Abstract:
An auxiliary energy brake system of automotive vehicles has a central pressure source and a hydraulic system which includes pressure medium lines, electrically controllable hydraulic valves, and pressure medium orifices. A pressure sensor is located at the outlet of the pressure source. The braking pressure in the wheel brakes (P Rb ) is established from the measured values (P meas ), determined by the pressure sensor, and from the hydraulic conductance (D) of the orifices of the hydraulic system in accordance with formula 
     
       P.sub.Rb f (D,P.sub.meas).

Description:
This application is the U.S. national-phase application of PCT International Application No. PCT/EP94/00981. 
     BACKGROUND OF THE INVENTION 
     This invention relates to an automotive vehicle auxiliary energy brake system having a central pressure source and a hydraulic system which includes pressure medium lines connecting the wheel brakes of the automotive vehicle with the pressure source, electrically controllable hydraulic valves, and pressure medium orifices. Brake systems of this type also include an electronic control unit for generating valve control signals and for controlling the braking pressure in the wheel brakes in dependence on measured pressure signals and a pressure sensor arranged at the outlet of the pressure source. 
     On principle, an auxiliary energy brake system of this type will always be needed if a wheel brake is to be activated independently of the actuation of the brake system by the driver. A known example of the application of such a system is traction slip control by brake management. In these systems, the braking pressure is controlled in response to the rotational behavior of the spinning or unstable wheels. 
     Further, there are brake systems conceivable and being developed which will be activated automatically as an accident-preventing measure as soon as the distance between an automotive vehicle and another vehicle in front of it or another obstacle becomes too small. Speed control systems likewise require brake management in some situations. For such systems, the rotational behavior of the wheels is not a suitable control parameter; intervention by means of control does not depend on the instability of any wheel. 
     Adjusting a preset braking pressure which is variable or depends on certain conditions is difficult. Control systems are known which work with a fixed pulse pattern and, if necessary, modify this pulse pattern in dependence on reactions of the vehicle. These systems are relatively inaccurate and are only able to effect rough pressure changes. Moreover, there is a relatively strong dependence of the braking pressure on the environmental and operating temperatures. 
     Besides, there are known control systems which cannot function without direct measurement of the braking pressure in the individual wheel brakes. In these systems, a plurality of pressure sensors is needed, namely at least one sensor per brake caliper which, moreover, must be arranged near the brake caliper. 
     German Patent DE-A-40 29 793 discloses a vehicular hydraulic brake which comprises a central pressure source and hydraulic valves by means of which both a boosting function and anti-lock control and traction slip control are possible. Moreover, this system provides driving dynamics adaptation within the normal braking range and can even function as a remote controlled brake system. For performing these functions, a hydraulic valve interrupts the pressure medium path from the master cylinder of the brake system to the wheel brakes and switches on the auxiliary pressure source. Then, the pressure at the outlet of the master cylinder, i.e., upstream of this point of separation, and the pressure metered in by the pressure source, i.e., the pressure downstream of the point of separation, are measured by pressure sensors and are evaluated for pressure control or pressure regulation. This requires a considerable expenditure. 
     SUMMARY OF THE INVENTION 
     The present invention provides an auxiliary energy brake system which permits continuous adjustment of a preset, variable braking pressure and requires a comparatively small manufacturing expenditure. The brake system displays a stable behavior over a wide operative range and is insensitive to variations in the hydraulic parameters, e.g., caused by temperature changes, aging, wear and tear, or manufacturing tolerances. 
     A brake system according to the present invention develops braking pressure in the wheel brakes as a function of the pressure at the outlet of a pressure source, measured by a pressure sensor arranged there, and of the hydraulic conductance (D) of the orifices of the hydraulic system. 
     In order to determine the control parameter (i.e., the braking pressure in the wheel brakes), the inventive auxiliary energy brake system only needs one pressure sensor which is arranged at the outlet of the pressure source. According to the present invention, it is possible to adjust the braking pressure in the wheel brakes to the desired value if the hydraulic conductance between the pressure source and the wheel brakes is known. The hydraulic conductance depends on pressure medium orifices provided in the hydraulic system or is adjustable to a particular value by additional components; the conductance can be computed from the design of the hydraulic system and from the characteristic curves of the valves and of the wheel brakes or can be determined empirically. A brake system according to the present invention is insensitive to the effects of changes in temperature. 
     Further characteristics, advantages, and applications of this invention will become evident from the following description of this invention, reference being made to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
