Abstract:
A method is for evaluating a coverage factor of a photon emission cone of a time of flight sensor. The method may include the steps of assigning a reference curve to the sensory providing a photon flux intensity as a function of time of flight; and acquiring a time of flight and a corresponding flux intensity with the sensor. The method may also include reading the intensity provided by the reference curve for the acquired time of flight, and providing an indication of the coverage factor based on the ratio between the acquired intensity and the read intensity.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to human-machine interfaces, and, more particularly, to sensing the position and motion of a hand or object relative to a screen. 
       BACKGROUND 
       [0002]    Many touch screens, such as those used in mobile phones and tablets, are equipped with proximity detectors. These detectors, often using infrared transmitters, are designed to detect simple gestures, such as the approach of an object. This detection is used, for example, to disable the touch screen function during a call when the phone is near the ear of the user. 
         [0003]    Infrared sensors typically use the brightness reflected by the target object. Thus, a distance evaluated using an infrared sensor is a rough estimate, since it depends on the reflectance and the tilting of the target object. 
         [0004]    Patent application US20130175435 discloses a proximity detection technique using a photon time of flight sensor. 
         [0005]      FIG. 1  schematically illustrates an implementation of a time of flight sensor as described in the aforementioned patent application. The sensor includes, on a printed circuit board  8 , an infrared radiation source  10  emitting photons in a cone  12 . A photon detector  14  is arranged on the printed circuit board close to the transmitter  10  for receiving photons reflected from a target object placed in the cone  16 . The detector  14  is based on so-called single photon avalanche diodes (SPAD). 
         [0006]    A control circuit, not shown, energizes the transmitter  10  with short duration pulses and observes the signal from the detector  14  to determine the elapsed time between each pulse and the return of a corresponding burst of photons on the detector  14 . The circuit thus measures the time of flight of the photons along a path  18  going from the transmitter  10  to the object  16  and returning to the detector  14 . The time of flight is proportional to the distance between the object and the detector, and does not depend on the intensity of the received photon flux, which varies depending on the reflectance of the object. 
         [0007]    A time of flight sensor of the type of  FIG. 1  thus makes it possible to accurately determine the distance of a target object relative to the sensor without being influenced by the reflectance of the object or parasitic phenomena that alter the intensity of the photon flux received by the detector, such as the interposition of a tinted glass. 
       SUMMARY 
       [0008]    It would be convenient to use one or more time of flight sensors to determine not only the distance of the object (z coordinate), but also its transverse position (x and y coordinates), or to detect a transverse movement of the object. 
         [0009]    A method is provided for evaluating a coverage factor of a photon emission cone or area of a time of flight sensor, comprising the steps of assigning a reference curve to the sensor, providing a photon flux intensity as a function of time of flight; acquiring a time of flight and a corresponding flux intensity with the sensor; reading the intensity provided by the reference curve for the acquired time of flight; and providing an indication of the coverage factor based on the ratio between the acquired intensity and the read intensity. 
         [0010]    The reference curve may be constructed in a calibration phase according to the steps of moving a reference object along the axis of the emission cone between two positions where the object completely covers the emission cone; during the movement, storing multiple time of flight values and the corresponding flux intensities; and constructing the reference curve from the stored values. 
         [0011]    The reference curve may be selected from a set of multiple reference curves assigned to different reflectance values, according to the steps of measuring changes in the intensity of the flux; storing a local maximum value of the intensity of the flux and the corresponding time of flight; and finding in the set of curves the curve that, for the stored time of flight, provides the intensity closest to the stored local maximum value. 
         [0012]    A method for detecting motion of an object may use a time of flight sensor, according to the steps of evaluating a coverage factor of the emission cone of the time of flight sensor; and determining that the object approaches the time of flight sensor when the coverage factor increases, or determining that the object moves away from the time of flight sensor when the coverage factor decreases. 
         [0013]    The method for detecting motion may use two time of flight sensors emitting photons in respective emission cones of parallel axes, according to the steps of evaluating a coverage factor of each emission cone; and detecting a motion of the object from a first cone to the second cone when the coverage factor of the second cone increases and the coverage factor of the first cone decreases. 
