Method for checking a PPG sensor of a hearing device and hearing device system

Abstract

Method for checking a PPG sensor (12) of a hearing device (1) having a rechargeable battery (10), wherein according to the method - a charger (22) with a receiving chamber (26) for the hearing device (1) is provided, wherein the charger (22) has a test environment for the PPG sensor (12), - the hearing device (1) is positioned in the recording room (26), and - a functional test of the PPG sensor (12) is performed using the test environment, wherein the test environment for the PPG sensor (12) comprises a test light source (36), wherein, as part of the functional test, the PPG sensor (12) is illuminated by means of the test light source (36) and a measure (S) subsequently output by the PPG sensor (12) P ) for a measured amount of light with a corresponding measure (S CL ) the amount of light emitted by the test light source (36) is compared, and / or wherein the test environment for the PPG sensor (12) comprises a test photodetector (38), wherein during the functional test the PPG sensor (12) illuminates the test photodetector (38) and a measure (S) subsequently output by the test photodetector (38) PT ) for a detected amount of light is compared with a corresponding measure (Sc) of the amount of light emitted by the PPG sensor (12).

Description

[0001] The invention relates to a method for testing a PPG sensor of a hearing device. Furthermore, the invention relates to a hearing device system.

[0002] Hearing devices are typically used to output an audio signal to the wearer's ear. This output is achieved via an output transducer, usually acoustically through sound waves transmitted via a loudspeaker (also called a "receiver"). Such hearing devices are frequently used as assistive listening devices (or simply hearing aids). They typically include an acoustic input transducer (especially a microphone) and a signal processor. This processor is designed to process the input signal (also called the microphone signal) generated from the ambient sound by the input transducer, using at least one user-specific signal processing algorithm, in such a way as to at least partially compensate for the wearer's hearing loss.Particularly in the case of a hearing aid, the output transducer can be a loudspeaker, a bone conduction receiver, or a cochlear implant, all of which are designed to mechanically or electrically couple the sound signal into the wearer's ear. The term "hearing devices" also includes devices such as tinnitus maskers, headsets, headphones, and similar devices.

[0003] Similar to the increasing use of features in so-called "wearables," such as fitness trackers, smartwatches, and the like, which use sensors to record bodily functions (e.g., pulse, movement, etc.), the use of such functions is also becoming more common in hearing aids. For example, body temperature, pulse, or similar data may be recorded. Such an additional use of hearing aids is particularly practical because, especially in the case of hearing aids, these devices are usually worn close to the body and often for relatively long periods or even continuously.

[0004] In addition to the use of familiar chest straps that detect the electrical signals of heart muscle contractions, so-called photoplethysmography sensors (PPG sensors) are also used to measure pulse. The measuring principle of such a PPG sensor is based on the fact that body tissue is irradiated with light from one or more predefined wavelength ranges (e.g., different bands of the near-infrared range and / or the visible wavelength range), and reflected or transmitted radiation is detected and used to determine tissue properties, in particular the current blood flow. The amount of light detected obviously influences the evaluation and determination of the tissue properties. Since hearing devices, especially hearing aids, are usually worn on or even in the ear, they are also known to contain bodily fluids, e.g.,exposed to sweat and / or cerumen, which can lead to a deterioration in the function of the PPG sensor.

[0005] From DE 10 2020 209 507 A1, it is known that, to check hearing aid functions, a hearing aid to be tested is placed in a charging case with a lockable charging compartment, a communication connection is established between the hearing aid and the charging case, an acoustic test signal is emitted and recorded by means of a microphone in the hearing aid. A signal processor in the hearing aid checks the transfer function for the test signal for an indication of a malfunction; if a malfunction is indicated, an error entry is made in a memory of the hearing aid and / or an error message is displayed by means of a display device in the charging case.

