# Patent

## Title
Touch sensing

## Abstract

A method and apparatus varying, by interval, a frequency of a drive signal applied to one electrode of each of a plurality of electrode pairs, select a frequency corresponding to the frequency of the drive signal, monitor changes in capacitance of each of the electrode pairs through receive signals at the selected frequency, from the other electrode of each of the plurality of electrode pairs; and determine a position of at least two objects, which are simultaneously on a touch device, according to the monitored capacitance changes.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 12/040,387, filed Feb. 29, 2008, which claims the priority benefit of U.S. provisional application no. 61/023,988, filed Jan. 28, 2008, each of which is incorporated herein by reference in its entirety for all purposes.


TECHNICAL FIELD
The present disclosure relates generally to user interfaces utilizing capacitive sensing.
BACKGROUND
User interfaces utilizing capacitive sensing include circuitry for detecting a capacitance change when a finger or other object is adjacent to a touch screen, touch pad, series of touch buttons, or other touch panel. The FIG. 1 shows an example electrode grid array 1 (also known as an electrode matrix) used by a touch sense device having a touch pad.
The array 1 includes rows 3 and columns 4 of electrodes overlaid by a dielectric (not shown). A charge is applied to both of the electrodes 3 and 4 by circuits 5 such that, if a finger or other conductive object is placed on the touch panel, the finger or other object increases capacitive coupling to ground.
The change in capacitance caused by the finger touching the touch panel can be detected by circuits 5 coupled to the electrodes 3 and 4. Ones of the circuits 5 that correspond to ones of the electrodes 3 and 4 that are closest to the finger measure different capacitance than the remaining circuits 5. These capacitive measurements can be used to identify which intersection of the electrodes 3 and the column electrodes 4 is closest to the finger, which can be used to pinpoint the position of the finger. This process is dynamic as the finger changes positions on the touch panel allowing the touch sense device to, for example, move a mouse pointer on a display to correspond with the finger movement.
The circuits 5 do not have the ability to resolve multiple presses, i.e. more than position on the touch panel are being touched simultaneously, or nearly simultaneously. The disclosure that follows solves this and other problems.
SUMMARY OF THE INVENTION
In one embodiment, an apparatus comprises a plurality of capacitors, each having a first electrode and a second electrode. The apparatus includes charging circuitry coupled to the first electrodes and sensing circuitry coupled to the second electrodes, the sensing circuitry configured to detect changes in capacitance across the capacitors responsive to movement of an input object relative to the apparatus. Interpolating circuitry identifies which one of the capacitors is nearest to the input object according to the detected capacitance changes.
The apparatus can also include multiplexing circuitry electrically interposed between the first electrodes and the charge circuitry, the multiplexing circuitry alternating which one of the first electrodes is electrically coupled to the charge generation circuitry according to a time division scheme.
The charge circuitry can include a pseudo random signal generator to provide a signal driving the first electrodes. The pseudo random signal generator provides a reference signal to a demodulator that is connected to the second electrodes. The demodulator demodulates a signal corresponding to the second electrodes according to the reference frequency generated by the pseudo random signal generator. A low pass filter to can be used to filter the output of the demodulator. An analog to digital converter can be used to sample the output of the low pass filter.
The apparatus can be coupled to a graphical user interface or other interface that allows a user to adjust at least one characteristic of circuit components used in the sensing circuitry and/or charge circuitry. The adjusted characteristics can be selected from the group comprising the center frequency of a pseudo random signal generator, a sampling rate of the analog to digital converter, and an operating characteristic of a low pass filter.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an electrode grid array used by a typical touch sense device.
FIG. 2A illustrates an apparatus to measure capacitance occurring at a junction point in an electrode grid array.
FIG. 2B illustrates a multi-touch capability of the apparatus shown in FIG. 2A.
FIG. 3 illustrates one example of the sensing circuitry shown in FIG. 2A.
FIG. 4 illustrates a graphical user interface for adjusting the circuitry shown in FIG. 3 to optimize performance with different hardware.
FIG. 5 illustrates a method for assembling and using the apparatus circuitry shown in FIG. 2A.
FIG. 6 illustrates another example of the sensing circuitry shown in FIG. 3.
FIGS. 7A and 7B show other capacitor arrangements that can be used with the sensing circuitry of FIG. 3.



