[0001 ] The present invention generally relates to the technical field of sensing and classifying persons that are present in a vehicle interior, and more specifically to a classifying in terms of mass of a person in a vehicle interior, using radar sensing. The present invention further relates to monitoring vital signs of a person that is present in the vehicle interior.

Background of the Invention

[0002] Seat occupant detection and/or classification devices are nowadays widely used in vehicles, in particular in passenger cars, for providing a seat occupant signal for various appliances, for instance for the purpose of a seat belt reminder (SBR) system or an activation control for an auxiliary restraint system (ARS). Seat occupant detection and/or classification systems include seat occupant sensors that are known to exist in a number of variants, for instance based on capacitive sensing. An output signal of the seat occupant detection and/or classification system is usually transferred to an electronic control unit of the vehicle to serve, for instance, as a basis for a decision to deploy an air bag system to the vehicle seat.

[0003] In automotive technology, for quite many applications it is mandatory to detect whether there is a person located on a seat or not. Beyond that, it could be favorable if a detected passenger could be classified in terms of body mass. For example, the mass information could be used for estimating and controlling the intensity of an air bag deployment. Other applications are conceivable as well, such as optimizing belt tensioners that are operated in case of an occurring accident. Further valuable information, usable as important input for Advanced Driver Assistance Systems (ADAS) could be provided by monitoring a vital sign of the detected person.

[0004] It is also known in the art to use radar technology for seat occupant detection systems. Occupancy sensors based on radar technology offer advantages in comparison to other occupancy detection methods as their operation is contact-free and unnoticeable for vehicle occupants. Moreover, radar sensors can easily be integrated in the vehicle interior, for example behind plastic covers and textiles.

[0005] By way of example, international application WO 2016/038148 A1 describes a method for sensing occupancy status within an automotive vehicle. The method uses a radar sensor system comprising an antenna system, at least one sensor and processing circuitry. The method includes illuminating, by using the antenna system, at least one occupiable position within the vehicle with an outgoing radar signal, and receiving, by using the at least one sensor, at least one sensor signal that has been reflected as a result of the outgoing radar signal. The method further comprises obtaining accelerometer data value from at least one accelerometer, wherein the accelerometer data contain information regarding vibration or motion of the automotive vehicle and supplies the accelerometer data to the processing circuitry. The processing circuitry is being operated for generating, based on the at least one sensor signal and on the accelerometer data, one or more occupancy status signals, wherein the occupancy status signal indicates a property that is related to the at least one occupiable position.

Object of the invention

[0006] It is an object of the present invention to provide a method of operating a radar sensor system for detecting and classifying a person, particularly in terms of mass, and/or for monitoring a vital sign of the person that is present in a vehicle interior. This object is achieved by the invention as claimed in claim 1 .

General Description of the Invention

[0007] In one aspect of the present invention, the object is achieved by a method of operation a radar sensor system for detecting and classifying a person with regard to its mass in an interior of a vehicle. The radar sensor system includes a radar transmitting unit having at least one radar transmitting antenna and being configured for transmitting radar signals towards at least one seat within the vehicle interior. Further, the radar sensor system comprises a radar receiving unit having at least one radar receiving antenna and being configured for receiving radar signals that have been transmitted by the radar transmitter unit and have been reflected by a person located at the at least one seat. Then, the radar sensor system includes an evaluation and control unit that is configured for evaluating Doppler infornnation from the radar signals received by the radar receiving unit and for receiving accelerometer data from at least one accelerometer device, wherein the accelerometer data contain information regarding vibration or motion of a body of the vehicle.

[0008] The at least one accelerometer device may form part of the radar sensor system. The at least one accelerometer device may also be associated to existing vehicle components, such as an airbag control unit. In this case a data connection has to be provided for transferring the accelerometer data from the at least one accelerometer device to the evaluation and control unit.

[0009] The phrase "being configured to", as used in this application, shall in particular be understood as being specifically programmed, laid out, furnished or arranged. The term "vehicle", as used in this application, shall particularly be understood to encompass passenger cars, trucks and buses.

