Patent GB-622125-A (GB)
Title: Impedance measuring device
IPC: G01R17/00
Filed: 1946-09-18
Abstract: 622,125. Impedance measurements; magnetic tests; indicating height of aircraft; determining physical qualities and dimensions of materials. DARDEL, G. F. VON. Sept. 18, 1946, No. 28047. Convention date, Sept. 19, 1945. [Class 37] An A.C. impedance bridge 9, Fig. 2, has its output terminals 8 connected through an amplifier 12 to its input terminals 7 so as. to produce oscillations, which are measured by an ammeter or voltmeter 13. When the bridge is balanced, the output voltage V2 is zero and no oscillations are produced. If the bridge is unbalanced the output voltage V2 is amplified and the amplified voltage V1 is applied to the input of the bridge, thereby increasing the output voltage V2. This continues until equilibrium is established owing to the decrease of amplification with increasing magnitude of the oscillations, thus increasing the sensitivity of the bridge. The amplifier should be tuned to the measuring frequency and gives a strong negative feedback for all frequencies which are not in the immediate neighbourhood of the measuring frequency so that the instrument is insensitive to harmonics, power frequency hum or circuit or valve noise. The vectorial components of the bridge unbalance change respectively the amplitude and frequency of the oscillations produced. These may be measured on separate instruments so as to obtain both components of the unbalance change. Fig. 5 shows the application to magnetic crack detection.. The impedance bridge contains two test coils 13, 18 and a potentiometer 15 forming the other two arms. For resistive balancing of the bridge, another potentiometer 16 may be used. The bar or tube to be examined is pushed through the coils 14, 18. The impedance of the coils and thus the unbalance of the bridge is constant if the bar is free from flaws and a constant reading is obtained. If there is a flaw a change of reading is obtained first in one direction and then in the other as the flaw passes through the two coils. The coils 14, 18 may be placed at a distance from one another, a standard bar being placed in one coil and bars to be tested being put into or pushed through the other coil. One coil may be connected to the rest of the apparatus by a flexible cable and may have an iron core, which can be pressed against the surface of an article to be examined. The thickness of conducting coatings on a base material with different electric or magnetic properties may similarly be measured. By reading both frequency and amplitude of the oscillations, two different properties of the material may be measured separately. Fig. 6 shows an altimeter for an aircraft in which the capacitance between two plates 23, 24 is measured by means of a bridge having ratio arms Z1, Z2 and a balancing condenser Z4. The arms Z1, Z2 may be the two parts of a tapped coil, which may be one winding of a transformer. In other applications, one or several of the impedances contain resistors, coils or condensers, the impedance of which changes with the distance between two points. If these elements are fastened to a body the apparatus functions as a strain gauge.