Abstract:
A system for determining the absolute position of a multi-turn shaft is disclosed. The shaft is mounted in an elliptically shaped wave generator. The wave generator is coupled to a flex spline which is deformed to match the wave generator. The exterior surface of the flex spline has a number of gear teeth. The flex spline is installed within a cylindrical, circular spline. The circular spline has an interior diameter which is slightly larger than the largest diameter of the flex spline. The interior surface of the circular spline has a number of teeth which interlock with the teeth of the flex spline. In such a manner, the rotation of the shaft will cause an angular displacement of the circular spline. A position sensor is located on the circular spline and serves to track the position of the circular spline which is proportional to the absolute position of the shaft.

Description:
FIELD OF INVENTION 
     This invention relates to a sensor to determine the rotational angle of a multi-turn shaft. More specifically, this invention relates to an angular sensor that determines the absolute position of a multi-turn shaft. 
     BACKGROUND 
     In the automotive and other fields, rotational control devices such as steering wheels may turn several times in order to enable a full range of operation. In the case of a steering wheel for example, the wheel is turned three times for the full range of turning the front wheels of a car. The angular position of the steering wheel must be measured and additionally the number of turns completed must also be determined in order to determine the real position of the front wheels. 
     In order to determine angular position, a potentiometer is attached to a circular resistor located around the shaft and a wiper contact is attached to one point on the circumference of the shaft. The resistance changes with the movement of the wiper contact and thus the voltage measured by the potentiometer changes as the shaft is rotated, providing a determination of the angle of the shaft. Additional circuitry is required to record the precise position of the shaft by staring the number of rotations in order to obtain the true position of the front wheels. Such circuitry unnecessarily complicates the sensor system. 
     Some present systems use mechanical solutions to magnify the effect of the angle of the shaft rotation. Such mechanical configurations include a relatively complex helical or spur gear system. Thus, the single rotation cycle of the shaft will translate in smaller increments of movements via the helical or spur gear. Unfortunately, the use of helical or spur gears adds to the complexity and expense of the steering assembly. Additionally, with increasing numbers of gears, mechanical backlash becomes problematic. 
     Thus, there exists a need for a simple absolute position sensor for a multi-turn shaft assembly. There is also a further need for an absolute position sensor that may determine the angular position of a shaft assembly without the use of processing circuitry to determine the number of turns of a shaft. Also, there exists a need for a position sensor that may be used in conjunction with an in-line, concentric assembly of a multi-turn shaft. Finally, there exists a need for a position sensor that may be used in conjunction with a gearing assembly that eliminates gear backlash. 
     SUMMARY OF THE INVENTION 
     The present invention may be embodied in an angular position sensor system for determining the absolute position of a rotating member. The sensor system has a drive assembly coupled to the rotating member. The drive assembly includes a wave generator having an elliptical shape. A flex spline is coupled to the wave generator. The flex spline has an exterior surface with teeth. A cylindrical spline has a diameter larger than the largest diameter of the flex spline and an interior surface with teeth which interlock with the teeth of the flex spline. The cylindrical spline rotates at a proportional angle of rotation when the rotating member is rotated a full revolution. A position sensor is operatively coupled to the drive assembly and outputs the angular absolute position of the rotating member. 
     The present invention may also be embodied in an angular position sensor system for determining the absolute position of a rotating shaft. The sensor system has a drive assembly coupled to the shaft. The drive assembly has a wave generator having an elliptical shape coupled to the shaft. A flex spline is coupled to the wave generator. The flex spline has an exterior surface with teeth. A cylindrical spline has a diameter larger than the largest diameter of the flex spline and an interior surface with teeth which interlock with the teeth of the flex spline. The cylindrical spline rotates at a proportional angle of rotation when the shaft is rotated a full revolution. A position sensor is operatively coupled to the cylindrical spline and outputs the angular absolute position of the shaft. 
     It is to be understood that both the foregoing general description and the following detailed description are not limiting but are intended to provide further explanation of the invention claimed. The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the method and system of the invention. Together with the description, the drawings serve to explain the principles of the invention. 
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of a rotational position sensor in conjunction with a harmonic drive assembly according to one embodiment of the present invention. 
     FIG. 2 is an exploded view of the rotational position sensor and harmonic drive assembly of FIG.  1 . 
