Ultrasonic transducers for position tracking

Ultrasonic transducers may be used to track the position of an object. For example, eyewear such as a pair of glasses or other head-mounted device may use ultrasonic transducers to track a user's eye position and gaze direction. To increase positioning accuracy over short-range distances, an array of ultrasonic transducers with different center frequencies may be formed on a common substrate. The closely spaced ultrasonic transducers with relatively small individual bandwidths may be used to simulate a single transducer with a larger bandwidth. Control circuitry may adjust the phases of the transducers in the array to steer the ultrasonic signal beam towards the user's eye and/or to receive an ultrasonic signal beam from the direction of the user's eye. The control circuitry may use time delay and/or phase delay measurement techniques to determine a distance and/or direction to the user's eye using the ultrasonic transducer arrays.

FIELD

This relates generally to wearable devices, and, more particularly, to wearable devices such as head-mounted devices and other eyewear.

BACKGROUND

Head-mounted devices and other eyewear may use gaze tracking circuitry to track a user's gaze.

It can be challenging to design gaze tracking circuitry that performs satisfactorily. If care is not taken, the gaze tracking circuitry may produce inaccurate measurements or may exhibit other performance limitations such as excessive power consumption.

SUMMARY

Ultrasonic transducers may be used to track the position of an object. For example, eyewear such as a pair of glasses or other head-mounted device may use ultrasonic transducers to track a user's eye position and gaze direction. To increase positioning accuracy over short-range distances, an array of ultrasonic transducers with different center frequencies may be formed on a common substrate. The closely spaced ultrasonic transducers with relatively small individual bandwidths may be used to simulate a single transducer with a larger bandwidth.

Control circuitry may adjust the phases and/or amplitudes of the transducers in the array to steer the ultrasonic signal beam towards the user's eye (and away from direct paths to receiving transducer arrays, if desired) and/or to receive a reflected ultrasonic signal beam from the direction of the user's eye. The control circuitry may use time delay measurement techniques, phase delay measurement techniques, and/or amplitude measurement techniques to determine a distance and/or direction to the user's eye using the ultrasonic transducer arrays.

The transducer arrays may include piezoelectric micromachined ultrasonic transducers, capacitive micromachined ultrasonic transducers, and/or other suitable type of ultrasonic transducers formed in a common substrate. The transducers may be provided with different center frequencies by forming cavities with different depths, cavities with different diameters, and/or cavities with different surface features.

Control circuitry may control each transducer in the array independently of one another, and/or the control circuitry may control subsets of transducers in the array (e.g., subsets with the same center frequency) independently of other sets of transducers in the array. One or more sets of transducers with the same center frequency may be placed adjacent to one another in the array to help disambiguate phase information. The remaining transducers in the array that are not used for phase disambiguation may be spaced farther apart from other transducers of the same center frequency.

DETAILED DESCRIPTION

Ultrasonic transducers may be used to track the position of one or more objects of interest. For example, eyewear such as a pair of glasses or other head-mounted device may use one or more ultrasonic transducers to track a user's eye position and gaze direction. The ultrasonic transducers may include capacitive micromachined ultrasonic transducers, piezoelectric micromachined transducers, and/or other suitable ultrasonic transducers for emitting and/or detecting acoustic signals. To increase positioning accuracy over short-range distances, multiple ultrasonic transducers with different center frequencies may be arranged in an array. An array of closely spaced ultrasonic transducers with relatively small individual bandwidths may be used to simulate a single transducer with a much larger bandwidth, without sacrificing performance. When the transducers in the array are properly phased, the array may be able to generate an ultrasonic signal pulse with a much shorter pulse length than that which could be produced with a single transducer alone. Being able to generate short ultrasonic signal pulses may be especially beneficial for determining short-range distances and for resolving two objects that are closely spaced together. For example, a phased transducer array may be able to resolve objects that are as close as one wavelength apart or other suitable distance.

If desired, the ultrasonic transducers with different center frequencies may be operated independently of one another to independently measure signals at different frequencies. The independently measured signals can then be used to estimate time of flight and path length to the desired level of accuracy.

In arrangements where the ultrasonic transducers are used for gaze tracking in a pair of glasses or other eyewear, multiple arrays of ultrasonic transducers may be distributed at different locations around each of the user's eyes. Each array may include ultrasonic transducers with different center frequencies. One or more of the arrays may emit ultrasonic signals towards the user's eye. The ultrasonic signals may reflect off of the user's eye and may be detected by one or more of the other arrays. Control circuitry may gather data from the transducer arrays to determine the user's eye position and/or gaze direction (e.g., using time delay measurement techniques, phase delay measurement techniques, amplitude measurement techniques, and/or other suitable measurement techniques).

