Patent Publication Number: US-8534416-B2

Title: Hydraulic steering

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/714,419 filed on Mar. 6, 2007, which claims foreign priority benefits under 35 U.S.C. §119 from German Patent Application No. 10 2006 010 697.0 filed on Mar. 8, 2006, the contents of which are incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The invention concerns a hydraulic steering with a steering unit with feedback behaviour, a steering motor connected to the steering unit via working pipes, a steering member and a feedback suppression device, which is located in at least one working pipe. 
     BACKGROUND OF THE INVENTION 
     Such a steering is known from, for example, DE 10 2004 021 531 A1. 
     Such a hydraulic steering is preferably used in tractors or other self-propelled working machines, which are driven in different environments. For example, a tractor must be able to drive on a public road to and from a field. Its real work is then performed, when it is driving in the field. When driving in the street a steering behaviour is desired that corresponds to that of a normal vehicle. This means that the driver must be able to feel a feedback of the steered wheels on the steering handwheel. For this purpose, it is necessary that the steering unit has feedback behaviour. Such a steering unit then also has the advantage that a “self alignment” of the vehicle can take place, that is, the steered wheels can automatically move to the straight forward position, which the driver learns by means of the turning of the steering handwheel. In many cases, such feedback behaviour is not desired when driving in the field. Otherwise, the driver would have to constantly act upon the steering handwheel or another steering member with a certain force. 
     In the steering shown in DE 10 2004 021 531 A1, the feedback suppression device has a valve that is located in a working pipe between the steering unit and the steering motor. This valve is electrically activated and opens, when the steering handwheel is activated. Therefore, a certain effort is required to activate this valve. 
     SUMMARY OF THE INVENTION 
     The invention is based on the task of changing the feedback behaviour of a hydraulic steering in a simple manner. 
     With a hydraulic steering as mentioned in the introduction, this task is solved in that the feedback suppression device has a valve arrangement with hydraulic control inlet. 
     The valve arrangement that forms a substantial part of the feedback suppression device is thus purely hydraulically activated. An additional electrical system is no longer required to change the feedback behaviour of the steering unit. Hydraulic fluid with the required pressure is available in a hydraulic steering anyway, so that only relatively few additional measures are required to either activate or deactivate the feedback suppression device. When the feedback suppression device is deactivated, the driver feels forces acting upon the steering motor on the steering member. When the feedback suppression device is activated, the driver does not feel the effects of such forces. 
     Preferably, the valve arrangement can also be opened by a pressure from the steering unit. With this embodiment it is ensured that the steering unit can also always act upon the steering motor, when the feedback suppression device is active. Thus, the steering unit has a higher priority than the feedback suppression device, so that the feedback suppression device does not have to be disconnected or deactivated to enable a steering. 
     Preferably, the control inlet is connected to an operation mode valve supplying the control inlet with a higher or a lower control pressure in dependence of its activation. The operation mode valve can simply be a change-over valve that supplies a higher control pressure to the control pressure inlet in one position. Depending on the design of the valve arrangement, this for example causes that the feedback suppression device is connected or activated. When, however, the operation mode valve is changed so that the control inlet of the valve arrangement is supplied with a lower control pressure, the feedback suppression device is deactivated. Of course, also a reversed switching is possible. 
     Preferably, the valve arrangement has a pretensioning device, which acts against the pressure at the control inlet. Thus, it is ensured that the valve arrangement and thus the feedback suppression device always assume a defined state. Depending on the effect of the control pressure at the control inlet, this causes that the pretensioning device either activates or deactivates the feedback suppression device. 
     Preferably, for each working pipe the valve arrangement has a valve that interrupts the working pipe in a predetermined state. When the working pipe is interrupted, a feedback of the steering motor upon the steering unit is no longer possible. When the valves are open, the desired feedback behaviour appears. The use of a valve in each working pipe has the advantage that the steering behaviour and also the feedback behaviour are “symmetrical”, as the lengths of the hydraulic pipes, which can change their volume when acted upon by a pressure, correspond to each other, when a corresponding valve is located in each working pipe. 
     Preferably, a series connection of two two-way valves is located between the two working lines, the connecting point of said two-way valves being connected to the control inlet, each two-way valve having an outlet that is connected to the valve in the working pipe not connected to the two-way valve in question. This is a relatively simple method of using the higher of the pressures in the working pipes to open the valve in the other working pipe, as long as the pressure in the working pipe is higher than the pressure at the control inlet. When this is not the case, the valves are acted upon via the pressure at the control inlet. 
