Abstract:
A hydraulic steering system ( 1 ) including a hydraulic steering unit ( 2 ) and an auxiliary device ( 3 ). Said The steering unit ( 2 ) feeds a hydraulic medium to a steering cylinder ( 5 ) according to the movement of a steering wheel ( 4 ). The auxiliary device ( 3 ) includes a valve unit ( 6 ) and also feeds a hydraulic medium to the steering cylinder ( 5 ) according to the movement of the steering wheel ( 4 ), in order to assist the steering movement. The hydraulic medium ejected or discharged from the steering cylinder ( 5 ) as a result of the hydraulic medium fed thereinto flows back into a tank ( 10 ) or to a pump ( 8, 9 ) through the steering unit ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a hydraulic steering system having a hydraulic steering unit and a power assist device. The hydraulic steering system is used, for example, in mobile machines in the construction and agricultural sector, but also in floor conveyor vehicles for example. 
   DISCUSSION OF THE PRIOR ART 
   For example, DE 38 37 395 A1 describes a hydrostatic power-assisted steering system for vehicles which has a power assist device. The power assist device, which is arranged very indirectly and mostly far away from the steering means, is subject to numerous fault influences. The hydraulic steering system with power assist device, also referred to as a hydrostatic power-assisted steering system, is not protected against malfunctions of the power assist device. 
   It is a disadvantage of the hydraulic steering system described by DE 38 37 395 A1 that malfunctioning of the power assist device can result in a steering behaviour which is not controllable by the driver. 
   SUMMARY OF THE INVENTION 
   It is therefore the object of the invention to provide a hydraulic steering system having an assist device which allows the vehicle to be controllably steered even in the event of malfunctioning of the assist device. 
   The object is achieved according to the invention by a hydraulic steering system with assist device having the features of claim  1 . 
   The measures set out in the subclaims relate to advantageous developments of the hydraulic steering-system according to the invention. 
   In particular, it is advantageous if the hydraulic medium flowing off from the steering cylinder is prevented from flowing back via the assist device or the valve unit by at least one blocking device, for example in the form of nonreturn valves, which permits the flow of hydraulic medium only in one direction. 
   It is additionally advantageous to configure the valve unit of the assist device as a proportional valve. The servo control can thereby take place even more precisely. 
   Furthermore, it is advantageous to regulate or control the servo control by an electronic control device in dependence on at least the steering movement. In particular, the servo control can thereby be made easily influenceable by further command variables, for example the vehicle speed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail below, by way of example, using an exemplary embodiment with reference to a schematic illustration. Corresponding components are provided here with corresponding reference symbols. 
       FIG. 1  shows a schematic illustration of a hydraulic steering system according to the invention; 
       FIG. 2  shows a more detailed schematic illustration of a first exemplary embodiment of the hydraulic steering system according to the invention; 
       FIG. 3  shows a more detailed schematic illustration of a second exemplary embodiment of the hydraulic steering system according to the invention; 
       FIG. 4  shows a schematic illustration of a second unblockable assist device, and 
       FIG. 5  shows a schematic illustration of a second exemplary embodiment of an unblockable assist device. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The hydraulic steering system illustrated in  FIG. 1  and denoted by the reference symbol  1  consists essentially of a steering unit  2 , an assist device  3 , a steering cylinder  5  and of a right line RL and a left line LL, which hydraulically connect the steering unit  2 , the assist device  3  and the steering cylinder  5 . 
   In the exemplary embodiment illustrated, the steering unit  2  comprises a steering means  4  embodied as a steering wheel, and a valve block  12 . Arranged on the steering means  4  is a sensor  11  which transmits data on, for example, the position of the steering means  4  via an electrical line  13  to a control device  7  of the assist device  3 . In the exemplary embodiment, the steering unit  2  or the valve block  12  is fed in a pressurised manner with hydraulic medium from a tank  10  and a first pump  8 , embodied for example as a gear pump, via a supply connection P of the valve block  12 . The valve block  12  additionally has a left connection L and a right connection R. The pressure prevailing at the supply connection P is transmitted via valves (not illustrated) in the valve block  12  either to the left connection L or the right connection R depending on the movement of the steering means  4 , whereby an amount of hydraulic medium typically in proportional relationship to the degree of turning of the steering means emerges from the left connection L or the right connection R. 
