Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
1999-04-13
2001-02-27
Zanelli, Michael J. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C180S422000
Reexamination Certificate
active
06195601
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for controlling the valve characteristic of a motor vehicle's power assisted steering system driven by an electric motor and hydraulic pump as a function of the current consumption of the electric motor.
BACKGROUND OF THE INVENTION
Electrohydraulic power-assisted steering systems of the generic class are well known. A servo steering valve is directly or indirectly coupled with a vehicle steering wheel and put under hydraulic pressure. On torsion, the hydraulic pressure is guided to one or the other side of a steering gear and provides steering power assistance. The hydraulic pressure is produced by a hydraulic pump, which is driven by an electric motor.
In principle, it is desirable in power-assisted steering systems to adjust the steering behavior as a function of the vehicle speed. Prior art describes methods and devices for selectively controlling the steering behavior of power-assisted steering systems. For example, methods are known for controlling the valve characteristic as a function of the steering rate in that the volumetric flow of the hydraulic pump to the steering valve is selectively varied. The reduced volumetric flow can be adjusted, for example, by an interior bypass valve that opens a bypass from the delivery side to the return flow side with increasing speed of the hydraulic pump.
For power-assisted steering systems of the aforementioned type the course of the servo assistance has a narrow valve characteristic for high volumetric flow rates. The driver then finds the power-assisted steering easy. At smaller volumetric flow rates, servo assistance is reduced and the driver finds the steering stiff.
Varying the displaced volume in order to control the valve characteristic is very simple to implement in electrohydraulic power-assisted steering systems. Changing the motor speed also changes the speed of the hydraulic pump, which at low speeds provides low and at high speeds high volumetric flow rates.
The speed of electric motors is changed simply by changing the input voltage. At constant vehicle voltage, for example, in the automobile industry, the motor speed is adjusted by pulse width modulation. Pulse width modulation produces small current pulses, whereby the length of the current pulses defines the motor speed.
Control systems of the described generic class are a more economical alternative to the control of valve characteristics. The driver, however, perceives the increase in the response threshold as a disadvantage. The response threshold defines the amount of torque at which servo assistance begins. This response threshold, also referred to as dead band, increases with smaller volumetric flow and thus impairs the steering precision of the steering assembly. Within the dead band, the driver is steering without any servo assistance only via the elastic torque rod. This decisively impairs the steering precision of the steering assembly. Desirable is a characteristic like the one known, for example, in hydraulic reaction systems, since the dead band of the valve characteristics in these power-assisted steering systems is kept approximately constant.
For example, in steering systems with hydraulic reaction, the steering characteristic of the steering valve is typically controlled by returning the existing operating pressure to a hydraulic-mechanical converter in the steering valve. Such steering assemblies advantageously exhibit a linear increase in servo assistance as a function of the input torque applied by the driver.
Furthermore, using an electrohydraulic converter makes it possible to set a speed-dependent valve characteristic. Such power-assisted steering systems are distinguished by light servo assistance at high speeds. Due to an increase in torsional rigidity, i.e. reduced relative torsion of the steering edges at a given torque, produced by the hydraulic-mechanical converter, the driver perceives steering stiffness. In the parking range, however, the driver finds steering advantageously easy.
The disadvantage of such steering assemblies with hydraulic reaction is their increased price compared to conventional power-assisted steering due to the required additional components such as the electrohydraulic converter, which is controlled by an electric control unit ECU. In addition, the steering valve design becomes more complex and expensive due to the hydraulic-mechanical converter. The economical standard valve of a conventional assisted-power steering system can no longer be used.
Based on the described prior art, the object of the present invention is to define a method that provides an economical way to control the valve characteristic in electrohydraulic power-assisted steering systems without impairing steering precision.
SUMMARY OF THE INVENTION
To attain this object, the invention proposes continuously to measure the actual current consumption and the actual speed of the electric motor up to a maximum speed and to compare these values with the values for the setpoint speed for the respective current consumption stored in a memory and in case of deviation to correct to the setpoint speed.
The method according to the invention is highly flexible since selecting the course of the setpoint speed over the current consumption makes it possible to set nearly all the most common valve characteristics following a minor adjustment of the control edges. The elimination of the components required for electrohydraulic and hydraulic-mechanical converters furthermore provides significant space and cost savings.
According to one proposal of the invention, the values for the setpoint speeds and the current consumption are stored in an electronic memory.
Servo assistance and the valve characteristic are dependent on the respective volumetric flow of the hydraulic pump at any rotation of the steering valve. The delivered volumetric flow results from the speed of the hydraulic pump, which corresponds to the driving speed of the motor. If the driver turns the steering valve, the pressure increases, which causes the current consumption of the motor to increase. The current consumption is determined by the ECU, for example, via the voltage drop of a resistor.
The characteristic of an electric motor typically shows a non-controlled constant or falling course of the speed over the driving torque. The driving torque and the current consumption of the electric motor thereby increase approximately proportionally with the hydraulic pressure delivered by the hydraulic pump. This relation results in a constant or falling course of the speed of the electric motor as a function of the hydraulic pressure.
The method according to the invention describes a speed control as a function of the current consumption of the electric motor such that the actual current consumption and the actual speed of the electric motor are continuously measured up to a maximum speed and compared with the values for the setpoint speed at the respective value of the current consumption stored in a memory and in case of deviation a correction to the setpoint speed is effected.
According to a further proposal of the invention, the speed of the electric motor is changed by a pulse width modulation process.
According to another proposal of the invention, the speed of the electric motor is controlled incrementally.
Through speed control and selection of the setpoint speeds, additional speed characteristics of the electric motor beyond prior art can be represented as a function of the driving torque, which makes it possible selectively to control the valve characteristic. The setpoint speed is selected based on a calculation of the deviation of an actual valve characteristic, which is established without speed control, compared to a setpoint valve characteristic.
For each rotation of the steering valve there is a restrictor cross section, which produces a defined hydraulic pressure at a given volumetric flow, i.e. at a given speed of the electric motor. The difference of the hydraulic pressures of an actual valve characteristic and a setpoint
Seidel Mathias
Spillner Robert
Gibson Eric M
Tarolli, Sundheim, Covell Tummino & Szabo L.L.P.
TRW Fabrwerksysteme GmbH & Co. KG
Zanelli Michael J.
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