Abstract:
A steering apparatus ( 10 ) for a vehicle having steerable road-engaging wheels comprises a rotatable steering wheel ( 11 ) and a sensor ( 12 ) which senses the rotational position of the steering wheel and generates a first signal corresponding to the sensed rotational position of the steering wheel ( 11 ). A first electric motor ( 14 ), when energized, resists rotation of the steering wheel ( 11 ). A second electric motor ( 20 ) is controlled by the signal generated by the sensor for sensing the rotational position of the steering wheel ( 11 ). A steering gear ( 130 ) is actuated by the second electric motor ( 20 ) to turn the steerable wheels of the vehicle. An electrical circuit ( 19 ) includes the first and second motors ( 14  and  20 ) in series.

Description:
TECHNICAL FIELD 
   The present invention relates to a steering apparatus for a vehicle having steerable road-engaging wheels. 
   BACKGROUND OF THE INVENTION 
   Integral hydraulic power steering gears are commonly used in trucks, heavy equipment such as earth-moving vehicles, and construction vehicles. “Integral” refers to a steering gear containing a manual steering mechanism, a hydraulic control valve assembly, and a hydraulic power cylinder integrated into a single unit. 
   The hydraulic power cylinder typically comprises a chamber divided into two chamber portions by a piston. The piston has a set of teeth which mesh with a sector gear fixed to an output shaft. The output shaft is connected via steering linkage to steerable wheels of a vehicle to steer the vehicle when the output shaft is rotated. 
   The hydraulic control valve assembly controls the flow of pressurized hydraulic fluid between a hydraulic pump and one of the chamber portions to control the direction and amount of steering. The valve assembly typically comprises two relatively rotatable valve elements, one of which is connected to a rotatable input shaft operatively coupled to the vehicle steering wheel. The other valve element is connected with a follow-up member, such as a ball screw drive, which rotates in response to movement of the piston. The ball screw drive provides a direct connection between the input shaft and the piston to allow for manual steering of the vehicle in the event of hydraulic fluid pressure loss. 
   In thee typical integral hydraulic power steering gear, the input shaft is connected to the vehicle steering wheel by one or more intermediate shafts. The intermediate shafts are usually relatively long and can be prone to excessive lash. It is desirable to eliminate the intermediate shaft from the vehicle steering system. The intermediate shaft can be eliminated, and thus there is no mechanical connection between the steering wheel and the steering gear. Such systems are known, and are commonly referred to as “steer-by-wire” systems. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a steering apparatus for a vehicle having steerable road-engaging wheels. The apparatus comprises a rotatable steering wheel and a sensor which senses the rotational position of the steering wheel and generates a first signal corresponding to the sensed rotational position of the steering wheel. A first electric motor, when energized, resists rotation of the steering wheel. A second electric motor is controlled by the signal generated by the sensor for sensing the rotational position of the steering wheel. A steering gear is actuated by the second electric motor to turn the steerable wheels of the vehicle. An electrical circuit includes the first and second motors in series. 
   One feature of the present invention is that the magnitude of the electrical current through the first and second motors may be substantially the same, and thus the torque produced by the first and second motors may be substantially the same. Thus, the driver of the vehicle experiences a road feel as though there was a mechanical connection between the steering wheel and the steering gear. 
   A further embodiment of the present invention is a steering apparatus for a vehicle having at least two steerable road-engaging wheels. The apparatus includes a vehicle steering wheel and a sensor for sensing the rotational position of the steering wheel and for generating a first signal corresponding to the sensed rotational position of the steering wheel. A first electric motor, when actuated, resists rotation of the steering wheel. A second electric motor and a third electric motor are controlled by the first signal. The second and third electric motors are associated with first and second steering gears, respectively, and actuate the first and second steering gears to turn respective steerable road-engaging wheels on either end of an axle, or to steer two axles with conventional tie rod linkage. An electrical circuit includes the second and third motors in parallel with each other and in series with the first electric motor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will be apparent to those skilled in the art to which the present invention relates from the following detailed description of preferred embodiments of the present invention made with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of a first embodiment of a steering apparatus embodying the present invention; 
       FIG. 2  is a cross-sectional view of a part of the steering apparatus of  FIG. 1 ; 
       FIG. 3  is a schematic block diagram of an electrical circuit of the steering apparatus shown in  FIG. 1 ; 
       FIG. 4  is a further schematic block diagram of the steering apparatus of  FIG. 1 ; 
       FIG. 5  is a schematic block diagram of a second embodiment of a steering apparatus embodying the present invention; and 
       FIG. 6  is a schematic view of an electrical circuit of a third embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is embodied in a steering apparatus generally designated  10  in FIG.  1 . The steering apparatus  10  includes a steering wheel  11  which is turned manually by the driver in the vehicle. A suitable sensor  12  senses the angular position of the steering wheel  11 . The sensor  12  provides an output signal dependent upon the amount of steering wheel turning and the angular position of the steering wheel. The position sensor  12  may be any suitable known sensor. The position sensor  12  provides an output signal which controls an electric motor  20 . 
