Capacitive sensing with low-frequency noise reduction

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 comprises inducing charge onto the key during a drive part of a measurement cycle, coupling a signal measurement capacitor to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor, determining from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle, and controlling the drive part, the signal measurement part and the noise measurement parts of the cycle to provide the charge sensing circuit with a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

BACKGROUND

Touch sensitive control devices are now prevalent on many electronic devices such as mobile phones, MP3 players, personal digital assistants as well as white goods such as cookers and freezers. This is because they are space saving in terms of an amount of “surface real estate” available to position user controls, robust in that there is a reduction in the amount of mechanical components required in their implementation and they can also be made to resist potentially harmful substances in an environment in which they are disposed. For the example of white goods, the presence of water and other aqueous substances is typically harmful to contact switches. Therefore, the touch sensitive switch can be disposed behind a protective layer preventing damage from being caused by the aqueous substances. Furthermore, a touch sensitive control can be disposed in front of a display screen such as for example an LCD display screen with the effect that a user can select a particular function by touching the screen at a position at which a particular menu option has been displayed.

There are various forms of touch sensitive controls which use a capacitive sensor to sense the presence of a body such as a user's finger. A touch sensitive capacitive sensor for example is disclosed in WO-97/23738. In WO-97/23738 a single coupling plate is provided and disposed to form a touch sensitive switch. The touch sensitive plate is referred to as a key. In accordance with this example, the key is charged using a drive circuit for a drive part of a measurement cycle and then this charge is measured by transferring the induced charge from the key by a charge detection circuit during a measurement part of the cycle. The charging and transferring parts of the cycle can vary widely and can be selected in accordance with the application concerned. The sensor can detect the presence of an object near the key as a result of a change in an amount of the charge induced onto the key, even in the presence of interfering substances.

Another form of touch sensitive control is disclosed in WO-00/44018. In this example a pair of electrodes are provided which act as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the pair of electrodes (labelled X) is driven with a drive circuit and the other of the pair of electrodes (labelled Y) is connected to a charge measurement circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-00/440018 several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor. Such two dimensional capacitive sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in the example of consumer electronic devices and domestic appliances. As indicated above, such two dimensional capacitive touch sensors can be used in conjunction with liquid crystal displays or cathode ray tubes to form such touch sensitive screens.

Although touch sensitive capacitive sensors, such as those described above and disclosed in the above-mentioned disclosures, have been successfully deployed in many applications, some applications can present a challenging environment for detecting a change in charge as a result of the presence of a body. For example, noise which may be present for a particular application can cause a disruption in accurately measuring an amount of charge transferred from a capacitively charged key for the various examples set out above.

SUMMARY

According to the present invention there is provided a method and apparatus for sensing the presence of a body from a change in an amount of charge present on a capacitively charged key. The method finds application for an example in which a key comprises a single plate which is charged first and then discharged, and for an example in which the key includes a drive plate X and a receiving plate Y, in which the amount of charge received on the Y plate is determined contemporaneously with the drive plate being charged by a drive voltage. The present invention also finds application in an example in which a plurality of key pairs form a key matrix.

The method comprises inducing charge onto the key during a drive part of a measurement cycle, coupling a signal measurement capacitor to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor, determining from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle, and controlling the drive part, the signal measurement part and the noise measurement parts of the cycle to provide the charge sensing circuit with a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

Embodiments of the present invention can provide a method and apparatus for improving an accuracy with which a signal measurement is taken from a capacitively charged key, and in particular for providing a way of removing or at least reducing the effects of noise, which may otherwise cause an erroneous reading to be made. In one example, the determining an amount of charge induced on the key by noise, includes coupling a noise measurement capacitor to the key for the noise measurement part of the measurement cycle either after the charge induced by the key has been transferred to the signal measurement capacitor or before the charge has been induced on the key during the drive part of the measurement cycle. The charge induced on the key may be transferred to the signal measurement capacitor by coupling the signal measurement capacitor to the key contemporaneously with the drive part of the measurement cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor during the signal measurement part of the measurement cycle, and determining an amount of charge present on the signal measurement capacitor.

In another example, the determining the amount of charge induced on the key by the noise, includes coupling the signal measurement capacitor to the key contemporaneously with the drive part of the cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor, and the determining from the noise measurement part of the measurement cycle the amount of charge induced on the key by the noise, includes either before the drive part of the measurement cycle or after the signal measurement part of the measurement cycle coupling the signal measurement capacitor to the key in reverse so that charge induced by noise on the key reduces the amount of charge induced on the measurement capacitor during the signal measurement part of the cycle.

