Patent Publication Number: US-2010108431-A1

Title: Steering system

Description:
TECHNICAL FIELD 
     The present invention relates to a steering system suitable for vehicles such as a construction machine. 
     BACKGROUND ART 
     An example of steering systems applied to vehicles such as a construction machine is a steering system described in Patent Document 1. In this steering system, when a steering handle is manipulated, a pilot flow rate is supplied from a steering motor unit to a directional control valve, and the directional control valve is switchingly operated based on the manipulation of the steering handle. When the directional control valve is switchingly operated, directions of pressure oil supply from a steering pump to a steering actuator are switched, and the vehicle is steered according to the manipulation of the steering handle. 
     In this type of steering system, a situation in which the operation of the steering actuator does not match the manipulation of the steering handle (hereinafter, “knob displacement”) may be caused by various factors such as an internal oil leakage and a disturbance. When such a knob displacement occurs, the vehicle does not travel according to intensions of an operator, which lowers operability of the applied vehicle. 
     Thus, in the technique described in Patent Document 1, a corrective hydraulic circuit including an electromagnetic valve unit is interposed between the directional control valve and the steering actuator. That is, in the technique described in Patent Document 1, when the knob displacement occurs, the electromagnetic valve unit of the corrective hydraulic circuit is actuated to supply a corrective pressure oil to the steering actuator, thereby correcting the knob displacement. 
     Patent Document 1: Japanese Patent Application Laid-open No. 2005-297924 
     DISCLOSURE OF INVENTION 
     Problem to be Solved by the Invention 
     In the technique described in Patent Document 1, the corrective pressure oil is supplied to the steering actuator. Thus, the correction is enabled when the operation of the steering actuator is smaller or later than the manipulation of the steering handle. However, when the operation of the steering actuator is larger or earlier than the manipulation of the steering handle, the correction is difficult. 
     The present invention has been achieved in view of the above circumstances, and an object of the present invention is to provide a steering system capable of constantly improving operability of a vehicle applied with the steering system. 
     Means for Solving Problem 
     According to an aspect of the present invention, a steering system with a directional control valve switchingly operating based on a pilot flow rate supplied from a steering motor unit when a steering handle is manipulated, that steers a vehicle according to a manipulation of the steering handle by controlling a supply of pressure oil to a steering actuator through the directional control valve includes an electromagnetic valve that supplies a correction pilot pressure to the directional control valve so as to correct a pilot pressure generated by the pilot flow rate from the steering motor unit when a command signal is given. 
     Advantageously, in the steering system, the steering actuator is a cylinder actuator that operates to extend or retract according to a direction of pressure oil supply, the directional control valve includes a spool being disposed movably relative to the sleeve and changes the direction of the pressure oil supply to the steering actuator according to a movement direction of the spool, and the spool includes a pilot-pressure receiving surface on which the pilot pressure generated by the pilot flow rate from the steering motor unit is applied, and a correction-pilot-pressure receiving surface on which the correction pilot pressure from the electromagnetic valve is applied. 
     EFFECT OF THE INVENTION 
     According to the present invention, there are provided the electromagnetic valve units that supply the correction pilot pressure to the directional control valve in a manner to correct the pilot pressure generated by the pilot flow rate from the steering motor unit when a command signal is given, and thus, when the correction pilot pressure is supplied from the electromagnetic valve units, the directional control valve can be operated. Accordingly, when the operation of the steering actuator is smaller or later than the manipulation of the steering handle, this can be corrected by supplying the correction pilot pressure in the same direction as that of the pilot pressure. When the operation of the steering actuator is larger or earlier, this can be corrected by supplying the correction pilot pressure in a direction opposite to that of the pilot pressure. As a result, it becomes possible to drive the steering actuator based on the manipulation of the steering handle, thereby enabling improvement of the operability of the applied vehicle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram of a steering system according to an embodiment of the present invention. 
         FIG. 2  is a plan view for conceptually depicting a vehicle applied with the steering system shown in  FIG. 1 . 
         FIG. 3  is an enlarged view of a directional control valve shown in  FIG. 1 . 
         FIG. 4  is a perspective view of relevant parts for conceptually depicting pilot-pressure receiving surfaces and correction-pilot-pressure receiving surfaces of the directional control valve shown in  FIG. 1 . 
         FIG. 5  is a flowchart of a correcting process performed by a controller shown in  FIG. 1 . 
