Abstract:
A position sensing device having an insulating board, a first conductive sensing element, a second conductive sensing element, an oscillator, conductive material, a differential amplifier, and a processor. The first and second spaced conductive sensing elements are on the insulating board. The oscillator generates an oscillating signals and the conductive material is spaced from the sensing elements. The conductive material receives the oscillating signal to create an electric field between the conductive material and the first and second sensing elements. The differential amplifier includes differential inputs that are connected to the conductive sensing elements. The signal levels at the differential inputs vary based on the position of a pointing object of a user relative to the first and second conductive sensing elements. The processor generates a position signal based on an output of the differential amplifier.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. § 119 (e) to U.S. provisional patent application No. 60/524,170, filed Nov. 21, 2003, which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to an input device. In particular, the present invention relates to a position sensing device for electronic devices. 
   BACKGROUND OF THE INVENTION 
   Various position sensing input devices are used today to allow an operator to interface with electronic devices such as a computer, PDA, mobile telephones, pocket organizers, portable gaming devices and the like. For instance, a cursor controlled by a mouse has become a necessary tool of the modern computer system. The mouse allows the operator to both operate the movement of an on-screen cursor and execute commands. However, a relatively large input device such as the mouse is impractical to be incorporated into small portable electronic devices. Moreover, such mechanically complex input device is expensive to manufacture and prone to breakage. 
   Therefore, it is desirable to provide a position sensing device that is accurate and yet small, durable and inexpensive to manufacture. 
   SUMMARY OF THE DISCLOSURE 
   According to the invention there is provided a position sensing device having an insulating board, a first conductive sensing element positioned on the insulating board and a second conductive sensing element positioned on the insulating board and spaced from the first conductive sensing element. An oscillator is also provided for generating an oscillating signal. Conductive material spaced from the conductive sensing elements is operable to receive the oscillating signal to create an electric field with the first and second conductive sensing elements. A differential amplifier having first and second differential inputs connected to the first and second conductive sensing elements is also provided. The signal levels at the differential inputs vary based on the position of a pointing object of a user relative to the first and second conductive sensing elements. A processor generates a position signal based on an output of the differential amplifier. 
   According to one embodiment of the invention, the first and second conductive sensing elements include a pair of elongated conductive triangular wedges facing each other. 
   According to another embodiment of the invention, the first conductive sensing element includes a plurality of spaced conductive pads that increase in size in a first direction. The second conductive sensing elements include a plurality of spaced conductive pads that are alternately positioned with the conductive pads of the first conductive sensing elements and that decrease in size in the first direction. 
   In a third embodiment of the invention, the first conductive sensing element includes a first set of curved conductors electrically coupled to each other. The second conductive sensing elements include a second set of curved conductors electrically coupled to each other and spaced from the first set. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exemplary schematic block diagram of a position sensing system in accordance with a present invention. 
       FIG. 2  illustrates the arrangement of conductive sensing elements in accordance with one embodiment of the invention. 
       FIG. 3A  is a graph showing the relative differential amplifier output versus the position of a pointing object according to the sensor arrangement of  FIG. 2 . 
       FIG. 3B  is a graph showing the relative signal on the inputs to a differential amplifier according to the arrangement of  FIG. 2  as a function of the position of a pointing object. 
       FIG. 4  shows the arrangement of conductive sensing elements in accordance with another embodiment of the invention. 
       FIG. 5  is a graph showing the relative signal on the inputs to a differential amplifier according to the arrangement of  FIG. 4  as a function of the position of a pointing object. 
       FIG. 6  illustrates a cross sectional diagram of a sensor arrangement of  FIG. 2  in accordance with the invention. 
       FIG. 7  shows the arrangement of conductive sensing elements according to another embodiment of the invention. 
       FIG. 8  illustrates the arrangement of the conductive sensing elements corresponding to the x-axis for the arrangement of  FIG. 7 . 
       FIG. 9  illustrates the arrangement of the conductive sensing elements corresponding to the y-axis for the arrangement of  FIG. 7 . 
       FIG. 10  is a graph showing the relative differential amplifier output versus the position of a pointing object according to the sensor arrangement of  FIG. 7 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring first to  FIG. 1 , reference numeral  10  generally designates a position sensing device comprising an insulating board  20  attached to a conductive layer. The conductive layer (see  32  in  FIG. 2 , for example) is below the insulating board  20 . The insulating board  20  may be part of a keyboard, mobile telephone, PDA or any other such device which may receive input from a user. In the present system  10 , the conductive layer receives an oscillating signal (AC Voltage) from an oscillator  40 . Generally the oscillating frequency transmitted to the conductive layer is in the range of 1-100 KHz, and preferably about 20 KHz. 
