U.S. patent application number 11/533372 was filed with the patent office on 2008-03-20 for sensorless start-up method for driving a brushless dc motor.
Invention is credited to Yen-Chuan Chang, Ying-Yu Tzou.
Application Number | 20080067961 11/533372 |
Document ID | / |
Family ID | 39103572 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080067961 |
Kind Code |
A1 |
Chang; Yen-Chuan ; et
al. |
March 20, 2008 |
SENSORLESS START-UP METHOD FOR DRIVING A BRUSHLESS DC MOTOR
Abstract
At first, a plurality of detection modes are applied in sequence
to a brushless DC motor, such that with respect to each of the
detection modes a composite magnetic field generated by a motor
current flowing through motor coils points in a different
direction. Then, each of peak values of the motor currents
generated by the detection modes is compared with respect to one
another so as to select, from the plurality of detection modes, one
detection mode which generates a largest one of the peak values of
the motor currents. An initial position of a rotor is determined on
a basis of a direction of a composite magnetic field generated by
the detection mode which generates the largest one of the peak
values of the motor currents. Based on the initial position of the
rotor, the brushless DC motor is driven to start rotating.
Inventors: |
Chang; Yen-Chuan; (Hsinchu
City, TW) ; Tzou; Ying-Yu; (Hsinchu City,
TW) |
Correspondence
Address: |
NORTH AMERICA INTELLECTUAL PROPERTY CORPORATION
P.O. BOX 506
MERRIFIELD
VA
22116
US
|
Family ID: |
39103572 |
Appl. No.: |
11/533372 |
Filed: |
September 20, 2006 |
Current U.S.
Class: |
318/432 |
Current CPC
Class: |
H02P 6/20 20130101 |
Class at
Publication: |
318/432 |
International
Class: |
H02P 7/00 20060101
H02P007/00 |
Claims
1-5. (canceled)
6. A sensorless start-up method for driving a brushless DC motor,
comprising: performing a plurality of detection modes in sequence,
such that with respect to each of the detection modes a composite
magnetic field generated by a motor current flowing through motor
coils points in a different direction, wherein each mode is
performed to last for a corresponding detection time such that the
motor current gradually increases to a peak value at an end of the
corresponding detection time; comparing each of the peak values of
the motor currents generated by the detection modes with respect to
one another so as to select, from the plurality of detection modes,
one detection mode which generates a largest one of the peak values
of the motor currents; determining an initial position of a rotor
on a basis of a direction of a composite magnetic field generated
by the detection mode which generates the largest one of the peak
values of the motor currents; and driving the brushless DC motor to
start rotating based on the initial position of the rotor, wherein:
the corresponding detection time of each of the odd-numbered
detection modes is implemented by a first group detection time
(T.sub.G1); the corresponding detection time of each of the
even-numbered detection modes is implemented by a second group
detection time (T.sub.G2); and the first group detection time
(T.sub.G1 ) and the second group detection time (T.sub.G2) satisfy
an equation as follows: T G 2 = - ( L R ) ln ( 4 ( - ( R L ) T G 1
) - 1 3 ) ##EQU00003## wherein L represents an equivalent
inductance of any of the motor coils and R represents an equivalent
resistance of any of the motor coils.
7. The sensorless start-up method according to claim 6, wherein:
the plurality of detection modes include twelve detection modes for
generating twelve composite magnetic fields, respectively, any
adjacent two of which have a difference of 30 degrees in
direction.
8. The sensorless start-up method according to claim 7, wherein:
the motor coils include a first phase coil, a second phase coil,
and a third phase coil, coupled together to form a Y-shaped
structure.
