- Email: [email protected]
Accuracy test and analysis for infrared search and track system
Optik 124 (2013) 2313–2317
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Accuracy test and analysis for infrared search and track system Hui-ming Qu ∗ , Dan Cao, Qi Zheng, Yuan-yuan Li, Qian Chen School of Electronic Engineering and Optoelectronics Technology, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
a r t i c l e
i n f o
Article history: Received 16 February 2012 Accepted 27 June 2012
Keywords: Infrared search and track (IRST) Small target track Tracking accuracy Azimuth Elevation
a b s t r a c t Infrared search and track (IRST) attracts many attentions nowadays. It is important to evaluate the performance of the detection and tracking performance for the development of IRST. This paper presents a tracking accuracy indoor test method for IRST. The proposal approach compares the simulated target trajectories with the tracks given by the IRST and calculates the dynamic tracking accuracy of the tested system. An infrared point target track simulator was built to simulate the actual target and missile trajectories. The movement of the simulator was driven by a high-precision stepper motor. A blackbody was used to simulate the infrared radiation of targets. The simulator was controlled by computer via RS-232 serial port. The simulator moving speed, direction and the operation angle value was managed in a software interface written in Visual Basic. The moving point target’s coordinates, azimuth and elevation angle trajectories were reconstructed through the algorithm of coordinate translation. A comparison between the simulated target track and IRST recorded trajectory was performed to get the tracking accuracy of tested system. The paper describes the tracking accuracy test system design and implementation in detail. The experimental test and result analysis are provided to verify the effectiveness. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction An infrared search and track (IRST) system, also known as the infrared surveillance system (IRSS) in some literature, is pointed to a class of a passive detection system based on infrared radiation for timely detecting and capturing the particular infrared radiation targets in the background. It is capable of providing identification of objects as well as accurate 3D tracks, and sending the alarm information of target orientation and threat level to the display system or weapon control system. IRST attracts extensive study interest nowadays [1–4]. The tracking process of IRST is a key contributor for many system performances: reaction time, ability to maintain tracks on maneuvering targets, tracking accuracy, angular discrimination performance, and behavior in presence of crossing targets [5]. It is important to evaluate the performance of tracking performance for the development of IRST system. In recent years, with the development of infrared focal plane devices, a variety of intelligent infrared search and track system was proposed, which plays a more and more important role in the aviation, aerospace, military and other fields. At the same time, it raises a challenge to the traditional testing methods [6,7]. The system’s target detection rate, false alarm rate and other major indicators of combat skills are heavily dependent on the details of the target signal and background characteristics. The traditional lab investigation cannot
∗ Corresponding author. Tel.: +86 25 84315869; mobile: +86 13601582591. E-mail address: [email protected] (H.-m. Qu). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.06.093
reflect the actual situation. That is to say the performance evaluation of IRST system need multifarious test in the field. However, the field test has many disadvantages, such as the difficult control of the test condition, expensive cost, poor mobility, limited number of tests and so on. Dynamic infrared scene simulation technology is developed in this background. It relies on high and new technology such as computer, information processing, microelectronics, what’s more. It innovates to the laboratory test method and makes up for the deficiency of the field test. By combining the large number of infrared simulation tests and fewer field test validation, infrared scene simulation has an important meaning for evaluating the system performance, shortening the development cycle and saving cost. It has been widely used during the development phases of IRST [7,8]. To address this need, a real-time infrared target simulator was developed and tracking accuracy test approach for IRST is proposed in this paper. The following sections briefly discuss the simulator of moving point targets and tracking accuracy test method in the lab. The principle of small target track accuracy test is discussed in Section 2. The laboratory setup and measurement methodology is described in Section 3. The experimental test and results analysis are mentioned in Section 4. The conclusion is given in Section 5. 2. The principle of small target track accuracy test Infrared search and track system is a device of measuring twodimensional angle in polar coordinates. Its measuring principle is dynamic track and measurement. After the moving targets in space have been captured, they will enter the field of an optical
