Patent Publication Number: US-6222522-B1

Title: Baton and X, Y, Z, position sensor

Description:
The present invention relates generally to man-machine interfaces and particularly to methods and systems for sensing the motion of a stylus, pointer, drum stick or baton over a surface. 
     BACKGROUND OF THE INVENTION 
     The present invention is a motion and position sensor that can be used as an electronic drum, much like an electronic keyboard is used with a musical synthesizer. The present invention can also be used for inputting one-dimensional, two-dimensional or three-dimensional data points into a computer or any other system. 
     The present invention uses a capacitive two-dimensional tablet as a position digitizer. Examples of other positional digitizers and capacitive two dimensional tablets are described in U.S. Pat. Nos. 4,705,919 (Dhawan), U.S. Pat. No. 3,999,012 (Dym), U.S. Pat. No. 4,087,625 (Dym et al.), and U.S. Pat. No. 4,659,874 (Landmeier). An earlier patent by the present inventor concerning a capacitive, two-dimensional tablet and position digitizer is U.S. Pat. No. 4,980,519 (Mathews), which is hereby incorporated by reference in its entirety as background information. 
     Objects of the present invention include providing improved uniformity in the accuracy over the X-Y-Z space in which a positioning member is moved, providing an antenna that allows for a position to be efficiently determined based on measured capacitances, and providing an antenna (i.e., capacitive tablet) that is readily fabricated on a two sided printed circuit board. 
     Another object of the present invention is to accurately detect the X, Y position of one or more batons or styluses, without regard to the strength of the signal emitted by the batons or styluses. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention is a radio signal actuated electronic position sensor which operates in conjunction with one or more batons. Each baton has a transmitter that transmits a distinct radio frequency signal at a position in space. The position sensor determines the current position of each baton transmitter in terms of X, Y, and Z coordinates. 
     To determine the position of each baton transmitter, the position sensor includes a tablet having a flat support member at a predefined position, with at least two pairs of electrodes coupled to the flat support member. Each of the electrodes is a separate antenna. A first pair of the electrodes is shaped so that the amount of capacitive coupling between each baton transmitter and the first pair of electrodes corresponds to the position of the baton transmitter with respect to the X axis. A second pair of the electrodes is shaped so that the amount of capacitive coupling between each baton transmitter and the second pair of electrodes corresponds to the position of the baton transmitter with respect to the Y axis. 
     In one pair of antenna electrodes, each of the electrodes is comprised of a plurality of wedge shaped regions. The width of each of the regions of one electrode decreases linearly in the X direction and the width of each region of the second electrode increases linearly in the X direction. The width of each of the regions of an electrode in this pair is the same for any given Y position. Thus as a positioning member moves across the surface of the sensor in the Y direction, the width of each of the regions of an electrode in this pair is the same. At X positions other than the X-dimension midpoint of the sensor, the width of the second electrode regions is different from the width of the first electrode regions. 
     The second pair of antenna electrodes are comprised of narrow rectangular electrode regions. The widths of these electrode regions are constant in the X direction. The widths of the electrode regions of a first antenna in the pair increase linearly with increasing Y, and the widths of electrode regions of the second antenna in the pair decrease linearly with increasing Y. In a preferred embodiment, the regions forming the first pair of antenna electrodes are interleaved with the regions forming the second pair of antenna electrodes. 
     Capacitance is a linear function of the area of an electrode. The ratio of the areas of the antenna electrodes of the first pair of electrodes depends linearly on the X position of a baton transmitter, and is independent of the Y position. The ratio of the local areas of the second pair of antenna electrodes depends linearly on the Y position of the baton transmitter, and is independent of the X position. Thus by measuring the capacitance values between the baton and the four antennae, the X, Y position of the baton can be computed with uniform accuracy anywhere in the X-Y plane. 
     The position sensor also has a CPU which receives signals from the electrodes, and uses those signals to compute the X, Y and Z coordinates of each baton. The Z position of the baton transmitter is inversely proportional to the total capacitive coupling between the baton and either pair of electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which: 
     FIG. 1 is a block diagram of a system incorporating the present invention. 
     FIG. 1A illustrates a cross-sectional view of a capacitive tablet. 
     FIG. 1B is a detailed block diagram of a system incorporating the present invention. 
     FIG. 2 illustrates a printed circuit board metalization pattern that forms the capacitive electrodes of the receiver antenna used in a preferred embodiment of the present invention. 
     FIG. 3 illustrates a printed circuit board metalization pattern that forms the ground layer for the receiver antenna according to a preferred embodiment of the present invention. 
     FIG. 4 illustrates a radio frequency amplifier. 
