Abstract:
Electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from one another by a moveable wall, a control valve arrangement, which can be actuated by means of an electromagnetic actuating device and via which a pressure difference between the pressure and the vacuum chamber can be set, wherein the control valve arrangement takes up a holding position, a pressure build-up position, a pressure reduction position in accordance with a current generated as correcting variable by an electronic control unit and flowing through the electromagnetic actuating device, a cylinder-piston arrangement, which is connected to the brake booster and comprises a piston, coupled to a brake pedal, of a hydraulic cylinder which has a hydraulic connection leading to a wheel brake, a sensor arrangement for detecting a signal which is correlated with the pressure of the hydraulic cylinder and which reproduces the controlled variable, wherein the electronic control unit establishes a command variable, taking account of the output signal of a sensor arrangement detecting the driver&#39;s braking requirement, and wherein the value of the correcting variable remains unchanged with respect to a value output directly beforehand if the control difference established from the command variable and the controlled variable does not deviate from a first predetermined tolerance range value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP99/10013 filed Dec. 16, 1999, which claims priority to German Patent Application No. 19858649.3 filed Dec. 18, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an electronically controllable brake booster. The invention relates in particular to an electronically controllable brake booster with a vacuum chamber and a pressure chamber, which are separated from one another by a moveable wall. A control valve arrangement can be actuated by means of an electromagnetic actuating device. A pressure difference between the pressure and the vacuum chamber can be set via the control valve arrangement. The control valve arrangement takes up a holding position, a pressure buildup position or a pressure reduction position in accordance with a current generated as correcting variable by an electronic control unit and flowing through the electromagnetic actuating device. A cylinder-piston arrangement is connected to the brake booster, this arrangement comprising a piston, coupled to a brake pedal, of a hydraulic cylinder which has a hydraulic connection leading to a wheel brake. A sensor arrangement serves to detect a signal which is correlated with the pressure of the hydraulic cylinder and which reproduces the controlled variable. A brake booster of this kind is described, for example, in DE 19527 493 A1, which is incorporated by reference herein, together with a learning process for the operating point setting. 
     The brake booster illustrated above is operated in a closed loop. This ensures that the electromagnetic actuating device is largely prevented from overshooting or undershooting. The control characteristic is subject to a variation range in the region of the two sloping branches due to interference effects caused, for example, by friction losses upon the armature and the valve components coupled thereto moving, or by tolerances of the spring arrangement, or by fluctuations of external reaction forces, which include in particular fluctuations in the compressive force component in the vacuum chamber when using a brake booster. This proves to be particularly problematic for an accurate setting of the “holding position”. If, for example, a change-over from “build-up” position to “holding position” is desired, it is possible for a change-over to occur immediately after “reduction position”, which results in an abrupt pressure drop which may have serious consequences, especially where safety-critical applications are concerned, as in the case of an electronically controlled brake booster. In order to counter this problem, according to-the teaching of this prior art, when changing over to “holding position” a current I 0  resulting as the arithmetic average value of the currents I 1  and I 2  is always set, as the arrangement is designed such that the lower limit of the variation range of the left-hand sloping branch and the upper limit of the variation range of the right-hand sloping branch do not overlap. 
     SUMMARY OF THE INVENTION 
     The present invention takes up the approach to a solution from DE 195 27 493 A1 and directly includes the driver&#39;s braking requirement in this concept. However the consideration of the pressure which is described in this prior art does not determine the object. 
     In order to solve this problem in a brake booster of the type initially described, a command variable is established by the electronic control unit, taking account of the output signal of a sensor arrangement detecting the driver&#39;s braking requirement, wherein the value of the correcting variable remains unchanged with respect to a value output directly beforehand if the control difference established from the command variable and the controlled variable does not deviate from a first predetermined tolerance range value. 
     The result of this measure is to “smooth” the actuation of the actuator without causing any impairment of the control quality. 
     In a preferred embodiment the value of the correcting variable remains unchanged with respect to a value output directly beforehand if a (the first or second) derivative of the control difference also does not deviate from a second predetermined tolerance range value. The control behaviour is also adapted to the dynamic behaviour of the overall arrangement by including the gradients of the control deviation. 
     Instead of using the derivative of the control difference, the value of the correcting variable may remain unchanged with respect to a value output directly beforehand if an intermediate variable established from a derivative of the command variable and a derivative of the controlled variable also does not deviate from a second predetermined tolerance range value. 