   A sensor or sensing element  30  is positioned on the insulating board  20 . The sensor  30  may consist of a series of conductive sensing elements positioned in varying arrangements on the insulating board  20 . The varying arrangements will be discussed in further detail later herein. When an oscillating signal is applied to the conductive layer, an electric field is created between the conductive layer positioned below the insulating board  20  and the conductive sensing elements  30 . The voltage and current that flows from the conductive sensing elements of the insulating board  20  to the conductive sensing elements of the sensor  30  can be measured. 
   The electric field can be disturbed by a body part such as a finger or any other pointing object utilized by an operator, thereby interfering with the current that flows from the conductive layer to the conductive sensing elements  30 . High impedance buffer differential amplifiers  42 ,  44  are connected to the outputs of the conductive sensing elements corresponding to x-axis data. The output from the high impedance buffer differential amplifier  42  and  44  are then inputted into a difference amplifier  50 . The output of the difference amplifiers  50  is fed via band-pass filters  54  and a synchronous demodulator/detector  58  to a first of the inputs of an analog-to-digital converter (ADC)  62 . Conductive sensing elements corresponding to y-axis data are connected to the two inputs, respectively, via a high impedance buffer amplifier  46  and  48 , and the output of the difference amplifier  52  is connected via a band-pass filter  56  and a synchronous demodulator/detector  60  to a second input of the analog-to digital converter  62 . The band-pass filters  54  and  56  each have a centre frequency which corresponds to the frequency of the oscillator  40 . The sensing system  10  also comprises a microprocessor  64 . The output of the analog-to-digital converter  36  is connected to an input of the microprocessor  46 . Other inputs such as a PDA or mouse may also be connected to the microprocessor  64 . Subsequently, a look up table  66  is utilized to convert the voltage signal to position values. 
   The synchronous detector/demodulator is a demodulator that runs at the same frequency as the input frequency. The simplest form of this is a rectifier. In the embodiment shown, since the oscillating frequency is known, the synchronous demodulator uses a switch that switches from positive to negative at the zero crossings in the input signal. The output for a sinusoidal input signal is simply a rectified sinusoidal. This effectively performs a demodulation on the signal—transferring the useful information (amplitude in the present case) from a high frequency down to DC. The high oscillating frequency signal allows the signal to propagate through the capacitive coupling of the sensing elements allows the input amplifiers to operate in a relatively noise free frequency band. Thus, the synchronous demodulator enables easy determination of the signal amplitude by a standard analog to digital converter. 
   As mentioned above, conductive sensing elements positioned on the sensor  30  can be situated in an X co-ordinate direction or the Y co-ordinate direction. The X co-ordinate direction is along the length of the insulating board  20 , and, as will be explained in more detail hereinafter, is thus able to detect the position of the operator&#39;s pointing object or body part in the X co-ordinate direction. The Y co-ordinate direction is in a direction perpendicular to the X co-ordinate direction. The sensor is able to detect the position of the operator&#39;s pointing object or body part (finger) in the Y co-ordinate direction. 
   The detection of the presence of a finger is performed by measuring the output of either conductive sensing element (x or y sensing element) or the output of the sum of the two sensing elements  30 . When a finger is in close proximity to the sensors  30 , the fields are interrupted and the total signal is reduced. A fixed or a noise riding threshold can be used to determine activation. Hysteresis can be used to reject spurious signals. This activation signal responds to approximately uniformly over the entire sensor area, and can generally be adjusted to be sensitive to actual touch of the dielectric surface only. In order to measure the signals accurately, a chip such as an AD7103 ASIC from Analog Devices, Inc. of Norwood, Mass. is used. 
   The conductive sensing elements are able to detect the strength (i.e. amplitude) of this field and, from this determine the position of the operator&#39;s right hand in the X and Y co-ordinate directions. This is done in conjunction with the difference amplifiers  50 ,  52  and the synchronous demodulators  58 ,  60 . Any extraneous signals are filtered out by the band-pass filters  54  and  56  and the synchronous demodulators  58 ,  60  provide analog outputs corresponding to the position of the operator&#39;s right hand in, respectively, the X and Y co-ordinate directions. The two analog signals, one provided by the synchronous demodulator  58  and the other by the synchronous demodulator  60 , are fed to the analog-to-digital converter  62 , which converts the two signals to a digital form. The microprocessor  64  serves to convert the signal into a suitable data bit-stream. The protocol of the bit-stream may be such as to emulate a standard mouse protocol required by a conventional software mouse driver resident in a PC. The bit-stream is fed to a look up table  66  or the like, and is interpreted by the computer as if it was reading data sent by a conventional mouse during normal mouse operation. 
