Patent Application
20090174675
Embodiments of the present invention are related to capacitive touch pads. More particularly, embodiments of the present invention are directed to a capacitive touch pad design and method for improving capacitive touch pad operation.
There exist problems with locating multiple fingers (or other input objects) using capacitive touch pads. There also exists a need to locate these input objects accurately enough to allow emulation of keypads or keyboards, such as those with small keys, using touch pad systems.
Capacitive touch pads can accept input from a variety of different objects, including fingers, pens, styli, and the like. For most capacitive touch pads, the input objects are conductive. However, capacitive touch pads can be made to accept non-conductive objects. For simplicity and clarity of explanation, the discussion below uses fingers as the example input objects. However, it is understood that any combination of different acceptable objects can produce the profiles used to ascertain the positions of these objects.
When two or more fingers touch or come into sufficient proximity to a capacitive touch pad utilizing a profile sensing scheme, the resulting capacitance profiles are approximately equal to the sums of the profiles that would be due to the fingers separately (i.e. the resulting profiles roughly superimpose the profiles that would result from each of the fingers if it was applied separately in time from any other fingers). In one implementation, a peak interpolation method is used to calculate the location of each finger. For improved interpolation accuracy, a representation of the capacitance profile of the first finger to arrive is saved. This saved profile representation is subtracted from later profiles obtained while a second finger is also interacting with the touch pad to yield modified profiles that isolate the portions of profiles due to the second finger. Even if the captured profile representation of the first finger is not perfectly accurate, subtracting it from a profile obtained with two fingers yields an adjusted profile that is better than the unadjusted profile for measuring the position of the second finger. Various techniques are used to improve the accuracy of the adjustment made to the multiple-finger profile based on the first-finger profile and other information available.
The major existing alternative for accurately locating multiple fingers on a capacitive sensor is known as a “capacitive imaging” sensor, which measures not just row and column capacitances but the separate capacitance of each point on the surface. Imaging sensors require more expensive electronics, higher data rates, and higher power than profile sensors. The present invention allows cheap and simple capacitance profile sensors to perform functions historically attributed to imaging sensors.
Some multi-finger applications for touch pads require that the two touching fingers be not just counted but located accurately. Great care is required in order to locate the fingers accurately enough to allow emulation of keypads or keyboards with very small keys. This invention provides a method for identifying and accurately locating fingers in the presence of multi-finger touch, with enhancements to improve accuracy by taking advantage of the special usage model of a keypad-like application.
This invention is especially suitable for touch pad applications where the fingers rarely move once placed, such as on-screen keyboards or keypads. Embodiments of the present invention include a method for determining locations of a plurality of objects contemporaneously interacting with a capacitive touch pad having a sensing region. The method includes generating a first capacitive profile associated with a first object and a second object contemporaneously in the sensing region and determining locations of the first and second objects with respect to the sensing region utilizing the first capacitive profile.
Embodiments of the present invention also include a capacitance sensing touch pad for determining locations of a plurality of objects. The capacitance sensing touch pad includes a capacitance profile generator coupled with the touch pad for generating a first capacitance profile associated with a first object proximate the touch pad and a position determiner coupled with the profile generator for determining a position of the first object with respect to the touch pad based on the first capacitance profile. In one embodiment, the capacitance profile generator generates a second capacitance profile associated with the first object and a second object simultaneously proximate the touch pad. In one embodiment, a profile adjuster is coupled with the profile generator for determining an adjusted capacitance profile based on the first and second capacitance profiles wherein the position determiner determines a position of the second conductive object with respect to the touch pad based on the adjusted capacitance profile.
The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Some profile capacitive touch pads, such as X-Y profile touch pads, measure the capacitance on each column and row electrode in a grid of sensor electrodes. These measurements of row and column electrode capacitances form X- and Y-axis capacitance profiles. Each measured value in the profile represents the total capacitance on one row or one column. A finger or other conductive object touching in the sensing region of the pad will increase the capacitances on the rows and columns that fall under or near the finger, producing a characteristic “bump” in each (X-and Y-axis, or Cartesian) profile. It is appreciated that the touch sensor could also be a “linear” sensor, one that produces a one dimensional profile for a single axis. Other touch pads can be designed to sense only along one dimension and produce such a one dimensional profile.
In this sensing scheme, the capacitance change due to a finger will typically be largest on the electrode nearest the center of the finger. If the electrodes are numbered consecutively in each axis profile, the electrode number of a finger's maximal electrode in the X-axis profile provides a rough estimate of the X coordinate of the location of the finger on the surface of the touch pad. Similarly, the number of the finger's maximal electrode in the Y-axis profile estimates the Y coordinate of the finger location.