         [0014]    A method for evaluating a position of an object may use a first pair of time of flight sensors emitting photons in respective emission cones of parallel axes, according to the steps of evaluating a coverage factor of each emission cone; and producing an abscissa of the object as the barycenter of the abscissas of the time of flight sensors, weighted by the coverage factors of the corresponding emission cones. 
         [0015]    The method may use a second pair of time of flight sensors arranged along an ordinate axis perpendicular to the axis, of abscissa, of the first pair of sensors, according to the steps of evaluating a coverage factor of each emission cone of the second pair of sensors; producing an ordinate of the object as the barycenter of the ordinates of the second pair of sensors, weighted by the coverage factors of the corresponding emission cones; and producing an elevation of the object based on the average of the times of flight provided by the sensors. 
         [0016]    An electronic device may include an infrared radiation source, an adjacent photon detector, and a control circuit configured to define the coverage factor as described above. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]    Other advantages and features will become more clearly apparent from the following description of particular embodiments of the invention provided for exemplary purposes only and represented in the appended drawings, in which: 
           [0018]      FIG. 1 , previously described, schematically illustrates a conventional implementation of a proximity sensor measuring the time of flight of photons; 
           [0019]      FIG. 2  illustrates an exemplary set of curves representing the intensity of a photon flux received by a time of flight sensor according to the distance and reflectance of the target object in accordance with the invention; 
           [0020]      FIGS. 3A and 3B  show an object moving in the emission cone of a time of flight sensor and corresponding coverage phases of the cone in accordance with the invention; 
           [0021]      FIG. 4  illustrates a technique for determining the coverage factor of the cone based on knowledge of the reflectance of the object and of the corresponding distance-intensity curve in accordance with the invention; 
           [0022]      FIGS. 5A and 5B  illustrate a first reflectance calibration technique for determining the cone coverage factor in accordance with the invention; 
           [0023]      FIGS. 6A and 6B  illustrate a second reflectance calibration technique for determining the cone coverage factor in accordance with the invention; and 
           [0024]      FIG. 7  illustrates an implementation of several time of flight sensors to determine the transverse position of an object of the respective coverage factors of the sensors in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In order to detect a transverse position change of an object with a time of flight sensor, it is proposed to combine the information of distance and intensity produced by the sensor. 
         [0026]      FIG. 2  illustrates an exemplary set of curves representing the intensity of the photon flux received by the detector  14  as a function of the distance D and the reflectance R of the target object. The intensity I may be expressed, as indicated, by the number of photons received by the detector for each transmitted pulse. Three curves are shown for respective reflectance values of 88%, 17% and 3%. 
         [0027]    For each curve, a maximum intensity value is reached at a distance of about 10 mm. This distance where the maximum value is reached depends on the offset between the emitter  10  and detector  14 . Below this distance, the object is too close to the emitter, whereby the reflected photons no longer reach the detector. From the maximum value, the intensity decreases substantially in inverse proportion to the distance. 
         [0028]    For a given distance, the amplitude is not proportional to the reflectance—it is not trivial to construct the curve for a given reflectance from curves known for other reflectance values. The inventors have however found that the intensity, for a given distance, is substantially proportional to the covered area of the photon emission cone by an object having a uniform reflectance. The measured value of the covered area may be used, as discussed below, to determine the transverse position of the object relative to the axis of the emission cone. 
         [0029]    In  FIG. 3A , it is assumed that a target object  16  moves from the left to the right before a time of flight sensor, passing through the axis of the emission cone  12  of the sensor. It is assumed that the object is flat and large enough to completely cover the cone when it is centered on the axis of the cone. 
         [0030]      FIG. 3B  illustrates various coverage phases of the cone during movement of the target object. The circles represent the useful section of the cone, i.e. the intersection of the cone by the plane of the object, and the black part of the circles represents the covered portion. The central circle corresponds to a total coverage, where the photon detector produces a maximum intensity. The other circles represent different degrees of partial coverage. The intensity produced by the photon detector is proportional to the covered surface. The coverage factor is defined hereinafter as the ratio between the measured intensity and a reference intensity corresponding to a total coverage. 
