Abstract:
Three-dimensional characterization wherein an object interacts capacitively with a resistive medium and the object&#39;s orientation, mass distribution and/or distance from the medium is characterized by electrodes distributed linearly around the medium&#39;s perimeter. Thus, three-dimensional characteristics are projected into two dimensions and sensed along a single dimension.

Description:
RELATED APPLICATION 
     This application claims the benefits of and priority to U.S. Provisional Patent Application No. 60/418,670, filed on Oct. 15, 2002, the entire disclosure of which is hereby incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under Contract No. CCR-0122419 awarded by the National Science Foundation. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the sensing of orientation, position and distribution of mass within a defined space, and in particular to a sensing system wherein a two-dimensional impedance distribution is characterized by electrodes distributed linearly around the perimeter of a resistive medium. 
     BACKGROUND OF THE INVENTION 
     Position sensors are used to provide inputs for a variety of electronic devices. Some of these sensors are electromechanical devices, such as the ubiquitous “mouse” that is used to provide position input signals to digital computers. Other sensors, which are non-mechanical, usually make use of electrostatic or magnetic fields to provide position information. An example of an electrostatic sensor is a capacitive button switch, which is actuated when the user places a finger thereon; in so doing the user effectively increases the capacitance of a capacitor, with the resulting increase in capacitive current being sensed to indicate actuation of the button. 
     Non-mechanical sensors are advantageous in that they have no moving parts and moreover are, in theory at least, not restricted to operation over a small area such as a mousepad or the like. Actually, however, because of configuration and sensitivity considerations, these sensors are limited to a small area; indeed, when they are used as “pushbuttons,” this is a desirable attribute of capacitive sensors. 
     Electromechanical sensors are limited by their construction to detection of specific types of user movements. For example, a mouse can detect position along a two-dimensional surface and transmit the user&#39;s actuation of “click” buttons mounted on the mouse; three-dimensional location and gestures other than the familiar button click, however, are beyond the mouse&#39;s capacity to detect. The prior electrostatic and magnetic sensors suffer from the same disabilities. 
     In fact, determining the position, mass distribution or orientation of an object within a defined space represents a highly complex problem. Solutions have been proposed for free space measurements; see, e.g., U.S. Pat. Nos. 5,844,415 and 6,066,954. However, these solutions require electrodes arranged throughout the space of interest. This may not be practical in all applications. For example, it may be inconvenient to distribute electrodes in spaces the size of a room, or the necessary locations may be physically inaccessible or render the electrodes susceptible to damage. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an object interacts capacitively with a resistive medium and the object&#39;s orientation, mass distribution and/or distance from the medium is characterized by electrodes distributed linearly around the medium&#39;s perimeter. As a result, three-dimensional characteristics are projected into two dimensions and sensed along a single dimension. Accordingly, the electrodes are conveniently placed within a wide variety of environments and contexts, and the approach scales quite well. It is found that a large number of electrodes is not necessary to determine the centroid of the mass with precision. The ability to characterize the distribution (e.g., the shape) of the mass, however, increases as electrodes are added. 
     In one aspect, the invention comprises a method of characterizing a three-dimensional position and/or a size of one or more electrically conductive masses within a defined space. In accordance with the method, a resistive medium (e.g., in the form of a sheet) is disposed such that a surface thereof is proximate to the space. A series of spaced-apart electrodes is connected to the resistive medium along the periphery thereof, and an AC signal is sent through at least some of the electrodes into the resistive medium, thereby capacitively coupling the mass or masses to the resistive medium. The result is creation of a charge distribution, affected by the at least one mass, in the medium. In one embodiment, the size and/or the position of the mass or masses relative to the surface is inferred based on this charge distribution. In another embodiment, a voltage difference is measured between an electrode through which an AC signal is sent and an electrode through which the AC signal is not sent. Based at least on this difference and knowledge of electrode position, a two-dimensional location of a centroid of the mass or masses relative to the resistive medium is inferred. Obviously, these two embodiments can be combined in a single system. 