   As shown schematically in  FIG. 1 , rotation of the steering wheel  11  causes rotation of a shaft  13  which is associated with the position sensor  12 . Also associated with the shaft  13  is an electric motor  14  which is constructed to resist turning of the shaft  13  by the driver of the vehicle. The electric motor  14  may be any suitable variable speed reversible electric motor. 
   The steering apparatus  10  is a steer-by-wire system. The steering apparatus  10  has no mechanical connection between the steering wheel  11  and a steering gear  130  which is operatively coupled with at least one steerable road-engaging wheel (not shown) on the end of a vehicle axle (not shown). The steering gear  130  may be of any suitable construction, but is preferably an integral hydraulic steering gear which includes a hydraulic motor and a directional control valve for actuating the hydraulic motor, as is known in the art. 
   The integral hydraulic power steering gear  130  includes a two-piece housing  132  ( FIG. 2 ) having a hydraulic power cylinder  134 . The power cylinder  134  comprises a chamber  136  divided into two chamber portions  138  and  140 , respectively, by a piston  142 . The piston  142  includes an inner bore  143  with a helical groove  144 . The piston  142  also has a set of external teeth  145  which mesh with a sector gear  146 . The sector gear  146  is fixed to an output shaft  148  which extends outwardly from the housing  132 . The output shaft  148  is connected to a pitman arm  125  ( FIG. 1 ) which, in turn, is connected via steering linkage  126  to the steerable wheels to steer the vehicle. As the piston  142  moves in the chamber  136 , the output shaft  148  is rotated to operate the steering linkage  126 , which turns the steerable wheels of the vehicle. 
   A hydraulic control valve assembly  150  ( FIG. 2 ) controls the flow of pressurized hydraulic fluid between a hydraulic circuit including a hydraulic pump (not shown) and one of the chamber portions  138  and  140  to control the direction and amount of steering. The valve assembly  150  is actuated by a rotatable input shaft  152 . The input shaft  152  is rotated by the electric motor  20 . 
   The valve assembly  150  comprises first and second valve members  154  and  156 , respectively. The first valve member  154  comprises a valve core  160  and the second valve member  156  comprises a valve sleeve  162 . The valve core  160  is located coaxially within the valve sleeve  162  and is supported for rotation by the valve sleeve. The valve core  160  is formed integrally as one piece with the input shaft  152 . The valve core  160  has oppositely disposed first and second end portions  164  and  166 , respectively, and a valve section  168  between the end portions. The first end portion  164  of the valve core  160  projects beyond the valve sleeve  162  and the second end portion  166  of the valve core lies within the valve sleeve. 
   The valve section  168  of the valve core  160  has a plurality of circumferentially spaced, axially extending grooves  170  as is known in the art. A first portion of the grooves  170  are fluidly connected with an internal passage  172  extending from the valve section  168  of the valve core  160  to the second end portion  166 . The internal passage  172  communicates via passages (not shown) with the return line of a hydraulic pump circuit (not shown). A second portion of the grooves  170  are in fluid communication with a plurality of passages  174  in the valve sleeve  162 . 
   The valve sleeve  162  has oppositely disposed first and second ends  180  and  182 , respectively. The valve sleeve  162  further includes a sleeve section  184  adjacent the first end  180  and a ball screw section  186  adjacent the second end  182 . An axially extending passage  188  extends from the first end  180  of the valve sleeve  162  through the sleeve section  184  and the ball screw section  186  to the second end  182 . 