In some embodiments a time period of the signal measurement part and the noise measurement part of the cycle are substantially equal. As such a contribution made by the noise during the signal measurement part of the measurement cycle will be represented by a sample of the noise determined from the noise measurement part of the measurement cycle, without any adjustment, scaling or adaptation. On the other hand, if the time period of the noise measurement part of the measurement cycle is different to the signal measurement part of the measurement cycle, then the noise measurement can be scaled in proportion with the difference in the time periods of the signal and noise measurement parts of the cycle. In some embodiments the noise measurement part of the measurement cycle includes a first period before the signal measurement part of the cycle and a second period after the signal measurement part of the cycle. As a result, if the noise is low frequency noise, then the average noise measurement taken between the first and second parts of the noise measurement part of the measurement cycle will provide a more accurate estimate of the amount of charge induced by the noise during the signal measurement part of the measurement cycle.

In some examples, in order to reduce a dwell time, inducing the charge onto the key during the drive part of the measurement cycle occurs before the coupling of the signal measurement capacitor to the key during the signal measurement part of the measurement cycle to the effect that the key is pre-charged before the charge induced on the key is transferred to the signal measurement capacitor. The dwell time is a time required to allow the charge transferred from the key to reach a steady state value.

Various further aspects and features of the present invention are defined in the appended claims.

DETAILED DESCRIPTION

As explained above there are various forms of touch sensors which can determine the presence of a body proximate the touch sensor as a result of a change of charge transferred from a key of the touch sensor. An example of such a touch sensor is shown inFIGS. 1A and 1B. The example shown inFIGS. 1A and 1Bcorrespond to an example in which a pair of electrodes form a touch sensor. As shown inFIG. 1Aa pair of electrodes100,104which form a drive or X plate and a receiving or Y plate in the following description are disposed beneath the surface of a touch sensitive control panel15. As shown inFIGS. 1A and 1Bthe touch sensor10is arranged to detect the presence of a body such as a user's finger20as a result of a change in an amount of charge transferred from the Y plate104. As shown inFIG. 1Awhen the X plate100is charged or driven by a circuit, an electric field is formed which is illustrated by the lines18and19both above and below the touch panel surface15as a result of which charge is transferred to the Y plate104. The X plate and the Y plate100,104form a capacitively charged key10. As shown inFIG. 1Bas a result of the disturbance of the electric field18due to the presence of the user's finger20the electric field above the surface of the control panel15is disturbed as a result of an earthing or grounding effect provided by the user's finger20as illustrated schematically by a ground symbol34.

An equivalent circuit diagram of the touch sensor shown inFIGS. 1A and 1Bis shown inFIG. 2. InFIG. 2equivalent capacitances are illustrated in the form of a circuit diagram. A capacitance formed between the X plate and the Y plate of the key100,104is a capacitance CE105. The presence of the body20has an effect of introducing shunting capacitances30and32, which are then grounded via the body20by an equivalent grounding capacitor22to the ground34. Thus the presence of the body20affects the amount of charge transferred from the Y plate of the key and therefore provides a way of detecting the presence of the body20.

FIG. 3provides an example circuit diagram which forms a touch sensor by sensing an amount of charge transferred from the X plate100shown inFIG. 2to the Y plate104and includes a charge measurement circuit which has been reproduced from WO-00/44018 in order to assist in the illustration of example embodiments of the present invention.

As shown inFIG. 3a drive circuit101is connected to the X plate of the key100and the Y plate of the key104is connected to an input106of a charge measurement circuit108, wherein the X and Y plates collectively form the capacitor105. The input106is connected to a first controllable switch110and to one side of a measuring capacitor CS112. The other side of the measurement capacitor112is connected via a second switch114to an output116of the measurement circuit108which is fed as a voltage VOUTto a controller118. In the circuit diagram shown inFIG. 3a convention has been adopted to show that a control input of each of the switches110,114is open for the control input “0” and closed for the control input “1”. The other side of each of the switches110,114is connected to ground, so that if the control input is “1” then the connecting input would be connected to ground. The operation of the touch sensor shown inFIG. 3including the function of the measurement circuit which is arranged to measure an amount of charge transferred from the X plate to the Y plate of the key104will now be explained with reference to the timing diagram shown inFIG. 4.

