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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of determining an electrical
angle in a synchronous motor and an apparatus for the same.
2. Description of the Related Art
As an apparatus for measuring an electrical angle with high precision in a
synchronous motor even in the cased state of a rotor or at a low-revolving
speed of the rotor, the applicant of the present invention has proposed an
apparatus for observing a behavior of electric current flowing through
each phase and determining an electrical angle based on the observed
behavior (JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-177788). In the case
of a three-phase synchronous motor, for example, the proposed apparatus
observes behaviors of electric currents flowing through any two phases,
thereby unequivocally determining the electrical angle.
The proposed apparatus can determine the electrical angle with high
precision even when the rotor is at a stop or at a low revolving speed.
Foreign noises, however, sometimes occur in the process of determining the
electrical angle in a synchronous motor with the proposed apparatus. The
following describes this drawback in detail. The proposed apparatus
carries out measurement while successively varying the combination of
phases to which the voltage is applied. It is assumed that a three-phase
synchronous motor is driven by making electric currents Iu, Iv, and Iw
flow through U, V, and W phases. As shown in FIGS. 18A and 18B, the
proposed apparatus returns the driving currents to zero at a predetermined
timing, applies a voltage between U-WV phases to measure an electric
current flowing through the U-phase at a timing m1 in FIG. 18B, applies
the voltage between V-WU phases to measure an electric current flowing
through the V-phase at a timing m2, and applies the voltage between W-UV
phases to measure an electric current flowing through the W-phase at a
timing m3. Since the sum of the electric currents flowing through the
three phases is equal to zero, electric current sensors are required at
two difference places, for example, in the U-phase and the V-phase.
In case that the respective phase currents are measured three times while a
voltage is applied between each combination of phase-coils, several
milliseconds are required for determination of the electrical angle. In
order to ensure a certain level of precision, measurement of the
electrical angle is required at every ten-odd milliseconds. The electric
currents for measurement are thus flown for a period of several
milliseconds at the intervals of ten-odd milliseconds as shown in FIGS.
18A and 18B. The known procedure returns the driving voltage applied to
the synchronous motor to zero before application of a voltage for
measurement, and resumes application of the driving voltage of the
original level after conclusion of the measurement. The foreign noises
occur in this process. The foreign noises become larger with an increase
in torque current to the synchronous motor.
SUMMARY OF THE INVENTION
The object of the present invention is to prevent the occurrence of foreign
noises while keeping the advantages of the proposed technique of
determining the electrical angle.
At least part of the above and the other related objects is realized by a
method of determining an electrical angle of a rotor unit in a synchronous
motor, which makes a multi-phase alternating current flow through a
plurality of phase-coils and causes the rotor unit to be rotated by an
interaction between a magnetic field created by the plurality of
phase-coils and a magnetic field created by a plurality of permanent
magnets. The method of the present invention includes the steps of:
applying a predetermined voltage for measurement to be superposed upon a
driving voltage applied to each phase-coil;
measuring an electric current flowing through each phase-coil in response
to the predetermined voltage for measurement and correcting the observed
value of electric current with an effect of the driving voltage, so as to
obtain a corrected value of electric current flowing through each
phase-coil; and
determining the electrical angle of the rotor unit based on the corrected
value of electric current flowing through each phase-coil.
In case that no driving current is supplied to each phase-coil of a
synchronous motor, the electrical angle in the synchronous motor can be
determined by applying a predetermined voltage for measurement to each
combination of phase-coils and observing a behavior of electric current
flowing through each phase-coil in response to the predetermined voltage
applied. In case that the predetermined voltage for measurement is applied
in the presence of the driving current supplied to the synchronous motor,
on the other hand, the observed behavior of electric current is varied by
the driving current. The method of the present invention accordingly
carries out correction corresponding to the driving current supplied to
the synchronous motor in the process of application of the predetermined
voltage for measurement. This procedure accordingly determines the
electric current flowing through each phase-coil in response to
application of the predetermined voltage for measurement, and determines
the electrical angle of the rotor unit based on the corrected value of
electric current flowing through each phase-coil.
