Aero-engine Tip Dynamic Clearance Detection And Realization

Blade tip clearance detection is an important part in the aircraft engines'research and maintenance work. Much work has been done in order to maximize the efficiency of aviation engines. One of these primary methods is adjusting the distance between the engine casing and blade tip. However, while the tip clearance's increment reduces the efficiency of the engine, and even causes engine surge, clearance's decrement will improve the efficiency by limiting the fuel leak, although such a small clearance will easily lead abrasion between blade tip and engine casing. More seriously, this abrasion may cause damages to the engine, affecting the safety and reliability of it, resulting in substantial economic loss, even threatening person's life. For all these reasons, setting up an appropriate distance of the blade tip clearance is crucial and essential. On the other hand, real-time dynamic detection of tip clearance has already become an important part in health monitoring project.Some methods are commonly used in the field of engine blade tip clearance testing, such as: eddy current method, microwave inducement, frequency capacitance way, fiber optical test and so on. A new method involved in this project is capacitance testing based on amplitude modulation (AM). By using high frequency analog circuits and high-speed data acquisition system, the amplitude of voltage on the tip clearance's capacitors is extracted (engine speed:12000r / min).In the research process, the tip clearance's capacitor is being precisely charged and discharged in the exact time. After that, there are some analog circuits extracting and rebuilding the signal, and finally the voltage differences will output to the next stage. There is a CPLD in this end stage, controlling the mode of AD converter, high-speed memory and PC interface by its logics. In the research work, speed is the first thing to think about, especially in the aspects of extracting speed, sampling rate and converting speed. Besides, low noise and offset amplifier circuits were involved, which making sure the infinitesimal differences on the tip clearance's capacitor can be extracted with less noise interference.After much experiments, the voltage changes on the capacitor is successfully extracted, which demonstrates the method of capacitance testing based on amplitude modulation feasible in engine blade tip detection. However, for the reasons of easy testing, debugging and modification, some control logic is formed by digital-chip structures, not completed in CPLD, in this early PCB edition. This causes some distortion of the signal waveforms, and directly influences the voltage value across the capacitor. But this will not happen in next PCB edition, because all of the control logics will be written into the CPLD.