[0010] The proposed method comprises at least steps of operate the evaluation and control unit for evaluating Doppler information from radar signals received by the radar receiving unit,

operate the evaluation and control unit for obtaining, from the received accelerometer data, motion quantities of the radar sensor system relative to an initial position of the radar sensor system, and

the following steps, which are to be carried out in an iterative manner:

- select temporary evaluation parameters of a predefined mechanical model representing a coupled motion of the at least one seat and the radar sensor system,

- based on the obtained motion quantities of the radar sensor system, the predefined mechanical model and the fixed temporary evaluation parameters, reconstruct a motion of the at least one seat,

- generate, based on the reconstructed motion of the at least one seat, temporary Doppler information from Doppler information evaluated from the received radar signals,

- calculate an objective function value for the temporary Doppler information, wherein the objective function is predefined and has an extreme value if the temporary Doppler information is strictly periodic and/or has constant extreme values different from zero, and

- compare the objective function value with at least one predetermined threshold value.

[001 1 ] The iterative steps are continued until the objective function value is equal to or less than the at least one predetermined threshold value. Then, an output signal is generated that is indicative of the latest temporary evaluation parameter that represents the mass of the person.

[0012] The invention is based on the insight that there will be a periodic motion reflected from a seat direction in case that the seat is occupied by a person. The periodic motion may for instance be generated by the person's breathing or heartbeat. If the seat was unoccupied, a periodic motion with a vanishing amplitude would be expected.

[0013] The periodic motion is disturbed by vibrational noise, which stems from exterior or interior events. For example, a vehicle driving over street bumps transfers a vibration caused by the bumps in a vehicle body into an interior of the vehicle, resulting in vibrational motions of a seat/person system. These movements superimpose the periodic motion corresponding to the breathing or heartbeat. The radar sensor system determines both the Doppler frequency shift of the periodic motion and the Doppler frequency shift of the vibration.

[0014] The method of operation according to the invention uses at least one accelerometer in order to track the acceleration caused by the exterior events, since the whole vehicle interior is mechanically linked to the person via the seat. The motion of this configuration is approximated by the predefined mechanical model using the acceleration data as an input. It is then possible to use the accelerator data to reconstruct the source of disturbance and to eliminate the Doppler frequency shift caused by exterior events from the observed Doppler information. It should be noted that the predefined mechanical model is understood to be any mechanical model suitable for representing a coupled motion of the at least one seat and the radar sensor system. It may e.g. comprise a predefined differential equation representing a coupled motion of the at least one seat and the radar sensor system. [0015] The calculated objective function value for the temporary Doppler information is a measure of the degree of elimination of the Doppler frequency shift caused by the exterior events. If the at least one predetermined threshold value for the objective function value is suitably selected such that the degree of elimination suffices, the mass of the person located at the at least one seat can be determined as one of the parameters of the predefined differential equation.

[0016] Preferably, the output signal can be transferred to an electronic control unit of the vehicle to serve, for instance, as a basis for a decision to deploy an air bag system to the vehicle seat and/or for estimating and controlling the intensity of an air bag deployment.

[0017] It is herewith noted that the proposed method is beneficially applicable to conventional Doppler radar sensor systems (CW-radars) as well as to frequency- modulated continuous wave (FMCW) radar sensor systems and phase-modulated continuous wave (PMCW) radar sensor systems.

[0018] It should be noted that the objective function should in general reflect vital sign characteristics. In preferred embodiments of the method, the objective function includes at least a term representing a measure of dispersion of local maxima and/or local minima of the temporary Doppler information. Thus, a low objective function value indicates a high degree of regularity of the temporary Doppler information, which corresponds to the motion generated by a person's breathing or heartbeat.

[0019] In preferred embodiments of the method, the objective function includes at least a term representing a measure of dispersion of time differences of local maxima and/or local minima of the temporary Doppler information. Thus, a low objective function value indicates a high degree of periodicity of the temporary Doppler information, which corresponds to the motion generated by a person's breathing or heartbeat.

[0020] Preferably, the objective function includes at least a term representing a measure of dispersion of time differences of local maxima and/or local minima of the temporary Doppler information as well as at least a term representing a measure of dispersion of time differences of local maxima and/or local minima of the temporary Doppler information. In this way, both the degree of regularity and the degree of periodicity can be exploited for classifying the person in the interior of the vehicle in terms of mass. The terms may be combined in a weighting function, for instance a weighting function having constant weighting factors.

[0021 ] The measure of dispersion may be selected as, without being limited to, a mathematical variance, a standard deviation, a coefficient of variation or a mean absolute distance. Generally, any other measure of dispersion may be used that appears suitable to those skilled in the art.

