Abstract:
A vehicle steering system transmission comprising a driver connected to a driver shaft, the driver shaft connected to an output shaft by a flexible link, a first sensor sensing the driver shaft, a control unit receiving a signal from the first sensor, and the control unit transmitting a signal to the driver to control an output shaft movement.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a vehicle steering system transmission, and more particularly, to a vehicle steering system transmission comprising a driver connected to a driver shaft, the driver shaft connected to an output shaft by a flexible link, a first sensor sensing the driver shaft, a control unit receiving a signal from the first sensor, and the control unit transmitting a signal to the driver to control an output shaft movement. 
       BACKGROUND OF THE INVENTION 
       [0002]    Electric power assist steering systems (EPAS) have been around since the 1960&#39;s. Hydraulic power assist steering has traditionally dominated the market. Hydraulic systems have high parasitic energy loss when the hydraulic pump is pumping, but power assist is not required. Early attempts to eliminate this parasitic loss involved fitting an electric motor to the pump and only driving the pump when necessary. 
         [0003]    Electric hydraulic assisted power steering systems use an electric motor to drive a hydraulic pump to feed a hydraulic power steering system. These systems are an intermediate step by the industry and their use will likely fade with the increased use of EPAS. EPAS systems allow realization of reduced noise, reduced energy use, active safety features, and adjustability to meet driving conditions. However, the use of these systems has remained limited until recent C.A.F.E. requirements became more difficult to meet. This is driving automotive manufactures to turn to EPAS systems more and more in an effort to improve vehicle fuel economy. EPAS systems eliminate the parasitic losses typically found in hydraulic assist power steering systems. System manufacturers such as Nexteer make claims of 6% fuel economy improvements. 
         [0004]    For example, one difficulty that slowed implementation of EPAS systems was meeting the power requirement with a 12 volt electric motor. Recently systems have been developed that successfully solve this problem. Further, all EPAS systems require a control module to sense driver input and control the electric motor to provide the desired assist. The control module measures driver input torque and uses this to determine the amount of assist required. Assist can be tuned to meet the drivers need depending on driving conditions. The system can even have a tunable “feel” available to the driver. 
         [0005]    Even though the main driver for automotive EPAS is fuel economy improvement, EPAS has additional benefits. The system can make steering assist available even when the vehicle&#39;s engine is not running. It also enables the use of the automatic parallel parking systems available today. 
         [0006]    There are two main types of EPAS systems; column assist and rack assist. Rack assist EPAS systems have an electric motor that is connected to the steering rack. The electric motor assists the rack movement usually through driving a lead screw mechanism. Column assist EPAS systems have an electric motor connected to the steering column. The electric motor assists the movement of the column shaft usually through a worm gear type arrangement. One advantage of these types of systems is the electric motor can be placed in the passenger compartment freeing up valuable space under the hood. This also keeps any sensitive electrical components out of the harsh under hood environment. 
         [0007]    Worm drive column assist systems are usually used in small cars where the assist power requirements are lower than what would be needed in a large heavy vehicle. These systems are limited by the speed of the steering wheel and the ratio of the worm drive. The steering wheel at its fastest speed rotates relatively slowly at approximately 60 rpm. With a 60 rpm speed of the steering wheel and a worm drive ratio of 15:1, the max speed of the electric motor would only be 900 rpm. Worm drives are limited to ratios under 20:1 because ratios higher than that cannot be back-driven. 
         [0008]    The steering system must be able to be operated with no power. This requires the worm drive be able to operate with the gear driving the worm (back-driven). Having a low motor speed and limited ratio worm drive causes the need for high torque motor. Even with a high torque motor, these types of systems have not been made successful on heavy vehicles. Small vehicles are light and require less steering effort thus enabling the use of these systems. Worm drive column assist EPAS systems are the lowest cost systems and thus also lend themselves to smaller less expensive vehicles. 
