GB2558768A

GB2558768A - Proximity detection

Info

Publication number: GB2558768A
Application number: GB1719627.0A
Authority: GB
Inventors: Kavli Tom; Jørgen Bang Hans
Original assignee: Elliptic Laboratories ASA
Current assignee: Elliptic Laboratories ASA
Priority date: 2016-11-28
Filing date: 2017-11-27
Publication date: 2018-07-18
Also published as: GB201719627D0; GB201620062D0

Abstract

A method for determining proximity of an external object to an electronic device comprising transmitting from an ultrasonic transmitter an ultrasonic probe signal and receiving at an ultrasonic receiver an ultrasonic response signal 36. The ultrasonic response signal 36 is compared to a library 32 of stored reference signals 34a-c, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence or presence of any external object proximate to the device. It is determined that the external object is proximate to the device if a difference between the ultrasonic response signal 36 and each (or at least one) of the library 32 of stored reference signals 34a-c exceeds or is below a threshold. Embodiments include a method for calibrating an electronic device having a proximity detector, comprising determining that the device is in a predetermined state; transmitting and receiving ultrasonic signals and storing the response signal in a library of stored reference signals. Further embodiments include: a method for calibrating an electronic device including determining at least a partial transfer function between the ultrasonic transmitter and receiver using the probe and response signals; detect a predetermined touch event before triggering transmission of an ultrasonic probe signal.

Description

(56) Documents Cited:

EP 1174732 A2 US 20160274732 A1 US 20120263019 A1

G01S 15/04 (2006.01)

WO 2017/137755 A2 US 20160154535 A1 US 20100080084 A1 (71) Applicant(s):

Elliptic Laboratories AS Akersgata 32, Oslo 0180, Norway (72) Inventor(s):

Tom Kavli Hans Jergen Bang (74) Agent and/or Address for Service:

Dehns

St. Bride's House, 10 Salisbury Square, LONDON, EC4Y 8JD, United Kingdom (58) Field of Search:

INT CL G01S, G06F, H04M Other: EPODOC, WPI (54) Title of the Invention: Proximity detection

Abstract Title: Determining proximity of an external object to an electronic device via ultrasonic transmitter and receiver (57) A method for determining proximity of an external object to an electronic device comprising transmitting from an ultrasonic transmitter an ultrasonic probe signal and receiving at an ultrasonic receiver an ultrasonic response signal 36. The ultrasonic response signal 36 is compared to a library 32 of stored reference signals 34a-c, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence or presence of any external object proximate to the device. It is |A| 4 determined that the external object is proximate to the ~ device if a difference between the ultrasonic response signal 36 and each (or at least one) of the library 32 of __ stored reference signals 34a-c exceeds or is below a threshold. *

Embodiments include a method for calibrating an / electronic device having a proximity detector, comprising 36 determining that the device is in a predetermined state; transmitting and receiving ultrasonic signals and storing the response signal in a library of stored reference signals. Further embodiments include: a method for calibrating an electronic device including determining at least a partial transfer function between the ultrasonic transmitter and receiver using the probe and response signals; detect a predetermined touch event before triggering transmission of an ultrasonic probe signal.

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At least one drawing originally filed was informal and the print reproduced here is taken from a later filed formal copy.

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- 1 Proximity Detection

The present invention relates to a proximity detection function of a portable electronic device, particularly detecting the proximity of an external object to a device using ultrasound.

Many modern portable electronic devices such as smartphones comprise a proximity sensor arranged to detect the presence of nearby external objects without requiring any physical contact. Such proximity sensors are typically utilised to turn off a display backlight in order to save power during phone calls and to deactivate the touch screen to prevent inadvertent touch inputs, e.g. by the user's cheek. A further application of such proximity sensors is to automatically deactivate the display and touchscreen when the device is placed in a bag or the user's pocket.

Most conventional proximity sensors found in devices such as smartphone make use of the emission and reception of infrared (IR) signals. If an external object is in the vicinity of the sensor, the emitted signal is partly reflected back towards the sensor. If the intensity of the response signal is above a certain threshold, it may be determined that an object is nearby. Typically the proximity sensor is placed towards the top edge of the smartphone close to the ear-piece (i.e. where a user places their ear during a phone call). The reason for this is to prevent false positive detections when the user is simply holding the phone in their hand or even during normal use of the touchscreen. Similarly proximity should not be detected when the phone is laying face up on a surface such as a table.

An alternative to IR based sensors is to use ultrasound. Similarly to IR-based sensors, the underlying principle is the active transmission of ultrasonic signals and the reception of signals reflected by objects in the vicinity of the device. An important advantage of ultrasound-based detection is that existing audio components can be leveraged obviating the need for additional hardware ultrasonic signals are not audible to the user and so they can be superimposed on the regular audio signals used during phone calls. Specifically the ear-piece speaker may be used as a transmitter while most ordinary microelectromechanical systems (MEMS) microphones can function as ultrasound receivers. This reduces

-2cost and simplifies the industrial design process. In order to prevent false positive detections when the user is operating the touchscreen and/or holding the phone, a microphone close to the ear-piece speaker, e.g. directly next to the ear-piece or on the top edge of the phone, is utilised.

Ultrasonic proximity detection operates on the premise that received signals that have been reflected by a nearby object (or reflector) will have a detectable difference when compared to a signal received that has no components that are due to any nearby objects. A conventional yet basic approach is to compare the received signal against a reference signal which resembles the received signal in the absence of a nearby reflector. However, the relative change in the received signal when a reflector is present is typically small since a direct signal component between the speaker and microphone (i.e. signals that travel directly from the transmitter to the receiver without being reflected by any external reflector) tends to dominate the echo signal component of the received signal (i.e. signals that travel indirectly from the transmitter to the receiver after being reflected by an external reflector). A reference signal that accurately matches the direct signal is therefore critical to ensure correct proximity detections.

