Patent Publication Number: US-2013234501-A1

Title: Piston-cylinder device and method for conducting a hydraulic fluid under pressure to an actuating device, actuating device for a vehicle brake system, and a method for actuating an actuating device

Description:
PRIOR ART  
     The invention relates to a piston-cylinder device for delivering a hydraulic fluid, in particular for a vehicle brake system. 
     Piston-cylinder devices of this type are known for various purposes for delivering hydraulic fluid under pressure to a thus actuated device. In many cases, piston-cylinder devices of this type are more or less oversized in terms of their displacement volume in order to have sufficient stroke or hydraulic volume available for different applications. 
     Master cylinders for brake assemblies are known, for example, which are highly oversized in the event of fading or air in the brake circuit. 
     However, brake control systems are known from DE 102007062839, for example, in which the tandem master cylinder (TMC) is smaller and redelivery devices with hydraulic accumulator or delivery pistons coupled to the master cylinder piston supply additional volume to the brake circuit with corresponding control. The first only allows limited redelivery volume and the second is expensive. 
     There are also control systems where pressure is generated using TMC according to DE 195 38 794 and DE 103 18 401, in which the TMC is controlled as a delivery piston in order to compensate for the volume discharged from the brake circuit when the pressure falls through intake. Refilling control is provided for this purpose. Intake is via the piston sealing collar, which is known to open at approx. 0.5 bar, and consequently the actual negative pressure for intake, as also described, is reduced. 
     An arrangement is described in DE 102008051316 in which the brake piston is returned as a result of directed brief negative pressure in the wheel cylinder thus eliminating residual friction. Additional solenoid valves are required in the connecting line from the THC to the reservoir for this purpose. 
     The object to be achieved by the invention is to produce a piston-cylinder device for delivering a hydraulic fluid, in particular for vehicle brakes, with which the disadvantages of the prior art can be eliminated in a simple and effective manner. 
     This object is achieved according to the invention in that the hydraulic fluid is supplied to the pressure chamber under pressure by means of an additional arrangement. 
     The solution according to the invention is to provide a simple and effective delivery facility with overpressure which is not restricted in terms of the required delivery volume and which allows a high delivery rate. 
     This solution is also extremely advantageous at low temperatures; 
     problems can arise in known systems at such temperatures since the intake rate decreases in proportion to increasing viscosity. The redelivery operation can therefore be kept very short and consequently the resulting release of air can be prevented even if there is negative pressure over an extended period of time. 
     Intermittent operation of the delivery arrangement is advantageous. Said arrangement can be used particularly in brake systems for different delivery pistons, preferably tandem master cylinders, where intake or delivery can be performed via sleeves or additional solenoid valves. The delivery arrangement can work separately for each hydraulics or preferably brake circuit. 
     Advantageous embodiments or designs of the invention and the associated further advantages result from the following embodiments or further claims respectively. 
     In an extended configuration of the TMC, the sleeves should no longer open when there is negative pressure and consequently negative pressure control using the TMC pistons is necessary for brake lining ventilation control without additional non-return valves to the reservoir. 
     A preferably electromagnetic preliminary pump between the reservoir and the tandem cylinder is suggested as delivery arrangement, which is actuated when the TMC piston (s) are returned. Magnetic force acts on the pump pistons during this intake phase and generates the required excess pressure. The turn on and off times of the magnets are adjusted according to piston movement. According to the prior art, different pump embodiments are conceivable with or without intake valves. Preferably, a pump without valves is suggested in which the pump piston is arranged in a similar way as in a TMC. Here the snifting bore lies behind the sleeve and the bore then closes after the collar has passed over it. The pressure load (already low pressure&lt;10 bar) can be optimised by moving the MC piston first before the pump piston start. The snifting bore is then already in the uncritical sleeve pressure range. 
     When using an electromagnetic preliminary pump, the aforementioned piston collar that is resistant to negative pressure can be dispensed with by activating the backing pump at the desired negative pressure control to control brake lining ventilation. This also closes the connection from the TMC to the reservoir. 