     In an alternative embodiment it may be ensured that a pressure relief valve is allocated to each valve that can be opened by both the pressure at the control inlet and the pressure in the respective other working pipe, and in the opened state connects a closing pressure side of the valve with the related working pipe. As long as the pressure relief valve is closed, the closed valve cannot open, as the pressure on the closing pressure side cannot be removed. This is not possible until the pressure relief valve opens. In this case, the pressure at the control inlet is on the one hand sufficient to open the valve, so that a steering with feedback behaviour is possible. On the other hand, the opening pressure can also be generated by the hydraulic fluid flowing back from the motor. 
     Preferably, a power assisted steering valve is connected in parallel to the steering unit, the steering valve being connected to the working pipes in an area between the feedback suppression device and the steering motor. The vehicle can then not only be controlled via the steering unit, but also via the steering valve. The steering valve can, for example, be a proportional valve. When the steering motor is controlled via the steering valve, the feedback suppression device is particularly advantageous, as it can prevent a movement of the steering member, which is caused by the activation of the steering motor by the steering valve. 
     Preferably, the operation mode valve is arranged in series with a pressure control valve. The pressure control valve ensures that the operation mode valve always passes on a predetermined pressure to the control inlet. This is a simple manner of ensuring that the control inlet is neither overloaded by receiving a too high pressure, nor that it is undersupplied, because the pressure is too low. When the pressure control valve is located between a high-pressure connection and the operation mode valve, it can also be ensured that also the operation mode valve only has to be dimensioned in accordance with a predetermined pressure. 
     Preferably, the steering unit has a load-sensing connection, whose pressure acts upon the valve arrangement against the pressure at the control inlet. Thus, by means of the pressure at the load-sensing connection the effect of the pressure on the control inlet can be neutralized again. As soon as the driver activates the steering member, for example the steering handwheel, thus activating the steering unit, a higher pressure occurs at the load-sensing connection, which usually corresponds to the highest pressure available at the steering unit. In a simple manner, this pressure is able to override the pressure at the control inlet, either directly or indirectly. 
     It is particularly preferred that the load-sensing connection is connected to an auxiliary valve that is located between a pilot control pipe and a connection with a higher pressure. The connection with the higher pressure can be the supply connection or the high-pressure connection of the steering unit. In this case, the activation of the valve arrangement will cause no large “consumption” of hydraulic fluid. Such consumption could cause a “hard point” in the steering. When the pressure at the load-sensing connection acts upon the auxiliary valve, only a small amount of hydraulic fluid is required to deflect the auxiliary valve or to change its position. This small amount can practically not be felt by the driver or operator. The auxiliary valve has the further advantage that the pressure at the load-sensing connection in the neutral position does not have to be set exactly at the lowest pressure or tank pressure, when the auxiliary valve has a spring or another resetting device acting against the pressure at the load-sensing connection. The pressure at the load-sensing connection would then at least have to overcome the force of the resetting device, before the auxiliary valve is opened. 
     Preferably, the steering unit has a working pressure connection, relieved in the neutral position of the steering unit to a low-pressure connection and carrying at a predetermined deflection of the steering unit the pressure ruling in the controlled working pipe, a pressure at the working pressure connection acting upon the valve arrangement against the pressure at the control inlet. The pressure at the working pressure connection is also called “S-pressure” or “S-signal”. A steering unit with such a working pressure connection is available at Sauer-Danfoss ApS, Nordborg, Denmark, under the name of OSP-EL. This S-signal has the property that, in the neutral position and in a small band around the neutral position, it is relieved to tank or another area with a low pressure. When deflecting to the left or to the right, the S-signal always receives the pressure available at the outlet side of the set of teeth of the steering unit, that is, practically the pressure available in the “left” or “right” working pipe. Thus, a hard point in the steering is avoided, as no hydraulic fluid is consumed by the load-sensing connection. Further, a clearly defined “non-feedback state” is available, as in the neutral position the S-signal is effectively set at the tank pressure. Further, the concept offers an advantage in the emergency steering situation, where a pressure built up in one of the two working pipes will provide an additional certainty for the forced opening of the valve unit. 
