Patent Publication Number: US-2022234565-A1

Title: Method for checking an electrohydraulic vehicle power brake system of an autonomously driving land vehicle for the presence of air in the brake fluid

Description:
FIELD 
     The present method relates to a method for checking an electrohydraulic power brake system of a land vehicle autonomously traveling on public roads for the presence of air in the brake fluid. 
     BACKGROUND INFORMATION 
     Autonomous driving up to a level 4 (the driver may be prompted to intervene) and level 5 (the highest level; no driver required) necessitates a power vehicle brake system offering redundancies, which excludes a complete failure of the vehicle brake system with a probability bordering on certainty without the need for a driver intervention. 
     German Patent Application No. DE 10 2014 220 440 A1 describes an electrohydraulic power brake system for a vehicle equipped with an operating brake unit and an auxiliary brake unit. Both brake units have a power brake-pressure generator with an electrically controllable pressure source and a brake-pressure control valve system for each wheel brake. The auxiliary brake unit is connected to the operating brake unit, and hydraulic wheel brakes are connected to the auxiliary brake unit so that with a passive auxiliary brake unit, the wheel brakes are able to be operated through the auxiliary brake unit by the operating unit, and in an interruption or a malfunction of the operating brake unit, they are operable using the auxiliary brake unit. The individually active brake unit controls the wheel brake pressures in the wheel brakes. 
     SUMMARY 
     A check method according to the present invention is provided for an electrohydraulic power brake system of a vehicle for autonomous driving up to levels 4 and 5 on public roads. Level 4 is also known as highly automated driving and means that the control of a vehicle is permanently assumed by an electronic system, and a driver is prompted to intervene only if the system is no longer able to manage the driving tasks. Level 5 is also known as a full automatization and requires no driver. 
     The vehicle brake system for carrying out the check method according to an example embodiment of the present invention has a first power brake-pressure generator, a second, redundant, power brake-pressure generator, and a manual brake master cylinder, which is also meant to be understood as an auxiliary brake master cylinder in this context, that is to say, as a brake master cylinder having a brake booster. A brake pressure for actuating one or more wheel brake(s) is usually generated by the first power brake-pressure generator. In an interruption or a malfunction of the first power brake-pressure generator, the brake pressure is generated by the second, redundant, power brake-pressure generator. In a non-autonomous driving operation, the brake master cylinder is used as a setpoint device for the brake pressure to be generated by the first power brake-pressure generator. In an interruption or a malfunction of the first power brake-pressure generator during non-autonomous driving, the brake pressure is also able to be generated by the brake master cylinder instead of the second power brake-pressure generator. 
     The hydraulic wheel brake(s) is/are connected via a first valve to a first power brake-pressure generator and via a second valve to the second, redundant, power brake-pressure generator. In an actuation of the wheel brake(s) by the first power brake-pressure generator, the first valve is open or is opened and the second valve is closed, so that the second power brake-pressure generator is hydraulically separated from the wheel brakes and preferably also separated from the first power brake-pressure generator. In an actuation of the wheel brake(s) by the second power brake-pressure generator, the second valve is open or is opened and the first valve is closed. 
     As is common in hydraulic vehicle brake systems, the brake master cylinder is connected in particular to a brake fluid reservoir at zero pressure and, via a suction valve and a return valve, to the second, redundant, power brake-pressure generator, which means that if the brake master cylinder is not actuated, the second power brake-pressure generator is able to aspirate brake fluid from the brake fluid reservoir through the brake master cylinder and through the suction valve and/or to return brake fluid from the vehicle brake system through the return valve into the brake master cylinder and, through the brake master cylinder, into the brake fluid reservoir. Via the return valve, the first power brake-pressure generator is likewise able to return brake fluid through the brake master cylinder into the brake fluid reservoir. It is also possible that a shared valve connects the brake master cylinder to the brake fluid reservoir rather than the suction valve and the return valve. If the brake master cylinder is actuated, it hydraulically separates itself, and thus also the second power brake-pressure generator, from the brake fluid reservoir because a brake master cylinder piston has crossed a compensating bore via which the brake master cylinder communicates with the brake fluid reservoir, and closes it as a result, or a central valve in the brake master cylinder piston is closed via which the brake master cylinder communicates with the brake fluid reservoir. 
     Preferably, the power vehicle brake system has a brake-pressure control valve system with the aid of which the wheel brake pressures in the wheel brake(s) are able to be regulated. A regulation may also refer to a control. The wheel brake pressure is preferably regulated individually in each wheel brake, but it is also possible to regulate wheel brake pressures in groups of wheel brakes or to jointly regulate the wheel brake pressure of all wheel brakes. A magnitude of the wheel brake pressure(s) and thus of brake forces of the wheel brakes is regulated. In addition, a traction-slip control is possible with the aid of the brake-pressure control valve system(s) and one of the two power brake-pressure generators. For instance, such slip-traction controls are anti-lock braking systems, drive-slip control systems and/or a driving dynamics control/electronic stability program, for which the abbreviations ABS, TC/ESP are commonly used. Such drive-slip controls are conventional and will not be described further. 
