# Patent

## Title
Multi-touch sensing method

## Abstract

A capacitance measurement sensor, having a voltage subtractor that rejects common signals between the columns or rows of a touch sensor matrix depending on which are driven and which are being sensed, is described.

## Background
RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No. 61/067,743 filed Feb. 29, 2008.
TECHNICAL FIELD
The present disclosure relates generally to touch sensor devices and, more particularly, to resolving multi-touches on a multi-touch sensor device.
BACKGROUND
One type of touchpad operates by way of capacitance sensing utilizing capacitance sensors. The capacitance detected by a capacitance sensor changes as a function of the proximity of a conductive object to the sensor. The conductive object can be, for example, a stylus or a user's finger. In a touch-sensor device, a change in capacitance detected by each sensor in the X and Y dimensions of the sensor array due to the proximity or movement of a conductive object can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitive sensor is processed by a processing device, which in turn produces electrical or optical signals representative of the position of the conductive object in relation to the touch-sensor pad in the X and Y dimensions. A touch-sensor strip, slider, or button operates on the same capacitance-sensing principle.
A first type of touchpad is composed of a matrix of rows and columns. Within each row or column there are multiple sensor elements, however, all sensor pads within each row or column are coupled together and operate as one long sensor element. The number of touches a touchpad can detect is not the same as the resolution of the touchpad. For example, even though a touchpad may have the capability to detect two substantially simultaneous touches with an XY matrix, the touchpad cannot resolve the location of the two substantially simultaneous touches.
One way to resolve the location of a second touch is if the touches arrive sequentially in time. If both touches arrive or are detected substantially simultaneously, however, there is no way to resolve which of the two pairs of potential locations constitute “actual” touches, instead of invalid touches (e.g., “ghost” touches). Thus, the two-axis touchpads are configured to resolve only a location of a single touch. Similarly, touch screens are designed to detect the presence and location of a single touch.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention are illustrated by way of example and not intended to be limited by the figures of the accompanying drawings in which like references indicate similar elements and in which:
FIG. 1A illustrates multiple finger presses on a touch pad according to an embodiment.
FIG. 1B illustrates one embodiment of a capacitive sensor matrix.
FIG. 2 illustrates multiple touch points that are to be resolved according to an embodiment.
FIG. 3 illustrates the coupling capacitance between rows and columns according to an embodiment.
FIG. 4 illustrates a capacitance measurement sensor having a differential amplifier according to one embodiment of the present invention.
FIG. 5 illustrates a capacitance measurement sensor having a differential amplifier according to an alternative embodiment of the present invention.
FIGS. 6a-6d illustrate the use of a single row as a reference for differential measurements according to an embodiment of the present invention.



