Sect. I Page 3 rotating equipment from the action of clutches, brakes, o r other mechanical components. F o r this application, connect the output of the tachometer generator to the vertical deflection plates of an oscilloscope. Since the data presented on the oscilloscope screen is usually nonrepetitive in nature, a photographic record is normally made. Torsional vibration, harmonic-ringing and the action of intermittent motions a r e shown as a function of time by variations in the height of the oscilloscope trace.
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is applied to the reading obtained on the Model 500B.
1-6
PHOTOELECTRIC TACHOMETRY
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Photoelectric tachometry pickups have three particular advantages: they a r e effective over a wide range of speeds; they a r e easily adaptable to a wide range of situations; they do not load a system under measurement. The Model 500B is designed for use with photoelectric transducers and a special connector (PHOTOTUBE), located on the front panel of the instrument, supplies the necessary bias voltage to a photocell of type 1P41 o r equal. This jack serves also as the signal input jack for this application. The Model 506A Tachometer provides for counting speeds o r revolutions over a wide range from about 300 RPM (5 RPS) to 300,000 RPM (5,000 RPS). The light source in the Model 506A Tachometer Pickup illuminates a moving part which is prepared with alternate reflecting and absorbing surfaces. The interrupted reflected light is picked up by the phototube and the electrical impulses generated a r e transmitted to the 500B. This system is positive in action and the danger of fractional o r multiple e r r o r s inherent in other measuring methods is eliminated. F o r best results, the size of the reflecting and absorbing surfaces should be approximately 3/4 in. square. This means that the shaft whose speed is to be measured should have a diameter of at least 1/2 inch. The speeds of smaller diameter shafts may be measured by installing a sleeve of larger diameter, o r by providing a rotating, reflecting, and absorbing surface at right angles to the plane of the shaft. Surfaces such as these a r e also used for increasing resolution in measurement of low RPM where the multiple absorbing and reflecting surfaces provide a large number of impulses per revolution. When this is done, a divisionfactor
Figure 1-3.
-hp- Model 506A Optical Tachometer Pickup
The -hp- Model 506A consists of a pair of shielded tubes, one of which contains an incandescent light source, while the other houses a Type 1P41 Phototube. These a r e equipped with condensing lenses and a r e so oriented that proper focus is obtained at a distance between 3 and 6 inches from the reflecting surface. The light source and phototube assembly is mounted on an adjustable stand for optimum positioning of both light source and phototube. The base of this stand contains a transformer which provides the proper voltage for operating the incandescent lamp. The phototube requires a bias voltage from +70 to +90 volts. This voltage is automatically supplied by the PHOTOTUBE jack on the -hp- Model 500B.