     FIG. 5 illustrates an oscillator and radio frequency signal detector. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, there is shown a motion and position sensing system  100 , which includes batons  102  and  104 , a position sensor  110 , and receiver electronics  130 . The sensor  110  detects the (X,Y,Z) position of each of the batons relative to its top surface  112 , and receiver electronics  130  generates corresponding position signals on signal line  114 . 
     In the preferred embodiment, the position sensing system  100  is used in conjunction with a music synthesizer or computer  120 , and the position signals transmitted over line  114  to the synthesizer are used to control various aspects of the synthesizer&#39;s operation. In a simple application, where the sensing system is used as a variable pitch drum, the X position of the baton  102  can be used to control the pitch of the drum beats generated by the synthesizer, the Y position of the baton  102  can be used to control the timbre of the drum, the velocity of the baton when it hits the surface  112  can be used to control the volume (also known as velocity) of the drum beats. 
     In other applications of the present invention, the apparatus  120  which receives the signals output by the sensing system  100  may be a computer. 
     The position sensor  110  has a housing  122 , which holds a sensing tablet  124  near its upper surface  112 , and the receiver electronics  130 . Referring to FIG. 1A, the sensing tablet  124  has four separate antennae formed on a conductive layer  132 . The conductive layer  132  and a ground layer  134  are separated by an thin insulator layer  136 , typically about a sixteen of an inch (0.16 cm) thick. In a preferred embodiment, the antennae (shown in FIG. 2) are thin copper patterns that are formed on a printed circuit board base using standard etching techniques, similar to those used to form the conductive patterns on printed circuit boards. 
     At the end of the baton is a small electrode which functions as a radio frequency transmitter antenna  142 . The amount of capacitive coupling between a baton transmitter antenna  142  and each of the four antennae in the sensor  110  is measured by measuring the strength of the RF signal received by each of the four antennae at the frequency which corresponds to that baton. 
     Referring now to FIG. 1B, and as described above, the sensor  110  contains four antennae  151 ,  152 ,  153  and  154 . The radio frequency signals received by each of these four antennae, via capacitive coupling between the batons and the antennae, are carried on lines  161 ,  162 ,  163  and  164  to a set of four RF amplifiers  168 , which generate four amplified antennae signals on lines  171 ,  172 ,  173  and  174 . 
     For each baton there is an oscillator and detector circuit  182  (for baton  102 ),  184  (for baton  104 ) which generates an RF signal at a distinct RF frequency, and which also detects the strength of the four antennae signals on lines  171 - 174  at that RF frequency. The detector  182  generates four dc voltage signals on lines  192  which correspond to the strengths of the four antennae signals at the RF frequency transmitted by baton  102 . Similarly, detector  184  generates four dc voltage signals on lines  194  which correspond to the strengths of the four antennae signals at the RF frequency transmitted by baton  104 . 
     The DC voltage signals which represent the strengths of the antennae signals from each of the baton are then read into a small digital computer or CPU  200  via an analog to digital converter  20  (ADC)  202 , which sequentially converts each of the voltage signals into a digitally encoded value. All eight voltage signals are read by the CPU  200  on a periodic basis (e.g., one thousand times per second) and then the CPU, under the control of software  204 , uses these received values to compute the X, Y, and Z coordinates of each of the batons  102  and  104 . These values are then converted into MIDI control parameters which are transmitted to a standard music synthesizer, such as the Yamaha DX 7 , which then responds to those MIDI control parameters by generating corresponding musical sounds. Alternately, they may be sent to a computer, which after processing the signals from the baton, sends commands to a music synthesizer. 
     In the preferred embodiment we use parameters X1, Y1 and Z1 to denote the (X,Y,Z) position of the transmitter at the end of the first baton  102 , and we use X2, Y2 and Z2 to denote the position of the transmitter at the end of the second baton  104 . Given these six parameters, the X1 and Y1 parameters may be used to control the frequency and volume of a first MIDI channel, and X2 and Y2 may be used to control the frequency and volume of a second MIDI channel. A MIDI event, comprising a note or drumbeat, is then played on each MIDI channel whenever the corresponding Z value reaches a predefined threshold value (e.g., Z=0), which corresponds to the baton hitting the surface  112  of the sensor  110 . 
     U.S. Pat. No. 4,980,519, incorporated by reference above, describes a technique for accurately determining the time at which a baton hits the surface of the capacitive sensor and for determining the velocity of the baton at the time of impact. 
     FIG. 2 illustrates three portions of the pattern  150  printed on the top surface of a printed circuit board, with electrode regions forming the four antennae  151 - 154  of sensor  110 . 
     The first pair of antennae  151 ,  152  (also sometimes called electrodes) are formed from a plurality of pairs of wedge shaped regions  151 A- 151 E and  152 A- 152 E, and the second pair of antennae  153 ,  154  are formed from a plurality of pairs of rectangular shaped regions  153 A- 153 E and  154 A- 154 E. As shown, the pairs of rectangular shaped regions are interleaved with the plurality of pairs of wedge shaped regions. 
