Advanced capacitive proximity sensor

A proximity sensor, and a portable device equipped therewith, with at least two sense electrodes, one influencing the other. By reading twice the capacity of one electrode, while either setting the potential of the counter-electrode to guard or letting it float, the sensor of the invention discriminates between a body part, or another electrically equivalent object, and low-permittivity objects.

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 in particular portable devices like cell phones that, being equipped with the inventive proximity sensors, are capable of discriminating body parts, like for example the head or one hand of the user, from low permittivity object, and recognizing the direction whence the detected body part is approaching.

DESCRIPTION OF RELATED ART

It is often desired to detect whether a body portion is at short distance of an apparatus. In the particular 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, in order to comply with SAR (Specific absorption Ratio) regulations. SAR is a measure of the amount of RF energy radiated in the human body when in close proximity 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.

Despite all the above advantages, conventional capacitive detectors alone provide little or no information about the distance and size of the approaching object. A body part at a given distance and an inanimate object, if it is large and close enough, can generate the same capacity rise, and would not be distinguishable.

This lack of discrimination of capacitive detectors may lead to wrong decisions in some situations. One example is when the phone or the portable device is placed on a support, like a table or a holder. In this case the phone may reduce RF power or disable the screen without actually needing to. Mitigating these false detections in conventional capacitive sensors is difficult.

BRIEF SUMMARY OF THE INVENTION

An aim of the present invention is to provide an advanced capacitive detector which may address the shortcomings of the conventional devices mentioned above, as set forth in the appended claims.

Although the invention is applicable to a large array of devices, for example laptops, tablets, e-readers, wearables, hearables, electronic measuring instruments, and also to non-portable devices, the present description will refer simply to a “mobile phone” for concision's sake. This should not be taken as a limiting feature of the invention though.

FIG. 1shows the relationship between the SAR as a function of the distance between the transmitter antenna and the user's body. Curve210may represent the Specific Absorption Rate associated with a conventional cellphone transmitting at full RF power. It is apparent that the absorption rate exceeds the statutory absorption rate limit (Max) for small distances.

Curve220represents the absorption rate in a phone associated with a phone whose RF power has been deliberately reduced. Clearly the rate complies with statutory limits but, since the power is lower, the connectivity of the phone will be degraded.

FIG. 2illustrates a solution to the above dilemma. The connected device is equipped with a proximity detector that measures the distance d to the user. A hardware or software processor in the device generates a logical signal when the distance sinks below a predetermined limit dtthat triggers a momentary reduction of the RF power, as shown by plot230. In this manner the device suffers no degradation of the connectivity when the distance d is large enough that the SAR is below the maximum acceptable limit, and the power is reduced only when it is required. AlthoughFIG. 2shows only one threshold dt, it should be understood that the device could determine the distance in relation to several threshold values, and reduce the power in several progressive steps, in a search of the best connectivity compatible with the SAR limit at any given distance.

FIG. 1shows schematically the structure of a capacity proximity detector, as it could be employed in the frame of the invention. The detector includes a sense electrode20connected to an input terminal IN for the determination of its capacity. The capacity of the electrode can be determined by applying a variable potential of determined amplitude V to the input terminal IN, integrating the input current to obtain the electric charge Q, which is related to the capacity by C=Q/V. The input potential can vary according a sinusoidal or square law, for example.

The capacitive sensor readout circuit80may include a capacity-to-voltage conversion unit53, that generate a voltage signal proportional to the capacity seen by the electrode20.

FIG. 4shows a simplified circuit that could be used for the purpose. The informed reader will recognize that the terminal IN is a low impedance node whose potential is the same, thanks to reaction, as the output of the voltage source47, which can be square, sinusoidal, or follow any suitable function, and that the amplitude of the output signal V is proportional to the capacity20. For additional information, the reader is directed to patent application EP2876407, which is hereby incorporated by reference. Other circuits fulfilling the same function as the one ofFIG. 4are available and comprised in the frame of the invention. The principle of operation of the capacitive sensor readout circuit80is that, the head and body of a user have a dielectric constant much higher than that of free space. Thus, when the user approaches the head or another body to the electrode,20its capacity increases by a tiny but measurable amount.

