CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based on and claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2005-331115, filed on Nov. 16, 2005, the entire content of which is incorporated herein by reference.  
       FIELD OF THE INVENTION  
       [0002]     This invention generally relates to a stabilizer control apparatus for a vehicle.  
       BACKGROUND  
       [0003]     A known stabilizer of a vehicle is provided between a right wheel and a left wheel of a front wheel side, a rear wheel side, or both thereof of a vehicle, and functions as a torsion spring when a relative displacement difference occurs in a suspension stroke between the right wheel and the left wheel. A stabilizer control apparatus is known as changing a torsional rigidity of the stabilizer. JP2000-289427A, for example, discloses a stabilizer control apparatus in which a torsion portion of a stabilizer is divided into a right portion and a left portion. In order to achieve both an effective roll control on a turning road, and the like, and a superior ride comfort during a straight-ahead running, respective ends of the right portion and the left portion face each other and between which a clutch mechanism is arranged for connecting and disconnecting the right portion and the left portion. The clutch mechanisms of a spline engagement type and of an operation pin type are disclosed in JP2000-289427A.  
         [0004]     Further, JP2000-289427A discloses a hydraulic cylinder device for allowing a connecting rod in a connecting rod assembly connected to a right suspension portion or a left suspension portion to freely elongate or contract, or for locking a movement of the connecting rod. When a vehicle is running at a high speed or turning, an operation piston of the cylinder device is locked so that the connecting rod assembly is prevented from elongating or contracting. On the other hand, when a vehicle is straight running at a low speed, the operation piston is allowed to move freely so that the connecting rod assembly as a hole can freely elongate or contract.  
         [0005]     Since the stabilizer control apparatus intends to improve a ride comfort in the straight running state and to prevent a roll motion in the turning state, the torsional rigidity can be changed by means of an operation of a drive such as a manual switch. Then, the torsional rigidity is immediately switched before a vehicle starts the roll motion, i.e. when the vehicle is straight running or the roll motion is slightly generated.  
         [0006]     According to the disclosed stabilizer control apparatus, when the right portion and the left portion of the torsion portion of the stabilizer is connected or disconnected by the clutch mechanism of spline engagement type or of operation pin type, a torsion is generated in the stabilizer control apparatus when the vehicle is stopped or straight running. The spline, the operation pin, and the like receive that torsion, which may prevent a smooth connection or disconnection of the clutch mechanism.  
         [0007]     Meanwhile in the case that the connecting rod is allowed to elongate or contract, or is locked by the cylinder device, a suspension stroke difference for changing the torsional rigidity of the stabilizer is difficult to be secured and otherwise a large apparatus is required.  
         [0008]     Thus, a need exists for a stabilizer control apparatus that can have a small structure and can immediately and appropriately switch a torsional rigidity.  
       SUMMARY OF THE INVENTION  
       [0009]     According to an aspect of the present invention, a stabilizer control apparatus includes a first torsion bar connected to one of the right wheel and the left wheel of the vehicle, a second torsion bar connected to the other one of the right wheel and the left wheel of the vehicle, and a connecting and disconnecting means including a rotation member disposed between the first torsion bar and the second torsion bar and rotating as a unit with the first torsion bar, and a housing accommodating therein the rotation member so as to be rotatable and forming two pressure chambers between the rotation member in a circumferential direction, the housing being in contact with the second torsion bar, the two pressure chambers being filled with a fluid. The connecting and disconnecting means generates a connecting state in which a movement of the fluid to the two pressure chambers is blocked so that a relative rotation between the first torsion bar and the second torsion bar is prohibited and a disconnecting state in which a movement of the fluid to the two pressure chambers is allowed so that the relative rotation between the first torsion bar and the second torsion bar is allowed. The stabilizer control apparatus further includes a switching means for switching a state of the connecting and disconnecting means between the connecting state and the disconnecting state.  