An illustrative system having a device with one or more ultrasonic transducers is shown inFIG.1. System10may include a head-mounted device such as eyeglasses14(sometimes referred to as glasses14, head-mounted device14, eyewear14, etc.). Glasses14may include one or more optical systems such as adjustable lens components22mounted in a support structure such as support structure12. Structure12may have the shape of a pair of eyeglasses (e.g., supporting frames), may have the shape of goggles, may form a housing having a helmet shape, or may have other configurations to help in mounting and securing the components of glasses14on the head of a user.

Adjustable lens components22may form lenses that allow a viewer (e.g., a viewer having eyes16) to view external objects such as object18in the surrounding environment. Glasses14may include one or more adjustable lens components22, each aligned with a respective one of a user's eyes16. As an example, lens components22may include a left lens22aligned with a viewer's left eye and may include a right lens22aligned with a viewer's right eye. This is, however, merely illustrative. If desired, glasses14may include adjustable lens components22for a single eye.

Adjustable lenses22may be corrective lenses that correct for vision defects. For example, eyes16may have vision defects such as myopia, hyperopia, presbyopia, astigmatism, higher-order aberrations, and/or other vision defects. Corrective lenses such as lenses22may be configured to correct for these vision defects. Lenses22may be adjustable to accommodate users with different vision defects and/or to accommodate different focal ranges. For example, lenses22may have a first set of optical characteristics for a first user having a first prescription and a second set of optical characteristics for a second user having a second prescription. Glasses14may be used purely for vision correction (e.g., glasses14may be a pair of spectacles) or glasses14may include displays that display virtual reality or augmented reality content (e.g., glasses14may be a head-mounted display). In virtual reality or augmented reality systems, adjustable lens components22may be used to move content between focal planes from the perspective of the user. Arrangements in which glasses14are spectacles that do not include displays are sometimes described herein as an illustrative example.

Glasses14may include control circuitry26. Control circuitry26may include processing circuitry such as microprocessors, digital signal processors, microcontrollers, baseband processors, image processors, application-specific integrated circuits with processing circuitry, and/or other processing circuitry and may include random-access memory, read-only memory, flash storage, hard disk storage, and/or other storage (e.g., a non-transitory storage media for storing computer instructions for software that runs on control circuitry26).

Glasses14may include input-output circuitry such as eye state sensors, range finders disposed to measure the distance to external object18, touch sensors, buttons, microphones to gather voice input and other input, sensors, and other devices that gather input (e.g., user input from viewer16) and may include light-emitting diodes, displays, speakers, and other devices for providing output (e.g., output for viewer16). Glasses14may, if desired, include wireless circuitry and/or other circuitry to support communications with a computer or other external equipment.

Control circuitry26may also control the operation of optical elements such as adjustable lens components22. Adjustable lens components22, which may sometimes be referred to as adjustable lenses, adjustable lens systems, adjustable optical systems, adjustable lens devices, tunable lenses, etc., may include fluid-filled variable lenses, may include Alvarez lenses, and/or may contain electrically adjustable material such as liquid crystal material, volume Bragg gratings, or other electrically modulated material that may be adjusted to produce customized lenses. Each of components22may contain an array of electrodes that apply electric fields to portions of a layer of liquid crystal material or other voltage-modulated optical material with an electrically adjustable index of refraction (sometimes referred to as an adjustable lens power or adjustable phase profile). By adjusting the voltages of signals applied to the electrodes, the index of refraction profile of components22may be dynamically adjusted. This allows the size, shape, and location of the lenses formed within components22to be adjusted.

Glasses14may include gaze tracking circuitry such as gaze tracking circuitry20. Gaze tracking circuitry20may include one or more sensors for tracking a user's eyes16. For example, gaze tracking circuitry20may include one or more digital image sensors (e.g. visible image sensors and/or infrared image sensors that gather images of the user's eyes), ultrasonic sensors, light-based sensors such as lidar (light detection and ranging) sensors, and/or other suitable sensors for tracking the location of a user's eyes. As an example, gaze tracking circuitry20may be used by control circuitry26to gather eye information such as cornea location, pupil location, gaze direction, and/or other information about the eye(s) of the viewer. The locations of the viewer's pupils and the locations of the viewer's pupils relative to specular glints from light sources with known positions or the rest of the viewer's eyes may be used to determine the locations of the centers of the viewer's eyes (i.e., the centers of the user's pupils) and the direction of view (gaze direction) of the viewer's eyes. If desired, gaze tracking circuitry20may also include a wavefront sensor that measures the aberrations of a user's eyes so that control circuitry26can adjust the optical properties of lens component22to correct the user-specific aberrations detected by the wavefront sensor.