The method of the present invention can apply the voltage for measurement
without returning the driving current flowing through the synchronous
motor to zero. This restricts a variation in electric current accompanied
by application of the voltage for measurement, thereby reducing the
occurrence of foreign noises. A variety of techniques are applicable to
correct the observed electric current with the effect of the driving
voltage. An arbitrary technique can be applied to eliminate the effect of
the driving current and determine the electrical angle with high
precision.
One of the available techniques is applied in a first apparatus for
determining an electrical angle of a rotor unit in a synchronous motor,
which makes a multi-phase alternating current flow through a plurality of
phase-coils and causes the rotor unit to be rotated by an interaction
between a magnetic field created by the plurality of phase-coils and a
magnetic field created by a plurality of permanent magnets. The first
apparatus of the present invention includes: measurement voltage
application means for applying a predetermined voltage for measurement to
each combination of phase-coils; electric current behavior observation
means for observing a behavior of electric current flowing through each
phase-coil, in response to the predetermined voltage for measurement that
is applied by the measurement voltage application means; storage means for
previously storing a relationship between electrical angle and behavior of
electric current flowing through each phase-coil in response to
application of the predetermined voltage for measurement, with respect to
each combination of phase-coils; correction means for correcting a
variation in observed behavior of electric current flowing through each
phase-coil with a driving current that flows through each phase in the
synchronous motor during the predetermined voltage is applied for
measurement; and electrical angle determination means for referring to the
relationship stored in the storage means and determining the electrical
angle of the rotor unit in the range of 0 to 2.pi., based on the behavior
of electric current corrected by the correction means.
In the first apparatus of the present invention, the storage means stores
the relationship between electrical angle and behavior of electric current
flowing through each phase-coil only in response to application of a
predetermined voltage for measurement, with respect to each combination of
phase-coils. The measurement voltage application means applies the
predetermined voltage for measurement to each combination of phase-coils,
and the electric current behavior observation means observes a behavior of
electric current flowing through each phase-coil in response to the
predetermined voltage for measurement applied by the measurement voltage
application means. The correction means corrects a variation in observed
behavior of electric current flowing through each phase-coil with a
driving current that flows through each phase in the synchronous motor
during the predetermined voltage is applied for measurement. The
electrical angle determination means refers to the relationship stored in
the storage means and determines the electrical angle of the rotor unit in
the range of 0 to 2.pi., based on the behavior of electric current
corrected by the correction means.
The structure of the first apparatus corrects the observed electric
current, which flows through each phase-coil in response to the
predetermined voltage for measurement, with the effect of the driving
current, in order to estimate the state in the absence of the driving
current and determine the electrical angle. This structure enables the
existing data representing the behaviors of electric currents to be
utilized for determination of the electrical angle.
Another technique of correcting the observed electric current with the
effect of the driving current is applied in a second apparatus for
determining an electrical angle of a rotor unit in a synchronous motor,
which makes a multi-phase alternating current flow through a plurality of
phase-coils and causes the rotor unit to be rotated by an interaction
between a magnetic field created by the plurality of phase-coils and a
magnetic field created by a plurality of permanent magnets. The second
apparatus of the present invention includes: voltage application means for
applying a predetermined voltage for measurement to each combination of
the phase-coils; electric current behavior observation means for observing
a behavior of electric current flowing through each phase-coil, in
response to the predetermined voltage for measurement applied by the
voltage application means; driving current detection means for measuring a
driving current supplied to each phase-coil at the time of application of
the predetermined voltage for measurement; storage means for previously
storing a relationship between electrical angle and behavior of electric
current flowing through each phase-coil in response to the predetermined
voltage for measurement that is applied to each combination of phase-coils
while a driving current is flown in the synchronous motor; and electrical
angle determination means for referring to the relationship stored in the
storage means and determining the electrical angle of the rotor unit in
the range of 0 to 2.pi., based on the observed behavior of electric
current and the measured driving current.