     FIG. 1 by way of schematical simplification, shows the main hydraulic components and the hydraulic circuit diagram of an auxiliary energy brake system according to this invention in combination with an anti-lock-controlled brake system; 
     FIG. 2 shows the hydraulic conductance equivalent of the hydraulic system of FIG. 1 with regard to pressure build-up and pressure reduction; 
     FIG. 3 schematically shows the control circuit of an auxiliary energy brake system according to this invention; and 
     FIG. 4 by way of a diagram, shows the pressure variation in the wheel brakes and at the outlet of the central pressure source in accordance with the present invention and with the brake system shown in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The brake system shown in FIG. 1 essentially consists of a dual-circuit hydraulic braking pressure generator 1, a complete anti-lock-control valve-and-hydraulic-system unit 2, a central pressure source 3 provided for the auxiliary energy brake system, and an additional valve unit 4 which serves to change over from the application of brake pressure from the pressure generator 1 the auxiliary energy brake system. Moreover, the additional valve unit 4 contains one spring-loaded accumulator 5, 5&#39; per hydraulic circuit I, II. Together with the appertaining valve pairs 6, 7; 8, 9, each spring-loaded accumulator 5, 5&#39; simulates pedal travel during auxiliary energy brake operation. 
     In the embodiment according to FIG. 1, a complete separation of the valve/hydraulic-system units 2, 3, 4 is shown for the sake of better understanding. For cost reasons, in practice, a multiple exploitation of individual hydraulic components, i.e., of the valves and of the energy supply, and thus a simplification of the overall hydraulic circuitry, might be preferred. 
     The pressure source 3 of the inventive auxiliary energy brake system essentially consists of an electric-motor-driven hydraulic pump 14, a hydraulic accumulator 15, and two 2/2 way valves 16, 17 designed as SG-valves which are normally closed. Moreover, a pressure limiting valve 18 and a pressure switch 19 are provided. Pressure switch 19 ensures the maintenance of a preset pressure in the system by switching the pump 14 on and off. Finally, a pressure sensor 20 is arranged at the outlet of pressure source 3. According to this invention, the value of the braking pressure in the individual wheel brakes 21-24 will be computed from the output signals (P meas ) of this pressure sensor 20. 
     The hydraulic system of the auxiliary energy brake system of FIG. 1 contains one or several orifices in the path from the central pressure source 3 to the individual wheel brakes 21 through 24, after the switching-over of the &#34;separating valves&#34; 10, 11; 12, 13. These orifices are designed as integral components of the valves 11, 13, EV 1  -EV 3  or as additional (non-represented) discrete components. The total hydraulic conductance of these orifices of each pressure medium path from the pressure source 3 to the respective wheel brake is marked D in the equations listed below; the hydraulic conductance D of the orifices can be computed or can be determined empirically. 
     FIG. 2 shows the hydraulic conductance equivalent of the hydraulic system of the inventive auxiliary energy brake system for both the pressure build-up and reduction phases. The orifices D build , D red  symbolize the hydraulic conductance of the orifices between the pressure source 3 and the wheel brakes 21-24 of the system of FIG. 1 for the pressure build-up (D build ) and for the pressure reduction (D red ) . The orifices D 1 , D 2  symbolize the hydraulic conductance in the connecting paths to the hydraulic accumulator 15 and the pressure compensation reservoir 25, respectively. The reference numeral of the pressure sensor 20 was selected so as to be the same as that of the corresponding element in the circuitry of FIG. 1. The wheel brake shown in FIG. 2 could be representative of any of the wheel brakes 21 through 24. 
     The control circuitry of FIG. 3 illustrates the mode of operation of the auxiliary energy brake system of FIG. 1. An electronic control unit 26 may be an integral component of the electronic system of the anti-lock control system. In a known manner, the nominal pressure P nom  is determined and preset for the control unit 26 by a sensor, a potentiometer, or by a digital signal in dependence on the application. This preset nominal value represents the desired braking pressure in the wheel brakes 21 through 24. Typically, the mathematical interrelationship between the preset nominal value P nom  and the braking pressure P Rb  in the individual wheel brakes 21 through 24 in many events varies in dependence on the respective brake characteristic curve, on the load on the wheel (front wheel or rear wheel) etc. According to this invention, the actual pressure value P act  is exclusively determined by measuring and evaluating the pressure P meas  at the outlet of the pressure source 3 by means of the pressure sensor 20. This invention utilizes the fact that the pressure in the wheel brakes P Rb  is a function of the hydraulic conductance D of the orifice and of the pressure P meas  adjusted and measured at the outlet of the pressure source 3. In FIG. 3, the hydraulic system containing the orifice having the hydraulic conductance D is symbolized by a block 28 comprising the two hydraulic units 4 and 2 (see FIG. 1). Connected to unit 2 are the wheel brakes, shown as &#34;Rb&#34; in FIG. 3. 
     In order to determine the control parameter P Rb , i.e., the pressure in the wheel brake, a non-linear state variable reconstruction is performed by means of a computing circuit or &#34;observer&#34; 27 (see FIG. 3). To this end, the following computing steps are carried out recursively: ##EQU1## 
     
