Electricity: measuring and testing – Magnetic – Plural tests
Patent
1981-12-31
1985-01-29
Strecker, Gerard R.
Electricity: measuring and testing
Magnetic
Plural tests
324238, 324262, G01N 2772, G01R 3300, G01N 2782
Patent
active
044969040
DESCRIPTION:
BRIEF SUMMARY
This invention relates to measurement apparatus and in particular to apparatus for the non-destructive testing of engineering materials and structures by eddy current measurement.
Eddy currents may be induced in conductive materials be setting up a varying magnetic field in the vicinity of the material. Such currents give rise to a field which opposes the inducing field and may be detected by a suitable transducer. The nature and structure of the material gives rise to a characteristic eddy-current pattern. Any perturbation of the pattern indicates an area of possible defect. A field inducing transducer coil may itself be used to examine the eddy current pattern by the impedance change induced by the opposing field.
Signals due to eddy currents are small and appear superimposed on larger signals due to other effects. Typically an eddy current signal at a transducer output appears as small perturbations about a substantially constant reference level signal. In order to remove the reference level signal to give a measurable signal two transducers are arranged in a conventional bridge, so that the reference level is balanced out. Such an arrangement is described in GB Patent Application No. 2 028 510 A in which transducer coils are opposingly mounted on a common former for rotation above a rivet fastener in engineering structure. The coils are connected in a bridge to balance out the reference level signal. A defect in a fastener hole for example causes a perturbation of the eddy current pattern in that region, which is detected during rotation as a coil passes the region. The opposing coil provides a reference level signal corresponding to sound structure at a reference location and a measurable signal due to the defect is produced at the bridge output. Measurement of signal components in phase and quadrature yields information about the nature of the defect.
In bridge arrangements the transducers forming the bridge must be carefully matched. At the signal levels of interest in eddy current measurement transducers must be particularly closely matched. The accuracy to which transducers can be matched imposes a limit on the performance of a system including a bridge arrangement.
In systems which detect differences in signal level between a reference and a measurement location transducer mismatch produces a direct error and many such systems employ additional circuitry to achieve bridge balance. A fundamental limitation of such systems having bridge arranged transducers is inability to reliably detect defects at both measurement and defect locations.
According to the present invention measurement apparatus for receiving an eddy current transducer signal which varies with respect to a reference level includes a signal generator for generating a signal at substantially the reference level, the output of the signal generator being combined with the transducer signal to produce a measurable output.
Where transducers require external excitation the signal generator may take the form of means for producing a signal which is proportional to or a function of the excitation signal.
The output of the signal generator is preferably predetermined. Alternatively the output may be defined during a calibration cycle which precedes a measurement cycle, the calibration cycle advantageously involving a plurality of iterative steps. Signal generator output may be set by determining in phase and quadrature components of the reference level.
It will be appreciated that the present invention overcomes the limitations of prior art eddy current measurement since the reference level may be set to correspond to a known defect free structure or the average value of typical samples or other preferred level. Hence problems associated with mismatch and mutual defects are not encountered with measurement apparatus in accordance with the present invention.
Signal levels are preferably set by control coefficients which control for example the output of the signal generator and excitation signal. Coefficients may advantageously be computed and
REFERENCES:
patent: 4322683 (1982-03-01), Vieira et al.
patent: 4326166 (1982-04-01), Pigeon et al.
Snow Walter E.
Strecker Gerard R.
The Secretary of State for Defence in her Britannic Majesty's Go
LandOfFree
Eddy current measurement apparatus for non-destructive testing i does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Eddy current measurement apparatus for non-destructive testing i, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Eddy current measurement apparatus for non-destructive testing i will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-508670