Close touch detection and tracking

A method of tracking touches at a touch-sensing surface may include, detecting an initial location of a first contact and an initial location of a second contact at the touch-sensing surface based on a first scan of a touch-sensing surface, detecting a plurality of signal levels caused by the first contact and the second contact during a second scan of the touch-sensing surface, identifying a first signal level of the plurality of signal levels as a local maximum, and locating a lost touch based on one or more signal levels associated with one or more unit cells within a fixed distance from a first unit cell associated with the local maximum.

TECHNICAL FIELD

This disclosure relates to the field of touch-sensors and, in particular, to the detection and tracking of close touches at a capacitive touch-sensing surface.

BACKGROUND

Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of one or more conductive objects, such as a finger. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes.

Another user interface device that has become more common is a touch screen. Touch screens, also known as touchscreens, touch windows, touch panels, or touchscreen panels, are transparent display overlays which are typically either pressure-sensitive (resistive or piezoelectric), electrically-sensitive (capacitive), acoustically-sensitive (surface acoustic wave (SAW)) or photo-sensitive (infra-red). The effect of such overlays allows a display to be used as an input device, removing the keyboard and/or the mouse as the primary input device for interacting with the display's content. Such displays can be attached to computers or, as terminals, to networks. Touch screens have become familiar in retail settings, on point-of-sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs where a stylus is sometimes used to manipulate the graphical user interface (GUI) and to enter data. A user can touch a touch screen or a touch-sensor pad to manipulate data. For example, a user can apply a single touch, by using a finger to touch the surface of a touch screen, to select an item from a menu.

DETAILED DESCRIPTION

The performance of a touchscreen may be affected by many factors. Such performance characteristics may include the precision with which a touch position may be located, linearity when tracking a moving touch, proper detection of multiple touches, and proper recognition of gestures.

One embodiment of an method for searching and processing touches at a touch-sensing surface may not be able to detected close touches, where the distance between touches is less than three times the sensor pitch. For example, when touches are close together, some touches may be lost and linearity may degrade when tracking the motion of the close touches.

FIGS. 3A and 3Billustrate the tracked paths of multiple touches at a touch-sensing surface, where the closeness of the touches causes inaccuracies in the calculated locations and paths, according to one embodiment. Specifically,FIG. 3Aillustrates the detected paths of two conductive objects each having a diameter of 11 millimeters and separated by a gap of 2 millimeters between their edges.FIG. 3Billustrates the detected paths of four 8 millimeter diameter conductive objects separated by a gap of 3 millimeters between their edges.

Because of the closeness of the conductive objects at the surface, the system may also have difficulty interpreting continuous moving touches. InFIG. 3A, for example, each of the separate line segments may be assigned a different touch identification (touch ID) even when they are caused by the same object.

One method for designing a touch-sensing system to more accurately detect and track close touches is to decrease the pitch size to less than two times the size of the object, such as a finger, that touches the surface. Alternatively, specialized processing may be used to detect “lost” touches and improve the linearity of a path of the touches when the touches are close together.

FIG. 4illustrates a “heat map”400including signal levels detected by unit cells of a touch-sensing surface, according to one embodiment. In one embodiment, each unit cell may be associated with an intersection between unique pairs of horizontal and vertical sensor elements, and the signal levels in the heat map400represent a difference between a count value measured at the corresponding intersection and a baseline count value for that intersection.FIG. 4illustrates a particular case where four conductive objects are contacting the touch-sensing surface, resulting in the illustrated signal levels.

One embodiment method for calculating locations of the conductive objects at the touch-sensing surface may calculate centroids based on local maxima identified from among the signal levels. Such a process may detect only three local maxima410from the signal levels in heat map400, despite the presence of four conductive objects at the touch-sensing surface. Thus, when touches are close together, some touches may be lost.

FIGS. 5A and 5Billustrate a heat map500resulting from two diagonally close touches at the touch-sensing surface and the corresponding touch location calculated using a standard local maxima detection method, respectively. For the heat map500, the distance between the two touches is less than three times the sensor pitch; thus, the local maxima detection method detects only local maximum510. This is interpreted as a single contact at the calculated touch location520.