DESCRIPTION OF EXAMPLE EMBODIMENTS
Several preferable examples of the present application will now be described with reference to the accompanying drawings. Various other examples of the invention are also possible and practical. This application may be exemplified in many different forms and should not be construed as being limited to the examples set forth herein.
The figures listed above illustrate preferable examples of the application and the operation of such examples. In the figures, the size of the boxes is not intended to represent the size of the various physical components. Where the same element appears in multiple figures, the same reference numeral is used to denote the element in all of the figures where it appears. When two elements operate differently, different reference numerals are used regardless of whether the two elements are of the same class.
Only those parts of the various units are shown and described which are necessary to convey an understanding of the examples to those skilled in the art. Those parts and elements not shown may be conventional and known in the art.
FIG. 2A illustrates an apparatus to measure capacitance occurring at a junction point in an electrode grid array.
The apparatus 200 includes sensing circuitry 15 to identify changing capacitance between the row and column electrodes 13 and 14 of the electrode grid array 11. What follows below with reference to FIG. 2A is an overview of one example of the sensing circuitry 15; a more detailed overview of the sensing circuitry 15 will be provided later with reference to other figures.
The sensing circuitry 15 includes signal generating circuitry 18, which provides a charge to the column electrodes 14. The multiplexing circuitry 17 selectively couples the column electrodes 14 to the circuitry 18 such that, at any given instant, the charge is being fed to only one of the column electrodes 14. In the present example, the cycle time for the multiplexing circuitry 17 is sixteen milliseconds, meaning that after sixteen milliseconds every column electrode 14 has received the charge. This sixteen millisecond cycle time can be different for other examples, and is hereinafter referred to as the “cycle time”. Although the charge is fed to the column electrodes 13 one at a time in the present example, other schemes whereby more than one column electrode 13 is charged at any given instant are also possible and practical. Although the charge is fed to the column electrodes 14 in the present example, it should be apparent that in other examples the charge could instead be fed to the row electrodes 13. During a cycle, unselected electrodes may be driven to a fixed potential, such as a circuitry ground.
In the present example, the sensing circuitry 15 also includes one instance of capacitive change detection circuitry 19 for each row electrode 13. For ease of illustration, only one of the instances of the circuitry 19 is connected to one row electrode, namely row electrode R0, but it should be apparent that connections exist between the other circuits 19 and the other row electrodes 13. As illustrated by each of the capacitor symbols 12 between each row/column junction, the circuitry 19 detects changes in capacitance between the corresponding row and column electrodes 13 and 14. For example, as a finger or other capacitive surface approaches the column electrode C0, one or more of the circuits 19 will detect a decrease in voltage and an increase in capacitance. Generally speaking, whichever one of the circuits 19 measures the greatest capacitive change is connected to the row electrode 13 closest to the approaching finger. This information, when combined with information about which column electrode 14 is being powered, can be used by interpolating circuitry 31 to determine which intersection of the rows and columns is closest to the finger. In an embodiment, the interpolating circuitry 31 identifies which one of the capacitors is nearest to the finger according to the detected capacitive changes.
Although the present example includes one circuit 19 for each row electrode 13 (parallel sensing), it should be apparent that the sensing can also use a time division multiplexing scheme where a multiplexer selectively connects a single instance of the circuit 19 sequentially through all row electrodes 13 (serial sensing). Also, although the circuits 19 detect the row electrodes 13 in the present example, it should be apparent that in other examples the circuits 19 could instead detect the column electrodes 14.
As described above, measuring capacitance between the row and column electrodes 13 and 14 allows a touch sense device to resolve a change in capacitance at a point, instead of at an entire row or entire column. This feature provides numerous advantages, some of which are described with reference to FIG. 2B.
