Abstract:
In a linear position encoder, a support is provided upon which a pair of phase quadrature windings are mounted. The windings are arranged to have a sinusoidal magnetic sensitivity characteristic along the length of the support. Mounted on a movable element there is a resonant circuit including a coil and capacitor that can magnetically couple with the windings. When the circuit is excited, it induces currents in the windings that are dependent upon the position of the circuit within a period T s  of the windings. An excitation and processing unit is provided to energize the circuit and to process the signals induced in the windings. The excitation and processing unit is operably coupled to an external high permeability rod extending along the measurement path in a first plane having the windings but removed therefrom. The rod has first and second excitation coils connected in series and wound around the rod and disposed at opposite ends defining a length of the rod.

Description:
BACKGROUND 
   The present invention relates to linear position encoders generally. The invention has particular although not exclusive relevance to non-contact linear position encoders. The invention is particularly suited for use in systems where there may be electromagnetic interference, particularly when used in automotive applications. 
   Many types of non-contact linear position sensors have been proposed. A system similar to the present invention is described in U.S. Pat. No. 5,815,091 which is incorporated herein in its entirety by reference. In particular, U.S. Pat. No. 5,815,091 discloses a system for use as a linear position encoder. The basic layout of U.S. Pat. No. 5,815,091 is illustrated in  FIG. 1 .  FIG. 1  shows an outside trace  16  which is connected to a signal generator  11  that generates a trace signal typically in the 0.1 to 10 MHz. range. This outside trace  16  becomes the excitation trace. When an excitation signal is generated in the excitation trace, an output from a pair of phase quadrature conductive windings  13  and  15  depicted as sine and cosine traces is zero volts if perfect symmetry is observed. When a circuit  10  which is resonate at the excitation frequency is placed over the circuit board having the sine and cosine traces the symmetry is distributed and signals are induced into the sine and cosine traces. The voltage level of the signals at the outputs of corresponding sine and cosine traces are the sine and cosine representative of the linear position of the resonate circuit  10  with respect to the stationary printed sine and cosine traces. 
   The system determines the position of the movable element (i.e., resonant circuit) relative to the stationary element (i.e., circuit board) by utilizing the variation in mutual inductance between the coil and the plurality of sine and cosine wave windings. More specifically, when the power source energizes the coil, a large voltage signal is induced in a sine and cosine wave windings if the coil is adjacent a high part thereof. Only a small voltage signal is induced in a winding if the coil is adjacent a low part thereof. Therefore, the 
   However, this system has a number of disadvantages that pose real world problems when implemented for use, particularly in automotive applications. Firstly, the system is not “balanced”, i.e. it is not immune to electromagnetic interference. The flux from the excitation loop trace easily interacts with conductive materials in its proximity. 
   Secondly, the ability to get a null or zero signal at the outputs of the sine and cosine traces without the resonate circuit present varies with the mounting conditions and nearby objects. Thirdly, the resonate frequency of the moveable board will change with temperature and the presence of nearby conductive objects with respect to the excitation frequency which will greatly change the induced signal. 
   SUMMARY OF THE INVENTION 
   When used as a translational position encoder, the invention may comprise a rack and pinion steering assembly having means for indicating the racks relative position for vehicle steering, said means being relative position indicating apparatus as aforesaid. The translational position encoder may be used to determine the relative position of other fixed and movable members in engineering and automotive systems. 
   In one embodiment, a position detector includes a sensing circuit extending over a measurement path for sensing alternating magnetic field oriented in a predetermined direction; an energizing circuit, different from the sensing circuit, for generating an energizing alternating magnetic field. The energizing circuit includes an external high permeability rod extending along the measurement path in a first plane having the sensing circuit but removed therefrom. The rod has first and second excitation coils connected in series and wound around said rod disposed at opposite ends defining a length of the rod. The detector also includes a resonator electromagnetically coupled to the sensing circuit and energizing circuit. At least one of the resonator and the sensing circuit are mounted for relative movement with respect to the other over the measurement path in the first plane that is substantially parallel to the predetermined direction. The resonator is operable, upon energization of the energizing circuit, to resonate and to generate an alternating magnetic field whose magnetic axis lies substantially in the predetermined direction, which resonator magnetic field induces an alternating signal in the sensing circuit. The resonator and sensing circuit are arranged so that the amount of electromagnetic coupling therebetween varies sinusoidally as a function of their relative positions, thereby causing the amplitude of the signal induced in the sensing circuit by the resonator magnetic field to vary in a similar sinusoidal manner as a function of the position of the resonator relative to the sensing circuit. 
   A method is also disclosed for detecting the position of first and second members which are mounted for relative movement along a measuring path. The method employs the position detector disclosed above. 
   The above described and other features are exemplified by the following figures and detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the figures wherein the like elements are numbered alike: 
       FIG. 1  is a perspective view of a linear positioning encoder of the prior art; 
       FIG. 2  is an n end view of a cross section cut from a position encoder track forming part of the encoder of  FIG. 1 ; 
       FIG. 3  is a plan view of an exemplary embodiment of a linear position encoder of the present invention; 
       FIGS. 4 and 5  are layers of printed conductors from which a practical encoder track may be formed; 
       FIG. 6  is a diagrammatic view of the resulting 2-layer printed encoder track; 
       FIGS. 7–9  show alternative configurations of the conductors that form the encoder track; 
       FIG. 10  is a schematic representation of preferred excitation and processing circuitry used to determine the position of the resonant circuit relative to the encoder track of  FIG. 3 ; and 
       FIG. 11  is a plan view of fine quadrature spiral windings on the circuit board of  FIG. 3  having a set of course quadrature spiral windings superimposed thereon. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a cross section view and  FIG. 3  is a plan view of a linear position sensor  8  embodying the present invention. As shown in  FIGS. 2 and 3 , there are a pair of sine and cosine phase quadrature conductive windings  13  and  15 , respectively, and a feedback trace loop  18  mounted on a support  5  and described more fully herein. In a simple form as illustrated in  FIG. 3 , windings  13 ,  15  and the feedback loop  16  optionally take the form of insulated wires of copper or other conductor adhered e.g. by an epoxy adhesive in the required pattern onto a substrate  5  of fiberglass or other flat insulating sheet material. In a more developed form as shown in  FIGS. 4–6  conductive patterns are formed in successive printed circuit board layers which are shown in  FIGS. 4 and 5 , each layer having conductors in a printed pattern as shown and extending between via holes. When the two layers are superimposed and connected through the via holes as shown in  FIG. 6  there is obtained a track of the required sinusoidal or “spiral” configuration. The same principle can be used to provide additional layers e.g. of other “spiral” conductors or coarse position indicating tracks as described in more detailed below. 
   Still referring to  FIG. 3 , each winding  13  and  15  starts from one end  5   a  of support  5  and follows a sinuous path therealong until it reaches the other end  5   b , where it returns back along support  5  following a sinuous path to the starting end  5   a . The sinusoidal forward and return paths that form each winding  13  and  15  have period T s  and are in antiphase, i.e. they are substantially 180° out of phase. The windings  13  and  15  shown in  FIG. 3  and described above, will hereinafter be referred to as “sine and cosine windings” since they visually resemble a sine and cosine waveforms relative to one another. The sine and cosine windings  13  and  15  are insulated from each other, either by using via holes to the other side of the support  5  at the cross-over points, or by using a laminated conductor-insulator structure. If a laminate structure is used, the support  5  should be non-magnetic and preferably non-conducting for reasons that will become apparent later. The sine and cosine windings  13  and  15  may be formed using any conductive wire, but are preferably formed by etching or other standard printed circuit board technique. The feedback loop  18  runs around the periphery of the support  5  and may comprise a single loop of conductor or alternatively may comprise many turns of conductive wire. 
   Each end of the sine and cosine windings  13  and  15  and the feedback loop  18  are connected to the excitation and processing unit  11 . As those skilled in the art will realize, in practice the excitation and processing circuit  11  can be provided by a power source and a single semiconductor integrated chip. 
     FIG. 3  also shows coils of wire  14  and  34 , and a capacitor  17  that together form a resonant circuit. Printed coil  14  and capacitor  17  are on board  10  that are mounted above board  5  as shown. As represented by arrows  19  and  30 , the resonant circuit board  10  is free to move along the length of the support  5 , i.e. along the x-axis of  FIG. 3 . Likewise, coil  34  is free to move along the length of ferrite rod  20 . Preferably, an axis of the coil  14  is orthogonal to the surface of the support  5  on which the sine and cosine windings  13  and  15  are mounted, as this provides the greatest magnetic coupling between the spiral windings  13  and  15  and the coil  14 . 
     FIGS. 7 ,  8 , and  9  show alternative configurations of the windings. In  FIG. 7  the windings have, when viewed overall, a hexagonal configuration, in  FIG. 8  they are triangular and in  FIG. 9  they are square waves. 
   Attempts to use the prior art design having the excitation loop in the printed circuit board for automotive use has been met with failure. In this disclosure, the excitation loop of the prior art is removed from the printed circuit board to an external high permeability rod  20 , such as ferrite, for example, which traps the generated flux. This configuration always provides a zero or null condition without resonate circuit board  10  being present. This configuration also incorporates use of a phase locked loop (PLL)  22  to adjust the excitation frequency to match the resonate frequency of the moveable resonant circuit board  10 . 
   More specifically in an exemplary embodiment, an excitation magnetic loop  24  is removed from the printed circuit board  5  which has the printed sine and cosine traces  13  and  15 , respectively. The new excitation circuit utilizes two coils  26  and  28  connected in series and placed at either end of rod  20  having a high permeability core and has little interaction with materials proximate thereto. Excitation coils  26  and  28  are wound in first planes corresponding to the z-axis and placed at a right angle to a second plane corresponding to a plane having the sine and cosine printed traces  13 ,  15  and an axis  30  of rod  20  lying therein corresponding to a plane defining PCB  5  which greatly reduces any unwanted interactions between the two elements. 
   In an exemplary embodiment and still referring to  FIG. 3 , coil  14  and capacitor  17  are preferably mounted on a PCB  32  and coil  14  is connected in series with a coil  34 . Capacitor  17  is connected across the total inductance of coil  14  and coil  34  to form a resonant tank circuit generally shown at  36 . Coil  34  is wound on a bobbin (not shown) around rod  20  such that it can move along the high permeability rod  20  between coils  26  and  28 . Coil  14  is preferably a planer coil typically of printed circuit board construction which is mechanically as well as electrically connected to coil  34 . 
   In operation, coil  34  picks up energy from the rod  20  and coil  14  inductively couples the energy into the printed traces  13  and  15 . The excitation drive is generated by a phase locked loop (PLL)  22  circuit which is initially oscillating near the designed resonate frequency of the moveable resonate circuit  36 . PLL  22  circuit is preferably an integrated circuit chip and is more preferably integrated with the excitation and processing unit  11  as shown in  FIG. 3 . The initial frequency is picked up by the feedback trace  18  and is connected to the PLL  22 . The feedback trace loop  18  to PLL  22  completes the loop and causes the frequency of the PLL  22  to change until the PLL frequency matches and phase locks to the frequency of the moveable resonate circuit  36 . 
   As the ambient temperature changes, the electrical properties of resonate tank circuit  36  elements change and affect the resonant frequency of circuit  36 . PLL  22  in turn is configured to change the excitation frequency to match the new temperature dependent resonate frequency of tank circuit  36 . Matching the excitation frequency in magnetic loop  24  to the resonate frequency of the moveable tank circuit  36  assures maximum transfer of energy to the printed sine and cosine traces  63 ,  65  on stationary board  5  with reference to  FIG. 11 . 
   If a multilayer printed circuit board is used for the sine and cosine trace board  5  then the sines and cosines of different periods can be printed on the same board. If the sines and cosines of different periods are used then course and fine resolutions are available (See  FIG. 11 ). 
   If the high and low frequency printed periods are not related by an integer then the physical phase relationship between the printed high and low frequency traces changes with linear position and very long position encoders can be constructed with very high resolution. In particular, if there are multiple periods of the low frequency period, then the low and the high frequency periods must not have an integer relationship. For example, if there were 5 low frequency periods and the ration between high and low was 5:1, then the low and high frequency periods are back in phase after one period of low frequency. More specifically, the high frequency goes through 5 five periods and the two different frequencies are back in phase and cycle repeats for every low period cycle. Thus, there is no way to determine which of the multiple low frequency periods the encoder is in. If, however, the ratio is 5.2:1, then after one low frequency period the high has gone 5.2 cycles. It takes in this case, five low frequency periods before the low and high frequencies are in phase again. 
   The operation of the sensor system shown in  FIGS. 3 and 11  will now be briefly described. When the position of coil  14  along the x-axis relative to the support  5  is to be determined, excitation current is applied to the excitation magnetic loop  24  from excitation processing unit  11 . The frequency of the excitation will lock at the resonate frequency of coil  14  of resonant circuit  36  inducing a voltage in each sine and cosine windings  63  and  65 . (See  FIG. 11 .) The magnitude of the voltage induced is dependent upon the position of the resonant circuit  36  along the x-axis. Therefore, by suitable processing of the voltages induced in the sine and cosine windings  63  and  65 , the position of the resonant circuit  36  within a period of the windings  63  and  65  can be determined. As will become apparent later, two phase quadrature spiral windings are required to give unambiguous readings over the whole period T s  of the sine and cosine windings  63  and  65 . In the present embodiment, absolute position is determined by using one period low frequency sine and cosine traces  63 ,  65  and a multiple period high frequency sine and cosine traces  13 ,  15 . For long encoders having high accuracy, a multiple period low frequency sine and cosine traces  63 ,  65  with a non-integer ratio between the high and low frequency traces is used as described above. 
   Although the operation of the circuit described above is in many respects similar to that of U.S. Pat. No. 5,815,091 for sensing linear displacement, using the sine and cosine windings  63  and  65  with removed excitation loop  24  provides several advantages. In particular, by removing the excitation from board support  5  to an external high permeability rod, such as a ferrite rod, the generated flux is trapped in the ferrite rod. The use of PLL  22  also allows matching the excitation frequency to the resonant frequency that is temperature dependent. Lastly, this design always provides a null or balanced circuit in the absence of resonant circuit  36 . 
   It ill be recognized that since the windings  63  and  65  are not digital in nature, i.e. they are continuously varying along the length of the support  5 , the resolution of the system has a theoretical infinite setability. In practice, however, the output signals are processed digitally and the resolution of the analog-to-digital converter (ADC) in the processing circuitry which digitizes the signals from windings  63 ,  65  will contribute to the resolution of the system. In addition, the system is relatively insensitive to dirt, dust, grease etc. that can affect the proper operation of optical type position sensors. 
   To determine where along the length of the sine and cosine spiral windings the resonant circuit  36  is, i.e. to determine the value of d within the measurement range T s , the signals from the sine and cosine windings  63  and  65  are processed in the excitation and processing unit  11  (See  FIG. 11  illustrating circuit  36 ).  FIG. 10  schematically shows excitation and processing circuitry that may be used to calculate the position of the resonant circuit  36  within the period T s  of the sine and cosine windings  63  and  65 . As shown in  FIG. 10  there is a signal generator  41  that generates the excitation signal as a voltage controlled output (VCO) that is applied to the excitation loop  24  via a low pass filter (LPF)  42 , buffer  43  and transformer  44 . More specifically, the excitation drive is generated by PLL  22  which is initially oscillating at a preselected resonant frequency of moveable resonant circuit  36 . The initial frequency  48  is picked up by the feedback trace loop  18  via coil  28  and connected to PLL  22  via a phase comparator  50  that also receives a driven current phase signal  52  from a current to voltage converter  54 . The feedback to PLL  22  completes the loop and causes the frequency of PLL  22  to change until the PLL frequency matches and phase locks to the frequency of the moveable circuit  36  as indicated by VCO_in in  FIG. 10 . As temperature changes the electrical properties of the elements of resonant circuit  36  change. PLL  22  changes frequency to match the new resonant frequency of moveable tank circuit  36 . The excitation and processing circuit shown in  FIG. 10  and described above is given by way of example only and should not be construed as limiting in any way. 
   Theoretically, the sine and cosine windings  63  and  65 , respectively, can have any period T s , and therefore the sensor can be of any length. However, as the period T s  of the windings increases, the resolution to which the detector can detect changes in position decreases. The reason is that small changes in position of the resonant circuit  36  within the period T s  of the sine and cosine windings only produce small changes in the sensor signals. Whether these small changes are detected or not, depends on the resolution of the analog-to-digital converter used in the processing circuitry, the signal to noise ratio of the received signal and the spatial accuracy of the windings. Usually, for a given application, the resolution of the ADC is fixed by other system parameters or by cost. Increased accuracy and resolution can be obtained by adding a set of higher frequency printed sine and cosine traces  13 ,  15 . The increased accuracy and resolution will closely follow the ratio of low to high frequency. The practical maximum frequency occurs when the printed length of a high frequency period is twice the width of printed coil  14 . 
   In the exemplary embodiments described above, a coarse and fine set of spiral windings along the length of the sensor are used to allow the system to keep track of the absolute position of the resonant circuit. An example of such an arrangement is schematically shown in  FIG. 10  and illustrated in  FIG. 11  which show part of a 2.4 m long support  5  which has a set of fine quadrature spiral windings  13  and  15  with period 200 mm, for example, and a set of coarse quadrature spiral windings  63  and  65  with one period of 2.4 m, for example, mounted thereon. The signals from the coarse spiral windings  63 ,  65  are used to determine the position of the resonant circuit within the coarse spiral period (the measurement range, i.e., between start and end), and the signals from the fine windings  13 ,  15  are used to improve the measurement accuracy and resolution. As shown in  FIG. 11  the fine and coarse set of windings  13 ,  15  and  63 ,  65  are superimposed on top of each other, and as in one embodiment, vias or the like are used at the conductor cross overs. For this solution to work, the coarse windings should be able to distinguish between the periods of the fine windings. If this is not possible, then one or more intermediate periodicity windings should be used. 
   The transducer of the present invention may be applied to a number of applications. Applications include valve position sensing, positioning of the rack in a rack and pinion steering system, cranes, shock absorber/ride height sensors and the like. 
   While the invention has been described with reference to an exemplary embodiment, it will be understood that by those skilled in the art the various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.