Patent Publication Number: US-2003226709-A1

Title: EPAS assembly with motion conversion sleeve

Description:
TECHNICAL FIELD  
       [0001] The present invention relates generally to an EPAS assembly and more particularly to an EPAS assembly including a sleeve for converting rotary motion to linear motion.  
       BACKGROUND OF THE INVENTION  
       [0002] Present automotive designs commonly incorporate power assisted steering. Typical power assisted steering systems utilize hydraulic pumps driven by the engine crank shaft. The steering system incorporates a control valve sensitive to the driver input force which when paired with the hydraulic system reduces the effort required to steer the vehicle. Hydraulic systems, unfortunately, can be inefficient and can tax engine power. This can result in an undesirable increase in fuel consumption as well as an undesirable reduction of vehicle performance. These as well as other deficiencies associated with hydraulic based power steering systems have driven engineers and designers to search for viable alternatives.  
       [0003] One alternative has been the development of electrically power assisted steering (also known as EPAS). Conventional EPAS systems utilize a torque sensor in communication with the steering column. The torque sensor communicates with the servo motor and control electronics. When the driver turns the steering column, the servo motor assists in turning of the steering gear in response to driver torque sensed by the torque sensor. This, in turn, reduces the force the driver need exert on the steering column.  
       [0004] A significant advantage presented by EPAS systems is the ability to improve a vehicle&#39;s fuel economy. The improvement of fuel economy is a benefit long recognized by the automotive industry. In addition, automotive performance can be improved since these power assisted steering systems no longer provide a physical drain on the engine crankshaft. This, in turn, can provide increased customer satisfaction with the vehicle purchase. Finally, EPAS systems can be designed and implemented in embodiments that can be significantly smaller than their equivalent hydraulic counterparts. Their smaller profile can increase the useful space within the engine compartment, thereby providing an added benefit to designers and manufacturers.  
       [0005] Although present EPAS systems provide these and a variety of other benefits to automotive steering design, they can also incorporate undesirable design elements. Once such negative design element arises in the area of communication between the steering column and the torque sensor. Complex gearing arrangements have been utilized, but these solutions are often bulky and expensive to manufacture. Additionally, many current designs result in undesirable frictional losses in the communication between the steering column and torque sensor. Thus, the present state of EPAS steering designs leaves considerable room for improvement. It would, therefore, be highly desirable to have a power steering system that incorporated the benefits associated with EPAS designs and further improved communication between the steering column and the torque sensor.  
       SUMMARY OF THE INVENTION  
       [0006] It is, therefore, an object of the present invention to provide an EPAS assembly with an improved communication linkage between the steering column shaft and the torque sensor.  
       [0007] In accordance with the objects of the present invention, an EPAS assembly is provided. The EPAS assembly includes a steering input shaft including at least one shaft pin and a pinion shaft including at least one pinion pin. The EPAS assembly further includes a sleeve element including at least one shaft slot and at least one pinion slot. The at least one shaft pin is positioned within the at least one shaft slot. The at least one pinion pin is positioned within the at least one pinion slot. The at least one shaft slot and the at least one pinion slot are oriented such that the relative rotary motion of the steering input shaft and the pinion shaft is converted to linear motion of the sleeve element. The present invention further includes a torque sensor in communication with the sleeve element, the torque measuring the linear motion of the shaft element.  
       [0008] Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a cross-sectional illustration of an embodiment of an EPAS assembly in accordance with the present invention;  
     [0010]FIG. 2 is a cross-sectional illustration of the embodiment of the EPAS assembly illustrated in FIG. 1, the cross-section taken along the lines  2 - 2  in the direction of the arrows;  
     [0011]FIG. 3 is a detailed illustration of an EPAS assembly in accordance with the present invention; and  
     [0012]FIG. 4 is a detailed illustration of a pin element for use in the EPAS assembly illustrated in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS(S)  
     [0013] Referring now to FIG. 1, which is an illustration of an EPAS assembly  10  in accordance with the present invention. The EPAS assembly  10  includes a steering linkage  12  in communication with a rack  14 . Rotation of the steering linkage  12  induces motion in the rack  14 , thereby providing steering control. The use of steering columns  12  and racks  14  are well-known in the art. Although a particular embodiment may be illustrated, a wide variety of individual embodiments will become obvious to one skilled in the art. The steering linkage  12  is comprised of a steering input shaft  16  and a pinion shaft  18 .  
     [0014] A torque sensor  20  is utilized to measure the torque applied to the input shaft  16 . This torque information is utilized by a control motor  22  (see FIG. 2) that in turn provides power assist to the pinion shaft  18 . Although the present invention contemplates a variety of drive assemblies  23  for transferring power assist from the motor  22  to the pinion shaft  18 , one embodiment, illustrated in FIG. 2, utilizes a worm drive  24  driven by the motor  22 . The worm drive  24  is in communication with the worn gear  26  that is, in turn, mounted in communication with the pinion shaft  18 . It should be understood that although one particular drive assembly  23  for imparting electric power assist to a steering linkage  12  has been described, a wide variety of modifications and alternatives would be obvious to one skilled in the art.  