     The size of the antenna regions relative to the printed circuit board have been enlarged for clarity. Antenna  151  is comprised of a plurality of wedge shaped regions  151 A- 151 E. For simplicity, only five regions have been shown, in one embodiment each antenna is comprised of approximately twenty-five regions. Similarly, antenna  152  is comprised of a plurality wedge shaped regions  152 A- 152 E. Each of the electrode regions comprising antenna  151  are coupled together using vias to a second layer of the printed circuit board by trace  161  illustrated in FIG.  3 . Each of the electrode regions comprising antenna  152  are coupled together by trace  162 . The width of each antenna region  151 A- 151 E decreases linearly in the X direction and the width of each antenna region  152 A- 152 E increases linearly in the X direction. The width of each of the regions  151 A- 151 E at a given X position is the same for any given Y position. This is also true for regions  152 A- 152 E. Thus as a positioning member moves across the surface of the sensor in the Y direction, the width of each of the regions  151 A- 151 E is the same. Similarly, the width of each of the regions  152 A- 152 E is the same. However, at X positions other than the center of the sensor, the width of regions  151 A- 151 E is different from the width of regions  152 A- 152 E. In other words, the widths of the antenna electrode regions  151 A- 151 E and  152 A- 152 E do not vary in the Y direction. 
     Antennae  153  and  154  are comprised of narrow rectangular electrode regions  153 A- 153 E, and electrode regions  154 A- 154 E respectively. The widths of these electrode regions are constant in the X direction. The widths of antenna electrode regions  153 A- 153 E decrease linearly with increasing Y, and the widths of electrode regions  154 A- 154 E increase linearly with increasing Y. 
     As indicated above, FIG. 2 is not drawn to scale. In a preferred embodiment, the combined width of each pair of electrode regions  153   x ,  154   x , including the space occupied by a thin nonconductive strip between the regions, is about 0.57 centimeters (0.225 inches). Similarly the combined width (at any and all X positions) of each pair of electrode regions  151   x ,  152   x , including the space occupied by a thin nonconductive strip between the regions, is about 0.57 centimeters (0.225 inches). The length of these regions will depend on the size of the capacitive tablet, but will typically be between fifteen and fifty centimeters, with all of the regions in a particular capacitive tablet having the same length. 
     The radio frequency signals received by each of these four antennae  151 - 154 , via capacitive coupling between the batons and the antennae, are coupled to receiver electronics  130  (FIG. 1) for processing. 
     Capacitance is measured by transmitting a low frequency signal from baton transmitter  142  and measuring the signal received at the antennae  151 - 154 . Capacitance is a linear function of the area of a capacitor&#39;s electrodes. Capacitance is also inherently a strongly non-linear function of the distance between two electrodes. Capacitance changes rapidly when the electrodes are close together and slowly when they are far apart. The ratios of the areas of the antenna electrode regions comprising antennae  151  and  152  depend linearly on the X position of baton transmitter  142 , and are independent of the Y position. The ratios of the local areas of the antenna electrode regions comprising antennae  153  and  154  depend linearly on the Y position of baton transmitter  142 , and are independent of the X position. Thus by measuring the capacitance values between baton transmitter  142  and the four antennae  151 - 154 , the X, Y position of baton transmitter  142  can be computed with uniform accuracy anywhere in the X-Y plane. To avoid granularity in the antennae output signals, the widths of the antennae electrode wedges and rectangles should be small compared with the dimension of the baton transmitter electrode  142 . This design criterion can be readily satisfied with conventional printed circuit board fabrication techniques. 
     Except for thin strips of insulation between the antennae  151 - 154 , the four antennae completely occupy the area of the sensor top surface  112 . The sum of the capacitances to all four antennae provides a measure of the Z position of baton transmitter  142 , e.g. the distance between the baton transmitter  142  and the sensor top surface  112 . This measure of Z is independent of the X and Y positions of the baton. 
     FIG. 3 illustrates a ground plane for the receiver antenna. In a preferred embodiment, the antennae  151 - 154  illustrated in FIG. 2 are formed on one side of a printed circuit board and the ground layer  160  is fabricated on the other side of the printed circuit board. Ground layer  160  consists of two large ground planes  165  and  166 , and two traces  161  and  162  between the ground planes. Trace  161  couples the electrode regions  151   x  of antenna  151 , and trace  162  couples the antenna electrode regions  152   X . 
     The equations relating capacitance measurements to the X and Y baton position, for the antenna geometry of the present invention, are straightforward and facilitate fast, efficient determinations of the baton position. The “local” capacitance between baton transmitter  142  and one of the receiver antennae  151 - 154  is proportional to the “area” of the antenna region at the particular value of X or Y. Thus, the equations relating capacitance to the X position are: 
     