     
    
    
     EXPLANATIONS OF LETTERS OR NUMERALS 
     
         
         
           
               200  directional control valve 
               201  sleeve 
               202   a ,  202   b  actuator port 
               203  input port 
               204   a ,  204   b  drain port 
               205  spool 
               206  neutral spring 
               207   a ,  207   b  correction-pilot-pressure receiving surface 
               208   a ,  208   b  pilot-pressure receiving surface 
               215   a ,  215   b  communication port 
               216  second short-circuit oil passage 
               220  first short-circuit oil passage 
               221  fixed throttle valve 
               230  valve driving unit 
               231   a  first pilot-pressure chamber 
               231   b  second pilot-pressure chamber 
               232   a ,  232   b  correction-pilot-pressure chamber 
               300  steering motor unit 
               310  steering motor 
               320  steering valve 
               321  pilot input port 
               322  pilot drain port 
               323   a ,  323   b  motor port 
               324   a ,  324   b  pilot output port 
               325  centering spring 
               330  steering handle 
               331  handle shaft 
               400  steering pump 
               500  oil tank 
               600   a ,  600   b  electromagnetic valve 
               602   a  first correction-pilot-pressure output passage 
               602   b  second correction-pilot-pressure output passage 
               700  controller 
             C 1 , C 2  cylinder actuator for steering 
           
         
       
    
     BEST MODE(S) FOR CARRYING OUT THE INVENTION 
     Exemplary embodiments of a steering system according to the present invention will be explained below in detail with reference to the accompanying drawings. 
     EMBODIMENTS 
       FIG. 1  is a circuit diagram of a steering system according to an embodiment of the present invention. The steering system exemplified herein is applied to a vehicle  100  used as a construction machine such as a wheel loader and a dump truck. Particularly, in the present embodiment, as shown in  FIG. 2 , the steering system is applied to the vehicle  100  which includes a linking pin  101  along a vertical axis at the center portion thereof and which is steered by placing a forward vehicle-body unit  110  and a backward vehicle-body unit  120  to form a bend with respect to each other about the linking pin  101 . The forward vehicle-body unit  110  and the backward vehicle-body unit  120  each include a pair of left and right wheels W, and are swingably linked to each other via the linking pin  101 . 
     Between the forward vehicle-body unit  110  and the backward vehicle-body unit  120 , cylinder actuators C 1  and C 2  for steering are arranged at both sides across the linking pin  101 . Each of the cylinder actuators C 1  and C 2  has one end, for example, the proximal end of a cylinder body, which is swingably supported about a vertical axis at a portion that is the front end of the backward vehicle-body unit  120 . The other end of each of the cylinder actuators C 1  and C 2 , for example, the distal end of an actuation rod, is swingably supported about a vertical axis at a portion that is the rear end of the forward vehicle-body unit  110 . 
     The cylinder actuators C 1  and C 2 , when holding a state of a mutually neutral position as shown in the upper portion of  FIG. 2 , for example, can move straight the vehicle  100  by placing the forward vehicle-body unit  110  and the backward vehicle-body unit  120  in a straight line. When the cylinder actuator C 1  is actuated to degenerate whereas the other cylinder actuator C 2  is actuated to expand, as shown in the lower portion of  FIG. 2 , for example, the forward vehicle-body unit  110  comes to a state of being bent to the left relative to the backward vehicle-body unit  120 . Accordingly, in this state, the vehicle  100  can be moved in the left direction in which the forward vehicle-body unit  110  is oriented. When the vehicle  100  is to be moved to the right, it suffices to operate the cylinder actuators C 1  and C 2  in the opposite direction. In the present embodiment, as shown in  FIG. 1 , head-side pressure chambers and rod-side pressure chambers are alternately connected by a pair of connection oil passages  1  and  2  so that the cylinder actuator C 1  and the other cylinder actuator C 2  are synchronized to operate in the opposite directions. Specifically, the rod-side pressure chamber of the first cylinder actuator C 1  located on the left side in  FIG. 1  and the head-side pressure chamber of the second cylinder actuator C 2  located on the right side in  FIG. 1  are connected to each other by the first connection oil passage  1 . Similarly, the head-side pressure chamber of the first cylinder actuator C 1  and the rod-side pressure chamber of the second cylinder actuator C 2  are connected to each other by the second connection oil passage  2 . 
     The steering system applied to the vehicle  100  includes a directional control valve  200 , a steering motor unit  300 , and a steering pump  400 . 
