Abstract:
A touch screen uses a combination of capacitive sensing and inductive sensing applied to the same sensor pattern. A capacitive sensor uses the electric field formed by the columns and rows of the sensor matrix. An inductive sensor uses the magnetic field formed by current flowing in column and row lines to induce an inductive pen. Using the same sensor lines, the magnetic field created by the oscillating inductive pen is detected. Both methods require no moving elements in the sensor and it is possible to combine both method of detections in the same sensor pattern. Using switch matrices, the sensor lines are operated in an open loop fashion for the capacitive detection mode, and are operated in a closed loop fashion for the inductive detection mode.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related to touch screens, and more particularly, to a touch screen that is capable of operating in a capacitive and inductive mode using the same sensor panel. 
       BACKGROUND OF THE INVENTION 
       [0002]    A capacitive touch screen can only receive input from a finger, but it gives a different experience to the user and enables multi-touch inputs. A resistive touch screen is able to receive inputs from both a finger and a stylus. However, a resistive touch screen requires more pressure to activate the detection, and a traditional 4/5/8 wire resistive touch screen only allows one point of detection. Although a capacitive touch screen gives a better experience (sensitivity, multi-touch, and other advantages), some users still prefer to use a stylus, especially for hand-writing recognition application. 
         [0003]    A schematic of a prior art inductive touch screen  100  is shown in  FIG. 1A . In the prior art, overlapped closed loop current lines  106  are used to induce an inductive pen at the resonant frequency of the pen. The touch screen  100  also includes a first oscillation current source and detector  102  coupled to the X-lines and a second oscillation current source and detector  104  coupled to the Y-lines. Once the pen is charged up, the same line is used to detect the magnetic field induced by an oscillating pen. The process is repeated for all closed loop paths in the X-axis and the Y-axis. To estimate the exact location of the pen, a weighted average algorithm is used. The sensor is implemented using wires and placed behind the touch screen. In  FIG. 1B , the closed loop current lines  108  are shown for the Y-axis only. 
         [0004]    A schematic of a prior art capacitive touch screen  200  is shown in  FIG. 2 . The prior art capacitive touch screen  200  includes a plurality of open loop X-lines and a plurality of open loop Y-lines. In  FIG. 2 , three X-lines X 1 -X 3  and five Y-lines Y 1 -Y 5  are shown. Any number of X-lines and Y-lines can be used, as is known in the art. In the prior art capacitive touch screen sensor, cross-capacitance between the X-lines and the Y-lines is measured. A finger touch causes the cross-capacitance on the touched intersection to change. A weighted average is then used to estimates the exact location of finger touch(es). The capacitive sensor is typically realized with an ITO layer (Indium Tin Oxide, which is transparent and conductive) and placed in front of the touch screen. 
         [0005]    While the capacitive touch screen and the inductive touch screen each have their respective advantages and disadvantages, what would be desirable is a touch screen that can combine both modes of operation in a single touch screen system. 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention, a touch screen uses a combination of capacitive sensing and inductive sensing applied to the same sensor pattern. A capacitive sensor uses the electric field formed by the columns and rows of the sensor matrix. An inductive sensor uses the magnetic field formed by current flowing in column and row lines to induce an inductive pen (with a resonant frequency formed by the corresponding inductance and capacitance). Using the same sensor lines, the magnetic field created by the oscillating inductive pen is detected. Both methods require no moving elements in the sensor and it is possible to combine both method of detections in the same sensor pattern. Using switch matrices, the sensor lines are operated in an open loop fashion for the capacitive detection mode, and are operated in a closed loop fashion for the inductive detection mode. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The above and other objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiment in conjunction with the accompanying drawings, wherein: 
           [0008]      FIG. 1A  is a schematic of a prior art inductive touch screen system; 
           [0009]      FIG. 1B  is a schematic of the closed loop current lines for the touch screen system of  FIG. 1A  for only the Y-axis; 
           [0010]      FIG. 2  is a schematic of a prior art capacitive touch screen sensor; 
           [0011]      FIG. 3  is a schematic of a portion of a capacitive touch screen sensor; 
           [0012]      FIG. 4  is a schematic of a charge amplifier for use with a capacitive touch screen sensor; 
           [0013]      FIG. 5  is a schematic of a combined inductive/capacitive touch screen system according to the present invention, in which the capacitive mode is illustrated; 
           [0014]      FIGS. 6 and 7  are data matrices of the touch screen showing the value of the cross-capacitance associated with all of the touch screen locations, and illustrating valid touches versus noise according to the present invention; 
           [0015]      FIG. 8  is a schematic of a combined inductive/capacitive touch screen system according to the present invention, in which the inductive mode is illustrated; 
           [0016]      FIG. 9  is a schematic of a combined inductive/capacitive touch screen system according to the present invention, in which a charging phase of the inductive mode is illustrated; and 
           [0017]      FIG. 10  is a schematic of a combined inductive/capacitive touch screen system according to the present invention, in which a measurement phase of the inductive mode is illustrated. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Referring now to  FIG. 3 , a portion  300  of a capacitive touch screen is shown, having a plurality of open loop X-lines X 1 -X 4 , and a plurality of open loop Y-lines Y 1 -Y 6 . A portion  302  of the touch screen is further highlighted where, for example, the third X-line crosses the fifth Y-line. The cross-capacitance between the two lines is shown in further detail. A multi-touch capacitive touch screen measures the cross-capacitance between the X-lines and the Y-lines. When a finger presses an intersection, the cross-capacitance is reduced. A charge amplifier is used to quantify the charge transferred by the cross-capacitance, and the value can be digitized. 