Gaze tracking information may be used as a form of user input, may be used to determine where, within an image, image content resolution should be locally enhanced in a foveated imaging system (e.g., in arrangements where device14is a head-mounted display), and/or may be used to determine where, within one or both of lenses22, vision correction should be locally enhanced in a foveated lens arrangement. In a foveated lens arrangement, control circuitry26may dynamically adjust lens components22so that the optical properties of portions of lens components22that align with a user's gaze are different than the optical properties of portions of lens components22that are outside of the user's gaze. For example, portions of lens components22that align with a user's gaze may be optically modulated to produce a first lens power, while the remaining portions of lens components22may be left optically unmodulated, may be optically modulated to produce a second lens power magnitude that is less than the first lens power magnitude, and/or may be optically modulated to produce a phase profile that is less spatially varied than the phase profile of portions of lens components22within the user's gaze. Control circuitry26may gather eye information from gaze tracking circuitry20and may adjust the optical properties of lens components22accordingly.

FIG.2is a top view of glasses14showing how gaze tracking circuitry may be used to track a user's gaze. As shown inFIG.2, gaze tracking circuitry20in glasses14may include one or more arrays24of ultrasonic transducers. Each array24may include multiple ultrasonic transducers38on a substrate such as substrate42. Ultrasonic transducers38within each array24may have different center frequencies such that, when used together, the array can collectively achieve the positioning accuracy of a wideband ultrasonic transducer without experiencing the reduction in quality factor that would normally result from a single broadband ultrasonic transducer. Arrays24may be mounted in support structure12and may be distributed at different locations around the user's eye16. There may be any suitable number of arrays24in glasses14(e.g., one, two, three, four, five, six, ten, fifteen, twenty, more than twenty, less than twenty, etc.), and each array24may include any suitable number of transducers38(e.g., five, eight, ten, fifteen, twenty, fifty, one hundred, two hundred, less than two hundred, more than two hundred, etc.).

During operation, one or more of arrays24such as array24A may be used to emit ultrasonic signals32. The ultrasonic signals32may reflect off of a user's eye16(e.g., may reflect off of cornea30of the user's eye16). One or more of arrays24such as array24B may be used to detect ultrasonic signals32after the signals reflect off of the user's cornea30. Using time-of-flight measurement techniques, control circuitry26may be used to determine the time that it takes for the emitted signal32to reflect back from eye16, which may in turn be used to determine the distance to eye16(e.g., the distance to the point of specular reflection on cornea30). As eye16rotates, control circuitry26may continue to monitor changes in distance to the point of specular reflection on the user's eye. For example, as the user's eye16moves in direction36, ultrasonic signals32′ from transducer array24A may reflect off of cornea30′ and may be detected by array24B. The time-of-flight of signals32′ may be different from the time-of-flight of signals32, due to the change in distance to the point of specular reflection of the user's eye16. Control circuitry26may monitor these changes in distance to determine the direction of the user's gaze during the operation of glasses14. If desired, the same array24may be used to emit and detect signals32. Arrangements in which multiple arrays24emit signals32and/or where multiple arrays24detect signals32may also be used. The example ofFIG.2is merely illustrative.

If desired, one or more of arrays24may be a phased transducer array. In a phased ultrasonic transducer array, beam steering techniques may be used in which ultrasonic signal phase and/or magnitude for each transducer in array24are adjusted to perform beam steering. Beam steering may be used to “illuminate” a particular area of interest with ultrasonic signals32. Beam steering may also be used to avoid illuminating certain areas with ultrasonic signals32(e.g., to avoid directly illuminating other arrays24and/or to avoid illuminating certain parts of the user's face). For example, a phased ultrasonic transducer array24may be configured to emit a concentrated beam of ultrasonic signals32that strikes cornea30but does not strike the user's eye brow. This type of beam steering arrangement may help improve gaze tracking accuracy by avoiding detecting significant reflections from surfaces around the user's eye16.

The use of time-of-flight based measurement techniques is merely illustrative. If desired, other time-based, amplitude-based, and/or phase-based measurement schemes such as time difference of arrival measurement techniques, angle of arrival measurement techniques, triangulation methods, and/or other suitable measurement techniques may be used to determine a location of the user's eye16using ultrasonic sensor arrays24. Arrangements in which other sensors such as visible light cameras, infrared light cameras, and/or proximity sensors (e.g., infrared proximity sensors or other proximity sensors) are used to gather eye location information, glint location information, gaze direction information, pupil shape information, and/or other eye information may also be used.

FIGS.3and4are cross-sectional side views of illustrative ultrasonic transducers of the type that may be included in gaze tracking circuitry20.

