Electricity: motive power systems – Constant motor current – load and/or torque control
Reexamination Certificate
1998-03-19
2001-06-12
Martin, David S. (Department: 2837)
Electricity: motive power systems
Constant motor current, load and/or torque control
C318S434000, C701S041000, C180S443000
Reexamination Certificate
active
06246197
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an electric power steering controller for use in automobiles.
2. Description of the Prior Art
It is known that, as shown in the following equation (1), when a voltage Va is applied to the coil of a DC motor having a brush, a counter electromotive voltage Ve is produced, a voltage obtained by subtracting the counter electromotive voltage Ve from the application voltage Va to the coil generates a coil current, thereby causing a voltage drop Vc in the coil, and, as shown in the following equation (2), the rotation speed of the motor is proportional to the counter electromotive voltage. Conversely speaking, the counter electromotive voltage is the difference between the application voltage to the coil of the motor and the voltage reduced by the coil. The voltage drop in the coil is expressed by the total of a voltage drop caused by coil resistance and a voltage drop caused by inductance which is proportional to a differential value of current as shown in the following equation (3). The frequency band at which the coil inductance appears as a voltage drop is high and can be generally expressed by the product of a coil resistance value and a coil current as shown in the following equation (3′) at a steering frequency range by a car driver except a current response.
Therefore, the rotation speed of the motor can be expressed by the following four equations.
Vc=Va−Ve
(1)
Ve=Ke·w
(2)
Vc=Ra·Ia+La·
(
dIa/dt
) (3)
Vc=Ra·Ia
(3′)
w=
(
Va−Ra·Ia
)/
Ke
(4)
In the above equations, Ve is a counter electromotive voltage, Va an application voltage to the coil, Vc a voltage drop in the coil, Ke a counter electromotive voltage constant, w a rotation speed of the motor, Ra a coil resistance, La a coil inductance and Ia a coil current.
For example,
FIG. 28
is a flow chart showing a method for detecting the rotation speed of a motor in a conventional electric power steering controller disclosed in Japanese Laid-open Patent Application No. Hei 8-175404. In
FIG. 28
, steps S
1
to S
5
are algorithm steps for detecting the rotation speed of the motor.
A description is subsequently given of the method for detecting the rotation speed of the motor in the conventional electric power steering controller.
In the conventional rotation speed detection method, a detection value of voltage applied to both terminals of the motor and a current value running through the coil are first read in step S
1
. In the subsequent step S
2
, a coil resistance equivalent value Rac and a counter electromotive voltage constant equivalent value Kec prestored in a ROM are read. In the next step S
3
, a rotation speed estimation value correction gain K
1
is determined based on the current value running through the coil with reference to a table prestored in the ROM. In the next step S
4
, a motor rotation speed estimation value west is calculated based on the following equation (5).
west=
K
1
·(
Vt
—
sns−Isns×Rac
)/
Kec
(5)
wherein west is a motor rotation speed estimation value, Vt_sns is a inter-terminal voltage measurement value, Isns a coil current measurement value, Rac a coil resistance equivalent value, Kec a counter electromotive voltage constant equivalent value, and K
1
a motor rotation speed estimation value correction gain.
In the subsequent step S
5
, the calculated motor rotation speed estimation value west is stored in a RAM. The above procedure is repeated to calculate the motor rotation speed estimation value west each time sampling is made for digital control.
The above rotation speed estimation correction gain K
1
is a coefficient which is set to be “1” when the coil current is small and approximate “0” as the coil current increases and which is used to prevent the influence of an error of a coil resistance value or an error caused by temperature variations from growing along with an increase in current.
Although the inter-terminal voltage Vt of the motor is handled as the application voltage Va to the coil in the prior art described above, the actual application voltage Va to the coil is obtained by subtracting a voltage drop Vdrop between a brush and a commutator from an inter-terminal voltage Vt as shown in the following equation (6).
Va=Vt−V
drop (6)
wherein Vt is an inter-terminal voltage and Vdrop is a voltage drop between the brush and the commutator.
It is known that the voltage drop Vdrop which is a function of a coil current as shown in
FIG. 27
changes according to the coil current in an extremely narrow range and changes its direction according to the direction of the coil current in a normal operation range. Since this voltage drop Vdrop is not taken into account in the prior art, an almost fixed error is always produced in the motor rotation speed estimation value west. Particularly, in a low rotation speed range, the influence of the error becomes relatively large and the accuracy of the motor rotation speed estimation value west lowers. Therefore, when control is made based on this rotation speed estimation value west, if the motor rotation speed is low and the steering torque of a driver is small, for example, when driving straight, the electric power steering controller oscillates and the steering wheel vibrates by itself though the driver does not steer a vehicle.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above problem of the prior art and it is an object of the present invention to provide an electric power steering controller which estimates the rotation speed of the motor for assisting a steering torque with accuracy and always enable stable operation regardless of the running state of a vehicle.
According to a first aspect of the present invention, there is provided an electric power steering controller in which voltage detection means outputs as an application voltage estimation value a value obtained by subtracting a correction voltage obtained based on a detection current value detected by current detection means from the inter-terminal voltage measurement value of a motor and motor rotation speed estimation means estimates a motor rotation speed based on the application voltage estimation value from the voltage detection means and the detection current value from the current detection means.
According to a second aspect of the present invention, there is provided an electric power steering controller in which a correction voltage at the voltage detection means is obtained with reference to voltage drop characteristics between the brush and commutator of the motor.
According to a third aspect of the present invention, there is provided an electric power steering controller in which a correction voltage at the voltage detection means is a value obtained by multiplying a voltage drop between the brush and commutator of the motor by a coefficient smaller than 1.
According to a fourth aspect of the present invention, there is provided an electric power steering controller in which a control unit calculates a target current using a differential signal obtained by differentiating the output of rotation speed estimation means.
According to a fifth aspect of the present invention, there is provided an electric power steering controller in which a correction voltage at the voltage detection means is a function whose upper limit is the maximum voltage drop value between the brush and commutator of the motor or a value obtained by multiplying it by a coefficient smaller than 1 and which is substantially proportional to the output of the current detection means below the upper limit and is saturated at the upper limit to become constant.
According to a sixth aspect of the present invention, there is provided an electric power steering controller in which a correction voltage at the voltage detection means is output by a relay circuit which is switched only by the sign of the output of the current detection
Inoue Noriyuki
Kifuku Takayuki
Kurishige Masahiko
Wada Shunichi
Yamamoto Munenori
Martin David S.
Mitsubishi Denki & Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
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