     The preliminary pump can also be used in a system configuration according to DE 103 18 401, for example, where intake is via the MC sleeve. Here, the preliminary pump effects a considerably shorter intake or refilling operation using excess pressure. In other system configurations, a TMC with 2/2 solenoid valve in the line from the brake circuit to the reservoir is activated. Here the preliminary pump can support the intake operation directly in the brake circuit or according to the sleeve configuration also parallel via the sleeve. There is also the option in said TMC configuration to simplify the design by only providing preferably a small snifting bore in the MC pistons. This reduces sleeve friction and there is less free travel of the TMC. 
     There is a further option with said TMC configuration to activate both preliminary pumps at the same time to control the redelivery operation by activating the 2/2 solenoid valve individually per brake circuit. The preliminary pump can be incorporated into the TMC housing or with the reservoir. 
     The free travel of the TMC to the point of application of the brake linings even taking account of the level course of the pressure volume characteristic is known to be extremely flat in the low pressure range. If the preliminary pump is activated at the same time, upon or prior to activation of the TMC, free travel can be reduced significantly which leads to a desired more rigid pedal characteristic. 
     The preliminary pump is also suitable in half-open systems with a slightly loose solenoid valve which is activated to reduce pressure in a reservoir. In such half-open systems, there is a safety problem in known solutions if a solenoid valve becomes loose releasing pressurising agents into the reservoir when there is a loss of pressure. Leakage flow can be countered advantageously in the invention by the pump that functions intermittently. 
     Since the TMC is located in the crash zone, the projecting magnet portion can be fixed such that it cannot be easily sheared in the event of a crash and thus does not act as a rigid barrier. 
     Since compared with the position of the push rod piston in the TMC, which is measured via the turning angle transmitter of the brake servo unit, the position of the accumulator chamber piston is not determined, said position can be determined using a simple contact-free sensor. Determining a region in which redelivery is made is sufficient. 
    
    
     
       Exemplary embodiments of the invention and their embodiments as well as further advantages and features are shown in the drawings and described below. 
         FIG. 1  shows a piston-cylinder device as part of an actuating arrangement for a vehicle brake system; 
         FIG. 2  shows a piston-cylinder device according to  FIG. 1 , however with an arrangement for multiplex operation; 
         FIG. 3  shows a pump integrated into a piston-cylinder device; 
         FIG. 4  shows a first embodiment of an actuating device for a vehicle brake system; 
         FIG. 5  shows a second embodiment of an actuating device with a distance simulator; 
     
    
    
       FIG. 1  shows the known arrangement of a TMC with housing reservoir  1 , power assist  2 , which can be a power assist with or without a pedal, i.e. with separate actuating unit or with or without travel simulator, housing  3 , two sealing sleeves  8  per piston, push rod piston  4 , accumulator chamber piston  5  with piston return spring  6 . The preliminary pump  9  is arranged in the connecting line between TMC  23  and reservoir  1 . The snifting bores  7  are applied in large numbers in pistons in modern TMC and are located behind the sealing sleeve when the piston is in its initial position. If the control arrangement that is not shown requires the redelivery of volume to the hydraulic circuits  4   a  and  5   a , then upon corresponding HCU valve control, with closing of the connection between the TMC and the wheel brake  2 , the piston is returned which, in the event of negative pressure, leads to an intake of brake fluid from the reservoir, for example. The TMC and piston respectively are controlled accordingly in this embodiment and in other embodiments for the redelivery of hydraulic volume. The preliminary pump  9  is activated at the same time or slightly later and generates the desired excess pressure to increase delivery rate or shorten the intake or delivery operation. A delivery pump is used preferably for each hydraulic circuit. 
     Valve circuits are used in the HCU, which effect a build-up of pressure via the inlet valves indicated and a reduction in pressure via outlet valve  10  into the corresponding return lines  11  to the reservoir. The volume extracted from the wheel cylinder for the purpose of reducing pressure must be generated by the TMC piston when the increase in pressure follows. Since the volume for ABS function is 3-5 cm 3 /s, at 20 s control time, the TMC needed to be far too big. Consequently, redelivery of the volume takes place in accordance with the criteria already described at intermittent intervals as described above. 
     The position of the push rod piston  4  is generally determined by the turning angle sensor of the brake servo unit  2  and consequently a position for redelivery can be specified here. The position is not reported to the accumulator chamber piston. Only the end position can be assessed from the pressure increase gradient of the pressure sensor  23  in the push rod piston circuit compared with the push rod piston position. 