FIGS. 5 and 6illustrate schematically the capacitive effect that lies behind the proximity sensor of the invention. InFIG. 5, the capacitive sensor is represented by a disk pad20on a printed circuit board137, surrounded by a ground ring25. The electrode20is preferably backed by a protective electrode124held at ground potential or at shield potential, to screen out noise coming from the circuit of the phone that lies below. Preferably, the conductive pads20,25are covered by a thin dielectric protective layer138. When the electrode20is connected to the IN terminal of the capacitive detector, its electric potential is different from that of the surrounding ground electrode, and an electric field, represented by the field lines, is generated.

When body part approaches, as inFIG. 6, it modifies the electric field, because it has a dielectric constant ε different from that of the surrounding air, and possibly also because of its electric conductivity. This records as a small change of the capacity C seen by the electrode20.

Importantly, the shape of the sense electrode20has little significance and the capacitive sensor would function as well with an electrode of arbitrary shape. The ground ring25and the shield124, although useful, are not essential, and the real shape of the electric field will in any case be very different from that represented, because the electrode will couple in complicated ways with all the components of the phone. In all cases, no matter what the final configuration will be, the capacity C of the electrode20will have a baseline value Cenv, constant or slowly varying together with the environmental characteristics like the temperature, that will increase slightly and momentarily by an amount CUser with the approach of a body part. Although the exact amount of the increase may be difficult to compute a priori exactly, it can be estimated by the formula below
Cuser=ε0εrA/d
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.

The relative permittivity of the human body is very high, due to its high water content, and is typically εr>80. The permittivity of most structural insulating materials, such as glass, FR4, plastic laminates and wood, between 2 and 8. Thus the capacitive detector of the invention will be considerably more sensitive to the human body than to other materials, but could still be misinterpret a large body of low permittivity as a part of human body if it is very close and fairy large.

Returning toFIG. 3, we have seen that the capacity change determined by the proximity of the user is superposed to a large baseline value that is constant, or drifts slowly. The sensor of the invention includes preferably an offset subtraction unit50that is arranged to subtract a programmable value to the total capacity before it is converted to a digital value in the ADC55, to enhance the proximity induced variations and utilize optimally the dynamic range of the latter.

In the drawing, the offset compensation unit50is represented as a separate block acting on an analog signal generated by the capacity-to-voltage converter53. Although this is a possible and favored implementation, it is not the only one; the invention is not limited to this embodiment, and the blocs of the schematics1should be interpreted as functional elements rather than physically separated entities. In variants, the subtraction of the offset could be carried out in the capacity-to-voltage converter53, or in the ADC55. Also, if the proximity detector readout circuit80comprises several input channels, as it will be detailed further, the offset compensation could be done in independent units for each channel, or in a shared compensation circuit.

Another difficulty in capacitive proximity detectors is that the input electrode20can pick up all sort of signal and disturbances generated in its environment, including those coming from the phone in which it is embedded. Although such disturbances can be filtered by signal processing, it is preferable to attenuate them from the start. To this purpose, the detector can provide a shield electrode23, below the sense electrode20, in order to screen it from the electronics inside the phone. Preferably, the sense electrode is connected to the output terminal, of a shield control unit51, which follows the variable potential of the input terminal IN. In this manner, the shield23does not contribute to the capacity seen by the electrode20. The shield electrode is represented below the sense electrode, but it could be positioned elsewhere.