InFIG. 4, four timing diagrams130,132,134,138are shown to illustrate the operation of the measurement circuit108shown inFIG. 3. A first timing diagram130represents the control input applied to the second switch114. Thus, on the left hand side, the logical value of the control input is shown, whereas on the right hand side the effect at the connecting point114.1is shown to be either “Z” in which the connecting point114.1is isolated or floating, or for a logical control input of 1 grounded. Similarly a timing diagram132illustrates for logical control input values “0” or “1” of a connecting point110.1at either floating (Z) or ground (0). A third timing diagram134shows a relative timing of a drive signal provided to the X plate100of the key in which case, in contrast to the timing diagrams130,132for the two switches110,114, the value of the timing diagram is an absolute value so that the left hand side illustrates that the voltage varies between 0V and the reference voltage V, which is the voltage used to charge the X plate100. The final timing diagram138provides an illustration of the example signal strength or voltage produced on the measurement capacitor112as a result of the opening and closing of the switches110,114and the driving of the X plate100in accordance with the timing illustrated by the timing diagrams130,132,134. The timing diagrams130,132,134,138will now be explained as follows:

InFIG. 4at a first point t1, the charge measurement circuit108is initialized with both the control inputs for the switches110,114being high (1) so that both the Y plate and the charge measurement capacitor112are set to ground and the X plate100of the key is at zero and therefore not being driven by the drive circuit101. Correspondingly, the output voltage across the charge measurement circuit112is at zero. At t2the logical input to the control switch114is set to zero thereby opening the switch and floating the connecting point114.1, which connects the output voltage116to one side of the measurement capacitor112.

At a next time t3the control input to the switch110is set low (0) thereby floating the connecting point110.1which is YAbefore at a time t4the drive circuit101drives the X plate of the key100to the reference voltage V. Then in order to charge the measurement capacitor CSfor a period S between t5and t6, the control input to the switch114is set high (1) thereby grounding YBto transfer charge induced on the Y plate of the key104onto the charge measurement capacitor112, until t6when the control input to the switch114is set to low (0), which again floats the connecting point114.1. After charging the measurement capacitor CSfor a first dwell time between t5and t6, at t7the control input to switch110is set high (1), thereby grounding the connecting point110.1, which is connected to the other side of the charge measurement capacitor CS112. As a result, the voltage across the measurement capacitor can be measured. The amount of charge transferred from the Y plate104onto the measurement capacitor CS112during the dwell time between t5and t6is represented as the output voltage VOUT.

At t8the drive circuit101goes low (0), which concludes a first measurement burst.

At t9the next measurement cycle of a measurement burst occurs. At t9the control input to the switch110goes low (0) thereby floating YA, before the drive circuit again drives the X plate100with a voltage “V”, at time t10. The measurement capacitor112is again charged from charge transferred from the Y plate104of the key onto the measurement capacitor112. As with the first burst at point t11the control input to the switch114goes high (1) thereby grounding the point114.1and driving charge onto the measurement capacitor until t12, when the control input to the switch114goes low, again floating YB. Thus again charge is transferred from the Y plate104during the dwell period between t11and t12, thereby increasing the voltage across the measurement capacitor CSas represented as the output voltage VOUT. At t13the control input to the switch110is set high (1) thereby grounding YAand at t14the drive circuit101goes low (0), which concludes the second measurement burst. Thus, as with the first burst an amount of charge has been transferred from the Y plate, which has then increased the voltage across the measurement capacitor112, which represents an amount of charge transferred from the Y plate.

After several bursts the amount of charge present on the Y plate transferred to the measurement capacitor112is consistent, thereby providing a representation of charge present on the key produced by the drive signal to the X plate100via the drive circuit101. The amount of charge on the measurement capacitor112is determined with the aide of a discharge resistor140. One side of the discharge resistor140is connected to the measurement capacitor and the other side SMP is connected to a discharge switch142. The discharge switch142receives a control signal from the controller118via a control channel144. The controller118is controlled so as to ground SMP, during measurement bursts and to discharge the measurement capacitor CS112through the discharge resistor140by connecting SMP to a voltage VDD. The controller118then determines an amount of charge present by counting a number of predetermined clock periods before the charge on the measurement capacitor CSis discharged to zero. The number of clock periods therefore provides a relative signal sample value for the respective measured charge signal.

In alternative embodiments, instead of arranging for the controller118to generate a predetermined number of measurement bursts and then measuring the charge present on the Y plate, the controller may operate to continue with the measurement bursts until a predetermined threshold voltage is reached. The number of measurement bursts required to reach the predetermined threshold then provides an indication of the amount of charge transferred from the X plate to the Y plate and therefore an indication of the electric coupling between them. Hence the presence of a body proximate the coupling will change the electric coupling and therefore the number of bursts required to reach the threshold, which can therefore be detected by the controller.

