Abstract:
A flow sensor includes sensors connected to a flexible membrane. The sensors detect ambient temperature, pressure, and flow rate of a medium. A method of sensing flow rate includes providing the flexible membrane; coupling the plurality of sensors to the flexible membrane; and detecting ambient temperature, pressure, and flow rate of the medium by the sensors. A flow sensing system includes the flow sensor, an operational amplifier, and a closed loop controller. The sensors are connected in a Wheatstone bridge configuration. The operational amplifier is connected to the Wheatstone bridge and outputs a pressure signal representative of the pressure of the medium. The closed loop controller is connected to the operational amplifier and controls a current through a heating element for a resistor in the bridge such that a voltage across the operational amplifier is substantially zero. The output of the closed loop controller represents the flow rate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to a flow sensor for measuring the flow rate of gaseous media, such as air, and relates more particularly to a multiple technology flow sensor suitable for measuring various physical characteristics of gaseous media, such as pressure, temperature, and flow rate, which may be used to enhance the accuracy of flow rate measurements.  
         [0003]     2. Description of the Prior Art  
         [0004]     There are essentially three prevailing methods used to measure the flow of gaseous media. The first is inferential flow measurement, which senses a difference in pressure across a restricted orifice. The second method uses a thermal sensor, which is also referred to as a constant temperature anemometer, to monitor temperature changes that are dependant upon the speed of the medium. This method is described in U.S. Pat. No. 6,470,741 to Fathollahzadeh, which is incorporated herein by reference. The third method utilizes displacement sensors that detect mechanical displacement of a portion of the sensor caused by the flow of gas.  
         [0005]     Each of these methods has different application ranges, as well as inherent advantages and disadvantages. Inferential flow measurement generally requires two pressure sensors and a restriction in flow. A temperature sensor is also typically required with this method to compensate for variations in pressure due solely to temperature fluctuations. However, the requirement of multiple sensors substantially increases the rate of failure and cost of installation. Thus, application of inferential flow measurement principles becomes practical in only limited circumstances.  
         [0006]     The remaining two flow sensing methods do not require multiple sensors, but have other drawbacks. Typically, thermal sensors are used for lower flow rates while displacement sensors are used for relatively higher flow rates. Selection of the most appropriate flow sensor for a particular application requires a detailed knowledge of the anticipated range of measurements, the potential physical characteristics of the medium, such as temperature and pressure, as well as the environmental characteristics of the location in which the medium is to be measured. Accordingly, use of either thermal sensors or displacement sensors generally requires a customized solution for each particular application.  
         [0007]     Conventional methods of measuring flow rate typically involve the use of separate dedicated sensors. These sensors are often located at significant distances from each other, which necessitate the use of external wiring and/or interface assemblies, as well as substantially increasing the size, cost, and space requirements of the system.  
         [0008]     In addition, most flow rate sensors do not compensate for the effect of temperature or pressure in the flow rate measurement. Accordingly, such measurements may be highly inaccurate, particularly when taken over a wide range of conditions.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to provide a multiple technology flow sensor and a method for determining flow rate that have generic applicability and do not require substantial modification over a wide range of pressure, temperature, and types of media.  
         [0010]     It is another object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that utilize both thermal and displacement sensing to measure temperature, pressure, and flow rate over an extended range of physical conditions.  
         [0011]     It is yet another object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that are able to simultaneously or sequentially characterize physical characteristics of gaseous media, such as temperature and pressure, which may then be used to compensate and significantly improve the accuracy of flow rate measurements.  
         [0012]     It is still another object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that reduce the size, cost, and space requirements of the sensor by incorporating multiple sensors within a single housing.  
         [0013]     It is a further object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that simplify the manufacture of the sensor.  
         [0014]     It is still a further object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that substantially eliminate external wiring and supplemental interfacing hardware requirements.  
         [0015]     It is yet a further object of the present invention to provide a multiple technology flow sensor and method for determining flow rate that significantly increase measurement accuracy by substantially reducing the effect of environmental factors on flow rate measurements.  