Digital processor65elaborates the digital signal generated by the ADC55and provides a proximity signal PROX based on the capacity of the electrode20. It is in communication with a host system, for example a mobile phone through a bus DB, and can be implemented by any form of wired or programmable logic. The digital processor65takes care of function like fine offset subtraction, noise filtering, and implements a decision algorithm that asserts the PROX signal when the capacitance read at the input IN1(as well as input IN2) is compatible with a given threshold. The digital processing will then produce another set of signals named OBJECTAand OBJECTBthat are asserted, for example whether the capacitance increase is judged to be a body part (head cheek, hand, lap) or an inanimate object. These are what the host can then use to determine if power should be lowered from the part of a user's body in proximity.

AlthoughFIG. 3represents only one antenna terminal, the proximity sensor could have several input groups INA, INB, . . . , each connected to different electrodes for different antenna terminals, possibly sharing a common ADC by a multiplexer.

Importantly, the capacitive sensor is capable of disconnecting the shield electrode (or at least one of the shield electrodes if there are many), leaving the corresponding shield input in a high-impedance state. In such a situation, the corresponding sense electrode will be floating.

Although figure represents the function of disconnecting the shield electrode by a switch S0, there are other ways to realize a terminal that can be set either to a desired voltage or to a high-impedance state, for example a logic three-state output (if the variable voltage47is a square signal), a transmission gate, or a CMOS switch, among others.

FIG. 7shows a possible realization of the sensor of the invention in a mobile phone100that has, concealed below a front dielectric layer, a sense electrode20connected to an input IN of the proximity detector, a shield electrode23connected to the SHIELD terminal, and, optionally, a ground guard pad25encircling the sense electrode20and the shield electrode23.

Preferably the mobile phone comprises more than one pair of electrodes. In the example shown inFIG. 7, the sense electrode20and the shield electrode23are placed close to the top of the phone, and another pair comprising a sense electrode21and a shield electrode24is placed at the bottom of the phone100. Possibly, the bottom sense electrode21is connected to both the capacitive proximity sensor80and to a RF transceiver through suitable decoupling elements, as shown inFIG. 8, and doubles as RF antenna.

As it can be appreciated, the shield electrodes23and24needs not be below the corresponding electrodes20and21, but could also be aside, as depicted, or partly covered, facing the front or the back of the phone, or in any position on the phone100.

The proximity detector of the invention is arranged to acquire two capacity measurements: the first one, denoted as Cmain is the capacity seen by the main sense electrode20while the auxiliary shield electrode23is held by the shield control unit at the same potential as the sense electrode20, and the second, denoted as Caux is the capacity seen by the sense electrode20while the shield electrode23is floating (the SHIELD output in a high-impedance state. Should the telephone include more than one sense/shield electrode pair, the same sequence can be repeated in each pair, for example the capacity of the lower sense electrode21will be measured first holding the shield electrode24at the same potential as the electrode whose capacity is measured, and then when the shield electrode24is floating.

The inventors have found that setting the auxiliary electrode23in a high-Z state changes the measured capacity in a manner that allow to discriminate between a body part and a low-permittivity object.FIG. 8plots the capacities Cmain (solid) and Caux (dashed) against time. The left part of the plot (interval213) corresponding to the approach of the user's hand, and the right part of the plot (interval215) corresponding to the approach of a low-permittivity object. It can be seen that, although the signals have similar amplitudes, Cmain and Caux reacts differently to the approach of the two objects. In this configuration, Cmain responds more to the body part than to the low-permittivity object, and inversely Caux. Advantageously, the digital processor65is arranged to determine whether the approaching object is a part of a human body rather than a low-permittivity object, based on the capacities Cmain and Caux.

This can be achieved by choosing a strategy that includes essentially all the genuine approaches and reject at least a large part of the low-permittivity bodies. For example, the OBJECTAor the OBJECTBsignal could be asserted when the first capacity and the second capacity, taken as coordinates in a two-dimensional plane, locate a point inside a predefined acceptance region, or when the ratio Cmain/Caux in a predetermined acceptance interval, or by another suitable selection algorithm based on Cmain and Caux.

REFERENCE SIGNS