         [0009]    Typical steering systems with worm drive assists are limited in their efficiency. EPAS systems must be designed to operate when there is no power available. Due to the nature of worm drive&#39;s tendency to lock up during back driving when ratios exceed approximately 20:1, worm drive EPAS systems efficiency is not greater than approximately 85% and nearer to 65% during back-driving conditions. 
         [0010]    Representative of the art is U.S. Pat. No. 8,327,972 which discloses a vehicle steering system transmission comprising a housing, an input shaft journalled to the housing, an electric motor connected to the housing and coupled to the input shaft, an output shaft journalled to the housing, the input shaft and the output shaft coupled by a first pair of sprockets having a first belt trained therebetween and having a first ratio, the first belt and first pair of sprockets comprising a helical tooth configuration, the input shaft and the output shaft coupled by a second pair of sprockets having a second belt trained therebetween and having a second ratio, and the input shaft and the output shaft coupled by a third pair of sprockets having a third belt trained therebetween and having a third ratio. 
         [0011]    What is needed is a vehicle steering system transmission comprising a driver connected to a driver shaft, the driver shaft connected to an output shaft by a flexible link, a first sensor sensing the driver shaft, a control unit receiving a signal from the first sensor, and the control unit transmitting a signal to the driver to control an output shaft movement. The present invention meets this need. 
       SUMMARY OF THE INVENTION 
       [0012]    The primary aspect of the invention is a vehicle steering system transmission comprising a driver connected to a driver shaft, the driver shaft connected to an output shaft by a flexible link, a first sensor sensing the driver shaft, a control unit receiving a signal from the first sensor, and the control unit transmitting a signal to the driver to control an output shaft movement. 
         [0013]    Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings. 
         [0014]    The invention comprises a vehicle steering system transmission comprising a driver connected to a driver shaft, the driver shaft connected to an output shaft by a flexible link, a first sensor sensing the driver shaft, a control unit receiving a signal from the first sensor, and the control unit transmitting a signal to the driver to control an output shaft movement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention. 
           [0016]      FIG. 1  is a perspective view of the system transmission. 
           [0017]      FIG. 2  is a perspective view of an intermediate sprocket. 
           [0018]      FIG. 3  is a perspective view of an intermediate sprocket. 
           [0019]      FIG. 4  is a perspective view of a steering system. 
           [0020]      FIG. 5  is a schematic diagram of the control system. 
           [0021]      FIG. 6  is a perspective view of an alternate embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0022]      FIG. 1  is a perspective view of the system transmission. The inventive device comprises sensor  1 , sensor  2 , sensor  3 . Sensor  1  is mounted to input shaft  4 . Sensor  2  is mounted to intermediate shaft  12 . Sensor  3  is mounted to intermediate shaft  23 . Flexible link  6  is trained between input shaft  4  and intermediate shaft  12 . Flexible link  7  is trained between intermediate shaft  12  and intermediate shaft  23 . Belt  8  is trained between intermediate shaft  23  and output shaft  10 . Each belt comprises a mechanical linkage between the respective sprockets. 
         [0023]    Input sprocket  4  is driven by an electric motor or any other device providing rotation and torque. Sensor  1  measures the angular position of input shaft  4 . Sensor  2  measures the angular position of intermediate shaft  12 . Sensor  3  measures the angular position of intermediate shaft  23 . Input shaft  4  has 20 teeth. Flexible link  6  and flexible link  7  may each comprise a toothed belt or a chain. 
         [0024]    Intermediate shaft  12  comprises a first sprocket  120  and a second sprocket  121 . Sprocket  120  has 43 teeth, sprocket  121  has 18 teeth (see  FIG. 2 ). Input shaft  4  and sprocket  120  are connected through flexible link  6 . Intermediate shaft  23  has first sprocket  230  and second sprocket  231 . Sprocket  230  has 41 teeth. Sprocket  231  has teeth. Sprocket  121  and Sprocket  230  are connected through flexible link  7 . Output shaft  5  has 73 teeth. Sprocket  231  and output shaft  5  are connected through flexible link  8 . 
