Patent Publication Number: US-2023150564-A1

Title: Tdr-based system for hand or body part positioning detection on an object, particularly on a steering wheel

Description:
TECHNICAL FIELD 
     The invention relates to a sensing system for hand or other body part positioning detection on an object, particularly on a steering wheel, a steering wheel with hand or body part positioning detection comprising such sensing system, a method of detecting a hand or body part positioning by operating such sensing system and a software module for automatically carrying out such method. 
     BACKGROUND 
     In the field of automotive vehicle sensor applications, it is known to employ sensors for so-called Hands off Detection (HOD) systems, in which one or more sensors provide information about whether a driver has his hands on a steering wheel of a vehicle or not. This information can be provided as input to an Automatic Driver Assistance System (ADAS) such as an Adaptive Cruise Control (ACC), which, based on the provided sensor signal, can alert the driver and remind him or her to take control of the steering wheel again. In particular, such HOD systems can be used in support to fulfill a requirement of the Vienna convention that the driver must remain in control of the vehicle at all times. HOD systems may as well be employed in a parking assistance system or an ADAS that is configured for evaluating a driver activity at high speed. 
     It is further known to employ capacitive sensors in vehicle HOD systems. 
     By way of example, WO 2016/096815 A1 proposes a planar flexible carrier for use in steering wheel heating and/or sensing of the presence of the driver&#39;s hand(s) on the steering wheel. The planar carrier, which can be employed for mounting on a rim of a steering wheel without wrinkles, comprises a portion of planar flexible foil of roughly rectangular shape having two longitudinal sides and two lateral sides. A length B of the lateral sides is 0.96 to 1.00 times the perimeter of the rim. A number of N cut-outs per unit length are provided on each of the longitudinal sides, wherein the cut-outs of one side are located in a staggered fashion relative to opposing cut-out portions on the opposite side. The determining of an optimum shape and size of the cut-outs is described. Further described is a heat carrier, a heating and/or sensing device and methods for their production. 
     Multi-zone HOD capacitive sensor systems are known in the art that comprise a plurality of distinct and independent sensing zones along the steering wheel. These HOD capacitive sensor systems are capable of distinguishing various ways of holding the vehicle steering wheel by the driver (one hand, two hands, angular position). 
     For instance, European patent EP 1 292 485 B1 describes a steering wheel for a vehicle, which comprises a steering ring, a hub, and at least one spoke connecting the steering ring and the hub. On the steering ring, sensors are arranged in a distributed manner along the circumference of the steering ring, extending over the entire length of the steering ring. The sensors may be configured for operating on a resistive, capacitive or inductive basis. The sensors are subdivided into a plurality of segments arranged one behind another in the longitudinal direction of the steering ring. The spacing between two segments of a sensor from one another in the longitudinal direction of the steering ring is smaller than a finger width, wherein the segments of the sensors, in the longitudinal direction of the steering ring, are shorter than a finger width. By that, the thumb and the fingers of the hand can be reliably distinguished from each other and a high spatial resolution is achieved. 
     Employment of other sensors has also been proposed for HOD applications. WO 2019/086388 A1 describes a system for detecting whether at least one hand of a user is on a steering wheel. The system is in general based on a time-domain reflectometry (TDR) measurement. The system comprises a signal line that extends from a first point to a second point and is disposed along at least a portion of a surface of the steering wheel. The system further includes a detection unit that is coupled to the first point. The detection unit is configured to send a time-dependent detection signal traveling along the signal line, to receive a reflected signal traveling along the signal line and to detect the presence of a hand on the surface based on the reflected signal. 
     SUMMARY 
     It is therefore an object of the invention to provide a sensing system of low complexity for hand or body part positioning detection on an object, particularly on a vehicle steering wheel, that is at least capable of distinguishing between scenarios with at least one hand or body part touching the object, no hand or body part touching the steering wheel and at least one hand grasping the object. 
     This object is achieved by a system for hand or body part detection on an object, particularly on a steering wheel, according to the claims, a steering wheel with hand or body part positioning detection according to the claims, and a method of detecting a hand positioning on an object, particularly on a steering wheel according to the claims. 
     In one aspect of the present invention, the object is achieved by a sensing system for hand or body part positioning detection on an object, particularly on a rim of a steering wheel, that comprises at least one electrically conducting signal line, a signal voltage source and a control and evaluation unit. 
