, and the detection of rotor position from back-EMF is getting difficult at the low speed. Furthermore, recently HDD is widely spread into the mobile applications such as portable PC and PDA. So, the stable starting and acceleration to nominal operating speed regardless of severe mechanical shock is utmost concern in these applications.
IAS 2004
2397
0-7803-8486-5/04/$20.00 © 2004 IEEE
(a) Equivalent circuit of the motor when the V1 voltage vector applied
(a) Measured DC-link current samples
Initial rotor position estimation is important not only for the stable starting but also for the maximum starting torque to reduce start-up time of HDD. One popular method to estimate the initial rotor position at standstill is utilization of the saturation effect of stator iron core due to the permanent magnet [1-5]. A stator inductance varies with the rotor position due to the saturation caused by rotor magnet, so does the rate of change of the current in the stator winding when a constant voltage is applied to the windings. By the measurement of the
II. PROPOSED INITIAL ROTOR POSITION ESTIMATION METHOD
A. Inductance variation due to the magnetic saturation The estimation of wenku.baidu.comhe rotor position is based on the
Wook-Jin Lee and Seung-Ki Sul
School of Electrical Engineering and Computer Science Seoul National University Seoul, Korea
wjlee@eepel.snu.ac.kr and sulsk@plaza.snu.ac.kr
Abstract— This paper presents a novel method to detect the rotor position of the BrushLess DC(BLDC) motor of a Hard Disk Drive(HDD) at standstill and a start-up method to accelerate the rotor up to a certain speed where the conventional position sensorless control methods based on the back EMF could work reasonably. The proposed initial rotor position estimation method is suitable to avoid the temporary reverse rotation or the starting failure of HDD. The principle of the estimation is based on the variation of the current response caused by the magnetic saturation of the stator core of BLDC motor when the current flows along the magnetic axis. This method can be implemented using only one current sensor at DC link of the inverter, that is prerequisite for HDD drive. It does not depend on the model of the motor, and it is robust to motor parameter variations. By the proposed method, it is demonstrated experimentally that a stable starting can be achieved even with severe mechanical disturbance.
Fig.1. Location of eight possible stationary voltage vectors for a VSI in the d-q plane according to the switching function (Sa, Sb, Sc)
rate of the change of current in the stator winding due to the change of inductance, it is possible to estimate the relative position between a rotor magnet and a stator winding. The inductance of the stator winding has its minimum value where the North pole of the rotor magnet lies in the corresponding stator winding axis in the case of the spindle motor of the HDD. Several directions of voltage vectors can be applied to the stator winding of the motor to identify the rotor position, and the direction that has minimum inductance can be decided as the d-axis direction [2]. Using a single current sensor in DC link of the inverter, with conventional method 60 degrees of resolution in the estimation of rotor position is possible at best, because the available voltage vectors with PWM inverter are restricted to the basic six nonzero voltage vectors. In Fig.1, the available voltage vectors with a 3-phase voltage source inverter are represented.
As the starting method from standstill to a certain low speed, various methods which inject extra voltage pulses periodically to measure the stator inductance varying with the rotor position were proposed. [4, 5] However, these may not be acceptable in HDD because of their complexity. As a simple and conventional method to rotate the motor at standstill, a predetermined voltage waveform had been applied. Then the rotor might rotate synchronously with the applied voltage [1, 6]. In this method, the electrical and mechanical parameters such as the torque constant, the friction coefficient, and the inertia of the rotor should be considered for the stable starting performance. Some unexpected disturbance or load torque during starting procedure may cause serious start failures [1].
(b) DC-link current waveform when a voltage vector applied
Fig.2. The inductance measurement method of the stator winding by applying one voltage vector during a sampling time(Ts)
Keywords-BLDC motor; HDD; sensorless; starting; saturation
I. INTRODUCTION
Recently sensorless operation of brushless dc(BLDC) motor using the back-EMF information, such as back-EMF zero crossing, is widely used and adequate for small or lowcost application such as Hard Disk Drive(HDD). But when the motor is at standstill or at low speed, it is impossible or very difficult to get the position information from back-EMF. Therefore, a special starting method is generally needed [1]. In HDD the power supply voltage is getting reduced, and the detection of rotor position from back-EMF is getting difficult at the low speed. Furthermore, recently HDD is widely spread into the mobile applications such as portable PC and PDA. So, the stable starting and acceleration to nominal operating speed regardless of severe mechanical shock is utmost concern in these applications.
IAS 2004
2397
0-7803-8486-5/04/$20.00 © 2004 IEEE
(a) Equivalent circuit of the motor when the V1 voltage vector applied
(a) Measured DC-link current samples
Initial rotor position estimation is important not only for the stable starting but also for the maximum starting torque to reduce start-up time of HDD. One popular method to estimate the initial rotor position at standstill is utilization of the saturation effect of stator iron core due to the permanent magnet [1-5]. A stator inductance varies with the rotor position due to the saturation caused by rotor magnet, so does the rate of change of the current in the stator winding when a constant voltage is applied to the windings. By the measurement of the
II. PROPOSED INITIAL ROTOR POSITION ESTIMATION METHOD
A. Inductance variation due to the magnetic saturation The estimation of wenku.baidu.comhe rotor position is based on the