Abstract:
A micro-electro-mechanical systems (MEMS) pixel for display and touch position sensing includes a substrate and a capacitive element. The capacitive element includes one or more pixels having a first conductive platelet above the substrate, and a second conductive platelet above and spaced apart from the first conductive platelet, the two platelets forming the capacitive element. A connection to each platelet provides for applying a voltage, wherein the platelet separation changes according to the applied voltage. A transparent dielectric plate, spaced apart from and positioned opposite the substrate, covers the at least one pixel. A capacitance sensing circuit attached to the connection to each platelet of the pixel senses changes in capacitance not resulting from the applied voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/194,412, filed Aug. 19, 2008, and assigned to the assignee hereof. The disclosure of the prior application is considered part of, and is incorporated by reference in, this disclosure. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    The present disclosure relates generally to a MEMS display and method of operation and, more particularly, to a MEMS display capable of position touch sensing. 
       BACKGROUND 
       [0003]    A number of display devices include touch position sensing to enable graphical interactive selection of features in a screen display application. There are several different approaches in the current art to accomplishing touch position sensing. For example, a resistive touch panel may use two layers of separated conductive material. Pressure to the top layer, by force of finger contact, for example, may deform the top layer, bringing it into contact with the lower layer. The contact location is computed by measuring the voltage at the contact point. However, this type of sensor is highly mechanical in nature and aging or fatigue in the conductive material may adversely affect the long term stability of such a device. 
         [0004]    Another touch sensor in use with display panels is based on capacitive sensing. For example, two orthogonal rows of conductive traces in layers separated by an insulating substrate and over-coated with an insulating and protective surface is known in the art. The capacitance between any two orthogonally crossing traces can be sensed. The proximity of, for example a finger, to any of the crossing traces causes a change in the sensed capacitance at that location. This occurs because the body of the user is substantially at ground potential with respect to one layer of traces, but not the other. However, the resolution for position location may be limited by the resolution of the traces. 
         [0005]    One form of capacitive sensing operates by deforming the spacing between the two layers of sensor electrodes, physically changing the capacitance. The electrodes do not make physical contact, but change proximity. Another form of capacitive sensing is non-contact; that is, by sensing the fringing field of the capacitance induced, for example, between a finger, a hand or grounded stylus in close proximity to a portion of the sensor array. 
         [0006]    Conventionally, such capacitive sensors are devices distinct and separate from and are placed over the display screen as an additional structure, which may incur additional manufacturing costs. Moreover, in order to make the electrodes substantially invisible to the human eye the electrodes are, in some embodiments, made very narrow, or made of transparent conductors such as, for example, indium tin oxide (ITO). 
         [0007]    In the current approaches described above, it is generally necessary to implement the touch sensor as a separate device either above or beneath the display. This may require additional manufacturing processes and increase the thickness of the display device. 
       SUMMARY 
       [0008]    Disclosed herein is a method and apparatus for sensing touch or proximity to an image display screen, wherein the display methodology is based on capacitive effects to provide the image. The image may be comprised of elements, such as pixels, and therefore, the capacitive property of the pixel is accessed to detect a presence or contact to the display by means of sensing circuitry in communication with the display. No additional structures or apparatus pertaining to the display structure beyond that required to provide the image are required. 
         [0009]    In an embodiment, a micro-electro-mechanical systems (MEMS) pixel for display and touch position sensing, includes a first conductive platelet and a second conductive platelet disposed opposite and electrically insulated from the first platelet, the first and second platelets forming a capacitor. The pixel includes an optical cavity having a gap dimension associated with the relative positions of the first and second platelets. Driving circuitry applies a voltage difference to the first and second platelets, wherein the separation between the platelets is changed by electrostatic attraction from a first position to a second position, changing the gap dimension of the associated optical cavity and the capacitance of the first and second platelets simultaneously. Sensing circuitry coupled to the first and second platelets determine the capacitance and/or change in the capacitance corresponding to the relative positions of the first and second platelets. 
         [0010]    In an embodiment, a method of sensing touch position in a MEMS display pixel, includes determining the capacitance state of the pixel. The pixel includes a first conductive platelet and a second conductive platelet disposed opposite and electrically insulated from the first platelet, the first and second platelets forming a capacitor. The method includes applying a difference voltage to the platelets to control a separation between the platelets and measuring the capacitance of the platelets corresponding to the separation. If the measured capacitance does not match the expected capacitance within a selected tolerance, a touch or proximity to contact condition is determined to be detected. 
