Patent GB-880184-A (GB)
Title: Improvements in or relating to measuring and testing methods and apparatus
Inventor: HAYNES ANTHONY CHARLES RENDELL
IPC: G01B7/06
Filed: 1957-04-26
Abstract: 880,184. Electric tests; measuring bridges. HAYNES &  HAYNES  Ltd. April 25, 1958 [April 26, 1957; Sept. 30, 1957], Nos. 13384/57 and 30488/57. Class 37. Apparatus for detecting and measuring variations in the thickness of a tube, rod, sheet, strip, &c. of a plastic or other dielectric material, or for detecting eccentricity of the bore of an elongated tubular product of such material, comprises a measuring capacitor whose dielectric is substantially constituted by the product under examination, and a second capacitor, which may be a variable pre-set reference capacitor or a second measuring capacitor whose electrostatic field traverses a portion of the tubular product circumforentially separated from that portion traversed by the field of the first measuring capacitor, both capacitors being series-connected in a normally balanced circuit supplied at high frequency, the output from said circuit being amplified and applied to a detecting device which provides indication of the unbalance of the circuit caused by variation from a predetermined standard thickness, or by eccentricity of the bore, of the product under examination. Measuring thickness.-As shown, Fig. 7, the output of a R.F. oscillator 20 is modulated by the output of an A.F. oscillator 22 in a suppressed-carrier modulator 24, the sideband waves being fed, in opposite phases, via screened leads 26, 28 to a detector head 30 which contains a measuring capacitor and a reference capacitor in series. The carrier wave is also transmitted, via an attenuating circuit 36, to be added to the detector head output in a variable-gain R.F. amplifier 34 before amplification, the output of the amplifier being passed to an A.F. detector network 38 which feeds an A.F. signal of the same frequency as the modulating wave via an A.F. amplifier 42 to a phase-sensitive detector 44, wherein it is compared with a reference signal derived from oscillator 22, the polarity and magnitude of the output of detector 44, which represent the in- or anti-phase condition of the received signal and its strength, being indicated on a centrezero meter 46. The detector network 38 also feeds back a D.C. signal to circuit 36 to vary the attenuation of the carrier wave input to amplifier 34; this D.C. signal is also supplied to a meter 40 for monitoring purposes. In a simpler embodiment (not shown), the unmodulated output of the H.F. oscillator 20 is fed, via the earth-balanced secondary of a transformer, to the detector head 30, and the error signal, having been amplified at 34, is applied to the phase-sensitive detector, wherein it is compared with oppositely phased reference voltages from the oscillator 20. Detector head. Figs. 11, 12.-This comprises a rectangular box 130 which is made in two halves 158, 160 which meet on a horizontal plane, and are hinged at 162, the side walls of the two halves having mutually registering semicircular cut-outs which form aligned openings 166 when the box is closed. The reference capacitor comprises two electrodes 170, 172, between which a cam 360 of dielectric material is movable by a knob 174 and a spindle 168 to adjust the reference capacitance, and the measuring capacitor comprises semicircular electrodes 192, 194, located coaxially with openings 166, the former electrode being made integral with electrode 172 of the reference capacitor and the output lead 32 being connected thereto, while the input leads 26, 28 from the suppressed-carrier modulator are connected to electrodes 170 and 194 of the reference and measuring capacitors respectively. The box 130 is supported via a universal joint on a counterpoised lever (not shown). A pair of external rollers 210 above each opening 166 maintain the material under test concentric with openings 166 and prevent it fouling the edges thereof. Measuring eccentricity.-A modified detector head 030, Fig. 14, comprises electrodes 540, 541 which are supplied with high-frequency energy via leads 26, 28, each electrode being associated with a pair of further electrodes 542, 544 and 543, 545, respectively, the electrodes being disposed around the tubular product under examination, as shown. Eccentricity of the product in a diametral plane ZZ produces an unbalance signal on output line 032; similarly, eccentricity in plane YY causes an unbalance signal on line 033. The modified detector head 030 is connected in a similar supply, amplification and detection circuit to that shown in Fig. 7, except that ganged.changeover switches provide for the alternative connection of output lines 032, 033, to the R.F. amplifier 34, and for the connection of the output of the A.F. amplifier 42 to the appropriate one of two phase-sensitive detector circuits with meters. In addition, the apparatus may include a thickness-responsive detector head, which may be enclosed in a common box with the modified detector head and a third phase-sensitive detector circuit may be provided to enable the mean thickness of the product to be monitored. Automatic means may be used to select the three monitoring circuits in sequence. Circuitry. Figs. 8a, 85.-The R.F. oscillator comprises a pentode 56 controlled by a crystal X58, and oscillating between screen and grid, the output being taken from a tuned coil in the anode circuit which is coupled to a cathode follower triode 70 which feeds the modulator. The A.F. oscillator comprises a pentode 72 with a phase-shift network between anode and grid, and a cathode follower triode 86 supplying a step-down transformer 88 which feeds the modulator and the phase-sensitive detector. A voltage stabilizing network 92 maintains the primary voltage of the transformer 88 substantially constant. The suppressed-carrier modulator comprises two modulator triodes 102, 104 to whose grids are applied the carrier wave with the same phase and the modulating wave with phase reversed as between the grids, and whose anodes are connected to opposite ends of the primary coil of a transformer 110, the secondary coil of which is balanced about earth and feeds sideband voltages of opposite phases to the series-connected measuring capacitor 54 and reference capacitor 52 in the detector head 30. The detector head output is applied, with the attenuated carrier wave, to the grid of a pentode 334 in the first R.F. amplifying stage, the gain of which is controlled by varying the potential applied by a potentiometer 356 to the suppressor grid of the pentode 334. The second R.F. amplifying stage comprises a pentode 370, the output of which is coupled to the A.F. detector network, comprising a diode 384, which is capacity coupled to the grid of a triode 394 in the A.F. amplifier, which also comprises a cathode follower triode 396 whose output is fed, via line 430, to the phase-sensitive detector. This comprises two diodes 432, 434, on whose cathodes are developed equal A.F. reference voltages of opposite phase from the transformer 88. Bias voltages are built up on capacitors C436, C438, equal to the sum and difference respectively of the reference and signal voltages, causing a current to flow through meter 46 which is determined as to amplitude and phase by the signal voltage. Terminals 440, 442 are for connection to a continuous recorder device (not shown). The attenuator circuit comprises two cathode follower triodes 402, 404 in cascade, the carrier wave being fed, via a phase-adjusting network 408, to the grid of the triode 402, and the amplitude of the output being controlled by the D.C. voltage supplied from the detector circuit and applied to the grids of both triodes 402, 404, so that maximum amplitude is achieved in the absence of any signal on the input of the detector circuit.