This invention relates generally to pressure sensors and more particularly to pressure responsive variable parallel plate capacitive transducers. Such transducers are shown and described, for example, in U.S. Pat. No. 4,716,492, assigned to the assignee of the present invention. In that patent a capacitive pressure transducer is shown having a thin ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a ceramic base, with metal layers deposited on respective opposed surfaces of the diaphragm and base to serve as capacitor plates arranged in predetermined closely spaced relation to each other to form a capacitor. Transducer terminals connected to the capacitor plates are arranged at an opposite surface of the transducer base and a signal conditioning electrical circuit connected to the transducer terminals is mounted on the transducer. A cup-shaped connector body of electrical insulating material is fitted over the electrical circuit and is secured to the transducer by a housing sleeve which has a port for exposing the transducer diaphragm to an applied fluid pressure. The diaphragm is movable in response to variations in fluid pressure applied to the diaphragm to vary the capacitance of the capacitor in accordance with the changes in applied pressure and the electrical circuit provides an electrical output signal corresponding to the applied pressure.
In order to maximize the economies of mass production to lower the transducer cost and thereby make such transducers economically feasible for a wide number of applications including many previously served by low cost mechanical transducers, a standard size package is selected small enough to be received in a large number of applications yet large enough to provide a reliable signal. The size of the package determines the maximum size of the capacitor plates which, along with the gap between the plates, determines the capacitance signal. This results in limiting the size of the capacitor plates to a smaller size than would be ideal for many applications and relying on the electrical circuit to properly condition the signal. The circuit on the other hand requires a minimum level of capacitance in order to effectively condition the output signal and this in turn affects the size of the gap required between the capacitive plates to produce the minimum capacitance level. In transducers of the type disclosed in the above referenced patent, gaps between plates were in the order of 1-17 microns and more recently gaps in the order of 1-30 microns have been used effectively.
Transducers made in accordance with one of the embodiments described employ a flat diaphragm element secured to a flat base in selectively spaced sealed relation thereto by disposing a layer of a mixture of glass frit including spacing elements such as spheres or rods of a selected diameter between the diaphragm and the base along their peripheries. The glass frit is selected to be fusible at a first temperature at which the spacing elements remain unfused and the mixture is then heated to the fusing temperature of the frit to secure the diaphragm to the base at a spacing therebetween determined by the diameter of the spacing elements.
The diaphragm and base are typically made by taking conventional material, such as alumina, provided in powdered form coated with an organic binder and pressing the material into generally cylindrical configurations of appropriate thicknesses in a die. The diaphragms and bases must then go through a series of screen printing and firing operations to apply the capacitor plates and associated electrically conductive traces and the spacing and sealing glass pattern.
Although the diaphragm and base members are typically processed in batches of thirty or more at a time the individual members must be loaded and aligned into fixtures a number of times during the processing. This results in a labor and/or capital intensive product. Further, in order to obtain consistent, repeatable results from one transducer to another it is important that the diaphragm and base have a high degree of flatness. As a result, the parts particularly the bases, must be subjected to a grinding operation which adds appreciable cost to the finished product.