In the example ofFIG.3, ultrasonic transducer38is a capacitive micromachined ultrasonic transducer. Ultrasonic transducer38ofFIG.3includes a movable membrane such as membrane56, a substrate50(e.g., a semiconductor substrate such as a silicon substrate and/or any other suitable type of substrate), and sidewalls54that together define a cavity such as cavity52. A pair of capacitive electrodes such as first electrode46on membrane56and second electrode48on substrate50may be located on opposing sides of cavity52. When control circuitry26applies an alternating voltage across electrodes46and48, the resulting electrostatic force causes membrane56to oscillate up and down in directions44(see, e.g., membrane56oscillating between positions P1 and P2 inFIG.3), thereby generating an acoustic wave having a frequency that corresponds to the frequency at which membrane56oscillates. Transducer38may also be used to detect ultrasonic waves, if desired. In particular, incident ultrasonic waves may cause membrane56to oscillate, which in turn may cause a change in capacitance across electrodes46and48. Control circuitry26may measure this change in capacitance to determine the frequency, amplitude, and/or phase of the incident ultrasonic waves.

In the example ofFIG.4, ultrasonic transducer38is a piezoelectric micromachined ultrasonic transducer. Ultrasonic transducer38ofFIG.4includes a movable membrane such as membrane56, a substrate50(e.g., a semiconductor substrate such as a silicon substrate and/or any other suitable type of substrate), and sidewalls54that together define a cavity such as cavity52. In a piezoelectric transducer arrangement, membrane56is formed from or coupled to a piezoelectric material. A pair of electrodes such as first electrode46and second electrode48may be located on opposing sides of membrane56. Control circuitry26may apply an alternating voltage across electrodes46and48which excites the piezoelectric material in membrane56, causing membrane56to oscillate up and down in directions44(see, e.g., membrane56oscillating between positions P1 and P2 inFIG.4), thereby generating an acoustic wave having a frequency that corresponds to the frequency at which membrane56oscillates. Transducer38ofFIG.4may also be used to detect ultrasonic waves, if desired. In particular, incident ultrasonic waves may cause membrane56to oscillate, which in turn causes the piezoelectric material to generate charge. Control circuitry26may measure a corresponding change in voltage across electrodes46and48to determine the frequency, amplitude, and/or phase of the incident ultrasonic waves.

The examples ofFIGS.3and4are merely illustrative. If desired, arrays24may include other types of ultrasonic transducers and/or may have ultrasonic transducers with different features and/or different structures from the examples ofFIGS.3and4. For example, a piezoelectric transducer38may include more than two electrodes and more than one layer of piezoelectric material, if desired (e.g., transducer38may include two layers of piezoelectric material and three layers of electrodes, or may include any other suitable number of piezoelectric layers and electrode layers).

FIG.5is a top view of an illustrative ultrasonic transducer array that may be used in gaze tracking circuitry20. As shown inFIG.5, transducer array24may include multiple transducers38on substrate42. Array24may include transducers38with different center frequencies. The center frequency of an ultrasonic transducer may refer to the frequency at the center of the frequency range of which the transducer is capable of operating. For example, array24may include transducers with two, three, four, five, six, seven, ten, more than ten, or less than ten center frequencies. The center frequencies of transducers38may, for example, be between 750 kHz and 1.25 MHz, between 500 kHz and 1.25 MHz, between 700 kHz and 1 MHz, between 800 kHz and 1.2 MHz, between 900 kHz and 1.1 MHz, between 750 kHz and 1.4 MHz. or between any other suitable frequency range. There may be any suitable number (e.g., one, two, three, four, five, more than five, less than five) of transducers38in array24for a given center frequency. If desired, each transducer38may have one or more natural oscillation frequencies that are used for the excitation or detection of ultrasound waves.

The bandwidth of each individual transducer38may be smaller than the collective bandwidth spanned by all of the transducers38in array24. The center frequencies of individual transducers38may be selected so that the collective bandwidth of the entire array24spans some or all of the desired frequency range (e.g., from 750 kHz to 1.25 MHz, from 700 kHz to 1 MHZ, from 800 kHz to 1.2 MHz, from 900 kHz to 1.1 MHz, from 750 kHz to 1.4 MHz, or any other suitable frequency range). The desired frequency range may depend on the range of distances to be measured. For example, to measure distances to objects that are within a few centimeters (such as a user's eye16), array24may span a frequency range of 750 kHz to 1.25 MHz (as an example).

Substrate42may have any suitable dimensions. For example, lateral dimensions L1 and L2 of substrate42may be between 2 mm and 2.5 mm, between 1 mm and 1.5 mm, between 1 mm and 3 mm, between 2 mm and 4 mm, and/or other suitable length. Dimensions L1 and L2 may be equal (so that substrate42has a square footprint) or unequal (so that substrate42has a rectangular footprint), or the footprint of substrate42may have other shapes (e.g., circular, oval, round, triangular, etc.).

Transducers38may be arranged in an evenly spaced grid of rows and columns on substrate42, or may be arranged with any other suitable pattern (e.g., unevenly spaced clusters, a random pattern, a non-grid pattern, etc.). The example ofFIG.5in which transducers38have a circular shape is merely illustrative. If desired, transducers38may be square, rectangular, oval, round, or any other suitable shape.