         [0010]     According to another aspect of the present invention, a stabilizer control apparatus includes a first torsion bar connected to one of the right wheel and the left wheel of the vehicle, a second torsion bar connected to the other one of the right wheel and the left wheel of the vehicle, an intermediate bar arranged between the first torsion bar and the second torsion bar, and a connecting and disconnecting means including a rotation member rotating as a unit with one end portion of the intermediate torsion bar and the first torsion bar, and a housing accommodating therein the rotation member so as to be rotatable and forming two pressure chambers between the rotation member in a circumferential direction, the housing fixed to the other end portion of the intermediate torsion bar and the second torsion bar, the two pressure chambers being filled with a fluid. The connecting and disconnecting means generates a connecting state in which a movement of the fluid to the two pressure chambers is blocked so that a relative rotation between the first torsion bar and the second torsion bar is prohibited and a disconnecting state in which a movement of the fluid to the two pressure chambers is allowed so that the relative rotation between the first torsion bar and the second torsion bar is allowed. A stabilizer control apparatus further includes a switching means for switching a state of the connecting and disconnecting means between the connecting state and the disconnecting state. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawings, wherein:  
         [0012]      FIG. 1  is a partial cross-sectional view of a stabilizer control apparatus according to a first embodiment of the present invention;  
         [0013]      FIG. 2  is a structural view of the stabilizer control apparatus including a cross-sectional view taken along the line II-II in  FIG. 1 .  
         [0014]      FIG. 3  is a flowchart of a control for switching a torsional rigidity of the stabilizer control apparatus;  
         [0015]      FIG. 4  is a graph showing a roll characteristic of a vehicle;  
         [0016]      FIG. 5  is a flowchart showing another example of the control for switching the torsional rigidity;  
         [0017]      FIG. 6  is a partial cross-sectional view of a stabilizer control apparatus according to a second embodiment of the present invention;  
         [0018]      FIG. 7  is a partial cross-sectional view of a stabilizer control apparatus according to a third embodiment of the present invention; and  
         [0019]      FIG. 8  is a graph showing a roll characteristic of a vehicle according to the third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Embodiments of the present invention will be explained with reference to the attached drawings.  FIGS. 1 and 2  show a structure of a stabilizer control apparatus  1  according to a first embodiment. The stabilizer control apparatus  1 , which is arranged between a right wheel and a left wheel of a front wheel side, a rear wheel side, or both thereof of a vehicle, includes a first torsion bar  11  connected to one of the right wheel and the left wheel, a second torsion bar  12  connected to the other one of the right wheel and the left wheel, and an intermediate torsion bar  12   a  arranged between the first torsion bar  11  and the second torsion bar  12 . According to the present embodiment, the intermediate torsion bar  12   a  is formed with the second torsion bar  12  as a unit, and of which a diameter is smaller than that of the second torsion bar  12 . A spline groove is formed at an end portion of the intermediate torsion bar  12   a  facing a rotor  21 .  
         [0021]     A rotary valve device as a connecting and disconnecting means, i.e. a rotary valve  20 , is disposed between the first torsion bar  11  and the second torsion bar  12 . In the rotary valve  20 , the rotor  21  (rotation member) is accommodated within a housing  22  so as to be rotatable, and a pair of vanes  21   b  are integrally formed on the rotor  21  in a circumferential direction thereof. As shown in  FIG. 2 , each vane  21   b  is arranged so as to be slidable to an inner periphery of the housing  22  via a seal member  21   s . In this case, alternatively, the vane  21   b  can be formed independently of the rotor  21  and then be connected to a shaft portion of the rotor  21 . Further, as shown in  FIG. 2 , the housing  22  includes support portions  22   e  extending to a rotation center of the rotor  21  so that the support portions  22   e  make slidably contact with an outer periphery of a shaft portion  21   a  of the rotor  21  via respective seal members  22   s . Then, pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b  are defined within the housing  22  in a circumferential direction. The pressure chambers C 1   a  and C 1   b  are in communication with each other via a continuous bore P 1   c  while the pressure chambers C 2   a  and C 2   b  are in communication with each other via a continuous bore P 2   c.    
