RECORDING LOW AND HIGH-FREQUENCY VIBRATIONS
Reproduced from a press release of Interkama International Congress to be held at Dusseldorf from 13th to 14th October 1965 - The Certificated Engineer April 1965.
Fifty years ago the electrical determination of energy fluctuations presented considerable difficulties. The instruments then available enabled an electrical value to be read in the form of indicator deflection, and the near-constant measurement could be plotted as a function of time by making frequent readings. Recording instruments featuring an ink-filled writing device which moved on a chart mounted on a rotating drum, permitted the automatic recording of slowly varying quantities - in a manner still employed for some procedures. Because of the great mechanical inertia of their mechanisms, the instruments could not, however, cope with rapidly varying quantities, ranging from a few vibrations a second to the technical alternating-current and audio frequencies up to the ultra frequency field.
To this end measuring elements were required whose moving parts entailed little or no friction, and a writing device capable of recording instantaneous values of rapidly varying quantities as a function of time or another electrical or mechanical quantity. Instruments having a moving system capable of extremely rapid vibration and of recording the vibrations are known as oscillographs, a word derived from Latin and Greek. They produce a graphic record of waveforms, illustrating the sequence of a test.
As many forms of energy can be comparatively easily converted into electrical voltages or currents, the oscillograph has much application in mechanics, acoustics, and optics, for analysing everything from engineering processes to musical sound. Oscillographs are the sole means of recording the manifold forms of mechanical vibration connected with unbalance and running truth, rotation, torsion and deflection, the loading and braking of mechanical systems, etc. They are also employed for the strain measurement of structural members and for recording the working cycles of automobile and aircraft engines. Increased knowledge is thereby obtained for the benefit of designers and operators.
It goes without saying that much the same applies to most of the electrical plant and equipment where applications in the audio, radio, and super-high fields are so numerous that they defy description. Oscillo-graphs have become indispensable for a multitude of purposes, from the measurement of all kinds of frequency curves, plotting valve characteristics, testing resonant circuits, transformers, filters, and their networks in radio and television research, to measuring duties in advanced pulse techniques.
Not least, they play a significant role in acoustics for the analysis of voice and speech and of sound curves, for measuring the acoustic properties of halls and theatres; in medicine, they permit the graphic representation of the action of the heart (electric cardiogram ), the cerebral circulation system and other variable processes in the human body and in animals; in geology, they enable cavities, ore deposits and rock masses of all kinds to be examined; in astronomy, they assist in the investigation of cosmic noise, the determination of the velocity of meteors and other celestial bodies; in meteorology, they are used to record earthquakes, storms, and atmospheric disturbances.
One is therefore justified in saying that oscillographs are firmly established as universal measuring instruments and that they have made an important contribution to research and development in widely varying fields. They are thus part and parcel of modern instrument engineering. As such they will, understandably, play a significant role at the Interkama, 1965- the International Congress and Exhibition for Instrumentation and Automation, to be held at Dusseldorf from 13th to 19th October. The user will be able to inspect instruments at present available and inform himself on manufacturers' advancements and refinements.
The individual types of oscillograph are classified mainly according to the manner of producing the trace. The frequency ranges of the three groups overlap to a certain extent. First, there is the oscillograph employing a capillary pen, for recording vibrational frequencies up to about 1 000 cis. Its measuring element features a single measuring loop tensioned between two permanent magnets and mounting a fine, flexible 'glass' capillary tube, which follows the movements of the loop. At the end of the tube is a nozzle having a 0.001 mm diameter outlet, through which a fine spray of coloured liquid is transferred to a moving chart. An oscillogram ready for direct evaluation is thereby produced. Recently, moving magnet elements have been incorporated; in this case, the capillary tube is adhesive-bonded to a cross-magnetized permanent magnet which is movably mounted in the field of two coils through which the measuring current is conducted.
Light-beam oscillographs can be employed for frequencies up to 15 000 cis, as their moving elements are of the loop or coil type, which are much smaller and have less inertia than the instruments employing a capillary pen. Instead of the capillary tube, the loop-type oscillograph incorporates a tiny mirror that reflects a light beam, and after passing through multiple optical systems it produces a trace on a moving photographic paper chart. With the moving-coil type instruments, the moving element comprises a coil suspended between tension strips and a mirror fixed above. The film of the trace has to be developed in a dark room or in a special automatic high-speed developer. Nearly all instruments can be provided with direct recording with ultra-violet light on special sensitive papers.
The big advantage of the capillary-pen and light-beam oscillographs is the compactness of their moving elements, several of which can be installed in a comparatively small housing to permit a plurality of traces to be simultaneously produced. Instruments currently available range from the portable versions having two or three channels to the large universal pen recorders with up to sixteen galvanometers, and light-beam types featuring up to fifty moving-coil elements. Some of the instruments are designed on the building block principle, which case they can be adapted or extended to suit specific requirements.
The cathode-ray oscillograph must be employed for the very high-frequency range. It consists essentially of a Braun tube, named after its inventor, in which is produced a beam of electrons. High voltage cold cathode tubes are used mainly in research and low voltage hot cathode tubes are now employed in many industrial applications. The electrons leaving the cathode are accelerated to the anode and two sets of plates mutually at right angles are used to deflect the beam which ultimately strikes a fluorescent screen and produces a spot of light the traverse of which produces a trace of varying intensity. The beam may also be magnetically deflected by coils suitably placed and supplied with current. To obtain a permanent record, the display must be photographed by a corresponding device. Cathode-ray oscillographs are available in many forms, ranging from medium-duty battery-operated portables to universal high-sensitivity models of exceptional bandwidth incorporating features for utmost operational convenience.