Wearable LED Sensor Device that Employs a Matched Filter to Generate Biometric Samples

An LED of a wearable sensor device can be driven less frequently while still obtaining an accurate biometric sample. By reducing how frequently the LED is driven, the power required to operate the wearable sensor device is likewise reduced. To allow the LED to be driven less frequently while still obtaining an accurate biometric sample, a known lighting profile of an LED can be employed with a matched filter to generate a biometric sample from a number of samples taken while the LED is lighting. In this way, the LED only needs to be lighted for a short period of time and a reduced amount of data is stored both of which minimize the power requirements of the wearable sensor device.

BACKGROUND

A wearable sensor device is a device worn by a user that is configured to monitor an action or characteristic of the user. For example, a wearable sensor device may include an accelerometer for detecting a user's movement and/or a biometric sensor for measuring the user's blood oxygen level or pulse rate. As wearable sensor devices become more commonplace, a primary design consideration is the power requirement of the wearable sensor device. If a wearable sensor device does not provide good battery life, it can be inadequate for monitoring a user's biometric characteristics. Accordingly, the present invention is directed to techniques for increasing the power efficiency of a wearable sensor device, and in particular, of a wearable sensor device that employs LEDs and a light sensor to detect a wearer's biometrics.

BRIEF SUMMARY

The present invention extends to wearable sensor devices and to methods performed by such wearable sensor devices which increase the power efficiency of the wearable sensor device. The present invention includes techniques for allowing an LED of a wearable sensor device to be driven less frequently while still obtaining an accurate biometric sample. By reducing how frequently the LED is driven, the power required to operate the wearable sensor device is likewise reduced.

To allow the LED to be driven less frequently while still obtaining an accurate biometric sample, the present invention can employ a known lighting profile of an LED in conjunction with a matched filter to generate a biometric sample from a number of samples taken while the LED is lighting. In this way, the LED only needs to be lighted for a short period of time and a reduced amount of data is stored both of which minimize the power requirements of the wearable sensor device.

In one embodiment, the present invention is implemented as a a wearable sensor device that includes a housing configured to allow the wearable sensor device to be worn on a portion of the body, and a circuit for producing biometric samples. The circuit includes a first LED secured to the housing in a manner that causes the first LED to face the portion of the body when the wearable sensor device is worn; a light sensor secured to the housing, the light sensor being positioned to receive light that is transmitted from the first LED and reflected from the portion of the body, the light sensor being configured to generate a first set of sensed samples representing an intensity of the reflected light during a first period of time; a matched filter configured to receive the first set of sensed samples and a known lighting profile of the first LED, the matched filter performing a weighted average of the first set of sensed samples and the known lighting profile to generate a first biometric sample for the first period of time; and a storage for storing the first biometric sample.

In another embodiment, the present invention is implemented as a method for generating a biometric sample. A first LED is powered on at a first time. A first set of sensed samples is received from a light sensor. The first set of sensed samples represents an intensity of light from the first LED that is incident on the light sensor over a first period of time after the first time. A known lighting profile of the first LED is received. A biometric sample is generated from the first set of sensed samples and the known lighting profile of the first LED.

In another embodiment, the present invention is implemented as a wearable sensor device that includes a housing configured to allow the wearable sensor device to be worn on a portion of the body, and a circuit for producing biometric samples. The circuit includes a first LED and a second LED that are each secured to the housing in a manner that causes the first LED and the second LED to face the portion of the body when the wearable sensor device is worn; a light sensor secured to the housing, the light sensor being positioned to receive light that is transmitted from the first and second LEDs and reflected from the portion of the body, the light sensor being configured to generate, during a first period of time, a first set of sensed samples representing an intensity of the reflected light of the first LED and a second set of sensed samples representing an intensity of the reflected light of the second LED; and a matched filter configured to receive the first set of sensed samples and a known lighting profile of the first LED and to receive the second set of sensed samples and a known lighting profile of the second LED, the matched filter configured to generate a first biometric sample and a second biometric sample for the first period of time, the first biometric sample being generated by performing a weighted average of the first set of sensed samples and the known lighting profile of the first LED, the second biometric sample being generated by performing a weighted average of the second set of sensed samples and the known lighting profile of the second LED.

DETAILED DESCRIPTION

FIG. 1illustrates an example of a bracelet100that can be configured to implement embodiments of the present invention. Although a bracelet configured to be worn around the wrist will be used to describe the present invention, it is noted that other types of wearable devices that can be worn on the wrist or other parts of the body can also be configured to perform embodiments of the present invention.

Bracelet100includes a red LED101aand an infrared (IR) LED101bthat are exposed on an inner surface of bracelet100. Accordingly, when bracelet100is worn by a user, red LED101aand IR LED101bwill emit red light and infrared waves (collectively referred to as “light”) onto the wearer's skin.