In some arrangements, it may be desirable to maximize the amount of space between transducers38that share the same center frequency. Maximizing the spacing between commonly configured transducers38in array24may increase the accuracy of distance measurements made with array24. Different rules regarding placement of the different subsets of transducers38on substrate42may be implemented to achieve the desired performance from array24. As an example, the convex hull of a given set of transducers38that share the same center frequency may cover at least 50% of array24, may cover at least 80% of array24, or may cover other suitable portions of array24. As another example, most pairs of transducers38that share the same center frequency may be separated by a transducer38of a different center frequency. These examples are merely illustrative. In general, transducers38may be placed in any suitable arrangement on substrate42.

Operation of transducers38may be controlled by control circuitry26. Substrate42may include interconnects40for conveying signals between transducers38and control circuitry26. For example, interconnects40may be used to convey driving signals from control circuitry26to transducers38and to convey sensor signals (e.g., sensor signals associated with ultrasonic waves that are detected by transducers38) from transducers38to control circuitry26.

If desired, transducers38in array24may be independently controlled from one another. For example, the frequency, phase, and pulse shape of the driving signal for a given transducer38may be different from other transducers38in array24. Each individual transducer38in array24may be independently controlled with different driving signals, or there may be subsets of transducers38(e.g., a subset that share the same center frequency or other suitable subset) that are controlled with the same drive signals but that are independently controlled from other subsets of transducers38. This is merely illustrative, however. If desired, transducers38may not be independently controlled and/or may be controlled with any other suitable driving scheme.

In some arrangements, transducers38may be driven by off-chip control circuitry. In this type of arrangement, interconnects40may include leads, contact pads, solder and/or other conductive elements for conveying signals between array24and control circuitry26that is separate from array24. In other arrangements, substrate42may be a multilayer substrate in which transducers38are stacked with a control circuitry layer (e.g., an application-specific integrated circuit layer) that includes control circuitry26. With this type of integrated control circuitry, interconnects40may include metal vias or and/or other conductive elements for conveying signals between transducers38and control circuitry26that is located in a different layer of substrate42. These examples are merely illustrative. If desired, interconnects42may include metal vias for conveying signals between different layers of substrate42and may also include contact pads for conveying signals between array24and external circuitry.

FIGS.6and7are cross-sectional side views of illustrative arrays of ultrasonic transducers with different center frequencies.

In the example ofFIG.6, array24includes transducers with different center frequencies such as transducer38-1with a first center frequency, transducer38-2with a second center frequency, and transducer38-3with a third center frequency. In piezoelectric micromachined ultrasonic transducers, the center frequency is determined at least in part by the dimensions of the cavity. If desired, transducers38may be provided with different center frequencies by using cavities52with different dimensions (e.g., different depths, different diameters, etc.). In the example ofFIG.6, transducers38-1,38-2, and38-3have cavities52with different respective depths D1, D2 and D3 to achieve the desired set of center frequencies. This may be achieved, for example, by forming membranes56on a first substrate (e.g., a substrate forming side walls54) and bonding the first substrate to a second substrate (e.g., substrate50) having pads of different heights. Photolithographic techniques (e.g., grayscale lithography) and/or substrate imprinting techniques may be used to create surfaces at different heights on substrate50. One or both of substrate50and side walls54may be formed from semiconductor material such as silicon or may be formed from any other suitable material.

In the example ofFIG.7, transducers38-1,38-2, and38-3have cavities52with the same depth but with different surface features such as surface features58to create the desired acoustic reflection phase at the center frequency. Surface features58may, for example, include protrusions60separated by recesses62. The size, pattern, height, shape, and/or other characteristic of surface features58may be varied for different transducers38to achieve the desired center frequency. Membranes56may be formed on a first substrate such as the substrate forming side walls54. In one illustrative arrangement, surface features58are formed on the substrate that forms side walls54(e.g., by processing the substrate to produce surface features58and/or by attaching surface features58to the substrate). In another suitable arrangement, surface features58are formed on substrate50(e.g., by processing substrate50to produce surface features58and/or by attaching surface features58to substrate50), and substrate50may be bonded to side walls54.

The transducers ofFIGS.6and7are merely illustrative examples of the types of structures that may be used to achieve different center frequencies in a transducer array. If desired, other structures may be used to produce an array of transducers with different center frequencies. For example, instead of varying the cavity depth as in the example ofFIG.6, transducers38may have uniform cavity depth (e.g., may all have a relatively short cavity depth) but with different lateral cavity dimensions (e.g., different diameters, different lengths and widths, etc.). Arrangements in which both cavity depth and the lateral dimensions of the cavity are varied may also be used. In general, any suitable technique for producing transducers with different center frequencies may be used.

