CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Stage of International Application No. PCT/SE2018/051084, filed Oct. 23, 2018, which claims priority to Swedish Patent Application No. 1751354-0, filed Oct. 31, 2017. The disclosures of each of the above applications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a biometric imaging system comprising ultrasonic transducers and to a method of controlling such a biometric imaging system.

BACKGROUND OF THE INVENTION

Biometric systems are widely used as means for increasing the convenience and security of personal electronic devices, such as mobile phones etc. Fingerprint sensing systems, in particular, are now included in a large proportion of all newly released personal communication devices, such as mobile phones.

Due to their excellent performance and relatively low cost, capacitive fingerprint sensors are used in an overwhelming majority of all biometric systems.

Among other fingerprint sensing technologies, ultrasonic sensing also has the potential to provide advantageous performance, such as the ability to acquire fingerprint (or palmprint) images from very moist fingers as well as to provide biometrics data beyond the surface of the skin. One class of ultrasonic fingerprint systems of particular interest are systems in which acoustic signals are transmitted from the periphery of a device member to be touched by a user, and a fingerprint (palm print) representation is determined based on received acoustic signals resulting from the interaction between the transmitted acoustic signals and the user's skin.

Such ultrasonic fingerprint sensing systems, which are, for example, generally described in US 2017/0053151 may allow for a larger sensing area, which may be optically transparent, without the cost of the fingerprint sensing system necessarily scaling with the sensing area.

Although the general principle of such ultrasonic fingerprint sensing is known, there appear to be remaining challenges to be overcome. For instance, it would be desirable to provide for cost-efficient mass production of ultrasonic transducer devices suitable for use in such ultrasonic fingerprint sensing systems.

SUMMARY

In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved ultrasonic fingerprint sensing and a method for controlling such a system.

According to a first aspect of the invention, there is provided a method for controlling a plurality of ultrasonic transducers in a fingerprint sensing system. The fingerprint sensing system comprises a cover plate having a sensing surface configured to be touched by a finger, and a plurality of ultrasonic transducers located at the periphery of the cover plate, wherein the plurality of ultrasonic transducers are configured to transmit an acoustic signal propagating in the cover plate, receive an ultrasonic signal having interacted with an object in contact with the sensing surface, and to determine properties of the object based on the received ultrasonic signal. The method comprises: in response to a first input, controlling the plurality of transducers such that at least a portion of the sensing surface has a first feature detection resolution; and in response to a second input, controlling the plurality of transducers such that at least a portion of the sensing surface has a second feature detection resolution, different from the first feature detection resolution.

The described fingerprint sensing system may be arranged in an electronic device such as a smartphone, tablet computer or the like, where the sensing surface also forms an exterior surface of the electronic device to be touched by a finger. The sensing surface may thus be the exterior surface of a touch sensitive display. The sensing system may also be used to acquire and identify palm prints of a user. The concept of fingerprint sensing may also be described as fingerprint imaging, biometric imaging and biometric sensing etc.

That the fingerprint sensing system has a controllable feature detection resolution should in the present context be interpreted to mean that at least one of the ultrasonic transducers is active and transmitting a signal which is capable of interacting with an object touching the sensing surface, and that an interaction signal is detectable by a transducer after interaction with the object. From the interaction signal, it is assumed that some features of the object touching the sensing surface can be determined.

The sensing surface is the entire surface of the fingerprint sensing system where it is theoretically possible to detect an object touching the surface. As an example, the sensing surface may be a display area in an application where the fingerprint sensing system is integrated in a display. However, it is equally possible that the theoretical sensing surface is smaller than such a display.

The present invention is based on the realization that it is desirable to be able to control the plurality of ultrasonic transducers according to various application requirements such that the resolution of the fingerprint sensing system can be controlled. The resolution of the fingerprint sensing can in general be assumed to be proportional to the degree of certainty with which a fingerprint can be identified. Accordingly, a low resolution offers a lower degree of certainty than what a higher resolution provides. However, since not all applications have the same requirements of a fingerprint sensing system, the described method can thereby provide a flexible solution were the resolution is controllable.

Moreover, by controlling the resolution in a controllable area, specific applications are not limited by a fixed fingerprint sensing area configuration but may instead control the fingerprint sensing system to provide fingerprint sensing according to the application requirements.