     FIG. 3 is a side cutaway view of the rotational position sensor and harmonic drive assembly of FIG.  1 . 
     FIGS. 4A-C are side cutaway views of the harmonic drive shown in FIG. 1 in various angles of rotation of the shaft. 
     FIG. 5 is a side view of an alternate embodiment of a position sensor for a multi-turn shaft. 
     FIG. 6 is a side view of an alternate embodiment of a harmonic drive assembly which incorporates a position sensor according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the present invention is capable of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. 
     The drawings and more particularly FIGS. 1-3 show a perspective view, an exploded perspective view and a side view of an absolute angle position sensor generally indicated at  10 , embodying the general principles of the present invention. The sensor  10  is mounted on a rotating member assembly such as a multi-turn shaft assembly  12 . The shaft assembly  12  is coupled to a mechanical pinion system  14  which is moved by rotating a shaft  16 . The pinion system  14  such as the front wheels of an automobile is moved in relative motion by rotating the shaft  16  several times. The absolute angle of the rotation of the shaft  16  determines the precise position of the pinion system  14  within a smaller range of motion. In this example the shaft  16  may be rotated approximately 1300 degrees (approximately three and half full rotations) in order to achieve the limited range of movement of the pinion system  14 . 
     The shaft assembly  12  has a harmonic drive type assembly  18  which is located on the shaft  16  and facilitates measurement of the absolute angular position of the pinion system  14  in relation to the steering assembly. The harmonic drive assembly  18  has a wave generator  20 , a flex spline  22  and a circular spline  24 . The wave generator  20  has an elliptical cam  26  which is enclosed in an anti-friction, ball bearing assembly  28 . The elliptical cam  26  has a back surface  30  which has a cylindrical socket hub  32 . The socket hub  32  has a socket  34  which has a slot  36  to insure that the shaft  16  is locked into the elliptical cam  26  and moves rotationally with the elliptical cam  26 . 
     The ball bearing assembly  28  is elliptically shaped with an annular groove  38  which holds a number of ball bearings  40 . The ball bearings  40  are lubricated in order facilitate the rotational movement of the wave generator  20 . The ball bearing assembly  28  has an exterior surface  42  which is mated with the flex spline  22 . 
     The flex spline  22  has a flexible, circular cup  44 . The circular cup  44  has an interior surface  46  which is the same shape as the exterior surface  42  of the wave generator  20  creating a locking fit between the wave generator  20  and the flex spline  22 . The circular cup  44  also has an exterior surface  48  which has a series of annular spline teeth  50 . 
     The greatest diameter of the circular cup  44  is slightly smaller than the circumference of the circular spline  24 . The circular spline  24  has a thick-walled, rigid ring  52  having an interior surface  54 . The interior surface  54  of the rigid ring  52  is circular in shape and has a series of internal spline teeth  56 . At two ends  58  and  60  of the greatest diameter of the circular cup  44  the annular spline teeth  50  of the flex spline  22  interlock with the internal spline teeth  56  of the ring  52 . The circular spline  24  also has an exterior surface  62  which has a magnet  64  which as will be explained below is used to determine the absolute angular position of the circular spline  24 . 
     The flex spline  22  has a number of lock points  70  which are located on its circumference. The flex spline  22  is connected to a protective cover  72  which has a number of mounting holes  74 . Screws or bolts (not shown) connect the protective cover  72  to the lock points  70  of the flex spline  22 . The protective cover  72  is mechanically coupled to a fixed, non-rotating reference point such as a steering column to fix the drive assembly  18  in place. 
     The harmonic drive assembly  18  is coupled to the shaft  16 . The interaction between the wave generator  20 , flex spline  22  and the circular spline  24  reduce the rotation of the shaft from 1300 degrees of travel of the wave generator to 20 to 30 degrees of travel for the circular spline  24 . The travel of the circular spline  24  is proportional to the movement of the pinion assembly  14  from the shaft  16 . In other words, turning the shaft  16  approximately three and a half times will rotate the circular spline  24  and the pinion assembly  14  approximately 30 degrees. Since the magnet  64  is located on the circular spline  24  rather than the shaft  16 , the absolute angular position of the spline  24  may be determined via the sensor  10 . In this case, the shaft  16  is coupled to a steering wheel while the pinion assembly  14  rotates the front wheels of a vehicle. 