9. The sensorless start-up method according to claim 8, wherein: at
a first detection mode of the twelve detection modes, an input
terminal of the first phase coil is coupled to a ground potential,
an input terminal of the second phase coil is coupled to a supply
voltage, and an input terminal of the third phase coil is coupled
to the supply voltage; at a second detection mode of the twelve
detection modes, the input terminal of the first phase coil is
coupled to the ground potential, the input terminal of the second
phase coil is coupled to the supply voltage, and the input terminal
of the third phase coil is floated; at a third detection mode of
the twelve detection modes, the input terminal of the first phase
coil is coupled to the ground potential, the input terminal of the
second phase coil is coupled to the supply voltage, and the input
terminal of the third phase coil is coupled to the ground
potential; at a fourth detection mode of the twelve detection
modes, the input terminal of the first phase coil is floated, the
input terminal of the second phase coil is coupled to the supply
voltage, and the input terminal of the third phase coil is coupled
to the ground potential; at a fifth detection mode of the twelve
detection modes, the input terminal of the first phase coil is
coupled to the supply voltage, the input terminal of the second
phase coil is coupled to the supply voltage, and the input terminal
of the third phase coil is coupled to the ground potential; at a
sixth detection mode of the twelve detection modes, the input
terminal of the first phase coil is coupled to the supply voltage,
the input terminal of the second phase coil is floated, and the
input terminal of the third phase coil is coupled to the ground
potential; at a seventh detection mode of the twelve detection
modes, the input terminal of the first phase coil is coupled to the
supply voltage, the input terminal of the second phase coil is
coupled to the ground potential, and the input terminal of the
third phase coil is coupled to the ground potential; at an eighth
detection mode of the twelve detection modes, the input terminal of
the first phase coil is coupled to the supply voltage, the input
terminal of the second phase coil is coupled to the ground
potential, and the input terminal of the third phase coil is
floated; a ninth detection mode of the twelve detection modes, the
input terminal of the first phase coil is coupled to the supply
voltage, the input terminal of the second phase coil is coupled to
the ground potential, and the input terminal of the third phase
coil is coupled to the supply voltage; at a tenth detection mode of
the twelve detection modes, the input terminal of the first phase
coil is floated, the input terminal of the second phase coil is
coupled to the ground potential, and the input terminal of the
third phase coil is coupled to the supply voltage; at an eleventh
detection mode of the twelve detection modes, the input terminal of
the first phase coil is floated, the input terminal of the second
phase coil is coupled to the ground potential, and the input
terminal of the third phase coil is coupled to the supply voltage;
and at a twelfth detection mode of the twelve detection modes, the
input terminal of the first phase coil is coupled to the ground
potential, the input terminal of the second phase coil is floated,
and the input terminal of the third phase coil is coupled to the
supply voltage.
10. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a sensorless start-up
method for driving a brushless DC motor.
[0003] 2. Description of the Related Art
[0004] Along with the advancement of technology, motors play a more
and more important role in everybody's daily life, such as spindle
motors of disc players, drive motors of image scanners, motors in
toys, and motors of windscreen wipers. Due to the lack of
commutating brushes, permanent magnet brushless DC motors is easier
to be taken care of and has a smaller size, a higher efficiency,
and other advantages, all of which render the brushless DC motors
widely applied in many areas.
[0005] A conventional method for driving the permanent magnet
brushless DC motor is to detect the position and rotating rate of a
rotor through the use of Hall sensors in order to effectively
perform the control on the position and rotating rate. However, the
Hall sensors tend to be influenced by the operational environment,
which adversely reduces the accuracy of detection. For this reason,
a sensorless driving method is introduced into the mainstream of
technological development. With respect to the sensorless driving
method, the position of the rotor is detected on the basis of BEMF
(back electromotive force). However, BEMF becomes almost zero when
the rotor is static or at a very low rate of rotation, resulting in
the impossibility of detecting the rotor's position. Therefore, one
of the most important issues is how to accurately and stably start
driving the motor by the sensorless method.
[0006] Among the proposed start-up methods, the open-loop control
is the most widely employed. Three tri-phase sinusoidal waves, each
of which has an increasing frequency, are directly applied to three
phase coils of the motor so as to raise the rate of rotation of the
motor. However, the start-up process may fail since the largest
torque may not be generated upon the start-up due to the lack of
knowledge about the rotor's position. In order to avoid such
failure of start-up, the stator's coils may at first be energized
for aligning the rotor in a predetermined direction, and then the
open-loop control is executed. Nonetheless, the energization of the
stator's coils may cause the rotor to reversely rotate, causing
limitations to the applicable range. Moreover, the third method is
to firstly detect the initial position of the rotor, and then
starts driving the motor by the open-loop control in accordance
with the rotor's initial position, thereby avoiding the failure of
the start-up and reverse rotation
[0007] Many techniques have been proposed to detect the initial
position of the rotor in accordance with a functional relationship
between the coil inductance and the rotor's position.
