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VS300 PULSE TRANSMITTER
The VS 300 is a pulse transmitter which converts the angular rotation of its input shaft into two separate electrical pulse trains, phase shifted by 90°. It is specifically designed for positive displacement volume flow metering applications in the oil and petrochemicals industry and is certified for use in hazardous areas. The VS 300 finds use in control systems applications as a general purpose, robust, industrial pulse transmitter.
The VS 300 pulser uses a chemically etched stainless steel disc with 100 radial slots to interrupt the infra-red emission from two solid state light sources. The resulting modulation is detected by two silicon photo-diodes whose electrical outputs are amplified, squared and then used to switch a pair of differential line drivers. The phase displacement between the two pulse streams is trimmed to 90°, +/- 10°.
To reduce the effects of P.D. meter chatter and spurious pulses generated by small angular oscillations of the input shaft when it is basically stationary, the pulser has a built in peg and slot mechanism designed to introduce 5° of lost motion. The rotation of the disc and its support are also lightly damped by the bearing friction and lubricant viscosity.
Heat generated by the internal voltage regulator is dissipated in the optical components to combat condensation and icing. The standard VS 300 supplied at 12V can operated down to -20°C.
The maximum input shaft speed is 1200 r.p.m. This yields two 90° separated pulse streams at 2kHz. The 20mA source/sink capability of the TTL differential drivers allows a typical twisted pair telephone cable up to two kilometres long to be satisfactorily driven.
Connections are made to the VS 300 by a factory-fitted length of flame-proof six core screened cable. A variety of adaptor kits is available for mechanically interfacing with all popular P.D. meter flanges and couplings.
Electrical Load
The VS 300 provides two separate differential electrical output pulses. The first pulse stream consists of two electrical signals which are relatively inverted i.e A and A bar. The second pulse stream is shifted 90° relative to the A stream and also consist of two electrical signals which are reatively inverted i.e. B and B bar.
Each of these four pulse streams is a typical TTL level relative to the 0V common (the black wire). The design intends the user to employ all four signals and to draw a differential load from each pair of about 10mA, typically into a pair of back-to-back opto couplers.
The consequence of this is that at all times there will be a total load current of at least 20mA (10mA from A-A bar and 10mA B-B bar). This current is then used by the internal regulator, which is thermally coupled to the optics, to raise the internal temperature sufficiently to combat the formation of ice and condensation down to -20°C. Failure to comply with this load current requirement may jeopardise the performance at low temperatures.
The required load current may be accomplished by a resistance of 270 ohms between A and A bar and 270 ohms between B and B bar.
It is possible to use the VS 300 as a TTL voltage source by using any four output signals as TTL levels relative to the 0V supply, but the recommended 270 ohm resistors must be fitted.
Technical Notes
The VS 300 is designed to be driven by applying a rotary torque to the input shaft by means of the roll pin. Side thrusts and end thrusts are reduced by this method. This method of drive is necessary because the input shaft's outer bearing is part of the safety flame-trap and does not guarentee satisfactory alignment with gears etc.. It is not recommended that gears or other devices should be attached to the input shaft because there would be no definite control of the various thrusts and tolerances.