Device for measuring a pressure

Abstract: An improved device for measuring pressures, comprising a silicon membrane or diaphragm (1) which is exposed, on its two faces, to two pressures (P1,P2) whose difference is to be measured. A resonant element (2) is provided on one face of the diaphragm, which is treated by selective chemical etching, and is mechanically coupled to the diaphragm so that its resonance frequency varies as a consequence of the deformation undergone by the diaphragm.

Description: The present invention relates to the measurement of pressures in the industrial field. Pressure measurements are used extensively in process instrumentation and in control systems.From the state of the art many techniques are well known for pressure measurements. One of the most used techniques consider the use of resonating sensors. These sensors use variations in the resonance frequency of a structure which is already oscillating due to the elastic deformations caused by external mechanical factors such as force, acceleration, pressure, etc., also of the differential-mode kind. Materials such as quartz, steel, or other metallic compounds are commonly used for the fabrication of these sensors. The use of these sensors is advantageous with respect of the other known techniques because they allows to achieve better performances in terms of accuracy, resolution, etc. In spite of this fact, conventional resonance sensors do not allow to achieve performances able to fullfil the needs of the modem measuring systems expecially in terms of resolution. In order to overcome this problem, some sensors using a resonant structure made with semiconductor materials have been proposed. Although those sensors have shown better performances, they still have a high cost and entail considerable construction difficulties owing to the complex system for the excitation of the resonant element and for signal acquisition. Finally those sensors require an overpressure protection system which is external to the sensor and is generally very complex and difficult to provide. Patent application EP 0456029A discloses a pressure measuring device comprising a resonating element arranged on a measuring diaphragm. Patent US 5553506 discloses a force sensor including a resonator mounted by means of a dielectric layer on a bending element. The paper "Intelligent Differntial Pressure Transmitter using Micro-Resonators" (by Tokuji Saigusa et al., SIGNAL PROCESSING AND SYSTEMS CONTROL, INTELLIGENT SENSORS AND INSTRUMENTATION, SAN DIEGO, Nov. 9 th -13 th , 1992, vol. 3, N° Conf. 18, November 9 th , 1992, INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS, pages 1634-1639, XP000378729) discloses a resonant silicon pressure sensor and an intelligent pressure transmitter incorporating this sensor. Finally, patent application GB 2162314A discloses a transducer for stress or strain, in which a resonant double fork sensor comprises a couple of tines arranged to vibrate in their common plane. The aim of the present invention is to provide a pressure measurement device, which is an improvement with respect to the above-mentioned conventional devices This aim is achieved by means of the present invention, which consists of a device for measuring a pressure comprising a diaphragm and a resonant element, mechanically coupled to the diaphragm, said diaphragm being installed inside a casing and being exposed, on its two faces, to two pressures (P1, P2) whose difference is to be measured and determines a movement of said diaphragm. The device, according to the present invention, further comprises excitation means for generating the oscillations of said resonant element (2) and means for detecting said oscillations. The device, according to the present invention, is characterized in that: said diaphragm is formed by a silicon membrane and comprises a cavity accommodating said resonant element, said resonant element being obtained monolithically from said diaphragm and being rigidly connected to said diaphragm with its ends; and said diaphragm is fitted on a post, which rises from a base of said casing, a channel being formed inside said post in order to expose one face of said diaphragm to one of said two pressures (P1, P2); and said resonant element is arranged to vibrate in a direction perpendicular to the movement of said diaphragm. The diaphgram can be formed monolithically from a silicon wafer by machining. Also the resonant element is obtained monolithically from said silicon wafer from which the diaphgram is obtained. Preferably, the resonant element or resonator is formed on the upper surface of said diaphragm. By appropriately machining a silicon structure, the invention allows to overcome the manufacturing difficulties of the prior art and to achieve mass-production of the pressure-sensitive diaphragm on a silicon wafer. The resonator excitation system and the signal processing circuits can also be integrated directly on the same silicon structure. Furthermore, according to the present invention, it is possible to integrate on the silicon membrane another sensing element (based on the same principle or on a different one) capable of measuring the common-mode pressure applied to the membrane. The present invention is now described with reference to the accompanying drawings, which relate to preferred but non-limitative embodiments of the invention, and wherein: figure 1 is a schematic view of the structure of a pressure sensor according to the present invention; figure 2 is a view of a preferred embodiment of the pressure sensor according to the present invention; figure 3 is a more detailed view of the diaphragm and of the resonator of the device according to the present invention; figure 4 is a perspective view of a resonator of the Double Ended Tuning Fork (DETF) type; figure 5 is a view of a preferred embodiment of the DETF resonator; figure 6 is a plan view of some possible arrangements of the