Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2000-08-17
2002-04-30
Masih, Karen (Department: 3619)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S800000, C318S805000, C318S798000, C318S812000, C318S606000, C318S438000
Reexamination Certificate
active
06380706
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electric power steering device, assisting a steering force applied by a driver of an automobile and so on by a motor.
2. Discussion of Background
Various methods of protecting an electric power steering device assisting a steering force, applied by a driver, by a motor are conventionally devised.
FIGS. 16 and 17
illustrate a conventional electric power steering device disclosed in Japanese Utility Model No. 2586020.
FIG. 16
is a control block chart illustrating the conventional electric power steering device.
In
FIG. 16
, numerical reference
1
designates a torque sensor detecting a steering force, applied by a driver; numerical reference
2
designates a speed sensor detecting a speed of a vehicle; numerical reference
3
designates a microprocessor; numerical reference
4
designates a motor driving circuit; numerical reference
5
designates a motor, driven by the motor driving circuit
4
to generate a steering assisting force; and numerical reference
6
designates a motor current detecting means detecting a current flowing through the motor
5
. Numerical reference
31
designates a steering force assisting current setting means determining the motor current in order to reduce the steering force by the driver; numerical reference
32
designates an inertia compensating current setting means determining the motor current in order to reduce an influence of a moment of inertia of the motor; and numerical reference
33
designates an upper limit motor current setting means determining an upper limit of the motor current in order to protect the motor driving circuit
4
from overheat and to maintain the motor current, wherein numerical references
31
through
33
are realized by a software in the microprocessor
3
.
FIG. 17
illustrates the upper limit motor current of the conventional electric power steering device.
Next, an operation of the conventional electric power steering device will be described.
When the driver steers a steering wheel, a steering force is detected by a torque sensor
1
, and a signal is inputted in the microprocessor
3
. The microprocessor
3
sets the steering force assisting current in the steering force assisting current setting means
31
to obtain an appropriate steering force based on a vehicle speed detected by the vehicle speed sensor
2
and the steering force. Further, the inertia compensating current is set by the inertia compensating current setting means
32
in order to reduce an influence of the moment of inertia of the motor and to improve a steering feeling. The steering force assisting current is limited to be the upper limit value or less, wherein the upper limit value is determined in accordance with a characteristic illustrated in
FIG. 17
in response to an integrated value of the motor current, detected by the motor current detecting means
6
, squared. Thus limited steering force assisting current and the inertia compensating current are added and fed for a feedback control so that the added value and the detected value of the motor current by the motor current detecting means
6
match. The motor
5
is driven by the motor driving circuit
4
.
In the conventional electric power steering device, a squared value of the current has a close relationship with a calorific value and is appropriate for an index of overheat protection. The upper limit of the motor current is determined in response to the integrated value of the motor current squared in the motor current upper limit value setting means
33
. However, a loss in heat generating portions of the motor and the controller is analoguous to a power function of the current, and an exponent of the power function is between the first power and the second power. Accordingly, especially in a large current range, when the overheat protection is conducted using the index of the current squared, there is a problem that an overheat is excessively protected. As a result, in case of parking a vehicle in a garage located in a narrow parking area by stationarily steering the steering wheel, there are problems that the steering assisting force becomes small, and the steering force by the driver is increased.
Another conventional device, which determines an upper limit of a motor current in response to an integrated value of the motor current to the first power, is also known. In this case, as disclosed in Japanese Utility Model No. 2586020, the upper limit value is not rational, and it is necessary to design the motor driving circuit
4
with a margin.
Hereinbelow, another conventional device will be described with reference to the figures.
FIG. 18
illustrates an equivalent circuit of a generally used d.c. motor.
In
FIG. 18
, numerical reference
7
designates a resistance of an armateur; numerical reference
8
designates an inductance of the armateur; and numerical reference
9
designates a resistance of a brush.
FIG. 19
illustrates a voltage drop in the brush of the d.c. motor illustrated in FIG.
18
.
In
FIG. 18
, provided that the motor current is represented by Im, and the voltage drop in the brush is represented by Vbr, a copper loss Pm of the motor is expressed by the following equation.
Pm=Ra*Im
2
+Vbr*Im, (Equation 1)
where
Pm denotes the copper loss of the motor (W);
Ra denotes the resistance of the armateur (&OHgr;);
Im denotes the current of the armateur (A); and
Vbr denotes the voltage drop in the brush (V).
As illustrated in
FIG. 19
, the voltage drop Vbr in the brush increases as the current Im of the armateur increases. When the current Im of the armateur becomes a predetermined value Im
1
or more, the voltage drop is saturated at a predetermined value Vbr
1
. In a large current range that current of armateur Im> predetermined value Im
1
, where a heat from the motor causes problems, the voltage drop Vbr in the brush becomes constant irrespective of the current Im of the armateur.
From FIG.
19
and Equation 1, it is possible to regard the copper loss Pm of the motor a sum of a term in proportion to the current Im squared of the amateur and a term in proportion to the current Im of the armateur to the first power. Therefore, the copper loss Pm of the motor is a power function of the current Im of the armateur as follows.
Pm≈C
1
*Im
n1
, (Equation 2)
Where
1≦n
1
≦2, and
C
1
denotes an arbitrary constant.
Thus the copper loss Pm of the motor is analoguous to Equation 2.
FIG. 20
illustrates the motor driving circuit of a conventional electric power steering controller.
In
FIG. 20
, numerical reference
4
designates a motor driving circuit composed of MOSFET Q
1
through Q
4
; numerical reference
5
designates a motor; and numerical reference
10
designates a battery.
FIG. 21
is a graph illustrating a waveform of a motor current of the motor driving circuit illustrated in
FIG. 20
, wherein MOSFET Q
1
and Q
4
are driven for PWM, and MOSFET Q
2
and Q
3
are turned off.
FIG. 22
illustrates a voltage drop of a parasitic diode MOSFET of the motor driving circuit in the conventional electric power steering device.
Next, an operation of the motor driving circuit illustrated in
FIG. 20
will be described. In a duration that MOSFET Q
1
and Q
4
are turned on, the motor current flows through a passage I
1
. In a duration that MOSFET Q
1
and Q
4
are turned off, parasitic diodes of MOSFET Q
2
and Q
3
are turned on, whereby the motor current flows through a passage I
2
. Provided that losses of MOSFET Q
1
through Q
4
respectively are P
1
through P
4
, and a switching loss is ignored, a loss Pd of the motor driving circuit
4
is expressed by following equations.
Pd=P
1
+P
2
+P
3
+P
4
(Equation 3)
P
1
=P
4
=&agr;*Ron*Im
2
(Equation 4)
P
2
=P
3
=(1−&agr;)VF*Im (Equation 5)
where
Pd denotes a loss (W) without the switching loss of the motor driving circuit;
P
1
denotes the loss (W) without the switching loss of MOSFET Q
1
;
P
2
Kifuku Takayuki
Nishimura Hiroshi
Ohmae Katsuhiko
Yamamoto Munenori
Masih Karen
Mitsubishi Denki & Kabushiki Kaisha
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