     The directional control valve  200  includes, as shown in  FIG. 3 , a sleeve  201  arranged with a pair of actuator ports  202   a  and  202   b , an input port  203 , and a pair of drain ports  204   a  and  204   b , and also includes a spool  205  movably arranged inside the sleeve  201 . In the directional control valve  200 , when the position of the spool  205  relative to the sleeve  201  is changed, connecting manners of the pair of actuator ports  202   a  and  202   b , the input port  203 , and the pair of drain ports  204   a  and  204   b  can be changed. The actuator ports  202   a  and  202   b  of the directional control valve  200  are individually connected to the connection oil passages  1  and  2  of the cylinder actuators C 1  and C 2 . Specifically, the first actuator port  202   a  located on the left side in  FIGS. 1 and 3  is connected to the first connection oil passage  1  whereas the second actuator port  202   b  located on the right side in  FIGS. 1 and 3  is connected to the second connection oil passage  2 . The input port  203  is connected to a discharge outlet of the steering pump  400  through a main supply-oil passage  3 . The drain ports  204   a  and  204   b  are connected to an oil tank  500  through drain oil passages  4  and  5 , respectively. 
     The directional control valve  200  is normally in a neutral state by a spring force of neutral springs  206 , and the pair of actuator ports  202   a  and  202   b , the input port  203 , and the pair of drain ports  204   a  and  204   b  are all held in closed states. 
     When the spool  205  is moved from this neutral state to the left side relative to the sleeve  201  in  FIGS. 1 and 3 , for example, the first actuator port  202   a  and the first drain port  204   a  located on the left side in  FIG. 1  establish communication with each other by a first communication oil groove  205   a  arranged in the spool  205 . At the same time, by a second communication oil groove  205   b  and a third communication oil groove  205   c  arranged in the spool  205 , the second actuator port  202   b  and the input port  203  establish communication with each other via an in-valve oil conduit  201   a  of the sleeve  201 . Accordingly, when the steering pump  400  is driven, a pressure oil supplied through the main supply-oil passage  3  to the input port  203  is passed through the second communication oil groove  205   b , the in-valve oil conduit  201   a , and the third communication oil groove  205   c  to be fed to the second actuator port  202   b , and further, the pressure oil is supplied to the head-side pressure chamber of the first cylinder actuator C 1  and the rod-side pressure chamber of the second cylinder actuator C 2 , respectively, from the second actuator port  202   b  through the second connection oil passage  2 . As a result, the first cylinder actuator C 1  operates to expand whereas the second cylinder actuator C 2  operates to degenerate. This brings the forward vehicle-body unit  110  in a state bent to the right relative to the backward vehicle-body unit  120 , for example. In this way, the vehicle  100  can be moved toward the right. In the state described above, the pressure oil in the rod-side pressure chamber of the first cylinder actuator C 1  and that in the head-side pressure chamber of the second cylinder actuator C 2  are drained through the first connection oil passage  1 , the first actuator port  202   a , the first communication oil groove  205   a , and the drain port  204   a , into the oil tank  500 . 
     On the other hand, when the spool  205  is moved from the neutral state to the right side relative to the sleeve  201  in  FIG. 1 , the first actuator port  202   a  and the input port  203  establish communication with each other by the first communication oil groove  205   a  and the second communication oil groove  205   b  in the spool  205 . At the same time, by the third communication oil groove  205   c  of the spool  205 , the second actuator port  202   b  and the second drain port  204   b  located on the right side in  FIG. 1  establish communication with each other. Accordingly, when the steering pump  400  is driven, the pressure oil supplied through the main supply-oil passage  3  to the input port  203  is passed through the second communication oil groove  205   b , the in-valve oil conduit  201   a , and the first communication oil groove  205   a  to be supplied to the first actuator port  202   a , and further, the pressure oil is supplied to the rod-side pressure chamber of the first cylinder actuator C 1  and the head-side pressure chamber of the second cylinder actuator C 2 , respectively, from the first actuator port  202   a  through the first connection oil passage  1 . As a result, the first cylinder actuator C 1  operates to degenerate whereas the second cylinder actuator C 2  operates to expand. This brings the forward vehicle-body unit  110  in a state bent to the left relative to the backward vehicle-body unit  120 , for example. In this way, the vehicle  100  can be moved toward the left. In the state described above, the pressure oil in the head-side pressure chamber of the first cylinder actuator C 1  and that in the rod-side pressure chamber of the second cylinder actuator C 2  are drained through the second connection oil passage  2 , the second actuator port  202   b , the third communication oil groove  205   c , and the drain port  204   b , into the oil tank  500 . 