Unfortunately, these techniques require either complicated
algorithms or the feedback of tri-phase voltages and currents. In
U.S. Pat. No. 6,946,808, entitled "Motor Drive Control Circuit And
Motor Drive Apparatus," the rotor's initial position is detected by
way of firstly applying a set of detection signals and then
determining the rotor's initial position in accordance with
responses of a common-terminal voltage of the stator's coils. Such
a technique has an advantage of being easily carried out by using
analog circuitry. However, it is difficult to retrieve the
common-terminal voltage because the common terminal of the stator's
coils is not qualified as a standard input/output interface of the
motor structure.
SUMMARY OF THE INVENTION
[0008] Regarding with the open-loop control for driving the motor,
it is unnecessary to 100%-precisely locate the initial position of
the rotor, and a range covering several tens of degrees of
electrical angle should be accurate enough for the open-loop
start-up control. Therefore, the present invention provides a
sensorless start-up method for driving a brushless motor, which has
advantages of using simple steps with a low cost by detecting the
rotor's initial position in accordance with peak values of motor
currents, without the requirement of positional sensors and
complicated algorithms. In one preferred embodiment, the resolution
of detecting the rotor's initial position may achieve 30 degrees of
electrical angle. The method according to the present invention
detects the rotor's position before starting of the motor, i.e.,
the rotor is prevented from rotating during the detection period.
The method according to the present invention overcomes the failure
of start-up and reverse rotation occurred in the conventional
methods.
[0009] According to one aspect of the present invention, a
sensorless start-up method is provided for driving a brushless
motor, including an initial position detection phase, an open-loop
control phase, and a close-loop control phase. The initial position
detection phase is used for detecting an initial position of a
rotor. The rotor is kept at a rate of rotation of zero during the
initial position detection phase. The open-loop control phase is
used for driving the rotor in accordance with the initial position
of the rotor such that the rate of rotation of the rotor gradually
increases to reach a predetermined threshold rate. The close-loop
control phase is used for driving the rotor such that the rate of
rotation of the rotor increases from the predetermined threshold
rate to reach a target rate.
[0010] The initial position detection phase includes three steps.
The first step is performing a plurality of detection modes in
sequence, such that with respect to each of the detection modes a
composite magnetic field generated by a motor current flowing
through motor coils points in a different direction. Each mode is
performed to last for a corresponding detection time such that the
motor current gradually increases to a peak value at an end of the
corresponding detection time. The second step is comparing each of
the peak values of the motor currents generated by the detection
modes with respect to one another so as to select, from the
plurality of detection modes, one detection mode which generates a
largest one of the peak values of the motor currents. The third
step is determining an initial position of a rotor on a basis of a
direction of a composite magnetic field generated by the detection
mode which generates the largest one of the peak values of the
motor currents.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The above-mentioned and other objects, features, and
advantages of the present invention will become apparent with
reference to the following descriptions and accompanying drawings,
wherein:
[0012] FIG. 1 is a chart showing a functional relationship between
the coil inductance and the rotor's electrical angle when the motor
current is zero;
[0013] FIG. 2 is a chart showing a functional relationship between
the coil inductance and the rotor's electrical angle when the motor
current exists;
[0014] FIG. 3 is a chart showing respective currents of two
different inductances in response to the same DC voltage;
[0015] FIG. 4 is a circuit diagram showing a sensorless start-up
control circuit for driving a brushless DC motor according to the
present invention;
[0016] FIG. 5 is a circuit diagram showing a first initial position
detection mode according to the present invention;
[0017] FIG. 6 is a circuit diagram showing a second initial
position detection mode according to the present invention;
[0018] FIGS. 7(a) to 7(l) are schematic diagrams showing twelve
initial position detection modes according to the present
invention, respectively;
[0019] FIG. 8 is a flow chart showing a sensorless start-up method
for driving a brushless DC motor according to the present
invention; and
[0020] FIG. 9 is a chart showing a variation of a rate of rotation
of a brushless DC motor during an initial position detection phase,
an open-loop control phase, and a close-loop control phase
according to the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] The preferred embodiments according to the present invention
will be described in detail with reference to the drawings.