[0022] In preferred embodiments of the method, the predefined differential equation can be expressed as

Mx(t) + Dx(t) + Sx(t) = Dy(t) + Sy(t), wherein x denotes a displacement of the at least one seat, y denotes a displacement of the interior of the vehicle and parameter M denotes the mass of the at least one seat and the person located at the at least one seat. Further parameters are damping coefficient D and stiffness coefficient S, representing a resilient and a damping member between the seat and the vehicle body, respectively, which are taken to be constant over time.

[0023] In this way, the coupled motion between the at least one seat and the radar sensor system can be described and solved in a mathematically simple way, for instance by applying the simple Euler method for solving differential equations. The mass of the person located at the at least one seat can thus be determined with sufficient accuracy and low effort.

[0024] In other embodiments, the predefined differential equation representing the coupled motion of the at least one seat and the radar sensor system can be of the non-linear type by defining non-linear characteristics or non-constant coefficients. In this case, numerical methods, which are well-known to those skilled in the art, can be applied for solving.

[0025] Preferably, the step of selecting temporary evaluation parameters of the predefined differential equation comprises to select each of the temporary evaluation parameters from an assigned predetermined range for the respective temporary evaluation parameter. By using the assigned predetermined ranges, for instance as starting points, a faster convergence of a solution resulting from the iterative execution of steps can be facilitated. [0026] For instance, parameters damping coefficient D and stiffness coefficient S may be a priori known but change over the lifetime of the vehicle. In this case it would be beneficial if the predetermined ranges assigned to each of these parameters would include the a priori known values. A size of each of the predetermined ranges may be chosen based on experience regarding an expected change of each one of the parameters over the lifetime of the vehicle.

[0027] Once the mass of the person located at the at least one seat is determined by the above method or from another system, a method in accordance with the invention can be extended for operating the radar sensor system for monitoring a vital sign of the person in the interior of the vehicle in preferred embodiments.

[0028] The method for monitoring a vital sign of the person in the interior of the vehicle comprises at least steps of operate the evaluation and control unit for evaluating Doppler information from radar signals received by the radar receiving unit,

operate the evaluation and control unit for obtaining, from the received accelerometer data, motion quantities of the radar sensor system relative to an initial position of the radar sensor system,

select (46) temporary evaluation parameters of a predefined mechanical model representing a coupled motion of the at least one seat (18) and the radar sensor system (10),

based on the obtained motion quantities of the radar sensor system (10), the predefined mechanical model and the selected temporary evaluation parameters, reconstruct (48) a motion of the at least one seat (18),

generate, based on the reconstructed motion of the at least one seat, corrected Doppler information from Doppler information evaluated from the received radar signals, and

evaluate the corrected Doppler information with regard to at least one vital sign parameter.

[0029] An output signal representing the evaluated vital sign parameter can be generated and can be transferred to an advised driver assistance system (ADAS) for further use. [0030] The vital sign can be formed by the person's breathing or heartbeat. The at least one vital sign parameter can be formed by, without being limited to, a fundamental frequency, a periodicity or an amplitude.

[0031 ] By the proposed method for monitoring a vital sign of a person, traffic safety can be increased. For parked passenger cars, in particular, it can be prevented that infants are erroneously left behind.

[0032] Preferably, the step of generating temporary Doppler information or corrected Doppler information from Doppler information evaluated from the received radar signals includes steps of based on the reconstructed motion of the at least one seat, generating a radar correction signal, and

processing the Doppler information evaluated from the received radar signals by applying the generated radar correction signal.

[0033] In this way, a fast and effective radar signal processing for generating temporary Doppler information can be facilitated.

[0034] In another aspect of the invention, a radar sensor system for detecting, classifying and/or monitoring a person in an interior of a vehicle is provided that includes a radar transmitting unit having at least one radar transmitting antenna and being configured for transmitting radar signals towards at least one seat within the vehicle interior,

a radar receiving unit having at least one radar receiving antenna and being configured for receiving radar signals that have been transmitted by the radar transmitter unit and have been reflected by a person located at the at least one seat, and

an evaluation and control unit that is configured for automatically executing the steps of the method disclosed herein.

[0035] The benefits described in context with the method proposed herein apply to the radar sensor system to the full extent.

