Abstract:
A method of manufacturing a low cost, high resolution, contactless, absolute rotational position sensor is disclosed. A coupling disc and a transceiver disc are the only required elements. The coupling disc can be manufactured as a simple two sided printed circuit board. A common 4 layer printed circuit board can act as the transceiver element and the circuit board for implementing the signal processing and output driver. The coupling disc capacitively couples drive signals originating on the transceiver disc to receiving tracks on the transceiver disc. Full 360 degree position decoding can be achieved. Potentials measured on the receiver nodes are processed to obtain an absolute position and a syndrome. The syndrome is a numerical result that can be used to predict the integrity of the position data. The maximum resolution of the sensor is 4N*(A/D resolution), where N is the larger of M and N above. Normally, resolution is limited to 2N*(A/D resolution) to allow for a halving of the coupling disc gap without accuracy degradation. A sensor conforming to the invention can tolerate several mils of axial movement or runout in the coupling disc with minimal effect on sensor accuracy.

Description:
SUMMARY OF INVENTION  
         [0001]    The invention is a configuration of a contactless absolute position sensor that has several advantages compared with other sensors of similar function. It can be fabricated at low cost from commercially available FR4 printed circuit boards assembled to tolerances typical of potentiometer manufacture. Potentiometers, a common example of a low cost absolute position sensor, wear relatively quickly due to the dragging of the contact along the resistive element. Furthermore, position information is often corrupted by contaminants which may be deposited on the sensing element or contact surface. Airborne particles, wear debris generated by the sliding contact, friction generated polymer, and atmospheric condensates are some of the many sources of contamination. Contactless absolute position sensors which offer high positional resolution, adequate environmental durability and high reliability, are usually costly. Expensive materials, low allowable assembly tolerances or component tolerances, expensive processes and a large number of manufacturing operations are some of the cost adders which plague these sensors.  
           [0002]    The invention, is comprised of a transceiver element and a coupling element. With the transceiver element and coupling element situated as specified, excitations applied to the transceiver element completely determines the rotational position of the coupling element within the specified range and resolution. The specified range can be up to 360 degrees. Excitation, Processing and Output drive circuitry can be included on the circuit board which is used as the transceiver disc.  
           [0003]    The invention comprises: A specific patterns of conductive areas on adjacent opposing surfaces of the coupling and transceiver element; A specification of how these conductive patterns are attached to form circuit nodes; A specification for applying potentials to two nodes of the transceiver element; An algorithm for generating an angular position value and reliability value from differential potentials measured between receiver nodes on the transceiver element. The sensor so comprised exhibits decreased sensitivity to: externally generated varying potentials; variations in the spatial separation of the two elements; variations from ideal form of element surfaces; offset and gain errors in the potential measuring device.  
           [0004]    The manner in which the invention achieves the benefits stated will be easily discernable from the detailed description. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0005]    [0005]FIG. 1 is a side sectional view of a rotary position sensor containing the invention features;  
         [0006]    [0006]FIG. 2 is a top view of the coupling disc of the sensor shown in FIG. 1;  
         [0007]    [0007]FIG. 3 is a bottom view of the transceiver disc of the sensor shown in FIG. 1;  
         [0008]    [0008]FIG. 4 is a simplified schematic diagram of the sensor shown in FIG. 1;  
         [0009]    [0009]FIG. 5 is a graph of selected transceiver node potentials versus time for an operating sensor;  
         [0010]    [0010]FIG. 6 is a graph of normalized differential transceiver node potentials versus position;  
         [0011]    [0011]FIG. 7 is a graph of normalized differential transceiver node potentials versus position;  
         [0012]    [0012]FIG. 8 defines an abbreviated notation for differences of transceiver node potentials at time  81 ;  
         [0013]    [0013]FIG. 9 defines an abbreviated notation for differences of transceiver node potentials at time  84 ;  
         [0014]    [0014]FIG. 10 is an equation defining the term A in the position equation;  
         [0015]    [0015]FIG. 11 is an equation defining the term B in the position equation;  
         [0016]    [0016]FIG. 12 is an equation defining the term C in the position equation;  
         [0017]    [0017]FIG. 13 is an equation defining the term D in the position equation;  