resonator and/or of the element that is sensitive to common-mode pressure on the diaphragm; and figure 7 is a schematic plan view of the excitation and acquisition systems. In the figures, the same reference numerals are used to designate identical or substantially corresponding components. With reference to figure 1, the sensor according to the present invention comprises a casing 10 which is preferably metallic and for example cylindrical and accommodates a membrane or diaphragm 1 which is suitable to be exposed to two pressures P1, P2 whose difference is to be measured. The casing 10 is provided with a base 11 and with a closure or cover 9 whereon it is possible to provide an optional opening 13 to allow one of the pressures (P1 in the figure) to access the diaphragm 1. A support or post 12, preferably made of silicon, rises from the base 11 of the casing; the diaphragm 1, which accommodates a resonant element 2, is mounted on the support. Inside the post 12 there is provided a channel 14 for exposing the lower surface of the diaphragm 1 to the other one of the two pressures (P2 in the figure). As shown in greater detail with reference to figure 3, the diaphragm 1 is constituted by a suitably machined silicon wafer and the resonant element 2 is formed at one of the two surfaces of the diaphragm. In order to provide insulation from common-mode pressure, it is possible to provide reinforcements, designated by the reference numerals 15, 16, 17 and 18, which are fixed both to the post 12 and to the external structure. With reference to figure 3, in the absence of pressure stresses on the faces, i.e., when the pressures on both faces of the diaphragm 1 are identical, a resonant element 2 can be made to vibrate with a suitable excitation system and has a resonance frequency of its own. When the two faces of the diaphragm 1 are exposed to different pressures P1 and P2, the diaphragm flexes upward or downward according to the difference between the pressures. In these conditions, by means of its couplings to the diaphragm, the resonator 2 is subjected to elastic deformations (i.e., non-permanent deformations) causing compression or traction and its resonance frequency varies. A detection system senses the oscillations of the resonant element and provides a signal having the same frequency of the detected oscillations. In order to avoid damping effects, the resonant element 2 is mounted at one of the faces of the diaphragm 1 in a cavity 7 formed on one of the two faces, and the cavity is kept in vacuum or in any case at a pressure of no more than 1.33 Pascall (10 -2 torr). The resonator 2 is obtained from the diaphragm 1, keeping its ends rigidly coupled to the diaphragm, and the cavity 7 is closed hermetically (in vacuum or in any case at very low pressure) by a layer of silicon 8 which preferably covers the entire corresponding surface of the diaphragm. The embodiment shown schematically in figure 2 further includes a protection against overpressures. The resonator 2 is more sensitive to compressions, and an excessive difference in pressure (overpressure) when P1 > P2 might cause it to buckle. According to the embodiment shown in figure 2, in order to limit the flexing of the diaphragm 1, such diaphragm is provided with a central protrusion or retainer 23 which protrudes from the lower surface 3 of the diaphragm 1 and is suitable to abut against a substrate 31 which is inserted between the post 12 and the diaphragm 1. In an upward region, the substrate 31 is joined to the edge of the diaphragm by means of a layer 25 of insulating material (preferably silicon oxide SiO 2 ), while in a downward region the substrate 21 is applied to the post by means of an annular layer 24 of insulating material (preferably pyrex). Preferably, the protrusion 23 is formed monolithically with the diaphragm 1. The dimensions of the raised portion are chosen so as to avoid appreciably altering the elastic deformation characteristics of the diaphragm. The substrate 31 is relatively rigid and has a hole 32 for keeping the lower face of the diaphragm connected to the channel 14. The abutment surface of the protrusion 23 is arranged peripherally on a plane which is parallel to the plane of the membrane and to the plane of the substrate 31. Figure 4 illustrates the structure of a resonator 2 according to the present invention, which is formed by a balanced resonator which is capable of minimizing the constraint reactions caused by the oscillations of the resonator, thus reducing the effect of the damping actions at the coupling points between the resonator and the diaphragm. The resonator 2 is formed by two parallel tines or arms 21, 22 which are joined at their ends and is known as DETF (Double Ended Tuning Fork); its lateral vibrations, i.e., the vibrations which are perpendicular to the movement of the diaphragm and are designated by the arrows F in figure 4, are used to detect the pressure. In the balanced resonator, the tines 21, 22 vibrate in phase opposition and at the constrained ends 24 the reactions to the motion of the two tines partially compensate each other, with a consequent lower dissipation of energy with respect to the case of a single vibrating prong. The balanced structure also allows several additional advantages, such as greater stability with respect to external influences, higher resolution, and reduction of the effect of long-term drifts. Advantageously, as shown in the preferred embodiment in figure 5, the DETF resonator is configured so as to have, at each end, two lateral protrusions 25 and a connecting portion 26 which are respectively wider and narrower than the central portion of the resonator. It is of course possible to use resonators with three or more parallel tines. Preferably, as shown in figure 6, the resonator 2 can be formed in various positions inside the diaphragm in order to achieve optimum sensitivity. More specifically, according to a first embodiment designated by the reference numeral 2B, it can be arranged substantially at the center of the diaphragm 1 and parallel to the plane of the diaphragm, in order to achieve maximum sensitivity. According to another embodiment, designated by the reference numeral 2A in figure 6, the resonator can be formed (or in any case positioned) at the edge of the diaphragm 1. This embodiment has lower sensitivity but offers the advantage of limiting the effects of overpressures. Moreover, according to the present invention, an additional sensing element, which need not be provided in the same manner and according to the same principle as described above, can be integrated on the silicon membrane and allows to measure the common-mode pressure applied to the membrane. According to a preferred embodiment, the resonator can be employed to measure the common-mode pressure by using a suitable electronic circuitry to detect the change of resistance of the resonator, said change being a consequence of the mechanical stress acting on the resonator itself and generated by the common-mode pressure. The device also has an excitation system for generating the oscillations of the resonator and a system for detecting them. In the embodiment shown schematically in figure 7, the excitation and acquisition systems are of the electrostatic or piezoelectric type and have two armatures or electrodes 27 and 28 which are parallel to the arms 21 and 22 of the resonator 2 and are insulated from the resonator by means of insulating layers 29 and 30. The detection system is of piezoresistive type and the voltage signal is detected by terminals 31 and 32. The detection can be carried out, for example, using a bridge circuit connected to an electronic differential stage. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims: A device for measuring a pressure, comprising a diaphragm and a resonant element (2), mechanically coupled to the diaphragm, said diaphragm being installed inside a casing (10) and being exposed, on its two faces, to two pressures (P1, P2) whose difference is to be measured and determines a movement of said diaphragm, said device further comprising excitation means for generating the oscillations of said resonant element (2) and means for detecting said oscillations, characterized in that : said diaphragm is formed by a silicon membrane (1) and comprises a cavity (7) accommodating said resonant element (2), said resonant element (2) being obtained monolithically from said diaphragm and being rigidly connected to said diaphragm with its ends; and said diaphragm is fitted on a post (12), which rises from a base (11) of said casing (10), a channel (14) being formed inside said post (12) in order to expose one face of said diaphragm to one of said two pressures (P1, P2); and said resonant element (2) is arranged to vibrate in a direction perpendicular to the movement of said diaphragm. A device, according to claim 1, characterized in that said diaphragm is formed monolithically from a silicon wafer by machining. A device, according to one or more of previous claims, characterized in that the pressure inside said cavity (7) is lower than 1.33 Pasced (10 -2 torr). A device, according to one or more of previous claims, characterized in that said resonant element (2) is formed by two parallel tines (21, 22), which are joined at their ends. A device, according to claim 4, characterized in that said resonant element (2) comprises two arms and comprises, at each end thereof, two lateral protrusions (31) and a connecting portion (32), which are respectively wider and narrower than a central portion of the resonant element (2). A device, according to one or more of previous claims, characterized in that it comprises a substrate (31), which is inserted between said post (12) and said diaphragm, and in that said diaphragm has a central protrusion (23), which is relatively rigid, protrudes from the surface of the diaphragm, and is suitable to abut against said substrate (31) in order to limit the flexing of the diaphragm. A device according to claim 6, characterized in that said substrate (31) comprises a hole (22) for keeping the lower face of the diaphragm connected said channel (14). A device according to claim 7, characterized in that said substrate (31) is joined, in an upward region, to the edge of the diaphragm by means of a layer (25) of insulating material, and in that in a downward region said substrate (31) is applied to said post (12) by means of an additional layer of insulating material (24). A device, according to claim 7, characterized in that said protrusion (23) is formed monolithically with the diaphragm and an abutment surface of the protrusion lies (23) on a plane, which is parallel to the surface of the diaphragm and to the surface of said substrate (31). A device, according to one or more of previous claims, characterized in that said excitation means of the resonant element (2) comprise an electrostatic system formed by fixed armatures or electrodes (27, 28) and moving armatures or electrodes formed by tines (21, 22) of the resonant element (2). A device, according to one or more of claims from 1 to 9, characterized in that said excitation means of the resonant element (2) comprise a piezoelectric system formed by fixed armatures or electrodes (27, 28) and moving armatures or electrodes formed by tines (21, 22) of the resonant element (2). A device, according to one or more of previous claims, characterized in that said excitation means also detect the oscillations. A device, according to one or more of previous claims, characterized in that said pressure is an absolute pressure or a differential pressure or a relative pressure.

Inventors: DE BORTOLI EROS, DE BORTOLI, EROS
Applicants: ABB KENT TAYLOR SPA, CONSORZ ABB SVILUPPO TECNOL, ABB KENT TAYLOR S.P.A., CONSORZIO ABB PER LO SVILUPPO TECNOLOGICO
Application year: 1997
Main classification: G01L9
IPC codes: G01L9