Capacitive proximity sensor with enabled touch detection

A capacitive proximity sensor circuit capable of distinguishing between instances of detected user proximity includes one or more guard electrodes, a first sensor, and a second sensor. The capacitive proximity sensor is installed in a device such that a first sensor faces a first component of the device, and the second sensor faces a second component of the device. The first and second sensors measure a capacitance to detect proximity of a user relative to the respective sensor. The guard electrode is provided to mitigate stray capacitance to reduce error in the capacitance measurements obtained by the first and second sensors.

FIELD OF THE INVENTION

This invention generally relates to proximity-sensing circuitry and, more particularly, to proximity-sensing circuitry implementing a capacitive sensor.

BACKGROUND

In various technological applications, it is oftentimes advantageous to sense the proximity of an object relative to a device. For example, in mobile phone applications, it may be advantageous to detect the proximity of a user's head to the phone's display, such as when the user is participating in a phone call, so that the display panel may be disabled and battery consumption thereby reduced.

One such solution for sensing the proximity of objects involves the use of an optical sensor. However, optical sensors tend to be cost-prohibitive and may be difficult to incorporate in various devices. Another solution for sensing proximity of an object involves the use of a capacitive sensor. However, conventional capacitive proximity-sensing technology is unsophisticated as it is unable to distinguish between an object held above the sensor (e.g., a user's head) and an object in contact with components of the device. For example, in mobile phone applications, a user touching the casing of the mobile phone, even at a distance from the sensor, causes the conventional capacitive proximity-sensing circuitry to incorrectly register proximity detection of an object. As such, conventional capacitive proximity-sensing technology has not been satisfactory for all conditions of use.

SUMMARY

The present disclosure provides a capacitive proximity sensing circuit comprising: one or more guard electrodes; first capacitive sensor circuitry operable to sense a first capacitance and produce a first capacitive sensor reading indicative of the sensed first capacitance, wherein the first capacitive sensor circuitry includes a first capacitive sensor coupled to a first side of at least one of the one or more guard electrodes; and second capacitive sensor circuitry operable to sense a second capacitance and produce a second capacitive sensor reading indicative of the sensed second capacitance, wherein the second capacitive sensor circuitry includes a second capacitive sensor coupled to a second side of at least one of the one or more guard electrodes.

Another embodiment provides a capacitive proximity sensing circuit comprising: one or more guard electrodes; a first sensor coupled to a first side of at least one of the one or more guard electrodes; a second sensor coupled to a second side of at least one of the one or more guard electrodes; first capacitive sensor circuitry coupled to the first sensor, the first capacitive sensor circuitry operable to sense user proximity with respect to the first sensor and to produce a first capacitive sensor reading indicative of the sensed user proximity with respect to the first sensor; and second capacitive sensor circuitry coupled to the second sensor, the second capacitive sensor circuitry operable to sense user proximity with respect to the second sensor and to produce a second capacitive sensor reading indicative of the sensed user proximity with respect to the second sensor.

Yet another embodiment provides an integrated circuit operable to detect user proximity with respect to a first component of a device and user proximity with respect to a second component of the device, the integrated circuit comprising: a first switched capacitive integrator circuit coupled towards the first component of the device, the first switched capacitive integrator circuit operable to sense a first capacitance and produce a first output signal in response to the sensed first capacitance; a second switched capacitive integrator circuit coupled towards the second component of the device, the second switched capacitive integrator circuit operable to sense a second capacitance and produce a second output signal in response to the sensed second capacitance; one or more capacitive plates, wherein at least one of the one or more capacitive plates is coupled on a first side to the first switched capacitive integrator circuit, and at least one of the one or more capacitive plates is coupled on a second side to the second switched capacitive integrator circuit; and output circuitry operable to receive the first output signal and second output signal, and to produce a third output signal operable to indicate detection of the user proximity with respect to one of the first switched capacitive integrator circuit or the second switched capacitive integrator circuit.

The foregoing and other features and advantages of the present disclosure will become further apparent from the following detailed description of the embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the disclosure, rather than limiting the scope of the invention as defined by the appended claims and equivalents thereof.

