Abstract:
A hydraulic steering assistance system for vehicles, including a power steering pump and a variable-ratio gear, particularly a booster drive for the power-steering pump wherein the power steering pump hydraulically powers both the hydraulic steering assistance system and the variable-ratio gear.

Description:
BACKGROUND 
     The present invention relates to a hydraulic steering assistance system (servo steering) for motor vehicles having a power-steering pump and a variable-ratio gear, in particular a booster drive for the power-steering pump. Under related art methods, the variable-ratio gear is adjusted by an electric drive, which has the disadvantage of placing heavy demands on the motor vehicle&#39;s electrical system and of making for actuators that are slow to respond and that have relatively large dimensions. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to devise a control device for the variable-ratio gear which will overcome these disadvantages. 
     This objective is achieved by a hydraulic steering assistance system (servo steering) for motor vehicles having a power-steering pump and a variable-ratio gear, in particular a booster drive for the power-steering pump, the power-steering pump hydraulically powering both the hydraulic steering assistance system, as well as a hydraulic system of the variable-ratio gear. In accordance with the present invention, the hydraulic system of the variable-ratio gear is hydraulically connected between the power-steering pump and a flow-control valve device for the steering assistance system. 
     It is a distinguishing feature of the hydraulic steering assistance system design of the present invention that the hydraulic system of the variable-ratio gear has at least one open-loop control, switching or closed-loop control device, in particular a valve or a restrictor, and a hydraulic control device, in particular a motor or a cylinder. A hydraulic steering assistance system is preferred in which the cylinder may be designed either as a single-action or double-action steering cylinder. 
     A steering assistance system is also preferred in which a position sensor or a speed sensor is additionally provided for the hydraulic control device of the variable-ratio gear. This has the advantage of enabling the transmission ratio of the variable-ratio gear to be controlled in a closed-loop control circuit. A hydraulic steering assistance system is also preferred, in which an electronic control device is provided for the open-loop control, switching or closed-loop control device. 
     A hydraulic steering assistance system has the feature that the hydraulic system of the variable-ratio gear is connected downstream of the power-steering pump in parallel to the steering assistance system. As a result, the pressure difference across the hydraulic system of the variable-ratio gear is equal to the pressure difference across the steering assistance system, while the volume flow of the pump is divided between the two systems. A hydraulic steering assistance system is also preferred, in which the switching, open-loop control or closed-loop control device of the hydraulic system of the variable-ratio gear is disposed in the main flow path of the power-steering pump. A hydraulic steering assistance system is also preferred, in which the open-loop control, switching or closed-loop control device is disposed in a bypass flow path (branched-off bypass flow) of the power-steering pump. Here, the advantage is derived that the open-loop control, switching or closed-loop control device may be designed to be smaller in size. 
     Another hydraulic steering assistance system according to the present invention has the feature that the hydraulic system of the variable-ratio gear is connected downstream of the power-steering pump in series with the steering assistance system. Here, the advantage is derived that the power output of the control device may be controlled independently of the pressure prevailing in the steering assistance system. A hydraulic steering assistance system is also preferred, in which a pressure sensor is disposed upstream of the open-loop control, switching or closed-loop control device. Here, the advantage is derived that a pressure-limiting function is able to be realized by the pressure sensor. 
     A hydraulic steering assistance system has the feature that the hydraulic control device of the variable-ratio gear automatically resets itself to a minimum speed of the variable-ratio gear in response to corresponding forces, in particular at zero pressure in response to spring forces, or in response to equal pressurization of different-sized effective pressure areas. A hydraulic steering assistance system is also preferred in which the variable-ratio gear constitutes a booster drive, which, at a low speed of the combustion engine, steps up the speed of the power-steering pump and, conversely, at a high speed of the combustion engine, steps down the speed of the power-assisted steering pump, preferably to the same speed as that of the combustion engine. 
     Another hydraulic steering assistance system has the feature that the open-loop control, switching or closed-loop control device of the variable-ratio gear is pilot controlled. Here, the advantage is derived that smaller magnets may be used for the pilot stage. 
     A hydraulic steering assistance system is also preferred in which the electronic control device for the variable-ratio gear considers or evaluates additional signals or driving state variables such as speed and/or steering-wheel angular velocity and/or transmission ratio of the steering gear and/or driving speed. 
     A hydraulic steering assistance system also has the feature that, in the case of a series circuit connection of the steering assistance system and the hydraulic control device of the variable-ratio gear, the open-loop control, switching or closed-loop control device is connected in parallel to the control device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is described in the following with reference to the figures, in which: 
         FIG. 1  shows an embodiment of the present invention having a valve being incorporated in the main flow path. 
