Testing device and testing method for non destructive detection of a defect in a test piece by means of an eddy current

1. Test set-up for non-destructive detection of a flaw in a device being tested by means of an eddy current, comprising:

an excitation coil adapted to act on a device being tested with an alternating electromagnetic field,

a receiving coil adapted to generate a coil signal as a function of a flaw in the device being tested,

an analog-digital converter having an input side which is coupled to the receiving coil,

a filter arrangement having an input side which is coupled to an output side of the analog-digital converter and is adapted for band-pass filtering and scan frequency reduction, and

a demodulator having an input which is coupled to an output of the filter arrangement

wherein the filter arrangement comprises a band-pass filter, an input of which is coupled to the analog-digital converter and an output of which is coupled to the demodulator to issue an input signal, and

wherein the filter arrangement further comprises a conversion system that is adapted to provide only one value as the demodulator input signal for scan frequency reduction of a first number of values of the input signal, whereby the first number is an integer greater than 1.

2. Test set-up according to claim 1 , wherein the analog-digital converter is adapted to produce a converter output signal with a converter scan frequency; wherein the filter arrangement is adapted to convert a converter output signal into a demodulator input signal having a reduced scan frequency which is smaller than the converter scan frequency by a reduction factor, and wherein the demodulator is adapted to demodulate the demodulator input signal.

3. Test set-up according to claim 1 wherein the analog-digital converter is adapted to provide an input signal having a first number of values during a second number of periods of the excitation signal, the first number and the second number being different integers, wherein the filter arrangement comprises a number of low-pass filter arrangements and wherein each of the values of the input signal is sent to a respective one of the low-pass filter arrangements.

4. Test set-up according to claim 3 , in which at least one of the low-pass filter arrangements comprises a short-term low-pass filter and a long-term low-pass filter which has a longer time constant compared to a time constant of the short-term low-pass filter.

5. Test set-up according to claim 1 , further comprising a band-rejection filter, an input of which is coupled to the demodulator.

6. Test set-up according to claim 1 , further comprising a signal processor, a generator that is coupled to the excitation coil and comprises the demodulator, the generator being adapted to issue a demodulator signal to a demodulator input of the demodulator.

7. Test set-up according to claim 6 , further comprising a digital-analog converter which is arranged between the generator and the excitation coil.

8. Test set-up according to claim 6 , further comprising a clock generator the output of which is coupled to a clock input of the signal processor and a clock input of the analog-digital converter.

9. Test method for non-destructive detection of a flaw in a device being tested by means of an eddy current, comprising the steps of:

acting on the device being tested with an electromagnetic alternating field by means of an excitation coil to which an excitation signal is sent,

generating a coil signal as a function of a flaw in the device being tested by means of a receiving coil,

providing a converter output signal by digitalization of the coil signal,

generating a demodulator input signal by filtering the converter output signal with a band-pass characteristic and scan frequency reduction, and

demodulation of the demodulator input signal by by means of a demodulator,

wherein the converter output signal is provided with a converter scan frequency and the demodulator input signal is provided with a reduced scan frequency which is smaller than the converter csan frequency by a reduction factor.

10. Test method for non-destructive detection of a flaw in a device being tested by means of an eddy current, comprising the steps of:

acting on the device being tested with an electromagnetic alternating field by means of an excitation coil to which an excitation signal is sent,

generating a coil signal as a function of a flaw in the device being tested by means of a receiving coil,

providing a converter output signal by digitalization of the coil signal,

generating a demodulator input signal by filtering the converter output signal with a band-pass characteristic and scan frequency fate reduction, and

demodulation of the demodulator input signal by means of a demodulator,

wherein the input signal is generated by means of a band-pass filter from the converter output signal or a signal derived from the converter output signal; and wherein a scan frequency reduction by a first number that is an integter greater than 1 is produced from values of the input signal, and wherein only one value is provided as the demodulator input signal.

11. Test method according to claim 10 wherein a scan frequency of the input signal is a rational multiple of an excitation frequency of the excitation signal, and wherein the input signal has a first number of values during a second number of periods of the excitation signal, the first number and the second number being different integers.