FIGS.8and9are graphs showing illustrative frequency coverage arrangements for an array of ultrasonic transducers (e.g., an array24in gaze tracking circuitry20).

In the example ofFIG.8, each curve64correspond to the transfer function (in the frequency domain) of an associated transducer38in a given array24. Curve66corresponds to the overall frequency coverage of the phased array24taken as a whole. In this example, the frequency coverage is relatively dense in the sense that the bandwidth B1 of an individual transducer38is larger than or comparable to the gap G1 between adjacent center frequencies. The number of transducers38with different center frequencies (and thus the number of curves64spanning the desired frequency range) may be greater or less than that shown inFIG.8. When array24has a relatively dense frequency coverage of the type shown inFIG.8, the entire bandwidth of curve66may be used efficiently during operation of array24.

In the example ofFIG.9, each curve68corresponds to the transfer function (in the frequency domain) of an associated transducer38in a given array24. Curve70corresponds to the overall frequency coverage of phased array24taken as a whole. In this example, the frequency coverage is relatively sparse in the sense that the bandwidth B2 of an individual transducer38is smaller than the gap G2 between adjacent center frequencies. The number of transducers38with different center frequencies (and thus the number of curves68spanning the desired frequency range) may be greater or less than that shown inFIG.9. When array24has a relatively sparse frequency coverage of the type shown inFIG.9, certain portions of the overall bandwidth of curve70(e.g., discrete portions centering around the individual center frequencies of transducers38) may be used more efficiently than other portions during operation of array24.

The examples ofFIGS.8and9are merely illustrative. If desired, the frequency coverage of arrays24may be denser than that shown inFIG.8, may be sparser than that shown inFIG.9, or may have a frequency coverage that is sparser than that ofFIG.8but denser than that ofFIG.9. In general, measurements from an array with relatively dense frequency coverage may have a greater signal-to-noise ratio than measurements from an array with relatively sparse frequency coverage.

FIG.10is a schematic diagram showing how angle of arrival (sometimes referred to as direction of arrival) measurement techniques may be used to determine the angle of arrival of incident ultrasonic signals. As shown inFIG.10, array24may include multiple transducers38(e.g., a first transducer38-1and a second transducer38-2). Transducers38-1and38-2may each receive an ultrasonic signal32after it reflects off of the user's eye16(FIG.2). Transducers38-1and38-2may be laterally separated by a distance A1, where transducer38-1is farther away from the user's eye than transducer38-2(in the example ofFIG.10). Therefore, ultrasonic signal32travels a greater distance to reach transducer38-1than it does to reach transducer38-2. The additional distance between the user's eye and transducer38-1is shown inFIG.10as distance A2.FIG.10also shows angles X and Y (where X+Y=90°).

Distance A2 may be determined as a function of angle Y or angle X (e.g., A2=A1 sin(X) or A2=A1 cos(Y)). Distance A2 may also be determined as a function of the phase difference between the signal received by transducer38-1and the signal received by transducer38-2(e.g., A2=(Δϕλ)/(2π), where Δϕ is the phase difference between the signal received by transducer38-1and the signal received by transducer38-2and λ is the wavelength of the received signal32). Control circuitry26may include phase measurement circuitry coupled to each transducer38to measure the phase of the received signals and identify a difference in the phases (Δφ). The two equations for A2 may be set equal to each other (e.g., A1 sin(X)=(Δφλ)/(2π)) and rearranged to solve for angle X (e.g., X=sin−1((Δφλ)/(2πA1)) or may be rearranged to solve for angle Y. As such, the angle of arrival may be determined (e.g., by control circuitry26) based on the known (predetermined) distance between transducer38-1and transducer38-2, the detected (measured) phase difference between the signal received by transducer38-1and the signal received by transducer38-2, and the known wavelength or frequency of the received signals32.

Because phase measurements are restricted to an interval between negative pi and positive pi, phase wrapping may occur which can, if care is not taken, lead to ambiguous phase difference measurements. Phase disambiguation may be achieved by performing non-uniform sampling and/or by clustering certain transducers together within array24. This type of arrangement is illustrated inFIG.11.

As shown inFIG.11, some transducers38with the same center frequency may be placed adjacent to one another within array24. For example, two, three or more transducers38A may have the same center frequency and may be clustered together within array24. If desired, one or more additional groups of transducers with the same center frequency such as transducers38B may be clustered together within array24(the center frequency of transducers38B and other groups of transducers used for phase disambiguation may be the same or different than the center frequency of transducers38A). The close spacing of transducers with the same center frequency (e.g., transducers38A, transducers38B, and any other clusters of transducers used for phase disambiguation) helps disambiguate phase information by ensuring that the signals detected by transducers38A occur close enough in time so that the true phase delay of an incoming signal can be determined. The remaining transducers38in array24that are not used for phase disambiguation may be spaced farther apart, as discussed in connection withFIG.5. The example ofFIG.11is merely illustrative, however. If desired, array24may not include any clusters of transducers with the same frequency.

