Proximity sensor and mobile wireless device

A capacitive proximity sensor for use in mobile devices such as smartphones and connected tables, in which it is used to switch off a display (70) when the device is brought to the ear, and to reduce selectively the RF power when the device is in close proximity to a body part of a user, in order to fulfil regulatory SAR limits. The capacitive sensor uses two electrodes (60, 30), the first of which may also serve as RF antenna, and the other is preferably on the back of the phone and is opposite the display.

FIELD OF THE INVENTION

The present invention concerns a proximity sensor and a method for detecting the proximity of a body portion. Embodiments of the present invention concern wireless mobile devices, like mobile phones, personal computer or tablets that are equipped with the proximity sensor of the invention and, based on proximity signals generated by the sensor, can inhibit a display and/or a touch sensitive panel, and/or adapt a RF power of a radio transmitter.

DESCRIPTION OF RELATED ART

It is often desired to detect whether a body portion is at short distance of an apparatus. In the special case of cell phones and wirelessly connected mobile device, (including tablets and other similar terminals). This form of proximity detection can be used as an input to the apparatus, but, for RF-emitting devices, it is known to use a proximity indication to adapt the instantaneous RF power, to comply with SAR (Specific absorption Ratio) regulations. SAR is a measure of the amount of RF energy radiated in the human body when close to a radio emitting device (phone, tablet, laptop, etc.).

Other useful functions of portable connected devices that rely on proximity detection are: disabling the touch screen of a portable phone when it is brought to the ear for a call, lest the user may trigger unwanted actions by touching the screen with the cheek or the ear, and switching the screen backlighting off to economize energy, in the same situation.

Sensors arranged for detecting a body near to an object, including inductive, optical, heat, and capacitive based sensors, are known. In the cell phone market, the most common method today is a capacitive based sensor to detect an object near the RF antenna.

Capacitive sensors are often realized as metallized pads on a PCB but, in many cases, an existing element such as an antenna (i.e. conductive line), can double as a capacitive detector, such that the detector can be added with no surface penalty.

An example of capacitive sensor for proximity sensing in a mobile communication device is described in patent application EP2988479, in the name of the applicant, whose content is hereby included by reference.

BRIEF SUMMARY OF THE INVENTION

According to the invention, these aims are achieved by means of the object of the appended claims. In particular, these aims are achieved by a proximity sensor for a portable device arranged for detecting a body portion of an user in proximity of the portable device, having a first electrode and second electrode, both capacitively couplable with a body part of a user of the portable outside of the portable device; a readout circuit operatively arranged for acquiring a first capacity seen by the first electrode and a second capacity seen by the second electrode, and a to generate a directional proximity signal, as well as a second, omnidirectional, proximity signal based on said first capacity and second capacity.

Advantageous variants of the invention include a sensor as detailed above in which the first and second proximity signals are used, in a portable device, to switch off and on a touch-sensitive display, and to reduce the power of a RF transmitter, to respect regulatory SAR limits. Further embodiments relate to the inclusion of decoupling elements, by which the second electrode can be connected at the same time to the proximity readout circuit and to the RF transceiver, and function at the same time as RF antenna; to noise subtraction, pedestal subtraction and drift compensation in a digital processing unit of the readout circuit.

Other advantageous claimed variants of the invention relate to the generation of a directional proximity signal based on the difference, or on a weighted combination or on the ratio of the capacities seen by the electrodes, or else when a vector having as coordinates the first and second capacity falls in a predetermined region in a cartesian plane.

Further, when the invention is applied to a portable smartphone with a touch-sensitive screen, the first electrode would preferably be on the top of the phone, while the second one would preferably be on the back of the phone itself, opposite the display. The second electrode and/or the first one may have a shield electrode below, and/or a guard electrode in proximity, or above. Preferably, the shield and guard electrodes can be connected selectively to a voltage source or left in a high-impedance floating state.

In the context of this invention the directions “up”, “down”, “front”, “back”, refer to the normal orientation of a portable device, such as a mobile phone, with the screen facing the user and the loudspeaker up.