   The control device  7  controls or regulates a valve unit  6  of the assist device  3  in dependence at least on the movement of the steering means  4 . The valve unit  6  is likewise fed in a pressurised manner with hydraulic medium from the tank  10  via a second pump  9  and via a supply connection P′. The valve unit  6 , configured as a proportional valve, introduces an amount of hydraulic medium corresponding to the control commands of the control device  7  into the left line LL via a left connection L′ and into the right line RL via a right connection R′ and/or builds up a corresponding pressure. The left line LL leads from the left connection L′ of the valve unit  6  to the left connection L of the steering unit  2 . The right line RL leads from the right connection R′ of the valve unit  6  to the right connection R of the steering unit  2 . 
   The right line RL has a right junction RA which is connected to a right steering-cylinder chamber RZ of the steering cylinder  5 . The left line LL has a left junction LA which is connected to a left steering-cylinder chamber LZ of the steering cylinder  5 . Accordingly, hydraulically the right connections R′ and R are connected in parallel and the left connections L and L′ are connected in parallel, it being the case that, for example, the left connections L and L′ in other exemplary embodiments (not illustrated) can each be connected via separate hydraulic lines to the left steering-cylinder chamber LZ and then are in permanent connection via the latter. The valve unit  6  is in connection with the tank  10  via a discharge connection T′. The steering unit  2  is in connection with the tank  10  via a discharge connection T of the steering unit  2 . 
   If, for example, the steering means  4  is turned to the right, the right line RL is fed from the right connection R of the steering unit  2  with an amount of hydraulic medium depending on the extent of the turning movement of the steering means  4 . In order, for example, to assist the steering action or ensure a steering action even in the event of a failure of the functioning of the valve block  12 , the control device  7  induces, owing to the movement of the steering means  4  transmitted by the sensor  11 , the valve unit  6  to feed hydraulic medium from the right connection R′ of the valve unit  6  into the right line RL. 
   Owing to the amount of hydraulic medium fed into the right line RL by the valve block  12  and the assist device  3 , the right steering-cylinder chamber RZ in the steering cylinder  5  increases, while the left steering-cylinder chamber LZ, opposite the right steering-cylinder chamber RZ, decreases as a result of the increase of the right steering-cylinder chamber RZ. As a result of the decrease of the left steering-cylinder chamber LZ, a corresponding part of the amount of hydraulic medium situated therein is displaced into the left line LL. Accordingly, if the steering means  4  is turned to the left, hydraulic medium flows out of the right steering-cylinder chamber RZ into the right line RL. 
   Arranged between the left junction LA and the left connection L′ of the valve unit  6  is a first blocking device, configured in this exemplary embodiment as a left nonreturn valve LR, which prevents hydraulic medium from being able to flow off via the valve unit  6  from the steering cylinder  5  or the left steering-cylinder chamber LZ and the left line LL, or from the system comprising steering cylinder  5  and left line LL. 
   Arranged between the right junction RA and the right connection R′ of the valve unit  6  is a second blocking device, configured as a right nonreturn valve RR, which prevents hydraulic medium from being able to flow off via the valve unit  6  from the steering cylinder  5  or the right steering-cylinder chamber RZ and the right line RL, or from the system comprising steering cylinder  5  and right line RL. 
   In the exemplary embodiment shown, the whole amount of hydraulic medium returned from the steering cylinder  5  is thus always returned via the steering unit  2  and not via the power assist device  3  or the valve unit  6 . The device which prevents the hydraulic medium from flowing back or permits it only in one direction, namely out of the system of steering cylinder  5  and lines RL and LL, may in other exemplary embodiments also be arranged, for example, in the valve unit  6  or formed by appropriately modified control pistons of the valve unit  6 . 
   In the event of a malfunction of the assist device  3  or the valve unit  6 , the valve unit  6  may suddenly attempt to convey hydraulic medium through the left connection L′ into the left line LL or through the right connection R′ into the right line RL. Without the features according to the invention of the hydraulic steering system  1 , in the exemplary embodiment the hydraulic medium displaced from the steering cylinder  5  could flow off via the respective other connection L′ or R′ of the valve unit  6 , resulting in unintentional or surprisingly severe and sudden steering movements of the vehicle, since countersteering by the driver can then only take place with a delay. 
   If the steering means  4  is in neutral position for example, an incorrect attempt by the assist device  3  to meter hydraulic medium through the left connection L′ of the valve unit  6  into the left line LL does not result in hydraulic medium flowing off from the right steering-cylinder chamber RZ, on the one hand since the hydraulic fluid cannot flow off from the right steering-cylinder chamber RZ via the right connection R of the steering unit  5  since this connection is closed by the neutral position of the steering means  4 , and on the other hand because the flowing-off of hydraulic medium via the right connection R′ of the valve unit  6  is prevented by the right nonreturn valve RR. There is thus no steering movement of the vehicle. The same applies analogously if the steering means  4  is in neutral position and there is an incorrect attempt by the power assist device  3  to meter hydraulic medium through the right connection R′ of the valve unit  6  into the right line RL. A neutral position in this context is also to be regarded as being outside the central position of the steering means, provided that the steering means remains in this position, i.e. does not perform a turning movement. 
   In the case of a hydraulic steering system  1  without the features according to the invention, the hydraulic medium flowing off from the right steering-cylinder chamber RZ would flow off via the right connection R′ of the incorrectly functioning or incorrectly controlled valve unit  6  and an unintentional steering movement would result. 
   If the steering means  4  in the exemplary embodiment shown is turned to the right for example, an incorrect attempt by the power assist device  3  to meter hydraulic medium through the left connection L′ of the valve unit  6  into the left line LL results in the amount of hydraulic medium fed from the valve unit  6  flowing off through the left connection L, simultaneously opened by the right position, of the steering unit  2 . The left connection L′ is as it were connected in an unpressurised manner to the tank  10  via the steering unit  2 . This produces only a very slight pressure effect of the hydraulic medium, fed through the left connection L′, on the steering cylinder  5 . The steering action of the steering system  1  in the right direction as a result of the right position of the steering means  4  survives, in weakened form, since the hydraulic medium metered from the right connection R of the steering unit  2  cannot flow off via the right connection R′, which is blocked by the right nonreturn valve RR, of the valve unit  6  and thus hydraulic medium is caused to flow off from the left steering-cylinder chamber LZ via the opened left connection L of the steering unit  2 . The same applies analogously if the steering means  4  is turned to the left and there is an incorrect attempt by the assist device  3  to meter hydraulic medium through the right connection R′ of the valve unit  6  into the right line RL. 
   In the case of a hydraulic steering system  1  without the features according to the invention, turning the steering means  4  to the right and an incorrect attempt by the assist device  3  to meter hydraulic medium through the left connection L′ of the valve unit  6  into the left line LL would result in the amount of hydraulic medium fed in by the right connection R of the steering unit  2  flowing off via the parallel-connected right connection R′ of the valve unit  6 , without a appreciable pressure being exerted on the right steering-cylinder chamber RZ. Neither of the hydraulic flows would thus exert any pressure and there would be no steering action of the steering system  1 . However, since the greater oil flow is often available to the power assist device  3 , a steering action directed opposite to the steering movement of the steering means  4  may result. 
   If the steering means  4  in the exemplary embodiment shown performs only a slight turn to the right for example, an incorrect attempt by the assist device  3  to meter hydraulic medium at the greatest pressure or in a maximum amount through the right connection R′ of the valve unit  6  into the right line RL results in an increased counterpressure in the left steering-cylinder chamber LZ, since the hydraulic medium can flow off only via the left connection L of the steering unit  2  owing to the left nonreturn valve LR blocking the flow-off. The movements of the steering cylinder  5  are thereby less unexpectedly rapid and remain controllable. 
   The same applies analogously if the steering means  4  is turned slightly to the left and there is an incorrect attempt by the power assist device  3  to meter hydraulic medium at the greatest pressure or in a maximum amount through the left connection L′ of the valve unit  6  into the left line LL. 
     FIG. 2  shows an exemplary embodiment of the hydraulic steering system according to the invention, in which the valve block  12  and the valve unit  6  are illustrated in a detailed hydraulic circuit. To avoid unnecessary repetition, reference symbols corresponding to  FIG. 1  are also used in  FIG. 2 , repeated description being dispensed with. 
   In the valve block  12  there is provided a steering valve  15  which cooperates with the steering means  4 . In dependence on a turning movement of the steering means  4 , steering valve  15  connects a supply connection P to the left connection L during a turning movement to the left and to the right connection R during a turning movement to the right. Simultaneously, the respective other connection is connected to the tank connection T via the steering valve  15 . Arranged between the supply connection P and the steering valve  15  is a spring-loaded nonreturn valve  16  which is arranged in the direction of the steering valve  15 . In the further course of the connection between the supply connection P and the steering valve  15  there is formed a first connecting duct  17  which connects the supply connection P to the tank connection T. Arranged in this first connecting duct  17  is a second nonreturn valve  18  which opens in the direction of the supply connection P. 
