Brushless motor and driving control device therefor

Details Brushless motor and driving control device therefor Brushless motor and driving control device therefor

2001-03-22

2002-10-22

Ro, Bentsu (Department: 2837)

Electricity: motive power systems

Limitation of motor load, current, torque or force

C318S254100, C318S721000

Reexamination Certificate

active

06469463

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a brushless motor having plural excitation phases as well as a driving control device for the brushless motor and, more particularly, to a brushless motor suited to a drive source for an electrically-operated power steering system as well as a driving control device for such a brushless motor.
2. Description of the Related Art
Brushless motors used as drive sources for the power steering systems of automobiles are motors having three or more excitation phases, and are driven by means of excitation currents of rectangular waveforms.
For example, in the case of a 5-phase brushless motor, a motor driving circuit rotationally drives its rotor by exciting 5-phase excitation coils “a” to “e” hereinafter referred to also as “a-phase” to “e-phase”) by a rectangular wave current while switching the coils “a” to “e” sequentially from phase to phase by a 4-phase excitation method of simultaneously exciting four phases, under control of a control circuit such as a microcomputer, the 5-phase excitation coils “a” to “e” being disposed to surround the outer circumferential surface of the rotary element(rotor) of the motor in the state of being spaced apart by an electrical angle of 72 degrees. In the 4-phase excitation method, motor currents flow in four phases from among five phases, and the coil resistances of the respective excitation coils are formed to be all equal so that currents can flow in the respective phases with good balance.
Such a motor driving circuit is normally made of ten field effect transistors(FETs). Among these ten transistors, each pair of two corresponding transistors are connected in series to form five series transistor circuits, and each of the series transistor circuits is connected between the positive and negative terminals of a power source, and the connection between the two transistors of each of the series transistor circuits is connected to each of the five excitation coils “a” to “e” interconnected by a Y-shaped star connection, thereby being connected to the coil circuit of the motor.
The direction and length of an excitation current(rectangular wave) which is supplied to each of the excitation coils from the motor driving circuit are as shown in
FIG. 1
by way of example with respect to the rotational angle(electrical angle) of the rotor. Specifically, the excitation coils are switched sequentially from phase to phase by an electrical angle of 36 degrees, thereby exciting one phase coil through an electrical angle of 144 degrees to continuously rotate the rotor. In
FIG. 1
, letting &thgr; be the electrical angle, (1) to (10) denote respectively the following intervals: 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°.
In this example, the a-phase current flows in the plus direction through the intervals (1) and (2), then returns to “0” in the interval (3), then flows in the minus direction through the intervals (4) to (7), then returns to “0” in the interval (8), and again flows in the plus direction through the intervals (9) and (10) and back in the interval (1). The b-phase current flows in the plus direction through the intervals (1) and (4), then returns to “0” in the interval (5), then flows in the minus direction through the intervals (6) to (9), then returns to “0”, in the interval (10), and again flows in the plus direction in the interval (1). The c-phase current flows in the minus direction in the interval (1), then returns to “0” in the interval (2), then flows in the plus direction through the intervals (3) to (6), then returns to “0” in the interval (7), and again flows in the plus direction through the intervals (8) to (10) and back in the interval (1). The d-phase current flows in the minus direction through the intervals (1) to (3), then returns to “0” in the interval (4), then flows in the plus direction through the intervals (5) to (8), then returns to “0” in the interval (9), and again flows in the plus direction in the interval (10). The e-phase current remains “0” in the interval (1), then flows in the minus direction through the intervals (2) to (5), then returns to “0” in the interval (6), then flows in the plus direction through the intervals (7) to (10), and again returns to “0” in the interval (1). Accordingly, at the boundary between each of the intervals (1) and (10)(at the time of switching performed every 36 degrees in electrical angle), two of the five excitation coils are switched in the mutually opposite directions.
This switching of such an excitation current is in principle represented by the rise or the fall of a rectangular wave as shown in FIG.
1
. However, actually, the waveform of the rise or the fall does not change perpendicularly to the horizontal axis and a certain period of time &Dgr;t(about three times the time constant of the motor circuit) is taken until the excitation current rises in the plus direction or falls in the minus direction.
For example, at the boundary between the intervals (8) and (9) of FIG.
1
(288 degrees in electrical angle), the a-phase current rises from “0” to a plus constant value, while the d-phase current falls from the plus constant value to “0”, and the b-phase current and the c-phase current remain at the minus constant value with the e-phase current remaining at the plus constant value.
FIG. 2
shows on an enlarged scale the variations in the waveforms at this boundary.
Specifically, the a-phase rise current gradually increases from “0” to the plus constant value during the time &Dgr;t, while the d-phase fall current decreases from the plus constant value to “0” during time &Dgr;t
1
shorter than the time &Dgr;t (smaller than the time constant of the motor circuit). During this time, the other three phases “b”, “c” and “e” remain unswitched. Letting i
a
, i
b
, i
c
, i
d
and i
e
represent respectively the five phase currents, the relationship of the following expression (1) is established among these currents:
i
a
+i
d
+i
c
=−(
i
b
+i
c
)=
I
  (1)
Accordingly, as the a- and d-phase currents vary as described above, the b-, c- and e-phase currents also vary. In other words, since the a-phase current and the d-phase current differ in current variation rate, the total value of these two phase currents does not become a steady value and the b- and c-phase currents vary as shown in
FIG. 2
, so that the e-phase current also varies during the time &Dgr;t. These current variations cause transient torque variations.
The reason why the current variation rates of two phase currents differ between their rises as well as their falls as described above is as follows.
Let “Vb” denote a power source voltage to be supplied to the motor driving circuit, and “Vn” denote a voltage provided at the central connection point of the star-connected excitation coils “a” to “e”. In addition, let (1) and (2) in
FIG. 2
denote the interval of the time &Dgr;t
1
and the interval of time &Dgr;t
2
(=&Dgr;t−&Dgr;t
1
), respectively.
In the interval (1), the d-phase(OFF-phase) current i
d
, which is switched from plus to “0”, lowers to zero(0) from half(I/2) of an energization current I supplied to the motor from the motor driving circuit, at a variation rate according to a voltage −Vn, a counter-electromotive voltage E
d
of the coil and the time constant of the motor circuit. At this time, letting V
OFF
denote a voltage to be applied to the OFF-phase equivalent circuit, V
OFF
=−Vn−Ed<0, and Vn approximates Vb/2. On the other hand, the a-phase(ON-phase) current i
a
, which is switched from “0” to plus, rises from zero(0) at a variation rate according to the voltages Vb and −Vn, a counter-electromotive voltage E
a
of the coil and the time constant of the motor circuit. At this time, letting “V
ON
” denote a voltage to be applied to the ON-pha

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