Electricity: measuring and testing – Magnetic – With compensation for test variable
Reexamination Certificate
2001-12-27
2003-04-01
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
With compensation for test variable
C324S234000
Reexamination Certificate
active
06541963
ABSTRACT:
The present invention relates in general to measuring technology and has particular reference to a differential eddy-current transducer.
FIELD OF THE INVENTION
The present invention can find application in mechanical engineering for non-destructive testing of current-conducting and/or ferromagnetic materials and products, namely, for measuring a distance to the surface, thickness of sheeting and coatings, dimensions of articles, as well as for control over production processes, in flaw detection, and in other fields of engineering and technology.
BACKGROUND OF THE INVENTION
At present there are numerous constructions of differential eddy-current transducers extensively used in diverse apparatus and instruments, and in measuring systems.
Most of the heretofore-known eddy-current transducers are characterized by the presence of an error in the results of measurements, the so-called additional error, caused by external factors. Many of the improvements in the construction of the known eddy-current transducers are aimed at increasing the accuracy of measurements by minimizing the additional error; nevertheless, at present this problem is far from being solved completely.
Known in the art presently is a differential eddy-current transducer for measuring mechanical (non-electrical) quantities (cf. DE #3,817,371 A1) which comprises two coils and a ferromagnetic core movable inside said coils and connected to the object under control. Depending on the position assumed by the core the inductance values of the coils are varied, a common lead-out of said coils being connected to one of the inputs of an LC-oscillator while two other lead-outs thereof are alternately connected, via an analog multiplexer, to a second input of said LC-oscillator. The oscillation frequency of the self-excited oscillator depends on the inductance value of the LC-circuit, i.e., on the fact which of the coils at a given instant of time is connected to the input thereof. The frequency of the output signal of the LC-oscillator is determined after connecting each of the two coils, whereupon a microcomputer calculates, against the difference between frequencies, the value of the quantity measured by the transducer.
The eddy-current transducer discussed before is characterized by a low operating speed, necessity to make use of multi-digit counters (since its deviation is relatively low), restricted resolution, as well as reasonably high complexity and, accordingly, high cost of hardware implementation.
Known in prior art is an eddy-current measuring system (cf. U.S. Pat. No. 5,541,510 A) for non-destructive testing of electrically conducting and/or ferromagnetic materials and products, comprising a generator supplying power to an impedance network, an amplitude detector, a phase detector, demodulators, a computing unit, and an eddy-current transducer having one search coil and establishing a resonant circuit together with a parallel-connected capacitor. Said measuring system enables measuring two parameters pertaining to the object under test. However, it is more than two parameters that actually affect the eddy-current transducer in this case.
For instance, when measuring a distance to the object being tested and a linear dimension (thickness) thereof, a possible change in at least two out of the three other parameters occur to be non-compensated, viz, conductance, magnetic permeability, and temperature, as well as, probably the rest of the parameters of the object involved (its linear dimensions inclusive).
Effect of said factors on the results of measurements is partly eliminated in an embodiment of the system comprising two differentially connected eddy-current transducers. However, such a variant of the system involves stricter requirements imposed on the eddy-current transducers used, that is, as to similar dimensions and electric parameters, which is hard-to-attain due to technological spread in characteristics.
Furthermore, it is necessary that both of the eddy-current transducers be positioned very precisely so that they are in a similar position with respect to the object under test and/or under similar environmental conditions.
Non-identity of the eddy-current transducers and their arrangement results in an incomplete compensation for change in uncontrollable factors and, accordingly, leads to errors in the results of measurements.
In addition, processing of signals generated by eddy-current transducers necessitates the use of a costly instrument amplifier. The aforesaid peculiar features of the system add to its complexity and cost.
For electrically connecting eddy-current and inductive transducers there are most extensively used bridge circuits, wherein one of the bridge diagonals is power-supplied from a source of sinusoidal voltage, and the other bridge diagonal is connected to the inputs of a differential amplifier after which an alternating voltage is subjected to phase-sensitive detection and filtration.
Such circuit designs are characterized by sophisticated balancing, certain non-linearity of an unbalanced bridge, temporal and temperature instability, influence of electromagnetic interference protection against which involves the use of, e.g., expensive coaxial cable.
The closest to the herein-proposed transducer is a differential eddy-current transducer described in a prospectus of the Kaman Instrumentation (USA) entitled “The Measuring Solution Handbook”, 1999, p.6 (also pp. 4 and 5), which transducer is also based on a bridge circuit.
Said known eddy-current transducer comprises a primary detector incorporating two similar search coils and an electronic unit comprising two capacitive and two resistive elements. Each of the search coils is shunted by a capacitive element together with which said coil forms a parallel resonant circuit. Both of such circuits are cut into adjacent arms of the bridge whose other two arms are in effect the resistive elements. The common point of the windings is grounded and an alternating voltage is applied to the common point of the resistive elements. An output signal generated by the circuit is picked off the common points of the resistors and coils.
To compensate for temperature instability is possible only in case of a balanced bridge, i.e., only with a fixed position of the object under test and an invariable value of the electromagnetic parameters of the object (that is, electrical conductance and magnetic permeability).
An incomplete identity of the parameters of the search coils and the presence of a spread in the parameters of the transducer circuitry components hampers selecting the capacitive elements for the resonant circuits and resistive elements for balancing the bridge arms. Hence complete balancing is practically impossible due to the fact that the bridge arms are formed by dissimilar elements. Even when the bridge is amplitude-balanced, a phase shift occurs which results, in case of phase-sensitive demodulation, in a “zero-drift” error, affected resolution and temperature instability of the output signal.
An upset bridge balance occurs in the course of measurements when the object under test produces an unsymmetrical effect upon the search coils, which also tells negatively on the accuracy of the results of measurement.
And finally, further conversion of a pickup signal requires a differential (instrument) amplifier which complicates measuring equipment as a whole and adds to the cost thereof.
BRIEF DESCRIPTION OF THE INVENTION
It is an essential object of the invention to enhance the accuracy of measurements by compensating for the additional error caused by external factors.
It is another object of the invention to enhance the resolving power and interference immunity of a differential eddy-current transducer.
The foregoing object is accomplished due to the fact that in a differential eddy-current transducer comprising a primary detector which is adapted to co-operate with the object under test and incorporates two similar search coils, each of which has a first output and a second output, a voltage being applied to said second output, and a
Mednikov Felix Matveevich
Mednikov Stanislav Felixovich
Nechaevsky Mark Lazarevich
Collard & Roe P.C.
Patidar Jay
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