         [0031]    The reference intensity depends on the distance and the reflectance of the target object. The reference intensity could thus be provided by one of the reference curves of  FIG. 2 , chosen according to the reflectance. 
         [0032]      FIG. 4  illustrates a technique for determining the coverage factor based on the knowledge of the reflectance R of the target object and the corresponding reference curve. The sensor produces a measured intensity Im and a measured distance Dm, 40 mm for example. Assuming that the reflectance of the object is R, the intensity i(R,40) is read from the curve for a distance of 40 mm. The coverage factor is then equal to C=Im/I(R,40). 
         [0033]    However, the reflectance of the target object is not necessarily known. The object is, in practice, the hand of the user, and its reflectance depends on various parameters, such as pigmentation of the skin and its surface state. The user could also wear gloves. Although the system could be designed for a manual adjustment of the reflectance, it is convenient to provide a calibration phase during which the reflectance of the object is evaluated. 
         [0034]      FIGS. 5A and 5B  illustrate a first technique to take into account the reflectance. The sensor is placed in a calibration phase, and the user is prompted to move the target object, or his hand, along the axis of the emission cone. 
         [0035]      FIG. 5A  illustrates the movement of the target object between a position near the sensor (in solid lines) and a remote position (in dotted lines). It is assumed that the object completely covers the emission cone throughout this movement. 
         [0036]      FIG. 5B  illustrates a reference intensity curve constructed during the calibration phase. While the object moves, the sensor control system periodically stores the current distance (current time of flight) Di and the corresponding intensity value Ii. At the end of the calibration phase, the control system has stored several (intensity, distance) pairs defining a reference curve adapted to the reflectance of the actual object used. 
         [0037]    During a normal use phase, the thus constructed reference curve is used according to the technique of  FIG. 4  to determine at any time the coverage factor of the emission cone of the sensor. The same reference curve may be used as long as the same target object is used. 
         [0038]      FIGS. 6A and 6B  illustrate a technique to take into account the reflectance automatically. 
         [0039]    During normal use, as illustrated in the upper part of  FIG. 6A , the user may swipe his hand in front of the sensor and happen to fully cover its emission cone. 
         [0040]    The lower part of  FIG. 6A  is a graph corresponding to this situation, illustrating an example of variation with time of the intensity (solid line) and the distance (dashed line) when the hand swipes over the sensor. During each swipe, the intensity reaches a local maximum Im. The system may be designed to regularly detect the local maxima and store the current maximum value Imax with the corresponding measured distance Dmax. 
         [0041]      FIG. 6B  shows a set of reference curves stored in the system, for various values of reflectance R 0 , R 1 , R 2  . . . The pair of values (Imax, Dmax) stored in the phase of  FIG. 6A  is used to select the reference curve serving for the calculation of the coverage factor. It is thus assumed that the maximum value Imax corresponds to a total coverage. As shown by way of example, the pair (Imax, Dmax) defines a point that belongs to the curve R 2 . This curve R 2  is then used to calculate the coverage factor according to the technique of  FIG. 4 . 
         [0042]    In practice, the number of stored reference curves is limited, and the point (Imax, Dmax) does not fall on one of the curves. In such a case, the nearest curve is selected. Alternatively, the curve to use may be interpolated between the two nearest curves. 
         [0043]    The coverage factor obtained by these techniques may be exploited in various human-machine interface applications. 
         [0044]    By using a single time of flight sensor, both a vertical and transversal proximity of the hand may be detected. The vertical proximity is quantitative and corresponds to the distance normally provided by the sensor. The transversal proximity is provided qualitatively by the coverage factor. A transversal approaching movement is detected when the coverage factor increases, and a transversal departing movement is detected when the coverage factor decreases. 
         [0045]    By using two time of flight sensors emitting photons in cones with parallel axes, a quantitative measure of the position of an object between both sensors may be provided, together with the direction of movement between the two sensors. 
         [0046]    A swipe of an object from a first sensor towards the second sensor may be detected when the coverage factor of the first sensor decreases as the coverage factor of the second sensor increases. The abscissa x of the object on the axis connecting the two sensors may be determined as the barycenter of the abscissas Xa, Xb of the two sensors, weighted by the coverage factors Cxa, Cxb of the sensors. In other words: 
         [0000]    
       
         
           
             x 
             = 
             
               
                 
                   Cxa 
                   · 
                   Xa 
                 
                 + 
                 
                   Cxb 
                   · 
                   Xb 
                 
               
               
                 Cxa 
                 + 
                 Cxb 
               
             
           
         
       
     
         [0047]    For an object of a certain size, the abscissa may locate the center of the object along the x-axis. 
         [0048]    As indicated above, the coverage factor may be expressed by C=Im/I(R,Dm), where Im is the measured intensity and I(R,Dm) is the intensity read for the measured distance Dm on the reference curve assigned to the reflectance R. If the distance of the object at the level of each sensor is the same, the term I(R,Dm) disappears from the expression of the abscissa x. In other words, assuming that the object has a surface parallel to the sensor support, the expression of the abscissa is independent of the reflectance. 
         [0049]      FIG. 7  shows a sensor configuration for locating an object, such as a hand, in space. Four time of flight sensors are arranged on a flat support such that their emission cones have parallel axes. A first sensor pair Xa, Xb is arranged along an abscissa axis x and the second sensor pair Ya, Yb is arranged along a perpendicular ordinate axis y. The sensors are preferably arranged in a rhombus, as shown. The center of the rhombus is then the origin of the coordinate system. 
         [0050]    With this configuration, the x-coordinate is expressed as above, by: 
         [0000]    
       
         
           
             x 
             = 
             
               
                 
                   Cxa 
                   · 
                   Xa 
                 
                 + 
                 
                   Cxb 
                   · 
                   Xb 
                 
               
               
                 Cxa 
                 + 
                 Cxb 
               
             
           
         
       
     
         [0051]    And the y-coordinate is expressed by: 
         [0000]    
       
         
           
             y 
             = 
             
               
                 
                   Cya 
                   · 
                   Ya 
                 
                 + 
                 
                   Cyb 
                   · 
                   Yb 
                 
               
               
                 Cya 
                 + 
                 Cyb 
               
             
           
         
       
     
         [0052]    Where Ya and Yb are the y-coordinates of sensors Ya and Yb, and Cya and Cyb the respective coverage factors of sensors Ya and Yb. 
         [0053]    These relationships are applicable to any pair of sensors of arbitrary coordinates in a same coordinate system. Thus, if the target object only covers sensors Xa and Ya, for example, its coordinates may be determined by applying these relationships with (Xb, Yb, Cxb, Cyb)=(0, 0, Cya, Cxa). 
         [0054]    The average elevation z of the object may be provided as the average of the distances produced by the four sensors. 
         [0055]    The transversal range of use of the sensor system depends on the distance (z) of the object, which determines the effective section of the emission cones. In a lower limit configuration, the object placed in the center of the sensor system is located outside the cones and is tangent thereto, as shown for sensor Xb. In a higher limit configuration, the object is located within the cones and is tangent thereto from the inside, as shown by a dotted circle corresponding to the cone of sensor Xa. A preferred configuration corresponds to the case where the useful sections of the cones are tangential.