Techniques for Locally Improving Signal to Noise in a Capacitive Touch Sensor

Abstract

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for digital signal processing (DSP) techniques for generally improving a signal-to-noise ratio (SNR) of capacitive touch sensors.

1 . A method performed by a data processing apparatus associated with a capacitive touch sensor, the sensor comprising trace lines arranged in rows and columns with a matrix configuration, the method comprising:
 conducting a first scan including scanning the columns of the capacitive touch sensor in an interlace pattern, wherein the interlace pattern includes a frame, wherein the frame comprises n number of subframes, where n is an integer;   using information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a capacitance from a row to a column;   using the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of columns upon which to focus a second and subsequent scan; and   scanning the subset of columns selected for the second and subsequent scan, wherein the first scan is associated with a first measurement, wherein the second scan is associated with a second measurement,   wherein the scanning of the subset of columns comprises:
 determining a target signal level and a noise level for the second scan; 
 determining a target signal to noise ratio; and 
 determining an integration period to achieve the target signal to noise ratio by utilizing a function that is an average of the second measurement and the first measurement. 
   
     
     
         2 . The method of  claim 1 , wherein the first scan comprises:
 determining a signal level and a noise level for the first scan; and   determining a target signal-to-noise ratio based on the signal level and the noise level determined for the first scan.   
     
     
         3 . The method of  claim 2 , the method further comprises:
 receiving signals for the first and second scans; and   determining a signal with a signal-to-noise ratio that is higher than the signal-to-noise ratios associated with the first or second scans by averaging the signals that are received for the first and second scans.   
     
     
         4 . The method of  claim 1 , wherein n is equal to 4, wherein the frame is configured to be at about 30 Hz and the subframes to be at about 120 Hz, wherein the interlace pattern comprises 16 columns per frame and 4 columns per subframe, wherein a latency of the sensor is about 120 Hz, and wherein the row to the column capacitance comprises a fringing capacitance. 
     
     
         5 . A method performed by a data processing apparatus associated with a capacitive touch sensor, the sensor comprising trace lines arranged in rows and columns with a matrix configuration, the columns being arranged as n sets of columns, where n is an integer, the method comprising:
 sequentially conducting a first scan of each of the n sets of columns of the capacitive touch sensor in an interlace pattern;   using information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a fringing capacitance, the fringing capacitance comprising a capacitance from a row to a column;   using the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of each of the n sets of columns upon which to focus a second and subsequent scan according; and   scanning the subset of each of the n sets of columns selected for the second and subsequent scan, wherein the first scan is associated with a first measurement, wherein the second scan is associated with a second measurement,   wherein the scanning of the each subset of the n sets of columns comprises:
 determining a signal level and a noise level for the second scan; 
 determining a target signal to noise ratio; and 
 determining an integration period to achieve the target signal to noise ratio by utilizing a function that is an average of the second measurement and the first measurement, and 
   wherein, for each set of columns and corresponding subsets of columns in the n sets of columns, the first scan and the second scan are conducted before commencing scanning on a subsequent set of columns and corresponding subsets of columns.   
     
     
         6 . The method of  claim 5 , for each subset of the n sets of columns, the method further comprises:
 scanning the subset of columns in the integration period;   obtaining the second measurement related to received signals of the scan of the subset of columns, wherein the second measurement is related to a second measurement-derived signal-to-noise ratio;   determining whether the subset of columns are configured to have at least a minimum signal-to-noise ratio;   determining, on the basis of the second measurement-derived signal-to-noise ratio, whether the second measurement-derived signal-to-noise ratio is less than the minimum signal-to-noise ratio for the subset of columns; and   upon determining that the second measurement-derived signal-to-noise ratio is less than the minimum signal-to-noise ratio for the subset of columns,
 performing another scan of the subset of columns; 
 obtaining another measurement related to received signals of the other scan of the subset of columns, and 
 averaging the measurement and the other measurement of the subset of columns to produce a combined measurement that has a property where a signal-to-noise ratio related to the combined measurement is higher than the signal-to-noise ratio related to either of the measurements of the subset of columns. 
   