     In an alternative embodiment it is ensured that the steering unit exists in the form of an “open-center” steering unit and has a pressure connection, which is connected to a counter-control connection, whose pressure acts upon the valve arrangement against the pressure at the control inlet. In this connection, you take advantage of the behaviour of an “open-center” steering unit. In the neutral position this steering unit has a connection between the high-pressure connection and the low-pressure connection, so that in the neutral position of this steering unit basically the same pressure rules at both the low-pressure connection and at the high-pressure connection. This pressure is not sufficient to open the valve arrangement. Not until the steering unit is activated, will the connection between the high-pressure connection and the low-pressure connection be interrupted, so that the pressure required to switch the valve arrangement to the feedback mode builds up very quickly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is described on the basis of preferred embodiments with reference to the drawings, showing: 
         FIG. 1  is a first embodiment of a hydraulic steering with feedback suppression device, 
         FIG. 2  is a modified embodiment of a feedback suppression device, 
         FIG. 3  is a third embodiment of the feedback suppression device, 
         FIG. 4  is a fourth embodiment of the feedback suppression device, 
         FIG. 5  is a fifth embodiment of the feedback suppression device, 
         FIG. 6  is a second embodiment of a steering, 
         FIG. 7  is a third embodiment of a steering, 
         FIG. 8  is a fourth embodiment of a steering, 
         FIG. 9  is a modification of the feedback suppression device according to  FIG. 8 , 
         FIG. 10  is a second modification of the feedback suppression device according to  FIG. 8 , 
         FIG. 11  is a fifth embodiment of a steering, 
         FIG. 12  is a sixth embodiment of a steering, 
         FIG. 13  is a seventh embodiment of a steering, 
         FIG. 14  is a schematic view explaining the S-signal, and 
         FIG. 15  is an eighth embodiment of a steering. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view of a steering  1  with a steering unit  2 , here in the form of a “closed-center” steering unit. With a closed-center steering unit a connection between a high-pressure connection P and a low-pressure connection T is interrupted in the neutral position. 
     The high-pressure connection P is connected to a pump  4  via a priority valve  3 , the pump  4  being formed as a variable pump. 
     Further, the steering unit  2  has the low-pressure connection T, which is connected to a tank  5 . 
     The steering unit  2  is activated by a steering member in the form of a steering handwheel  6 . In a manner known per se it has a set of teeth  7  having, for example, an externally toothed gear wheel and an internally toothed gear ring and serves as measuring motor. By means of a directional valve, not shown in detail, pressurised hydraulic fluid can be supplied to one of two working pipes L, R. The working pipes L, R are connected to a steering motor  8 , here in the form of a piston-cylinder unit. 
     Between the steering unit  2  and the steering motor  8  is located a feedback suppression device  9 . The feedback suppression device  9  has a valve arrangement  10 , which is, in the embodiment according to  FIG. 1 , provided with a non-return valve  11 ,  12  that can be opened in each working pipe L, R. 
     Via a pilot control pipe  13  the working pipe L is connected to the non-return valve  12  in the other working pipe R. Via a pilot control pipe  14  the working pipe R is connected to the non-return valve  11  in the other working pipe L. The pressure in the pilot control pipes  13 ,  14  opens the non-return valves  11 ,  12  so that a flow can pass them in a direction from the steering motor  8  to the steering unit  2 . 
     Further, the non-return valves  11 ,  12  are connected to a control inlet C, so that a pressure ruling at the control inlet C can open the two non-return valves  11 ,  12 . 
     The control inlet C is connected to an operation mode valve  15 , which connects the control inlet C to either the tank  5  or a reference pressure P ref . In the shown position of the operation mode valve  15  the control inlet C is relieved to the tank  5 . The operation mode valve  15  can be activated mechanically, hydraulically, electrically or otherwise. 
     With such a steering  1 , in which the feedback suppression device  9  comprises the valve arrangement  10  and the operation mode valve  15 , it is possible to set either a non-feedback operation or a feedback operation. 
     In the shown position of the operation mode valve  15 , the operation of the steering is non-feedback. The two non-return valves  11 ,  12  are closed, so that no hydraulic fluid can flow from the steering motor  8  via the connections A, B into the valve arrangement  10 . Accordingly, there is no feedback on the steering unit  2 . 