DETAILED DESCRIPTION
In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.
Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment. It should be noted that the apparatus and methods may be discussed herein in relation to a touch pad for ease of explanation purposes and that embodiments of the present invention are not limited to use just with touch pads but may also be used with other touch sensitive devices such as a touch screen.
An apparatus and method of capacitive sensing utilizing a capacitance sensor measurement circuit is described. In one embodiment, the capacitance sensor measurement circuit includes a voltage subtractor (e.g., a differential amplifier) that rejects common signals between the columns or rows of the touch sensor matrix depending on which are driven and which are being sensed. Differential scanning may also reduce the number scans that may be used to cover an entire touch panel, for example, (N−1)*M scans may be deployed to cover a panel with N columns and M rows of sensors. For example, with a matrix of 10×15 sensing elements, one touch sensor may use 150 scans whereas one embodiment of the present invention may use, for example, 135 scans.
FIG. 1A illustrates multiple simultaneous finger presses on a touchpad according to an embodiment. The touchpad 100 senses an increase in capacitance attributable to each finger press 110. The capacitance is measured along each vertical column and each horizontal row. For example, when there are two simultaneous finger presses 110 on a touchpad 100, there are two columns 115, 120 that detect an increase in capacitance. The increase in capacitance is illustrated by the taller bars along the x-axis near columns 115 and 120. Likewise, there are two corresponding rows 125, 130 that also detect an increase in capacitance, which is similarly illustrated. As a result, there are four intersections where a column and row both detect an increase in capacitance. These intersections represent potential touch locations. The potential touch locations are evaluated to determine which locations are “actual touch” locations and which are invalid touches, also referred to as “ghost touch” locations or “phantom” locations 135.
FIG. 1B illustrates one embodiment of a capacitive sensor matrix 150. The row conductors (R0-RN) and column conductors (C0-CN) are arranged to overlap, creating a matrix of intersections. Various methods of capacitive sensing, including those that are capable of multi-touch sensing, resolve the capacitance at the junction between a row and a column. In addition, various known circuitry may be used to implement detection of capacitive sensor activation. For example, such detection may be performed by utilizing a capacitive switch relaxation oscillator (CSR). The CSR may be coupled to an array of sensor elements using a current-programmable relaxation oscillator, an analog multiplexer, digital counting functions, and high-level software routines as discussed in further detail below. However, it should be noted that there are various known methods for measuring capacitance with a capacitance sensor. The present embodiments are not limited to using relaxation oscillators, but may include other methods known in the art, such as current versus voltage phase shift measurement, resistor-capacitor charge timing, capacitive bridge dividers, charge transfer, sigma-delta modulators, charge-accumulation circuits, or the like. Additional details regarding these alternative embodiments are not included so as to not obscure the present embodiments.
FIG. 2 illustrates multiple touch points that are to be resolved using embodiments of the present invention and FIG. 3 illustrates the coupling capacitance between rows and columns according to embodiments of the present invention. When a conductive object, such as a finger, is placed at an intersection (Point-1-Point-N) of rows R0-RN and columns C0-CN in touchpad matrix 200, the row to column coupling capacitance is changed at that intersection. The coupling capacitance 301 is illustrated between the rows R0-RN and columns C0-CN in touchpad matrix 300. A capacitance sensor is used to measure the capacitance change when a conductive object is placed on or near the touchpad. The change in capacitance at the intersection is used to determine the location of the conductive object, as described further below.
FIG. 4 illustrates a capacitance measurement sensor 400 having a differential amplifier 410 according to one embodiment of the present invention. In one embodiment, the capacitance measurement sensor 400 includes a drive circuit configured to generate a drive signal. In one embodiment, the drive circuit is a pseudo-random sequence generator (PRS) 420 that provides an alternating current (AC) signal that is coupled across two sensor capacitances (Csensor1 and Csensor2) 430, 440. The sensor capacitances are coupled between the rows and columns of the touch pad sensor matrix illustrated in FIG. 3.
The intersection of a column with a row, for example column C0 with row R1, forms a Csensor. The impedance of a Csensor 430, 440 in series with the resistor R (e.g., R1 or R2) 435, 445 forms a voltage divider. In one illustrative embodiment, the intersection of column C0 with row R1 forms Csensor1 430. The intersection of column C0 with an adjacent row, for example row R2, forms Csensor2 440. Thus, a voltage divider is formed by Csensor1 430 and resistor R1 435. For a given frequency F, Csensor1 430 will have an impedance Z1=1/(F*Csensor1). The voltage across resistor R1 435 can then be calculated to be:




  
    
      
        
          VR
          1
        
        =
        
          
            
              
                R
                1
              
              
                
                  R
                  1
                
                +
                
                  Z
                  1
                
              
            
            *
            
              V
              PRS
            
          
          =
          
            
              
                R
                1
              
              
                
                  R
                  1
                
                +
                
                  1
                  
                    F
                    *
                    
                      Csensor
                      1
                    
                  
                
              
            
            *
            
              V
              PRS
            
          
        
      
    
    
      
        (
        1
        )
      
    
  




Therefore, a change in Csensor1 will directly result in a change in voltage across R1 (VR1).