As explained in WO-00/44018 a charge subtraction capacitor is provided to subtract charge from the Y plate of the key104and the measurement capacitor to ensure that there is a linear transfer of charge onto the measurement capacitor112to provide an accurate measurement. Further explanation is therefore provided in WO-00/44018 the content of which is incorporated herein by reference.

One advantage of the measurement circuit shown inFIG. 3is that, using the same principles of construction and operation, a matrix of touch sensitive switches can be formed, so that a user can select either a plurality of different positions on a touch sensitive screen, for example, or a plurality of different functions in dependence upon the position of the user's finger for example with respect to the matrix of points. For example,FIG. 5has been largely reproduced from WO-00/44018.

InFIG. 5drive circuits101.1,101.2,101.3,101.4are arranged to drive different sensor points205which with example shown inFIG. 5forms an N=4×M=4 array. Thus, as shown correspondingly inFIG. 6a control panel with sixteen touch sensitive points is provided which can be used to either form the touch sensitive screen or a control panel with multiple selection control switches.

As shown inFIG. 5each of the drive circuits101.1,101.2,101.3,101.4is controlled by controller118.1to drive each of the corresponding lines X1, X2, X3, X4in the same way as the X plate100is driven inFIG. 3and represented inFIG. 4. The output of the coupling capacitor at each of the points205are connected to one side of measuring capacitors112.1,112.2,112.3,112.4which are arranged to measure an amount of charge present on the Y plate Y1, Y2, Y3, Y4providing output signals116.1,116.2,116.3,116.4to detect the presence of an object in the same way as the operation of the circuit shown inFIG. 3andFIG. 4. More details for the operation of such a matrix circuit are disclosed in WO-00/44018.

Although the touch sensor described above with reference toFIGS. 1 to 6provides an effective touch sensor which can be used for many applications, there is a desire to use such touch sensors in increasingly challenging environments. For example, the use of a touch sensor on a mobile phone can create a technical problem because there is a variety of disturbing noise signals produced by radio frequency radiation by radio frequency signals and by modulators within the mobile phone. Similarly, on a television, switching noise as a result of switching LCD displays and pixels within the display on and off can produce rectangular noise. Sinusoidal noise, such as that produced by sinusoidal electricity may also be present, which can affect the amount of charge detected on a key. An example of low frequency noise is shown inFIG. 7.

InFIG. 7a plot is shown of signal strength or amplitude which may be voltage or charge measured with respect to time. As shown inFIG. 7various points220are shown to indicate points at which burst measurements are taken for a touch sensor such as those shown inFIGS. 4 and 5. As will be appreciated, as a result of sinusoidal noise represented by a line222, an amount of charge transferred from a key by a measurement capacitor of the measurement circuit (such as those shown inFIGS. 3 and 5) will vary and therefore could in some circumstances cause a false measurement of the presence of a body.

As will be appreciated, in some examples, both sinusoidal noise and rectangular noise will be present so that the plot of signal amplitude with respect to time for a combination of sinusoidal noise shown inFIG. 7and switching noise is as shown inFIG. 8. Thus, as will be appreciated in some example embodiments a technique for obviating or at least reducing the effects of sinusoidal or mains noise and a technique for obviating or at least reducing the effects of switching noise with higher frequency components, which is disclosed in co-pending U.S. application Ser. No. 12/466,230, can be combined together to improve a likelihood of correctly detecting the presence of a body proximate the touch sensitive sensor key in the presence of both sinusoidal noise and switching noise.

An example embodiment of the present invention which is arranged to reduce the effects of sinusoidal noise on a touch sensor is shown inFIG. 9. InFIG. 9a key forming a touch sensor10.1includes an X plate100.1and a Y plate104.1. The key is arranged to detect the presence of a body proximate the charge sensor in the way explained above with reference toFIGS. 1 to 6. In order to reduce the effects of sinusoidal noise on the key10.1a measurement circuit300is provided, which includes a first measurement capacitor302referred to as CSNfor measuring noise and a second measurement capacitor304referred to as CSSfor measuring the signal and noise present on the key. The measurement circuit also includes switches306,308,310having respective logical inputs312,314,316, which are each controlled by a controller318, which may be embodied with a micro-controller. There is also a switch320, which performs the operation of the drive circuit101as shown inFIGS. 3 and 5. The switch320has a logical control input322also connected to the micro-controller318. Nodes324,326,328,330adjacent respective switches306,308,310,320are also indicated as well as discharge resistors360,362for discharging capacitors CSSand CSNand associated switches364,365, which are controlled by the micro-controller318. As for the description of the measurement circuit shown inFIG. 3, a convention has been adopted such that if the logical control inputs312,314,316,322are low (0) then the switch is open and if the logical control input is high (1) then the switch is closed. The operation of the touch sensor shown inFIG. 9will now be described with reference toFIGS. 10, 11 and 12.