The use of multiple transducers38in phased transducer array24allows beam steering arrangements to be implemented by controlling the relative phases and magnitudes (amplitudes) of the ultrasonic signals conveyed by the transducers. Control circuitry26may, for example, include phase shifter circuits and/or circuitry for adjusting the magnitude of the ultrasonic signals. The circuitry within control circuitry26that controls the phase and/or magnitude of drive signals for arrays24may sometimes be referred to as beam steering circuitry (e.g., beam steering circuitry that steers the beam of ultrasonic signals transmitted and/or received by phased transducer array24).

Control circuitry26may adjust the relative phases and/or magnitudes of the transmitted signals that are provided to each transducer38in phased transducer array24and may adjust the relative phases and/or magnitudes of the received signals that are received by phased transducer array24. Control circuitry26may, if desired, include phase detection circuitry for detecting the phases of the received signals that are received by phased transducer array24. The term “beam” or “signal beam” may be used herein to refer to ultrasonic signals that are transmitted and/or received by phased transducer array24in a particular direction. The signal beam may exhibit a peak gain that is oriented in a particular pointing direction at a corresponding pointing angle (e.g., based on constructive and destructive interference from the combination of signals from each transducer in the phased transducer array). The term “transmit beam” may sometimes be used herein to refer to ultrasonic signals that are transmitted in a particular direction whereas the term “receive beam” may sometimes be used herein to refer to ultrasonic signals that are received from a particular direction.

If, for example, control circuitry26provides control signals to transducers38to emit ultrasonic signals32having a first set of phases and/or magnitudes, the transmitted signals32may form a transmit beam that illuminates (i.e., covers) both the cornea30and sclera28of the user's eye16, as shown in the example ofFIG.12. If control circuitry26provides control signals to transducers38to emit ultrasonic signals32having a second set of phases and/or magnitudes, the transmitted signals may form a transmit beam that is concentrated on the cornea30and not the sclera28of eye16, as shown in the example ofFIG.13.

Control circuitry26may also operate array24to receive signals from a given range of angles. This may be achieved by adjusting the phase and magnitude of transducers38so that ultrasonic signals32are received from a particular direction. In other arrangements, this may be achieved post-measurement by changing the gain of signals received from a given region to amplify those signals relative to signals received from other regions.

If desired, control circuitry26may actively adjust control signals for transducers38in real time to steer the transmit or receive beam in different desired directions over time. For example, gaze tracking circuitry20may include a camera that captures images of the user's eye, which in turn may be used to determine the location of a glint on the user's eye. Control circuitry26may control arrays24based on eye information gathered with the camera so that beam shaping and beam steering can be performed accordingly (e.g., so that signals32are steered towards the location of the glint on the eye and/or received from the location of the glint on the eye). If desired, the ultrasonic signal beam may also be steered away from other arrays24(e.g., to avoid a scenario in which a receiving array24detects a signal that travels directly to the receiving array24from a transmitting array24without reflecting off the user).

During operation, control circuitry26may operate one or more arrays24as a transmitting array and one or more arrays24as a receiving array. For example, control circuitry26may control the phases of transducers38in a first array24(e.g., a transmitting array) to transmit a short pulse towards the user's eye and may control the phases of transducers38in a second array24(e.g., a receiving array) in a similar manner (e.g., such that the receiving array would generate the same short pulse if used for transmission). Control circuitry26may also, if desired, control the phases of the transmitting array24and the receiving array24to direct the ultrasonic signal beam to (or receive the ultrasonic signal beam from) an area of interest and thereby limit detected reflections from other areas. Active phasing may be achieved digitally or in analog. In digital phasing arrangements, control circuitry26may use a system clock to shift the time and phase of the driving signal to transducers38. If desired, each transducer38may have an associated analog to digital converter circuit and phasing can be achieved computationally post-measurement.

Control circuitry26may, if desired, measure the actual resonance frequency of transducers38and may adjust drive signals for transducers38to compensate for any detuning that may occur in transducers38(e.g., detuning that may occur as a result of manufacturing effects or environmental conditions). This is merely illustrative. If desired, control circuitry26may not calculate or compensate for resonance frequency errors.

