AN EDDY CURRENT TEST PROBE USING AN OSCILLATOR MOUNTED IN A DIGITALLY MANIPULATABLE HOUSING

• US 3,723,861 A
• Filed: 09/28/1970
• Issued: 03/27/1973
• Est. Priority Date: 09/28/1970
• Status: Expired due to Term

• Current Assignee: Everett L. Samples
• Original Assignee: Everett L. Samples
• Inventors: Samples, Everett L.
• Primary Examiner(s): Corcoran, Robert J.
• Application Number:
  US05/075,977
• Time in Patent Office: 911 Days
• Field of Search: 324/37,40,41,3 331/146,112
• US Class Current: 324/237
• CPC Class Codes: G01N 27/9006 Details, e.g. in the structure or functioning of sensors

Abstract

A device for eddy current testing of metals for cracks and the like including a sensing coil disposed in a contoured contact tip carried at one end of a hollow, cylindrical housing which houses electrical circuitry and a battery for powering the device. The electrical circuit is closed by plugging an earphone into a jack in the side of the housing, and the circuitry includes a blocking oscillator connected by a center tap to the sensing coil so that this single coil forms a portion of the tuned circuit of the oscillator. The blocking frequency developed by the circuit is in the audible range and is adjusted for purposes of metals testing by the use of a potentiometer connected in the control circuit. A change of the developed blocking frequency, as evidenced by a change in the signal heard at the earphone, is indicative of the presence of an anomaly in the metal under test.

Claims

1. An instrument for non-destructively testing metal comprising:

a transistorized blocking oscillator including;

a tuned tank circuit including a center tapped detector coil and a capacitor providing parallel inductive and capacitive reactance;

a regenerative feedback circuit; and

a control circuit including a battery, a transistor, a potentiometer, and a variable trimming resistor for varying the blocking frequency of said blocking oscillator;

a housing containing said battery, said feedback circuit, said control circuit, and a portion of said tank circuit, said housing having an earphone jack opening in one side thereof and connected in said control circuit for closing said control circuit when an earphone is plugged into said jack; and

a probe mounted on one end of said housing and containing said detector coil, said probe being a generally conically shaped contact member having a rounded tip and having said coil positioned in immediate juxtaposition to said tip.

2. An instrument for non-destructively testing metal as defined in claim 1 and further characterized as including an earphone plugged into said jack and closing said control circuit for responding to the blocking frequency of said oscillator.

3. An instrument as defined in claim 1 wherein said center tapped coil is disposed in convolutions around the axis of said contact member in a bore in said contact member provided therefor, and further including a soft iron core disposed concentrically in said coil and extending into a counterbore extending from said bore toward said rounded tip along the axis of said contact member.

4. An instrument as defined in claim 1 and further characterized to include:

a rotatable blocking frequency selector knob located outside said housing; and

a shaft connected at one end to said frequency selector knob and extending through said housing for connection at its other end to the movable tap of said potentiometer.

5. An instrument as defined in claim 1 and further characterized to include a pulse shaping circuit comprising a resistor and capacitor connected in parallel between said earphone jack opening and the emitter of said transistor in the emitter circuit of the transistor for limiting current flow in said emitter circuit and shaping the pulses of the blocking frequency developed by said blocking oscillator.

6. In an eddy current testing instrument of the type including a detector coil for responding to fluctuations in eddy currents induced in a metallic test specimen, and an oscillator circuit developing an output frequency varying with the inductance of said detector coil, the improvement which comprises:

a cylindrical, digitally manipulatable housing containing said oscillator, and having an earphone jack in one side thereof, said jack being connected in said oscillator circuitry to provide switch contacts closeable upon plugging an earphone into said jack;

a battery disposed in said housing and connected in said oscillator circuit for powering said oscillator circuit;

a frequency varying element disposed in said oscillator circuit for varying the output frequency thereof;

said frequency varying element comprising a variable resistance, and a capacitor cooperating to provide selective variation of the output frequency of said oscillator circuit; and

a digitally manipulatable frequency selector knob rotatably mounted on the outside of said housing and connected through said housing to said frequency varying element to facilitate selective variation of the output frequency of said oscillator circuit; and

a tapered probe mounted on one end of said housing and adapted for containment of a detector coil in the tip of said probe; and

an iron core detector coil mounted in said tapered probe immediately adjacent the tip thereof.

7. An eddy current testing circuit comprising:

a battery source of power;

a common emitter transistor connected to said battery;

an iron core detector coil arranged to be disposed in inductive relationship to metal to be tested;

a capacitor connected in parallel to a first portion of said detector coil through a center tap to form a tuned circuit developing a normal testing frequency;

a direct collector-to-base feedback circuit connected between said center tap of said coil and the base of the transistor and including, in series, a second capacitor and a second portion of said detector coil;

means included in the base circuit of said transistor for determining with said second capacitor, the value of a blocking audio frequency developed in said testing circuit, said means including;

a potentiometer; and

a trimming resistor connected in series with said potentiometer;

an earphone connected in the emitter circuit of said transistor and responsive to said blocking audio frequency to emit an audible signal indicative of changes in the inductive reactance of said iron core detector coil; and

a pulse shaping circuit comprising a resistor and capacitor connected in parallel between said earphone and the emitter of said transistor in the emitter circuit of the transistor for limiting current flow in said emitter circuit and shaping the pulses of the blocking frequency developed by said blocking oscillator.