Abstract:
An electrohydraulic steer-by-wire steering system has an operator interface for specifying a turning angle, which is associated with a sensor for the acquisition of a position or movement of the operator interface. An electronic control device is connected to a first electrohydraulic circuit and to a second electrohydraulic circuit and set up in order to actuate the two electrohydraulic circuits as a function of a signal from the sensor. The system further includes a steering actuator for adjusting the turning angle of a steerable wheel. The steering actuator has two chambers each of which is connected simultaneously to the two electrohydraulic circuits.

Description:
RELATED APPLICATIONS 
     This application is a continuation application of International Application Serial No. PCT/EP2014/058217, which has an international filing date of Apr. 23, 2014, and which claims the benefit of German Application Ser. No. 102013209467.1, filed on May 22, 2013. The disclosures of these aforementioned applications are hereby expressly incorporated by reference in their entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates to an electrohydraulic steering system and more particularly to a steer-by-wire electrohydraulic steering system. 
     BACKGROUND 
     Many different types of machines, such as agricultural work vehicles, are usually steered by means of hydraulic steering systems having a continuously driven hydraulic pump that is connected via valve devices with a steering cylinder. The valve devices are hydraulically controlled by means of an operator interface (steering wheel) or sensors, for example, for the acquisition of plant rows in order to move the work vehicle on a desired path. To allow sufficiently rapid steering reactions, the conveyance volume of the continuously running hydraulic pump has to be dimensioned so that it is sufficiently large. This steering arrangement thus requires a relatively large amount of energy and contributes considerably to the losses in the powertrain of the work vehicle. 
     Moreover, in the case of forklifts, for example, so-called steer-by-wire steering systems are used, which by means of sensory registration of the turning position of an operator interface and of an electronic control device, adjust the turning angle of the steered wheels electromechanically, electrohydraulically or electropneumatically without mechanical connection between the operator interface and the steered wheels. Such a steering system may include a sensor that acquires the turning movements of the operator interface and transmits corresponding signals to a control device. Depending on the detected steering intention, the control device drives an electric motor whose output shaft is connected to a hydraulic pump, which in turn applies pressure to the chambers of the steering cylinder. Here, driving power for the electric motor is only needed if a turning movement is to be carried out. 
     In steer-by-wire steering systems, redundancy has to be provided to ensure functioning even if a component of the steering system fails. For this purpose, two electric motors and two hydraulic pumps are provided in a conventional embodiment, and the latter can be connected as desired by means of switching valves to the respective associated chamber of the steering cylinder. In the case of failure of an electric motor or of a pump, the steering cylinder is thus connected to the other pump that is still in running order. In another embodiment, the steering cylinder has four chambers, two of which are connected to one pump and the others of which are connected to the other pump. In this embodiment as well, only one of the two hydraulic circuits works at a time in each case. 
     Other conventional steer-by-wire steering devices from the automotive sector have two steering cylinders which are adjusted by associated electrohydraulic circuits, wherein, in normal operation, the two circuits are operated at the same time. In the case of failure, only one of the circuits is then still active for emergency operation. 
     In other conventional systems, it may be a disadvantage that the redundancy provided, within each case an electric motor and a hydraulic pump (and, in the embodiment with the four-chamber steering cylinder, also half of the steering cylinder), is without effect in normal operation. The steering speed and reaction is limited by the capacity of a single hydraulic circuit, which consequently has to be dimensioned to be sufficiently large so that a sufficient steering speed is achieved. The buyer thus acquires a number of expensive components and takes them along day after day in his/her vehicle although in most cases they are of no use to him/her, since they do not contribute to the steering action. This disadvantage may be avoided but at the expense of the use of two steering cylinders. 
     Therefore, the present disclosure provides an electrohydraulic steer-by-wire steering system which, in spite of redundancy, requires only one steering cylinder and exploits the capacity of the two hydraulic circuits in normal operation. 
     SUMMARY 
     In one embodiment of this disclosure, an electrohydraulic steer-by-wire steering system is provided with an operator interface for specifying a turning angle, which is associated with a sensor for the acquisition of a position or movement of the operator interface. An electronic control device connected to the sensor is connected to a first electrohydraulic circuit and to a second electrohydraulic circuit and set up in order to activate the two electrohydraulic circuits as a function of the signals of the sensor. A steering actuator for adjusting the turning angle of at least one steerable wheel has two chambers which, in normal operation of the steering system, are each connected to the two electrohydraulic circuits at the same time. 
