Pressure-based fluid flow sensor

A volumetric fluid flow sensor (100) includes a flow channel (120) for flowing a fluid therein; and a diaphragm (110) having an outer surface within the flow channel (120). The diaphragm (110) includes at least one flow disrupting feature mechanically coupled to or emerging from the outer surface of the diaphragm (110). A sensing structure (126) is coupled to the diaphragm (110) for generating a sensing signal responsive to a pressure signal on the diaphragm (110).

FIELD OF THE INVENTION

Embodiments of the present invention relate to diaphragm-based fluid flow sensors.

BACKGROUND

Both gas and liquid flow can be measured in volumetric or mass flow rates (such as liters per second or kg/s). These measurements can be converted between one another if the density of the material is known. However, the density of the material is not always known, such as in the case the composition of the fluid is unknown.

Volumetric flow sensors can be embodied as mechanical flow meters including rotometers and pith-ball indicators. Such mechanical flow meters are relatively large mechanical assemblies. Monolithic volumetric flow sensors are available that create a pressure differential from fluid flow normal to a planar stress-gauge diaphragm with a hole in it. Such a sensor, commonly referred to as an orifice plate, is placed in the flow and constricts the flow. Monolithic volumetric flow sensors use the same principle as the venturi meter in that the differential pressure relates to the velocity of the fluid flow (Bernoulli's principle).

Other devices for volumetric flow sensing are based on various configurations for sensing a flow-induced pressure differential. Well known microbridge structures achieve tangential sensing of mass flow only, require a well controlled heater for accuracy, and because of the heater, operate at higher levels of power dissipation. What is needed is new flow sensing devices having reduced complexity for lower cost and/or more robust designs.

SUMMARY

A volumetric fluid flow sensor according to an embodiment of the invention comprises a flow channel for flowing a fluid therein; a diaphragm having an outer surface within the flow channel, wherein the diaphragm comprises at least one flow disrupting feature mechanically coupled to or emerging from the outer surface of the diaphragm. The flow disrupting feature increases the pressure on the diaphragm compared to a conventional diaphragm which is uniformly planar. The increased pressure increases the deflection of the diaphragm and thus the available output signal provided by a sensing structure that provides an output signal based on the deflection of the diaphragm. As used herein, the term “flow disrupting feature” refers to a feature that is mechanically coupled to or emerging from the outer surface of the diaphragm that increases the pressure on the diaphragm ≧20% at one or more diaphragm locations as compared to the pressure obtained from a conventional planar diaphragm.

A sensing structure is coupled to the flow disrupting feature. The sensing structure generates a sensing signal responsive to a pressure signal on said diaphragm. In one embodiment, the sensing structure comprises a plurality of piezoresistive elements, such as arranged in a Wheatstone bridge configuration, which produces an electrical sensing signal. The sensing signal can produce a sensing signal that is proportional or nearly proportional to the pressure on the diaphragm induced by the fluid flow.

A method for sensing volumetric flow of a fluid comprises flowing a fluid over a diaphragm having an outer surface within a flow channel, wherein the diaphragm comprises at least one flow disrupting feature mechanically coupled to or emerging from said outer surface. A sensing signal is generated responsive to a pressure signal from a portion of said diaphragm. A flow rate of the fluid is determined from the sensing signal. The flow disrupting feature can partially obstruct a flow of the fluid to create a pressure differential between opposite sides of the diaphragm, along a direction of the flow. In another embodiment, the flow disrupting feature deflects the flow of the fluid to produce a force on the diaphragm.

DETAILED DESCRIPTION

The present Inventors have discovered that fluid flowing across the top of a pressure sensor comprising a stress sensitive diaphragm that is modified to include an attached flow disrupting, protruding feature produces a pressure on the diaphragm that is significantly enhanced (e.g. ≧20%, and typically ≧40%) as compared to the pressure provided by a conventional planar diaphragm. Depending on the geometry of the flow disrupting feature, the gas or liquid flowing around the flow disrupting feature can produce, for example: a pressure differential tangential to the diaphragm due to obstruction of the flow and thereby delivering a moment to the diaphragm, a pressure differential directed normal to the diaphragm resulting from an airfoil effect and thereby delivering a force that is normal to the diaphragm, and forces delivered to the diaphragm caused by deflection of the fluid such as that produced by an aileron, or a resultant of any combination of these effects. The pressure signal can be related to the flow rate using calibration data that can be generated empirically or by simulation.

In the case of piezoresistor-based sensing elements, these extra force-generating effects produces additional stresses on the piezoresistors which increases the output signal of the sensor. The piezoresistors can be located and oriented on the diaphragm in a manner that maximizes the output signal in response to an individual stress pattern produced by a corresponding individual flow disrupting feature shape. Although generally described using piezoresistors, embodiments of the present invention can use the diaphragm deflection to provide other useful measurables. For example, capacitive elements can be used for sensing. In the case of capacitive elements, one of the plates of the capacitor comprises or is supported by the diaphragm. Sensing elements can also include light-based generally fiber optic-based sensors, such as photoelastic, intensity-based, or interferometric-based. In yet another embodiment, the sensing can be Doppler-based.