FIG. 6Aillustrates paths of two diagonally close conductive objects as tracked using an embodiment of a standard local maxima detection method. As illustrated inFIG. 6A, the smoothness and continuity of the paths are degraded as a result of the close touch condition.FIG. 6Billustrates paths of four diagonally close conductive objects as tracked using an embodiment of a standard local maxima detection method. The paths as illustrated inFIG. 6Balso suffer breaks in continuity and degraded linearity due to the closeness of the touches.

In one embodiment, a standard local maxima detection process may determine that a signal level B is a local maxima if A<B≧C, according to the diagram inFIG. 7. InFIG. 7, i and j represent indexes of column and row sensor elements. Thus, the signal level B is at the center of a 9×9 grid of unit cells corresponding to the intersections between sensor elements j−1, j, and j+1 and column sensor element i−1, i, and i+1.

FIG. 8Aillustrates a set of signal levels resulting from two conductive objects contacting the touch-sensing surface, according to an embodiment. For example, the signal levels inFIG. 8Amay be caused by two 7 mm diameter conductive objects separated by a 3.5 mm gap between edges. The local maxima are detected at the unit cells having signal levels39and46; these unit cells correspond to the “B” unit cell inFIG. 7.

FIG. 8Billustrates the resulting detected touch locations when the standard local maxima detection process is used with the signal levels ofFIG. 8A. Here, the signal levels are accurately interpreted as two separate touches at contact locations810and811.

FIG. 9Aillustrates a set of signal levels resulting from two close touches at the touch-sensing surface. Here, the standard local maxima detection method detects one conductive object instead of two.FIG. 9Billustrates the contact location910that is detected based on the signal levels illustrated inFIG. 9A.

FIG. 10Aillustrates signal levels resulting from three conductive objects at the touch-sensing surface, where the conductive objects are sufficiently spaced apart so that the standard local maxima detection method accurately reports all three touch locations. Here, the detected local maxima have the values31,32, and41.FIG. 10Bshows the corresponding contact locations1011,1010, and1012, respectively.

FIG. 11Aillustrates signal levels resulting from three conductive objects at the touch-sensing surface, where two of the touches are close together. Here, the standard local maxima detection method only detects the signal levels32and41as local maxima, resulting in the contact locations1110and1111, as illustrated inFIG. 11B.

In one embodiment, a method for accurately locating and tracking close touches at a touch-sensing system may identify unit cells associated with the local maxima in a set of signal levels, then analyze the signal levels of unit cells surrounding the unit cells of the local maxima. In one embodiment, a lost touch may be identified based on analysis of unit cells that are a specific number of unit cells (such as two unit cells) away from the unit cells where the local maxima are detected.

In one embodiment, a close touch tracking method may maintain a list of local maxima that potentially correspond to actual touches, then remove local maxima from the list that are attributable to other touches.

FIG. 1illustrates a block diagram of one embodiment of an electronic system100including a processing device110that may be configured to measure capacitances from a touch sensing surface116including a capacitive touch sensor pattern as described above. The electronic system100includes a touch-sensing surface116(e.g., a touchscreen, or a touch pad) coupled to the processing device110and a host150. In one embodiment, the touch-sensing surface116is a two-dimensional user interface that uses a sensor array121to detect touches on the surface116.

In one embodiment, the sensor array121includes sensor elements121(1)-121(N) (where N is a positive integer) that are disposed as a two-dimensional matrix (also referred to as an XY matrix). The sensor array121is coupled to pins113(1)-113(N) of the processing device110via one or more analog buses115transporting multiple signals. In this embodiment, each sensor element121(1)-121(N) is represented as a capacitor.

In one embodiment, the capacitance sensor101may include a relaxation oscillator or other means to convert a capacitance into a measured value. The capacitance sensor101may also include a counter or timer to measure the oscillator output. The processing device110may further include software components to convert the count value (e.g., capacitance value) into a sensor element detection decision (also referred to as switch detection decision) or relative magnitude. It should be noted that there are various known methods for measuring capacitance, such as current versus voltage phase shift measurement, resistor-capacitor charge timing, capacitive bridge divider, charge transfer, successive approximation, sigma-delta modulators, charge-accumulation circuits, field effect, mutual capacitance, frequency shift, or other capacitance measurement algorithms. It should be noted however, instead of evaluating the raw counts relative to a threshold, the capacitance sensor101may be evaluating other measurements to determine the user interaction. For example, in the capacitance sensor101having a sigma-delta modulator, the capacitance sensor101is evaluating the ratio of pulse widths of the output, instead of the raw counts being over or under a certain threshold.