         [0018]    [0018]FIG. 14 is an equation defining the normalized A;  
         [0019]    [0019]FIG. 15 is an equation defining the normalized B;  
         [0020]    [0020]FIG. 16 is an equation defining the normalized C;  
         [0021]    [0021]FIG. 17 is an equation defining the normalized D;  
         [0022]    [0022]FIG. 18 is the definition of the interval position function f( );  
         [0023]    [0023]FIG. 19 is the definition of 0 degrees position;  
         [0024]    [0024]FIG. 20 is the definition of the rotational angle φ;  
         [0025]    [0025]FIG. 21 is the definition of fractional position P;  
         [0026]    [0026]FIG. 22 is the definition of numerical operator INT( );  
         [0027]    [0027]FIG. 23 is the definition of numerical operator FRACT( );  
         [0028]    [0028]FIG. 24 is the equation used to calculate fractional position P;  
         [0029]    [0029]FIG. 25 is an equation used to estimate the fractional position P;  
         [0030]    [0030]FIG. 26 is an equation which implicitly defines the error term Syndrome;  
         [0031]    [0031]FIG. 27 is a relation defining the allowable limits of the error term;  
         [0032]    [0032]FIG. 28 is an expression for evaluating the term Q in FIG. 24;  
         [0033]    [0033]FIG. 29 is the definition of the function H( ) occurring in FIG. 28; 
     
    
     DETAILED DESCRIPTION  
       [0034]    The Rotary capacitive sensor shown in FIG. 1 is a particular embodiment of a sensor which includes the invention. It includes a coupling disc  1 , which is bonded to a rotor hub  2 . Also comprising the sensor is a transceiver disc  4  which is bonded to the sensor housing  5 . The embodiment shown is intended to be attached to a rotating shaft which extends orthogonally from a flat surface. Details of the device to which the sensor is attached are irrelevant except to note that when so attached, the coupling disc may rotate about an axis centered in bore  3  and orthogonal to surface  6 . The housing  5  and rotor hub  2  are an example of a physical mechanism used to prevent any motion, other than rotation about the shaft axis, of the coupling disc  1  relative to the transceiver disc  4 . In the particular embodiment, the rotor hub  2  and the housing  5  are aluminum. When these two parts consist of materials with high electrical conductivity, they decrease the sensitivity of sensor output to external varying potentials. The housing may be connected electrically to circuit node  43 , shown in FIG. 4 to minimize external potentials relative to this node.  
         [0035]    [0035]FIG. 2 shows the surface of the coupling disc adjacent to the transceiver disc. FIG. 4 is a schematic representation of the sensor. Elements within the dashed rectangle  44  in FIG. 4 are physically realized on the coupling disc.  
         [0036]    The coupling disc contains four annular tracks. Inner tracks copper annulus  7  and copper annulus  8  function as the capacitor plates  7  and  8  shown in FIG. 4. The area of plates  7  and  8  are approximately equal. The next annular track consists of 10 identical annular copper segments spaced by 36 degrees. Every other segment comprises capacitor plate  9  and is electrically connected to node  11  as shown in FIG. 4. The remaining 5 annular copper segments  10  function as capacitor plate  10  and are electrically connected to node  12 . The outer most track of the coupling disc consists of 64 equally spaced identical annular copper segments. Every other segment functions as capacitor plate  13  and is a part of electrical node  11 . The remaining 32 copper segments function as capacitor plate  14  and are connected to electrical node  12 . The top surface of the coupling disc is a contiguous sheet of copper except where etched away to provide isolation from connecting traces. This copper sheet decreases output sensitivity to stray fields. In the particular embodiment, the coupling disc is fabricated from 0.063 inch thick FR4 epoxy glass. In general, the top copper sheet must not result in excessive cross coupling of node  11  to node  12 .  
         [0037]    Connection of the segments are made with 0.007 inch wide traces. In general, the area of the connecting traces is kept to a minimum. Ideally, the capacitive coupling between node  11  and each of transceiver nodes  17 ,  18 ,  19  and  20  shown in FIG. 3 is identical to the capacitive coupling between node  12  and each of the nodes  17 ,  18 ,  19  and  20  when the coupling disc has been rotated 36 degrees. Ideally the capacitive coupling of node  11  with each of transceiver nodes  21 ,  22 ,  23  and  24  shown in FIG. 3 is identical to the capacitive coupling of node  12  with each of the transceiver nodes  27 ,  22 ,  23  and  24  when the coupling disc is rotated 5.625 degrees. Non-ideal features in this regard are made as small as practical.  
         [0038]    [0038]FIG. 3 shows the metalization pattern of the transceiver disc. The metalization consists of annular areas  15  and  16  which oppose areas  7  and  8 , respectively, on the coupling disc. These metalized areas function as the capacitor plates  15  and  16  shown in FIG. 4.  