     In another aspect, the invention comprises an apparatus for characterizing a three-dimensional position and/or a size of one or more electrically conductive masses within a defined space. The apparatus comprises a resistive medium having a surface locatable proximate to the space, a series of spaced-apart electrodes connected to the resistive medium along its periphery, an AC source, and circuitry for performing operations leading to the desired characterization. In one embodiment, the apparatus comprises control circuitry for causing the AC source to send an AC signal through at least some of the electrodes and into the resistive medium, thereby capacitively coupling the mass or masses to the resistive medium and creating a charge distribution, affected by the at least one mass, in the medium, as well as circuitry for sensing the charge distribution and, based thereon, inferring at least one of the size and the position of the at least one mass relative to the surface. In another embodiment, the apparatus comprises circuitry for measuring a voltage difference between an electrode through which an AC signal is sent and an electrode through which the AC signal is not sent, and circuitry for inferring a two-dimensional location of a centroid of the mass or masses relative to the resistive medium based at least on locations of the electrodes and the voltage difference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing discussion will be understood more readily form the following detailed description of the invention, when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  schematically illustrates a general case of two-dimensional projective measurement. 
         FIG. 2  schematically illustrates a two-electrode sensing arrangement for measuring position along a one-dimensional resistive element. 
         FIG. 3  is a schematic view of a four-electrode measurement system. 
         FIG. 4  shows the current density and equipotentials in a resistive medium with two active electrodes. 
         FIGS. 5A and 5B  topographically depict the charge distribution the resistive medium due to the presence of masses of different sizes and/or distances from the resistive medium. 
         FIGS. 6A and 6B  illustrate the manner in which different sets of electrodes may be activated in accordance with the invention. 
         FIG. 7  is a schematic diagram of switch logic that may be used to alter the modes in which the various electrodes operate. 
         FIG. 8  illustrates the general principle of a single projective measurement using two current paths. 
         FIG. 9  schematically illustrates a circuit implementing one branch of the measurement bridge shown in FIG.  8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , a simple position sensor  10  is arranged to sense a characteristic of an electrically conductive mass  12  within a defined space using a resistive medium  14  disposed proximate to the space. For example, the resistive medium  14  may be a surface bounding one side of the space, e.g., the underside of a table or the wall of a room. The resistive medium  14  may be composed of, for example, carbon-loaded plastic. 
     As illustrated, the position sensor  10  includes a representative series of electrodes, E 1 , E 2 , E 3 , physically or otherwise connected to the medium  14  along one periphery  16  thereof, and a series of electrodes, E 1 ′, E 2 ′, E 3 ′, along a second periphery  16 ′ opposed to the periphery  16 . For present purposes, electrodes E 1 , E 2 , E 3  are “sending” electrodes that receive current from a power source and inject it into the medium  14 , and electrodes E 1 ′, E 2 ′, E 3 ′, are “receiving” electrodes that are used to sense current and voltage. An AC signal is applied to the sending electrodes E 1 , E 2 , E 3 . As a result, the conductive mass  12  is capacitively coupled to the resistive medium  14 . As shown  FIG. 1 , the conductive mass  12  can be modeled as a resistive element capacitively coupled to ground (with the quality of the connection to ground determining the impedance of the capacitor  20 ). Each electrode E 1 , E 2 , E 3 , and E 1 ′, E 2 ′, E 3 ′, is effectively connected to the grounded mass  12  by a corresponding implicit resistive element R 1 , R 2 , R 3 , and R 1 ′, R 2 ′, R 3 ′. The values of the various resistors, which represent the portions of the resistive medium  14  through which current travels to and from the sending and receiving electrodes via mass  12 , naturally depend on the respective distances of the mass  12  from the electrodes. 