DETAILED DESCRIPTION OF THE EMBODIMENTS

This application incorporates by reference for all purposes U.S. patent application Ser. No. 13/539,731, entitled “Directional Capacitive Proximity Sensor with Bootstrapping,” filed Jul. 2, 2012.

The present disclosure provides a capacitive proximity sensor circuit capable of distinguishing between instances of detected user proximity. The capacitive proximity sensor disclosed herein is generally described in various embodiments in which the capacitive proximity sensor is installed in a mobile phone device, wherein advantages of the proximity sensing may include distinguishing user proximity detected above the plane of the sensor (e.g., the user touching a section of the phone's display screen located above the sensor) from user proximity detected at a remote location on the phone (e.g., the user touching the external casing of the phone). It should be appreciated that the capacitive proximity sensor disclosed herein may be applied to other devices such as, for example, GPS devices, tablet computers, mobile media players, remote controls, and various other devices, and may provide proximity sensing for reasons other than those discussed herein.

FIGS. 1A and 1Billustrate an example embodiment of the disclosed capacitive proximity sensor100, wherein the sensor100is shown inFIG. 1Afrom a front view and inFIG. 1Bfrom a side view along line A-A ofFIG. 1A. The sensor100, includes a guard electrode110, a first sensor120disposed on a first side of the guard electrode110, and a second sensor130disposed on a second (opposite) side of the guard electrode110. In general, the first and second sensors120and130are each designed to measure a capacitance to detect proximity of a user relative to the respective sensor. The guard electrode110is provided to mitigate stray capacitance to reduce error in the capacitance measurements obtained by the first and second sensors120and130. In some embodiments, this may be achieved by a “bootstrapping” technique wherein the guard electrode110is controlled with a low impedance output applied to a known voltage (or known voltage waveform), thereby shielding the first and second sensors120and130from interfering signals and reducing effect of the guard electrode110on the sensors.

FIGS. 2A and 2Billustrate an example circuit diagram200and corresponding timing diagram250, respectively, for an example embodiment of the disclosed capacitive proximity sensor circuitry100. In the embodiment illustrated inFIG. 2A, the guard electrodes are labeled as elements110,510and520. Accordingly, the guard electrodes are labeled as element110to indicate a first embodiment wherein the guard electrodes comprises a single structure (although shown as two separate structures), as shown inFIGS. 1A and 1B, whereas the guard electrodes are labeled as elements510and520indicate a second embodiment, discussed with respect toFIGS. 5A and 5B, wherein the guard electrodes comprise two separate structures, as shown inFIGS. 5A and 5B. Similarly, the sensors inFIG. 2Aare labeled as elements120/515and130/525, wherein element120/515indicates that the sensor may be the first sensor120shown inFIGS. 1A and 1Bor may be the first sensor515shown inFIGS. 5A and 5B, and element130/525indicates that the sensor may be the second sensor130shown inFIGS. 1A and 1Bor may be the second sensor525shown inFIGS. 5A and 5B. As such, the circuit200and timing diagram250illustrated inFIGS. 2A and 2Bmay be applied to the embodiment illustrated inFIGS. 1A and 1B, as well as the embodiment illustrated inFIGS. 5A and 5B. However, for the sake of simplicity, the circuit200and timing diagram250are described below with respect to the embodiment illustrated inFIGS. 1A and 1B, although it should be appreciated that the description of the circuit200and timing diagram250may also apply to the embodiment illustrated inFIGS. 5A and 5B.

As shown inFIG. 2A, the guard electrode110(510) is coupled to a first voltage VOL1via switch202, and is coupled to a second voltage VOL2via switch204. Guard electrode110(520) is coupled to the first voltage VOL1via switch203, and is coupled to the second voltage VOL2via switch205. The first sensor120is coupled to the second voltage VOL2via switch206, and the second sensor130is coupled to the second voltage VOL2via switch207. The circuit200further includes switch208operable to couple the first sensor120to a first inverting amplifier210at a first inverting input node212, and switch214operable to couple the second sensor130to a second inverting amplifier216at a second inverting input node218. In the embodiment illustrated inFIG. 2A, the first and second sensors120and130operate independent of each other. InFIG. 2A, switches202-205are coupled to the guard electrode110(510and520), however, it should be appreciated that, in some embodiments, a single set of switches (e.g., switches202and204) may be used.