         [0016]     A flow sensor formed in accordance with the present invention, which incorporates some of the preferred features, includes a flexible membrane and a plurality of sensors. The plurality of sensors is operatively connected to the flexible membrane. At least one of the plurality of sensors is adapted for detecting ambient temperature, pressure, and the flow rate associated with the medium.  
         [0017]     A method of sensing flow rate of a medium in accordance with the present invention, which incorporates some of the preferred features, includes the steps of providing a flexible membrane, coupling at least one of a plurality of sensors operatively to the flexible membrane, and detecting ambient temperature, pressure, and flow rate of the medium by at least one of the plurality of sensors.  
         [0018]     A flow sensing system formed in accordance with the present invention includes the flow sensor, an operational amplifier, and a closed loop controller. The plurality of sensors includes at least four resistors operatively connected in a Wheatstone bridge configuration that are used to measure ambient temperature, pressure, and flow rate associated with a gaseous medium. The operational amplifier is operatively connected to the Wheatstone bridge and outputs a pressure signal representative of the pressure of the medium. The closed loop controller is selectively connected to the operational amplifier and controls an electrical current through a heating element for one of the resistors in the Wheatstone bridge, such that a voltage across the inputs of the operational amplifier is substantially zero. The output of the closed loop controller is representative of the flow rate of the medium.  
         [0019]     These and other objects, features, and advantages of the invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a top view of a multiple technology flow sensor formed in accordance with the present invention.  
         [0021]      FIG. 2  is a side cross-sectional view of the multiple technology flow sensor formed in accordance with the present invention.  
         [0022]      FIG. 3  is a top view of a substrate including a plurality of flow sensors formed in accordance with the present invention prior to cutting to form individual devices or chips.  
         [0023]      FIG. 4  is a block diagram of the multiple technology flow sensor formed in accordance with the present invention, which is operatively coupled to a preferred embodiment of signal processing hardware.  
         [0024]      FIG. 5   a  is a side cross-sectional view of a first embodiment for mounting the multiple technology flow sensor formed in accordance with the present invention.  
         [0025]      FIG. 5   b  is a side cross-sectional view of a second embodiment for mounting the multiple technology flow sensor formed in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]     In order to measure the flow rate of media, which is the volume or mass per unit time, additional physical characteristics, such as temperature and pressure, are preferably used to enhance the precision of the flow measurement. A multiple technology flow sensor formed in accordance with the present invention measures the flow of media, such as a gas, liquid, slurry, composition, and the like, but preferably air. The sensor detects the amount of electrical current required to maintain a body at a uniform temperature, and compensates this measurement with temperature and pressure information, which is also detected simultaneously or sequentially by the sensor.  
         [0027]      FIG. 1  shows a top view of a preferred embodiment of the flow sensor  10 , which includes a Wheatstone bridge disposed on a wafer  16 . The Wheatstone bridge preferably includes four resistors or sensing elements R 1 , R 2 , R 3 , and R 4 , at least a portion of which measure the flow rate (O), pressure (p), and temperature (T) of the media. The flow of media is preferably directed across the sensors, as indicated by arrow A in  FIGS. 1, 2 ,  5   a , and  5   b.    
         [0028]      FIG. 1  also shows a preferred placement of the four resistors R 1 , R 2 , R 3 , R 4 , at least some of which function as strain gauges during a pressure measurement or thermal sensors during a temperature measurement, on a membrane  12 . When used as strain gauges, the resistors R 1 , R 2 , R 3 , and R 4  are preferably configured in a full bridge configuration. Resistors R 1  and R 4  are preferably located near the edge of the membrane or diaphragm  12 , and resistors R 2  and R 3  are preferably located near the middle of the membrane  12 .  
         [0029]     As shown in  FIG. 2 , the membrane  12  preferably covers a cavity  14  in the wafer  16 , which provides a flexible surface that is displaced in response to changes in the absolute pressure of the medium being measured. This displacement is detected by resistors R 1 , R 2 , R 3 , and R 4  when acting as strain gauges.  