A second voltage divider is formed with Csensor2 440 and resistor R2 445. As a finger, or other conductive object, comes near the sensor area, the capacitance changes due to the coupling to ground (Agnd). When the capacitance decreases, less of the PRS signal is dropped across the resistors (e.g., R1 and R2). It should be noted that in alternative embodiments, other elements may be used instead of resistors to form the voltage dividers, for example, capacitors.
The drive signal is applied to the first and second voltage dividers and a difference signal is generated based on a difference between an output of the first voltage divider and an output of the second voltage divider. In this embodiment, the capacitance measurement sensor has a differential amplifier 410 as a voltage subtraction circuit. The differential amplifier in this first stage rejects noise signals common to both sensor capacitors (e.g., Csensor1 and Csensor2). The differential amplifier is coupled so as to subtract the voltage present on two adjacent sensors. In one embodiment, a first input of the voltage subtractor is coupled to the first voltage divider and a second input is coupled to the second voltage divider. The voltage subtractor is configured to generate a difference signal based on a difference in capacitance signals from the first and second capacitance sensors. With no finger on the sensor panel, the voltages VR1 and VR2 will be roughly equal. The differential amplifier 410 will subtract the two equal AC voltages and have a near zero output. In one embodiment, this near zero output will be passed through an amplitude detect circuit coupled to receive the difference signal from the voltage subtractor and the drive signal and a lowpass filter 460 and then an analog to digital converter (ADC) 470 will measure zero, as discussed in further detail below. In one embodiment the amplitude detect circuit is a demodulator 450. The amplitude detect circuit is configured to reject noise in the difference signal that is out of phase with the drive signal. The filter 460 is coupled to an output of the amplitude detect circuit to reject out of band noise that may be in phase with the drive signal. The ADC 470 is coupled to an output of the filter. In an alternative embodiment, the differential amplifier 410 may be replaced with another type of voltage subtractor. In one alternative embodiment, for example, the voltage subtractor may be composed of two analog-to-digital blocks. The subtraction is performed by digital processing and then converted back to an analog output signal. As another example, the subtractor may be composed of two voltage-to-current converters, where the currents are subtracted before being converted to a difference voltage.
In one embodiment, when a finger is present on the sensor panel close to Csensor1, the capacitance of Csensor1 may be different than that of Csensor2. This results in VR1 being larger than VR2. The output of the differential amplifier 410 will be the difference between VR1 and VR2. This output may be a small amplitude signal very similar to the PRS drive signal.
The difference between the two sensor capacitor signals (i.e., the output of the differential amplifier) and the PRS drive signal are provided to a demodulator 450. The demodulator 450 rejects noise in the difference signal that is out of phase with the PRS drive signal using synchronous rectification. In one embodiment, such noise rejection is performed by multiplying the difference signal by the sign of the PRS signal, to perform a synchronous rectification of the difference signal. The rectified signal is a pulsing DC signal that represents the peak-to-peak amplitude of the AC signal input to the demodulator 450. This type of demodulator can be thought of as a coherent modulator. It accepts signal components that are in phase with the touch panel drive signal. Out of phase or non-correlated noise signals are chopped. In an alternative embodiment, the demodulator may be replaced with another type of amplitude detection circuit.
In one embodiment, the output of the demodulator 450 is coupled to a low pass filter (LPF) 460. The rectified signal output from the demodulator 450 is lowpass filtered by the LPF 460. The LPF 460 rejects out of band noise that may be in phase with the drive signal. Low pass filtering rejects high frequency noise, and non-correlated low frequency signals that have been frequency shifted by the demodulator to be high frequency noise. The output of the LPF 460 is then measured by an analog-to-digital converter (ADC) 470. The ADC converts the difference signal to a digital signal. When the finger is close to Csensor1, the ADC 470 will measure a positive value. When the finger is close to Csensor2, the ADC 470 will measure a negative value. It should be noted that the design of the PRS, LPF, demodulator and ADC are well known in the art.