Each of the inverting amplifiers210and216receive the first voltage VOL1as a reference voltage at non-inverting inputs220and222, respectively, and each include a feedback loop coupled between the amplifier output and the respective first and second inverting input nodes212and218. As explained in greater detail below, the first inverting amplifier210produces a first output signal231indicative of a capacitance measured by the first sensor120, and the second inverting amplifier216produces a second output signal232indicative of a capacitance measured by the second sensor130. Each feedback loop comprises a feedback switch224/226coupled in parallel with a feedback capacitor Cf, wherein the value of the feedback capacitor Cf inversely affects the magnitude of a voltage swing of the respective output signal231/232in a negative direction. In some embodiments, the feedback capacitor Cf may vary from 1 pF to 100 pF depending upon system requirements; however, it should be understood that one skilled in the art may choose a feedback capacitor having a particular value to produce a desired output signal voltage swing magnitude for a given charge on the feedback capacitor.

As shown inFIG. 2A, the circuitry coupled to the first sensor120is substantially identical to the circuitry coupled to the second sensor130. Thus, the respective first and second output signals231and232are produced in substantially the same manner. As such, the circuit200and timing diagram250are generally discussed herein with respect to a single sensor (i.e., the first sensor120or the second sensor130) and the output signal (i.e., first output signal231or second output signal232) produced by the inverting amplifier corresponding to the respective sensor (i.e., first inverting amplifier210or second inverting amplifier216). Thus, it should be understood that the timing diagram250illustrated inFIG. 2B, and the operation of the embodiments as described herein, may be applied to the first sensor120and the corresponding circuitry producing the first output signal231, as well as to the second sensor130and the corresponding circuitry producing the second output signal232.

The circuit200incorporates a bootstrapping technique wherein the guard electrode110forms a capacitor with the adjacent sensor120/130in order to isolate the sensor120/130from surrounding conductive materials (such as other circuitry on the PCB near the sensor), thereby limiting the sensor120/130to detecting a capacitance from a location substantially above the sensor120/130. As shown inFIG. 2Aand further described below, the bootstrapping technique is provided by controlling the guard electrode110with known voltages VOL1and VOL2during a two-phase operation of the circuit200. Accordingly, stray capacitance is mitigated and resulting errors in the capacitance measurements obtained by the sensor120/130are reduced.

To illustrate operation of the disclosed capacitive proximity sensor100,FIGS. 2A and 2Bpresent an example embodiment in which the circuit200detects proximity of a user's finger230using a two-phase operation, wherein a first group of switches (switches204,205,206,207,224and226generally identified inFIG. 2Aas Switch A and referred to herein as Switches A) and a second group of switches (switches202,203,208and214generally identified inFIG. 2Aas Switch B and referred to herein as Switches B) are operated in alternating fashion. As the user's finger230approaches the first or second sensor120or130(i.e., interferes with a fringe electric field of the sensor), the finger230acts as a virtual ground and forms a virtual capacitance C1between the finger230and respective sensor120/130. The virtual capacitance C1is inversely proportional to the distance between the finger230and the respective sensor120/130and, in some embodiments, may range from 10 fF to 10 pF depending upon the size of the sensor120/130. Thus, the closer the finger230is to the sensor120/130, the greater the virtual capacitance C1. As the circuit200alternates between the first and second phases, the virtual capacitance C1is transferred to the feedback capacitor Cf, as explained in greater detail below. The charge on the feedback capacitor Cf (i.e., the virtual capacitance C1transferred at the second phase) affects the magnitude of the voltage swing occurring on the respective output signal231/232. Thus, the magnitude of the voltage swing occurring on the output signal231/232may be measured to detect proximity of the finger230.

