Abstract:
A brake system in which a hydraulic fluid can be conveyed from an accumulator via a valve into individual wheel brake cylinders, the hydraulic fluid being conveyed by a pump into the accumulator, having a pressure sensor arranged on the outlet side of the pump for detecting pressure pulsations in the hydraulic fluid arising in the operation of the pump and having an arrangement for evaluating the pressure pulsations in order to obtain a measuring signal for controlling and/or monitoring the pump.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a brake system as well as to a method for controlling and/or monitoring a brake system pump. 
     BACKGROUND INFORMATION 
     German Patent 195 48 248 A1 describes a method and a device for controlling a pump of an electrohydraulic brake system. In the brake system, a hydraulic fluid is conveyed from an accumulator via a valve means to the individual wheel brake cylinders, the hydraulic fluid being conveyed to the accumulator by a pump. In order that the loading of the accumulator by the pump be as noiseless as possible, the pump can be driven at a mark-to-space ratio that can be stipulated in accordance with need. 
     German Published Patent Application No. 196 38 196 describes a system for monitoring a brake system having a controllable hydraulic pump that is located in a hydraulic circuit, and having at least one solenoid valve whose operating state can be altered in accordance with a control signal. In this context, by altering the operating state of the solenoid valve, the resistance to flow in the hydraulic circuit is influenced. Monitoring elements are provided which, when predetermined operating conditions exist, actuate a display device to indicate a fault, as a function of a detected slowing of the hydraulic pump in varying operating states of the solenoid valve. 
     For safety reasons, the pressure supply, is monitored especially in electrohydraulic brake systems. For this purpose, the absolute system pressure is continually monitored with respect to threshold values, as is the pressure and the pressure change rate when the accumulator is loading. While the pressure is being regulated (for example, in the context of an anti-locking system or an anti-spin regulation system), the result is that the accumulator undergoes a volume drain, which is impossible or at best very difficult to measure. When there is a simultaneous reloading of the accumulator, it proves impossible in conventional systems to carry out a precise monitoring of the pump effectiveness. In conventional systems, conclusions about the operation or the effectiveness of the pump can only be formed on the basis of a pressure increase in the accumulator. 
     In order to avoid generating noise, a pump of this type is not driven at 100% during a loading operation, but is generally operated in a clocked manner. However, during travel, a noise generated by the clocked pump is noticeable and disturbing, in particular when rotational speeds change, whereas noise is significantly less noticeable at a constant rotational speed. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to create a brake system in which the pump operation can be reliably monitored in a simple manner. In addition, it is the goal to make available a brake system that produces noise at as low a level as possible. 
     According to the present invention, it is possible, in particular, to operate an electrohydraulic brake system such that the pump effectiveness, i.e., particularly the pump rotational speed and the pump performance, can be evaluated and monitored even during a simultaneous volume drain from the accumulator. In addition, by determining the pump rotational speed, which is made simple by the present invention, a phase-regulated driving of the pump is possible in order to minimize the disturbing noises by improving the pump clocking. During the operation of the pump, pressure pulsations are generated whose temporal curve mirrors the periodic opening of the discharge valve of the pump, i.e., in an electrohydraulic brake system, for example, the accumulator loading pump. In this context, the period duration of the pulsations or the corresponding measuring signal corresponds to the duration of one revolution of the pump. The maximum fluctuation level is a measure for the pump performance at a preselected elasticity on the pump outlet side and at a preselected temperature of the hydraulic fluid or of the pressure medium. In a typical brake system, e.g., an electrohydraulic one, the clocked pump operates, for example, at rotational speeds of roughly 1500-3000 rpm, which corresponds to a period duration of 20-40 ms. 
     According to one preferred embodiment of the brake system according to the present invention, the pressure sensor is arranged directly at the outlet of the pump. As a result of this arrangement, and despite the presence of elasticities which are caused, for example, by reservoirs provided in the brake system and/or by bore holes, pressure pulsations can be measured in a very precise and reliable manner. 
     According to one preferred embodiment of the method according to the present invention, a smoothing-out, as well as an offset compensation, is carried out on a measuring signal obtained as a result of detecting the pressure pulsations. At pump rotational speeds of 1500-3000 rpm, it is possible to smooth out the signal, for example, by reading in the pressure sensor signal sufficiently frequently, for example, every 2 ms. An offset compensation can be achieved as a result of the fact that this signal has subtracted from it a signal that is filtered over a long term, for example, the average value of the signal over the immediately preceding 40-80 ms. In this context, the time duration between two positive zero crossings is a measure for the period duration. The maximum value, or the amplitude of the signal obtained in this manner, depending on the temperature of the pressure medium, is a direct function of the pump performance. For example, by comparing the measured signal values with the stored table values, it can be evaluated as to whether the pump performance conforms with the specified values, and therefore whether the pump is functioning normally. 