FIG. 10shows six timing diagrams according to an example operation of the touch sensor shown inFIG. 9in which four measurements are performed, two for signal plus noise and two for noise only. A first of the timing diagrams340.1represents the electrical value at a connecting point324or YBNon the switch306. A second of the timing diagrams342.1represents an electrical value at a connecting point328or YBSon the switch310. A third of the timing diagrams344.1represents the electrical value at a connecting point326or YAon the switch308. A fourth timing diagram346.1represents the electrical value at a connecting point330or X on the switch320. Fifth and sixth timing diagrams348.1,350.1represent the output voltage taken across the noise measurement capacitor302and the signal measurement capacitor304respectively. Thus, the output voltages across the noise measurement capacitor CSNis taken between terminals YAand YBNand the signal measurement voltage VBSacross the signal measurement capacitor CSSis taken between the terminals YAand YBSshown inFIG. 9. The timing diagram shown inFIG. 10will now be explained as follows:

At a first time t1switches306,308,310are all closed (1) grounding connecting points324,326,328. Moreover, the switch320for operating the drive circuit101is 0 thereby also grounding the input to the X plate100.1. As such, time t1represents an initialization or reset of the touch sensor.

At time t2the logical inputs to switches306,308,310are all set low (0) thereby floating the connecting points324,328,326.

At time t3, in a first part of the measurement cycle, the X drive plate100.1of the key10.1is driven high by applying a logical high value (1) to the control input322, in preparation for driving the charge onto the measurement capacitor CSS. This step will be referred to as pre-charging X or the drive plate of the key, and has an advantage that it reduces dwell time, which is the time required to ensure that charge, which is transferred from the key has settled to a steady state value on the measurement capacitor CSS, thereby allowing a faster measurement cycle. A faster measurement cycle can be used to provide a more responsive touch sensor.

At time t4the logical input326to the switch310is set high (1), thereby grounding the connecting point328(YBS) which is one side of the measurement capacitor304. The grounding of the measurement capacitor CSShas an effect of transferring the charge, which has been induced on the Y plate104of the key onto the measurement capacitor CSSfollowing the principles already described with reference toFIGS. 1 to 6.

Between times t4and t5charge induced onto the Y plate104.1is transferred onto the measuring capacitor304because connecting point328is driven to ground by the switch310being closed. At t5the logical input316to the switch310is driven low (1) thereby floating the connecting point328which ends measurement of the signal plus noise on the key10.1.

At time t6with YAand YBSfloating the logical input to switch306is driven high (1), thereby grounding YBN, which is one side of the noise measurement capacitor CSN302. Between times t6and t7noise induced in the Y plate104.1of the key10.1is transferred to the noise measurement capacitor CSNbecause the charge induced onto the Y plate by the drive signal346.1has already been transferred to the signal measurement capacitor304.

At time t7the switch306opens with a low signal (0) on the logical input channel312. One side of the noise measuring capacitor302CSNis then floating which completes the noise measurement signal collection.

Between times t2and time t8a connecting input314to the switch308is driven low (0) thereby floating YAso that charge can be transferred onto the signal measuring capacitor304and then the noise measurement capacitor302. Thus, setting the logical input314high (1) at time t8grounds YAthereby ending the measurement cycle. At t8therefore a potential difference between YAand YBN(connecting points326to324) represents an amount of noise induced on the key as a result of charge accumulated on the measurement capacitor302. Similarly, with the grounding of YA(connecting point326) an amount of charge transferred onto the measurement capacitor304following the signal CSSrepresents an amount of signal plus noise present on the key. Accordingly, as shown in the upper timing plots348.1to350.1for the measurement voltages, voltage values are shown, which are proportional to the noise only and signal plus noise respectively.

At time t9the logical input314to the switch308is set low (0), thereby floating YA.

