1、 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 parameters precisely one needs an accurate measuring sys
2、tem. 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 projectile by acoustic measurement. However, the SPI does
3、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 systems have been proposed 3-5. These systems hav
4、e 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. Another system, called the electro-optical projectile
5、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 directed to a one-dimensional beam expander system
6、 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 grid system.From the aspect of speed and position mea
7、surement, 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 construct a two-dimensional positioning system. We even p
8、ropose 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 50m/s to 1200m/s, has been constructed and tested.
9、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 deviation of position accuracy is less than 1mm.2 B
10、asic 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 arranged into an L-shaped photodiode array. EOTS u
11、ses 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 construct an optical gate. Although there is a distance be
12、tween 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 of EOTSFig. 3 shows the shot position of the proje
13、ctile 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 illustration clarity, only the photodiode array and
14、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 activated by the disturbance. In the first optica
15、l 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 certain photodiodes, numbered from j to k, are act
16、ivated 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 decided. After the two angles have been measured, theFig
17、. 3 Illustration of shot-position calculationshot position of the projectile is deduced in Cartesian coordinates as （4） （5）If S is the distance between the two light screens, then the average speed v for the projectile passing through the distance S is given by （6）where T is the time interval for th
18、e 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 diameter w and the beam expanding angle is shown in Fig. 4.
19、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 cylindrical mirror, the received laser power at each photo
20、diode 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 channel contains a photodiode, a linear amplifier, a b
21、and-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 other light sources nearby. The comparator compares t
22、he 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 circuit. Input coming from the analogue channel is fed
23、 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 data from the influence of shock waves behind the projec
24、tile, 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 stops the counter. Moreover, the NAND gate U2 passes a
25、n 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 relates the time interval T in eqn. 6. Besides, every
26、 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 projectile.Fig. 5 Block diagram of digital signal processin
27、g 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 in v, S and T, respectively. In EOTS, photodiodes are
28、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 waveforms of an analogue channel when a projectile passes
29、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 is considered to be constant along the x-axis. Refer
30、ring 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 voltage 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 v
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