the one microsecond signal from the oscillator. The counter, which operates at speeds up to one Ground Testing of million counts per second, incorporates six decade counting channels, the output being clearly dis played by six rows of ten lamps, switched by high-speed relays. The counter responds to each Airborne Equipment negative half-cycle of the oscillator signal, which it continues to count until a second pair of con tacts, this time incorporated in the test com High Acceleration Rate Test Facility for Small Components ponent itself, makes a circuit which 'closes' the electronic gate, so cutting off the oscillator input OME of the delicate components of airborne to the counter. The latter unit therefore displays equipment have to withstand very high exactly the number of cycles occurring during the accelerations during certain phases of flight. interval between the arrest of the test chassis and There are also g-operated devices which must the operation of the device mounted on it. Since react within a very brief interval when subjected the oscillator output is at one microsecond, this to g above a certain value. The testing of such count gives the time interval directly in micro components during development presents a diffi seconds. cult problem because of the extremely short periods during which the required conditions Accuracy obtain. A neat method of obtaining reliable Since the counter is a digital unit, no error is answers to such problems has been devised by introduced into the result of this end of the Wilkinson Sword Ltd., Southfield Road, London, W.4, in association with Graviner Manufacturing measuring chain. The accuracy is dependent only on the accuracy and stability of the oscillator, Co. Ltd. at their Colnbrook, Slough, Bucks, and on the precision with which the gating action factory, where development of temperature- and is carried out. The oscillator stability is closer acceleration-sensitive components for airborne than 1 part in 106, approaching 1 in 107 after equipment is carried out. settling down, the former figure corresponding to One such development task involved a com repeatability of the order of one microsecond per ponent which had to resist a certain fairly high second. The electronic switch did at first introduce level of g, yet complete an electrical circuit with a complication by its very speed of response. It out fail within an extremely small fraction of a will, in fact, operate to open and close the second of being subjected to a g-value exceeding measuring circuit on successive signals only a few the limit level. The brevity of the interval pre microseconds apart. Inconsistencies in initial test cluded the use of conventional inertial methods results were found to be due to the fact that the of g-measurement, and at the same time presented contacts on the test apparatus, on closing the great difficulties in the measurement of the vital primary circuit, produced a damped train of time interval itself. After careful experiment a high-frequency oscillations, which is a common technique was devised which made use of an feature of many types of switch. These oscillations electronic interval-measuring equipment of high confused the electronic switch, which produced accuracy, with modifications to the circuit to corresponding gating signals to the counter in make it receptive to the abrupt signal from the quick succession, thus creating irregularities in component under test. the actual measured period. This difficulty was The required g-forces were developed by drop overcome by inserting a simple cold cathode question. The chassis is decelerated at the end of ping the device from the 40-ft. high tower shown gating circuit before the electronic switch. Since the fall by the steel bar projecting downwards, in FIG. 1, which was a modification of a flood the switch need not respond to signals in rapid so that it penetrates some way into the cylindrical lighting column. The component under test is succession, it could be made to open on receipt lead block beneath it. The overall deceleration mounted on a special chassis, as shown in FIG. 2, of the first pulse from the contacts, and to remain rate of the complete chassis is interpreted em which is raised on steel cables and can be dropped open until the 'close' signal was received a con pirically, but accurately, from the height of fall in free fall from any distance up to the full siderable fraction of a second later. This modifica and the depth of penetration of the steel rod into height of the tower. The device shown in the tion completely eliminated the difficulty, and the the lead. Measuring the speed of actuation of the picture is a simple inertia switch which, for the equipment was proved to respond to artificially test specimen when subjected to the required g sake of illustration, replaces the component in created intervals of as little as two microseconds, is the more difficult part of the operation. read off the counter simply as '2'. Electronic Timer Fire Extinguisher Development The time interval involved is too brief to use In another aircraft industry application, the any conventional form of duration measurement. equipment is used to determine the minimum Instead, advantage is taken of the fact that it is voltage required to set off automatic fire extin in practice relatively easy, using modern crystal- guishers, which use an extinguishing fluid con controlled oscillators, to control the frequency tained in vessels pressurized by nitrogen at 250 lb./ of an alternating current within very fine limits. sq. in., and released by an electrically-fired ex If the test device is used to switch the output of plosive charge. such an oscillator on and off, at the beginning and end of the delay period, a count of the number of cycles of accurately known frequency occurring within the period will give an accurate measure of the time interval. The measurement is carried out automatically by the equipment shown in FIG. 3. Made by Airmec Ltd., High Wycombe, Bucks, it consists of a Type 213 standard oscilla tor (the upper unit at the right), of a Type 202 electronic switch (below it) which accurately 'gates' the oscillator in response to signals from the test component, and a Type 865 electronic counter (at the left) which counts the number of cycles of high-frequency current admitted to it by the gating switch. The equipment works in the following way. When the test chassis contacts the lead block, a contact mounted on the former completes a circuit which 'opens' the electronic gate. This unit, which responds to a wide range of input signal values with a pulse width down to one-tenth microsecond, produces an output pulse which starts the electronic counter, to which is also applied July 1960 205
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jul 1, 1960
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