The reference signal can either be fixed or updated dynamically during operation of the device. However, since the background signal cannot be expected to be constant over time, it is not considered sufficient to rely on a single fixed reference signal. A dynamic reference signal can adapt to changes in the response of both the microphone and the speaker. Such changes can occur gradually due to mechanical wear or degradation of the components or on a shorter time scale due to changes in temperature, ambient pressure and other environmental factors. However, reliable estimation can only occur when the background signal (i.e. the combination of the direct signal and any unwanted ambient signals) is observed directly without reflectors in close proximity to the speaker-microphone arrangement. When the device is covered, changes in the background signal cannot be discerned from changes in the echo signal. Thus the reference signal should not be updated when the phone is held up to the user’s head during a call or is in the user’s pocket. This can cause the reference to become outdated if the phone is covered for a long time or if the environmental conditions change abruptly.

- 3The reference signal is also likely to be inaccurate immediately after the proximity detection function has been initiated.

When viewed from a first aspect, the present invention provides a method for determining proximity of an external object to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; comparing the ultrasonic response signal to a library of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determining that said external object is proximate to the device if a difference between the ultrasonic response signal and each of the library of stored reference signals exceeds a threshold.

This first aspect of the invention extends to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; compare the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determine that an external object is proximate to the device if a difference between the ultrasonic response signal and each of the plurality of stored reference signals exceeds a threshold.

The first aspect of the invention further extends to a non-transitory, computerreadable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal;

- 4compares the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determines that an external object is proximate to the device if a difference between the ultrasonic response signal and each of the plurality of stored reference signals exceeds a threshold.

Thus it will be appreciated that in accordance with the invention an ultrasonic response signal is compared against a library of stored reference signals rather than just a single one. This may be advantageous as the device does not therefore need to rely on a single reference signal to be updated. While large variations in the background signal may be observed, the Applicant has appreciated that the direct signal component typically varies in a repeatable and consistent manner in response to particular environmental conditions (such as temperature and pressure). Using a number of stored (or viable) reference signals that are known a priori and which correspond to the device being unobstructed by any external objects under a variety of conditions may greatly aid proximity detection. If there is no sufficiently close match between the received response signal and any of the stored reference signals, it can be determined with reasonable confidence that an external object is proximate to the device.

It will of course be appreciated that in practice, the signals that are received even when no external object is proximate to the device may not match any of the stored reference signals exactly, and the difference between the received signal and the stored reference signals should exceed a particular threshold in order to make the determination regarding proximity.

While the number of viable direct signals is potentially infinite, the Applicant has appreciated that they can be well approximated using only a relatively small finite number of signals. In particular, a relatively small set of reference signals can represent all of those likely to be encountered in practice with a tolerable error margin. It will be appreciated by those skilled in the art that the term tolerable as used herein should be understood to mean that the difference between an ultrasonic response signal and the closest stored reference signal is smaller than echoes from nearby objects.

- 5The comparison of the received ultrasonic response signal and the library of stored reference signals to determine whether or not there is any significant match may be achieved using any suitable metric known in the art perse. Such a metric may be applied to the signals in the time-domain or, alternatively, in the frequencydomain after computing the Fourier transform, performing pulse compression or applying any suitable transform. The difference metric may be applied to the signals themselves or to feature vectors derived from the signals, wherein the feature vectors have fewer degrees of freedom associated therewith. It will be of course be appreciated that any such transforms and/or feature vector calculations may form part of the metric. For example, a Euclidean distance approach, known in the art perse, may be taken in order to generate a total difference score between the ultrasonic response signal and each of the library of stored reference signals, and any score below a particular threshold may be deemed a match.

An alternative to using echo-free background references is to use reference signals corresponding to various reflection scenarios. That is to say that proximity may be detected directly rather than being detected indirectly by testing for the absence of proximity as in the first aspect of the present invention. Thus when viewed from a second aspect, the present invention provides a method for determining proximity of an external object to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; comparing the ultrasonic response signal to a library of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and determining that said external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the library of stored reference signals is below a threshold.

- 6This second aspect of the invention extends to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; compare the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and determine that an external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the plurality of stored reference signals is below a threshold.

The second aspect of the invention further extends to a non-transitory, computerreadable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal; compares the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and determines that an external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the plurality of stored reference signals is below a threshold.

This approach may in some circumstances be able to separate between various types of proximity events such as the device being held to the user’s head, the device lying in a pocket or that the device is lying face down on a table, etc. This would allow the device to respond differently in each case.

It will be appreciated by those skilled in the art that the use of stored reference signals can greatly aid proximity detection if an accurate set of reference signals can be obtained. For example where the reference signals represent background

- 7signals, as in accordance with the first aspect of the invention, the viable background signals that can be encountered are largely unique to each device and will also to some extent change over its lifetime. The Applicant has appreciated that it is advantageous to utilise a continual process in which background references are captured and stored over the lifetime of the device. Similar considerations apply where reference signals representing reflections are used. Thus, in at least some embodiments of either aspect of the invention, the stored reference signals are updated. The library of stored reference signals may be an ever-increasing set (within the limits of the device's memory); replaced as an entire set periodicially; replaced on a first-in-first-out (FIFO) basis; or any other such method of updating the library.

In accordance with aspects of the invention a determination of proximity, or presence of an object proximate to the device, is made. It will be appreciated by those skilled in the art that this relates to the presence or absence of an object which is essentially static relative to the device such as a user’s head or a bag or pocket, Whilst such objects may exhibit small, involuntary movements (e.g. significantly less than the dimensions of the device) such movements do not affect the fact that the object is proximate to the device. This is to be contrasted with gesture detection which relies on detection of predetermined movements of an object (typically a user's hand).

