A Study Of The Capacitive Sensor For Electromagnetic Absorbing Coating Thickness Measurement

Thickness measurement and check of radar absorbing coat (RAC) is very important for their theoretical study and application. In this thesis, several common coating thickness theory and methods are introduced. According on the electromagnetic absorber peculiarity and the structure characteristics of RAC, RAC painted on aluminium alloy base, a new high-performance, capacitance-based thickness measuring system for RAC has been proposed.In the pole structure designing, adding an equipotential shielding protect pole around the test pole of the coated capacitive sensor and accordingly using drive cable technique to reduce effectively the influence of the edge effect and cable capacitance on measuring. Also making signal collecting circuit and capacitive sensor as a unit block to reduce stray capacitance and using PCB technologic to fabricate the pole to reduce further the influence of the edge effect. All these measure are able to reduce the influence of capacitive sensor disadvantage factor during pole design. This thesis made extensive analysis on these measures and presented a new kind of coated capacitance sensor.The stability of the kernel circuit used to convert and amplify the input signal is of most importance. This thesis analyzed various factors affecting the stability of the instrument. In order to implement high accuracy measurement, the relevant methods were presented. Such as: by using a new capacitance-voltage converter (CVC) frame-ASIC, CAV424, shielding and a low-cost RISC microcontroller (MCU), three-signal approach associated with difference technology, the system can be automatic balanced and most of the errors are eliminated. Since the output signal from the signal-conditioning ASIC is industry-standard voltage signal and A/D converter is embedded in the MCU, the output of the ASIC can directly be connected to a microcontroller. As using Atmega8 MCU, the system has not to use external ADC, brown-out detection circuitry, a crystal. So this system is very simple.This thesis introduced the structure of system hardware and software. In the peripheral unit designing, keyboard unit and the interface circuit with MCU and accordingly software are important content. And some abbreviated schematic figure of the main peripheral unit interface circuit with MCU and the system main software flow chart are presented in this thesis.This thesis presented a serial of methods of how to debugging, calibration and test this thickness measurement instrument. The test proved that the performance index of instrument met the design requirement.