At time t10the drive circuit represented by the switch320is driven high by logical input330thereby driving the X plate100.1with a voltage V which causes the measuring capacitor304to charge via the Y plate104.1. This is a pre-charging of the X plate, which reduces the dwell time, in preparation for the next measurement cycle which again starts at t11where a logical input316to switch310is driven high (1), thereby grounding YBS, which is the other side of the signal measurement capacitor304.

Again, there is a period in which the measuring capacitor304is charged by charge transferred from the Y plate104.1of the key between t11and t12when the switch310opens as a result of a logical zero applied to the control input316. Similarly, as with time points t6and t8time points t13and t14define a period where a logical one is applied to the control input112of the switch306thus grounding YBN, so that the measuring capacitor CSNis charged by noise present on the key. At time point t15the switch308is closed thereby grounding YA, and at point t16the X plate100.1is grounded so that a voltage is presented across the signal measuring capacitor304and the noise measuring capacitor302, which provide a stable value as a result of a second dwell period on both the noise and the signal capacitors302and304.

Thus as can be appreciated from the operation of the circuit which forms part of the touch sensor shown inFIG. 9,FIG. 10provides an illustration of double dwell example where the signal is measured first.

As explained above for the touch sensor shown inFIGS. 3 and 4, one technique for measuring the charge value on the signal measurement capacitor CSS304and the noise measurement capacitor CSN302is to discharge each of the capacitors through respective discharge resistors360,362. The discharge resistors360,362are respectively connected to ground, during charging and connected to VDDwhen discharging. The charging and discharging is arranged using the control switches364,365, which receive a control signal from the controller318via a control channel366. Thus the controller respectively discharges each of the signal measurement capacitor CSS304and the noise measurement capacitor CSN302via the corresponding discharge resistors360,362respectively by controlling the switches364,365and determines an amount of charge present on each, by counting a number of predetermined clock periods before the charge is discharged to zero. The number of clock periods therefore provides a relative signal sample value for the respect signal and noise samples. The micro-controller318can then adjust the value of the signal plus noise, by applying the noise value from the signal plus noise value to get a more accurate measurement of the signal caused by the charge being transferred from the Y plate of the key10.1. The presence of the noise may produce a positive or negative effect on the value of the signal plus noise, so that if the noise produces a positive charge, then the signal plus noise charge is reduced, whereas if the noise produces a negative charge, then the signal plus noise is increased in accordance with the value of the noise charge.

It will be appreciated from the explanation provided above that other parts may be applied to the measurement circuit300, shown inFIG. 9such as a charge subtraction circuit which is described in WO-00/44018.

In alternative embodiments, instead of arranging for the controller318to generate a predetermined number of measurement bursts and then measuring the charge present on the Y plate, the controller may operate to continue with the measurement bursts until a predetermined threshold voltage is reached. The number of measurement bursts required to reach the predetermined threshold then provides an indication of the amount of charge transferred from the X plate to the Y plate and therefore an indication of the electric coupling between them. Hence the presence of a body proximate the coupling will change the electric coupling and therefore the number of bursts required to reach the threshold, which can therefore be detected by the controller.

A second example of an operation of the measurement circuit300of the touch sensor shown inFIG. 9is presented inFIG. 11. InFIG. 11there is again a double dwell shown for both signal plus noise and noise only. However, in contrast to the example shown inFIG. 10the noise measurement period occurs first. InFIG. 11, four timing diagrams are shown. A first timing diagram340.2provides a logical example of the control input312of the switch306, a second timing diagram342.2represents the logical value of the control input316of the switch310, a third timing diagram344.2represents a logical value of the control input314to the switch308and a fourth timing diagram346.2represents the logical value of the control input322to the switch320representing the operation of the drive circuit101. Also shown inFIG. 11is a value of an output voltage signal between YAand YBN348.2and an output voltage signal between YAand YBS350.2to represent the output measurements of noise and signal plus noise as for the example illustrated inFIG. 10. The timing diagram shown inFIG. 11operates correspondingly to those shown inFIG. 10. However, since the noise measurement takes place first then after the initialization at time t1the switch306is closed between t2and t3grounding YBNtherefore driving any noise induced onto the Y plate104.1onto the charge noise measurement capacitor302before the drive signal X is applied to the X plate by closing the switch320at t4. As before, the X plate is pre-charged to reduce the dwell time when YBS350.2is grounded and the signal measurement capacitor304is being charged. As for the example shown inFIG. 10between times t5and t6the switch310is closed to ground YBSwhilst switch320is closed driving and the X plate100.1of the key is being driven to a voltage V. At time t6the switch310is then opened floating YBSWhilst the noise measurement capacitor and signal plus noise measurement capacitor302,304are being driven, YAis floating with a logical low (0) being applied to the control input314. However, at time t7the logical input314to the switch308goes high (1) therefore closing the switch308thus grounding YAand presenting voltages at the output of the measurement circuit for the noise between YAand YBNand the signal plus noise between YAand YBSas shown in the timing diagrams348.2,350.2. Thus the second dwell periods between t9and t10and t11and t12occur in the same way as the first dwell period and so these will not be described again.

