1、Electro-optical target system for position and speed measurementAbstractThis paper introduces an electro-optical target system(EOTS) covering the speed range from subsonic to supersonic. This microcomputer-based system has a novel structure and shows the capability of precisely detecting the positio
2、n as well as the velocity of small caliber projectiles in real time. A prototype EOTS whose target area is 1m2 has been constructed and tested. A speed accuracy of better than 0.3% was achieved. A position accuracy, mainly dependent on the spacing between photodiodes in EOTS, of better than 1mm on a
3、 target area of 1m2was also accomplished.Keywords: External ballistics, No contact measurement, Electro-optical techniques, Position measurement, Speed measurement1 IntroductionThe speed and position measurements of projectiles are two important items in ballistic research. To determine these parame
4、ters precisely one needs an accurate measuring system. A conventional method, namely the hanging up(and taking down) of target discsl, though accurate in position measuring, is time consuming. A shot-position indicator(SPI), described in Reference 2, can measure the position of a high speed projecti
5、le by acoustic measurement. However, the SPI does not provide the speed information; neither does the conventional method. Besides, the SPI is used within the limits of supersonic projectiles.To measure the speed and position of projectiles rapidly and simultaneously, different electro-optical based
6、 systems have been proposed 3-5. These systems have the ability to cover the speed range from subsonic to supersonic. One system, called the target measurement system(TMS)3, uses vertical and horizontal banks of light sources to form two perpendicular light grids that construct the target area. Anot
7、her system, called the electro-optical projectile analyzer4, uses the same principle as TMS, but simplifies light sources with fiber optics bundles and a single light source in each light grid. The other system, called the electronic yaw screen(EYS)5, uses a solid state laser that is collimated and
8、directed to a one-dimensional beam expander system to form a fan-shaped light screen. This light screen then is reflected by a mirror to construct a portion of the target area. The light screen is more precise than the light grid because there is no dead zone in the target area as with the light gri
9、d system.From the aspect of speed and position measurement, we take advantage of the above systems and propose a novel system; the electro-optical target system(EOTS)6. We use a cylindrical mirror that reflects the incident laser beam into a 90 fan-shaped light screen. Two such light screens constru
10、ct a two-dimensional positioning system. We even propose a bent cylindrical mirror to generate a 90 light screen with a few degrees extended in a direction normal to the light screen to reduce the sensitivity to vibrations.A prototype EOTS, whose target area is 1m2 and measured speed range is from 5
11、0m/s to 1200m/s, has been constructed and tested. A speed range of up to 5000m/s can also be expected according to the simulation results from the electronic circuit using PSpice7. Finally, a nine-point testing result from a 0.38in. pistol is shown in this paper. The result shows that the standard d
12、eviation of position accuracy is less than 1mm.2 Basic principle of EOTSFig. 1 shows the optical configuration of EOTS. A laser beam from a He-Ne laser is directed onto a cylindrical mirror. The reflected laser beams create a fan-shaped light screen and are directed onto photodiodes that are neatly
13、arranged into an L-shaped photodiode array. EOTS uses two laser sources, two cylindrical mirrors and two photodiode arrays, which are arranged on the opposite sides of the EOTS body to form two fan-shaped light screens. Each light screen is combined with its own signal processing circuit to construc
14、t an optical gate. Although there is a distance between the two parallel light screens, viewed from a distance point, these fan beams intersect in a region of space called the target area (Fig. 2). A projectile can be measured only if it travels through this target area.Fig. 1 Optical configuration
15、of EOTSFig. 3 shows the shot position of the projectile is calculated. The target area, for the convenience of illustration, is a square of dimension D on each side. The number of photodiodes on the L-shaped photodiode array is 2N. Each photodiode is numbered in order, as shown in the figure. For il
16、lustration clarity, only the photodiode array and the cylindrical mirror of the first optical gate are shown. The projectile is considered to be incident normally to the first and to the second optical gate in sequence. When the projectile blocks the light screens, the respective photodiodes will be
17、 activated by the disturbance. In the first optical gate, the laser beam from the cylindrical mirror to each photodiode makes a unique angle with the y-axis. This angle is measured counter-clockwise from the axis. The angle with respect to a photodiode, numbered n, can be calculated as (1)and (2)If
