Abstract:
A method for capacitive sensing comprises the steps of tagging a transmitting signal by modulating a sub-carrier on said signal using state of the art modulation techniques; demodulating said subcarrier out of useful/received signal to prove validity of said signal.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to the technical field of capacitive measurement circuits and more specifically to a capacitive measurement system having one or more electrodes, in which the characteristics of a conductive body such as shape and location are determined by means of capacitive coupling via the electrically conductive body. 
     BACKGROUND OF THE INVENTION 
     Capacitive measurement and/or detection systems have a wide range of applications, and are among others widely used for the detection of the presence and/or the position of conductive body in the vicinity of an electrode of the system. A capacitive sensor, called by some electric field sensor or proximity sensor, designates a sensor, which generates a signal responsive to the influence of what is being sensed (a person, a part of a person&#39;s body, a pet, an object, etc.) upon an electric field. A capacitive sensor generally comprises at least one antenna electrode, to which an oscillating electric signal is applied and which in response emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which could comprise the one or more antenna electrodes themselves—at which the influence of an object or living being on the electric field is detected. 
     The technical paper entitled “Electric Field Sensing for Graphical Interfaces” by J. R. Smith, published in Computer Graphics I/O Devices, Issue May/June 1998, pp 54-60 describes the concept of electric field sensing as used for making non-contact three-dimensional position measurements, and more particularly for sensing the position of a human hand for purposes of providing three dimensional positional inputs to a computer. Within the general concept of capacitive sensing, the author distinguishes between distinct mechanisms he refers to as “loading mode”, “shunt mode”, and “transmit mode” which correspond to various possible electric current pathways. In the “loading mode”, an oscillating voltage signal is applied to a transmit electrode, which builds up an oscillating electric field to ground. The object to be sensed modifies the capacitance between the transmit electrode and ground. In the “shunt mode”, which is alternatively referred to as “coupling mode”, an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode is measured, whereby the displacement current may be modified by the body being sensed. In the “transmit mode”, the transmit electrode is put in contact with the user&#39;s body, which then becomes a transmitter relative to a receiver, either by direct electrical connection or via capacitive coupling. 
     The capacitive coupling is generally determined by applying an alternative voltage signal to a capacitive antenna electrode and by measuring the current flowing from said antenna electrode either towards ground (in the loading mode) or into the second electrode (receiving electrode) in case of the coupling mode. This current is usually measured by means of a transimpedance amplifier, which is connected to the sensing electrode and which converts a current flowing into said sensing electrode into a voltage, which is proportional to the current flowing into the electrode. 
     Due to this measurement principle, these capacitive measurement systems are generally quite sensitive to parasitic electrical fields, which may disturb the electrical field generated around the antenna electrode and thus influence the capacitive detection. Such parasitic electrical fields may be generated by all kinds of active transmitters (electrical devices etc), which accordingly have the potential to negatively impact the performance of capacitive detection systems. 
     OBJECT OF THE INVENTION 
     The object of the present invention is therefore to reduce the influence of active transmitters on the detection performance. 
     GENERAL DESCRIPTION OF THE INVENTION 
     In order to overcome the abovementioned problems, the present invention proposes a method for capacitive sensing, said method comprising the steps of
         tagging a transmitting signal by modulating a sub-carrier on said signal using state of the art modulation techniques;   demodulating said subcarrier out of useful/received signal to prove validity of said signal.       

     In one embodiment such a method for capacitive sensing comprises for instance the steps of:
         generating a control signal and supplying said control signal to a transmitter, said control signal causing said transmitter to generate a transmitting signal;   detecting a response signal, said response signal being responsive to a transfer behaviour of a transfer channel for said transmitting signal;   determining at least one characteristic of said transfer behaviour from said response signal;       

     said method further comprising the further steps of:
         tagging said transmitting signal by modulating a sub-carrier signal on said transmitting signal;   validating the result of said determining step by demodulating said sub-carrier signal out of said response signal.       

     Said validating step preferably further comprises the step of determining at least one characteristic of said transfer behaviour from said demodulated response signal. Furthermore, said step of tagging said transmitting signal preferably comprises the steps of
         generating a carrier signal and a subcarrier signal,   supplying said carrier signal and said sub-carrier signal to a first modulator, and   supplying the output signal of said first modulator as control signal to said transmitter.       