[0022] In a permanent magnet brushless DC motor, a rotor is made of
a permanent magnet and a stator is made of coils. Subjected to the
influence of magnetic poles of the permanent magnet, the inductance
of the stator's coil is not fixed, but a function of an electrical
angle of the rotor. FIG. 1 is a chart showing a functional
relationship between the coil inductance and the rotor's electrical
angle when the motor current is zero. When the coil is aligned in
parallel with either the north pole (NP) or the south pole (SP) of
the permanent magnet, the coil is subjected to magnetic saturation,
causing a reduction of the coil inductance.
[0023] As soon as a current flows through the coil, a magnetic
field is induced by the coil. In this case, the functional
relationship between the coil inductance and the rotor's electrical
angle is affected due to the magnetic interaction between the
coil's magnetic field and the permanent magnet's field. FIG. 2 is a
chart showing a functional relationship between the coil inductance
and the rotor's electrical angle when the motor current exists.
When the current is applied to the coil such that the magnetic
field generated by the coil has the south pole aligned with respect
to the south pole of the permanent magnet, the coil inductance
becomes larger than the case where no current is applied because
the magnetic saturation is relaxed due to the opposite directions
of the two magnetic fields. When the current is applied to the coil
such that the magnetic field generated by the coil has the south
pole aligned with respect to the north pole of the permanent
magnet, the coil inductance becomes smaller than the case where no
current is applied because the magnetic saturation is enhanced due
to the same direction of the two magnetic fields.
[0024] The motor coil may be considered equivalent to a combination
of a resistor, an inductor, and a BEMF, which are coupled together
in series. When the motor is static, the BEMF is zero. If a DC
voltage is applied to the motor coil, a gradually-increasing coil
current is generated. Different coil inductances cause different
increasing rates of coil current. In other words, a smaller coil
inductance results in a larger increasing rate of coil current.
FIG. 3 is a chart showing the respective currents of two different
inductances in response to the same DC voltage. Referring to FIG.
3, it is assumed that a DC voltage V.sub.dc is applied to each of
inductors L.sub.1 and L.sub.2 from time T0 to T1. If the inductor
L.sub.1 is smaller than the inductor L.sub.2, a current response 31
of the inductor L.sub.1 has a larger slope and a current response
32 of the inductor L.sub.2 has a smaller slope. Therefore, at time
T1, a current peak IP.sub.1 flowing through the inductor L.sub.1 is
larger than a current peak IP.sub.2 flowing through the inductor
L.sub.2.
[0025] FIG. 4 is a circuit diagram showing a sensorless start-up
control circuit 40 for driving a brushless DC motor M according to
the present invention. Referring to FIG. 4, the sensorless start-up
control circuit 40 includes a switching circuit 41, a driving
circuit 42, a switch control circuit 43, a current detecting
circuit 44, an initial position detecting circuit 45, and a
start-up circuit 46. The initial position detecting circuit 45
applies in sequence twelve sets of initial position detecting
signals PD to the switch control circuit 43. In response to the
twelve sets of initial position detecting signals PD, the switching
circuit 41 is operated with twelve initial position detection
modes, respectively, for applying a supply voltage V.sub.sup to a
brushless DC motor M. Each of the twelve initial position detection
modes is operated with a corresponding detection time, during which
a motor current I.sub.m is generated to gradually increase. At an
end of each corresponding detection time, the current detecting
circuit 44 detects the peak value of the motor current I.sub.m and
feedbacks the detecting result to the initial position detecting
circuit 45. Through comparing the twelve peaks of the motor
currents I.sub.m respectively generated by the twelve initial
position detection modes, the initial position detecting circuit 45
selects the largest peak from the twelve peaks of the motor
currents I.sub.m. On a basis of the initial position detection mode
that generates the largest peak of the motor current I.sub.m, the
initial position detecting circuit 45 determines the initial
position of the rotor. Afterwards, the initial position detecting
circuit 45 applies an initial position indicating signal POS to the
start-up circuit 46. On a basis of the rotor's initial position
indicated by the initial position indicating signal POS, the
start-up circuit 46 performs a two-phase operation for driving the
brushless DC motor M. At first, the start-up circuit 46 performs an
open-loop control phase for driving the brushless DC motor M to
reach a predetermined threshold rate of rotation. Then, the
start-up circuit 46 performs a close-loop control phase for driving
the brushless DC motor M to reach a target rate of rotation
specified by user's commands.