Conventional capacitive touch pads use an interpolation method to calculate the location of a finger on the pad to a resolution much finer than the physical spacing of the electrodes. One such method, called “peak interpolation,” applies a mathematical formula to a maximal capacitance value and its neighboring values in a profile to estimate the precise center of the capacitance “bump” due to a finger.
When two objects are interacting contemporaneously with a touch sensing system, such as when two fingers are placed on a touch pad, peak interpolation can be applied separately about the peak of each finger “bump” to determine the independent positions of the respective fingers. This works well if the fingers are spaced relatively far apart so that the profile bumps due to the two fingers do not overlap.
In one embodiment, each “bump” can be defined as the vicinity of a “peak” electrode higher in capacitance than its neighboring electrodes (a local maximum of capacitance) and whose capacitance value exceeds some threshold chosen based on the desired touch sensitivity of the sensor. Fluctuations due to electrical noise and electrode sensitivity variation can cause this simple method to falsely count a single finger as two bumps.
Various alternative embodiments are known that can eliminate such artifacts. One such method looks for groups of adjacent electrodes all of which exceed a threshold; another method processes the profiles to reduce fluctuations before searching for bumps. Any method for identifying finger bumps in a capacitance profile may be used with the present invention. However, the present invention may permit a candidate second bump to be isolated and subjected to additional criteria such as a “Z” calculation before being accepted as a second finger. For this reason, the simple definition of “peaks” and “bumps” will suffice for use with the present invention despite its potential for artifacts.
One embodiment of the invention uses three-value peak interpolation. However, the invention is not limited to three-value peak interpolation; any method that calculates the position of a finger from a set of capacitance values can be used.
For example, a centroid calculation can be used as the interpolation method for the present invention. Peak interpolation can be also used because it is simple yet relatively immune to hover effects. This is useful, for example, in systems designed to ignore other objects hovering from the touch pad at a distance beyond a threshold, or to distinguish between touch and hover or different levels of hover. For example, if a second finger is not yet touching the pad but is held near enough to create a small amount of capacitance, this extra capacitance will tend to perturb a centroid calculation that combines measurements from the entire pad surface. Extra capacitance from a hovering finger will have less effect on peak interpolation, which combines measurements from only the neighborhood of the intended finger. In general, local interpolation methods (those that examine only electrodes in the vicinity of the finger) are preferable when locating multiple fingers on a touch pad.
In some applications, the accuracy achievable by applying peak interpolation independently to each finger bump may suffice. For example, this would be true if the fingers are expected to be held a certain distance apart in both (e.g. X and Y) dimensions of a two-dimensional input system (e.g. X-Y touch pad). It would also be true if the distance between the fingers is needed in only the more-distant dimension. For example, a “two-finger pinch” gesture can be implemented that depends on changes in the distance between two fingers but not on the absolute positions of the fingers.
In this “pinch” gesture, the user moves the two fingers closer together or farther apart to perform some action in the user interface such as adjusting the zoom level of a user interface window or adjusting the volume of an audio output. The “pinch” gesture can be implemented on an X-Y capacitance profile sensor device by defining the pinch distance as the greater of the distance between finger bumps in the X-axis profile and the distance between finger bumps in the Y-axis profile. Even if the fingers are held as shown in
However, other applications may require the locations of two fingers to be determined accurately regardless of the placement of the fingers. For such applications, independent peak interpolation may perform poorly because the fingers may be near enough for their bumps to overlap in at least one axis.
When the two fingers are near to each other, the capacitance profile that results is approximately equal to the electrode-wise sum of the profiles due to each finger individually. This is a consequence of fact that the capacitance of two capacitors connected in parallel is equal to the sum of their capacitances. Even if the sensor device measures a mildly non-linear function of capacitance, it often suffices to approximate the combined profile as a simple sum.
Even if the finger 102 on the right in
To resolve the positions of two overlapping fingers accurately, the present invention uses the changes over time in the profiles to disambiguate the two fingers. The techniques of this invention work well in applications where each finger can be assumed to hold in a steady position once it has been placed on the touch pad.
The user of the capacitive keypad 200 might use two fingers to touch two separate keys at the same time. For example, the user may press a modifier key such as Shift 231 or Ctrl 323 together with another key. Multiple key presses can also occur when the user presses a new key before releasing a previously typed key. This situation, known as “two-key rollover,” often arises during rapid typing. In each scenario, it is imperative that the positions of both fingers be interpolated accurately.