     To determine whether air is present in the brake fluid or whether the brake fluid is compressible in general, an example embodiment of the present invention provides a method that has three consecutive phases. Possible additional phases prior to, between or following the phases of the present invention are not excluded. In a first phase, the first valve is opened, the second valve is closed, and a first brake pressure is generated by the first power brake-pressure generator, which is subsequently lowered again, preferably using the first power brake-pressure generator. The first brake pressure is used for operating the wheel brake(s), which means the friction brake pads of the wheel brake(s) are pressed against a brake disk or disks, brake drum or drums, and/or, in general, against a brake element, so that an air gap until the friction brake pads make contact with the brake element(s) is overcome. The first brake pressure is reduced only to such an extent that the friction brake pads continue to rest against the brake element(s) so that no play in the wheel brakes is created that would prevent the determination as to whether air is contained in the brake fluid or, more generally, whether the brake fluid is compressible, or which would at least make such a determination more difficult. 
     In a second phase of the check method according to an example embodiment of the present invention, the suction valve and the return valve will be or are closed, which means that the brake master cylinder is hydraulically separated, or will be hydraulically separated, from the second power brake-pressure generator, and a second brake pressure is generated by the first power brake-pressure generator. The second brake pressure may have the same magnitude as the first brake pressure or be greater; preferably, the second brake pressure is lower than the first brake pressure. After the second brake pressure has been generated, the first power brake-pressure generator is set to idling again so that it neither increases nor decreases the brake pressure. By opening the return valve, the brake master cylinder is subsequently connected to the first power brake-pressure generator and/or to the second power brake-pressure generator, so that the brake pressure is lowered again. A precondition is that the brake master cylinder is not actuated so that the brake fluid is able to flow through the brake master cylinder into the brake fluid reservoir. At the end of the second phase as well, the brake pressure is lowered only to such an extent that the friction brake pads of the wheel brakes remain in contact with the brake element(s) so that no play is created in the wheel brake(s). 
     In a third phase of the check method according to an example embodiment of the present invention, the return valve is closed again, the suction valve is opened, and a third brake pressure is generated by the second power brake-pressure generator, which is preferably as high as the second brake pressure, but may also be higher or lower than the first brake pressure or the second brake pressure. 
     A second brake fluid volume, which is supplied or displaced by the first power brake-pressure generator for the generation of the second brake pressure, is compared with a third brake fluid volume supplied or displaced by the second power brake-pressure generator for the generation of the third brake pressure. The supplied or displaced brake fluid volumes are able to be measured or calculated, for instance based on a piston travel, a piston area and possibly the number of piston travels of the power brake-pressure generators. This does not exclude other possibilities for ascertaining the supplied or displaced brake fluid volumes. In the event that the third brake fluid volume is too large for the generated third brake pressure or, conversely, the third brake pressure is too low for the brake fluid volume supplied or displaced for its generation, then a compressibility of the brake fluid in the region of the second power brake-pressure generator and/or the brake master cylinder has to be assumed. 
     The method according to the present invention makes it possible to determine whether air is present in the brake fluid in the region of the brake master cylinder and/or the second, redundant, power brake-pressure generator, which will normally not manifest itself when the vehicle brake system is actuated by the first power brake-pressure generator but has an adverse effect on the operation of the vehicle brake system when the power vehicle brake system is to be actuated by the second power brake-pressure generator and/or the brake master cylinder in the event of an error or an interruption of the first power brake-pressure generator. 
     The method according to the present invention is preferably carried out exclusively when a vehicle equipped with the power vehicle brake system is at a standstill and when the brake master cylinder is not being actuated. 
     In particular, both power brake-pressure generators are electromechanical brake-pressure generators which have an electric motor which drives a hydraulic pump, especially a piston pump, for the generation of a brake pressure, or which displaces a piston in a cylinder of a piston-cylinder unit. 
     Advantageous embodiments and further refinements of the present invention are disclosed herein. 
     All features provided in the description and the figures are able to be realized individually or in basically any combination in exemplary embodiments of the present invention. Embodiments of the present invention that do not have all but only one or more features of embodiments are essentially possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       Below, the present invention will be described in greater detail with the aid of an embodiment illustrated in the figure. 