[0006] GB 2 532 745 A describes a portable device that measures a user's core body temperature and other vital parameters (e.g., respiration, pulse oximetry, movement, ECG, BCG). The device comprises an earpiece that is held in the ear canal and has an infrared thermopile at its innermost end. The thermopile is positioned as close as possible to the eardrum. A channel is provided for audio transmission, which can be via a loudspeaker. Calibration of the thermopile may be provided. The earpiece may include a reflector to reflect infrared signals onto the thermopile when it is positioned parallel to the ear canal. Two earpieces may be provided. The earpiece communicates wirelessly with a remote device, such as a watch, smartphone, computer, tablet, or internet hub, and can be used in various situations (e.g.,in healthcare, sports, the military or the fire brigade).

[0007] CN 1 11 067 499 A describes an apparatus for testing PPG devices. The apparatus comprises a dark box, a black card insertion device, a black card drive mechanism, a gray card insertion device, a gray card drive mechanism, a light source blocking piece, a light source blocking mechanism, and a control device. A position for inserting a PPG device is formed on an upper plate of the dark box. A through-hole is formed in the PPG insertion position. The black card drive mechanism is used to drive the black card insertion device to move a black card to or from a position opposite the through-hole in the dark box.The drive mechanism for the gray card is used to move a gray card toward or away from a position opposite the through-hole in the dark box. The drive mechanism for the light source blocking device is used to move the light source blocking device toward the through-hole to block the light source of the PPG device placed at the PPG device placement position, or away from the PPG device placement position.The control device is used to control the drive mechanism of the black card, the drive mechanism of the grey card, and the drive mechanism of the light source locking device, and to control the PPG device to emit light when the black card and grey card move to a position corresponding to the through-hole, or when the light source locking device blocks the light source of the PPG device.

[0008] The invention is based on the objective of being able to better monitor the function of a hearing device.

[0009] This problem is solved according to the invention by a method having the features of claim 1. Furthermore, this problem is solved according to the invention by a hearing device system having the features of claim 9. Advantageous and partly inventive embodiments and further developments of the invention are set out in the dependent claims and the following description.

[0010] The method according to the invention serves to test, in particular to test the functionality of, a PPG sensor of a hearing device that has a rechargeable battery. According to the method, a charger with a receiving compartment for the hearing device is provided. Preferably, the charger is designed as a lockable box, so that during normal charging operation, the receiving compartment containing the hearing device is closed. The charger also has a test environment for the PPG sensor. The hearing device is positioned in this receiving compartment, and then a functional test of the PPG sensor is performed using the test environment.

[0011] The charger and test environment thus advantageously create at least approximately repeatable—and sufficiently stable for the purpose of the procedure described here and below—and therefore comparable environmental conditions for the hearing device. This facilitates, or even initially enables, functional testing.

[0012] In a preferred method variant, a user of the hearing device receives a message regarding the functionality of the PPG sensor at least when a functional test result falls below a predefined threshold. In this case, the user is specifically warned that the PPG sensor is no longer functioning correctly. Since malfunctions of the PPG sensor in a hearing device are frequently caused by contamination of the PPG sensor with bodily fluids, particularly due to at least a partial blockage of the beam path between a light source and a photodetector of the PPG sensor, a recommendation for cleaning the PPG sensor is optionally issued in addition to the warning.

[0013] It is conceivable that, during the functional test, a wall of the recording chamber is used as a reflective surface for the PPG sensor. In this case, the hearing device performs the functional test as a self-test. Preferably, the hearing device has a controller that is configured (particularly through programming or circuitry) to perform this self-test automatically. Specifically, the PPG sensor emits a test signal via its light source, detects a reflection signal (occurring particularly due to reflection of the test signal at the wall) using its photodetector, and then evaluates the functionality of the PPG sensor. The wall of the recording chamber preferably has predetermined reflective properties. For example, the wall has a reflective surface to minimize optical attenuation (and / or scattering) of the test signal.However, a white (optionally glossy) and / or matte surface is also conceivable. Optionally, the reflective properties of the hearing aid could be transmitted from the charger to the hearing aid when it is inserted into the recording space (e.g., as part of a pairing process).

[0014] For evaluation, the amount of light emitted as part of the test signal is compared with the amount of light measured for the reflection signal, and in particular with a limit value. If the emitted and detected amounts of light differ too greatly (i.e., if the difference between these two amounts of light falls below the aforementioned limit value), this is considered a limited function, and the aforementioned warning is preferably issued.

