Patent Publication Number: US-2010108800-A1

Title: Object detection system having an image detection system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority, under 35 U.S.C. §119, of German patent application DE 10 2008 046 362.0, filed Sep. 9, 2008; the prior application is herewith incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an object detection system having an image detection system with an imaging detector, a position detection system having a position detector and optics which guide incident radiation onto both detectors. 
     Various optical systems for the detection of a target and for keeping it in view are known for guiding unmanned missiles in the direction of a target. In the case of a passive guidance system, the target can be selected by an operator before the missile is launched, and a reference image of the target scene, with the marked target, can be passed to the missile. During target approach, the target is detected by the missile on the basis of the reference image, which is highly up-to-date because its age is only a few seconds, and the missile can steer itself autonomously to the target. 
     In the case of a semi-active laser guidance system, the target selected by an operator is illuminated with a marking laser, and a position detection system in the missile detects the angle offset of the illuminated spot relative to its field of view by imaging the radiation reflected from the illuminated spot onto the position detector. The extent of the angle offset is in this case determined by the position or the orientation of the imaged illuminated spot on the radiation-sensitive surface of the position detector. The missile is steered in the direction of the illuminated spot as a function of the determined angle offset, and is thus guided to the target. In this case, the target illumination must be maintained until the missile reaches the target. The target can be illuminated by an observer in an advanced position. Pulsed radiation and pulse repetition rates of about 10-20 Hz are typically used for illumination, with the pulse repetition rate being used to code the laser designator, in order to make is possible to approach the correct target even when there are a plurality of illuminated targets in the seeker field of view. The pulse code of the target illuminator is transmitted to the missile before launch. By way of example, one position detection system is disclosed in commonly assigned German published patent application DE 10 2004 029 343 A1 and its counterpart U.S. Pat. No. 7,304,283 B2. 
     In order to reduce the danger to an illuminator, for example an observer in an advanced position, it is known for the target to be illuminated for only a short time, for example one second, and for the target to be assigned to the missile in this way. The missile has the characteristic, in the sense of a dual-mode system, of using an imaging system to identify targets which have been marked with a laser target illuminator. The missile can be guided passively to the target with the aid of the image detection system once the target has been transferred by the target illumination and the position detection or, to be more precise, the angle offset of the missile with respect to the target has been determined by means of the position detection system in the missile. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide an object detection system, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which specifies such a system by means of which an object can be reliably detected as a target and can be tracked in a target scene, so as to allow a missile to be reliably steered to the target. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, an object detection system, comprising: 
     an image detection system with an imaging detector; 
     a position detection system with a position detector; and 
     optics disposed to guide incident radiation along a beam path onto said imaging detector and onto said position detector; 
     wherein said imaging detector and said position detector are disposed one behind another in the beam path. In particular, the two detectors are disposed adjoining one another. 
     In other words, the objects of the invention are achieved by an object detection system of the type mentioned initially in which the two detectors are arranged one behind the other, in particular adjacent to one another, in the beam path. The beam path is therefore first of all guided to one of the detectors, and the same beam path is then guided to the other detector. Arranging the detectors one behind the other makes it possible to save valuable physical space, and the optics can be used not only for the imaging detector but also for the position detector, thus making it possible to achieve a system of little complexity, and of compact design. 
     The two detectors are expediently arranged adjacent to one another, for example immediately adjoining one another, or separated from one another only by a layer, for example an adhesive layer or an optically active layer, such as a filter. 
     The detectors are advantageously mounted such that they cannot move with respect to a missile housing. 
     The imaging detector is preferably mounted on the cooling unit of a cooler, in order to increase its sensitivity. By way of example, the cooler may be a cold finger. It is feasible, of course, to also provide a cooling capability for the position detector. 
     With a clever design, the optics have an optical element which not only images the beam path on the imaging detector but also guides it to the position detector. The optical element therefore guides the same beam path both onto the imaging detector and onto the position detector. Radiation which is guided onto the position detector can therefore also be guided onto the imaging detector. It is expedient to arrange the last beam-forming or beam-deflecting optical element in the beam path in front of the detectors, and this may be a lens, a mirror, a prism or planar optics. The beam path which is imaged on the imaging detector is expediently completely guided to the position detector. The beam path can be imaged on the imaging detector by arranging the imaging detector on an image plane in the beam path. 