In one embodiment, the processing device110further includes processing logic102. Operations of the processing logic102may be implemented in firmware; alternatively, it may be implemented in hardware or software. The processing logic102may receive signals from the capacitance sensor101, and determine the state of the sensor array121, such as whether an object (e.g., a finger) is detected on or in proximity to the sensor array121(e.g., determining the presence of the object), where the object is detected on the sensor array (e.g., determining the location of the object), tracking the motion of the object, or other information related to an object detected at the touch sensor.

In another embodiment, instead of performing the operations of the processing logic102in the processing device110, the processing device110may send the raw data or partially-processed data to the host150. The host150, as illustrated inFIG. 1, may include decision logic151that performs some or all of the operations of the processing logic102. Operations of the decision logic151may be implemented in firmware, hardware, software, or a combination thereof. The host150may include a high-level Application Programming Interface (API) in applications152that perform routines on the received data, such as compensating for sensitivity differences, other compensation algorithms, baseline update routines, start-up and/or initialization routines, interpolation operations, or scaling operations. The operations described with respect to the processing logic102may be implemented in the decision logic151, the applications152, or in other hardware, software, and/or firmware external to the processing device110. In some other embodiments, the processing device110is the host150.

In another embodiment, the processing device110may also include a non-sensing actions block103. This block103may be used to process and/or receive/transmit data to and from the host150. For example, additional components may be implemented to operate with the processing device110along with the sensor array121(e.g., keyboard, keypad, mouse, trackball, LEDs, displays, or other peripheral devices).

The processing device110may reside on a common carrier substrate such as, for example, an integrated circuit (IC) die substrate, or a multi-chip module substrate. Alternatively, the components of the processing device110may be one or more separate integrated circuits and/or discrete components. In one embodiment, the processing device110may be the Programmable System on a Chip (PSoC™) processing device, developed by Cypress Semiconductor Corporation, San Jose, Calif. Alternatively, the processing device110may be one or more other processing devices known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable device. In an alternative embodiment, for example, the processing device110may be a network processor having multiple processors including a core unit and multiple micro-engines. Additionally, the processing device110may include any combination of general-purpose processing device(s) and special-purpose processing device(s).

In one embodiment, the electronic system100is implemented in a device that includes the touch-sensing surface116as the user interface, such as handheld electronics, portable telephones, cellular telephones, notebook computers, personal computers, personal data assistants (PDAs), kiosks, keyboards, televisions, remote controls, monitors, handheld multi-media devices, handheld video players, gaming devices, control panels of a household or industrial appliances, or other computer peripheral or input devices. Alternatively, the electronic system100may be used in other types of devices. It should be noted that the components of electronic system100may include all the components described above. Alternatively, electronic system100may include only some of the components described above, or include additional components not listed herein.

FIG. 2is a block diagram illustrating one embodiment of a capacitive touch sensor array121and a capacitance sensor101that converts changes in measured capacitances to coordinates indicating the presence and location of touch. The coordinates are calculated based on changes in measured capacitances relative to the capacitances of the same touch sensor array121in an un-touched state. In one embodiment, sensor array121and capacitance sensor101are implemented in a system such as electronic system100. Sensor array220includes a matrix225of N×M electrodes (N receive electrodes and M transmit electrodes), which further includes transmit (TX) electrode222and receive (RX) electrode223. Each of the electrodes in matrix225is connected with capacitance sensing circuit201through demultiplexer212and multiplexer213.

Capacitance sensor101includes multiplexer control211, demultiplexer212and multiplexer213, clock generator214, signal generator215, demodulation circuit216, and analog to digital converter (ADC)217. ADC217is further coupled with touch coordinate converter218. Touch coordinate converter218may be implemented in the processing logic102.

The transmit and receive electrodes in the electrode matrix225may be arranged so that each of the transmit electrodes overlap and cross each of the receive electrodes such as to form an array of intersections, while maintaining galvanic isolation from each other. Thus, each transmit electrode may be capacitively coupled with each of the receive electrodes. For example, transmit electrode222is capacitively coupled with receive electrode223at the point where transmit electrode222and receive electrode223overlap.