The present invention is not limited to keypad applications. Any use for a touch pad in which two or more fingers must be placed accurately will benefit from this invention. For example, the invention could be used for a touch screen that displays large or small icons or other controls.
When the second finger arrives, the interpolated position of the first finger will shift as the measurements of the electrodes marked with arrows 340 and 341 in
The position calculation for the second finger is also perturbed by the presence of the first finger. The first finger might remain present throughout the period of presence of the second finger. For example, the first finger could hold a Shift key while the second finger types and releases a letter key. For this reason, it may not be possible to capture a clear picture of the second-finger profile directly from the capacitance measurements; every profile measurement that includes the second finger also includes the first finger.
In conventional touch pads, a baseline profile is stored and then subtracted from the currently measured profile to remove background capacitance. These conventional touch pads take great pains to capture the baseline profile only when no finger is present. The present invention may include the usual calibration and baseline profile processing of a conventional touch pad, however, the present invention also captures an additional profile that deliberately includes the effects of capacitance due to the first finger. This additional captured profile is the one marked with “x”s 430 in
An interpolation method is applied to the adjusted profile to calculate the position of the second finger. Again, any interpolation method may be used, not necessarily the same method that was used to locate the first finger. The adjustment to the profile could also be incorporated into the formula for interpolating the second finger position instead of being done as a distinct step. For simplicity, one embodiment of the invention uses a distinct profile adjustment step (scaling or some other modification of the profile) followed by the same kind of three-value peak interpolation method that is used to locate the first finger.
In actual practice, the first finger rarely remains completely motionless as the second finger touches the pad. For instance, in a standard touch pad implementation, the capacitance due to a finger, and hence the height of the finger bump, rises as the finger lands more and more firmly on the sensor device's surface. Fingers may touch in rapid succession, so the first-finger profile must be captured soon after the first finger touches in order to ensure that it is largely free of second-finger capacitance. But if the first-finger profile or a representation of the first-finger profile is saved very early, when the first finger is initially detected, then the saved image of the first-finger bump is likely to be much smaller than the same bump will be by the time the second-finger interpolation is performed. Subtracting a saved profile with a much smaller bump will only partially erase the first finger, and thus the second-finger position calculation will still be perturbed.
It is possible to record many finger profiles throughout the time between the arrival of the first finger and the arrival of the second finger, and then to choose the best one retrospectively once the second finger is detected. However, it may be that none of the recorded profiles capture a full-sized first-finger bump with no presence of the second finger, especially if the user types rapidly with two hands, or if the user uses two fingers of the same hand and the hand as a whole moves in the action of placing the second finger. Also, it may not be feasible to record many profiles in the memories of the small chips that are typically used to operate touch pad sensor devices. Instead, one embodiment of this invention captures a single, very early first-finger profile and then computes the adjusted profile by subtracting a scaled version of the saved profile.
Similarly, it may be beneficial to limit the scale factor to some maximum such as 10.0 in order to avoid numerical overflows in case unusual usage patterns violate the assumptions of the scaling algorithm.
If the fingers overlap in one axis as shown in
The X-axis electrodes together cover the same surface area as the Y-axis electrodes, so a doubling of finger capacitance sensed by one axis must necessarily correspond with a doubling of capacitance sensed by the other axis. The X- and Y-axis bumps might not change in perfect unison due to inaccuracies or nonlinearity in the capacitance measurements, or because the first finger has shifted its position since it was captured, but the adjustment will generally be close enough to allow acceptably accurate interpolation of the second-finger position.
Although this invention can be used for applications where the finger is not expected to move once placed on the pad, nevertheless it is good for the performance to degrade gracefully if the first finger moves unexpectedly. When subtracting the scaled first-finger capacitance from the present capacitance, the resulting value for any electrode is forced to zero if the difference would have been negative. This ensures that although the adjustment step may undesirably erode the bump of the second finger if the first finger moves, it will not produce a dramatically unrealistic profile such as an “inverted bump” that might cause gross malfunction in subsequent calculations.
Alternatively, the scale factor could be allowed to drop all the way to 0.0 when the first finger seems to have moved from its original location. This alternative embodiment might be preferable for applications in which fingers are more likely to move once placed, and reliably sensing at least the presence and general location of a second finger is more important than locating the second finger with optimal accuracy.
If the touch pad's sensor measurements are susceptible to additive common offsets or noise, it is best to remove these additive offsets before applying the methods of this invention, in order for the multiplicative scaling of the saved profile to work effectively. Techniques for removing common offsets are well-known in the art, such as subtracting the lowest value in the profile from the entire profile, or subtracting the value of a reference electrode that is not exposed to touch.
