METHOD OF ADJUSTING THE DIRECTION OF THE SIGHTING AXIS OF A TELEVISION SYSTEM..
Smelkov Vyacheslav Mikhailovich, candidate of technical sciences, associate professor
METHOD OF ADJUSTING THE DIRECTION OF THE VIEWING AXIS OF A TELEVISION SYSTEM
An analog method of optical-electronic image scaling and original technical solutions based on it were published earlier [1, 2, 3]. A distinctive feature of the method is the use of a beam splitter and two CCD matrices as photodetectors in the TV camera.
The undoubted advantages of the proposed method include:
- high speed of the “zoom” operation due to the elimination of the time for adjusting the focal length due to the refusal to use a varifocal lens;
- no loss of resolution for an enlarged image compared to the digital scaling method.
The beam splitter device ensures that the direction of the viewing axis of the television system is such that at the input the optical axis of the conventional “wide-angle” television camera coincides with the optical axis of the conventional “narrow-angle” television camera.
It should be recognized that the presence of light separation leads to noticeable losses in the energy (light) sensitivity of the television system, especially in the enlarged image channel. These losses can be considered unacceptable for television systems that are designed to search for, detect and track remote objects.
The rejection of a beam splitter inevitably leads to spatial separation of the optical axes of the television cameras, for example, to separation horizontally with the coincidence of the vertical. However, given that the magnitude of this basic separation is much less than the distance of removal of the television system from the object, and the parallelism of the optical axes is observed, the error in determining the coordinates of the target can be neglected.
A well-known example of such a solution is the implementation of a television system for observing Halley's comet [4, p. 147]. It contained two television cameras on board the spacecraft: a «narrow-angle» (with a long-focus lens) and a «wide-angle» (with a short-focus lens), with the latter being used as a television system targeting sensor. Domestic charge-coupled device (CCD) matrices were used as photodetectors in each of the television cameras. In addition to the television targeting sensor, the television system was equipped with a second, analog targeting sensor. Both targeting sensors separately or jointly controlled the spacecraft's rotating platform, which made it possible to change the direction of the television system's sighting axis both in the orbital plane and in the plane perpendicular to it. The television system's electronic unit, implementing two operating modes: standby and main, ensured the switching of image signals from each of the television cameras for subsequent recording of video information on the on-board tape recorder, transmission of the accumulated information to Earth and reproduction of the image on the screen of the video monitor of the Mission Control Center.
However, in comparison with the space project, for relatively simple and mass-produced special equipment products, control of the rotating platform, and, consequently, the direction of the sighting axis of the television system, can be incorporated a priori from only one (non-television sensor) guidance.
This, in turn, leads to the need to set strict requirements for the placement of the television system on the rotary platform, in particular, for the direction of the sighting axis of the television system relative to the landing plane of its base. For a two-camera television system, this means increased requirements for the parallelism of the optical axes of the television cameras to each other and their parallelism relative to the landing plane of the base. In this case, the permissible error in the direction of the sighting axis must be confirmed by metrological testing during the process of setting up the television system at the manufacturer.
In the technical solution proposed below, the adjustment of the direction of the sighting axis of the two-camera television system is carried out using:
- reflective and electronic tables of the “grid field” type, the parameters of which take into account the basic horizontal distance between the geometric centers of the photodetectors of the television cameras of the present television system;
- laser target designator, the radiation from which is produced through a groove made in the base of the television system and allows with high precision (limited only by the capabilities of the technology for manufacturing the groove itself) to ensure parallelism of the laser probe to the base of the television system.
The structural diagram of the device for performing technological adjustment (alignment) of the direction of the sighting axis of the television system is shown in Fig. 1. The device comprises a reflective table (1) installed in the plane of an object of a television system consisting of a first (wide-angle) television camera (2), a second (narrow-angle) television camera (3), a video signal switch (4) and a video monitor (5); a laser target designator (6) and an electronic table generator (7), wherein the laser target designator (6) forms a visible spectrum spot (10) in the plane of the reflective table (1) through a groove (8) made in the base (9) of the television system, the outputs of the television cameras (2) and (3) are connected respectively to the first and second inputs of the video signal switch (4), the output of the television camera (3) is connected to the external synchronization input of the television camera (2), and the output of the video signal switch (4) is connected through the electronic table generator (7) to the video monitor (5).
Fig. 1. Structural diagram of the device for performing technological adjustment of the direction of the sighting axis of the television system
The reflective table (1) is used as an optical test when performing the adjustment process of the television system.
An example of the implementation of the reflective table (1) is shown in Fig. 2.
Fig. 2. Test table “mesh field”
The table contains 20 cells horizontally and 12 cells vertically. The geometric center of the table is marked by point “O”. To the left of the center of “O” with a horizontal shift of two cells is point “A”, and to the right, also with a shift of two cells is point “B”. Point “C” is marked with a vertical shift downwards relative to the center of “O” by three cells. The size of one cell horizontally and vertically corresponds to one-fourth of the value of the basic horizontal spacing of the optical axes of television cameras. Let us assume that the value of the specified basic distance is 68 mm, then the size of the cell is (17×17) mm.
Both television cameras (2) and (3) must be synchronized in Genlock mode with frequency and phase binding of horizontal and vertical scans to the receiver synchronization signal (RSS) or to the full television signal from one of the television cameras or from an external source. In Fig. 1, Genlock mode is provided by feeding a composite signal from camera (3) to the external synchronization input of camera (2).
In the proposed solution, the same value of the field of view angle should be set for each of the television cameras. In practice, this can be ensured by using CCD matrices with the same size and target format in each of the television cameras, for example, with a diagonal size of 1/2 inch and a 4/3 format, and also by using a zoom lens (vario lens) as a lens for one of the cameras.