         FIG. 1  shows a hydraulic circuit diagram of an electrohydraulic power vehicle brake system, based on which the method according to an example embodiment of the present invention will be described. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Electrohydraulic power vehicle brake system  1  shown in  FIG. 1  is intended for a land vehicle, i.e., a passenger car, which travels autonomously up to level 4 or 5 on public roads. Level 4 refers to autonomous driving in which the driver may be prompted to intervene, and level 5, the highest level, refers to autonomous driving that does not require any driver intervention. Vehicle brake system  1  is a dual-circuit vehicle brake system. 
     Power vehicle brake system  1  has a first electromechanical power brake-pressure generator  2 , a second, redundant electromechanical power brake-pressure generator  3  and a manual brake master cylinder  4 . In the standard case, a brake pressure is generated by the first power brake-pressure generator  2 , and when an interruption or a malfunction of first power brake-pressure generator  2  has occurred, the brake pressure is generated by the redundant second power brake-pressure generator  3  and/or brake master cylinder  4 . In non-autonomous driving, brake master cylinder  4  is used as a setpoint adjuster for the brake pressure to be generated. In the exemplary embodiment, a pedal travel of brake pedal  16  is measured with the aid of a pedal travel sensor  24 . 
     First power brake-pressure generator  2  has a piston-cylinder unit  6  including a piston  7 , which is displaceable in a cylinder  10  of piston-cylinder unit  6  by an electric motor  8  via a mechanical reduction gear unit and a helical gear  9  in order to generate a hydraulic brake pressure. Cylinder  10  is connected to two brake circuits of vehicle brake system  1  via a first valve  11  in each case. 
     Second power brake-pressure generator  3  has a hydraulic pump  12 , e.g., a piston pump, for each brake circuit, which can be jointly driven by an electric motor  13  in order to generate a brake pressure. In each brake circuit, intake sides of hydraulic pumps  12  are connected to brake master cylinder  4  via a suction valve  14 , and pressure sides of hydraulic pumps  12  in each brake circuit are connected to brake master cylinder  4  via a return valve  15 . 
     Brake master cylinder  4  is a dual circuit brake master cylinder, which is manually operable using a brake pedal  16 . It has a non-pressurized brake fluid reservoir  17  to which brake master cylinder  4  is directly connected, that is to say, especially without an interconnected valve. 
     In a brake circuit, a piston-cylinder unit having a piston, acted upon by a spring, is connected to brake master cylinder  4  via a simulator valve  18  as a pedal travel simulator  19 . 
     In the exemplary embodiment, vehicle brake system  1  has four hydraulic wheel brakes  20 , of which two in each case are assigned to a brake circuit. 
     Each wheel brake  20  has an intake valve  21  and a discharge valve  22 . Via intake valves  21 , wheel brakes  20  are connected to first valves  11  of first power brake-pressure generator  2 , and they are connected to the pressure sides of hydraulic pumps  12  of second power brake-pressure generator  3  via a second valve  23  in each brake circuit. By way of first valves  11 , second valves  23  and return valves  15  of second power brake-pressure generator  3 , first power brake-pressure generator  2  is connectable to brake master cylinder  4 . Via discharge valves  22 , wheel brakes  20  are connected to brake fluid reservoir  17 . 
     Intake valves  21  and discharge valves  22  form wheel brake pressure-control valve systems by which wheel-brake pressures in each wheel brake  20  are able to be individually regulated, a regulation also denoting a control. Slip-traction controls such as an anti-lock braking control, a drive-traction control and a driving dynamics/electronic stability program, for which the abbreviations ABS, ATC and ESP are commonly used, are realizable with the aid of intake valves  21  and discharge valves  22 . Such traction controls are conventional and will not be described here. 
     First valves  11 , suction valves  14 , return valves  15 , simulator valve  18 , intake valves  21 , discharge valves  22  and second valves  23  are 2/2 way solenoid valves, return valves  15 , intake valves  21  and second valves  23  being open in their currentless basic positions and first valves  11 , suction valves  14 , simulator valve  18  and discharge valves  22  being closed in their currentless basic positions. Other embodiments are possible. 
     The method according to the present invention for checking power brake system  1  of a vehicle for the presence of air in the brake fluid is preferably carried out exclusively in stationary vehicles and with a non-actuated brake master cylinder  4 , so that brake master cylinder  4  communicates with brake fluid reservoir  17 . With open first valves  11  and closed second valves  23 , a first brake pressure p 1  of up to approximately 100 bar, for instance, is then generated by first power brake-pressure generator  2 . The brake pressure is generated by displacing piston  7  in cylinder  10  of piston-cylinder unit  6  of first power brake-pressure generator  2  in a first direction without return travel. Because of open intake valves  21 , first brake pressure p 1  acts on hydraulic wheel brakes  20  so that their brake pads rest against their brake elements such as their brake disks or brake drums. This overcomes an air gap in wheel brakes  20  so that wheel brakes  20  do not take on brake fluid by overcoming the air gap between the brake pads and the brake elements during the further check. Discharge valves  22  of wheel brakes  20  remain closed. 