         v(k)=v(k-1)+T.sub.o ·Q(k) 
    
     
         P.sub.Rb (k)=f(v(k)). 
    
     &#34;Q(k)&#34; is the volume flow rate in the system at the sampling time k. &#34;v(k)&#34; is the pressure medium volume in the wheel brake at the sampling time k. &#34;P Rb  (k)&#34; is the developed pressure in the wheel brake at the sampling time k. It should be noted that, in the top equation, the absolute value of the square root is calculated to avoid obtaining an &#34;imaginary number.&#34; Also, in that equation, the term after the square root is meant to indicate that the sign (positive or negative) of the volume flow rate will be the sign of the measured pressure at time k minus the developed pressure at time k-1. The sign indicates the direction of flow, with a positive sign meaning that flow occurs toward the wheel brakes and a negative sign meaning that flow occurs from the wheel brakes. 
     Thus, by measuring the pressure P meas  upstream of an orifice whose conductance is marked D, the volume flow rate Q(k) is estimated which flows in the system. Then the braking pressure P Rb  is determined, with the characteristic curve of the system being taken into account. 
     This reconstructed (or mathematically developed) wheel braking pressure P Rb  forms the actual value P act  for the control unit 26 which computes the operating time of the valves 16, 17 (see FIG. 1) of the pressure source 3, in particular of the 2/2 way valve 16 controlling the pressure build-up. The operating time is computed so that the braking pressure will approach or achieve the nominal pressure P nom . This computation likewise follows a hydraulic model (cf. FIG. 2) which in mathematical terms is to be described as follows (v again is the pressure medium volume in the wheel brake; v o  is the pressure medium volume at the starting time of the computation): 
     
         P.sub.Rb =av.sub.o +bv.sub.o.sup.2 
    
     
         P.sub.nom =a(v.sub.o +T.sub.v Q)+b(v.sub.o +T.sub.v Q).sup.2 
    
     
         P.sub.nom -P.sub.Rb =a T.sub.v Q+b T.sub.v.sup.2 Q.sup.2 +2b T.sub.v Qv.sub.o ##EQU2## 
    
