Patent GB-804339-A (GB)
Title: Improvements in or relating to thickness gauges
IPC: G01B7/06
Filed: 1957-01-25
Abstract: 804,339. Electric tests. GENERAL MOTORS CORPORATION. Jan. 25,1957 [March 2, 1956], No. 2732/57. Class 37. [Also in Group XL (c)] A gauge for measuring the thickness of one electrically conductive lamina on a body of another comprises a variable frequency oscillator arranged to sweep continually a predetermined frequency band and coupled to an inductance for indicating eddy currents in the layer under test and means for indicating changes in the load on the oscillator as the eddy current penetration reaches greater depth as the oscillator frequency is reduced. In the particular arrangement described the oscillator is loaded by a coil which induces eddy currents in the material under test while the frequency at which oscillation ceases as the frequency varies indicates the thickness of the lamina. As shown in Fig. 4 the valve 30 forms part of an oscillator having five tank circuits comprising coils 118, 152, 158, 160 and 162 selected by a switch 104. Corresponding circuits 32, 34, 36, 38, 40 in the grid circuit are selectable by switch 56. Each of the coils in the anode circuit is coupled to the " probe " which consists of a shield coil placed on the lamina being measured by inductively coupled low impedance windings 124. The output of the oscillator is taken from variable potentiometer 168 in the cathode circuit and applied to the vertical deflector plates of oscilloscope 164. The frequency of the oscillator is swept through a range by applying a sawtooth voltage through the grid coil in use to the grid of the oscillator thus varying the transconductance of the valve. The voltage is generated by a double triode 174 which acts as a flip-flop to generate square wave pulses. These pulses are differentiated, the negative voltage peaks are short-circuited by rectifier 222 and the positive peaks trigger a thyratron 176 through which a condenser 238 discharges. The sawtooth voltage thus generated is fed through a number of resistors 90 and having a slider at a different setting. The particular resistor described may be selected by switch 78 and the frequency range of the oscillator adjusted by varying the position of the slider. The sawtooth is also used to control the horizontal deflection of the oscilloscope. The cessation of oscillation as the frequency is varied produces a step on the trace which is an indication of the material thickness. Probe construction (Fig. 5).-The probe consists of a shielding body 250 of copper in which is a polystyrene block 258 held in position by a clamping screw through the side of body 250. A central bore 252 in the block holds a coupling for a shielded line which is connected by lead 264 to coil 268 wound on the end of the block. The other end of the coil is earthed to the body 250. An indexing lip 251 enables the body to be applied to curved surfaces. In a modification (Figs. 7 and 8, not shown) the shield consists of an annular cap which fits over the polystyrene block and the coil assembly may be moulded in place by filling the cavity with insulating material after the wires are wound around the end of the body 258.