         FIG. 2  shows an embodiment of the present invention having a valve being incorporated in the bypass flow path. 
         FIG. 3  shows an embodiment of the present invention having a variable restrictor. 
         FIG. 4  shows an embodiment of the present invention having a position sensor for the hydraulic cylinder. 
         FIG. 5  shows an embodiment of the present invention having a pilot controlled throttle valve. 
         FIG. 6  shows a variant of  FIG. 5 , the return flow being directed from valve  104  to the steering gear. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates the adjustment of the booster drive by a hydraulic cylinder, a parallel circuit connection being provided between the system of the booster drive and the steering assistance system. A power-steering pump  1  communicates via a hydraulically actuated, variable-ratio gear  3 , thus the booster drive, with a driving engine  5 , typically the combustion engine. At low speeds of combustion engine  5 , for example in the idling state, and to respond to high volumetric flow demands of the servo steering, it is intended that booster drive  3  drive power-steering pump  1  at a higher speed to enable it to supply an appropriate volumetric flow for the high steering-wheel angular velocities of the servo steering. At higher combustion engine speeds, power-steering pump  1  only requires lower volumetric flows, so that the power-steering pump may be driven at a reduced speed, and booster drive  3  may, therefore, again be stepped down accordingly. This state may suffice in most operating points of the motor vehicle. 
     Power-steering pump  1  delivers its volume flow via a connecting line  7  to a valve device  9 . Valve device  9  may be an infinitely variable valve, such as a proportional valve, for example, but also a valve actuated by pulse-width modulation or, optionally, even a switching valve. Actuating magnet  11  of valve  9  is activated by an electronic controller  13 . In addition, valve  9  communicates via a line  15  with a hydraulic cylinder  17  which constitutes the control device for booster drive  3 . The hydraulic cylinder essentially includes a piston  19  having a large effective piston area  21 , a smaller piston-ring area  23  and a return spring  27 . Where appropriate, other cylindrical designs may likewise be used. Large effective piston area  21  communicates via line  15  with valve  9 ; smaller ring area  23  via a line  29  with reservoir  31 . Thus, pressure is not able to build up on ring area  23 , rather the restoring force acting against a pressure force on effective piston area  21  may only be effected by return spring  27 . Cylinder  17  is connected via a connecting rod  33  to booster drive  3 , in which a certain eccentric adjustment mechanism is typically adjusted from the “centric” position to the “maximum eccentric” position. In the open/close valve position of valve  9  shown here, which is reached in response to the action of a spring  35  against magnet  11 , which, in this case, is de-energized, connecting line  15  leading to cylinder  17  communicates likewise at zero pressure via a connecting line  37  with reservoir  31 . Thus, in the pressureless position, the control device for booster drive  3  including cylinder  17  is switched to a setting at which the speed of the power-steering pump is not stepped up, rather, as the case may be, at which it is equal to the speed of combustion engine  5 . As soon as valve  9  or magnet  11  is energized via electronic control device  13 , valve  9  is switched against the force of spring  35  into the other position, or is adjusted to a corresponding intermediate position in such a way that connecting line  7  of power-steering pump delivers both to supply line  15  of the hydraulic cylinder, as well as to a line  39  extending to steering assistance device  49 . Disposed subsequently thereto is a flow control device for power-steering pumps, as is known from the related art, having a main-flow restrictor  41 , at which a pressure difference is generated to adjust a pressure regulator  43 , pressure regulator  43  allowing the volume flow not needed for the steering operation to flow off to reservoir  31 , i.e., back into the suction side of pump  1 . In addition, the known flow control device for servo steering systems includes a pilot restrictor  45  and a pressure-limiting pilot valve  47 , which make it possible, when a maximum pressure is reached, for the flow-control valve system to function as a pilot controlled pressure-limiting system, and for the complete volume flow of power-steering pump  1  to be returned to reservoir  31 , i.e., into the suction side of power-steering pump  1 . Thus, the volume flow of power-steering pump  1  is divided into an adjusting volume flow for hydraulic cylinder  17  and a delivery volume flow QV which is directed to servo steering  49 . With regard to the circuit configuration in  FIG. 1 , it is worth mentioning that the system of booster drive  3 , thus cylinder  17 , is connected in parallel to steering assistance device  49 , and control valve  9  is disposed in main-flow line  7  of power-steering pump  1  in a series circuit upstream of flow-control valve system  41 ,  43 . 
     Another circuit configuration is shown in  FIG. 2 . In principle, it does not differ in most of its elements from the illustration of  FIG. 1 , except for the fact that, here, a control valve  51  is located in a bypass channel  53  leading out from main flow line  7 . This means that valve  51  merely needs to be designed to accommodate the magnitude of volume flow to supply control cylinder  17 , since the remaining volume flow directed to steering assistance device  49  does not need to flow through valve  51 , as it does in  FIG. 1  through valve  9 . Thus, in this circuit configuration, a smaller valve  51  may be used. All of the other elements correspond in their function to the elements of  FIG. 1  and will not be described again for the sake of avoiding repetition. 