     It will be appreciated, that in accordance with one embodiment of the present invention, the method for capacitive sensing comprises the steps of
         generating a carrier signal   generating a first sub-carrier signal,   supplying said carrier signal and said first sub-carrier signal to a first modulator, and   supplying the output signal of said first modulator as a first control signal to a transmitter, said first control signal causing said transmitter to generate a first transmitting signal;   detecting a first response signal, said first response signal being responsive to a transfer behaviour of a transfer channel for said first transmitting signal;   determining at least one characteristic of said transfer behaviour from said first response signal;   said method further comprising the further steps of validating the result of said determining step by   generating a second sub-carrier signal,   supplying said carrier signal and said second sub-carrier signal to said first modulator, and   supplying the output signal of said first modulator as a second control signal to said transmitter, said second control signal causing said transmitter to generate a second transmitting signal;   detecting a second response signal, said second response signal being responsive to a transfer behaviour of said transfer channel for said second transmitting signal;   determining at least one characteristic of said transfer behaviour from said first response signal;   demodulating said sub-carrier out of said second response signal; and   determining at least one characteristic of said transfer behaviour from said demodulated second response signal.       

     In a preferred embodiment of this method said first sub-carrier signal is a DC signal and said second sub-carrier signal is a time variant signal. In another possible embodiment, the method further comprises the steps of modulating known information on said sub-carrier signal, and demodulating of said known information out of said sub-carrier to further confirm origin of said response signal. Said known information comprises e.g. a binary protocol. In case of a detected interference one or more bit values in said binary protocol may be varied in order to increase robustness of the measurement. 
     In other possible embodiments of the invention, the method further comprises frequency hopping of said carrier signal and/or said sub-carrier signal to ensure system availability in case of detected interference and/or variation of transmitted information in case of detected interference. 
    
    
     
       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 drawings, wherein: 
         FIG. 1  is a simplified block diagram showing the components of a state-of-the-art coupling-mode capacitive detection system; 
         FIG. 2  is a schematic view of a coupling mode capacitive detection system in an application for the occupant detection in a vehicle seat; 
         FIG. 3  is a simplified block diagram showing the components of a state-of-the-art loading-mode capacitive detection system; 
         FIG. 4  is a schematic view of a loading mode capacitive detection system in an application for the occupant detection in a vehicle seat; 
         FIG. 5  is a simplified block diagram showing the components of a first embodiment of a coupling-mode capacitive detection system according to the present invention; 
         FIG. 6  is a schematic view of the capacitive detection system of  FIG. 5  in an application for the occupant detection in a vehicle seat; 
         FIG. 7  is a simplified block diagram showing the components of a second embodiment of a coupling-mode capacitive detection system according to the present invention; 
         FIG. 8  is a schematic view of the capacitive detection system of  FIG. 7  in an application for the occupant detection in a vehicle seat; 
         FIG. 9  is a simplified block diagram showing the components of a first embodiment of a loading-mode capacitive detection system according to the present invention; 
         FIG. 10  is a schematic view of the capacitive detection system of  FIG. 9  in an application for the occupant detection in a vehicle seat; 
         FIG. 11  is a simplified block diagram showing the components of a second embodiment of a loading-mode capacitive detection system according to the present invention; 
         FIG. 12  is a schematic view of the capacitive detection system of  FIG. 11  in an application for the occupant detection in a vehicle seat; 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Today&#39;s coupling-mode capacitive detection systems often correspond to the block diagram indicated in  FIG. 1 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 4 
                 receiving signal 
               
               
                 6 
                 sensing unit 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 36 
                 control signal 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                   
               
             
          
         
       