Clock generator214supplies a clock signal to signal generator215, which produces a TX signal224to be supplied to the transmit electrodes of touch sensor121. In one embodiment, the signal generator215includes a set of switches that operate according to the clock signal from clock generator214. The switches may generate a TX signal224by periodically connecting the output of signal generator215to a first voltage and then to a second voltage, wherein said first and second voltages are different.

The output of signal generator215is connected with demultiplexer212, which allows the TX signal224to be applied to any of the M transmit electrodes of touch sensor121. In one embodiment, multiplexer control211controls demultiplexer212so that the TX signal224is applied to each transmit electrode222in a controlled sequence. Demultiplexer212may also be used to ground, float, or connect an alternate signal to the other transmit electrodes to which the TX signal224is not currently being applied. In an alternate embodiment the TX signal224may be presented in a true form to a subset of the transmit electrodes222and in complement form to a second subset of the transmit electrodes222, wherein there is no overlap in members of the first and second subset of transmit electrodes222.

Because of the capacitive coupling between the transmit and receive electrodes, the TX signal224applied to each transmit electrode induces a current within each of the receive electrodes. For instance, when the TX signal224is applied to transmit electrode222through demultiplexer212, the TX signal224induces an RX signal227on the receive electrodes in matrix225. The RX signal227on each of the receive electrodes can then be measured in sequence by using multiplexer213to connect each of the N receive electrodes to demodulation circuit216in sequence.

The mutual capacitance associated with each intersection between a TX electrode and an RX electrode can be sensed by selecting every available combination of TX electrode and an RX electrode using demultiplexer212and multiplexer213. To improve performance, multiplexer213may also be segmented to allow more than one of the receive electrodes in matrix225to be routed to additional demodulation circuits216. In an optimized configuration, wherein there is a 1-to-1 correspondence of instances of demodulation circuit216with receive electrodes, multiplexer213may not be present in the system.

When an object, such as a finger, approaches the electrode matrix225, the object causes a decrease in the measured mutual capacitance between only some of the electrodes. For example, if a finger is placed near the intersection of transmit electrode222and receive electrode223, the presence of the finger will decrease the charge coupled between electrodes222and223. Thus, the location of the finger on the touchpad can be determined by identifying the one or more receive electrodes having a decrease in measured mutual capacitance in addition to identifying the transmit electrode to which the TX signal224was applied at the time the decrease in capacitance was measured on the one or more receive electrodes.

By determining the mutual capacitances associated with each intersection of electrodes in the matrix225, the presence and locations of one or more conductive objects may be determined. The determination may be sequential, in parallel, or may occur more frequently at commonly used electrodes.

In alternative embodiments, other methods for detecting the presence of a finger or other conductive object may be used where the finger or conductive object causes an increase in measured capacitance at one or more electrodes, which may be arranged in a grid or other pattern. For example, a finger placed near an electrode of a capacitive sensor may introduce an additional capacitance to ground that increases the total capacitance between the electrode and ground. The location of the finger can be determined based on the locations of one or more electrodes at which a change in measured capacitance is detected.

The induced current signal227is integrated by demodulation circuit216. The rectified current output by demodulation circuit216can then be filtered and converted to a digital code by ADC217.

A series of such digital codes measured from adjacent sensor or intersections may be converted to touch coordinates indicating a position of an input on touch sensor array121by touch coordinate converter218. The touch coordinates may then be used to detect gestures or perform other functions by the processing logic102.

In one embodiment, the capacitance sensor101can be configured to detect multiple touches. One technique for the detection and location resolution of multiple touches uses a two-axis implementation: one axis to support rows and another axis to support columns. Additional axes, such as a diagonal axis, implemented on the surface using additional layers, can allow resolution of additional touches.

FIG. 12is a flow diagram illustrating a process1200for detecting contacts at a touch-sensing surface such as touch-sensing surface116, according to one embodiment. In one embodiment, the operations of process1200may be performed by a processing device such as processing device110. Alternatively, the process1200may be performed in the host150, or the operations of process1200may be divided between the host150and processing device110.

The touch detection process1200begins at block1201, where the processing device performs a scan of the touch-sensing surface. In one embodiment, the scan of the touch-sensing surface is performed by measuring a signal level from each unit cell of the touch-sensing surface. Each of the signal levels may be, for example, difference count values representing the difference between a measured count value and a baseline count value for the unit cell. From block1201, the process1200continues at block1203.