     The at least one electrically conducting signal line is arrangeable to extend along a part of a surface of the object with a priori knowledge about a relation between a distance of any portion of the at least one signal line from a reference point and information on a position on the object. 
     The signal voltage source is operatively connectable to each signal line and is configured for providing a time-dependent measurement signal to be traveling along the respective connected signal line. 
     The control and evaluation unit is operatively connectable to each signal line and is configured for:
         receiving the measurement signal after being at least partially reflected by at least one portion of the signal line to which the measurement signal has been provided,   determining a position or positions on the object of the portion or portions of the signal line at least partially reflecting the measurement signal, and   determining a hand or body part positioning on the object based on the determined position or positions of the portion or portions of the signal line.       

     The term “signal”, as used throughout this application, shall be understood to mean an electric or electromagnetic signal. The phrase “configured to”, as used in this application, shall in particular be understood as being specifically programmed, laid out, furnished or arranged. The term “(electrically) connected”, as used in this application, shall particularly be understood as being electrically connected by a galvanic connection or a capacitive or inductive coupling. 
     In general, the sensing system for hand or body part positioning detection according to the invention is based on time-domain reflectometry (TDR), and the provided time-dependent measurement signal intended to be traveling along the respective connected signal line is suitable for TDR measurements. The signal shape of the time-dependent measurement signal may depend on the specific application. 
     The provided time-dependent measurement signal will be traveling along the respective connected signal line and will at least partially be reflected from any impedance discontinuity along the signal line. Such impedance discontinuities can temporarily be generated by a hand or one or more fingers of the hand or a body part being positioned in proximity to the signal line. The nature of the generated impedance discontinuity or impedance discontinuities is determined by the nature of the hand or body part positioning on the object. 
     With the a priori knowledge about a relation between a distance of any portion of the at least one signal line from a reference point and information on a position on the object, the proposed sensing system is enabled to determine positions of detected impedance discontinuities on the object. This information can be exploited for determining hand or body part positioning scenarios on the object, such as one or two hands touching or grasping an object such as a steering wheel. The proposed sensing system for hand or body part positioning detection on an object can be of low complexity and can at least be capable of distinguishing between scenarios with at least one hand or body part touching the object, no hand or body part touching the object and at least one hand grasping the object, such as a steering wheel. 
     Preferably, each signal line is designed to have a predefined uniform characteristic impedance. In this way, impedance discontinuities generated by a touch or grasp of the hand of the operator of the steering wheel stand out better against any other potential impedance disturbances. 
     Preferably, the proposed sensing system is intended for hand or body part positioning detection on an object that is formed by an automotive vehicle steering wheel and forms part of a steering wheel of an automotive vehicle. The term “automotive vehicle”, as used in this patent application, shall particularly be understood to encompass passenger cars, trucks, semi-trailer trucks and buses, although application to other vehicles such as aircrafts (yoke) or watercrafts is also contemplated. Further, the proposed sensing system can be employed for hand or body part positioning detection on steering devices of construction vehicles or agricultural vehicles or the like. 
     The at least one signal line comprises a first section, which is arrangeable to extend along a part of a surface of the rim of the steering wheel that is facing an operator of the steering wheel, and a second section, which is arrangeable to extend along a part of a surface of the rim of the steering wheel that is facing away from the operator of the steering wheel, wherein the first section and the second section are electrically connected in series. 
     It is further noted herewith that the terms “first”, “second”, etc. are used in this application for distinction purposes only, and are not meant to indicate or anticipate a sequence or a priority in any way. 
     With this configuration, a touch of only one of the surfaces of the rim of the steering wheel by a hand or other body part would generate an impedance discontinuity and, by that, a reflected signal only in either the first portion or the second portion of the signal line, whereas a grasping hand of the operator of the steering wheel would generate two reflections of the measurement signal provided to the signal line at the same position on the rim. Two reflections of the measurement signal provided to the signal line that are detected at different positions on the rim can be attributed to two hands touching the steering wheel. By that, a reliable distinction between a positioning of a touching hand or touching hands and a positioning of a grasping hand or grasping hands of the operator of the steering wheel can be accomplished in an especially easy manner. 