In order for the stored reference signals to be useful, they should be captured when there are no reflectors proximate to the device (where background reference signals are used) or when there are reflectors proximate (where proximity references are used). However, this creates a circular problem - as the purpose of the reference signals is to detect proximity, it is difficult to know when it is appropriate to capture such reference signals. The Applicant has appreciated that it is advantageous to have an independent mechanism that helps to ensure that the device is in a proximity or proximity free state (as appropriate) before acquiring a new reference signal. Accordingly, in some embodiments of either of the foregoing aspects, the method further comprises:

determining that the device is in a predetermined state;

transmitting from the ultrasonic transmitter an ultrasonic test probe signal;

receiving at the ultrasonic receiver an ultrasonic test response signal; and

- 8storing said ultrasonic test response signal as one of said library of stored reference signals.

This is novel and inventive in its own right and thus when viewed from a third aspect, the present invention provides a method for calibrating an electronic device having a proximity detector comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

determining that the device is in a predetermined state; transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; and storing said ultrasonic response signal in a library of stored reference signals.

The third aspect of the invention extends to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

determining that the device is in a predetermined state; transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; and store said ultrasonic response signal in a library of stored reference signals.

This third aspect of the invention further extends to a non-transitory, computerreadable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

determines that the device is in a predetermined state; transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal; and stores said ultrasonic response signal in a library of stored reference signals.

Those skilled in the art will appreciate that a method in accordance with this third aspect of the invention may be used to acquire reference signals to be used to confirm either the presence or absence of external objects proximate to the device. In a set of embodiments the predetermined state comprises a state in which it is

- 9determined that no external object is proximate to the device. It will also be appreciated by those skilled in the art that the library of stored reference signals acquired in accordance with this third aspect may have further uses beyond confirming the presence or absence of external objects proximate to the device by direct comparison as described with reference to the foregoing first and second aspects. By way of non-limiting example only, the library of stored reference signals may instead be used to train a parameterised model of the transmitter-receiver response of a device which may provide a full set of viable reference signals. Alternatively, the library of stored reference signals may be interpolated in order to obtain an estimate of a wider set of reference signals than those captured directly.

Where the library of stored reference signals represent those where a reflector is nearby, the step of determining that the device is in the predetermined state comprises the device being used for a telephone call. In some potentially overlapping embodiments, the step of determining that the device is in the predetermined state comprises the device determining from a camera that a light level has suddenly fallen below a threshold. This may indicate to the device that it has been placed next to a user's head or in a pocket and would be a good time to obtain a new reference signal.

In an alternative set of embodiments the step of determining that the device is in the predetermined state comprises determining that no external objects are proximate to the device.

In some embodiments of any of the foregoing aspects, the library of stored reference signals is used to normalise a transfer function between the ultrasonic transmitter and the ultrasonic receiver. In some such embodiments, the magnitude of the stored reference signals may be normalised and subsequently used to normalise received ultrasonic signals such that consistent thresholds can be used when making comparisons of received signals to the stored reference signals, e.g. when determining whether or not an external object is proximate to the device in accordance with either of the first and second aspects respectively.

- 10More generally it has been appreciated that determining a transfer function between the ultrasonic transmitter and the ultrasonic receiver when the device is in a proximity-free state is novel and inventive in its own right and thus when viewed from a fourth aspect the invention provides a method for calibrating an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

determining that no external objects are proximate to the device; transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; determining at least a partial transfer function between said ultrasonic transmitter and receiver using said ultrasonic probe and response signals.

Transmitting a probe signal when the device in a proximity free state gives an estimate of the direct path signal between the transmitter and receiver as there are no strong reflections potentially interfering with the direct path signal. This can therefore provide an indication of the transfer function between them.

As used herein the transfer function should be understood as the input-output relationship between the transmitter and receiver. A full transfer function T will give an output signal r for any input signal s (i.e. r = T(s)). In accordance with the present invention it is possible to determine at most a partial transfer function for the same set of frequencies as contained in the probe signal. In particular the ultrasonic probe signal cannot give information about frequencies in the audible range. However this does mean that some probe signals will give more information than others. For example a chirp signal will theoretically give an input-output relationship for all signals in the frequency range of the chirp

In a set of embodiments, the fourth aspect of the invention comprises storing calibration information, wherein said calibration information depends on the transfer function. This means that the direct path signal may be used to calibrate the transmitter/receiver pair for subsequent use in proximity or gesture sensing. Indeed as used herein the term ‘calibration’ should be understood to mean any change, adjustment or update that occurs after production of the device to an algorithm used to detect proximity of an object to the device or to detect a gesture performed by an input object. This would include adding reference signals, adjusting transmit power,

- 11 adjusting internal parameters and adjusting the transmit signal definition (e.g. setting the frequency)

The purpose of the calibration is, typically, to ensure that the transmitter and receiver perform close to their intended specification in spite of individual differences between transmitters and receivers (for example making sure that the maximum detection range is similar).

The advantage of using the direct-path signal to calibrate the device is that it can be re-measured over time and that changes are mostly due to the transmitter and receiver itself. In contrast, for received echoic signals it is difficult to distinguish changes in the transmitter and receiver response from changes in the configuration of reflecting objects.