     [0015] Although a variety of EPAS assemblies are known in the prior art, the present invention provides a unique and novel approach to the interaction between steering linkage  12  and the torque sensor  20 . The steering input shaft  16  and the pinion shaft  18  are not formed integrally as part of a single shaft. Instead, the present invention includes a sleeve  28  that provides communication between the steering input shaft  16  and the pinion shaft  18 . The sleeve  28  provides dual functionality to the EPAS assembly  10 . The sleeve  28  allows rotation imparted to the steering input shaft  18  to be transmitted to the pinion shaft  18 . In addition, however, the sleeve  28  allows rotational differences between steering input shaft  16  and the pinion shaft  18  to be translated into linear motion. This provides a compact and efficient EPAS assembly  10  and can allow for a reduced profile EPAS housing  30  that reduces engine compartment spacing requirements.  
     [0016] Although it is contemplated that a sleeve element  28  may be constructed in a variety of fashions to translate differential rotation between the steering input shaft  16  and the pinion shaft  18  into linear motion, one embodiment is detailed in FIG. 3. The sleeve element  28  includes at least one helical slot  32  and at least one axial slot  34  formed into opposing ends of the sleeve  28 . Although the at least one helical slot  32  and at least one axial slot  34  may be formed in a variety of combinations, the EPAS assembly  10  is preferably formed with two helical slots  32  formed in the sleeve  28  180° apart and two axial slots  34  formed in the sleeve  28  180° apart. Additionally, although placement of the helical slots  32  in relation to the axial slot  34  may be varied, one embodiment contemplates placement of the axial slots  34  in a position approximately 90° from the placement of the helical slots  32 . Finally, although FIG. 3 illustrates the helical slots  32  as shaft slots  33  providing communication with the steering input shaft  16  and the axial slots  34  as pinion slots  35  providing communication with the pinion shaft  18 , it should be understood that these relationships can easily be reversed.  
     [0017] The present invention further includes at least one input shaft pin  36  positioned within the at least one helical slot  32 . The input shaft pins  36  can be mounted to the steering input shaft  16  in a variety of fashions. In one embodiment the input shaft pin  36  are pressed into the steering input shaft  16 . In other embodiments, however, a variety of alternative attachment methodologies would become obvious to one skilled in the art. Similarly, the present invention includes at least one pinion pin  38  mounted on or pressed into the pinion shaft  18 . The pinion pin  38  is positioned within the at least one axial slot  34  and providing communication between the sleeve  28  and the pinion shaft  18 . This structural arrangement allows a rotational difference between the input shaft  16  and the pinion shaft  18  to be translated into linear motion of the sleeve  28 .  
     [0018] The present invention can further include bearing elements  39  added to the input shaft pins  36  and the pinion pin  38 . The use of bearings  39  can be utilized to reduce the friction associated with motion of the pins  36 ,  38  within the slots  32 ,  34 . This allows smoother linear movement of the sleeve  28 . The linear motion of the sleeve  28  is translated into sensed torque by communication between the sleeve element  28  and the torque sensor  20 . Although it is contemplated that this communication may be accomplished through a variety of methods, one embodiment contemplates the use of a circumferential guide  40  formed into the sleeve element  28 . The circumferential guide  40  engages a tongue  42  formed as a portion of the torque sensor  20 . Linear motion of the tongue  42  is translated into a sense torque valued by the torque sensor  20 . Additionally, a tongue element  42  can be utilized to provide resistance to linear motion of the sleeve element  28 . This allows a minimum value of torque to be applied to the steering input shaft  16  before differential rotation of the steering input shaft  16  and the pinion shaft  18  is realized.  
     [0019] Although the torque profile controlling the differential rotation between the steering input shaft  16  and the pinion shaft  18  may be accomplished through a variety of methods, the described use of the torque sensor  20  in communication with the sleeve element  28  may provide a variety of benefits. One such benefit is that the power assist profile of the EPAS assembly  10  may be easily modified or adjusted through a modification of substitution of the torque sensor  20 . This provides a convenient and expedient method of altering the power steering profile in comparison to the complex steering systems often associated with prior EPAS designs. In addition, this may create a flexible EPAS assembly  10  that may be manufactured to be usable in a variety of applications as opposed to single application designs commonly utilized in the industry. It should be understood, however, that although the torque sensor in combination with the sleeve element  28  has been described as controlling the differential rotation between the steering input shaft  16  and the pinion shaft  18  (in relation to torque imparted on the input shaft  16 ), a variety of other arrangements may be utilized to control the differential rotation between the input shaft  16  and the pinion shaft  18 .  
     [0020] It is furthermore contemplated that a variety of the elements comprising the EPAS assembly  10  may be adjusted to modify the functional profile of the EPAS assembly  10 . The helical angle  44  of the helical slots  32  may be varied to test the linear travel distance of the sleeve element  28 . In addition, helical stops  46  and axial stops  48  may further limit the magnitude of sleeve  28  travel. It is also contemplated that the profile of the circumferential guide  40  may also be modified to create a desired profile for movement of the tongue  42  of the torque sensor  20 . Although each of these elements may be modified independently, it is known that the sleeve  28  travel is a function of the magnitude and direction of driver torque, torque sensor  20  resistance, helical and axial stop locations  46 ,  48 , and the helical angle  44  of the helical slot  32  cut into the sleeve  28 . Thus, a combination of elements may be modified in concert in order to provide a functional profile of the EPAS assembly  10 .  
     [0021] While particular embodiments of the invention have been shown and described, numerous variations and alternative embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.