An Accurate Method to Determine the Muzzle Leaving Time of Guns

Transcription

1 Sensors & Transdcers 4 by IFSA Pblishing, S. L. An Accrate Method to Determine the Mzzle Leaving Time of Gns H. X. Chao, M. Go, H. S. Hang, X. Y. Gao, S. L. Li, W. B. D Northwest Institte of Mechanical and Electrical Engineering, 799, Xianyang, Shanxi, P. R. China Information Center of China North Indstries Grop Corporation, 89, Beijing, China gqpianpian@6.com Received: 8 Agst 4 /Accepted: October 4 /Pblished: December 4 Abstract: This paper states the importance of determining the mzzle leaving time of gns with a high degree of accracy. Two commonly sed methods are introdced, which are the high speed photography method and photoelectric transdcer method, and the advantage and disadvantage of these two methods are analyzed. Frthermore, a new method to determine the mzzle leaving time of gns based on the combination of high speed photography and synchronized trigger technology is present in this paper, and its principle and ncertainty of measrement are evalated. The firing experiments shows that the present method has distingish advantage in accracy and reliability from other methods. Copyright 4 IFSA Pblishing, S. L. Keywords: Mzzle leaving time, High speed photography, Photoelectric, Synchronos trigger.. Introdction The conception of mzzle leaving time is defined as the time when the projectile leaves mzzle of a firing gn. This parameter plays an important role in many research fields, sch as gn s firing accracy analysis, gn s dynamics analysis and gn s test and measrements. When stdying the factors that inflence the gn s firing accracy, sch as the initial distrbance of the mzzle and the projectile s in bore movement rle, the mzzle leaving time is needed. When analyzing the strctre response of gns, dynamics, the mzzle leaving time has essential effect on gn s vibrate dynamic modle [-4]. Besides, in gn s firing test, many parameters, sch as the shock wave of the blast, the displacement and the velocity of gn s organ, need the nified benchmark to record the time process. Therefore, acqiring the precise vale of mzzle leaving time has important giding significance and great engineering application vale.. The Commonly Used Methods of Mzzle Leaving Time Several methods are commonly sed in the gn s research field all over the world, sch as the mzzle cord on-off method, photoelectric transdcer method, and the high speed photography method [5]. The mzzle cord on-off method is niversal and primeval, in which a cord of copper line is bond at the center of end mzzle srface of a gn, and DC crrent flows throgh the cord. When the gn fires, the cord is broken by the projectile s head, the DC crrent is ct to zero and a sdden voltage altar may occr in the circit, ths voltage altar indicating the mzzle leaving time. The trait of this method lies in its 4

2 simplicity and convenience, bt it is vlnerable to the leakage gas of the mzzle, and t in many cases, the cord is broken by the gas before the projectile reaches the mzzle, so the se rate of this method is decreasing. Another adopted method in abroad is mzzle pressre method. In this method, by clamping a pressre probe to the gn tbe approximately 5 mm from the first mzzle brake exit vent [6], the pressre signal is provided to determine the mzzle leaving time. This method reqires excellent shock wave protection which is hard to be flly meet. Crrently, the photoelectric method and high speed photography method are two mainly sed methods... Photoelectric Transdcer Method The principle of photoelectric transdcer method is based on photoelectric transformation. The photoelectric transdcer is composed of lens, photosensitive nit, and signal processing circit. Its principle diagram is shown in Fig.. When measring, the transdcer is located apart from the gn and directly facing the mzzle, the image of the mzzle is clearly focsed on the photosensitive nit by adjsting the focal of lens. When the gn fires and the projectile leaves the mzzle, the fire light or the heavy smoke may case the changes in the nmber of the photoelectron on the photosensitive nit, hence prodcing a voltage signal to indicate the mzzle leaving time. Fig.. The mzzle photoelectric signal obtained by the photoelectric transdcer. However, the method also has its inherent falt. Since the occrring time of mzzle firing flame is different depend on the type of gn, the featre point of mzzle leaving time is different. For example, some mm gn s firing flame occrs after the projectile flies apart from the mzzle, as shown in Fig., and some 55 mm gn s firing flame occrs when half of the projectile is still in bore, as shown in Fig. 4. (a) The mzzle smoke Fig.. The principle sketch of electric transdcer method. - gn tbe, - fire or smoke, projectile, 4 lens, 5 - observation window, 6 - photosensitive nit. (b) The end of the projectile leaving the mzzle The trait of this method lies in its simplicity in operation. Since either firing flame or mzzle smoke can case obvios signal, the sdden change of the signal cased by the firing flame is sed to determine the mzzle leaving time. The mzzle photoelectric signal obtained by the photoelectric transdcer is shown in Fig., in which the point A indicates the mzzle leaving time. (c) The mzzle firing flame Fig.. The process of projectile passes throgh a mm gn mzzle. 4