     When, however, the steering unit  2  is activated, for example to deflect the steering motor  8  to the left, the working pipe L is pressurised by the steering unit  2 . The pressure in the working line L opens the non-return valve  11  in the working pipe L immediately, so that hydraulic fluid can flow to the left working chamber of the steering motor  8 . The fluid displaced from the right working chamber of the steering motor  8  is then displaced to the connection B of the valve arrangement  10 . From here, it can flow off, as the non-return valve  12  in the working pipe R has been opened via the pilot control pipe  13 . 
     As soon as the pressure from the steering unit  2  subsides, the two non-return valves  11 ,  12  close and the steering unit  2  is isolated from the steering motor  8 . 
     When, however, the operation mode valve  15  is moved in the other direction, the control inlet C is acted upon by a reference pressure P ref , which opens both non-return valves  11 ,  12 . In this case, the hydraulic fluid displaced from the steering motor  8  is supplied directly into the steering unit  2 , where it causes a reaction that can usually also be felt at the steering handwheel  6 . 
     The steering unit  2  also has a load-sensing connection LS, which is on the one side connected to the priority valve  3  and on the other side to the pump  4 . Via a branch line  16  the priority valve  3  is connected to a further hydraulic consumer, not shown in detail, which also reports its working pressure to the pump  4  via a two-way valve  17 . 
     The steering  1  works exclusively with hydraulic signals. The valve arrangement  10  of the feedback suppression device  9  is set by activation of the operation mode valve  15  in such a manner that a feedback of the steering motor on the steering unit  2  is either possible or not possible. 
       FIG. 2  shows a modified embodiment, in which same or similar elements have the same reference numbers as in  FIG. 1 . 
     Two two-way valves  18 ,  19  are connected in series between the two working pipes L, R. A connecting point  20  between the two two-way valves  18 ,  19  is connected to the control inlet C. The two-way valve  18 , which is connected to the working pipe L, has an outlet, which is connected via the pilot control pipe  13  to the non-return valve  12  that is located in the other working pipe R. In a similar manner, the two-way valve  19 , which is connected to the other working line R, has an outlet, which is connected via the pilot control pipe  14  to the non-return valve  11  in the first working pipe L. 
     The mode of functioning is similar to that in  FIG. 1 . When, for example, the working pipe L is pressurised by the steering unit  2 , the non-return valve  11  is opened immediately by the pressure in the working pipe. The non-return valve  12  in the other working pipe is opened, as the pressure in the working pipe L is led to the non-return valve  12  via the two-way valve  18  and the pilot control pipe  13  in such a manner that the non-return valve  12  opens. In this case, the control inlet C is at a low pressure, that is, the operation mode valve  15  is in the position shown in  FIG. 1 . 
     When, however, the control inlet C is brought to a higher pressure by means of the operation mode valve  15 , the two two-way valves  18 ,  19  change over and pass the pressure from the pressure outlet C on to directly open the two non-return valves  11 ,  12 . 
       FIG. 3  shows an embodiment, in which the non-return valves  11 ,  12  have been integrated in slides  21 ,  22  of slide valves. The slides  21 ,  22  are pressed into the shown position by springs  23 ,  24 , as long as the pressure at the control inlet C is lower than a pressure corresponding to the force of the springs  23 ,  24 . In this case, the valve arrangement  10  according to  FIG. 3  works exactly like the valve arrangement  10  according to  FIG. 2 . In the closed state the non-return valves  11 ,  12  prevent hydraulic fluid from getting to the steering unit  2  from the steering motor  8 . An activation of the steering motor  8  by the steering unit  2 , however, is possible. 
     When the operation mode valve  15  is switched from the position shown in  FIG. 1 , the two slides  21 ,  22  are displaced to the position not shown against the force of the springs  23 ,  24 , so that a passage through the working pipes L, R from the steering motor  8  to the steering unit  2  is possible. Also the pressures at the connections A, B of the valve arrangement  10  act in the same direction as the pressure from the control inlet C to open the valves in the working pipes L, R. 
     The valves  11 ,  12  have a venting to the environment of the spring chambers, in which the springs  23 ,  24  are located. This has the advantage that these valves  11 ,  12  do at the same time act as shock valves, so that the valves located in a following steering unit can still be used. 
     The embodiment of the valve arrangement  10  in  FIG. 4  corresponds to that of  FIG. 3 . Merely the connection between the connections A, B and the slides  21 ,  22  are missing. 
     In the embodiment of the valve arrangement according to  FIG. 5  the two springs  23 ,  24  load the slides  21 ,  22  so that both working pipes L, R are open, when the operation mode valve  15  is in the position shown in  FIG. 1  and the control inlet C is pressure relieved accordingly. 
     In the embodiments according to the  FIGS. 1 to 4  a non-feedback operation is set, when no pressure or only a small pressure is available at the pressure inlet C. 
     In the embodiment according to  FIG. 5 , however, a feedback operation is set, when no pressure or only a small pressure is available at the pressure inlet C. 
     When a high pressure is available at the pressure inlet C, that is a pressure exceeding the force of the springs  23 ,  24 , the slides  21 ,  22  are displaced so that the non-return valves  11 ,  12  are positioned in the working pipes L, R. In this case a feedback of the steering motor  8  on the steering unit  2  is no longer possible. 
     However, a steering through the steering unit  2  is possible. When, for example, the working pipe L is exposed to pressure, the non-return valve  11  opens and the slide  22  is taken to the position shown in  FIG. 5  by the pressure in the pilot control pipe  13 , so that hydraulic fluid can flow off from the steering motor  8 . 
     Whereas in the embodiments according to  FIGS. 1 to 5  the valve arrangement  10  is designed so that it has a valve for each working pipe L, R, this is not the case in the embodiment according to  FIG. 6 . Here the valve arrangement  10  has only one single valve with a valve slide  22 , which is pressed to the opening position by the spring  24 , when the pressure at the control inlet C is relieved to the tank  5 . When, however, the operation mode valve  15  is switched over, the control inlet is supplied with the reference pressure P ref , so that the slide  22  is taken to the closing position and interrupts the working pipe R. 
     It is favourable, when this valve is located as close to the steering unit  2  as possible. 
     In the embodiment according to  FIG. 6 , the steering unit  2  is made as an open-center steering unit, that is, in the neutral position the steering unit  2  has a connection between the pressure connection P and the tank connection T. The pressure connection P is connected to the slide  22  via a counter-control connection  31 , so that the pressure in the counter-control connection  31  acts in the same direction as the force of the spring  24 . 
     The force of the spring  24  corresponds to, for example, a pressure of 7 bar. Also the springs  23 ,  24  in the embodiments according to the  FIGS. 3 to 5  can correspond to a pressure of 7 bar. The reference pressure P ref  is, for example, 13 bar, so that the slide  22  is displaced to the closing position, when the operation mode valve  15  is switched to the position not shown. In this case a feedback of the steering motor  8  on the steering unit  2  is not possible. 
     When, however, the steering unit  2  is activated, the connection between the pressure connection P and the tank connection T is interrupted, so that the pressure at the pressure connection P rises very quickly to the outlet pressure of the pump  4 . At least together with the force of the spring  24  this outlet pressure exceeds the pressure P ref , so that for the steering the slide  22  can be taken to the opening position. 
     In the embodiment according to  FIG. 7  a closed-center steering unit  2  is used, in which a connection between the pressure connection P and the tank connection T does not exist in the neutral position either. 
     The steering unit  2  according to  FIG. 7  has a load-sensing connection LS, which acts upon the slide  22  of the valve arrangement  10  via the pilot control pipe  13  in the same direction as the spring  24 . 
     In the shown switching position of the slide  22  the working pipe R is open for passage. Due to the position of the operation mode valve  15  the pressure at the tank  5  rules at the control inlet C. In this case a feedback steering behaviour occurs. 
     When the operation mode valve  15  is switched over, the reference pressure P ref  reaches the control inlet C and switches the slide  22 , so that the working pipe R is interrupted. In this case a feedback of the steering motor  8  on the steering arrangement  2  is not possible. When, however, the steering arrangement  2  is activated, the pressure at the load-sensing connection LS increases and together with the spring  24  it opens the working pipe R in that the slide  22  is displaced to the position shown. 
     In  FIG. 7  the reference pressure P ref  is set by means of a pressure control valve  25 , for example at a value of 13 bar. The pressure control valve  25  has a valve slide  26 , which is loaded in the opening direction by the force of a spring  27  and the pressure at the tank connection T. In the closing direction the valve slide  26  is loaded by the pressure at the outlet of the pressure control valve  25 . The inlet of the pressure control valve  25  is connected to the pressure connection P. 