     
     A control unit  35  generates a control signal  36  necessary for the transmitter  1  to generate a transmitting signal  2  and for a useful signal extractor  40  to convert the receiving signal  4  into the useful signal  37 . The transmitting signal  2  passes through a transfer channel  3 . The transfer channel  3 , e.g. a complex impedance Z(jw), has a certain transfer behaviour. Said transfer behaviour directly impacts the receiving signal  4  and thus the useful signal  37 , which is evaluated by the control unit  35 . Depending on the useful signal&#39;s properties, the control unit  35  concludes on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw), and derives information that are delivered to the environment via the data output signal  38 . 
     Transmitter  1 , useful signal extractor  40  and control unit  35  may (but must not necessarily) be configured as one unit  6 . 
     Disturbing influences  30  like external radiation (EMI) might also enter the transfer channel  3 . These influences cause a disturbing signal part in the receiving signal  4  and, thus, a disturbing signal in the useful signal  37 . The disturbing signal part in the useful signal  37  can have the potential to degrade the determination of the transfer behaviour of the transfer channel  3 . As a consequence, the information derived out of the useful signal  37  by the control unit  35  might be wrong. 
     A typical application of the detection system of  FIG. 1  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 2 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 an electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 11 
                 receiving electrode 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 AC current 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 control unit 
               
               
                 36 
                 measurement signal 
               
               
                 38 
                 data output 
               
               
                 39 
                 measured AC current 
               
               
                   
               
             
          
         
       
     
     The capacitive system is assembled in a vehicle seat  32 . Its purpose is to determine the status of occupation of said vehicle seat to adapt the airbag deployment in case of a crash. 
     A control unit  35  outputs a measurement signal to an AC voltage source  7 , which drives a corresponding voltage at the transmitting electrode  8 . A receiving electrode  11  is connected to ground potential  13  via a low ohmic current meter  12 . Due to the difference in potential between transmitting electrode  8  and receiving electrode  11 , an electrical field  9  forms and causes the AC current  14  to flow. Said AC current can be of constant or of varying frequency in case where the frequency of the AC voltage source  7  varies as well. Said current is measured by the low-ohmic current meter  12  and evaluated in e.g. phase angle and amplitude to determine whether there is a person  10  sitting on the vehicle seat  32  or not. Also the variation over frequency of said phase angle and amplitude of said current can be subject of evaluation. The detected status of seat occupation is delivered to the airbag control unit of the vehicle via the data output  38  to adapt the airbag deployment in case of a crash. 
     Disturbing electrical fields  31  coupled into the person  10  or into the receiving electrode  11  can cause a disturbing signal part in the measured AC current  14  beyond the useful part which has its basic origin in the AC voltage  7  applied to the transmitting electrode  8 . The disturbing signal part in the AC current  14  causes a disturbing part in the measured AC current  39  evaluated by the control unit  35 . Said disturbing part can have the potential to trigger a wrong classification, e.g. a person is detected if no person is positioned on the passenger seat or the person is not detected although it is sitting on the seat. This wrong classification can cause a danger in case where e.g. a child seat is detected as a person, which possibly enables the airbag for deployment in case of a crash. 
     State-of-the-art loading-mode capacitive detection systems usually correspond to the block diagram indicated in  FIG. 3 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 6 
                 sensing unit 
               
               
                 13 
                 ground potential 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 36 
                 control signal 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                 41 
                 transmitter internal signal 
               
               
                   
               
             
          
         
       
     
     A control unit  35  generates a control signal  36  necessary for the transmitter  1  to generate a transmitting signal  2  and for the useful signal extractor  40  to convert the transmitter internal signal  41  into the useful signal  37 . The transmitting signal  2  passes through a transfer channel  3 . The transfer channel  3 , e.g. a complex impedance Z(jw) to ground potential  13 , has a certain transfer behaviour. The transmitter internal signal  41  is in direct dependence to the transfer behaviour of the transfer channel  3 . Changes on the transmitting signal  2  or on the transfer channel  3  directly impact the transmitter internal signal  41  and, thus, the useful signal  37 , which is evaluated by the control unit  35 . Depending on the properties of the useful signal  37 , the control unit  35  concludes on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw) to ground potential  13  and derives information that are delivered to the environment via the data output signal  38 . Transmitter  1 , useful signal extractor  40  and control unit  35  can be, but must not be, implemented as one unit  6 . 
     Disturbing influences  30  like external radiation (EMI) might enter the transfer channel  3 . These influences cause a disturbing signal part in the transmitter internal signal  41  and, thus, a disturbing signal in the useful signal  37 . The disturbing signal part in the useful signal  37  can have the potential to degrade the determination of the transfer behaviour of the transfer channel  3 . As consequence, the information derived by the control unit  35  out of the useful signal  37  might be wrong. 
     A typical application of the detection system of  FIG. 3  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 4 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage source 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 transmitting current 
               