[0036] Preferably, the evaluation and control unit comprises a processor unit and a digital data memory unit to which the processor unit has data access. In this way, the steps of the method disclosed herein can be performed within the radar sensor system to ensure a fast and undisturbed signal processing and evaluation.

[0037] If the radar sensor system further includes at least one accelerometer device that is configured for providing the accelerometer data to the evaluation and control unit, a short signal path can be established for providing the accelerometer data without data sharing, and fast signal processing can be accomplished.

[0038] In preferred embodiments of the radar sensor system, the at least one accelerometer device is an integral part of the radar sensor system. In this way, a radar sensor system with a compact design can be provided. Moreover, a systematic error for the accelerometer data due to a spatial separation of the accelerometer device and the radar sensor system can be avoided.

[0039] It will be appreciated, that in a possible embodiment, the radar sensor system may comprise least a first accelerometer device and a second accelerometer device, both said at least first and second accelerometer devices configured for providing the accelerometer data to the evaluation and control unit. It could for instance be very useful, to mount said first accelerometer device to the vehicle seat while the second accelerometer device is an integral part of the radar sensor system and mounted in the radar unit.

[0040] Preferably, at least one accelerometer device comprises at least one accelerometer sensor that is designed as a micro-electromechanical system (MEMS). In this way, the accelerometer data can readily be provided in an economic and part-saving manner.

[0041 ] In yet another aspect of the invention, a software module for controlling an automatic execution of the method disclosed herein is provided.

[0042] The method steps to be conducted are converted into a program code of the software module, wherein the program code is implementable in a digital memory unit of the radar sensor system and is executable by a processor unit of the radar sensor system. Preferably, the digital memory unit and/or processor unit may be a digital memory unit and/or a processing unit of the evaluation and control unit of the radar sensor system. The processor unit may, alternatively or supplementary, be another processor unit that is especially assigned to execute at least some of the method steps.

[0043] The software module can enable an automatic, robust and reliable execution of the method and can allow for a fast modification of method steps and/or parameters.

[0044] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

[0045] It shall be pointed out that the features and measures detailed individually in the preceding description can be combined with one another in any technically meaningful manner and show further embodiments of the invention. The description characterizes and specifies the invention in particular in connection with the figures.

Brief Description of the Drawings

[0046] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:

Fig.1 schematically shows a radar sensor system in accordance with the invention for detecting and classifying a person in an interior of a vehicle in a side view, installed in the vehicle,

Fig. 2 is a mechanical equivalent of the configuration pursuant to Fig. 1 ,

Fig. 3a is a flowchart of a possible embodiment of the method for detecting and classifying, in accordance with the invention,

Fig. 3b is a flowchart of a possible embodiment of the method for monitoring a vital sign, in accordance with the invention,

Fig. 4a is a plot of a simulation of temporary Doppler information with a deviation of 30% from the real mass,

Fig. 4b is a plot of a simulation of temporary Doppler information with a deviation of 10% from the real mass,

Fig. 4c is a plot of a simulation of temporary Doppler information with the exact mass, Fig. 4d is a plot of a simulation of temporary Doppler information with eliminated Doppler frequency shift caused by exterior events,

Fig. 5 is a plot of evaluated Doppler information from radar signals received by the radar receiving unit,

Fig. 6 is a plot of temporary Doppler information obtained from the evaluated

Doppler information pursuant to Fig. 5 and a mass temporary evaluation parameter of 100 kg,

Fig. 7 is a plot of temporary Doppler information obtained from the evaluated

Doppler information pursuant to Fig. 5 and a mass temporary evaluation parameter of 73.24 kg,

Fig. 8 visualizes an evolution of the mass temporary evaluation parameter for consecutive iterations, and

Fig. 9 visualizes an evolution of the objective function values for the consecutive iterations pursuant to Fig. 8.

Description of Preferred Embodiments

[0047] Fig. 1 schematically illustrates a radar sensor system 10 in accordance with the invention for detecting and classifying a person 28 in an interior 16 of a vehicle 14 in a side view. The radar sensor system 10 is installed in the vehicle 14, which is designed as a sedan passenger car.