     Further in such embodiments, the at least one signal line is preferably shaped as a meandering pattern such that a maximum lateral dimension between adjacent turning points of the meandering pattern is more than 25% and less than 50% of a circumferential length of the cross-section of the rim of the steering wheel. In this way, the first section of the signal line is readily arrangeable to extend along a major part of the surface of the rim that is facing an operator of the steering wheel, and the second section of the signal line is readily arrangeable to extend along a major part of the surface of the rim that is facing away from the operator of the steering wheel. It will be appreciated that by the meandering pattern of the signal line, a total length of the signal line that is arrangeable to extend along a part of a surface of a rim of the steering wheel can be enlarged compared to a straight shape, by which a requirement with regard to time resolution can be alleviated. Further, using a meandering pattern can make it easier to cover a large part of the surface of the rim of the steering wheel so as to avoid any “dead zones”, meaning areas in which a touch or even a grasp of a hand of a steering wheel operator could be undetected by the sensing system. 
     Preferably, the meandering pattern comprises half-circle shaped turns connected by straight line portions. In this way, an appropriate signal line of easy design, requiring little manufacturing effort, can be provided. 
     In preferred embodiments of the sensing system, at least one signal line is designed as a microstrip line or a coplanar waveguide, either of which comprises a flexible dielectric carrier. Microstrip lines and coplanar waveguides are well-known solutions for designing electric transmission lines as signal lines with a well-defined desired uniform impedance. The flexible dielectric carrier can beneficially allow to install the signal line on the surface of the rim of the steering wheel with little wrinkling and almost unnoticeable to the operator of the steering wheel. 
     Preferably, at least one signal line is terminated by at least one lumped impedance that is equal to the characteristic impedance of the signal line. In this way, reflections of the provided measurements signal at an end of the signal line can be avoided and do not superpose reflections at impedance discontinuities generated by a touch or grasp of the hand of the operator of the steering wheel. 
     The time-dependent measurement signal does not need to be a pulse signal as is often used in TDR, but can be a continuous signal as well. In preferred embodiments of the sensing system, the signal voltage source is configured for providing a pulse-shaped signal, a frequency-swept signal, a pseudo random phase-shift keyed signal or a pseudo random signal. This can provide design freedom for appropriately covering a wide range of applications. 
     In the case of a frequency swept sine wave, the control and evaluation unit may be configured to apply a Fast Fourier Transform (FFT) to the received reflected measurement signal. In the case of a pseudo random phase-shift keyed signal or a pseudo random signal, the control and evaluation unit may be configured to apply a cross correlation between the provided measurements signal and the received reflected measurement signal. 
     In a further aspect of the invention, a steering wheel with hand or body part positioning detection is provided. The steering wheel comprises an embodiment of the sensing system for hand or body part positioning detection as disclosed herein, wherein the at least one electrically conducting signal line is arranged to extend along a part of the surface of the rim of the steering wheel. The benefits described in context with the sensing system applies to the proposed steering wheel to the full extent. 
     In particular, the proposed steering wheel is applicable with advantage in the automotive sector; i.e. for use in a vehicle. However, it is also contemplated to employ the proposed steering wheel for aircrafts and sea crafts. 
     In another aspect of the invention, a method of detecting a hand or body part positioning on an object, particularly on a rim of a steering wheel, by operating a sensing system is provided, wherein the sensing system comprises: at least one electrically conducting signal line, which is arranged to extend along a part of a surface of the object with a priori knowledge about a relation between a distance of any portion of the at least one signal line from a reference point and information on a position on the object; a signal voltage source that is operatively connectable to each signal line and a control and evaluation unit that is operatively connectable to each signal line. 
     The method comprises at least steps of:
         by operating the signal voltage source, providing to a signal line a time-dependent measurement signal intended to be traveling along the respective connected signal line,   operating the control and evaluation unit for receiving the measurement signal after being at least partially reflected by at least one portion of the signal line to which the measurement signal has been provided,   operating the control and evaluation unit for determining a position or positions on the object of the portion or portions of the signal line that has or have at least partially reflected the measurement signal, and   operating the control and evaluation unit for determining a hand or body part positioning on the object based on the determined position or positions of the portion or portions of the signal line.       

     The benefits described in context with the sensing system applies to the proposed method to the full extent. 