18、certain photodiodes, numbered from j to k, are activated by projectiles, then the shot-position angle 1, is given by (3)Fig. 2 Intersections of the two light screens in the target areaSimilarly, the shot-position angle of the second optical gate 2, measured clockwise from the minus y-axis, is decide
19、d. After the two angles have been measured, theFig. 3 Illustration of shot-position calculationshot position of the projectile is deduced in Cartesian coordinates as (4)and (5)If S is the distance between the two light screens, then the average speed v for the projectile passing through the distance
20、 S is given by (6)where T is the time interval for the projectile to pass through distance S.3 Configuration of EOTS3.1 Optical system of EOTSWe use a He-Ne laser directed onto a cylindrical mirror to create a light screen. The relation among the laser beam diameter d, the cylindrical mirror diamete
21、r w and the beam expanding angle is shown in Fig. 4. This relation can be calculated as (7)To create a light screen of which equals 90, the ratio of w to d is 2.8. Because the He-Ne laser beam has Gaussian distribution and each photodiode on the photodiode array has a different distance to the cylin
22、drical mirror, the received laser power at each photodiode is not constant. This will influence the speed accuracy of EOTS (see Fig. 6 and Section 4.1).3.2 Analogue circuitryEOTS has 2N analogue channels in each of its two optical gates. Every analogue channel has the same structure. Each analogue c
23、hannel contains a photodiode, a linear amplifier, a band-pass filter and a comparator. The linear amplifier amplifies the signal coming from the photodiode. The band-pass filter filters noises such asFig.4 Laser beam directs on a cylindrical mirrorbugs flying through the light screen and flicker of
24、other light sources nearby. The comparator compares the output V0 , coming from the filter with a threshold voltage VTH. If V0 is higher than VTH, then the comparator will activate a flip-flop (FF) to change the state.3.3 Digital circuitryFig.5 is the block diagram of the digital signal processing c
25、ircuit. Input coming from the analogue channel is fed to a relative FF. When the projectile blocks the light screen of the first optical gate, the state-changed FFs will make the output of the NAND gate U1 change state. The U1 locks all FFs of the first optical gate to protect genuine projectile dat
26、a from the influence of shock waves behind the projectile, and starts the counter U5 that operates at a clock frequency of 10MHz. As the projectile blocks the light screen of the second optical gate, the circuit of the second optical gate functions as the circuit of the first optical gate did, but s
27、tops the counter. Moreover, the NAND gate U2 passes an interrupt signal (INT) to the central processing unit (CPU) while U5 is being stopped. The CPU then recognizes the interrupt request, picks the projectile data up, and resets U5 and all FFs for the next shot, in sequence. In Fig. 5, the counter
28、relates the time interval T in eqn. 6. Besides, every photodiode is assigned a specific FF and every FF is given a relative address. Therefore, the CPU will be able to identify which photodiode generates the signal, to decide the impact position of eqns. 1-5, and to calculate the speed of the projec
29、tile.Fig. 5 Block diagram of digital signal processing circuit4 Accuracy of EOTS4.1 Accuracy of speed measurementThe accuracy of projectile velocity measurement with sky-screens has been deduced by Hartwig8 as (8)where parameters were the same as eqn. 6 used. v, S and T are values of maximum error i
30、n v, S and T, respectively. In EOTS, photodiodes are directed by nonuniform optical power, as described in Section 3.1, which implies that different analogue channels will have different response times, as though they are activated in the same way. Fig. 6 describes the typical input and output wavef
31、orms of an analogue channel when a projectile passes through the light screen. The dotted line is theFig. 6 Typical input and output waveform of analogue channelresponse of the weaker optical input with respect to the solid line. In this Figure, the optical power density directed onto the photodiode
32、 is considered to be constant along the x-axis. Referring to the solid line, the projectile touches the light screen at T1 and entirely blocks laser beams at T2; the activated photodiode current ID drops from IDH to IDL. The output voltage V0 of the analogue channel then rises to a saturation voltag
33、e Vsat. The counter is not triggered until V0 is larger than VTH. The interval from T1 to the time that V0 equals VTH is called the response time tr From Fig. 6, we can realise that a different input power variation with time will produce a different output response time tr. Therefore, the T of eqn.
