Abstract:
A variable position sensor has a stationary portion and a moveable portion. A plurality of plates are positioned on one of the members with the surfaces of the plates forming segments of a larger surface. A single plate is mounted on the other of the two members. One pole of an electric potential is applied to a first of the plurality of plates on the one member and the second pole of the electric potential is applied to the single plate on the other member. Similarly a pole of an electric potential is separately applied to each of the other of the parallel plates and to the one plate. When the one plate is positioned adjacent one of the plurality of plates a capacitance is formed between the adjacent plates, and by detecting the capacitance the position of the one plate can be determined.

Description:
The present invention relates to an improved position sensor, which may be either angular or linear, and operates without requiring a brush to make electrical contact between the parts, and specifically to a variable position sensor employing capacitance to determine position. 
   BACKGROUND OF THE INVENTION 
   An angular position sensor detects the angular orientation of a rotating shaft with respect to a housing and normally consists of a rotor fitted on the shaft and a housing which surrounds the rotor. Most existing angular position sensors have an annular resistive element that surrounds the rotor and a brush that makes electrical contact with the rotor to measure changes in resistance to determine the angular orientation of the rotor. 
   Linear position sensors are employed to measure linear movement of one object with respect to another. Most existing linear position sensors include a generally planar resistive member along which moves the distal end of a wiper. For both angular and linear position sensors employing resistance, an electrical circuit is provided to measure the change in resistance that occurs as the wiper moves along the resistive member to thereby determine the position of the wiper with respect to the resistor. 
   Where a wiper moves along a resistive member, the wiper may cause small particles of material to be loosened from the resistive member and the loosened particles may cause contamination of the surrounding environment in which the detector is positioned. For example, if the detector is used to detect the amount of liquid in a container, the particles loosened by the wiper may contaminate the liquid. Alternately, particles in the surrounding environment in which the detector is located may interfere with the movement of the wiper with respect to the resistor and cause the device to provide an incorrect reading of the position of the wiper. Furthermore, the movement of the wiper across the resistive surface over a long period of time may alter the resistive qualities of the resistive material, or wear the resistive material away altogether such that the device no longer provides an accurate reading of position. 
   To avoid the problems that occur in which variable position sensors that rely upon contact between a stationary member and a moving member, it is desirable that the parts be made so as not to require such contact. Efforts have been made to use magnetism and magnetic fields to provide a position sensor that does not require physical contact between a stationary part and a moveable part, but such magnetically operated position sensors may not be useable in circumstances where the magnetic field will interfere with other adjacent devices. 
   Variable capacitors have also been used as position sensors. Such variable capacitors have a stationary plate or electrode and a parallel moveable electrode. The moveable electrode is moveable between a first position in which the two electrodes are overlapping, or aligned with each other so as to maximize capacitance to a second misaligned position in which the electrodes are not overlapping so that capacitance is minimized. Such variable capacitance requires a relatively large amount of space to provide the same degree of accuracy as resistor type position sensor for several reasons. One is that it is difficult to measure small changes in capacitance and another is that enough space must be provided to allow the plates to become totally misaligned from each other. In the case of an angular position sensor, existing variable capacitors are only rotatable through one hundred eighty degrees, wherein many uses require accurate measurement through three hundred sixty degrees of rotation. 
   It would be desirable, therefore, to provide a variable position sensor that will accurately provide a reading of the position of a first member that is moveable with respect to a second member and does not rely upon physical contact between the members. 
   SUMMARY OF THE INVENTION 
   The present invention is embodied in a variable position sensor having a first stationary member and a second moveable member. A plurality of spaced electrically conductive plates or electrodes are positioned on one member, with the plurality of plates defining the segments of a surface. Where the device measures angular position, the surface defined by the plurality of plates is a circular disc or a hollow cylinder, and where the device measures linear position, the surface is a plane. 
   An electrically conductive plate or electrode is also positioned on the second member such that movement of the second member with respect to the first member will move the electrically conductive electrode on the second member adjacent to successive ones of the plurality of spaced electrically conductive electrodes on the first member. 