               
                 21 
                 optional shield electrode 
               
               
                 26 
                 person&#39;s capacitance to ground 
               
               
                   
                 potential 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 control unit 
               
               
                 36 
                 measurement signal 
               
               
                 38 
                 data output 
               
               
                 39 
                 measured AC current 
               
               
                   
               
             
          
         
       
     
     The capacitive system is assembled in a vehicle seat  32 . Its purpose is to determine the status of occupation to adapt the airbag deployment in case of a crash. 
     A control unit  35  outputs a measurement signal  36  to an AC voltage source  7 , which drives a corresponding voltage at the transmitting electrode  8  and, optionally, to a shield electrode  21 . The electrodes are assembled in a vehicle seat  25 . Due to the difference in potential between transmitting electrode and ground potential  13 , an electrical field  9  forms and causes the AC current  14  to flow. Said AC current can be of constant or of varying frequency in case where the frequency of the AC voltage source  7  varies as well. Said current is measured by the low-ohmic current meter  12  and evaluated in e.g. phase angle and amplitude to determine whether there is a person  10  sitting on the vehicle seat  32  or not. Also the variation over frequency of said phase angle and amplitude of said current can be subject of evaluation. The detected status of seat occupation is delivered to the airbag control unit of the vehicle via the data output  38  to adapt the airbag deployment in case of a crash. 
     Disturbing electrical fields  31  coupled into the person  10  or into the transmitting electrode  8  can cause a disturbing signal part in the measured AC current  39  beyond the useful part which has its basic origin in the AC voltage  7  applied to the transmitting electrode  8 . Said disturbing part can have the potential to trigger a wrong classification, e.g. a person is detected if no person is positioned on the passenger seat or the person is not detected although it is sitting on the seat. This wrong classification can cause a danger in case where e.g. a child seat is detected as a person, which possibly enables the airbag for deployment in case of a crash. 
     The drawback of the shown measurement concepts concerning the disturbing influence of external radiation is that once a disturbance is able to pass through the useful signal extractor  40  in  FIG. 1  and  FIG. 3 , it is difficult for the control unit  35  to explicitly distinguish between the part of the useful signal  37  having its origin in the transmitting signal  2  and the part caused by the disturbance itself. 
     In order to alleviate this situation, the present invention proposes to rely on active tagging of the transmitting signal. If it is possible to tag the transmitting signal with e.g. certain information, it is possible, after extraction of the information e.g. in the useful signal extractor, to identify which part of the useful signal has its origin in the transmitting signal and which part of the useful signal is caused by a disturbance. 
       FIG. 5  shows a block diagram of a coupling mode capacitive detection system with active tagging of the transmitting signal by subcarrier modulation. The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 4 
                 receiving signal 
               
               
                 6 
                 sensing unit 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                 51 
                 first modulator 
               
               
                 53 
                 first demodulator 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 57 
                 received subcarrier 
               
               
                 60 
                 output of the first modulator 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and a subcarrier signal  54  and feeds it into the first modulator  51 . The output  60  of said modulator sources the transmitter  1 , which generates the transmitting signal  2 . The transmitting signal  2  passes the transfer channel  3 , e.g. a complex impedance Z(jw), and superposes with disturbances having its origin in the disturbing influence  30 . The useful signal extractor  40  receives the receiving signal  4 , extracts the useful signal  37  and feds it into the first demodulator  53 . Said demodulator is synchronized to the carrier  55 , demodulates the received subcarrier  57  out the useful signal  37  and outputs it to the control unit  35 . 
     By applying e.g. a DC signal as subcarrier signal  54  to the first modulator  51 , the output signal  57  of the first demodulator can be evaluated in e.g. phase angle and amplitude to conclude on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw). This result is to be confirmed for validity since the disturbing influence  30  might have caused a wrong measurement result. 
     Validation of the measurement result is performed by the control unit  35  by applying a time variant subcarrier signal  54  to the first modulator  51 , checking for subcarrier existence in the demodulated signal  57  and by assessing the properties of the demodulated subcarrier signal  57 , e.g. its phase angle or its amplitude in relation to the carrier amplitude. 
     If the subcarrier check proves that the measurement of the transfer behaviour of the transfer channel  3  was valid and not disturbed by a disturbing influence  30 , the control unit derives information out of the transfer channel property measurement result and sends it to the environment via the data output signal  38 . 
     A typical application of the detection system of  FIG. 5  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 6 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 11 
                 receiving electrode 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 AC current 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 control unit 
               
               
                 38 
                 data output 
               
               
                 42 
                 mixer 
               
               
                 43 
                 mixer 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 57 
                 received subcarrier 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and subcarrier signal  54  and feeds it into the mixer  43 . The voltage source  7  drives a voltage at transmitting electrode  8  which is in direct relation to the output of mixer  43 . The difference in potential between transmitting electrode  8  and receiving electrode  11 , which is connected to GND via a low ohmic current meter  12 , causes an electrical field  9  to form, resulting in a complex impedance Z(jw) between transmitting electrode  8  and receiving electrode  11 . A person  10 , whose presence on the vehicle seat  32  shall be detected, influences this impedance. 
     Due to the complex impedance Z(jw) between both electrodes, an AC current  14  flows from transmitting electrode  8  to receiving electrode  11 . Said current is measured by the current meter  12  and is mixed by mixer  42  with the carrier signal  55 . The control unit  35  characterizes the complex impedance Z(jw) in e.g. phase angle, absolute value or frequency dependency by evaluating signal  57  while keeping subcarrier signal  54  at DC. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz to the first mixer  43 , checking for subcarrier existence in the demodulated signal  57  and by assessing the properties of the demodulated subcarrier signal  57 , e.g. its phase angle or its amplitude in relation to the absolute value of Z(jw). 
     The embodiment shown in  FIG. 5  may be further enhanced to further increase the selectivity of the system by tagging the transmission signal by means of binary protocol transmission. Such an embodiment of the capacitive sensing system is represented in  FIG. 7 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 4 
                 receiving signal 
               
               
                 6 
                 sensing unit 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                 50 
                 second modulator 
               
               
                 51 
                 first modulator 
               
               
                 52 
                 second demodulator 
               
               
                 53 
                 first demodulator 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 56 
                 binary protocol 
               
               
                 57 
                 output of the first demodulator 
               
               
                 59 
                 output of the second modulator 
               
               
                 60 
                 output of the first modulator 
               
               
                 61 
                 demodulated binary protocol 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and a subcarrier signal  54 . The carrier signal  55  is fed into the first modulator  51 . The subcarrier signal  54  is fed into the second modulator  50 . The output  59  of the second modulator  50  is connected to the input of the first modulator  51 . The output  60  of the first modulator  51  is connected to the input of the transmitter  1 . The control unit  35  applies a binary protocol  56  to the input of the second demodulator  50 . The transmitting signal  2  passes the transfer channel  3 , e.g. a complex impedance Z(jw), and superposes with disturbances having its origin in the disturbing influence  30 . The useful signal extractor  40  receives the receiving signal  4 , extracts the useful signal  37  and feeds it into the first demodulator  53 , which is synchronized to the carrier  55 . Said demodulator demodulates the received subcarrier  57  out the useful signal  37  and inputs it to the second demodulator  52 , which is synchronized to the subcarrier signal  54  and which demodulates the binary protocol  61 . Said binary protocol is sent to the control unit  35 . 
     By applying e.g. a binary ‘one’ as binary protocol and a DC signal as subcarrier signal  54  to the second modulator  50 ; the output signal  61  of the second demodulator can be evaluated in e.g. phase angle and amplitude to conclude on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw). This result is to be confirmed for validity since the disturbing influence  30  might have caused a wrong measurement result. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. sinusoidal subcarrier signal  54  of e.g. 1 KHz and a binary protocol  56  sequence to the second modulator  50 . The control unit  35  can check for subcarrier existence and properties in the demodulated binary protocol signal  61  during constant bit values of the sent binary protocol  56 . The properties of the received subcarrier signal can be evaluated in e.g. its phase angle or its amplitude in relation to the carrier amplitude. In addition, to further increase the selectivity of the detection system, the demodulated binary protocol  61  can be compared with the sent binary protocol  56 . 
     If the subcarrier check and the binary protocol check prove that the measurement of the transfer behaviour of the transfer channel  3  was valid and not disturbed by a disturbing influence  30 , the control unit derives information out of the transfer channel property and sends it to the environment via the data output signal  38 . 
     A typical application of the detection system of  FIG. 7  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 8 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 11 
                 receiving electrode 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 AC current 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 control unit 
               