         [0019]    Referring now to  FIG. 4 , a charge amplifier  400  is shown for use in conjunction with a capacitive touch screen or combination capacitive/inductive touch screen according to an embodiment of the present invention. Charge amplifier  400  includes a differential amplifier or operational amplifier  402  having a positive input for receiving a VREF reference voltage. An input signal  404  represents the signal input supplied by the user, and capacitor CC represents the cross-capacitance as shown in  FIG. 3 . A feedback impedance including resistor Rx and capacitor Cx is coupled between the output  408  and the negative input of amplifier  402 . In operation, a rising edge signal  404  is applied to a representative X-axis line Xn. The cross-capacitance (CC) transfers charge to the corresponding Y-axis line Yn. The charge amplifier  402  amplifies the charge and stores it across amplifier capacitance Cx and Rx discharge the capacitor Cx slowly. The width of a voltage glitch (“t”) at the output  408  of the amplifier  402  is proportional with the cross capacitance with some degree of non-linearity. 
         [0020]    Referring now to  FIG. 5 , a capacitive/inductive touch screen system  500  is shown, and in the capacitive operating mode. Touch screen system  500 , according to the present invention, includes a touch screen sensor  510 , which is a plurality of open-loop X-lines and Y-lines in the capacitive mode. Sensor  510  can be an ITO sensor mounted on the surface of the touch screen. Touch screen system also includes a first switch matrix  502 , coupled to a first end of the Y-lines, and under control of a switch control block  512 . Switch matrix  502  includes switches Y 1 -Yn corresponding to Y-lines Y 1 -Yn. A second switch matrix  504  is coupled to a first end of the X-lines, and control of a switch control block  514 . Switch matrix  504  includes switches X 1 -Xn corresponding to X-lines X 1 -Xn. A third switch matrix  506  is coupled to a second end of the Y-lines. Switch matrix is coupled to amplifier  522 , measurement block  520 , and processing block  516  for providing an output signal. Switch matrix  506  is also coupled to a “charge-to-delay” and digitizing block  518 , which is also coupled to processing block  516 . A fourth switch matrix  508  is coupled to a second end of the X-lines, and is coupled to a sourcing current source  526  and a sinking current source  528 . Switch matrix  508  is also coupled to amplifier  524 . 
         [0021]    To measure the cross-capacitance in the capacitive operating mode of touch screen system  500 , two X-lines and two Y-lines are used. Two lines are used to increase the active area and sensitivity. Two lines are also used because the inductive mode of operation (explained below) requires thin and closely spaced lines in the X-axis and Y-axis to increase resolution. During the capacitive cycle, all of the switches in the switch matrices  502  and  504  are open. In operation, the scanning sequence used in the capacitive operating mode is as follows:
       (1) Connect lines X 1 -X 2  to charging signal generator  530  and measure the transferred charge at lines Y 1 -Y 2 , Y 3 -Y 4 , Y 5 -Y 6 , . . . , Yn-Yn+1 in sequential order.   (2) Connect lines X 3 -X 4  to charging signal generator  530  and measure the transferred charge at Y 1 -Y 2 , Y 3 -Y 4 , Y 5 -Y 6 , . . . , Yn-Yn+1 in sequential order.   (3) Connect lines Xn-Xn+1 to charging signal generator  530  and measure the transferred charge at Y 1 -Y 2 , Y 3 -Y 4 , Y 5 -Y 6 , . . . , Yn-Yn+1 in sequential order.       