     In preferred embodiments of the method, the step of determining a hand or body part positioning on an object includes determining a hand or body part positioning that is equivalent to a single-touch positioning if only one position of the signal line that has at least partially reflected the measurement signal on the rim has been determined. 
     In preferred embodiments of the method, if at least two positions of the signal line that have at least partially reflected the measurement signal on the object have been determined, the step of determining a hand or body part positioning on an object includes:
         comparing a first determined position on the object of a first portion of the signal line at least partially reflecting the measurement signal to a second determined position on the object of a second portion of the signal line at least partially reflecting the measurement signal,   determining a hand or body part positioning equivalent to touch positionings if the first determined position differs from the second determined position by at least a predefined difference threshold value,   determining a hand or body part positioning equivalent to a grasping hand positioning if the first determined position differs from the second determined position by less than the predefined difference threshold value, and   executing the above steps for all possible pairings of determined positions on the object.       

     In this way, from a received reflected measurement signal that includes at least two reflections, a hand or body part positioning can reliably be distinguished to be either equivalent to touch positionings or to grasping hand positionings. 
     In preferred embodiments of the method, a refined hand or body part positioning detection on an object can be obtained. In these embodiments, the step of determining a hand or body part positioning on the object further includes:
         comparing an amplitude of a reflected and received measurement signal with a predefined amplitude threshold value that depends on the determined hand or body part positioning being equivalent to a touch positioning or a grasping hand positioning,   determining a refined hand or body part positioning that is equivalent to a strong touch positioning if the determined hand or body part positioning is equivalent to at least a single-touch positioning and if the amplitude is larger than the predefined amplitude threshold value for a touch positioning, and   determining a refined hand or body part positioning that is equivalent to a strong grasp positioning if the determined hand or body part positioning is equivalent to a grasping hand positioning and if the amplitude is larger than the predefined amplitude threshold value for a grasping hand positioning.       

     Another refined hand or body part positioning detection on an object can be obtained in preferred embodiments of the method, in which the step of determining a hand or body part positioning on the object further includes:
         comparing a signal width of a reflected and received measurement signal with a predefined signal width threshold value,   determining a refined hand or body part positioning that is equivalent to a touch positioning of multiple fingers if the determined hand or body part positioning is equivalent to at least a single-touch positioning and if the signal width is larger than the predefined signal width threshold value,   determining at refined hand or body part positioning that is equivalent to full hand grasp positioning if the determined hand or body part positioning is equivalent to a grasping hand positioning and if the signal width is larger than the predefined signal width threshold value.       

     In preferred embodiments of the method, with the object being formed by a rim of a steering wheel, the step of determining a hand or body part positioning on the object further includes determining a body part positioning equivalent to a knee-driving positioning if the determined hand or body part positioning is equivalent to at least a single-touch positioning and if the position of the signal line that has at least partially reflected the measurement signal on the rim has been determined to be located at a lowest position of the rim in the current position of the steering wheel. This can allow for detection of a misuse condition in which the driver tries to steer the vehicle with the knees. 
     In preferred embodiments of the method, with the object being formed by a rim of a steering wheel, the step of determining a hand or body part positioning on the object further includes determining a manipulation situation if at least one of the following conditions is fulfilled:
         more than four positions on the object of portions of the signal line at least partially reflecting the measurement signal have been determined, and   a hand or body part positioning equivalent to a single-touch positioning has been determined for at least a predefined time period.       

     By detecting a fulfillment of the first condition, a manipulation condition can be detected in which another person than the driver is also touching or even grasping the rim of the steering wheel. By detecting a fulfillment of the second condition, a manipulation condition can be detected in which the rim of the steering wheel is fixed in an impermissible manner. 
     In yet another aspect of the invention, a non-transitory, computer-readable medium storing a software module for controlling automatic execution of steps of an embodiment of the method disclosed herein is provided. 
     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 (such as the computer-readable medium) of the sensing system for hand positioning detection on a steering wheel, and is executable by a processor unit of the sensing system. Preferably, the digital memory unit and/or processor unit may be a digital memory unit and/or a processing unit of the control and evaluation unit of the sensing 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. 