[0048] The person 28 in the interior 16 of the vehicle 14 is the driver, who is located at and is occupying a seat 18 of the vehicle 14, namely the driver's seat. The vehicle 14 is shown to be driving on a road 38 having a vertical road profile 40, which is shown in Fig. 1 in an exaggerated manner for clarity purposes. The roughness of the vertical road profile 40 causes vertical motion 34 of vehicle wheels 20. The vertical motion 34 of the wheels 20 is transferred to a vehicle body 22 via a vehicle suspension system (not shown), generating forced vibrations of the seat 18 and the person 28 occupying the seat 18 (defined as seat/person system 32, Fig. 2). Additional forced vibrations of the seat/person system 32 are induced by mechanical vibrations 36 of a vehicle engine (Fig. 1 ).

[0049] Thus, a breathing motion 30 of the person's chest is superimposed by the forced vibrations of the seat/person system 32 that are mainly induced by the vertical road profile 40 and the vehicle engine. Other exterior sources that may as well induce forced vibrations of the seat/person system 32 are, for instance, strong winds or heavy oncoming traffic passing in close distance.

[0050] The radar sensor system 10 includes a radar transmitting unit and a radar transceiver antenna that is arranged in front of the person 28 at an inside of a roof 24 of the vehicle 14, and is directed towards the driver's seat. The radar transceiver antenna is configured for transmitting radar signals towards the person's chest. The radar transceiver antenna is further configured for receiving radar signals that have been transmitted by the radar transmitting unit and have been reflected by the person's chest.

[0051 ] The radar sensor system 10 moreover comprises an evaluation and control unit that is configured for evaluating Doppler information from the radar signals received by the radar receiving unit. To this end, the evaluation and control unit comprises a processor unit and a digital data memory unit to which the processor unit has data access. As the breathing motion 30 of the person's chest is superimposed by the forced vibrations of the seat/person system 32, the evaluated Doppler information is also a superposition of Doppler information generated by the person's breathing motion 30 and Doppler information generated by the forced vibrations of the seat/person system 32 induced by the vertical road profile 40 and the vehicle engine.

[0052] The evaluated Doppler information decoded by the sensor system 10 can be thought to be of the following simplified form:

ΘΧρ(2?Π (f _ext t t + f _br eathmg (t i)) (I)

[0053] In this formula, f _ext is the Doppler frequency resulting from movements caused by external sources, and f _brea thing is the Doppler frequency induced by the breathing motion 30 of the person's chest.

[0054] Fig. 2 shows a mechanical equivalent of the configuration pursuant to Fig. 1 that is used for dynamically describing a displacement x of the seat/person system 32 and a displacement }/ of the vehicle body 22, which is caused by the vertical road profile 40, the vehicle engine and other exterior sources. A mechanical connection between the seat/person system 32 and the vehicle body 22 is considered by a spring damper system 26 having a damping member with damping coefficient D and a stiffness member with stiffness coefficient S disposed between the seat/person system 32 and the vehicle body 22, respectively. Damping coefficient D and stiffness coefficient S are defined to be constant over time in this mechanical equivalent.

[0055] The mechanical equivalent of Fig. 2 can therefore be described by the following linear, second-order ordinary differential equation with parameters M, D and S:

Mx t) + Dx t) + Sx t) = Dy t) + Sy t) (II)

[0056] Furthermore, the radar sensor system 10 includes an accelerometer device 12 that is arranged at the inside of the vehicle roof 24 (Fig. 1 ). The accelerometer device 12 comprises a three-axis accelerometer sensor that is designed as an on-chip micro-electromechanical system. A digital data link (wireless or by wire connection), indicated in Fig. 1 as a dashed line, is provided between the accelerometer device 12 and the evaluation and control unit. The accelerometer device 12 is configured to provide digital accelerometer data to the evaluation and control unit via the digital data link. The accelerometer data contain information regarding vibration or motion of the vehicle body 22. The evaluation and control unit is configured to receive digital accelerometer data from the accelerometer device via the digital data link.

[0057] In the embodiment of the radar sensor system 10 illustrated in Fig. 1 , the accelerometer device 12 is arranged near a top of the inside of the vehicle roof 24, spaced from the balance of the radar sensor system 10. In another embodiment of the radar sensor system, the accelerometer device 12 may be an integral part of the radar sensor system, and, in this way, may be arranged close to the radar transceiver antenna.

[0058] A result of a simulation illustrated in Figs. 4a to 4d shows that a regularity and periodicity of Doppler information becomes the more pronounced the better the parameters of the second-order ordinary differential equation (II) approximate the real quantities.