If desired, control circuitry26may phase the entire array24of transducers38, combining all of the different center frequencies to form a short ultrasonic signal pulse. In other arrangements, control circuitry26may phase only transducers38in array24with the same center frequency. For example, each set of transducers38with the same center frequency in array24may be phased to produce an ultrasonic signal that illuminates the desired area and/or to receive an ultrasonic signal beam from a known range of angles. Control circuitry26may determine the phase and/or amplitude of the received signal from each set of phased transducers38with the same center frequency in array24. Based on the phase and/or amplitude of the received signal at the different frequencies covered by array24, control circuitry26may determine the time delay and/or phase delay of the emitted signal (and thus the location of the user's eye).

FIG.14is a top view of illustrative gaze tracking circuitry20. In the example ofFIG.14, only two arrays24are shown, but there may be additional arrays24surrounding eye16, if desired. During operation, control circuitry26may configure array24T to be a transmitting array and array24R to be a receiving array. Array24T may include transducers38with different center frequencies such that the emitted ultrasonic signal beam includes signals with different frequencies. For example, the ultrasonic signal beam may include ultrasonic signal32-1, ultrasonic signal32-2, and ultrasonic signal32-3. Signals32-1,32-2, and32-3may have different frequencies and/or may each include multiple frequencies (e.g., with different amplitudes). The ultrasonic signal beam may bounce off of one or more specular reflection points such as specular reflection points C1 (e.g., on the user's face74). C2 (e.g., on cornea30), and C3 (e.g., on sclera28). Because transmitting array24T is located a few centimeters away from the reflection points, control circuitry26may treat signals32-1,32-2, and32-3as originating from a point source (instead of originating from transducers at different locations), if desired. Control circuitry26may analyze signals32-1,32-2, and32-3received by array24R to determine the location of reflection points C1, C2, and/or C3.

FIG.15is a top view of illustrative gaze tracking circuitry20showing how multiple receiving arrays may be used to detect a reflected ultrasonic signal emitted from a single transmitting array. In the example ofFIG.15, only three arrays24are shown, but there may be additional arrays24surrounding eye16, if desired. During operation, control circuitry26may configure array24T to be a transmitting array and arrays24R to be receiving arrays. Array24T may include transducers38with different center frequencies such that the emitted ultrasonic signal beam includes signals with different frequencies. For example, the ultrasonic signal beam may include ultrasonic signal32-1and ultrasonic signal32-2. Signals32-1and32-2may have different frequencies and/or may each include multiple frequencies (e.g., with different amplitudes). The ultrasonic signal beam may bounce off of one or more specular reflection points such as specular reflection points C1 (e.g., on cornea30), and C2 (e.g., on sclera28). Control circuitry26may analyze signals32-1and32-2received by arrays24R to determine the location of reflection points C1 and/or C2.

If desired, control circuitry26may tune the transmission properties (e.g., phases and/or amplitudes) of array24T to reduce the amplitude of signals directly transmitted from transmitting arrays24T to receiving arrays24R. In the event that direct transmission from transmitting arrays24T to receiving arrays24R remains too large, one or more additional transmitting arrays24T may be used to transmit a signal (e.g., steered away from the user's eye) that partially or completely nullifies the direct transmission from transmitting arrays24T to receiving arrays24R, if desired.

Cornea30and sclera28may be approximately spherical. If desired, control circuitry26may determine the locations of reflection points C1 and/or C2 as a function of the sphere center and/or sphere radii of cornea30and/or sclera28. For example, the location of reflection point C1 may be determined as a function of cornea center cs.

FIG.16is a top view of illustrative gaze tracking circuitry20showing how a single transmitting array and a single receiving array may be used to determine a location of the center of the cornea. In the example ofFIG.16, only two arrays24are shown, but there may be additional arrays24surrounding eye16, if desired. During operation, control circuitry26may configure array24T to be a transmitting array and array24R to be a receiving array. Array24T may include transducers38with different center frequencies such that the emitted ultrasonic signal beam includes signals with different frequencies. For example, the ultrasonic signal beam may include ultrasonic signal32-1and ultrasonic signal32-2. Signals32-1and32-2may have different frequencies and/or may each include multiple frequencies (e.g., with different amplitudes). The ultrasonic signal beam may bounce off of one or more specular reflection points such as specular reflection points C1 (e.g., on cornea30) and C2 (e.g., on sclera28). When approximating the cornea as a spherical shape, a bisecting line separating equal angles θ between the emitted signal path and the reflected signal path (reflected from point C1, for example) will pass through the cornea center location cs. If the radius rsof cornea30is known, control circuitry26may analyze signals32-1and32-2received by array24R to determine the location of cornea center cs. Control circuitry26may use the cornea radius rsand cornea center location csto solve for the gaze direction.

The examples ofFIGS.14,15, and16are merely illustrative. If desired, control circuitry26may employ other techniques to gather information about the user's eye during operation of glasses14. For example, control circuitry26may combine path length information from several reflection paths off of the cornea and/or sclera to determine gaze angle. This technique may be advantageous in situations where path length can be measured with more precision than the incident angle of the reflected signal.