       
         C1=K(1−X) 0&lt;X&lt;1 
       
     
     
       
          C2=K(X) 
       
     
     
       
         
           
             
               
                 
                   
                     
                       R 
                       X 
                     
                     = 
                     
                       
                         C1 
                         C2 
                       
                       = 
                       
                         
                           ( 
                           
                             1 
                             - 
                             X 
                           
                           ) 
                         
                         X 
                       
                     
                   
                    
                   
                       
                   
                    
                   
                     X 
                     = 
                     
                       1 
                       
                         ( 
                         
                           1 
                           + 
                           
                             R 
                             X 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
         
         
             
         
       
     
     where C1 and C2 arc the measured capacitance values between the baton transmitter  142  and the receiver antennae  151  and  152  respectively, R X  is the ratio of these capacitance values, and the width of the active region of the capacitive tablet has been normalized to one. In other words, the value X computed using the equations shown above is a value between 0 and 1 that indicates the X position of the baton as a percentage of the width of the active region of the capacitive tablet. (The width of the capacitive table can also be described as the length of the individual antennae regions on the capacitive tablet.) The position X′ of the baton, measured in unit of inches, centimeters or the like is obtained by multiplying X by the width of the active region of the capacitive tablet. 
     Similarly to compute the Y position of a baton, the following equations are used: 
     
       
         C3=K(1−Y) 0&lt;Y&lt;1 
       
     
     
       
          C4=K(Y) 
       
     
     
       
         
           
             
               
                 
                   
                     
                       R 
                       Y 
                     
                     = 
                     
                       
                         C3 
                         C4 
                       
                       = 
                       
                         
                           ( 
                           
                             1 
                             - 
                             Y 
                           
                           ) 
                         
                         Y 
                       
                     
                   
                    
                   
                       
                   
                    
                   
                     Y 
                     = 
                     
                       1 
                       
                         ( 
                         
                           1 
                           + 
                           
                             R 
                             Y 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
         
         
             
         
       
