CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention relates to torque sensors and particularly transducers for providing an electrical signal indicative of the changes in torsional strain which a torsion member undergoes when subjected to a varying applied torque or moment. Such sensors and particularly transducers for sensing torsional strain are particularly desirable in systems where a shaft is subjected to varying torque during operation of the system including static, quasi-static and dynamic torque variation. Such applications include steering shafts for motor vehicles and rotor shafts in motors or power transmission shafts. 
     Heretofore, torque transducers for shafts have employed magneto elastic elements positioned on the shaft and sensing coils for detecting the change in the magnetic field as the magneto elastic element has been subjected to torsional strain. However, the difficulties in attaching the magneto elastic element to the shaft in a manner which could reliably transmit the shaft strain to the magneto elastic element created problems which rendered mass production of the device prohibitive. 
     A known transducer employing a piezoelectric element is that employing a piezoelectric element mechanically constrained and electrically connected to receive the output of the piezoelectric element upon the transducer being subjected to high impulse loads. Such a device is shown and described in U.S. Pat. No. 4,835,436 issued to H. S. Lew and is unsuitable for applications where a low torsional strain rate and/or low magnitude torque is applied to the torsion member, and is particularly not suitable for automotive steering shaft applications. 
     In quasi-static applications such as for an automotive steering shaft, it has further been desired to provide an electrical indication of the angular movement of the shaft; and, heretofore this has required a separate angle position sensor. 
     Thus it has long been desired to provide a way or means of sensing and providing an electrical indication of torsional strain in a member subjected to an applied torque in a manner that provides a high degree of sensitivity and relatively high resolution of the electrical signal output in response to changes in the torsional strain on the member. It has further been desired to provide such a torque sensor or transducer which provides an electrical indication of rotary angular position, which is easy to manufacture and install in high volume mass production and which is robust in service and relatively low in manufacturing cost. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a torque sensor or transducer which provides a continuous electrical indication of the torsional strain which a member undergoes when subjected to an applied torque. The transducer of the present invention employs a piezoelectric element potted onto a prepared area of the torsion member to be strained; and, the piezoelectric element is capacitively coupled via a rotary capacitor to an external detection circuit employing an oscillator. The rotary capacitor has a variable capacitance for indicating the angular deflection thereby eliminating the need for a separate angular position sensor. The piezoelectric element has the electrodes thereof covered and resilient dielectric spacers provide an air gap between the electrodes and the cover forming a subassembly to improve the signal output of the sensor at its resonant frequency. The piezoelectric element subassembly is secured to the prepared area preferably a recess, by a strain transmitting potting medium. In the presently preferred practice, the detection circuit includes an oscillator and detects changes in the resonant frequency of the piezoelectric element as a measure of the strain to which the torsion member is subjected. 
     The present invention provides a simple to manufacture and robust piezoelectric torque transducer which is relatively low in cost and easy to manufacture in high volume production and is particularly suitable for attachment to a quasi-statically strained element as, for example, a vehicle steering shaft or to a rotating shaft such as a rotating power transmission shaft or motor shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axonometric view of the transducer of the present invention installed on a torsion member; 
     FIG. 2 is a section view taken along section-indicating lines  2 — 2  of FIG. 1; 
     FIG. 3 is an exploded view of the transducer of FIG. 1; 
     FIG. 4 is an exploded view of another embodiment of a transducer; 
     FIG. 5 is a cross-section of another embodiment of the transducer installed on a torsion member; 