     It has proven to be advantageous to operate the pump in a clocked manner and to drive it at a time point that can be stipulated, in accordance with a zero crossing and/or an extreme value of the smoothed-out or offset-compensated measuring signal. As result of this measure, it is possible to minimize structure-born sound generation from the point of view of noise intensity. The pump can be operated particularly quietly if it is driven in a phase-correct manner. 
     According to the present invention, it is possible, in a simple manner, to monitor the efficiency of the pump on the basis of the level of the detected pressure pulsations, or of the amplitude of the measuring signal that is generated from this source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a block diagram to illustrate the elements of an electrohydraulic brake system according to the present invention. 
     FIG. 2 depicts a flowchart to illustrate the method according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the depicted brake system, a brake pedal is designated as reference numeral  100 . Via the brake pedal, pressure can be built up in a master brake cylinder  110 . Using a pedal travel sensor  118 , the motion of the brake pedal can be detected. Master brake cylinder  110  is in contact with a reservoir  115 . Master brake cylinder  110  is connected to a safety valve  120 , which in the depicted position is located in its non-current-receiving state. A pedal travel simulator  125  is connected parallel to the safety valve. 
     In the connecting line between master brake cylinder  110  and safety valve  120 , or pedal travel simulator  125 , a pressure sensor  130  is arranged that makes available a signal which registers pressure PHZ in the master brake cylinder. 
     In the non-current-receiving state, safety valve  120  enables the connection between the master brake cylinder and discharge valves  141  and  142 . The discharge valves, also in their non-current-receiving state, are connected in the pass direction and they enable the connection to the wheel brake cylinders. 
     Discharge valve  141  is assigned to wheel brake cylinder VR of the right front wheel, and discharge valve  142  is assigned to wheel brake cylinder VL of the left front wheel. The pressure in the wheel brake cylinders can be measured by sensors  151 ,  152 . 
     In addition, the wheel brake cylinders are in contact with an accumulator  185 , via intake valves  161  and  162  and a check valve  170 . The pressure in accumulator  185  can be measured using a pressure sensor  180 . Intake valve  161  is assigned to the right front wheel, and intake valve  162  is assigned to the left front wheel. 
     Accumulator  185  is also in contact, via intake valves  163  and  164 , with wheel brake cylinder HL of the left rear wheel and with wheel brake cylinder HR of the right rear wheel, respectively. The wheel brake cylinders of the left rear wheel and of the right rear wheel are in turn in contact with reservoir  115  via discharge valves  143  and  144 , respectively. 
     Discharge valves  141  and  142 , via safety valve  120 , can also be brought into contact with reservoir  115 . 
     A pump  190 , driven by a pump motor  195 , conveys the hydraulic fluid from reservoir  115  into accumulator  185 . 
     On the outlet side of pump  190 , i.e., between pump  190  and reservoir  185 , a further pressure sensor  200  is provided. Using this pressure sensor  200 , pressure pulsations in the hydraulic fluid caused by the operation of the pump can be detected. The temporal curve of the pressure pulsations mirrors the periodic opening of the discharge valve (undepicted in detail) of pump  190 . The determined pressure fluctuation signals can be fed to a control unit  300 , which carries out an appropriate signal processing. This control unit  300  is advantageously a control unit that controls and regulates the entire operation of the depicted electrohydraulic brake system, i.e., the driving of the pump, the other pressure sensors, and the valves. Input signal lines and output signal lines of control unit  300  are designated as  301  and  302 , respectively. For the sake of the clarity of the drawing, the signal lines that communicate with signal lines  301 ,  302  and are connected to pressure sensor  200  or to the other elements of the depicted brake system are not depicted in detail. 
     The depicted electrohydraulic brake system operates as follows: 
     In normal operation, safety valve  120  receives current. Safety valve  120  enables the connection between reservoir  115  and the discharge valves and interrupts the connection between master brake cylinder  110  and discharge valves. When the driver actuates brake pedal  100 , then sensor  118  determines the signal that corresponds to the pedal travel of brake pedal  100  and/or sensor  130  delivers a pressure signal reflecting the pressure in the master brake cylinder. 