In the second apparatus of the present invention, the storage means stores
the relationship between electrical angle and behavior of electric current
flowing through each phase-coil in response to a predetermined voltage for
measurement that is applied to each combination of phase-coils while a
driving current is flown in the synchronous motor. Upon condition that the
voltage application means applies the predetermined voltage for
measurement to each combination of phase-coils the electric current
behavior observation means observes a behavior of electric current flowing
through each phase-coil and the driving current detection means measures a
driving current supplied to the synchronous motor. The electrical angle
determination means refers to the relationship stored in the storage means
and determines the electrical angle of the rotor unit in the range of 0 to
2.pi., based on the observed behavior of electric current and the measured
driving current. This structure does not have the step of correcting the
observed electric current in response to the application of the
predetermined voltage for measurement, thereby further shortening the time
period required for determination of the electrical angle.
These and other objects, features, aspects, and advantages of the present
invention will become more apparent from the following detailed
description of the preferred embodiments with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram schematically illustrating structure of a motor
control apparatus 10 as a first embodiment according to the present
invention;
FIG. 2 schematically illustrates structure of a three-phase synchronous
motor 40 with a stator unit 30 incorporated therein;
FIG. 3 is an end view illustrating the relationship between the stator unit
30 and a rotor unit 50 of the three-phase synchronous motor 40;
FIG. 4 shows an equivalent circuit of the three-phase synchronous motor in
the first embodiment;
FIG. 5 is a graph showing a transient response of U-phase current Iu(t)
when a voltage El is applied between the U-VW phases;
FIG. 6 is a graph showing the relationship between the driving current and
the amount of attenuation that is stored as a table 122A in the memory 122
of the first embodiment;
FIG. 7 is a graph showing the relationship between the coil inductance and
the electrical angle .theta.;
FIG. 8 shows a technique of determining the inductance based on the
behavior of electric current under the condition of application of a
predetermined voltage;
FIG. 9(A) and FIG. 9(B) are graphs showing the relationship between the
electrical angle and the inductance;
FIG. 10 shows the relationship between the stator unit 30 and the rotor
unit 50 of the three-phase synchronous motor 40 in a linear configuration;
FIG. 11 is a flowchart showing an electrical angle determination routine
carried out in the first embodiment;
FIG. 12 is a graph showing a curve of the observed phase current;
FIG. 13 is a graph showing the observed phase currents plotted against the
voltage applied;
FIG. 14 is a graph showing variations in electric currents when voltages of
opposed polarities are applied between the U-VW phases;
FIG. 15 is a graph showing a method of specifying the range of the
electrical angle by taking advantage of a magnetic saturation;
FIG. 16 is a block diagram schematically illustrating structure of another
motor control apparatus 310 as a second embodiment according to the
present invention;
FIG. 17 is a block diagram schematically illustrating structure of still
another motor control apparatus 410 as a third embodiment according to the
present invention; and
FIGS. 18(A) and 18(B) are graphs showing a prior art technique of
determining the electrical angle.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Some modes of carry out the present invention are described below as
preferred embodiments. Referring to FIGS. 2 and 3, general structure of
the three-phase synchronous motor 40 is described first. The three-phase
synchronous motor 40 includes a stator unit 30, a rotor unit 50, and a
casing 60 for accommodating the stator unit 30 and the rotor unit 50
therein. The rotor unit 50 has permanent magnets 51 through 54 attached to
the periphery thereof. A rotating shaft 55 arranged on the axial center of
the rotor unit 50 is supported rotatably by bearings 61 and 62 disposed in
the casing 60.