     An interim position can only be estimated from the control signal as described. 
     It is expedient for safety reasons to determine the position of the accumulator chamber piston  5  via a target  14  using a sensor which can be determined easily using an Hall sensor and permanent magnet as target  14  in the piston. These means can be used to achieve rapid redelivery where the pistons are in a safe position and where there is still sufficient volume for emergency braking in the event of the malfunction of the brake servo unit  2 , via the brake pedal for example. Preferably a brake servo unit with travel simulator according to DE102005018649 is used here to which full reference is made herein for disclosure purposes. 
     The embodiment according to  FIG. 2  has the same design in respect of brake servo unit and TMC. The HCU is designed in accordance with so-called multiplex operation as described in DE 102005055751 to which full reference is made herein for disclosure purposes, and thus the outlet valve is not necessary since pressure is built up and reduced through corresponding piston control. These systems require an additional 2/2-way solenoid valve  15  in the connection between brake circuit  4   a  and  5   a  and the reservoir via line  11 . This valve is necessary, for example, for so-called free travel clearance as described in DE 102005055751 to which full reference is made herein for disclosure purposes, upon which volume from the brake circuit is discharged into the reservoir via the solenoid valve  15  so that the push rod piston moves forwards and there is no collision with the indicated brake pedal or its push rod respectively. If there is redelivery in this system, this can be performed via said solenoid valve  15  and the line  11 . Since rapid snifting or redelivery takes place via the preliminary pump  9  via the solenoid valves  15  for both circuits, only a small snifting bore  7   a  can be used which considerably improves sleeve friction as the main cause of THC malfunction. There is also correspondingly less free travel. This small snifting bore ensures temperature adjustment in a stationary vehicle. In this context, the sealing sleeve  8  can be more rigid in design, i.e. resistant to negative pressure. This is advantageous for brake lining ventilation control using negative pressure in order to save on two solenoid valves in the connecting line to the reservoir. Alternatively, this is not necessary if the preliminary pump  9  is activated during this operation. Thus the connection between the TMC and the reservoir  1  is separate. By means of corresponding control of the valves in the HCU and the TMC pistons, the brake pistons can be controlled individually via negative pressure in order to prevent greater friction on the brake lining. 
     Redelivery can take place via the separate preliminary pump  9  individually per brake circuit. Both backing pumps can be activated together to save costs; the brake circuits are controlled individually via the 2/2 solenoid valves  15 . 
     The option for redelivery on initial braking has already been described above. 
       FIG. 3  describes the arrangement and design of the preferably electromagnetically activated pump, integrated into the TMC housing in this exemplary embodiment. This is particularly advantageous if the pump pistons with seal have a similar design to the TMC pistons. The piston bore can be made using the same or similar tools in a clamping device. 
     In  FIG. 3  the pump is shown principally from the outside having pistons  16 , return spring  17  and seal  24 . The pump pistons are in the starting position where the snifting bore  7   a  is connected to the piston chamber  4   a ,  5   a  via the radial groove  25  and the intake channel  26  to the reservoir. When the solenoid  20  is activated, magnetic flux is generated accordingly via the magnetic circuit  19  and the rotor  22  effects the corresponding magnetic force on the pistons  16  to control the pressure. 
     The rotor is mounted twice in the bearing sleeve  18  and front bearing  18   a  which is integrated into the solenoid body. The magnetic circuit can have the standard poles to generate greater initiating force. The magnetic circuit can be designed as round or flat from laminated panels which reduces magnetic losses, saves construction space and improves response time. In this arrangement the magnet housing projects into the space at risk in the event of a crash. Therefore, the housing flange or attachment  21  can be designed such that this zone is soft for the crash sequence, i.e. can be sheared. 
     The TMC can be considerably smaller in size in a rapid delivery arrangement since it can actually only be designed for the fall-back level at approx. 100 bar. If higher pressure is needed with the brake servo unit, for fading, for example, a higher pressure level of 150 bar can be redelivered in approx. 50 ms. This dwell time has a negligible effect on the braking distance which is good for the chassis in the event of long delays, a transient effect for further pressure control. 