[0015] According to one embodiment of the invention, the test environment for the PPG sensor includes a test light source. In a particularly advantageous method variant, during the functional test, the PPG sensor is illuminated by the test light source, and a measurement of the detected light quantity output by the PPG sensor is compared with a corresponding measurement of the light quantity emitted by the test light source. This allows the function of the PPG sensor, and in particular its photodetector, to be checked independently of the PPG sensor's light source. This is because it can be assumed with a sufficiently high probability that the test light source will not degrade or will degrade significantly more slowly than the PPG sensor (in particular, it will be hardly exposed to contamination), so that an unexpectedly low detected light quantity indicates a functional limitation of the PPG sensor.

[0016] Preferably, a test light source similar to the light source of the PPG sensor, which preferably emits the same or at least a sufficiently similar light spectrum, is used as the test light source.

[0017] In a particularly useful further development, the test light source is controlled with a predetermined value of an electrical control variable, especially a current, to illuminate the PPG sensor. This control variable is used specifically as a measure of the emitted amount of light. As a measure of the detected amount of light, a photocurrent of the photodetector, generated, for example, by one or more photodiodes, is preferably used. The "photocurrent" is, in particular, the current output by the photodetector in response to incident light. Optionally, the values ​​(or at least one) of these two measures or quantities are normalized for better comparability.

[0018] According to the invention, the test environment for the PPG sensor comprises, in addition to or as an alternative to the aforementioned test light source, a test photodetector. During the functional test, the PPG sensor advantageously illuminates this test photodetector, particularly by means of its light source. A measure of the detected light quantity then output by the test photodetector is compared with a corresponding measure of the light quantity emitted by the PPG sensor (e.g., the control variable for the light source of the PPG sensor).

[0019] Preferably, a photodetector similar to that of the PPG sensor is used as the test photodetector.

[0020] In a suitable further development, the PPG sensor, in particular its light source, is controlled with a predetermined value of an electrical control variable, especially a current, to illuminate the test photodetector. This control variable is used as a measure of the emitted amount of light. Accordingly, in this case, the photocurrent described above (here referred to as the "test photocurrent") is used as a measure of the amount of light detected by the test photodetector. Optionally, the values ​​(or at least one) of these two measures or quantities are normalized for better comparability.

[0021] In a particularly useful advanced training scenario, the test environment includes both the test light source and the test photodetector. In this case, the light source of the PPG sensor illuminates the test photodetector, and vice versa. This advantageously allows the function of the photodetector and the light source to be checked independently of each other – at least under the assumption that both the test light source and the test photodetector are functioning correctly.

[0022] Optionally, in the above case, the recording chamber is designed to be radiation-absorbing, e.g., matte black, to prevent reflection of the light emitted by the PPG sensor's light source onto its own photodetector. Alternatively or additionally, the illumination of each photodetector is staggered to avoid interfering with the other.

[0023] In another advantageous variant of the procedure, a brightness measurement is determined as a measure of the distance between the PPG sensor and the test photodetector and / or the test light source during a preliminary test for the functional test using the PPG sensor and / or the test photodetector. Preferably, in this case, the functional test is only performed if the brightness measurement meets a predetermined criterion. If the test environment comprises only the test light source, the brightness measurement is determined using the PPG sensor, specifically its photodetector. If the test environment comprises only the test photodetector, the test photodetector is illuminated using the PPG sensor, specifically its light source.In the event that the test environment includes both the test light source and the test photodetector, two brightness measurements are optionally determined and compared, and optionally averaged.

[0024] The procedure described above is particularly useful when hearing aids are used, optionally hearing aids with in-the-ear loudspeaker units that incorporate the PPG sensor. In this case, the loudspeaker unit is often not as easily and consistently positioned in the recording space as a comparatively larger hearing aid body. The brightness measurement, and in particular the resulting indication of the distance between the PPG sensor and the corresponding test unit, thus allows verification of whether the arrangement of the hearing aid, and optionally the loudspeaker unit, is sufficient for conducting the functional test in the current case. If the brightness measurement does not meet the criterion, especially if its value falls below a threshold value that defines the criterion, then, for example...A message was issued to the user indicating that the hearing device should be reinserted or reinserted differently into the recording room.

[0025] In a preferred embodiment of the process, the measurements and quantities described above—specifically, the measurement of the amount of light detected, the measurement of the amount of light emitted, and / or the brightness measurement—are transmitted from the hearing aid to the charger for comparison with the corresponding measurement or predefined criterion. The charger then preferably uses a controller to compare these measurements and quantities with the corresponding measurement or criterion. This controller is configured to perform the comparisons and evaluations described above, including controlling and / or reading the test equipment. In other words, the evaluation of the functional test is performed on the charger's side by means of a suitable controller.

[0026] Alternatively, the evaluation can also be carried out on the side of the hearing device, preferably by means of a suitably equipped and trained controller.

[0027] The hearing device system according to the invention comprises a hearing device including a PPG sensor and a rechargeable battery, as well as a charger comprising a receiving chamber for the hearing device and a test environment for the PPG sensor. Furthermore, the hearing device system includes at least one controller configured to perform the method described above, preferably automatically, optionally with user interaction. Preferably, the hearing device and the charger are the respective devices described above within the context of the method. The controller is part of the hearing device or the charger.

[0028] Preferably, both devices each have a controller that is optionally configured to take over at least part of the functional test. For example, the hearing aid's controller is configured to process the signals from the PPG sensor, e.g., to condition them, and output them for transmission to the charger.

[0029] The hearing aid system thus incorporates all the advantageous enhancements resulting from the preceding description, including the resulting physical features – e.g., the test light source and / or the test photodetector – equally in corresponding optional enhancements. The resulting advantages are therefore also accrued to the hearing aid system.

[0030] The conjunction “and / or” is to be understood here and in the following in particular as meaning that the features linked by means of this conjunction can be formed both jointly and as alternatives to each other.

[0031] Furthermore, the following method represents an additional variant or an invention independent of the method described above. A method for checking a temperature sensor of a hearing device which has a rechargeable battery, wherein according to the method - a charger with a receiving compartment for the hearing device is provided, the charger having a test temperature sensor, - the hearing device is positioned in the recording room, - a temperature measurement is recorded using the temperature sensor (of the hearing device) and the test temperature sensor, - the two temperature measurements are compared with regard to their values, and - depending on a difference between the values ​​of the two temperature measurements, a further measure is taken. B Procedure according to point A, wherein the two temperature measurements (in particular their values) are transmitted between the hearing aid and the charger. Preferably, the temperature measurement from the hearing aid is transmitted to the charger. C Procedure according to point A or B, As a further measure, the temperature sensor of the hearing aid (more commonly referred to as the "test temperature sensor") is calibrated (or referenced) to the value of the test temperature sensor. In other words, the temperature sensor of the hearing aid is specifically set to the value of the test temperature sensor. This is done under the assumption that the test temperature sensor is subject to less stress that could negatively affect the measurement and should therefore provide more reliable values ​​compared to the temperature sensor. D Procedure according to one of points A to C, where, based on a temporal development, at least one of the temperature measured values ​​is monitored until a temperature stabilization occurs in the recording chamber. Specifically, the process waits until the values ​​of the corresponding (test) temperature sensor no longer change (or only change to a negligible extent), thus ensuring a steady-state (i.e., stable, preferably static) thermal condition in the recording chamber. E Procedure according to one of points A to D, wherein a resistive temperature sensor or an infrared temperature sensor is used as the temperature sensor of the hearing device, and conversely, an infrared temperature sensor or a resistive temperature sensor is used as the test temperature sensor. In particular, the photodetector of the PPG sensor described above, which is advantageously sensitive to infrared radiation, is used as the infrared temperature sensor. F Procedure according to point E, The infrared temperature sensor is arranged such that the corresponding temperature measurement is detected at the measuring point of the resistive temperature sensor. In particular, the recording space is designed such that, when the hearing aid is positioned as intended within the recording space, the temperature sensor and the test temperature sensor are arranged opposite each other. Optionally, the recording space is designed such that the hearing aid can only be positioned in one spatial orientation within the recording space. This arrangement of the infrared temperature sensor allows it—especially since it can detect the temperature over a free distance of up to a few millimeters (approximately 2 to 5)—advantageously to detect the temperature at the same location as the resistive temperature sensor. G Procedure according to one of points A to F, in particular E or F, as a further measure, the response behavior of the temperature sensor to a temperature change is modeled as a function of the temporal development of the value of its temperature measurement quantity as well as the value of the test temperature sensor, in particular by means of the infrared temperature sensor.

