Force balanced pressure sensors.
Gogoi, Bishnu Prasanna
1998
Abstract
In this dissertation, the area of research that was explored was the concept of using force balancing or actuation balance to realize pressure sensors that are less sensitive to variations in material properties and manufacturing variations. Some of the key concepts developed were the principle of exact equivalence to achieve the force balance and a force multiplication scheme that enables the use of low voltages to balance large atmostpheric pressures (10$\sp5$Pa). These concepts make it attractive to develop practical control circuits and sensing devices that operate in closed loop mode. Two different devices that use these ideas were designed, fabricated and tested in this dissertation. The first device, fabricated with a 20 mask process, used an electrostatic actuator 200 x 200$\mu{\rm m}\sp2$ size, which required 25 volts to balance a pressure of 100 kPa. The zero pressure capacitance measured was 0.342pF with a full scale capacitance change of 70 fF for one atmosphere of pressure. The second device, fabricated with a 15 mask process used a hermetically sealed actuator 200 x 200$\mu {\rm m}\sp2$ in size with a 0.5$\mu$m gap and required a voltage of 12 volts to balance one atmosphere of pressure. The nominal capacitance at zero pressure was 2.12 pF, with a full scale capacitance change of 100 fF for 100 kPa change in pressure. The theory of electrostatic levitation as applied to a force balanced pressure application was first developed and the voltage requirements along with the stability limits were determined. The design issues associated with the practical implementation along with the physical limitations imposed by the fabrication process were discussed. A design method for the force balanced pressure sensor was then developed and actual dimensional parameters were obtained. Low voltage force balanced pressure sensors were fabricated using surface micro-machining techniques. A number of new processing techniques were developed to fabricate these devices. These devices were tested electrically to validate and characterize the device performance. It has been shown that low voltages in the order of 10-20 volts can be used to balance one atmosphere of pressure. In addition, the development of a force balanced pressure sensor system was described. An electromechanical model of the system was developed using the mechanical performance characteristics along with the requirements of the drive circuitry necessary to achieve the force balancing function. The system behavior was simulated using the design parameters and the differential equations defining the system behavior. The effect of the different parameters on the system performance is discussed. In this book, several interfacing circuits are proposed for closed loop applications.Subjects
Actuation Exact Equivalence Force Balanced Pressure Sensors
Types
Thesis
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