FIG. 2B illustrates a multi-touch capability of the apparatus shown in FIG. 2A.
Referring now to FIG. 2B, such measurements allow multi-touch sensing. Multi-touch sensing allows, for example, a touch sense device to detect a plurality of finger presses occurring in a single cycle time. For example, a touch sense device using the sensing circuitry 15 can, for example, identify the presence of a finger touching the touchpad at a position corresponding to Point-N while identifying another finger touches the touchpad at a different position at Points 1-3 during a same cycle time. This multi-sense capability enables new applications, such as a touch screen allowing a user to input data using both hands, and a touch screen that is capable of detecting a capacitive image according to differing capacitance measurements.
Besides multi-touch, the sensing circuitry 15 provides other advantages. Point-based capacitance sensing can provide better accuracy than previous sensing that detected capacitive change in entire rows and/or columns Also, point based touch sensing only requires charging one set of the electrodes, not both, which can reduce power consumption.
FIG. 3 illustrates one example of the sensing circuitry shown in FIG. 2.
The sensing circuitry 15 includes a modulated signal generator 21 to drive electrode 13 with an Alternating Current (AC) signal. For reasons that will be explained in greater detail later, preferably a Pseudo Random Signal generator (PRS) is selected as the modulated signal generator 21. Although the generator 21 drives the row electrode 13, in other examples the generator 21 instead drives the column electrode 14.
The signal output by the generator 21 is coupled across the capacitive structure 22, which includes electrodes 13 and 14 and a dielectric layer separating electrodes 13 and 14. The capacitance across the capacitor structure 22 changes according to the presence or absence of a finger on the touchpad adjacent to the electrode 14. For example, when a finger is adjacent to the electrode 14, the coupling to the input stage (the buffer/amplifier) decreases.
The remaining circuit elements 23-27 measure the change in capacitance across the capacitor element 22. The demodulator 25 receives a signal directly from the generator 21, which is used to select a passing frequency for the demodulator 25. For example, when the signal generator 21 outputs a signal at 200 kHz, the demodulator 25 receives such indication and passes 200 kHz signals from the buffer/amplifier 24. Passing the 200 kHz signals (as opposed to all signals) can prevent noise radiated from other circuits near or in the touch sense device from providing false indications of the presence or absence of a finger adjacent to the electrode 14.
The resistor 23 may not be needed depending on the resistance to ground. The ground can be an analog ground or a circuit ground. The low pass filter 26 can be used to remove ripple from the output of the demodulator 25. The Analog to Digital Converter (ADC) 27 can be used to convert the signal into digital form.
Referring again to the signal generator 21, preferably a PRS is used. A PRS operates by varying a pulse width and duty cycle. The output signal varies both in frequency and the amount of time the different frequencies are used. In the present example, the signal generator 21 is a PRS varying around a center frequency of 200 kHz.
As mentioned previously, several advantages are realized by using a PRS for the signal generator 21. Using a pseudo randomly varying signal output, or even a randomly varying signal output, greatly reduces the likelihood of neighboring circuit elements disrupting operation by radiating energy at a same frequency as the sensing element 15. Accordingly, such radiated energy is unlikely to pass the demodulator 25, thereby removing crosstalk. Thus, the use of a PRS instead of non-random signal generators such as a Pulse Code Modulator (PCM) minimizes the possibility of a false indication of the presence or absence of a finger.
Also, the random nature of the PRS minimizes the effect of Electro Magnetic Interference (EMI). When the touch sense device is adjacent or coupled to another electronic component such as a cell phone or Personal Digital Assistant (PDA), this feature can reduce interference with those devices.
It should be apparent that the above benefits may be realized by using any type of varying signal generator 21, whether it is random, pseudo random or otherwise varying intermittently. A Pulse-Width Modulator (PWM) may be used instead of the PRS as an alternate design. It should also be apparent that numerous modifications are possible to the above example circuit, for example it is possible to implement an ADC having the demodulator and the low-pass filter integrated therein.
FIG. 4 illustrates a graphical user interface for adjusting the circuitry shown in FIG. 3 to optimize performance with different hardware.