At block1203, the processing device identifies local maxima from among the signal levels acquired at block1201. For example, the firmware within the processing device may identify local maxima by analyzing, for each unit cell, signal levels from the adjacent unit cells. In one embodiment, the process for identifying the local maxima may use the comparison A<B≧C, as previously described with reference to the diagram inFIG. 7.

In one embodiment, the touch-sensing system interprets a signal level as indicating a touch if the signal level exceeds a touch threshold. The processing device may also assign a touch ID to a location or unit cell associated with each of the local maxima. Thus, the process1200may continue at block1205upon detecting at least one local maxima that exceeds a touch threshold. In one embodiment, if the touch threshold is not exceeded, the touch-sensing surface may be scanned again.

At block1205, the processing device evaluates whether the number of touches has changed from a previous scan of the touch-sensing surface. The number of local maxima detected at block1203that exceed the touch threshold may be interpreted as touches. The number of touches may be compared with an initial number of touches as determined by a previous scan of the touch-sensing surface, which may have been performed at block1201during a prior execution of the process1200.

At block1207, the processing device performs a process for finding lost touches and assigning touch IDs to the lost touches. In one embodiment, the processing device may determine a location of a lost touch based on signal levels associated with a subset of unit cells surrounding a unit cell where a local maximum was found.

FIG. 13Aillustrates an embodiment of a lost touch detection process1300that may be performed at block1207. The lost touch detection process1300begins at block1301. At block1301, the processing device identifies a subset of unit cells associated with sensor elements that are two unit cells away from a unit cell where a local maximum was detected. In other words, the subset includes unit cells associated with sensor elements located in the range of 2 times the sensor pitch from the unit cell where the local maximum was detected. In an alternative embodiment, the range may be some other fixed distance greater or less than 2 times the sensor pitch. From block1301, the process1300continues at block1303.

At block1303, the process1300identifies a second unit cell having a highest signal level from among the subset of unit cells. From block1303, the process1300continues at block1305.

At block1305, the processing device evaluates whether the highest signal level identified at block1303exceeds the touch threshold. If the signal level exceeds the touch threshold, this unit cell having the highest signal level is treated as a lost touch, and the process1300continues at block1307. Otherwise, the process1300continues at block1309.

At block1307, the processing device assigns a touch ID to the unit cell having the highest signal level, as determined at block1303. In one embodiment, the processing device may choose a touch ID to assign depending on the locations of touch IDs from a previous scan. For example, a previous scan may detect a number of touches that may each be assigned a unique touch ID. The processing device may assign to the identified “lost touch” one of the touch IDs from the previous scan that was nearest the same location in the previous scan, or may assign a touch ID that was lost since the previous scan. From block1307, the process1300continues at block1309.

At block1309, the processing device identifies the next unit cell at which a local maximum was detected. Execution of blocks1301-1307is then repeated using this next unit cell to locate any additional lost touches. In this way, the process1300may be repeated until it has been performed for all of the local maxima identified at block1203of process1200.

FIG. 13Billustrates a heat map400, also illustrated inFIG. 4, according to an embodiment. InFIG. 13B, the local maxima1351,1352, and1354are detected using the local maxima detection process, according to block1203. In one embodiment, the local maxima1351,1352, and1354are detected using the process for identifying local maxima as described with reference toFIG. 7.

For heat map400, a lost touch search process that analyzes sensor elements that are two elements away from the local maxima may identify a lost touch1353. In one embodiment, the process may analyze unit cells1350, which are associated with sensor elements that are two elements away from the local maximum1354. Because the signal level 78 is the highest signal level among the unit cells1350, the corresponding unit cell is identified as a location of a lost touch.

In one embodiment, when the subset of unit cells includes two or more signal levels that are equal, the process may select one based on the position of the unit cell relative to the central local maximum unit cell. For example, a closer unit cell may be selected as a lost touch over a unit cell having an equal signal level that is farther away from the central unit cell. Thus, the unit cell1353may be identified as the location of a lost touch instead of the unit cell directly above it, which also has a signal level of 78. From block1207, the process1200continues at block1209.