     In these formulas, 
     a, b are model parameters (i.e., constants) of the wheel brake; 
     T v  is the sum of the valve operating times; 
     P nom  is the nominal pressure value; 
     P Rb  is the estimated pressure in the brake. 
     Different hydraulic conductances D build , D red  of the orifices apply in the pressure build-up phase and in the pressure reduction phase. The difference in the hydraulic conductances is caused by, for instance, the fact that the non-return valves provided in the hydraulic-system unit 2 (see FIG. 1) only act in one direction of the pressure medium flow. Assuming a pressure build-up with a hydraulic conductance D build  from a pressure source with P accumulator  of 160 bar (see FIG. 2), by approximation, the following equation applies ##EQU3## to the volume flow rate Q in the pressure build-up phase. 
     Correspondingly, the relationship ##EQU4## applies to pressure reduction into a pressureless pressure compensation reservoir 25 (see FIG. 1). 
     If the result of subtraction of the two roots is greater than 0, a pressure build-up will be initiated. If result is negative, the pressure will be reduced via the valve 17 (see FIG. 1) of the pressure source 3. 
     The inventive control or regulation of the braking pressure by solely measuring the pressure at the outlet of the pressure source 3, while exploiting the hydraulic conductance D, D build , D red  of the orifices of the hydraulic system, is characterized by a high-degree stability and independence of temperature influences, aging, variable valve switching times etc. With no pressure measurement taking place at the wheel and thus one sole pressure sensor (20) being required for the overall system, the manufacturing expenditure is relatively small. 
     Apart from the applications mentioned at the beginning, auxiliary energy brake systems of the inventive type are also suited for driving stability control, for systems where the instruction for actuation is exclusively transmitted electrically (brake-by-wire systems), or for brake systems in electric vehicles. 
     The aforementioned equations can also be simplified by linearizing the interrelationships which will lead to simplified computer systems, with accuracy being sufficient. 
     In a linearized system, the following relationship applies: 
     
         P.sub.Rb (k)=F(P.sub.lin).P.sub.Rb (k-1)+(1-F(P.sub.lin)).P.sub.meas +Fak(P.sub.lin).Q.sub.o 
    
     with F, Qo, Fak being functions of the linearized point of the known pressure-volume characteristic curve of a wheel brake or of a hydraulic system. 
     There applies 
     
         T.sub.v =K.sub.r (P.sub.lin).(P.sub.nom -P.sub.Rb), 
    
     to the linearized control unit, with ##EQU5## for the pressure build-up phase and with ##EQU6## for the pressure reduction phase. K r  (P lin ) build  and K r  (P lin ) red  are referred to as proportional amplification factors of the linearized control unit. 
     The values of F, Fak, Q o  and K r  can be stored in a table in order to reduce the required number of computing operations. 
     The variation of the curves of FIG. 4 illustrates the pressure control of the auxiliary energy brake system according to this invention. Illustrated are the pressure P meas  measured at the outlet of the pressure source 3, the pressure in the wheel brakes of the rear wheels P HA , and the pressure in the wheel brakes of the front wheels P VA , as a function of time t. Moreover, the pressure P model  established in the computer is outlined stepwise by a dotted line. In the illustrated example, the pressure in the wheels of the rear axle (P HA ) changes with a considerably greater gradient than the pressure in the wheels of the front axle (P VA ). 
     In this simplified example of FIG. 4, a nominal pressure P nom  of 20 bar is preset. At time t o , the pressure build-up valve 16, of pressure source 3, is open (see FIG. 1). The control unit (reference numeral 26 in FIG. 3) computes the valve operating times required for achieving the preset nominal pressure. When the &#34;model pressure &#34; P model  reaches the value derived from the preset nominal pressure at time t 1 , the control unit will switch to &#34;pressure keep-up.&#34; At time t 2 , in the course of control, a pressure reduction occurs. Subsequently, the pressures in the rear and front wheel brakes will continue to approach the nominal pressure P nom . After time t 3 , there will be no further valve actuation as the adjusted pressure is within control accuracy. Thus, control is stable and works quite accurately.