     The software module can enable a robust and reliable execution of the method and can allow for a fast modification of method steps. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     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 embodiments of the invention in particular in connection with the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 illustrates a sensing system in accordance with an embodiment of the invention for hand or body part positioning detection on an object formed by a rim of a steering wheel in an installed state, 
         FIG.  2    schematically illustrates the electrically conducting signal line of the sensing system pursuant to  FIG.  1   , 
         FIG.  3    schematically illustrates the sensing system pursuant to  FIG.  1    in a scenario with two hands touching the steering wheel (double touch positioning), 
         FIG.  4    is a schematic plot of reflected measurement signals received by the sensing system pursuant to  FIG.  1    in the scenario pursuant to  FIG.  3   , 
         FIG.  5    schematically illustrates the sensing system pursuant to  FIG.  1    in a scenario with one hand grasping the steering wheel (grasping hand positioning), 
         FIG.  6    is a schematic plot of reflected measurement signals received by the sensing system pursuant to  FIG.  1    in the scenario pursuant to  FIG.  5   , 
         FIG.  7    schematically illustrates the sensing system pursuant to  FIG.  1    in a scenario with two hands grasping the steering wheel (grasping hand positioning), 
         FIG.  8    is a schematic plot of reflected measurement signals received by the sensing system pursuant to  FIG.  1    in the scenario pursuant to  FIG.  7   , 
         FIG.  9    schematically illustrates the sensing system pursuant to  FIG.  1    in a scenario with one hand touching the steering wheel and one hand grasping the steering wheel (mixed hand positioning), 
         FIG.  10    is a schematic plot of reflected measurement signals received by the sensing system pursuant to  FIG.  1    in the scenario pursuant to  FIG.  9   , 
         FIG.  11    is a flow chart of a method of detecting a hand positioning on a steering wheel by operating the sensing system pursuant to  FIG.  1   , and 
         FIGS.  12  to  14    show optional steps of the method pursuant to  FIG.  11   . 
     
    
    
     In the different figures, the same parts are always provided with the same reference symbols or numerals, respectively. Thus, they are usually only described once. 
     DETAILED DESCRIPTION 
       FIG.  1    schematically illustrates a sensing system  10  for hand or body part positioning detection on an object formed by a rim  40  of a steering wheel  38  in an installed state. For clarity purposes, only the rim  40  or steering ring of the steering wheel  38  is shown, which further comprises at least one spoke that connects the rim  40  to a steering column via a hub in a manner known per se. The steering wheel  38  may be employed in a vehicle designed as a passenger car, but may as well be employed in an aircraft or a watercraft. 
     The sensing system  10  comprises an electrically conducting signal line  12 , a signal voltage source  26  and a control and evaluation unit  28 . 
     The signal line  12  of the sensing system  10  is schematically illustrated in  FIG.  2   . The signal line  12  may be shaped as a meandering pattern, which comprises a plurality of half-circle shaped turns connected by straight-line portions. The signal line  12  may be designed as a coplanar waveguide having an electrically conductive center line and two electrically conductive return lines, which are arranged in an equally spaced manner at both sides of the center line. The center line and the return lines may be attached to a flexible dielectric carrier  14  such as a polymeric foil, for instance by applying a screen printing method. In this way, the signal line  12  is designed to have a predefined uniform characteristic impedance, which can be laid out by varying the geometry and relative positions of the center line and the return lines, as is well known in the art. 
     The flexible dielectric carrier  14  may be backed by an electrically conductive ground plane serving as an additional return line of the coplanar waveguide. 
     The signal line  12  may be open-ended as shown in  FIG.  2   , but it may also be terminated by a lumped impedance that is equal to the characteristic impedance so as to avoid reflections at its end. In the open-ended case a total reflection is to be expected, which can be used as a time reference mark. 
     A maximum dimension between adjacent turning points  16  of the meandering pattern of the signal line  12  is adapted to a circumferential length of a cross-section of the rim  40  of the steering wheel  38  such that a maximum lateral dimension between adjacent turning points  16  of the meandering pattern is more than 25% and less than 50% of a circumferential length of the cross-section of the rim  40  of the steering wheel  38 , and in this specific embodiment may be about 30% of the circumferential length. 
