Patent GB-836723-A (GB)
Title: Improvements in or relating to the electromagnetic testing of materials
Inventor: HASLETT ROY WILLIAM GEORGE
IPC: G01N27/90
Filed: 1956-10-04
Abstract: 836,723. Electric tests. KELVIN & HUGHES Ltd. Nov. 12, 1957 [Oct. 4, 1956], No. 30333/56. Class 37. The invention is concerned with plotting a Forster eddy current graph, an example of which is shown in Fig. 1. A conductor under test is situated in a coil energized with a variable frequency current. As the frequency varies Ls/Lo is plotted against #R/#Lo where Lo is the inductance of the empty coil, #R is the change in resistance of the coil when a rod is present at a frequency F, from a basic resistance of an empty coil of Ro and # = 2##. The invention is concerned with deriving these quantities Ls/Lo and #R/#Lo and with depicting them or the screen of a cathode-ray tube. As shown (Fig. 2), the output of a 20 kc. oscillator 1 is mixed with the output of a variable frequency oscillator at 3 to give a range of 200 cycles/sec. to 9 kcs./sec. The output is amplified and applied to the specimen chain consisting of an inductance 5 and the test coil 6. The inductance 5 has a high " Q " and is approximately 1 Henry which is high compared with the inductance of the test coil. A signal developed across coil 6 is again mixed with the output of oscillator 2 to restore it to 20 kcs./sec. Since the current through chain 5, 6 is almost wholly inductive the voltage at terminal 8 is in phase with the applied voltage, #Ls being much greater than Ro+#R. The current through the chain is also proportional to 1/#. Thus the in-phase component between terminal 8 and earth is proportional to Ls/Lo since Lo is constant and the quadrature component is proportional to R+#R/#. The quadrature component is selected by phasesensitive rectifier 11 fed with a 20 kc. reference signal from frequency changer 13, phase-shifted through 90 degrees. The in-phase component is selected in phase-sensitive rectifier 12 fed with reference signal from frequency changer 13. These signals are fed respectively to the X and Y plates of the C.R.T. but the D.C. signal proportional to Ro+#R/# is corrected by subtracting Ro/# before applying it to the X plates. The signal Ro/# is derived across a small resistor 22 in series with inductance 21, the reactance of which is always high compared to resistance 22. Thus the voltage across 22 is proportional to Ro/# and is subtracted from Ro+#R/# in the pushpull amplifier. By varying the calibrated oscillator a series of points may be produced on the screen of the C.R.T. and may be marked and subsequently joined to give the F÷rster graph. In the embodiment of Fig. 3, a crystal oscillator is used and the high Q inductances are turned in effect into ideolized inductances. As shown, a 50 cycle to 10 kc. signal is fed to a chain comprising the test coil 6 indicator 5 at least two orders greater than 6 and having an impedance much greater than all the components in the chain and a resistor R2. The current when Ec is the applied voltage, L is the inductance of 5 and R2 is the total resistance of the chain but R2 is negligible hence A voltage Eo exactly is phase with the voltage across a pure conductance is required for the reference circuit and it is necessary to subtract IRc. A voltage IRc is developed across resistor R2 amplified and phase-inverted at 30 and fed to a subtractor 29 also fed with a voltage proportional to Ec from potentiometer circuit r 1 , r 2 . The voltage across the test coil and resistor R2 is of the form Hence there is an emergency component proportional to Ls as required which is separated as described for Fig. 2. The in-phase component is corrected by the subtraction of (R2+Ro)/#.