It should be noted that in alternative embodiments, the capacitance measurement circuit illustrated in FIG. 4 may include other circuit blocks such as modulation blocks, amplifiers or filters that may be used, for example, to improve the measurement of the circuit. Alternatively, other configurations for a capacitance measurement circuit having a differential amplifier may be used. For example, in one alternative embodiment, the PRS generator may be replaced with another type of drive circuit such as a pulse width modulator (PWM) or a clock (CLK) circuit. FIG. 5 illustrates an alternative embodiment of a capacitance measurement circuit 500 where the drive circuit 520 is a pulse width modulator circuit. In this embodiment, a bandpass filter 580 is included between the output of the differential amplifier and the input of the demodulator. The bandpass filter rejects out of band noise in the difference signal.
In another alternative embodiment, a single row (or column) may be used as a reference for all other rows (or columns) being sensed, as illustrated in FIGS. 6a-6d. FIG. 6a shows a touchpad matrix 610 with adjacent row difference measurements using adjacent rows R0 and R1. FIG. 6b shows a touchpad matrix 620 with adjacent row difference measurements using adjacent rows R1 and R2. Similarly, any one row of the touchpad matrix may be used as a reference for any other row. In the examples of FIGS. 6c and 6d, row R0 is used as the fixed reference row for the difference measurement against row R1 of touchpad matrix 630 in FIG. 6c and row R2 of touchpad matrix 640 in FIG. 6d. 
Some embodiments may include the ability to perform voltage mode sensing that is insensitive to panel row and column resistance. Additionally, differential measurement that is insensitive to parasitic capacitance of a touch panel and differential measurement that reduces the specified measurement dynamic range, thereby providing an optimal signal-to-noise ratio at a faster scan rate, are possible.
Although the present invention has been described with reference to specific embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. An apparatus comprising:
a drive circuit to generate a drive signal on a first plurality of sensor elements oriented in a first direction;
a second plurality of sensor elements oriented in a second direction, wherein intersections of the first plurality of sensor elements and the second plurality of sensor elements form a plurality of sensor locations;
a first voltage divider coupled to a first sensor element of the second plurality of sensor elements and coupled to ground, wherein the first voltage divider is configured to decrease a first voltage at the first sensor element to a second voltage at an output of the first voltage divider;
a second voltage divider coupled to a second sensor element of the second plurality of sensor elements and coupled to the ground, wherein the second voltage divider is configured to decrease a third voltage at the second sensor element to a fourth voltage at an output of the second voltage divider;
a voltage subtractor comprising a first input coupled to the output of the first voltage divider and a second input coupled to the output of the second voltage divider, the voltage subtractor configured to generate a difference signal based on a difference in measured signals on the first sensor element and the second sensor element, wherein the measured signals represent mutual capacitances at the intersections of the first sensor element and the second sensor element, wherein, responsive to proximity of a conductive object closer to the first sensor element than the second element, the difference signal is a positive signal, and wherein, responsive to proximity of the conductive object closer to the second sensor element than the first element, the difference signal is a negative signal; and
an amplitude detect circuit configured to receive the drive signal and to receive the difference signal, the amplitude detect circuit configured to reject noise in the difference signal that is out of phase with the drive signal.