     FIG. 6 is a pictorial representation of the transducer installed with capacitive coupling; 
     FIG. 7 is an alternate arrangement of the embodiment of FIG. 6; 
     FIG. 8 is a block diagram of the electrical circuitry of the system employing the transducer of the present invention; 
     FIG. 9 is an electrical schematic of the system for the installed transducer of the present invention; 
     FIG. 10 is a section view taken along section indicating line  10 — 10  of FIG. 6 of the capacitive coupling electrodes providing angular position; 
     FIG. 11 is an axonometric view of the rotor electrode of FIG. 7 for indicating angular position; and, 
     FIG. 12 is a block diagram of the signal processing for the rotary angle position sensing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 through 3, a transducer subassembly indicated generally at  10  and includes a piezoelectric crystal element  12  having a pair of electrodes  14 ,  16  in the form of plates or disks formed on opposite faces of the crystal  12  as, for example, by ion sputtering of conductive material thereon. In the present practice of the invention the electrodes are formed of gold material; however, it will be understood that other suitable conductive materials may be used. A pair of resilient annular dielectric spacers  18 ,  20  are disposed on opposite sides of the element  12 ; and, a cover member  22 ,  24  is disposed over each of the spacers and electrodes and registers against respectively one of the spacers  18 ,  20  to define an air gap  26 ,  28 , above each of the electrodes  14 ,  16 . In the presently preferred practice, the annular spacers  18 ,  20  are formed of elastomeric material; and, in particular, fluorosilicone elastomer has been found satisfactory provided that the hardness does not exceed  70  Durometer on the Shore “A” scale. The cover members  22 ,  24  may be formed out of suitable plastic or metallic material. The assembly  10  may be held together temporarily by suitable adhesive, as for example, a spray tack material. An electrical lead  30 ,  32  is attached respectively to each of the electrodes  14 ,  16 , preferably by weldment such as, for example, by soldering. 
     Referring to FIG. 1, a torsionally strained element  34  has a recess  36  formed therein which comprises the prepared area into which the subassembly  10  is received and supported from the bottom thereof by suitable spacers denoted by reference numeral  38 . The subassembly  10  is then potted in place in recess  36  by a suitable strain transmitting potting compound such as, for example, epoxy resinous material. In the present practice of the invention, Bisphenol A resin with cyclohexylamine or hexahydrophthalic hardener type epoxy material having an upper service temperature limit of about 500° F. and a modulus of elasticity, preferably of at least 5.6×10 8  PA at 150° C. with a glass transition temperature preferably of at least 150° C. have been found satisfactory; however, it will be understood that other potting materials may be employed as the strain transmitting medium. It will be understood that the edges of the piezoelectric crystal element  12  are in direct contact with the potting material for strain transmission therebetween. In the present practice of the invention, it has been found satisfactory to form the spacers  38  from ceramic material. The subassembly  10  is disposed centrally with respect to the prepared area  36 . 
     In the present practice of the invention, the piezoelectric element  12  has the axis of maximum strain sensitivity identified by a flat surface  40  on the periphery thereof which is employed for orienting the subassembly  10  with the edge  40  disposed at an angle of about 45 degrees to the longitudinal axis of the torsion member  34 ; or, to the axis of the torque to be applied to the member  34 , which axis is denoted by reference numeral  42  in FIG.  1 . Upon installation of the subassembly  10  into recess  36 , the recess is filled with the potting compound which, upon hardening or curing is of sufficiently high modulous of elasticity to transmit strain in the torsion member  34  to the crystal element  12 . 
     In the present practice of the invention, the torsion member is formed of medium carbon steel such as SAE 1045 steel and has a diameter of about 0.75 inches (19 mm); and, the recess  36  is in the form of a flat bottom counter bore having a diameter of about 0.60 inches (15.2 mm) and a depth of about 0.28 inches (7.1 mm). However, it will understood that the particular size of the torsion member  34  and the transducer element may be varied in accordance with the practice of the invention in which the diameter of recess  36  is about 80% of that of the diameter of the torsion member  34 . 
     Referring to FIG. 4, an alternate embodiment of the transducer subassembly is indicated generally at  50  and includes a piezoelectric crystal element  52  preferably having a generally disk-shaped configuration having electrode layers  54 ,  56  deposited on opposite faces thereof. The embodiment of FIG. 4 has resilient annular spacers  58 ,  60  disposed over the electrodes  54 ,  56 ; and, the spacers  58  have a generally rectangular cross-section or gasket-like configuration. A pair of flat disk-like or wafer configured covers  62 ,  64  are disposed respectively over the spacers  58 ,  60  defining an air gap respectively between the faces of the electrodes  56 ,  54  and the inner surface of the covers  62 ,  64 . 
     Referring to FIG. 5 another embodiment of the invention is illustrated in which a transducer subassembly indicated generally at  70  has a piezoelectric element  72  with a generally disk-like configuration having electrodes deposited on opposite surfaces thereof, one of which is illustrated in FIG.  5  and denoted by reference numeral  74  and which have respectively electrical leads  76 ,  78  attached thereto. A pair of annular resilient spacers  80 ,  82  are disposed over the electrodes and a thin wafer-like cover  84 ,  86  is disposed over each of the spacers  80 ,  82  respectively. The subassembly  70  is received in a recess  88  formed in a torsion member  90 ; and, the recess is filled with a suitable strain transmitting potting material  92 . The subassembly  70  is spaced from the bottom of the counterbore  88  by suitable spacers  94  disposed thereabout. In the present practice of the invention, it has been found satisfactory to form the spaces  94  of ceramic material. 
     Referring to FIG. 6, the torque sensor assembly of the present invention is indicated generally at  100  and includes a transducer subassembly  102  installed in a recess  104  provided in a torsion member  106 . If desired, the transducer subassembly  102  may comprise any of the configurations of subassemblies  10 ,  50 , or  70  and which is potted in prepared area by strain transmitting potting (not shown in FIG.  6 ). 
     The transducer subassembly  102  has electrical leads  108 ,  110  extending from the potting material and lead  108  is attached to a capacitor plate  112  mounted on torsion member  106  and disposed axially spaced on one side of the subassembly  102 . The remaining lead  110  is connected to an oppositely disposed capacitor plate  114  provided axially spaced on the side of the subassembly  102  opposite plate  112 . It will be understood that plate  114  is similarly mounted on the torsion member  106  for movement therewith. 
     A second pair of stationary capacitor plates are provided, with one plate disposed concentrically about the torsion member  106  and spaced axially closely adjacent plate  112  as denoted by reference numeral  116 . Plate  116  has an electrical lead  117  connected to the electrode surface thereof. The other plate  118  of the pair is disposed in axially closely spaced arrangement with respect to plate  114 . Plate  118  is supported externally of the torsion member  106  by a support structure or base  120  and has an electrical lead  119  connected to the electrode surface of plate  118  for external circuit connection. In similar fashion, plate  116  is supported externally of torsion member  106  by a support structure  122 . Plate  116  is illustrated with a clearance hole  124  which has the torsion member  106  received therethrough in full clearance arrangement such that member  106  may be freely rotated with respect thereto. It will be understood that plate  118  likewise has a clearance hole formed centrally therethrough as shown in dashed outline in FIG.  6  and denoted by reference numeral  126  such that member  106  may be freely rotated with respect to plate  118 . If desired, for additional circuit capacitance, a plurality of capacitive coupling plate pairs may be employed. 
     Referring to FIG. 7, another embodiment of a system employing the torque transducer of the present invention is indicated generally at  130  and includes a torsionally strained member  132  which has a recess  134  therein with a torque transducer subassembly  136  having electrical leads  138 ,  140  attached thereto and extending therefrom with the subassembly  136  received in recess  134  and potted therein with a strain transmitting potting material (not shown). 