The graphical user interface 40 includes software dials or interfaces 41-44 for adjusting characteristics of the sensing circuitry 15 according to characteristics of hardware used with the sensing circuitry 15. It may be advantageous to manufacture the sensing circuitry 15 as a modularized component that can be used with a wide variety of touch panels and electrode grid arrays. In such a scheme, the graphical interface 40 can be used to adjust the characteristics of the aforementioned PRS 21, the ADC 27, and the LPF 26, as well as any other components of the sensing circuitry. The graphical interface 40 can be displayed on a display attached to a general purpose computer that is connected to the sensing circuitry via a Universal Serial Bus (USB) or other connection.
The interface 41 can be used to adjust the center frequency of the PRS 21. Adjusting the center frequencies can help tune the sensing circuitry 15 to the particular electrode grid array and touch surface to which the sensing circuitry is connected. The amount of distance there is between electrode layers in the electrode array, and the amount of distance there is between the touch surface and the electrodes, may affect an optimal center frequency. The interface 41 can be used to manually test different center frequencies to determine, empirically, which center frequency produces the desired results with the particular hardware.
Also, if the interface 41 can be used to adjust the center frequency to reduce the effect of radiated energy from the sensing circuitry 15 to other unrelated components that may be placed in close physical proximity to the sensing circuitry 15. For example, in a cell phone with a touch panel, the center frequency PRS 21 may need to be adjusted to avoid interference with the cell phone's transceiver.
The interface 42 can be used to adjust the sample rate of the ADC 27 to correspond with the set center frequency of the PRS 21. In most scenarios, a one to one correspondence has been empirically shown to be preferable, e.g. if the center frequency is two hundred Hertz the ADC 27 sample rate should be set to two hundred Hertz. However, it can be advantageous to set the sample rate to be any multiple of the center frequency, for example twice or four times the center frequency, dependent on the hardware characteristics.
The interface 43 can be used to adjust multiple characteristics of the LPF 26, such as the cutoff frequency, whether the LPF 26 provides amplification to the signal, the roll off of the LPF 26, etc. The characteristics of the LPF 26 may be tuned to correspond with circuit characteristics including the center frequency of the PRS 21.
The characteristics of the LPF 26 may also be tuned according to characteristics of the associated touch device. For example, the LPF's 26 cutoff frequency should be tuned below the PRS 21 center frequency. The LPF 26 should not be tuned so low that it restricts the ADC sample rate.
One or more of the additional interfaces 44 can be used to change any other characteristics of the sensing circuitry. Selectively coupling the resistor 23 between ground and analog ground can provide performance differences. For example, using the interface 44 to select an analog ground produces a full wave rectified signal input into the demodulator 25, while using the interface 44 to select a circuit ground produces a half wave rectified input signal. Selecting between these input signals may be helpful depending on the features of the analog to digital converter used. The interface 44 can also be used to vary the value of the resistor 23 based on circuit characteristics, which can affect responsiveness of the circuitry at the price of accuracy, and vice versa.
The software controls 41-44 may allow a user to slide a bar to vary characteristics, turn a software dial, or input numerical values to be used. It should be apparent that, although the present example uses a graphical user interface 40 with software controls 41-44, other examples may utilize physical knobs or other non-software controls to adjust the various characteristics as discussed above.
FIG. 5 illustrates a method for making and using the sensing circuitry shown in FIG. 3.
A process 500A for making the sensing circuitry is illustrated. In block 501, a dielectric layer is formed between two sets of electrodes, the electrodes arranged in a grid with the first set being arranged non-parallel to the second set. In block 502, a touchable surface/overlay or other insulating layer is provided adjacent to one of the electrode sets.
In block 503, a signal generator is electrically coupled to one or more electrodes of one of the sets. The signal generator can be an intermittently varying signal generator, either random or pseudo random, or a signal generator that emits a constant signal. In block 504, the remaining set of electrodes is connected to sensing circuitry to measure capacitance between electrodes from the first and second sets.