As known in the art, a typical pressure sensor structure comprises a diaphragm which is exposed to one pressure on one side and a control pressure on the other side. The flexing of the diaphragm is then measured and correlated to the pressure. As also known in the art, this type of sensor can be formed using conventional semiconductor (e.g. silicon) integrated circuit processing using etching techniques (e.g. plasma etching), with, as one example, piezoresistive elements formed generally at locations within the diaphragm via ion implantation. The piezoresistive elements will vary their resistance in accordance with the stress placed on the diaphragm, allowing measurement of the pressure with an electrical circuit. Alternately, in the case of capacitive sensing, a chip can be bonded to the top of the sensor and capacitive changes can be measured instead of piezoresistive changes.

As described above, volumetric flow sensors according to embodiments of the invention modify typical pressure sensor structures by adding at least one attached flow disrupting protruding feature on the stress sensitive diaphragm. In one embodiment a horizontal pressure gradient is created across the diaphragm producing a moment in the structure, described below relative toFIG. 3A. In an alternate embodiment a vertical pressure gradient is created by an airfoil producing a vertical force on the diaphragm, described below relative toFIG. 4A. In yet another embodiment, a pressure gradient is generated by a wedge shaped feature mechanically coupled to an outer surface of the stress sensitive diaphragm described below relative toFIG. 5.

FIG. 1shows a cross sectional view of a volumetric fluid flow sensor100according to an embodiment of the invention having a flow disrupting feature comprising a bump105mechanically coupled to or emerging from an outer surface of a stress sensitive-sensitive diaphragm110. Stress sensitive diaphragms are conventionally used for pressure sensing, not for flow sensing according to embodiments of the invention. Diaphragm110is supported by a frame116which in one embodiment is formed from an integral substrate118, and in another embodiment diaphragm110and frame116are disposed on another substrate (e.g. bonded to), generally referred to as a constraint substrate118. Cavity region122is between diaphragm110and substrate118. Flow channel120allows fluid to flow over diaphragm110and interact with bump105which experiences forces responsive to fluid flow. Flow channel120can comprise a tube has a length sufficient to extend beyond the area of diaphragm110in both the length and width dimension. The flow disrupting bump105delivers flow-induced stress signals directly to the pressure transducer diaphragm110, and creates a pressure differential (gradient) along the diaphragm thereby imparting a moment or force on the diaphragm.

Two piezoresistive sensing elements126are shown (2 other piezoresistive resistors normally present to form a Wheatstone Bridge are not shown) which are generally formed within the diaphragm110, laterally placed from the bump105, such as by ion implantation. The piezoresistive sensing elements126perform a transducer function by generating an electrical sensing signal responsive to a pressure signal received. As shown inFIG. 2described below, signal detection circuitry128can be coupled to receive the electrical sensing signal or other sensing signal in the case of other sensing structures and is operable for determining a flow rate of the fluid from the sensing signal. Alternatively, volumetric fluid flow sensor100can simply provide a sensing signal, such as an electrical signal at bond pads connected to respective piezoresistive elements126.

Diaphragm110generally has a thickness ranging from about 5 μm to about 30 μm. In some configurations the diaphragm is square shaped with linear dimensions ranging from 3 to 10 mm. Diaphragm110can be generally formed from materials including silicon (e.g. single crystal silicon, epitaxially grown silicon, polysilicon) and silicon nitride Flow disrupting features according to embodiments of the invention, such as bump105shown inFIG. 1, can be formed using several known techniques. As described above, flow disrupting features according to embodiments of the invention can comprise the same material as diaphragm110, or can comprise a different material. For example, a “bump” of material can be deposited or micromachined on top of a pressure transducer diaphragm110. Slab shaped flaps can be patterned and etched out of a variety of thin film materials, such as silicon nitride or silicon dioxide. Patternable polymers such as conventional photoresists, polyimides, and PMMA, can be use to form a smooth bump on a surface. Shapes formed from these materials can be altered by reflowing, such as by using thermal and optical (UV) methods. Glasses and other ceramics or metals can be applied to the diaphragm surface as a thick film through a silk screening process. These can be glazed in place or reflowed to give a desired shape. Another method is to directly modify the diaphragm material by plasma etching. Deep Reactive Ion Etching (DRIE) tool makers offer tools that can vary a plasma to micromachine lens shapes, which could serve as an available flow disrupting feature shape. Micro inkjet technology is also available to deposit bumps of material and microlenses. Techniques utilizing combinations of these manufacturing processes can be used to machine shapes out of the diaphragm material.

The flow disrupting feature105can comprise a variety of other shapes or a plurality of shapes. For example, an elongated tetrahedral shape can be used for improved performance. The dimensions of the flow disrupting feature can be varied. Generally, the dimensions are on the order of tens or hundreds of microns. In the case of a uniform bump, such as bump105, in one embodiment the height is 70 μm, the and the length and width is approximately 300 μm. Other exemplary dimensions are provide in the Examples described below.

