Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Patent
1995-10-12
1998-06-02
Nguyen, Tan Q.
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
701 42, 180442, 180443, 180446, B62D 506
Patent
active
057616272
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention pertains to a power-assisted steering system with a servo-valve that is open at its center and serves for supplying an actuator element with hydraulic pressure, and with a pump that is driven by an electric motor and serves for supplying the servo-valve with hydraulic fluid, with the power consumption of the electric motor being controlled by an electronic unit as a result of the comparison between operating parameters that are monitored by sensors and fixed reference values.
One known type of servo-valve that is utilized in power-assisted steering systems is the servo-valve with a so-called open center. In the neutral position of a servo-valve with an open center, hydraulic fluid is pumped at low pressure through the open valve into a reservoir by means of a pump. When turning the input shaft of the servo-valve relative to the valve sleeve, hydraulic pressure is supplied to a hydraulic motor such that the movement of the steering rack of the steering system is assisted in one of the two possible directions. In these known servo-valves with an open center, the pump for supplying hydraulic fluid to the servo-valve is driven by the motor of the vehicle via a V-belt. One disadvantage of these known servo-valves with an open center is that a continuous, strong flow of the hydraulic medium is maintained even in the neutral position of the servo-valve because the pump is directly connected to the motor of the motor vehicle. Consequently, some of the power generated by the motor is lost due to the dynamic pressure and the volume flow. One additional disadvantage of known power-assisted steering systems is that the power-assistance of the steering system is deactivated as soon as the motor of the motor vehicle is turned off, e.g., when towing a stranded vehicle.
DE-OS 3,622,217 discloses a power-assisted steering system in which an electric motor is used for driving the pump. In this known power-assisted steering system, the hydraulic pressure is monitored by means of a pressure sensor, with the pump drive motor being switched into at least one of three positions, namely on, minimum and maximum, in dependence on the hydraulic pressure.
One additional power-assisted steering system of this type is known from DE-OS 3,920,862. This system also utilizes an electric motor for driving the pump, with said electric motor being additionally controlled by a signal that depends on the speed of the motor vehicle.
These known power-assisted steering systems are based on the idea that the electric motor only needs to be supplied with a minimum of energy as long as the servo-valve is situated in the neutral position. In this condition, the electric motor only need drive the pump such that a minimum hydraulic pressure is maintained. When turning the steering wheel, a higher hydraulic pressure is required, i.e., the electric motor is switched into an operating state of higher power consumption. In this case, it is, in principle, desired that this switching take place in dependence on an input torque that is applied to the input shaft of the servo-valve. In order to realize this with respect to the switching technology, it is, for example, possible to measure the power consumption of the electric motor, which represents a measure of the required hydraulic pressure. Below a certain nominal value of the hydraulic pressure, the electric motor operates in the lower power consumption range, with the switching into the higher power consumption range taking place once the nominal pressure value has been reached.
However, all these known power-assisted steering systems have the disadvantage that a torque-dependent switching point cannot be insured. The progression of the required pressure as a function of the input torque has the shape of a parabola. It is known that this parabola which normally-intersects the pressure axis initially shows only a flat increase below a certain limiting pressure. Slight differences between such systems, e.g., due to manufacturing tolerances or wear, can already infl
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Seidel Mathias
Vile David
Nguyen Tan Q.
TRW Fahrewrksysteme GmbH & Co. KG
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