         [0025]    To operate properly, the initial starting position of the three shafts  4 ,  12 ,  23  with sensors  1 ,  2 ,  3  must be known as well as the position of the output shaft  5 . From this starting position, the position of each shaft can be measured as a result of rotation of input shaft  4 . The angular position of each sprocket can be calculated since we know the drive ratio at each stage and the overall drive ratio. 
         [0026]    For example, given:
       θ 1 =angle of input shaft  4  from its starting position (in turns where one turn=360 degrees)   N n =ratio of drive to each sprocket from the input sprocket   θ n =angle of the shaft from its starting position (in turns)       
 
         [0030]    The angular position of each shaft can be determined with the following equation: 
         [0000]      θ n   =N   n θ 1  
 
         [0031]    For a rotation of the input shaft  4  of 20 revolutions, intermediate shaft  12  rotates 
         [0000]      θ 12 =(20/43)(20)
 
         [0000]      θ 12 =9.3023 turns
 
         [0032]    The fractional portion of the number of turns is then converted to an angular position. 
         [0000]      0.3023 turns=0.3023*360 degree/turn=108.28 degrees 
         [0033]    This result represents the number of degrees of rotation from the original starting position of intermediate shaft  12 . Since the drive ratio at each sprocket in the drive is known, one can determine the position of each shaft from its starting position. 
         [0034]    The inventive device is configured such that each position of output shaft  5  corresponds to only one unique position of the three other shafts. If power is lost in the system, when power returns, the position of output shaft  5  can be determined by the position of the three shafts with sensors. If a drive is not limited in the number of revolutions of the output shaft, the positions of the individual shafts will eventually return to their respective starting positions and the shaft relative positions will repeat. 
         [0035]    In devices such as electric assist steering systems, where the output shaft is the vehicle steering shaft it is advantageous to know the position of the steering shaft. To avoid losing the known position of the output shaft, the arrangement of the drive ratios must be such that the number of turns required to cause repeating of the positions exceeds the total number of turns in the drive. 
         [0036]    In the system described above, one can compare the original starting position of each shaft with its current position. When all three current positions align with their original position within the tolerance zone of the sensor, the unique position is then repeating itself. The tolerance of the position sensors expands the possible positions of each shaft during sensing of shaft position. 
         [0037]    For example, if sensor  1  indicates input shaft  4  is at a position 5 degrees clockwise from its original position, sensor  2  can only indicate a position for intermediate shaft  12  that is equal to a whole number of turns of input shaft  4  plus the 5 degrees of rotation from its starting position multiplied by the fixed ratio of drive stage 1. For this consideration, assume all starting positions correspond to the vertical position. For example, if input shaft  4  rotates 5 degrees, intermediate shaft  12  must only have rotated 5 degrees×(20/43)=2.325 degrees. If input shaft  4  rotated one turn plus 5 degrees, intermediate shaft  12  must have rotated (360+5)×(20/43)=169.767 degrees. This same logic holds true for the position of intermediate shaft  23 . Continuing with the same example, intermediate shaft  23  must have rotated 5 degree×(20/42)×(19/43)=1.027 degrees for a 5 degree rotation of input shaft  4 . For one turn plus 5 degrees rotation of input shaft  4 , intermediate shaft  23  rotates 75.013 degrees. 
         [0038]    For each position of input shaft  4 , there is only one corresponding position of both intermediate shaft  12  and intermediate shaft  23 . These positions are used mainly to indicate the number of turns of input shaft  4 . For example, if sensor  3  on intermediate shaft  23  indicates an angle of 16.4 degrees, and sensor  1  indicates an angle of 5 degrees and sensor  2  indicates and angle of 169.7 degrees, the number of turns that input shaft  4  turned corresponds to 44 turns. So the position of output shaft  5  is determined as (44 turns×(360 deg/turn)+5 degrees)×(20/43)(19/34)(18/73)=802.9 degrees. This corresponds to two turns plus 82.95 degrees or 2.23 turns from its start position. 
         [0039]    The accuracy of the final position of output shaft  5  is determined by the accuracy of the sensor  1  on input shaft  4 . The location of output shaft  5  is known with an accuracy that is equal to the accuracy of sensor  1  divided by the drive ratio. For example, if sensor  1  has a position tolerance of 5 degrees on the above drive with an overall ratio of 19.73, the position of output shaft  5  can be determined within 5/19.73 degrees or 0.25 degrees. This sensor arrangement gives one the ability to gain significant position accuracy relative to the accuracy of individual position sensors. This arrangement provides improved accuracy when compared to placing a sensor on the output shaft without changing the accuracy of the sensor itself. 
         [0040]    For a given sensor tolerance, one must include this tolerance in determining the number of revolutions of input shaft  4  required to cause the shaft positions to repeat. Analysis of the inventive device shows that with a position tolerance of 5 degrees on each sensor, the shaft positions repeat every 258 revolutions of input shaft  4 . This equates to 13.07 revolutions of output shaft  5 . To track accurately the position of output shaft  5 , its travel must be limited to less than 13.07 total revolutions in order to avoid repeating the shaft positions. 
         [0041]    To maximize the total rotations between repeats of the shaft positions, the ratios of each stage should be arranged such that they are not whole numbers. For example, in the inventive device, the ratio of drive stage 1 is 43/20=2.15, the ratio of drive stage 2 is 43/19=2.26, and the ratio of drive stage 3 is 73/18=4.05. Changing drive stage 1 such that its ratio is the nearest whole number, for example 40/20=2, causes the shaft positions to repeat every 68 revolutions of input shaft  4  given the same sensor accuracy. The ratios of the drive should be arranged such that the repeating pattern of the positions of the shafts is large enough to exceed the number of revolutions required for the output shaft. 
         [0042]    In the examples, each sensor has a 5 degrees accuracy tolerance. If one increased the accuracy of sensor  1  to 2.5 degrees one could double the tolerance on the other two sensors ( 2 , 3 ) and maintain the same number of revolutions (258) until the shaft positions repeated. This would also improve the accuracy of detecting the position of output shaft  5  from 0.25 degrees to 0.125 degrees of error. 
         [0043]    Doubling the accuracy of sensor  1  to 2.5 degrees and maintaining the 5 degree tolerance on sensors  2  and  3  increases the number of revolutions until the shaft positions repeat to 1849. This improves the total number of revolutions of the device to 93.7 from 13.07 in the example here. 
         [0044]    The accuracy of sensor  1  determines the overall positional accuracy of the drive, hence, sensor  2  and sensor  3  can have twice the tolerance of sensor  1  and the device can still maintain the same number of revolutions to repeat the shaft positions. 
         [0045]    For the arrangement of the sensor  1 , sensor  2 , sensor  3  to be effective in determining the position of the output shaft  5 , the ratios of the drive stages must be such that one revolution of input shaft  4  causes at least one of the other shafts to move to a position that is outside its possible previous position inclusive of the tolerance of the sensor. 
         [0046]    For example, given:
       T=1 turn of input shaft   N=Ratio to interested shaft   E=sensor error (in turns)       
 
         [0050]    Then:
       TN&gt;E   1(20/43)&gt;5/360   0.4651&gt;0.01389       
 
         [0054]      FIG. 4  is a perspective view of a steering system. Output shaft  5  is the steering shaft for a steering system  80 . Steering wheel  51  is connected to steering shaft  5 . Shaft  5  is connected to a steering rack  60 . Steering rack  60  is connected to the vehicle wheels for steering, not shown. In this embodiment, motor  40  comprises a Johnson Electric EPS-B77. The motor listed herein is only an example and is not intended to limit the scope or use of other suitable motors. 