Bracelet100also includes a light sensor102that is exposed on the inner surface of bracelet100. Light sensor102is positioned adjacent LEDs101a,101bso as to be able to capture light (i.e., both red light and infrared waves) that is emitted by LEDs101a,101band reflected from the wearer's body. By sensing light that is reflected from the wearer's body, one or more biometrics of the wearer can be determined. These biometrics can include, for example, heart rate, heart rate variability, respiratory rate, oxygen saturation, pulse pressure, systemic vascular resistance, arterial stiffness, stroke volume, systolic blood pressure, diastolic blood pressure, cardiac output, and left ventricular ejection fraction.

A primary design consideration for a wearable device is its battery life. In bracelet100, the primary components that consume power are the LEDs. Accordingly, if the LEDs can be driven less frequently, the battery life of bracelet100will be increased. However, driving the LEDs less frequently would typically reduce the quality of the samples or require substantial and expensive circuitry to produce quality samples. The present invention is primary directed to these issues, or in other words, the present invention provides a way to drive the LEDs less frequently while still obtaining quality biometric samples.

FIG. 2illustrates a block diagram of a circuit200that can be used to generate biometric samples from LEDs101a,101beven when the LEDs are only powered on for short periodic durations. Circuit200includes red LED101a,IR LED101b,and light sensor102as depicted inFIG. 1. As described above, light sensor102is configured to sense light that is emitted from LEDs101a,101band reflected from the wearer's body. Light sensor102provides an output (e.g., a voltage) that represents the intensity of light that is incident on the light sensor. Light sensor102can be configured to provide a first output representing the intensity of red light and a second output representing the intensity of the infrared waves. Readings of the outputs of light sensor102will be referred to herein as “sensed samples” and are to be distinguished from biometric samples which are generated from sensed samples as will be described below.

The remaining components of circuit200are configured to allow LEDs101a,101bto be driven at short periodic intervals while still generating useful biometric samples. For purposes of the current discussion, it will be assumed that circuit200is configured to generate a single biometric sample per LED every 33 ms (i.e., to produce biometric samples at a frequency of 30 Hz). However, biometric samples could equally be produced at different frequencies in accordance with embodiments of the present invention. Accordingly, for every cycle that LEDs101a,101bare powered on, a single biometric sample will be generated based on sensed samples of red LED101aand a single biometric sample will be generated based on sensed samples of IR LED101b.

Because biometric samples are produced at a frequency of 30 Hz, circuit200can include a processing unit201for driving LEDs101a,101bat a frequency of 30 Hz. Processing unit201can therefore power on LEDs101a,101bevery 33 ms (e.g., by employing an interrupt service routine). At each cycle, processing unit201can drive LEDs101a,101bfor a specified duration to allow a sufficient number of sensed samples to be obtained. This specified duration can correspond with the amount of time that it takes for LEDs101a,101bto fully power up (i.e., to reach full intensity after being in an off state). In some embodiments, this duration may be approximately 3 ms which is slightly longer than the amount of time it takes for LEDs101a,101bto reach full intensity (which is typically around 1.5 ms).

Circuit200can include memory202in which sensed samples are temporarily stored. In conjunction with driving LEDs101a,101beach cycle, processing unit201can also initiate direct memory access to cause a number of sensed samples to be written to memory202each cycle. For example, every 33 ms, direct memory access may be enabled to cause 20 sensed samples per LED to be stored in memory202. Accordingly, a total of 600 sensed samples would be generated per LED per second. In a particular embodiment, the sampling interval employed to obtain sensed samples may be approximately 0.068 ms such that the 20 sensed samples are obtained over a period of approximately 1.36 ms.

As stated above, a single biometric sample per LED is generated for each cycle. Therefore, a biometric sample generated for a given cycle is based on the 20 corresponding sensed samples obtained during the given cycle. Although it may be possible to do a simple averaging of the 20 sensed samples or to take the highest value from the 20 sensed samples to generate the biometric sample, the present invention employs a matched filter to generate the biometric sample from the sensed samples as a weighted average as will now be described. By employing a matched filter to implement a weighted average, an accurate biometric sample can be generated even though the biometric sample is based on sensed samples obtained while the LED is powering on.

FIG. 3illustrates example lighting profiles of red LED101aand IR LED101b.These lighting profiles represent how the intensity of the light emitted by the LEDs increases when the LEDs are powered on. In other words, these lighting profiles can represent the output of light sensor102that would be generated when light sensor102senses unobstructed light emitted from LEDs101a,101bwhile the LEDs are powered on. In the graph inFIG. 3, each LED is powered on at time 0 ms at which point the intensity of each LED is 0. Each LED has a delay before any light is emitted. For example, red LED101ais shown as having a delay of approximately 0.2 ms while IR LED101bis shown as having a delay of approximately 0.7 ms. After these delays, the intensity of the light emitted by each LED increases quickly until reaching a steady-state value of approximately 1000.