 
         [0025]    Referring now to  FIGS. 6 and 7 , all X-line and Y-line intersections are ideally measured to create a data matrix. The touch location is then estimated using weighted average formula. The cross-capacitance in each intersection will form of matrix where a threshold is applied. In the example of  FIGS. 6 and 7 , the value “6” is taken as a cut-off value, wherein all value below the threshold are forced to a value of zero. A group of non-zero locations with a member bigger than two locations is considered as a valid finger touch. A group smaller or equal to two will be considered as noise. The value of a valid finger touch group&#39;s member can be used to calculate the finger location by using weighting average algorithm. In the example shown in  FIG. 6 , a first group of data values includes nine data values as shown, a second group of data values includes a single data value, and a third group of data values includes four data values. As shown in  FIG. 7 , the first and third groups are considered valid finger touches. 
         [0026]    Referring now to  FIG. 8 , the inductive touch screen mode of operation is illustrated.  FIG. 8  is substantially the same as  FIG. 5 , but note the inclusion of a stylus  802  and the switch positions of switch matrices  502  and  504 , which are different than in the capacitive mode of operation, and are explained in further detail below. Also note that there is current flowing in the X-lines and Y-lines. According to the present invention, the same ITO sensor pattern  510  is used for inductive touch screen detection. The inductive sensing method is divided into two phases, a charging phase and a measurement phase. 
         [0027]    Referring now to  FIG. 9 , during the charging phase, shorting switch matrix  504  connects switches X 2  and X 4  (Xn and Xn+2) to form a closed loop conductor while the current generator with switch matrix  508  generates AC current in the closed loop lines with a frequency that is the same as the resonant frequency of the stylus. Generation of alternating current is performed by switch matrix  508  by connecting current sources  526  and  528  to lines X 2  and X 4  in an alternating sequence. This current will be stopped when the charging period is complete. Stylus  802  comprises an inductor and a capacitor in series. When stylus  802  is induced with magnetic flux that is in the same frequency of the L-C resonant frequency coming from the X 2  and X 4  sensor lines, it will get charged up and store the energy. If the magnetic flux generated by the sensor lines is stopped, the stylus  802  will dissipate the energy stored and will oscillate for some period of time. This oscillation produces a magnetic flux, which can be detected by the X-lines. The amount of energy stored depends on the position of the stylus  802  with respect to the X 2  and X 4  lines. The maximum energy developed is when stylus is positioned right in the centre of the closed loop. 
         [0028]    Referring now to  FIG. 10 , during the detection phase, shorting switch matrix  502  shorts lines Y 3  and Y 5  (Yn and Yn+2) to form a closed loop conductor to catch magnetic flux generated by the stylus  802 . These lines are then connected to amplifier  522  to amplify the signal. The amplified signal is then fed to a rectifier and capacitor in the measurement block  520  to get a DC level that can be measured by an ADC. The amount of magnetic flux caught by the Y 3  and Y 5  closed loop lines depends on the position of the stylus. Maximum energy is caught when the stylus  802  is right on the center of the conductor loop. 
         [0029]    According to the present invention, the configuration of the conductor lines (Xn and Yn) during the charging and detection phases of the inductive operating mode is in a particular sequence to form overlapping closed-loop lines. For example, a particular sequence could be: Y 1 -Y 3 , Y 2 -Y 4 , Y 3 -Y 5 , . . . Yn-Yn+2. For each Y-axis closed loop lines, detection is performed in all X-lines combinations. After all X-Y lines intersections are measured, the data is available in the form of a data matrix, and the location of the stylus  802  can be calculated using a similar method to that of the capacitive touch screen mode of operation. 
         [0030]    According to the present invention, a combination of a capacitive and inductive touch screen has been shown. Using the detection method of the present invention, a combined capacitive and an inductive touch screen is possible using single sensor pattern. The capacitive and inductive detection can be performed in time-sharing basis, hence both finger or stylus may be detected at the same time. 
         [0031]    While the invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular application to the teachings of the invention without departing from its scope. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.