## 2 Methodology
### Experimental Setup for Design of the Infrared Imaging-Based Diagnostics System  
In order to design an infrared imaging-based diagnostic system for use with turbojet engines and to construct a working knowledge base, a small iSTC-21v turbojet engine, developed in the Laboratory of Intelligent Control Systems of Aircraft Engines at the Faculty of Aeronautics, Technical University of Kosice (Figure 9), was used [62]. This is a single-stream, single-shaft turbojet engine with a single-stage one-sided radial compressor, an annular combustion chamber, and a single-stage non-cooled gas turbine with a variable exhaust nozzle with full authority intelligent electronic digital control system developed from the TS-21 engine (Ljulka-Saturn, Rybinsk, Russia). A schematic of the measurement and data acquisition system is shown in Figure 10, and the arrangement of the camera is shown in Figure 11[62]. Figure 10: Schematic of the measurement and data acquisition system used in this study. Figure 9: Views of the iSTC-21v turbojet engine (left) and its starting process (right) in a laboratory. A digital data acquisition system was used to observe the state of the engine [62]. Various sensors and data acquisition (DAQ) devices serve this purpose and send all the data to the control personal computer (PC). This PC contains a control system created in the LabVIEW (National Instruments Corporation, Austin, TX, USA) and Matlab/Simulink (MathWorks, Natick, MA, USA) software which is able to control and visualize the engine's performance, save and track all measured parameters, and update the control algorithms. For the design of the IRT diagnostics system, the following parameters are utilized:  
* n (rpm)--speed of the engine's shaft. * T\({}_{2}\) (\({}^{\circ}\)C)--air temperature at the output of the radial compressor. * T\({}_{3}\) (\({}^{\circ}\)C)--gas temperature at the input to the gas turbine. * T\({}_{4}\) (\({}^{\circ}\)C)--gas temperature at the output of the gas turbine. These parameters represent the inner state and temperature of the engine. The interior temperature was measured by a thermal sensor contained in the engine. The temperature of the engine's casing was measured by a thermal camera; this temperature can be correlated with the inner temperature although it has different dynamics. To measure the thermal temperature field around the engine, a FLIR Thermovision A40 thermal imaging camera was used (FLIR Systems, Wilsonville, OR, USA), and a stationary IR camera was used for scanning during long measurement processes, where continuous measurement in real time is necessary. The IR detector of the camera is a \(320\times 240\) focal plane array uncooled microbolometer. Its spectral range is 7.5 to 13 \(\upmu\)m, its instantaneous field of view (IFOV) is 1.3 mrad, its noise equivalent temperature difference at 60 Hz is 0.08 \({}^{\circ}\)C at 30 \({}^{\circ}\)C, its measurement uncertainty is \(\pm\)2 \({}^{\circ}\)C or \(\pm\) 2%, its temperature sampling frequency is 60 Hz, and its observable temperature range is from \(-\)40 to 1500 \({}^{\circ}\)C. The camera was controlled via the PC using the ThermaCAM Researcher software (FLIR Systems, Wilsonville, OR, USA) through the FireWire interface. One of the settings when capturing a video is the choice of color palette, which is used to convert the levels of the infrared spectrum into the visible spectrum. This selection is mainly important when there are small temperature differences between parts of an object or its surroundings. In the case of an object with a wide range of temperatures, such as a turbojet engine, the standard iron-bow palette is a suitable option [63]. As the engine is a dynamic object, the displacement of its thermal field needs to be evaluated continuously. The camera allows uncompressed video to be captured at 25 frames per second in audio video interleave (AVI) format using the selected color palette. The video can then be decomposed into single frames in uncompressed bitmap (BMP) format.