Method and apparatus for testing projected capacitance matrices and determining the location and types of faults

A method, system and apparatus is described for measuring a sensor, comparing measured values of a sensor to a reference value, adjusting a calibration parameter in response to the comparing of measured values to a reference value and determining sensor integrity based on the value o the adjusted parameter.

TECHNICAL FIELD

The present disclosure relates generally to touch sensors and, more particularly, to capacitive touch sensors.

BACKGROUND

Capacitive touch sensors are susceptible to manufacturing defects, wear and breakage over the life of the end product. Changes in the capacitive properties of a panel from those used during development or over the life of the project can impair performance or create a defective interface. Previous methods for determining the manufacturing quality of a capacitive touch panel included optically scanning the panel for defects and physically, mechanically engaging the panel. Mechanical detection of defects is low and requires precision robotic test equipment. Optical scanning is prone to mistakes and good panels can be rejected as falsely defective and defective panels can be falsely passed as good.

Other fault detection methods rely on external circuitry, mechanical test structures and different methods relative to the sensing method to determine and locate faults. Such methods increase system complexity and add additional failure mechanisms in the system test, which decreases reliability of the tests and increases costs for production through decreased yield.

DETAILED DESCRIPTION

In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be evident, however, to one skilled in the art that the embodiments may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail or are shown in block diagram form in order to avoid unnecessarily obscuring an understanding of this description.

Touch panel fault detection circuits and method are described. Fault detection can be run as part of the manufacturing process or during operation of the touch panel in response to a command from and external controller, based on timing, start up or in response to detected trauma to the touch panel.

Embodiments of the present invention allow for the determination of faults in a measurement circuit for the mutual capacitance of two or more electrodes. Capacitance measurement can be performed with a single pair of electrodes or with the use of a multiple electrode system.FIG. 1shows a capacitive sensor100comprising a single pair of electrodes E1101and E2102situated close to each other. Electrodes E1101and E2102have self capacitances to a voltage potential Ce1110and Ce1120, respectively. The voltage potential to which the self capacitance are may be ground. Electrodes E1101and E2102have a mutual capacitance Cm115between them.

There are various circuit implementations that may be used for performing capacitance measurement.FIG. 2illustrates a single mutual capacitance measurement circuit200to measure Cm215.

The operation of the circuit may be described in several stages, which are repeated in sequence. Table 1 includes the switching sequence of switches for the circuits shown inFIG. 2.

Table 1: Switching sequence of switches and the voltages across capacitors Cm215, Cfilt250, Ce1210and Ce2220shown inFIG. 2.

FIG. 2illustrates one embodiment of a capacitance measurement circuit200built around an operational amplifier260. The capacitance measurement circuit ofFIG. 2also functions as a low pass filter (LPF) due to the presence of the filter capacitor Cfilt250in the amplified feedback path. The output voltage VSis proportional to the input current IS. The circuit ofFIG. 2operates continuously such that ADC conversion can be started any time after transient signals have stabilized. It should be noted that the buffer input for buffer240can be connected to Vreffor the circuit illustrated inFIG. 2, taking into account that both operational amplifier260inputs have approximately the same potential.

FIG. 2further illustrates parasitic mutual capacitance current compensation using a programmable current source230as a programmable current offset in the capacitance measurement circuit200according to one embodiment. The current output of programmable current source230is a calibration parameter that is used to detect the sensor integrity arising from the physical characteristics of the sensor. The physical characteristics of a sensor are derived from the manufacturing process or trauma that may affect the operation of the sensor during its life.

The measurement circuit300ofFIG. 3is configured to measure a mutual capacitance matrix315that is coupled to the measurement circuit300through buses301and302.

The circuit ofFIG. 3measures a matrix315of mutual capacitances, which can each be represented by Cm415in the unit cell400ofFIG. 4. A mutual capacitance Cm415exists between row and column conductors Ce1410and Ce1420, respectively. INROW401, INCOL404, OUTROW403and OUTCOL402are coupled to other unit cells in the matrix. For example, an OUTROWof a first unit cell is coupled to an INROWof a horizontally adjacent unit cell and an OUTCOLis coupled to an INCOLof a vertically adjacent unit cell.

A matrix of unit cells such as unit cell400illustrated inFIG. 4is illustrated inFIG. 5. The unit cells510(1,1) through510(N,M) ofFIG. 5are arranged in an N×M mutual capacitance matrix500, wherein the inputs of each row are coupled to a first bus501and the inputs of each column are coupled to a second bus502. The outputs of each row and column have a capacitance to ground.