[0026] In the embodiment shown in FIG. 4, the brushless DC motor M
has three phase coils U, V, and W, coupled together to form a
Y-shaped structure. The switching circuit 41 has three high-side
switches Q1, Q3, and Q5 and three low-side switches Q2, Q4, and Q6.
An input terminal A of the coil U is coupled to the supply voltage
V.sub.sup through the high-side switch Q1, and is coupled to the
ground potential through the low-side switch Q2. An input terminal
B of the coil V is coupled to the supply voltage V.sub.sup through
the high-side switch Q3, and is coupled to the ground potential
through the low-side switch Q4. An input terminal C of the coil W
is coupled to the supply voltage V.sub.sup through the high-side
switch Q5, and is coupled to the ground potential through the
low-side switch Q6.
[0027] The switch control circuit 43 applies a switch control
signal SC to the driving circuit 42. In response to the switch
control signal SC, the driving circuit 42 generates six switching
signals S1 to S6 for controlling the high-side switches Q1, Q3, and
Q5 and the low-side switches Q2, Q4, and Q6, respectively.
[0028] The initial position detecting circuit 45 applies in
sequence twelve sets of initial position detecting signals PD to
the switch control circuit 43. Each set of the initial position
detecting signals PD determines a specific operation mode defined
by a combination of ON/OFF states of the high-side switches Q1, Q3,
and Q5 and the low-side switches Q2, Q4, and Q6, for controlling
whether the input terminals A, B, and C of the three phase coils U,
V, and W are coupled to the supply voltage V.sub.sup the ground
potential, or the floating potential, i.e., being floated.
Therefore, the twelve sets of the initial position detecting
signals PD determine twelve initial position detection modes. As
for each mode, a composite magnetic field with a different
direction is generated because the motor current I.sub.m flows
through the three phase coils in a different pattern of current
paths.
[0029] For example, FIG. 5 is a circuit diagram showing a first
initial position detection mode according to the present invention.
Since the switches Q1, Q4, and Q6 are turned OFF and the switches
Q2, Q3, and Q5 are turned ON, the input terminal A is coupled to
the ground potential, the input terminal B is coupled to the supply
voltage V.sub.sup, and the input terminal C is coupled to the
supply voltage V.sub.sup. At this mode, the motor current I.sub.m
flows into the coils V and W through the terminals B and C and then
flows out of the coil U through the terminal A, such that the
composite magnetic field generated by the motor current I.sub.m has
a direction as pointed by the arrow 50.
[0030] For example, FIG. 6 is a circuit diagram showing a second
initial position detection mode according to the present invention.
Since the switches Q1, Q4, Q5, and Q6 are turned OFF and the
switches Q2 and Q3 are turned ON, the input terminal A is coupled
to the ground potential, the input terminal B is coupled to the
supply voltage V.sub.sup, and the input terminal C is floated. At
this mode, the motor current I.sub.m flows into the coil V through
the terminal B and then flows out of the coil U through the
terminal A, but no current flows through the coil W, such that the
composite magnetic field generated by the motor current I.sub.m has
a direction as pointed by the arrow 60. As apparently seen from the
comparison between FIGS. 5 and 6, the arrow 60 is 30 degrees
clockwise with respect to the arrow 50.
[0031] FIGS. 7(a) to 7(l) are schematic diagrams showing twelve
initial position detection modes according to the present
invention, respectively, determined by the twelve sets of initial
position detecting signals PD. At a first initial position
detection mode shown in FIG. 7(a), the coil U is coupled to the
ground potential GND, the coil V is coupled to the supply voltage
V.sub.sup, and the coil W is coupled to the supply voltage
V.sub.sup. At a second initial position detection mode shown in
FIG. 7(b), the coil U is coupled to the ground potential GND, the
coil V is coupled to the supply voltage V.sub.sup, and the coil W
is coupled to the floating potential FLT (i.e., being floated). At
a third initial position detection mode shown in FIG. 7(c), the
coil U is coupled to the ground potential GND, the coil V is
coupled to the supply voltage V.sub.sup, and the coil W is coupled
to the ground potential GND. At a fourth initial position detection
mode shown in FIG. 7(d), the coil U is coupled to the floating
potential FLT, the coil V is coupled to the supply voltage
V.sub.sup, and the coil W is coupled to the ground potential GND.