The rotor unit 50 is constructed by laying a plurality of rotors 57, which
are punched out of a thin plate of non-directional electromagnetic steel,
one upon another. As clearly shown in FIG. 3, each rotor 57 has four
salient poles 71 through 74, which are arranged perpendicular to one
another. After the plurality of rotors 57 are laid one upon another, the
rotating shaft 55 is pressed into the laminated rotors 57 to tentatively
join the laminated rotors 57. The rotors 57 composed of the
electromagnetic steel have insulating layers and adhesive layers formed on
the surface thereof. Heating the laminated rotors 57 to a predetermined
temperature causes the adhesive layers to be fused and joined with one
another.
After the assembly of the rotor unit 50, the permanent magnets 51 through
54 are arranged between the salient poles 71 through 74 on the periphery
of the rotor unit 50 and attached along the axis of the rotor unit 50. The
permanent magnets 51 through 54 are magnetized in the direction of
thickness. In the assembly of the rotor unit 50 and the stator unit 30,
for example, the permanent magnets 51 and 52 create a magnetic path Md
passing through the rotors 57 and stators 20 (see the one-dot chain line
in FIG. 3).
Like the rotors 57, a plurality of stators 20 constituting the stator unit
30 are punched out of a thin plate of non-directional electromagnetic
steel. As shown in FIG. 3, each stator 20 has twelve teeth 22. Stator
coils 32 that make the stator unit 30 create a revolving magnetic field
are wound on the teeth 22 to run through slots 24 formed between the teeth
22. The stator 20 has bolt holes for receiving fixation bolts 34 formed in
the periphery thereof, although the bolt holes are omitted from the
illustration of FIG. 3.
The stator unit 30 is assembled by heating and fusing the adhesive layers
in the state that the plurality of plate-like stators 20 are laid one upon
another and pressed against one another. The stator coils 32 are then
wound on the teeth 22 of the stators 20. The assembled stator unit 30 is
set in the casing 60, and the fixation bolts 34 are inserted into the bolt
holes and clamped, so as to fix the stator unit 30 to the casing 60. The
rotor unit 50 is subsequently set in the casing 60 to be rotatably
supported by the bearings 61 and 62 of the casing 60. This completes the
three-phase synchronous motor 40.
When exciting electric current is flown through the stator coils 32 of the
stator unit 30 to create a revolving magnetic field, a magnetic path Mq is
created to pass through the adjoining salient poles, the rotors 57, and
the stators 20 as shown in FIG. 3. In the drawing of FIG. 3, an axis `d`
represents an axis of the magnetic flux that is created by the permanent
magnets 51 through 54 and passes through the rotor unit 50 in the
diametral direction, whereas an axis `q` represents an axis of the
magnetic flux that is created by the stator coils 32 of the stator unit 30
and passes through the rotor unit 50 in the diametral direction. In this
embodiment having four poles, the axes `d` and `q` electrically have an
angle of 90 degrees.
Referring to FIG. 1, the following describes structure of the motor control
unit 10. The motor control unit 10 includes a motor currents-regulating
circuit 100 for receiving a torque command value from outside and
regulating three-phase motor currents (U, V, and W phases) of the
three-phase synchronous motor 40, and ammeters 102, 104, and 106 for
respectively measuring a U-phase electric current Iu, a V-phase electric
current Iv, and a W-phase electric current Iw of the three-phase
synchronous motor 40. The motor control unit 10 further includes a
current-torque converter 110 for converting the observed values of U-phase
electric current Iu and the V-phase electric current Iv to torques; three
analog-digital converters (ADC) 112, 114, and 116 for converting the
observed values of electric currents to digital data, a one-chip
microprocessor (CPU) 120 for executing arithmetic operations to determine
an electrical angle, and a memory 112 for storing tables used for the
arithmetic operation of an electrical angle. The torques converted by the
current-torque converter 110 are summed up and subtracted from a torque
command valuer so as to feedback control the actual torque in the
three-phase synchronous motor 40. The motor currents-regulating circuit
100 has a voltage application unit 130 on its output stage. The voltage
application unit 130 determines a voltage applied between the respective
coils of the three-phase synchronous motor 40, in order to obtain the
respective phase currents Iu, Iv, and Iw based on the torque command
value. The voltage application unit 130 receives a control output from the
CPU 120, so that the voltage applied to the respective coils of the
three-phase synchronous motor 40 can be regulated externally. As shown in
FIG. 1, the memory 122 has two tables stored therein; a table 122A used
for correcting observed values of electric currents and a table 122B used
for calculating an electrical angle from the corrected values of electric
currents.
