Abstract:
There is provided a system for forming an optical screen, including a continuous wave or pulsed laser transmitter for transmitting a beam of radiation at a predetermined wavelength and forming a planar or curved surface to be traversed by a moving object, at least one receiver including an array of detectors for receiving reflected or scattered beam radiation from the object and directing it towards the detectors for producing a signal, and a detection logic receiving the signal and determining parameters selected from the group of spatial position, velocity and direction of propulsion of them moving object. A method for detecting a moving object is also provided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to optical screen systems and methods for forming optical screens and for detecting and identifying objects traversing the screens. In addition, the present invention is concerned with systems and methods for detecting the position of an object passing through the optical screen. 
     BACKGROUND OF THE INVENTION 
     The exact positioning, time and velocity of an object, such as a projectile, relative to, or passing through, a real surface or an imaginary surface, such as an active optical screen, is important for determining the timing and flight trajectory of the object. Such screens have applications in the study of the dynamics of projectiles and in the protection of stationary or moving targets against projectiles sent toward the targets. 
     Some of the common methods for determining position, time and velocity are as follows:
         i) fast photography of the projectile, using two consequent exposures at a known time delay, and   ii) consumable screens which are torn by the projectile, namely, an electrical or optical conductor screen or screens placed in the trajectory of the projectile.       

     The first of the above methods requires a bully and expensive fast camera, whereas the second method is low priced, but requires replacement after every event and cannot perform as a permanent, multi-shot, measurement set up. The need for a re-usable, multi-shot, small and simple system calls for a novel system and method. 
     Optical or laser screens were proposed in the past. Generally, such systems fall into two categories of systems: a) active systems based on signal transmission towards an object and detection of signals reflected or scattered from the object, and b) passive systems that do not utilize energy transmission towards a target to be detected. A screen for traffic warning, according to their colour, is described in Japanese Patent Application No. 6,119,592. Similar kinds of screens proposed for use by pilots, such as landing strip guides, are described in DE Patent 19930096. U.S. Pat. No. 5,554,262 describes a system using a permanent screen, having a laser transmitter on one side and a laser receiver on the other, for maintaining the right position of paper edge in papermaking machines. The use of optical screens made of optical lines running back and forth between two mirrors is described in U.S. Pat. Nos. 4,097,800 and 6,259,365, where any interruption of the screen results in the same signal, providing no geometrical resolution. Temporally scanning the beam between various fixed mirrors, to enable some dimensional resolution is described in U.S. Pat. No. 4,855,608, wherein a polygon scanning device is operated, enabling the scan of very slow moving objects, depending on the scanning velocity. U.S. Pat. No. 4,185,192 discloses a passive optical system utilizing two detectors oriented in such a way that their optical axes and cone shaped fields-of-view intersect, thereby creating an overlapping region between these fields-of-view. By affecting this, only those signals from the detectors, which are received simultaneously, are thereby indicative of the fact the detected light comes from the overlapping volume. Another example of a passive optical system for determining the presence of an object by utilizing the forming of a scene at the intersection of two optical paths associated with two detectors, is disclosed in U.S. Pat. No. 4,317,992. U.S. Pat. No. 4,396,945 discloses a technique that can be utilized in either an active or passive system for determining the position of an object in space. This technique is based on the determination of the intersection of a line with a plane or with another line. U.S. Pat. No. 4,590,410 discloses a system, which is aimed at detecting small objects. In this case, multiple light emitters and multiple light detectors are utilized. According to the technique described in U.S. Pat. Nos. 4,724,480, a system is composed of at least one projector generating a non-planar light and at least one camera, oriented such that the optical axes of the camera and projector intersect. In this way, the projector associated with a first object and the region of intersection associated with a second object, can be aligned. U.S. Pat. No. 6,943,337 discloses a system, where a laser forms a screen like plane, and camera-like detectors projecting substantially perpendicular to the plane of the screen, are faced at a selected area of the screen, to detect scattered light from an object passing through the screen. This geometry provides no information on the object velocity or orientation. None of the above systems enable the detection of the required parameters for detecting fast moving objects such as projectiles. 
     An optical screen for detection of position, time of passage and velocity of an object, e.g., a projectile, through the optical or light screen, has to have the following properties:
         a) a capability of two and/or three-dimensional positioning;   b) a velocity determining capability to distinguish between slow and fast moving objections;   c) a multi-directional capability, or ability to measure the penetration angle;   d) not to be affected by sunlight or stray light, and lighted or shaded areas, and   e) able to detect objects above specified sized in diameter and/or length.       

     DISCLOSURE OF THE INVENTION 
     It is therefore a broad object of the present invention to provide a system and method for forming an optical screen and receiver for collecting and determining data concerning the presence of a moving object, or time of passage, or angle of crossing the screen, or velocity of moving objects, such as projectiles, through the optical or light screen. 