       FIG. 2  is a completely schematic depiction of the system  10  shown in  FIG. 1 , but for simplicity considered only for a single sending electrode E 1  and a single receiving electrode E 1 ′. The resistances R 1  and R 1 ′ are in series, and their relative magnitudes depend on the position of the mass  12  relative to the electrodes E 1 , E 1 ′. The effect is equivalent to current flowing through a potentiometer with a grounded wiping contact, with the voltage at each endpoint (i.e., electrode), V 1 , and V 2 , determined by its distance from the wiper. The quantity R src  represents the internal impedance of the power source  25 . This quantity is large relative to the resistances R 1  and R 1 ′ so that the power source  25  effectively behaves as a current source. 
     The voltage, V C , across capacitor  20  and the capacitance, C, of the capacitor  20 , may be determined from the knowledge of the frequency ω of the power source  25  and the total current, I, injected into the resistive medium  14 , given that: 
             I   =         I   1     +     I   2       =           2   ⁢     V   src       -     V   1     -     V   2         R   src       =       V   C     ⁢   ω   ⁢           ⁢   C                 (   1   )             
 
Therefore it is possible to obtain R 1  and R 2  in terms of observables as 
               R   1     =         V   1     -     V   C         I   1               (   2   )                 R   2     =         V   2     -     V   C         I   2               (   3   )               C   =         2   ⁢     V   src       -     V   1     -     V   2         ω   ⁢           ⁢     R   src     ⁢     V   C                 (   4   )             
 
     Accordingly, the position of the mass  12  (or, more accurately, the centroid of the mass  12 ) as between electrodes E 1  and E 1 ′ may be determined by the relative values of resistances R 1 , R 1 ′ (as indicated by the voltages at electrodes E 1 , E 1 ′). 
     In more realistic systems, the electrodes are distributed evenly around the periphery of the resistive medium  14 ; thus, as shown in  FIG. 3  for a rectangular medium, one of a series of electrodes  30 ,  32 ,  34  and  36  is connected to each edge of the resistive medium  14 . It should be understood, however, that the number of electrodes can be increased to enhance system resolution, as described below. As a result, the electrodes are arranged in a linear path around the resistive medium  14 . 
     In  FIG. 3 , the power source  25 , having a given frequency and impedance R src , is shown driving each side of a resistive divider  40  as described above and defined by the electrodes  32 ,  36  into a capacitive load  20  (representing the mass  12 ). (Connection to the electrodes  30 ,  34  is discussed below.) The voltages V 1 , V 2  developed at the ends of the resistive element (i.e., at the electrodes  32 ,  36 ) is proportional to the resistance between the corresponding end of the resistive divider and the contact capacitance of the capacitive load  20 , providing a proportional measurement of the capacitive contact position of the mass. The quantity R 2 −R 1  is sensed by an amplifier arrangement  45 . The inverting terminal of an operational amplifier  47  is connected to, e.g., the electrode  36  and the other electrode (i.e., electrode  32 ) is connected to the non-inverting amplifier terminal. A resistor  50  bridges the non-inverting input terminal and the output terminal. A leakage resistor  52  precedes the inverting input terminal of the operational amplifier  47 . The voltages at the electrodes  32 ,  36  are proportional to the resistances R 1 , R 2 , so the output of the amplifier  47  reflects the magnitude of this difference and may be used to determine the values of R 1  and R 2 . Accordingly, the accuracy with which the centroid of the mass  12  can be localized as between two opposed electrodes depends primarily on the accuracy with which the voltage difference between the electrodes can be measured. The number of electrodes around the perimeter of the medium  14  is less critical, as long as that number is adequate for the shape of the medium. For example, with even a single electrode on each side of a square medium  14 , the centroid of the mass  12  can be localized with reasonable accuracy given sufficient precision in making voltage measurements. 
