GB648981A - An electric circuit for the measurement of linear dimensions according to the capacity principle - Google Patents

Abstract

648,981. Electric domensional measurements. SKODA WORKS, NATIONAL CORPORATION. Dec. 30, 1947, No. 34651. Convention date, Jan. 21, 1943. [Class 37] [Also in Group XXXVIII] Fig. 1 shows a capacity plug gauge having a cylindrical body in which insulated electrodes a, b are mounted 180 degrees apart. The electrode diameter Dk is accurately dimensioned, and oversize ends act as guides so that the electrodes do not contact the surface to be measured either electrically or mechanically. The two electrodes together with the conductive mass of the work constitute two condensers in series whose total capacity is a function of the clearance between the aperture and the gauge. This capacity C1+C2 is connected across condenser C3 to tune inductance L1 in the grid circuit of a pushpull H.F. oscillator I (Fig. 3), the output of which is loosely coupled into the grid circuit of a second H.F. oscillator II whose frequency is variable by a condenser C4 and a switchable condenser bank C5 - C9. The beat frequency in the anode circuit of the second oscillator is detected by earphones or loudspeaker T so that its frequency may be varied to synchronize with that of the first oscillator, which is a function of the clearance between the gauge and the aperture. The switchable and variable condensers may be calibrated in aperture dimensions. Synchronism between the two oscillators may be indicated by applying their respective outputs to the X and Y plates of a C.R.O. (Fig. 4, not shown). In a modification (Fig. 5, not shown), a fixed frequency oscillator is inductively coupled to a tuned circuit whose frequency is variable by the gauge capacity and which is in turn coupled to a detector and galvanometer. Capacity variations with differing gauge clearances vary the amplitude of the detector signal and the galvanometer deflection, which may be dimensionally calibrated. The invention is applicable to caliper gauges for measuring the thickness of metal sheets.