[0032] Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. The drawing shows: Fig. 1. A hearing device in a schematic side view, Fig. 2 in a schematic side view a hearing aid system that adjusts the hearing aid according to Fig. 1 and a charger, and Fig. 3-5 in view according to Fig. 2 each a further embodiment of the hearing aid system.

[0033] Corresponding parts and sizes are always marked with the same reference symbols in all figures.

[0034] In Fig. Figure 1 is a hearing device in the form of a hearing aid, specifically a hearing aid worn behind the ear by a user (referred to here as "BTE 1"). The BTE 1 comprises a housing 2 in which electronic components of the BTE 1 are arranged. These electronic components include, for example, two microphones 4, a signal processor 6, and a battery module 8, which in turn contains a rechargeable battery. During normal operation of the BTE 1, the microphones 4 serve to receive ambient sound and convert it into electrical input signals (also called "microphone signals"), which are processed by the signal processor 6 (also referred to as the "controller") (in particular, filtered, frequency-dependently amplified, and / or attenuated, etc.).The processed input signals are then output to a loudspeaker (not shown) and converted into sound signals, which are then passed on to the user's hearing.

[0035] The BTE 1 also features a photoplethysmography sensor, or "PPG sensor 12" for short, which in this embodiment is also installed in the housing 2. The PPG sensor 12 serves to determine, for example, the pulse, and optionally also the oxygen saturation, of the user of the BTE 1. For this purpose, the PPG sensor 12 includes a light source, in this case an LED unit 16, which is configured to emit light of several frequency bands, but usually at least light in the near-infrared range. The PPG sensor 12 also includes a light sensor 18 ("photodetector"), which detects incident light. In normal operation, the light sensor 18 detects radiation emitted by the LED unit 16 and reflected by body tissue, for example, the wall of a blood vessel in the user. Based on the intensity profile detected, the user's pulse, for example, can then be determined.

[0036] In Fig. Figure 2 shows a hearing aid system 20 in more detail. This system comprises the BTE 1 described above and a charger 22. The latter is used to recharge the rechargeable battery of the BTE 1. For this purpose, the charger 22 has a receiving compartment 26 in which the BTE 1 is placed in its intended state of charge (see Figure 2). Fig. 2) is positioned. The charger 22 provides a test environment for the BTE 1, specifically for its PPG sensor 12. Furthermore, the charger 22 has a lid 28 by means of which the receiving chamber 26 can be closed.

[0037] An example of a procedure for a functional test of the PPG sensor 12 is described below using the following: Fig. 2 is described in more detail. In this process, the BTE 1, specifically its signal processor 6, performs a self-test of the PPG sensor 12. The test environment in this example serves as follows: Fig. 2. To provide conditions for the self-test of the PPG sensor 12 that are as repeatable and stable as possible. For this purpose, a wall of the receiving chamber 26, which in the intended insertion state of the BTE 1 in the receiving chamber 26 is equipped with a reflective surface 30 of a specified "quality". For example, this surface 30 is white but not glossy, in order to allow light scattering in as many directions as possible ("diffuse scattering / reflection"). During the self-test, the signal processor 6 controls the PPG sensor 12 to generate a light signal ("test signal") L using the LED unit 16. S to output a predetermined amount of light (or radiation intensity). Using the light sensor 18, at least a portion of the light signal L reflected from the surface 30 is detected. S The amount of light detected is measured by a photocurrent S output by the light sensor 18. PIts value is used. It is compared with a measure of the emitted light quantity, specifically a control variable, in particular a control current Sc, by means of which the LED unit 15 is controlled. If the value of the photocurrent S exceeds P If a limit value Gs, which is predetermined depending on the surface properties 30, is exceeded, it can be assumed that the PPG sensor 12 is functioning correctly. If this limit value Gs is undershot, the signal processor 6 sends a message to the user of the BTE 1. In this message, the signal processor 6 informs the user that the PPG sensor 12 is not functioning as intended, e.g., that it is dirty.

