Abstract:
An airflow sensor apparatus for measuring flow rate includes a pitot tube with a bypass channel wherein the pitot tube extends halfway into a flow channel in order to reduce a pressure drop. One or more upstream taps can be spaced along the pitot tube facing into a direction of a flow stream which directs the flow to the bypass channel. At least one or more downstream taps can be located to face perpendicular to the direction of flow, such that the fluid after passing over a flow sensor passes through the downstream tap(s). The upstream tap senses stagnation pressure and the down stream tap senses static pressure which is exerted in all directions in the flow channel in order to determine a velocity pressure based on a difference between pressures.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments are generally related to sensor methods and systems. Embodiments are also related to airflow sensors for medical ventilator applications. Embodiments are additionally related to airflow sensors with ultra low-pressure drop. 
       BACKGROUND OF THE INVENTION 
       [0002]    Flow rate control mechanisms are used in a variety of flow systems as a means for controlling the amount of fluid, gaseous or liquid, traveling through the system. The flow control mechanisms can be utilized to regulate flow rates in systems such as ventilators and respirators where, for example, it may be desirable to maintain a sufficient flow of breathable air or provide sufficient anesthetizing gas to a patient in preparation for surgery. 
         [0003]    MEMS based flow sensors can be utilized for measuring such flow rates in a variety of commercial, industrial and medical applications. In medical applications, for example, it is often required to accurately measure the flow rates of fluids introduced intravenously to medical patients and thereby control the flow rate of such fluids. In such applications, flow control is an inherent aspect of proper operation, which can be achieved in part by utilizing the flow sensors to measure the flow rate of fluid within the flow system. 
         [0004]    Ventilators are medical devices for delivering a breathing gas to a patient. Usually, ventilators employed in hospital critical care units provide a supply of air enriched with oxygen for inspiration by the patient, and may conventionally include controls for either assisting or controlling breathing, exhaled volume indicators, alarms systems, positive end expiratory pressure valves, pressure indicators, gas concentration monitors, flow indicators, and heated humidifiers for warming and humidifying the breathing gas. Ventilators used in home care are often used to treat obstructive sleep apnea and supply positive air pressure to assist breathing. Manufacturers of medical ventilator equipment require an ultra low pressure drop to insure efficient blower operations. 
         [0005]    The majority of prior art airflow sensors utilized for medical ventilators operate on a principle of a flow restrictor, traversing the air stream and measuring the pressure at a number of locations in the duct. The static pressure drives a sample of airflow through a bypass channel where the flow rate is measured. 
         [0006]    An alternate technology uses a pitot tube having a probe with an open tip, which is inserted, into the flow field in order to measure a static pressure. The static pressure is an increasing, continuous function of the airflow rate within the tube. The pitot tube extends completely through the main channel of the sensor therefore presents a barrier to the oncoming flow. The problem associated with these sensors is that the sensor itself is responsible for a certain amount of turbulence in the flow channel. Sensor-generated turbulence causes an increase in pressure drop across the sensor as well as noise in the duct system. 
         [0007]    Based on the foregoing it is believed that a need exists for an improved airflow sensor that reduces pressure drop and that is adapted to reduce obstruction to the flow. It is believed that the improved flow sensor disclosed herein can address these and other continuing needs. 
       BRIEF SUMMARY 
       [0008]    The following summary is provided to facilitate an understanding of some of the innovative features unique to the embodiments disclosed and is not intended to be a full description. A full appreciation of the various aspects of the embodiments can be gained by taking the entire specification, claims, drawings, and abstract as a whole. 
         [0009]    It is, therefore, one aspect of the present invention to provide for improved sensor methods and systems. 
         [0010]    It is another aspect of the present invention to provide for improved airflow sensor with low-pressure drop. 
         [0011]    The aforementioned aspects and other objectives and advantages can now be achieved as described herein. An airflow sensor apparatus for measuring flow rate includes a pitot tube with a bypass channel wherein the pitot tube extends halfway into a flow channel in order to reduce a pressure drop. One or more upstream taps can be spaced along the pitot tube facing into a direction of a flow stream which directs the flow to the bypass channel. At least one or more downstream taps can be located to face perpendicular to the direction of the flow, such that the fluid after passing over a flow sensor passes through the downstream tap(s). The upstream tap senses stagnation pressure and the down stream tap senses static pressure which is exerted in all directions in the flow channel in order to determine a velocity pressure based on a difference between pressures. 