         [0022]     As shown in  FIG. 2 , the vanes  21   b  are arranged so as to face each other relative to the shaft portion  21   a  and to form substantially 180 degrees from each other. The support portions  22   e  are arranged so as to extend from the inner periphery of the housing  22  towards the shaft portion  21   a  and to form substantially 180 degrees from each other. Accordingly, the pressure chambers C 1   a  and C 1   b  are arranged in opposite directions from each other relative to the shaft portion  21   a . In the same way, the pressure chambers C 2   a  and C 2   b  are arranged in opposite directions from each other relative to the shaft portion  21   a . That is, pairs of the pressure chambers each pair having the equal pressure are formed in the opposite directions from each other relative to the shaft portion  21   a . Thus, during the pressure control, the rotor  21  is prevented from being pressed to one side in the circumferential direction of the housing  22 . An appropriate relative rotation can be maintained between the rotor  21  and the housing  22 .  
         [0023]     As shown in  FIG. 1 , the housing  22  includes three cylindrical-shaped members  22   a ,  22   b , and  22   c . The shaft portion  21   a  of the rotor  21  is rotatably supported by one end of the cylindrical member  22   a  via one of bearings  22   j  and one of seal members  22   r . Further, the cylindrical member  22   b  is arranged so as to sandwich, together with the cylindrical member  22   a , the vanes  21   b  of the rotor  21 . A second end of the shaft portion  21   a  of the rotor  21  is rotatably supported by the cylindrical member  22   b  via the other one of bearings  22   j  and the other one of seal members  22   r . Then, the cylindrical member  22   c  is arranged between the cylindrical members  22   a  and  22   b  in such a manner that the vanes  21   b  of the rotor  21  are surrounded by the cylindrical. member  22   c , and is welded along a contact portion with the rotor  21  in the circumferential direction, thereby forming the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b  in the circumferential direction within the housing  22  as shown in  FIG. 2 .  
         [0024]     As shown in  FIG. 1 , a cylindrical-shaped tightening member  12   c  is welded and connected to the other end of the cylindrical member  22   a . The cylindrical tightening member  12   c  is spline connected to the second torsion bar  12 . In addition, a recess portion is formed on a surface of a first end of the shaft portion  21   a  of the rotor  21  and into which an end portion of the intermediate torsion bar  12   a  is received and spline connected. On the other hand, a cylindrical tightening member  1   c  is welded and connected to the second end of the shaft portion  21   a  of the rotor  21 . The tightening member  11   c  is spline connected to the first torsion bar  11 .  
         [0025]     The first torsion bar  11  and the second torsion bar  12  are rotatable as a unit via the intermediate torsion bar  12   a  and the shaft portion  21   a  of the rotor  21 . The housing  22  is rotatable as a unit with the second torsion bar  12 . In addition, the housing  22  is supported by the shaft portion  21   a  of the rotor  21  that is rotatably supported within the housing  22 . That is, consequently, the housing  22  is supported by the first torsion bar  11  so as to be rotatable relatively thereto. The first torsion bar  11  and the second torsion bar  12  are assembled onto a vehicle body (not shown) by means of mounts MT, respectively, shown by a chain double-dashed line in  FIG. 1 . In this case, alternatively, the rotor  21  can be connected to one end of the intermediate torsion bar  12   a  while the housing  22  can be fixed to the other end of the intermediate torsion bar  12   a . Further, the rotor  21  can be integrally formed with the first torsion bar  11  or the intermediate torsion bar  12   a.    