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The coordinate of missile position expressed as Eq. (4):
⎧ x = r cos(ωt), ⎪ ⎨ m ⎪ ⎩
ym = L,
(4)
zm = −r sin(ωt),
Then
⎧ xt − xo −r cos(ωt) ⎪ , ⎨ tgAt = y − y = L t o xm − xo r cos(ωt) ⎪ ⎩ tgAm =
measurement system, and be locked by the servo in the track system. The system tracks the targets steadily and ensures they are always at the view field of the optical measurement system. Meanwhile, the tracking system records the targets’ deviation relative to system boresight: azimuth deviation and elevation deviation. In addition, the measurement system gives the azimuth angle and elevation angle of the boresight synchronously. Hence, the targets’ actual angle position information in the coordinate system of a search and track system is given as following equation: E = Ee + E
A = Ae + A
(1)
(2)
where Ee , Ae are the the boresight azimuth and elevation angle of IRST system. E, A are the azimuth deviation and elevation angle deviation of target to the IRST boresight. The Cartesian coordinate system for the trajectory data is shown in Fig. 1 in which z is the altitude coordinates. It shows motion relationship between the moving point targets and IRST system in the same space coordinates. Point O in the figure is the axes intersection point of coordinate system which located in the entrance pupil center of imaging system. Plane XOY is the datum plane for elevation angle. Plane YOZ is the datum plane for azimuth angle. Point T stands for the simulated target; M stands for the simulated missile. They are symmetrical to the center of a circle. When the stepper motor moving with the angular velocity ω and time t, the target turn an arc track which the corresponding centering angle ˛ equal to ωt. Where ˛ is the angle that the simulated target turned from the datum plane XOY, E is the elevation angle of the small targets, and A is the azimuth angle of the small targets. When the searching system tracks the targets, its optical axis coincides with the line OT. If the target moves with angle ˛ from one point to another, the search and track system will track the targets synchronously, and get the targets’ azimuth angle A and elevation angle E timely. In this space coordinates system, axis center of the search and track system: O(x0 , y0 , z0 ) = O(0, 0, 0), The center coordinate of simulated target T: T(xt , yt , zt ), The center coordinate of simulated missile M: M(xb , yb , zb ), The distance between the moving plane and XOZ plane is L, yt = yb = L. The target turned an angle ˛ = ωt, at the angular velocity of ω, then the coordinate of target position expressed as following:
⎧ x = −r cos(ωt), ⎪ ⎨ t ⎪ ⎩
yt = L, zt = r sin(ωt),
ym − yo
⎧ ⎪ ⎪ ⎨ tgEt =
Fig. 1. Schematic diagram of target trajectory coordinate system.
(3)
=
⎪ ⎪ ⎩ tgEm =
(5)
.
L
zt − zo
=
(xt − xo )2 + (yt − yo )2 zm − zo
(xm − xo )2 + (ym − yo )2
r sin(ωt)
=
[r cos(ωt)]2 + L2 −r sin(ωt)
,
[r cos(ωt)]2 + L2
(6) .
To take an inverse tangent to Eqs. (5) and (6), the azimuthal position and elevation angle of the target and missile are given below.
⎧ −r cos(ωt) ⎪ ⎨ At (t) = arctg , L
(7)
⎪ ⎩ Am (t) = arctg r cos(ωt) . L
⎧ r sin(ωt) ⎪ ⎪ ⎨ Et (t) = arctg 2
,
[r cos(ωt)] + L2 −r sin(ωt)
⎪ ⎪ ⎩ Em (t) = arctg
[r cos(ωt)]2 + L2
(8) .
The azimuth and elevation angle At (t), Am (t), Et (t), Em (t) changed with time t at the angular velocity ω. The above azimuth and elevation angle are simulation target and missile position coordinates. IRST system tracks the simulator and and elevation angle of the provides the real-time tracking azimuth (t) . By comparing the target At (t), Et (t) and missile Am (t), Em actual value of dynamic simulation target and IRST tracking value, we can calculate the tracking precision both the target and missile. ıAt (t) = At (t) − At (t)
(9)
Et (t) − Et (t)
(10)
ıEt (t) =
Am (t) − Am (t)
(11)
ıEm (t) = Em (t) − Em (t)
(12)
ıAm (t) =
The relative error in tracking the target and missile simultaneously can also be calculated in Eq. (13):
ıAtm (t) = Atm (t) − Atm (t) , (t) . ıEtm (t) = Etm (t) − Etm
(13)
where Atm (t) is the azimuth difference between the simulated target and missile in time t, Atm (t) is the azimuth difference between target and missile tracked by IRST in time t, Etm (t) is the elevation (t) difference between the simulated target and missile in time t, Etm is the elevation difference between target and missile tracked by IRST in time t. 3. Laboratory setup and measurement methodology The laboratory setup of the tracking performance test for IRST was shown in Fig. 2. It consists of tested IRST system, a blackbody, missile and target track simulator, control computer and relevant software. The missile and target track simulator is composing of rotary table and tripod. The rotary table with two pin-holes is used
H.-m. Qu et al. / Optik 124 (2013) 2313–2317
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Fig. 2. The layout of the laboratory setup.