Further, the wording “directional” when applied to a proximity detection, indicates that the signals are processed in such a way that conductive bodies in a determined direction, for example in front of the electrode, are detected with higher sensitivity than other conductive bodies of similar characteristics in other directions, for example behind the electrode. This directional sensitivity may include combining the readings of two or more electrodes.

Although the present invention is applicable to vast array of devices including, but not limited to, cell phones, tablets, and laptop computers, examples of this description may refer simply to a phone. This should not be construed as a limitation of the invention, but merely as an example focusing on a particular implementation for concision's sake.

The functioning of a capacitive proximity detector suitable for the invention will now be recalled referring to figures. As illustrated inFIG. 1, capacitive sensors may use, as sense electrode, a conductive layer20on a printed circuit board or PCB157. In the example, the electrode20is surrounded by a grounded ring-shaped guard electrode25, backed by a shield electrode23, and covered by a dielectric overlay158, but none of these features is essential, as the shape that can be round, as shown or any other shape. The guard electrode25may be used to enhance the directivity of the detection, shielding the main electrode laterally. In possible variants, the guard electrode25may cover in part the main electrode20, or else it may be configured as a grid overlaying the main electrode. By selectively connecting the guard electrode to the ground, or the same potential as the sense electrode20, one can change the distance at which the electric field extends above the electrodes, and the detection range with that.

In free space, far from other conductive bodies, the capacitance of the sense electrode will have a baseline value: Csensor=Cenv, determined from the electric induction between the electrode and all the surrounding conductors.

A conductive body in the proximity of the sensor, as the finger shown inFIG. 2, modifies the electric field distribution and, in general, induce an increase of the capacitance of the sense electrode: Csensor=Cenv+Cuser.

It is well to realize that the increase Cusermay be much less than the baseline capacitance Cenv. In a typical case, Cusermay be 1% of Cenv, or even less. On the other hand, Cenvis hard to predict or simulate reliably, because it depends from several uncontrollable effects. Nevertheless, Cusercan be estimated by the formula below

CUser=ɛ0⁢ɛr⁢Ad
where A is the common area between the two electrodes, hence the common area between the user's finger/palm/face and the sensor electrode20, d their distance, and ε0, εrdenote the absolute and relative dielectric permittivity. Conductive effects are neglected.

FIG. 3illustrates a portable device, for instance a smartphone40The phone40includes a touch-sensitive screen panel70on a front side, as well as a microphone105and a loudspeaker110below, respectively above the screen70. A desirable function of the phone is that, when the device is brought to the ear for a telephone conversation, the screen70should be disabled to reduce power consumption, and any contact between the screen and the user's cheek should be ignored.

The portable device40necessarily connects with other devices in the neighbourhood and/or in the internet by some form of radio connection. This typically includes Wi-Fi (IEEE 802.11), and/or Bluetooth®, and/or a mobile telephone network. When the user is close to the radio source, its tissues absorb energy from the electromagnetic radiofrequency field; the absorption rate (SAR) is limited by specific regulations. Another desired function of the portable device40is that the RF power should be limited when a user is nearby, to comply with these rules without compromising connectivity at all times.

The portable device40comprises at least two electrodes for capacitively measuring proximity of a part of the user's body. A first electrode30is also used as RF antenna for radio communication and is usually mounted near the top of the device. A second electrode60is used for spatial discrimination and directivity and may be mounted on the back of the phone, behind and/or opposite the touch screen70.

A same conductor can be used both as capacitive sense electrode and RF antenna by means of suitable decoupling.FIG. 4illustrates a possible arrangement in which a conductor30is connected both to a RF transceiver57by means of a high-pass network—symbolized by the capacitor C—and to a proximity detection circuit80through a low-pass network—symbolized by an inductor L. When the proximity detection circuit senses that the user is close to the antenna30, it informs the host100that causes the RF transmitter57to reduce the emission power. In a concrete implementation, the antenna may have a complex structure, with a plurality of segments, and shield and guard electrodes may be added, without departing from the general concept.

FIG. 6shows a proximity detection circuit80arranged to read the capacities seen by two electrodes30and60as shown in the device ofFIG. 3. The electrodes may have back shields33,63that are permanently grounded, or connected to a shield control unit51that allows to tie them at a desired voltage or keep them floating. The second electrode60has a guard65(depicted here as a grid) whose capacity is read by a separate readout channel. Although the circuit80is drawn with three readout channels for reading the capacities seen by three electrodes, this is not a limiting feature of the invention and the number of channels could be lower, or higher, as needed.

