Abstract:
A movable or removable seat for a motor vehicle is disclosed which preferably takes the form of a re-movable vehicle seat ( 1 ). The removable seat incorporates a first inductor ( 17 ) which forms part of an inductive coupling between the removable seat and a second inductor ( 21 ) which is mounted on the motor vehicle. The first inductor forms part of a resonant circuit which incorporates at least one capacitor. The resonant frequency of the resonant circuit is variable in response to a sensed parameter such as, for example, the position of a head-rest of the seat, the inclination of the back-rest of the seat and such like. A central unit ( 23 ) generates a square wave which excites the resonant circuit. A signal is transmitted from the resonant circuit to the central unit ( 23 ) which analyses the resonant frequency of the resonant circuit.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Great Britain patent application 0310723.2, filed May 9, 2003 and PCT/SE2004/000703, filed May 7, 2004. 
     FIELD OF THE INVENTION 
     THE PRESENT INVENTION relates to a movable or removable seat for a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     Many vehicles are presently manufactured which are provided with one or more removable seats. The seats may be removed from the vehicle should the driver of the vehicle wish, for example, to carry a large or bulky load. Also vehicles are provided with movable seats, to permit the seat position to be adjusted, and other movable components, such as pillar-loop height adjusters for safety-belts. 
     There is often a requirement for a vehicle seat to incorporate one or more sensor switches which are utilised to control safety devices intended to provide protection for an occupant of the seat in the event that an accident should occur. For example there may be a requirement for a weight-sensitive switch which indicates the presence of an occupant on the seat. Also there may be a requirement for a buckle switch to sense whether the occupant of the seat is, or is not, using a safety-belt. Signals derived from such switches may be essential for the proper control of the deployment of a safety device such as an air-bag. Many other types of switches may be provided to sense parameters relating to the seat and/or the occupant of the seat in very sophisticated cases. 
     If a vehicle seat is permanently fixed in position, each switch can be connected by fixed wiring to a central control unit which is utilised to control the deployment of safety devices within the vehicle. It is preferred that for diagnostic purposes a resistor should be connected in parallel with the switch so that the status of the wires can be checked from time-to-time without the switch being closed. Thus the resister creates a resistance valve in the switch circuit which can be measured as indicating the proper connection with the circuit despite the switch being in an open condition. 
     If, however, the vehicle seat is to be a removable seat, it is not possible to utilise fixed wiring between sensor switches within the seat and the central control unit. Whilst, it would be possible to provide a wiring arrangement involving a connector to be connected and disconnected each time the seat is placed in the vehicle or removed from the vehicle, an arrangement of this type is not preferred, since users of the vehicle may well forget to effect the necessary connections. It is, consequently, preferred to use a contactless-transmission of information relating to the status of switches within the seat using, for example, an inductive coupling in the form of a transformer. Even if a vehicle seat is a conventional adjustable seat it may be preferred to use some sort of wireless connection between the seat and the vehicle. For a component such as a pillar-loop with a height adjuster, which may have a sensor to sense the height of the loop, it may be difficult to make a permanent wiring connection when installing the pillar-loop, and again a wireless connection to a sensor in such a pillar-loop may be preferred. 
     Here it is to be understood that when a switch is provided with a resistor in parallel for diagnostic purposes as described above, what is sensed when the switch is closed is effectively a change in resistance. The circuit goes from a high resistance circuit, with a resistance value equivalent to that of the nominal value of the resistor that is in parallel with the switch to a low resistance circuit. 
     If there is, however, a contactless-transmission within the circuit as constituted, for example, by a transformer, the coupling factor of the transformer will effect the measured value of the resistance of the circuit. Where, for example, a transformer is constituted by two parts, one being a part on a seat and one being part on the motor vehicle, the actual value of the coupling factor may be unpredictable, and the coupling factor may be relatively low. It is thus very difficult to determine accurately the resistance of the combination of a switch and resistor in a motor vehicle seat when there is a contactless-transmission between the seat and a central control unit. 
     The present invention seeks to provide an improved movable or removable seat for a motor vehicle. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a removable seat for a motor vehicle, wherein the seat incorporates a first inductor to form part of an inductive coupling between the unit and a second inductor mounted on the motor vehicle, the first inductor forming part of a resonant circuit which incorporates at least one capacitor, the resonant frequency of the resonant circuit being variable in response to a sensed parameter. 
     Advantageously, the variable parameter is the position of a height-adjustable belt deflector. 
     Conveniently, the parameter is the position of a head-rest. 
     Preferably, the parameter is the inclination of a back-rest. 
     Advantageously, the parameter is the condition of part of a safety-belt system. 
     Conveniently, the parameter is a condition of a safety-belt retractor. 
     Preferably, the parameter is a condition of a safety-belt buckle. 
     Advantageously, the parameter is weight applied to the squab of the seat. 