as two point target which simulate the distance target and missile. Rotary table is driven by a stepper motor. The simulator is used for testing track performance of IRST. Blackbody was placed behind rotary table to simulate the temperature difference between targets and backgrounds. The control and user interface software fulfill two main functions. The first one is setting parameters and controlling the stepper motor movement. Another is gathering information of target output and calculating the trajectory generation, accuracy and outcome assessment. The control and test software are installed in the display and control computer. The target simulator includes two symmetrical holes in the dial take the place of simulated target and missile. Under the action of the driving force in the motor, the dial move around the center axes. An extension blackbody used to simulate temperature difference of target and background. Its temperature ranges from room temperature to 400◦ C. Its effective radiation area is 120 mm. The temperature resolution is 0.1 ◦ C. The temperature control precision reach to ± (0.15 + 0.002|t|) ◦ C. These parameters satisfy the requirements of experimental operation under the indoor temperature conditions. The temperature of blackbody is convenient to adjust and control target and background difference. It can provide appropriate target radiation to adapt different experimental environment conditions. Due to testing procedures depend strongly on accurate infrared target, background and atmosphere models [9]. The influence of atmosphere attenuation can be omitted in lab test. The simulated target and missile’s motion were driven by a high-precision stepper motor. A control PC and relative software manage the motor through serial port JMDM RS-485/232. The high-precision stepper motor includes five parts as follows: the serial port controller, serial communication lines, the stepper motor driver, 220 V convert to 24 V power supply module and a two-phase stepper motor. Stepper motor is an open-loop control component to put the pulse signal into angular displacement or linear displacement. In the case of non-overloaded the motor’s speed and stopping position depend on the pulse frequency and pulse number regardless of the impact of load changes. One pulse signal push the motor turning a one-step angle. There are linear relationship between the pulse number and motor step angle. There are only periodic error of the stepper motor and no accumulation of errors. These make the stepper motor simple and accurate in the speed, position and other control areas. The step angle in this system is 0.18◦ per pulse. The driver can achieve 256 subdivisions. It means that one pulse can make the stepper motor turn to the accuracy of 0.007◦ , converted to radians of 0.1 mrad. The search and track system is expected to reach the target track accuracy of 0.3 mrad. According to the error theory the test system can ensure high-precision target track simulation. The maximum speed
of the stepper motor reaches up to 90,000◦ per second. By speed the target movement step by step the maximum track speed and maneuverability of IRST can be evaluated. Hence it can fully meet the test requirements of high precision and maximum track speed for the search and track system. The blackbody and motor are fixed by the iron plate and supported by precision tripod, which can strictly control the height and level. The target track simulator system managed by a control PC through RS-232 serial port. The control software and user-interface programmed in VB MSComm. All signals convert to TTL level in chip MAX232 for stepper motor control. User interface software provides parameter settings used in the general operating applications (such as the stepper motor’s running length, running speed, running direction, output delay setting, the initial speed setting, manual speed settings, etc.). The PC serial port control and communication protocol was programmed with VB source code. The system controls the synchronous time and achieve target coordinates display as well as the synchronous dynamic trajectory by adding new timer. At the same time, the dynamic trajectory coordinates can be given on the specified interval angle. The tracking accuracy of the tested system can be educed by comparing the target trajectory coordinates given by IRST with simulation track coordinates. 4. Experimental test and results analysis 4.1. Experimental test In this tracking performance test, the small target track simulator is fixed at a distance L to search and track system. The distance L is 830 cm from rotation dial to the plane XOZ in Fig. 1. The radius of point target rotation track is 5 cm. The pin-hole diameter is 1 mm.Under that distance the small hole can be treated as point target imaging from the IRST. Through the parameters setting the simulator turns the target 90◦ , 135◦ and 180◦ respectively. At the same time the search and track system tracks and records the target’s real-time azimuth A, elevation angle E and their deviation A and E. Put the parameter r = 5 cm, L = 830 cm, ωt = 90◦ , ωt = 135◦ , ωt = 180◦ into Eqs. (7) and (8) respectively, we get the azimuth and elevation angle when the target turned an angle. The simulation software records the azimuth and elevation angle in the form of coder under different angular velocity and display the simulation target track on the user interface. The comparison between the simulated target track and the IRST tracking trajectory are shown in Figs. 3–8. Fig. 3 shows the azimuth difference when simulator turns from 0◦ to 90◦ . The x-coordinate is sampling numbers and y-coordinate
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H.-m. Qu et al. / Optik 124 (2013) 2313–2317
Fig. 3. Comparison between simulation and tracking azimuth when target move from 0 to 90◦ .
Fig. 4. Comparison between simulation and tracking elevation when target move from 0 to 90◦ .
