The present invention relates to an electronically controllable brake booster. In particular, the present invention relates to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from each other by a movable wall, a control valve arrangement which can be actuated by means of an electromagnetic actuation means, and by means of which a pressure difference between the pressure chamber and the vacuum chamber can be adjusted, with the control valve arrangement, as a function of a current flowing through the electromagnetic actuation means, assuming a holding position in which the current ranges between a higher value and a lower value without the control valve arrangement leaving its holding position, a first pressure changing position in which the current is higher than the higher value, and a second pressure changing position in which the current is lower than the low value.
From DE 195 27 493 A1 an electromagnetic actuation means is known which comprises a solenoid coil which can be subjected to a control current and an armature which is associated with the solenoid coil and which is adapted to perform movements which are a function of a control current flowing through the solenoid coil and a spring arrangement which biases the armature in the opposite direction.
Therein, a holding position is defined as a manipulated variable, which the armature assumes at a holding current through the solenoid coil. This holding current flowing through the solenoid coil can be changed to a higher value or to a lower value without the armature leaving the holding position.
In addition, both the higher and the lower current value are dimensioned in such a manner that interfering influences on the magnetic and spring forces actuating the armature do not bring the armature into an actuated position which differs from the holding position.
For this purpose, the higher current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is increased by a predetermined current step in a stepwise manner and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the second actuated position.
The lower current value is determined in such a manner that a value which is related to the position of the armature in the holding position is determined; the control current is decreased by a predetermined current step in a stepwise manner, and output as a manipulated variable to the solenoid coil of the electromagnetic actuation means, until the value which is related to the position of the armature in the holding position changes by a predetermined value towards the first actuated position.
From this document is it also known that the control valve arrangement can reliably be brought into the holding position if the arithmetic mean value of the currents is selected for the holding current.
However, the xe2x80x9cLearning of the working pointsxe2x80x9d covered in DE 195 27 493 A1 is limited in that the decisive currents for the lower limit and the upper limit must be learned and stored at each commencement of a trip upon switching on the ignition or in periodic time intervals during driving. Upon controlling the control valve arrangement, the currents which have been learned in this manner are used for specifying the first manipulated variable in order to achieve an adequate control behavior.
However, the pressure difference acting upon the movable wall of the brake booster, which can be adjusted to different values depending upon the desired control, is not considered quantitatively.
The pressure difference adjusted at the movable wall is also acting immediately upon the valve body, the valve seat, and the valve element, which also leads to a shift of the currents for the lower limit and for the upper limit.
The invention deals with the problem which results from the shift of the upper and lower current values that are necessary in order to retain the control valve arrangement in its holding position.
It is the object of the present invention to eliminate this disadvantage so that the control behavior is further improved.
The invention thereby utilises, in particular, the finding that the pressure variation or the pressure difference, respectively, between the two chambers is a parameter of considerable influence on the behavior of the control valve arrangement in the operation of the brake booster. The behavior of the control valve arrangement, however, is also influenced by the actual area of the movable wall, because the force acting on the control valve arrangement is the product of the pressure difference at the movable wall and the area of the movable wall. Due to the fact that the area of the movable wall is subject to production-related variations and is also series-dependent, it is not possible, in particular for a fast control, to work with absolutely accurate approximate values.
As a solution, the invention proposes that a value be sensed in the pressure chamber at two different pressure levels, which is characteristic for the pressure difference between the vacuum chamber and the pressure chamber, with the control valve arrangement in each case being in the holding position and, starting from the holding position, the current through the electromagnetic actuation means in each case being modified in such a manner that the control valve arrangement obtains the first pressure changing position or the second pressure changing position, respectively, and with the current values for the changeover from the holding position into the first pressure changing position at a high pressure level, for the changeover from the holding position into the first pressure changing position at a low pressure level, and/or for the changeover from the holding position into the second pressure changing position at a high pressure level, and for the changeover from the holding position into the second pressure changing position at a low pressure level being evaluated in order to determine a characteristic for the operation of the brake booster, according to which the current values for the upper and lower limit of the holding position of the control valve arrangement at different pressure levels are adjusted.
In a further development of the invention, the determination of the current for the upper and lower limit for the holding position of the control valve arrangement can also be effected for more than two pressure levels. In this case, not only a linear interpolation is performed between the determined points. Rather, depending on the number of interpolation points, even more accurate adjustments of the current values between the minimum and the maximum pressure level can be made.
These additional interpolation points in the pressure level characteristic can be desirable because significant non-linearities can occur due to interfering influences, such as e.g. friction losses in the movement of the armature and the valve components coupled with same, tolerances of the first and second spring arrangement, temperature variations, or variations of external reaction forces which, in particular, include variations of the pressure force component in the vacuum chamber.
In a preferred embodiment, an axial displacement of the movable wall is effected by the pressure difference between the pressure chamber and the vacuum chamber, which drives a housing of the control valve arrangement.
In this specific configuration, the electromagnetic actuation means is arranged in the housing, which comprises a solenoid and an armature, with the armature being movable relative to the housing in the axial direction under the assistance of a first spring arrangement.
In the housing interior, a valve arrangement is formed which connects or separates the vacuum chamber and the pressure chamber, with a valve body of the valve arrangement being biased by a second spring arrangement towards a valve seat or a valve element, respectively, of the valve arrangement.
In a preferred embodiment of the invention, the hydraulic or pneumatic pressure generated in a master cylinder downstream of the brake booster is evaluated as the characteristic parameter for the pressure difference.
According to a particularly preferred embodiment, the first pressure changing position is a pressure build-up position and the second pressure changing position is a pressure relief position.
The following description in which reference is made to the accompanying drawings, serves to further explain characteristics, variation possibilities, and properties of the invention.