     In the directional control valve  200  described above, the both ends of the spool  205  protrude from the sleeve  201 , and correction-pilot-pressure receiving surfaces  207   a  and  207   b  and pilot-pressure receiving surfaces  208   a  and  208   b  are formed at the ends, respectively. As shown in  FIG. 4 , the correction-pilot-pressure receiving surfaces  207   a  and  207   b  are end surfaces of a small diameter unit  209  (ø=d 1 ) forming columns at the both ends of the spool  205 , and are formed in circles to have the same area. The pilot-pressure receiving surfaces  208   a  and  208   b , which are larger in diameter than the small diameter unit  209 , are end surfaces of the spool  205  (ø=d 2 &gt;d 1 ), and formed in a toroidal surface shape that is obtained by eliminating therefrom a center portion where the small diameter unit  209  extends. Also the pilot-pressure receiving surfaces  208   a  and  208   b  are configured to have the same area. 
     The steering motor unit  300  includes a steering motor  310  and a steering valve  320 , as shown in  FIG. 1 . In the steering motor unit  300 , when the steering valve  320  is switchingly operated via a steering handle  330 , a connecting manner of a pilot input port  321 , a pilot drain port  322 , a pair of motor ports  323   a  and  323   b , and a pair of pilot output ports  324   a  and  324   b  can be changed. The pilot input port  321  is connected to a discharge outlet of the steering pump  400  through a unit-supply oil passage  7  including a depressurizing valve  6 . The depressurizing valve  6  depressurizes the pressure oil discharged from the steering pump  400  to a predetermined pressure, and feeds the depressurized pressure oil to the steering motor unit  300 . There is not an absolute need of the depressurizing valve  6 , and thus it is also possible to configure in such a manner that the pressure oil discharged from the steering pump  400  is directly fed to the steering motor unit  300 . 
     The pilot drain port  322  is connected to the oil tank  500  through a pilot-drain oil passage  8 . The pair of motor ports  323   a  and  323   b  is connected to a pair of distribution outlets arranged in the steering motor  310 . The pair of pilot output ports  324   a  and  324   b  is connected to pilot-pressure-supply oil passages  9   a  and  9   b , respectively. In  FIG. 1 , for the sake of convenience, a linear valve is shown as the steering valve  320 . Practically, however, the steering valve  320  is configured as a two-layer rotary valve including a spool connected to a handle shaft  331  of the steering handle  330  and a sleeve that surrounds the spool. The steering motor  310  is configured to make rotation drive based on the manipulation of the steering handle  330 . 
     In the steering motor unit  300 , the steering valve  320  is normally in a neutral state due to a spring force of centering springs  325 , and the pilot input port  321 , the pilot drain port  322 , the pair of motor ports  323   a  and  323   b , and the pair of pilot output ports  324   a  and  324   b  are each held in a closed state. 
     When the steering handle  330  is manipulated to rotate from this neutral state in one direction by a predetermined amount, the steering valve  320  is moved to a first position VL located on the left side in  FIG. 1 , for example. Thereby, the pilot input port  321  and the first motor port  323   a  located on the left side in  FIG. 1  establish communication. At the same time, the second motor port  323   b  and the second pilot output port  324   b  each located on the right side in  FIG. 1  establish communication. Moreover, the steering motor  310  is rotated in one direction based on the manipulation of the steering handle  330 . Accordingly, the pressure oil discharged from the steering motor  310  is output from the second pilot output port  324   b . As a result, it becomes possible to supply a pilot flow rate through the second pilot-pressure-supply oil passage  9   b  connected to the second pilot output port  324   b . In the state described above, the first pilot output port  324   a  located on the left side in  FIG. 1  and the pilot drain port  322  establish communication, and thus the pressure oil in the first pilot-pressure-supply oil passage  9   a  connected to the first pilot output port  324   a  is drained into the oil tank  500 . 
     The pressure oil output from the second pilot output port  324   b  to the second pilot-pressure-supply oil passage  9   b  is based on a manipulation amount for the steering handle  330 . After outputting the pressure oil of a flow rate based on the manipulation amount for the steering handle  330 , the steering valve  320  is restored to the neutral state by the centering springs  325 , irrespective of the position of the steering handle  330 . Accordingly, because the pilot input port  321 , the pilot drain port  322 , the pair of motor ports  323   a  and  323   b , and the pair of pilot output ports  324   a  and  324   b  are each closed again, the supply of the pressure oil from the second pilot output port  324   b  is stopped. 
     On the other hand, when the steering handle  330  is manipulated to rotate from the neutral state in the other direction by a predetermined amount, the steering valve  320  is moved to a second position VR located on the right side in  FIG. 1 . Thereby, the pilot input port  321  and the second motor port  323   b  establish communication. At the same time, the first motor port  323   a  and the first pilot output port  324   a  establish communication. Moreover, the steering motor  310  is rotated in the other direction based on the manipulation of the steering handle  330 . Accordingly, the pressure oil discharged from the steering motor  310  is output from the first pilot output port  324   a , and as a result, the pilot flow rate can be supplied through the first pilot-pressure-supply oil passage  9   a  connected to the first pilot output port  324   a . In the state described above, the second pilot output port  324   b  and the pilot drain port  322  establish communication, and as a result, the pressure oil in the second pilot-pressure-supply oil passage  9   b  connected to the second pilot output port  324   b  is drained into the oil tank  500 . 
     Similarly to the case described above, the pressure oil output from the first pilot output port  324   a  to the first pilot-pressure-supply oil passage  9   a  based on a manipulation amount for the steering handle  330 . After outputting the pressure oil based on the manipulation amount of the steering handle  330 , the steering valve  320  is restored to the neutral state by the centering springs  325 , irrespective of the position of the steering handle  330 . Accordingly, because the pilot input port  321 , the pilot drain port  322 , the pair of motor ports  323   a  and  323   b , and the pair of pilot output ports  324   a  and  324   b  are each closed again, the supply of the pressure oil from the first pilot output port  324   a  is stopped. 
     The steering pump  400  is a variable-displacement hydraulic pump. In the present embodiment, a hydraulic pump including a load-pressure-sensitive driving unit  410  that changes a displacement based on a load pressure during the supply of the pressure oil from the directional control valve  200  to the cylinder actuators C 1  and C 2  is applied as the steering pump  400 . The load-pressure-sensitive circuit is not necessarily limited to the configuration above described. For example, it is also possible to configure the load-pressure-sensitive circuit including a fixed pump and an unload valve that is actuated based on a load pressure. 
     On the other hand, in the steering system, as shown in  FIG. 3 , a first short-circuit oil passage  220  and a pair of communication ports  215   a  and  215   b  are arranged in the directional control valve  200 , and also, valve driving units  230  are provided at the both ends of the sleeve  20 , respectively. 
     The first short-circuit oil passage  220  is formed at a portion extending from one end of the sleeve  201  to the other end thereof in the directional control valve  200 , and has a fixed throttle valve  221  at an intermediate portion of the first short-circuit oil passage  220 . As obviously shown in  FIG. 1 , one end of the first short-circuit oil passage  220  is connected to the first pilot-pressure-supply oil passage  9   a  and the other end thereof is connected to the second pilot-pressure-supply oil passage  9   b . It is possible to supply a pressure oil from the steering motor unit  300  through the first pilot-pressure-supply oil passage  9   a  and the second pilot-pressure-supply oil passage  9   b.    
     The communication ports  215   a  and  215   b  are arranged at the both ends of a portion facing the spool  205  in the sleeve  201 . The second pilot-pressure-supply oil passage  9   b  is connected to the first communication port  215   a  located on the left side in  FIGS. 1 and 3 , whereas the first pilot-pressure-supply oil passage  9   a  is connected to the second communication port  215   b  located on the right side in  FIGS. 1 and 3 . In the pair of communication ports  215   a  and  215   b , a second short-circuit oil passage  216  and a third short-circuit oil passage  217  are arranged as conceptually shown in  FIG. 3 . The second short-circuit oil passage  216  and the third short-circuit oil passage  217  both have an opening area larger than that of the first short-circuit oil passage  220 . The second short-circuit oil passage  216  communicates between the communication port  215   a  and a second pilot-pressure chamber  231   b  whereas the third short-circuit oil passage  217  communicates between the communication port  215   b  and a first pilot-pressure chamber  231   a . The both passages  216  and  217  can communicate between the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  through communicative notches  218   a  and  218   b  formed at the ends of the spool  205 . 