     On the basis of at least one of these signals, which reflect the input of the driver, as well as of any further operational variables, control unit  300  determines the driving signals for impacting on intake valves  161 ,  162 ,  163 , and  164  as well as on discharge valves  141 ,  142 ,  143 , and  144 . 
     When pump motor  195  receives current, pump  190  is driven, and it conveys hydraulic fluid from reservoir  115  into accumulator  185 . The consequence of this is that the pressure in accumulator  185  rises, as measured by pressure sensor  180 . By opening intake valves  161  through  164  and by closing discharge valves  141  through  144 , the pressure in the wheel brake cylinders is increased in accordance with the input of the driver. By opening the discharge valves and closing the intake valves, the pressure in the wheel brake cylinders can be decreased in accordance with the pedal actuation. 
     It is particularly advantageous to measure the pressure in the wheel brake cylinders using pressure sensors  151  through  154 . In this case, pressure regulation and/or fault monitoring is possible. 
     Pedal travel simulator  125  brings it about that the driver feels on brake pedal  100  an appropriate force, which would arise in a corresponding actuation of the brake pedal in a conventional brake system. 
     In the event of the failure of the device, safety valve  120  loses its current and enables the connection between master brake cylinder  110  and wheel brake cylinders of front wheels VL, VR. Thus the driver, via the brake pedal, has direct influence on the wheel brake cylinders of the front wheels. 
     To a sufficient extent pump,  190  conveys hydraulic fluid into the accumulator so that sufficient brake pressure is available. The monitoring of the pump operation can be carried out using pressure sensor  200 . It should be noted that it is possible to dispense with the aforementioned pressure sensor  180  because, on the basis of the signal, filtered over a long term, from pressure sensor  200 , information exists regarding the loading of the accumulator. In this context, the period duration of the measured pressure pulse signal corresponds to the pump revolution period. The maximum fluctuation level, i.e., the amplitude of the measuring signal, represents a measure for the pump performance at a preselected elasticity on the pump outlet side and at a preselected temperature of the pressure medium. Using appropriate signal processing procedures to be carried out in control unit  300 , continual monitoring of the pump performance is therefore possible, especially during a simultaneous volume drain from reservoir  185 . The temperature of the electrohydraulic brake system, i.e., especially the temperature of the hydraulic fluid, can be measured, for example, using a temperature measuring device provided in pressure sensor  200 . Temperature measuring devices of this type can also be provided in further pressure sensors used in the depicted brake system. A temperature signal measured in this manner can also be fed to the control unit so that the measured pressure pulse signal for a given temperature can be compared with table values stored in the control unit. Therefore, in a simple and inexpensive manner, it can be determined whether the pump performance conforms to the stored, i.e., specified, values. Providing pressure sensor  200 , as in the present invention, also permits a phase-regulated driving of pump  190 , as a result of which it is possible to minimize the noise generated by pump  190 . Due to the fact that the rotational speed of pump  190  can be measured in a simple and reliable manner by pressure sensor  200 , this rotational speed can be used as a reference variable in the pump regulation. In general, by arranging pressure sensor  200  on the outlet side of pump  190 , the efficiency of the pump can be continually monitored, and the pump can be operated in a low-noise manner at a substantially constant rotational speed and/or in phase-correct driving. 
     The method according to the present invention is once again depicted in the flowchart of FIG.  2 . In this context, in a step  101 , hydraulic fluid is conveyed into accumulator  185  by pump  190 . Immediately thereafter, in a step  102 , a determination is carried out using pressure sensor  200  of the pressure pulsations arising in the operation of the pump. In a further step  103 , on the basis of an evaluation of the detected pressure pulsations, a measuring signal for controlling and monitoring the pump performance is obtained. In a subsequent step  104 , a smoothing out and/or an offset compensation of the obtained measuring signal is carried out. On the basis of this smoothed-out or offset-compensated measuring signal, in a step  105 , the driving of the pump takes place at a time point that can be stipulated in accordance with a zero crossing and/or an extreme value of the measuring signal. 
     The device and the method are not limited to the electrohydraulic brake system employed in the exemplary embodiment. Rather, the present invention can be used in any brake systems in which, at the outlet of the element conveying the pressure medium, especially a pump, pressure pulsations can be measured, for example, in a sensory manner and thus evaluated.