[0059] Fig. 4a is a plot of the result of a simulation of temporary Doppler information with a deviation of 30% from the real mass, 5% from the real stiffness coefficient S, 5% from the real damping coefficient D and 5% for the real acceleration value.

[0060] Fig. 4b is a plot of a simulation of temporary Doppler information with a deviation of 10% from the real mass, 5% from the real stiffness coefficient S, 5% from the real damping coefficient D and 5% for the real acceleration value.

[0061 ] Fig. 4c is a plot of a simulation of temporary Doppler information with the exact mass, 5% deviation from the real stiffness coefficient s, 5% from the real damping coefficient D and 5% for the real acceleration value.

[0062] Fig. 4d is a plot of a simulation of temporary Doppler information with completely eliminated Doppler frequency shift caused by exterior influences.

[0063] In the following, an embodiment of a method of operation the radar sensor system 10 for detecting and classifying a person 28 in an interior 16 of a vehicle 14 will be described with reference to Fig. 3a, which provides a flowchart of the method. In preparation of operating the radar sensor system 10, it shall be understood that all involved units and devices are in an operational state and configured as illustrated in Fig. 1 .

[0064] In order to be able to carry out the method automatically and in a controlled way, the evaluation and control unit is equipped with a software module. The method steps to be conducted are converted into a program code of the software module. The program code is implemented in the digital data memory unit of the evaluation and control unit and is executable by the processor unit of the evaluation and control unit. All predetermined/predefined values, thresholds, ranges and functions mentioned in this description reside in the digital data memory unit of the evaluation and control unit and can readily be retrieved by the processor unit of the evaluation and control unit.

[0065] Referring now to Fig. 3a, as a first step 42 of the method, the evaluation and control unit is operated for evaluating Doppler information from radar signals received by the radar receiving unit. In another step 44, the evaluation and control unit is operated for obtaining, from received accelerometer data, motion quantities given by velocity and position of the radar sensor system 10 relative to an initial position of the radar sensor system 10. [0066] The following steps of the method are to be executed in an iterative manner until a predefined condition is fulfilled.

[0067] In another step 46 of the method, temporary evaluation parameters of a predefined differential equation representing a coupled motion of the seat/person system 32 and the radar sensor system 10 are selected. The predefined differential equation is given by the linear, second-order ordinary differential equation (II) presented above.

[0068] Based on the obtained motion quantities of the radar sensor system 10, the predefined differential equation (II) and the fixed temporary evaluation parameters, a motion of the seat/person system 32 is reconstructed in another step 48. As a next step 50, temporary Doppler information is generated from Doppler information evaluated from the received radar signals, based on the reconstructed motion of the seat/person system 32.

[0069] In a next step 52 of the method, an objective function value for the temporary Doppler information is calculated. The objection function is predefined and comprises the coefficient of variation of local extrema of the temporary Doppler information relative to the corresponding mean extrema as a measure of dispersion of local maxima and/or local minima of the temporary Doppler information. It has an extreme value, more specifically a minimum value, if the temporary Doppler information is strictly periodic and/or has constant extrema that are different from zero.

[0070] The calculated objective function value is compared with a predetermined threshold value in a next step 54. The predefined condition is that the calculated objective function value shall be equal to or less than the predetermined threshold value. If the condition is not met, execution of iterations continues. If the condition is fulfilled, further execution of iterations is stopped, and an output signal is generated that is indicative of the latest temporary evaluation parameter that represents the mass of the person 28 in another step 56. As parameter M in equation (II) represents the mass of the seat/person system 32, for obtaining the mass of the person 28 the a priori known mass of the seat 18 needs to be subtracted from the latest temporary evaluation parameter. [0071 ] Figs. 5 to 9 illustrate the effect of the method. Based on the article "Generation of Random Road Profiles" by Feng Tyan et al., Journal of Advanced Engineering 4.2 (2009), a road profile of roughness class C (average) was simulated by using N = 1000 different frequencies, and a velocity of the vehicle of 15 m/s has been chosen. Further, the following temporary evaluation parameters have been selected:

M = 100 kg, selected from an assigned predetermined range o†30 kg≤M≤ 200 kg

S = 4000 N/m

D = 200 Ns/m

[0072] The values for S and D have been chosen following a proposal by the article "Optimal Seat and Suspension Design for a Half-Car with Driver Model Using Genetic Algorithm" by Wael Abbas et al., Intelligent Control and Automation 4 (2013), and the article by C. B. Patel et al., "Modelling and vibration analysis of a road profile measuring system", International Journal of Automotive and Mechanical Engineering 1 (2010). Artificial errors of 5% on the stiffness constant S and on the damping constant D have been set, and these parameters have been kept constant throughout the execution of the method.