     The communicative notches  218   a  and  218   b  are groove-shaped notches formed on the outer peripheral surface at the both ends of the spool  205  in a manner to run along an axial direction thereof. An end of the communicative notch  218   a  on the side that is close to the first pilot-pressure chamber  231   a  opens at a stepped portion between the spool  205  and the small diameter unit  209 , and is always communicated with the first pilot-pressure chamber  231   a . The other end of the communicative notch  218   a  on the side that is close to the communication port  215   a  is placed at a position spaced from the communication port  215   a  when the spool  205  is in a neutral state. The end is arranged at a position that enables to be communicated with the communication port  215   a  when the spool  205  moves by a predetermined amount to the right in  FIGS. 1 and 3 . The communicative notch  218   a  is formed in such a shape that an area communicated with the communication port  215   a  is progressively increased as a movement amount of the spool  205  to the right becomes greater. Likewise, an end of the communicative notch  218   b  on the side that is close to the second pilot-pressure chamber  231   b  opens at a stepped portion between the spool  205  and the small diameter unit  209 , and is always communicated with the second pilot-pressure chamber  231   b . The other end of the communicative notch  218   b  on the side that is close to the communication port  215   b  is placed at a position spaced from the communication port  215   b  when the spool  205  is in a neutral state. The end is arranged at a position that enables to be communicated with the communication port  215   b  when the spool  205  moves by a predetermined amount to the left in  FIGS. 1 and 3 . The communicative notch  218   b  is formed in such a shape that an area communicated with the communication port  215   b  is progressively increased as a movement amount of the spool  205  to the left becomes greater. 
     With cooperation of the second short-circuit oil passage  216 , the third short-circuit oil passage  217 , and the communicative notches  218   a  and  218   b , neither the communicative notch  218   a  nor the communicative notch  218   b  are communicated with the communication ports  215   a  and  215   b  when the spool  205  is in a neutral state relative to the sleeve  201 . Thus, the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  are held in a state isolated from each other. On the other hand, when the spool  205  is moved either to the left or right relative to the sleeve  201 , when the spool  205  is moved to the right side in  FIGS. 1 and 3 , for example, the communicative notch  218   a  positioned on the left side is communicated with the communication port  215   a . Accordingly, the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  are communicated with each other through the communicative notch  218   a , the communication port  215   a , and the second short-circuit oil passage  216 . Likewise, when the spool  205  is moved to the left in  FIGS. 1 and 3 , the communicative notch  218   b  positioned on the right side is communicated with the communication port  215   b , and thus the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  are communicated with each other through the communicative notch  218   b , the communication port  215   b , and the third short-circuit oil passage  217 . 
     The valve driving unit  230  is for moving the spool  205  relative to the sleeve  201  of the directional control valve  200 . The valve driving unit  230  includes the first pilot-pressure chamber  231   a  and a first correction-pilot-pressure chamber  232   a  at one end of the sleeve  201 , and the second pilot-pressure chamber  231   b  and a second correction-pilot-pressure chamber  232   b  at the other end of the sleeve  201 . 
     The first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  house the pilot-pressure receiving surfaces  208   a  and  208   b  of the sleeve  201 , respectively. The first pilot-pressure-supply oil passage  9   a  from the steering motor unit  300  is branch-connected to the first pilot-pressure chamber  231   a  whereas the second pilot-pressure-supply oil passage  9   b  from the steering motor unit  300  is branch-connected to the second pilot-pressure chamber  231   b . In the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b , the neutral springs  206  are respectively housed. 
     The first correction-pilot-pressure chamber  232   a  and the second correction-pilot-pressure chamber  232   b  house the correction-pilot-pressure receiving surfaces  207   a  and  207   b  of the spool  205  at portions further outside the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b . The first pilot-pressure chamber  231   a  and the first correction-pilot-pressure chamber  232   a  adjacent to each other, and the second pilot-pressure chamber  231   b  and the second correction-pilot-pressure chamber  232   b  adjacent to each other are respectively separated by partition walls  233 . 
     As shown in  FIG. 1 , the steering system further includes a pair of electromagnetic valve units  600   a  and  600   b  and a controller  700 . When a command signal is respectively given thereto, the electromagnetic valve units  600   a  and  600   b  individually output pressure oils for applying correction pilot pressures corresponding to the command signal. The first electromagnetic valve unit  600   a  located on the left side in  FIG. 1  has an output port  601   a , which is connected via a first correction-pilot-pressure output passage  602   a  to the first correction-pilot-pressure chamber  232   a  whereas the second electromagnetic valve unit  600   b  located on the right side in  FIG. 1  has an output port  601   b , which is connected via a second correction-pilot-pressure output passage  602   b  to the second correction-pilot-pressure chamber  232   b . Correction pilot pressures output from the electromagnetic valve units  600   a  and  600   b  are pressure oils that are supplied from the steering pump  400  through the unit-supply oil passage  7 . 