     
     
         7 . A method performed by a data processing apparatus associated with a capacitive touch sensor, the sensor being located in a system comprising a liquid crystal display, the method comprising:
 determining a noise frequency in the capacitive touch sensor;   identifying that the noise frequency is a function of a frequency of the liquid crystal display; and   determining an excitation frequency for the sensor as a function of the determined noise frequency, wherein determining the excitation frequency comprises:
 selecting an initial excitation frequency for the sensor; 
 computing a cross-correlation between the noise frequency and the initial excitation frequency over an integration period, wherein the computation of the cross-correlation is presentable in a sinc-like waveform with at least one peak and at least two nulls; and 
 selecting the excitation frequency for the sensor by selecting a frequency at one of the nulls in the sinc-like waveform and assigning the determined excitation frequency to be a same frequency as the frequency at the selected null. 
   
     
     
         8 . The method of  claim 7 , wherein the noise frequency is within a range of about 30 kHz to about 135 kHz, the capacitive touch sensor comprises a maximum transmit voltage of about 200V, and the capacitive touch sensor is configured to provide a current to travel through a user that is on an order of about tens of microamps. 
     
     
         9 . The method of  claim 7 , wherein the capacitive touch sensor comprises a front end interface, the method further comprising demodulating a waveform at an output of the front end interface of the capacitive touch sensor, wherein the waveform comprises the cross-correlation of the noise frequency against the initial excitation frequency. 
     
     
         10 . The method of  claim 7 , further comprising:
 measuring a level of noise in the sensor; and   setting an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         11 . The method of  claim 10 , further comprising:
 continuously measuring the level of noise in the sensor; and   continuously adjusting a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         12 . The method of  claim 7 , further comprising determining a plurality of orthogonal excitation waveforms for the sensor, wherein at least one of the orthogonal excitation waveforms comprises the selected excitation frequency,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the determined noise frequency.   
     
     
         13 . A method performed by a data processing apparatus associated with a capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration, the sensor being located in a system comprising a liquid crystal display, the method comprising:
 identifying a noise frequency;   generating an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the generation of the excitation waveform comprises:
 in a frequency domain, specifying an initial excitation waveform; and 
 converting the initial excitation waveform from the frequency domain into the excitation waveform in a time domain by using a Fourier transform in the conversion; and 
   transmitting the excitation waveform across at least one of the trace lines.   
     
     
         14 . The method of  claim 13 , further comprising:
 measuring a level of noise in the sensor; and   setting an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         15 . The method of  claim 14 , further comprising:
 continuously measuring the level of noise in the sensor; and   continuously adjusting a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         16 . The method of  claim 13 , further comprising determining a plurality of orthogonal excitation waveforms for the sensor,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the identified noise frequency.   
     
     
         17 . A method performed by a data processing apparatus associated with a capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration, the sensor being located in a system comprising a liquid crystal display, the method comprising:
 identifying a noise frequency;   generating an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the generation of the excitation waveform comprises:
 selecting an initial excitation waveform; 
 selecting an algorithm corresponding to a finite impulse response filter; and 
 generating the excitation waveform by applying the algorithm corresponding to the finite impulse response filter to the initial excitation waveform; and 
   transmitting the excitation waveform across at least one of the trace lines.   
     
     
         18 . The method of  claim 17 , further comprising:
 measuring a level of noise in the sensor; and   setting an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         19 . The method of  claim 18 , further comprising:
 continuously measuring the level of noise in the sensor; and   continuously adjusting a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         20 . The method of  claim 17 , further comprising determining a plurality of orthogonal excitation waveforms for the sensor,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the identified noise frequency.   
     