         [0030]     Resistors R 1  and R 4  preferably exhibit a positive elongation or are elongated by displacement of the membrane  12 , which occurs in response to an increase in the pressure of the medium. Resistors R 2  and R 3  preferably exhibit a negative elongation or are compressed by displacement of the membrane  12 , which occurs in response to an increase in the pressure of the medium.  
         [0031]     The positive elongation of resistors R 1  and R 4  is preferably designed to be about equivalent to the negative elongation of resistors R 2  and R 3  to simplify compensation for these quantities in the full Wheatstone bridge. Equivalence of these elongations is preferably achieved through placement of the resistors R 1 , R 2 , R 3 , and R 4  on the membrane  12 , which may be determined by, for instance, computer modeling and/or simulation.  
         [0032]     The elongation of resistors R 1 , R 3  and R 2 , R 4  creates a measurable output signal from the full bridge circuit. As shown in  FIG. 1 , resistors R 1  and R 4  are preferably oriented longitudinally, that is, with the longest dimensions of R 1  and R 4  being substantially parallel to the direction of flow A. Resistors R 2  and R 3  are preferably oriented transversely, that is, with the longest dimensions of R 2  and R 3  being substantially perpendicular or positioned across the direction of flow A. However, it is anticipated that the resistors R 1 , R 2 , R 3 , and R 4  may be disposed in any orientation and/or position on the membrane  12  while remaining within the scope of the present invention.  
         [0033]     At least a portion of the Wheatstone bridge is also preferably used as a constant temperature anemometer to measure the mass flow rate of the medium. During the measurement of flow rate, resistor R 1  is preferably used as a hot film sensing element, resistor R 2  is preferably used as a temperature sensing element, and resistors R 3  and R 4  are preferably passive with respect to changes in temperature and are used to complete the bridge circuit.  
         [0034]     The resistor R 2  is preferably used to measure the ambient temperature, which may then be used to compensate flow rate measurements. Resistor R 2  is also used as a general-purpose temperature-sensing element for the entire flow-pressure-temperature (QpT) device formed in accordance with the present invention.  
         [0035]     The Wheatstone bridge preferably also includes additional trimming resistors, such as trimming resistor R 5  shown in  FIGS. 1, 2 , and  4 , which are preferably cut to yield a desired resistance. These trimming resistors are preferably used to compensate for offset voltages due to imbalances in the branches of the Wheatstone bridge.  
         [0036]      FIG. 4  shows a block diagram of a preferred embodiment of a flow sensing system, which includes the flow sensor  10  formed in accordance with the present invention operatively coupled to an embodiment of signal processing hardware  11 . The embodiment of the signal processing hardware  11  shown in  FIG. 4  is intended to illustrate one example of how the flow sensor  10  of the present invention may be utilized to process measured data so that the advantages of the flow sensor  10  may be realized, but is not intended to limit the scope of the present invention or the scope of alternative embodiments of the signal processing hardware  11 .  
         [0037]     An electrical current is preferably applied to heating element  22 , which is used to heat resistor R 1 , and maintain a substantially constant temperature difference between resistor R 1  and the ambient temperature. The amount of current required to maintain resistor R 1  at a constant temperature differential is preferably used as a measure of the flow rate and reflected in the output of a closed loop controller  32 . Resistor R 2  is preferably of the type PT 1000, which exhibits a positive temperature coefficient of about 100.  
         [0038]     A MEMS (Micro-Electro-Mechanical System) structure of the flow sensor formed in accordance with the present invention will now be described. MEMS refers to the integration of mechanical and electrical elements on a common silicon substrate by utilizing microfabrication techniques. The electronic circuits are preferably fabricated using IC (Integrated Circuit) processes, such as CMOS (Complementary Metal Oxide Semiconductor), bipolar, or BICMOS (Bipolar Complementary Metal Oxide Semiconductor) processes. The micromechanical components are preferably fabricated using compatible micromachining processes, which selectively etch away portions of the silicon wafer or add new structural layers to form mechanical and electromechanical devices.  