The measurement circuit ofFIGS. 2 through 5is used to calibrate and determine the type and location of faults in the mutual capacitance matrix315. The method for initializing and executing the Built-In Self-Test (BIST) is shown inFIG. 6 through 8.

FIG. 6illustrates a flowchart for the initialization and execution of the BIST. First, the touchpad is initialized in block610. After the touch panel is initialized the panel receives a “run BIST” command in block620. The “run BIST” command can come from an external controller or can be set as a command based on a timer. In one embodiment, the “run BIST” command of block620can result from a manufacturing process command to identify defective touch panels before they are assembled into finished units. In another embodiment, the “run BIST” command of block620can be set to a timer and repeated at an interval to maintain the calibration parameters and perform an automated self-diagnostic of the touch panel. In another embodiment, the “run BIST” command of block620can be sent to the touch panel in response to a trauma to the device. A trauma may be that the device was dropped. In such a case, a diagnostic of the touch panel may identify a fault caused by the trauma and alert the user that the device has diminished performance and requires service. The embodiments described here are not meant to be an exhaustive list of situations in which a “run BIST” command may be sent to the touch panel. Rather, they are merely examples of the situations for which a “run BIST” command would be appropriate. The BIST routine is run in block630and the results of the BIST are output in block640.

FIG. 7illustrates a flowchart for the calibration of sensors or sensor groups and the fault detection routine as it is integrated into the BIST. The first sensor is calibrated in block710. The calibration step is then repeated for the remaining sensors in block720. Block720is intended to be indicative of the repeatedly calibrating all sensors. Calibration values for sensor groups are reported in block730. The Pass/Fail status for each sensor group is determined in block740and the failed sensor groups are sorted into fault types in block750. The number of faults in each fault type is counted in block760. Pass/Fail information, both the type and number, is sent to a host (FIG. 11,1180) in block770.

FIG. 8illustrates a flowchart for one embodiment of the calibration routine according to the present invention. The current source successive approximation register (SAR) is initialized in block810. The SAR current setting is set to an initial value of 80h in block820. The SAR count variable is set to 0 in block830. The SAR count variable (and the threshold for it) defines how many times the SAR routine is run during calibration. The capacitance measurement conversion is run according to Table 1 in block840. In block850the SAR variable is incremented. In decision block860, the SAR Variable is compared to a threshold. If the SAR variable is greater than the threshold, the calibration routine is complete and the SAR value is stored in block870. If the SAR variable is less than the threshold, the SAR current setting is updated in block880and the capacitance measurement circuit is run again in block840.

FIGS. 9A through 9Dillustrate possible fault types for a mutual capacitance touch panel900according to the embodiments. In an embodiment, each of the intersections of all row and column electrodes are measured, but for the purposes of clarity, only a single point of measurement is shown in the figures.FIG. 9Aillustrates a first fault type wherein there exists a short910between the measured column electrode920and either a row electrode930that is not part of the measured pixel941and is therefore coupled to ground or a shield layer (not shown). A short910to the row electrode930that is not part of the measured pixel941or a short942to a shield electrode970, wherein the shield970is grounded, will act as a resistive ground connection, since the row electrode is grounded when it is not coupled to the receive circuit905. The output voltage Vs225, therefore cannot be trimmed to a normal level and the maximum output value is recorded.

Table 2 shows the pixel current source calibration values for a 16×11 array with a short to ground fault as shown inFIG. 9A.

Table 2 (as well as Tables 3 and 4) show an array of 16 columns and 11 rows. For the ease of explanation,FIGS. 9A through 9Dillustrate only three columns and 4 rows. To illustrate a 16×11 matrix would be unnecessarily confusing. Tables 2 through 4 andFIGS. 9A through 9Dare intended to be representative examples that do not necessary map directly to each other. Additionally, the values in each cell of Tables 2-4 indicate the programmable current source230necessary for the correct current offset.

High output values of “FE” (254in decimal) corresponding to the current offset from programmable current source230determined by controller280and stored in the memory290on three rows indicate that there is at least one short on three rows. These shorts are to either the shield layer970or a column since three rows show values of “FE.” The method for this determination is shown inFIG. 10and described below.

FIG. 9Billustrates a second fault type wherein there exists a short912between the measured column electrode920and a row electrode932that is part of the measured pixel. A short912between the measured column electrode922and the row electrode932that is part of the measured pixel943prohibits the output voltage from reaching an expected level, keeping the current from the current source230ofFIG. 2negative or very low and the value for that pixel “00.”

Table 3 shows the pixel current source calibration values for an array with a fault as shown inFIG. 9B.

Output values of “00” are detected for three pixels, indicating that there are three shorts between columns and rows, one each on rows1,2and10.

FIG. 9Cillustrates a third fault type wherein there is a crack in the mutual capacitance electrodes or a manufacturing defect in the touch panel interconnections. A crack in the electrodes or a manufacturing defect in the touch panel interconnections yields a correctly trimmed output voltage but low and diminishing sensitivity. The lower the current that is able to calibrate the output voltage is a product of lower conductivity and thus lower current through the electrodes, which can be the result of a crack in the metal bridges between sensing electrodes, breaks in the electrodes themselves or a bad etch in the patterning of the touch panel903.

Table 4 shows the pixel current source calibration values for an array with a fault as shown inFIG. 9C.

Diminishing output values are detected on five rows. In each case an expected value is detected, followed by a lower value and a still lower value in the third pixel. Thereafter, pixels have a lower output value indicating that the bad etch or crack in the IDAC pattern is located between the pixel950with the expected value and the first lower value. For example, in the second row, the fault lies between the first and second pixels. In the tenth row, the fault lies between the fifth and sixth pixels.

FIG. 9Dillustrates a fourth fault type wherein there is a crack or poor trace quality between the sensing circuitry and the touch panel904itself. In this embodiment the output voltage can be trimmed to a normal level but the sensitivity is low for an entire column due to the increased source impedance of the TX drive signal.

Table 5 shows the pixel current source calibration values for an array with a fault as shown inFIG. 9D.

Low output values for entire columns indicate that there are bad connections for columns 4 through 8.

The four fault types illustrated inFIGS. 9A through 9Dand exemplified in Tables 2 through 5 are distinguished from each other according to the method illustrated inFIG. 10. The SAR output is compared to a reference value indicating a high output in block1010. In this embodiment, and 8-bit SAR is used and the high output value is set to 254 (FEh). In another embodiment, a different resolution of SAR can be used as well as a different high output value. If the SAR output is equal to FEh, it is determined that there is a short to a grounded row or to the shield electrode as shown inFIG. 9Aand block1015. If the SAR output does not equal FEh, the SAR output is compared to a low value of 0 (00h) in block1020. If the SAR output is equal to 00h, it is determined that there is short to a between and column and a row at the point of measurement, as shown inFIG. 9Band block1025. If the SAR output does not equal 00, it is compared to the previous pixel SAR output value in block1030. If the SAR output for the measured pixel is less than the SAR output for the previous pixel it is then compared to a first threshold value in block1040. If the SAR output value is less than the previous pixel and less than a first threshold, the previous pixel is compared to the pixel before it in block1050. If the previous pixel is less than the pixel before it, the previous pixel is compared to a second threshold in block1060. In the previous pixel is less than the pixel before it and the previous pixel is less than a second threshold value, it is determined in block1065that there is a crack or a poor connection in the touch panel903. If any of steps1030through1060are “no,” the SAR output for an entire column is compared to a third threshold in step1070. If the SAR output for an entire column is less than the third threshold, it is determined in block1075that there is a crack or a poor connection in the connection between the sensing circuit and the touch panel904inFIG. 9. If the output of block1070is “no,” the connection between the sensing circuit and the touch panel901,902,903or904is identified as having no failure in block1080.

A system that executes this test process can have several configurations shown inFIGS. 11A through 11D.FIG. 11Aillustrates an embodiment wherein the controller1140for this test process may be integrated into the touch panel control circuitry1190. System1100comprises touchpanel control circuitry1190coupled to host1180. Touch panel control circuitry1190includes capacitive sensor1110which is coupled to capacitance measurement circuit1120. Capacitance measurement circuit1120is coupled to calibration circuit1130. Controller1140is coupled to calibration circuit1130and capacitance measurement circuit1120and coupled to the host1180.

FIG. 11Billustrates an embodiment, system1101, wherein the touch panel control circuit1195is coupled to the controller1140through a bed-of-nails tester1150. The bed-of-nails tester1150couples the controller to appropriate locations on the touch panel control circuit1195.

FIG. 11Cillustrates an embodiment, system1102, wherein the capacitance sensor1110, calibration circuit1130and capacitance measurement circuit1120are located on a printed circuit board1160. The controller1140is coupled to the printed circuit board1160and to the host1180.

FIG. 11Dillustrates an embodiment, system1103, wherein the capacitance sensor1110, capacitance measurement circuit, calibration circuit and controller are all located on the same substrate1170for the capacitance sensor.

In the foregoing specification, the invention has been described with reference to specific example embodiments thereof. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.