Abstract:
A power-assisted steering system ( 10 ) for a vehicle that has steerable front wheels ( 26  and  28 ) that are inflated. The system ( 10 ) includes a manually operated steering member ( 12 ) for steering input from a vehicle operator. A series of components ( 14-22 ) relays forces between the steering member ( 12 ) and the steerable front wheels ( 26  and  28 ). A motor ( 32 ) provides an assist steer force to steer the steerable wheels ( 26  and  28 ) in response to a force applied to the steering member ( 12 ). At least one tire pressure sensor (e.g.,  66 ) monitors an inflation pressure of one of the front wheels (e.g.,  26 ). A controller ( 42 ) determines and accordingly controls a compensating steer force to the steerable wheels ( 26  and  28 ), responsive to the monitored inflation pressure, which attenuates force transmitted to the steering member ( 12 ) due to front wheel inflation condition.

Description:
TECHNICAL FIELD 
     The present invention is directed to power-assisted steering systems, and is specifically directed to systems that attenuate an undesirable torque or pulling force at a manually operated steering member. 
     BACKGROUND OF THE INVENTION 
     Power-assisted steering systems are known in the art. One example type of a powered-assisted steering system is a system that includes an electrical power assist motor and that is commonly referred to as an electric assist steering system. Typically, an electrical assist steering system that utilizes a rack and pinion gear set provides assistance force by using the electric motor to either (i) apply rotary force to a steering input shaft connected to the pinion gear, or (ii) apply linear force to a steering member that has the rack teeth thereon. The electric motor in such a system is controlled in response to sensed force (e.g., torque) applied to a manually operated steering member (e.g., a vehicle steering wheel) by an operator, and possibly other sensed parameters (e.g., vehicle speed). 
     The torque applied by the vehicle operator to the steering wheel is sensed via a torque sensor. Torque sensors for power-assisted steering systems are known in the art. A typical torque sensor for a power-assisted steering system is operatively connected between a steering input shaft and an output shaft. The input shaft is connected to the steering wheel and the output shaft is connected to the pinion of the rack and pinion steering gear set. The torque sensor includes a torsion bar connecting the input shaft to the output shaft. The torque sensor further includes a rotary position sensor adapted to monitor the amount of relative rotation between the input shaft and the output shaft that occurs as torque is applied to the steering wheel. The amount of relative rotation is functionally related to the strength of the torsion and the amount of steering torque applied to the steering wheel. 
     Due to the connection between the steering wheel and the rack and pinion steering gear set that is provided by the input shaft, the torsion bar, and the output shaft, torque from the rack and pinion steering gear set may also be transmitted to the steering wheel. Typically, a certain amount of torque (e.g., a resisting force) is desirable to provide the vehicle operator a certain amount of steering feel. However, torque applied to the steering wheel as a result of some types of vehicle events may be undesirable. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect, the present invention provides a power-assisted steering system for a vehicle that has steerable front wheels that are inflated. The system includes a manually operated steering member for steering input from a vehicle operator. Means relays forces between the steering member and the steerable front wheels. Means provides an assist steer force to steer the steerable wheels in response to a force applied to the steering member. Means monitors an inflation pressure of one of the front wheels. Means provides a compensating steer force to the steerable wheels, responsive to the monitored inflation pressure, which attenuates force transmitted to the steering member due to front wheel inflation condition. 
     In accordance with another aspect, the present invention provides a method of controlling a power-assisted steering system for a vehicle that has steerable front wheels that are inflated. Forces are relayed between a manually operated steering member and steerable front wheels. An assist steer force to steer the steerable wheels is provided in response to a force applied to the steering member. An inflation pressure of one of the front wheels is monitored. A compensating steer force is provided to the steerable wheels, responsive to the monitored inflation pressure that attenuates force transmitted to the steering member due to front wheel inflation condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic block diagram illustrating a power-assisted vehicle steering system in accordance with the present invention; and 
     FIG. 2 is a flow chart for a process performed within the system shown in FIG.  1  and in accordance with the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     A power-assisted vehicle steering system  10  in accordance with the present invention is shown in FIG.  1 . In the illustrated example, the steering system  10  is a direct-connection electric assist steering system. However, it is to be appreciated that the present invention is applicable to other vehicle power steering systems. The system  10  includes a steering wheel  12  that is manually operated by an operator (not shown) of a vehicle (not shown in total) within which the system is provided. 