     
         21 . A system comprising:
 a data processing apparatus; and   a capacitive touch sensor configured to interact with the data processing apparatus, the sensor comprising trace lines arranged in rows and columns with a matrix configuration, and   the system is configured to:
 conduct a first scan with the sensor that includes scanning the columns of the capacitive touch sensor in an interlace pattern, wherein the interlace pattern includes a frame, wherein the frame comprises n number of subframes, where n is an integer; 
 utilize information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a capacitance from a row to a column; 
 utilize the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of columns upon which to focus a second and subsequent scan; and 
 scan the subset of columns selected for the second and subsequent scan, 
 wherein when scanning the subset of columns the system is configured to:
 determine a signal level and a noise level for the second scan; and 
 determine a signal-to-noise ratio based on the signal level and the noise level determined for the second scan and relating to the areas of the sensor that had the change in capacitance. 
 
   
     
     
         22 . The system of  claim 21 , wherein the system is configured to conduct the first scan by:
 determining a signal level and a noise level for the first scan; and   determining a signal-to-noise ratio based on the signal level and the noise level determined for the first scan.   
     
     
         23 . The system of  claim 22 , the system is further configured for:
 receiving signals for the first and second scans; and   determining a combined signal-to-noise ratio that is higher than the signal-to-noise ratios associated with the first or second scans by averaging the signals that are received for the first and second scans.   
     
     
         24 . The system of  claim 21 , wherein n is equal to 4, wherein the frame is configured to be at about 30 Hz and the subframes to be at about 120 Hz, wherein the interlace pattern comprises 16 columns per frame and 4 columns per subframe, wherein a latency of the sensor is about 120 Hz, and wherein the row to the column capacitance comprises a fringing capacitance. 
     
     
         25 . A system comprising:
 a data processing apparatus; and   a capacitive touch sensor that is configured to interact with the data processing apparatus, the sensor comprising trace lines arranged in rows and columns with a matrix configuration, the columns being arranged as n sets of columns, wherein n is an integer,   the system is configured to:
 sequentially conduct a first scan of each of the n sets of columns of the capacitive touch sensor in an interlace pattern; 
 utilize information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a fringing capacitance, the fringing capacitance comprising a capacitance from a row to a column; 
 utilize the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of each of the n sets of columns upon which to focus a second and subsequent scan according; and 
 scan the subset of each of the n sets of columns selected for the second and subsequent scan, wherein the first scan is associated with a first measurement, wherein the second scan is associated with a second measurement, 
 wherein the system is configured for scanning of the each subset of the n sets of columns by:
 determining a target signal level and a noise level for the second scan; 
 determining a target signal-to-noise ratio; and 
 determining an integration period to achieve the target signal to noise ratio by utilizing a function that is an average of the second measurement and the first measurement, and 
 
   wherein, for each set of columns and corresponding subsets of columns in the n sets of columns, the system is configured such that the first scan and the second scan are conducted before commencing scanning on a subsequent set of columns and corresponding subsets of columns.   
     
     
         26 . The system of  claim 25 , wherein for each subset of the n sets of columns, the system is configured to:
 scan the subset of columns in the integration period;   obtain the second measurement related to received signals of the scan of the subset of columns, wherein the second measurement is related to a second measurement-derived signal-to-noise ratio;   determine whether the subset of columns are configured to have at least a minimum signal-to-noise ratio;   determine, on the basis of the second measurement-derived signal-to-noise ratio, whether the second measurement-derived signal-to-noise ratio is less than the minimum signal-to-noise ratio for the subset of columns; and   upon determining that the second measurement-derived signal-to-noise ratio is less than the minimum signal-to-noise ratio for the subset of columns,
 perform another scan of the subset of columns; 
 obtain another measurement related to received signals of the other scan of the subset of columns, and 
 average the measurement and the other measurement of the subset of columns to produce a combined measurement that has a property where a signal-to-noise ratio related to the combined measurement is higher than the signal-to-noise ratio related to either of the measurements of the subset of columns. 
   