     In a preferred embodiment of the invention the control valve arrangement can be brought by a first current value of the correcting variable into the pressure build-up position, in which the connection between the vacuum chamber and the pressure chamber is blocked and the connection between the pressure chamber and the atmosphere is open, so that a pressure difference is built up or increased at the moveable wall, by a second current value of the correcting variable into the pressure holding position, in which the connection between the vacuum chamber and the pressure chamber is blocked and the connection between the pressure chamber and the atmosphere is blocked, so that a pressure difference acting at the moveable wall is maintained, or by a third current value of the correcting variable into the pressure reduction position, in which the connection between the vacuum chamber and the pressure chamber is open and the connection between the pressure chamber and the atmosphere is blocked, so that a pressure difference acting at the moveable wall is reduced by a pressure equalisation procedure. 
     According to the invention, the correcting variable assumes the first current value if the command variable is greater than the controlled variable, the second current value if the command variable is substantially equal to the controlled variable, and the third current value if the command variable is less than the controlled variable. 
     In a preferred embodiment of the invention an additional proportional and/or integral and/or differential controller serves to convert the control difference into an auxiliary correcting variable so as to superimpose this on the correcting variable, preferably in additive fashion 
     In a development a linear transmission member with a dead zone may be connected upstream of the additional proportional and/or integral and/or differential controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further properties, advantages, features and possible variations of the invention are illustrated on the basis of the following description of a currently preferred embodiment of the invention with reference to the drawings, in which 
     FIG. 1 shows in schematic form a vehicle brake system with an electronically controllable brake booster, 
     FIG. 2 shows in schematic form a graph for illustrating the control characteristic of an electronically controllable brake booster, 
     FIG. 3 shows in schematic form a loop for operating according to the invention an electronically controllable brake booster, 
     FIG. 4 shows in schematic form a flow diagram for illustrating the operation according to the invention of an electronically controllable brake booster, and 
     FIG. 5 shows in schematic form two time graphs for illustrating the operation according to the invention of an electronically controllable brake booster. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the vehicle brake system represented schematically in FIG. 1 a brake pedal  1  serves to actuate a brake pressure transmitter unit  2  via an actuating element. The brake pressure transmitter unit  2  comprises a brake cylinder  25 , in which a piston  28  defines a pressure chamber  29 . The pressure chamber  29  is supplied with brake fluid from a reservoir  27 . A brake line  3  leads from the pressure chamber  29  to a wheel brake  4  of the vehicle. 
     An anti-lock braking control device and/or traction control device ABS/TC is/are disposed in the brake line  3  between the brake pressure transmitter unit  2  and the wheel brake  4 . The anti-lock braking control device and/or traction control device ABS/TC comprise(s) in a known manner, inter alia, valve and pump arrangements which are activated by an electronic control unit ECU in order to modulate the pressure in the wheel brake  4 . This takes place in accordance with the rotational behaviour of a vehicle wheel which is associated with the wheel brake  4 , this behaviour being detected by means of a sensor  41  and fed to the electronic control unit ECU. 
     The brake pressure transmitter unit  2  comprises a brake booster  21  in order to boost the actuating force introduced by the driver via the brake pedal  1 . A moveable wall  22  divides the brake booster  21  into a vacuum chamber  23  and a pressure chamber  24 . The vacuum chamber is connected to a vacuum source Vac, which is not shown in detail, in order to produce the vacuum. The vacuum which is produced in the intake pipe in accordance with the principle is available for this in a vehicle equipped with an Otto engine. 
     However a vehicle driven by a diesel engine or an electric motor requires an additional vacuum pump as vacuum source Vac. Upon actuating the brake pedal  1 , the brake booster functions in a known manner through the admission of atmospheric pressure to the pressure chamber  24 , so that a pressure difference, which assists the actuating force introduced at the brake pedal  1 , acts at the moveable wall  22 . In the non-actuated state the vacuum chamber  23  and the pressure chamber  24  are connected together and therefore pressure-equalised, so that no pressure difference acts at the moveable wall  22 . 
     The brake booster  21  may also be controlled electronically through an electromagnet arrangement  26 . The possibility of controlling the brake booster  21  electronically also enables braking to be executed automatically, i.e. independently of any actuation of the brake pedal  1 . This may serve to execute, for example, anti-lock braking control, vehicle movement dynamics control or distance control. A sensor arrangement  11  is provided in order to detect variables which are related to the actuation of the brake pedal  1 , such as, e.g. the pedal travel, the pedal force or the pedal actuation speed, for evaluation in the electronic control unit ECU in order also to brake in emergency situations, in which case, for example, a value exceeding a certain pedal actuation speed serves as criterion. A desired pressure signal pSOLL is generated as command variable in the electronic control unit ECU from the signal(s) of one or more sensor(s). 