A process 500B for using the sensing circuitry includes blocks 505-507. In block 505, the sensing circuitry is used to determine whether there is a change in capacitance between the electrodes of the first and second set. If there is a capacitance change in box 506, in block 507 an interpolating component implemented as circuitry or a processor executing stored instructions identifies the presence of a finger or other object on a corresponding portion of the touchable surface.
FIG. 6 illustrates another example of the sensing circuitry shown in FIG. 3.
The additional example of the sensing circuitry 15 shown in FIG. 3 provides a more simplified circuit whereby the electrode 14 is connected to the ADC 27, with only a pull down resistor 23 positioned between them. This simplified version of the sensing circuitry 15 omits several elements, which reduces costs in exchange for performance. The additional example of the sensing circuitry 15 uses a modulated signal generator 22 instead of the PRS generator.
FIGS. 7A and 7B show other capacitor arrangements that can be used with the sensing circuitry of FIG. 3.
In the arrangement shown in FIG. 7A, the capacitors 98 correspond to a touch button device or other touch device besides a trackpad touch device. The capacitors 98 are arranged in a line, which may be referred to as a 1×3 capacitor array having one column and three rows. Each of the capacitors 98 includes one electrode electrically coupled to a charge providing device such as a PRS and another electrode coupled to the sensing circuitry. As a finger or other object approaches the capacitor arrangement, there is a greater capacitive change associated with one of the capacitors 98 than the remaining capacitors. It should be apparent that the electrodes of each capacitor 98 may be directly coupled to the PRS and sensing circuitry, or coupled to the PRS and sensing circuitry through intervening multiplexing circuitry.
In the arrangement shown in FIG. 7A, the capacitors 99 correspond to a touch button device or other touch device besides a trackpad touch device. The capacitors 99 are arranged in a 2×3 capacitor array having two columns and three rows. Each of the capacitors 99 includes one electrode electrically coupled to a charge providing device such as a PRS and another electrode coupled to the sensing circuitry. As a finger or other object approaches the capacitor arrangement, there is a greater capacitive change associated with one of the capacitors 99 than the remaining capacitors. It should be apparent that the electrodes of each capacitor 99 may be directly coupled to the PRS and sensing circuitry, or coupled to the PRS and sensing circuitry through intervening multiplexing circuitry.
Several preferable examples have been described above with reference to the accompanying drawings. Various other examples of the invention are also possible and practical. The system may be exemplified in many different forms and should not be construed as being limited to the examples set forth above.
The figures listed above illustrate preferable examples of the application and the operation of such examples. In the figures, the size of the boxes is not intended to represent the size of the various physical components. Where the same element appears in multiple figures, the same reference numeral is used to denote the element in all of the figures where it appears.
Only those parts of the various units are shown and described which are necessary to convey an understanding of the examples to those skilled in the art. Those parts and elements not shown are conventional and known in the art.
The system described above can use dedicated processor systems, micro controllers, programmable logic devices, or microprocessors that perform some or all of the operations. Some of the operations described above may be implemented in software and other operations may be implemented in hardware.
For the sake of convenience, the operations are described as various interconnected functional blocks or distinct software modules. This is not necessary, however, and there may be cases where these functional blocks or modules are equivalently aggregated into a single logic device, program or operation with unclear boundaries. In any event, the functional blocks and software modules or features of the flexible interface can be implemented by themselves, or in combination with other operations in either hardware or software.
Having described and illustrated the principles of the invention in a preferable embodiment thereof, it should be apparent that the invention may be modified in arrangement and detail without departing from such principles. I claim all modifications and variation coming within the spirit and scope of the following claims.