         [0026]     As shown in  FIGS. 1 and 2 , continuous bores P 1   a  and P 2   a  are formed so as to open towards the pressure chambers C 1   a  and C 2   a , respectively, and to which a communication passage  30  is connected. A first magnetic on-off valve  41  is provided at the communication passage  30  that brings the pressure chambers C 1   a  and C 2   a  to communicate with each other. In addition, an accumulator  50  is also connected to the communication passage  30  via a second magnetic on-off valve  42 . Further, plugs  51  and  52  are provided at the communication passage  30 . A system oil, such as a pressurized fluid that is pressurized to a predetermined pressure level, is sealingly enclosed, via the first and second magnetic on-off valves  41  and  42 , the accumulator  50 , the plugs  51  and  52 , in the communication passage  30 , and consequently the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b . The plugs  51  and  52  are retained in a closed position except after the system oil is enclosed, such as at a time of oil change, or the like. Since the communication passage  30  can be constituted by a flexible tube and thus an easy piping is available, the magnetic on-off valves  41  and  42 , and the like can be arranged in an appropriate position away from the rotary valve  20 , which may achieve a remote control thereof Accordingly, a degree of freedom for mounting the stabilizer control apparatus  1  in a vehicle may be increased.  
         [0027]     Even if the system oil in the communication passage  30  and the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b  is expanded due to heat, the pressure fluctuation caused thereby is absorbed by the accumulator  50 . Thus, the appropriate heat expansion compensation is achieved. Further, air may be mixed in when the system oil is supplied from the plugs  51  and  52 , thereby generating air form due to which fluctuation of oil pressure may occur. However, the system oil introduced to the accumulator  50  via the magnetic on-off valve  42  in an open position is brought to a predetermined pressure level. For example, even if the pressure in the communication passage  30  is decreased due to breakage of air form, the communication passage  30  is in communication with the accumulator  50  with the magnetic on-off valve  42  in the open position, thereby retaining the oil pressure within the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b  to the predetermined pressure level.  
         [0028]     As shown in  FIG. 2 , the magnetic on-off valves  41  and  42  are controlled to open or close by a stabilizer electronic control unit ECU within an electronic control device  100 . A manual switch MS is connected to the stabilizer electronic control unit ECU so that a torsional rigidity of the stabilizer control apparatus  1  can be changed by a switch operation of a driver. In the electronic control device  100 , detection signals of a steering angle of a steering wheel (not shown), a vehicle speed, and the like are input to a lateral acceleration estimation portion YG where then a lateral acceleration of a vehicle (Gy) is estimated and calculated on the basis of the input detection signals of a steering angle, a vehicle speed, and the like. When the estimated and calculated lateral acceleration (Gy) exceeds a predetermined value (in fact, before the lateral acceleration exceeds the predetermined value), the magnetic on-off valves  41  and  42  are brought to the closed position by the stabilizer electronic control unit ECU.  
         [0029]     Further, the stabilizer electronic control unit ECU is connected to a communication bus (not shown) by means of which the stabilizer electronic control unit ECU can share a processing information in an electronic control unit for other control systems such as a brake electronic control unit, detection signals from various sensors such as vehicle heights on right and left sides obtained by a height sensor provided on right and left sides of a vehicle. A sub-routine of a common control for changing the torsional rigidity in the stabilizer control apparatus  1  is explained with reference to FIGS.  1  to  4 .  
