Patent GB-709312-A (GB)
Title: Improvements relating to apparatus for measuring the amplitude of a mechanical vibration
Inventor: MARSH CHARLES DONALD
IPC: G01H1/00
Filed: 1952-01-12
Abstract: 709,312. Electric indicating systems. FERRANTI, Ltd. Jan. 12, 1953 [Jan. 12, 1952], No. 991/52. Class 40 (1). Apparatus for determining the amplitude of vibration of a mechanical member comprises a capacitance variable with the position of the member which is included in the capacitative component of a bridged T network energised by a constant amplitude alternating voltage where frequency is high relative to the vibration frequency, to produce an output signal modulated by the vibration which is rectified and applied to an amplitude measuring device; and from which is derived a control signal corresponding to the mean value of the rectified voltage, operative to maintain constant the mean value of the vibration variable capacitance irrespective of the mean position of the vibrational member; means being provided to adjust the capacitance variations to lie within the linear portion of the network response curve. In Fig. 1 an earthed electrode 11 secured to a member 12 mechanically vibratable at e.g. 5000 c/s for testing, e.g. electronic valves or discharge tubes confronts a rigid insulated electrode 13 and the interelectrode capacitance 10 is connected in parallel with a variable capacitance 24 connected between earth and the junction of equal inductances 16, 17 whose free ends are bridged by a variable resistance 20, to form a bridged T network 15 energised by a generator 26 at a high frequency (e.g. 500 kc/s). The output of the network is transformer coupled to a pentode amplifier 29, which is transformer coupled to a full wave diode rectifier 35 whose demodulated signal passes through a low pass filter 36 and is measured by load resistor 37 and series micro-ammeter 38. The negative going unidirectional component at the live end 39 of the load resistor is combined with a stabilised positive potential of 150 volts on potentiometer 40 whose slider 44 is connected to the grid of a cathode follower pentode 45 the cathode load 46 of which is returned to a stabilised negative potential of -70v, so that the mean value of the cathode output voltage applied over load 50 to an oscillograph (not shown) determining the vibrational amplitude is variable between +150 and -60v. Microphonic vibrations of the electron tube under test may be displayed on the same or an adjacent oscilloscope. The cathode voltage is integrated by a long time constant RC network 60, 61 and amplified by a power tetrode 63 to energise an electromagnet 64 whose flexibly mounted armature 65 carries the moving plate 67 of a capacitor 67, 68 whose capacitance is thus controlled by the D.C. component of the cathode voltage of pentode 45 and is connected in parallel with the vibrational capacitance 10 and varible capacitance 24, which is adjusted so that the total mean capacitance corresponds to the midpoint of the linear portion of the output voltage/shunt capacitance curve of the bridged T network Fig. 2 (not shown). The vibrational variations of the capacitance then extend over the linear portion of the curve, and variations in the mean position of the vibrating electrode 11 are compensated by corresponding variations of the capacitance 25 responsive to the integrated control signal. The oscilloscope may be calibrated for vibrational amplitude by successively imposing known displacements on the electrode 11 while the capacitance 25 is maintained constant; the trimmer 24 being readjusted after each displacement to restore the working point to the middle of the linear part of the response curve, as indicated by microammeter 38. Both electrodes of capacitor 10 may be fixed and the vibrations applied to vary the insertion of a dielectric plate therebetween, and the compensating capacitance 25 may be eliminated by arranging the electromagnet 64 to vary the position of a flexibly mounted plate of capacitor 10. An auxiliary control signal dependent on the modulation amplitude may be applied to stabilise the amplitude of the applied mechanical vibration, and where the unit under test is to be subjected to constant acceleration over a range of frequencies whereby amplitude varies inversely with frequency, this signal may be applied to adjust the vibrational capacitor so that its displacement sensitivity varies directly with frequency to stabilise the main control signal.