     It is a further object of the present invention to provide an optical screen and detector system, capable of operating outdoors while compensating for sunlight and reflected sunlight acting as a disturbing radiation source, having radiation or signal values similar to, or even larger than, the operating optical radiation or signals. 
     In accordance with the present invention there is therefore provided a system for forming an optical screen, comprising at least one continuous wave or pulsed laser transmitter for transmitting a beam of radiation at a predetermined wavelength and forming at least one planar or curved surface to be traversed by a moving object; at least one receiver including an array of detectors for receiving reflected or scattered beam radiation from said object and directing it towards at least one of said detectors for producing a signal, and a detection logic means receiving said signal and determining parameters selected from the group of spatial position, velocity and direction of propulsion of said moving object. 
     The invention further provides a method for detecting a moving object, comprising providing a system for forming an optical screen, comprising at least one continuous wave or pulsed laser transmitter for transmitting a beam of radiation at a predetermined wavelength and forming at least one planar or curved surface to be traversed by a moving object; at least one receiver including an array of detectors for receiving reflected or scattered beam radiation from said object and directing it towards at least one of said detectors for producing a signal, and a detection logic means receiving said signal and determining parameters selected from the group of spatial position, velocity and direction of propulsion of said moving object; transmitting at least one beam of radiation towards the estimated direction of movement of the object, to form a screen to be traversed by said object; detecting reflected/scattered radiation from said object and producing a signal of the detected radiation; feeding the signal to said logic means, and determining data relating to said object based on the detected signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures, so that it may be more fully understood. 
       With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
       In the drawings: 
         FIG. 1  is a schematic illustration of a laser transmitter, according to the invention; 
         FIGS. 2A to 2E  are schematic illustrations of various laser transmitters, according to the present invention; 
         FIG. 3  is a schematic illustration of a receiver, according to the present invention; 
         FIG. 4  is a schematic illustration of an optical screen system, according to the present invention; 
         FIG. 5  is a schematic illustration of a preferred embodiment of a system including one transmitter and two receivers; 
         FIGS. 6A and 6B  are schematic views of a transmitter and a receiver having a common fan-out point; 
         FIGS. 7A and 7B  are schematic views of the system according to the present invention, providing velocity detection capability, utilizing a single screen; 
         FIG. 8  is an experimental diagram of times of penetration of a projectile through an optical screen, according to the present invention, and 
         FIGS. 9A and 9B  are schematic views of a further embodiment according to the present invention; 
         FIGS. 10A to 10C  are schematic illustrations of one or more systems according the present invention, mounted in a cylindrical body. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates a laser transmitter  2 , spreading a laser beam into a contiguous surface  4  or periodically intermitted beams  6 , forming a screen  8 . The laser transmitter  2  uses an optical light spreading device, such as spherical and/or cylindrical and/or diffractive lenses, for transmitting a beam of radiation at a predetermined wavelength, to a predetermined location and direction, towards e.g., a passing projectile. The surface of the screen can be planar or curved in any other configuration. Examples of the various ways to spread or fan-out the laser beam for forming screens of various configurations sizes and colours, are shown in  FIGS. 2A to 2E . 
       FIG. 2A  is an example of a conical screen, e.g., required for protection of objects around their whole circumference, having a laser  2  emitting a CW or pulsed beam  10  that impinges on a conical mirror  12 , and spreads into a three-dimensional conical screen  14 .  FIG. 2B  is a two or three-dimensional screen, composed of discrete beams  16  formed by a laser  2  emitting a beam  10  and passing through a diffractive grating or other diffractive optics  18  spreading the single beam  10  into a plurality of beams  16 .  FIG. 2C  is a spatially, continuous screen  8  formed by a laser  2  emitting a beam  10  passing through a converging spherical or cylindrical lens  20 , spreading the light into plane screen  22 .  FIG. 2D  is a spatially continuous screen formed by a laser  2  emitting beam  10  passing through a diverging spherical or cylindrical lens  24 , spreading the light into plane screen  26 .  FIG. 2E  depicts a cylindrical screen, formed by a laser  2  emitting beam  10  passing through an inner conical reflector  28  and an outer conical reflector  30 , spreading the light into a cylindrical or conical screen  32 . The colour of the screen can also be determined by selecting the wavelength of the laser&#39;s beam. 
     Referring to  FIG. 3 , there is shown a schematic view of a receiver  34 , which includes an array of detectors  36  located at an image plane P, and a detection logic means  38 . Each detector  36  accepting radiation, e.g., in the form of a solid cone  40  via a converging optics, e.g., a lens  42 . The signal outputs for the array of detectors  36  are electrically transmitted to the detection logic means  38  to be further processed, as will be discussed hereinafter. The lens  42  can be suitably coated to form a filter, or preceded or followed by a colour filter (not shown), to reduce noise and receive selective laser colour. The various kinds of optics used for the transmitters  2  shown in  FIGS. 2A to 2E  can serve as the detector&#39;s optics as well. 
