Advanced capacitive proximity sensor

Abstract

Advanced capacitive proximity sensor. A proximity sensor, and portable device equipped with said proximity sensor, having at least two sensing electrodes, one affecting the other. By reading the capacitance of one electrode twice, when setting the potential of the counter electrode to protection or letting it float, the sensor of the invention discriminates between a body part or another electrically equivalent object and a low dielectric constant object.

Description

Technical Field

[0001] This invention relates to proximity sensors and methods for detecting the proximity of body parts. Embodiments of the invention particularly relate to portable devices, such as cellular phones, equipped with an inventive proximity sensor capable of distinguishing body parts, such as a user's head or a hand, from objects with low permittivity and identifying the direction in which the detected body part is approaching. Background Technology

[0002] It is often desirable to detect whether a body part is within a short distance of a device. In the particular case of cellular phones and wirelessly connected mobile devices (including tablets and other similar terminals), this form of proximity detection can be used as input to the device. However, for RF emitting devices, it is known to use proximity indicators to adapt to instantaneous RF power in order to comply with SAR (specific absorption ratio) rules. SAR is a measurement of the amount of RF energy radiated into the human body when closely approaching a radio emitting device (telephone, tablet, laptop, etc.).

[0003] Other useful features of portable connectivity devices that rely on proximity detection include disabling the phone's touchscreen when the phone is held to the ear for a call to prevent users from triggering unwanted actions by touching the screen with their cheek or ear, and turning off the screen backlight to save energy in the same situation.

[0004] Sensors are known for being deployed to detect the proximity of a body to an object, including inductive, optical, thermal, and capacitive sensors. In the cellular phone market, the most common method today is the use of capacitive sensors to detect the proximity of objects to an RF antenna.

[0005] Capacitive sensors are often implemented as metallized pads on a PCB, but in many cases, existing components such as antennas (i.e., conductive lines) can double as capacitive detectors, allowing detectors to be added without surface cost.

[0006] An example of a capacitive sensor for proximity sensing in a mobile communication device is described in the applicant’s patent application EP2988479, the contents of which are incorporated herein by reference.

[0007] Despite all the advantages mentioned above, conventional capacitive detectors alone provide almost no information about the distance and size of approaching objects. At a given distance, a body part and an inanimate object, if large enough and close enough, can produce the same rise in capacitance and will not be indistinguishable.

[0008] This lack of discrimination in capacitive detectors can lead to incorrect decisions in some situations. One example is when a phone or portable device is placed on a support such as a table or holder. In this case, the phone might reduce its RF power or disable its screen when it doesn't actually need to be. Mitigating these false detections in conventional capacitive sensors is difficult. Summary of the Invention

[0009] The object of the present invention is to provide an advanced capacitive detector that overcomes the shortcomings of the conventional devices described above, as set forth in the appended claims.

[0010] While this invention is applicable to a wide array of devices, such as laptops, tablets, e-readers, wearable devices, hearable devices, and electronic measuring instruments, and also to non-portable devices, for the sake of brevity, this specification will simply refer to "mobile phone." However, this should not be considered a limiting feature of the invention. Attached Figure Description

[0011] The invention will be better understood with the help of the description of embodiments given by way of example and illustrated in the accompanying drawings, wherein: Figure 1 and Figure 2 SAR levels from portable connected devices were plotted; Figure 3 A capacitive proximity sensor in a portable connected device is illustrated schematically. Figure 4 It is a simplified, idealized circuit used to convert capacitance into voltage. Figure 5 and Figure 6 The diagram illustrates the capacitance change of a capacitive proximity detector. Figure 7 A portable connection device including a capacitive sensor is shown according to one aspect of the present invention. Figure 8 Possible connections are shown, where the RF antenna also functions as a capacitive sensor. Figure 9The capacitance read by the sensor of the present invention is plotted when a portable device, including the sensor of the present invention, first approaches a part of a human body and then approaches an inanimate object. Detailed Implementation

[0012] Figure 1 The relationship between SAR as a function of the distance between the transmitter antenna and the user's body is shown. Curve 210 can represent a specific absorption rate associated with a conventional cellular phone transmitting at full RF power. It is clear that the absorption rate exceeds the statutory absorption rate limit (Max) for short distances.

[0013] Curve 220 represents the absorption rate in a telephone associated with a telephone whose RF power has been intentionally reduced. Clearly, this rate complies with legal limits; however, due to the lower power, the telephone's connectivity will be degraded.