     The electromagnet arrangement  26  actuates a control valve, which is not shown in detail here, in order to bring the brake booster  21  into different control positions (I., II., III.): 
     into a first so-called “build-up position” (I.), in which the connection between the vacuum chamber  23  and the pressure chamber  24  is blocked and the connection between the pressure chamber  24  and the atmosphere is open, so that a pressure difference is built up or increased at the moveable wall  22 , 
     or into a second so-called “holding position” (II.), in which the connection between the vacuum chamber  23  and the pressure chamber  24  is blocked and the connection between the pressure chamber  24  and the atmosphere is blocked, so that a pressure difference acting at the moveable wall  22  is maintained, or 
     into a so-called “reduction position” (III.), in which the connection between the vacuum chamber  23  and the pressure chamber  24  is open and the connection between the pressure chamber  24  and the atmosphere is blocked, so that a pressure difference acting at the moveable wall  22  is reduced by a pressure equalisation procedure. 
     In order to bring the control valve into the different control positions (I., II., III.) for the purpose of modulating the pressure difference at the movable wall  22 , the electronic control unit ECU passes a control current I TARGET , which forms the correcting variable, through the electromagnet arrangement  26 , with the variation in the control current I TARGET  being effected, for example, by pulse width modulation. A magnetic force is in the process exerted on the armature of the electromagnet arrangement  26 , which force brings the armature into a position to which the resulting control positions (I., II., III.) correspond. 
     The brake pressure p ACTUAL  which is produced in the pressure chamber  29  of the brake cylinder  25  and introduced into the brake line  3  represents the controlled variable and is detected by means of a sensor  31  and transmitted to the electronic control unit ECU in order to control the brake pressure p ACTUAL  in accordance with a desired pressure value and/or pressure variation p TARGET  by adjusting the control current I TARGET  flowing through the electromagnet arrangement  26 . 
     The travel is plotted along the abscissa and the magnetic force which is exerted on the armature of the electromagnet arrangement  26  and is produced in accordance with the control current I TARGET  along the ordinate in the graph according to FIG.  2 . This is an idealised schematic representation relating to an operating range which is designed such that there is a proportional interrelationship between magnetic force and control current. The control characteristic of the electronically controllable brake booster  21  is also plotted. This control characteristic has a total of three branches. A current range I REDUCTION &lt;I TARGET &lt;I BUILD-UP  is associated with a certain armature position x0 in the case of the vertical branch, the position x0 corresponding exactly to the holding position (II.). The sloping branch adjoining the vertical branch to the left applies to a current I TARGET &gt;I BUILD-UP  and represents the build-up position (I.), while the sloping branch extending from the vertical branch to the right applies to a current I TARGET &lt;I REDUCTION  and characterises the reduction position (III.). 
     Due to interference effects caused by, for example, friction losses, tolerances, temperature fluctuations or fluctuations of external reaction forces, which include in particular fluctuations in the compressive force component in the vacuum chamber  23  of the brake booster  21 , the control characteristic is subject to a variation range in the region of the sloping branches, which leads to a displacement of the operating points I REDUCTION  and I BUILD-UP . In order to counter this problem, a current resulting as the arithmetic average value of the currents I REDUCTION  and I BUILD-UP  is preferably set for I HOLD . This and, in particular, a process for learning the currents I REDUCTION  and I BUILD-UP  determining the operating points are described in DE 195 27 493 A1. 
     The operation of the electronically controllable brake booster  21  in a closed loop is represented in FIG.  3 . Here a controlled variable, the brake pressure p ACTUAL  produced in the brake cylinder  25 , which originates from the controlled member, the brake pressure transmitter unit  2 , is continuously detected and compared with a command variable, the desired pressure value p TARGET . The result of this comparison is a control difference xd, which is fed to a control device. The correcting variable which is output by the control device is the control current I TARGET  flowing through the electromagnet arrangement  26 . The interference variables z are predominantly the above-mentioned effects caused by friction, tolerances, temperature and reaction forces. 
     The control device in the first place comprises a three-stage controller RO, which—in accordance with the control difference xd—initially carries out a “coarse setting or presetting” of the correcting variable I TARGET , according to which I TARGET  is set equal to I BUILD-UP  if p TARGET  is greater than p ACTUAL , I TARGET  is set equal to I HOLD  if p TARGET  is (substantially) equal to p ACTUAL , I TARGET  is set equal to I REDUCTION  if p TARGET  is less than p ACTUAL . 
     The control difference xd is in each case compared with a threshold value xs according to the weighting functions represented in FIG.  3 . The control device also comprises a controller R of the analogue or digital type which, for example, exhibits proportional and/or integral and/or differential control behaviour. The controller R is connected in parallel with the three-stage controller RO in order to carry out the “fine control” of the correcting variable I TARGET . A linear transmission member with a dead zone TZ is connected upstream of the controller R, which is preferably in the form of a P 1  controller. The dead zone TZ is dimensioned such that a control difference not exceeding the holding current is removed, while a control difference corresponding to the build-up or reduction current is fed unchanged to the controller R. 