     Conveniently, the resonant circuit includes at least one capacitor associated with a switch to switch the capacitor in and out of the circuit. 
     Preferably, the seat incorporates a plurality of capacitors each with a respective switch to enable the capacitors to be switched individually in and out of the circuit. 
     Advantageously, the resonant circuit is a passive circuit. 
     Conveniently, the seat is provided in combination with a vehicle, the vehicle being provided with said second inductor to co-operate with the first inductor to form a contactless-transmission, the second inductor being associated with a signal generator and analyser to generate an exciting signal to excite the resonant circuit and to analyse signals from the resonant circuit. 
     Preferably, the resonant circuit has two resonant frequencies depending upon the condition of a switch, and the signal generator is configured to generate a signal such that the circuit has resonant frequencies which are odd harmonics of the base frequency of the generated signal. 
     Advantageously, the signal generator is adapted to generate a plurality of separate signals, each signal having a different frequency which is a possible resonant frequency of the circuit. 
     The separate signals may all be generated simultaneously, but conveniently, the signal generator is configured to generate successive signals of different possible resonant frequencies. 
     In one embodiment, the signal generator and analyser is configured to generate a signal during one period of time and analyse a signal during a succeeding period of time. In one embodiment the unit is configured to take an average of the analysed signals and compare the phase of the coverage with the phase of the signal generated by the signal generator. 
     Alternatively the signal generator and analyser may be configured to generate a signal and analyse a signal simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  is a diagrammatic view of a vehicle seat and part of a vehicle, 
         FIG. 2  is a sensor circuit diagram together with some wave forms illustrating features of one seat in accordance with the invention, 
         FIG. 3  is a view corresponding to  FIG. 2 , but with alternate wave forms, 
         FIG. 4  is a view corresponding to  FIG. 2 , but illustrating a modified embodiment of the invention, 
         FIG. 5  is a view illustrating wave forms associated with the sensor circuit of  FIG. 4 , 
         FIG. 6  is a further view illustrating wave forms associated with the sensor circuit of  FIG. 6 , 
         FIG. 7  is an alternative view illustrating wave forms for purposes of comparison, 
         FIG. 8  illustrates a modification of part of the sensor circuit of  FIGS. 2 ,  3  and  4 , 
         FIG. 9  is a view corresponding to  FIG. 7  showing a further modification, 
         FIG. 10  is a view corresponding to  FIG. 7  showing a further modification, and 
         FIG. 11  is a view corresponding to  FIG. 7  showing a further modification. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to  FIG. 1  of the accompanying drawings, a vehicle seat  1  is illustrated, the seat being in the form of a removable seat which can be removed from and re-mounted in a motor vehicle by the driver of the vehicle. The seat includes a squab  2  or seat cushion and a back-rest  3 . The back-rest  3  is pivotable relative to the squab  2  about an axis  4 . The back-rest  3  carries a head-rest  5 . The head-rest  5  is connected to the back-rest by means of a supporting arm  6 . The position of the head-rest  5  may be adjusted by moving the supporting arm  6  upwardly and downwardly. 
     The seat  1  is provided with a seat-belt  7 , the seat-belt  7  being mounted on a retractor reel  8 . Part of the safety-belt  7  passes through an adjustable-height loop deflector  9 . A buckle  10  is provided to receive a tongue provided on the safety-belt. 
     Various sensor switches are provided within the seat  1 . A first sensor switch  11  is mounted within the squab  2  of the seat and is a weight-responsive sensor switch. The switch  11  is thus closed when an occupant is seated on the seat  1 . 
     The buckle  10  incorporates a switch  12  which is closed when the seat-belt tongue is inserted in the buckle  10 , thus showing that the occupant of the seat is wearing the seat-belt  7 . 
     The adjustable-height loop deflector  9  is provided with a sensor switch  13  which is closed in an appropriate manner to indicate the height of adjustment of the loop-deflector. The back-rest  3  is provided with a sensor switch  14  responsive to the degree of inclination of the back-rest  3  relative to the squab  2 . The retractor  8  is provided with a sensor switch  15  which is closed in dependence along the length of safety-belt withdrawn from the retractor. The head-rest  5  is associated with a sensor switch  16  which is closed in dependence upon the degree of movement of the head-rest. 
     Whilst reference has been made to many simple switches, it is to be appreciated that many of the described switches may be replaced by sensors which, instead of providing a simple “on-off” output, provide a gradually increasing output, as will be described hereinafter in greater detail. 
     The seat  1  is provided with an inductor  17  and, as will be described hereinafter, the various switches and sensors are provided in a circuit which incorporates the inductor  17 . 
       FIG. 1  also illustrates part of the floor  20  of a motor vehicle on which the seat  1  is to be mounted. The floor  20  is provided with an inductor  21  which is to co-operate with the inductor  17  provided on the seat, to form a transformer or a contactless-transmission arrangement extending between the seat and the vehicle. The inductor  21  is connected by means of a wire  22  to a central control unit  23 . The control unit  23  is configured to control deployment of various safety devices, such as the safety device  24 . 
       FIG. 2  illustrates, schematically, sensor circuitry present within the vehicle and within the seat, but, for the sake of simplicity, the circuitry in the seat only includes one sensor switch. 
     Considering  FIG. 2  in more detail showing a sensor circuit  18  in accordance with this invention, the inductor  17  which is present on the seat  1  forms part of a tuned resonant circuit, the circuit including a first capacitor  25  connected in parallel with the inductor  17 , and also including the series connection of a second capacitor  26  and a switch  27  connected in parallel with the combination of the inductor  17  and the first capacitor  25 . The switch  27  is thus effective in switching the second capacitor  26  in (as shown in  FIG. 3 ) and out (as shown in  FIG. 2 ) of the resonant circuit depending upon whether the switch  27  is open or closed. 
     It will be appreciated that the sensor circuit  18  shown in  FIG. 2  is such that with the switch  27  open, a resonant circuit is formed having a frequency determined by the inductance L 1  for the inductor  17  and the capacitance of the capacitor  25 . However, when the switch  27  is closed, the circuit  18  will have a second resonance frequency determined by the inductance L 1  and the combination of the capacitors  25  and  26 . 
       FIG. 2  also illustrates the inductor  21  provided on the floor  20  of the motor vehicle and shows a signal processing circuit  28  that is connected to the inductor  21 . The signal processing circuit  28  may be located immediately adjacent the inductor  21  with the output of that circuit being connected to the central control unit  23 , or alternatively the signal processing circuit  28  may form part of the central control unit  23 . 
     The signal processing circuit  28  incorporates a signal generator and signal analyser  29 . The signal generator and analyser  29  is connected, together with a resistor  30 , in circuit with the inductor  21 . Also connected in circuit with the inductor  21  is a pulse-shaper  31 , for example in the form of a Schmidt trigger circuit which is connected to convert sinusoidal waves present within the inductor  21  into a square pulse-wave. 
     The signal generator and analyser  29  is adapted to generate a square wave current signal having a frequency F 0  as shown at  32 . The square wave is transmitted inductively from the inductor  21  to the inductor  17 , and thus excites the resonant circuit. The resonant circuit resonates with a voltage frequency F 5  as shown at  33 . The voltage curve is almost a pure sine wave. Signals from the resonant circuit are transmitted back from the inductor  17  to the inductor  21 . Signals are passed from the inductor  21  to the signal shaping circuit  31 . The input  34  to the signal shaping circuit is a sine wave voltage equivalent to the signal  33  but with added spikes generated whenever there is a change of polarity of the square wave  32 . The signal  33  is shaped by the signal-shaping circuit  31  to provide a square wave  35  that has the frequency F 5  of the fifth harmonic of the square wave F O . This wave  35  is analysed by the signal generator and analyser  29 . 
     Here it is to be noted that in the described embodiment with the switch  27  open, the resonant frequency of the resonance circuit provided within the vehicle seat is the fifth harmonic F 5  of the frequency F 0 , whereas with the switch  27  closed (as illustrated in  FIG. 3 ), the resonant frequency of the circuit in the seat is the third harmonic F 3  of the frequency F 0 . 
     Thus, with the switch  27  closed, as in  FIG. 3 , the frequency of the signals  33 ,  34  and  35  are all F 3 , the third harmonic of the originally generated square wave signal  32 . 
     It is thus to be understood that in operation of the sensor circuit  18  illustrated in  FIGS. 1 and 2 , the signal generator and analyser  29  will initially generate the square wave F 0  which will be transmitted from the inductor  21  to the inductor  17  which will cause the resonant circuit to resonate. The circuit  18  will resonate at an appropriate resonant frequency depending upon whether the switch  27  is open or closed. If the switch is open (as illustrated in  FIG. 2 ), the resonant circuit resonates at the fifth harmonic F 5  of the frequency F 0 . A signal will be transmitted from the inductor  17  to the inductor  21 , and that signal will be shaped by the shaping circuit  31  with a resultant square wave being fed to the signal generator and analyser  29 . The signal generator and analyser  29  will be able to determine whether the received signal is the third harmonic ( FIG. 3 ) or the fifth harmonic ( FIG. 2 ) of the frequency F 0 , and will thus be able to determine whether the switch  27  is opened or is closed. 
     It is thus to be appreciated that the arrangement disclosed enables the condition of the switch to be sensed, but also enables a diagnostic check of the relevant circuitry to be carried out. 
       FIG. 4  is a figure provided for explanatory purposes. The circuitry shown in  FIG. 4  is generally equivalent to that shown in  FIG. 2  save that a controllable switch  36  is provided connected between one end of the inductor  21 , the signal generator and analyser  29 , and the pulse-shaping circuit  31 . Various points on the sensor circuit  18  are identified by letters A to D. 
       FIG. 5  is a graphical figure illustrating the wave forms present at the points A to D in operation of the circuit  18  during two periods of time T 1  and T 2 . During the first period of time T 1 , the signal generator and analyser  29  is actually transmitting a signal, and the switch  36  is connected to the resistor  30 , whereas in the subsequent period of time T 2  the signal generator and analyser  29  is analysing signal and the switch  36  is connected to the pulse-shaping circuit  31 . 
     Initially, when the signal generator and analyser  29  is generating a square-wave outlet signal at a frequency F 0 , as can be seen at point A, which is one of the leads to the inductor  21 , there is a strong square-wave. At point B which is part of the resonant circuit mounted in the seat, there is a strong sinusoidal wave. At point C, which is the input to the shaping circuit  31 , there is no signal and at point D, which is the output of the shaping circuit  31 , there is again no signal. At the end of time T 1 , the condition of the switch  36  is changed and the signal generator and analyser  29  commences an analysing function. At the beginning of the time T 2 , there is a still strong sinusoidal wave present at point B within the resonant circuit mounted on the seat. This wave does, however, decay with time. This wave is inductively coupled from the inductor  17  to the inductor  21  and, as can be seen, at point A there is a somewhat smaller, but no less discernible sinusoidal wave form which again decays with time. It can be seen that as the switch  36  is now connected to the input of the shaping circuit  31 , the wave form at point C is essentially identical to the wave form at point A. The output of the shaping circuit  31  is a square wave form which facilitates analysis of the signal by the signal analyser  29 . 
     Turning to  FIG. 6 , it can be seen that the signal transmission and signal analysis procedures may be completed many times. As can be seen, the phase of the square-wave signal at point D as generated by the shaping circuit  31  is typically in phase with the immediately preceding signal at point A as generated by the signal generator and analyser  29 . Successive signals over n cycles of operation may be averaged, and the average signal, on being compared with the original square wave, is in phase with the original square wave. 
     In  FIG. 7  the same procedure is repeated, but the resonant circuit is not present and a spurious signal is present. It can be seen that the averaged analysed signal is not in phase with the original square wave. Indeed, the averaged signal is little more than “noise”. The signal generator and analyser  29  is thus adapted to compare the phase of the received signal from the shaping circuit  31  with the phase of the recently transmitted signal. This helps identify any spurious signals from external perturbations. 
     In a variant of the above-described invention, it is envisaged that the signal generator and analyser  29  could be configured to generate a separate wave for each possible resonant frequency of the resonant circuit. These separate waves could be generated successively. 
     Whilst the invention has been described with reference to one particular form of resonant circuit  18 , it is to be appreciated that many different types of resonant circuit may be utilised.  FIG. 8 , for example, shows a modified sensor circuit  18 A in which the inductor  17  is connected to a series connection of the two capacitors  25  and  26 , with one of the capacitors  26  being effectively shorted out by the switch  27 . The switch  27  therefore again permits the capacitor  26  to be switched in or out of the resonant circuit. Of course, many other configurations of resonant circuit may be utilised. 
     So far the description has only concentrated on one switch whereas, in  FIG. 1 , it was shown that a single seat may incorporate a large number of switches.  FIG. 9  illustrates a sensor circuit  18 B where the inductor  17  is connected in a resonant circuit which incorporates a first capacitor  25 , and which additionally includes a number of individually switched supplementary capacitors including a capacitor  40  with an associated switch  41 , a capacitor  42  with an associated switch  43  and a capacitor  43  with an associated switch  44 . The value of the capacitors will be chosen so that regardless of the combination of switches which are closed, a clearly identifiable resonant frequency will be achieved. It is to be understood that any number of capacitors may be present in a circuit of this type. 
       FIG. 10  illustrates a sensor circuit  18 C in which the inductor  17  is associated with a single capacitor  45 , that capacitor being a variable capacitor. A variable capacitor may be used in place of the switch  14 , for example, with the capacitance of the capacitor being adjusted in accordance with the degree of rake at the back of the seat. The variable capacitor  45  may be used also, for example, in place of the switch  13  which was associated with the height adjustable deflector loop  9 . 
       FIG. 11  illustrates a sensor circuit  18 D which includes the inductor  17  and the capacitor  25 , and also a variable inductor  46 . The variable inductor may again be utilised where a variable signal is to be provided. 
     It is to be understood, therefore, that in the vehicle seat of the invention there is a resonant circuit, the resonant frequency of which varies in dependence upon the condition of the vehicle seat. In each of the embodiments described, the resonant circuit provided on the seat has no active components. The resonant circuits are therefore each passive. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying drawings.