The example timing diagram250inFIG. 2Billustrates the two-phase operation of the corresponding circuit200illustrated inFIG. 2Afor a duration of three cycles. The timing diagram250includes a sensor waveform252corresponding to either the first sensor120or the second sensor130, a guard electrode waveform254corresponding to the guard electrode110, and three output signal waveforms256,257and258each corresponding to either the first output signal231or the second output signal232generated for one of three conditions. The first output signal waveform256corresponds to an output signal231/232generated when the finger230is not present, the second output signal waveform257corresponds to an output signal231/232generated when the finger230is slightly detected, and the third output signal waveform258corresponds to an output signal231/232generated when the finger230is strongly detected.

During the first phase, Switches B are opened and Switches A are closed. When Switches B are opened, the sensor120/130is disconnected from the respective amplifier210/216and feedback capacitor Cf, and the guard electrode110is disconnected from VOL1. When Switches A are closed, VOL2is applied to the sensor120/130and to the guard electrode110. VOL2biases the sensor120/130to charge the virtual capacitor C1when the finger230is within the fringe electric field of the sensor120/130. Additionally, the feedback capacitor Cf is shunted by the respective feedback switch224/226causing the feedback capacitor Cf to discharge (i.e., reset) as the amplifier210/216is reset. Thus, if a finger230was detected during the previous cycle, the output signal231/232returns to VOL1(the voltage at the non-inverting input220/222of the amplifier210/216), as shown during phase one of the second and third cycles of output signal waveforms257and258inFIG. 2B. Otherwise, the output signal231/232remains unchanged as shown by output signal waveform256.

During the second phase, Switches A are opened and Switches B are closed. When Switches A are opened, the sensor120/130and guard electrode110are disconnected from VOL2, and the shunt provided by switch224/226is removed. When Switches B are closed, the charge from the virtual capacitor C1(if any) is transferred to the feedback capacitor Cf, and VOL1is applied to the guard electrode110. As the charge dissipates from the virtual capacitor C1, the charge at the sensor120/130approaches VOL1, as shown by the sensor waveform252inFIG. 2B. The respective first and second inverting amplifiers210and216receive the charges on the respective feedback capacitors Cf, and operate as integrators to generate the respective first and second output signals231and232.

As mentioned above, the charge on the feedback capacitor Cf affects the magnitude of the voltage swing on the respective output signal231/232. If no finger230is detected, there is no charge transferred from the virtual capacitor C1to the feedback capacitor Cf, and there is no change to the output signal231/232, as shown by the output signal waveform256. If the finger230is slightly detected, a smaller charge is transferred from the virtual capacitor C1to the feedback capacitor Cf, and a lesser voltage swing ΔVa is produced on the output signal231/232, as shown by the output signal waveform257. If the finger230is strongly detected, a larger charge is transferred from the virtual capacitor C1to the feedback capacitor Cf, and a greater voltage swing ΔVb is produced on the output signal231/232, as shown by the output signal waveform258. For example, in one embodiment, VOLT =1.65V, VOL2 =3.3V, AVa =10mV and AVb =50mV.

In accordance with the foregoing, the respective first and second output signals231and232may be sampled during the second phase to determine proximity detection of the finger230by the respective first and second sensors120and130. In some embodiments, processing circuitry may be implemented to evaluate the respective first and second output signals231and232. An example of such an embodiment is illustrated inFIG. 3, wherein processing circuitry310such as, for example, an analog-to-digital converter, receives, at a first input312, the first output signal231from the first amplifier210and receives, at a second input314, the second output signal232from the second amplifier216, and produces an output signal316. In the embodiment illustrated inFIG. 3, the processing circuitry310may evaluate the magnitude of the respective first and second output signals231and232to determine proximity detection of the finger230by one, both, or neither of the respective first and second sensors120and130. In other embodiments, the processing circuitry310may evaluate whether a stronger proximity was detected by a particular one of the sensors. For example, in accordance with this embodiment, the output signal316may indicate whether or not the first sensor120detected a greater proximity than the second sensor130. In some embodiments, the processing circuitry output signal316may be utilized by additional circuitry (not shown) to perform various operations in response to the detected user proximity. Examples of such operations are described below with respect toFIGS. 4A,4B and4C.

