Capacitive Sensing with High-Frequency Noise Reduction

Abstract

A method includes performing a measurement burst which generates signal sample values from measurement cycles. Each measurement cycle includes inducing charge onto a key during a drive part of the measurement cycle, measuring during a signal measurement part of the measurement cycle the charge induced on the key during the drive part of the measurement cycle, and generating a signal sample value to represent the charge measured from the key. The method includes comparing the value of the signal sample values of the measurement burst with a determined range of accepted values between a determined maximum and a determined minimum value, and processing the signal sample values, which are outside the determined accepted range to the effect that the presence of the body can be determined only from a change in the value of one or more of the signal samples which are within the determined accepted range.

1 . A method of sensing the presence of a body from a change in an amount of charge present on a capacitively charged key, the method comprising
 performing a measurement burst which generates a plurality of signal sample values from a plurality of measurement cycles, each of the measurement cycles including,   inducing charge onto the key during a drive part of the measurement cycle,   measuring during a signal measurement part of the measurement cycle the charge induced on the key during the drive part of the measurement cycle,   generating a signal sample value representing the charge measured from the key during the signal measurement part of the measurement cycle,   comparing the value of the plurality of the signal sample values of the measurement burst with a determined range of accepted values between a determined maximum and a determined minimum value, and   processing any of the plurality of signal sample values, which are outside the determined accepted range to the effect that the presence of a body can be determined only from a change in the value of one or more of the plurality of signal samples which are within the determined accepted range.   
     
     
         2 . A method as claimed in  claim 1 , wherein the processing the signal sample values includes
 adapting the signal sample values, which exceed the predetermined maximum or which are lower than the determined minimum.   
     
     
         3 . A method as claimed in  claim 1 , wherein the processing the signal sample values includes
 discarding signal sample values from the measurement burst, which are outside the accepted range, and   determining whether the body is present from a change of a mean value of the remaining signal sample values of the measurement burst, which are within the accepted range.   
     
     
         4 . A method as claimed in  claim 1 , wherein the processing the signal sample values includes
 limiting a rate of change between the value of the signal samples.   
     
     
         5 . A method as claimed in  claim 4 , wherein the limiting of the rate of change between the value of the signal sample values includes
 determining a rate of change between the value of the signal sample values at a first time,   determining an upper limit on the rate of change of the signal sample values in accordance with the determined rate of change at the first time,   limiting the rate of change of the signal sample values in accordance with the determined upper limit,   determining a rate of change between the value of the signal sample values at a second time after the first time,   adapting the upper limit on the rate of change of the signal sample values in accordance with the rate of change at the second time with respect to the rate of change at the first time, and   limiting the rate of change of the signal sample values in accordance with the adapted upper limit.   
     
     
         6 . A method as claimed in  claim 5 , wherein the processing of the signal sample values includes
 averaging the signal sample values.   
     
     
         7 . A method as claimed in  claim 1 , comprising
 generating, during a test phase, a plurality of signal sample values from one or more measurement bursts, each measurement burst including a plurality of measurement cycles, each of the measurement cycles generating one of the signal sample values, and   setting the determined range of accepted values between the determined maximum and the determined minimum value based on the value of the signal samples generated during the test phase.   
     
     
         8 . An apparatus for sensing the presence of a body from a change in an amount of charge present on a cap actively charged key, the apparatus comprising
 a driving circuit coupled to the key and operable to induce charge onto the key,   a charge sensing circuit which includes a signal measurement capacitor, and   a controller, the controller being operable   to control the driving circuit and the charge sensing circuit to generate a signal sample value for each of a plurality of measurement cycles forming a measurement burst by inducing charge onto the key during a drive part of the measurement cycle, measuring during a signal measurement part of the measurement cycle the charge induced on the key during the drive part of the measurement cycle, and generating a signal sample value to represent of the charge measured from the key during the signal measurement part of the measurement cycle,   to compare the value of the plurality of the signal sample values of the measurement burst with a determined range of accepted values between a determined maximum and a determined minimum value, and   to process any of the plurality of signal sample values, which are outside the determined accepted range to the effect that the presence of the body can be determined only from a change in the value of one or more of the plurality of signal samples which are within the determined accepted range.   
     
     
         9 . An apparatus as claimed in  claim 8 , wherein the controller is operable to adapt the signal sample values, which exceed the determined maximum or which are lower than the determined minimum. 
     
     
         10 . An apparatus as claimed in  claim 8 , wherein the processing the signal sample values includes
 discarding those of the plurality of signal sample values from the measurement burst, which are outside the accepted range, and   determining whether the body is present from a change of a mean value of the remaining signal sample values of the measurement burst, which are within the accepted range.   
     
     
         11 . An apparatus as claimed in  claim 8 , wherein the controller is operable to process the signal sample values by
 limiting a rate of change between the value of the signal samples.   
     
     
         12 . An apparatus as claimed in  claim 11 , wherein the controller is operable to limit the rate of change between the value of the signal sample values by
 determining a rate of change between the value of the signal sample values at a first time,   determining an upper limit on the rate of change of the signal sample values in accordance with the determined rate of change at the first time,   limiting the rate of change of the signal sample values in accordance with the determined upper limit,   determining a rate of change between the value of the signal sample values at a second time after the first time,   adapting the upper limit on the rate of change of the signal sample values in accordance with the rate of change at the second time with respect to the rate of change at the first time, and   limiting the rate of change of the signal sample values in accordance with the adapted upper limit.   
     