As a further measure to avoid capturing a hovering second finger as part of the first-finger profile, the preferred embodiment applies the adjustment step only to the electrodes in the vicinity of the first-finger peak. As presently preferred, the first-finger peak electrode and its three nearest neighbors on each side are adjusted for each axis, but more distant electrodes are not adjusted. The number of electrodes adjusted is chosen based on the largest likely size of a finger in the intended application. Adjusting just a subset of the electrodes also allows further memory savings for implementation in small chips. Alternatively, the more-distant electrodes can be adjusted but with a reduced scale factor.
The presently preferred embodiment captures the actual profile capacitances of the electrodes in the vicinity of the first finger, but equivalent alternatives are possible that use a simplified or processed first-finger bump to adjust the profiles. For example, an artificial bump could be calculated based on the known typical shapes of finger bumps and the previously calculated position of the first finger. This alternative is likely to do a poorer job of canceling the first finger than would a scaled version of the actually recorded first-finger profile; however, an artificial bump may be preferable if memory resources are extremely scarce.
The first-finger profile is preferably captured each time a first finger touches the pad, and also each time a second finger is removed from the pad leaving just one finger remaining. For example, if finger A touches the pad, and then finger B touches the pad, and then finger A leaves the pad, finger B is now the sole finger and should play the role of “first finger” for purposes of interpolating any finger C that touches the pad while finger B is still present.
If the first finger might have moved from its original position, and neither axis profile shows evidence of a second finger, it may be desirable to recapture the first-finger profile periodically. For applications that do not expect the first finger to move once placed, it should suffice to capture the profile for a given first finger just once.
The finger position can be calculated just once when a finger is first detected, or, in some applications, it is preferable to recalculate the finger position for as long as it is present in order to track a moving finger. The profile adjustment technique of the present invention assumes the first finger will remain stationary when two fingers are present, but the finger can be detected and tracked by conventional touch pad algorithms when only one finger is present.
For example, many touch pads calculate a “Z” value in addition to any calculation of position coordinates, and they compare this Z value to a threshold with hysteresis in order to detect the finger. In one embodiment, Z is a representation of the height or area of the finger bump. There have been multiple formulas used to derive this Z value. Touch pads using the present invention could continue to apply these Z-based methods for detecting the first finger.
The simplest way to determine when a second finger is present is to check for a bump of sufficient height in each of the adjusted profiles in each axis. However, this simple method is easily fooled; for example, if a single finger touches down in one place and then slides to a significantly different position, the finger bump will reappear in the adjusted profile and could be mistaken as a second finger. To avoid this problem, the present invention checks the adjusted profile for a second finger bump only if the unadjusted profile shows signs of two distinct finger bumps in at least one axis.
Various methods can be used for this determination, such as counting distinct peaks in the profile, or counting distinct regions in the profile that exceed a threshold value. Alternatively, the presence of a second finger may be validated by checking that new bumps appear in the adjusted profile while the original first-finger peak electrodes still show substantial measurements in the unadjusted profile.
Once examination of the unadjusted profiles shows evidence of two fingers, any of the conventional methods for detecting a finger on a touch pad can be applied to the adjusted profiles in order to confirm the presence of a second finger. For example, a second Z value can be calculated based on the adjusted profiles and compared against a suitable threshold with hysteresis.
When two fingers are present it is possible to track motion of the second finger provided that the first finger remains stationary; this is unlikely to be useful in a keypad application, but it could be a realistic usage pattern in a different kind of application that can make use of the present invention. For example, one finger could be held steady on an icon or command button while the other finger is moved to operate an on-screen scroll bar. Or a second finger could be rotated about a fixed first finger to produce a “pivot gesture” for rotating or otherwise adjusting the contents of a window.
If two fingers touch the pad simultaneously, so that one set of measured profiles along all axes of the touch pad show no fingers and the very next set of measurements show signs of two finger bumps in at least one axis, then there is no way to capture a profile of a first finger. In this case, the present embodiment falls back to operating without profile adjustment. For example, an X-Y embodiment interpolates around each bump in the unadjusted profile, using the same X (or Y) coordinate for both fingers if the X-axis (or Y-axis) profile has only one bump. In some applications such as typing on keyboards, where there is a known maximum reasonable typing speed, a suitable alternative would be to measure successive profiles at a high enough rate to resolve all reasonable finger transitions, and to ignore as invalid a second finger that arrives simultaneously with a first finger within the same measurement period.