To employ these lighting profiles to generate biometric samples from the sensed samples, the sensed samples and the lighting profiles can be represented as vectors having the same dimensions. In the current example, each vector would therefore comprise 20 entries. For example, vectors representing the known lighting profile of red LED101a(hereinafter “Red Template”) and the known lighting profile of IR LED101b(hereinafter “IR Template”) can be as follows:

As shown, in some embodiments, the template values can be scaled down (e.g., by a factor of 10) to prevent overflow during the matched filter processing.

Circuit200can be configured to provide the red template and IR template as inputs to matched filter203as shown inFIG. 2. At each cycle, processing unit201can be configured to access memory202to obtain the 20 sensed samples per LED and provide the sensed samples in the form of a vector. For example, example vectors containing sensed samples for red LED101a(hereinafter “Red Samples”) and sensed samples for IR LED101b(hereinafter “IR Samples”) for a particular cycle can be as follows:

Matched filter203can be configured to first perform a dot product on the template and sample vectors. For example, matched filter203can produce the dot product of Red Template and Red Samples to generate a result corresponding to red LED101aand can produce the dot product of IR Template and IR Samples to generate a result corresponding to IR LED101b.Matched filter203can then divide the dot product by the number of sensed samples to generate the biometric sample as a weighted average. In some embodiments, the dot product can be divided by an integer multiple of the number of samples to reduce the size of the biometric sample. By reducing the size of the biometric samples (e.g., by three bits), less power can be required to transmit and store the biometric sample in storage204.

Accordingly, matched filter204can be configured to perform the following calculation:

where X is an integer multiple of the number of samples such as 200. Using 200 as the value of X, the biometric sample that would be generated from Red Template and Red Samples would therefore be 4672.

By employing the known lighting profiles in this calculation, the dynamic range of the biometric samples is increased while minimizing any effect that noise in the sensed samples will have on the calculation. For example, if it is assumed that the sensed samples are each represented as 11 bit values, matched filter203can yield a 14 bit value for the biometric sample. Employing the templates in the dot product calculation ensures that this increase in 3 bits represents a fixed gain that minimizes the presence of noise in the reflected light.

In some embodiments, the leading zeros in the templates can also be used to identify when bracelet100is not being worn. If bracelet100is not being worn when an LED cycle is initiated, the initial sensed samples may have a non-zero value due to ambient light incident on light sensor102. In such cases, circuit200can be configured to detect that bracelet100is not being worn and can enter into a power saving mode in which the LEDs are not cycled on.

In some embodiments, processing unit201can be configured to monitor the values of the sensed samples to determine if the gain and offset of light sensor102and/or the power level of LEDs101a,101bshould be adjusted. To maximize power savings, processing unit201can be configured to monitor the values of the sensed samples to determine if the values are sufficiently high, and if so, can lower the amount of power that is output to drive one or both of the LEDs. If the values of the sensed samples become too low, processing unit201can then increase the power level thereby increasing the intensity of the light emitted by the LEDs. This process can be continually performed to ensure that the LEDs are driven with just enough power to produce suitable sensed samples.

If adjusting the power level of the LEDs does not produce satisfactory increases in the values of the sensed samples, the gain of light sensor102can be increased. Also, if the baseline of the sensed samples moves, the offset of light sensor102can be adjusted accordingly. This control loop can be implemented to ensure that the quality of the sensed samples is maintained while also minimizing the amount of power that circuit200consumes.

FIG. 4provides a flowchart of an example method400generating a biometric sample. Method400will be described with reference toFIG. 2and the example templates and sensed samples provided above.

Method400includes a step401of powering on a first LED at a first time. For example, red LED101aor IR LED101bcould be powered on at a first time.

Method400includes an act of receiving, from a light sensor, a first set of sensed samples representing an intensity of light from the first LED that is incident on the light sensor over a first period of time after the first time. For example, matched filter203can receive the Red Samples vector or the IR Samples vector.

Method400includes an act403of receiving a known lighting profile of the first LED. For example, matched filter203can receive the Red Template vector or IR Template vector.

Method400includes an act404of generating a biometric sample from the first set of sensed samples and the known lighting profile of the first LED. For example, matched filter203can generate a biometric sample from Red Samples and Red Template or from IR Samples and IR Template.

The present invention has been described as employing a known lighting profile to generate a biometric sample. However, the same process for generating a biometric sample can be employed with a template that is not a known lighting profile. In other words, the values within the template do not need to correspond with the intensity of light emitted by the LED as it is powered on. For example, a generic template configured as a vector of twenty values could be employed in place of Red Template and/or IR Template in the above described example. For purposes of this description and the following claims, therefore, the term “template” should be construed as a vector of values whether or not the values represent a known lighting profile of an LED.