34、 8 should include tr, for EOTS, where tr, is the worst-case difference, i.e., the largest tr of the first optical gate minus the smallest tr of the second optical gate. Table 1 lists the simulation results of tr of the analogue circuit with respect to different projectile velocities using PSpice.Tab
35、le 1 Simulation results of t, respect to projectile speedProjectile speedm/strs2002.1310000.4450000.084.2 Accuracy of position measurementConsidering an EOTS structure in Fig. 3, if a photodiode, numbered n, is activated by a projectile, the exact shot-position angle e, will be within the range (9)o
36、r (10)Referring to eqns. 1 and 2, eqns. 9 and 10 express that the worst-case deviation of e, is caused by half-photodiode-spacing shift of measuring ambiguity. Thus, the deviations of the y-axis and z-axis can be deduced as (11)and (12)Where 1 and 2 are of the first and the second optical gate, resp
37、ectively. (13) (14) (15)and (16)It is obvious from eqns 9-16 that N has to be increased as the position accuracy needs to be better for a same size of D.Fig. 2 shows the intersection of the two light screens in the target area where intervals between photodiodes are considered to be constant. As ind
38、icated in this Figure, different detector positions will produce different resolutions. Fig. 7 shows simulation results of the worst-case deviations on the y-axis (or z-axis). The shot-position angle 1, is fixed at 45 and the activated photodiode of the second optical gate varies from number 40 to 3
39、60 on an EOTS with D = 1000mm and 2N = 400. Fig. 7 shows deviations on the y-axis (or z-axis) of less than 1mm corresponding to certain photodiodes that are numbered approximately from 120 to 280. A pentagon-shaped region, which is shown in Fig. 2 and has an accuracy better than 1mm, also correspond
40、s to those photodiodes.Fig. 7 The worst-case deviation on y-axis and z-axis as 1 is fixed at 455 Experimental resultsA prototype EOTS was used in the experiments. The main specifications of the system are listed as follows: d = 0.81mm, w = 2.5mm, S = 635mm, D = 1000mm, 2N = 384 and laser output opti
41、cal power P = 7.5mW. We hung up a paper target behind EOTS for comparison. Fig. 8 shows the y and z coordinates of nine impact points from a 0.38in. pistol. The impact positions and the velocities were measured by EOTS. In Fig. 8, the two crosses at the bottom indicate the positions of cylindrical m
42、irrors. Table 2 compares the results generated by EOTS with the measurement results from the paper target. The standard deviation is less than 1mm.6 Discussion and conclusionsThis paper presents a novel electro-optical target system for small calibre projectiles. Position and speed data can be gener
43、ated instantaneously by the microcomputer-based control unit with the addition of appropriate software. The most accurate region of positioning, which is a pentagon-shaped area, distributes over the centre of the target area. The accuracy of position and speed measurement has been analyzed in this p
44、aper. To improve the speed accuracy, we should reduce the influence of the response time difference. To increase the distance between the two light screens, of course, is another method to improve the speed accuracy, but the position accuracy will become worse. To improve position accuracy, the phot
45、odiode array which has less space between two adjacent photodiodes is suggested.Fig. 8 The computer printout of EOTS, origin is shifted to centre of the target areaThe measured speed range of EOTS is from subsonic to supersonic. A speed accuracy of better than 0.3% is accomplished. With a different
46、design concept, EOTS need not synchronized with the firing signal as EYS. It is always ready for any advancing projectile as the power of EOTS has been turned on.Fig. 9 Laser beam directs on a bent cylindrical mirrorIf a slightly bent cylindrical mirror were used (Fig. 9), the light screen could ext
47、end a few degrees in the x-direction. This makes optics alignment easier and insensitive to vibrations. However, the surface quality of the cylindrical mirror is critical to the uniformity of the fan-shaped beam in the x-direction. The nonuniformity of the fan beam in the x-direction will enhance the sensitivity to vibrations.Table 2 List of results measured by EOTS and by artificialImpact pointsDistance measured by (mm)Impact pointsDistance measured by (mm)FromToEOTSA
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