[0014] Figure 2 The diagram illustrates a solution to the aforementioned dilemma. The connected device is equipped with a distance measurement function to the user. The proximity detector. As shown in Figure 230, the hardware or software processor in the device detects a predetermined limit to the instantaneous drop in distance below the trigger RF power. A logic signal is generated at that time. In this way, when the distance... It is large enough that when the SAR is below the maximum acceptable limit, the device does not suffer from connectivity degradation, and power is only reduced when needed. Although Figure 2 Only one threshold is shown. However, it should be understood that the device can determine distances related to several thresholds and reduce power in several asymptotic steps in the search for optimal connectivity compatible with SAR limitations at any given distance.

[0015] Figure 3 The structure of a capacitive proximity detector is schematically shown, as it can be used within the framework of this invention. The detector includes a sensing electrode 20 connected to an input terminal IN for determining the capacitance of the sensing electrode 20. This is achieved by using a variable potential to determine the amplitude. The input current is integrated at the input terminal IN to obtain the charge. The capacitance of the electrodes can be determined, which involves The capacitance. For example, the input potential can vary according to a sine curve or a square law.

[0016] The capacitive sensor readout circuit 80 may include a capacitor-to-voltage conversion unit 53, which generates a voltage signal proportional to the capacitance seen by the electrode 20.

[0017] Figure 4 A simplified circuit that can be used for this purpose is shown. A well-informed reader will recognize that terminal IN is a low-impedance node whose potential is due to the same reaction as the output of voltage source 47, which can be square, sinusoidal, or any suitable function thereof, and the amplitude of the output signal V is proportional to capacitor 20. For additional information, the reader is directed to patent application EP2876407, which is incorporated herein by reference. Figure 4 Other circuits with the same function are available and included within the framework of this invention. The operating principle of the capacitive sensor readout circuit 80 is that the user's head and body have a dielectric constant much higher than that of free space. Therefore, when the user brings their head or other body part close to the electrode 20, its capacitance increases by a small but measurable amount.

[0018] Figure 5 and Figure 6 The capacitive effect located behind the proximity sensor of the present invention is schematically illustrated. Figure 5 In this design, the capacitive sensor is represented by a disk pad 20 on a printed circuit board 137, surrounded by a ground ring 25. Electrode 20 is preferably supported by a protective electrode 124 held at a ground potential or a protective potential to screen out noise from the circuitry of the telephone located below. Preferably, the conductive pads 20, 25 are covered by a thin dielectric protective layer 138. When electrode 20 is connected to the IN terminal of the capacitive detector, its potential differs from that of the surrounding ground electrode, generating an electric field represented by field lines.

[0019] When body parts are close together, such as in Figure 6 In this way, it modifies the electric field because it has a different dielectric constant than the surrounding air. And possibly because of its conductivity. This is recorded as the capacitance seen by electrode 20. Small changes.

[0020] Importantly, the shape of the sensing electrode 20 is almost meaningless, and capacitive sensors will operate with electrodes of arbitrary shapes. While the grounding ring 25 and protection 124 are useful, they are not necessary, and the true shape of the electric field will in any case be very different from the represented shape because the electrode will be coupled to all components of the phone in a complex manner. In all cases, regardless of the final configuration, the capacitance of electrode 20... Having a baseline value Cenv, whose constant or environmental characteristics, such as temperature, change slowly, this will increase slightly and instantaneously with the approach of a body part by an amount CUser. While the exact amount of this increase may be difficult to calculate precisely in the priori, it can be estimated using the following formula. in It is the common area between the two electrodes, therefore the common area between the user's finger / palm / face and the sensor electrode 20. Their distance and , This represents the absolute and relative dielectric constant. Conductive effects are ignored.

[0021] Because of the high water content of the human body, its relative permittivity is very high, and it is usually... Most structural insulating materials, such as glass, FR4, laminated plastics, and wood, have dielectric constants between 2 and 8. Therefore, the capacitive detector of this invention will be considerably more sensitive to the human body than to other materials; however, if a large, low-dielectric-constant object is very close and fairly large, it may still be mistaken for part of the human body.

[0022] Return to Figure 3 We have seen capacitance changes determined by user proximity superimposed on a large baseline value, which is constant or slowly drifting. The sensor of the present invention preferably includes an offset subtraction unit 50, which is arranged to subtract a programmable value from the total capacitance before it is converted to a digital value in the ADC 55, to enhance proximity sensing variation and optimally utilize the latter's dynamic range.

[0023] In the accompanying drawings, the offset compensation unit 50 is shown as a separate block, which acts on the analog signal generated by the capacitor-to-voltage converter 53. While this is a possible and advantageous implementation, it is not the only one; the invention is not limited to this embodiment, and the illustration is merely schematic. Figure 1 The blocks should be interpreted as functional elements rather than physical entities. In a variant, offset subtraction can be performed in the capacitor-to-voltage converter 53 or in the ADC 55. Furthermore, if the proximity detector readout circuit 80 includes several input channels, as will be described in further detail, offset compensation can be performed for each channel in an independent unit or in a shared compensation circuit.