     The steering wheel  12  is operatively connected to a rack and pinion gear set  14 . Specifically, the steering wheel  12  is connected to an input shaft  16  and a pinion gear of the rack and pinion gear set  14  is connected to an output shaft  18 . The input shaft  16  is operatively connected to the output shaft  18  through a torsion bar  20 . The torsion bar  20  twists in response to applied torque thereby permitting relative rotation between the input shaft  16  and the output shaft  18 . Stops (not shown) limit the amount of such relative rotation between the input and output shafts  16  and  18  in a manner well known in the art. 
     The pinion gear has helical teeth (not shown) that are meshingly engaged with straight cut teeth (not shown) on a rack gear of the rack and pinion gear set  14 , wherein the rack gear is located on a rack member  22  (e.g., a linear motion member). The rack member  22  is steerably coupled to front wheels  26  and  28  of the vehicle. Thus, it to be understood that the front wheels  26  and  28  are the steerable wheels of the vehicle. 
     The coupling between the rack member  22  and the front wheels  26 ,  28  is via a steering linkage (not shown) in a known manner. When the steering wheel  12  is turned, the rack and pinion gear set  14  converts the rotary motion of the steering wheel into linear motion of the rack member  22 . When the rack member  22  moves linearly, the front wheels  26 ,  28  pivot about their associated steering axis and the vehicle is steered. 
     An electric assist motor  32  is drivingly connected to the rack member  22 . When the electric assist motor  32  is energized, it provides steering assist so as to aid in the rotation of the steering wheel  12  by the vehicle operator. In one example, the electric assist motor  32  is drivably connected to the rack member  22  with a ball-nut drive arrangement. When the electric assist motor  32  is energized, a rotor moves which, in turn, rotates a nut portion of the ball-nut drive arrangement. When the nut rotates, a plurality of ball bearings, within the ball-nut drive arrangement, transfer a linear force to the rack member  22 . The direction of rack member movement is dependent upon the direction of drive force provided by the electric assist motor  32 . 
     The provision of electrical energy to the electric assist motor  32  for driving the motor in either direction is via power switches that are operatively connected  36 . The power switches  34  are operatively connected  38  to a source of electrical energy B+(e.g., a vehicle battery). The power switches  34  are operatively connected  40  to be controlled by a controller  42  of the steering system  10 . 
     The controller  42  utilizes a plurality of sensory inputs to make determinations regarding electric motor control (i.e., actuation direction and force amount). A steering position sensor  44  is operatively connected  46  to the controller  42  to provide a first input to the controller. The steering position sensor  44  is operatively connected  48  across the input shaft  16  and the output shaft  18 . The steering position sensor  44  provides an electrical signal that has a value indicative of relative rotational position between the input shaft  16  and the output shaft  18 . Thus, in one respect, the steering position sensor  44 , in combination with the torsion bar  20 , form a torque sensor  50  (shown in phantom). Accordingly, the output signal of the steering position sensor  44  is indicative of the steering torque applied to the steering wheel  12  by the vehicle operator. 
     At least one motor sensor  54  is operatively connected  56  to the electric assist motor  32  and operatively connected  58  to the controller  42  for providing additional input to the controller. In one example, a plurality of motor sensors  54  is provided. One motor sensor is a rotor position sensor that is operatively connected to the motor rotor of the electric assist motor  32  and to the motor stator of the motor. The function of the rotor position sensor is to provide an electrical signal indicative of the position of the rotor relative to the stator. Another motor sensor is a current sensor that senses the current flow through the electric assist motor  32  as an indication of the force being applied by the motor. 
     Other sensory input to the controller  42  for use in determining control of the electric assist motor  32  may include any useful sensed vehicle parameter. For example, vehicle speed may be utilized within the calculation to determine the amount of assist force to be applied via operation of the electric assist motor  32 . The other sensory input is provided by one or more sensors  60  that are operatively connected  62  to the controller  42 . 
     It is to be appreciated that each the ground engaging wheels (only the front wheels  26 ,  28  shown) of the vehicle includes a tire that is inflated to have a predetermined desired inflation pressure. When one of tires of the ground-engaging wheels has a reduction in inflation pressure from the desired pressure value, the loss of pressure may create a pulling effect on the steering system  10 . In particular, the loss of inflation pressure at a tire may create a pull or torque on the steering wheel  12 . This is especially manifest if the tire inflation pressure loss occurs at one of the front steerable wheels  26 ,  28  of the vehicle. 
     For example, a loss of pressure in at left front wheel  26  will cause the vehicle to be pulled toward the left direction. The pulling effect is manifest as a counterclockwise rotational force on the steering wheel  12  due to the interconnection between the steering wheel and the front steerable wheels  26 ,  28 . The rotational force of the steering wheel  12  could cause difficulty for the vehicle operator. 