As mentioned above, in some embodiments, the calibration information may be used to vary a transmission power of the ultrasonic signal transmitted for proximity or gesture sensing. The adjustment of the transmission power may, in some arrangements, provide a mechanism for maintaining the amplitude of signals that arrive at the receiver via the direct signal path at a substantially constant value. Varying the transmission power in this way may also advantageously ensure that the ultrasonic transmitter is not transmitting signals at a higher power than strictly necessary, thus reducing power consumption. Alternatively the transmitted signal could be kept at a fixed amplitude and the calibration used to adjust the amplification applied to the received signals and/or to adjust the detection threshold used in the proximity or gesture sensing algorithm.

It will be appreciated that using estimates corresponding to signals having travelled via such a direct signal path in order to calibrate the device may require a high degree of correlation between the direct signal travelling from the ultrasonic transmitter to the ultrasonic receiver and the average strength of the ultrasonic response signals (i.e. any reflected or echo signals). This is not necessarily the case if the ultrasonic transmitter has a highly irregular transmit pattern or if the transmitter and receiver are collocated (e.g. are located in a common cavity).

There may be internal reflections that cancel or amplify the direct signal at certain frequencies which can cause the received echo signal to be strong while the direct

- 12path signal is comparably weak or vice versa. When the direct path signal does not provide a desirable indication of the average strength of the received response signals, it may be necessary to estimate the average strength (or a certain percentile of the strength) directly in order to calibrate the device. However, this typically requires a long record of historical data to give reliable estimates and often results in poor accuracy.

In some potentially overlapping embodiments, determining the transfer function comprises comparing amplitudes of a plurality of frequency components of the ultrasonic response signal. The relative amplitudes of the plurality of frequency components of the ultrasonic response signal (i.e. the spectral amplitudes of the echo) may be compared to one another and/or to amplitudes of a corresponding plurality of frequency components of the ultrasonic probe signal which may be known a priori. The Applicant has appreciated that comparing the relative strengths of different frequency components in the received response signal may require less historic data than determining the absolute spectral amplitudes. While the absolute spectral amplitudes of the received response signal will depend heavily on the distance to and the size of any reflector, the frequency composition of the received signal will typically be affected to a lesser degree. In some embodiments, the ultrasonic probe signal comprises a multi-frequency probe signal. By transmitting a multi-frequency probe signal (i.e. an ultrasonic probe signal having multiple ultrasonic frequency components), the most sensitive frequency, frequencies, or range of frequencies may be identified.

In the simplest case of a proximity of gesture detection system that uses a single frequency signal, it may be important to identify the most suitable frequency to use. This may simply be the frequency that results in the strongest received echo signal, however other parameters such as the noise level may also have a significant impact on performance. A quantity of particular interest is the direct path signal level. The direct signal will often be purposefully removed for analysis, for example by subtracting the response signal previously captured in the predetermined state. However, an error resulting from a difference between the expected and the actual signals received via the direct path may be strongly correlated with the overall level of the signal. It is therefore preferable to select a frequency that balances a strong echo signal level with a weak direct signal level with a view to optimising

- 13performance. The optimal frequency will usually be slightly different between individual devices due to process variations and may also change over time, e.g. due to wear and tear, temperature, etc. It may therefore be advantageous to reselect the operating frequency multiple times over the life time of the device.

In some embodiments of any of the foregoing aspects, determining that no external objects are proximate to the device comprises instructing a user to ensure that the device is in said predetermined state. In embodiments where the library of stored reference signals represent those without a nearby reflector, this may comprise instructing the user to remove any external objects from the vicinity of the device.

In contrast, where the library of stored reference signals represent those with a nearby reflector, this may instead comprise instructing the user to place the device next to their head or in their pocket. This may, for example, be carried out by providing the user with a visual warning on a display of the device or an audible message produced by a speaker of the device.

However, requiring user interaction for the acquisition of reference signals may not provide a desirable user experience and thus in at least some preferred embodiments, the step of determining that the device is in the predetermined state is performed automatically. In some such embodiments where the library of stored reference signals represent those without a nearby reflector, the step of determining that the device is in the predetermined state comprises the device being changed between a locked state and an unlocked state. When the device is unlocked (i.e. brought from a dormant locked state in which many or all of the functions of the device are disabled to an unlocked state in which these functions are enabled), for example when a user inputs a passcode or presents their fingerprint, it is typically the case that the device will be in the user's hand and free of obstructions proximate to the transmitter and receiver. As such, it can be deduced that it is a suitable time to obtain a new reference signal.

In some potentially overlapping embodiments, the step of determining that the device is in the predetermined state comprises determining that a particular button provided on the device has been pressed. This may, for example, be a home button on a smartphone that returns the user to the main menu or home screen of the smartphone.

- 14ln some further potentially overlapping embodiments, the step of determining that the device is in the predetermined state comprises determining that an application has been launched on the device. Launching an application on a device may involve pressing an icon on a touchscreen of the device, from which the device may infer that it is in the hand of the user who is looking at the screen rather than holding it up to their head. This could be triggered by launching any application or only one or more particular applications.

The Applicant has appreciated that there are other events that are indicative of the device being in a proximity free state and that one or more of these may be used when determining that the device is in such a state. While these may be relied on with a certain degree of confidence, in some embodiments the step of determining that the device is in the predetermined state comprises determining an orientation of the device from an accelerometer and/or gyroscope and determining that the device is in said state only if said orientation of the device is a given range. This advantageously permits the acquisition of new reference signals only if the device is oriented in a particular manner. By way of non-limiting example, it may be determined that if the device is upside-down, it is likely to be in a user's pocket and thus a reference signal should not be acquired at that time.

Transmitting a probe signal when the device in a proximity free state gives an estimate of the direct path signal between the transmitter and receiver. Measuring the direct path signal is a practical and efficient way to track changes in the speaker/microphone response over time or to make comparisons between different devices. This is in contrast to echo signals which depend on the exact configuration of the reflecting objects which is hard to repeat at a later time.

