Patent GB-736570-A (GB)
Title: Improvements in or relating to comparison micrometers of the electrical capacity type
IPC: G01B7/12
Filed: 1953-07-10
Abstract: 736,570. Electric indicating systems. POSSATI, M., GIARDINO, L., and FAENZA, V. July 10, 1953, No. 19233/53. Class 40 (1). In an electric capacitor type micrometer for continuous or non-continuous dimensional checking, e.g. of internal diameters, two feelers 8, 9 carrying diamond contacts 53, 54 are adjustable along and secured to respective circular coaxial bars 1, 2 each secured to the frame 7 by two parallel blade springs 3-4, 5-6 to form two similar rectangular assemblies 7-3-1-4, 7-5-2-6, the measuring capacitor plates 12, 13 being secured to the bars 2, 1 respectively. Initially, the rectangular assemblies are locked by rotating a spring biased button 67 anticlockwise, causing levers 43, 44 to rock against their biasing springs 65, 66 and engage elements 21, 32 on the respective assemblies to hold them in the static position in which the springs 3-6 are unstressed, adjustable biasing springs 19 balancing out the bending effect on the springs 3, 4 due to the masses of the assemblies. In this position the feelers 8, 9 are adjusted axially along their respective bars 1, 2 until the contacts 53, 54 engage the inner bore of a standard part, and are then secured in position, Fig. 2 (not shown). The button 68 is now released. It is rocked clockwise by its biasing spring 70 whereupon the levers 43, 44 rock to release the rectangular assemblies. The indicator, see below, is now zeroized by adjustment of a trimmer condenser Ca. The gauge may now be used for checking-test pieces or may be used to indicate continuously deviations of test pieces from the standard internal diameter. Normally energized electromagnets 38, 42 may be released to hold a reading by locking the rectangular assemblies. This is effected by clamping strip elements 24, 28 of the movable assemblies between rigid frame plates 25, 29 and springs 26, 30 moved to clamping position by biasing springs 36 acting on the armatures 33, 41. Earthed screening plates 14, 15 for the measuring capacitor'are fixed to the respective rectangular assemblies and the instrument may be sealed in a casing 7 through which the bars 1, 2 protrude. Measuring circuit. Fig. 4. The output of an oscillator V1 stabilized in amplitude by a valve V6, is fed over a coaxial cable to a potential divider consisting of a preset condenser Cp, the measuring condenser Cm and the trimmer Ca. The voltages across Cp and the series combination Cm-Ca, respectively, are rectified in opposed diodes V2, V3 to produce a D.C. output proportional to the difference between those two voltages. This difference is zero when the trimmer Ca is adjusted to zeroize the indicator S. Any D.C. output produced by deviation of a measured part from standard is fed over the coaxial cable to the grid of a cathode follower V8 forming with V9 the first stage of a balanced amplifier. The outputs of V8, V9 are fed respectively to triodes V5, V4, the ammeter indicator S being bridged across the anodes and protected against overload by a tube V7. The tubes V9, V4  compensate for mains variations. A switch Sr normally closed during a test is linked with the work so as to be opened when the feelers lose contact with the work. This introduces a coupling Rr-Cr between the stages V8, V5 having a large time constant so that the meter reading remains steady for short breaks in contact of the feelers.