     The object detection system may be a component of a seeker head of a missile. The image detection system is used to detect an image on which an object which may be marked as a target is imaged. The imaging detector may be a point detector, a line detector or a matrix detector. In the case of a point detector or line detector, the image which reproduces the object can be recorded sequentially by scanning and can be assembled to form the complete image. The position detection system is used to detect a position or to determine an angle offset of the object relative to a coordinate system which, for example, is firmly linked to a missile axis. The position detector can thus be designed such that it outputs angle coordinates which correspond to the offset angle of the target being aimed at, relative to the fixed coordinate system. The angle offset may be detected once, a plurality of times, or continuously. For example, the optics can be carried by the object, corresponding to the position, and a line of sight spin rate can be detected, which is used to steer the missile. Alternatively, in the case of optics which are arranged in a fixed position in the missile, the missile can be steered on the basis of the position itself, with the position being maintained on the missile axis, for example by appropriately steering the missile. 
     Radiation for which the imaging detector is sensitive can advantageously pass through the position detector. The two detectors may be arranged one behind the other in the beam path, without the rear detector being shadowed. Transmissibility is achieved with a transmission level of at least 50%, in particular at least 80%. 
     In a further advantageous embodiment of the invention, the two detectors are arranged on the image plane of the beam path. Both the object and the illumination spot can be imaged in focus on both detectors. In this context, the image plane is understood as being a plane with a thickness at right angles to the optical axis of the beam path which is no greater than 10% of the focal length of the beam path on the image plane, in particular 3%. 
     In order to produce the position detection system, a lateral effect detector is advantageous, which is expediently arranged rigidly in front of the imaging detector. A lateral effect detector may have a very compact design and may be designed to be transmissive for medium infrared and far infrared, which means that an imaging detector which operates in these wavelength ranges can be arranged behind the lateral effect detector, without shadowing. High-purity silicon is advantageous as a detector substrate for the position detector. Furthermore, a lateral effect detector can be made very large, for example up to (20 mm) 2 , as a result of which its field of view covers a wide angle range. A wide field of view can be used for reliable target detection since a large angle scatter can occur in the case of an indirect launch with a ballistic flight path in the direction of the target. 
     A further advantage of lateral effect detectors, detectors with a transmissively radiation-sensitive surface, is that they can be operated without cooling, and dispensing with a cooling capability makes it possible to save both the costs required for this purpose and physical space. 
     The two detectors advantageously cover fields of view of different size. While the field of view of the position detector is advantageously large, for example with a diameter of at least 5°, and preferably of at least 15°, the field of view of the imaging detector can be kept small, that is to say for example less than 5° or even less than 1°, since the alignment of the narrow field of view with the target can be carried out by the position detector. The narrow field of view makes it possible to achieve high angle resolution of the imaging detector. 
     It is, of course, also feasible to design the fields of view of the two detectors to be the same size. This offers the advantage that increased functionality can be achieved. For example, if only an image of inadequate quality can be obtained using the imaging detector, the position of the target with respect to the missile can be determined once again using the position detector. This therefore provides a mutual monitoring capability between the two detectors. The result which one of the detectors produces can therefore be checked by the result which the other detector produces. This allows the missile to be guided particularly reliably to the target. 
     The field of view of the imaging detector is expediently located in the field of view, in particular centered in the field of view, of the position detector. A simple optics geometry can be achieved by arranging the imaging detector centered with respect to the position detector. 
     It is also proposed that the position detector be mounted rigidly on a housing of the imaging detector. There is no need for additional holding elements, and the system can be designed to be compact. 
     A high degree of compactness is likewise achieved if the position detector forms an inlet window of the imaging detector. This can be coated with a spectral filter, expediently on the side facing away from the imaging detector, in order to filter the radiation to the imaging detector. 
     Irrespective of its position, the spectral filter is expediently designed such that it has a transmission window in the wavelength range of the position detector, a transmission window in the wavelength range of the imaging detector, and an opaque area between the two transmission windows. A single spectral filter can be used for both detectors, thus allowing the object detection system to be kept compact. 
     If the two detectors are each connected to their own cooling unit, with the cooling units being arranged in one another, then the object detection system can likewise be designed to be compact and simple. 
     For exact detection of the position of the target being aimed at, it is advantageous for detector outputs of the position detector to be connected to amplifier electronics via a coupling capacitor in order to suppress background radiation, thus resulting in a bias T circuit. 
     The detector outputs of the position detector preferably have a DC bias voltage applied to them, in order to increase the speed of the detector and thus to widen the bandwidth. If the already mentioned coupling capacitor is in this case arranged downstream from the DC bias voltage supply, then it can not only ensure suppression of background radiation but also outputting of the DC bias voltage and/or AC coupling. 