Transmitting an ultrasonic probe signal when there is no external object proximate to the device (i.e. a proximity free state) does not have to be used for calibration. It may also be used to identify potential faults associated with the ultrasonic transmitter or receiver. An unusually weak or strong signal, or the presence of Total Harmonic Distortion (THD), when the device is determined to be in a proximity free state may be indicative of the transmitter or receiver being damaged. If it is determined that the transmitter and/or receiver is damaged, the device may prompt

- 15a user to service the device. Information regarding the occurrence of such faults may be stored for later use, e.g. by the manufacturer or a service centre to gather information about the life-time performance of the components.

In light of the above the Applicant has appreciated that a device which is able to transmit a probe signal in a state when there is a good chance that there is no external object proximate to the device is more generally novel and inventive in its own right. Thus when viewed from a fifth aspect the invention provides an electronic device comprising at least one ultrasonic transmitter, at least one ultrasonic receiver and a touch-sensitive display screen, the device being programmed: detect a predetermined touch event, trigger a transmission of an ultrasonic probe signal from the ultrasonic transmitter in response to said predetermined touch event; and receive an ultrasonic response signal derived from said ultrasonic probe signal at the ultrasonic receiver.

The predetermined touch event is conveniently one which indicates that there is a good chance that there is no external object proximate to the device. As set out above in accordance with the previous aspects of the invention, in a set of embodiments the predetermined touch event comprises one which is used to unlock the device from a secure state or activate it from a low power state. In other embodiments it could comprise pressing a button or launching an application as previously discussed. In a set of embodiments the predetermined touch event comprises one or more selected from the group including: pressing a button on the device; inputting a passcode or unlock pattern, reading a fingerprint or launching an application. The predetermined touch event could be detected by the touchsensitive display screen or by a separate button or touch-sensitive zone.

Certain embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figs. 1A-C show a conventional infrared proximity sensor and a typical application;

Fig. 2 shows a conventional ultrasonic proximity sensor that utilises the earpiece and microphone of a smartphone;

- 16Fig. 3 shows an ultrasonic proximity sensor in accordance with an embodiment of the present invention wherein an ultrasonic transmitter and receiver pair are disposed in a recessed cavity;

Fig. 4 illustrates a method of detecting proximity in accordance with an embodiment of the present invention;

Fig. 5 further illustrates the method shown in Fig. 4 when an object is proximate to the device;

Figs. 6A-D show a method of acquiring reference signals by requesting a user removes all objects from the vicinity of a smartphone;

Figs. 7A-C show a method of acquiring reference signals automatically upon a user pressing a button on a smartphone;

Figs. 8A-C show a method of acquiring reference signals automatically upon a user launching an application on a smartphone; and

Fig. 9 illustrates a further method of detecting proximity in accordance with another embodiment of the present invention.

Figs. 1A-C illustrates a typical application of a conventional infrared proximity sensor 8 provided on a smart phone 2. The smart phone 2 comprises a touch screen display 4 and an infrared proximity sensor 8 that is arranged to deactivate the display 4 if the smart phone 2 is, for example, held next to a user’s head 12.

The infrared proximity sensor 8 is arranged to transmit and receive infrared (IR) signals. If an external object such as the user’s head 12 is in the vicinity of the sensor, the transmitted IR signals are partly reflected back towards the sensor 8. If the intensity of the received reflected signal is above a particular threshold, the smart phone 2 determines that an object is nearby and dims or disables the display 4.

Fig. 1A shows the smart phone 2 in the user’s hand 6 as the smart phone 2 receives a call as shown by the display 4. The user stretches his thumb 10 to press an answer button 13 on the display 4 in order to accept the call. It should be noted that, while the user’s thumb 10 partly obstructs the front of the smart phone 2, this does not cause the proximity sensor 8 to disable the display 4.

Fig. 1B illustrates the smart phone 2 during the call after it has been answered.

The smart phone 2 is being held in the palm of the user’s hand 6. As the front of

- 17the smart phone 2 is relatively unobstructed in this case, the proximity sensor 8 determines that no object such as the user’s head 12 is proximate to the smart phone 2 and the display 4 remains enabled. However, as shown in Fig. 1C, when the smart phone 2 is brought to the side of the user’s head 12, the proximity sensor 8 determines that the reflected infrared signals that it receives exceed a predetermined threshold and thus disables the display 4 as shown by the cross hatching.

Fig. 2 shows an alternative proximity sensor arrangement that utilises ultrasound instead of IR. In this case the ear piece 14 and the microphone 20 that are typically used for listening and speaking during a call are utilised to transmit and receive ultrasonic probe signals 16 and ultrasonic response signals 18 respectively. The smart phone 2 can determine from the received response signals 18 whether an object such as the user’s head 12 is proximate to the smart phone 2. This may be achieved using, for example, the round-trip time-of-flight (TOF) of the ultrasonic signals, the amplitude of the received response signals 18, the spectral profiles of the received signals 18 or any other such metric known in the art perse.

Rather than having the ultrasonic transmitter 14 and the ultrasonic receiver 20 disposed at different positions on the smart phone 102, Fig. 3 shows an alternative arrangement in which the ultrasonic transmitter 14 and the ultrasonic receiver 20 are disposed within a common cavity 24 recessed from a front surface 26 of the smart phone 102.

The smart phone 102 is arranged such that ultrasonic signals transmitted by the transmitter 14 are reflected by an object such as the user’s head 12 and are received by the receiver 20, shown by the indirect path 28. It will be appreciated that there will likely be a number of possible such paths, but only one is shown in Fig. 3 for the sake of clarity.