The following describes the principle and actual procedure of determining
an electrical angle of the rotor unit 50 in the three-phase synchronous
motor 40 of the above construction with the motor control unit 10
discussed above. FIG. 4 shows an equivalent circuit diagram of the
three-phase synchronous motor 40. In case that a predetermined voltage El
like a step function is applied between the U-phase and the VW-phase of
the three-phase synchronous motor 40, an electric current flowing between
the U-VW phases represents a transient response depending upon an
inductance component L of the circuit. The graph of FIG. 5 shows an
example of the transient response. The inductance L of the circuit is
defined as a function of an electrical angle .theta. of the rotor unit 50.
In case that the rotor unit 50 is not rotated and kept in a still state,
the electrical angle, that is, the angle of the axis `d` electrically
formed with the axis `q` in the rotor unit 50, determines the inductance L
of the circuit.
The electric current Iu(t) flowing through the equivalent circuit shown in
FIG. 4 (hereinafter referred to as the U-phase current) has a response
defined by Equation (1) given below:
Iu(t)={1-exp(-Rt/L)}E1/R (1)
wherein exp() represents an exponential function, and R and t respectively
denote the impedance of the circuit and the time. The U-phase current
Iu(t) shows a gentle increase under the condition of a large inductance L.
Measurement of the electric currents at a predetermined time point after
the application of the voltage gives the inductances L and eventually
determines the electrical angle .theta..
When the voltage is applied for the measurement of the inductance L during
operation of the three-phase synchronous motor 40, that is, in the state
that a driving current flows through the U-phase, the U-phase current
Iu(t) is affected by the driving current and attenuated from the transient
response defined by the above Equation (1). The relationship between the
amount of attenuation and the driving current can be measured in advance.
The graph of FIG. 6 shows an exemplified relationship between the amount
of attenuation and the driving current in the case of the three-phase
synchronous motor 40 used in this embodiment. The amounts of attenuation
corresponding to known driving currents of the respective phases can be
read from the graph. Proper U-phase current Iu(t), V-phase current Iv(t),
and W-phase current Iw(t) are determined by adding the amounts of
attenuation to the respective phase currents Iu, Iv, and Iw observed. The
graph representing the relationship between the amount of attenuation and
the driving current of each phase is stored in the table 122A of the
memory 122.
Inductances Lu, Lv, and Lw of the U, V, and W phases are calculated from
the variations in electric currents Iu, Iv, and Iw of the respective
phases and eventually determine the electrical angle. Inductances of all
the three phases are not necessary for the determination of the electrical
angle, but inductances of any two phases are sufficient. In this
embodiment, the inductances Lu and Lv are measured in advance for a
variety of electrical angles and stored in the memory 122. The graph of
FIG. 7 shows the observed relationship as an example. The curve of solid
line represents the observed inductance between the U-VW phases plotted
against the electrical angle, whereas the curve of broken line represents
the observed inductance between the V-WU phases plotted against the
electrical angle. The relationship between the observed inter-coil
inductance and the electrical angle is stored in the table 122B used for
determining the electrical angle according to the corrected value of
electric current in the memory 122.
A concrete procedure first applies a voltage between the U-VW phases to
measure an inductance L2, and then applies a voltage between the V-WU
phases to measure an inductance L1. Only one angle of the rotor unit 50,
that is, only one electrical angle, in the range of 0 to 2.pi. corresponds
to the inductance L2 between the U-VW phases and the inductance L1 between
the V-WU phases. This is the electrical angle of .alpha.1=.beta.1.