         [0012]    The upstream taps and the downstream taps average the pressure in the bypass channel in order to provide a more accurate reading of the flow in the flow channel. The pitot tube extends halfway into the flow channel hence the obstruction to the flow channel and the pressure drop can be reduced. In addition, this technique of sensing velocity pressure eliminates the need to add pressure drop to the system to measure flow such as with a flow restrictor or orifice. The velocity pressure can be sensed electronically utilizing the flow sensor or ultra low-pressure sensor. The orientation of the upstream taps and the downstream in this configuration produces the difference between stagnation pressure and drag pressure, which can be correlated to the flow of the fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the embodiments and, together with the detailed description, serve to explain the embodiments disclosed herein. 
           [0014]      FIG. 1  illustrates a perspective view of an airflow sensor apparatus, which can be adapted for use in implementing a preferred embodiment; 
           [0015]      FIG. 2  illustrates a cut-away sectional view of the airflow sensor apparatus, in accordance with a preferred embodiment; 
           [0016]      FIG. 3  illustrates a cross sectional view of an airflow sensor, in accordance with a preferred embodiment; and 
           [0017]      FIG. 4  illustrates a cross sectional view of an airflow sensor, in accordance with a preferred embodiment; and 
           [0018]      FIG. 5  illustrates a cut-away sectional view of an exemplary airflow sensor apparatus, in accordance with a preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate at least one embodiment and are not intended to limit the scope thereof. 
         [0020]    Referring to  FIG. 1  a perspective view of an airflow sensor apparatus  100  is illustrated, in accordance with a preferred embodiment. The airflow sensor apparatus  100  includes a housing  110  defining a flow channel  125  into which an entering fluid may flow from a flow system. Note that as utilized herein the term “fluid” can refer to a liquid or a gas. The flow channel  125  can be defined by a flow channel wall  115 . The flow channel  125  preferably has a cross-sectional shape and size compatible with that of existing flow systems, such as to fit conical connector as used in ventilators and respirators. A pitot tube  130  extends halfway into the flow channel  125 . The pitot tube  130  includes one or more upstream taps  141 ,  142 ,  143 ,  144 ,  146 ,  147 ,  148  and  149  shaped with a circular cross-sectional shape that can be oriented upstream in flow channel  125  to face into the direction of fluid (e.g., gas) flow. 
         [0021]    The taps  141 ,  142 ,  143 ,  144 ,  146 ,  147 ,  148  and  149  can be implemented as upstream taps that face into the flow of fluid. As utilized herein, the term “tap” can refer to a small opening that permits flow of a liquid or gas. The pitot tube  130  additionally includes one or more downstream taps (not shown) that face perpendicular to the fluid flow or opposite the direction of the flow. The downstream taps function as exhaust taps for the bypass channel. 
         [0022]    Referring to  FIG. 2  a cut-away sectional view of the airflow sensor apparatus  100  shown in  FIG. 1  is illustrated, in accordance with a preferred embodiment. The fluid can pass through the flow channel  125  in the direction as indicated by arrow  126  via an inlet end  155 , with the fluid exiting the flow channel  125  at an outlet end  165 . A flow sensor die  190  is disposed in a bypass-sensing channel  170  parallel to the flow channel  125 . The sensor die  190  and the flow channel  125  are located adjacent which is protected by a cover  116 , which in turn is situated within a housing  110 . A cover  116 , disposed against the rear side of the substrate  117  opposite the sensor die  190  protects the sensor die  190  from environmental effects. 
         [0023]    The pitot tube  130  is disposed in the flow channel  125  and aligned perpendicular to the flow channel  125 . The pitot tube  130  has a leading edge  131  and is preferably sharpened as seen in  FIG. 1  such that the fluid smoothly enters the tube  130  thus minimizing or substantially reducing turbulence and droplet shear. The pitot tube  130  extends halfway into the flow channel which reduces pressure drop and obstruction to flow. The upstream taps  141 , 142 , 143  and  144  in the pitot tube  130  leads to the low resistance flow channel  125 , which can direct bypass flow to the sense die  190 . After passing over the sense die  190 , the bypass flow of fluid continues in a low resistance flow path and exhausts through downstream taps  146 ,  147 ,  148  and  149 , which are oriented opposite the direction of flow. 
         [0024]    The fluid flows through the flow channel  125  in the direction indicated by arrow  126 , a portion of the fluid flows through the upstream taps  141 ,  142 ,  143  and  144  in the pitot tube  130  to the bypass channel  170  so that the flow sensor die  190  can measure the flow rate of the fluid in the flow channel  125  indirectly without being exposed to the damage or fluctuating conditions existing in typical flow channels. 
         [0025]    Referring to  FIG. 3 , a cut-away sectional view of the airflow sensor apparatus  200  is illustrated wherein downstream taps  146 ,  147 ,  148  and  149  are shown oriented away from the flow direction. 
