Abstract:
A multi-dimensional position sensor includes at least three curved triangular-shaped sense inductors and a movable shaft that incorporates a conductive material. The distance of the movable shaft from the sense inductors varies as the shaft is moved in a two-dimensional plane. The variation in distance causes a variation in the inductance of each of the triangular-shaped inductors and this variation in inductance may be used to determine the physical position of the movable shaft through means of triangulation calculations.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to joysticks and, more particularly, to a two-dimensional position sensor using a movable shaft, incorporating a conductive material, surrounded by at least three, curved triangular-shaped sense coils. The physical position of the shaft is determined by triangulation, by detecting the change in the self-inductance of the sense coils as the shaft moves.  
         BACKGROUND OF THE INVENTION  
         [0002]    Joysticks are used in many military, industrial, and commercial applications to control movement of an aircraft, vehicle, object on a video screen, etc. Most joysticks convert the angular movement of a control shaft into movement along an X and Y axis, using a mechanical linkage to translate the motion. The displacement of the shaft in each direction is detected by means of mechanical switches, variable resistors, or optical sensors. Usually the greater the resolution required in detecting the shaft position, the greater the cost of the precision sense elements, such as optical sensors, required to detect tiny changes in the shaft position.  
           [0003]    A number of joystick systems using inductive sense elements have been developed. U.S. Pat. No. 4,685,678 describes a joystick system where position is determined through use of a pair of inductors that operate with a movable slug. The movement of the slugs, by the joystick handle, causes a change in inductance. The inductors produce signals proportional to the position of the slugs in two dimensions.  
           [0004]    U.S. Pat. No. 4,855,704 describes a system that utilizes two induction coils, and a spherical induction body secured to the joystick. As the joystick is moved, the location of the induction body relative to the sensors changes the inductance of the sensors.  
           [0005]    Finally, U.S. Pat. No. 5,598,090 describes a joystick system that uses biasing springs as inductors. The biasing springs position the joystick in a neutral position. Movement of the joystick compresses or extends the springs, changing their inductance. All of these approaches incorporate multiple, movable components, or mechanical elements to translate the joystick position into two dimensions.  
           [0006]    U.S. Pat. No. 5,949,325 describes a joystick system where the joystick is secured to a conductive rubber transducer. As the joystick is moved about, the curved rubber transducer is deflected and contacts conductors on a printed circuit board. This approach eliminates many of the mechanical moving parts required to translate angular motion into two axes, however, it uses direct contact between the joystick and sensing elements.  
           [0007]    In view of the above, there is a need in the art for a joystick with no moving parts other than the joystick shaft. Further, it would be desirable to eliminate the need to mechanically translate the motion of the shaft into an X and Y direction. It would be further desirable to detect the position of the joystick shaft without physical contact to the shaft which, in combination with the elimination of moving parts, provides improved reliability and durability. It is also desirable to detect the joystick shaft position with significant precision to provide increased resolution using low cost sensors.  
         SUMMARY OF THE INVENTION  
         [0008]    The needs described above are in large measure met by an inductive joystick of the present invention. The inductive joystick has no moving parts other than the joystick shaft, and utilizes low-cost printed sense inductors. The shaft position is detected by use of a triangulation algorithm that can detect at least 72 discrete positions within the sphere of movement of the shaft. There is no physical contact with the joystick shaft.  
           [0009]    Specifically, the inductive joystick of the present invention includes a shaft that is provided at a first end with a layer of a cone-shaped conductive material. A tubular housing surrounds the first end and contains at least three curved sense inductors that surround the movable shaft. The sense inductors are preferably triangular in shape. The distance of the movable shaft from the sense inductors varies as the shaft is moved in a two dimensional plane. The variation in distance between the conductive material and the sense inductors causes variation in the inductance of each of the triangular shaped inductors and this variation in inductance is used to determine the physical position of the movable shaft through use of triangulation calculations.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a diagram showing the construction of an inductive joystick incorporating a plurality of triangular-shaped sense coils surrounding a movable shaft fitted with a conductive cone.  
         [0011]    [0011]FIG. 2 is a block diagram of the inductive joystick that incorporates an inductive sensory apparatus and a microprocessor controlled successive approximation A/D converter.  
         [0012]    [0012]FIG. 3 is a detailed circuit diagram of the inductive joystick electronics. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    Construction of an inductive joystick  200  of the present invention is shown in FIG. 1. In this particular configuration, three triangular sense coils  100  (only two may be seen) are curved about and evenly spaced around the circumference of a non-conductive cylinder  202 . Each of triangular sense coils  100  is preferably printed with conductive ink, as a flex circuit, onto mylar or other suitable material that allows the coils  100  to be curved about the cylindrical shape of cylinder  202 . Sense inductors  100  are triangular shaped because the self inductance of such coils varies linearly in proportion to the position of proximate conductive material along the coil axis.  