Fig. 5. Comparison between simulation and tracking azimuth when target move from 0 to 135◦ .
is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 4 shows the elevation difference when simulator turns from 0◦ to 90◦ . Fig. 5 shows the azimuth difference when simulator turns from 0 to 135◦ . The x-coordinate is sampling numbers and y-coordinate is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 6 shows the elevation difference when simulator turns from 0 to 135◦ . Fig. 7 shows the azimuth difference when simulator turns from 0 to 180◦ . The x-coordinate is sampling numbers and y-coordinate is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 8 shows the elevation difference when simulator turns from 0◦ to 180◦ . The experimental tests are also performed when the simulator turned to the same point but in different angular velocity. Take the average of tracking azimuth and elevation angle among different speed trial and compare with the homologous
Fig. 6. Comparison between simulation and tracking elevation when target move from 0 to 135◦ .
Fig. 7. Comparison between simulation and tracking azimuth when target move from 0 to 180◦ .
Fig. 8. Comparison between simulation and tracking elevation when target move from 0 to 180◦ .
simulation values, the tracking error of the tested IRST system are following.
⎧ ıA90◦ = 0.0110◦ − 0◦ = 0.0110◦ = 0.19 mrad ⎪ ⎪ ⎪
⎪ ⎪ ⎪ ıE90◦ = 0.3360◦ − 0.3452◦ = 0.0092◦ = 0.16 mrad ⎪ ⎪ ⎪
⎪ ⎨ ıA135◦ = 0.2490◦ − 0.2440◦ = 0.0050◦ = 0.09 mrad
⎪ ıE135◦ = 0.2380◦ − 0.2441◦ = 0.0061◦ = 0.11 mrad ⎪ ⎪ ⎪
⎪ ⎪ ⎪ ıA180◦ = 0.3315◦ − 0.3452◦ = 0.0137◦ = 0.24 mrad ⎪ ⎪ ⎪
⎩ ıE180◦ = 0.0143◦ − 0◦ = 0.0143◦ = 0.25 mrad
From the above tracking errors of azimuth and elevation in the different angular position ωt, we know the tracking accuracy of tested IRST is better than 0.3 mrad. The maximum tracking speed and maneuverability of IRST can also be evaluated through change the moving speed of simulation target. Hence the established
H.-m. Qu et al. / Optik 124 (2013) 2313–2317
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setup and proposed method can fulfill the tracking performance evaluation and indoor test for search and track system in the development phase.
proposed test method. They can make the tracking performance test and evaluation in the laboratory conditions. That is great significance in the development of IRST system.
4.2. Error analysis
Acknowledgments
The errors of the small target track simulator include the systemic errors caused by simulator fabrication and assembly, the vibration of motor movement and the boresight calibration both the simulator and tested IRST. The simulator assembly and movement precisions satisfy the requirements through device parameter chosen. The boresight of the simulator and tested IRST was calibrated with a laser boresight calibration device before the test procedure. According to above experimental conditions, the slight vibration of the motor contributes 0.009 mrad maximum error. Comparing with the IRST tracking error and simulator step error it can be ignored and does not affect the experimental results. As a result, the small target track simulator and proposal test procedure can ensure the accuracy requirements for IRST indoor tests.
This work was supported by National Natural Science Foundation of China (grant no. 61171164), the National Defense Pre-research Foundation of China (grant no. 62201050103). We thank the anonymous reviewers for their constructive comments.
5. Conclusion A dynamic infrared target simulator is designed and implemented. High-precision infrared target track are simulated which employs the motion disk driven by stepper motor. Combined with software control, the simulator produces accuracy target track which is used for tracking trial and performance evaluation for IRST system. The simulation track accuracy was better than 0.1 mrad of the designed system. An indoor test approach for IRST tracking performance evaluation is proposed based on the built system. Various experimental tests have performed with IRST system. The experimental results verified the function of the established setup and
References [1] S. Kim, Y. Yang, B. Choi, Realistic infrared sequence generation by physics-based infrared target modeling for infrared search and track, Opt. Eng. 49 (11) (2010), 116401-1-9. [2] S.U. De Ceglie, M. Lo Moro, R. Vita, A. Neri, G. Barani, M. Cavallini, C. Quaranta, G. Colombi, A bi-spectral panoramic IRST – results from measurement campaigns with the Italian Navy, Proc. SPIE 7660 (2010), 766007-1-13. [3] S. Kim, J. Lee, Scale invariant small target detection by optimizing signal-toclutter ratio in heterogeneous background for infrared search and track, Pattern Recogn. 45 (2012) 393–406. [4] X. Sui, Q. Chen, L. Bai, Detection algorithm of targets for infrared search system based on area infrared focal plane array under complicated background, Optik 123 (2012) 235–239. [5] D. Maltese, O. Deyla, G. Vernet, Carole Preux Gisèle Hilt, P.-O. Nouguès, New generation of naval IRST: example of EOMS NG, Proc. SPIE 7660 (2010), 7660041-12. [6] G. Tidhar, S. Nadav, M. Danino, IRST tests standardization for panoramic IR sensors, Proc. SPIE 4030 (2000) 86–95. [7] L.E. Guzzetti, L. Busnelli, Infrared search and track and imaging system: testing in the laboratory and during flight, Proc. SPIE 7300 (2009), 73000S-1-12. [8] M. Scheuer, FLIR/IRST of the European fighter aircraft, Proc. SPIE 4714 (2002) 1–10. [9] P.B.W. Schwering, IRST testing methodologies: maritime infrared background simulator, Proc. SPIE 6206 (2006), 620621-1-12.
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Accuracy test and analysis for infrared search and track system Hui-ming Qu ∗ , Dan Cao, Qi Zheng, Yuan-yuan Li, Qian Chen School of Electronic Engineering and Optoelectronics Technology, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
a r t i c l e
i n f o
Article history: Received 16 February 2012 Accepted 27 June 2012
Keywords: Infrared search and track (IRST) Small target track Tracking accuracy Azimuth Elevation
a b s t r a c t Infrared search and track (IRST) attracts many attentions nowadays. It is important to evaluate the performance of the detection and tracking performance for the development of IRST. This paper presents a tracking accuracy indoor test method for IRST. The proposal approach compares the simulated target trajectories with the tracks given by the IRST and calculates the dynamic tracking accuracy of the tested system. An infrared point target track simulator was built to simulate the actual target and missile trajectories. The movement of the simulator was driven by a high-precision stepper motor. A blackbody was used to simulate the infrared radiation of targets. The simulator was controlled by computer via RS-232 serial port. The simulator moving speed, direction and the operation angle value was managed in a software interface written in Visual Basic. The moving point target’s coordinates, azimuth and elevation angle trajectories were reconstructed through the algorithm of coordinate translation. A comparison between the simulated target track and IRST recorded trajectory was performed to get the tracking accuracy of tested system. The paper describes the tracking accuracy test system design and implementation in detail. The experimental test and result analysis are provided to verify the effectiveness. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction An infrared search and track (IRST) system, also known as the infrared surveillance system (IRSS) in some literature, is pointed to a class of a passive detection system based on infrared radiation for timely detecting and capturing the particular infrared radiation targets in the background. It is capable of providing identification of objects as well as accurate 3D tracks, and sending the alarm information of target orientation and threat level to the display system or weapon control system. IRST attracts extensive study interest nowadays [1–4]. The tracking process of IRST is a key contributor for many system performances: reaction time, ability to maintain tracks on maneuvering targets, tracking accuracy, angular discrimination performance, and behavior in presence of crossing targets [5]. It is important to evaluate the performance of tracking performance for the development of IRST system. In recent years, with the development of infrared focal plane devices, a variety of intelligent infrared search and track system was proposed, which plays a more and more important role in the aviation, aerospace, military and other fields. At the same time, it raises a challenge to the traditional testing methods [6,7]. The system’s target detection rate, false alarm rate and other major indicators of combat skills are heavily dependent on the details of the target signal and background characteristics. The traditional lab investigation cannot
∗ Corresponding author. Tel.: +86 25 84315869; mobile: +86 13601582591. E-mail address: [email protected] (H.-m. Qu). 0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.06.093
reflect the actual situation. That is to say the performance evaluation of IRST system need multifarious test in the field. However, the field test has many disadvantages, such as the difficult control of the test condition, expensive cost, poor mobility, limited number of tests and so on. Dynamic infrared scene simulation technology is developed in this background. It relies on high and new technology such as computer, information processing, microelectronics, what’s more. It innovates to the laboratory test method and makes up for the deficiency of the field test. By combining the large number of infrared simulation tests and fewer field test validation, infrared scene simulation has an important meaning for evaluating the system performance, shortening the development cycle and saving cost. It has been widely used during the development phases of IRST [7,8]. To address this need, a real-time infrared target simulator was developed and tracking accuracy test approach for IRST is proposed in this paper. The following sections briefly discuss the simulator of moving point targets and tracking accuracy test method in the lab. The principle of small target track accuracy test is discussed in Section 2. The laboratory setup and measurement methodology is described in Section 3. The experimental test and results analysis are mentioned in Section 4. The conclusion is given in Section 5. 2. The principle of small target track accuracy test Infrared search and track system is a device of measuring twodimensional angle in polar coordinates. Its measuring principle is dynamic track and measurement. After the moving targets in space have been captured, they will enter the field of an optical