         [0030]     As shown in  FIG. 3 , in Step  101 , the lateral acceleration (Gy) is estimated in the aforementioned manner. Then, in Step  102 , an operation status of the manual switch MS is determined. When it is determined that the manual switch MS is turned off, a process proceeds to a main routine (not shown). When it is determined that the manual switch MS is turned on, the process proceeds to Step  103  in which the first magnetic on-off valve  41  and the second magnetic on-off valve  42  are brought to the open position. As a result, as shown in  FIG. 2 , the pressure chamber C 1   a  in communication with the pressure chamber C 1   b  via the continuous bore P 1   c  is connected to the pressure chamber C 2   a  via the continuous bore P 1   a,  the communication passage  30 , the magnetic on-off valve  41 , and the continuous bore P 2   a , and also to the pressure chamber C 2   b  via the continuous bore P 2   c . At this time, since the communication passage  30  is connected to the accumulator  50  via the magnetic on-off valve  42  in the open position, the oil pressure in the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b  is maintained at the predetermined pressure level. In Step  104 , the rotary valve  20  is in a disconnecting state (free state) and thus the torsional rigidity by the first torsion bar  11 , the second torsion bar  12 , and the intermediate torsion bar  12   a  is obtained. The roll characteristic of a vehicle at this time is shown by a in  FIG. 4 .  
         [0031]     In Step  105 , the lateral acceleration (Gy) estimated in the aforementioned manner is compared with a predetermined value K 1 . When the lateral acceleration (Gy) exceeds the predetermined value K 1 , it is determined that a high torsional rigidity is required. Then, in Step  106 , the magnetic on-off valves  41  and  42  both are brought to the closed position. As a result, the communication between the pressure chambers C 1   a  and C 1   b,  and the pressure chambers C 2   a  and C 2   b  is cut off or prohibited. Then, in Step  107 , the rotary valve  20  is in a connecting state (locked state), thereby adding the torsional rigidity by the housing  22  of the rotary valve  20  to the torsional rigidity by the first torsion bar  11 , the second torsion bar  12 , and the intermediate torsion bar  12   a . That is, as shown in  FIG. 4 , the roll characteristic of a vehicle at this time is switched to b at a point of K 1  in  FIG. 4 , b being obtained by superimposing the roll characteristic of c on the roll characteristic of a. Accordingly, before the actual lateral acceleration exceeds the predetermined value, a relative rotational position between the first torsion bar  11  and the second torsion bar  12  is adjusted to a desired position according to the control for switching the torsional rigidity based on the estimated lateral acceleration (Gy), thereby controlling a roll angle of a vehicle to a target value (explanation for controlling the target value is omitted). When the lateral acceleration (Gy) is determined to be equal to or smaller than the predetermined value K 1  in Step  105 , it is determined that the connecting state of the rotary valve  20  is not required and thus the process proceeds to the main routine with the magnetic on-off valves  41  and  42  both in the open position.  
         [0032]     According to the stabilizer control apparatus  1  with the aforementioned structure, which is arranged between a right wheel and a left wheel of a front wheel side, a rear wheel side, or both thereof of a vehicle, when different stroke inputs are made to the right wheel and the left wheel, the torsion is generated in the first torsion bar  11 , the second torsion bar  12 , and the intermediate torsion bar  12   a , i.e. the stabilizer control apparatus  1 . Then, a force for returning the torsion, i.e. torsion spring force, is generated. The rotary valve  20  is intermittently controlled in response to the running state of the vehicle calculated on the basis of the operation of the manual switch MS or the aforementioned sensor signal and then the torsional rigidity is changed. In this case, the status of the rotary valve  20  is switched by the system oil, thereby preventing occurrence of irregular sound and achieving a smooth connection or disconnection of the stabilizer control apparatus  1 . In addition, the rotary valve  20  can be easily switched to the connecting state not only when the vehicle is straight running state but also when running on the uneven surface or rough road, thereby achieving a smooth switching of the torsional rigidity.  
         [0033]     Further, according to the present embodiment, when a difference between the respective vehicle heights on the right side and the left side obtained on the basis of the vehicle height signal that is detected in the aforementioned manner is equal to or greater than a predetermined value, the magnetic on-off valves  41  and  42  are brought to the open position by the stabilizer electronic control unit ECU. Thus, even if the vehicle is shifted from running on the uneven surface to the straight running with the rotary valve  20  in the connecting state, the vehicle can keep a stable running state without inclining.  
