Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
2001-12-27
2003-12-16
Chin, Gary (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C180S443000, C180S446000
Reexamination Certificate
active
06665599
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system for controlling electric power steering.
2. Description of Related Art
FIG. 6
shows a conventional system for controlling electric power steering.
FIG. 7
is a block diagram showing the control in the system for controlling the electric power steering.
As illustrated in
FIG. 6
, a pinion
2
is placed on one end of an input shaft
1
associated with a steering wheel W. A rack is provided on a rod
4
associated with wheels
3
R,
3
L at two end portions. The pinion
2
on the input shaft
1
engages the rack
5
on the rod
4
.
An electric motor S is associated with a reduction gear
7
having an output shaft on which a pinion is provided. The pinion also engages the rack
5
on the rod
4
. A controller CT is connected to steering-torque detection means
8
for detecting steering torque acting on the input shaft
1
.
As illustrated in
FIG. 7
, the controller CT includes basic assist command-value determination means
9
for determining a basic assist command-value in response to a steering torque signal from the above sensor
8
, and steering-torque differentiation command-value determination means
10
for differentiating the steering torque detection signal.
The controller CT further includes output control means
11
for controlling the outputs of the electric motor
6
. The output control means
11
includes motor-current amplification means
12
for amplifying an input signal, and digital-to-analog conversion means
13
for converting a digital signal into an analog signal. For example, the means
11
can be a pulse width modulation circuit made up of a combination of the motor-current amplification means
12
and the DA conversion means
13
.
In order to detect the outputs of the electric motor
6
, the output control means
11
has output wiring which is connected to motor current detection means
14
. In order to return a motor-current detection signal from the motor-current detection means
14
to the controller CT, the controller CT incorporates analog-to-digital conversion means
15
. In other words, the AD conversion means
15
converts the motor-current detection signal in analog form into a digital signal.
In this way, the motor-current detection signal is sent back to the controller CT under the feedback control.
With the above configuration, the control system operates for controlling the electric motor
6
as follows.
As illustrated in
FIG. 7
, the steering-torque detection signal from the steering-torque detection means
8
is inputted to the basic assist command-value determination means
9
. Then, the basic assist command-value determination means
9
outputs a basic assist command-value signal in accordance with the received steering-torque detection signal. The steering-torque detection signal is also inputted to the steering-torque differentiation command-value determination means
10
. Then, the steering-torque differentiation command-value determination means
10
outputs a steering-torque differentiation command-value signal in accordance with the received steering-torque detection signal. The steering-torque differentiation command-value signal is added to the above basic assist command-value signal to generate a motor current command-value signal.
The basic assist command-value signal and the steering-torque differentiation command-value signal are digital signals.
The motor current detection means
14
detects motor current outputted from the output control means
11
, and sends the detected motor-current detection signal back to the input of the output control means
11
via the AD conversion means
15
placed in the controller.
The motor current detection signal is added to the above motor current command-value signal to generate a motor current control signal. The motor current control signal is inputted to the motor current amplification means
12
in the output control means
11
. The motor current amplification means
12
amplifies the received motor current control signal, and then inputs the amplified motor current control signal to the DA conversion means
13
.
The DA conversion means
13
outputs motor current according to the amplified motor current control signal to the electric motor
6
.
In this way, the system for controlling the electric power steering controls the currents of the electric motor
6
in accordance with the steering-torque detection signal and the motor current detection signal.
In the above system for controlling the electric power steering, the motor current command-value signal is inputted in digital form to the controller CT. That is, the motor current detection signal is converted into a digital signal for addition to the motor current command-value signal.
Thus, it is necessary to convert the motor current detection signal from analog form into digital form. The AD conversion means
15
converts from an analog signal into a digital signal as follows.
FIG. 8
shows a graph illustrating the relationship between the input signal before the AD conversion and the output signal after AD conversion where the motor current detection signal is concerned. Specifically, in
FIG. 8
, the horizontal axis indicates the input signal of the motor current detection signal and the vertical axis indicates the output signal thereof.
The solid line A in
FIG. 8
represents the output when an analog signal represented by the two-dot chain line B is inputted to the AD conversion means
15
, and forms a staircase shape as illustrated in FIG.
8
. The reason why the output signal forms such a staircase shape will be described next.
The AD conversion means
15
includes a conversion circuit with 10-bit resolution. Specifically, the number of divisions of a digital signal is 2 raised to the tenth power=1024 bits. The motor current is detected in the range of from −80A to +80A in agreement with the output range of the motor current. Hence, the amount of change in the motor current per bit results in 160A/1024 bits=0.156A/bit.
Since the amount of change in the motor current per bit is 0.156A/bit, 1 bit is shifted every time the motor current detection signal as an input signal changes by 0.156 A. When 1 bit is shifted, the motor current detection signal output sharply increases by 0.156 A. In other words, if the motor current detection signal inputted in analog form continuously changes, the motor current detection signal outputted in digital form takes on discrete values only every 0.156 A.
The points of the sharp changing of the output signal, e.g., points a, band c in
FIG. 8
, are hereinafter referred to as “change points”.
The fact that the motor current detection signal after the AD conversion takes on discrete values for the amount of change as described above, may produce the following problems
When a vehicle travels straight ahead, the driver holds the steering wheel W around the middle position. At this point, the driver typically grips the steering wheel W loosely, and also, to be exact, he/she repeatedly turns the steering wheel W by extremely small degrees in a right or left direction. In other words, the driver oscillates the steering wheel win the right-left direction by extremely small degrees about the middle position.
Such turning of the steering wheel W by extremely small degrees as described above, causes the motor current detection signal to vary in amplitude between positive values and negative values about the zero point. If the above change point is within the amplitude range of the motor current detection signal, the output signal after the AD conversion repeats a sharp increase and decrease every time the motor current detection signal passes through the change point.
Further, as described above, the output signal after the AD conversion is added to the motor current command-value signal to form the motor current control signal. Hence, if the output signal after the AD conversion repeats a sharp increase and decrease, the motor current control signal also repeats a sharp increase and decrease. In
Ishikawa Hisazumi
Kogiso Yoshinori
Chin Gary
Kayaba Industry Co. Ltd.
Steinberg & Raskin, P.C.
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