According to one embodiment of the invention, controlling the plurality of ultrasonic transducers may comprise controlling each of the transducers to be in an on-state or in an off-state. Thereby, it is not required that all of the transducers are active at a given time. This in turn leads to a possibility of saving power since only the transducers required in a selected configuration need be active, i.e. in an on-state, thereby making it possible to minimize the power consumption. A further advantage of using a lower number of transducers when performing imaging is that the number of received signals is correspondingly lower, which in turn reduces the power consumption in the readout and signal processing circuitry. In applications where the emitting transducers are activated sequentially during an image capture, i.e. where only one transducer at the time is active (multiple receivers may be active simultaneously also during sequential activation of emitters), the time it takes to capture an image is reduced. Accordingly, there are several advantages related to reducing the number of active transducers in an image capture sequence.

According to one embodiment of the invention, the method may further comprise controlling a power level of each ultrasonic transducer in a continuous range between an off-state and a maximum power state. Thereby, additional flexibility and control is provided since each transducer can be carefully controlled to provide the required fingerprint sensing properties for a given application and the resolution of the fingerprint sensing can be controlled by controlling the power of the individual transducers.

In simplified terms, the resolution of the fingerprint sensing system scales with the power provided to the transducers where a higher power leads to a larger amplitude of the generated acoustic signal, i.e. a stronger signal, which in turn leads to a stronger acoustic response in the interaction signal. Thereby, the minute details of the fingerprint which would have been otherwise buried under noise floor can be detected successfully.

It should also be noted that it is important to control the power to each individual transducer so that the desired resolution is achieved at the selected area portion of the sensing surface. The power to each individual transducer is typically provided as pulsed power, and the time averaged power to a transducer can be controlled by controlling the pulse amplitude, pulse width (i.e. duty cycle) and pulse repetition frequency.

According to one embodiment of the invention, the first feature detection resolution and the second feature detection resolution may advantageously be selected from a group of predetermined feature detection resolutions comprising: a touch detection resolution mode, a low resolution fingerprint detection mode, a high resolution fingerprint detection mode, and an enrolment mode. By describing distinct modes of feature detecting resolution based on the detectable feature, implementation of different controlled resolutions may be facilitated. In a touch detection resolution mode, it is sufficient that it is possible to determine when an object touches the sensing surface. The feature detection resolution mode may thereby be seen as a sleep mode which can detect a touch. The, feature detection resolution mode can thereby be implemented in a “tap-to-wake” application where the power consumption of the fingerprint sensing device is minimized while still being able to wake up when touched.

A low resolution fingerprint mode can be used when it is desirable to verify the fingerprint but when lower accuracy is acceptable and/or when lower power consumption is required. Correspondingly, the high resolution fingerprint detection mode is advantageously used when a high accuracy is required and where the power consumption is less important. An application example utilizing both the high and low resolution modes could be an electronic device where a user initially is verified using the high resolution mode, and where subsequent verifications are performed intermittently using the low resolution mode to verify that it is still the same user which uses the device.

The mode providing the highest possible feature detection resolution may be referred to as an enrolment mode where it is desirable to capture a fingerprint with the highest resolution possible to form a fingerprint template comprising as much detail as possible. A low resolution mode can also be used as a so called “touch detection resolution mode” which in this regard can be compared with capacitive touch detection. For higher resolution modes, the sensing system operates at resolutions enabling fingerprint detection at industry standards. A fingerprint detection resolution mode may vary between 100 dpi up to 1000 dpi based on the security level required. Depending on the configuration of the ultrasonic transducers and associated readout circuitry even higher resolutions may be achievable.

According to one embodiment of the invention the first input and/or the second input may be a command to form an area of the sensing surface having a resolution selected from the group of predetermined feature detection resolutions. Accordingly, a specific area of the sensing surface, which may be the full sensing surface area or part of the sensing surface area, can be formed based on a command issued by an application so that a control unit controls the plurality of ultrasonic transducers to form the requested area having the requested properties. It can be assumed that the capabilities of the fingerprint sensing system in terms of possible resolutions and area control are known to an application operating in an electronic device comprising the fingerprint sensing system.

According to one embodiment of the invention, the first input and/or the second input may be generated by the detection of an object touching the sensing surface. Thereby, the electronic device comprising the fingerprint sensing system may be configured to automatically switch from a first resolution mode to a second resolution mode in response to an object touching the sensing surface. This embodiment may be exemplified by the above described tap-to-wake functionality, where the wake-up action is followed by a fingerprint imaging and verification step.