         [0039]    In the third annular track from the center is 8 annular segments  17 ,  18 ,  19  and  20  that match annular segments  9  and  10  of the coupling disc in size and radial position. They function as the capacitor plates attached to nodes  17 ,  18 ,  19  and  20  as shown in FIG. 4. The 8 annular segments are composed of 4 segment pairs, with the spacing between segments of a pair being 36 degrees. Adjacent pairs are spaced 90 degrees apart.  
         [0040]    The fourth annular track from the center contains 48 annular segments  21 ,  22 ,  23  and  24 . Each segment matches segments  13  and  14  of the coupling disc in size and radial position. These metalized areas function as the capacitor plates attached to nodes  21 ,  22 ,  23  and  24  as shown in FIG. 4. The 48 annular segments are composed of 24 segment pairs, with the spacing between segments of a pair being 5.625 degrees. Adjacent pairs are spaced by 14.0825 degrees or 19.6875 degrees. The 19.6875 degree spacing occurs 4 times at 90 degrees apart.  
         [0041]    To operate the sensor a voltage potential  25  must be applied as shown in FIG. 4. Also, a potential  26  must be applied to the receiving plates  17 ,  18 ,  19 ,  20 ,  27 ,  22 ,  23  and  24  thru resistors  27 ,  28 ,  29 ,  30 ,  31 ,  32 ,  33  and  34 . The value of potential  26  is one half the potential  25 . In the specific device described resistors  27  through  34  have a value of 4.99K ohms. The resistor values should be equal within practical limitations. The value of potential  25  in the specific device described is 5 volts. As shown in FIG. 4, the potential  25  is connected to drive plate  15  thru resistor  37  in series with switch  41  and to drive plate  16  through resistor  38  in series with switch  42 . Drive plates  15  and  16  are also connected to the zero potential node  43  by resistor  35  in series with switch  39  and resistor  36  in series with switch  40 , respectively. For the specific device, resistors  35 ,  36 ,  37  and  38  have a value of 899 ohms. Switches  39 ,  40 ,  41  and  42  are implemented using an integrated circuit of type 74HC4066.  
         [0042]    The potential on drive plates  15  and  16  is varied by opening and closing switches  39 ,  40 ,  41 , and  42  at specified times. Initially all switches are open. At time  80  indicated in FIG. 5 switches  41  and  40  are closed. At time  82 , switches  41  and  40  are opened. At time  83  switches  39  and  42  are closed. At time  85  switches  39  and  42  are opened. The switching sequence is repeated for as long as sensor output is required. For the specific sensor described, the switching sequence is repeated every 4 usecs. The potential waveforms shown as  88  and  89  in FIG. 5 are expected on plates  15  and  16  respectively while the specified switching sequence is performed.  
         [0043]    The information required to compute the receiver disc position is obtained by measuring potential differences at the times  81  and  84  as shown in FIG. 5. The specific differences measured are: potential at node  17  minus potential at node  18 ; potential at node  19  minus potential at node  20 ; potential at node  21  minus potential at node  22 ; potential at node  23  minus potential at node  24 . Waveforms  86  and  87  are the expected potentials vs. time for nodes  22  and  21  respectively with the sensor at 0 degrees. The sensor is at zero degrees in FIG. 1, FIG. 2 and FIG. 3.  
         [0044]    The circuit board for the specific device includes ground plates to stabilize the capacitance of transceiver plates  15 ,  16  and  17  through  24  with respect to ground. The ground plates also reduce unintended cross coupling. The ground plates are electrically connected to node  43 . In the particular device, a ground plate of annular shape is separated from plates  15  and  16  by 0.2 mm. The resulting capacitance in conjunction with resistors  35 ,  36 ,  37  and  38  largely determine the charge and discharge rate of plates  15  and  16 . It is important to stabilize the charge rate of plates  15  and  16  so that the time when the receiver nodes attain their maximums can be accurately predicted. The ground plates above receiver plates  17  through  24  are separated by about 1.2 mm. The choice of separation will depend on available layer positions in the circuit board, measuring device sensitivity, nominal coupling capacitance and measuring device input impedance. The discharge resistors  27  through  34  are also factors in predicting the time maximum receiver node amplitudes occur.  45  is intended to represent circuitry which processes the receiver node potentials into the desired form of output  46 . This circuitry may be included as part of the transceiver disc.  
         [0045]    The four potential differences are converted to an angular position by the equation of FIG. 24. The units, direction and origin of the position so calculated are defined by FIG. 21, FIG. 20 and FIG. 19 respectively. The relative angular location of potential difference minimums generated from the third and fourth tracks are determined by the relative positioning of their respective plates when the discs are manufactured. The alignment of minimums for differences A and C as shown in FIG. 6 and FIG. 7 is for computational convenience only. The solution is a continuous function of the differential node potentials provided the error term is limited as shown in the equation of FIG. 27. The error term, referred to as Syndrome, is implicitly defined by the equation in FIG. 26. Small syndrome amplitude is an indication of reliable sensor data. The Syndrome is seen to be the difference in the position calculated using the equation in FIG. 25 from the position calculated using the equation of FIG. 24.  
         [0046]    [0046]FIG. 28 is a solution for the term Q appearing in the equation of FIG. 24. In FIG. 29, a function of the incremental position terms appearing in the equation of FIG. 28 is defined. FIG. 22 and FIG. 23 define numerical operators appearing in the equations of FIG. 28 and FIG. 29. The evaluation of the incremental position terms is given by the function of FIG. 18. The incremental position terms are a function of the normalized potential difference terms as defined by FIG. 14, FIG. 15, FIG. 16 and FIG. 17. The difference terms prior to normalization are defined in FIG. 10, FIG. 11, FIG. 12, and FIG. 13. The abbreviated notation used in FIG. 10 through FIG. 13 is defined in FIG. 8 and FIG. 9.  
         [0047]    Many of the stated benefits can be verified by examining the position calculation. From the equations in FIG. 10 through FIG. 13, it follows that potential variations that are applied equally to both nodes of a node pair will not alter the output. This remains true as long as the measurement is a linear function of the applied potentials. Static offsets in the measuring device are also canceled since the potential difference at time  84  is subtracted from the potential difference at time  81 .  
         [0048]    From the equations in FIG. 14 through FIG. 17 it follows that signal attenuations which affect both pair sets in a track equally do not affect position. For example, if plates attached to nodes  23  and  24  are attenuated by the same factor K as plates attached to nodes  21  and  22 , then the left-hand side of equations of FIG. 10 and FIG. 11 will be attenuated by factor K. It follows that the left-hand side of equations given in FIG. 14 and FIG. 15 remain unchanged.  
         [0049]    Coupling of receiver nodes equally disposed between plates  7  and  8  is not required to be equal for accurate sensor operation. An increased coupling to either of plate  7  or  8  will exist on both nodes of the node pair and therefore cancel. This is an important feature since it is difficult to guarantee equal coupling to plates  7  and  8 .  
         [0050]    It is important to understand the theory of sensor operation so that optimal choices can be made when fabricating sensors of various sizes, resolutions, materials and circuitry. The principal assumption on which the position calculations are based is: The capacitive coupling of a receiver node to the opposed coupling disc node varies linearly with the overlapping area.  
         [0051]    In particular, the coupling of node  21  to node  7  is maximal when segments of node  21  are directly opposed to segments of node  13 . The coupling of node  21  to node  7  is minimal when the segments of node  21  are directly opposed to segments  14 . The coupling of node  21  to node  7  is half way between minimal and maximal when the segments of node  21  are equally disposed between segments  13  and  14 . When the principal assumption holds, FIG. 6 is a graph of the potential differences A, B versus position, as defined in equations of FIG. 10 and FIG. 11. FIG. 7 is a graph of the potential differences C, D as defined in equations FIG. 12 and FIG. 13. The peak values have been normalized to one. The graphs show that the absolute value of A summed to the absolute value of B is constant for all positions. This is the basis for the normalization equations of FIG. 14, FIG. 15, FIG. 16 and FIG. 17.  
         [0052]    A fractional position spanning a rotation from A at a local minimum to the next minimum can be determined from the graph of FIG. 6 for a given A, B. A similar statement holds for FIG. 7 given C and D. FIG. 18 which gives the fractional position function, is the mathematical equivalent of this statement. This function repeats N times when traversing the full sensor range with A and B as arguments. The function repeats M times with C, D as the arguments. The equations in FIG. 24 and FIG. 25 are a statement of this property. For real world measurements, these two equations are unlikely to agree exactly due to limited measurement resolution and imperfect linearity. We attribute the discrepancy to error in the lower resolution track as indicated in FIG. 26.