         [0014]    It should be noted that the apex  108  of each of triangular sense coils  100  overlaps the area of the adjacent triangular sense coil  100  where the coil turns run perpendicular, designated by dashed line  110 , to imaginary coil axis, designated by dashed line  102 , which is the non-linear sensing portion of each triangular sense coil  100 . Alternatively, a portion of each triangular sense coil  100  may be bent at a right angle, such that the non-sensing area of each triangular sense coil  100  extends perpendicularly to the side of cylinder  202  and only the linear sensing portions of each of triangular sense coils  100  are evenly spaced, without overlap, around the circumference of cylinder  202 .  
         [0015]    Cylinder  202  is maintained in a stationary position about a movable shaft  204  that is preferably comprised of a shaft handle  206  and a cone  208 . Cone  208  is covered in a conductive material, preferably a highly conductive silver paint. The angle of cone  208  is such that as shaft  204  of joystick  200  is rotated about at its maximum extremity, the edge of cone  208  becomes parallel to the wall of cylinder  202 . Shaft  204  is centrally supported by a base  210  that allows shaft  204  to pivot and rotate 360 degrees. Base  210  is preferably of a configuration to enable storage of the circuitry that comprises the inductive joystick sensory system.  
         [0016]    While cylinder  202  is preferably fitted with three triangular sense coils  100 , depending upon the signal strength from sense coils  100  when shaft  204  is in its idle, center position, it is usually desirable to use a fourth external coil as a reference coil (the signal strength while in the idle, center position is dependent upon the conductivity of the cone material and the diameter of cylinder  202  surrounding shaft  204 ). The reference coil is preferably identical in inductance to triangular sense coils  100 . Alternatively, in certain configurations, any of the three triangular sense coils  100  fitted about cylinder  202  may be used as the reference coil.  
         [0017]    [0017]FIG. 2 shows a block diagram of the inductive joystick sensory system. The three position sensing triangular sense coils  100  and one reference coil  102  are used as inductors in an LC oscillator  22  circuit. The negative excursion of the oscillator  22  output is clamped to ground and the positive peak detected by a peak detector  28 , to convert the oscillation amplitude to a DC level. When the microcontroller  26  selects the reference coil  102  through a coil multiplexor  24 , a feedback control loop  32  adjusts the drive voltage to the LC oscillator  22 , by means of a voltage comparator  34  and peak detector/buffer  36 , to clamp the signal from the reference coil to a fixed amplitude. This allows the system to compensate for changes in oscillation amplitude due to temperature, supply voltage, and component tolerance variations. The software within microcontroller  26  than samples the signals from each of the three triangular sense coils  100  which are digitized by a low cost successive approximation A/D converter  32 .  
         [0018]    Referring to the detailed inductive joystick electronic circuit diagram of FIG. 3, operational amplifier  60  forms part of the successive approximation A/D converter  32  comprising a comparator whose reference voltage is set by microprocessor  26 , through a D/A converter  200 . D/A converter  220  utilizes a summing amplifier  222 , feedback resistor  223 , and a precision resistive ladder network  224 . The microcontroller  26  outputs digital words to the inputs of precision resistive ladder network  224  feeding summing amplifier  222  using a binary search algorithm until the output of the D/A converter matches the coil signal. The software of microcontroller  26  triangulates the position of shaft  204  by comparing the digitized signals from each of the three triangular sense coils  100  against a stored table of nominal signal values for various shaft positions, and runs a closeness of fit algorithm to determine the current position of shaft  204 . The purpose of the closeness of fit algorithm is to find the nominal shaft position which most closely matches the measured coil signals with minimal error. The position of shaft  204  may then be output by microcontroller  26  to control a video game, machine, etc. It is possible to detect at least forty-eight (48) discrete positions around the perimeter of the largest circle circumscribed by the rotation of shaft  204  of joystick  200 , with a proportional number of intermediate positions also detectable.  
         [0019]    The inductive nature of operation of joystick  200  provides desirable advantages over that of mechanical, resistive, or optical joystick approaches. Specifically, joystick  200  provides excellent resolution at low cost and high reliability because there are no moving parts other than center shaft  204 , and sensing of the joystick position requires no mechanical or physical contact with center shaft  204 .  
         [0020]    An alternative embodiment of joystick  200  comprises mounting three triangular sense coils  100  around the surface of an inverted cone surrounding shaft  204 . In this case, the conductive portion of shaft  204  can be cylindrical in shape, rather than cone shaped; the operation of joystick  200  remains as substantially described above.  
         [0021]    The present invention may be embodied in other specific forms without departing from the spirit of the essential attributes thereof; therefore, the illustrated embodiments should be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.