A further example operation of the touch sensor shown inFIG. 9is provided inFIG. 12. Timing diagrams are again provided for switch306,310and308as well as the control input of the switch320,340.3,342.3,344.3,346.3. Also shown are timing diagrams for the output voltage VBNbetween YAand YBN348.3and the output voltage VBS350.3between YAand YBS. For the example shown inFIG. 12, there are two noise measurement periods and one signal plus noise measurement period. Furthermore, as can be observed inFIG. 12the duration of the noise measurement periods between time t2and t3and t7and t8are half the signal plus noise measurement period between t5and t6. The operation of the touch sensor as represented by the timing diagrams inFIG. 12corresponds to those shown inFIGS. 11 and 12and so for brevity the description will not be repeated. However, as can be seen the second noise measurement period between t7and t8can be performed whilst the X plate is driven between t5and t9. In contrast, the first noise measurement period between times t2and t3occurs when the X plate is grounded.

The example shown inFIG. 12represents an arrangement in which the signal plus noise measurement is made either side of two noise measurements. This arrangement has an advantage of producing a better average noise value if for example the noise signal is varying between a time at which the measurements are taken. For example, as shown inFIG. 13which provides a plot of signal value against time for charge induced on the key10.1, the signal level is rising with time as a result of the low frequency noise. Therefore, by taking the noise measurement two points either side of the signal measurement360and362the signal plus noise measurement364and averaging the two noise measurements360,362, a better representation of the average noise present during the signal plus noise measurement period364is provided, therefore providing a more accurate measurement of the signal when the noise measurement is subtracted from the signal.

FIG. 14provides a further example of a touch sensor which is arranged to include a measurement circuit for canceling or at least reducing the effect of sinusoidal or sinusoidal noise. InFIG. 14a measurement circuit400includes a single measurement capacitor410. The measurement circuit400also includes switches412,414,416. A logical control input419,420,422,426of each of the switches412,414,416,424is connected to a micro-controller418. Again, a convention has been adopted to the effect that the switches412,414,416are closed with a logical “1” and open with a logical “0” on each of the control inputs412,420,422. As before, a drive circuit is represented by a switch424which has a logical control input426connected to the micro-controller418.

A set of timing diagrams are shown inFIG. 15to represent the operation of the touch sensor shown inFIG. 14. A first diagram440represents the control input480on the switch412, a second timing diagram442represents the control input on the switch414, a third timing diagram444represents the control input422on the switch416and a fourth timing diagram446represents the control input426to the switch424forming the drive circuit for the X plate of the key10.1. Finally a fifth timing diagram represents the output voltage measured between the point YAand ground.

In contrast to the operation of the circuit shown inFIG. 9, the circuit shown inFIG. 14only uses a single measurement capacitor410to measure both noise and signal plus noise. A switch412is provided to reverse the direction of the flow of the charge from the Y plate of the key104.1. Turning to the timing diagrams inFIG. 15at time t1an initialization occurs in which the switches414,416are closed grounding YAand YB. At this point the X plate is set to ground and the switch412is closed connecting the Y plate104.1to YA, that is one side of the capacitor410. At time t2, the switch414is open thereby floating YBand at time t3the switch416is also open thereby floating YA. At time t4the X plate is driven by closing switch424and connecting the X plate to the drive voltage V with the switch412closed, and at time t5the control input420on switch414is set to a logical high closing the switch414thereby grounding YB, which drives charge from the Y plate104.1. Thus, as for the previous examples the X plate is pre-charged. Between t5and t6charge is transferred from the Y plate to the measurement capacitor410as for the operation of the other measurement circuits described above.

At t7, the control switch412is opened with a logical zero (0) applied to the control input418, thereby connecting the Y plate104.1to the other side of the measurement capacitor410. At t8the switch416is closed thereby grounding YAand driving charge, which results from noise into a second side of the measurement capacitor410, than the first side of the measurement capacitor410on to which charge was transferred when the signal plus noise was measured as represented by arrows450,452. Thus, between t8and t9charge as a result of noise is driven onto the other side of the measurement capacitor. At t10the X plate is connected to ground to complete one measurement cycle.

