Abstract:
A differential pressure sensing system is provided. The sensing system includes a membrane layer having a channel extending diametrically therein, and including one or more cavities provided radially outbound of the channel and at least one resonant beam disposed in the channel and configured to oscillate at a desired frequency. The system further includes sensing circuitry configured to detect oscillation of the at least one resonant beam indicative of deformation in the membrane layer.

Description:
BACKGROUND 
   The present invention relates generally to sensors and, more particularly, to resonant differential pressure sensors. 
   Traditional differential pressure sensors are designed to determine the differential pressure between the two sides of the sensor. By way of example, traditional differential pressure sensors detect the differential pressure between two regions of interest by evaluating the net effect of the pressure forces of the two regions on a component or components of the sensor. When employed in harsh industrial environments, traditional pressure sensors often require a more robust construction. For example, if a differential pressure sensor is exposed to relatively high-pressure and/or high-temperature environments, the exposed components of the pressure sensor benefit from a construction robust enough to accommodate these conditions. 
   However, such traditional differential pressure sensors, the features and attributes that facilitate operation in such high pressure (i.e., harsh) environments can negatively impact the resolution of the sensor. That is to say, traditional differential pressure sensors that are robust enough to withstand high-pressure environments, for example, cannot detect the pressure differential between the two regions of interest in orders of magnitude less than the pressure difference in the environment. For example, resonating differential pressure sensors robust enough to withstand pressures of 5000 pounds per square. inch (psi), and beyond, generally do not have sufficient resolutional capabilities to detect a pressure differential of +/−10 psi, for instance. This is because traditional resonating pressure sensors contain vacuum within the closed enclosure between the diaphragms of the pressure sensor, and therefore with high pressures acting on the each of the diaphragm, the diaphragms may tend to bulge inside. 
   Thus, there is a need for a pressure sensing system and method that can provide differential pressure sensing capabilities with high resolution, while withstanding high line-pressures, for instance. 
   SUMMARY 
   In accordance with one aspect of the present technique, a differential pressure sensing system is provided. The system comprises a membrane layer having a channel extending diametrically therein, and including one or more cavities provided radially outbound of the channel and at least one resonant beam disposed in the channel and configured to oscillate at a desired frequency. The system further includes sensing circuitry configured to detect oscillation of the at least one resonant beam indicative of deformation in the membrane layer. 
   In another embodiment of the present technique a differential pressure sensor is provided that comprises a fixed support structure and a first membrane layer and a second membrane layer coupled to the fixed support structure. The first and second membrane layers cooperate to define at least one cavity therein. The sensor further includes a resonant member disposed within the cavity and configured to oscillate at a resonant frequency and one or more mesas bonded on the first and the second membrane layers and coupled to the resonant member. The mesas are configured to transmit deformations generated in the first membrane layer to the second membrane layer and to the at least one resonant member. 
   In alternate embodiment of the present technique, a method of manufacturing a differential pressure sensor is provided. The method comprises disposing a first membrane layer including a first channel in a fixed support structure and disposing a second membrane layer including a second channel in the fixed support structure, such that the first and second membranes form a membrane and the first and second channels form a closed channel. The closed channel extends to peripheral portions of the membrane. The method further comprises disposing at least one resonant beam slidably within the closed channel and one or more cavities etched within the peripheral portions of the membrane. The one or more pillars support the at least one resonant beam within the one or more cavities at the peripheral portions of the membrane. 
   These and other advantages and features will be more readily understood from the following detailed description of preferred embodiments of the invention that is provided in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatical view of a differential pressure sensing system, in accordance with an exemplary embodiment of the present technique; 
       FIG. 2  is a cross-sectional view of a differential pressure sensor, in accordance with an exemplary embodiment of the present technique; 
       FIG. 3  is an exploded view of a differential pressure sensor, in accordance with an exemplary embodiment of the present technique; 
       FIG. 4  is a cross-sectional view of a differential pressure sensor, in accordance with an exemplary embodiment of the present technique; 
       FIG. 5  is a cross-sectional view of a differential pressure sensor, in accordance with an embodiment of the present technique. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   In the subsequent paragraphs, an approach for measuring pressure differential within an industrial system will be explained in detail. The approach described hereinafter provides and facilitates measurements of high-resolution differential pressure in high line-pressure environments. As will be appreciated by those of ordinary skill in the art, and as a preliminary matter, line-pressure is the pressure force independently acting on a diaphragm, while the difference between the line-pressures acting on the two surfaces of a diaphragm (in a single diaphragm differential pressure sensor) or on the two diaphragms (of a two-diaphragm differential pressure sensor) is called differential pressure. The various aspects of the present technique will be explained, by way of example only, with the aid of figures hereinafter. 