It can also be seen from Fig. 3 that some signals simply travel directly from the transmitter 14 to the receiver 20 as shown by the direct path 30. These direct signals cause problems as they are difficult to distinguish from the signals that have travelled via the indirect path 28, particularly as they are likely to be stronger than the reflected signals since they have propagated a shorter distance and have not

- 18undergone any absorption as a result of (typically imperfect) reflection. As will be explained below however, a more accurate determination of proximity can be achieved using a library of reference signals.

Fig.4 illustrates a method for determining whether or not an object is proximate to a device such as a smart phone 102 in accordance with an embodiment of the present invention. Here the smart phone 102 includes within its internal memory a library 32 of reference spectra 34a-c. These reference spectra 34a-c correspond to the frequency spectra of the ultrasonic response signals that would typically be obtained if there were no objects in the vicinity of the smart phone 102 under a variety of different environmental conditions such as temperature, local pressure and other environmental factors. The acquisition of these reference spectra 34a-c will be described in further detail below with reference to Figs. 6-8. When an ultrasonic response signal 18 is received by the ultrasonic receiver 20, a received frequency spectrum 36 is generated which corresponds to the amplitudes of various frequency components within the ultrasonic response signal 18. This received frequency spectrum 36 can then be compared to the reference spectra 34a-c to determine whether or not an external object such as the user’s head 12 is proximate to the smart phone 102.

It can be seen that the received frequency spectrum 36 does not closely match either of the first two reference spectra 34a and 34b, however, the received frequency spectrum 36 is a relatively close match to the third reference spectrum 34c. This allows the smart phone 102 to determine that it is unlikely that there are any external objects, such as the user’s head 12, proximate to the smart phone 102 with a reasonable degree of certainty.

By way of contrast, Fig. 5 shows a received frequency spectrum 38 corresponding to a received ultrasonic response signal 18 that has been reflected by an external object such as the user’s head 12. It can clearly be seen that this received frequency spectrum 38 does not match any of the reference spectra 34a-c and thus does not correspond to any of the viable scenarios in which there is no object proximate to the smart phone 102. Thus the smart phone 102 determines that it is not currently in a proximity free state and may dim or disable the touch screen

- 19display 4 on the assumption that the touch screen 4 is not needed at this time, e.g. because the user has placed the smart phone 102 next to his head 12.

Fig. 6A-D show a simple method for obtaining the reference spectra as described previously. In Fig. 6A the smart phone 102, held in the palm of the user's hand 7, displays a notification message 42 on the touch screen 4. This notification message 42 may instruct the user to remove any obstructions from the vicinity of the smart phone 102 in order to establish an obstruction free zone 44 as shown in Fig. 6B. The message 42 may indicate to the user that they may keep the smartphone 102 held in the palm of their hand 7 during the acquisition of the reference signals, as it is likely that the smartphone 102 will be held in such a manner during regular operation. It will of course be appreciated that the smartphone 102 may instead be placed on a different surface such as a desk while acquiring the reference spectra. Once the smart phone 102 is satisfied that there are no external objects in the vicinity of the smart phone 102, for example after a predetermined time or user confirmation, the proximity sensor 46 transmits an ultrasonic probe signal 48 as shown in Fig. 6C.

Objects in room and walls etc. will reflect the ultrasonic probe signal 48 and the proximity sensor 46 may potentially receive a number of ultrasonic reflected signals 50. The smart phone 102 can store these received signals 50 as reference signals (either directly or as a parameterised characteristic of the reference signals such as a spectral profile of frequency components) for comparison when performing the proximity detection method described previously with reference to Figs. 4 and 5.

While asking the user to remove any obstruction from the immediate vicinity of the smart phone 102 is simple, the applicant has appreciated that this does not necessarily lead to an optimal user experience. Figs. 7A-C illustrate an alternative method for obtaining reference signals that requires no such user intervention. In Fig. 7A, the user places his hand 6 such that his index finger 52 presses a home button 54 provided on the smart phone 102, which is being held in the palm of the user's other hand 7. The pressing of this home button 54 may for example return the user to a main menu of the smart phone 102. The home button 54 may also be equipped with a fingerprint scanner arranged to identify whether the user is authorised to access the smart phone 102.

- 20The smart phone 102 assumes from the pressing of this home button 54 that it is unlikely that the smart phone 102 is currently being held against the user’s head 12 and is instead likely to be held in one of the user’s hands 6, 7. The proximity sensor 46 is instructed to transmit an ultrasonic probe signal 48 accordingly as shown in Fig. 7B and to receive ultrasonic reflected signals 50 as shown in Fig. 7C and as described previously with reference to Figs. 6B and 6C respectively. This provides the user with a more ‘seamless’ user experience in which the proximity sensor 46 simply functions without requiring any intervention.

Figs. 8A-C show a further method by which the smart phone 102 may ensure with a reasonable degree of confidence that no objects are in its immediate vicinity.

Fig. 8A shows the user placing his hand 6 such that his index finger 52 presses an application icon 56 on the main menu in order to load a particular application such as a messaging application or a calendar application. Similarly to the case described with reference to Fig. 7A, the smart phone 102 can be reasonably sure that it is not being held up to the user’s head 12 or placed in a pocket and is instead likely to be held in one of the user’s hands, e.g. the palm of the user's other hand 7. The smart phone 102 thus instructs the proximity sensor 46 to transmit an ultrasonic probe signal 48 as shown in Fig. 8B and as described previously. The proximity sensor 46 then receives ultrasonic reflected signals 50 as shown in Fig. 8C, and these ultrasonic reflected signals 50 may be used to generate new reference signals to be stored in the library 32.