   Now referring to  FIG. 2 , the operation of the position sensing device  10  will be described according to one exemplary embodiment of the present invention. FIG.  2  illustrates the conductive sensing elements  72 , 74  formed in a triangular wedge arrangement  70  which comprise the sensing element  30 . The oscillator  40  provides an oscillating signal to the conductive layer  32  positioned beneath the insulating board  20  (not shown). Complementary opposing conductive sensing elements  72  and  74  formed in a triangular wedge are positioned on the sensing device  30  above the insulating board  20 . In a downward direction, conductive strip  72  decreases in width in a triangular format and conductive strip  74  increases as illustrated. 
   The conductive layer  32  receiving the oscillating signal from the oscillator  40  is in close proximity to and separated from the conductive sensing elements  72 ,  74  by a thin insulating board  20 . Thus, oscillator field lines are generated from the conductive layer  32  to the conductive sensing elements  72 ,  74 . When an operator places a pointing object such as his finger tip on the surfaces of the wedge shaped conductive sensing elements  72  and  74 , simultaneous covering of portions of both conductive sensing elements results in interrupting the electric field coupling in that region of the wedge shaped conductive sensing elements. Although the finger can physically touch the conductive sensing elements  72 , 74 , a thin insulation coating (see  FIG. 6 ) in the embodiment shown isolates the finger from the conductive sensing elements. 
   Placing the pointing object or finger near the bottom of the sensor  30  results in covering a larger area of the wedge shaped conductive sensing elements  74  connected to input B and a smaller area of the conductive sensing element  72  connected to input A. This results in interrupting the field lines coupling to B in a far greater amount than that for input A. The result is that the input signal to A is now greater than the input to B causing the resulting output of the differential amplifier  50  to swing more positive if input A is the positive input to the differential amplifier  50 . As the finger moves up along the wedge shaped conductive sensing elements, the amount of coverage decreases on the wedge shaped conductive sensing element  74  connected to B and increases on the wedge shaped conductive sensing element  72  connected to A. The resulting output due to the motion is to cause the output of the differential amplifier  50  to swing from positive to negative in a generally linear fashion. 
     FIG. 3A  is a graph illustrating the linear relationship between the output of the differential amplifier  50  and the finger position over the sensing device  70 . As illustrated, as the finger is moved from the bottom of the sensing device at position  1  to the top at finger position  21 , the output of the amplifier  50  increases from −1 to +1. As a result, the position of the finger can be accurately determined. The table of values used to generate the graph can be used as the look up table  66 . 
     FIG. 3B  illustrates a graph showing the relative signal on Inputs A and B as a finger moves from bottom to top. As the finger is moved from bottom to top, signal A increases from negative to positive, as signal B decreases from positive to negative in a generally linear fashion. 
   Now turning to  FIG. 4 , an illustration of another embodiment of the present invention is shown. Specifically,  FIG. 4  illustrates the conductive sensing elements  82 - 96  formed as rectangular strips  80  which comprise the sensing element  30 . The oscillator  40  provides an oscillating signal to the conductive layer  32  positioned beneath the insulating board  20  (not shown). Conductive sensing elements  82 - 96  connected to one input of a differential amplifier  52  increase in size in a downward direction. Complementary conductive sensing elements  98 - 112  connected to the other input of the differential amplifier  52  decrease in size in a downward direction. The conductive sensing elements  82 - 96  are interposed with the conductive sensing elements  98 - 112  in an alternating manner. 
   As explained previously, signals are provided via an oscillator  40  to the rectangular shaped conducting layer  32 , which is positioned below the conducting elements  82 - 112  and separated by a thin insulator. Thus, the oscillator field lines couple to the conductive sensing elements  82 - 112 . Similar to the embodiment of  FIG. 2 , while the finger can physically touch the conductive sensing elements  82 - 112 , a thin insulation coating (see  FIG. 6 ) isolates the finger from the conductive sensing elements. 
   When an operator places a pointing object or finger on the surfaces of the sensor  30 , simultaneously covering adjacent portions of each conductor results in interrupting the coupling in that region of the conductive sensing elements  82 - 112 . 