3 (a) The projectile goes ot the mzzle In the test, the high-speed photography is placed in the right place on the side of the gn, and its lens focs on the mzzle. A switch for the trigger of the high speed photography is connected to the data recording machine. Dring the firing, the switch triggers the high speed photography and the switch signal is transferred to the data recording machine at the same time. By sbseqent image processing, the passing frame nmber between the trigger time and the mzzle leaving time is nmbered as s, sppose the frame sample rate is f, ths the interval between the switch time and the mzzle leaving time is determined as t ( s )/ f. The sketch is shown in Fig. 6. (b) The mzzle firing flame Fig. 4. The process of projectile passes throgh a 55 mm gn mzzle. Besides, the rising edge of the signal of the transdcer is abot.5 ms, as shown in Fig., ths the specific location of mzzle leaving time on the crve is hard to accrately determine. In this method, the resoltion of mzzle leaving time is in ms magnitde and it makes little sense... High Speed Photography Method As the sampling rate of the high speed photography increases, a method based on high speed photography it is widely sed to determine mzzle leaving time in recent years. The principle of this method is shown in Fig. 5. Fig. 6. The sketch of the relationship between the mzzle leaving time and the trigger time. The precision of the high speed photography method are sbject to two main factors. One is the frame sample rate; another is the time resoltion of the trigger switch. Under the field condition, the maxim frame sample rate can be set at thosand frames per second, that is, the time resoltion can be controlled within. ms, which can meet the need of measrement accracy. However, the precision of trigger switch is difficlt to be conveniently controlled. Since the sal trigger switch is designed to switch between on and off state by a mechanism device, the corresponding signal is not a clean step signal, which sally has intermittent p and down edges. As shown in Fig. 7, the twitters can last more than ms, ths the mzzle leaving time is hard to determine. Even thogh some newly developed high speed photography can send its reglated trigger plse when being triggered, this trigger plse also has some twitters nder the field condition, so frther efforts is needed to improve the time precision. 5 4 Fig. 5. The sketch of high speed photography method. 6.8ms Fig. 7. The trigger signal of on-off switch. Time [s] 44