Fig. 9 shows an alternative embodiment of the present invention in which proximity is detected directly rather than indirectly by checking for the absence of a proximity free state as described herein before. In Fig. 9 the library 32’ is arranged to store a number of reference spectra 34a’-c’ that correspond to ultrasonic response signals typical of different situations wherein an external object is placed proximate to the smart phone 102. An incoming ultrasonic response signal may have its spectrum 38’ compared to each of the reference spectra 34a’-c’ in order to determine whether it matches a known proximity state. For example in Fig. 9, the received frequency spectrum 38’ is a close match to the second reference spectrum 34b’ and this may by way of example indicate that the smart phone 102 is being held between the shoulder and cheek of the user.

- 21 In order to evaluate the response for different frequencies, a multi-frequency probe signal may be used. This may, by way of example only, be a sinusoidal sweep starting at a lower frequency A and ending at an upper frequency /₂ in a given time span T and with a given amplitude a such that:

β - f.

p(t) = a sin(2nf₁t -I--——t² ) , 0 < t < T.

By computing the Fourier Transform of the received response signal, the relative strengths of the various frequency components may be identified. To evaluate any signals having travelled via a direct path, there should be no close reflectors to the transmitter and receiver when the probe signal is transmitted. As described with reference to Figs. 7A -C, for example this may be ensured with high probability if the probe signal is transmitted when the user presses a particular button (e.g. a home button) on the device. Similarly, by transmitting a probe signal when an external object is proximate to the device as described with reference to Figs. 8A C, for example, the frequency composition of the received response signal may be identified.

It should be noted that if only one measurement is relied upon certain frequencies can be amplified or cancelled due to the particular shape of a given external object. Several different measurements taken at different times may be averaged in order to obtain a more accurate result.

If %(/) denotes the Fourier transform of the average echo signal and X_d(f) the frequency transform of direct path signal, the operating frequency /* can be chosen maximize the ratio the difference |X_e(/)|² - 1/0,(/)1² , or some other \^xd(f )1 suitable combination of//,(/) and X_d(f). In some embodiments, a noise and/or an interference level is used to determine an operating frequency to be used when transmitting the ultrasonic probe signal.

Thus it will be seen the embodiments of the present invention provide an improved method for proximity detection that does not rely on updating a single reference signal but instead makes use of an array of reference signals typical of different

-22operating conditions. It will be appreciated by those skilled in the art that the embodiments described above are merely exemplary and are not limiting on the scope of the invention.

Claims

Claims

1. A method for determining proximity of an external object to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; comparing the ultrasonic response signal to a library of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determining that said external object is proximate to the device if a difference between the ultrasonic response signal and each of the library of stored reference signals exceeds a threshold.
2. The method as claimed in claim 1, comprising using the library of stored reference signals to normalise a transfer function between the ultrasonic transmitter and the ultrasonic receiver.
3. The method as claimed in claim 1 or 2, further comprising: normalising the magnitude of the stored reference signals; and using the normalised magnitude of the stored reference signals to normalise received ultrasonic signals such that consistent thresholds are used when making comparisons of received signals to the stored reference signals.
4. The method as claimed in any preceding claim, further comprising: determining that the device is in a predetermined state;

transmitting from the ultrasonic transmitter an ultrasonic test probe signal; receiving at the ultrasonic receiver an ultrasonic test response signal; and storing said ultrasonic test response signal as one of said library of stored reference signals.
5. The method as claimed in claim 4, comprising instructing a user to ensure that the device is in said predetermined state.

- 246. The method as claimed in claim 4, wherein the step of determining that the device is in the predetermined state is performed automatically.
7. The method as claimed in claim 6, wherein the step of determining that the device is in the predetermined state comprises the device being changed between a locked state and an unlocked state.
8. The method as claimed in claim 6 or 7, wherein the step of determining that the device is in the predetermined state comprises determining that a particular button provided on the device has been pressed.
9. The method as claimed in any of claims 6 to 8, wherein the step of determining that the device is in the predetermined state comprises determining that an application has been launched on the device.
10. The method as claimed in any of claims 6 to 9, wherein the step of determining that the device is in the predetermined state comprises determining an orientation of the device from an accelerometer and/or gyroscope and determining that the device is in said state only if said orientation of the device is a given range.
11. An electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; compare the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determine that an external object is proximate to the device if a difference between the ultrasonic response signal and each of the plurality of stored reference signals exceeds a threshold.
12. A non-transitory, computer-readable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

- 25transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal; compares the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to absence of any external object proximate to the device; and determines that an external object is proximate to the device if a difference between the ultrasonic response signal and each of the plurality of stored reference signals exceeds a threshold.
13. A method for determining proximity of an external object to an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; comparing the ultrasonic response signal to a library of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and determining that said external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the library of stored reference signals is below a threshold.
14. The method as claimed in claim 13, comprising using the library of stored reference signals to normalise a transfer function between the ultrasonic transmitter and the ultrasonic receiver.
15. The method as claimed in claim 13 or 14, further comprising: normalising the magnitude of the stored reference signals; and using the normalised magnitude of the stored reference signals to normalise received ultrasonic signals such that consistent thresholds are used when making comparisons of received signals to the stored reference signals.
16. The method as claimed in any of claims 13 to 15, further comprising: determining that the device is in a predetermined state;

transmitting from the ultrasonic transmitter an ultrasonic test probe signal;