     Next, the brake pressure is lowered again by a return travel of piston  7  in cylinder  10  counter to the first direction. In particular, the brake pressure is lowered to the ambient pressure or a slightly higher pressure, which ensures that the brake pads of wheel brakes  20  remain applied to the brake elements. 
     After the brake pressure has been lowered by the return travel of piston  7  in cylinder  10  of piston-cylinder unit  6  of first power brake-pressure generator  2 , first valves  11  remain open and second valves  23  are also opened. Suction valves  14  and return valves  15  of second power brake-pressure generator  3  remain open or are closed so that no brake fluid is able to be displaced into brake master cylinder  4 . The renewed displacement of piston  7  in cylinder  10  of piston-cylinder unit  6  of first power brake-pressure generator  2 , once again in the first direction, raises the brake pressure again, which means that a second brake pressure p 2  is generated. Second brake pressure p 2  may have the same magnitude or a higher magnitude than first brake pressure p 1 , but second brake pressure p 2  is preferably lower than first brake pressure p 1 . In the exemplary embodiment, second brake pressure p 2  amounts to approximately 40 bar or, expressed in more general terms, amounts to approximately one-half the first brake pressure p 1  or less. 
     A brake fluid volume V 2  required for generating second brake pressure p 2  and supplied by first power brake-pressure generator  2  or displaced from cylinder  10  is measured or ascertained. In the exemplary embodiment, second brake fluid volume V 2  required for generating second brake pressure p 2  is calculated as the product of the travel of piston  7  in cylinder  10  in the first direction, multiplied by a piston surface area. 
     Then, return valves  15  are opened so that brake fluid flows from cylinder  10  of piston-cylinder unit  6  of first power brake-pressure generator  2  and from wheel brakes  20  into brake master cylinder  4  and through non-actuated brake master cylinder  4  into non-pressurized brake fluid reservoir  17 , so that brake pressure p drops to ambient pressure again or to a level slightly above ambient pressure. 
     After the renewed lowering of brake pressure p, return valves  15  are closed again and suction valves  14  are opened instead. Return valves  15  are closed before the friction brake pads of hydraulic wheel brakes  20  lift off the brake elements. Piston  7  in cylinder  10  of piston-cylinder unit  6  of first power brake-pressure generator  2  is no longer moved until the end of the check method. Using second power brake-pressure generator  3 , a third brake pressure p 3  is now generated, which is higher, equal to or lower than first brake pressure p 1  or second brake pressure p 2 . In the exemplary embodiment, third brake pressure p 3  is approximately as high as second brake pressure p 2 . Measured or ascertained is a third brake fluid volume V 3 , which is required for raising the brake pressure to third brake pressure p 3  or for generating third brake pressure p 3 . In the exemplary embodiment, this is accomplished in that the number of revolutions of electric motor  13  of the two hydraulic pumps  12 , which are piston pumps in the exemplary embodiment, or the number of piston travels is multiplied by the piston travels, which means the piston travels and the piston surface areas. 
     Second brake fluid volume V 2  is compared with third brake fluid volume V 3 . If the brake fluid is incompressible, then second brake fluid volume V 2  and third brake fluid volume V 3  are equal when second brake pressure p 2  and third brake pressure p 3  are equal and the lowered brake pressures p are equal prior to the generation of second brake pressure p 2  by first power brake-pressure generator  2  and prior to the generation of third brake pressure p 3  by second power brake-pressure generator  3 . 
     If brake pressures p, p 2 , p 3  differ, then this must be taken into account when comparing brake fluid volumes V 2 , V 3 . In practice, small deviations of brake fluid volumes V 2 , V 3  are produced by elasticities of vehicle brake system  1 . If third brake fluid volume V 3  is considerably greater than second brake fluid volume V 2 , then air in the brake fluid in the region of second power brake-pressure generator  3  has to be assumed. 
     If air is present in the brake fluid in the region of first power brake-pressure generator  2  and/or wheel brakes  20 , then this increases brake fluid volume V 2  required for generating second brake pressure p 2  and also third brake fluid volume V 3  required for generating third brake pressure p 3 . A second brake fluid volume V 2  increased because of air in the brake fluid, for instance, is determined when second brake pressure p 2  is generated by first power brake-pressure generator  2  in that second brake fluid volume V 2  required for this purpose and supplied or displaced by first power brake-pressure generator  2  is compared with the generated second brake pressure p 2 . If generated second brake pressure p 2  is too low in comparison with brake fluid volume V 2  required for its generation, then the brake fluid is compressible, which means that it contains air or that some other error has occurred in vehicle brake system  1 . 
     A check of a compressibility of the brake fluid in vehicle brake system  1  is carried out, which may lead to the conclusion that air is present in the brake fluid.