     The steering actuator is accordingly activated in normal operation by two electrohydraulic circuits. Its two chambers are connected to the two circuits, so that the two circuits actively contribute to the steering. If one of the circuits fails, the other circuit is always still controlling the steering actuator. In this manner, the two electrohydraulic circuits are normally used for the steering, although only a single steering actuator with only two chambers is provided. In the case of failure, the steering speed is indeed reduced but still sufficient, which has the advantage that the operator can detect the failure and react with an appropriately adapted operating mode and visit a repair workshop. The present steering system thus combines the rapid steering speed of the prior steering systems having two steering cylinders with the simplicity of the prior steering system having only one steering cylinder. 
     In one embodiment, the electrohydraulic circuits each have an electric motor that is controlled by the control device, whose direction of rotation can be reversed, and which can be driven with a modifiable rotational speed, and a hydraulic pump conveying hydraulic fluid in the two rotation directions, the connections of which in each case are, or can be, connected to a chamber of the steering actuator. This embodiment saves energy, since the driving power for the electric motor is needed only when a turning movement actually occurs. 
     For the decoupling of the two electrohydraulic circuits, it is proposed to connect the connections of the hydraulic pumps in each case, such as by means of spring-loaded check valves associated with said hydraulic pumps, to the chambers of the steering actuator. As a result, particularly in the case of a failure of an electrohydraulic circuit, the other circuit still continues to be active, without the hydraulic fluid being able to flow into the failed circuit, since this is prevented by the check valves. Instead of the check valves, it is also possible to use any other isolation valves of choice which, in the case of failure, separate the affected electrohydraulic circuit from the steering actuator. For this purpose, it is possible to use, for example, conventional pressure-controlled valves which can be controlled by pressure differences between the two circuits. The check valves or isolation valves moreover can also be used in electrohydraulic circuits that have a permanently driven hydraulic pump and are connected by electromagnetic stop valves controlled by the control device, or by any other valves to the chambers of the steering actuator. 
     A storage tank for hydraulic fluid can be connected in each case to an inlet of the hydraulic pump for the supply of hydraulic fluid or in each case via a check valve to the two connections of the two hydraulic pumps. As a result, the hydraulic pump, when needed, can be supplied with additional hydraulic fluid, or excess hydraulic fluid can be discharged again into the storage tank. 
     In addition, the steering actuator-side connections (outlets) of the check valves arranged between the connections of the hydraulic pumps and the chambers of the steering actuator can each be connected to a first connection of a stop valve, the second connection of which can be connected or is connected to a storage tank for hydraulic fluid. The stop valve may be a so-called lowering brake valve. The stop valve may be controlled by the hydraulic pump-side pressure of the respective other check valve of the respective electrohydraulic circuit, or by the pressure applied at its first connection. The stop valve ensures that the pressure in the respective chamber of the steering actuator that contracts during the turning is dissipated at an appropriate flow rate, since a pressure outflow restricted through the check valve that connects the contracting chambers to the suctioning connection of the hydraulic motor, and which is now closed. 
     The second connections of the two stop valves can be connected via a filter to the storage tank for hydraulic fluid, with which a pressure relief valve can be connected in parallel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned aspects of the present disclosure and the manner of obtaining them will become more apparent and the disclosure itself will be better understood by reference to the following description of the embodiments of the disclosure, taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a top view of an agricultural work vehicle with an electrohydraulic steer-by-wire steering system; 
         FIG. 2  is a simplified diagram of the steering system of the agricultural work vehicle of  FIG. 1 ; and 
         FIG. 3  is a hydraulic diagram of the steering system of the agricultural work vehicle of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present disclosure described below are not intended to be exhaustive or to limit the disclosure to the precise forms in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present disclosure. 