## Claims

1. An apparatus comprising:
a pseudo random signal generator to provide a first signal to a first electrode, to vary a frequency of the first signal, wherein the first electrode is configured to form a first capacitance with a second electrode;
a demodulator to receive the first signal from the second electrode, and to receive, from the pseudo random signal generator, a reference signal indicative of the frequency of the first signal, wherein the reference signal is used to select a passing frequency used by the demodulator to pass a portion of the first signal from the second electrode; and
detection circuitry to detect, using a demodulated first signal, a change in the first capacitance of the first electrode and the second electrode, wherein the change in the first capacitance is responsive to a presence of a first input object proximate the first electrode and second electrode, the detection circuitry further to detect, using another demodulated first signal, a change in a second capacitance of a third electrode and a fourth electrode, wherein the change in the second capacitance is responsive to a presence of a second input object proximate the third electrode and the fourth electrode, wherein the presence of the first input object is concurrent with the presence of the second input object, and wherein the other demodulated first signal corresponds to the first signal provided to the third electrode by the pseudo random signal generator, the third electrode configured to form the second capacitance with the fourth electrode.

2. The apparatus of claim 1, further comprising multiplexing circuitry to alternate which of the first electrode or a third electrode receives the first signal according to a time division scheme.

3. The apparatus of claim 1, wherein the first signal comprises an alternating current signal.

4. The apparatus of claim 1, wherein the demodulator is to vary the passing frequency to correspond with the frequency of the first signal, wherein the demodulated first signal includes the portion of the first signal at the passing frequency.

5. The apparatus of claim 1, wherein the change in the first capacitance comprises a change in capacitance at an intersection of the first electrode and the second electrode.

6. The apparatus of claim 5, wherein the first electrode and the second electrode form an electrode grid array, the first electrode being non-parallel with the second electrode.

7. An apparatus comprising:
a pseudo random signal generator to provide a first set of electrodes with drive signals that vary in frequency, the first set of electrodes to form capacitance with a second set of electrodes; and
detection circuitry comprising a demodulator, the demodulator to receive, from the pseudo random signal generator, reference signals indicative of the frequency of the drive signals, wherein the reference signals are used to select passing frequencies used by the demodulator to pass portions of receive signals of the second set of electrodes, wherein the receive signals of the second set of electrodes correspond to the drive signals provided to the first set of electrodes by the pseudo random signal generator, the detection circuitry to use demodulated receive signals to detect changes in the capacitance, wherein the detection circuitry is configured to detect a change in a first capacitance between a first electrode, of the first set of electrodes, and a second electrode, of the second set of electrodes, using a first demodulated receive signal and detect a change in a second capacitance between a third electrode, of the first set of electrodes, and a fourth electrode, of the second set of electrodes, using a second demodulated receive signal.

8. The apparatus of claim 7, further comprising an interpolating component to interpolate a position of an object relative to the first set of electrodes and the second set of electrodes, based on the detected changes in capacitance.

9. The apparatus of claim 7, wherein the first set of electrodes and the second set of electrodes are included in at least one of a touchpad or a touchscreen.

10. The apparatus of claim 7, further comprising multiplexing circuitry to alternate, according to a time division scheme, which electrode of the first set of electrodes is to receive the drive signals.

11. The apparatus of claim 7, wherein the detection circuitry is to vary the passing frequencies for the receive signals responsive the reference signals from the pseudo random signal generator that indicates the frequency of the drive signals.

12. The apparatus of claim 7, wherein a center frequency of the pseudo random signal generator is adjustable through a graphical user interface.

13. A method comprising:
providing, by a pseudo random signal generator, a drive signal to one electrode of each of a plurality of electrode pairs, wherein a frequency of the drive signal is varied;
receiving, at a demodulator and from the pseudo random signal generator, a reference signal indicative of the frequency of the drive signal to the one electrode of each of the plurality of electrode pairs, the reference signal used to select a passing frequency used by the demodulator to pass a portion of a receive signal;
monitoring changes in capacitance of each of the electrode pairs in view of the receive signal, the receive signal corresponding to the drive signal provided to the one electrode of each of the plurality of electrode pairs and received from the other electrode of each of the plurality of electrode pairs; and
determining, in view of the monitored changes, a position of at least two objects concurrently proximate a touch device, wherein a first change of capacitance of a first electrode pair of the plurality of electrode pairs is responsive to a presence of a first object of the at least two object, and a second change of capacitance of a second electrode pair of the plurality of electrode pairs is responsive to a present of a second object of the at least two objects.

14. The method of claim 13, wherein providing the drive signal to the one electrode of each of a plurality of electrode pairs is provided in view of a time division multiplexing scheme, the method further comprising:
determining a plurality of contact positions on a touch device, the plurality of contact positions determined within a same cycle of the time division multiplexing scheme.

15. The method of claim 13, further comprising
adjusting a center frequency of the drive signal using a graphical user interface.

16. The method of claim 13, further comprising
adjusting a sample rate of an analog to digital converter that digitizes the monitored changes in the capacitance to correspond with an adjusted center frequency.

17. The method of claim 13, further comprising
changing a cutoff frequency of a low pass filter in view of an adjusted center frequency.

18. The method of claim 13, further comprising at least one of changing a cutoff frequency of a low pass filter in view of a responsiveness of a position determination or adjusting a resistance of a pull down resistor in view of an accuracy of the position determination.

19. The method of claim 13, wherein the frequency of the drive signal is varied after an interval comprises varying the frequency of an alternating current signal.