     
     where C3 and C4 are the measured capacitance values between the baton transmitter  142  and the receiver antennae  153  and  154  respectively and the length of the active region of the capacitive tablet has been normalized to one. The value Y computed using the equations shown above is a value between 0 and 1 that indicates the Y position of the baton as a percentage of the length of the active region of the capacitive tablet. The position Y′ of the baton, measured in unit of inches, centimeters or the like is obtained by multiplying Y by the length of the active region of the capacitive tablet. 
     The Z coordinate can also be calculated. Z is a monotonic decreasing function of the sum of the signals from all four antennae. Although this function is strongly nonlinear, it can easily be represented as a table lookup function in the CPU&#39;s baton signal processing software  204 . The accuracy of the computed Z coordinate is dependent on knowledge of the signal strength of the signal transmitted by the baton, whereas the accuracy of the computer X and Y coordinates is independent of the strength of the signal transmitted by the baton. 
     The same calculations are used to determine the (X2, Y2, Z2) position of the second baton  104 , except that the measured capacitance values (i.e., measured voltages) used in this second set of calculations correspond to the strength of the signals received at the frequency transmitted by the second baton  104 . 
     FIG. 4 shows the Quad-radio frequency amplifier  168  used in the receiver electronics  130 . The quad RF amplifier  168  has four RF amplifiers  212 A- 212 D. Each amplifier  212  is a three stage pre-amplifier circuit suitable for amplifying the range of RF signals used in the preferred embodiment. The input to each of the RF amplifiers  212  is coupled to a respective one of the four antenna output lines  161 ,  162 ,  163  and  164 . The output lines  171 ,  172 ,  173 ,  174  from the four RF amplifiers  212  go to the oscillator and detector circuits  182  and  184 . 
     FIG. 5 shows an oscillator and signal detector  182 , including an oscillator  220  which generates a periodic signal having a period of 13.2 microseconds, which corresponds to a frequency of 75.76 Khz. The oscillator  220  is coupled to four RF signal detectors  222 A- 222 D as well as baton  102 . Thus, the same RF signal from the oscillator  220  is used to generate the RF signal transmitted by the baton  102  and to drive a set of four RF signal detectors  222 A- 222 D. 
     In a system with more than one baton, for each distinct baton  102 , there is a corresponding oscillator  220  and set of four RF signal detectors  222 A- 222 D. Each baton uses a distinct RF frequency so that the signals from each baton can be separately detected. While a preferred embodiment has just two batons, other embodiments could use three, four or more batons, each with its own oscillator having a distinct RF frequency and a corresponding set of RF signal detectors. 
     Each radio frequency signal detector circuit  222  includes an input voltage divider  224 , followed by an operational amplifier  226  (e.g., an LF347), followed by a sampling circuit  228  (e.g., an LF13202) which samples an amplified input signal on line  230  at the running frequency of the oscillator circuit  220 . Input line  230  is coupled to output line  171  of one of the RF amplifiers, such as the one shown in FIG.  4 . The input lines of the other three RF signal detector circuits  222 B,  222 C and  222 D are coupled to the output signal lines  172 ,  173  and  174  of the other RF amplifiers. 
     The output of the sampling circuit  228  is filtered by an RC low pass filter  232  so as to produce a stable DC output signal on line  192 , which is proportional to the strength of the RF signal received by one of the antennae. 
     In a preferred embodiment receiver electronics  130  provides X, Y and Z position information to music synthesizer  120  at predetermined time intervals. Alternatively, receiver electronics  130  can send a continuous stream of information to the music synthesizer. In this case, a program will compute X, Y and Z (for each baton) and will send these data to a computer at each sampling time. The continuous stream of information can be used to control nonpercussive timbres such as violin timbres. For example, X might control loudness of the sound and Y the strength of the vibrato. 
     In many applications, the values of X and Y are meaningful only if the baton is close to the sensor surface. The program can be made to send X and Y information (i.e., values) to the computer only when Z is less than a specified threshold. In this way, the computer will not be overloaded with meaningless information. 
     In yet other variations of the program, the program can be arranged so that one baton sends trigger signals to the computer and another baton sends a continuous stream of X, Y and Z values. Alternatively, the same baton can send both triggers and a continuous stream of X, Y and Z values to a computer. If desired, X and Y values can be sent only while the baton is close to the surface of the drum immediately subsequent to generating a trigger signal. Thus the trigger can be used to initiate a note and the subsequent X and Y data can be used to shape the sound of the note. 
     In some embodiments of the present invention, the receiver electronics  130  (FIG. 1) or computer  200  (FIG. 1B) compute only the X and Y coordinates of one or more moveable positioning members (e.g., batons  102 ,  104 ). Also, in alternate embodiments of the present invention, the receiver electronics  130  (FIG. 1) or computer  200  (FIG. 1B) may compute the position of the moveable positioning member in terms of an alternate coordinate system, such as a radial coordinate system. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.