     The value of the reference pressure P ref  can then be set by selecting the force of the spring  27 . 
       FIG. 8  shows that the pump  4  is a variable capacity pump, which is controlled by the pressure at the load-sensing connection LS. Of course, also here an embodiment is possible with a pump  4  having a fixed displacement, that is, an embodiment as in  FIG. 7 . 
     In each working pipe the valve arrangement  10  has a non-return valve  11 ,  12 . Further, in each working pipe the valve arrangement  10  has a valve slide  21 ,  22  arranged in parallel, which is pressed into the opening position against the force of the springs  23 ,  24  (shown). In this case a feedback steering is possible, as a direct connection between the steering motor  8  and the steering unit  2  is permanently available. 
     Only when the operation mode valve  15  is switched so that the reference pressure P ref  reaches the control inlet C, the slides  21 ,  22  are displaced against the force of the springs  23 ,  24 , so that the working pipes L, R are interrupted. In this case there is no feedback of the steering motor  8  on the steering unit  2 . 
     However, the steering unit  2  can still activate the steering motor  8 . In the “forward” direction the non-return valve  11 ,  12  in the related working pipe is opened. In the “return” direction the slide  22  is opened by the pressure at the load-sensing connection LS. 
     The  FIGS. 9 and 10  show modified embodiments of the valve arrangements. 
     In the embodiment according to  FIG. 9  the non-return valves  11 ,  12  are integrated in the slides  21 ,  22 . 
     In the embodiment according to  FIG. 10  non-return valves are not available. The feedback suppression occurs exclusively through the slides  21 ,  22 , which are opened by the force of the springs  23 ,  24  to ensure feedback behaviour, when the control inlet C is not under pressure. When, however, by means of the operation mode valve  15  pressure is acting upon the slides  21 ,  22 , the slides move against the force of the springs  23 ,  24  in the closing direction. In this case, an opening of the working pipes L, R is only possible, when the steering unit  2  generates a correspondingly high pressure at the load-sensing connection LS, which exceeds the pressure at the control inlet C. 
       FIG. 11  shows a steering  1 , which substantially corresponds to that in  FIG. 7 . Further, it has a steering valve  28 , which is located in parallel to the steering unit  2  between the pressure connection P and the tank connection T on the one side and the steering motor  8  on the other side. The steering valve  28  ends in the working pipes L, R between the valve arrangement  10  and the steering motor  8 , so that the fluid from the steering valve  28  is not influenced by the valve arrangement  10 . 
     When the pressure at the load-sensing connection LS is used to activate the valve arrangement  10  directly, a so-called “hard point” may occur during steering. At the instant, when the valve arrangement  10  is activated, a small amount of hydraulic fluid will be missing, which can be felt at the steering handwheel  6 . 
     To remedy this problem, the embodiment according to  FIG. 12 , which otherwise corresponds to the embodiment according to  FIG. 8  that is supplied with a valve arrangement according to  FIG. 10 , is provided with an auxiliary valve  29 . The auxiliary valve  29  is activated by the pressure at the load sensing connection LS and then creates a connection between the pressure connection P and the pilot control pipe  13 , so that the slides  21 ,  22  are acted upon by the pressure of the pressure connection P against the pressure at the control inlet C. The auxiliary valve  29  is switched by an auxiliary valve spring  30 , so that the pilot control pipe  13  is connected to the tank  5  again. 
     This embodiment has two substantial advantages. Firstly, the amount of hydraulic fluid required to switch over the auxiliary valve  29  is substantially smaller than the amount of hydraulic fluid required to switch over the two slides  21 ,  22 . Accordingly, no “hard point” occurs during steering. 
     Secondly, with this embodiment it is no longer required that the load sensing connection LS carries the same pressure as the tank  5 , when the steering unit  2  is in the neutral position. As long as the pressure at the load-sensing connection LS does not exceed the force of the spring  30 , the auxiliary valve  29  remains in the position, in which the pilot control pipe  13  is isolated from the pressure connection P, so that the positions of the slides  21 ,  22  controlled by the operation mode valve  15  remain unchanged. 