     The controller  700  outputs the command signal to the electromagnetic valve units  600   a  and  600   b  based on a detection signal from a handle angular sensor  701  that detects a manipulated angle of the steering handle  330 , and a detection signal from a vehicle-body angular sensor  702  that detects a vehicle body angle formed between the forward vehicle-body unit  110  and the backward vehicle-body unit  120 . In the controller  700 , manipulation amounts (handle angles) of the steering handle  330  and appropriate vehicle-body angles formed between the forward vehicle-body unit  110  and the backward vehicle-body unit  120  corresponding to the handle angles are previously stored in its own memory (not shown) in a mutually associated manner. 
       FIG. 5  is a flowchart of a correcting process performed by the controller  700  shown in  FIG. 1 . With appropriate reference to the flowchart, an operation of the steering system will be described below. 
     In the steering system configured as described above, when the steering handle  330  is held in a straight-ahead state, for example, the steering valve  320  of the steering motor unit  300  is held in a neutral state, and accordingly, also the directional control valve  200  is held in a neutral state. As a result, the pair of cylinder actuators C 1  and C 2  maintains the current state without operating to extend and retract, and thus the vehicle  100  continues a straight-ahead movement. 
     When the steering handle  330  is manipulated, a pilot flow rate corresponding to the manipulation amount is applied to the directional control valve  200  and the valve driving unit  230  through the pilot-pressure-supply oil passage  9   a  or  9   b  according to a manipulated direction. 
     Assuming here that the pilot flow rate is supplied to the directional control valve  200  and the valve driving unit  230  from the steering motor unit  300  through the first pilot-pressure-supply oil passage  9   a , for example, the pilot flow rate acts as a pilot pressure on the first pilot-pressure chamber  231   a , and acts as a pilot pressure on the second pilot-pressure chamber  231   b  through the first short-circuit oil passage  220 . The pilot flow rate is then drained through the second pilot-pressure-supply oil passage  9   b  into the oil tank  500 . 
     In this example, the pilot pressure acting through the first short-circuit oil passage  220  on the second pilot-pressure chamber  231   b  is smaller than the pilot pressure acting on the first pilot-pressure chamber  231   a . As a result, due to a difference between the pilot pressure acting through the first pilot-pressure chamber  231   a  on the pilot-pressure receiving surface  208   a  and that acting through the second pilot-pressure chamber  231   b  on the pilot-pressure receiving surface  208   b , the spool  205  starts moving to the right relative to the sleeve  201  in  FIG. 1 . When the spool  205  starts moving relative to the sleeve  201 , the communicative notch  218   a  located on the left side of the spool  205  establishes communication with the communication port  215   a  at a time point when the movement amount reaches a previously set value. Thus, the first pilot-pressure chamber  231   a  and the second pilot-pressure chamber  231   b  establish communication therebetween through the communicative notch  218   a , the communication port  215   a , and the second short-circuit oil passage  216 , and as a result, the flow rate of the pressure oil that has passed through the second short-circuit oil passage  216  increases. This further enlarges the difference between the pilot pressure acting through the first pilot-pressure chamber  231   a  on the pilot-pressure receiving surface  208   a  and that acting through the second pilot-pressure chamber  231   b  on the pilot-pressure receiving surface  208   b , and thereby, also the movement amount in the same direction of the spool  205  relative to the sleeve  201  is increased. 
     When the spool  205  is moved to the right relative to the sleeve  201  in  FIG. 1 , the first actuator port  202   a  and the input port  203  establish communication with each other by the first communication oil groove  205   a  and the second communication oil groove  205   b  of the spool  205 , as described above. At the same time, by the third communication oil groove  205   c  of the spool  205 , the second actuator port  202   b  and the second drain port  204   b  establish communication with each other. Accordingly, the pressure oils are supplied from the steering pump  400  through the first actuator port  202   a  and the first connection oil passage  1  to the rod-side pressure chamber of the first cylinder actuator C 1  and the head-side pressure chamber of the second cylinder actuator C 2 , respectively. As a result, the first cylinder actuator C 1  operates to degenerate whereas the second cylinder actuator C 2  operates to expand. This brings the forward vehicle-body unit  110  in a state bent to the left relative to the backward vehicle-body unit  120 , for example. In this way, the vehicle  100  can be moved to the left. 
     During the operation described above, the controller  700  is in a state of monitoring the displacement of the steering handle  330  through the handle angular sensor  701 , as shown in  FIG. 5  (Step S 101 ). When it is detected by the handle angular sensor  701  that the steering handle  330  is manipulated from this state (YES at Step S 101 ), the controller  700  obtains a current vehicle body angle through the vehicle-body angular sensor  702  (Step S 102 ), and further compares the obtained vehicle body angle and the vehicle body angle corresponding to the handle angle previously stored in the memory to calculate the knob displacement (Step S 103 ). 
