Research On Rotor-Stator Clearance Measurement Based On Microwave Sensing

Rotor-stator clearance is an important state parameter of aero-engine,gas turbine and other large rotating machines,which directly reflects the safety state of equipment.Online clearance measurement can be used to optimize the design of large rotating machinery and is the basis of fault diagnosis and predictive maintenance.However,terrible working conditions such as high temperature,corrosion,pollution and narrow space bring great challenges to online clearance measurement.Traditional millimeter-level distance measurement methods,such as optical fiber method,capacitance method and eddy current method,cannot meet requirements.Microwave method is able to achieve high-precision and non-contact measurement of axial gap and blade tip clearance,with characteristics of high temperature resistance,gas corrosion resistance and small probe volume.This paper has taken deep research on microwave sensing from mechanism,measurement model,influencing factors and realization form.Measurement models and methods were proposed separately by microstrip and resonator.Measurement systems were successfully developed with microstrip applied to axial gap and resonator applied to blade tip clearance,which realized high-precision measurement.The main work is as follows:（1）To meet the requirement of rotor-stator clearance measurement applied to narrow space,microwave ranging methods based on pulse,frequency and phase were analyzed,and its mathematical models were built.Method based on phase was chosen to carry out high-precision online measurement through error analysis of different methods.Characteristis of microwave sensors,such as size,directionality,and high-temperature resistance were analyzed according to application environment.As a result,microstrip is determined to measure axial gap,and resonant is determined to measure blade tip clearance with temperature extremely high.（2）A kind of sensor structure with the microstrip patch antenna was designed for clearance measurement.The radiation field model of antenna was built using the cavity model to obtain key dimension parameters.The structure of microstrip sensor was optimized through electromagnetic simulation and its impedance matching was realized.In another way,the sensor structure with the open cavity to support high temperature was designed.The electromagnetic field model with the open cylindrical resonant and target coupled was built,so the working mode was set to TE₁₁₁.The sensor’s transition section was designed to bear high temperature by high impedance compensation.Parameters like characteristic impedance and cut-off frequency of the coaxial cable were used to obtain the optimal compensation value of the stepped transition section by axial misalignment,which ensures the impedance matching of the cable,the transition section,and the resonant cavity.So the distance between the coaxial cable and the sensor’s end face was extended,and the end face of the sensor can bear 1300℃uninfluenced by the coaxial cable.Simulation results showed that the measured phase changed monotonously with the clearance,which verified that the designed sensors can be used to measure clearance.（3）A signal adjusting method of axial gap was proposed by double heterodyne quadrature demodulation of signals of the measuring circuit and the reference circuit.The system structure was designed with the measurement path referring to the reference path using two local signal sources of different RF frequencies,so the intermediate frequency component of down-conversion signal was generated,which avoided the influence of DC deviation and scintillation noise to the axial gap measurement signal which varies slowly and continuously.The measurement error of axial gap was analyzed using the model of orthogonal demodulation signals.Phase noise model of PLL loop was built,and the loop width was optimized by simulation to limit the phase noise of the signal source,which brought high frequency stability.Meanwhile,the measurement range of axial clearance was extended using both of the phase and intensity signals of the axial gap.Results of simulations and experiments showed that the maximum relative error of signal source’s carrier frequency was controlled below0.55%.（4）Accuracy of the phase solution is easily affected by the interference of signals,which are the same frequency with the measuring signal,using the traditional arctangent method.A signal calibration method of clearance measurement was proposed to avoid the influence of interference caused by sensor end reflection,stator stray reflection,etc.Taking factors like end-face reflection,stray reflection,RF crosstalk and amplitude-phase imbalance into consideration,the mathematical models of measurement signals including in-phase（I）and quadrature（Q）were built.The relationship between clearance values and the signals of I and Q was obtained by the calibration method of traversing clearance at equal intervals.A complex signal was constructed based on the I and Q signals,and the amplitude components of the dominant frequency,image frequency and DC frequency were extracted according to the signal spectrum to build the high-precision calibration model of clearance signals.So the phase calibration curve can become monotonic in the full range,and the linearity of the curve is significantly improved.Results of the axial gap calibration experiments showed that the proposed method can extend the axial gap measurement range from12.65mm to 18.5mm.Using the piecewise polynomial fitting method,the maximum fitting deviation of the calibration curve in the full range was controlled below 25μm,and the curve’s linearity was better than 1.39‰.（5）A dynamic measurement method of blade tip clearance signals was proposed by adaptively intercepting phase regions,which can solve the problem of phase wrapped caused by unshrouded blades rotating.The intensity and phase signal models of the blade tip clearance of flat-top straight blade were built.Influence of the blade tip morphology such as blade thickness and cavity was simulated to analyze its effects on the intensity and phase signal curves.The peak region of the phase signal was adaptively intercepted using the intensity signal,and peak position was estimated by correlation matching.The signal peak value was obtained accurately by second-order polynomial fitting,and relationship between the peak value and the blade tip clearance was calibrated,which achieved the high-precision measurement of blade tip clearance based on the single-peak and complex multi-peak microwave sensing signals.（6）Two new kinds of microwave sensors were invented made by the microstrip patch antenna and the open cavity separately.Prototypes of the measurement systems were built to carry out experiments.The proposed microwave sensor of open resonant cavity can bear temperature up to 1300℃.A prototype of the axial gap measurement system has been developed,whose measurement range is 0.5～18.5mm,and the measurement accuracy is better than 30μm（at 0.5～3mm）and better than 1%（at3～18.5mm）.A prototype of the blade tip clearance measurement system has been developed too,whose measurement range is 0.5～3mm,and the dynamic measurement accuracy is better than 40μm at a speed of 3000r/min.Experimental results verified the effectiveness of the proposed methods.