The laser target designator 6 can be the LCU device manufactured by the Belarusian Optical and Mechanical Association «BELOMO», with a laser radiation wavelength of 645 nm and creating a red light point on the object [http://matrix 1984 narod. ru].
The groove (8) is intended for channeling the laser radiation probe in the base 9 of the television system, in a direction parallel to its mounting plane. The groove (8) can be made by precision milling.
The electronic table generator (7) is designed to form an electrical signal “grid field” in a format equal to the frame format of the photodetectors of television cameras. In our example, this format is 4/3, and the electronic table contains 16 cells horizontally and 12 cells vertically. Obviously, the size of one cell in units of time of standard line scanning is: 52/16 = 3.25 μs.
The input signal for the generator (7) is the full television signal from the output of the video signal switch (4) with a peak-to-peak amplitude of (1±0.2) V on a load of (75±3.75) Ohm. At the output of the generator (7), on a load of (75±3.75) Ohm, a full television signal with a peak-to-peak amplitude of (1±0.2) V of the total image is generated, the components of which are the input video signal and the “grid field” signal. The second signal is preferably formed by replacing the first signal corresponding to the coordinate. It is also advisable to provide positive and negative polarity of the “grid field” signal with the possibility of operational switching. The circuit solution of the generator (7) can be implemented on the basis of a PIC processor.
Let us consider the technological process of adjusting the direction of the sighting axis of a two-camera television system, using the structural diagram in Fig. 1.
Television cameras (2) and (3) operate simultaneously in the frequency and phase synchronization mode of the frame and line scans from the composite signal of camera (3).
The video signal switch (4) sends the full television signal from the television camera (2) or from the television camera (3) to the input of the electronic table generator (7) on an external command. In the generator (7), a marker signal “grid field” is added to the video signal. The total image signal is reproduced on the video monitor (5).
First, the position of the reflective table (1) is oriented so that when looking at it, the controller can record the spot from the laser designator at point “C”.
Then they begin to analyze the television images. Let us assume that the video signal from the first television camera (2) is switched to the output of the television system.
The image of the area of the reflective table (1) located on the right and limited by the benchmarks of this fragment is entered into the raster of the photodetector of the first camera (Fig. 2). The format of this area is 4/3, and its geometric center coincides with point “B” on the table (1). In this case, the number of observed cells of the table (1) horizontally is 16, and vertically – 12 and corresponds to a similar number of marker cells from the generator of the electronic table (7). Note that the basic distance between points “A” and “B” occupies four cells, which in units of time is 3.25×4 = 13 μs.
Then, using the elements for adjusting the angular movement along the horizontal and vertical, provided in the design of the television camera (2) for the “lens – photodetector” set, the maximum alignment of the observed center of the television image “B” of the table (1) with the center of the electronic table is achieved, and the cells of the table image (1) – with the marker cells from the generator (7). The ideal result of image alignment is shown in Fig. 3a.
a)
b)
Fig. 3. a – image from the video monitor screen from the first television camera (2); b – image on the video monitor screen from the second television camera (3)
Next, without changing the spatial position of the reflective table (1), the image signal from the second television camera (3) is switched to the output of the television system. In this case, the controller should observe the image of another area of the table (1) on the left on the video monitor screen (5), with the center at point “A” and limited by the benchmarks of this fragment. The number of cells of the observed television image is similar to the previous switching of the video signal and is 16×12 with a 4/3 format. Then, similarly, using the angular displacement adjustment elements for the “lens – photodetector” assembly of the camera (3), the maximum alignment of the observed center “A” with the center of the electronic table is achieved, and the cells of the optical test image with the marker cells, ideally achieving the result shown in Fig. 3b.
Let's conduct an engineering assessment of the technical result of the proposed solution.
The value of the “residual” angular displacement of the direction of the sighting axis after the adjustment is completed can be determined by the relations:
(1),
where b is the maximum value of the “residual” horizontal image misalignment in mm;
S is the distance from Table 1 to the television camera in mm.
(2)
where a– the maximum value of the “residual” vertical misalignment of images in mm;
S – the distance from Table 1 to the television camera in mm.
Let a CCD matrix with the number of elements 768(H)x576(V) be used as a photodetector in television cameras, and the set value of the field of view for its two angular values (HxV) is (12×7.8) degrees.
If we accept that with an acceptable misalignment of images the minimum value of optical change is 2 elements of photodetector resolution in both directions, then the value of gg will be 0.031 degrees (0.55 mrad), and the value of gv will be 0.027 degrees (0.47 mrad). These parameters can be considered working for performing the adjustment.
Therefore, if, upon completion of the adjustment, the calculation using relations (1) and (2) yields values of “residual” angular displacements that are greater than 0.55 mrad and 0.47 mrad, respectively, this means that the adjustment is not complete and must be continued to achieve the required accuracy.
In conclusion
The accuracy of adjustment in the direction of the sighting axis of the television system according to the proposed method fundamentally depends on the information capacity of the CCD matrices. Therefore, to reduce the value of the “residual” angular displacement of the sighting axis, it is necessary to increase the number of photodetector elements in both directions.
Literature
1. Smelkov V.M. Television camera for covert surveillance and automated security.//Special equipment, 2001, No. 3, p. 20 – 23.
2. Smelkov V.M. Security television camera: a new solution using the optical-electronic scaling method.//Special equipment, 2002, No. 6, pp. 12 – 15.
3. Smelkov V.M. Security television camera with selective scaling: a new solution.//Special equipment, 2003, No. 3, pp. 17 – 20.
4. Tsytsulin A.K. Television and space. St. Petersburg, ETU “LETI” Publishing House, 2003.