FIG. 2illustrates a depiction of 4 piezoresistive elements126within the diaphragm110including flow disrupting bump feature105for measuring the pressure differential. The piezoresistor elements126are shown connected to signal detection circuitry128, such as ASIC-based circuitry. The Wheatstone Bridge could be configured, such as guided by simulation, both electrically and physically, for maximum response to the resulting stress gradient. Symmetrical stresses would then appear as a common mode signal, offering rejection to uniform pressure changes and symmetrical package induced stresses.

FIG. 3Ais a top-side view of a volumetric fluid flow sensor300(flow channel not shown) according to an embodiment of the invention having a flow disrupting tear-drop shaped feature305mechanically coupled to the outer surface and near the center of a stress sensitive diaphragm110. Piezoresistors126are formed within the diaphragm110. Beyond the area of the diaphragm110, is a relatively thick portion of the die330(e.g. 200 to 400 μm thick). Bond pads331-334are coupled to the piezoresistors126by the electrical traces339shown. In operation, fluid impinging on tear drop305applies a moment near the center of the diaphragm110.

FIG. 3Bis a top-side view of the volumetric fluid flow sensor shown inFIG. 3Aassembled to include a flow channel120and a package345that together with flow channel surrounds sensor300, shown as reference350, according to an embodiment of the invention. The flow channel120extends beyond the diaphragm110shown inFIG. 3A, but does not extend to reach the bond pads331-334. Bond wires341-344are shown for contacting bond pads331-334and coupling the signals provided to circuitry such as ASIC circuitry, within the package345, for processing the sensor signal. The wire bonds341-344are outside the flow channel120, which generally leads to enhanced reliability as compared to wire bonds which are conventionally inside the flow channel.

FIG. 4Ais a top-side view of a volumetric fluid flow sensor400(flow channel not shown) according to an embodiment of the invention having a flow disrupting airfoil shaped feature405mechanically coupled to the outer surface near the center of a stress sensitive diaphragm110.FIG. 4Bis a cross sectional view of sensor400shown inFIG. 4A.

FIG. 5is a top-side view of a volumetric fluid flow sensor500according to an embodiment of the invention having a flow disrupting wedge-shaped feature505mechanically coupled to the outer surface of a stress sensitive diaphragm110, along with bond pads331-334coupled to the piezoresistors126. The fluid flow around the wedge505increases the pressure on the diaphragm110near the center of the diaphragm110, and has shown improved performance as compared to a uniform slab or bump, such as bump105described above relative to flow sensor100inFIG. 1.

EXAMPLES

The examples provided below are non-limiting examples provided to only show particular embodiments of the present invention. Unless noted otherwise, the fluid was air and, the air flow was 1 m/sec, and the diaphragm was a silicon diaphragm 16 μm thick, and had a length and width of 1.7 mm. The sensing structure comprised piezoresistors. Simulations used Ansys software (Ansys, Inc. 275 Technology Drive Canonsburg, Pa. 15317. Reference data was generated by simulating air flow over a conventional flat diaphragm.

FIGS. 6Aand B show the diaphragm deflection and Von Mises stress produced by the air flow, respectively.FIGS. 7Aand B show the diaphragm deflection and Von Mises stress produced by the air flow over a diaphragm with a bump according to an embodiment of the invention. The dimensions of the bump were 600 μm width (in the flow direction), 250 μm length (perpendicular to the flow direction) and 100 μm height. The flow around the bump produces a larger pressure in this section of the diaphragm compared to the rest of the diaphragm. A larger pressure on the diaphragm increases the deflection on it and thereby increases the output signal. The bump on the diaphragm was found to increase the stresses on the diaphragm by 57.7% as compared to the conventional flat diaphragm. This extra stress was found to increase the sensor output signal by 44%.

FIGS. 8Aand B show the diaphragm deflection and Von Mises stress produced by the air flow over a diaphragm with a wedge, according to an embodiment of the invention. The dimensions of the wedge were 600 μm width (in the flow direction) 200 μm length (perpendicular to the flow direction) and a height linearly increasing from zero to 100 μm. The wedge was found to increase the diaphragm stress by 93.4% as compared to the conventional flat diaphragm. This extra stress was found to increase the sensor output signal by 66.5%.

The shape and dimensions of the diaphragm, flow disrupting feature and fluid channel can be modified to further increase the pressure sensor's output signal. Such modifications can be based on analysis and prototype testing which can be used to adjust the size and shape of the components to increase the output signal.

This invention can be embodied in other forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be had to the following claims rather than the foregoing specification as indicating the scope of the invention.

In the preceding description, certain details are set forth in conjunction with the described embodiment of the present invention to provide a sufficient understanding of the invention. One skilled in the art will appreciate, however, that the invention may be practiced without these particular details. Furthermore, one skilled in the art will appreciate that the example embodiments described above do not limit the scope of the present invention and will also understand that various modifications, equivalents, and combinations of the disclosed embodiments and components of such embodiments are within the scope of the present invention.