Abstract:
A differential eddy current sensor apparatus having a balance bridge to measure impedance unbalance between a pair of sensor coil which have a conductive target centered therebetween. When the target is moved toward one of the sensors the resultant impedance unbalance is detected at the bridge and demodulated by a demodulator unit to provide an analog output signal which is proportional to the target misalignment.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates broadly to a eddy current sensor apparatus, and in particular to a differential eddy current sensor apparatus for edge phasing of movable or deformable mirror segments. 
     The state of the art of edge sensing apparatus is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. Patents: 
     U.S. Pat. No. 3,397,357 issued to Metzker et al on Aug. 13, l968; 
     U.S. Pat. No. 3,961,243 issued to Schulz on June 1, 1976; and 
     U.S. Pat. No. 4,731,579 issued to Petersen et al on Mar. 15, 1988. 
     The Metzker et al patent discloses an inductive apparatus for centering a punch within its die. An annular housing which fits into the die cavity and has central passage to receive the punch. The housing contains two balanced inductors spaced equidistant from and on opposite sides of the desired path for the punch and connected to two legs of a balanced bridge circuit. When the punch is off-center, the impedances of the two inductors are unequal and the bridge circuit is unbalanced. The unbalanced bridge circuit is electrically detected and indicated. 
     The Schulz patent is directed to an inductive displacement transducer using a current measuring bridge circuit including a pair of adjustable electrical resistors coupled in series relationship in one half thereof, and a pair of series coupled induction coils and a movable magnetic armature in the other half of the bridge circuit. The improvement comprises a pair of series coupled electrical shunt resistors, each coupled to one of the induction coils. 
     The Petersen et al patent describes a magnetic position indicator using a pair of magnetic fields which are juxtaposed to each other such that they have opposite polarity and are of such a field strength that they provide for a generally linear variation in field strength extending over an elongated path which traverses the abutment of said fields. 
     In the prior art, edge sensors are utilized to measure or maintain incremental distances between structural elements. For example, edge sensors may be utilized to provide positional information about mirror segments which are part of large depoyable and deformable mirror. The edge sensors are utilized to maintain the shape of the desired optical surface and are needed to correct for phasing errors between the mirror segments. For complete control of such a mirror, the relative motion of each mirror segment with respect to its neighbor in three linear dimensions must be known. 
     The present differential eddy current sensor apparatus provides an edge sensing unit which can achieve sub-micro inch stability, avoidance of joint creep and insensitivity to temperature variation/gradients, gravity loading, and vibration disturbances. 
     SUMMARY OF THE INVENTION 
     The present invention utilizes an inductive edge sensor measuring apparatus wherein a balanced bridge circuit is utilized in conjunction with a pair of sensing coils to determine the impedance variation that is caused by eddy currents which are induced in a conductive metal target positioned between the pair of sensor coils. 
     It is one object of the present invention, therefore, to provide an improved differential eddy current sensor apparatus. 
     It is another object of the invention to provide an improved differential eddy current sensor apparatus wherein the relative motion of each segment with respect to its neighbor is determined. 
     It is still another object of the invention to provide an improved differential eddy current sensor apparatus wherein positional information between adjacent segments is measured. 
     It is a further object of the invention to provide an improved differential eddy current sensor apparatus wherein positional information between segments is utilized to achieve sub-micro inch stability. 
     It is still a further object of the invention to provide an improved differential eddy current sensor apparatus which is insensitive to temperature variation/gradients. 
     It is yet another object of the invention to provide an improved differential eddy current sensor apparatus which is insensitive to gravity loading and vibration disturbances. 
     It is still another object of the invention to provide an improved differential eddy current sensor apparatus which is economical to produce and utilizes conventional, currently available components that lend themselves to standard mass production manufacturing techniques. 
     These and other advantages, objects and features of the invention will become more apparent after considering the following description taken in conjunction with the illustrative embodiment in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of the differential eddy current sensor apparatus according to the present invention, and 
     FIG. 2 is a plan view of the differential eddy current sensor apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a schematic diagram of the differential eddy current sensor apparatus. An oscillator unit 10 is connected between ground and a balanced bridge unit 12. The balanced bridge unit 12 comprises resistors R1, R2 and capacitors C1, C2. The junction point of capacitors C1, C2 is connected to ground. The junction point of resistors R1, R2 is connected to the oscillator unit 10. A demodulator unit 14 is connected between the junction point of resistor R1 and capacitor C1, and the junction point of resistor R2 and capacitor C2. A metallic target 18 which can be moved in the directions shown by arrow 16 is positioned between first and second sensor coils, S1, S2. The first and second sensor coils S1, S2 are respectively connected by a shielded cable 20 to junction points A and B. 
     Turning now to FIG. 2, there is shown a plan view of the differential eddy current sensor apparatus. The sensing coils S1, S2 which are supported in glass plates 30, 32, are positioned on opposite sides of a metallized copper target 18. Sensor coils S1, S2 are respectively connected to shielded cables 60a, 60b, as described above with respect to FIG. 1. Cables 60a, 60 b may comprise a commercially available cable such as RG-178 cable. In the present example, the target 18 is described as being constructed of copper, however, it should be well understood that any commercially available conductive material may be utilized as the target material. The metallized target 18 is attached to both sides of the glass plate 34. The target supporting structure which comprises plate 34, spacer 36 and pad 38, is attached to mirror segment 44 a constant force mounting means 42. The constant force mounting means 42 may comprise a conventional screw or bolt and a spring. A super invar insert 40 is positioned within pad 38. 
     The coil supporting structure which comprises plates 50a, 50b, spacers 52a, 52b and pad 54, is attached to mirror segment 58 by a constant force mounting means 62. The constant force mounting means 62 may comprise a conventional screw or bolt and a spring. A super invar insert 56 is positioned within pad 54. 
     The differential eddy current sensor apparatus provides a measuring system which uses the principle of impedance variation that is caused by eddy-currents induced in a conductive metal target that is positioned near each sensor coil. The electromagnetic coupling between each coil and the target is dependent upon the gap between the elements. Since the coils comprise a pair of sensors which are in opposite legs of an instrumentation bridge circuit in FIG. 2 the bridge is balanced and the differential output is zero when the sensors are at equal distances from the metal target. This bridge circuit implementation also effectively cancels the temperature effects which are common to the sensors and cables of a pair. When the target is moved away from one sensor and toward the opposite sensor, the coupling is no longer equal, thus causing an impedance unbalance between the sensors. This unbalance which is detected at the bridge, is amplified and demodulated to provide an analog output signal that is directly and linearly proportional to the first 600 microns of metal movement or misalignment. Beyond this distance, the output is nonlinear but can be used to detect the direction of misalignment. The mechanical design aspects in the implementation of the sensor head for edge phasing of the segments, as shown in FIG. 2, is extremely critical to achieve sub-micro inch stability. The important considerations are avoidance of joint creep and insensitivity to temperature variation/gradients, gravity loading, and vibration disturbances. In order to minimize stability errors which are due to temperature variations or gradients, zero expansion Zerodur glass is used for all mechanical components. To further insure against creep with time and also to provide a low contact thermal resistance, the glass components are: polished flat to 0.25 μm or better; assembled by wringing the flat surfaces together; and clamped with a constant force mounting by using belleville type spring washers under the screw heads. The coil assemblies are integrally mounted to the Zerodur glass plates. Pure copper is also locally metallized on the target plate. 
     The differential eddy current sensor apparatus has characteristics which are as follows: 
     
         ______________________________________Static Resolution and Repeatibility               ≦.25 nm (.01μ inches)Measuring Range     ±10 milsNon-Linearity over ± 10 mil range               &lt;±5μ inchesNon-Linearity over ± 1 mil range               &lt;±1μ inchThermal Sensitivity Shift, ± 1° F.               &lt;±.33μ inchesLong Term Stability &lt;±5μ inches/monthSensitivity         1 mv/μ inchCoil Power Dissipation               10μ watts______________________________________ 
    
     Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.