At a fifth initial position detection mode shown in FIG. 7(e), the
coil U is coupled to the supply voltage V.sub.sup, the coil V is
coupled to the supply voltage V.sub.sup, and the coil W is coupled
to the ground potential GND. At a sixth initial position detection
mode shown in FIG. 7(f), the coil U is coupled to the supply
voltage V.sub.sup, the coil V is coupled to the floating potential
FLT, and the coil W is coupled to the ground potential GND. At a
seventh initial position detection mode shown in FIG. 7(g), the
coil U is coupled to the supply voltage V.sub.sup, the coil V is
coupled to the ground potential GND, and the coil W is coupled to
the ground potential GND. At an eighth initial position detection
mode shown in FIG. 7(h), the coil U is coupled to the supply
voltage V.sub.sup, the coil V is coupled to the ground potential
GND, and the coil W is coupled to the floating potential FLT. At a
ninth initial position detection mode shown in FIG. 7(i), the coil
U is coupled to the supply voltage V.sub.sup, the coil V is coupled
to the ground potential GND, and the coil W is coupled to the
supply voltage V.sub.sup. At a tenth initial position detection
mode shown in FIG. 7(j), the coil U is coupled to the floating
potential FLT, the coil V is coupled to the ground potential GND,
and the coil W is coupled to the supply voltage V.sub.sup. At an
eleventh initial position detection mode shown in FIG. 7(k), the
coil U is coupled to the ground potential GND, the coil V is
coupled to the ground potential GND, and the coil W is coupled to
the supply voltage V.sub.sup. At a twelfth initial position
detection mode shown in FIG. 7(l), the coil U is coupled to the
ground potential GND, the coil V is coupled to the floating
potential FLT, and the coil W is coupled to the supply voltage
V.sub.sup.
[0032] If the composite magnetic field generated by the first
initial position detection mode shown in FIG. 7(a) has a direction
referred to as 0 degree of electrical angle, then the composite
magnetic fields generated by the second to twelfth initial position
detection modes shown in FIGS. 7(b) to 7(l) have directions
referred to as 30, 60, 90, 120, 150, 180, 210, 240, 270, 300, and
330 degrees of electrical angle, respectively. In other words, the
composite magnetic fields generated by any two adjacent modes of
the twelve initial position detection modes have the directions
with a difference of 30 degrees of electrical angle with respect
one another.
[0033] For each of the initial position detection modes, the
different direction of the composite magnetic field causes a
different equivalent inductance of the motor coils, as described
above with reference to FIG. 2, and therefore the increasing rate
of the motor current I.sub.m changes in accordance with the
different equivalent inductance of the motor coils When the
composite magnetic field of the coils and the permanent magnet's
field of the rotor become closer to the same direction, the
equivalent inductance of the motor coils becomes smaller. The
smaller the equivalent inductance of the motor coils becomes, the
larger the peak is reached by the motor current I.sub.m at an end
of the corresponding detection time of the initial position
detection mode.
[0034] Referring back to FIG. 4, the sensorless start-up control
circuit 40 is provided with the current detecting circuit 44,
coupled to the switching circuit 41, for detecting the motor
current I.sub.m. For example, the current detecting circuit 44 may
be implemented by a resistor R.sub.sen coupled in series between
the commonly-connecting point of the low-side switches Q2, Q4, and
Q6 of the switching circuit 41 and the ground potential. Therefore,
a potential difference generated by the motor current I.sub.m
flowing through the resistor R.sub.sen may be used as a current
detection signal DI for being applied to the initial position
detection circuit 45.
[0035] Hereinafter is described in detail a sensorless start-up
method for driving a brushless DC motor according to the present
invention with reference to FIG. 8. In a step SS1, a plurality of
detection modes are sequentially performed, such that with respect
to each of the detection modes a composite magnetic field generated
by a motor current flowing through motor coils points in a
different direction. Each mode is performed to last for a
corresponding detection time such that the motor current gradually
increases to a peak value at an end of the corresponding detection
time. In a step SS2, each of the peak values of the motor currents
generated by the detection modes is compared with respect to one
another so as to select, from the plurality of detection modes, one
detection mode which generates a largest one of the peak values of
the motor currents. In a step SS3, an initial position of a rotor
is determined on a basis of a direction of a composite magnetic
field generated by the detection mode which generates the largest
one of the peak values of the motor currents. In a step SS4, based
on the initial position of the rotor, the brushless DC motor is
driven to start rotating.