At block1211, the processing device calculates a centroid for each of the touches, including the local maxima and the recovered lost touches. The centroid calculation process may calculate a centroid position based on the signal levels of unit cells around a unit cell at which a local maximum or lost touch is detected. In one embodiment, the calculated centroid locations may be used to track the movement of a touch over the course of several scans.

FIGS. 14A and 14Billustrate paths of conductive objects as tracked across a touch-sensing surface using a lost touch seeking process, according to one embodiment.FIG. 14Aillustrates the paths of two close conductive objects moving along two diagonals of the touch-sensing surface.FIG. 14Billustrates the paths of four close conductive objects, such as fingers, moving together diagonally across the surface.

The conductive objects resulting in the paths ofFIGS. 14A and 14Band the paths illustrated inFIGS. 6A and 6B, may be similar; however, the paths illustrated inFIGS. 14A and 14Bmore accurately represent the continuity of contact between the conductive objects and the touch-sensing surface as a result of identifying lost touches. Referring back toFIG. 12, the process1200continues at block1213from block1211.

At block1213, the processing device determines whether a close touch condition exists by comparing distance between the centroid locations of the touches with a close touch threshold. In one embodiment, the close touch threshold may be three times the sensor pitch, such that a close touch condition is detected if a distance between any pair of touches is less than three times the sensor pitch. In one embodiment, each of the touches that has another touch nearby may be flagged as a close touch. For example, a “CloseFinger” flag associated with each touch ID may be used to identify close touches.

In one embodiment, the process1200may check the “CloseFinger” flag upon the next execution of block1205, such that the lost touch seeking operations1207are performed only when close touches are detected. From block1213, if close touches are detected the process1200continues at block1215. Otherwise, the process1200continues at block1217, where the touch locations are reported to a host or other device.

At block1215, the processing device may adjust the positions of the centroids calculated at block1211to smooth the paths of the conductive objects as they are tracked over the course of successive scans of the touch-sensing surface. In one embodiment, such a smoothing process may analyze the centroid locations to find a virtual center for a group of close touches.

FIG. 15illustrates an embodiment of a smoothing process1500for smoothing the path of close touches in motion, as tracked by a touch-sensing surface.

In one embodiment, the process1500is used only when specific conditions are detected. In one embodiment, the process1500may be used only when close touches are detected at block1213, or when the number of close touches is less than four. In one embodiment, the process1500may be invoked only when the distance between the most distant of the touches is less than a certain threshold.

In one embodiment, the process may use touch centroid positions as calculated by a centroid calculation process according to block1211of process1200.

At block1501, the process1500may calculate the distance between the centroid locations of close touches, along with their relative angle.FIG. 16Aillustrates these parameters in further detail, according to one embodiment.

As illustrated inFIG. 16A, the dashed lines connect original centroid locations, such as location1604. The distance1602designates a distance between two touch locations from the same scan and having different touch IDs. The angle1603designates an angle formed by a line segment between two touch locations having different touch IDs.

At blocks1503and1505, the distance is tested against a threshold distances MIN and MAX. In one embodiment, MIN (as seen in block1507) represents a theoretical minimum distance between two touches at which the touch-sensing system can recognize two touches instead of one. In one embodiment, MIN is approximated according to Equation 5 below, for the exemplary case of a sensor array having a sensor pitch of 16 pixels.
Min2_Dist=√{square root over (162+162)}≈23  (Equation 5)
Thus, in one embodiment, the operation of blocks1505and1507set Δ according to Equation 6.
Δ=Min2_Dist, if Δ<Min2_Dist  (Equation 6)

If the distance is greater than MAX, the calculation of new positions is bypassed and the process1500continues to block1515. From blocks1505and1507, the process1500continues at block1509.

At block1509, the processing device may calculate offset values for the corrected positions of close touches according to the following Equations 7 and 8:
ΔX=Δ×cos(α)/2  (Equation 7)
ΔY=Δ×sin(α)/2  (Equation 8)
Here, ΔX and ΔY represent the X and Y offsets of the corrected touch location relative to a virtual center between the two original touch locations. From block1509, the process1500continues at block1511.

At block1511, the processing device calculates the location of the virtual center (Xc, Yc) as a middle point between two touch locations. In one embodiment, the method chosen for calculating the virtual center may depend on the number of close touches detected. In one embodiment, if two or three close touches are detected, a 5×5 or 7×7 centroid calculation process may be used to determine the virtual center.