     According to the invention a postprocessor L is connected between the control device RO, R and the controlled member  2 . The postprocessor L adapts the correcting variable I TARGET  to the dynamic behaviour of the command variable p TARGET  and the controlled variable p ACTUAL . For this purpose the gradient of the control difference is (continuously) established in order to (continuously) determine the deviation xd* thereof and process this in the postprocessor L. The correcting variable I TARGET , which is available at the input side of the postprocessor L, is then adjusted in accordance with the deviation xd* and/or the control deviation xd, so that a correcting variable I TARGET * adapted to the dynamic behaviour is output at the output side of the postprocessor L. 
     The control device RO, R and the postprocessor L are preferably constructed as a component part of the electronic control unit ECU. As the electronic control unit ECU is usually equipped with at least one microcomputer, the control device RO, R and the postprocessor L may easily be implemented in software—i.e. without any expenditure on circuitry. 
     FIG. 4 represents a possible implementation of the postprocessor L as a flow diagram. The sequence represented in the flow diagram is executed cyclically as follows: 
     The first-question to be asked is whether the control deviation xd is less than the threshold value xs. 
     The following question is whether the deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt lies within a threshold range |xs*|. 
     If the answer to one of the questions is “no”, i.e. the deviation xd* lies outside of the threshold range |xs*| or the control deviation xd is greater than the tolerance range value xs, then the correcting variable I TARGET (n) which is output by the control device RO, R in the current cycle is stored as I TARGET (n−1) for the next control cycle, and the correcting variable I TARGET (n) which is output in the current cycle is taken up and output unchanged as new correcting variable I TARGET *. 
     If the answer to both questions is “yes”, i.e. the deviation xd* lies within the tolerance range |xs*| and the control deviation xd is greater than the tolerance range value xs, then the correcting variable I TARGET (n−1) which is stored in the preceding cycle is output as new correcting variable I TARGET *. 
     As a result of implementing the postprocessor L in this way, the correcting variable I TARGET  is maintained constant during time segments in which the deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt lies within the tolerance range xs*f. The correcting variable is in this case maintained at the value which was last stored under I TARGET (n−1) in the “NO” interrogation path. In contrast, the correcting variable I TARGET (n) currently output by the control device RO, R remains uninfluenced and is output as I TARGET * during time segments in which the deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt lies outside of the tolerance values |xs*|. 
     Because the correcting variable is maintained constant over relatively long time segments during control, the actuator, which comprises the electromagnet arrangement  26  and the control valve, which is not represented in detail, is also adjusted less frequently. The substantial reduction in adjustment work affords the important advantage of a particularly convenient control procedure overall with respect to noise development, for the control valve remains in a certain control or open position during the constant phases of the correcting variable. Related to the vehicle brake system, this results in a particularly harmonious or low-noise brake pressure variation. 
     It is obvious that an averaging process over several preceding values can also be carried out when storing the correcting variable under I TARGET (n−1), providing the possibility of compensating for individual “mavericks” which could have adverse effects on the control performance. It is also obvious that the range of the tolerance value |xs*| and/or the tolerance range value |xs|, with which the control deviation xd is compared, can be controlled in adaptive fashion before or during the control procedure in order to adapt the response behaviour of the postprocessor L to different operating points of the electronically controllable brake booster. 
     The operation according to the invention is illustrated in FIG. 5 on the basis of two time graphs. The variation in time of the command variable p TARGET  and the controlled variable p ACTUAL  is represented in the top time graph, while the variation in time of the correcting variable I TARGET  is represented in the bottom time graph. The command variable p TARGET  is increased like a ramp from the instant to, and the correcting variable I TARGET  assumes the build-up current I BUILD-UP  instantaneously at the instant t 0  in order to control the controlled variable p ACTUAL . 
     The controlled variable p ACTUAL  extends below the command variable p TARGET  in the time segment  0 -t 1 . The deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt lies within the tolerance value |xs*| from the instant t 1 , so that the correcting variable I TARGET  is maintained constant in the time segment t 1 -t 2 . At the instant t 2  the deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt or xd/dt comes to lie outside of the tolerance value |xs*|, so that the control process recommences with the correcting variable I TARGET  assuming the reduction current I REDUCTION  instantaneously in order to control the controlled variable p ACTUAL . The controlled variable p ACTUAL  extends above the command variable p TARGET  in the time segment t 2 -t 3  until the deviation xd* of the gradients p TARGET /dt, p ACTUAL /dt or xd/dt again lies within the tolerance value |xs*| at the instant t 3 , so that the correcting variable I TARGET  is maintained constant again from the instant t 3 . The correcting variable I TARGET  is not maintained constant during the time segments t 0 -t 1  and t 2 -t 3 , although the deviation xd* of the gradients of the control difference is almost zero in some instances. This is due to the fact that the control deviation xd is greater than its tolerance value xs during the time segments t 0 -t 1  and t 2 -t 3 , as has already been illustrated on the basis of FIG.  4 . 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.