   The device further includes a detecting circuit for detecting capacitance between the one electrode on the one hand and one of the plurality of electrodes on the other hands. Each of the plurality of electrodes has its own connecting wire, and the position of the moveable electrode with respect to the plurality of electrodes is determined by the circuit that detects a change in capacitance with respect to the one of plurality of electrodes that is adjacent the one electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be had after a reading of the following detailed description taken in conjunction with the drawings wherein: 
       FIG. 1  is a side elevational view of a linear position sensor employing the present invention; 
       FIG. 2  is a top view of the device shown in  FIG. 1 ; 
       FIG. 3  is a cross-sectional view of the device shown in  FIG. 1  taken through line  3 — 3  thereof; 
       FIG. 4  is a fragmentary side view of the device shown in  FIG. 1  with the slide removed to a position between two adjacent stationary electrodes; 
       FIG. 5  is a schematic view of a circuit for use with the embodiment shown in  FIG. 1 ; 
       FIG. 6  is a cross-sectional view of an angular position sensor employing the present invention; 
       FIG. 7  is a front view of the stationary disc used in the embodiment shown in  FIG. 6 ; 
       FIG. 8  is a front view of the disc on the rotating shaft of the embodiment shown in  FIG. 5 ; 
       FIG. 9  is a schematic view of a circuit for use with the embodiment shown in  FIG. 6 ; 
       FIG. 10  is a partially cross-sectional view of another embodiment of an angular position sensor employing the invention; and 
       FIG. 11  is a cross-sectional view of the embodiment of  FIG. 10  taken through line  11 — 11  thereof. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1 ,  2 , and  3 , a linear position sensor  10  includes a generally planar body  12  having an electrically nonconductive insulator  14  on the upper surface thereof, and mounted linearly along the upper surface  14  are a plurality of spaced electrically conductive plates  16 A,  16 B, . . .  16 N, . . .  16 Z with each plate having a planar upper surface. The planar upper surfaces of all the plates  16 A,  16 B . . .  16 N, . . .  16 Z define the segments of the same plane. Each plate, of which plate  16 N is representative of all such plates, is rectangularly shaped with parallel opposing ends  17 ,  18  that define a width  19 , and parallel opposing sides  20 ,  21  that define a length  22 . Also, as depicted in  FIG. 2 , each plate  20 N has a spacing  24  between the ends  17 ,  18  thereof and the ends of the adjacent plate  16 P. 
   The linear position sensor  10  also includes a slide bar  26  retained at the ends thereof by posts  28 ,  30  mounted perpendicular to the upper surface of the body  12 . The elevations of the posts  28 ,  30  at opposite ends of the slide bar  26  are chosen to space the lower surface of the moveable plate  36  a fixed distance from the surface defined by the stationary plates  16 A,  16 B, . . .  16 N, . . .  16 Z. The slide bar  26  has a generally rectangular cross-section, and fitted around the slide bar  26  is a slide  32  having a generally rectangular central opening  34  with dimensions a little larger than those of the slide bar  26  to thereby permit longitudinal movement with a minimum of resistance. Attached to the slide  32  and positioned between the slide bar  26  and the plates  16 A,  16 B, . . .  16 N is a moveable electrically conductive plate  36  having opposing parallel ends  38 ,  39  defining a length  40  substantially equal to the length  22  of the various stationary plates  16 N and having opposing sides  42 ,  43  defining a width  44  approximately equal to the width  19  of the various plates  16 N plus the length of one spacing  24  between any two adjacent plates  16 N. 
   Referring briefly to  FIG. 5 , the moveable plate  36  is connected to ground  41 . As the slide  32  moves along the slide bar  26 , the moveable plate  36  is successively moved opposite one after another of the stationary plates  16 A, . . .  16 B, . . .  16 N. Where the moveable plate  36  moves opposite one of the stationary plates  16 N, the moveable plate  36  and the opposing stationary plate  16 N form the electrodes of a capacitor and a measurable capacitance is created in the stationary plate  16 N with respect to ground. 
   Referring further to  FIGS. 1 and 2 , mounted to one of the side posts  30  and to the slide bar  26  is a retainer  45  below which is attached a fixed electrically conductive plate  46  having sides  47 ,  48  and outer ends  49 ,  50  that are equal to the length  19  and width  22  of all the plates  16 N. The fixed plate  46  is also spaced from the end plate  16 Z a distance that is equal to the spacing of the moveable plate  36  from the plane determined by the various plates  16 A,  16 B, . . .  16 N, . . .  16 Z. The position of the fixed plate  46  is immediately opposite the end stationary plate  16 Z and the plates  16 Z,  46  become the electrodes of a calibration capacitor equal to the maximum capacitance formed between the moveable plate  36  and any one of the remaining fixed plates  16 A,  16 B, . . .  16 N. 