At t11the control switch412is again closed connecting YAand the first side of the measuring capacitor410to the Y plate104.1. As such, between t12and t13and t14and t15a second dwell period is provided for the measurement of signal plus noise and noise.

Referring to the timing diagram for the output voltage448, it can be seen that at t5, after YBis again floated an output voltage appears as a result of the signal plus noise on the measuring capacitor410. During the dwell period for the measurement of noise between times t8and t9, because the control switch412is at a logical zero (0), the noise causes charge from the Y plate104.1to flow in the opposite direction to that of the signal plus noise into the measurement capacitor412with a result of reducing the voltage by an amount corresponding to the amount of noise. As a result noise is subtracted from the measurement so that after the first dwell time a first indication of the signal value is provided at the output VOUT. Again after a second dwell period, corresponding to a second measurement cycle, the signal plus noise is again transferred to the measuring capacitor and noise is arranged to remove the charge as a result of reversal of the switch412so that a final output voltage VOUTat YAcorresponds to the value of the signal.

Although the noise is shown inFIG. 15to have increased the signal plus noise measured charge, and is therefore subtracted from the signal plus noise, to form an estimate of the signal without the noise, it will be appreciated that this is just an example. As will be appreciated, the noise may induce a negative charge onto the measurement capacitor and so the signal plus noise measurement may be increased, when the noise is taken into account.

The example embodiment described with reference toFIG. 14has an advantage in reducing a number of pin outs provided on the measurement circuit400compared to that of the measurement circuit300, which can assist in deploying the measurement circuit on a system where a number of output pins is limited. Furthermore, the processing, which is required to subtract the noise measurement from the signal measurement does not need to be implemented within the micro-controller since this is formed as a result of the charge transfer provided by the measurement circuit400in the analogue domain.

An example illustration of the operation of the present technique is provided inFIGS. 16, 17 and 18.FIG. 16provides an illustration of the general method of sensing the presence of a body from a change in an amount of charge present on a capacitively charged key, according to an example embodiment. The steps of the method shown inFIG. 16are summarized as follows:

S1: Charge is induced onto the key during a drive part of a measurement cycle.

S2: A signal measurement capacitor is then coupled to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor.

S4: From a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle is determined, and

S6: The drive part, the signal measurement part and the noise measurement parts of the cycle are controlled to provide the charge sensing circuit with a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

As shown inFIG. 17, one example of the step S2includes

S8: coupling the signal measurement capacitor to the key contemporaneously with the drive part of the measurement cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor at the end of the signal measurement part of the measurement cycle, and

S10: determining an amount of charge present on the signal measurement capacitor.

Then an example of the operations of step S4, through which an amount of charge induced on the key by noise during the noise measurement part of the measurement cycle, includes

S12: coupling a noise measurement capacitor to the key for the noise measurement part of the measurement cycle either after the charge induced by the key has been transferred to the signal measurement capacitor or before the charge has been induced on the key during the drive part of the measurement cycle, and

S14: determining an amount of charge induced on the noise measurement capacitor.

Then the step S6, would include step S16: subtracting the noise measurement provided by the charge on the noise measurement capacitor from the signal measurement provided by the charge on the signal measurement capacitor.

As shown inFIG. 18, another example of the operation of step S2, through which an amount of charge induced on the key by the signal, may include

S18: coupling the signal measurement capacitor to the key contemporaneously with the drive part of the cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor. Furthermore, for this example, another example of step S4for determining from the noise measurement part of the measurement cycle the amount of charge induced on the key by the noise, may include

S20: either before the drive part of the measurement cycle or after the signal measurement part of the measurement cycle, coupling the signal measurement capacitor to the key in reverse so that charge induced by noise on the key reduces the amount of charge induced on the measurement capacitor during the signal measurement part of the cycle.

Further aspects and features of the present invention are defined in the appended claims. Various modifications may be made to the example embodiments described above without departing from the scope of the present invention. In particular, although the above description has been made with reference to a matrix touch sensor which includes a key having an X plate and a Y plate where the X plate is driven and charge is measured on the Y plate, the present invention also finds application where only a single plate is provided in the touch sensor which is first charged in a charge cycle and then discharged in a measurement cycle such as that disclosed in WO-97/23738.