     Many functions can be performed with this preliminary pump at low cost. 
     The invention relates to an actuating arrangement for a vehicle braking system which, advantageously, can have a delivery device as described above and below. 
     A further hydraulic piston-cylinder unit is provided here, which can be actuated by the actuating arrangement and the first piston-cylinder unit can be actuated by means of the servo unit in order to feed hydraulic fluid into the brake circuit. 
     An actuating device is already known from the “Brake Manual”, 1 st  edition, Vieweg Verlag, wherein the servo unit is a vacuum brake servo unit. A hydraulic aggregate (HCU) has an inlet valve and an outlet valve on each wheel brake. Furthermore, accumulator chambers are assigned to the brake circuits in the HCU and a redelivery pump driven by an electric motor is provided to feed the brake fluid in the accumulator chambers back to the TMC. The redelivery pump is a piston pump which causes pressure pulsations in the TMC. Additional damper chambers are provided to reduce the associated noise. Although parallel pressure control in the wheel brakes is possible with this device, it is expensive overall and is usually combined with a vacuum brake servo unit. However, this does not match the general trend which will be based on electric brake servos in the future. 
     The solution according to the invention involves an actuating device for vehicles which manages without a vacuum brake servo unit. A redelivery pump is also unnecessary in this solution thereby eliminating the problems associated with such pumps. 
     An accumulator is expediently provided in the actuating device and consequently the hydraulic fluid can be redelivered from the accumulator to the brake circuit. This configuration allows individual pressure reduction. 
     The servo unit advantageously has an electromotive drive wherein a gearing mechanism can be provided which in particular is coupled to the piston in the first piston-cylinder unit in a positive-fit or force-fit manner and consequently movements of the gearing mechanism in both directions are transferred to the pistons. 
     According to further embodiments the actuating device is connected to a travel simulator. Said travel simulator can be connected to a pressure chamber in the further piston-cylinder unit. 
     A mechanism that can be activated by the actuating arrangement, which has two elements that can be moved relative to each other between which an elastic element is arranged, is provided in further embodiments. 
     A further hydraulic line can expediently be provided in which a valve arrangement is activated, leading from the hydraulic line leading to the wheel brakes to the brake fluid reservoir in order to enable free travel clearance. 
     The actuating device  31  for a vehicle brake system shown in  FIG. 4  has an actuating arrangement  32  which is provided in particular with a brake pedal  33  which is swivel-mounted on a bearing pedestal  34  and to which a push rod  65  is linked. The actuating arrangement  32  acts on a mechanism, which has a first piston-cylinder unit  36 , a servo unit  37  and a transmission unit  38 , which transfers the pedal force via the push rod  35  to the push rod piston  44 . Furthermore, a hydraulic control unit (HCU)  39 , various sensors and an electronic control unit ECU (not shown here) are provided. 
     The first piston-cylinder unit  36  has a housing  40  which is connected to the servo unit  37 . Two pistons  43 ,  44  are arranged in the housing in an axially displaceable manner. The first piston (FP)  43  forms a first pressure chamber  45  and the second piston (PRP)  44  a second pressure chamber  46  and both thus form a tandem master cylinder (TMC). The pistons  43 ,  44  are supported on the housing and against each other via springs  47 ,  48 . Openings  49 ,  50  are provided in the housing which lead to hydraulic lines  51 ,  52  that are connected to the HCU  39 . Further openings  55 ,  56  in the housing  40  are sealed relative to the pistons  43 ,  44  and guide hydraulic lines to a brake fluid reservoir  53  at normal pressure. The piston  44  has recesses on both sides one of which receives the end of the spring  48 . 
     The servo unit  37  connected to the first piston-cylinder unit  36  has a housing  40  in which an electric motor  61  with stator  62  and rotor  63  is arranged wherein the latter is rotatably mounted in the housing via bearings. A gearing mechanism or a mechanism for converting the rotational movement of the rotor  63  into a linear movement is arranged concentrically in the rotor. Said mechanism has a ball screw  64  here, which is arranged in the rotor in a torque proof and axially displaceable manner and which acts together with a spindle nut  64   a,  which is fixedly attached to the rotor. A push rod  65  is mounted on the spindle  64 ; a magnetic coupling can be provided on the end of said push rod facing the actuating mechanism. The front end of the ball screw is arranged here in the recess of the piston  44  which projects into the housing  60 . The spindle  64  is fixedly attached to the pistons  44  via a magnetic coupling on the front end of the screw and consequently movements of the spindle in both directions are transferred to the pistons. A sensor  54  is provided to determine the rotational movement of the rotor  63 . 