         [0055]      FIG. 5  is a schematic diagram of the control system. Sensor  1  transmits an angular position signal for shaft  4  to the steering control unit  500 . Sensor  2  detects an angular position of shaft  12  and transmits a signal to steering control unit  500 . Sensor  3  detects an angular position of shaft  23  and transmits a signal to steering control unit  500 . In addition, a torque sensor  400  detects a torque load applied to shaft  10  by a user. Torque sensor transmits a signal to steering control unit  500 . Steering control unit  500  processes the signals from the position sensors ( 1 , 2 , 3 ) and torque sensor to determine a control requirement for the motor  40 . Unit  500  then transmits a control signal to the motor  40 , which in turn applies a boost torque to shaft  10 . Motor  40  is connected to vehicle power  600 . Sensors  1 , sensor  2 , sensor  3  comprise Gill Blade 360 Rotary Sensors. Torque sensor  400  comprises a TT Electronics model SX-4428. The sensors listed herein are only examples and are not intended to limit the scope or use of other suitable sensors. 
         [0056]      FIG. 6  is a perspective view of an alternate embodiment. In an alternate embodiment, the inventive device comprises sensor  1 , sensor  2 , sensor  3 . Sensor  1  is mounted to input shaft  4 . Sensor  2  is mounted to intermediate shaft  12 . Sensor  3  is mounted to intermediate shaft  23 . Input shaft  4  is driven by an electric motor  40  or any other device providing rotation and torque. Sensor  1  measures the angular position of input shaft  4 . Sensor  2  measures the angular position of intermediate shaft  12 . Sensor  3  measures the angular position of intermediate shaft  23 . Input shaft  4  comprises a gear  40  with 20 teeth. 
         [0057]    Intermediate shaft  12  consists of first gear  120  and second gear  121 . Gear  120  has 43 teeth. Gear  121  has 18 teeth. Each gear pair between shafts comprises a mechanical linkage. Input shaft  4  drives gear  120 . Intermediate shaft  23  has first gear  230  and second gear  231 . Gear  230  has 41 teeth. Gear  231  has 18 teeth. Gear  121  drives gear  230 . Output gear  50  has 73 teeth. Gear  231  drives output shaft  5 . 
         [0058]    Except as described as follows, the calculations for the gear drive system are the same as for the belt drive system. 
         [0059]    Since the sensors have a tolerance, one must include this tolerance in determining the number of revolutions of input shaft  4  required to cause the shaft positions to repeat. Analysis of the inventive device shows that with a position tolerance of 5 degrees on each sensor, the shaft positions repeat every 387 revolutions of input shaft  4 . This equates to 19.49 revolutions of output shaft  5 . To fully track the position of output shaft  5 , its travel must be limited to less that this 19.49 total revolutions to avoid repeating the shaft positions. 
         [0060]    To maximize the total travel between repeats of the shaft positions, the ratios of each stage of the drive should be arranged such that they are not whole numbers. For example, in the inventive device, the ratio of drive stage 1 is 43/20=2.15, the ratio of drive stage 2 is 43/19=2.26, and the ratio of drive stage 3 is 73/18=4.05. Changing drive stage 1 such that its ratio is the nearest whole number, for example 40/20=2, causes the shaft positions to repeat every 32 revolutions of input shaft  4  given the same sensor accuracy. The ratios of the drive should be arranged such that the repeating pattern of the positions of the shafts is large enough to exceed the number of revolutions required for the output shaft. 
         [0061]    In the examples given here, the sensors each have the same 5 degrees accuracy tolerance. If the accuracy of sensor  1  is increased to 2.0 degrees, one could increase the tolerance on the other two sensors to 5.0 degrees and maintain the same number of revolutions (387) until the shaft positions repeated. This would also improve the accuracy of the position of output shaft  5  from 0.25 degrees to 0.125 degrees of error. 
         [0062]    Doubling the accuracy tolerance of sensor  1  to 2.5 degrees and maintaining the 5 degree tolerance on sensors  2  and  3  improves the number of revolutions for which the shaft positions repeat to 1763. This improves the total number of revolutions of the device to 88.77 from 19.49 in the given example. The accuracy of sensor  1  determines the overall positional accuracy of the drive. Sensors  2  and  3  can have greater tolerance and the device can maintain the same number of revolutions to repeat the shaft positions. 
         [0063]    Although forms of the invention have been described herein, it will be obvious to those skilled in the art that variations may be made in the construction and relation of parts without departing from the spirit and scope of the invention described herein.