     The behavior of the circuit shown in  FIG. 3  is illustrated in FIG.  4 . In the absence of the mass  12 , charge in the homogeneous resistive medium would be distributed symmetrically with concentrations at the electrodes  32 ,  36 , and voltages at these electrodes would be equal. Capacitive coupling of the mass  12  disrupts this symmetry (i.e., it alters the impedance distribution of the system, which in turn dictates the charge distribution). The degree to which charge is drawn to the region of the mass  12  depends on the quality of its connection to ground, its size and its distance (if any) from the resistive medium  14 . 
     The asymmetry in voltage between the electrodes  32 ,  36  is a direct measure of the position of the capacitively coupled grounding load (i.e., the mass  12 ). If the load were centered on the sheet, then similar voltages would develop at the electrodes  32 ,  36  (e.g., 4.0 V and −4.0 V), while if the load were at the left edge of the medium  14 , one would expect to more highly skewed voltage measurements (e.g., −8 V and 0 V, respectively, at the electrodes  32 ,  36 ). In other words, the current flowing through the medium  14  establishes a potential gradient (indicated by the solid lines representing isopotential contours) while the mass  12  provides a movable ground reference that determines the voltages that develop at the electrodes  32 ,  36 . 
       FIGS. 5A and 5B  topographically depict the charge distribution in the resistive medium  14  for two different cases. For a given mass, as the distance between the mass  12  and the resistive medium  14  increases, the charge distribution becomes more diffuse and its concentration in the region of the mass  12  less pronounced as shown in FIG.  5 A. Alternatively, as depicted in  FIG. 5B , when the distance between the mass  12  and the resistive medium  14  decreases, the charge distribution narrows and its concentration in the region of the mass  12  becomes more pronounced. Similarly, a large but poorly coupled mass may produce the charge distribution shown in  FIG. 5A  even if proximate to the resistive medium  14 , while a small, well-coupled mass may produce the charge distribution shown in  FIG. 5B  even if relatively distant from the resistive medium  14 . Thus, the distance from the mass  12  to the resistive medium  14  may be estimated if the size of mass  12  is known, or the size of the mass  12  may be estimated if its distance from the resistive medium is known. 
     Unlike localizing the centroid of the mass  12 , the ability to characterize the charge distribution depends strongly on the number of peripheral electrodes surrounding the resistive medium  14 . The more electrodes that are employed, the greater will be the resolution with which the charge distribution can be characterized. In order to maximize the resolution for a given number of electrodes, measurements can be obtained sequentially using different sets of electrodes. 
     In  FIG. 6A  an AC source  60  is connected to the resistive medium  14  via the left-side electrodes  36   1 ,  36   2 ,  36   3 ,  36   4  and the right-side electrodes  32   1 ,  32   2 ,  32   3 ,  32   4 . Current-sensing devices generically indicated at  61 , connected individually to each of the electrodes  32 ,  36 , measure current through these electrodes. These measurements provide information used to characterize the impedance distribution in the medium  14 . (Not shown in  FIGS. 6A and 6B  is the capacitively coupled mass that causes the charge distribution to become asymmetric.) 
       FIG. 6B  illustrates that the AC source  60  may then be connected to the upper electrodes  30   1 ,  30   2 ,  30   3 ,  30   4  and the lower electrodes  34   1 ,  34   2 ,  34   3 ,  34   4 . Measurements of current through these electrodes further contribute to an accurate characterization of the impedance distribution in the medium  14 . It should be emphasized that although it is natural to simultaneously activate all electrodes on opposed sides of a rectangular medium as illustrated, in fact the sets of electrodes activated at any one time may depend on various factors, including the desired resolution and the shape of the medium  14 . 
       FIG. 7  depicts representative switch logic used to select the various electrodes that are connected to the power source and to measure the voltage on (to determine the centroid of the mass  14 ) and the current through (to characterize the charge distribution) each active electrode. The circuit includes the AC source  60  and a switch matrix  64  under the control of a computer  70  including a memory  72  and a processor  74 . The memory unit  72  of the computer  70  stores both data and executable programming instructions. In the simplest approach, these instructions cause the processor  74  to operate the switch matrix  64  to sequentially couple different ones of the electrodes to the AC source  60  and other electrodes to voltage and current measurement circuitry generically indicated at  76 . The measurement circuitry may be in the form of hardware (as discussed below) or, instead, the electrodes may be connected directly to the processor  74  via a multiple-port analog-to-digital converter. In the latter case, the processor is programmed to measure the sensed current and voltage levels. In any case, the measurements are stored in a memory  72  and analyzed to characterize the charge distribution. 