     The transmission unit  38  is mounted on the servo unit housing. This forms a recess  70  which receives the back end of the ball screw and the push rod  65 . A space  66  is formed in the cylinder which receives a piston  67 . 
     The piston  67  forms a central extension  68  which projects through an opening in the base of the cylinder  69  into the recess  70  in order to act together with the push rod  65 . 
     The piston  67  forms a cylindrical recess in which an element  71  is arranged in an axially displaceable manner. An elastic member  72 , for example a disc spring, flat spring or a rubbery elastic element or similar is arranged between the cylinder and the element  71 . Two distance sensors  73 ,  74  are also provided in the transmission unit  38  which can be used to measure the distances covered by the piston  67  or the element arranged thereon  71 . The corresponding values are delivered to the ECU in order to control the servo unit via the differential values proportional to pedal force. The push rod  35  in the actuating arrangement is connected to the element  71  via a universal joint and consequently movements are transmitted in both directions. 
     The HCU  39  provided between the TMC  36  and the wheel brakes FL, FR, RL, RR has various valves which are controlled by the ECU. Each of the wheel brakes is connected to a pressure chamber in the TMC. A currentless, open 2/2 way magnetic valve  75  is activated in this connection. A currentless, closed 2/2 way magnetic valve  76  is arranged in a connection leading from the wheel brake via a return valve  77  and via one of the hydraulic lines  51 ,  52  to the corresponding pressure chamber of the TMC and thus to the brake fluid reservoir  53 . Furthermore, an accumulator chamber  78  is arranged in this connection upstream of the return valve  77 . The valve configuration described above for a wheel brake FL is provided accordingly for the other wheel brakes FR, RL and RR as shown in the drawing. 
     The function of the device shown in  FIG. 4  is described below based on the starting position shown: 
     Pressure builds up in the pressure chambers of the TMC  36  on activation of the device such that brake fluid can flow via the open valves  75  to the wheel brake cylinders thereby activating the wheel brakes. If the ABS is active, for example, the pressure can be kept constant by closing the valve  75  or reduced by opening the valve  76 . When pressure is reduced, the brake fluid flows into the accumulator chamber  78 . At certain intervals when the accumulator chamber is almost full, the TMC is returned via the servo unit drive as a result of which the accumulator chamber is emptied if the inlet valves are closed. The activation and corresponding control of the TMC to empty the accumulator can make a return pump, as normally used in such systems in the known cases, redundant. The inlet valves are designed such that they operate even in the event of great differential pressure on both sides. A return valve that is generally operated in parallel is not provided in said inlet valves. 
     In the event of a brake servo unit malfunction, foot power can be transmitted directly to the pistons  44  via the piston  67  or push rod  68  and push rod  65 . 
     In the device shown in  FIG. 5 , the actuating arrangement, servo unit and TMC are substantially the same as in the device according to  FIG. 4  and consequently no detailed description will be provided in this respect. 