[0024] Another challenge in capacitive proximity detectors is that the input electrode 20 can pick up all kinds of signals and interference generated in its environment, including those from the telephone embedded within it. While such interference can be filtered out through signal processing, it is preferable to attenuate it from the outset. For this purpose, the detector can provide a shield electrode 23 below the sensing electrode 20 to shield it relative to the electronics inside the telephone. Preferably, the sensing electrode is connected to the output terminal of the protection control unit 51, which conforms to the variable potential of the input terminal IN. In this way, the shield 23 does not contribute to the capacitance seen by the electrode 20. The shield electrode is indicated below the sensing electrode, but it can be located elsewhere.

[0025] Digital processor 65 processes the digital signal generated by ADC 55 and provides a proximity signal PROX based on the capacitance of electrode 20. It communicates with a host system, such as a mobile phone, via bus DB and can be implemented using any form of wired or programmable logic. Digital processor 65 handles functions such as fine offset subtraction and noise filtering, and implements a decision algorithm that asserts the PROX signal when the capacitance read at input IN1 (and input IN2) is compatible with a given threshold. The digital processing then generates the asserted signal named OBJECT. A and OBJECT B Another set of signals, such as an increase in capacitance, is determined whether it originates from a body part (head, cheek, hand, knee) or an inanimate object. These are things the host can then use to determine whether power should be reduced from nearby parts of a user's body.

[0026] Although Figure 3 This indicates a single antenna terminal, but proximity sensors can have several input groups IN. A IN B ..., each of the different electrodes connected to different antenna terminals may share a common ADC through a multiplexer.

[0027] Importantly, capacitive sensors can disconnect the protection electrodes (or at least one of them, if there are many), maintaining the corresponding protection input in a high-impedance state. In such cases, the corresponding sensing electrode will be floating.

[0028] Although the diagram illustrates the function of disconnecting the protection electrode via switch S0, there are other ways to achieve a terminal that can be set to the desired voltage or to a high-impedance state, such as, among other things, a logic three-state output (if the variable voltage 47 is a square signal), a transmission gate, or a CMOS switch.

[0029] Figure 7 A possible implementation of the sensor of the present invention in a mobile phone 100 is shown, which has a sensing electrode 20 hidden under the front dielectric layer and connected to the input IN of the proximity detector, a protection electrode 23 connected to the SHIELD terminal, and optionally, a grounding shield pad 25 surrounding the sensing electrode 20 and the protection electrode 23.

[0030] Preferably, the mobile phone includes more than one pair of electrodes. Figure 7 In the example shown, sensing electrode 20 and protection electrode 23 are positioned near the top of the phone, and another pair, including sensing electrode 21 and protection electrode 24, is positioned at the bottom of the phone 100. Possibly, the bottom sensing electrode 21 is connected to both the capacitive proximity sensor 80 and the RF transceiver via suitable decoupling elements, as shown in [example missing]. Figure 8 As shown, it also functions as an RF antenna.

[0031] As can be understood, the protective electrodes 23 and 24 need not be lower than the corresponding electrodes 20 and 21, but may also be adjacent to, as depicted, or partially covered, facing the front or back of the telephone, or at any location on the telephone 100.

[0032] The proximity detector of the present invention is arranged to acquire two capacitance measurements: the first, denoted as Cmain, is the capacitance seen by the main sensing electrode 20 when the protection control unit holds the auxiliary protection electrode 23 at the same potential as the sensing electrode 20; and the second, denoted as Caux, is the capacitance seen by the sensing electrode 20 when the protection electrode 23 is floating (the protection (SHIELD) output is in a high-impedance state). If the telephone includes more than one sensing / protection electrode pair, the same sequence can be repeated in each pair; for example, the capacitance of the lower sensing electrode 21 will first be measured while the protection electrode 24 is held at the same potential as the electrode whose capacitance is measured, and then measured when the protection electrode 24 is floating.

[0033] The inventors have discovered that setting the auxiliary electrode 23 in a high Z state allows for a change in the measured capacitance in a manner that distinguishes between body parts and objects with low dielectric constants. Figure 9The capacitances Cmain (solid line) and Caux (dashed line) are plotted over time. The left portion of the graph (interval 213) corresponds to the approach of a user's hand, and the right portion (interval 215) corresponds to the approach of a low-dielectric-constant object. It can be seen that although the signals have similar amplitudes, Cmain and Caux respond differently to the approach of the two objects. In this configuration, Cmain responds more strongly to body parts than to low-dielectric-constant objects, and vice versa for Caux. Advantageously, the digital processor 65 is arranged to determine whether the approaching object is part of a human body rather than a low-dielectric-constant object based on the capacitances Cmain and Caux.