         [0011]    A MEMS display includes an array of pixels arranged in columns and rows, wherein each pixels comprises a first conductive platelet and a second conductive platelet disposed opposite and electrically insulated from the first platelet. The first and second platelets form a capacitor. Each pixel corresponds to an optical cavity having a gap dimension associated with the relative positions of the first and second platelets. The display includes an array driver controller comprising a column line for each column of pixels, a row line for each row of pixels, a column driver circuit, a row driving circuit, and a sensor controller circuit. The column driver circuit provides a processor controlled first voltage to each column line wherein the first conductive platelet of each pixel in a column is electrically connected to the corresponding column line. The row driver circuit provides a processor controlled second voltage to each row line wherein the second conductive platelet of each pixel in a row is electrically connected to the corresponding row line. The sensor controller circuit is configured to sense a capacitance between the first and second platelet in each pixel. 
         [0012]    A method of sensing proximity and/or touch position in a capacitive MEMS display includes addressing an image to an array of pixels in the capacitive MEMS display and determining a state of the each of the pixels corresponding to the addressed image. An expected value of capacitance is specified for each pixel corresponding to the state of the pixel. A tolerance value is specified as a matching condition for an acceptable range of the specified capacitance. The capacitance value of each pixel is measured and compared to the expected capacitance value. A touch or proximity contact has been detected if the difference in the measured and expected capacitance exceeds the matching condition specified by the tolerance value, and the difference value is stored in a processor memory with a corresponding location of the pixel in the array. A touch or proximity contact has not been detected if the difference in the measured and expected capacitance value is equal to or less than the tolerance value, and a null value for the difference is stored in the processor memory with a corresponding location of the pixel in the array. The stored difference and null values are processed to determine a touch or proximity contact location. 
         [0013]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description disclosed that follows may be better understood. Additional features and advantages will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0015]      FIG. 1  is a block diagram showing an exemplary wireless communication system in which an embodiment of the invention may be advantageously employed; 
           [0016]      FIG. 2  is a cross-section view of two capacitive MEMS display pixels, according to an embodiment of the disclosure; 
           [0017]      FIG. 3A  is an equivalent circuit of a single capacitive MEMS display pixel in proximity to a grounded object (e.g., finger), according to an embodiment of the disclosure; 
           [0018]      FIG. 3B  is a plot illustrating the dependence of effective capacitance on proximity to an external grounded object, according to the equivalent circuit of  FIG. 3A ; 
           [0019]      FIG. 4  is a flow diagram of a method of sensing touch and proximity using a capacitive MEMS display pixel; 
           [0020]      FIG. 5  is a block diagram of a capacitive MEMS touch sensing display, according to an embodiment of the disclosure; and 
           [0021]      FIG. 6  is a flow diagram of a method of determining touch location in a capacitive MEMS touch sensing display, according to an embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIG. 1  shows an exemplary wireless communication system  100  in which an embodiment of the disclosure may be advantageously employed. For purposes of illustration,  FIG. 1  shows three remote units  120 ,  130 , and  150  and two base stations  140 . It will be recognized that typical wireless communication systems may have many more remote units and base stations. Remote units  120 ,  130 , and  150  include capacitance-based displays with touch sensing  125 A,  125 B, and  125 C, respectively, which are embodiments of the invention as discussed further below.  FIG. 1  shows forward link signals  180  from the base stations  140  and the remote units  120 ,  130 , and  150  and reverse link signals  190  from the remote units  120 ,  130 , and  150  to base stations  140 . 
         [0023]    In  FIG. 1 , remote unit  120  is shown as a mobile telephone, remote unit  130  is shown as a portable computer, and remote unit  150  is shown as a fixed location remote unit in a wireless local loop system. For example, the remote units may be cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment. Although  FIG. 1  illustrates remote units according to the teachings of the invention, the invention is not limited to these exemplary illustrated units. The invention may be suitably employed in any device which includes a display with touch sensing. 
         [0024]    U.S. Pat. No. 7,321,457 issued Jan. 28, 2008, to HEALD, the disclosure of which is herein expressly incorporated by reference in its entirety, discloses a MEMS interferometric modulator (iMOD) display element currently being used for active display. The MEMS display is a capacitive device. Herein, a method and system of providing a capability to sense and provide touch position location based on the capacitance properties of the device are disclosed. In one or more embodiments described herein, no additional sensing structures need be added to the display. Additional circuitry coupled to the display elements may be adapted to obtain and evaluate the sensed signals and determine touch location. 