FIGS. 4A,4B and4C illustrate various examples of operation of an embodiment in which the disclosed capacitive proximity sensor100is incorporated in a mobile phone device400having a touch screen405disposed on the front surface of the phone400, and an external casing410disposed along the back and side surfaces of the phone400. As shown inFIGS. 4A,4B and4C, the capacitive proximity sensor100is located near the top402of the phone400and is positioned along a plane substantially parallel to the touch screen405such that the first sensor120is disposed towards (i.e., facing) the front surface of the phone400, and the second sensor130is disposed towards the back surface of the phone400. Readings are taken from each of the first sensor120and the second sensor130to detect proximity of an object relative to the respective first and second sensors120and130. The positioning of the first sensor120towards the front surface of the phone400enhances the first sensor's120capability to detect user proximity generally along the front surface of the phone400. The positioning of the second sensor130towards the back surface of the phone400enhances the second sensor's130capability to detect user proximity generally along the external casing410located along the back and side surfaces of the phone400. In some embodiments, as described with respect to each of the examples, the readings from the first and second sensors120and130may be interpreted by additional logic/circuitry (such as that shown inFIG. 3), and an operation may be performed or an instruction for performing an operation may be provided by the additional circuitry in response to the readings from the proximity sensor circuitry100.

FIG. 4Aillustrates a first example of operation wherein a user touches or approaches (shown inFIG. 4Aby dashed circle415) the casing410at a location near the bottom404of the phone400. Since the touch415is located on the casing410and at a substantial distance from the first sensor120, the first sensor120records a low reading. The distance of the touch415from the second sensor130also causes the second sensor130to record a low reading. However, because the touch415is located on the casing410of the device, the reading of the second sensor130may be greater than the low reading of the first sensor120. In accordance with the example illustrated inFIG. 4A, the additional circuitry ofFIG. 3may be provided in some embodiments to interpret the readings of the respective first and second sensors120and130to determine that the detected user proximity is not located above the capacitive proximity sensor100. Accordingly, in some embodiments, the additional circuitry may determine that the phone400is not raised to the user's head, and thus, no related actions, such as disabling the display, may be performed.

FIG. 4Billustrates a second example of operation wherein a user touches or approaches (shown inFIG. 4Bby dashed circle425) the front surface of the phone400at a location above the capacitive proximity sensor100. As shown inFIG. 4B, the touch425is located on the front surface of the phone400and, therefore, is closer to the first sensor120than the second sensor130. Additionally, the guard electrode110shields the second sensor130from sensing the touch425. Accordingly, the reading from the first sensor120is greater than the reading from the second sensor130. In accordance with the example illustrated inFIG. 4B, the additional circuitry ofFIG. 3may be provided in some embodiments to interpret the readings of the respective first and second sensors120and130to determine that the detected user proximity is located above the capacitive proximity sensor100. Accordingly, in some embodiments, the additional circuitry may determine that the phone400is raised to the user's head, and thus, one or more related actions, such as disabling the display, may be performed.

FIG. 4Cillustrates a third example of operation wherein a user touches or approaches the phone400at a first location430along the casing410near the bottom404of the phone400(similar to the user touch/approach415discussed above with respect toFIG. 4A), and at a second location435above the capacitive proximity sensor100(similar to the user touch/approach425discussed above with respect toFIG. 4B). In the embodiment illustrated inFIG. 4C, the first and second sensors120and130both register readings from the user touches430and435, wherein the readings registered by the respective first and second sensors120and130are the sum of the capacitance measured by each respective sensor in response to each of the user touches430and435. Thus, the first sensor120registers a reading comprised of the low capacitance measured by the first touch430(as discussed above with respect toFIG. 4A) and the high capacitance measured by the second touch435(as discussed above with respect toFIG. 4B). Similarly, the second sensor130registers a reading comprised of the capacitance measured by the first touch430(as discussed above with respect toFIG. 4A) and the capacitance measured by the second touch435(as discussed above with respect toFIG. 4B). Accordingly, the reading from the first sensor120is greater than the reading from the second sensor130. In accordance with the example illustrated inFIG. 4C, the additional circuitry ofFIG. 3may be provided in some embodiments to interpret the readings of the respective first and second sensors120and130to determine that a detected user proximity is located above the capacitive proximity sensor100. Accordingly, in some embodiments, the additional circuitry may determine that the phone400is raised to the user's head, and thus, one or more related actions may be performed.

