FEATURES OF TECHNOLOGICAL EQUIPMENT FOR ALIGNMENT OF THE SIGHTING AXIS OF A TELEVISION SYSTEM..
Vyacheslav Mikhailovich SMELKOV, Candidate of Technical Sciences, Associate Professor
FEATURES OF TECHNOLOGICAL EQUIPMENT FOR ALIGNMENT OF THE SIGHTING AXIS OF A TELEVISION SYSTEM
In [1], a method for adjusting the direction of the sighting axis of a two-camera television system was proposed. The direction of the sighting axis of a television system is understood to be the orientation of the angle of the field of view of each of the television cameras (“wide-angle” and “narrow-angle”) or the position of the optical axes of the television cameras. It is assumed that in space relative to the base of the television system, the optical axes of the television cameras coincide vertically and are separated horizontally by the value of the base distance.
Setting the adjustment task means that after its completion the optical axes of the television cameras should be parallel to each other and parallel to the landing plane of the television system base. A distinctive feature of the method described in work [1] is the need to set the same values of the field of view angles for each of the cameras during adjustment, which is easily achieved by aligning the focal lengths of the camera lenses, i.e. using at least one zoom lens as a lens.
However, in practice, the task of adjusting the sighting axis can be formulated so that even when adjusting the sighting axis, the operational values of the focal lengths of the lenses used are preserved, and the use of a zoom lens is excluded.
In this paper, to perform this task, it is proposed to use the structural diagram of the adjustment device shown in Fig. 1. Unlike option [1], the following changes have been made to the construction of individual elements of the diagram:
— de facto and during adjustment, the first television camera 2is “wide-angle”, and the second TV camera 3 is “narrow-angle”;
— a new “grid field” test table is used as a reflective table 1;
— an electrical signal generator “central cross” is used as a generator of an electronic table 7.
Fig. 1. Structural diagram of the device for
performing technological adjustment of direction
sighting axis of the television system
New test table 1is designed taking into account both possible variants of the relative positioning of television cameras based on the television system. It contains two large (left and right) overlapping “grid field” tables, limited by benchmarks and shifted horizontally relative to each other by the value of the basic spacing of the geometric centers of the photodetectors, which is a multiple of the dimensions of the cell of the large table, as well as two small “grid field” tables (left and right), limited by benchmarks and located symmetrically relative to its geometric center. Each large and small table has a width-to-height ratio equal to the format of the photodetector; the dimensions of the cell of the large table are multiples with a coefficient k of the dimensions of the cell of the small table; the number of cells M horizontally for each large table is determined by the ratio:
M = m · 1/k · f2/f1, (1)
and the number of cells N vertically — by the ratio:
N = n · 1/k · f2/f1, (2)
where m and n are the corresponding number of cells horizontally and vertically for each small table;
k is the cell multiplicity coefficient;
f1 and f2 are the focal lengths for the “wide-angle” and “narrow-angle” cameras.
Example of the implementation of a reflective table 1for the value of the basic spacing of geometric centers in 4 large cells, the ratio of focal lengths (f2/f1) equal to 4, and the cell multiplicity coefficient (k) equal to 2, is shown in Fig. 2.
Fig. 2. Test table “grid field”
The table contains 20 large cells horizontally and 12 large cells vertically. The geometric center of the table is marked by point “O”. To the left of the center “O” with a horizontal offset of two large cells is point “A”, and to the right, also with an offset of two large cells is point “B”. Point “C” is marked with a vertical offset downwards relative to the center “O” by four large cells on the table. The left large table is marked with horizontal wedge-shaped benchmarks, and the right large table with vertical wedge-shaped benchmarks. The size of one large cell horizontally and vertically is taken to be equal to one fourth of the value of the basic horizontal spacing of the optical axes of the television cameras. Let us assume that the value of the specified basic distance is 68 mm, then the size of the large cell is (17×17) mm.
Two small tables (left and right), limited by diamond-shaped benchmarks, are located symmetrically relative to the geometric center “O”. Each of these tables contains 8 small cells horizontally and 6 small cells vertically. The dimensions of a small cell are half the size of the corresponding dimensions of a large cell, i.e. in our example they are (8.5×8.5) mm.
Generator 7is designed to generate an electrical signal, the “central cross”. The vertical and horizontal dimensions of the “cross” occupy the entire height and width of the monitor screen, respectively. At the output of the 7 generator, 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 “central cross” signal. Schematic diagram of the 7 generatorcan be implemented on the basis of the PIC16F873-201/SP processor, which is well-known on the Russian market.
Let's consider the process of adjusting the direction of the sighting axis of the television system, using the structural diagram in Fig. 1.