[0073] The three articles mentioned above shall hereby be incorporated by reference in their entirety with effect for those jurisdictions permitting incorporation by reference.

[0074] Fig. 5 is a plot of evaluated Doppler information from radar signals received by the radar receiving unit. The received radar signals have been reflected from a point target. A vertical displacement is caused by the simulated road profile.

[0075] Fig. 6 is a plot of temporary Doppler information shown as an FFT (Fast Fourier Transformation) curve obtained from the evaluated Doppler information pursuant to Fig. 5 and a mass temporary evaluation parameter of 100 kg, damping coefficient D of 210 Ns/m and stiffness coefficient of 4200 N/m. The dashed line represents a mean of the maxima and a mean of the minima, respectively. The dotted line represents a linear interpolation between the maxima and the minima, respectively. The calculated objective function value is 68.9121 . [0076] Fig. 7 is a plot of temporary Doppler information shown as an FFT curve obtained from the evaluated Doppler information pursuant to Fig. 5 and a mass temporary evaluation parameter of 73.238 kg, damping coefficient D of 210 Ns/m and stiffness coefficient of 4200 N/m with a calculated objective function value of 0.79273. The dashed line represents a mean of the maxima and a mean of the minima, respectively. The dotted line represents a linear interpolation between the maxima and the minima, respectively.

[0077] Fig. 8 visualizes an evolution of the mass temporary evaluation parameter for consecutive iterations. As can be taken from Fig. 8, the mass temporary evaluation parameter quickly converges to a stable value, and so do the objective function values for the consecutive iterations pursuant to Fig. 8, whose evolution is visualized in Fig. 9.

[0078] Fig. 3b is a flowchart of a possible embodiment of the method for monitoring a vital sign, in accordance with the invention. Once the driver's mass has been determined by the method visualized in Fig. 3a, a method for monitoring a vital sign of the driver can be started.

[0079] As one step 42 of the method, the evaluation and control unit is operated for evaluating Doppler information from radar signals received by the radar receiving unit as described before. In another step 44, the evaluation and control unit is again operated for obtaining, from the received accelerometer data, motion quantities given by velocity and position of the radar sensor system 10 relative to an initial position of the radar sensor system 10.

[0080] Then, in another step 48, a motion of the at least one seat 18 is reconstructed by using the latest temporary evaluation parameters of the predefined differential equation as fixed parameters, based on the obtained motion quantities of the radar sensor system 10, the predefined differential equation and the fixed parameters.

[0081 ] Based on the reconstructed motion of the at least one seat 18, corrected Doppler information from Doppler information evaluated from the received radar signals is generated in another step 58. [0082] In the next step 60, the corrected Doppler information with regard to at least one vital sign parameter, which is given by a breathing frequency, is evaluated by the evaluation and control unit.

[0083] An output signal representing the evaluated breathing frequency is generated in the next step 62. The output signal can be transferred to an ADAS for further use.

[0084] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

[0085] Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality, which is meant to express a quantity of at least two. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.

List of Reference Symbols

10 radar sensor system

12 accelerometer device

14 vehicle

16 vehicle interior

18 seat

20 wheel

22 vehicle body

24 vehicle roof

26 spring damper system

28 person

30 breathing motion

32 seat/person system

34 vertical motion of wheel

36 vehicle engine vibrations

38 road

40 vertical road profile

Method steps:

42 evaluate Doppler information from received radar signals

44 obtain motion quantities of the radar sensor system

46 select temporary evaluation parameters

48 reconstruct motion of seat/person system

50 generate temporary Doppler information

52 calculate objective function value

54 compare calculated objective function value with predetermined threshold value

56 generate output signal representing person's mass

58 generate corrected Doppler information

60 evaluate corrected Doppler information regarding vital sign parameter

62 generate output signal representing vital sign parameter

D damping constant

M mass of seat/person system

S stiffness constant