     As shown in FIGS. 4A-4C, the rotation of the circular spline  24  is reduced to a small angle via the gearing arrangement between the wave generator  20 , flex spline  22  and the circular spline  24 . FIG. 4A shows the wave generator  20  in a first position relative to the circular spline  24 . As the shaft  16  is rotated, the annular spline teeth  50  at the elliptical diameter of the flex spline  22  interlock with the corresponding interior teeth  56  of the circular spline  24 . As the shaft  16  and the flex spline  22  is rotated as shown in FIG. 4B, the circular spline  24  is rotated proportionally. The rotation of the shaft  16  at 180 degrees as shown in FIG. 4C causes the circular spline  24  to be rotated by one gear tooth on the circular spline  24  or approximately 4.1 degrees. If the shaft  16  is rotated a full 360 degrees, the circular spline  24  will be rotated by two gear teeth or approximately 8.2 degrees. It is to be understood that the gearing ratio may be adjusted by adjusting the number of gear teeth in the flex spline  22  and the circular spline  24 . 
     Returning to FIGS. 1-3, the magnet  64  moves with the circular spline  24 . Measurement of the angle of the magnetic field of the magnet  64  will determine the absolute angular position of the circular spline  24 . The sensor  10  measures the angle of the magnetic field and includes a magnetic field transducer unit  80 . The magnetic field transducer unit  80  is coupled to a processing unit  82  which reads the magnetic field output from the transducer unit  80 . The detector  80  may be any appropriate magnetic field detector such as magneto-resistance, GMR, Hall effect or other type of magnetic field sensor. The processing unit  82  has an output interface  84  which may be coupled to further processing electronics such as a chassis computer for the purpose of vehicle stability control in this example. 
     The transducer unit  80  in this embodiment is a transducer integrated circuit such as a KMZ43 magnetic field sensor manufactured by Philips Electronics. The transducer unit  80  outputs an electronic signal responsive to the detected magnetic field direction from the magnet  64  in FIGS. 1-3. However any appropriate magnetic field sensor may be used. The transducer unit  80  has a pair of magnetic field transducers which are magneto-resistive elements in this example and detect the direction of the magnetic field generated by magnet  22 . 
     As will be explained below, the transducer unit  80  outputs a pair of varying sinusoidal signals from the magnetic field transducers which are representative of the angle direction of the detected magnetic field from the magnet  64 . The signal processor unit  82  reads the sinusoidal signals output from the transducer unit  80  and converts them into a digital linear output. The signal processor unit  82  in the preferred embodiment is a UZZ9000 sensor conditioning electronic unit manufactured by Philips Electronics. However, any appropriate hardware or software configuration may be used to process the raw signals from the transducer unit  80  to output a linear signal. The signal processor unit  82  calculates the arctangent of the angle based on the sine and cosine of the magnetic field direction determined from the sensing elements of the transducer unit  80  which measure the magnetic field angle of the magnet  64  and thus the angular position of the circular spline  24 . 
     Alternatively, the shaft  16  could be coupled to the circular spline  24  causing the wave generator  20  to turn at a reduced gearing ratio. In such a configuration, the positions sensor would be installed on the wave generator  20 . It is to be understood that any application which requires determination of the absolute position of a rotating member may use the principles explained above. 
     Also, any reduction gearing applications may use the principles illustrated above. The harmonic drive assembly  18  provides input and output gears in the form of the wave generator  20 , flex spline  22  and circular spline  24  which are concentric thus eliminating the need for more complex gearing in reduction gearing application. The spline teeth  50  and  56  are precisely machined and due to the interference between them, backlash is negligible. Additionally, the multiple tooth engagement between the flex spline  22  and the circular spline  24  allows high output torque to be achieved with negligible backlash. 
     The harmonic drive assembly  18  may be configured to fix the wave generator  20 , the flex spline  22  or the circular spline  24 . The other two components are coupled to rotating members and are used to cause the rotation reduction. In such configurations, the position sensor components would be located on the component which has the reduced rotation. For example, if the wave generator  20  were fixed to a non-rotating base, gear reduction could be achieved from a rotating member coupled to the flex spline  22  to reduce rotation of the circular spline  24 . The absolute angle position sensor in such a configuration would be located on the circular spline  24 . Alternatively, gear reduction could be achieved from a rotating member coupled to the circular spline  24  to reduce rotation of the flex spline  22 . The absolute angle position sensor in such a configuration would be located on the flex spline  22 . 