     The power-assisted steering system  10  in accordance with the present invention operates to attenuate force that is transmitted to the steering wheel  12  due to front wheel inflation pressure loss. Specifically, the system  10  includes tire pressure sensors  66  and  68  that are operatively connected to sense tire inflation pressure at the wheels. In illustrated example, only the tire pressure sensors at the front wheels of the vehicle are shown because the rear wheels have been omitted. Each tire pressure sensor (e.g.,  66 ) senses the inflation pressure within the associated tire. 
     Information regarding the inflation pressure is communicated  70 ,  72  to the controller  42 . In the illustrated example, dashed lines indicate the communication  70 ,  72 . The communication  70 ,  72  of inflation pressure information to the controller  42  may be by any suitable means and is dependent upon the construction and configuration of the tire pressure sensors  66 ,  68 . 
     In one embodiment, each tire pressure sensor (e.g.,  66 ) may be located within a respective tire (as shown in the illustrated example). The tire pressure sensor (e.g.,  66 ) communicates with a portion of the vehicle outside of the tire via a radio frequency transmission. The vehicle portion that receives the radio frequency signal may be a communications portion of an overall vehicle system communication bus that is operatively connected to the controller. The dashed line communication  70 ,  72  of FIG. 1 thus represent the radio transmission, the communications portion, and the communication bus. Alternatively, the radio communication may be provided directly to the controller  42  via a suitably provided receiver operatively connected to the controller. Again, the dashed line communication  70 ,  72  represent such a conveyance. It is to be appreciated that other tire sense arrangements may be utilized including a hard-wired arrangement that receives pressure information via a rotary electrical or magnetic connection. 
     Upon the occurrence of a loss of pressure that would create a pulling effect, the controller  42  utilizes the sensory information indicating the loss of pressure to modify or compensate the amount of force that the electric assist motor  32  provides to the rack member  22 . Specifically, the controller  42  modifies the control signals provided to the power switches  34  that control flow of electrical energy through the electric assist motor  32 . 
     In one example, a processor within the controller  42  performs an algorithm that utilizes the pressure information to determine electric assist motor control. For example, the algorithm accesses a low tire pressure look-up table within a memory of the controller  42  to retrieve a compensation or modification value associated with the pressure value and the wheel that is experiencing the pressure loss. It is to be appreciated that the compensation or modification of force provided by the electric assist motor  32  is via an addition or subtraction of an adjustment amount from the force otherwise applied. 
     In addition, an alert device  76  is operatively connected  78  to receive a signal from the controller  42  upon the occurrence of an adjustment or modification due to a low tire pressure condition. The alert device  76  is provided within the vehicle to provide notice to the vehicle operator of the low tire pressure condition and the compensation or adjustment that is occurring within the steering system  10 . In one example, the alert device  76  may be an indicator light located on an instrument panel (not shown) of the vehicle and/or may be an audio sound producing mechanism (not shown). 
     A flow chart for a process  100  performed in accordance with the present invention is shown in FIG.  2 . The process  100  is initiated at step  102  and proceeds to step  104 . At step  104 , the various sensory inputs (e.g., the signals from the steering position sensor  44 , the motor sensor(s)  54 , the other sensor(s)  60 , and the tire pressure sensors  66 ,  68 ) are monitored to derive the sensory information therefrom. At step  106 , it is determined whether a low tire pressure condition exists at one of the monitored tires. 
     If the determination at step  106  is negative (i.e., a low tire pressure does not exist), the process  100  proceeds from step  106  to step  108 . At step  108 , motor drive control is determined without a compensation or adjustment due to tire inflation condition. The process  100  proceeds from step  108  to step  110  in which the power switches  34  that provide electrical energy to the electric assist motor  32  are controlled accordingly. Upon completion of step  110 , the process  100  loops to repeat step  104 . 
     If a low tire pressure condition exists, the determination at step  106  is affirmative. Upon an affirmative determination at step  106  (i.e., a low tire pressure), the process  100  proceeds from step  106  to step  112 . At step  112 , the motor drive control is determined and then compensated or adjusted via use of sensed tire pressure. In pertinent part, the controller  42  determines which tire is experiencing a pressure loss and the amount of pressure that is being experienced. The controller  42  then proceeds to the look-up table to retrieve a compensation value. At step  114 , the alert device  76  is activated such that the vehicle operator is made aware of the condition. The process  100  then proceeds to step  110  in which the power switches are controlled using the compensated determinations. 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.