     When the amount of knob displacement calculated at Step S 103  is out of an acceptable range previously set, that is, when the vehicle body angle between the forward vehicle-body unit  110  and the backward vehicle-body unit  120  is displaced beyond the acceptable range relative to the manipulation amount of the steering handle  330  (YET at Step S 104 ), the controller  700  calculates a correction amount of the spool  205  to render the amount of knob displacement zero (Step S 105 ). The controller  700  further transmits a correction-pilot-pressure output command signal to the corresponding electromagnetic valve unit  600   a  or  600   b  to move the spool  205  according to the calculated correction amount (Step S 106 ). 
     As a result, from the electromagnetic valve unit  600   a  or  600   b  to which a command signal is given, the correction pilot pressure is applied to the correction-pilot-pressure chamber  232   a  or  232   b , and according thereto, the spool  205  of the directional control valve  200  is moved. At this time, when the vehicle body angle is smaller than the manipulation amount of the steering handle  330 , the correction pilot pressure is applied from the electromagnetic valve unit  600   a  or  600   b  in the same direction as that of the pilot pressure so that the pilot pressure from the steering motor unit  300  is increased. When the vehicle body angle is larger than the manipulation amount of the steering handle  330 , the correction pilot pressure that flows in a direction opposite to that of the pilot pressure is applied from the electromagnetic valve unit  600   a  or  600   b  so that the pilot pressure from the steering motor unit  300  is decreased. Accordingly, even when the vehicle body angle is either larger or smaller than the manipulation amount of the steering handle  330 , the manner of supplying the pressure oil to the cylinder actuators C 1  and C 2  is changed so that the vehicle body angle formed between the forward vehicle-body unit  110  and the backward vehicle-body unit  120  is corrected to an appropriate angle corresponding to the manipulation amount of the steering handle  330 . This significantly improves the operability of the vehicle  100 . 
     When the amount of knob displacement calculated at Step S 103  is within the acceptable range previously set, that is, when the forward vehicle-body unit  110  and the backward vehicle-body unit  120  form a bend based on the manipulation amount of the steering handle  330  (NO at Step S 104 ), the controller  700  ends the current process without performing the succeeding processes, and causes the procedure to return. As a result, in the steering system, the directional control valve  200  is operated according to the pilot pressure supplied from the steering motor unit  300 , and further, the cylinder actuators C 1  and C 2  are operated by the pressure oil supplied and controlled by the directional control valve  200 . Accordingly, in this case also, the vehicle  100  is steered based on the manipulation amount of the steering handle  330 . 
     In the embodiment described above, the manipulation amount of the steering handle  330  detected by the handle angular sensor  701  is compared with the vehicle body angle detected by the vehicle-body angular sensor  702 . However, the present invention is not limited thereto. For example, the correction pilot pressure can be set by arranging a stroke sensor  800  that detects the movement amount of the spool  205  relative to the sleeve  201  in the directional control valve  200 , and comparing the manipulation amount of the steering handle  330  detected by the handle angular sensor  701  with the movement amount of the spool  205  detected by the stroke sensor  800 . 
     In the embodiment described above, the cylinder actuator is exemplified as the steering actuator. However, the cylinder actuator is not necessarily needed. 
     Moreover, in the embodiment described above, the steering system applied to the construction machine is exemplified. However, the present invention can be also applied to other vehicles. In this case, the present invention is not limited to the vehicle that is steered by forming a bend with the forward vehicle-body unit and the backward vehicle-body unit. That is, it is possible to achieve identical operational effects also in a case that the present invention is applied to a steering system configured such that an orientation of steered wheels is changed relative to the vehicle. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention is useful for a steering system of a vehicle such as a construction machine, and enables to operate the directional control valve by supplying the correction pilot pressures from the electromagnetic valve units. Thus, when the operation of the steering actuator is smaller or later than the manipulation of the steering handle, this can be corrected by supplying the correction pilot pressure in the same direction as that of the pilot pressure. When the operation of the steering actuator is larger or earlier, this can be corrected by supplying the correction pilot pressure in a direction opposite to that of the pilot pressure. As a result, it becomes possible to drive the steering actuator based on the manipulation of the steering handle, thereby enabling improvement of the operability of the applied vehicle.