       FIG. 1  shows an agricultural work vehicle  10  in the form of a tractor, which has a vehicle chassis  12  which is supported on steerable front wheels  14  and driven rear wheels  16 . A combustion engine (not shown) is used for driving the elements of the work vehicle  10  that can be driven. A work place for an operator is located in a cabin  18 , from where the operator can specify the turning angle of the steerable wheels  14  by means of an operator interface  20  in the form of a steering wheel. The operator interface  20  works together with a sensor  22  which acquires the rotation angle of the operator interface  20  or changes of the rotation angle of the operator interface  20  about a rotation axis. The sensor  22  can be constructed, for example, as an incremental angle sensor with a light barrier or magnetic field sensors. The operator interface  20  could also be implemented as a joystick or similar device. 
     The sensor  22  is electrically connected to an electrohydraulic control arrangement  24 , which in turn is connected by two hydraulic lines  28 ,  30  to the two chambers  36 ,  38  of a steering actuator  26  in the form of a hydraulic cylinder. The piston rods  32  of the steering actuator  26  adjust a tie rod  40  in a lateral direction, which in turn controls the angle of the steerable wheels  14  pivotably attached to a front axle  42 , that is to say the turning angle. A return coupling sensor  34  reports the respective actual turning angle of the work vehicle  10  back to the control arrangement  24 . 
       FIG. 2  shows a simplified diagram of the steering system of the work vehicle  10  of  FIG. 1 . The electrohydraulic control arrangement marked overall with  24  has an electronic control device  44  which receives signals pertaining to the turning angle desired by the operator from sensor  22  and, from the return coupling sensor  34 , signals pertaining to the actual turning angle of the steerable wheels  14 . The supply of current to the control device  44  is ensured by the on-board power supply system of the work vehicle  10 . The supply of the chambers  36 ,  38  of the steering actuator  26  with hydraulic fluid occurs by means of two separate electrohydraulic circuits  46 ,  48 . 
     The first electrohydraulic circuit  46  has a first electric motor  50  whose output shaft  52  is used for driving a first hydraulic pump  54 . A first connection  56  of the first hydraulic pump  54  is connected via a spring-loaded check valve  58  to a connection point  60  to which a first hydraulic line  30  is also connected, which leads to the first chamber  38  of the steering actuator  26 . A second connection  62  of the first hydraulic pump  54  is connected via a spring-loaded check valve  64  to a connection point  66  to which a second hydraulic line  28  is also connected, which leads to the second chamber  36  of the steering actuator  26 . 
     The second electrohydraulic circuit  48  may have a second electric motor  68  whose output shaft  70  is used for driving a second hydraulic pump  72 . A first connection  72  of the second hydraulic pump  54  is connected via a spring-loaded check valve  78  to the connection point  60  to which the first hydraulic line  30  is also connected, which leads to the first chamber  38  of the steering actuator  26 . A second connection  80  of the second hydraulic pump  72  is connected via a spring-loaded check valve  82  to the connection point  66  to which the second hydraulic line  28  is also connected, which leads to the second chamber  36  of the steering actuator  26 . 
     The electric motors  50 ,  68  can be driven in two rotational directions and at modifiable speeds. For example, they can be implemented as a direct-current motor. Analogously, the two hydraulic pumps  54 ,  72  can convey their hydraulic fluid in the two directions. The hydraulic pumps  54 ,  72  can be implemented as fixed displacement pumps or vane pumps. Accordingly, if the operator turns the operator interface  20  to the right, in order to steer to the right (see  FIG. 1 ), the control unit  44  causes the electric motors  50 ,  68  to turn in a rotation direction which leads to filling of the second chamber  36  of the steering actuator  26  and emptying of the first chamber  38  of the steering actuator  26 . Analogously, the control unit  44  causes the electric motors  50 ,  68  to turn in a rotation direction which leads to filling of the first chamber  38  of the steering actuator  26  with the hydraulic fluid and emptying of the second chamber  36  of the steering actuator  26 . Here, the signal of the return coupling sensor  34  is used in order to ensure that the actual turning angle coincides with the desired turning angle. The adjustment speed of the steering actuator  26  is determined by the predetermined target speed at which the operator operates the operator interface. The speed of the electric motors  50 ,  68  and thus the flow rate in the hydraulic lines  28 ,  30  depend on this target speed. As a result of the parallel operation of the two electrohydraulic circuits  46 ,  48 , one achieves in normal operation, i.e., when the two electrohydraulic circuits  46 ,  48  are operational, a relatively high steering speed. The applying of pressure to the single steering actuator  26  by means of only two chambers  38 ,  36  acting in opposite steering directions keeps the expense within manageable limits. 
     The check valves  58 ,  64 ,  78 ,  82  each open only when a higher pressure is applied at their inlets, i.e., indicated at the top of  FIG. 2 , which are connected to the connections  56 ,  62 ,  74 ,  80  of the hydraulic pumps  54 ,  72 , than the pressure at the outlets indicated at the bottom of the drawing. As a result, in the case of a failure of one of the electrohydraulic circuits  46 ,  48 , failure of the other electrohydraulic circuit  48 ,  46  can be prevented. In the case of failure of an electrohydraulic circuit  46  or  48 , the steering system accordingly still remains functional, although with half the steering power. This has the additional advantage that the user can easily notice the failure and react to it. Other valves (not included in the drawing of  FIG. 2 ) may provide a return flow of the hydraulic fluid from the respective contracting chamber of the steering actuator  26  toward the suction-side connection of the hydraulic pumps  54 ,  72 . In one non-limiting example, wherein closed circuits  46 ,  48 , as diagrammatically represented in  FIG. 2 , or open circuits  46 ,  48  are conceivable; the hydraulic fluid from the respective contracting chamber of the steering actuator  26  may go into a storage tank  84  and from there reach a suction-side connection of the hydraulic pumps  54 ,  72 , as shown in  FIG. 3 . 
       FIG. 3  shows the electrohydraulic control arrangement of  FIG. 2  with further refinements. A storage tank  84  for hydraulic fluid is connected in each case via lines  86  to an inlet  88  of the hydraulic pumps  54 ,  72 , by means of which, if necessary, hydraulic fluid can be supplied to the hydraulic pumps  54 ,  72 . In addition, the storage tank  84  is connected via a check valve  90 ,  92 ,  94 ,  96  in each case to the two connections of the two hydraulic pumps  54 ,  72 . If, at one of the connections of the hydraulic pumps  54 ,  72 , a pressure is applied that is lower than that in the storage tank  84 , hydraulic fluid is automatically resupplied to the affected connection of the hydraulic pump  54 ,  72 , in order to prevent cavitation effects. The storage tank  84  can be associated with the steering system, or the storage tank of the on-board hydraulic system can also be used by the steering system, wherein the supply would have to be ensured by appropriate valves. 
     Moreover, in total, four stop valves  98 ,  100 ,  102 ,  104  implemented as proportional valves are provided. The inlets (indicated at the bottom in  FIG. 3 ) of the stop valves  98 ,  100 ,  102 ,  104  are each connected to an outlet of one of the check valves  58 ,  64 ,  72  and  82 . The outlets (indicated at the top in  FIG. 3 ) of the stop valves  98 ,  100 ,  102 ,  104  are each connected to one of two connection points  106 ,  108 , which in turn are connected via a filter  110 ,  112  and a pressure relief valve  114 ,  116  connected parallel to the filter  110 ,  112 , to the storage tank  84 . The stop valves  98 ,  100 ,  102 ,  104 , which are pretensioned by springs in the closed position, are controlled by the pressure at their inlet and by the pressure at the inlet of the respective other check valve  64 ,  58 ,  82 ,  78  of the respective circuit  46  or  48 . For this purpose, their control chambers are connected via control lines to the inlets of the check valves  64 ,  58 ,  82 ,  78 . The opening pressure necessary for opening the stop valve  98 ,  100 ,  102 ,  104  in the control chamber connected directly to the inlet of the stop valve  98 ,  100 ,  102 ,  104  (owing to smaller surface area) is higher than that in the control chamber connected to the respective other check valve  64 ,  58 ,  82 ,  78  of the respective circuit  46  or  48 . 