     Electrical lead  138  is connected to a cylindrical electrode layer  142  provided on a circular plate  144  attached to the torsion member  132 . A second cylindrical electrode  146  is disposed on a plate  148  attached to torsion member  132  and located axially spaced from recess  134  on a side opposite the plate  144 . An outer annular support member or ring  150  is supported by an external stationary support structure  152  and has a second cylindrical electrode  154  provided on the inner periphery thereof which electrode  154  is disposed in radially spaced arrangement of the electrode surface  142  at the same axial station along torsion member  132  such that electrodes  142  and  154  form a capacitor. Plate electrode  154  has an electrical lead  155  connected thereto for external circuit connection. 
     A second annular support structure or ring  156  is disposed at the same axial station as electrode surface  146  and is supported by a stationary supporting base  158 . Annular structure or  156  has an electrode plate or surface  160  disposed about the inner periphery thereof which is radially spaced from the electrode surface  146  and forms an annular capacitor therewith. Electrode surface  160  also has an electrical lead  161  connected thereto for external circuit connection. 
     Referring to FIG. 8, a block diagram of the electrical detection system employed for the present invention indicates that the installation of the torque transducer on the torsion member and capacitive coupling at  100 ,  130  as comprising any of the subassemblies indicated generally at  10 ,  50 ,  70 ,  102 ,  136  and has the stationary capacitive plates  116 ,  154  and  118 ,  160  respectively connected respectively by leads  117 ,  155  and  119 ,  161  to an oscillator  170  which has its output connected to one input of a mixer  172 . Mixer  172  receives at its other input a signal from the output of a reference oscillator  174  which has a reference piezoelectric element connected thereto. The output of the mixer  172  is fed through a low pass filter  176  which filters out the heterodyne frequencies and provides a signal comprising the difference between the outputs of the drained strained piezoelectric element and the reference piezoelectric element  171  which signal is fed through a signal conditioner  178  which converts the signal to a series of pulses at the output  180  thereof. 
     Referring to FIG. 9, the transducer  100 ,  130  provides outputs through rotary capacitor CC 1  which comprises plates  118 ,  160  and capacitor CC 2  which comprises plates  116 ,  154  and plates  112 ,  142 . The outputs, from plates  160 ,  118  and  116 ,  154  are connected to input terminals  182 ,  184  of oscillator  170 . The oscillator  170  utilizes resistors R 1 , R 2  and capacitors C 1 -C 3  and diodes D 1  and D 2  connected to the base of switch Q 1  which has its output connected to L 1  and through R 5  to one input of the mixer  172 . 
     A reference piezoelectric element or crystal  171  is connected to input terminals  186 ,  188  of the reference oscillator  174  which comprises diodes D 3 , D 4  and resistors R 3 , R 4  and capacitors C 2 , C 5 -C 6  connected to the base of switch Q 2  which has its output connected to L 2  and through R 6  to the other input of mixer  172 . 
     Oscillator  170  receives a supply voltage V CC  from power supply  198  at terminals  190 ,  192 ; and, oscillator  174  is powered by V CC  at terminals  194 ,  196  from the power supply  198  which has voltage outputs V CC  A, B, C and comprises device U 2 , coils L 3 , L 4  and capacitors C 11 , C 12  and C 13 . 
     Mixer  172  provides an input to the low pass filter circuit  176  which comprises capacitor C 8 , resistor network R 7  through R 13  and capacitor C 10  and device U 1 A. The filter network  176  provides inputs to comparator U 1 B for conditioning the signal to a series of positive frequency modulated pulses at the output  180  thereof. The change in frequency of the mixer output is thus an indication of the change in torsional strain in the torsion element. Values of the circuit components are set forth in Table I below. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Capacitor 
                 μFd 
                 Resistor 
                 Ohms 
                 Device 
                 Type 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CC1-2 
                 15 
                 PF 
                 R1, 4 
                 390 
                 D1-4 
                 1N 148 
               
               
                 C1, 2, 9, 10 
                 100 
                 PF 
                 R2, 4 
                 130K 
                 Q1, 2 
                 2N 3904 
               
               
                 C3, 5 
                 47 
                 PF 
                 R5, 6 
                 1K 
                 U1A, 1B 
                 MC34072 
               
               
                 C4, 6 
                 0.1 
                   
                 R7, 8 
                 1.0K 
                 U2 
                 78L05 
               
               
                 C7 
                 47 
                 PF 
                 R9 
                 10K 
                 MXR 
                 SRA-1 
               
               
                   
                   
                   
                   
                   
                   
                 (Micro Circuit) 
               
               
                 C8 
                 0.22 
                   
                 R10, 11 
                 1.15K 
                 L3, 4 
                 4.7 μH 
               
               
                 C11-13 
                 0.33 
                   
                 R12 
                 39.2K 
                 MI 
                 SRA-1 
               
               
                   
                   
                   
                 R13 
                 37.4K 
                 XTAL 
                 NE 612 
               
               
                   
                   
                   
                   
                   
                   
                 (Phillips) 
               
               
                   
               
             
          
         
       
     
     Operationally, the piezoelectric elements  100 ,  130  and the reference element  171  are placed in the feedback loops of separate oscillators, respectively  170 ,  174 ; and, the change in properties of the piezoelectric elements under torsional strain will cause a corresponding shift in the output frequency. The frequency of the reference oscillator  174  is chosen close to that of the unstressed transducer piezoelectric element such that there is an offset between the two frequencies. The output of both oscillators  170 ,  174  are fed into the mixer  172  which produces the heterodyne frequencies comprising the sum, difference and product of the oscillator outputs. Mixer  172  passes only the difference of the two oscillator outputs. The reference oscillator  174  is tuned such that there is an offset with the output of oscillator  170  at zero torsional strain on the piezoelectric element. The advantage of the arrangement of the present invention is that only a low frequency signal in the kilohertz range need be processed; and, thus the cost of the circuit is minimized. In the present practice of the invention the piezoelectric element has a resonant frequency of about 5 megahertz and the oscillators have a frequency of about 15 megahertz. The change in frequency of the piezoelectric crystal element is then easily detected with inexpensive circuitry. 
     Referring to FIG. 10, the rotary capacitive plate  114  is shown as having the electrode surface  115  thereon configured to have a spiral edge as denoted by reference numeral  115  which, in cooperation with a sector electrode having radial edges (not shown) on the stationary plate  118  provides a linear ramp output which provides an indication of the rotary or angular position of the plate  114 . 
     Referring to FIG. 11, the rotary plate  144  of the embodiment of FIG. 7 is illustrated wherein the electrode  142  has a helical or axial ramp edge  143  which provides an axially varying width to the electrode  142  and thus provides a linear ramp output or capacitive value to the capacitor as the plate  144  is rotated and thereby provides an electrical indication of the angular position of plate  144  with respect to the stationary ring  150 . The capacitive coupling of the present invention thus includes an inherent provision for providing an electrical signal indicative of the angular or rotational position of the torsion member. It will be understood that the ramp surface electrode may alternatively be provided on the stationary electrode for the capacitor. It will also be understood that although the ramp electrode has been illustrated for only one of the capacitors that both capacitors may be so arranged to thus provide a signal which averages the change in capacitance of both capacitors either by vector summing techniques or ratioing. 
     Referring to FIG. 12, a block diagram of the electrical signal processing for the angle position sensing is indicated wherein at step  200  the capacitors are excited by the piezoelectric element; and, at step  202 , a vector couplet is formed of the capacitance from the first and second electrode pairs. At step  204  the monitor circuit or detection circuit converts the capacitance to voltage or digital logic. The voltage or logic from step  204  is then operated on by an algorithm of step  206  wherein the difference of the capacitances from the two electrode pairs is determined and employed as an indicator of angular position. Alternatively, the ratio of the capacitances of the two electrode pairs may be taken and used as an indication of angular position. 
     The present invention thus provides a unique and novel torque transducer utilizing a piezoelectric element having an axis of maximum strain sensitivity which is oriented at about 45 degrees to the axis of a torsion member subjected to an applied torque. The piezoelectric element is sandwiched between covers providing an air gap over the electrodes thereof by resilient spacers and is potted into a recess formed in the torsion member. The electrical leads from the piezoelectric element are coupled to an external stationary detection circuit by a rotary capacitive coupling which also serves as a rotary position angle sensor. The arrangement of the present invention advantageously offers a transducer which is simple to assemble, low in manufacturing cost, robust and provides a relatively high degree of torque sensitivity and signal resolution. 
     Although the invention has hereinabove been described with respect to the illustrated embodiments, it will be understood that the invention is capable of modification and variation and is limited only by the following claims.
A method of manufacturing a piezoelectric torque transducer is provided, comprising the steps of (a) forming a prepared area on a surface of a torsion member adapted to be strained by an applied torque; (b) providing a piezoelectric element having an axis of maximum strain sensitivity and disposing first and second electrodes on opposite faces of the element, respectively, and attaching an electrical lead to each electrode; (c) connecting electrical leads to the electrodes; and (d) disposing the element on the prepared area and orienting the axis of maximum strain sensitivity on the member and securing the element to the prepared area with a material selected from a group consisting of (i) adhesive material and (ii) potting material. The step of disposing the piezoelectric element includes configuring the piezoelectric element in a plate-shaped configuration and disposing a resilient annular member on each opposite face of the plate-shaped element and overlaying each of the annular members with a protective cover.