         [0039]     The flow sensor  10  formed in accordance with the present invention is preferably manufactured using silicon planar technology and micromachining by techniques similar to those described in U.S. Pat. No. 5,144,843 to Tamura et al., which is incorporated herein by reference. One functional element of the flow sensor  10  shown in  FIGS. 1 and 2  is the thin silicon membrane  12 , which is preferably etched from bulk silicon by either an isotropic or anisotropic wet etching process. The resistors R 1 , R 2 , R 3 , and R 4  are disposed above the membrane  12 , and are preferably designed, oriented, and positioned to optimize an output signal representing the pressure of the medium.  
         [0040]     The resistors R 1 , R 2 , R 3 , and R 4  are preferably formed from a layer that is deposited on the silicon wafer  16  by a PVD (Physical Vapor Deposition) or CVD (Chemical Vapor Deposition) process. This layer, and consequently the resistors R 1 , R 2 , R 3 , and R 4 , are preferably manufactured from platinum, nickel/chromium, or doped polysilicon. Photolithographic techniques are preferably used to define the dimensions of the resistors R 1 , R 2 , R 3 , and R 4 . An electrically isolating layer  20 , which may be formed from silicon dioxide, is preferably used to separate the resistors R 1 , R 2 , R 3 , and R 4  from the membrane  12 .  
         [0041]     The area below the resistor or hot-film sensing element R 1  is preferably occupied by the heating element  22 , as shown in  FIGS. 1 and 2  which is preferably a resistive element. The heating element  22  preferably includes a thin film layer of platinum or polysilicon, which is deposited below the resistor R 1 . The heating element  22  is preferably isolated from the resistor R 5  on its top side by the isolating layer  20 . The heating element  22  is also preferably isolated from the semiconductor chip or wafer  16  on its bottom side by another isolating layer  24 , which may also be manufactured from silicon dioxide.  
         [0042]     As shown in  FIG. 2 , the wafer  16  is preferably bonded to a substrate  26  by a wafer bonding technique, following which the wafer  16  and substrate  26  are cut to form individual devices or chips  27 , as shown in  FIG. 3 . As shown in  FIG. 2 , the substrate  26  functions as a passive mechanical support for the wafer  16  and the components disposed thereon, and provides a base for the cavity  14 . Both the wafer  16  and the substrate  26  are preferably manufactured from silicon. The wafer bonding technique is preferably performed in a vacuum, which creates the evacuated cavity  14  having a pressure Po that is equal to about zero.  
         [0043]     The cavity  14  preferably enables pressure to be measured by resistors R 1 , R 2 , R 3 , and R 4  in response to deflection of the membrane  12 . The cavity  14  also provides thermal isolation between the resistors R 1 , R 2 , R 3 , and R 4 , as well as between the wafer  16  and the resistors R 1 , R 2 , R 3 , and R 4  during flow rate measurements. In this way, heat applied to the resistor R 1  is efficiently transferred directly to the flow of the medium by convection. Accordingly, cross sensitivities between pressure and temperature are substantially eliminated, which significantly enhances accuracy.  
         [0044]     In the full bridge configuration of the flow sensor  10  shown in  FIG. 4 , the hot film sensing element resistor R 1  is preferably heated to and maintained at a temperature of about 300° C. during flow rate measurements by the separate heating element  22 , which is also shown in  FIGS. 1 and 2 . When not using R 1  as a flow sensor, the heating element  22  beneath R 1  is preferably turned off and R 1  is allowed to cool to ambient temperature, which preferably takes about 8 milliseconds. As described above, the remaining resistors R 2 , R 3 , and R 4  are preferably thermally isolated from the hot film element resistor R 1  by the membrane  12 . The shape and dimensions of the membrane  12  are preferably determined by the requirements of a particular application, such as range and sensitivity.  