     The stop valves  98 ,  100 ,  102 ,  104  are implemented as lowering brake valves. On the one hand, they open when the pressure at one of the connection points  60  or  66  increases and they lead the hydraulic fluid via the filter  110  or  112  (or in case of clogging of the filter  110  or  112  via the pressure relief valve  114  or  116 ) into the storage tank  84 . As a result, a reduction of the volume of the respective chamber  38  or  36  of the steering actuator  26 , which contracts during a turning movement, becomes possible, since the check valve  64 ,  58 ,  82 ,  78  connected to the respective contracting chamber  38  or  36  of the steering actuator  26  is closed in each case. On the other hand, the stop valves  98 ,  100 ,  102 ,  104  also open due to the pressure of the respective expanding chamber  38  or  36  of the steering actuator  26 , which is acquired upstream of the then opening check valve  58 ,  64 ,  78 ,  82  and which is supplied to the control chamber of the stop valve  98 ,  100 ,  102 ,  104 . 
     In normal operation, the functioning of the steering system is as follows. If no turning movement is desired, the electric motors  50 ,  68  and also the hydraulic pumps  54 ,  72  stand still and no hydraulic fluid flows and the steering actuator  26  stands still. In any case, the check valves  58 ,  64 ,  78 ,  92  and  90  to  96  may allow a pressure difference still remaining from a previous turning movement to be dissipated. 
     When the operator now operates the operator interface  20  in order to steer to the right, the control device  44  causes the electric motors  50 ,  68  to turn in the direction which leads to pressure building up at the connections, i.e., indicated on the left in  FIG. 3 , of the hydraulic motors  54  and  72 . This pressure leads to the check valves  58  and  78  opening and the hydraulic fluid reaching the first chamber  38  of the steering actuator  26  via the connection point  60  and the hydraulic line  30 . The piston  32  shifts to the left and the wheels  14  are steered to the right. Due to the pressure at the inlets of the check valves  58 ,  78  and the pressure that builds up in the contracting second chamber  36  of the steering actuator  26 , which also reaches the control chambers of the stop valves  100  and  104  via the hydraulic line  28  and the connection point  66 , the aforementioned stop valves open and the hydraulic fluid flows via the stop valves  100  and  104  and the filters  110 ,  112  or the pressure relief valves  114 ,  116  into the storage tank  84 . The supply of the hydraulic pumps  54 ,  72  with hydraulic fluid occurs via the inlets  88  or the check valves  92 ,  96 . The respective other stop valves  98 ,  102  and the check valves  64 ,  82  remain closed as a result of the lack of applied pressures sufficient for the opening. 
     When the operator now analogously operates the operator interface  20  in order to steer to the left, the steering device  44  causes the electric motors  50 ,  68  to turn in the other direction, which leads to a pressure building up at the connections of the hydraulic motors  54  and  72  indicated on the right in  FIG. 3 . This pressure leads to the check valves  64  and  82  opening and the hydraulic fluid reaching the second chamber  36  of the steering actuator  26  via the connection point  66  and the hydraulic line  28 . The piston  32  shifts to the right and the wheels are steered to the left. Due to the pressure at the inlets of the check valves  64 ,  82  and the pressure that builds up in the contracting first chamber  38  of the steering actuator  26 , which also reaches the control chambers of the stop valves  98  and  102  via the hydraulic line  30  and the connection point  60 , said stop valves open, and the hydraulic fluid flows via the stop valves  98  and  102  and the filters  110 ,  112  or the pressure relief valves  114 ,  116  into the storage tank  84 . The supply of the hydraulic pumps  54 ,  72  with hydraulic fluid occurs via the inlets  88  or the check valves  90 ,  94 . The respective other stop valves  100 ,  104  and the check valves  58 ,  78  remain closed due to the lack of applied pressures that are sufficient for the opening. 
     In the case where one of the electrohydraulic circuits  46 ,  48  fails, no pressure can build up in this circuit. The check valves  58 ,  64 ,  78 ,  82  prevent the hydraulic fluid from flowing out of a functioning circuit into a defective circuit and they allow the further operation of the steering system with only one remaining functioning circuit. The functional failure of the defective circuit  46  or  48  can be acquired by the control device  44  by means of corresponding sensors, for example, for the rotation of the electric motors  50 ,  68  or the pressures in the circuit, and optionally the electric motor  50  or  68  of the defective circuit  46  or  48  is switched off automatically. 
     While embodiments incorporating the principles of the present disclosure have been described hereinabove, the present disclosure is not limited to the described embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of the disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this disclosure pertains and which fall within the limits of the appended claims.