2. The apparatus of claim 1, wherein the amplitude detect circuit is configured to perform synchronous rectification of the difference signal to reject noise in the difference signal that is out of phase with the drive signal, wherein the amplitude detect circuit is configured to multiply the difference signal by the sign of the drive signal to perform the synchronous rectification.

3. The apparatus of claim 2, further comprising:
a filter coupled to an output of the amplitude detect circuit to reject out of band noise that may be in phase with the drive signal; and
an analog to digital converter coupled to an output of the filter.

4. The apparatus of claim 3, wherein the filter comprises a low pass filter.

5. The apparatus of claim 2, wherein the amplitude detect circuit is a demodulator and wherein the demodulator is configured to reject noise in the difference signal using synchronous rectification.

6. The apparatus of claim 2, wherein the drive circuit comprises a pseudorandom sequence generator (PRS).

7. The apparatus of claim 2, wherein the drive circuit comprises a pulse-width modulator (PWM) circuit.

8. The apparatus of claim 2, further comprising a band pass filter coupled between the voltage subtractor and the amplitude detect circuit.

9. The apparatus of claim 1, wherein the voltage subtractor is a differential amplifier.

10. A method comprising:
providing a drive circuit to generate a drive signal on a first plurality of sensor elements oriented in a first direction;
providing a second plurality of sensor elements oriented in a second direction, wherein intersections of the first plurality of sensor elements and the second plurality of sensor elements form sensor locations;
applying the drive signal to a first voltage divider coupled to a first sensor element of the second plurality of sensor elements and coupled to ground;
decreasing a first voltage at the first sensor element to a second voltage at an output of the first voltage divider;
applying the drive signal to a second voltage divider coupled to a second sensor element of the second plurality of sensor elements and coupled to the ground;
decreasing a first voltage at the second sensor element to a second voltage at an output of the second voltage divider;
generating a difference signal based on a difference in the measured signals on the first sensor element and the second sensor element, wherein the measured signals represent mutual capacitances at the intersections of the first plurality of sensor element elements and the second plurality of sensor element elements, wherein, responsive to proximity of a conductive object closer to the first sensor element than the second element, the difference signal is a positive signal, and wherein, responsive to proximity of the conductive object closer to the second sensor element than the first element, the difference signal is a negative signal; and
rejecting noise in the difference signal that is out of phase with the drive signal through multiplying the difference signal by the sign of the drive signal.

11. The method of claim 10, wherein generating a difference signal comprises subtracting the output of the second voltage divider from the output of the first voltage divider.

12. The method of claim 10, wherein the rejecting of the noise in the difference signal that is out of phase with the drive signal includes multiplying the difference signal by the sign of the drive signal to perform a synchronous rectification of the difference signal.

13. The method of claim 12, wherein rejecting noise comprises using synchronous rectification.

14. The method of claim 10, further comprising rejecting out of band noise in the difference signal that is in phase with the drive signal.

15. The method of claim 14, wherein rejecting out of band noise comprises low-pass filtering the difference signal.

16. The method of claim 10, further comprising converting the difference signal to a digital signal.

17. A system comprising:
a capacitive sensor array comprising columns and rows of sensor elements, wherein the intersections of the columns and the rows form sensor locations, and wherein the columns of sensor elements form a first plurality of sensor elements and the rows of sensor elements form a second plurality of sensor elements;
a first voltage divider coupled to a first sensor element of the second plurality of sensor elements and coupled to ground, wherein the first voltage divider is configured to decrease a first voltage at the first sensor element to a second voltage at an output of the first voltage divider;
a second voltage divider coupled to a second sensor element of the second plurality of sensor elements and coupled to the ground, wherein the second voltage divider is configured to decrease a third voltage at the second sensor element to a fourth voltage at an output of the second voltage divider;
a voltage subtractor comprising a first input coupled to the first voltage divider and a second input coupled to the second voltage divider, the voltage subtractor configured to generate a difference in measured signals on the first sensor element and the second sensor element, wherein the measured signals represent mutual capacitances, responsive to a drive signal, at the intersections of the first sensor element and the second sensor element, wherein, responsive to proximity of a conductive object closer to the first sensor element than the second element, the difference signal is a positive signal, and wherein, responsive to proximity of the conductive object closer to the second sensor element than the first element, the difference signal is a negative signal; and
an amplitude detect circuit coupled to the voltage subtractor and configured to reject a signal generated by both a column and a row of the capacitive sensor array, which is out of phase with the drive signal.

18. The system of claim 17, wherein a single column of the capacitive sensor array is a reference for one or more other columns sensed.

19. The system of claim 18, wherein a single row of the capacitive sensor array is a reference for one or more other rows sensed.