The electrodes are connected to a capacity-to-voltage sub-circuit53through the multiplexer54that reads all the channels IN0-IN2one by one in turn. The switches S0-S1are preferably operated synchronously with the multiplexing cycle. The multiplexer is advantageous but is not an essential feature of the invention: parallel implementations are also possible.

The capacity-to-voltage sub-circuit53converts the capacity seen by the electrode into a suitable voltage signal and is followed by a programmable offset subtraction sub-circuit50. As already mentioned, the proximity signal Cuseris superposed to a much larger baseline value Cenvthat is neither known in advance nor stable. The digital processor65can estimate the value of the base capacity and compensate for it by programming suitable values in the offset subtraction sub-circuits50. The compensated capacity signals are converted into numeric values by the ADC55.

The proximity detection circuit80may communicate with a host processor100my means of a suitable data bus DB and produces one directional proximity logic flag (PROX) that signal the host that the operation of the screen70should be inhibited, and an omnidirectional SAR flag indicating that a part of the user's body is close to the antenna30, and the RF power should be adapted.FIG. 8illustrated this method in a workflow graph.

FIG. 5show in simplified fashion a possible architecture of the treatment carried out by the digital processor65for each channel. The raw samples R(n) from the ADC are treated by an input filter210to remove noise and yield the “useful” signal U(n). Input filter210can be a linear low-pass filter, or a nonlinear one. The drift estimation unit212determines a drift component superposed to the U(n) samples, which is subtracted at215. Finally,235stands for additional processing including debouncing, overflow/underflow detection (which may trigger a baseline compensation), and so on.

The processed samples D(n) may be fed to a discriminator218to generate proximity flags or combined with the reading of other electrodes for directional detection, as described below.

Importantly, the proximity detector80generates a directional proximity signal (PROX) based on the capacities seen by the first electrode30, respectively by the second electrode60. Different objects in various spatial relationship with the phone will affect the capacities seen by the first and second electrode in different measure, and this can be used for a directional discrimination.

When the user approaches the phone to the ear, for example, the capacities seen by the first and second electrode will both increase, in a given proportion. This should be discriminated against other situations which should not trigger a proximity signal like device grips, when the user holds the telephone in the hand, and loads the back electrode60considerably more than the top electrode30, and finger slides, when the device slides a finger on the screen while holding the phone.

In a possible variant of the invention, the proximity sensor decides whether a proximity flag is to be raised based on the ratio, between the capacities seen by the first and the second electrode, or on their difference, or on a linear combination of the first and second capacity.

In a preferred variant, the proximity detection circuit80decides whether raising a proximity flag based on the position in a cartesian plane of a vector having the first and second capacity as components.FIG. 7plots this situation. D− denotes the capacity of the back electrode60and D+ that of the top electrode30. Device grip events cluster in a specific region265, while finger slide events fall mostly in region267. When the phone is brought to the ear, the capacities seen by both electrodes rise following, within a given approximation, curve250. The detection circuit80is arranged to raise a proximity trigger when the vector defined by D+ and D− lies in the region255(white) between the lower threshold line260and the upper threshold line262. Lower and upper thresholds are preferably programmable by the host system or preloaded in a firmware of the proximity circuit80and may be parameterized as piecewise linear functions, or in any other way.

At the same time as the directional proximity signal (PROX), the proximity circuit generates also a unidirectional proximity signal (SAR) that is used to reduce the RF power. In a favourable variant, the SAR signal is generated based on the capacity seen by the top electrode30that is also the RF antenna. This can be done simply by comparing the D+ digital signal against a threshold. More sophisticated processing is also possible and may consider, for example:the rate of change of the D+ signal. A user's approach is expected to be gradual. Abrupt changes are indication of a noise event, or of the intervention of a load-matching circuit, if the electrode30is also the RF antenna.The variance of the D+ signal. Little variation indicate that the telephone may be resting on a table rather than close to a body part.Contribution of the D− signal coming from the other electrode.

REFERENCE SYMBOLS USED IN THE FIGURES