   Within the valve block  12 , additionally the left connection L and the right connection R are connected to one another via a second connecting duct  19  and a third connecting duct  20 . A third nonreturn valve  21  and a fourth nonreturn valve  21 ′ are arranged in the second connecting duct  19 , the third nonreturn valve  21  opening in the direction of the left connection L and the fourth nonreturn valve  21 ′ opening in the direction of the right connection R. 
   A first differential pressure limiting valve  22  and a second differential pressure limiting valve  22 ′ are arranged in the third connecting duct  20 . The two differential pressure limiting valves  22  and  22 ′ are each spring-loaded. Bypassing the steering valve  15 , the tank connection is furthermore connected to the third connecting duct  20  via a bypass line  23 , the bypass duct  23  opening out at a point of the third connecting duct  20  which opens out between the first and second differential pressure limiting valves  22  and  22 ′, respectively. In the further course, the bypass duct  23  connects the third connecting duct  20  to the second connecting duct  19 , the connecting point between the bypass duct  23  and the second connecting duct  19  again opening out between the third and fourth nonreturn valves  21 ,  21 ′. 
   If, for example, the pressure in the left line LL increases, the first differential pressure limiting valve  22  opens from a specific differential pressure set by the spring of the first differential pressure limiting valve  22 . To this end, the differential pressure limiting valve  22  is subjected to the pressure prevailing in the left line LL, on the one hand against the force of the spring. In the opposite direction and with an equal action to the spring, the first differential pressure limiting valve  22  is subjected to the pressure prevailing in the third connecting duct  20 . As soon as the first differential pressure limiting valve  22  opens, pressure medium is carried off from the left line LL in the direction of the bypass line  23  via the first differential pressure limiting valve  22 . The carried-off pressure medium flows on via the bypass line  23  on the one hand via the second connecting duct  19  and the thereupon opening fourth nonreturn valve  21 ′ in the direction of the right connection. In addition, the pressure medium carried off from the left line LL is fed via the bypass line  23  to the tank connection T and discharged into the tank  10 . An equalisation of the pressure prevailing in the left line LL and the right line RL therefore takes place via the first differential pressure limiting valve  22  and the fourth nonreturn valve  21 ′, provided that the steering means  4  is not turned during the described pressure increase in the left line LL and hence the steering valve  15  is in a position in which connections are disconnected from one another. 
   Through the pressure equalisation in the left line LL and the right line RL, an equalisation of the pressure is also achieved in the left cylinder chamber LZ and the right cylinder chamber RZ. Through the equalisation of this pressure, the forces acting on the steering system cancel each other out, so that there is no unintentional steering movement. If, owing to a turning movement of the steering means  4 , either the left connection L or the right connection R is connected to the tank connection T via the steering valve  15 , part of the volume flow carried off via this connection is removed and carried off via a throttle  24 . Downstream of the throttle  24  a control pressure connection LD is formed on the valve block  12 . The delivery rate of the first pump  8 , which is of adjustable design in  FIG. 2 , is adjusted via the control pressure present at the control pressure connection LD. To avoid the occurrence of excessive control pressures, a line branch in which a third differential pressure limiting valve  25  is arranged is provided between the throttle  24  and the control pressure connection LD. By means of the third differential pressure limiting valve  25 , the control pressure connection LD is relieved in the direction of the tank connection T upon the occurrence of a differential pressure exceeding a specific, adjustable value. 
   The valve unit  6  is embodied in the figure as a 4/3-way valve. In the rest position of the valve unit  6  as illustrated in  FIG. 2 , the supply, tank, left and right connections P′, T′, L′ and R′ are disconnected from one another. Without driving by the control device  7 , the valve unit  6  is kept in this rest position by two springs acting in opposite directions on the valve unit  6 . A first electromagnet  26  or a second electromagnet  26 ′ can be supplied with a signal via the control device  7 . The first electromagnet  26  subjects the valve unit  6  to a force in the direction of a first end position of the valve unit  6 . In this first end position of the valve unit  6 , the supply connection P′ is connected to the left connection L′ of the valve unit  6 , so that pressure medium is fed into the left line LL via the second pump  9 . 
