Patent GB-1303229-A (GB)
IPC: G01B7/02
Filed: 1971-08-06
Abstract: 1303229 Electro-physical measurements; measuring capacitance ADE CORP 6 Aug 1971 [17 Aug 1970] 36999/71 Headings G1N G1U The invention relates to a capacitor type probe, particularly for gauging dimensions of dielectric material (using a conducting base plate), the probe having a MEASURING ELECTRODE 50 and a BALANCING ELECTRODE 68 also mounted in a conductive housing 40 whereby the reference capacitor formed by the balancing electrode and housing is subjected to the same conditions e.g. temperature as the measuring electrode. These electrodes are arranged in a bridge circuit with capacitors 86, 88 and connected by a diode switching ring 74. The construction may include a guard ring. CONSTRUCTION Several embodiments are given; in Fig.2, a dielectric annulus 56, 66 and conducting ring 58, 64 may surround the electrodes. A ductile filler 60, such as solder may be used between the parts. A moisture-proof cover 98 may seal the tip. It is suggested that the measuring electrode may initially be thicker and machined down until its capacitance to the housing is substantially the same as that of the balancing electrode. ALTERNATIVE CONSTRUCTIONS Figs.3a,b, (not shown), the probe may be of rectangular or circular cross section, and may have a curved end-face Fig.4, (not shown), Fig.5, (not shown). The diode ring may be mounted between the electrodes and the leads brought out to a co-axial plug on top of the housing. Fig.6, (not shown). The electrode may be formed as a deposit on dielectric discs e.g. of glass by bonding sputtering, electro-forming etc. Fig.9, 10 (not shown), a guard ring may be similarly deposited around the electrodes. Fig.8, (not shown), the probe may comprise a double walled container, the inner wall acting as a guard electrode, and the bridge circuit may be in a separate container connected by co-axial cable. Figs.13, 14, (not shown) the probe tip may have three side-by-side electrodes or, Fig.15 five electrodes may be used, the outer pair providing a reference or compensation signal for the distance of a base-plate. CIRCUITS Fig.11, shows the measuring capacitor 236 and balancing capacitor 238 connected by a diode ring 244 to upper bridge arms 240, 242 shown as resistors but may be capacitors. As the value of capacitor 236 varies a residual D.C. signal component builds up at points 246, 248. A.C. blocking chokes 250, 252 are provided in the meter circuits. Fig.12 shows an arrangement providing an output proportional to distance (instead of inversely proportional). Two bridge circuits similar to Fig.11 are used. The output of the first measuring bridge 266 is maintained at a constant reference value 272 by differential amplifier 270 which regulates the excitation of an oscillator 268 energizing the bridge. The oscillator output is applied also to an identical dummy bridge 274 with fixed capacitors. A mathematical explanation is given. Fig.16, (not shown), comprises a circuit for processing signals from the probe of Fig.15, the two outer electrodes being connected to a common bridge circuit (330-331). Three possible inter-connections between the bridge outputs and logic elements are provided by Figs. 17a, b, c. (not shown). Fig.18 is an alternative circuit for use with the probe of Figs.13, 14, The outer electrodes are connected in parallel to give C1 + C3 from bridge 408 and after inversion provides compensation for thickness t of a di-electric strip being monitored, to a width output w.