If four touches are present, the virtual center may be the middle point between the two touches separated by the greatest distance. Alternatively, the four touch locations may be treated as two pairs of touches, and 5×5 centroid calculations may be performed on the pairs.

In one embodiment, the distance Δ between touches is calculated according to Equations 9 and 10 below.

Δ=δ⁢⁢Maxcos⁡(α)⁢⁢where(Equation⁢⁢9)δ⁢⁢Max=δ⁢⁢X,if⁢⁢δ⁢⁢X>δ⁢⁢Y,⁢δ⁢⁢Max=δ⁢⁢Y,if⁢⁢δ⁢⁢X<δ⁢⁢Y,and⁢⁢α=arctan⁢⁢δ⁢⁢Xδ⁢⁢Y(Equation⁢⁢10)
FIG. 16Bis a diagram further illustrating the distance Δ between two touches (x1, y1) and (x2, y2), angle α, and the components of distance Δ, δY and δX.

The corrected positions of the two touches (X′1, Y′1) and (X′2, Y′2) are calculated using the Equations 11 below.
X′1=Xcent±ΔX
Y′1=Ycent±ΔY
X′2=Xcent∓ΔX
Y′2=Ycent∓ΔY(Equation 11)
where
ΔX=Δ×cos(α)
ΔY=Δ×sin(α)
Here, the ΔX and ΔY represent the distances along the X and Y axes, respectively, between the original touch positions and their corresponding corrected touch positions.

FIGS. 17A and 17Billustrate the paths of conductive objects in motions, as tracked across the touch-sensing surface and corrected according to process1500.FIG. 17Aillustrates paths of two 11 mm diameter conductive objects separated by a gap of 2 mm between edges.FIG. 17Billustrates the paths of four 8 mm diameter conductive objects separated by a gap of 3 mm between edges. From block1513, the process1500continues at block1515.

At block1515, the process1500may return to process1200, having performed path smoothing operations according to block1215of process1200.

In one embodiment the distance Δ between the close touches may be corrected to further compensate for the “gravity” effect between close touches according to Equation 12 below.

Δ′=Δ*k⁢⁢where⁢⁢k=const×f⁡(1Δ⁢)(Equation⁢⁢12)
Here, Δ′ represents the corrected value of Δ, and const is a constant. In one embodiment, ƒ is a non-linear function that depends on the size and shape of the unit cells. In one embodiment, ƒ may be determined empirically. From block1215, the process1200continues at block1217.

At block1217, the processing device the processing device may report the detected touch locations to a host, such as host150as illustrated inFIG. 1. From block1217, the process1200continues back to block1201. Thus, the process1200repeats over the course of sequential scans of the touch-sensing surface, allowing the touch-sensing surface to locate and track the movements of close touches.

FIG. 18illustrates an alternate embodiment of a lost touch detection process1800that may be used when close touches are detected, for example, at block1205. Process1800may be performed by a processing device such as processing device110. In one embodiment, the operations of process1800may correspond to the operations of block1207, as illustrated inFIG. 12.

At block1203, also illustrated as part of process1200, the processing device identifies local maxima from a set of signal levels acquired from a scan of the touch-sensing surface. The local maxima may then be added to a list of local maxima at block1801. From block1801, the process1800continues at block1803.

At block1803, the processing device identifies local maxima from the set of signal levels using an alternate set of “light” criteria. In one embodiment, the light version of the local maxima detection process may analyze fewer signal levels around the central unit cell than the standard version. For example, the processing device may compare the signal level from the central unit cell with signal levels of unit cells that are horizontally and vertically adjacent to the first unit cell, but omit signal levels of unit cells that are diagonally adjacent.

This is illustrated inFIG. 19, according to one embodiment, for a central unit cell B at location (i, j). In one embodiment, the light local maxima detection process may analyze the unit cells A at locations (j, i−1), (i, j−1), and unit cells C at locations (j, i+1), (i, j+1) to test for the inequality expressed by Equation 13 below:
A<B≧C(Equation 13)
If the inequality of Equation 13 is evaluated as true, then B is interpreted as a local maximum by the light local maxima detection process.

According to one embodiment,FIGS. 20A and 20Billustrate a set of signal levels from a scan of the touch-sensing surface and touch locations resulting from processing of the signal levels using the light local maxima detection process, respectively. With reference toFIG. 20A, the light local maxima detection process (as described with reference toFIG. 19) detects two local maxima at the unit cells having signal levels28and42.

According to one embodiment,FIGS. 21A and 21Billustrate another set of signal levels from a scan of the touch-sensing surface and touch locations resulting from processing of the signal levels using the light local maxima detection process, respectively. With reference toFIG. 21A, the light local maxima detection process detects three local maxima at the unit cells having signal levels30,32, and41.

From block1803, the process1800continues at block1805. At block1805, the processing device may add the local maxima found using the light local maxima detection process at block1803to the list of local maxima. From block1803, the process1800continues at block1807.

At block1807, the processing device removes local maxima that are part of another local maxima from the list of local maxima. In one embodiment, a local maximum may be identified as part of another local maximum if it is within a threshold distance from a unit cell corresponding to the other local maximum. In one embodiment, the threshold distance may be equal to the sensor pitch.

In one embodiment, the processing device identifies a local maximum to be ignored by analyzing each local maximum identified using the light local maxima detection process (block1803) and determining whether it is within a threshold distance of a local maximum detected using the standard local maximum detection process (block1203). If so, the processing device may then remove the analyzed local maximum from the list of local maxima.

In an alternative embodiment, the threshold distance may apply to the sensor elements, such that a local maximum (from block1803) may be removed from the list if it is associated with a sensor element that is within the threshold distance from the other local maxima (from block1203).

In one embodiment, a local maximum (from block1803) may be considered part of another local maximum (from block1203) if the unit cell where the local maximum (from block1803) is found is within the set of unit cells analyzed when identifying the other local maximum (from block1203) as a local maximum. For example, with reference toFIG. 7, the unit cells A and C may be considered part of the local maximum at unit cell B since the unit cells A and C were analyzed to determine that unit cell B is a local maximum.

In one embodiment, if any local maximum is detected that is a part of another local maxima, it is ignored by being removed from the list of local maxima. The removed local maxima are therefore not processed as touches.

FIG. 22Aillustrates the removal of a local maximum according to an embodiment. In one embodiment, the standard local maximum detection process (at block1203) used on a set of signal levels from a scan of the touch sensing surface may detect local maxima2201and2203. The light local maximum detection process (at block1803) may detect local maxima2201,2202, and2203.

In one embodiment, the local maximum2202may be ignored by being removed from the list of local maxima. In one embodiment, local maxima may be removed according toFIG. 22Band Equation 14:
(LM1≧LM2)&&((LM2+LM2/4)≦(A+B))  (Equation 14)
Thus, if Equation 14 is evaluated to be true, the local maximum LM2is ignored. In the example ofFIG. 22A, the local maxima2201and2202correspond to LM1and LM2, respectively.

In an alternative embodiment, Equation 15 below may be evaluated to determine whether the local maxima LM2is ignored.
(LM1≧LM2)&&((LM1+LM2)≦[2×(A+B)+(A+B)/2])  (Equation 15)
Thus, if Equation 15 is evaluated to be true, the local maximum LM2is ignored.

AlthoughFIG. 22Billustrates LM2as being situated diagonally to the bottom right of LM1, LM2may be positioned at any of the other three corners of LM1for local maxima that are detected at those positions.

FIGS. 23A and 23Billustrate the paths of touches in motion as tracked across the touch-sensing surface, according to one embodiment. In contrast withFIGS. 6A and 6B,FIGS. 23A and 23Billustrate the effects of the local maxima detection process1800; specifically, the continuity of each path is preserved and touch IDs are stable for the duration of the touch.

Embodiments of the present invention, described herein, include various operations. These operations may be performed by hardware components, software, firmware, or a combination thereof. As used herein, the term “coupled to” may mean coupled directly or indirectly through one or more intervening components. Any of the signals provided over various buses described herein may be time multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit components or blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be one or more single signal lines and each of the single signal lines may alternatively be buses.

Certain embodiments may be implemented as a computer program product that may include instructions stored on a computer-readable medium. These instructions may be used to program a general-purpose or special-purpose processor to perform the described operations. A computer-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The computer-readable storage medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory, or another type of medium suitable for storing electronic instructions.

Additionally, some embodiments may be practiced in distributed computing environments where the computer-readable medium is stored on and/or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the transmission medium connecting the computer systems.