       FIG. 8  illustrates another approach to measuring comparative voltage levels V 1 , V 2  between two electrodes. This approach scales well and may be implemented according to the architecture shown in FIG.  7 . As shown in  FIG. 8 , an analog bridge measurement circuit  90  uses comparators to obtain a time-domain measurement of the current flowing in each arm of the bridge. At time t=0, a voltage step V 0  (i.e., V step =V 0 ) is applied to the top of the bridge. The current flowing down one arm of the bridge will depend on the resistance in that arm and the voltage difference across the arm, i.e. I(t)=(V 0 −V C (t))/(R+R 1 ), where V C (t) is the voltage developed across the load capacitance, R is the source impedance, and R 1  is the variable resistance corresponding to the distance between the source electrode and the capacitive load. V C (t) is proportional to I(t).
   V   C ( t )= V   0 −( R+R   1 ) I ( t )  (6) 
and that it follows an exponential characteristic
   V   C ( t )= V   0 (1− e   t/R     par     C )  (7) 
where R par  is the total parallel resistance feeding C. Once again, R&gt;&gt;R 1  and R&gt;&gt;R 2 , so R par ≈R/2, so V C (t) is relatively insensitive to R 1  and R 2 . However, the comparators in  FIG. 8  measure
 
  V   1   =V   C ( t )+ R   1   I ( t )
 and   V   2   =V   C ( t )+ R   2   I ( t )  (8) 
against some reference voltage V ref . The comparators will trigger at different times that depend on R 1  and R 2 . Comparing these times t 1  and t 2  facilitates computation of R 1  and R 2 , and thus the position of the capacitive load along the axis defined by the two electrodes.
 
     To utilize this approach in the context of the present invention, each arm of the bridge shown in  FIG. 8  may be implemented in the form of the the circuit  95  in FIG.  9 . The circuit  95  includes the explicit source impedance R as well as the implicit load C and sheet resistance R n . The comparator is replaced by a Schmitt trigger  97 , the output of which is disabled when the electrode is not active. This boundary-scan circuit  95  is repeated at each electrode around the perimeter of the resistive medium, and all of the circuits are wired in series. A serial bit stream is provided at one end to the input line SDI by the computer  70  (see  FIG. 7 ) and passes through an upper set of flip-flops  100  and thereafter to subsequent circuits  95  via the output line SDO. Three global control signals SCK (serial data clock), SLD (serial data latch) and DRV (drive active electrodes) are provided to sequence the measurement operation, while the time-domain measurement is returned from the active measuring electrode as a logic edge on the global return bus THR (threshold). The SLD line and the outputs of the upper flip-flops  100  serve as inputs to a lower set  102  of flip-flops. 
     In operation, the computer  70  feeds serial data into the SDI line (clocked by SCK) to configure each circuit  95  in the chain. Once all data have been fed into the chain formed by the linked upper sets  100  of flip-flops, the data are latched into the lower sets  102  of flip-flops by a pulse on the SLD line. The charging time measurement is then initiated by driving the DRV line high and awaiting a rising (or falling) edge on THR. Thus, to implement the time-domain bridge measurement, two electrodes are configured to drive their outputs high at the given signal, and one of these two drives the THR output of its circuit  95 . This obtains the time-domain measurement for one arm of the bridge. The measurement is then repeated with the THR output enabled for the other electrode. 
     Having shown the preferred embodiments, one skilled in the art will realize that many variations are possible within the scope and spirit of the claimed invention. It is therefore the intention to limit the invention only by the scope of the claims.