     
     
         27 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor configured to interact with the data processing apparatus; and   a liquid crystal display,   the system is configured to:
 determine a noise frequency in the capacitive touch sensor; 
 identify that the noise frequency is a function of a frequency of the liquid crystal display; and 
 determine an excitation frequency for the sensor as a function of the determined noise frequency, wherein when determining the excitation frequency the system is further configured to:
 select an initial excitation frequency for the sensor; 
 compute a cross-correlation between the noise frequency and the initial excitation frequency over an integration period, wherein the computation of the cross-correlation is presentable in a sinc-like waveform with at least one peak and at least two nulls; and 
 select the excitation frequency for the sensor by selecting a frequency at one of the nulls in the sinc-like waveform and assigning the determined excitation frequency to be a same frequency as the frequency at the selected null. 
 
   
     
     
         28 . The system of  claim 27 , wherein the noise frequency is within a range of about 30 kHz to about 135 kHz, the capacitive touch sensor comprises a maximum transmit voltage of about 200V, and the capacitive touch sensor is configured to provide a current to travel through a user that is on an order of about tens of microamps. 
     
     
         29 . The system of  claim 27 , wherein the capacitive touch sensor comprises a front end interface, the system is configured to demodulate a waveform at an output of the front end interface of the capacitive touch sensor, wherein the waveform comprises the cross-correlation of the noise frequency against the initial excitation frequency. 
     
     
         30 . The system of  claim 27 , the system is configured to:
 measure a level of noise in the sensor; and   set an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         31 . The system of  claim 30 , the system is configured to:
 continuously measure the level of noise in the sensor; and   continuously adjust a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         32 . The system of  claim 27 , the system is configured to determine a plurality of orthogonal excitation waveforms for the sensor, wherein at least one of the orthogonal excitation waveforms comprises the selected excitation frequency,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the determined noise frequency.   
     
     
         33 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor to interact with the data processing apparatus, the capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration; and   a liquid crystal display,   the system is configured to:
 identify a noise frequency; 
 generate an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the system is configured to generate the excitation waveform by:
 in a frequency domain, specifying an initial excitation waveform; and 
 converting the initial excitation waveform from the frequency domain into the excitation waveform in a time domain by using a Fourier transform in the conversion; and 
 
 transmit the generated excitation waveform across at least one of the trace lines. 
   
     
     
         34 . The system of  claim 33 , the system is configured to:
 measure a level of noise in the sensor; and   set an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         35 . The system of  claim 34 , the system is configured to:
 continuously measure the level of noise in the sensor; and   continuously adjust a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         36 . The system of  claim 33 , the system is configured to determine a plurality of orthogonal excitation waveforms for the sensor,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the identified noise frequency.   
     
     
         37 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor to interact with the data processing apparatus, the capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration; and   a liquid crystal display,   the system is configured to:
 identify a noise frequency; 
 generate an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the system is configured to generate the excitation waveform by:
 selecting an initial excitation waveform; 
 selecting an algorithm corresponding to a finite impulse response filter; and 
 generating the excitation waveform by applying the algorithm corresponding to the finite impulse response filter to the initial excitation waveform; and 
 
 transmit the generated excitation waveform across at least one of the trace lines. 
   
     
     
         38 . The system of  claim 37 , the system is configured to:
 measure a level of noise in the sensor; and   set an initial threshold for detecting a touch from a user of the sensor based on the level of measured noise.   
     
     
         39 . The system of  claim 38 , the system is configured to:
 continuously measure the level of noise in the sensor; and   continuously adjust a threshold for detecting the touch from the user of the sensor based on the level of continuously-measured noise.   
     
     
         40 . The system of  claim 37 , where in the sensor is further configured to determine a plurality of orthogonal excitation waveforms for the sensor,
 wherein the sensor is configured for simultaneous transmission of the plurality of orthogonal excitation waveforms, and   wherein the plurality of orthogonal excitation waveforms are all orthogonal to the identified noise frequency.   
     