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H.-m. Qu et al. / Optik 124 (2013) 2313–2317
The coordinate of missile position expressed as Eq. (4):
⎧ x = r cos(ωt), ⎪ ⎨ m ⎪ ⎩
ym = L,
(4)
zm = −r sin(ωt),
Then
⎧ xt − xo −r cos(ωt) ⎪ , ⎨ tgAt = y − y = L t o xm − xo r cos(ωt) ⎪ ⎩ tgAm =
measurement system, and be locked by the servo in the track system. The system tracks the targets steadily and ensures they are always at the view field of the optical measurement system. Meanwhile, the tracking system records the targets’ deviation relative to system boresight: azimuth deviation and elevation deviation. In addition, the measurement system gives the azimuth angle and elevation angle of the boresight synchronously. Hence, the targets’ actual angle position information in the coordinate system of a search and track system is given as following equation: E = Ee + E
A = Ae + A
(1)
(2)
where Ee , Ae are the the boresight azimuth and elevation angle of IRST system. E, A are the azimuth deviation and elevation angle deviation of target to the IRST boresight. The Cartesian coordinate system for the trajectory data is shown in Fig. 1 in which z is the altitude coordinates. It shows motion relationship between the moving point targets and IRST system in the same space coordinates. Point O in the figure is the axes intersection point of coordinate system which located in the entrance pupil center of imaging system. Plane XOY is the datum plane for elevation angle. Plane YOZ is the datum plane for azimuth angle. Point T stands for the simulated target; M stands for the simulated missile. They are symmetrical to the center of a circle. When the stepper motor moving with the angular velocity ω and time t, the target turn an arc track which the corresponding centering angle ˛ equal to ωt. Where ˛ is the angle that the simulated target turned from the datum plane XOY, E is the elevation angle of the small targets, and A is the azimuth angle of the small targets. When the searching system tracks the targets, its optical axis coincides with the line OT. If the target moves with angle ˛ from one point to another, the search and track system will track the targets synchronously, and get the targets’ azimuth angle A and elevation angle E timely. In this space coordinates system, axis center of the search and track system: O(x0 , y0 , z0 ) = O(0, 0, 0), The center coordinate of simulated target T: T(xt , yt , zt ), The center coordinate of simulated missile M: M(xb , yb , zb ), The distance between the moving plane and XOZ plane is L, yt = yb = L. The target turned an angle ˛ = ωt, at the angular velocity of ω, then the coordinate of target position expressed as following:
⎧ x = −r cos(ωt), ⎪ ⎨ t ⎪ ⎩
yt = L, zt = r sin(ωt),
ym − yo
⎧ ⎪ ⎪ ⎨ tgEt =
Fig. 1. Schematic diagram of target trajectory coordinate system.
(3)
=
⎪ ⎪ ⎩ tgEm =
(5)
.
L
zt − zo
=
(xt − xo )2 + (yt − yo )2 zm − zo
(xm − xo )2 + (ym − yo )2
r sin(ωt)
=
[r cos(ωt)]2 + L2 −r sin(ωt)
,
[r cos(ωt)]2 + L2
(6) .
To take an inverse tangent to Eqs. (5) and (6), the azimuthal position and elevation angle of the target and missile are given below.
⎧ −r cos(ωt) ⎪ ⎨ At (t) = arctg , L
(7)
⎪ ⎩ Am (t) = arctg r cos(ωt) . L
⎧ r sin(ωt) ⎪ ⎪ ⎨ Et (t) = arctg 2
,
[r cos(ωt)] + L2 −r sin(ωt)
⎪ ⎪ ⎩ Em (t) = arctg
[r cos(ωt)]2 + L2
(8) .
The azimuth and elevation angle At (t), Am (t), Et (t), Em (t) changed with time t at the angular velocity ω. The above azimuth and elevation angle are simulation target and missile position coordinates. IRST system tracks the simulator and and elevation angle of the provides the real-time tracking azimuth (t) . By comparing the target At (t), Et (t) and missile Am (t), Em actual value of dynamic simulation target and IRST tracking value, we can calculate the tracking precision both the target and missile. ıAt (t) = At (t) − At (t)
(9)
Et (t) − Et (t)
(10)
ıEt (t) =
Am (t) − Am (t)
(11)
ıEm (t) = Em (t) − Em (t)
(12)
ıAm (t) =
The relative error in tracking the target and missile simultaneously can also be calculated in Eq. (13):
ıAtm (t) = Atm (t) − Atm (t) , (t) . ıEtm (t) = Etm (t) − Etm
(13)
where Atm (t) is the azimuth difference between the simulated target and missile in time t, Atm (t) is the azimuth difference between target and missile tracked by IRST in time t, Etm (t) is the elevation (t) difference between the simulated target and missile in time t, Etm is the elevation difference between target and missile tracked by IRST in time t. 3. Laboratory setup and measurement methodology The laboratory setup of the tracking performance test for IRST was shown in Fig. 2. It consists of tested IRST system, a blackbody, missile and target track simulator, control computer and relevant software. The missile and target track simulator is composing of rotary table and tripod. The rotary table with two pin-holes is used
H.-m. Qu et al. / Optik 124 (2013) 2313–2317
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Fig. 2. The layout of the laboratory setup.