       FIG. 3  depicts an adjustment of the booster drive by a hydraulic motor, in this case, a series circuit connection being provided between the hydraulic system of the booster drive and the steering assistance system. The hydraulic system of booster drive  3  between driving engine  5  and power-steering pump  1  is represented here by a restrictor  64  and a hydraulic motor  60  which, as the case may be, is able to be operated in both directions of rotation by return devices (not shown here) and is connected via a connecting line  62  to pressure-outlet line  7  of the power-steering pump, as well as via a second line  66  to line  39  upstream of the flow-control valve system. Disposed in the main line between line section  7  and line section  39  is a variable restrictor  64 , for example a proportional restrictor valve, which, in this case, may also be adjusted by a suitable control electronics  68 . Thus, in this case, the system, including hydraulic motor  60  and the adjusting element, namely variable restrictor  64 , is arranged in series upstream of flow-control valve  41 ,  43 , downstream of line  39 , the volume flow at flow-control valve  41 ,  43  then being able to flow to steering assistance device  49  or via pressure regulator  43  back to reservoir  31 , i.e., back into the suction side of pump  1 . In contrast to the parallel circuits of the hydraulic system of the booster drive for steering assistance device  49  shown in  FIGS. 1 and 2 , in which the pressure, which is automatically adjusted in steering assistance device  49  in response to the forces acting at the wheels, also affects control system  17  of booster drive  3 , thus the pressure prevailing in steering assistance device  49  is equal to the pressure prevailing in control system  17  of booster drive  3 , in the circuit configuration in  FIG. 3 , the pressures of the two systems are realized in such a way that the pressure prevailing in steering assistance device  49  is added to the pressure difference of the system, including hydraulic motor  60  and variable restrictor  64 , and both pressure differences, together, produce the pressure at pump  1 . The advantage of this circuit configuration is that the requisite pressure difference for actuating hydraulic motor  60  may be set via variable restrictor  64  independently of the pressure difference in steering assistance device  49 . By employing a pressure sensor  70 , it is possible, for example, to additionally safeguard the maximum pressure to the effect that, in response to exceedance of the maximum pump pressure, variable restrictor  64  opens completely, preventing any further drop in the pressure difference across the system and thereby allowing the pressure-limiting valve, which includes pilot valve  47  and pressure regulator  43 , to become active at maximum pressure. 
     Another circuit diagram for a system of the booster drive is shown in  FIG. 4 , where, to begin with, hydraulic motor  60  of  FIG. 3  has been replaced with a double-action control cylinder  80 . Control cylinder  80  communicates by way of its large effective piston area  81  via line connection  82  with pump outlet line  7 , while a smaller ring area  83  communicates via a line connection  84  with line section  39  upstream of flow-control valve  41 ,  43 , but downstream of a variable restrictor  86 , which, in response to actuation by an electronic control device  88 , again allows an appropriate pressure difference to be produced across control cylinder  80 . However, a control cylinder having two surface areas of equal size may also be used. It is even beneficial when the ring area at the rear piston lateral face is equal in size to the front lateral face, since this ensures that the flow volume is not “swallowed” by the cylinder, thus ruling out any negative effects on the steering performance due to insufficient flow volume. In addition, control cylinder  80  has a sensor device  92 , which may be used, for example, to sense the lift of stroke of cylinder  80  and is thus able to transmit a displacement signal  90  indicative thereof to electronic control device  88 . Thus, this displacement-sensor feedback  90  allows operation of a closed-loop control circuit, which holds cylinder  80  in a specific position, so that the transmission ratio of booster drive  3  is infinitely adjustable in response to this control device. A control loop of this kind would likewise be conceivable in  FIG. 3  at hydraulic motor  60  if appropriate speed sensors were provided at the gear output, respectively speed or angle-of-rotation sensors at hydraulic motor  60 . All other functions correspond to the functions already presented with reference to  FIG. 3 . 
     In contrast to  FIG. 4 , in  FIG. 5 , variable restrictor  86  schematically represented in  FIG. 4  has been replaced with a pilot controlled throttle valve which includes a main stage  100  and a pilot stage  102 . By providing a smaller pilot stage  102 , it is possible to precontrol a main stage  100  for large volume flows using a lower-power control magnet  104 , thereby eliminating the need for large magnets for actuating valves of equivalent size. To this end, the pressure in line  82 , respectively  7 , upstream of the actual throttling point  108  of the main stage, acts on main stage  100  via effective piston area  106 , while a spring-chamber piston area  110  communicates via a pilot restrictor  112  with the pressure in line  82 . Thus, in response to opening of pilot stage  102 , the pressure falls off in the spring chamber at area  110  and through restrictor  112  and thus also allows the main stage to be opened. Thus, the throttling device is pilot controlled by a pilot valve  102  in the manner of a classical pressure regulator  100 , in response to opening of pilot valve  102 , the pressure being allowed to decrease at a pilot restrictor  112 , and, as a result, main throttle  100  reducing the size of throttling point  108  in response to the higher pressure acting on piston lateral face  106  as compared to the lower pressure on piston lateral face  110 . All other functions are described in the preceding figures. A pressure-limiting function may also be realized at pressure chamber  106  by using a pilot restrictor (not shown here) and an additional pressure-limiting pilot valve (such as pilot  47  in  FIG. 1 ), and by combining the same with the action of the flow controller, including elements  41 ,  43 ,  45 ,  47 . 
     The circuit configuration in  FIG. 6  differs from that of  FIG. 5  in that the return flow of pilot valve  102  is not directed into the reservoir, but to the steering gear, thus downstream of main-flow restrictor  41 . To that end, return line  120  leads back to point of intersection  122  downstream of main-flow restrictor  41 . This circuit configuration has two advantages: On the one hand, the pressure prevailing across valve  102  does not vary within an overly broad range, making the control simpler. On the other hand, the flow volume is supplied to consuming device  49  (the steering gear) and is not delivered to the reservoir, which, in certain circumstances, has a positive effect on the steering feel. 
     In all of the electrical control units presented in the descriptions, the adjustment of the variable-ratio gear may be influenced according to the circumstances, as a function of different driving state variables, such as the speed of the combustion engine, the steering-wheel angular velocity, the transmission ratio of the steering gear or the driving speed of the motor vehicle, so that the transmission ratio of the booster drive may be adjusted, and the speed of pump  1  may be varied relative to the speed of driving engine  5 , typically of a combustion engine. The inherent advantage of the variants which provide for a series circuit connection of the hydraulic system of booster drive  3  to steering assistance device  49  is that the power output of the hydraulic adjusting element, such as motor  60  or cylinder  80 , may be controlled via variable restrictors  68 ,  86 ,  100  independently of the system pressure of steering assistance device  49 . If an angular-position or displacement sensor  92  or speed sensor is additionally used in the hydraulic control mechanism of the booster drive, the position of piston  81  or of hydraulic motor  60 , and thus the transmission ratio of the booster drive may be utilized as the input variable for control unit  88 . This makes it possible to control the transmission ratio of the booster drive. This applies in principle to all of the circuit configurations presented here. The systems presented here also have a decisive advantage over known electrical systems, that the electrical systems are slow, require large control elements and place heavy demands on the vehicle electrical system. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  power-steering pump 
           3  hydraulically actuated, variable-ratio gear 
           5  driving engine (combustion engine) 
           7  connecting line 
           9  valve device 
           11  actuating magnet 
           13  electronic controller 
           15  line leading to the hydraulic cylinder 
           17  hydraulic cylinder 
           19  piston of the hydraulic cylinder 
           21  large effective piston area of the hydraulic cylinder 
           23  small piston-ring area of the hydraulic cylinder 
           27  return spring of the hydraulic cylinder 
           29  line leading from the small ring area  27  to the reservoir  31   
           31  reservoir 
           33  connecting rod to the booster drive 
           35  spring of valve  9   
           37  connecting line from valve  9  to reservoir  31   
           39  line extending to steering assistance system  49   
           41  main-flow restrictor 
           43  pressure regulator 
           45  pilot restrictor 
           47  pressure-limiting pilot 
           49  servo steering, steering assistance device 
           51  control valve 
           53  bypass channel 
           60  hydraulic motor 
           62  connecting line leading to the hydraulic motor 
           64  variable restrictor 
           66  second connecting line leading to the hydraulic motor 
           68  control electronics 
           70  pressure sensor 
           80  control cylinder 
           81  large effective piston area 
           82  connecting line of the large effective piston area 
           83  smaller piston-ring area 
           84  connecting line of the smaller piston-ring area 
           86  variable restrictor 
           88  electronic control device 
           90  displacement signal 
           92  sensor device 
           100  main stage of the precontrolled throttle 
           102  pilot stage of the precontrolled throttle 
           104  control magnet of the pilot stage 
           106  effective piston area of the main stage 
           108  throttling point of the main stage 
           110  spring chamber at effective piston area of the main stage 
           112  pilot restrictor 
           120  return line of valve  102   
           122  line junction (point of intersection)