Bridgeless system for directly measuring complex impedance of an eddy current probe

• US 5,055,784 A
• Filed: 04/05/1990
• Issued: 10/08/1991
• Est. Priority Date: 12/07/1987
• Status: Expired due to Term

• Current Assignee: American Research Corporation Of Virginia
• Original Assignee: American Research Corporation Of Virginia
• Inventors: Jaeger, Douglas J., Groger, Howard P.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US07/504,698
• Time in Patent Office: 551 Days
• Field of Search: 324/233, 324/234, 324/236-241, 324/607, 324/649, 324/654, 364/482, 364/483
• US Class Current: 324/233
• CPC Class Codes: G01N 27/9046 by analysing electrical signals

Abstract

An all digital eddy current measurement system is described and illustrated in which an eddy current probe is driven by a driving signal and voltage signals representing the current through and voltage across the probe coil are used to calculate the magnitude and phase angle of a complex probe impedance. Digitization of the voltage signals is controlled by a control logic system which is run separately from but initiated by a microprocessor, the latter of which functions to analyze the acquired data and calculate impedance magnitude and phase angle values therefrom.

Claims

1. An eddy current measuring system comprising:

an eddy current measuring probe having an electromagnetic coil;

a reference resistor connected in series with said electromagnetic coil for producing a reference resistor value;

means for applying a first driving signal to said coil;

means for producing second and third voltage signals respectively representing a probe current in said coil and a probe voltage across said coil, said second and third signals being produced during the time said driving signal is applied to said coil;

means for analog-to-digital converting said second and third signals;

means for determining a plurality of magnitude and phase angle values representing the complex impedance of said probe from said converted second and third signals; and

storing means connected to said determining means for receiving and storing said plurality of magnitude and phase angle values from said determining means wherein said converted values from said second signals represent probe current values and said converted values from said third signals represent probe voltage values and for storing said reference resistance value received from said reference resistor and for providing said reference resistance value to said determining means;

wherein said means for determining produces a plurality of digital signal values representing probe current by dividing sample values of said second voltage signal by said reference resistance value wherein each said second voltage sample is represented by a voltage V₂ across said reference resistance R as I=V_2/R and for producing a plurality of digital values representing said third voltage signal where each said third voltage value is represented by a voltage (V₁ -V₂), wherein V₁ is the voltage across a serial interconnection of said coil and said reference resistance, said voltages V₁ and V₂ being said voltage signals representative of the instantaneous current (I) through said coil and the instantaneous voltage (V₁ -V₂) across said coil.

2. A measuring system as in claim 1, wherein said means for determining the magnitude of said impedance determines RMS values for said voltage signals, said RMS values being used to produce a value representing the magnitude of the probe impedance.

3. A measuring system as in claim 1, wherein said stored digitized second signal represents probe current and wherein said means for determining phase angle comprises means for producing a probe voltage signal from at least one of said stored digitized second and third signals, means for defining a first reference point on one of the stored digitized second and probe voltage signals, means for finding a corresponding reference point on the other of the stored digitized second and probe voltage signals, means for determining the time difference between the two reference points, means for dividing the time difference by the period of one of said stored digitized second and probe voltage signals and means for multiplying the result by 2π

.

4. A system as in claim 1, wherein said means for determining the magnitude of said impedance determines an RMS voltage value for said stored digital signals and uses the RMS voltage values to produce a value representing the magnitude of probe impedance.

5. A system as in claim 4, wherein said means for determining determines a plurality of impedance magnitude values from respective pairs of stored samples of said stored digital signals.

6. A measuring system as in claim 1, further comprising means providing coordinate data representing the instantaneous position of said probe relative to an object;

and

means for associating a determined magnitude and phase angle for said complex impedance with coordinate data representing the position of said probe at which said magnitude and phase angle were determined.

7. A measuring system as in claim 1, wherein said means for applying comprises:

means for creating a digital waveform,

means for storing said digital waveform; and

means for digital-to-analog converting said stored digital waveform into an analog probe driving signal.

8. A measuring system as in claim 7, wherein said creating means is able to change the shape, amplitude and frequency of said digital waveform.

9. A measuring system as in claim 1, further comprising at least one other probe for detecting a characteristic of said object,

means for deriving a signal representing said object characteristic at a particular location of said object, and

means for associating said signal representing said object characteristic at said particular location with a determined value of magnitude and phase angle for the complex impedance of said first probe for said location.