[0038] Optionally, a comparison is made with the limit value Gs in a controller 32 of the charger 22. In this case, the signal processor transmits the value of the control current Sc and the photocurrent S. P by means of a transmitter 34 to the controller 32 of the charger 22.

[0039] In Fig. Figure 3 describes an alternative embodiment of the hearing aid system 20 and the method used to test the function of the PPG sensor 12. In addition to the surface 30 described above, the charger 22 also has a test light source 36. This light source is identical in design to the LED unit 16 of the PPG sensor 12 and can therefore emit light of the same wavelength(s). Furthermore, the test light source 36 is positioned in the receiving chamber 26 such that, when the BTE 1 is positioned as intended, it is opposite the PPG sensor 12.

[0040] This test light source 36 is optionally used in addition to or as an alternative to the procedure described above to test the light sensor 18 separately as part of the functional check. For this purpose, the charger 22 emits a test light signal L by means of the test light source 36. Tin the direction of the PPG sensor 12. If this is detected by the light sensor 18, the BTE 1 sends the determined photocurrent S. P to the controller 32 of the charger 22. This compares the photocurrent S P (whose value) with its own control current S output to the test light source 36 CL (specifically, its value). If the values ​​of both quantities are sufficiently similar, the controller 32 concludes that the light sensor 18 is functioning correctly. Otherwise, the controller 32 issues a warning. If the self-test described above is subsequently performed with a functioning light sensor 18 and the limit value Gs is undershot, this indicates a malfunction of the LED unit 16. A corresponding warning is then issued.

[0041] In Fig. Figure 4 describes an alternative embodiment of the hearing aid system 20 and the method used to test the function of the PPG sensor 12. In addition to the above, Fig. In addition to the test light source 36 described in Section 3, the charger 22 also has a test photodetector (referred to here as "test light sensor 38"). This is identical in design to the light sensor 18 of the PPG sensor 12 and thus has the same detection characteristics. The test light sensor 38 is also positioned in the recording space 26 such that, when the BTE 1 is positioned as intended, it is opposite the PPG sensor 12.

[0042] In addition to the one based on Fig. In the procedure described in 3, the light signal L will be displayed during the functional test. S emitted by the LED unit 16 and at least partially received by the test light sensor 38. A corresponding test photocurrent S PTThe controller 32 compares the control current Sc for the LED unit 16, which was transmitted to the controller 32 by the BTE 1. This allows the function of the LED unit 16 and the light sensor 18 to be tested independently of each other.

[0043] In an optional variant, the controller 32 is configured to perform a self-test for the test light source 36 and the test light sensor 38 by operating them similarly to the PPG sensor, e.g., when the BTE 1 is not inserted. In this case, the test is performed similarly to the test based on... Fig. The self-test described in section 2 was performed.

[0044] Optionally, several test light sources 36 are arranged in the recording chamber 26. To determine which test light source 36 is most "opposite" to the PPG sensor 12, i.e., specifically, which one is closest to it, the individual test light sources 36 are activated one after the other, and the intensity measured in each case is recorded. The test light source 36 that elicits the "strongest" response is assumed to be the closest.

[0045] In Fig.Figure 5 shows another embodiment of the hearing aid system 20. The BTE 1 includes a temperature sensor 50, which is designed as a resistive temperature sensor. This sensor serves to determine the body temperature of the user of the BTE 1 when worn as intended. The charger 22 includes an infrared temperature sensor, or IR sensor 52. As described above for the test light source 36, this sensor is also arranged opposite the temperature sensor 50. After the BTE 1 is placed in the charger 22, specifically in the receiving chamber 26, the temperature at the location of the temperature sensor 50 is measured by the IR sensor 52. After an adjustment period, the end of which is indicated by the fact that the measured temperature values ​​no longer change, the temperature values ​​measured by the temperature sensor 50 and the IR sensor 52 are compared.If these values ​​differ, the temperature sensor 50 is set to the value of the IR sensor 52, thus being "calibrated" to it.

[0046] The subject matter of the invention is not limited to the embodiments described above. Rather, further embodiments of the invention can be derived by a person skilled in the art from the foregoing description. In particular, the individual features of the invention and their various configurations described with reference to the different embodiments can also be combined with one another in other ways. Reference symbol list 1 BTE 2 cases 4 microphones 6 Signal processor 8 battery module 12 PPG sensors 16 LED unit 18 light sensor 20 hearing aid systems 22 charger 26 Recording room 28 lids 30 surface 32 Controller 34 Transmitter 36 Test light source 38 Test light sensor 50 temperature sensor 52 IR sensors L S Light signal L T Test light signal Sc control current S P Photocurrent Gs limit value S CL Control current S PT Test photocurrent

Claims

[1] Method for testing a PPG sensor (12) of a hearing device (1) having a rechargeable battery (10), wherein according to the method - a charger (22) having a receiving space (26) for the hearing device (1) is provided, wherein the charger (22) has a test environment for the PPG sensor (12), - the hearing device (1) is positioned in the receiving space (26), and - a functional test of the PPG sensor (12) is carried out using the test environment, wherein the test environment for the PPG sensor (12) comprises a test light source (36), wherein, as part of the functional test, the PPG sensor (12) is illuminated by means of the test light source (36) and a measurement (S P ) for a detected amount of light with a corresponding measure (S CL ) is compared with the amount of light emitted by the test light source (36), and / or wherein the test environment for the PPG sensor (12) comprises a test photodetector (38), wherein, as part of the functional test, the PPG sensor (12) illuminates the test photodetector (38) and a measurement (S PT ) for a detected amount of light is compared with a corresponding measure (Sc) of the amount of light emitted by the PPG sensor (12). [2] Method according to claim 1, wherein a user of the hearing device (1) is given a message regarding the functionality of the PPG sensor (12) at least when a result of the functional test falls below a predetermined limit value (Gs). [3] Method according to claim 1 or 2, wherein a test light source (36) similar to a light source (16) of the PPG sensor (12) is used as the test light source. [4] Method according to claim 1, wherein the test light source (36) is provided with a predetermined value of an electrical control variable (SCL ), in particular a current, for illuminating the PPG sensor (12), and wherein the control variable (S CL ) is used as a measure of the amount of light emitted. [5] Method according to one of claims 1, 2 or 4, wherein a test photodetector (38) similar to a photodetector (18) of the PPG sensor (12) is used as the test photodetector. [6] Method according to claim 1, wherein the PPG sensor (12), in particular a light source (16) of the PPG sensor (12), is controlled with a predetermined value of an electrical control variable (Sc), in particular a current, for illuminating the test photodetector (38), and wherein the control variable (Sc) is used as a measure of the amount of light emitted. [7] Method according to one of claims 1 to 6, wherein, as part of a preliminary test for the functional test, a brightness measurement variable is determined by means of the PPG sensor (12) and / or the test photodetector (38) as a measure of a distance between the PPG sensor (12) and the test photodetector (38) and / or the test light source (36), in particular wherein the functional test is only carried out if the brightness measurement variable satisfies a predetermined criterion. [8] Method according to one of claims 1 to 7, wherein the measure of the detected amount of light (S P ), the measure of the amount of light emitted (Sc) and / or the brightness measurement for comparison with the corresponding measure (Sc, S P , S CL , S PT ) or with the predetermined criterion from the hearing device (1) to the charger (22) and by the charger (22) by means of a controller (32) set up for this purpose with the respectively corresponding measure (Sc, SP , S CL , S PT ) or criterion is compared. [9] Hearing device system (20) comprising a hearing device (1) comprising a PPG sensor (12) and a rechargeable battery (10), as well as a charger (22) comprising a receiving space (26) for the hearing device (1) and a test environment for the PPG sensor (12), and at least one controller (6, 32) which is configured to carry out the method according to one of claims 1 to 8.

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