     When approaching the target—with the target being illuminated uniformly at the same time—the irradiation intensity (which is detected by the position detector) of the positioning emitter becomes stronger. In order to avoid reaching the saturation range of the position detector, it is advantageous for the object detection system to have an amplifier for signals from the position detector, which amplifier is designed for variable gain matching. 
     In addition, the object detection system advantageously comprises a control means for controlling the position detection and, expediently, the image processing of the image detection system. 
     If the control means has a memory in which a position calibration of the position detector is stored, then this makes it possible to ensure that little mechanical adjustment effort is required for the positioning system by means of an electronic calibration. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an object detection system having an image detection system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. The drawing and the description relate to numerous features in combination, which a person skilled in the art will expediently also consider individually and will combine them to make further worthwhile combinations. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  shows a schematic illustration of an object detection system with Cassegrain optics and with two detectors, attached to one another, on the image plane of the optics; and 
         FIG. 2  shows a schematic circuit illustration of a position detector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown a seeker head  2  in the front part of a missile  4  with a viewing window  6  in the form of a dome, behind which an object detection system  8  is arranged. The system  8  contains optics  10  in the form of Cassegrain optics with two mirrors  12 ,  14 , by means of which radiation from an object scene  16  with a target  18  is imaged in one beam path  20  onto a detector system  22 . The detector system  22  comprises a position detector  24 , also referred to as a positioning detector  24 , and an imaging detector  26 , which is arranged in the beam path  20  immediately behind the position detector  24 . The mirror  14  is an optical element which not only guides the beam path  20  onto the imaging detector  26  but also guides the same beam path and/or the same radiation onto the detector  24 , for example radiation in the far infrared, which passes through the detector  24  and is guided onto the detector  26 . 
     The object detection system  8  comprises a gyro system on air bearings with a gyro  28  which is monitored by a control means  30 , which is also used as evaluation electronics for the two detectors  24 ,  26 . The gyro  28  is connected to the optics  10 , which image the incident target radiation onto the large-area position detector  24  and the considerably smaller imaging detector  26 . Optics  10  pass the radiation to the two detectors  24 ,  26  and therefore, because of the different detector sizes, cover two different fields of view. The imaging detector  26  is seated on a cold finger  32  and has only a smaller field of view. The larger position detector  24  is connected to a housing  34  of the imaging detector  26 , which is passed around the cold finger  32 . The two detectors  24 ,  26  are mounted rigidly relative to the housing of the missile  4 , via the housing  34 . 
     The imaging detector  26  is in the form of a line detector, whose individual images are assembled in a scanning mode to form the overall image of the object scene  16 . The metallic housing  34  is used for mounting the position detector  24 . The position detector  24  is sensitive in the near infrared spectral range, and detects laser radiation from a target marker which emits in the near infrared, and derives from this the angle offset of the target being aimed at. 
     The position detector  24  is a lateral effect detector. The infrared light which falls on its active area generates a photocurrent which flows away in the direction of the p-doped and n-doped regions. In contrast to a simple photodiode, the detector  24  has a plurality of electrical contacts, however. This leads to splitting of the photocurrent at the electrodes, which are arranged at the side, as a function of the position of the light spot. The position in the x and y directions can be determined by forming the current difference between two opposite electrodes. Normalization of the total current makes the position signal independent of the incident light intensity. 
     The viewing window  6 , which is in the form of a dome and acts as a protection apparatus against external influences, may be used as the first optical element, for example as a lens, for imaging the object scene  16  onto the two detectors  24 ,  26 . It is composed of a material which has good transmission both for the near infrared and for the medium and far infrared, and which at the same time is very strong. For example, zinc-sulphide Cleartran®, a water-free form of zinc sulphide with a relatively broad transmission range from 0.5 to 14 μm, is very highly suitable for the spectral range from the near infrared to the far infrared. 
     A spectral bandpass filter  38  is fitted to the position detector  24 , to be precise on its side facing the mirror  14 , in order to suppress background radiation and for interference suppression. The filter  38  is opaque in the near infrared wavelength range, except for the specific wavelength range of the marking laser, which can pass through the filter. Medium infrared and long-wave infrared can pass through the filter. 
