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A New Starting Method of BLDC Motors without
Position Sensor
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 6, NOVEMBER 2006
Wook-Jin Lee, Student Member, IEEE, and Seung-Ki Sul, Fellow, IEEE
Student: Nai-Chiu Hsu
Adviser: Ming-Shyan Wang
Date : 18th-Dec-2009
Department of Electrical Engineering
Southern Taiwan University
Outline
Abstract
Introduction
Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
B. Initial Rotor-Position Estimation Method
C. Optimal Duration Ts of the Voltage Vectors
Proposed startup method
A. Accelerating Procedure
B. Implementation of the Proposed Startup Method
Experimental Results
A. Experimental Setup
B. Experimental Results
Conclusions
References
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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 startup method to accelerate the rotor up to a certain speed
where the conventional position sensorless control methods based on
the back electromotive force could work reasonably.
The principle of the estimation is based on the variation of the current
response caused by the magnetic saturation of the stator core of the
BLDC motor when the current flows along the magnetic axis.
This method can be implemented using only one current sensor at the
dc link of the inverter, which is a prerequisite for the HDD drive.
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Introduction
Recently, sensorless operation of a brushless dc(BLDC) motor using
the back-electromotive-force (EMF) information, such as back-EMF
zero crossing.
But, when the motor is at a standstill or at low speed, it is impossible
or very difficult to get the position information from the back EMF.
Therefore, a special starting method is generally needed.
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Introduction
Initial rotor-position estimation is important not only for the stable
starting but also for the maximum starting torque to reduce the start-up
time of the HDD.
One popular method to estimate the initial rotor position at standstill is
the utilization of the saturation effect of the stator iron core due to the
permanent magnet.
A stator inductance varies with the rotor position due to the saturation
caused by the rotor magnet, as 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 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.
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
The estimation of the rotor position is based on the nonlinear
magnetization characteristics of the stator core.
Therefore, if a stator winding is close to the magnetic pole of the rotor,
the rate of the change of the current in the stator winding flowing in
the magnetizing direction is large compared with that in the opposite
direction because of the magnetic saturation of the stator core.
Thus, the value of the current would be different according to the rotor
position if a constant voltage vector from the PWM inverter is applied
to the stator winding of the motor for a constant time period.
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
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).
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
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).
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
Fig. 2. Measurement of the inductance of the stator winding by applying
one voltage vector during a sampling time (Ts).
(a) Equivalent circuit of the motor when the V1 voltage vector is applied.
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
Fig. 2. Measurement of the inductance of the stator winding by applying
one voltage vector during a sampling time (Ts).
(b) DC-link current waveform when a voltage vector is applied.
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Proposed initial rotor-position-estimation method
A. Inductance Variation Due to the Magnetic Saturation
Fig. 3. Stator inductance as a function of rotor position.
(a) Measured dc-link current samples. (b) Measured stator-winding inductance.
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Proposed initial rotor-position-estimation method
B. Initial Rotor-Position Estimation Method
TABLE I
PARAMETERS OF THE TESTED MOTOR
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Proposed initial rotor-position-estimation method
B. Initial Rotor-Position Estimation Method
The estimation method in this paper consists of two processes.
One is a coarse estimation process, which is the conventional process,
and the other is a fine process, which is proposed in this paper.
Fig. 4. Coarse estimation result
(gray region indicates the estimated rotor position by the test).
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Proposed initial rotor-position-estimation method
B. Initial Rotor-Position Estimation Method
Fig. 5. Fine estimation result.
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Proposed initial rotor-position-estimation method
B. Initial Rotor-Position Estimation Method
Therefore, for this case, the first process is performed again after
changing the voltage vectors, for example from V1 and V4 to V2 and
V5.
Even with the consideration of this kind of the worst case, the number
of the applied voltage vector is at maximum five.
Thus, in the worst case, the total time to estimate the rotor position
with a 30◦ resolution is around 5 ms if the time interval between
samples is set to 1 ms.
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Proposed initial rotor-position-estimation method
B. Initial Rotor-Position Estimation Method
Fig. 6. Flowchart of a whole estimation process.
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Proposed initial rotor-position-estimation method
C. Optimal Duration Ts of the Voltage Vectors
The time duration of the voltage vectors for the estimation process is
an important parameter, because the proposed method basically uses
the current response of the motor winding.
The current response depends on the time constant of the motor winding.
As the time constant of each phase is almost the same if the stator core
is not severely saturated by the magnet, which is the usual case, the
time duration of the voltage vectors should be decided to maximize the
current difference between the voltage vectors.
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Proposed initial rotor-position-estimation method
C. Optimal Duration Ts of the Voltage Vectors
Fig. 7. Current rising with two different time constants.
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Proposed initial rotor-position-estimation method
C. Optimal Duration Ts of the Voltage Vectors
i1 (t )  VDC / Req (1  e
i2 (t )  VDC / Req (1  e