   Referring briefly again to  FIG. 5 , each of the individual stationary plates  16 A,  16 B, . . .  16 N, . . .  16 Z is connected by a suitable connector  54 A,  54 B, . . .  54 N, . . .  54 Z to a sensing circuit  56  of the type known in the art to detect and measure capacitance. A sensing circuit  56  may be provided for each of the stationary plates  16 A,  16 B . . .  16 N . . .  16 Z as shown or they may be encapsulated into a single ICU  58 . Where the moveable plate  36  is adjacent one of the stationary plates  16 N, a measurable capacitance will be detected with respect to such plate  16 N. Since the position of the stationary plate  16 N in which capacitance is detected is known the position of the moveable plate  36  can be determined. 
   Referring to  FIGS. 1 ,  2  and  3 , one aspect of the invention is that the width  44  of the outer ends  42 ,  43  of the moveable plate  36  is equal to the sum of the width  19  of one of the stationary plates  16 N plus one spacing  24 , the distance between any two adjacent plates  16 A,  16 B, . . . .  16 N. When the moveable plate  36  is positioned directed opposite one of the stationary plates  16 N as shown in  FIGS. 1 and 2 , the capacitance provided by plate  16 N will be equal to the capacitance detectable to plate  16 Z. As shown in  FIG. 4 , when the slide  32  incurs an incremental movement along the slide bar  26 , a portion of the moveable plate  36  will move away from plate  16 N and an equally sized portion of the plate  36  will then move opposite a portion of the adjacent plate  16 P. When this occurs, the capacitance detectable for plate  16 N will be reduced by a fixed amount. Also, with the reduction of the capacitance in plate  16 N, a small capacitance, corresponding to the amount of the reduction in the capacitance of plate  16 N, will now be detected in the adjacent plate  16 P. By calculating the fractional decrease of capacitance detected from plate  16 N and calculating the fractional increase in the capacitance now detectable in plate  16 P, a relatively precise determination of the linear position of the slide  32  with respect to the two plates  16 N,  16 P can be determined. 
   The capacitance between a pair of parallel spaced plates varies in response to changes in the surrounding environment such as temperature, humidity and air pressure. To compensate for such environmental changes, the stationary plate  16 Z and the fixed plate  46  positioned opposite thereto form a detectable capacitance that is used by the sensing circuit  56  to calibrate the maximum capacitance that can be formed between one of the stationary plates  16 A,  16 B, . . .  16 N and the moveable plate  36 . The maximum capacitance, as determined by the calibrating plates  16 Z,  46 , is used to calculate the fraction by which capacitance of a plate  16 N is reduced and the fraction by which the capacitance of an adjacent plate  16 M is increased. The ratio of the capacitance detected in the two plates  16 M,  16 N is then used to precisely determine the position of the slide  32 . 
   Referring to  FIGS. 6 through 9 , the concepts of the present invention may also be employed in an angular position sensor  60 . The angular position sensor  60  has a planar disc-shaped stationary member  62  made of an electrically insulating material having a central opening  64  through which a rotating shaft  66  extends. Mounted on the shaft  66  and spaced a short distance from the surface of the stationery member  62  is a rotating disc  67  made of an electrically insulating that is locked to the shaft  66  for rotation therewith. 
   Referring to  FIGS. 6 and 7 , the surface  69  of the stationary member adjacent the rotating disc  67  is planar. Positioned on the surface  69  and surrounding the central opening  64  is an annular electrically conductive calibrating ring  68  having a circular outer edge  70 . Also mounted on the surface  69  and spaced radially outward of the outer edge  70  of the ring  68  and extending around the circumference thereof, are a plurality of generally trapezoidally shaped electrically conductive plates  72 A,  72 B, . . .  72 N each of which is electrically insulated from adjacent plates and from the ring  68 . Each plate  72 N has converging sides  74 ,  75 , an inner arcuate end  76 , and an outer arcuate end  77 . The sides  74 ,  75  of each plate  72  are spaced from the parallel side of an adjacent plate  72 M by a fixed angular distance  78 . The inner arcuate ends  76  of all the plates  72 A,  72 B, . . .  72 N define a first circle concentric with the central opening  64  and the outer ends  77  of all the plates  72 A,  72 B, . . .  72 N define a second circle concentric with the first and with the central opening  64 . Also the converging sides  74 ,  75  of the various plates  72 A,  72 B, . . .  72 N are segments of radii extending between the inner and outer circles defined by the arcuate segments  76 ,  77 . 