     As shown in  FIG.  1   , the electrically conducting signal line  12  is arranged to extend along a major part of a surface of the rim  40  of the steering wheel  38 . The signal line  12  comprises a first section  18 , which is arranged to extend along a major part of a surface  20  of the rim  40  of the steering wheel  38  that is facing an operator of the steering wheel  38 , i.e. usually the driver of the vehicle. The signal line  12  further comprises a second section  22 , which is arranged to extend along a major part of a surface  24  of the rim  40  of the steering wheel  38  that is facing away from the operator of the steering wheel  38 . The first section  18  and the second section  22  of the signal line  12  are electrically connected in series without an impedance discontinuity. 
     The signal line  12  is installed and arranged on the surface of the rim  40  of the steering wheel  38  with a priori knowledge about a relation between a distance of any portion of the signal line  12  from a reference point and information on a position on the rim  40 . Any point of the signal line  12  may be chosen as the reference point. A position of any portion of the signal line  12  on the rim  40  may be defined by specifying the surface, i.e. either the surface  20  facing towards the operator or the surface  24  facing away from the operator, and by specifying a center angle with respect to a zero angle position. 
     The signal voltage source  26  is operatively connected to the signal line  12  and is configured for providing a time-dependent measurement signal to be traveling along the connected signal line  12 . In this embodiment, the signal voltage source  26  is designed as an integral part of the control and evaluation unit  28 , sharing the same housing for improved signal processing and control by the control and evaluation unit  28 . In other embodiments, the signal voltage source  26  may be designed as a separate unit with appropriate signal and control lines to the signal line  12  and the control and evaluation unit  28 . 
     In this specific embodiment, the signal voltage source  26  is designed for providing pulse-shaped signals, but in other embodiments, the signal voltage source may be configured for providing a frequency-swept signal, a pseudo random phase-shift keyed signal or a pseudo random signal. 
     The control and evaluation unit  28  is operatively connected to the signal line  12  as well as to the signal voltage source  26 . The control and evaluation unit  28  may comprise a microcontroller that includes a digital data memory unit  32 , a processor unit  30  with data access to the digital data memory unit  32  and a control interface  34 . As will be explained below, the control and evaluation unit  28  is configured for receiving the measurement signal after being at least partially reflected by at least one portion of the signal line  12  to which the measurement signal has been provided, for determining a position or positions on the rim  40  of the portion or portions of the signal line  12  at least partially reflecting the measurement signal, and for determining a hand positioning on the steering wheel  38  based on the determined position or positions of the portion or portions of the signal line  12 . 
     In the following, an embodiment of the method of detecting a hand or body part positioning on the object formed by the rim  40  of the steering wheel  38  by operating the sensing system  10  pursuant to  FIG.  1    will be described with reference to  FIGS.  1  and  3  to  10    and to  FIG.  11   , which provides a flow chart of the method. In preparation of operating the sensing system  10 , it shall be understood that all involved units and devices are in an operational state and configured as illustrated in  FIG.  1   . 
     In order to be able to automatically execute the method, the control and evaluation unit  28  comprises a software module  36 . The method steps to be conducted are converted into a program code of the software module  36 . The program code is implemented in the digital data memory unit  32  of the control and evaluation unit  28  and is executable by the processor unit  30  of the control and evaluation unit  28 . Alternatively, the software module  36  may as well reside in and may be executable by another control unit of the vehicle, and established data communication means between the control and evaluation unit  28  and the vehicle control unit would be used for enabling mutual data transfer. 
     With reference to  FIGS.  1  and  11   , in a first step  50  of the method, by operating the signal voltage source  26 , a time-dependent measurement signal is provided to the signal line  12 . The measurement signal is intended to be traveling along the connected signal line  12 . Without any hand positioning on the steering wheel  38  and with the signal line  12  being terminated with a lumped impedance, no reflection is expected at all. The provision of the time-dependent measurement signal may be controlled by the control and evaluation unit  28 . In other embodiments, the provision of the time-dependent measurement signal may be used as a trigger signal for the control and evaluation unit  28  for commencing execution of the following steps. 
     In another step  52  of the method, the control and evaluation unit  28  is operated for receiving the measurement signal after being at least partially reflected by at least one portion of the signal line  12  (also referred to as received reflected measurement signal). 