   The first end  180  of the valve sleeve  162  includes first and second lugs (not shown) that are disposed in diametrically opposed cut-outs (not shown) in the valve core  160 . Upon rotation of the valve core  160  of between 20° and 8° relative to the valve sleeve  162 , the lugs engage the cut-outs in the valve core to cause the valve sleeve to be rotated along with the valve core. Such rotation of the valve sleeve  162  causes the piston  142  to move axially in the chamber  136  and, hence, allows for manual steering of the vehicle even if a loss in hydraulic fluid pressure has occurred. 
   The sleeve section  184  of the valve sleeve  162  includes the plurality of passages  174  which extend from the outer circumference of the sleeve section to the inner circumference. The passages  174  communicate with an annular chamber  190  in the housing  132  which is fluidly connected to the hydraulic pump. A plurality of axially extending grooves  192  are formed in the inner surface of the valve sleeve  162  as is known in the art. The grooves  192  fluidly communicate with the second portion of the grooves  170  in the valve core  160 . Further, a first portion of the grooves  192  in the valve sleeve  162  are fluidly connected via passages (not shown) with the first chamber portion  138  in the housing  132 , and a second portion of the grooves  192  fluidly connected via passages (not shown) with the second chamber portion  140  in the housing. As is known in the art, when the valve core  160  is rotated relative to the valve sleeve  162 , hydraulic fluid is ported through the grooves  170  and  192  and associated passages to one of the chamber portions  138  and  140 , while the hydraulic fluid is vented from the other chamber portion, thereby causing the piston  132  to move accordingly. 
   The ball screw section  186 -of the valve sleeve  162  includes a helical groove  194  formed on its outer periphery. A plurality of balls  196  are located in the helical groove  140 . The balls  196  are also located in the helical groove  144  in the bore  143  formed in the piston  142 . As is well known in the art, axial movement of the piston  142  causes the ball screw portion  186  to rotate which, in turn, causes the rest of the valve sleeve  162  to rotate. 
   A torsion bar  198  connects the valve core  160  and the valve sleeve  162 . One end of the torsion bar  198  is connected by a pin  200  to the valve section  168  of the valve core  160 , while the other end of the torsion bar extends through the passage  188  in the valve sleeve  162  and is connected by a pin  202  adjacent the second end  182  of the valve sleeve. 
   From the above description it should be apparent that actuation of the motor  20  causes rotation of the valve core  160  of the steering gear  130  relative to the valve sleeve  162 . Rotation of the valve core  162  causes axial movement of the piston  142  in one direction or the other. Axial movement of the piston  142  results in rotation of the sector gear and the pitman arm  125 , thereby causing the road-engaging steerable wheels to turn laterally of the vehicle. 
   An output position sensor  60  senses the output position of the steering gear  130  and, as a result, senses the position of the steerable road-engaging wheels. The output position sensor  60  may be any suitable position sensor including an optical sensor or an electrical sensor. 
   The electric motors  14  and  20  are preferably of identical construction and a current flowing through one of the motors will provide an output torque which is equal to the same current flowing through the other motor. The apparatus  10  includes an electrical circuit  19  shown schematically in FIG.  3 .  FIG. 3  illustrates that the windings  14   a  and  20   a  of the electric motors  14  and  20 , respectively, lie in series in the circuit  19 . The power source for the circuit  19  shown in  FIG. 3  is preferably the battery  68  of the vehicle. 
   As illustrated in  FIG. 3 , the steering wheel position sensor  12  provides an output signal to an electronic control unit (ECU)  75 . The ECU  75  then determines the desired road wheel position as a function of the steering wheel position. As shown in  FIG. 4 , the ECU  75  has a variable ratio function  74  which calculates the demanded road wheel position based on the steering wheel position. The variable ratio function  74  permits a non-linear relationship between road wheel position and steering wheel position. The variable ratio function  74  can use algorithms or lock-up tables to perform the calculation of road wheel position. The ECU  75  also receives a signal from the road wheel position sensor  60 . The ECU  75  will determine any errors between the steering wheel position and the road wheel position and actuate a motor drive circuit  76  depending upon the position of the road wheels versus the position that the ECU is commanding the road wheels to take. 