Measurement of the phase currents Iu and Iv accordingly results in
determining the electrical angle .theta. of the rotor unit 50. In the
above discussion, determination of the inductance L between specific
phases depends upon a time period TO that has elapsed before the electric
current flowing between the specific phases reaches a predetermined level
as shown in FIG. 5. In accordance with another possible procedure,
determination of the inductance may depend upon a value of electric
current Im that is obtained when a predetermined voltage is applied
between specific phases for a predetermined time period, as shown in FIG.
8. The latter technique shown in FIG. 8 is used for the actual
determination of the electrical angle, since the required processing time
is fixed.
The observed curves shown in the graph of FIG. 7 have different
inductance-electrical angle characteristics in the range of 0 to .pi. and
in the range of .pi. to 2.pi.. The inductance-electrical angle curve,
however, essentially shows identical behaviors in the range of 0 to .pi.
and in the range of .pi. to 2.pi. as shown in the graph of FIG. 9.
Calculation of the inductances L2 and L1 between the respective phases
from the phase currents Iu and Iv gives two solutions that satisfy the
relationship. In this embodiment, the magnetic relationship between the
stator unit 30 and the rotor unit 50 is made unsymmetrical, so that the
inductance L and the electrical angle .theta. hold an unsymmetrical
relationship as shown in FIG. 7. In this embodiment, the unsymmetrical
magnetic relationship between the stator unit 30 and the rotor unit 50 is
realized by applying magnetic elements 86 through 89 to the magnetized one
faces of the permanent magnets 52 and 54 and the surfaces of the salient
poles 72 and 74. FIG. 10 shows the stator unit 30 and the rotor unit 50 of
the three-phase synchronous motor 40 in a linear configuration for the
better understanding. In the rotor unit 50, the magnetic elements 86 and
88 are arranged respectively on the lower faces of the permanent magnet 52
and the permanent magnet 54, whereas the magnetic elements 87 and 89 are
respectively attached to the surfaces of the salient poles 72 and 74. The
pair of permanent magnets 51 and 52 constituting the magnetic poles have
different characteristics in the presence of the magnetic elements 86 and
87 and in the absence of the magnetic elements 86 and 87. The relationship
between the electrical angle and the electric current flowing through each
phase accordingly shows different behaviors in the first 180 degrees
without the magnetic elements 86 and 87 (0 to .pi.) and in the next 180
degrees with the magnetic elements 86 and 87 (.pi. to 2.pi.).
The above description refers to the outline of the technique that applies a
predetermined voltage between the U-VW phases and between the V-Wu phases,
measures the electric currents flowing through the U phase and the V
phase, and determines the electrical angle .theta. based on the observed
values of electric currents. The following describes a process of
electrical angle determination carried out by the CPU 120 of this
embodiment. FIG. 11 is a flowchart showing an electrical angle
determination routine executed by the CPU 120. The electrical angle
determination routine is activated at every 16 msec by a timer
interruption and concluded in approximately 1 msec. FIG. 12 is a graph
showing an observed curve of the U-phase current, and FIG. 13 shows
measurement of electric current variations .DELTA.I of the respective
phases in this embodiment. The lower portion of FIG. 13 corresponds to
expansion of an area `A` in FIG. 12.
When operation of the three-phase synchronous motor 40 is required in
response to a Power-ON, the CPU 120 successively applies a predetermined
voltage between the respective coils, measures inductances between the
coils, and determines the electrical angle .theta. of the ceased rotor
unit 50 based on the principle shown in FIGS. 5 and 7. The CPU 120 makes a
driving current flow through each phase according to the electrical
angle.theta., so as to actuate the three-phase synchronous motor 40. The
CPU 120 subsequently carries out the process shown in the flowchart of
FIG. 11 to determine the electrical angle of the rotor unit 50, while
making the electric current flow through each phase and driving the
three-phase synchronous motor 40.