as two point target which simulate the distance target and missile. Rotary table is driven by a stepper motor. The simulator is used for testing track performance of IRST. Blackbody was placed behind rotary table to simulate the temperature difference between targets and backgrounds. The control and user interface software fulfill two main functions. The first one is setting parameters and controlling the stepper motor movement. Another is gathering information of target output and calculating the trajectory generation, accuracy and outcome assessment. The control and test software are installed in the display and control computer. The target simulator includes two symmetrical holes in the dial take the place of simulated target and missile. Under the action of the driving force in the motor, the dial move around the center axes. An extension blackbody used to simulate temperature difference of target and background. Its temperature ranges from room temperature to 400◦ C. Its effective radiation area is 120 mm. The temperature resolution is 0.1 ◦ C. The temperature control precision reach to ± (0.15 + 0.002|t|) ◦ C. These parameters satisfy the requirements of experimental operation under the indoor temperature conditions. The temperature of blackbody is convenient to adjust and control target and background difference. It can provide appropriate target radiation to adapt different experimental environment conditions. Due to testing procedures depend strongly on accurate infrared target, background and atmosphere models [9]. The influence of atmosphere attenuation can be omitted in lab test. The simulated target and missile’s motion were driven by a high-precision stepper motor. A control PC and relative software manage the motor through serial port JMDM RS-485/232. The high-precision stepper motor includes five parts as follows: the serial port controller, serial communication lines, the stepper motor driver, 220 V convert to 24 V power supply module and a two-phase stepper motor. Stepper motor is an open-loop control component to put the pulse signal into angular displacement or linear displacement. In the case of non-overloaded the motor’s speed and stopping position depend on the pulse frequency and pulse number regardless of the impact of load changes. One pulse signal push the motor turning a one-step angle. There are linear relationship between the pulse number and motor step angle. There are only periodic error of the stepper motor and no accumulation of errors. These make the stepper motor simple and accurate in the speed, position and other control areas. The step angle in this system is 0.18◦ per pulse. The driver can achieve 256 subdivisions. It means that one pulse can make the stepper motor turn to the accuracy of 0.007◦ , converted to radians of 0.1 mrad. The search and track system is expected to reach the target track accuracy of 0.3 mrad. According to the error theory the test system can ensure high-precision target track simulation. The maximum speed
of the stepper motor reaches up to 90,000◦ per second. By speed the target movement step by step the maximum track speed and maneuverability of IRST can be evaluated. Hence it can fully meet the test requirements of high precision and maximum track speed for the search and track system. The blackbody and motor are fixed by the iron plate and supported by precision tripod, which can strictly control the height and level. The target track simulator system managed by a control PC through RS-232 serial port. The control software and user-interface programmed in VB MSComm. All signals convert to TTL level in chip MAX232 for stepper motor control. User interface software provides parameter settings used in the general operating applications (such as the stepper motor’s running length, running speed, running direction, output delay setting, the initial speed setting, manual speed settings, etc.). The PC serial port control and communication protocol was programmed with VB source code. The system controls the synchronous time and achieve target coordinates display as well as the synchronous dynamic trajectory by adding new timer. At the same time, the dynamic trajectory coordinates can be given on the specified interval angle. The tracking accuracy of the tested system can be educed by comparing the target trajectory coordinates given by IRST with simulation track coordinates. 4. Experimental test and results analysis 4.1. Experimental test In this tracking performance test, the small target track simulator is fixed at a distance L to search and track system. The distance L is 830 cm from rotation dial to the plane XOZ in Fig. 1. The radius of point target rotation track is 5 cm. The pin-hole diameter is 1 mm.Under that distance the small hole can be treated as point target imaging from the IRST. Through the parameters setting the simulator turns the target 90◦ , 135◦ and 180◦ respectively. At the same time the search and track system tracks and records the target’s real-time azimuth A, elevation angle E and their deviation A and E. Put the parameter r = 5 cm, L = 830 cm, ωt = 90◦ , ωt = 135◦ , ωt = 180◦ into Eqs. (7) and (8) respectively, we get the azimuth and elevation angle when the target turned an angle. The simulation software records the azimuth and elevation angle in the form of coder under different angular velocity and display the simulation target track on the user interface. The comparison between the simulated target track and the IRST tracking trajectory are shown in Figs. 3–8. Fig. 3 shows the azimuth difference when simulator turns from 0◦ to 90◦ . The x-coordinate is sampling numbers and y-coordinate
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Fig. 3. Comparison between simulation and tracking azimuth when target move from 0 to 90◦ .