Let's assume that 2 is used as a television camera.The VNI-702 camera module manufactured by ZAO EVS (St. Petersburg) is used, and the focal length of the lens is 30 mm. The photodetector of this module is a CCD matrix with the number of elements 768 (H) x 576 (V) and a target size of 1/2 inch or (6.4 x 4.8) mm at a 4/3 format. Therefore, the operational value of the angular field of view of the first TV camera is 12 (H) x 7.8 (V) degrees.
Let the same camera module be used as TV camera 3, but the focal length of the lens is 120 mm. Then the operational value of the angular field of view of the second TV camera will be 3 (H) x 2 (V) degrees.
As a video monitor 5It is recommended to use a monitor with a signal-to-light converter based on a liquid crystal display, which, in comparison with a cathode-ray tube kinescope, has no coordinate distortions of the raster.
The laser module KLM-650/5 from the company «FTI-Optronik» (St. Petersburg) can be used as a laser target designator 6, which provides a laser radiation wavelength of 650 nm, an initial beam diameter of no more than 3.4 mm and a laser radiation power of at least 5 mW.
Channel 8is designed to set the direction of laser radiation from the target designator 6. Channel 8 can be made in the form of a “groove” in the base 9 of the television system using precision milling.
TV cameras 2 and 3 operate simultaneously in the frequency and phase synchronization mode of the frame and line scans from the synchronization signal of the receiver of camera 2.
Video signal switch 4, upon external command, feeds to the input of generator 7the full television signal from camera 2 or from camera 3. In generator 7, a marker signal “central cross” is added to the video signal. The total image signal is reproduced on video monitor 5.
First, the position of the reflective table 1 is oriented so that when looking at it, the traffic controller can record the spot from the laser pointer at point “C”.
Then we proceed to the analysis of television images. Let us assume that the video signal from the “wide-angle” television camera 2 is switched to the output of the television system.
By smoothly adjusting the position of the reflective table 1 “forward and backward” within small limits, we inscribe the image of its large right table into the raster of the photodetector using reference points (Fig. 3). The dimensions of the inscribed area are 16(H)x12(V) large cells. The format of this area is 4/3, and its geometric center coincides with point “B” on the table 1.
Then, using the adjustments of the mechanism for angular movement of the direction of its optical axis provided in the design of the television camera 2, the maximum alignment of the observed center of the television image “B” of the reflective table 1 with the geometric center of the electronic cross is achieved.
Fig. 3. Image from the video monitor screen
from the “wide-angle” TV camera
Next, without changing the spatial position of the reflective table 1, the image signal from the “narrow-angle” TV camera 3 is switched to the output of the television system. At the same time, on the video monitor screen 5the image of the small left table with the center at point “A” and limited by diamond-shaped benchmarks should be observed (Fig. 4). The format of this area is 4/3, and the number of small cells of the observed television image is 8(H)x6(V). Then, similarly, using the adjustments of the mechanism for angular movement of the direction of the optical axis of the “narrow-angle” camera 3, the maximum alignment of the observed center “A” with the geometric center of the electronic cross is achieved.
Fig. 4. Image from the video monitor screen
from the “narrow-angle” television camera
The alignment of the television system's sighting direction is considered complete if, when switching television images, the centers «A» and «B» of the reflective table in them consistently coincide with the center of the electronic marker, and the projection of the laser probe is maintained at point «C».
Let's conduct an engineering assessment of the technical result of the proposed invention.
When combining the centers of the table “A” and “B” with the electronic center, the accuracy of the direction of the sighting axis of the television system is determined by the thickness of the electronic marker horizontally and vertically, which is 2 elements of resolution in each direction.
For the angular field of view of the “wide-angle” camera 2 horizontally, amounting to 12 angular degrees, and the applied photodetector with the number of elements in the row 768, this provides the following value of error (D) in the non-parallelism of sighting:
D = (12/768 · 2 · p/180 · 1000) mrad. (3)
As a result, we obtain a sighting direction error value of 0.54 mrad.
It is obvious that for a “narrow-angle” camera, for which the angular field of view is 3 angular degrees, the error value (D) with the sighting direction adjustment performed will be four times smaller, i.e. about 0.14 mrad.
As conclusions
- From relation (3) it follows that for both cameras the sighting error will be the smaller, the higher the information capacity of the photodetector.
- To increase the objectivity of the result of the television system adjustment, it is advisable to supplement the structural diagram of the device for its implementation with a personal computer that ensures the measurement of the “residual” angular displacements of the direction of the sighting axis based on the input signal of the total image.
Literature
1. Smelkov V.M. Method of adjusting the direction of the sighting axis of the television system./Special equipment, 2004, No. 5, pp. 14 – 18.