     
         13 . An apparatus as claimed in  claim 12 , wherein the controller is operable to process the signal sample values by averaging the signal sample values. 
     
     
         14 . An apparatus as claimed in  claim 8 , wherein the controller is operable
 to generate, during a test phase, a plurality of signal sample values from one or more measurement bursts, each measurement burst including a plurality of measurement cycles, each of the measurement cycles generating one of the signal sample values, and   to set the determined range of accepted values between the determined maximum and the determined minimum value based on the value of the signal samples generated during the test phase.   
     
     
         15 . A touch sensitive control panel including a key matrix comprising a first plurality, N of input lines, a second plurality, M, of output lines and N times M keys, each of the N times M keys being arranged to sense the presence of a body and each of the N times M keys is disposed adjacent a respective intersection of an input line and an output line and comprises a driven plate (X) connected to one of the N input lines and a receiving plate (Y) connected to one of the M output lines,
 each of the N input lines being connected to a respective drive circuit, and   each of the M output lines being connected to a respective charge sensing circuit, each of the charge sensing circuits including a signal measurement capacitor, wherein the key matrix includes a controller, the controller being operable   to control the driving circuits and the charge sensing circuits to generate, for each of the N times M keys, a signal sample value for each of a plurality of measurement cycles forming a measurement burst by inducing charge onto the drive plate of each of the keys during a drive part of the measurement cycle, measuring during a signal measurement part of the measurement cycle the charge induced on the sensing plate of each of the keys during the drive part of the measurement cycle, and generating the signal sample value to represent the charge measured from each of the keys during the signal measurement part of the measurement cycle,   to compare the value of the plurality of the signal sample values of the measurement burst with a determined range of accepted values between a pre-determined maximum and a determined minimum value, and   to process any of the plurality of signal sample values, which are outside the determined accepted range to the effect that the presence of the body can be determined only from a change in the value of one or more of the plurality of signal samples which are within the determined accepted range.   
     
     
         16 . A touch sensitive control panel as claimed in  claim 15 , wherein the controller is operable to adapt the signal sample values, which exceed the determined maximum or, which are lower than the determined minimum. 
     
     
         17 . A touch sensitive control panel as claimed in  claim 15 , wherein the processing the signal sample values includes
 discarding signal sample values from the measurement burst, which are outside the accepted range, and   determining whether the body is present from a change of a mean value of the remaining signal sample values of the measurement burst, which are within the accepted range.   
     
     
         18 . A touch sensitive control panel as claimed in  claim 15 , wherein the controller is operable to process the signal sample values by
 limiting a rate of change between the value of the signal samples.   
     
     
         19 . A touch sensitive control panel as claimed in  claim 18 , wherein the controller is operable to limit the rate of change between the value of the signal sample values by
 determining a rate of change between the value of the signal sample values at a first time,   determining an upper limit on the rate of change of the signal sample values in accordance with the determined rate of change at the first time,   limiting the rate of change of the signal sample values in accordance with the determined upper limit,   determining a rate of change between the value of the signal sample values at a second time after the first time,   adapting the upper limit on the rate of change of the signal sample values in accordance with the rate of change at the second time with respect to the rate of change at the first time, and   limiting the rate of change of the signal sample values in accordance with the adapted upper limit.   
     
     
         20 . A touch sensitive control panel as claimed in  claim 18 , wherein the controller is operable to process the signal sample values by averaging the signal sample values. 
     
     
         21 . A touch sensitive control panel as claimed in  claim 15 , wherein the controller is operable
 to generate, during a test phase, a plurality of signal sample values from one or more measurement bursts, each measurement burst including a plurality of measurement cycles, each of the measurement cycles generating one of the signal sample values, and   to set the determined range of accepted values between the determined maximum and the determined minimum value based on the value of the signal samples generated during the test phase.   
     
     
         22 . A method of sensing the presence of a body proximate a touch sensitive control panel, the touch sensitive control panel including a key matrix comprising a first plurality, N of input lines, a second plurality, M, of output lines and N times M keys, each of the N times M keys being disposed adjacent a respective intersection of an input line and an output line and comprises a driven plate (X) connected to one of the N input lines and a receiving plate (Y) connected to one of the M output lines,
 each of the N input lines being connected to a respective drive circuit, and   each of the M output lines being connected to a respective charge sensing circuit, each of the charge sensing circuits including a signal measurement capacitor, the method comprising   controlling the driving circuits and the charge sensing circuits to generate, for each of the N times M keys, a signal sample value for each of a plurality of measurement cycles forming a measurement burst by inducing charge onto the drive plate of each of the keys during a drive part of the measurement cycle, measuring during a signal measurement part of the measurement cycle the charge induced on the sensing plate of each of the keys during the drive part of the measurement cycle, and generating a signal sample value to represent of the charge measured from each of the keys during the signal measurement part of the measurement cycle,   comparing the value of the plurality of the signal sample values of the measurement burst with a determined range of accepted values between a determined maximum and a determined minimum value, and   processing any of the plurality of signal sample values, which are outside the determined accepted range to the effect that the presence of a body can be determined only from a change in the value of one or more of the plurality of signal samples which are within the determined accepted range.