Fig. 4. Comparison between simulation and tracking elevation when target move from 0 to 90◦ .
Fig. 5. Comparison between simulation and tracking azimuth when target move from 0 to 135◦ .
is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 4 shows the elevation difference when simulator turns from 0◦ to 90◦ . Fig. 5 shows the azimuth difference when simulator turns from 0 to 135◦ . The x-coordinate is sampling numbers and y-coordinate is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 6 shows the elevation difference when simulator turns from 0 to 135◦ . Fig. 7 shows the azimuth difference when simulator turns from 0 to 180◦ . The x-coordinate is sampling numbers and y-coordinate is azimuth in degree. The green line is simulator value and the blue line is IRST tracking value. Fig. 8 shows the elevation difference when simulator turns from 0◦ to 180◦ . The experimental tests are also performed when the simulator turned to the same point but in different angular velocity. Take the average of tracking azimuth and elevation angle among different speed trial and compare with the homologous
Fig. 6. Comparison between simulation and tracking elevation when target move from 0 to 135◦ .
Fig. 7. Comparison between simulation and tracking azimuth when target move from 0 to 180◦ .
Fig. 8. Comparison between simulation and tracking elevation when target move from 0 to 180◦ .
simulation values, the tracking error of the tested IRST system are following.
⎧ ıA90◦ = 0.0110◦ − 0◦ = 0.0110◦ = 0.19 mrad ⎪ ⎪ ⎪
⎪ ⎪ ⎪ ıE90◦ = 0.3360◦ − 0.3452◦ = 0.0092◦ = 0.16 mrad ⎪ ⎪ ⎪
⎪ ⎨ ıA135◦ = 0.2490◦ − 0.2440◦ = 0.0050◦ = 0.09 mrad
⎪ ıE135◦ = 0.2380◦ − 0.2441◦ = 0.0061◦ = 0.11 mrad ⎪ ⎪ ⎪
⎪ ⎪ ⎪ ıA180◦ = 0.3315◦ − 0.3452◦ = 0.0137◦ = 0.24 mrad ⎪ ⎪ ⎪
⎩ ıE180◦ = 0.0143◦ − 0◦ = 0.0143◦ = 0.25 mrad
From the above tracking errors of azimuth and elevation in the different angular position ωt, we know the tracking accuracy of tested IRST is better than 0.3 mrad. The maximum tracking speed and maneuverability of IRST can also be evaluated through change the moving speed of simulation target. Hence the established
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setup and proposed method can fulfill the tracking performance evaluation and indoor test for search and track system in the development phase.
proposed test method. They can make the tracking performance test and evaluation in the laboratory conditions. That is great significance in the development of IRST system.
4.2. Error analysis
Acknowledgments
The errors of the small target track simulator include the systemic errors caused by simulator fabrication and assembly, the vibration of motor movement and the boresight calibration both the simulator and tested IRST. The simulator assembly and movement precisions satisfy the requirements through device parameter chosen. The boresight of the simulator and tested IRST was calibrated with a laser boresight calibration device before the test procedure. According to above experimental conditions, the slight vibration of the motor contributes 0.009 mrad maximum error. Comparing with the IRST tracking error and simulator step error it can be ignored and does not affect the experimental results. As a result, the small target track simulator and proposal test procedure can ensure the accuracy requirements for IRST indoor tests.
This work was supported by National Natural Science Foundation of China (grant no. 61171164), the National Defense Pre-research Foundation of China (grant no. 62201050103). We thank the anonymous reviewers for their constructive comments.
5. Conclusion A dynamic infrared target simulator is designed and implemented. High-precision infrared target track are simulated which employs the motion disk driven by stepper motor. Combined with software control, the simulator produces accuracy target track which is used for tracking trial and performance evaluation for IRST system. The simulation track accuracy was better than 0.1 mrad of the designed system. An indoor test approach for IRST tracking performance evaluation is proposed based on the built system. Various experimental tests have performed with IRST system. The experimental results verified the function of the established setup and
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