Abstract:
The relative position of two members is determined based upon a detected relative intensities of first and second colors of light. The intensity balance associated with relative occlusion of first and second color filter portions on one member by a mask on the other member. An exemplary use is in rotational position sensing within a limited rotation range. With a given compliance between the two members, the position sensing can be used as a proxy for torque sensing.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This patent application relates to a sensor technology which may be utilized in power-assist wheelchairs such as those shown in U.S. patent application Ser. No. 09/886,874 (the &#39;874 application) entitled “POWER ASSIST VEHICLE” that was filed on Jun. 21, 2001, the disclosure of which is incorporated by reference in its entirety herein, and claims priority to 60/361,820, file date Mar. 4, 2002. 

   BACKGROUND OF THE INVENTION 
   (1) Field of the Invention 
   This invention relates to position sensing, and more particularly to rotational position sensing within a restricted range. 
   (2) Description of the Related Art 
   Many torque sensing means exist including many optical sensors. Examples of optical sensors are given in U.S. Pat. Nos. 3,938,890, 4,502,681, 5,723,974, 5,907,106, 5,918,286, and 6,318,817. 
   BRIEF SUMMARY OF THE INVENTION 
   Relative angular position of first and second elements may be determined by detecting a balance between first and second colors of light passed from a source through a mask and a filter, one of which is substantially fixed relative to the first element and the other of which is substantially fixed relative to the second element. The balance is indicative of the relative orientation of the filter and mask and thus of the elements. The relative orientation of the elements may be indicative of a torque between the elements in view of a given compliance between the elements. 
   A key application is detecting the torque between a wheelchair handrim and its associated wheel. The sensor measures the deflection of the handrim relative to the wheel. A compliant coupling has the appropriate stiffness so the maximum angular deflection corresponds to the desired maximum torque. Most of the sensor is advantageously in the body of the chair, with minimal elements on the wheel/handrim. The sensor may allow the drive and sensing systems to be substantially independent. The sensor permits a small deflection or torque to be measured with no electrical connection between the body of the chair and the rotating wheel. By using an optical method to sense and transmit the data, the sensor may avoid the problems found in other non-contact methods including slip rings, radio links, inductive links, and/or capacitive links. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic longitudinal sectional view of a sensor according to principles of the invention. 
       FIG. 2  is a semi-schematic open end view of a light source and photo detector cup of the sensor of FIG.  1 . 
       FIGS. 3A &amp; B  are respective plan views of the shadow mask and the color wheel of the sensor of FIG.  1 . 
       FIGS. 4A-C  respectively show the superimposed mask and wheel with the wheel turned fully to the left, centered, and fully to the right. 
   

   Like reference numbers and designations in the various drawings indicate like elements. 
   DETAILED DESCRIPTION 
     FIG. 1  shows a wheelchair drive wheel  20  and coaxial handrim  22 . The wheel and handrim have concentric inner and outer hub tube portions  30  and  32  having a common axis  500 . The wheel hub tube  30  is carried within a main drive tube  34 . The drive tube  34  is held for rotation about the axis  500  relative to the body of the wheelchair (e.g., relative to the structural components  36  of a transmission unit) via bearings  38 . The drive tube is coupled to a motor (not shown) to drive the wheel about the axis  500 . Various non-illustrated details of the wheelchair, including control electronics and algorithms, may be similar to those disclosed in the &#39;874 application. A quick release mechanism (not shown) permits the wheel and handrim assembly to be disengaged from the transmission unit via extraction of the wheel hub tube  30  from the drive tube  34 . The wheel and handrim are rotationally coupled via a compliance  40  (e.g., an elastomer or a coil spring). Upon relative rotation of the handrim and wheel from an initial neutral orientation, the compliance produces an increasing resistive torque at least up until a substantially hard stop at each end of a restricted range of angular motion. An exemplary range is +/−1° from the neutral orientation. An exemplary peak measurable torque magnitude is 10-20 lb-ft at the hard stop. At its inboard end, the tube  30  carries a shadow mask  44  (described below). 
   At its own inboard end, the handrim hub tube  32  carries a color wheel  46  in close facing proximity to or contacting the shadow mask  44 . Behind (outboard of) the color wheel  46 , the handrim hub tube  32  carries a radially symmetric corner reflector  48  having an inboard-facing reflective surface. In a preferred embodiment, the reflector  48  is mounted to the outboard surface of the color wheel. The color wheel and shadow mask are advantageously as close as possible to minimize distortion without quite contacting each other so as to control wear and damage. 
   Inboard of the shadow mask  44 , the transmission unit carries a light source and sensor assembly  50 . The exemplary assembly  50  comprises a cup-shaped housing  52  having outer and inner walls  54  and  56 , respectively. Between the walls, the housing  52  carries a light source in the form of a plurality of green and red light emitting diodes (LEDs)  58  and  60 . Within the wall  56  the housing carries a light sensor or photo detector in the form of a plurality of photodiodes  62 . Shown in further detail in  FIG. 2 , the photodiodes  62  include photodiodes  62 A having a preferential sensitivity for green light and photodiodes  62 B having a preferential sensitivity for red light. The preferential sensitivity may be achieved via use photodiodes having particular chemistries sensitive to such light or via use of neutral photodiodes having filters which substantially filter all but green and red light, respectively. The LEDs are shielded from the photodiodes by the wall  56  acting as a baffle. LEDs make excellent sources for this application as they are monochromatic sources, are stable for a long time, and run cool (e.g., they draw about 75 milliwatts). 
     FIGS. 3A &amp; B  respectively show the shadow mask and the color wheel. The shadow mask has plurality of portions  70  substantially opaque to light in general (or at least to the two selected colors (red and green in the example)) and another plurality of portions  72  substantially transparent (or translucent) thereto. The exemplary portions are formed as alternating radially-extending sectors of a disk, each sector being of substantially identical circumferential span. Similarly, the color wheel  46  includes translucent or transparent portions,  74  and  76  which, respectively pass green and red light, filtering out other colors. For illustration the mask and wheel are shown with 5° segments. A preferred wheelchair encoder has 2° segments providing a 2° range (+/−1° from a center or neutral point) as described below. 
     FIG. 4B  shows the shadow mask and color wheel in superposition in a neutral orientation. In this orientation, the respective interfaces between the sectors of the two are offset from each other by half the angular pitch of the sectors. In this orientation, each opaque sector  70  occludes one half of each of an adjacent pair of green and red sectors  74  and  76  while each transparent sector  72  permits passage of light from one half of each of an adjacent pair of sectors  74  and  76 . If substantially equal amounts of green and red light are projected through the superposed combination, and the wheel viewed in a sufficiently defocused manner, the net result is a perceived yellow color. If, from this neutral orientation, the wheel and mask are turned relative to each other by half of the pitch, the opaque sectors  70  will occlude only one group of the green or red sectors  74  and  76  while the transparent sectors  72  will pass light only from the other. Thus in  FIG. 4A  the relative rotation of is such that only red light may pass through the combination while in  FIG. 4C  it is rotated by the same amount in the opposite direction so that only green light may pass. 
   In operation, the color wheel rotates with the handrim and the shadow mask rotates with the drivewheel. Light from red and green LEDs initially travels outboard where it encounters the shadow mask. Both colors will pass through the portions  72  and be blocked by the portions  70  providing an initial 50% reduction in total passed light of the two colors. Passed light then encounters the portions  74  and  76 , the respective colors of light passing only through their respective portions  74  and  76  to the extent such portions are not occluded. Depending on the angular displacement of the red/green color wheel relative to the shadow mask, the balance of red/green filtering changes. Thus the net proportion or balance of the two colors passed through the color wheel will depend on the relative displacement. After passing through the color wheel, the light is reflected by an outer portion of the reflector radially inward and then by a central portion back inboard. The reflected light then passes again through the color wheel and mask. Since there is the same degree of occlusion, this second pass does not affect the color balance although it does reduce the intensity. The light is then detected by the photodiodes  62 , the balance of output of the green photodiodes  62 A and red photodiodes  62 B being indicative of the balance of light passed and, therefore, the direction and degree of rotation. To the eye, the color would change from red, to red-yellow, to yellow, to yellow-green, to green as the handrim moved from −1 degree to 1 degree. In view of the known compliance (whether linear or otherwise) the photodiodes&#39; output may thus be used to determine the torque between the handrim and wheel. 
   The returned amount of light will be small. The photodiode outputs will be small so the amplifiers (not shown) will likely require very high gain. Electronics (not shown) are advantageously mounted with the sensor cup to reduce noise. Some type of optical gain (inexpensive lenses) could reduce the needed electrical gain. 
   The sensor should be very immune from electrical noise, and should radiate no EMI. 
   The system may have an analog output, appearing the same as the output from an instrumentation amplifier as used with a strain gauge, or could have a digital output. 
   The shadow mask and the color wheel can rotate as a unit relative to the wheelchair without changing the relative proportions of red and green light transmitted. If the intensity of the light source (but not its spectrum) changes, the red/green ratio does not change. The system would be fairly insensitive to changes in the transmission of light, so if the boundary between the drive unit and the wheel got a little dirty, the estimate of deflection would not change. 
   The entire sensing assembly can advantageously be removed from the drive (e.g., integrated motor/transmission unit) without opening the drive. 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, modifications to the color wheel and shadow mask may be made. For example, one of the two passes through the portions  70 ,  72 ,  74 , and  76  might be eliminated such as by providing apertures or additional transparent areas in the shadow mask and color wheel. This might help increase sensitivity. One could use other complementary colors, the receiver could be on the outside with the source on the inside, and one could also use a white light source. The drive wheel mounting arrangement may be such that a shaft protrudes from the gearbox onto which the wheel is placed, or that a shaft protrudes from the wheel and is inserted into the gearbox. One or more unfiltered photodiodes (sensitive to both color components) may be used by alternately illuminating the red and green LED&#39;s and measuring the photodiode output synchronously at very high speed (e.g., several kHz or more). Accordingly, other embodiments are within the scope of the following claims.