     
         41 . A system comprising:
 a data processing apparatus; and   a capacitive touch sensor configured to interact with the data processing apparatus, the sensor comprising trace lines arranged in rows and columns with a matrix configuration; and   means for conducting a first scan with the sensor that includes scanning the columns of the capacitive touch sensor in an interlace pattern, wherein the interlace pattern includes a frame, wherein the frame comprises n number of subframes, where n is an integer;   means for utilizing information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a capacitance from a row to a column;   means for utilizing the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of columns upon which to focus a second and subsequent scan; and   means for scanning the subset of columns selected for the second and subsequent scan,   and for scanning the subset of columns, the system includes:
 means for determining a signal level and a noise level for the second scan; and 
 means for determining a signal-to-noise ratio based on the signal level and the noise level determined for the second scan and relating to the areas of the sensor that had the change in capacitance. 
   
     
     
         42 . A system comprising:
 a data processing apparatus; and   a capacitive touch sensor that is configured to interact with the data processing apparatus, the sensor comprising trace lines arranged in rows and columns with a matrix configuration, the columns being arranged as n sets of columns, wherein n is an integer;   means for sequentially conducting a first scan of each of the n sets of columns of the capacitive touch sensor in an interlace pattern;   means for utilizing information generated as a consequence of the first scan to identify areas of the sensor that experienced a change in a fringing capacitance, the fringing capacitance comprising a capacitance from a row to a column;   means for utilizing the detection of the areas of the sensor that experienced the change in the capacitance to inform selection of a subset of each of the n sets of columns upon which to focus a second and subsequent scan according; and   means for scanning the subset of each of the n sets of columns selected for the second and subsequent scan,   wherein the system is configured for scanning of the each subset of the n sets of columns by means for:
 determining a signal level and a noise level for the second scan; and 
 determining a an integration period to achieve the target signal to noise ratio, when the second measurement is averaged with the first measurement, and 
   wherein, for each set of columns and corresponding subsets of columns in the n sets of columns, the system is configured such that the first scan and the second scan are conducted before commencing scanning on a subsequent set of columns and corresponding subsets of columns.   
     
     
         43 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor configured to interact with the data processing apparatus; and   a liquid crystal display,   means for determining a noise frequency in the capacitive touch sensor;   means for identifying that the noise frequency is a function of a frequency of the liquid crystal display; and   means for determining an excitation frequency for the sensor as a function of the determined noise frequency, wherein for determining the excitation frequency the system includes:
 means for selecting an initial excitation frequency for the sensor; 
 means for computing a cross-correlation between the noise frequency and the initial excitation frequency over an integration period, wherein the computation of the cross-correlation is presentable in a sinc-like waveform with at least one peak and at least two nulls; and 
 means for selecting the excitation frequency for the sensor by selecting a frequency at one of the nulls in the sinc-like waveform and assigning the determined excitation frequency to be a same frequency as the frequency at the selected null. 
   
     
     
         44 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor to interact with the data processing apparatus, the capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration; and   a liquid crystal display;   means for identifying a noise frequency;   means for generating an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the system is configured to generate the excitation waveform at least by means for:
 in a frequency domain, specifying an initial excitation waveform; and 
 converting the initial excitation waveform from the frequency domain into the excitation waveform in a time domain by using a Fourier transform in the conversion; and 
   means for transmitting the generated excitation waveform across at least one of the trace lines.   
     
     
         45 . A system comprising:
 a data processing apparatus;   a capacitive touch sensor to interact with the data processing apparatus, the capacitive touch sensor comprising rows and columns of trace lines arranged in a matrix configuration; and   a liquid crystal display,   means for identifying a noise frequency;   means for generating an excitation waveform to transmit across at least one of the trace lines in the sensor, wherein the excitation waveform is generated such that the excitation waveform is orthogonal to the identified noise frequency, wherein the excitation waveform is generated such that noise at the identified noise frequency is rejected in the excitation waveform, the system is configured to generate the excitation waveform by means for:
 selecting an initial excitation waveform; 
 selecting an algorithm corresponding to a finite impulse response filter; and 
 generating the excitation waveform by applying the algorithm corresponding to the finite impulse response filter to the initial excitation waveform; and 
   means for transmitting the generated excitation waveform across at least one of the trace lines.