       FIG. 2  shows a schematic circuit diagram of the lateral effect detector  24  and amplifier electronics connected to it. The detector  24  comprises four signal outputs  42 , which are each connected to reading electronics  40 , only one of which is illustrated in  FIG. 2 , for the sake of clarity. The reading electronics  40  are likewise connected to the control means  30 , which are also provided for target guidance and thus for steering the missile  4 . The irradiation of light onto a spot  44  on the detector  24  initiates a signal at each of the signal outputs  42 . The strength of the respective signal depends on the intensity of the light irradiated onto the spot  44  and the position of the spot  44  within the area  46  of the detector  24 . The closer the spot  44  is to one of the signal outputs  42 , the stronger is the signal at this signal output  42 , and the weaker the signal is at the opposite signal output  42 . If the spot  44  is positioned precisely at the center point of the area  46 , the four signals are all equally strong. 
     Because of the use of the continuous light-sensitive area  46 , the detector  24  can easily be calibrated electronically. The control means  30  have a memory in which a position calibration of the position detector  24  is stored. This position calibration includes the discrepancy between the optical axis and the position on the area  46  at which all four signals are the same. 
     The reading electronics  40  comprise in each case a bias-voltage source  48 , with the bias-voltage sources  48  having a positive bias voltage applied from two opposite signal outputs  42 , for example of +15 V, and with the bias voltage sources  48  of the two other signal outputs  42  having a corresponding negative voltage applied, corresponding to the p-doping and n-doping. In order to decouple the bias voltage from the amplifier electronics  50 , each of the reading electronics devices  40  has a coupling capacitor  52 . The pulses from the marking laser result in an alternating current at the signal outputs  42 , as a result of which the coupling capacitor  52  does not lead to any signal interruption. This alternating-current coupling of the detector  24  to the amplifier electronics  50  is used to reduce the background component, and to output the DC bias voltage. A controllable resistor  56  is connected across an amplifier element  54 , thus making it possible to vary the signal gain, controlled by the control means  30 . The signal can thus be reduced as the missile approaches the target, thus making it possible to avoid overdriving of the detector  24  and of the amplifier electronics  50 . 
     The currents from the signal outputs  42  are supplied via the reading electronics  40  to signal processing electronics which, for example, may be arranged in the control means  30  or between the control means  30  and the reading electronics  40 . These signal processing electronics digitize the signals and process them as a function of the functional phase, that is to say as a function of whether the signal is intended to be found as such with the aid of the transfer code, or whether the aim is to find offset angles, by means of a specific algorithm. For a digital interface, these status or offset signals are passed to the autopilot. A further electrical interface is used for the operating voltage supply. 
     The object detection system  8  may be operated as follows. In an initialization phase, all the hardware and software functions of the detectors  24 ,  26  and of the electronics are activated by the control means  30  and are switched to the basic state. In addition, the frequency code with which the target is being illuminated by the marking laser is passed to the control means  30 . The initialization phase may be initiated, for example, by a launch. 
     In a subsequent first acquisition phase, the control means  30  in conjunction with the position detector  24  and with the aid of the code search for the marking laser light. For this purpose, all pulses which exceed a threshold value are detected. A pulse sequence of three to six pulses is required for reliable synchronization, depending on the algorithm. 
     In the subsequent first tracking phase, the current offset angles produced by the detector  24 —for example in the form of two mutually perpendicular vectors—are determined precisely with respect to a coordinate system that is fixed to the seeker head, and are used for slaving the gyro system. For example, the optics  10  can be guided in the direction of the identified target on the basis of the offset angle, with the gyro  28  identifying the movement of the optics  10  and the control means  30  aligning the missile  4  in the direction of the target  18 , on the basis of corresponding control-surface signals. 
     If the optics  10 —or in the case of rigid optics  10 , the missile  4 —are or is at least essentially aligned with the target  18 , the second acquisition phase starts, in which the imaging detector  26  identifies the target  18 . For this purpose, the instantaneous offset angles are transferred to the infrared image of the detector  26 , with target marking thus being carried out, thus uniquely defining the target  18 . In the second, subsequent tracking phase, the offset angles of the target  18  are determined by means of image processing algorithms by the control means  30 , for slaving of the optics  10 , from which angles the gyro  28  is used to determine a line of sight spin rate, which is used to guide the missile  4 . This phase may continue until the target  18  is reached. 
     After the second acquisition phase, that is to say after identification of the target  18  by the image processing, the missile  4  can be guided to the target  18  both with the aid of the imaging detector  26  and the imaging processing and—when the target is being marked—solely by the position detector  24  and the corresponding reading electronics  40 . The approach phase can therefore be carried out both using the semi-active laser system and using the imaging system. In consequence, the target guidance is particularly insensitive to disturbances. Alternatively, after the image processing acquisition phase, the marking of the target  18  by the marking laser can be ended, and the missile  8  can be guided to the target  18  solely with the aid of the imaging detector  26  and the image processing.