10. A probe impedance measuring system comprising:

a probe comprising an electromagnetic coil;

means for generating a probe driving signal;

means for applying said driving signal to said probe coil;

means for receiving signals representing a voltage across said coil and a current passing through said coil when said coil is driven by said driving signal;

means for sampling and analog-to-digital converting said signals representing coil voltage and coil current to produce digital samples of said signals representing coil voltage and current;

means for storing said digital samples;

control logic means for controlling the sampling and converting means to produce said digital samples; and

processor means for controlling said generating means and applying means so that said driving signal is generated and applied to said probe coil, for initializing operation of said control logic means so that said signals representing coil current and coil voltage are converted to said digital samples, and for calculating a complex probe impedance from said stored digital samples of said signals representing coil voltage and current.

11. A system for directly measuring the magnitude and phase angle of a complex coil impedance comprising:

a probe comprising an electromagnetic coil;

a reference resistor connected in series to said electromagnetic coil for producing a reference resistor value;

a driving signal generator adapted to apply a probe driving signal to said coil;

means operative when said driving signal is applied to said coil for obtaining voltage signals representative of the instantaneous current through said coil and the instantaneous voltage across said coil;

analog to digital converter means for converting said voltage signals representative of instantaneous current through said coil and said instantaneous voltage across said coil respectively into first and second digital signals, each comprising a plurality of digital samples;

means for determining a value of a magnitude and phase angle of a complex impedance of said coil from said first and second digital signals, said means for determining a value of the magnitude and phase angle of said complex impedance doing so by extracting a plurality of sets of digital samples, one from each of said first and second digital signals, and using each said set of extracted digital samples to form a respective magnitude value and a respective phase angle value, thereby forming a plurality of magnitude and phase angle values corresponding to said plurality of sets of digital samples; and

storing means connected to said determining means for storing said plurality of magnitude and phase angle values representing said first and second digital signals received from said determining means and for receiving and storing said reference resistance value and for providing said reference resistance value to said determining means;

wherein said determining means produces said second digital signals by dividing sample values of said second digital signal representing said probe current by said reference resistance value wherein each said probe current value is represented by a voltage V₂ across a reference resistance R as I=V_2/R and said second digital signals are represented by a voltage (V₁ -V₂), where V₁ is the voltage across a serial interconnection of said electromagnetic coil and said reference resistor, wherein said voltages V₁ and V₂ being said voltage signals representative of the instantaneous current (I) through said coil and the instantaneous voltage (V₁ -V₂) across said coil.

12. A system as in claim 11, wherein one of said voltage signals represents instantaneous coil current and said means for determining said phase angle comprises:

means for defining a first reference point on one of a) said voltage signals which represents instantaneous probe current and b) a signal formed from said voltage signals which represents the voltage across said probe coil, means for finding a corresponding second reference point on the other signal, and means for determining the time difference between the two reference points, for dividing the difference by the period of one of said signals, and for multiplying the result by 2π

.

13. A system as in claim 11, further comprising means providing coordinate data representing the position of said probe relative to an object;

and

means for associating a determined magnitude and phase angle for said complex impedance with coordinate data representing the position of said probe at which said magnitude and phase angle were determined.

14. A system as in claim 11, further comprising means for changing the characteristics of said probe driving signal.

15. A system as in claim 11, wherein said generator comprises:

means for creating a digital waveform,

means for storing said digital waveform; and

means for digital-to-analog converting said stored digital waveform into an analog probe driving signal.

16. A system as in claim 11, further comprising means for sensing a temperature and provided adjacent said probe, means for receiving an output signal from said temperature sensor, said means for determining the magnitude of said complex impedance correcting a determined magnitude value in accordance with the output of said temperature sensor.

17. A system as in claim 16, wherein said means for determining the phase angle of said probe impedance corrects a determined phase angle value in accordance with the output of said temperature sensor.

18. A system as in claim 11, further comprising at least one other probe for detecting a characteristic of said object,

means for deriving a signal representing said object characteristic at a particular location of said object, and

means for associating said signal representing said object characteristic at said particular location with a determined value of magnitude and phase angle for the complex impedance of said first probe for said location.

19. A system as in claim 15, wherein said creating means is able to change the shape, amplitude and frequency of said digital waveform.

20. A system as in claim 11, wherein said probe further comprises another coil positioned adjacent said electromagnetic coil, and said system further comprises means for generating a driving signal for said another coil which is either in phase or out of phase with said driving signal for said electromagnetic coil.

21. A system as in claim 11 further comprising:

means for forming an average magnitude value from a stored plurality of magnitude values; and

means for forming an average phase angle value from a stored plurality of phase angle values.

22. A system as in claim 11 further comprising means for acquiring additional probe data, said analog-to-digital converter means converting said additional probe data into a third digital signal and means for storing said third digital signal.

23. A system as in claim 22 wherein said additional probe data represents temperature data, said system further comprising means for correcting each of said plurality of magnitude and phase angle values by using said temperature data.

24. A system as in claim 11 wherein said pair of voltage signals are respectively taken from across said series connection and from across said reference resistance.