     In a next step  54 , the control and evaluation unit  28  is operated to determine a position or positions on the rim  40  of the portion or portions of the signal line  12  that has or have at least partially reflected the measurement signal. This is obtained by using the a priori knowledge about the relation between a distance of any portion of the signal line  12  from the reference point and information on a position on the rim  40 . 
     In further steps then, the control and evaluation unit  28  is operated to determine a hand positioning on the steering wheel  38  based on the determined position or positions of the portion or portions of the signal line  12 . 
     In case only one position on the rim  40  of the portion of the signal line  12  that has partially reflected the measurement signal has been determined, the control and evaluation unit  28  determines a hand positioning equivalent to a single-touch positioning in a step  56 . 
       FIG.  3    schematically illustrates the sensing system  10  in a scenario with two hands  42  touching the steering wheel  38  (double touch positioning), wherein one hand  42  is touching the surface  20  of the rim  40  that is facing the operator and the other hand  42  is touching the surface  24  of the rim  40  that is facing away from the operator. 
       FIG.  4    is a schematic plot of reflected measurement signals  44  received by the sensing system  10  pursuant to  FIG.  1    in the scenario pursuant to  FIG.  3   . The ordinate of the plot relates to a signal amplitude, and the abscissa of the plot relates to a circumferential position on the rim  40 , starting from the reference point to a portion of the signal line  12 , as is determined by the control and evaluation unit  28 . 
     From the received reflected measurement signal  44 , two positions of the signal line  12  that have partially reflected the measurement signal on the rim  40  have been determined. In a step  60  of the method, the control and evaluation unit  28  compares the first determined position on the rim  40  of the first determined portion of the signal line  12  partially reflecting the measurement signal to the second determined position on the rim  40  of the second portion of the signal line  12  partially reflecting the measurement signal. 
     As becomes clear from  FIG.  4   , the two determined positions differ. More specifically, they differ by an amount that is larger than a predefined difference threshold value Δ. For this result of the step  60  of comparing, the control and evaluation unit  28  determines a hand positioning equivalent to touch positionings in another step  62 . 
     All predefined values, threshold values and conditions mentioned herein may reside in the digital data memory unit  32  of the control and evaluation unit  28  and can readily be retrieved by the processor unit  30  of the control and evaluation unit  28 . 
       FIG.  5    schematically illustrates the sensing system  10  pursuant to  FIG.  1    in a scenario with one hand  42  grasping the steering wheel  38  (grasping hand positioning). Again, two positions of the signal line  12  on the rim  40  that have partially reflected the measurement signal have been determined from the received reflected measurement signal  44  ( FIG.  6   ). In contrast to the received reflected measurement signal  44  in  FIG.  4   , the two positions determined match within the predefined difference threshold value A. For this result of the step  60  of comparing, the control and evaluation unit  28  determines a hand positioning equivalent to a grasping hand positioning in another step  64 . 
       FIG.  7    schematically illustrates the sensing system  10  pursuant to  FIG.  1    in a scenario with two hands  42  grasping the steering wheel  38  (grasping hand positioning).  FIG.  8    is a schematic plot of reflected measurement signals  44  received by the sensing system  10  pursuant to  FIG.  1    in the scenario pursuant to  FIG.  7   . 
     From the received reflected measurement signal  44 , four positions on the rim  40  of the signal line  12  that have partially reflected the measurement signal have been determined. As described before, in a step  58  of the method the control and evaluation unit  28  selects a pairing of a first determined position and a second determined position, compares the first determined position on the rim  40  of a first portion of the signal line  12  partially reflecting the measurement signal to the second determined position on the rim  40  of a second portion of the signal line  12  partially reflecting the measurement signal in a step  60  and determines a hand positioning based on a result of the step  60  of comparison. The step  60  of comparison is executed for all possible pairings among the four determined positions on the rim  40 . If two positions determined match within the predefined difference threshold value Δ, the control and evaluation unit  28  determines  64  a hand positioning equivalent to a grasping hand positioning for the pairing of positions under consideration. This is the case for two pairings of positions determined, and the control and evaluation unit  28  determines  64  a hand positioning equivalent to a grasping positioning by two hands  42 . 
       FIG.  9    schematically illustrates the sensing system  10  pursuant to  FIG.  1    in a scenario with one hand  42  touching the steering wheel  38  and one hand  42  grasping the steering wheel  38 , i.e. a mixed hand positioning.  FIG.  10    is a schematic plot of reflected measurement signals  44  received by the sensing system  10  pursuant to  FIG.  1    in the scenario pursuant to  FIG.  9   . 
