Control apparatus for eddy current non-destructive testing using a digital compensating circuit

Claim 1

A control apparatus utilizing eddy currents and comprising: a sensor contituted by a primary winding and a secondary winding, a sinusoidal exciting voltage generator having a given frequency F, said generator supplying the primary winding, a correcting circuit for eliminating the residual voltage supplied by the secondary winding of the sensor, said circuit comprising a compensating voltage generator having an output delivering a compensating voltage at frequency F of the exciting voltage and having a regulatable amplitude and phase, and a differential amplifier having two inputs, one receiving the compensating voltage and the other the voltage supplied by the sensor and one output supplying a corrected measuring voltage, a circuit for analyzing the corrected measuring voltage, said circuit comprising two samplers operating at the frequency of the generator and in phase quadrature with respect to one another, said samplers being controlled by pulses processed by an appropriate means connected to the generator, said samplers having each an input connected to said output of said differential amplifier, means for displaying the two signals supplied by the two samplers, wherein: (A) the exciting voltage generator is of the digital type and comprises an oscillator transmitting pulses at a stabilised frequency Fs, a first counter of maximum capacity C connected to the oscillator, a first memory containing a Table of C digital values of the sine function, said memory being addressed by the counter content, a first digital-analog converter connected to the memory, said converter supplying a sinusoidal voltage of frequency F=Fs/C which is applied to the primary winding, (B) the correcting circuit comprises: (i)--a first regulatable means for selecting a number between 0 and C, a first comparator with two inputs, one connected to said first means and the other to the output of the first counter of the generator, and an output supplying a pulse when the numbers received by the two inputs are equal, (ii)--a second counter identical to the first, said counter receiving the pulses supplied by the oscillator of the digital generator and having a zeroing input connected to the output of the first comparator, (iii)--a second memory containing a Table of C of digital values of the sine function, said memory being addressed by the content of the second counter, a second digital-analog converter connected to the memory and a regulatable potentiometer having an input connected to the output of the converter and an output which is said output of said compensating voltage generator and which is connected to one input of said differential amplifier, (C) the means for generating the control pulses of the samplers comprises: (i)--a second means for selecting a number between 0 and C, a second comparator with two inputs, one connected to said second means and the other to the first counter of the generator and an output supplying a pulse when the two numbers received by the inputs are equal, said output being directly connected to one of the samplers.