Capacitive pressure sensors are well known in the prior art. Such sensors typically include a fixed element having a rigid, planar conductive surface forming one plate of a substantially parallel plate capacitor. A displacable (relative to the fixed element) conductive member, such as a metal foil diaphragm, forms the other plate of the capacitor. Generally, the diaphragm is edge-supported so that a central portion is substantially parallel to and opposite the fixed plate. Because the sensor generally has the form of a parallel plate capacitor, the characteristic capacitance C of the sensor is approximated by the equation: ##EQU1##
where .epsilon. is the permittivity of the material between the parallel plates, A is the surface area of the parallel plate and d represents the gap between the plates. The characteristic capacitance is inversely proportional to the gap between a central portion of the diaphragm and the conductive surface of the fixed element. In order for there to permit a pressure differential across the diaphragm, the region on one side of the diaphragm is sealed from the region on the opposite side.
In practice, the diaphragm elasticity is selected so that pressure differentials across the diaphragm in a particular range of the interest cause displacements of the central portion of the diaphragm. These pressure differential-induced displacements result in corresponding variations in the gap, d, between the two capacitor plates, and thus in capacitance variations produced by the sensor capacitor. For relatively high sensitivity, such sensors require large changes of capacitance in response to relatively small gap changes. Regarding equation (1), if .epsilon. and A are held constant, the greatest slope of the d verses C plot occurs when d is small. Thus for the greatest sensitivity, the gap is made as small as possible when the device is in equilibrium and the sensor is designed so that the gap d decreases as pressure is applied. The multiplicative effect of .epsilon. and A increases the sensitivity of the d to C relationship, so .epsilon. and A are maximized to achieve the highest possible sensitivity.
In high pressure applications, the diaphragm must have a relatively small diameter and be relatively rigid to prevent rupture at the high pressure interface. The small diameter reduces A in equation (1) relative to conventional sensors, and the rigidity of the diaphragm reduces the range of d. These characteristics together tend to reduce the sensitivity of conventional sensors in high pressure applications.
There is therefore a need for a high pressure capacitive sensor that can attain the levels of sensitivity currently demonstrated by conventional capacitive sensors used in relatively low pressure environments.
It is therefore an object of the invention to provide a high pressure capacitive sensor that can attain the levels of sensitivity currently demonstrated by conventional capacitive sensors which are designed to operate in relatively low pressure environments.
Other objects and advantages of the present invention will become apparent upon consideration of the appended drawings and description thereof.