   Referring generally to  FIG. 1 , pressure sensing techniques will be described by reference to a pressure sensing system designated generally by numeral  10 . It should be appreciated, however, that the pressure sensors described below may find application in a range of settings and systems, and that its use in the pressure sensing application discussed is but one such application.  FIG. 1  illustrates a diagrammatical view of an exemplary pressure sensing system  10  that has a differential pressure sensor  12  for use in an industrial application. By way of example, industrial applications include but are not limited to pressure differential measurements in piping applications, oil drilling applications, vessels, and chemical manufacturing applications, to name but a few. The industrial applications also may include pipelines, pressure chambers, flow devices, or similar applications. The differential pressure sensor  12  is disposed in an industrial unit  14 , such as a pressure chamber, a flow device, a pump line, or a mixing chamber, or similar applications as will be appreciated by those skilled in the art in view of the present discussion. 
   The industrial unit  14  includes a first region  16  and a second region  18  at pressures that may be same or different from one other. The two regions  16  and  18  are isolated from one another by a barrier  20 . The sensor  12  measures the pressure differential between the two regions  16  and  18 . The sensor  12  is believed to be capable of measuring low differential pressure in the range of about 0.1 pound square inch (psi) to about 15 psi and is believed to be robust enough to withstand high static pressures of about 1000 psi to about 5000 psi, if not beyond. By determining the net effect of pressure forces between the first and second regions (i.e.  16  and  18 , respectively) on a component or a series of components of the pressure sensor  12 , the sensor  12  determines the difference in pressure between the two regions  16  and  18 . Indeed, the exemplary sensor  12 , as discussed in more detail below, presents features that facilitate measurement of relatively low pressure differential (e.g. about +/−10 psi) as well as withstanding relatively high pressure environments (e.g. about +/−5000 psi). Moreover, sensor  12  presents features that facilitate sensing higher pressure differentials as well. 
   The system  10  includes other functional components associated with the pressure sensing components, such as control circuitry  22 , sensing circuitry  24 , and processing circuitry  26 . The control circuitry  22  coupled to the sensor  12  is adapted to facilitate excitation of one or more resonating devices of the sensor  12  to oscillate each resonating device at its natural resonant frequency. The details of this excitation and the resultant oscillations are discussed further below. The sensing circuitry  24  detects deformations of a membrane layer by measuring the changes in the oscillations of resonating devices present in the sensor  12 , as the changes in the oscillations of the resonating devices have been corresponded with the deformation of the membrane layer as would be appreciated by those of ordinary skill in the art. Output data from the sensing circuitry  24  is then processed by the processing circuitry  26  to generate a value indicative of the pressure differential measured by the sensor  12 . The system  10  further includes communication circuitry  28 , a database  30 , and a remote monitoring center  32 . The database  30  is configured to store information pertinent and beneficial to the system  10 , such as information generated about pressure differential in the environment and predefined information about the sensor  12 , for example. The database  30  also is configured to store information from the sensing circuitry  24  or the processing circuitry  26 , as may be needed for a particular application or use. As discussed further below, the database  30  may be located locally or remotely, such as, for example, at the remote monitoring center  32 . 
   In the exemplary embodiment, the communication circuitry  28  receives data signals  34  from the processing circuitry  26  and transmits the data signals to a remote location, such as the illustrated remote monitoring center  32 . The communication circuitry  28  comprises hardware and/or software that enables the communication circuitry  28  to communicate the data signals  34  to the remote monitoring center  32 . In various embodiments, the communication circuitry  28  is configured to communicate the data signals to the remote monitoring center  32  in accordance with a cellular protocol, a wireless protocol, a radio frequency protocol, and the like. Of course, those of ordinary skill in the art will appreciate that any type of communication protocols can be employed. 