   If, in contrast, the second electromagnet  26 ′ is supplied with a control signal by the control device  7 , the electromagnet  26 ′ moves the valve unit  6  in the direction of its second end position, in which the supply connection P′ is connected to the right connection R′. In the second end position of the valve unit  6 , pressure medium is thus drawn from the tank  10  by the second pump  9  and delivered into the right line RL. The right line R′ or left line LL respectively not connected to the supply connection P′ remains closed, so that pressure medium cannot be discharged via the valve unit  6 . The blocking device which prevents the hydraulic medium from flowing back, is thus directly integrated in the valve unit  6 . To protect the hydraulic system, a further nonreturn valve is formed in the connection between the second pump  9  and the supply connection P′ of the valve unit  6 . 
   As already stated in relation to  FIG. 1 , the control signals which are sent to the first electromagnet  26  or the second electromagnet  26 ′ are determined by the control device  7  in dependence on a number of parameters. To this end, for example, a voltage is supplied to the control device by the sensor  11  via the electrical line  13 . The output voltage of the sensor  11  depends here, for example, on the angle of rotation and a speed of rotation, which correspond to the turning movement of the steering means  4 . The control signals output by the control device  7  can furthermore be influenced by a first selector switch  27  and a second selector switch  28 . The change of the hydraulic intensification achieved by the assist device  3  can thus be varied to take account of specific operating situations. For example, the first or second selector switch  27 ,  28  can be used to effect a changed control response owing to a machine moving on or off the road. The steering angle resulting from a specific turning movement of the steering means  4  can thus be adapted, for example, to different driving speeds off or one the road. 
   Furthermore, the parameter sets of the control device  7  can be adapted via a service device  29 . 
   The second exemplary embodiment, illustrated in  FIG. 3 , of the hydraulic steering system according to the invention differs from the exemplary embodiment of  FIG. 2  in that a modified valve unit  6 ′ is used in the assist device  3 . In contrast to the valve unit  6 , in the first end position in the modified valve unit  6 ′ not only the supply connection P′ is connected to the left line LL, but additionally the right line RL is connected to the tank connection T′. If, in contrast, the modified valve unit  6 ′ is deflected from its rest position illustrated in  FIG. 3  in the direction of the second end position, on the one hand the supply connection P′ is increasingly connected to the right line RL and on the other hand the left line LL is increasingly connected to the tank connection T′. This makes it possible for hydraulic medium to flow back via the modified valve unit  6 ′, outside the rest position of the modified valve unit  6 ′. Downstream of the modified valve unit  6 ′, a separate blocking device  30  is provided in the line leading to the tank  10  in order to prevent the hydraulic medium from flowing back. 
   In order to enable even better account to be taken of the particular operating situation, the separate blocking device  30  may also be of unblockable design. Accordingly, hydraulic medium can be returned to the tank  10  both via the steering valve  15  of the valve block  12  and via the valve unit  6 ′ of the assist device  3 , in dependence on the particular driving situation. 
   Unblockable blocking devices are, however, not only possible in the separate blocking device  30  as illustrated in  FIG. 3 . 
   Starting from, for example, the arrangement of the left nonreturn valve LR and the right nonreturn valve RR as shown in the first exemplary embodiment of  FIG. 1 , the backflow can take place via the valve unit  6  by unblocking the two nonreturn valves LR and RR. To this end, a modified design of the left nonreturn valve LR′ and the right nonreturn valve RR′ is used. The modified nonreturn valves LR′ and RR′ can be subjected, via a first unblocking line  31  and a second unblocking line  32  respectively, to a hydraulic pressure which moves the two nonreturn valves into their opened position irrespective of the pressure conditions in the left line LL or the right line RR. In this case, the two nonreturn valves LR′ and RR′ can be driven either jointly or independently of one another, the pressure set in the first unblocking line  31  and the second unblocking line  32  being settable independently of one another. 
   An alternative is illustrated in  FIG. 5 . Here, a nonreturn valve arranged between the tank connection T′ of the valve unit  6  is likewise to be moved into its opened position via an unblocking line  33 . It is thus possible for the left line LL or the right line RL to be respectively relieved in the direction of the tank  10  via the tank connection T′, the assignment taking place via the valve unit  6 . A nonreturn valve  34  is likewise formed on the supply side, upstream of the supply connection P′. This nonreturn valve  34  opens in the direction of the valve unit  6  and thus protects the second pump  9  from any impermissible pressure increase that may occur. 
   The invention is not restricted to the exemplary embodiment. The features of the exemplary embodiment may be combined with one another in any desired way.