When the program enters the routine of FIG. 11, the CPU 120 first reads a
value of electric current Iu1 flowing through the U phase at the start of
measurement at step S200, and applies a predetermined voltage (voltage for
measurement) E1 like a step function between the U-VW phases at step S205.
As shown in FIG. 13, application of a voltage for measurement in addition
to a driving voltage varies the electric current flowing between the U-VW
phases. The CPU 120 then reads a value of electric current Iu2 flowing
through the U phase at a time point when application of the voltage for
measurement is terminated at step S210, and calculates a variation
.DELTA.Iu before and after the application of the voltage for measurement
at step S215. The ADC 112 converts the analog outputs of the ammeters 102
to digital data, and the CPU 120 reads the digital data as observed values
of electric currents. The above procedure is repeated for the V phase and
the W phase. The CPU 120 reads values of electric currents Iv1 and Iw1 at
the start of measurement (steps S220 and S240), applies the predetermined
voltage for measurement (steps S225 and S245), reads values of electric
currents Iv2 and Iw2 at the termination of application (steps S230 and
S250), and calculates variations .DELTA.Iv and .DELTA.Iw (steps S230 and
S255). Inductances of at least two phases are required for determination
of the electrical angle as described above. The processing of, for
example, steps S240 through S255 for the W phase may thus be omitted.
These variations .DELTA.Iu, .DELTA.Iv, and .DELTA.Iw are measured in the
presence of the driving current and are attenuated from the proper
variations in the absence of the driving current. For the purpose of
correction, the CPU 120 refers to the table 122A stored in the memory 122,
reads the amounts of attenuation corresponding to the electric currents
Iu1, Iv1, and Iw1 at the start of measurement, and adds the amounts of
attenuation to the respective variations .DELTA.Iu, .DELTA.Iv, and
.DELTA.Iw. The CPU 120 accordingly obtains the values of phase currents
Iu, Iv, and Iw that are equivalent to the measurements in the absence of
the driving current at step S260.
The CPU 120 then determines the electrical angle based on the respective
phase currents corrected by the amounts of attenuation at step S270. This
process, in principle, follows the procedure discussed below. The
inductances Lu, Lv, and Lw are obtained first through calculation from the
corrected values of electric currents or otherwise by referring to a
pre-stored table. The electrical angle .theta. corresponding to the three
inductances Lu, Lv, and Lw is then read from the other table 122B stored
in the memory 122. In accordance with a concrete procedure, the three
calculated inductances Lu, Lv, and Lw are compared with inductance data
Lu0, Lv0, and Lw0 of the respective phases stored in the table 122B, and
an electrical angle .theta.0 giving a least sum of the differences is read
from the table 122B. It is a practical way to treat one piece of
inductance data as spare data, since all the three inductances may not
always be measured properly. By way of example, a certain combination of
calculated inductances Lu and Lv are compared with inductance data Lu0 and
Lv0 stored in the table, and the corresponding electrical angle .theta.0
is read upon condition that the sum of the differences is not greater than
a predetermined value. In case that the sum of the differences is greater
than the predetermined value, on the contrary, another combination of
calculated inductances Lv and Lw are compared with inductance data Lv0 and
Lw0.
The standard procedure calculates the inductances from the values of
electric currents of the respective phases and then determines the
electrical angle based on the obtained inductances. The procedure of this
embodiment, however, omits the process of calculating the inductances from
the observed values of electric currents and refers to the table 122B
stored in the memory 122, in order to directly read the electrical angle
corresponding to the values of electric currents. The processing of step
S270 in the flowchart of FIG. 11 compares the corrected values of electric
currents Iu, Iv, and Iw with electric current data Iu0, Iv0, and Iw0
stored in the table 122B, in order to determine the electrical angle.
As discussed above, the procedure of this embodiment does not require the
torque current to be returned to zero when the step voltage El is applied
to each phase. This shortens the time period required to prepare for
measurement as well as the time period required to return the electric
current to the original level after the termination of measurement. Namely
the procedure of the embodimen | | |