         [0025]      FIG. 2  shows a cross-section of an embodiment of a pair of MEMS-based interferometric light modulator (iMOD) display pixels  200   a  and  200   b . A single display pixel, such as a pixel  200   a , includes two parallel conductive platelets, i.e., a bottom platelet  22   a  ( 22   b  for pixel  200   b ) and a top platelet  24   a  ( 24   b  for pixel  200   b ), respectively. Both bottom and top platelets  22   a ,  22   b ,  24   a  and  24   b  include at least a conductive layer (not shown) which may serve at least as an electrode, reflective surface, or both. Alternatively, reflective and conductive layers may be provided separately. The top platelet  24   a  is spaced apart from bottom plate  22   a  by supporting pillar  26 . The display pixel elements  200   a  and  200   b  are disposed adjacent to a supporting base  21 , which may be, for example, a silicon substrate or a glass substrate, but may include other substrate materials. Alternatively the display pixel elements may be supported by a transparent dielectric cover plate  20  disposed above the top platelets  24   a  and  24   b . Cover plate  20  also protects and electrically isolates pixel  200  from external charge. The cover plate  20  may be, for example, the screen or outer shield of a display. 
         [0026]    When a driving voltage bias is changed from V=0 to V=VD and is applied between platelets  22   b  and  24   b , the electrostatic field produced will generate an attractive force to change the spacing between the platelets, as shown by spacing from a zero bias voltage for platelets  22   a  and  24   a , relative to the spacing shown for V=VD for platelets  22   b  and  24   b . In an embodiment as shown in  FIG. 2 , platelet  22   b  deforms toward platelet  24   b . However, in other embodiments platelet  24   b  could deform toward platelet  22   b , or both could deform toward each other. One or both of the platelets may be associated with an optical cavity. In one embodiment, the optical cavity is defined by the space between the platelets. Alternatively, in another embodiment, the optical cavity is defined by the space between one platelet and another reflecting surface outside and apart from both platelets. The volume of the optical cavity changes as the spacing between the platelets change. The associated optical cavity is further defined by two reflecting surfaces spaced apart and having specified reflection and transmission properties at each reflecting surface to enhance constructive or destructive interference of light in a selected wavelength range. 
         [0027]    Through proper selection of the transmissive and reflective properties of the reflecting layers of the platelets, the net reflectivity of the pixel in a destructive interference state may be as low as approximately 1%-2%, or lower at the selected wavelength range, giving the appearance of a black pixel. Conversely, when the optical cavity is in a second state, where the optical path length corresponds to constructive interference, pixel brightness may approach 90%, or more, i.e., a bright pixel at the selected wavelength range. 
         [0028]    In either of the two states—relaxed or collapsed—the two electrodes of the platelets form a capacitor that may be approximated as two parallel plates separated by a gap  29  which may include air and dielectric layer material. In the relaxed (“off”) state the capacitance may be denoted as Cr, and in the collapsed (“on”) state the capacitance may be denoted by Cc. Because parallel plate capacitance is approximately inversely proportional to the gap  29 , it can be seen that Cc&gt;Cr. The pixel will have a measured capacitance of one or the other of these two values Cc or Cr, depending on the pixel state (collapsed or relaxed). For simplicity, we may refer to the pixel capacitance as C, for either state. 
         [0029]    In the embodiment of  FIG. 2 , assume that bottom platelet  22   a  ( 22   b ) is at a relative electrical ground potential (an arbitrary designation, such as the device case potential). In a hand held portable device, such as remote units  120 ,  130  ( FIG. 1 ), with a display comprised of an array of capacitive MEMS pixel elements  200  covered by a transparent screen  20 , the device user is effectively at case ground potential, and a source of considerable mobile charge. Bringing a finger or conductive stylus grounded to the user in contact or proximity (“proximity contact”) with the cover plate  20  over a pixel creates an additional effective “extra” capacitance Cx between the top platelet  24   a  ( 24   b ) and relative ground. 
         [0030]      FIG. 3A  represents an equivalent circuit approximation of a single pixel and finger contributions to total capacitance. At distances large compared to the pixel gap the finger capacitance Cx is effectively zero, so only the pixel capacitance is apparent. When a finger or grounded stylus, for example, is brought in proximity or contact with cover plate  20  above the pixel, the effective external capacitance increases to a maximum Cx=Cxmax, limited by the closest proximity of the finger to the pixel by the thickness of cover plate  20 . The corresponding total effective capacitance is approximately the sum of the two capacitances in parallel, i.e., C′=C+Cx(d), where d corresponds approximately to a distance between the finger and top platelet  24   a  ( 24   b ). 
         [0031]      FIG. 3B  represents the change in effective capacitance C′ as a function of the distance between the finger (or grounded stylus) and the pixel. A sensing circuit connected to the pixel top platelet and bottom platelet may then measure C′. Assuming that the state of the pixel is known, and therefore the expected value of C (either Cr or Cc) is known within a certain accuracy tolerance ∈, a difference in the measured capacitance from one of the expected values may be determined to indicate that a region of the display area containing the pixel is being touched or that close proximity to contact is evident. 