   Now referring to  FIG. 5 , placing a finger near the bottom of the elements  82 - 112  results in covering a larger area of the conductive sensing elements  98 - 112  connected to input B and a smaller area of the conductive sensing elements  82 - 96  connected to input A. This results in interrupting the field lines coupling to B in a far greater amount than that for input A. The result is that the input signal to A is now greater than the input to B causing the resulting output of the differential amplifier to swing more positive (if input A is the positive input to the differential amplifier). As the operator&#39;s finger moves up along the rectangular shaped conductive sensing elements  82 - 112 , the amount of coverage decreases on the conductors connected to B and increases on the conductors connected to A. The result is that the output of the amplifier  52  swings from negative to positive in a generally linear fashion as the finger moves from the bottom to top as illustrated in  FIG. 5 . 
     FIG. 6  illustrates a cross sectional diagram of the sensing device  20 ,  30  arrangement of  FIG. 1 , in accordance with the invention. In the embodiment shown, the sensor system  120  consists of four layers: the top layer is a dielectric/insulating layer  122 , isolating the sensor layer  124  below from the user&#39;s finger above  126 . The second (sensor) layer  128  is again isolated from the bottom (source) layer by another dielectric/insulating layer  130 . The layer structure may be in the form of a PC board, flexible PC board, insulating film printed with conductive ink or any other combination of conductors and dielectric layers. 
   A known AC oscillating voltage is applied to the source layer  128  which creates an electric field in the dielectric layers  122  and  130  above between the source layer and the sensor layer  124 . Due to the gaps between the sensor conductors, the field extends through the sensor layer and also through the dielectric layer above. When a user&#39;s hand is placed above, it interferes with this electric field and causes a change in the current flowing from the source layer  128  to the sensor layer  124 . 
   Now turning to  FIG. 7 , a sensor arrangement  140  contains curved conducting elements that are shaped in concentric semicircles with two separate sensors in both the X and the Y directions. A first set of complementary opposing concentric semi circles are joined together by thin interconnections  142 , 144  as inputs for x-direction and a second set of complementary opposing concentric semi circles, which are interleaved with the first set of semi circles but which do not contact the first set, are joined together by thin interconnections  146 ,  148  as inputs for y-direction. In the embodiment shown, the diameter of the sensor arrangement  140  is in the range of 30 millimeters. The source layer  128  may be a solid conductive plane or a grid or other shaped surface. As illustrated in  FIG. 7 , four sensor segments are connected close to the center of the sensor system  120 . The X sensors are to the left and right and the Y sensors are at the top and bottom. 
     FIG. 8  illustrates the sensor with only the x-axis connected conductive sensing elements. As shown, the left side positioned conductive sensing elements are inputted into the differential amplifier  230  and the right side positioned conductive sensing elements are inputted into differential amplifier  240 . The output from amplifiers  230  and  240  are then transmitted into differential amplifier  250 . A synchronous demodulator  260  then receives the input signal which removes the frequency components of the original oscillating signal which is then converted into a digital signal via an analog-to-digital converter  270 . 
   Now looking at  FIG. 9 , a similar illustration is shown depicting the conductive sensing elements connected in a y-axis arrangement. The top portion of the conductive sensing elements is connected to electrode  310  and the bottom portion of the conductive sensing elements is connected to electrode  320 . The signals received by electrode  310  are inputted into differential amplifier  330  and the signals received by electrode  320  are inputted into differential amplifier  340 . The output from amplifiers  330  and  340  are then transmitted into differential amplifier  350 . A synchronous demodulator  360  then receives the input signal which is then converted into a digital signal via the analog-to-digital converter  270 . 
     FIG. 10  illustrates a graph of a relative output of the synchronous demodulator  260  as a function if distance in millimeters. When a user places his finger on top of the sensor to cover a portion of the sensing elements, the field between the source and sensor is interrupted and the signal measured on the sensor is reduced. Thus, when the finger is placed to the left of center, the signal on the left hand X sensor is reduced more than that of the right hand sensor and the difference between these two signals is negative. As the finger is moved towards the right, the right hand X signal is reduced more and the left hand X signal is reduced less. The difference increases and when the finger is on the right hand side, the difference signal is approximately equal in magnitude, but opposite in sign from the case at the extreme left. The differential signal change with respect to position reaches a saturation level once the finger has moved completely away from one of the two sides. Thus, for example, if the diameter of the sensor arrangement  140  is 30 millimeters, a usable distance may be +10 millimeter and −10 millimeter from the origin/center as shown in  FIG. 10 . The saturation effect can be overcome; and thereby the linear range extended, by varying the spacing between the sensor segments. 
   The foregoing disclosure and description of the disclosed embodiments are illustrative and explanatory thereof, but to the extent foreseeable, the spirit and scope of the invention are defined by the appended claims.