- 26receiving at the ultrasonic receiver an ultrasonic test response signal; and storing said ultrasonic test response signal as one of said library of stored reference signals.
17. The method as claimed in claim 16, comprising instructing a user to ensure that the device is in said predetermined state.
18. The method as claimed in claim 16, wherein the step of determining that the device is in the predetermined state is performed automatically.
19. The method as claimed in claim 18, wherein the step of determining that the device is in the predetermined state comprises the device being changed between a locked state and an unlocked state.
20. The method as claimed in claim 18 or 19, wherein the step of determining that the device is in the predetermined state comprises determining that a particular button provided on the device has been pressed.
21. The method as claimed in any of claims 18 to 20, wherein the step of determining that the device is in the predetermined state comprises determining that an application has been launched on the device.
22. The method as claimed in any of claims 18 to 21, wherein the step of determining that the device is in the predetermined state comprises determining an orientation of the device from an accelerometer and/or gyroscope and determining that the device is in said state only if said orientation of the device is a given range.
23. An electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; compare the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and

- 27determine that an external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the plurality of stored reference signals is below a threshold.
24. A non-transitory, computer-readable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal; compares the ultrasonic response signal to a plurality of stored reference signals, wherein said stored reference signals comprise ultrasonic response signals corresponding to presence of one or more external objects proximate to the device; and determines that an external object is proximate to the device if a difference between the ultrasonic response signal and at least one of the plurality of stored reference signals is below a threshold.
25. A method for calibrating an electronic device having a proximity detector comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

determining that the device is in a predetermined state; transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal; and storing said ultrasonic response signal in a library of stored reference signals.
26. The method as claimed in claim 25, wherein the step of determining that the device is in the predetermined state comprises the device being used for a telephone call.
27. The method as claimed in claim 25 or 26, wherein the step of determining that the device is in the predetermined state comprises the device determining from a camera that a light level has suddenly fallen below a threshold.

- 2828. The method as claimed in any of claims 25 to 27, wherein the step of determining that the device is in the predetermined state comprises determining that no external objects are proximate to the device.
29. The method as claimed in claim 28, wherein the step of determining that no external objects are proximate to the device comprises instructing a user to ensure that the device is in said predetermined state.
30. The method as claimed in claim 28, wherein the step of determining that the device is in the predetermined state is performed automatically.
31. The method as claimed in claim 30, wherein the step of determining that the device is in the predetermined state comprises the device being changed between a locked state and an unlocked state.
32. The method as claimed in claim 30 or 31, wherein the step of determining that the device is in the predetermined state comprises determining that a particular button provided on the device has been pressed.
33. The method as claimed in any of claims 30 to 32, wherein the step of determining that the device is in the predetermined state comprises determining that an application has been launched on the device.
34. The method as claimed in any of claims 30 to 33, wherein the step of determining that the device is in the predetermined state comprises determining an orientation of the device from an accelerometer and/or gyroscope and determining that the device is in said state only if said orientation of the device is a given range.
35. An electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the device being arranged to:

determining that the device is in a predetermined state; transmit from the ultrasonic transmitter an ultrasonic probe signal; receive at the ultrasonic receiver an ultrasonic response signal; and store said ultrasonic response signal in a library of stored reference signals.

- 2936. A non-transitory, computer-readable medium comprising instructions that, when executed by a suitable processor, cause the processor to operate an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, such that the device:

determines that the device is in a predetermined state; transmits from the ultrasonic transmitter an ultrasonic probe signal; receives at the ultrasonic receiver an ultrasonic response signal; and stores said ultrasonic response signal in a library of stored reference signals.
37. A method for calibrating an electronic device comprising at least one ultrasonic transmitter and at least one ultrasonic receiver, the method comprising:

determining that no external objects are proximate to the device; transmitting from the ultrasonic transmitter an ultrasonic probe signal; receiving at the ultrasonic receiver an ultrasonic response signal;

determining at least a partial transfer function between said ultrasonic transmitter and receiver using said ultrasonic probe and response signals.
38. The method as claimed in claim 37, comprising storing calibration information, wherein said calibration information depends on the transfer function.
39. The method as claimed in claim 37, comprising using the calibration information to vary a transmission power of the ultrasonic signal transmitted for proximity or gesture sensing.
40. The method as claimed in any of claims 37 to 39, wherein determining the transfer function comprises comparing amplitudes of a plurality of frequency components of the ultrasonic response signal.
41. The method as claimed in any of claims 37 to 40, comprising the ultrasonic probe signal comprises a multi-frequency probe signal.
42. The method as claimed in any of claims 37 to 41, comprising instructing a user to ensure that the device is in said predetermined state.

- 3043. The method as claimed in any of claims 37 to 41, wherein the step of determining that the device is in the predetermined state is performed automatically.
44. The method as claimed in claim 43, wherein the step of determining that the device is in the predetermined state comprises the device being changed between a locked state and an unlocked state.
45. The method as claimed in claim 43 or 44, wherein the step of determining that the device is in the predetermined state comprises determining that a particular button provided on the device has been pressed.
46. The method as claimed in any of claims 43 to 45, wherein the step of determining that the device is in the predetermined state comprises determining that an application has been launched on the device.
47. The method as claimed in any of claims 43 to 46, wherein the step of determining that the device is in the predetermined state comprises determining an orientation of the device from an accelerometer and/or gyroscope and determining that the device is in said state only if said orientation of the device is a given range.
48. An electronic device comprising at least one ultrasonic transmitter, at least one ultrasonic receiver and a touch-sensitive display screen, the device being programmed: detect a predetermined touch event, trigger a transmission of an ultrasonic probe signal from the ultrasonic transmitter in response to said predetermined touch event; and receive an ultrasonic response signal derived from said ultrasonic probe signal at the ultrasonic receiver.
49. The electronic device as claimed in claim 48, wherein the predetermined touch event comprises one which is used to unlock the device from a secure state or activate it from a low power state.
50. The electronic device as claimed in claim 48 or 49, wherein the predetermined touch event comprises one or more selected from the group

- 31 including: pressing a button on the device; inputting a passcode or unlock pattern, reading a fingerprint or launching an application.

Mr Euros Morris

1 May 2018