               
                 38 
                 data output 
               
               
                 42 
                 mixer 
               
               
                 43 
                 mixer 
               
               
                 44 
                 mixer 
               
               
                 45 
                 mixer 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 56 
                 binary protocol 
               
               
                 61 
                 demodulated binary protocol 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55 , a subcarrier signal  54  and a binary protocol signal  56 . Subcarrier signal  54  and binary protocol signal  56  are fed into mixer  44 . The output of mixer  44  is fed into mixer  43  together with carrier signal  55 . The voltage source  7  drives a voltage at transmitting electrode  8  which is in direct relation to the output of mixer  43 . The difference in potential between transmitting electrode  8  and receiving electrode  11 , which is connected to GND via a low ohmic current meter  12 , causes an electrical field  9  to form, resulting in a complex impedance Z(jw) between transmitting electrode  8  and receiving electrode  11 . A person  10 , whose presence on the vehicle seat  32  shall be detected, influences this impedance. 
     Due to the complex impedance Z(jw) between both electrodes, an AC current  14  flows from transmitting electrode  8  to receiving electrode  11 . This current is measured by the current meter  12  and is mixed by mixer  42  with the carrier signal  55 . The output of mixer  42  is mixed again with the subcarrier signal  54  by mixer  45 . The output of mixer  45  is input to control unit  35 . 
     The control unit  35  characterizes the complex impedance Z(jw) in e.g. phase angle, absolute value or frequency dependency by evaluating signal  61  while keeping subcarrier signal  54  at DC and the binary values of the binary signal  56  constant. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz and a binary protocol  56  to mixer  44 . The control unit  35  can check for subcarrier existence and properties in the demodulated binary protocol signal  61  during constant bit values of the sent binary protocol  56 . The properties of the received subcarrier signal can be evaluated in e.g. its phase angle or its amplitude in relation to the absolute value of impedance Z(jw). In addition, the demodulated binary protocol  61  can be compared with the sent binary protocol  56 . 
     The tagging principle shown in  FIG. 5  can be also applied to the topology indicated in  FIG. 3 .  FIG. 9  shows how ‘active transmitting signal tagging by subcarrier modulation’ can be applied to a loading mode capacitive sensor system. The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 6 
                 sensing unit 
               
               
                 13 
                 ground potential 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                 41 
                 transmitter internal signal 
               
               
                 51 
                 first modulator 
               
               
                 53 
                 first demodulator 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 57 
                 output of the first demodulator 
               
               
                 60 
                 output of the first modulator 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and subcarrier signal  54  and feeds it into the first modulator  51 . The output  60  of said modulator sources the transmitter  1 . In order to output the transmitting signal  2 , the transmitter generates an internal signal  41 , which is made accessible for the useful signal extractor  40 . The transmitting signal  2  passes the transfer channel  3 , e.g. a complex impedance Z(jw) to ground potential  13 , and superposes with disturbances having its origin in the disturbing influence  30 . The useful signal extractor  40  extracts the useful signal  37  out of the transmitter internal signal  41  and feds it into the first demodulator  53 . Said demodulator is synchronized to the carrier  55 , demodulates the subcarrier component  57  out the useful signal  37  and outputs it to the control unit  35 . 
     By applying e.g. a DC signal as subcarrier signal  54  to the first modulator  51 , the output signal  57  of the first demodulator can be evaluated in e.g. phase angle and amplitude to conclude on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw). This result is to be confirmed for validity since the disturbing influence  30  might have caused a wrong measurement result. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz to the first modulator  51 , checking for subcarrier existence in the demodulated signal  57  and by assessing the properties of the demodulated subcarrier signal  57 , e.g. its phase angle or its amplitude in relation to the absolute value of impedance Z(jw). 
     If the subcarrier check proves that the measurement of the transfer behaviour of the transfer channel  3  was valid and not disturbed by a disturbing influence  30 , the control unit derives information out of the transfer channel properties and sends it to the environment via the data output signal  38 . 
     A typical application of the detection system of  FIG. 9  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 10 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage source 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 transmitting current 
               
               
                 21 
                 optional shield electrode 
               
               
                 26 
                 person&#39;s capacitance to ground 
               
               
                   
                 potential 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 control unit 
               
               
                 38 
                 data output 
               
               
                 42 
                 mixer 
               
               
                 43 
                 mixer 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 57 
                 received subcarrier 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and subcarrier signal  54  and feeds it into the mixer  43 . The voltage source  7  drives a voltage at transmitting electrode  8  and optional at shield electrode  21  which is in direct relation to the output of mixer  43 . Due to the difference in potential between transmitting electrode  8  and ground potential  13 , an electrical field  9  forms, resulting in a complex impedance Z(jw) between transmitting electrode  8  and ground potential  13 . A person  10 , whose presence on the vehicle seat  32  shall be detected, influences this impedance. Said impedance causes the AC current  14  to flow, which is measured by the low-ohmic current meter  12  and mixed with carrier signal  55 . The control unit  35  characterizes the complex impedance Z(jw) in e.g. phase angle, absolute value or frequency dependency by evaluating signal  57  while keeping subcarrier signal  54  at DC. 
     Validation of the measurement result is performed by control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz to the first mixer  43 , checking for subcarrier existence in the demodulated signal  57  and by assessing the properties of the demodulated subcarrier signal  57 , e.g. its phase angle or its amplitude in relation to the absolute value of Z(jw). 
     The tagging principle shown in  FIG. 7  can be also applied to the topology indicated in  FIG. 3 .  FIG. 11  shows how ‘active transmitting signal tagging by binary protocol transmission’ can be applied to a loading mode topology. The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1 
                 transmitter 
               
               
                 2 
                 transmitting signal 
               
               
                 3 
                 transfer channel 
               
               
                 6 
                 sensing unit 
               
               
                 13 
                 ground potential 
               
               
                 30 
                 disturbing influence 
               
               
                 35 
                 control unit 
               
               
                 37 
                 useful signal 
               
               
                 38 
                 data output 
               
               
                 40 
                 useful signal extractor 
               
               
                 41 
                 transmitter internal signal 
               
               
                 50 
                 second modulator 
               
               
                 51 
                 first modulator 
               
               
                 52 
                 second demodulator 
               
               
                 53 
                 first demodulator 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 56 
                 binary protocol 
               
               
                 57 
                 output of the first demodulator 
               
               
                 59 
                 output of the second modulator 
               
               
                 60 
                 output of the first modulator 
               
               
                 61 
                 demodulated binary protocol 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55  and subcarrier signal  54 . The carrier signal  55  is fed into the first modulator  51 . The subcarrier signal  54  is fed into the second modulator  50 . The output  59  of the second modulator  50  is connected to the input of the first modulator  51 . The output  60  of the first modulator  51  is connected to the input of the transmitter  1 . The control unit  35  applies a binary protocol  56  to the input of the second demodulator  50 . The transmitting signal  2  passes the transfer channel  3 , e.g. a complex impedance Z(jw) to ground potential  13 , and superposes with disturbances having its origin in the disturbing influence  30 . The useful signal extractor  40  extracts the useful signal  37  out of the transmitter internal signal  41  and feds it into the first demodulator  53 . Said demodulator is synchronized to the carrier  55 , demodulates the subcarrier component  57  out the useful signal  37  and inputs it to the second demodulator  52 , which is synchronized to the subcarrier signal  54  and which demodulates the binary protocol  61 . Said binary protocol is output to the control unit  35 . 
     By applying e.g. a binary ‘one’ as binary protocol and a DC signal as subcarrier signal  54  to the second modulator  50 ; the output signal  61  of the second demodulator can be evaluated in e.g. phase angle and amplitude to conclude on the transfer behaviour of transfer channel  3 , e.g. on the impedance Z(jw). This result is to be confirmed for validity since the disturbing influence  30  might have caused a wrong measurement result. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz and a binary protocol  56  to the second modulator  50 . The control unit  35  can check for subcarrier existence and properties in the demodulated binary protocol signal  61  during constant bit values of the sent binary protocol  56 . The properties of the received subcarrier signal can be evaluated in e.g. its phase angle or its amplitude in relation to the absolute value of impedance Z(jw). In addition, to further increase the selectivity of the detection system, the demodulated binary protocol  61  can be compared with the sent binary protocol  56 . 
     If the subcarrier check and the binary protocol check prove that the measurement of the transfer behaviour of the transfer channel  3  to ground potential  13  was valid and not disturbed by a disturbing influence  30 , the control unit derives information out of the transfer channel properties and sends it to the environment via the data output signal  38 . 
     A typical application of the detection system of  FIG. 11  relating to the field of capacitive occupant detection system in an automotive vehicle is shown in  FIG. 12 . The different elements represented in this figure are: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 7 
                 AC voltage source 
               