         [0034]     For example, the switching control by the stabilizer control apparatus  1  can be performed as shown in  FIG. 5 . First, in Step  201 , the operation state of the manual switch MS is determined. When it is determined that the manual switch MS is turned on, the process returns to the main routine, contrary to the operation in Step  102 . When it is determined that the manual switch MS is turned off, the rotary valve  20  is in the connecting state and then in Step  202 , an amount of height fluctuation is specified on the basis of the height signal by the aforementioned height sensor. The amount of height fluctuation specifies a degree of fluctuation of the vehicle height. For example, the amount of height fluctuation is specified as the number of times the amount of change of vehicle height exceeds a predetermined value within a predetermined time period. Then, in Step  203 , it is determined whether or not the amount of height change is equal to or greater than a predetermined level. Precisely, it is determined whether or not the number of times the amount of height change is equal to or smaller than a predetermined number. Accordingly, it is determined whether or not the vehicle is running on the uneven surface or on the rough road, i.e. rough road determination is performed. When the change amount of vehicle height is equal to or greater than the predetermined level, it is determined that the vehicle is running on the rough load. When the change amount of vehicle height is less than the predetermined level, it is determined that the vehicle is in the normal running state.  
         [0035]     In Step  203 , when it is determined that the change amount of vehicle height is equal to or greater than the predetermined level and thus the vehicle is running on the rough road, the process proceeds to Step  204  in which the magnetic on-off valves  41  and  42  are brought to the open position. Then, in Step  205 , the rotary valve  20  is in the disconnecting (free) state. On the other hand, in Step  203 , when it is determined that the change amount of vehicle height is smaller than the predetermined level and thus the vehicle is in the normal running state, the rotary valve  20  is kept in the connected state as is operated by the manual switch MS and not brought to the disconnecting state. According to such switching control, even if a driver of the vehicle selects the connecting state of the stabilizer through the manual switch MS, the rotary valve  20  is brought to the disconnecting state if it is determined that the vehicle is running on the rough road, thereby maintaining a comfortable ride quality even when running on the rough road.  
         [0036]      FIG. 6  shows a second embodiment of the stabilizer control apparatus l. A communication passage  301  is connected to the continuous bore P 1   a  opening towards the pressure chamber C 1   a,  and a communication passage  302  connected to the continuous bore P 2   a  opening towards the pressure chamber C 2   a . The communication passages  301  and  302  are each made by a flexible tube, for example, and to which accumulators  501  and  502  are connected via a third magnetic on-off valve  421  and a fourth magnetic on-off valve  422 , respectively. Further, plugs  521  and  522  are provided at the communication passages  301  and  302 , respectively, as in the same way as the plugs  51  and  52  in  FIG. 1 . Accordingly, the system oil as the pressurized fluid is tightly and sealingly enclosed in the communication passages  301  and  302 , and then the pressure chambers C 1   a,  C 1   b,  C 2   a , and C 2   b . The magnetic on-off valves  421  and  422  are controlled to open or close by the stabilizer electronic control unit ECU in the electronic control device  100 .  
         [0037]     The manual switch MS is connected to the stabilizer electronic control unit ECU and thus the torsional rigidity of the stabilizer control apparatus  1  can be changed by the switch operation of a driver. Further, according to the second embodiment, since the magnetic on-off valves  421  and  422  are individually controlled to open or close, the pressure in the pressure chambers C 1   a  and C 1   b,  and the pressure in the pressure chambers C 2   a  and C 2   b  can be individually and appropriately controlled in response to each environmental change such as an ambient temperature. Thus, the rotor  21  is rotated in a circumferential direction so as to be set in a predetermined initial position by individually controlling the magnetic on-off valves  421  and  422  to open or close in response to the environmental change of the rotary valve  20 , for example. In addition, the pressure in the pressure chambers C 1   a  and C 1   b,  and the pressure in the pressure chambers C 2   a  and C 2   b  can be equal to each other so that the rotor  21  is prevented from being pressed on one side of the housing  22 , thereby maintaining an appropriate relative rotation between the rotor  21  and the housing  22 . The other structure of the second embodiment is same as that of the first embodiment shown in  FIG. 1  and thus substantially same parts or components shown in  FIG. 6  bear the same numbers in  FIG. 1 .  