[0036] FIG. 9 is a chart showing a variation of a rate of rotation
of a brushless DC motor during an initial position detection phase,
an open-loop control phase, and a close-loop control phase
according to the present invention. Referring to FIG. 9, the
sensorless start-up method for driving the brushless DC motor
according to the present invention includes: an initial position
detection phase, an open-loop control phase, and a close-loop
control phase. From time T0 to T1, the initial position detection
phase is used for detecting the initial position of the rotor.
During the initial position detection phase, the rate of rotation
of the rotor is kept zero, i.e., the rotor is static. From time T1
to T2, the open-loop control phase is used for driving the rotor in
accordance with the initial position of the rotor, such that the
rate of rotation of the rotor gradually increases to reach a
predetermined threshold rate VTH. From time T2 to T3, the
close-loop control phase is used for driving the rotor such that
the rate of rotation of the rotor gradually increases from the
predetermined threshold rate VTH to reach a target rate VF
specified by user's commands.
[0037] As described above, the sensorless start-up method for
driving the brushless DC motor according to the present invention
includes a step of determining an initial position of a rotor by
comparing the peak values of the motor currents generated by the
twelve detection modes with respect to one another so as to select,
from the twelve detection modes, one detection mode which generates
a largest one of the peak values of the motor currents. In order to
expand the relative difference between the peaks of the motor
currents I.sub.m generated by the detection modes so as to enhance
the accuracy of the initial position detection, it seems better for
each mode to last for a longer corresponding detection time. As for
any two first-order RL systems, if two system time constants are
similar, then the current responses of the two systems generate the
largest difference under a condition that the period for applying
the DC voltage signal is set equal to the system time constant. But
if the system time constant is too large such that the period for
applying the DC voltage signal becomes too long, the motor current
reaches a high enough level for causing the motor to start
rotating. For this reason, the corresponding detection time of each
position detection mode should be limited within a range that
prevents the rotor from rotating since the rotor is required to
remain static during the initial position detection phase according
to the present invention.
[0038] The twelve initial position detection modes may be
categorized into two groups. The first group consists of the
detection modes shown in FIGS. 7(a), 7(c), 7(e), 7(g), 7(i), and
7(k), with the same structural characteristic that any two phase
coils are coupled in parallel and then together coupled in series
to the remaining phase coil. As a result, the motor coils in the
first group provide an equivalent resistance of 1.5*R and an
equivalent inductance of 1.5*L wherein L represents the equivalent
inductance of any one of the motor coils and R represents the
equivalent resistance of any one of the motor coils. The second
group consists of the detection modes shown in FIGS. 7(b), 7(d),
7(f), 7(h), 7(j), and 7(l), with the same structural characteristic
that any two phase coils are coupled in series and the remaining
phase coil is floated. As a result, the motor coils in the second
group provide an equivalent resistance of 2*R and an equivalent
inductance of 2*L. Therefore, the first group has a current
response i.sub.G1(t) as shown in Equation (1) and the second group
has a current response i.sub.G2(t) as shown in Equation (1):
i G 1 ( t ) = V sup 1.5 R ( 1 - - ( R L ) t ) ( 1 ) i G 2 ( t ) = V
sup 2 R ( 1 - - ( R T ) t ) ( 2 ) ##EQU00001##
[0039] In order to eliminate the unfairness due to the difference
between the equivalent inductances and resistances of the first and
second groups, the detection time T.sub.G1 applied in the first
group and the detection time T.sub.G2 applied in the second group
must satisfy Equation (3):
i.sub.G1(T.sub.G1)=i.sub.G2(T.sub.G2) (3)
[0040] From Equation (3) is derived Equation (4):
T G 2 = - ( L R ) ln ( 4 ( - ( R L ) T G 1 ) - 1 3 ) ( 4 )
##EQU00002##
[0041] Therefore, after the detection time T.sub.G1 applied in the
first group is chosen, the detection time T.sub.G2 applied in the
second group is determined in accordance with Equation (4).
[0042] While the invention has been described by way of examples
and in terms of preferred embodiments, it is to be understood that
the invention is not limited to the disclosed embodiments. To the
contrary, it is intended to cover various modifications. Therefore,
the scope of the appended claims should be accorded the broadest
interpretation so as to encompass all such modifications.
* * * * *