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Aviation History
1964
1964 - 0762.PDF
FLIGHT International, 19 March 1964 443 INDUSTRY International Flight Systems Products Company News Great Britain Sperry Digital Data System Full infor- mation has now been released by Sperry in London concerning the digital data acquis- ition system reported in Flight International for February 20. Development has reached the stage where Sperry are able to demon- ~ strate an exhibition model of the system in ?" bread-board form handling inputs from a Z. synchro, a thermocouple and a strain gauge, converting them to digital form, recording and playing back through tape and wire recorders, printing-out on an electric typewriter or presenting on a trace recorder. The equipment is offered for use • during flight testing as well as for accident I and maintenance recording. The data acquisition system has been matched to all the main types of transducer —E-type pick-off, potentiometer, synchro or synchrotel, thermocouple, resistance "f thermometers and strain gauge—and can be recorded on either tape or wire deck. The recording element is in fact a relatively minor portion of the system because digitized signals are not susceptible to degradation by recorder characteristics. Playback in digital or analogue form by various methods has also been demonstrated, and Sperry intend to offer a playback ser- vice for the benefit of the smaller operator, as well as equipment for "quick-look" in- spection. Sperry have adopted 16-bit words for each sample, including a word synchroniz- ation signal and a parity bit, to provide an accuracy of 0.1 per cent, sufficient to meet future accuracy requirements in accident and maintenance recording or flight-test work, and capable of absorbing the accuracy of the inputs themselves. Three thousand six hundred samples can be taken every 12sec. A 256-channel system could be accommodated in four boxes of approxi- mately short JATR size, or a 64-channel system in a single £ATR case. All elec- tronics are transistorized and arranged in printed-circuit modules plugged into racks within the cases. A six-channel system— MoA minimum requirement—with a cap- acity for 24 channels would cost about £1,800, the subsequent cost per channel decreasing in proportion as their number increased. Sperry are now ready to produce specific systems tailored to the requirements of individual operators. All basic develop- ment work is complete, as the exhibition layout amply demonstrates. Sperry decided on digital recording on the basis of their own experience with a.c. to d.c. conversion, electro-mechanical and solid- state converters in various data acquisition systems. Digital methods, they say, virtually eliminate signal loss and are insensitive to variations in recording speed, voltage levels and so on. A parity bit is included in each word as a basic check of correct signal, but the recording of binary numbers in the form of "on" or "off" signals makes loss or degradation of signals very unlikely. The inputs are digitized at a very early stage in the chain of events, so that the original accuracy is as far as possible preserved. Digital conversion is by passing the trans- ducer inputs into pulse duration converters —there are individual designs for each type of input—which transform the quantitative level into an equivalent duration of pulse. A crystal-controlled multi-vibrator, which controls the timing of all operations in the system, produces a high-frequency square wave which is gated during a single pulse so that the number of pulses counted is pro- portional to the length of the original pulse. The general layout is shown in the accom- panying diagram. The information in the counter is transferred to a register after each count and the states of the ten stages are recorded continuously on ten parallel tracks on tape. A parity bit is on an 11th track, word synchronization signal on a 12th and frame synchronization on a 13th. The Schematic layout of Sperry's digital acquisition system, described on this page K|L|MIN|O|P|R|S|T|U|V
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