NEW APPROACHES FOR UTILIZING PLANAR INDUCTIVE SENSORS FOR GAP MEASUREMENT PROXIMITY AND LUBRICANT OIL WEAR DEBRIS MONITORING A Dissertation Presented to The Graduate Faculty of the University of Akron In Partial Fulfillment Of the Requirements for the Degree of Doctor of Philosophy Dian Jiao May, 2021 i NEW APPROACHES FOR UTILIZING PLANAR INDUCTIVE SENSORS FOR GAP MEASUREMENT PROXIMITY AND LUBRICANT OIL WEAR DEBRIS MONITORING Dian Jiao Dissertation Approved: Accepted: Advisor Department Chair Dr. Jiang Zhe Dr. Sergio Felicelli Committee Member Interim Dean of the College Dr. Jae-Won Choi Dr. Craig Menzemer Committee Member Dean of the Graduate School Dr. Shengyong Wang Dr. Marnie Saunders Committee Member Date Dr. Hongbo Cong Committee Member Dr. Yi Pang ii ABSTRACT Planar inductive sensors have been widely used in non-contact displacement measurement applications. Due to their advantages such as low cost, easy installation, high accuracy, and stability in harsh environments, planar inductive sensors are typically used to measure turbine blade tip clearance, the displacement of metal parts in belts, and the movement of robots/manipulators. However, the planar inductive sensors has a lot of limitations. First, when the material, size, or shape of the target is changed, the calibration curve needs to be rebuilt since the eddy current varies in different materials. Second, the inductive sensor which has a high sensitivity always needs a high frequency excitation signal. This means that when using the inductive sensor in industrial applications, several high performance support instruments need to be used with the sensor, such as high sampling rate data acquisition system and multi-channel power source. All these limitations cause a lot of inconvenience while using the planar inductive sensor. To overcome the above problems, we first presented a new method for planar inductive sensors to measure the gap between the sensor and a non-ferrite metallic target. The eddy current on the target plate was modeled as a virtual coil. The mutual inductance between the sensing coils and the virtual coil was calculated. From our analysis, we found that when the target material is changed, the new calibration curve can be obtained by adding a constant to the base calibration curve. To verify the validity of the method, three planar inductive sensors with different dimensions were manufactured and used to measure iii the gap from four different non-ferrite targets (Cu, Al, Zn, and Ti). Results showed that the new calibration method has a small error of 3.2% in the 500 – 5000 μm measurement range. Second, in order to extend the new calibration method to any shape/size of the target, we presented an improved method to measure the gap from an irregular and narrow target. The magnetic field distribution on an irregular and narrow target was numerically studied, suggesting the induced eddy current on the target can be modeled as a virtual planar coil. The gap was calculated from the mutual influence of the sensing and virtual coils. We found that the calibration curves are parallel for targets of different materials. Therefore, for a narrow and irregular target, the calibration curves corresponding to different materials can be obtained by adding a material constant to the base curve. To validate the approach, three planar coils of different sizes were tested with four metallic turbine blade shaped targets. Results showed that the measured gaps matched well with the actual gaps, with a maximum error of about 3.703%. Finally, a multi-sensing system using planar inductive sensors was designed to monitor the wear debris in lubrication oil, which is indicative of a machine’s health status. Several unique features were applied to the design of the sensing system including 1) parallel sensing via multiple sensing channels in order to improve the detection throughput, 2) use of an under sampling method in order to significantly reduce the amount of data to be collected, and 3) a new multi-layer structure to increase the sensitivity of the sensing system (25 μm diameter iron particles with 1470 μm inner diameter sensing tube). Testing results indicate that the amount of data size is dramatically reduced by nearly 20 times without scarifying the sensitivity. The sensor array is suitable for online wear debris monitoring. iv TABLE OF CONTENTS Page LIST OF FIGURES ........................................................................................................... ix LIST OF TABLES ........................................................................................................... xiv CHAPTER I. INTRODUCTION AND RESEARCH OBJECTIVES .................................................. 1 I.1 Introduction ............................................................................................................ 1 I.2 Motivation .............................................................................................................. 2 I.3 Research Objectives ............................................................................................... 5 I.4 Summary ................................................................................................................ 7 II. BACKGROUND AND LITERATURE REVIEW ....................................................... 9 II.1 Review of Non-Contact Displacement/Position Measurement Method ............... 9 II.1.1 Capacitive Method and Measurement Systems ........................................ 10 II.1.2 Optical Sensors and Measurement Systems ............................................. 11 II.1.3 Ultrasonic Method and Measurement System .......................................... 13 II.2 Inductive Proximity Sensors ............................................................................... 15 II.2.1 Inductive Proximity Sensor Based on a 3D Sensing Coil ........................ 16 II.2.2 Working Principle of 3D Inductive Proximity Sensors ............................ 17 v II.2.3 Inductive Proximity Sensor Based on a Planar Sensing Coil ................... 20 II.2.4 Working Principle of a Planar Inductive Proximity Sensor ..................... 21 II.2.5 Traditional Calibration Process for Inductive Proximity Sensor .............. 23 II.3 Study of Mathematical Model of Planar Coil Induction ..................................... 25 II.4 The Displacement Measurement Requirements from Narrow and Irregular Target ............................................................................................................. 29 II.5 Study of Wear Debris Monitoring Sensor........................................................... 31 II.5.1 Study of Wear Debris Sensors Based on Gill Sensor ............................... 33 II.5.2 Study of Wear Debris Sensors Based on Inductive Sensing .................... 35 III. NEW CALIBRATION METHOD FOR PLANAR INDUCTIVE SENSORS BASED ON CALCULATING MUTUAL INDUCTANCE .......................................................... 40 III.1 Calculation Method Based on Mathematical Model ......................................... 43 III.2 Method to Solve the Gap from Mutual Inductance ........................................... 53 III.3 Experimental Setup and Preparation ................................................................. 54 III.4 Measurement and Calculation Results ............................................................... 57 III.5 Validation of the Gap Measurements with the 푁1/푁2- 푑 Curves ...................... 63 III.6 Discussion .......................................................................................................... 69 III.7 Summary ............................................................................................................ 71 IV. A NEW APPROACH FOR GAP MEASUREMENT FROM NARROW AND IRREGULAR TARGETS ................................................................................................. 74 IV.1 Mathematical Model Analysis ........................................................................... 76 vi IV.1.1 FEA Method ............................................................................................ 76 IV.1.2 Working Principle ................................................................................... 82 IV.2 Experimental Setup ........................................................................................... 87 IV.3 Results and Discussions .................................................................................... 90 IV.3.1 Calibration ............................................................................................... 90 IV.3.2 Error Analysis ......................................................................................... 97 IV.4 Summary .......................................................................................................... 102 V. AN ADVANCED SENSOR ARRAY FOR WEAR DEBRIS MONITORING IN LUBRICANT OIL .......................................................................................................... 104 V.1 Design Concept for Single-Sensing Channel .................................................... 107 V.1.1 Design of Multi-layers Planar Inductive Sensing Coil ........................... 107 V.1.2 Measurement Circuit .............................................................................. 111 V.1.3 Under-sampling Signal Processing Method ........................................... 116 V.2 Device Design ................................................................................................... 119 V.3 Device Calibration