     Unlike as in  FIG. 4 , in the embodiment according to  FIG. 5 , a hydraulic line  80  is provided, which connects the pressure chamber  66   a  via a currentless, open 2/2 way solenoid valve  86  and a hydraulic line  84  to the reservoir  53 . The hydraulic line  84  is connected to the hydraulic line  52  via a currentless, closed 2/2 way valve  89  which leads to the pressure chamber  46  of the TMC, wherein the solenoid valve serves as a distance simulator. Openings  81 ,  82  are provided in the TMC to connect the line  84  and a corresponding line  83 , where said openings open out into a line provided in the wall of the TMC, said line is connected to the reservoir via a hydraulic line. The line provided in the wall has a groove here which is sealed in relation to the TMC piston on both sides by means of seals. The connection with the reservoir  53  can also made via lines that do not lead through the TMC. Currentless, closed 2/2 way solenoid valves are arranged in the hydraulic lines  83 ,  84 . When the hydraulic preliminary pump  9  is used, the return lines  83   a  and  84   a  lead directly to the reservoir  53 . A pressure sensor  90  is also provided in the line  84 . A hydraulic travel simulator  85  is also provided in this configuration which is connected to the line  80  via a currentless, open 2/2 way solenoid valve  86  and an arrangement  87  with throttle valve and return valve. The distance simulator  85 , which has a piston that is moveable in a cylinder against a spring, generates the desired reaction on the pedal force in this configuration in accordance with the spring characteristic of the distance simulator spring. The arrangement  87  with throttle valve and return valve serves for speed and direction-dependent restriction for the purpose of good response characteristics. A piston travel sensor  91  with sensor target  92  is arranged on the piston on the TMC and the reservoir  53  is equipped with an air pump  94  and a return valve  95 . 
     The function of the device shown in  FIG. 5  is described below: 
     When the device is activated by the driver, the piston  67  in the figure is displaced to the left resulting in the build-up of pressure in the pressure chamber  66   a  and via the line  80  in the connected travel simulator. Depending on the pressure desired by the driver or the resulting braking effect, the servo unit becomes active as a result of the actuation of the engine and the gearing mechanism, which acts on the pistons  44  by means of the recirculating ball screw such that pressure builds up in the pressure chambers and in the brake circuits accordingly. The solenoid valves  75  and  76  (and the corresponding solenoid valves which are assigned to the other wheel brakes) act in terms of building-up, maintaining and reducing pressure, by opening and closing in a known manner in order to perform functions such as ABS and ESP. The TMC acts as described in  FIG. 1  as a return pump. The decrease in pressure does not occur in the configuration according to  FIG. 5  in an accumulator chamber, however, but rather via lines  58 ,  84  via the TMC into the reservoir  53 . There is an option for design reasons to provide two connections for the return line to the reservoir  53 . 
     The volume of hydraulic fluid corresponding to the decrease in pressure is discharged from the brake circuit and then delivered again via the movement of the TMC piston. For safety reasons, in the event of malfunction of the brake servo unit, there must always be enough hydraulic fluid in the master cylinder piston chambers or pressure chambers respectively. Consequently, following respective piston movement or upon indirect evaluation of the volume when pressure decreases, for example on the basis of the pressure decrease time, pressure level from pressure model and temperature are returned according to the piston. There is an intake of hydraulic fluid volume into the piston chamber when the inlet valve (s)  75  is closed. 
     Intake via the valves  88 ,  89  is possible even at lowest negative pressure. The solenoid valves  88 ,  89  are preferably provided with a large cross section for this purpose in order to keep intake resistance low. This reduces the intake time. It is a significant advantage here that in each control mode, pressure build-up or pressure reduction, the pressure is maintained for a brief period in order to perform the intake operation so that sufficient volume reaches the piston chambers again. Preferably, however, in a pressure maintenance stage the intake operation is performed at least for the front wheels. 
     The volume discharged from the wheel cylinder circuit is supplemented again by piston movement and the intake operation. The position of the pressure circuit piston  44  is known via the turning angle sensor  54  in the engine. Conversely, the position of the floating piston  43  can only be determined via the pressure using previous diagnosis and the aforementioned estimation of volume. Therefore, the travel sensor  91  can be provided in an expedient manner in order to establish the position of the piston  43 . To simplify matters, evaluation of the position is sufficient which allows adequate residual volume for an increase in pressure even with fading. Preferably an echo sensor with a permanent magnet can be deployed on the piston. 
     To reduce the intake time, a pressure source  94 , in particular a compressed air pump can also be provided which generates pressure in the brake fluid reservoir  53  or in the connecting line to the TMC. This can, expediently, be effective in any braking action or in ABS operation. A return valve  95  is built into the cover of the reservoir  53  for this purpose which closes in the event of excess pressure. Alternatively, a delivery arrangement or backing pump can also be used for this purpose, as described particularly with reference to  FIGS. 1 to 3  and indicated in  FIG. 5  at  9  by a dotted line. 
     Intake can be used not only for the ABS operation described, but also to reduce the size of the TMC where there is an intake of additional volume in the infrequently high pressure range. 
     When pressure is reduced in the system via the brake pedal, the previous excess volume intake is discharged by assessing the piston position and pressure via the solenoid valve  76  or  89  in order to prevent sleeve damage in the TMC. 
     The system with distance simulator can be designed, in contrast to the one in  FIG. 5 , such that the ABS effect and the associated oscillation of the TMC pistons have no retrospective effect on the brake pedal. Both brake circuits with the associated hydraulic lines  51 ,  52 , each with a 2/2 way solenoid valve, are connected to the brake fluid reservoir  53  via return lines in the TMC for this purpose. The valves are opened if the piston  67  with the extension  68  has no free travel to the push rod  65  which can be established by evaluating the signals from the distance sensors  73 ,  74  and  54 . In this case, hydraulic free travel clearance is initiated wherein the distance between piston and extension  68  respectively and push rod  54  is altered when volume is discharged from the pressure chamber  45  and  46  separately or in parallel via the 2/2 solenoid valves  88 ,  89  into the brake fluid reservoir  23 . This function, which is also described in detail in DE 10 2010 045 617.9, to which reference is made here, is particularly expedient or necessary in ABS at low friction coefficient or recuperation too if the driver depresses the pedal further and the TMC piston has to travel further back in order to reach a low pressure level. A portion of the volume can be recovered again in the brake circuit if, for example, the friction coefficient changes from low to high. A small retrospective pedal effect can also be generated intentionally from the piston movement for free travel clearance purposes in order to indicate the use of ABS control to the driver. Guide the return movement of the piston to free travel=0 here and then the free control described will be controlled to the desired value or distance respectively. 
     LIST OF REFERENCE SIGNS  
       1  Reservoir 
       2  Power assist 
       3  TMC housing 
       4  Push rod piston 
       4   a  Push rod circuit 
       5  Accumulator chamber 
       5   a  Accumulator chamber circuit 
       6  Return spring 
       7  Snifting bores 
       7   a  Small snifting bore 
       8  Sealing sleeves 
       9  preliminary pump 
       10  Outlet valve 
       11  Return line to reservoir 
       12  Line to wheel brake 
       13  Position sensor 
       14  Sensor target 
       15  2/2 solenoid valve 
       16  Pump piston 
       17  Return spring 
       18  Rotor mounting  1   
       18   a  Rotor mounting  2   
       19  Magnet yoke 
       20  Solenoid 
       21  Magnet attachment 
       22  Magnet rotor 
       23  Pressure sensor 
       24  Seal 
       25  Radial groove 
       26  Intake channel 
       31  Actuating device 
       32  Actuating arrangement 
       33  Brake pedal 
       34  Bearing pedestal 
       35  Push rod 
       36  Piston-cylinder unit (TMC) 
       37  Servo unit 
       38  Transmission unit or piston-cylinder unit respectively 
       39  Hydraulic control unit (HCU) 
       40  Housing 
       43  Piston (floating) 
       44  Piston (push rod) 
       45  Pressure chamber 
       46  Pressure chamber 
       47  Spring 
       48  Spring 
       49  Opening 
       50  Opening 
       51  Hydraulic line 
       52  Hydraulic line 
       53  Brake fluid reservoir 
       54  Sensor 
       60  Housing 
       61  Electric motor 
       62  Stator 
       63  Rotor 
       64  Ball screw 
       65  Push rod 
       66  Pressure chamber 
       67  Piston 
       68  Extension 
       69  Cylinder base 
       70  Recess 
       71  Element 
       72  Elastic member 
       73  travel sensor 
       74  travel sensor 
       75  2/2 way solenoid valve 
       76  2/2 way solenoid valve 
       77  Return valve 
       78  Accumulator chamber 
       81  Opening 
       82  Opening 
       83  Hydraulic line 
       84  Hydraulic line 
       85  travel simulator 
       86  2/2 way solenoid valve 
       87  Throttle return valve arrangement 
       88  2/2 way solenoid valve 
       89  2/2 way solenoid valve 
       90  Pressure sensor 
       91  Piston travel sensor 
       92  Sensor target 
       93  Pressure source 
       94  Air pump 
       95  Return valve