Electricity: motive power systems – Positional servo systems – Pulse-width modulated power input to motor
Reexamination Certificate
2000-02-09
2002-06-04
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
Pulse-width modulated power input to motor
C318S132000, C318S139000, C318S245000, C318S254100, C318S434000, C318S700000, C318S800000, C318S805000, C318S811000
Reexamination Certificate
active
06400116
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a motor drive control system suitable for use for controlling the drive of a motor, such as a brushless motor or a linear motor, each of which has a plurality of excitation phases, through the use of rectangular waves.
BACKGROUND ART
For example, a brushless motor, which has been employed as a drive source for a power steering apparatus of a motor vehicle, is a motor having three or more excitation phases, with driving being performed by rectangular excitation currents.
In the case of a five-phase brushless motor, in general, a motor drive circuit is made to rotationally drive a rotor of this motor in a manner to cause excitation with rectangular currents while successively switching five-phase (a-phase, b-phase, c-phase, d-phase and e-phase) exciting coils a to e, disposed to surround an outer circumferential surface of the rotor in a state separated by an electrical angle of 72°, by phases according to a four-phase exciting method of performing the four-phase excitation simultaneously under control by a control circuit comprising a microcomputer or the like. In this four-phase exciting method, motor current always flows to four of five phases, while, for supplying the current to each of the phases in a well-balanced condition, all the resistances of the exciting coils are designed to be equal to each other. Incidentally, in the four-phase exciting method for the five-phase brushless motor, of the five phases, a phase to which the motor current flows is referred to as an “ON-phase”, and a phase to which it does not flow is called an “OFF-phase”.
Such a motor drive circuit is made up of 10 field effect transistors (FETs). These 10 transistors constitute 5 series transistor circuits, each of which is constructed by connecting two transistors, corresponding to each other, in series, and are connected between the positive and negative terminals of a power source. Further, the connecting parts of the two transistors of each of the series transistor circuits are coupled to the external terminals of 5 exciting coils “a” to “e”, connected in a Y-configuration, thus establishing a connection with a coil circuit of the motor.
For instance, the direction and length of an exciting current to be supplied from this motor drive circuit to each of the exciting coils is as shown in
FIG. 1
with respect to the value of a rotating angle (electrical angle) of the rotor. That is, the exciting coils are successively switched by phases at an interval of 36° in electrical angle, and each phase coil is excited for 14° in electrical angle so that the rotor rotates continuously. In
FIG. 1
, when the electrical angle is taken as &thgr;, the intervals of 0≦&thgr;<36°, 36°≦&thgr;<72°, 72°≦&thgr;<108°, 108°≦&thgr;<144°, 144°≦&thgr;<180°, 180°≦&thgr;<216°, 216°≦&thgr;<252°, 252°≦&thgr;<288°, 288°≦&thgr;<324° and 324°≦&thgr;<360° are expressed by (1), (2), . . . , (10).
In this instance, the a-phase current flows in the positive direction in the intervals (1) and (2), becomes “0” in the interval (3), flows in the negative direction in the intervals (4) to (7), becomes “0” in the interval (8), and again flows in the positive direction in the intervals (9), (10) and (1). The b-phase current flows in the positive direction in the intervals (1) to (4), becomes “0” in the interval (5), flows in the negative direction in the intervals (6) to (9), becomes “0”, and again flows in the positive direction in the interval (1). The c-phase current flows in the negative direction in the interval (1), becomes “0” in the interval (2), flows in the positive direction in the intervals (3) to (6), becomes “0” in the interval (7), and again flows in the negative direction in the intervals (8) to (10) and (1). The d-phase current flows in the negative direction in the intervals (1) to (3), becomes “0” in the interval (4), flows in the positive direction in the intervals (5) to (8), becomes “0” in the interval (9), and again flows in the negative direction from the interval (10). The e-phase current assumes “0” in the interval (1), flows in the positive direction in the intervals (2) to (5), becomes “0” in the interval (6), flows in the positive direction in the intervals (7) to (10), and again becomes “0” in the interval (1). Accordingly, at the boundary (the switching point at every 36° in electrical angle) of each of the intervals (1) to (10), two of five exciting coils are switched in opposite directions.
Although this switching of the excitation current can be expressed in principle by the leading edges and trailing edges of the rectangular waves as shown in
FIG. 1
, in fact the leading edges and the trailing edges do not vary at right angles with respect to its horizontal axis, and some amount of time &Dgr;t (approximately three times the time constant of the motor circuit) is needed until the exciting current rises in the positive direction or falls in the negative direction. For example, at the boundary (288° in electrical angle) between the interval (8) and the interval (9) in
FIG. 1
, the a-phase current rises from “0” to a positive constant value, while the d-phase current falls from a positive constant value to “0”, and both the b-phase and c-phase currents assume a negative constant value, and even the e-phase current is at positive constant value. The variations at these boundary portions are shown enlarged in FIG.
2
.
In detail, the a-phase rising (first transition) current increases gradually from “0” to the positive constant value for the time &Dgr;t, while the d-phase falling (last transition) current decreases from the positive constant value to “0” for time &Dgr;t
1
shorter than the time &Dgr;t (less than the time constant of the motor circuit). At this time, the other three phases, the b-phase, c-phase and e-phase are not intended to vary. When the five-phase currents are expressed with i
a
, i
b
, i
c
, i
d
and i
e
, the following relationship occurs among these currents.
i
a
+i
d
+i
e
=−(
i
b
+i
c
)=
I
(1)
Thus, when the a-phase and d-phase currents vary as mentioned above, the b-phase, c-phase and e-phase currents also vary. That is, since the rates of current change in the a-phase and the d-phase differ from each other, the sum of the two phase currents does not assume a constant value, and as a result of the variation of the b-phase and c-phase currents shown in
FIG. 2
, the e-phase current also varies for the aforesaid time &Dgr;t. This current variation causes a transient torque variation.
The above-mentioned difference between the rates of current change of the rise and fall of the two phase currents is based upon the following principle. First, let it be assumed that a power supply voltage to be given to the motor drive circuit is taken to be Vb and the voltage at the central connection point of the exciting coils “a” to “e” which are connected in a radiating arrangement is taken as Vn. Further, the interval of the time &Dgr;t
1
is indicated by {circle around (1)} and the interval of the time &Dgr;t
2
(=&Dgr;t−&Dgr;t
1
) is indicated by {circle around (2)}.
In the interval indicated by {circle around (1)}, the d-phase (OFF-phase) current i
d
, switching from the positive to “0”, falls from half (I/2) of energizing current I, to be supplied from the motor drive circuit to the motor, to “0” at a rate of change depending upon −Vn, a reverse electromotive voltage Ed of the coil and a time constant of the motor circuit. At this time, if a voltage to be applied to an OFF-phase equivalent circuit is taken to be V
OFF
, V
OFF
=−Vn−E
d
<0, and Vn becomes nearly Vb/2. On the other hand, the a-phase (ON-phase) current i
a
rises from “0” at a rate of change depending upon a voltage Vb, −Vn, a reverse electromotive voltage E
a
of the coil and a time constant of the motor circuit. At this time, if a voltage to be applied to an ON-phase equivalent circuit is taken to be V
ON
, V
ON
=Vb·Duty1 (a d
Chen Hui
Matsuda Kouji
Nappi Robert E.
NSK Ltd.
Smith Tyrone
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