         [0032]    Various sensing circuitry and methods may be provided to sense a change in capacitance. In one embodiment (not shown), the capacitance may be coupled to an inductive reference element L and a feedback amplifier circuit to function as an oscillator, which operates at the L-C resonance frequency determined by the effective capacitance C′ associated with a pixel. Each state of the pixel (relaxed or collapsed) will have an associated expected oscillator frequency in the absence of externally coupled capacitance. A measured oscillation frequency that is different from the expected oscillation frequency indicates a touch contact or proximity to contact is evident. The inductor value may be chosen so that the oscillating frequency of the resonant circuit formed is well above a frequency range associated with scanning an array of display pixels. The embodiment indicated above for measuring capacitance and determining touch is exemplary and not intended to be exhaustive. 
         [0033]      FIG. 4  is a flow diagram of an exemplary method of sensing capacitance using a capacitive MEMS display pixel element  200 . Block  420  determines the state of the pixel, for example, by the value of the applied voltage between the platelets. Block  421 , based upon the determined state of the pixel results, selects a known value of capacitance corresponding to the state of the pixel. This state may be Cr or Cc. Because manufacturing processes may often have tolerance limits on dimensions, compositions, etc., block  422  determines a tolerance limits to establish an acceptable capacitance range, e.g., C±∈. Block  423  measures the capacitance of the pixel to a measured value C′. C′ may be within the tolerance limit of s or not. Block  424  compares C′ and C. If the absolute value difference in measured and expected values, i.e., |C′−C| is equal or less than ∈ then block  425  indicates a “no touch” condition. If the absolute value difference between the measured and expected capacitance exceeds the tolerance limit ∈ then block  426  indicates that a touch (or proximity) contact has been detected. 
         [0034]      FIG. 5  is a block diagram illustrating one embodiment of a capacitive MEMS touch sensing display system  500 . The display system  500  includes a processor  510 , which may be any special or general purpose single or multi-chip processor, and associated memory  518 . The processor  510  is configured to communicate with an array driver  511 . In one embodiment, the array driver  511  includes a row driver circuit  513  and a column driver circuit  514  that provide signals to a display array  515 . The display array  515  is made up of pixels, such as pixels  200 . In one embodiment, the array driver  511  includes a sensing controller circuit  512  in communication with the display array  515 . 
         [0035]    In some embodiments, upper platelets  24   a  ( 24   b ) ( FIG. 2 ) are patterned into parallel strips, and may form row electrodes  516 , and the lower platelets  22   a  ( 22   b ) are patterned into parallel strips, and may form column electrodes  517  in the display system  500 . Alternatively, the lower platelets may be patterned to form rows and the upper platelets may be patterned to form columns. 
         [0036]    In the embodiment shown in  FIG. 5 , the sensing controller  512  communicates with the pixels through the row driver circuit  513  and the column driver circuit  514 . In another embodiment, the sensing controller may communicate directly with the row and column electrodes  516  and  517 , respectively. 
         [0037]      FIG. 6  shows one embodiment  600  of a flow diagram of a method of determining touch location in a capacitive MEMS touch sensing display. Block  610  addresses an image to the display array  515  ( FIG. 5 ). Block  611  then scans the display array  515  with the sensing controller  512 . The pixels in the display array  515  can be identified by indices i,j if the display array  515  is laid out, for examples, in rows and columns, and the capacitance sensing method is asserted on a pixel-by-pixel basis. A capacitance sensing measurement is associated with each pixel location, e.g., Xi,Yj. Blocks  612 - 618  are substantially the same as blocks  420 - 426  of the method  400  ( FIG. 4 ), and are not discussed further. 
         [0038]    If block  617  indicates a “no touch” condition, then block  619  sets the value of |C′−C| to a null value for the corresponding pixel i,j at location Xi,Yj, and block  620  stores the null value with the corresponding location in memory, such as the memory  518  of  FIG. 5 . 
         [0039]    Block  621  determines if the scan is complete. If not, the method  600  continues at block  611  by sensing a next pixel (e.g., at Xi+k,Yj+1) and repeating blocks  612 - 618 . 
         [0040]    If block  618  indicates a touch condition, then block  620  stores the capacitance difference as determined by block  616  in correspondence with the position Xi,Yj of the pixel i,j. The method  600  then continues, as discussed above, with block  621  determining if the entire array has been scanned. 
         [0041]    When block  621  determines that scanning is complete, block  622  processes the stored touch sensing data in memory to determine any touch location. For example, because a finger contact may indicate contact detection at a cluster of pixels, the data may be processed to determine a central contact position, based on various weighting calculations, which are well known in the image and signal processing arts. The processor  510 ,  FIG. 5 , may then initiate logical processes based on the touch location information so obtained to enable graphical interactive selection of features in a screen display application. 
         [0042]    Although specific circuitry has been set forth, it will be appreciated by those skilled in the art that not all of the disclosed circuitry is required to practice the invention. Moreover, certain well known circuits have not been described, to maintain focus on the invention. 
         [0043]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.