       FIG. 13  shows a further amended steering  1 , in which the valve arrangement  10  corresponds to that in  FIG. 12 . 
     The design of the steering unit has changed. It has a working pressure connection S, from which an “S-pressure” or an “S-signal” can be obtained, which will be explained by means of  FIG. 14 . 
     In the neutral position ( FIG. 14   a ) the S-signal corresponds to the pressure at the low-pressure connection T. This connection also remains in a small band around the neutral position. In this case, the pressure at the load-sensing connection LS corresponds to the pressure at the tank connection T. 
     When the steering unit  2  is activated so that the left working pipe L is exposed to pressure ( FIG. 14   b ), the S-pressure or the S-signal corresponds to the pressure in the left working pipe L. 
     When the right working pipe R is exposed to pressure ( FIG. 14   c ), the S-signal corresponds to the pressure in the right working pipe R. 
     Also with this working pressure connection S a hard point in the steering is avoided, as no pressure from the load-sensing connection LS is required to activate the valve arrangement  10 . Further, it gives a well defined non-feedback operation, as in the neutral position the S-signal is effectively set at the pressure at the tank connection T. 
     Further, a steering arrangement with the working pressure connection S is advantageous in an emergency steering situation, where the pressure built up at the working pressure connection S provides further safety for the opening of the valves in the working pipes L, R, at least in one of the working pipes L, R. 
       FIG. 15  shows an eight embodiment of a steering  1 , in which a pressure relief valve  31 ,  32  is allocated to each valve  11 ,  12 . Each pressure relief valve is retained in a closed position by a spring  33 ,  34 . 
     In the direction against the springs  33 ,  34  acts the pressure at the outlet of a two-way valve  35 ,  36 . Each two-way valve  35 ,  36  is on the one side connected to the respectively other working pipe R, L, that is the working pipe to which the pressure relief valve  31 ,  32  is not connected. On the other side the two-way valves  35 ,  36  are connected to the control inlet C. 
     The valve slides  21 ,  22  of the valves  11 ,  12  are loaded in the closed position by the two springs  23 ,  24 . 
     On the opposite side of the slide  21 ,  22 , that is, in the closing direction, acts the pressure at the control inlet C or the pressure in the working pipe L, R allocated to the valve  11 ,  12 , respectively. 
     Also in the opening direction acts a pressure at the outlet of the valves  11 ,  12 . This pressure originates from the motor  8 , when hydraulic fluid is displaced from the motor  8 . 
     Via a throttle  37 ,  38  the outlets of the valves  11 ,  12  are connected to the side of the valve slides  21 ,  22 , upon which also the springs  23 ,  24  act. At the same time, this side is connected to a pressure relief valve  31 ,  32 . 
     This steering  1  now works as follows: 
     When a pressure is available at the control inlet C, which exceeds the force of the springs  23 ,  24 , the pressure at the control inlet C opens the pressure relief valves  31 ,  32 , so that the slides  21 ,  22  of the valves  11 ,  12  are moved to the opening position. Then the steering  1  works with feedback behaviour. 
     When the pressure at the control inlet C drops so that it is below a pressure corresponding to the force of the springs  23 ,  24 , the valves  11 ,  12  are closed, as shown. 
     When, now, the steering arrangement  2  is activated, for example to expose the working pipe R to pressure, the valve  12  allocated to this working pipe R is opened, as the pressure in the working pipe R acts upon the slide  22  in the opening direction. 
     At the same time the pressure relief valve  31 , which is allocated to the other valve  11 , is opened via the two-way valve  35 . Accordingly, hydraulic fluid flowing back from the motor  8  can act upon the slide  21  in the opening direction. The pressure here is higher than the pressure in the closing direction, as the hydraulic fluid flowing off through the pressure relief valve  31  causes a pressure drop at the throttle  37 . Accordingly, the valve  11  is also opened and the motor  8  can be activated. 
     In all embodiments the steering unit  2  in itself has feedback behaviour, that is, a pressure change at its outlets, which are connected to the working pipes L, R, causes that the steering member  6  moves in dependence of the amount of hydraulic fluid flowing in or out. This feedback behaviour is desired in many cases, but undesired in many other cases. To enable connection or disconnection of this feedback behaviour without changing the steering unit  2 , the feedback suppression device  9  can be operated in a purely hydraulic manner. 
     While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.