[0034] This can be achieved through a selection strategy that essentially encompasses all genuine approaches to and at least rejects the majority of low dielectric constant bodies. For example, the OBJECT can be asserted when the first and second capacitors, treated as coordinates in a two-dimensional plane, position the point within a predefined acceptance region, or when the ratio of Cmain / Caux is within a predetermined acceptance interval, or based on Cmain and Caux by another suitable selection algorithm. A or OBJECT B Signal.

[0035] Reference number 20 Sensing Electrodes 21 Sensing Electrode 23. Protective and auxiliary electrodes 24. Protective and auxiliary electrodes 25 Shielding Electrodes 47 Voltage Source 50 offset compensation 51 Protection and Control 53 Capacitor-to-Voltage Converter 55 ADC 65 digital processor 70 monitors 80 Capacitive Sensor Circuit 85 Analog Interface 95 RF transceiver 100 Mainframe systems, mobile phones, portable devices 124 Back Electrode 137 Printed Circuits 138 Covering 210 Specific Absorption Rate / Total RF 213. Physical proximity of users 215 Approaching objects with low dielectric constant 220 Specific absorption rate / Reduced RF 230 Specific Absorption Rate / Smart

Claims

1. A portable device including a proximity sensor arranged to detect the proximity of a body part to the portable device and to reject inanimate objects, the sensor comprising: A pair of electrodes having a main electrode and an auxiliary reference electrode, the main electrode facing the outside of the portable device and held at a potential to a variable voltage source; a readout circuit including an output terminal connectable to the auxiliary reference electrode and set to a desired voltage or a high impedance state; and the readout circuit being operablely arranged to acquire a first capacitance of the main electrode by applying the same potential to both the main electrode and the auxiliary reference electrode, and to acquire a second capacitance of the main electrode when the auxiliary reference electrode is set to a high impedance state, the high impedance state of the auxiliary reference electrode causing the second capacitance to respond differently to objects with a high dielectric constant than to objects with a low dielectric constant; a determining unit configured to generate a proximity signal based on the first and second capacitances when the relationship between the first and second capacitances indicates that the approaching object has a dielectric constant substantially equal to that of the human body; the portable device being configured to cause a momentary drop in the RF power emitted by the portable device when the proximity signal is active; The auxiliary reference electrode acts as a protective electrode to shield the main electrode relative to the electronics inside the portable device.

2. The portable device of claim 1, wherein the main electrode is a conductive pad and the auxiliary reference electrode is a conductive element placed next to the main electrode.

3. The portable device of claim 1, wherein the determining unit is operatively arranged to generate a proximity signal when a first capacitor and a second capacitor, which are treated as coordinates in a two-dimensional plane, locate a point within a predefined receiving area.

4. The portable device of claim 1, wherein the determining unit is operatively arranged to generate a proximity signal when the ratio between the first capacitor and the second capacitor is within a predetermined interval.

5. The portable device of claim 1, comprising a first pair of electrodes positioned near the top of the device and a second pair of electrodes positioned at the bottom of the device.

6. The portable device of claim 1, configured to determine the distance between a body part and a proximity sensor and to reduce RF power in several progressive steps.

7. A method for detecting the proximity of a body part to a portable device and reducing SAR emissions from the portable device, the portable device having a main electrode and an auxiliary electrode placed near the main electrode, the main electrode facing outwards from the portable device, the method comprising: Switch the auxiliary electrode to the same electrode potential as the main electrode; Obtain the first capacitance of the main electrode; Switch the auxiliary electrode to a high impedance state; The second capacitance of the main electrode is obtained, and the high impedance state of the auxiliary reference electrode makes the second capacitance respond to a high dielectric constant object differently from the object responding to a low dielectric constant object, relative to the first capacitance. The dielectric constant of the approaching object is determined based on the relationship between the first capacitor and the second capacitor, which is substantially equal to that of the human body. as well as When the dielectric constant of the approaching object is determined to be substantially equal to that of the human body, it causes a momentary drop in the RF power emitted by the portable device, thereby reducing SAR emissions; The auxiliary reference electrode acts as a protective electrode to shield the main electrode relative to the electronics inside the portable device.

8. The method of claim 7, further comprising determining the distance between the body part and the proximity sensor and reducing the RF power in several progressive steps.

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