Abstract:
A position sensor for determining a set position thereof is disclosed. The sensor comprises means for generating a time-varying magnetic field, an electrically conductive element positioned such that said time-varying magnetic field generates eddy currents therein and at least one pick-up for generating a signal in response to said time-varying magnetic field. Movement of the conductive element relative to the pick-up alters the magnetic coupling between the field generator and the pick-up allowing the set position to be determined unambiguously.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a position sensor which may be used, for example, as an electronic controller or a rotary switch, in particular for controlling a cooking hob.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many rotary switches are used in the field of household goods in order to adjust washing programs in washing machines or temperature settings in electric cookers. Such rotary switches suffer, however from several drawbacks.  
           [0003]    A significant drawback of currently existing rotary switches is that they are generally not free from wear and tear, and may suffer from deterioration in performance during their effective life. A further disadvantage is that they are generally complex to produce and complicated to contact with a control panel.  
           [0004]    Selector switches may be incorporated on a control panel. Such switches have, however, a limited lifetime and they lack the flexibility of a rotary switch since the number of positions is limited by the geometric construction. Furthermore, as the number of positions increases, so the complexity of the switch also increases, increasing the cost. If sensors are employed which produce a sensor signal in response to a change of field caused by rotation of the rotary switch, this disadvantage can be overcome. Such sensors are, however, generally temperature sensitive, expensive and often show a hysterisis, which limits the number of switch positions. An example of such a field sensing switch would be one which incorporates a capacitive mechanism, wherein the capacitance is altered by the rotation of the selector. Such an arrangement is not suitable in many situations because of its sensitivity to dampness. Sensors which use the phenomenon of induction are resistant to dampness.  
           [0005]    Inductive position sensors are known in the field of electric motors and the like for determining the angular position of the rotor. An example is described in U.S. Pat. No. 5,621,179. A rotary position sensor which employs planar coils is described in U.S. Pat. No. 4,507,638. In the arrangement described therein, a rotatable plate is interposed between drive and sensing coils. The rotatable plate is fabricated from a material which blocks a magnetic field induced in the drive coil from reaching or activating the sensing coil. To provide position sensitivity, the rotatable plate may have a cam-like shape. Alternatively, a plurality of drive and sense coils may be used having a distribution around the operating angle. Pulsed or A.C. signals are applied to the drive coils, which may be in the form of a planar coil on a printed circuit board.  
           [0006]    A further position sensor is described in U.S. Pat. No. 5,239,288. The sensor described therein comprises a stator assembly and a rotor assembly. Each of the stator and rotor assemblies comprise a plurality of layers upon which one or more multi-turn coils are arranged. An A.C. signal is applied to one of the stator windings which results in a current being generated in the rotor windings. As a result, a signal which is dependent on the angular position of the rotor is generated in further coils of the stator.  
         SUMMARY OF THE INVENTION  
         [0007]    In particular, the present invention provides a position sensor for determining a set position thereof, characterized in that the sensor comprises:  
           [0008]    a time-varying magnetic field generator;  
           [0009]    an electrically conductive element positioned such that said time-varying magnetic field generates eddy currents therein;  
           [0010]    at least one pick-up for generating a signal in response to said time-varying magnetic field; and  
           [0011]    a signal processor arranged to receive said signal and to determine the set position of the position sensor,  
           [0012]    wherein the conductive element is movable relative to the at least one pick-up such that the signal generated by said pick-up has a magnitude which is dependent on the position of said conductive element, the position of the conductive element corresponding to the set position of the sensor.  
           [0013]    The sensor may comprise at least two pick-ups, with signals from the two pick-ups being processed in combination to determine a sensor set position. Certain embodiments of the invention, in particular a linear position sensor require only a single pick-up for the sensor set position to be determined. The pick-ups are preferably in the form of an inductively coupled sensor loops having a combined projected area which is less than a projected area of a drive loop.  
           [0014]    In a particular embodiment, the magnetic field is generated by a circular drive loop, which may be spiral in design and detected by two sensor loops which may each also be spiral, generally in the form of a sector. Several sensors may be multiplexed together, with a single processor determining the set position of each sensor. If desired, an isolation switch can be incorporated into the sensor to provide a positive disconnection, for safety reasons. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 is a cross section of a rotary position sensor of the invention;  
         [0017]    [0017]FIG. 2 is a schematic view of a sensor of the invention showing drive and sensor coil arrangements together with associated circuitry;  
         [0018]    [0018]FIG. 3 shows the FIG. 2 arrangement with a shielding element overlying the coils; and  
         [0019]    [0019]FIG. 4 shows a further view of a drive and sensor coil arrangement.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    Referring to FIGS. 1 and 2 there is shown a rotary position sensor  10  of the invention. The position sensor  10  comprises a drive coil  12  and two sensor coils  14  and  16 . A conductive shielding plate  18  is attached to a rotatable spindle  20 . In the embodiment shown in FIG. 1, the shielding plate  18  is attached to a control knob  22  made of plastic.  
         [0021]    The drive coil  12  and the sensor coils  14  and  16  are shown as being single-turn loops with connectors  24  at the end of the loops. Alternatively, one or more of the coils could be multi-turn coils, having a spiral-like structure. The drive coil  12  has a shape which approximates to a full circle, whilst the sensor coils  14  and  16  are in the form of a sector, with an enclosed area approximately one third of that of the drive coil  12 . As shown in FIG. 3, the shielding plate  18  is also in the form of a sector which has a surface area which is approximately equal to one third of that of the drive coil  12 . The drive coil  12  and the sensor coils  14  and  16  are preferably tracks defined on a printed circuit board. The shielding plate is preferably punched out from normal steel sheet, bonded to the control knob  22  and the rotatable spindle  20  by injection moulding.  
         [0022]    The shielding plate preferable is in the shape of a sector of a circle. It should, however, be noted that the shape of the shielding plate may be any form which may be rotated around the rotatable spindle  20 . The surface area of the shielding plate should not be less than one third of the area inside of the drive coil.  
         [0023]    The connectors  24  of the drive coil  12  are connected to an A.C. current source  26 . Voltages induced in the sensor coils  14  and  16  are converted into respective D.C. signals by respective converters  28 . These D.C. signals are then processed by a microcontroller  30  which includes analog to digital conversion capability.  
         [0024]    The current source  26  provides a time varying current to the drive coil  12  of approximately 5 mA which generates a time varying magnetic flux Φ. This time varying magnetic flux has components Φ 1  and Φ 2  penetrating the sensor coils  14  and  16  respectively. These magnetic fluxes induce a current in each of the sensor coils  14  and  16  which leads to a potential difference being generated across a resistance in the coils. This induced potential difference is directly proportional to the amplitude of the imposed current in the drive coil  12  and the drive frequency.  
         [0025]    The shielding plate  18 , which acts as a conductive element, lies in the magnetic field generated by the drive coil  12 . As a result, eddy currents are generated in the shielding plate  18  and therefore a magnetic field is generated which opposes the magnetic field generated by the drive coil  12 . The superposition of these fields means that the magnetic fields experienced by the two sensor coils  14  and  16  are reduced when the shielding plate  18  is the vicinity thereof. The exact positioning of the shielding plate  18  therefore leads to a predictable influence on the potential difference generated in the two sensor coils  14  and  16 , with the greater the coverage of the coil, the lower the induced voltage across the coil. An analysis of the potential differences generated by the sensor coils leads to a determination of the position of the shielding plate  18 , and therefore the control knob  22 .  
         [0026]    The oscillating current source  26  preferably comprises a low-cost CMOS inverter. The oscillator clock could have a frequency of 4, 8 or 12 MHz, or another frequency. This alternating voltage is applied to the drive coil  12  as a drive current via an output current limiting resistor (not shown).  
         [0027]    The potential differences across the sensor coils  14  and  16  are each transformed into a D.C. voltage by means of a simple amplitude detector comprising a Schottky diode, two resistors and two capacitors which can then be passed to the analog to digital port of the microcontroller  30 . Should the amplitude of the signals generated by the sensor coils  14  and  16  be too low, perhaps because of a small surface area, they can be amplified by a low cost operational amplifier.  
         [0028]    The necessary computational analysis for determining the set position of the sensor  10  will now be described.  
         [0029]    Defining the D.C. signals generated across the sensor coils  14  and  16  as U 1  and U 2  respectively, a relative induced signal measure I is calculated where I=U 1 /U 2 . A total induction measure G is also calculated where G=U 1 +U 2 .  
         [0030]    The values of I and G are used to determine the position of the shielding plate  18  by comparing them with data stored in a look-up table. Whilst the relative signal I provides a generally good basis for determining the relative position of the shielding plate  18 , it may be ambiguous. For example, when the shielding plate is in the uppermost position shown in FIG. 3, the signals U 1  and U 2  are approximately equal. This will also be the case when the shielding plate is in the lowermost position. Since I is the same in both cases, the position of the shielding plate can not be determined from I exclusively in these situations. In such situations, the total induction measure G is used to determine absolute position.  
         [0031]    Because the electronic components can be arranged in close proximity, the influence of temperature on the detected signals is equal such that the temperature has little or no effect on the relative measure I. The temperature is likely to have an influence on the total measure G and therefore this is used only to determine the coarse position of the shielding plate (whether in quadrant II/III or I/IV of FIG. 3). For this purpose, any temperature dependence of G will not affect the determination of sensor position.  
         [0032]    Manufacturing tolerance effects in the shielding plate  18  and the coils  12 ,  14  and  16  and particularly the associated electronics are taken into account in an initial calibration where I and G are calculated with the sensor set in a zero position. This calibration can be performed automatically.  
         [0033]    In a preferred embodiment, the position sensor  10  is arranged to have twenty-four set positions, equally distributed around the circumference of the drive coil  12 . If desirable, a notched wheel can be attached to the spindle  20  to give a tactile indication of these positions. The number of set positions is limited by the sensitivity of the electronics and the processing capability of the microcontroller. If an increase in the number of positions is subsequently desired, the program stored in the microcontroller could be up-dated or a replacement microcontroller inserted, and if appropriate the notched wheel exchanged.  
         [0034]    Whilst the preferred embodiment incorporates two sensor coils, the number of sensor coils could be greater than this number. For such arrangements, the ratios of the induced signals of adjacent coils could be used to determine the position of the shielding plate and the position of the shielding plate may be determined unambiguously by such ratio measurements. Where desirable, a separate on/off switch, operated by the control knob  22 , can be incorporated into the position sensor to provide a positive disconnection. A switch suitable for such purposes might be a D 4  switch available from Cherry GmbH.  
         [0035]    Where several position sensors are grouped together on a single unit, the sensor coils can be multiplexed to the two peak detectors, thereby reducing the overall cost. Should there be more than one unit, then the drive coils can be connected in series.  
         [0036]    The position sensor of the present invention is suitable for incorporation in control panels for cookers to control, for example, the setting of an electric hob element and other household machines. The temperature independence and lack of sensitivity to moisture being particularly beneficial in this application. Applications in the automobile industry would be suitable with the insensitivity to the surrounding environment being relevant here.  
         [0037]    Whilst the position sensor has been described with reference to a rotary sensor, the principles of operation are extendable to linear position sensors, for example sliding controls. For such an application, a rectangular drive coil of length l and two adjacent rectangular coils of length l/2 would operate in conjunction with a shielding plate of length l/2.  
         [0038]    The described position sensor is stable, meaning that even after a power failure, the switch position may be determined, with the determination of the switch position not being made by monitoring incremental changes in position. The sensors are generally temperature insensitive and resistant to dampness and because of the low resistance of the coils they are not sensitive to parasitic electromagnetic influences. Furthermore such sensors have very low electromagnetic interference characteristics.  
         [0039]    As described above, it may be preferable for either the drive coil or the sensor coils, or all the coils to comprise multi-turn coils. Such an arrangement is shown in FIG. 4.  
         [0040]    Whereas many alterations and modifications of the present invention will be apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting.