BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for adjusting the null value of a linear variable differential transformer (LVDT) 
     2. General Background and State of the Art 
     A linear variable differential transformer (LVDT) is a displacement transducer that produces an electrical signal proportional to the displacement of a moveable core (armature) within a cylindrical transformer. The transformer consists of a central, primary coil winding and two secondary coil windings on opposite ends of the primary winding. The coil windings are coaxial. The armature preferably is nickel-iron and is positioned within the coil assembly. The core provides a path for magnetic flux linking the primary coil to the secondary coils. 
     When the primary coil is energized with an alternating current, a cylindrical flux field is produced over the length of the armature. This flux field produces a voltage in each of the two secondary coils that varies as a function of the armature position. Armature movement moves the flux field into one secondary and out of the other causing an increase in the voltage induced in one secondary and a corresponding voltage decrease in the other. The secondary coils are normally connected in series with opposing phase. The net output of the LVDT is the difference between the two secondary voltages. When the armature is positioned symmetrically relative to the two secondaries (the “null” position), the differential output is approximately zero, because the voltage of each secondary is equal but of opposite phase. 
     Subjecting a transducer to pressure can move an LVDT armature through a linkage. As pressure increases, the armature moves toward one secondary winding and away from the other. This yields a voltage difference that can be proportional to the pressure on the transducer. Consequently, this voltage output can measure pressure and position. 
     Nearly all LVDTs that are designed for aircraft or missile applications are wound on an insulated stainless steel spool, magnetically shielded and enclosed in a stainless steel housing using welded construction. The armature is normally made from a 50% nickel-iron alloy and brazed to a stainless steel extension. Secondary leads are usually shielded to minimize channel-to-channel crosstalk for multi-channel units and to shield components from RF energy. 
     The length and diameter of an LVDT must be sufficient to allow adequate winding space for achieving the desired electrical performance, support any pressure requirement and withstand the environmental shock, vibration and acceleration. Where physical size is limited, electrical performance must be flexible. Although the LVDT is basically a simple device, the operating characteristics and electrical parameters are complex and depend to a large extent on the physical limitations. 
     The minimum diameter of the transformer housing will depend on electrical performance criteria for the excitation frequency being used and housing wall thickness required to support a pressure requirement. Armature diameters less than 0.110 in. (2.8 mm) (metric conversions are approximate) are easily damaged and are not recommended. The armature and the probe to which the armature is attached mounts within a tubular member. The probe and tube through which the armature moves should be slightly larger in diameter than the armature to protect the armature from rubbing against the tube. 
     The full scale or span is the displacement range of the LVDTs armature for which the electrical performance is required and is referred to as the electrical stroke. Since an LVDT is normally, although not necessarily, a center null device (zero output occurs at mid-stroke), the range or stroke is normally specified as a plus and minus displacement from the null position. The full stroke (100% of the stroke) is the total end-to-end stroke, and the full scale output (100% of the output) is the total end-to-end output voltage. 
     An LVDT requires an AC voltage for operation. This excitation could be provided by aircraft buss power or an excitation source specifically designed for an LVDT. In today&#39;s aerospace and aircraft industry, multi-channels with individual excitation sources are often used to obtain the highest possible system reliability. 
     An LVDT&#39;s output voltage is pro portional to the voltage applied to the primary. System accuracy depends on providing a constant input to the primary or compensating for variations of the input by using ratio techniques. The output can be taken as the differential voltage or, with a center tap, as two separate secondary voltages whose difference is a function of the displacement. If the sum of the secondary voltages is designed to be a specific ratio of the difference voltage, overall accuracy significantly improves. 
     Resolution of an LVDT is the smallest change in armature position which can be detected as a change in the output voltage. Sub-micro-inch resolution is not uncommon with LVDTs. In practice, the resolution is usually less than the noise threshold of external circuitry or resolution of the equipment used to measure the output. 
     Where system reliability requires more than one output signal for redundancy, up to four independent LVDTs can be packaged in a single transducer assembly. Coil placement may be in series or grouped side-by -side as a cluster. Multiple LVDT&#39;s in one housing require less space, weight, installation time and cost less than separately mounted LVDTs. Dual LVDT assemblies are two coils combined, in tandem or parallel. The choice of the configuration could be limited by the length or diameter of the envelope available for the installation. Triple LVDT assemblies are usually combined in parallel. The tandem configuration is excessively long for strokes above one inch. Finally, quad LVDT assemblies are nearly always combined in parallel. 
     Having an accurate starting point for the LVDT is necessary for accurate measurement. That starting point has the armature centered and the differential output is zero. When that occurs, the armature is in its null position. 
     The prior art recognizes the advantages of being able to adjust that null position. Examples include. Kather, U.S. Pat. No. 5,804,962 (1998), and Maples, U.S. Pat. No. 4,543,732 (1985). 
     INVENTION SUMMARY 
     In some cases it is desired to use an LVDT to determine the position of a linkage in a pressurized zone, and to be able to adjust the null position of the LVDT. Adjusting the null position may be accomplished by having the magnetic element of the LVDT move within a sealed tube, with the tube extending outward from the pressurized zone. Adjusting the null point of the LVDT without removing the sealed tube is desirable. It also is important not to place torsional stress on the tube that holds the pressure while the null is being adjusted. 
     One object, therefore, of the present invention is to provide an LVDT with simplified null adjustments. One problem with adjusting the null position relates to the assembler. It is an object of the present invention to enable an assembler to assemble a sensor, pressurize the system and then adjust the null position. One problem with some LVDT is that adjustments to the null position apply torque to one or more of the parts. That torque can lead to inaccurate measurements and could rupture the seal of the sealed tube in which the magnetic element moves. Therefore, it is an object of the present invention to provide a null adjustment mechanism that does not apply torque to the sealed tube. Another object of the present invention is to disclose and provide a method for adjusting the null position of an LVDT such that the coil assembly is moved relative to the LVDT assembly. 
     Many systems for adjusting the null position do so by positioning the armature. It is an object of the present invention to avoid moving the armature during the adjustment procedure. 
     In the present invention, the LVDT coils themselves are mounted for movement relative to an armature that remains fixed during the null adjusting step. The tube in which the armature mounts and the armature itself are fixed, and the support for the coils can be adjusted axially relative to the armature and its supporting tube. This adjustment is preferably through a threaded connection at the outer end of the coil assembly. Springs bias the coil support to maintain tightness in the system and to avoid backlash. 
     In one illustrative embodiment of the invention, the linear variable differential transformer (LVDT) assembly with nulling adjustment comprises a housing defining a pressure barrier between first and second zones. An armature tube is sealed to the housing at its inner end. The tube has an outer end that extends into the second zone and a central section. An LVDT multiple coil assembly extends around the central section of the tube and is movable longitudinally relative to the tube. A magnetic armature mounts on a mechanical linkage and extends into the tube within the multiple coil assembly. A fixed support in the housing extends around the tube near the tube&#39;s outer end. The coil assembly is adjustable axially along the tube and armature. A spring biases the coil assembly away from the fixed support toward the inner end of the tube. In one embodiment, the adjusting mechanism includes a threaded member secured to the coil assembly and extending along the outer end of the tube through the fixed support. An adjusting nut on the other side of the fixed support from the housing engages the threaded member and bears on the fixed support as a result of the biasing force of the spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side, sectional view of one embodiment of the linear variable differential transformer assembly with nulling adjustment of the present invention. 
     FIG. 2 is a plan view of the linear variable differential transformer assembly of the present invention. 
     FIG. 3 is a partial schematic of the armature and coils of the linear variable differential transformer assembly of the present invention. 
     FIG. 4 is a circuit diagram for linear variable differential transformer assembly of the present invention. 
     FIG. 5 is an end view showing a housing using three linear variable differential transformer assemblies of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     LVDT  10  comprises an armature  20  that moves longitudinally in the direction of the arrows in FIG.  3 . The armature is iron or a nickel/iron alloy which affects magnetic flux or it is naturally magnetic. The armature connects to a shaft  22 , which acts as a linkage to a pressure producing source. Increases or decreases in measured pressure act through linkage  22  on armature  20  to move the armature longitudinally. Two electrical coils, a center, primary coil  132 , and first and second secondary coils  134  and  136  form a coil assembly  130 . The coil assembly surrounds the armature  20  (FIG.  3 ). 
     The armature&#39;s stroke length varies with its environment. In the exemplary embodiment, the stroke length can range from 0.1 inc. (2.5 mm) to as much as 14 in. (356 mm). At the smaller stroke length, an armature of 0.67 in. (17 mm) yields acceptable results. For the longer stroke length, a 4 in. (102 mm) armature provides acceptable readings. 
     Alternating current is applied between terminals  138  and  139  (FIG.  4 ). Applicant recommends an alternating current at 1,800 to 3,500 Hz for good performance based on typical armature lengths. The flow of alternating current through primary winding  132  generates magnetic flux, which is coupled to secondary windings  134  and  136  through armature  20  (FIG.  4 ). The magnetic flux produces a voltage in each secondary winding. The two secondary windings normally have opposing phases. When the armature moves longitudinally (vertically in FIG.  4 ), it increases the voltage in one of the secondary windings and causes a corresponding voltage decrease in the other secondary winding. The voltage between points A and B over secondary winding  134  is compared to the between points C and D over secondary winding  136 . The difference between the two secondary voltages is the net output of the LVDT. 
     When the armature  20  is in a position evenly spaced between the two secondary windings  134  and  136 , the differential output is approximately zero. That is the null position of the LVDT. 
     Turning to the structure of the exemplary embodiment, the LVDT assembly  10  of the present invention mounts within a housing  50  (FIG.  1 ). The housing defines a pressure barrier between ambient pressure in a first zone outside the housing and internal pressure within the housing in a second zone  52 . The housing is preferably stainless steel or other corrosion-resistant material. The housing&#39;s shape is determined, in part, by its environment. 
     The housing has a front wall  54 . A fitting  56  extends outward from the front wall (to the left in FIG.  1 ). A bore  58  extends through the fitting  56  and continues as bore  63  through the front wall  54  into the second zone  52 . The housing also has a channel  60  through an extension  61  of the housing. The channel is angled over part of its length to exit perpendicular to the front wall  54  of the housing. The channel houses a conduit  62 , which contains electrical leads  64  to the LVDT (FIG.  1 ). After assembly, the channel is partially filled with a potting compound for sealing the channel. 
     A tube  12  for the LVDT is sealed to the housing. The tube is cylindrical in the exemplary embodiment. As discussed below, core or armature  20  also is cylindrical and is within tube  12 . The cylindrical shape minimizes friction between the tube and armature and prevents the armature from applying torque to the tube. Thus, while applicant contemplates possibly using tubes and armatures without a circular cross-section, those shapes are not as desirable. The tube preferably is stainless steel. 
     The tube has a central section and inner and outer ends. In the exemplary embodiment, the inner end  26  of the tube extends through bore  63  in the housing The tube is sealed at the bore to maintain a pressure barrier between the first and second zones. The tube has an annular ridge  28 , which rests in a corresponding cut-out portion  30  in the housing (FIG.  1 ). 
     A support  34  mounts within the housing and extends around the outer end  36  of the tube  12  (FIG.  1 ). One or more pins  38  may extend radially through the support. The support is welded or otherwise attached to the rear end  40  of the housing. The central section  42  of the tube  12  extends into the second zone  52 . 
     The LVDT multiple coil assembly  130  (FIG. 1) extends around the central section  42  of the tube  12 . The coils mount on spool assembly  140  (FIGS.  1  and  2 ). The spool assembly is tubular and has an.inner wall  142  that surrounds the central section  42  of tube  12 . The inside diameter of inner wall  142  is slightly greater than the outside diameter of the central section  42  of the tube  12 . These dimensions permit the spool assembly  140  to move longitudinally along the tube. The spool assembly  140  has two dividers  146  and  148  (FIG.  2 ). The dividers separate the primary coil  132  from the two secondary coils  134  and  136 . The primary coil surrounds region  150 , and the secondary coils surround regions  152  and  154  (FIG.  2 ). 
     Dividers  146  and  148  are of Teflon or other plastic material. Likewise, Teflon washers  156  and  158  are at the outer ends of the regions  152  and  154  for the secondary windings. Floating washers  160  and  162  separate the end Teflon washers  156  and  158  from end cap  164  and floating washer  166  (FIG.  2 ). A thin cover  170  (shown only in FIG. 1) may extend over the coils between cap  164  and floating washer  166  adjacent to end cap  173 . The Teflon washers  156 ,  146 ,  148  and  158  support cover  170 . As FIG. 1 shows, end cap  173  is spaced from support  34 . A pair of spring washers  184  mount in the space between end cap  173  and support  34 . These spring washers urge the coil assembly to the left in FIG.  1 . 
     A portion  172  of the coil assembly extends to the right of support  34  and, consequently, also extends beyond the right termination of tube  42 . Portion  172  has a threaded end  174 . An adjusting nut  176 , which rests against support  34 , is threaded to threaded end  174  (FIG.  1 ). The pin  38  engages a slot  39  along the length of the threaded end  174 . This engagement guides the spool assembly  140  longitudinally. 
     Those of ordinary skill will appreciate current techniques for winding transformer windings. Wire with thin but effective insulation is wound tightly into coils around the outside of spool assembly  140 . Neither the wires nor the coils are shown in FIG. 2, however. Although wire with a cylindrical cross-section is commonly used in transformers, in some instances one would want to use wire with a square or rectangular cross-section to maximize the number of coils within a given volume. 
     An end cap  178  covers the right side of the housing. An O-ring  180  within channel  182  provides a seal for the end cap  178 . The end cap is removable to allow access to threaded nut  176 . Applicants anticipate that the right side (FIG. 1) of the housing likely will be cylindrical. The end cap  178 , therefore, also will be cylindrical. Depending on the environment, a non-cylindrical cross-section may be used. 
     Armature  20  contains a magnet  44  or other material, such as iron or iron/nickel alloy capable of affecting the flux in coils  132 ,  134  and  136 . The armature also has a linkage  46  extending outside the housing. The linkage moves in response to force from a transducer, usually a pressure transducer. As linkage  46  moves, the magnet  44  within armature  20  moves the same distance. That movement creates a measurable voltage difference between the two secondary coils  134  and  136  (FIG.  3 ). 
     As previously stated, the armature must be in the null position at rest. After the device is assembled, the assembler electrically activates the device and reads any voltage difference between the two secondary coils. If any exist, the assembler adjusts the position of the coil assembly by tightening or loosening nut  176  on threaded end  174  of the coil assembly. Normally, this is done before end cap  178  is assembled onto the housing. If the end cap is already on the housing, it can be removed. 
     Using threaded nut  176  to adjust the spool assembly is one way of effecting that movement. Such an arrangement is usually a good way to cause precise longitudinal movements. Other devices could be used in place of the threaded nut. For example, one could use a slider with an inclined surface acting on the spool assembly. Pulleys and gear assemblies are feasible but add complexity to the adjustment. 
     Spring washers  184  maintain a force on the coil assembly to the left (FIG.  1 ). Rotating the nut  176  in one direction pulls the spool assembly  140  to the right against the spring forces of the washers, and turning the nut in the other direction moves the coil assembly to the left. In either case, spring washers  184  maintain force against the coil assembly so that the assembly stays in place and does not move longitudinally except during adjustment. 
     For redundancy and increased accuracy, multiple armatures and coil assemblies can be used. Multiple assemblies tend to compensate for any error in a single LVDT. FIG. 5 shows one possible arrangement. There, housing  200  mounts three LVDTs  202 ,  204  and  206  in a triangular pattern. They are spaced apart sufficiently to allow the respective tightening nuts  208 ,  210  and  212  to turn without interfering with each other. 
     While the specification describes particular embodiments of the present invention, those of ordinary skill can devise variations of the present invention without departing from the inventive concept.
A linear variable differential transformer (LVDT) assembly comprises a housing. A tube extends into the housing. An armature mounts inside of the tube and can move longitudinally within the tube. A coil assembly, which includes a primary coil and two secondary coils mounts on the outside of the tube. The coil assembly has adjustable connection with the housing. Consequently, the coil assembly can be adjusted longitudinally with respect to the armature until the LVDT is in the null position. That occurs when the differential voltage from alternating current through the secondary coils is zero.