Some applications might take no special action when a finger leaves the touch pad. For example, a 12-key phone keypad might only need to record the arrivals of fingers on keys. For applications that do need to act upon the removal of a second finger, this event can be marked when the number of finger bumps reduces to 1 on all axes (e.g. both axes of a two-dimensional profile touch pad). To determine which one of the two fingers was removed and which one remains, the coordinates of the remaining finger can be calculated and compared against the last-known positions of the two fingers. Provided that successive profiles are measured rapidly compared to the speed of typical finger motions, the remaining finger can be identified as the nearest of the prior two fingers.
If one finger leaves the pad while another simultaneously touches the pad, the number of finger bumps will remain the same (at “one bump”) from one set of measurements to the next. In the present embodiment, this situation is distinguished from ordinary motion of a single finger by checking for an impossibly large jump in at least one (e.g. X or Y) calculated finger coordinate from one measurement to the next.
Once calculated, the interpolated finger coordinates may be used in whatever way is appropriate to the specific application. For example, in a simple QWERTY keyboard emulation using an X-Y touch pad, each time a first or second finger touches down, its X and Y coordinates could be calculated and compared against the bounding boxes of the various virtual keys to decide which key was pressed. The appropriate letter is typed or the appropriate Shift-like modifier is activated depending on the key. When a finger leaves the pad, no action need be taken except for deactivating any Shift-like modifier that was activated by the finger's arrival.
If the application calls for the simultaneous location of three or more fingers, the methods just disclosed can be extended in a straightforward way. For example, each time the number of finger bumps computed from the unadjusted profile increases or decreases, the saved profile can be updated from the latest profile. When the number of finger bumps increases from two to three, the saved profile will therefore reflect both of the first two fingers, allowing the third finger to be revealed through an adjustment method. However, it will usually suffice to locate just two fingers accurately because it is hard for a user to place more than two fingers on a small touch pad with great accuracy.
The techniques of the present invention may allow more reliable counting of multiple fingers on the touch pad even in applications that do not require the positions of the respective fingers to be calculated accurately.
The techniques just described can be implemented as part of the basic processing of a touch pad device, in which case the calculated finger coordinates will typically be reported to a host in the form of packets or device registers. A variety of alternative implementation methods are possible and also fall within the scope of this invention; for example, profile data could be sent to a host processor and some or all of the processing of profiles into calculated positions could be performed in host software. Or, the calculated coordinates could be converted into keypad key identifiers before transmission to a host. Or, the profile adjustment operation could be implemented as part of the hardware that measures and delivers capacitance profiles to higher-level processing.
Table 1 shows an outline of an example implementation of one embodiment of this invention. This is only an example, and many equivalent implementations are possible.
At 602, 600 includes generating a first capacitance profile associated with a first object and a second object contemporaneously in a sensing region of a capacitance sensing touch pad. In one embodiment, local interpolation is performed on the capacitance profile.
At 604, 600 includes determining locations of the first and second objects with respect to the sensing region utilizing the first capacitive profile.
In one embodiment, 602 includes determining capacitance values associated with the first and second objects with respect to a first axis of the sensing region and 604 includes determining locations of the first and second objects in the first axis.
In one embodiment, 602 includes determining capacitance values associated with the first and second objects with respect to a second axis of the sensing region and 604 includes determining locations of the first and second objects in the second axis.
In one embodiment, 600 further includes determining a relationship between the locations in the first axis and the second axis and using the relationship to control a user interface.
In one embodiment, capacitance sensing touch pad 702 is coupled with a capacitance profile generator 704. In one embodiment, the capacitance sensing touch pad includes capacitance sensors in one or more axis. The capacitance profile generator 704 generates a first capacitance profile associated with a first object proximate the touch pad. The capacitance profile generator also generates a second capacitance profile associated with the first object and a second object simultaneously proximate the touch pad 702.
A position determiner 706 is coupled with the capacitance profile generator 704 for determining a position of an object with respect to the sensing region of the touch pad 702 based on the first capacitance profile.
A profile adjuster 708 is coupled with the profile generator for determining an adjusted capacitance profile based on the first and second capacitance profiles. The position determiner 706 determines the positions of the first and second objects based on the adjusted capacitance profile.
Example embodiments of the subject matter are thus described. Although the subject matter has been described in a language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.
This application claims priority to the copending provisional patent application, Ser. No. 61/010,644, Attorney Docket Number SYNA-20080104-A2.PRO, entitled “LOCATING MULTIPLE OBJECTS ON A CAPACITIVE TOUCH PAD,” with filing date Jan. 9, 2008, assigned to the assignee of the present application, and hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61010644 | Jan 2008 | US |