According to one embodiment of the invention, the method may further comprise; receiving a first input comprising a command to control the plurality of ultrasonic transducers to form a touch detection mode area corresponding to at least a portion of the sensing surface; detecting an object in contact with the sensing surface in the touch detection area; estimating a contact area of the object; and where the second input comprises increasing the resolution of the contact area to a resolution required for fingerprint detection. Accordingly, it is possible to automatically capture a fingerprint image from any part of the sensing surface in response to a touch on the sensing surface. Thereby, a user authentication can be performed automatically in response to a user placing a finger on any part of the sensing surface.

According to one embodiment of the invention the method may further comprise controlling the plurality of ultrasonic transducers such that active transducers form a non-uniform distribution around the circumference of the cover plate. Accordingly, it is possible to control each transducer individually to achieve the desired resolution in the desired area portion of the sensing surface.

According to one embodiment of the invention, the method may further comprise controlling the plurality of ultrasonic transducers to dynamically change a resolution of at least one of the first area portion and the second area portion. Thereby, it is possible to seamlessly change from one resolution to another without a user noticing any difference in appearance. Moreover, the resolution may be changed between different of the above described modes, or it may be changed gradually and arbitrarily between different resolutions.

According to one embodiment of the invention, the cover plate may advantageously be the cover glass of a display panel, and the display panel may further comprise a capacitive touch panel. Thereby, the described fingerprint sensing system can be implemented in touch panel displays such as LCD or OLED displays of the types commonly used in smartphones, tablet computers and the like.

According to one embodiment of the invention, the display panel may further comprise a capacitive touch panel. The method may the further comprise: by the capacitive touch panel, detecting an object in contact with the sensing surface; estimating a contact area of the object; and controlling the resolution of the contact area to a resolution required for fingerprint detection. Accordingly, the capacitive touch panel may be used both to detect an object touching the sensing surface and to estimate a contact area between the object and the sensing surface. Assuming that the object is a finger, the resolution of the estimated are can be controlled to be sufficiently high to capture a fingerprint. This means that a user may place a finger at any point on the sensing surface and have the fingerprint captures and verified by means of the described ultrasonic fingerprint sensing system.

According to one embodiment of the invention, a portion of the sensing surface having the first or second feature detection resolution is smaller than the total sensing surface. Accordingly, it is possible to form sub-areas, or sub regions, of the total sensing surface which has a different resolution for the remaining surface.

According to a second aspect of the invention, there is provided a fingerprint sensing system comprising: a cover plate having a sensing surface configured to be touched by a finger; a plurality of ultrasonic transducers connected to the cover plate and located at the periphery of the cover plate, wherein the plurality of ultrasonic transducers are configured to transmit an acoustic signal propagating in the cover plate, receive an ultrasonic signal having interacted with an object in contact with the sensing surface, and to determine properties of the object based on the received ultrasonic signal; and a control unit. The control unit is configured to: in response to a first input, control the plurality of transducers such that at least a portion of the sensing surface has a first feature detection resolution; and in response to a second input, controlling the plurality of transducers such that at least a portion of the sensing surface has a second feature detection resolution, different from the first feature detection resolution.

The ultrasonic transducers may for example be piezoelectric transducers connected to a control unit by electrodes arranged on opposing sides of a piezoelectric element such that by controlling the voltage of the two electrodes, an ultrasonic signal can be generated which propagates in the cover plate.

According to one embodiment of the invention, the plurality of ultrasonic transducers may advantageously be connected to a bottom surface of the cover plate, the bottom surface being opposite of the sensing surface. A fingerprint sensing system comprising ultrasonic transducers connected to the bottom of the cover plate may be easier to manufacture.

According to one embodiment of the invention, the plurality of ultrasonic transducers may be connected to a side surface of the cover plate, the side surface being perpendicular to the sensing surface. Thereby, the ultrasonic signals can be generated and directly propagate in the desired direction.

According to one embodiment of the invention, the cover plate is configured to form a back surface of an electronic device. Accordingly, the fingerprint sensing system can be provided as part of a glass surface or any other equivalent material where no display is present.

Additional effects and features of the second aspect of the invention are largely analogous to those described above in connection with the first aspect of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