   A further schematic block diagram of the preferred embodiment of the invention is shown in FIG.  4 . The ECU  75  further includes a control compensation circuit  77  which provides an output signal to the motor drive circuit  76 . The motor drive circuit  76  provides electrical current to the windings  14   a  and  20   a  of the motors  14  and  20 , respectively. The motor  14  resists rotation of the steering wheel to provide operator feel, and the motor  20  drives the steering gear  130  and, in particular, the valve core  60  in order to turn the steerable wheels as commanded by the ECU  75 . 
   Since the motors  14  and  20  are preferably identical in construction and since their windings  14   a  and  20   a , respectively, are in series, the torque applied by the motor  20  to the steering gear is also applied by the motor  14  to the steering shaft  13  in order to resist turning of the steering shaft and provide feel to the operator of the vehicle. Since the coils  14   a  and  20   a  of the motors  14  and  20 , respectively, are in a series, the torque provided by the motor  20  to the steering gear  130  is substantially identical to the torque applied by the motor  14  to the steering wheel  11  to resist the turning of the steering wheel. Thus, even though there is no mechanical connection between the steering wheel  11  and the hydraulic steering gear  130 , the torque applied by the motor  14  to the steering wheel  12  makes the operator feel as though there is a mechanical connection between the steering wheel  11  and the hydraulic steering gear  130 . 
   Another embodiment of the present invention is illustrated in FIG.  5 . As shown in  FIG. 5 , there are separate steering gears  130 A and  130 B; i.e., a respective steering gear for each of two respective steerable road wheels, or, alternatively, a respective steering gear for ends of two respective vehicle axles. A motor  100  drives steering gear  130 A for one steerable wheel, and a motor  101  drives steering gear  130 B for another steerable wheel. In the embodiment shown in  FIG. 5 , there is a road wheel position sensor  105  associated with steering gear  130 A and a second road wheel position sensor  107  associated with steering gear  130 B. Also, there is a steering wheel or steering wheel position sensor  109  in the embodiment of FIG.  5 . 
   The steering wheel position sensor  109  and the two road wheel position sensors  105  and  107  provide output signals to an electronic control unit (ECU)  110 . The ECU  110  is provided with power from the vehicle battery  112 . The ECU provides an output signal to the motors  100  and  102 . Specifically, the ECU provides an output signal to motor drive circuits  115  and  117  which are associated with the motors  100  and  102 , respectively. 
   As illustrated in  FIG. 5 , the motors  100  and  102  are in parallel in the circuit, and the motors  100  and  102  are in series with the steering wheel motor  14 . Since the motors  100  and  102  are in parallel with each other, the current that flows through the motor windings is summed at the juncture  120  where the current flows into the steering wheel motor  14 . The steering wheel motor  14  will thus have a current that flows through it that is equal to a total of the currents flowing through the two road wheel steering motors  100  and  102 . Thus, the steering wheel motor  14  will apply a torque to the steering shaft  13  which is equal to the sum of the torques applied by the motors  100  and  102  to the steerable wheels, respectively. As a result, the operator will experience a resistance to turning of the steerable wheels that is almost identical to the torque which is applied by the motor  100  to the steering gear  130 A with which it is associated plus the torque applied by the motor  102  to the steering gear  130 B with which it is associated. 
   The system illustrated in  FIG. 5  may function to turn one steerable wheel a different angular distance than another steerable wheel is turned. Thus, one of the motors  100  or  102  would be actuated differently than the other. As a result, perfect Ackerman steering can be achieved. 
   A further modification of the present invention is illustrated in FIG.  6 . The modification illustrated in  FIG. 6  is a modification of the embodiment shown in FIG.  5 . The modification shown in  FIG. 6  includes a resistor  120  which is a variable resistor in parallel with the steering wheel motor  14 . The resistor  120 , being a variable resistor and being in parallel with the steering wheel motor  14 , will carry some current depending upon the magnitude of the resistance. Thus, the current which flows through the coils of the steering wheel motor  14  will not be equal to the sum of the current which flows through the coils of the motors  100  and  102 . Thus, the steering wheel motor  14  will not apply a torque to the steering wheel  11  which is the sum of the torques applied by the motors  100  and  102  to the steering gears  130 A and  130 B, respectively, with which they are respectively associated. Thus, the operator of the steering mechanism will feel a reduced torque compared to the embodiment of FIG.  5 . 
   In view of the description above, those skilled in the art will become aware of modifications and changes which may be made in the present invention, and such modifications and changes are intended to be covered by the appended claims.