       FIG. 4  is a schematic view of a system consisting of a laser transmitter  2  and receiver  34 , showing a laser ray  44  impinging on a cylindrical object  46  penetrating the optical screen  8  and reflecting and/or scattering light  48  into the array of detectors  36 . The position of the object  46  is detected by a single detector on the array, covering the dotted solid cone  40 . 
       FIG. 5  schematically illustrates an embodiment utilizing a single transmitter  2  and two receivers  34 ,  34 ′. Here, when the laser beam impinges on an object in the area  50 , signals from reflected/scattered light will appear in one detector  36  on the right receiver  34 ′ and on detector  36  on the left receiver  34 , defining the position of the object in the screen plane  52 . 
       FIGS. 6A and 6B  show the schematic views of a receiver  34  and a transmitter  2  having a common fan-out point, and capable of locating the azimuthal position, determined by a lighted single detector. The radial distance to the object  46  can be estimated by the amplitude of the reflected signal, if the size and reflectivity of the penetrating object is known. As seen in  FIG. 6A , the transmitted beam  34  passes through a mirror  54  and a beam splitter  56 , and the reflected beam passes through the beam splitter  56  and lens  42 , to be received by a receiver  34  and a detector  36 . 
     A modification of the embodiment of  FIGS. 6A and 6B  is shown in  FIGS. 7A and 7B , providing velocity detection capability of a moving object, utilizing a single detection logic, where the signal  58 , reflected/scattered from the object  46 , moving in the direction of arrow A, is measured temporally. When the length of the object  46  is known or estimated, the velocity of the moving object is the length thereof divided by the time elapsed from the penetration to, and exit of the object  46  from, the screen  8 . With such a system, it is possible to calculate various parameters and obtain on-time data concerning a moving object. For example, for velocity detection of a moving object, by using a single optical screen, the velocity is calculated by the detected time lag between the input signal at the start of the penetration of the object, and the last signal at its exit time from the screen. Knowing or estimating the projectile length, the velocity equals the length of object/time lag between the two signals. 
     The actual measurement of this process is illustrated in  FIG. 8 . Similar single or multi-array systems, having many fan-out points, can perform in the same way. Seen is an experimental result of penetration time signals of a cylindrical object having a length of 20 cm, into an optical planar screen, according to the present invention. The time lag from the beginning to the end of penetration is about 2 ms, counting from zero to 2 ms. The laser is a 808 nm wavelength, 250 mW power, CW diode unit, followed by a cylindrical lens, φ=7 mm, f=10 mm, made of BK-7 glass, and the detector is a large radiant area, high speed, high sensitivity Silicon PIN photodiode, preceded by a plano-convex, φ=25 mm, f=25 mm, AR coated lens. The geometry of the system is like the one described with reference to  FIG. 3 . It is clearly seen that the velocity of the projectile is:
 
velocity=length of projectile/time lag=20 cm/2 ms=100 m/s.
 
       FIGS. 9A and 9B  are schematic views of the two systems of  FIGS. 7A and 7B , according to the present invention, providing velocity detection capability utilizing a detection logic, where the signal  58 , reflected/scattered from an object  46  moving in a direction of arrow A, is measured temporally, once traversing a first screen  8  and then traversing a second screen  8 ′. The distance between the screens  8 ,  8 ′ is known, and the velocity of the object  46  is therefore its length divided by the time elapsed from the penetration of the object  46  through the first screen to the second screen. The spatial disposition of the object on the first screen  8  and the spatial disposition of the object of the second screen  8 ′, provides information with regard to the angular or trajectory direction. 
     When two spaced-apart screens are formed, it is possible to calculate the velocity of the moving object, using the time lag between the signals obtained by the object traversing each screen and knowing the distance between the two screens. 
     For three-dimensional detection of a moving body, there are formed at least two screens and the inclination of the object is determined by the relative position that the object traverses each screen and by knowing the distance between the screens. 
     Referring to  FIGS. 10A to 10C  there is seen a transmitter  2  and a receiver  34  mounted in a non-shielding, transparent, cylindrical body  60 . The beam transmitted by a single transmitter  2  forms a screen  8 , extending perpendicular to the axis of the cylinder and having a coverage angle, as shown in this Figure. A plurality of transmitters  2  and receivers  34  (not shown) can cover the entire circumference of the cylindrical body  60 , namely, covering a 360° angle, as shown in  FIG. 10B . This can be effected by using e.g., four or six, or any number of transmitters and receivers mounted in cylinder  60 . The double-hatched area  62  shown in  FIG. 10A , corresponds to the area covered by a single transmitter and receiver. 
     The method according to the present invention also facilitates mounting at least one optical screen-producing system in a cylindrical body  60 , or similar non-shielding, transparent body, and launching it towards a moving object. When the body is in proximity to the object, there is formed at least one screen in the direction of movement of the object and the reflected or scattered radiation is detected by the receiver as described hereinbefore, for determining data concerning the moving object. 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrated embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.