     From the received reflected measurement signal  44 , three positions on the rim of the signal line  12  that have partially reflected the measurement signal have been determined. As described before, in a step  58  of the method the control and evaluation unit  28  selects a pairing of a first determined position and a second determined position, and in a step  60  of the method the control and evaluation unit  28  compares the first determined position on the rim  40  of a first portion of the signal line  12  partially reflecting the measurement signal to a second determined position on the rim  40  of a second portion of the signal line  12  partially reflecting the measurement signal; and determines a hand positioning based on a result of the step  60  of comparison. The step  60  of comparing is executed for all possible pairings among the three determined positions on the rim  40 . 
     As becomes clear from  FIG.  10   , two of the three positions determined match within the predefined difference threshold value Δ. For these positions under consideration, the control and evaluation unit  28  determines  64  a hand positioning equivalent to a grasping hand positioning. The third of the three positions differs by an amount that is larger than the predefined difference threshold value Δ from each of the other two positions. For this result of the step  60  of comparing, the control and evaluation unit  28  determines  62  a hand positioning equivalent to a touch positioning for the third position. 
     For all hand positionings determined for the scenarios as shown in  FIGS.  3 ,  5 ,  7  and  9   , the control and evaluation unit  28  can perform optional steps to determine refined hand positionings. 
     For the scenarios shown in  FIGS.  5  and  7    and the grasping hand positioning determined in the scenario of  FIG.  9   , the step of determining a hand positioning on the steering wheel may further include to operate the control and evaluation unit  28  to compare an amplitude of the received reflected measurement signal  44  with a predefined amplitude threshold value for a grasping hand positioning. A refined hand positioning that is equivalent to a strong grasp positioning is determined by the control and evaluation unit  28  if the amplitude is larger than the predefined amplitude threshold value for grasping hand positioning, otherwise a standard grasping hand positioning is determined. 
     Also optionally, the step of determining a hand positioning on the steering wheel may further include to operate the control and evaluation unit  28  to compare a signal width of the received reflected measurement signal  44  with a predefined signal width threshold value. A refined hand positioning that is equivalent to full hand grasp positioning is determined by the control and evaluation unit  28  if the signal width is larger than the predefined signal width threshold value, otherwise a standard grasping hand positioning is determined. 
     For the scenario shown in  FIG.  3    and the touch positioning determined in the scenario of  FIG.  9   , the step of determining a hand positioning on the steering wheel  38  may further include to operate the control and evaluation unit  28  to compare an amplitude of the received reflected measurement signal  44  with a predefined amplitude threshold value for a touch positioning. A refined hand positioning that is equivalent to a strong touch positioning is determined by the control and evaluation unit  28  if the amplitude is larger than the predefined amplitude threshold value for a touch positioning, otherwise a standard touch positioning is determined. 
     Also optionally, the step of determining a hand positioning on the steering wheel  38  may further include to operate the control and evaluation unit  28  to compare a signal width of the received reflected measurement signal  44  with a predefined signal width threshold value. A refined hand positioning that is equivalent to a touch positioning of multiple fingers is determined by the control and evaluation unit  28  if the signal width is larger than the predefined signal width threshold value, otherwise a standard touch positioning is determined. 
       FIGS.  12  and  13    illustrate further optional steps of the method that can allow for misuse detection during driving situations. As one further optional step of the method ( FIG.  12   ), a body part positioning equivalent to a knee-driving positioning can be determined  66  if the determined hand or body part positioning is equivalent to at least a single-touch positioning and if the position of the signal line  12  that has at least partially reflected the measurement signal on the rim  40  has been determined to be located at a lowest position of the rim  40  in the current position of the steering wheel  38 . 
     As yet another optional step of the method ( FIG.  13   ), the step of determining  56 ,  62 ,  64  a hand or body part positioning includes determining a manipulation situation  68  if at least one of the following conditions is fulfilled:
         more than four positions on the object of portions of the signal line  12  at least partially reflecting the measurement signal have been determined, and   a hand or body part positioning equivalent to a single-touch positioning has been determined for at least a predefined time period T.       

     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. 
     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.