It should be appreciated that, in some embodiments, the additional logic/circuitry may incorporate one or more sensor-reading threshold values for determining a given range of distance of the detected user proximity from the respective sensor. For example, a first threshold value may be used to determine whether the detected user proximity is close enough to a respective sensor to warrant a first action, and a second threshold value may be used to determine whether the detected user proximity if far enough from the respective sensor to warrant a second action. For example, in one embodiment, the first threshold value may be set such that when the reading from the first sensor120indicates a stronger proximity detection than the reading from the second sensor130, and also exceeds the first threshold value, the additional circuitry determines that the detected user proximity is located directly above the first sensor120and, therefore, may disable the display screen. Additionally, when the reading from the first sensor120indicates a stronger proximity detection than the reading from the second sensor130, but does not exceed the first threshold value, the additional circuitry determines that the detected user proximity is located on the touch screen of the phone and, therefore, may not disable the display screen, but rather, may increase the brightness of the display screen. In another example, the second threshold value may be set such that when readings from either the first sensor120or second sensor130indicate a proximity detection that does not exceed the second threshold value, the additional circuitry determines that the detected user proximity may be ignored.

As mentioned above, an additional embodiment of the disclosed capacitive proximity sensor circuitry is illustrated inFIGS. 5A and 5B, wherein the sensor500is shown inFIG. 5Afrom a front view and inFIG. 5Bfrom a side view along line A-A ofFIG. 5A. The embodiment illustrated inFIGS. 5A and 5Bis similar to the embodiment illustrated inFIGS. 1A and 1Band described above except that the sensor500is comprised of two guard electrodes, each having a single sensor disposed thereon. Thus, the capacitive proximity sensor circuit500includes a first guard electrode510having a first sensor515, and a second guard electrode520having a second sensor525. The capacitive proximity sensor circuit500operates similar to the sensor circuit100described above with respect toFIGS. 1A,1B,2A,2B,3,4A,4B and4C, except that the first and second sensors515and525may be placed at different locations within a device, as explained below with respect toFIG. 6.

FIG. 6illustrates an example of an embodiment in which the capacitive proximity sensor500is incorporated in a mobile phone device600having a touch screen605disposed on the front surface of the phone600, and an external casing610disposed along the back and side surfaces of the phone600. As shown inFIG. 6, the first and second sensors515and525(and respective guard electrodes510and520) may each be placed at different locations on the phone600. For example, inFIG. 6, the first guard electrode510and first sensor515are located towards the top left corner612of the phone600, and are positioned along a plane substantially parallel to the touch screen605such that the first sensor515is disposed towards the front surface of the phone600. Additionally, the second guard electrode520and second sensor525are located towards the bottom right corner614of the phone600, and are positioned such that the second sensor525is disposed towards the back surface of the phone600.

As discussed above, readings are taken from each of the first sensor515and the second sensor525to detect proximity of an object relative to the respective first and second sensors515and525. The positioning of the first sensor515towards the front surface of the phone600enhances the first sensor's515capability to detect user proximity generally along the front surface of the phone600. The positioning of the second sensor525towards the back surface of the phone600enhances the second sensor's525capability to detect user proximity generally along the external casing610located along the back and side surfaces of the phone600. In some embodiments, the readings from the first and second sensors515and525may be interpreted by additional logic/circuitry (such as that illustrated inFIG. 3), and an operation may be performed or an instruction for performing an operation may be provided by the additional circuitry in response to the readings from the proximity sensor circuitry500.

It should be appreciated that the various embodiments provided herein are intended to provide one or more examples for illustrating and/or describing the disclosed capacitive proximity sensor circuitry. As such, the disclosed capacitive proximity sensor circuitry is not limited to the devices, positions, locations, or orientations provided in the example embodiments. Additionally, the additional circuitry discussed herein is not limited to the operations, features, or functions disclosed herein, and may provide advantages other than those discussed herein.