               
                 8 
                 transmitting electrode 
               
               
                 9 
                 electrical field 
               
               
                 10 
                 person whose presence is to be 
               
               
                   
                 detected 
               
               
                 12 
                 low-ohmic current meter 
               
               
                 13 
                 ground potential 
               
               
                 14 
                 transmitting current 
               
               
                 21 
                 optional shield electrode 
               
               
                 26 
                 person&#39;s capacitance to ground 
               
               
                   
                 potential 
               
               
                 31 
                 disturbing electrical field 
               
               
                 32 
                 vehicle seat 
               
               
                 35 
                 is a control unit 
               
               
                 38 
                 data output 
               
               
                 42 
                 mixer 
               
               
                 43 
                 mixer 
               
               
                 44 
                 mixer 
               
               
                 45 
                 mixer 
               
               
                 54 
                 subcarrier signal 
               
               
                 55 
                 carrier signal 
               
               
                 56 
                 binary protocol 
               
               
                 61 
                 demodulated binary protocol 
               
               
                   
               
             
          
         
       
     
     The control unit  35  generates a carrier signal  55 , a subcarrier signal  54  and a binary protocol signal  56 . Subcarrier signal  54  and binary protocol signal  56  are fed into mixer  44 . The output of mixer  44  is fed into mixer  43  together with carrier signal  55 . The voltage source  7  drives a voltage at transmitting electrode  8  and optional at shield electrode  21  which is in direct relation to the output of mixer  43 . Due to the difference in potential between transmitting electrode and ground potential  13 , an electrical field  9  forms, resulting in a complex impedance Z(jw) between transmitting electrode  8  and ground potential  13 . A person  10 , whose presence on the vehicle seat  32  shall be detected, influences this impedance. Said impedance causes the AC current  14  to flow, which is measured by the low-ohmic current meter  12  and mixed with carrier signal  55 . The output of mixer  42  is mixed again with the subcarrier signal  54  by mixer  45 . The output of mixer  45  is input to control unit  35 . The control unit  35  characterizes the complex impedance Z(jw) in e.g. phase angle, absolute value or frequency dependency by evaluating signal  61  while keeping subcarrier signal  54  at DC and the binary values of the binary signal  56  constant. 
     Validation of the measurement result is performed by the control unit  35  by applying e.g. a sinusoidal subcarrier signal  54  of e.g. 1 KHz and a binary protocol  56  to mixer  44 . The control unit  35  can check for subcarrier existence and properties in the demodulated binary protocol signal  61  during constant bit values of the sent binary protocol  56 . The properties of the received subcarrier signal can be e.g. its phase angle or its amplitude in relation to the absolute value of impedance Z(jw). In addition, the demodulated binary protocol  61  can be compared with the sent binary protocol  56 . 
     It will be noted that the system availability of the above described embodiments in case of detected interference my be further improved. As result of a detected interference (means taken only if interference was detected) or in general (means are always taken, even if no interference was detected), one or more of the following measures may be applied: 
     1) carrier frequency hopping in case of a detected interference 
     2) subcarrier frequency hopping in case of a detected interference 
     3) variation of bit values in binary protocol in case of a detected interference 
     4) combinations of 1) . . . 3)