         [0038]     According to the structure in  FIGS. 1 and 6 , the first torsion bar  11  and the second torsion bar  12  are connected to each other via the intermediate torsion bar  12   a.  Thus, the stabilizer control apparatus  1  provides the torsional rigidity even if the rotary valve  20  is in the disconnecting state. In addition, if the rotary valve  20  fails to operate in the disconnecting state, the stabilizer control apparatus  1  can maintain a predetermined torsional rigidity. Meanwhile, according to a third embodiment shown in  FIG. 7 , the torsional rigidity is nil when the rotary valve  2  is in the disconnecting state. The first torsion bar  11  and the second torsion bar  12  are completely in a free state. That is, the first torsion bar  11  and the second torsion bar  12  are separated from each other, as the intermediate torsion bar  12   a  is not provided. Instead of the cylindrical member  22   a  of the housing  22 , a cylindrical member  22   x  is used. Then, the first torsion bar  11  and the second torsion bar  12  are connected or disconnected by means of the rotary valve  20 . Accordingly, when the rotary valve  20  is in the disconnecting state, the first torsion bar  11  and the second torsion bar  12  are separated from each other. The other structure of the third embodiment is same as that of the first embodiment and thus substantially same parts or components according to the third embodiment bear the same numbers.  
         [0039]     When the rotary valve  20  is in the disconnecting state (free state), the roll characteristic “d” shown in  FIG. 8  is acquired. That is, the torsional force of the first torsion bar  11  and the second torsion bar  12  is not added and the roll characteristic is only achieved by a base coil spring (not shown). Thus, the stabilizer function can be cancelled especially when the vehicle is running on the rough road. When the lateral acceleration (Gy) exceeds a predetermined value K 2 , the rotary valve  20  is brought to the connected state, thereby switching the roll characteristic from d to b′ at a point of K 2  in  FIG. 8 . The roll characteristic of b′ is obtained by superimposing c′ on d. According to the third embodiment, a relative rotational position between the first torsion bar  11  and the second torsion bar  12  can be also adjusted to a desired position by means of an immediate switching control based on the lateral acceleration (Gy).  
         [0040]     According to the aforementioned embodiments, the stabilizer control apparatus  1  with a small structure can immediately and smoothly switch the torsional rigidity. For example, even in the case that the rotary valve  20  is changed to the connecting state not only when the vehicle is in the straight running state but also when the vehicle is running on the rough road, the smooth switching of the torsional rigidity of the stabilizer can be achieved. In addition, a use of fluid in the rotary valve  20  prevents generation of irregular sound and achieves smooth connection or disconnection.  
         [0041]     Further, according to the aforementioned first and second embodiments, connecting of the first torsion bar  11  and the second torsion bar  12  can be conducted in an appropriate relative rotational position because of the intermediate torsion bar  12   a , and a predetermined torsional rigidity can be assured even while the rotary valve  20  is in the disconnecting state.  
         [0042]     The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.
A stabilizer control apparatus includes a first torsion bar, a second torsion bar, and a connecting and disconnecting means including a rotation member rotating as a unit with the first torsion bar, and a housing accommodating therein the rotation member so as to be rotatable and forming two pressure chambers, the two pressure chambers being filled with a fluid. The connecting and disconnecting means generates a connecting state in which a movement of the fluid to the two pressure chambers is blocked so that a relative rotation between the first torsion bar and the second torsion bar is prohibited and a disconnecting state in which a movement of the fluid to the two pressure chambers is allowed so that the relative rotation between the first torsion bar and the second torsion bar is allowed.