     If the circular spline  24  were fixed to a non-rotating base, gear reduction could be achieved from a rotating member coupled the flex spline  22  to reduce rotation of the wave generator  20 . The absolute angle position sensor in such a configuration would be located on the wave generator  20 . Alternatively, gear reduction could be achieved from a rotating member coupled to the wave generator  20  to reduce rotation of the flex spline  22 . The absolute angle position sensor in such a configuration would be located on the flex spline  22 . 
     FIG. 5 shows a second type of position sensor  100  which may be used in conjunction with the multi-shaft assembly  12  in FIG.  1 . Like elements to the multi-shaft assembly  12  in FIG. 1 are labeled identically in FIG.  5 . The position sensor  100  has a potentiometer  102  which is coupled to an arcuate resistor  104 . The arcuate resistor  104  is located on the outer surface of the circular spline  24 . One lead of the potentiometer  102  is coupled to one end of the resistor  104 . The other lead of the potentiometer  102  is coupled to the circular spline  24 . A wiper electrical contact  106  is located on the outer surface of the circular spline  24  to create electrical contract between the circular spline  24  and the face of the resistor  104  to complete the circuit. 
     The resistor  104  is of a sufficient arcuate length to provide continuous contact with the wiper electrical contact  106  through the course of rotating the shaft  16 . As the shaft  16  rotates, a smaller degree of rotation of the circular spline  24  occurs. The rotation of the circular spline  24  moves the wiper electrical contract  106  along the resistor  104  thus changing the voltage measured by the potentiometer  102 . The voltage measured by the potentiometer is therefore proportional to the angular position of the circular spline  24 . By providing a longer resistor, any range of angular movement between 1 and 360 degrees may be determined based on the rotation of the multi-turn shaft. 
     A cross sectional side view of a second type of harmonic drive assembly  200  is shown in FIG.  6 . The drive assembly  200  has a wave generator  202 , a rotating circular spline  204 , a fixed circular spline  206  and a flexible spline  208 . A shaft  210  is attached to the wave generator  202  via a socket  212 . The rotation of the shaft  210  thus rotates the wave generator  202  and the flexible spline  208 . 
     The wave generator  202  has an annular slot  214  which has ball bearings  216  to facilitate the rotation of the wave generator  202 . The wave generator  202  is elliptical in shape with a diameter which is just smaller than the diameter of the rotating circular spline  204  and the fixed circular spline  206 . The exterior surface of the flex spline  208  has a number of teeth which interlock with teeth which are located on the interior surface of the rotating circular spline  204 . Similarly to the assembly explained above, the rotation of the shaft  210  causes the wave generator  202  to rotate thus causing the teeth of the flex spline  208  to move around the interior teeth of the rotating circular spline  204 . The rotating circular spline  204  moves in proportion to the number of turns applied to the shaft  210 . The fixed circular spline  206  forces the flex spline  208  to rotate as their respective teeth are meshed. The rotation of the flex spline  208  is then coupled to the rotating circular spline  204 . 
     This system functions identically to the previous system except the flex spline  208  is coupled to a rotating member and rotates as a unit. In the system described in FIGS. 1-3, either the wave generator  20  or the circular spline  24  may be constrained from movement to provide a fixed reference point instead of the flex spline  24 . However, this arrangement results in a relatively long axial profile. The drive assembly  200  results in a more compact axial profile. 
     A magnet  220  is mounted on the exterior surface of the rotating circular spline  204 . The magnetic field emitted by the magnet  220  may be measured to determine the angular position of the rotating circular spline  204 . Of course other types of position sensors may be used to determine the angular position of the rotating circular spline  204 . 
     Of course, the present invention may be employed in any application which requires determination of angular position of a shaft or reduction gearing using harmonic drive assemblies. Examples in the automotive field include throttle position sensors, gas and brake pedal position sensors, suspension position sensors and window position sensors. The present invention may also be used in similar fields such as a throttle grip position sensor on a motorcycle. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the present invention without departing from the spirit or scope of the invention. Thus, the present invention is not limited by the foregoing descriptions but is intended to cover all modifications and variations that come within the scope of the spirit of the invention and the claims that follow.