t
1

t
2
)
(1)
)
(2)
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Proposed initial rotor-position-estimation method
C. Optimal Duration Ts of the Voltage Vectors
i (t )  i2 (t )  i1 (t )
VDC / Req (e
ln
TS  lim
 0

t
1
e

t
2
(3)
)
1
1  
1
1

1   1
 1
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(4)
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Proposed initial rotor-position-estimation method
C. Optimal Duration Ts of the Voltage Vectors
Therefore, the optimal duration of the voltage vector should be set
around the average time constant of the stator windings.
If the duration of the voltage pulse to estimate the rotor position is too
long, the rotor cannot maintain the standstill and starts to rotate.
Since the best choice of the duration of the voltage pulse is the same as
an electrical time constant of the stator coil (τ ), it should be checked in
an off-line manner whether the rotor rotates or not when a voltage
pulse whose duration is the same as τ is applied.
If the rotor rotates, the duration should be decreased appropriately,
which should be shorter than the time constant.
Otherwise, the stator-coil time constant τ can be used as the duration.
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Proposed startup method
If the initial rotor position has been identified with the proposed
method, the motor should be accelerated from the standstill up to a
certain speed where the back EMF is large enough.
In the case of the tested motor described in Table I, the speed that
should be reached with the proposed sensorless control method is
about 174 r/min, because the back-EMF signal amplitude should be at
least about ±40 mV for a reliable back-EMF detection.
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Proposed startup method
A. Accelerating Procedure
Fig. 8. Real rotor position (solid axes) and estimated rotor position (dashed lines).
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Proposed startup method
A. Accelerating Procedure
Fig. 8. Real rotor position (solid axes) and estimated rotor position (dashed lines).
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Proposed startup method
A. Accelerating Procedure
Fig. 8. Real rotor position (solid axes) and estimated rotor position (dashed lines).
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Proposed startup method
B. Implementation of the Proposed Startup Method
Fig. 9. DC-link current waveform and voltage vectors.
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Experimental Results
A. Experimental Setup
Fig. 10. Experimental system configuration.
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Experimental Results
B. Experimental Results
Fig. 11. DC-link current during the initial rotor-position estimation.
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Experimental Results
B. Experimental Results
Fig. 11. DC-link current during the initial rotor-position estimation.
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Experimental Results
B. Experimental Results
Fig. 12. Initial rotor-position estimation result.
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Experimental Results
B. Experimental Results
Fig. 13. Estimation error in electrical degree of the proposed method
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Experimental Results
B. Experimental Results
Fig. 14. Rotor-position estimation result during startup.
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Experimental Results
B. Experimental Results
Fig. 15. (a) Startup characteristic (acceleration up to about 600 r/min).
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Experimental Results
B. Experimental Results
Fig. 15. (b) Smooth restarting after a mechanical shock disturbance
with the proposed startup method.
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Experimental Results
B. Experimental Results
Fig. 15. (c) Failure of startup with the conventional open loop method.
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Conclusions
In this paper, a new starting method, consists of an initial
rotor-position estimation and a start-up from a standstill to a certain
speed where the back-EMF-based sensorless control can reliably work.
The whole drive system needs only one current sensor in the dc-link
side, like the conventional HDD control chips.
The proposed initial rotorposition- estimation method can estimate the
rotor position at a standstill, with two times better resolution compared
to that of the conventional method, that is 30◦ in electrical angle,
The proposed startup method utilizes the stator inductance variation
due to the rotor magnetic flux so that it does not rely on the motor para
meters.
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References
[1] N. Matsui, “Sensorless PM brushless dc motor drives,” IEEE Trans.
Ind.Electron., vol. 43, no. 2, pp. 300–308, Apr. 1996.
[2] S. Nakashima, Y. Inagaki, and I. Miki, “Sensorless initial rotor
position estimation of surface permanent magnet synchronous motor,”
IEEE Trans.Ind. Appl., vol. 36, no. 6, pp. 1598–1603, Nov./Dec. 2000.
[3] M. Tursini, “Initial rotor position estimation method for
PMmotors,” IEEETrans. Ind. Appl., vol. 39, no. 6, pp. 1630–1640,
Nov./Dec. 2003.
[4] G. H. Jang, J. H. Park, and J. H. Chang, “Position detection and
startup algorithm of a rotor in a sensorless BLDC motor utilizing
inductance variation,” Proc. Inst. Electr. Eng.—Electr. Power Appl.,
vol. 149, no. 2,pp. 137–142, Mar. 2002.
[5] M. Schroedl, “An improved position estimation for sensorless
controller permanent magnet synchronous motor,” in Proc. EPE Conf.,
1991,pp. 418–423.
[6] C. Vertemara. (1999, May). 12 V Disk Drive Power Combo IC,
STMicroelectronics.Appli. Note AN1139. [Online]. Available:
http://www.st.com
[7] M. A. Jabbar, “Disk drive spindle motors and their controls,” IEEE
Trans.Ind. Electron., vol. 43, no. 2, pp. 276–284, Apr. 1996.
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Thank you for your attention.
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