   Referring to  FIGS. 6 and 8 , the rotating disc  67  has a generally planar surface  80 , and mounted on the surface  80  so as to be directed toward the plates  72 A,  72 B, . . .  72 N is a trapezoidal plate  82  having converging sides  83 ,  84  and an arcuate outer end  85 . The inner end of the trapezoidal plate  82  joins to the outer circumference of an annular portion  86  which is positioned opposite the surface of the calibrating ring  68  on the stationary member  62 . The surface area of annular portion  86  and the surface area of the calibrating ring  68  are equal to the surface area of any one of the trapezoidal plates  72 A,  72 B, . . .  72 N such that the adjacent plates  68 ,  86  form the electrodes of a calibration capacitor having a capacitance equal to the capacitance formed between the rotating trapezoidal plate  82  and any one of the stationary plates  72 N when the rotating plate  82  is positioned directly opposite one of the stationary plates  72 N. 
   Referring further to  FIGS. 7 and 8 , the angular spacing  87  between the converging sides  83 ,  84  of the trapezoidally shaped rotatable plate  82  is equal to the angular spacing  88  between the converging sides  74 ,  75  of the outer edge of any of the trapezoidal plates  72 N plus the angular distance  78  between any two of the plates  72 A,  72 B, . . .  72 N. Accordingly, as the rotating disc  67  turns with the shaft  66 , the rotating plate  82  will be positioned opposite successive ones of the stationary plates  72 A,  72 B, . . .  72 N. Also, each time the rotating plate  82  moves to a position where it is offset and angle with respect to any one of the stationary plates  72 A,  72 B, . . . .  72 N, such as plate  72 A, the rotating plate  82  will move over an equal angular portion of the adjacent plate  72 B. The capacitance detectable in the first plate  72 A will then be reduced by a given amount and capacitance will be detectable in the adjacent plate  72 B equal to the capacitance lost from the first plate  72 A. 
   Referring to  FIG. 9 , the rotating plate  82  and annular surface  86  on the rotating disc  67  are connected to ground  47 . Each of the plates  71 A,  72 B, . . .  72 N is connected by separate connectors to a monitoring circuit  89  for measuring the capacitance of each of the plates  72 A,  72 B, . . .  72 N. By providing a comparison circuit such as circuit  56  described above that compares the capacitance between plate  72 A and  72 B with the capacitance of the calibrating ring  68  and ring  86 , a relatively accurate angular position of the rotating plate  82  with respect to the stationary member  62  can be determined. Also, the rotational position sensor  60  can be rotated and will give readings through 360 degrees. 
   Referring to  FIGS. 10 and 11 , in similar fashion, the present invention can be employed in a cylindrically shaped angular position sensor. In this embodiment, the angular position of a rotating shaft  90  is measured by providing a cylindrical insulating sleeve  92  around a rotor  93  on the shaft  90 . Extending across a small portion of the outer surface of the sleeve  92  is an electrically conductive plate  94 . The plate  94  is electrically connected by a connector  96  to the metal shaft  90  which in turn is connected to ground, not shown. 
   Surrounding the rotating shaft  90  is a cylindrical shaped stator  98  having a plurality of spaced electrically conductive plates  100 A,  100 B, . . .  100 N around the inner surface thereof with the surfaces of the various plates  100 A,  100 B, . . .  100 N forming segments of a cylindrical surface spaced a short distance from the cylindrical surface defined by the plate  94 . As was the case with the linear position sensor, each of the conductive plates  100 A,  100 B, . . .  100 N is connected to a sensing circuit similar to sensing circuit  56  described above. 
   As shown in  FIG. 11 , the arcuate length  102  of the rotating plate  94  is equal to the arcuate length  104  of any one of the stationery plates  100 N plus the arcuate spacing  106  between any two plates  100 A,  100 B, . . .  100 N. 
   To provide a calibrating capacitance, an annular electrically conductive ring  108  is provided on the rotor  93  axially displaced from the rotating plates  94 . Mounted on the stator  98  and radially outward of the conductive ring  108  is a second annular plate  110  with the areas of the concentric plates  108 ,  110  being substantially equal to the area of each of the conductive plates  100 A,  100 B, . . .  100 N such that the concentric electrically conductive plates  108 ,  110  form a calibrating capacitance as has been previously described. As described with respect to the linear position sensor, the calibrating capacitance can be used to determine fractions of the capacitance between the rotating plate  94  and any one of stationary plates  100 N. From a determination of the fractional capacitances, the angular position of the rotating plate  94  can be more accurately determined. 
   While the present invention has been described with respect to three embodiments, it will be appreciated that many variations and modifications may be made without departing from the true spirit and scope of the invention. It is therefore the intent of the appended claims to cover all such modifications and variations which fall within the true spirit and scope of the invention.