         [0026]    Referring to  FIG. 4  a cross sectional view of an airflow sensor  300  is illustrated, in accordance with a preferred embodiment. The sensor  300  includes an upstream sensing element  191 , a downstream sensing element  192  and a central heating element  193 . The heating element  193  and sensing elements  191  and  192  comprises of resistive thin films (not shown), which comprise an electrical bridge whose output is analogous to the differential pressure applied to the sensor apparatus  100  illustrated in  FIGS. 1-2 . The sensing elements  191  and  192  can be implemented as, for example, a MEMS type airflow sensor. It can be appreciated, of course, that the sensing elements  191  and  192 , may be configured in the context of other types of sensor designs, not merely MEMS-type configurations. The fluid will flow across the upstream sensing element  191 , the downstream sensing element  192  and the heating element  193 . Under no flow conditions, the upstream sensing element  191  and the downstream sensing element  192  would both read the same temperature due to the heating element  193 , i.e., both sensors would have the same measured resistance values. 
         [0027]    As the fluid enters the upstream port  150  of the pitot tube  130 , the upstream sensing element  191  senses the average sensor impact pressure of flowing fluid to establish a high pressure value resulting in a reduction of temperature. The downstream sensing element  192  senses low pressure, which forms an exhaust port  180  for the bypass channel  170  resulting in an increase of temperature. The change in temperatures produces a corresponding change in the resistance values of the sensor  300 . The sensor  300  transforms the respective high and low fluid pressures into an electrical signal whose character is a function of the differential pressure (DP), that is the difference between the sensed high and low fluid pressures. Upstream taps  141 ,  142 ,  143  and  144  and the downstream taps  146 ,  147 ,  148  and  149 , such as those illustrated in  FIGS. 2-3 , can average the pressure in the bypass channel  170  in order to provide a more accurate reading of the flow in the flow channel  125 . 
         [0028]    Note that as utilized herein the acronym “MEMS” refers generally to term “Micro-electro-mechanical Systems”. MEMS devices refer to mechanical components on the micrometer size and include 3D lithographic features of various geometries. They are typically manufactured using planar processing similar to semiconductor processes such as surface micromachining and/or bulk micromachining. These devices generally range in size from a micrometer (a millionth of a meter) to a millimeter (thousandth of a meter). At these size scales, a human&#39;s intuitive sense of physics do not always hold true. Due to MEMS&#39; large surface area to volume ratio, surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass. 
         [0029]    MEMS devices can be fabricated using modified silicon fabrication technology (used to make electronics), molding and plating, wet etching (KOH, TMAH) and dry etching (RIE and DRIE), electro discharge machining (EDM), and other technologies capable of manufacturing very small devices. MEMS sometimes go by the names micromechanics, micro machines, or micro system technology (MST). While the inserted position of the sensor shown in  FIGS. 1-3  are preferred for the illustrated design of pitot tube  130 , other configurations are possible and may even be favored for pitot tubes of different design and configuration. 
         [0030]    Referring to  FIG. 5  cut-away sectional view of the airflow sensor apparatus  400  is illustrated, in accordance with an alternative, but preferred embodiment. Note that in  FIGS. 1-4 , identical or similar parts or elements are generally indicated by identical reference numerals. The design of apparatus  400  thus includes one or more upstream taps  141 ,  142 ,  143  and  144 , which face into the flow direction  126  as shown in  FIG. 2  that is to be measured. The upstream taps  141 ,  142 ,  143  and  144  can direct the flow of fluid to the sensor die  190  (e.g., MEMS sensor). The sensor die  190  and the bypass channel  170  are located adjacent a housing  110 . 
         [0031]    The pitot tube  130  upstream taps  141 ,  142 ,  143  and  144  creates a pressure drop across the upstream port  150  and down stream port  160  of the bypass channel  170  which facilitates fluid flow into the bypass channel  170 . This pressure drop, or pressure differential, is dependent on pitot tube  130  geometry and increases with flow rate. Furthermore, the fluid in the flow channel  125  will have an increasingly turbulent flow as the flow rate of the fluid increases, i.e., an increasing non-uniform pressure and velocity across a given plane orthogonal to the direction of flow. In response, by reducing the pitot tube  130  to half of the flow channel  125  reduces pressure drop, straightens and luminaries the flow in the flow channel  125 , thereby reducing turbulence. The pitot tube  130  reduces turbulence by forcing the fluid to flow through other half of the flow channel  125 . The pitot tube  130  can incorporate an up and/or downstream flow straightener(s) ( 131 ) protruding into the flow to enhance flow stability. 
         [0032]    It will be appreciated that variations of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.