   Referring now to  FIG. 2 , a cross-sectional view of one exemplary embodiment of a differential pressure sensor  36 , for use in the pressure sensing system, such as system  10 , is illustrated. The pressure sensor  36  includes a fixed support structure  38  for providing structural support to two membrane layers  40  and  42 , which function as diaphragms. The fixed support structure  38  may be constructed as a circular structure, a rectangular structure, a square structure, or any closed or open structure that can facilitate pressure differential measurements between two pressurized environments. In this embodiment, the two diaphragms  40  and  42  are coupled to the fixed support structure  38  so that a cavity  44  is formed and defined by these structures. The cavity  44  may be evacuated to enclose a vacuum region inside. 
   A resonant member or a resonant beam  46  is disposed within the cavity  44 . The resonant beam  46  may be excited via an electrostatic actuator embedded in the control circuitry  22 , shown in  FIG. 1 . The resonant beam  46  is thus made to oscillate at its natural resonant frequency. However, when the resonant beam  46  is subjected to strain, such as the strain resulting from a deformation due to differential pressure, the resonant frequency of oscillation of the resonant beam shifts from the natural resonant frequency. This change or shift in the resonant frequency may be calibrated to read an amount of force, such as for example, pressure, weight, stress, and the like. The sensor  36  also includes a set of mesas  48  and  50 . The resonant beam  46  is sandwiched between mesas  48  and may be symmetrically located along the centre of the sensor  36 , while passing through mesa  50 . Although, there are three sets of mesas shown in  FIG. 2 , any number of mesas may be constructed. As illustrated, central mesas  50  may be constructed to cover a larger area than the area covered by the peripheral mesas  48 . The larger area of central mesas  50  transfers the force component acting perpendicularly to the membrane layer  40  to membrane layer  42 , and vice-versa, thereby allowing a deformation of the resonant member  46  proportional to the difference in pressures acting on the two membrane layers. It will be appreciated by one of ordinary skill in the art that larger the area of the mesas, the lesser the losses due to line-pressure sensitivity of the membrane layers  40  and  42 . 
   It is worth noting that within the cavity  44 , mesas  48  may be asymmetrically placed between the fixed support structure  38  and mesa  50 . The asymmetrical placement of mesas  48  may be based on the line-pressure sensitivity. This mesa asymmetry facilitates removal of line-pressure sensitivity of the two membrane layers  40  and  42 . 
   Referring to  FIG. 3 , an exploded view of a differential pressure sensor  37  is illustrated. As shown, the pressure sensor  37  includes a fixed support structure  38 , which supports two membrane layers or diaphragms  40  and  42 . The diaphragms  40  and  42  and the fixed support structure  38  enclose vacuum pressure within the cavity formed therein. The cavity includes masses  52  that are coupled to the fixed support structure  38  by beam supports  54 . Through the masses  52 , one or more resonant members  56 , such as resonant beams, are arranged. The diaphragms  40  and  42  and the masses  52  are be bonded together using mesas  48 . As illustrated, the central mesa  50  is larger compared to the other mesas  48 , and this mesa  50  is a single structure coupling the diaphragms  40  and  42 . This structure facilitates transmission of the force acting due to pressure applied on diaphragm  40  to counter the force acting on diaphragm  42 , or vice-versa. Thus, if P 1  is the pressure acting on diaphragm  40 , and P 2  is the pressure acting on diaphragm  42 , and if P 1 &gt;P 2 , the diaphragms  40  and  42  will move downwards in tandem, thereby causing masses  52  to move laterally in proportion to the difference in pressure, i.e., P 1 ˜P 2 . This lateral movement of masses  52  would result in the resonant members  56  being strained. 
   The resonant members  56  are energized via an electrostatic actuator embedded in the control circuitry  22 , shown in  FIG. 1 . The resonant members  56  have a natural resonant frequency with which they oscillate when excited or energized. However, when the resonant members  56  are subjected to strain, the resonant frequency of oscillation of the resonant members shifts from the natural resonant frequency. This change or shift in the resonant frequency is calibrated to read an amount of force, such as for example, pressure, weight, stress, and the like. 