         [0045]     The heating element  22  preferably includes a platinum layer, which is deposited below resistor R 1  and isolated from resistor R 1  by the isolating layer  24 . Electrical connections to the heating element  22  are preferably brought to the edge of the wafer  16  and isolated from the remaining components in the Wheatstone bridge.  
         [0046]     The ambient temperature is preferably measured directly from resistor R 2  by using switches  30 A and  30 B, which are preferably controlled by the computer  42 , as indicated by a dashed line  48 . Switches  30 A and  30 B selectively either connect resistor R 2  with the remaining circuitry in the Wheatstone bridge or connect resistor R 2  in parallel across a voltage source  46 . The resistance of R 2  determines the voltage at node B, which is preferably input to an analog-to-digital converter (ADC)  38  through a multiplexer  36 . The computer  42  preferably uses a digital value, which is obtained from the ADC  38 , corresponding to this voltage to determine the ambient temperature.  
         [0047]     As shown in  FIG. 4 , resistors R 1 , R 2 , R 3 , and R 4  are preferably connected in a Wheatstone bridge configuration with trimming resistor R 5 . Resistors R 1  and R 4  are connected at node P 1 , resistors R 3  and R 4  are connected at node P 4 , resistors R 1  and R 5  are connected at node P 3 , and resistors R 2  and R 3  are connected at node P 2 . A voltage source  44  is preferably connected in parallel across nodes P 1  and P 2 .  
         [0048]     Nodes P 3  and P 4  are preferably connected to the inverting and non-inverting terminals of an operational amplifier  28 , respectively. The output of the operational amplifier  28  is preferably connected to a closed loop controller  32  through a switch  34 , which is preferably controlled by the computer  42  as indicated by the dashed line  48 . The output of the operational amplifier  28  is also connected to the ADC  38  through the multiplexer  36 , which is preferably controlled by the computer  42 .  
         [0049]     The output of the closed loop controller  32  is preferably connected to a current driver circuit  44 , which selectively provides current to the heating element  22  that maintains resistor R 1  at the desired temperature. The output of the closed loop controller  32  is also preferably input to the ADC  38  through the multiplexer  36 , under the control of the computer  42 , so that the computer  42  is able to selectively monitor the flow rate of the medium.  
         [0050]     The sensor formed in accordance with the present invention preferably measures temperature (T), pressure (p), and flow (O) in a sequential multiplexed process. The first step of the process preferably includes obtaining a pressure measurement from the media at room temperature using resistors R 1 , R 2 , R 3 , and R 4 .  
         [0051]     The second step of the process preferably includes measuring the ambient temperature using resistor R 2 , as described above. The results of the temperature and pressure measurements may then be used to compensate for cross-sensitivities between temperature, pressure, and flow. To provide optimal compensation for the effects of ambient temperature, the temperature of the resistor R 2  is preferably about equal to the ambient temperature, which minimizes the loss of thermal energy.  
         [0052]     Temperature measurements provided by resistor R 2  are preferably used to compensate for flow rate measurements obtained from resistor R 1 . For example, during a flow rate measurement, resistor R 1  is preferably heated to and maintained at a substantially constant temperature of about 300° C. The temperature of resistor R 1  is determined by its resistance, which is measured by the Wheatstone bridge. The flow of media cools resistor R 1 , which then requires a specific amount of current to maintain the resistor R 1  at 300° C. This value of current represents the flow rate.  
         [0053]     However, the specific amount of current required to counteract the cooling effect of the media flow depends on the ambient temperature. For instance, as the ambient temperature increases, less current is required to maintain resistor R 1  at 300° C. Thus, ambient temperature is used in accordance with the present invention to compensate for the amount of current required to maintain the resistor R 1  at 300° C. This ensures that flow rate measurements remain substantially independent of the ambient temperature.  
