Design and Analysis of a Re-Entrant Microwave Cavity and Its Application for a High-Performance Accelerometer
Accelerometers with high resolution are required for tracking and navigation in launch vehicles, spacecrafts, missiles and other strategic applications. Pendulous integrating gyroscopic accelerometer and force balanced accelerometers cover the need for high accuracy guidance and navigation system. However, they require high precision fabrication, high piece part count and hence have low reliability. Moreover, these sensors are very expensive. The flexure mass accelerometer utilises a microwave resonator to provide inertial grade output. It also requires a complex fabrication process for the development and assembly of the proof mass. Moreover, its sensing electronics is complex and requires a large number of components. In this thesis, a microwave resonator is investigated, which offers a solution to the requirement of a high precision accelerometer for an inertial navigation system with low complexity, simple fabrication and high reliability. A differential re-entrant cavity sensor has been integrated with low noise cavity stabilized oscillator-based sensing electronics to achieve a high resolution and scale factor accelerometer. The cavities for the proposed accelerometer can be easily fabricated with conventional machining techniques. An improved approach is proposed in the present work to calculate the resonance frequency, its sensitivity, and quality factor of re-entrant cavities, having diverse taper shapes of the re-entrant post. The proposed closed-form expressions for resonance frequency are based on the chain matrix approach for tapered coaxial transmission lines. Analytical expression for various cases are used to evaluate the quality factor of these cavities. The calculated results for the resonance frequency, its sensitivity and quality factor have been verified with electromagnetic simulations and experiments, and the error in the proposed closed-form expression is found to be < 4%. It is demonstrated that the cavity with a post having a taper towards the re-entrant gap-end is more sensitive to gap variations as compared to straight post and previously reported inverted tapered post cavity. A displacement sensor and mechanically tunable resonator, based on a tapered post re-entrant cavity, is fabricated in this present work. The deflection of a thin diaphragm was evaluated numerically from the measured resonance frequencies and the proposed expressions derived in this work. This design of the sensor is found to be immune to manufacturing tolerance in the re-entrant gap. This thesis further discusses the design, fabrication and measurement results of an inertial grade microwave straight post re-entrant cavity-based accelerometer. A straight post cavity is chosen so as to reduce the piece part count and hence improve reliability of the sensor A scale factor model for the re-entrant cavity-based sensor is developed. It is then used for a parametric study of cavity’s resonance frequency, sensitivity and Q-factor. The design parameters of the sensor are optimized to achieve 5 times better scale factor as compared to previously reported work. The radio frequency (RF) readout for this high sensitivity accelerometer is designed using a low noise cavity-stabilized oscillator (CSO), which is frequency locked to the cavity. The CSO has an inherently low close-in phase noise and is simple in implementation. This method of locking the oscillator to the cavity does not require an active carrier suppression technique or an isolator, as reported in previous work. A mathematical model of phase noise of the CSO is developed to design this for an accelerometer resolution of 1 µg. The proposed accelerometer is evaluated after integrating the differential re-entrant cavity-based sensors with its CSO based sensing electronics. The measured phase noise of CSO is 47dB better, at 100 Hz offset, than the free running voltage controlled oscillator (VCO) used. The fabricated accelerometer achieved a scale factor of 2.5 MHz/g and a bias stability of 1ng with a measurement range of ±1 g. The achieved accelerometer performance is without any external control on the operating temperature and environmental condition. This is one of the best bias stability and scale factors reported to date in a similar genre of accelerometer fabricated using conventional machining. The proposed design can be used for low-cost, compact inertial grade accelerometers. This research focused on a high-performance applications of reentrant cavities. Results from this research indicate similar cavities and sensing circuits can be designed for diverse applications.