     FIG. 4  is a cross-sectional view of an alternative embodiment of a differential pressure sensor  58 . The pressure sensor  58  includes a fixed support structure  38 , disposed on which are two diaphragms or membrane layers  60  and  62 . The diaphragms  60  and  62  are supported by and bonded to a large mesa structure  64 . During fabrication, the fixed support  38  and the mesa structure  64  may include two layers of the fixed support and two layers of mesa structure that are then bonded to yield a single fixed support and a single mesa structure. A resonant beam or a resonant member  66  passes through a channel within the mesa  64 . This resonant member  66  is constructed such that it is supported on either ends on two pillar-like smaller mesas  68 . 
   The functioning of the sensor  58  is similar to the pressure sensor embodiments described above with reference to  FIGS. 2 and 3 . When a pressure of magnitude P 1  acts over the diaphragm  60 , and a pressure of magnitude P 2  acts over the diaphragm  62 , where P 1 &gt;P 2  for example, the two diaphragms  60  and  62  will move downwards together along with the mesa structure  64 . Mesa structure  68  facilitates the resonant member  66  to sway sideward, instead of directing the force due to the difference in pressures P 1 ˜P 2  to cause the resonant member  66  to move downwards. Thus, high precision readings of the pressure differential P 1 ˜P 2  may be acquired. It may be noted that if the central mesa structure  64  were not large enough, then there may be chances of local depressions in the diaphragms  60  and  62 , because the line-pressures P 1  and P 2  may be very high of the order of thousands of psi in magnitude, and the cavity formed by the fixed support structure  38  and the diaphragms encloses vacuum pressure. Therefore, it will be appreciated by one of ordinary skill in the art that the central mesa  64  may be constructed to extend throughout the cavity formed by the fixed support structure  38  and the diaphragms  60  and  62  to reduce stresses. Thus, in one embodiment, mesa  64  may function as a single membrane layer with a channel therein that allows a resonant member to pass through. Such a structure will have a pancake-like structure that has a channel diametrically extending to the peripheral walls. 
     FIG. 5  is a cross-sectional view of an alternative embodiment of a differential pressure sensor  70  with a single membrane layer. As described earlier with reference to  FIG. 4 , the pressure sensor  70  also includes a fixed support structure  38 , one diaphragm  72  that is formed by extending the central large mesa structure, and a resonant member  66  that rests on two smaller pillar-like mesa structures  68 . During fabrication, the fixed support  38  and the diaphragm  72  (or extended mesa structure) may include two layers  74  and  76  that are bonded to yield a single fixed support  38  and a single mesa structure  72 . The pressure sensor  70  functions in the same manner as the pressure sensor of  FIG. 4 . However, in sensor  70 , the diaphragm or the membrane layer  72  has grooves  78  on surfaces of the diaphragms, as shown. These grooves  78  are similar to a corrugated pattern that allows the movement of the diaphragm  72  for facilitating higher differential pressure sensitivity. As previously described, mesas  68  may be asymmetrically positioned inside grooves  80 , as illustrated. This asymmetric mesa placement reduces line-pressure sensitivity. 
   In sensor  70 , the central mesa is extended towards the fixed support structure  38 . Thus, sensor  70  has only a single pancake-like membrane layer  72  that includes a channel within, through which the resonant member  66  passes. Vacuum cavity  44  may also be present, in which the resonant member  66  can slide laterally. 
   It may be noted that fixed support structure  38 , diaphragms  40 ,  42 ,  60 ,  62 , and  72 , mesas  48 ,  50 ,  64  and  68 , masses  52 , support beams  54 , and resonant members  46 ,  56  and  66  may be fabricated using a semiconductor material, such as silicon (Si), silicon carbide (SiC), gallium nitride (GaN), and the like. Resonant members  46 ,  56  and  66  include one or more piezo-resistive or piezo-capacitive elements that allow strain measurement. However, in other embodiments, metallic components or a combination of metallic and semiconductor components can be used too. 
   Although the embodiments illustrated and described hereinabove represent only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, the pressure sensor embodiments  12 ,  36 ,  58 , and  70  may be employed in harsh environments, such as satellites, robots, avionic applications, and robots, among others. Furthermore, the sensor embodiments may be driven by circuitries known to those of ordinary skill in the art, which can actuate the resonant members, and correlate the deformation in the resonant members to pressure differentials. 
   Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.