4 . A New Method to Determine the Mzzle Leaving Time of Gns Based on the photoelectric transdcer and high speed photography, and with the application of synchronized trigger technology, an accrate method to determine the mzzle leaving time of gns is present in the paper. The prime instrments for the new method inclde the photoelectric transdcer, the high speed photography, and a synchronos device. The synchronos device is developed by the project grop in which the athor of this paper is involved... The Synchronos Device In order to precisely acqire the mzzle leaving time of gns, a synchronos device is designed to receive the photoelectric signal which is transformed by photoelectric transdcer. In the gn test, a photoelectric transdcer is located apart from the gn and directly facing the mzzle and its otpt signal is transferred into the synchronos device. Before the firing, the light level of backgrond is trned into corresponding voltage level by the photoelectric transdcer, and is sent to the synchronos device. According to this signal level, the synchronos device adjsts its trigger voltage level to keep ot signal stable. When the gn fires, the voltage level of the photoelectric transdcer srpass the trigger level of the synchronos device, and the comparator circit in the synchronos device exectes voltage reversal, ths triggering the high speed photography and sending a step signal to the data recording machine to realize the synchronizing... The Principle and the Implement of This New Method The principle of the present method is shown in Fig. 8. In the test, the photoelectric transdcer and the high speed photography are placed on each side of the gn, and both being aimed at the mzzle. When the gn fires, the photoelectric transdcer exectes the mzzle flame light-to-electricity conversion. Meanwhile, the video movement of the mzzle is recorded by the high speed photography. The synchronos device is pt in the nearby test room and is pre-adjsted by testers. When preparation is ready, and the all parameters are set, the high speed photography starts to circlate recording. When the gn fires, the synchronos device receives the signal sent by the electric transdcer, it atomatically trigger the high speed photography and simltaneosly sent the step signal to the data acqisition machine. The step signal is as the time benchmark for the other electrical measring signal. In the end, the mzzle leaving time is accrately determined by the process which is shown in Fig. 6. Fig. 9 shows a real step signal and a photoelectric signal in a field test. Fig. 8. The sketch of the new method. Fig. 9. The trigger signal and the photoelectric signal of a real gn fire test. The application of the new method avoids the reading error, which is cased by the twittering of the on-off switch, and ensres the precision of the time base. As the mzzle leaving time is close to the atomatically trigger time, the nmber of the intercept frames can be directly set and fixed, ths the workload for artificial interpretation is alleviated and the working efficiency is increased. Owing to the precise location and good consistence, the low-end high speed photography, which has small storage capacity, may woks at a rather high speed sample rate... Analysis on the Uncertainty of Mzzle Leaving Time The formla for the mzzle leaving time t is: t t t, () 45

5 where t is the trigger time, t is the interval between the switch time and the mzzle leaving time. These two parameters are weekly related, it can be considered that the correlation coefficient is zero, so the variance eqation is ( t ) ( t ) ( t ) () c The component of ncertainty and evalation is shown in Table. t ( ) t ( ) Table. Component of ncertainty and evalation. The sorces of ncertainty trigger time of synchronos device trigger response time of high speed photography resoltion of frame rate of photography niform niform K Parameters Component Distribtion Uncertainty (μs).58. normal.5 As the components in table are independent, the combined standard ncertainty is ( t ) ( ) c i.45 (μs), (4) i choosing the confidence level as 95 percent, the coverage factor k=, then the expended ncertainty of mzzle leaving time t is: U ( ) P t k ( ) c t =7 (μs) (5) 4. Conclsions Up to now, the new method which ses a photoelectric transdcer as trigger sorce to atomatically trigger high speed photography is the highest accrate measrement to determine the mzzle leaving time of gns. It is applicable to varios types of mzzle radis. Based on the recent experiment condcted by the projectile grop, the method is reliable and convenient. Meanwhile, the synchronos device can trigger several high speed photography simltaneosly, and with specifically designed photoelectric circit, it can sent step signal to gn s servo control system, ths a nified time base is constrcted for all the signals in the gn s test field. References []. Ma F-Qi, Chen Yn-Sheng, Do Ying-Xian, Gn and atomatic weapon, Beijing Institte of Technology Press, Beijing,, pp. -6. []. Wang Bao-Yan, Li Ki-W, Dynamics of atomatic weapon mechanism, National Defense Indstry Press, Beijing,, pp. -. []. W San-Ling, Wen Bo, Y Yong-Qiang, Test of gn dynamics, National Defense Indstry Press, Beijing, 4, pp [4]. Wang Bao-Yan, Shao Shiao-Jn, Smmarization of the measrement method for mzzle vibration responses, Jornal of Gn Lanch & Control, 9,,, pp. -6. [5]. Wang Bao-Yan, Chao Hong-Xiao, Measrement Methods for Mzzle-leaving Time of Projectile, Acta Armamentarii,, 6,, pp [6]. J. Garner, B. Gidos, B. Patton, Mzzle Motion Measrements for the M98 When Fired at and 45 Degrees, in Proceedings of the th U.S. Army Gn Dynamics Symposim, Astin, Texas, -6 April. 4 Copyright, International Freqency Sensor Association (IFSA) Pblishing, S. L. All rights reserved. ( 46