Patent GB-762955-A (GB)
Title: Apparatus for investigating the natural frequency of vibration of parts
Inventor: HARRISON SAMUEL MARTIN
IPC: G01H13/00
Filed: 1953-07-29
Abstract: 762,955. Electrical indicating systems. NAPIER & SON, Ltd., D. Oct. 28, 1954 [July 29, 1953], No. 21097/53. Class 40 (1). Apparatus for measuring the natural frequency of vibration of a turbine blade or similar part comprises an electromagnetic exciter which when A.C. energized induces eddy currents in and so vibrates the turbine blade. The movement of the turbine blade affects an electrical pick-up which transmits signals to apparatus which indicates resonant vibration of the turbine blade. In the embodiment shown, Fig. 1, an electromagnetic exciter 3, energized by an oscillator-amplifier 1, 2, vibrates the blade 4. The latter may form one plate of a condenser whose other plate is a copper foil 12, Fig. 2, embedded in an insulating medium 13 upon the head of the exciter 3. This condenser 4/12 forms a resonant circuit in conjunction with a coil 14 and the latter is inductively coupled to a grid dip oscillator which together with the resonant circuit is denoted by the numeral 5 in Fig. 1. The X and Y plates of an oscilloscope 6 receive respectively the outputs of oscillator-amplifier 1, 2 and grid dip oscillator 5. The reference 10 represents a power pack. In use the grid dip oscillator 5 is tuned to a frequency which approaches the resonant frequency of the resonant circuit 4/12 and 14 while the blade 4 is not subject to resonant vibration. The blade 4 is then vibrated by the exciter 3 over a range of frequencies so that the resonant circuit 4/12 and 14 variably absorbs power from the grid dip oscillator 5 and the resonant frequencies of the blade 4 are observed by watching the oscilloscope 6 for the points where the amplitude of vibration of the blade 4 rises and reaches peaks in relation to the impulses supplied by the exciter 3. Measuring frequencies of the exciter 3 at which resonant vibration of the blade 4 is obtained.- The pulses from oscillator-amplifier 1, 2 feeding the exciter 3 are also fed to a vibrator operating a pulse counting mechanism 7 and the latter is switched on for an exact time interval by selected first and second triggering pulses from pulse unit 9 controlled by tuning fork 8. Modifications.-The resonant circuit may comprise a condenser plate carried by a separate support, Fig. 3 (not shown), or may comprise a pick-up coil 16a, Fig. 4, mounted on the exciter support 18 so as to be electrically affected by movement of the turbine blade 4. The coil 16a co-operates with a variable condenser 17 to form the resonant circuit. Also, if desired, the oscilloscope could be arranged to be influenced only by the changes in the power absorbed from the grid dip oscillator with changes in the frequency of the resonant circuit as the blade vibrates and such an arrangement would be similar to the disclosure in Specification 762,759. In a further modification a crystal pick-up responds to vibration of blade 4 and feeds voltage pulses to one set of plates of the C.R.O. whilst the pulses fed to the exciter 3 are applied to the other plates. 762,962. Magnetic sound recording and reproducing apparatus. KUHNKE, H. Oct. 23,1953, No. 29324/53. Class 40 (2). A magnetic recording apparatus is provided with one or more associated endless magnetic records to allow recorded passages to be repeated, e.g. to facilitate typing of recorded speech. The main record T<SP>1</SP> may have erase, recording, and reproducing heads L<SP>1</SP>, S<SP>1</SP>, H<SP>1</SP>. A transfer device U, which may comprise separate reproducing and recording heads or a direct magnetic coupling, transfers the signals without amplification from the record T<SP>1</SP> to the endless record T2 which is provided with erase and reproducing heads L<SP>2</SP>, H<SP>2</SP>. In a modification (Fig. 3, not shown) a single erase head and a single reproducing head are used jointly for each record. The speed of the record T<SP>2</SP> may be varied.