         [0054]     The final step in the process in accordance with the present invention preferably includes measuring the flow rate by applying a current to the heating element  22 , which is shown in  FIGS. 1-3 , to heat resistor R 1 . The time required to raise the temperature of resistor R 1  from room temperature to the desired temperature is in the range of about 1-5 milliseconds.  
         [0055]     As described above, resistors R 1 , R 2 , R 3 , and R 4  are deformed in proportion to the pressure of the medium. The full Wheatstone bridge is used to determine this pressure. The voltage between nodes P 3  and P 4  preferably represents the absolute pressure of the medium and is inputted to the operational amplifier  28 . Thus, the voltage at the output of the operational amplifier  28  is applied to the multiplexer  36 , which selectively provides an analog signal representing pressure to the ADC  38 . The ADC  38  then digitizes this analog signal and provides the corresponding digital signal representing pressure to the computer  42 .  
         [0056]     The computer  42  preferably provides correction, compensation, and/or calibration of parameters provided by the ADC  38 , such as sensor offset, gain, temperature sensitivity, non-linearity, and calibration coefficients. These parameters are preferably programmed into memory  40 , such as non-volatile flash memory, and are available for use in compensation algorithms performed during subsequent measurements.  
         [0057]     The current necessary to keep resistor R 1  at a constant temperature differential with respect to the ambient temperature is used to measure the flow rate Q. For flow rate measurements, switch  34  is preferably closed and the multiplexer  36  is selected to direct the output of the closed loop controller  32  to the ADC  38 . The closed loop controller  32  preferably operates to ensure that the voltage between nodes P 4  and P 3  is maintained at about zero by selectively controlling the amount of current delivered to the heating element  22 , and thus the resistance of resistor R 1 . The output of the closed loop controller  32  is selected by the computer  42 , via the multiplexer  36 , for digital conversion by the ADC  38 . The output of ADC  46  is then preferably provided to the computer  42  as a digital representation of the flow rate.  
         [0058]     As shown in  FIG. 5   a , the flow sensor  10  may be incorporated into an in-line module  50 , to which segments of a conduit  52  are attached. Additional components and wiring to these components are preferably located internal and/or external to the module  50 . Alternatively, the flow sensor  10  may be mounted by any known means to an internal surface of the conduit  52 , as shown in  FIG. 5   b , or suspended within the conduit  52 . Interconnection between components in the flow sensor  10  and components external to the conduit  52  are preferably made using wires  54  extending from contacts  56  on the sensor  10  through a sealed orifice  58  in the conduit  52  to components located external to the conduit  52 .  
         [0059]     Therefore, the multiple technology flow sensor and method for determining flow rate formed in accordance with the present invention have generic applicability and do not require substantial modifications over a wide range of pressure, temperature, and flow rates. Further, the flow sensor and method for determining flow utilize both thermal and displacement sensing to measure temperature, pressure, and flow rate over an extended range of conditions. In addition, the multiple technology flow sensor and method for determining flow rate are able to simultaneously or sequentially characterize physical characteristics of gaseous media, such as temperature and pressure, which may be used to compensate for and significantly improve the accuracy of flow rate measurements.  
         [0060]     Unifying the manufacturing processes for all sensors in the flow sensor formed in accordance with the present invention permits many of these processes to be performed simultaneously. Enclosing each of the sensors in a single housing substantially simplifies and reduces the cost of manufacturing the device when compared with conventional systems having discrete sensors. Combining multiple sensors also substantially reduces the size and space requirements of the resulting device since intermediate space, which is normally required for external wiring, can be eliminated.  
         [0061]     Miniaturization of the flow sensor formed in accordance with the present invention enhances dynamic properties, such as reaction time and bandwidth. Integrating multiple sensors in the same device enables multiplexing of signals representing flow, pressure, and temperature without the need for additional wiring and/or interface hardware, much of which requires redundancy in conventional multi-sensor systems. Localizing multiple sensors within one device also standardizes and reduces the impact of environmental factors when compared with their effect on multiple sensors located at potentially diverse sites.  
         [0062]     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention.