WO1992021940A1

WO1992021940A1 - Mass air flow sensor

Info

Publication number: WO1992021940A1
Application number: PCT/US1992/004424
Authority: WO
Inventors: Stewart D. Johnson
Original assignee: Honeywell Inc.
Priority date: 1991-05-28
Filing date: 1992-05-27
Publication date: 1992-12-10

Abstract

A fluid flow sensor, or mass flow sensor, is provided with a means for directing particulate matter away from a semiconductor flow sensor. In addition, the mass flow sensor of the present invention is provided with a housing structure that allows the flow sensor to be disposed proximate a central region of a main conduit. The housing structure is provided with an upstream port and a downstream port which divert a flow of fluid in thermal communication with a flow sensor which is disposed within a chamber provided in a housing structure. A second directing means disposed within the housing structure causes the diverted stream to pass in intimate thermal communication with the flow sensor. Additionally, a fluid deflector is provided to prevent the diverted flow stream from abruptly striking the leading, or upstream, edge of a carrier device to which the flow sensor is attached. This deflector assists in the prevention of turbulent flow that could otherwise be caused by the fluid striking the upstream edge of the carrier apparatus.

Description

MASS AIR FLOW SENSOR

BACKGROUND OF THE INVENTION

Field of the Invention: The present invention is generally related to the field of mass air flow sensors and, more particularly, to a sensor which utilizes a technique for preventing particulate matter from damaging the flow sensing apparatus. Description of the Prior Art:

Many devices for measuring the velocity or mass flow of a fluid are known to those skilled in the art. Two techniques which can be used to sense the velocity or mass flow of a fluid comprise the hot wire anemometer and the microbridge devices which utilize semiconductor technology. United States Patent 4,501,144, which issued to Higashi et al on February 26,

1985, discloses a flow sensor that comprises thin film heat sensors and a thin film heater which are disposed on a generally insulative substrate. The sensors and heater are supported by a base with the sensors disposed on opposite sides of the heater and arranged in close proximity to the heater. United States Patent 4,478,077, which issued to Bohrer et al on October 23,

1984, discloses a flow sensor with a pair of thin film heat sensors and a thin film heater. It comprises a semiconductor body with a depression therein and a structure connecting the heater and the sensors to the body and bridging the depression so that at least a major portion of the heater and the sensors are out of contact with the body. The sensors are disposed on opposite sides of the heater.

United States Patent 4,478,076, which issued to Bohrer on October 23, 1984, describes a flow sensor which is generally similar to that described in United States Patent 4,478,077. The heater is operated at a constant temperature above ambient temperature under both flow and no-flow conditions. This patent also discloses a flow sensor wherein a portion of the depression is devoid of a side wall. United States Patent 4,914,742, which issued to Higashi et al on April 3,

1990, describes a thin film orthogonal micro sensor for airflow measurement. In the particular application that is described in this patent, the microbridge airflow sensor has a sealed and etched cavity beneath the silicon nitride diaphragm so that the cavity is not susceptible to contamination from residual films or other materials accumulating within the cavity. The cavity thermally isolates the heater and detectors which are encapsulated in the diaphragm. The cavity is fabricated by front side etching of ^"the silicon wafer. Narrow slots are made through the silicon nitride diaphragm to expose a thin film rectangle of aluminum. The first etch removes the aluminum leaving a four hundred angstrom very shallow cavity under the diaphragm. Anisotropic etch is then introduced into the shallow cavity to etch the silicon pit.

United States Patent 4,548,078, which issued to Bohrer et al on October 22,

1985, discloses an integral flow sensor and channel assembly that comprises a flow sensor with a sensing element integral to a semiconductor body. The assembly further comprises support structure for supporting the flow sensor. The support structure is provided with a first surface. An enclosed flow channel comprises the first surface formed into a groove running below the sensing element. The flow channel comprises an inlet and an outlet for providing flow across the sensing element. The sensing element is located in the flow channel between the inlet and the outlet. The support structure comprises apparatus for flip-chip mounting the semiconductor body.

The semiconnector body is flip-chip mounted so that the sensing element is positioned over the groove in the support structure.

United States Patent 5,006,421, which issued to Yang et al on April 9, 1991, describes a metalization system for heater and sensor elements. This patent describes a device which comprises a substrate and a metalized sensor/heater element having a temperature coefficient of resistance of at least two thousand parts per million. This patent also discloses methods of fabricating these types of devices.

United States Patent 4,872,339, which issued to Gerhard et al on October 10, 1989, discloses a mass flow meter. The mass flow measurement device described in this patent includes a circuit which provides a digital output without using a separate analog-two-digital converter. The air flow measurement circuit includes a bridge circuit in which a voltage is supplied to the bridge to control current through one or more sensing elements in order to keep the temperature and resistances of the sensing elements constant. A comparator is used to sense the balance of the bridge and the output of the comparator is applied to an input port of a microcomputer. The micro computer generates a rectangular wave pulse with modulated signal whose duty cycle is increased or decreased in response to the output of the comparator.

While the above described patents all incorporate semiconductor technology to create the microbridge structures that support and insulate the sensors and heaters from the main support structure of the sensor, it should be clearly understood that flow sensors need not be made of semiconductor devices. For example, the hot wire anemometer is well know to those skilled in the art and is described below in association with Figures 1 and 2. Fluid flow sensors are commonly employed by providing a diverted stream of fluid from a main flow stream and passing the diverted stream through or over the flow sensor. Generally, some means is used to cause the diverted flow to be separated from the main flow of fluid and the measured speed or mass flow of the diverted stream is then used to estimate the mass flow of the main fluid stream. The diverted flow is generally taken from a side wall of the main conduit and variations in the main flow conduit diameters are used to create a pressure differential that induces the diverted stream to, or divert, flow away from the main fluid stream and flow through or over a mass flow sensor. Predetermined relationships are used to extrapolate the data obtained from the diverted flow and to provide a mathematically derived value representing the velocity or mass flow of fluid passing through the main fluid conduit.

Several severe problems exist with respect to fluid flow sensors known to those skilled in the art. First, when fluid is diverted from a region proximate the walls of a main conduit, the extrapolation necessary to determine the mass flow of the fluid of the main stream is difficult and not always accurate because of the different conditions affecting the flow proximate the walls of a conduit and the flow at the central portion of the conduit. In addition, many known devices for diverting and sensing the mass flow of a fluid to create disturbances in the main stream conduit and affect the flow as a result of their shape and position in the fluid stream. In addition, the presence of the sensing devices in the main fluid stream usually creates a pressure drop and reduces the overall efficiency of the system being monitored.

Perhaps the most severe disadvantage of the techniques known to those skilled in the art is the fact that calibration of known sensors must generally be performed by passing the full operational flow rate through a main conduit while calibrating the sensor which is sensing the mass flow of only the diverted stream. This more difficult and costly calibration procedure is necessary because of the inter-relationship between the sensing apparatus and the flow characteristics of the main conduit. The sensing apparatus can not simply be calibrated by using the much reduced flow of the diverted stream without the presence of the flow through the main conduit because the separation of these two components would create an artificial condition that would not permit accurate calibration to be performed because it differs from the actual condition under which the sensor will actually be used.

Another problem that exists in flow sensors known to those skilled in the art is the fact that particulate matter in the fluid stream may be moving at extremely high velocities as they pass the flow sensor. If a fluid borne particle moving at a high rate of speed strikes the fluid sensor, severe damage may occur and the sensor may be disabled. This is particularly true if the sensor is a semiconductor device such as a microbridge structure. SUMMARY OF THE INVENTION

The present invention addresses several problems that exist with mass flow sensors known to those skilled in the art. First, it disposes the flow sensor at a region proximate the center of the main fluid flow stream. This avoids problems that otherwise exist if the sensor is disposed proximate one of the walls of the main stream conduit or obtains its diverted stream of fluid from the wall portion of the conduit. To avoid adverse effects on the main flow stream, the present invention provides a support structure which is streamlined to minimize pressure drop caused by the inclusion of the present invention in the main stream. Additionally, the present invention provides a generally conical shape at the upstream portion of the sensor housing to divert paniculate matter away from the flow sensor. To further concentrate the flow of the diverted stream past the flow sensor, an additional flow directing mechanism is provided inside the housing to reduce the cross section of the fluid passage through which the diverted stream flows at a region proximate the flow sensor. To avoid air turbulence that could be caused by the diverted stream striking the upstream edge of the semiconductor flow sensor, a fluid aligning plate is provided to direct the diverted flow stream along streamlines which are generally in or above the plane of the upper surface of the semiconductor flow sensor while avoiding the upstream edge of the flow sensor. The fluid flow sensor in a preferred embodiment of the present invention comprises a means for measuring the mass flow rate of a fluid. In a most preferred embodiment of the present invention, the measuring means comprises a semiconductor device, such as a microbridge structure, which incorporates two resistors and a heater. The resistors are responsive to temperature change and the heater is disposed between the two resistors with one resistor being located upstream from the heater and the other resistor being located downstream from the heater. It should be clearly understood that the arrangement of resistors and heater within the mass air flow sensor is not critical to the operation of the present invention. Alternative embodiments of the present invention could utilize a single heater and a single resistor with the resistor being disposed downstream from the heater. Many other configurations of circuit components can also be used in conjunction with the present invention. The most preferred embodiment of the present invention also comprises a means for disposing the measuring means within a central portion of a main fluid conduit, wherein the disposing means is attached to the conduit and extends from a wall of the fluid conduit inwardly toward the central portion. The present invention also comprises a means for directing particulates away from the measuring means while permitting the generally particulate-free fluid to pass in thermal communication with the measuring means. The disposing means comprises a housing that is shaped to receive the measuring means within it, the housing member being shaped to form the directing means at its upstream portion. A second directing means, used to direct fluid to flow in thermal communication with the measuring means, is disposed within the housing member with the measuring means being disposed within this second directing means. In one particularly highly preferred embodiment of the present invention, the second directing means comprises a means for reducing the cross sectional area of the diverted fluid stream which passes through the second directing means proximate the measuring means. One embodiment of the present invention also comprises a flow aligning device that is used as a means for preventing fluid from flowing against an upstream edge of the measuring means. If the fluid stream in the diverted portion of the flow is permitted to strike the upstream edge of the sensor, which extends above a circuit board, the fluid could possibly be forced into a turbulent flow behavior as a result of the protrusion of the flow sensor, or measuring means, into the stream. The turbulent flow could very likely distort the measurements provided by the flow sensor and result in potentially unreliable information. By disposing a thin aligning plate upstream from the flow sensor, the present invention conditions the diverted flow to pass along a path which is generally level with or above the upper surface of the fluid sensor. This causes the diverted fluid stream to avoid striking the upstream edge and increasing the turbulence of the flow. The housing structure of the present invention comprises first and second ports that permit flow to be diverted from the main stream into a second stream passing through the housing structure. The first port is associated in fluid communication with an upstream end of the second directing means and the second port is associated in fluid communication with a downstream end of the second directing means. In other words, fluid from the main stream passes through the first port into the internal structure of the housing, through the second directing means and in thermal communication with the flow sensor and, lastly, out of the housing structure through the second port and back into the main stream of fluid. BRIEF DESCRIPTION OF THE DRAWING

The present invention will be more fully understood from a reading of the description of the preferred embodiment in conjunction with the drawings, in which;

Figure 1 illustrates a fluid flow sensing device known to those skilled in the art; Figure 2 is a sectional view of Figure 1 ;

Figure 3 shows a typical method for providing a diverted fluid stream; Figure 4 illustrates the basic configuration of a semiconductor fluid mass flow sensor;

Figure 5 illustrates an internal flow directing means of the present invention; Figure 6 is a sectional view of the device of Figure 5;

Figure 7 illustrates a fluid deflecting means of the present invention; Figure 8 illustrates the second directing means of the present invention associated with a circuit board having a flow sensor disposed thereon;

Figure 9 shows the two half portions of the housing structure of the present invention;

Figure 10 illustrates the housing structure of the present invention; Figure 11 shows the paniculate deflecting means of the present invention; Figure 12 shows the present invention disposed for operation within a fluid conduit; and Figures 13-16 illustrate alternative shapes that can be used as the means for directing particulates away from the measuring means of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Throughout the description of the preferred embodiment of the present invention, like reference numerals will be used to identify like objects. Figure 1 illustrates a fluid flow sensor that is generally known to those skilled in the art. A main fluid conduit is provided which has two diametric dimensions. A first diameter conduit 10 directs fluid axially through the device toward a face portion 12 which defines the difference in cross sectional area between the first diameter conduit 10 and a second diameter conduit 14. As the fluid is directed from the portion of the conduit having the first diameter to the portion of the conduit having the second diameter, a diverted flow of fluid is caused to pass through a bypass conduit 16. The bypass conduit 16 directs a diverted flow of fluid past a hot wire anemometer (identified schematically by reference numeral 18) prior to directing the diverted fluid flow back into the main conduit at a position downstream from the hot wire anemometer 18.

A hot wire anemometer is an instrument designed to measure fluid velocities and turbulence by heat conduction from a very thin wire several thousandths of an inch in diameter. The sensing element and probe are extremely small and cause a minimum of disturbance to the diverted fluid flow. The wire, normally tungsten, is supported between two needle-like supports and is heated by an electrical current passing though it. The wire is then cooled through forced convection by the flowing fluid under test. Since the electrical properties of the wire change with temperature and the heat transfer is a function of velocity, either the voltage or current supplied to the wire may be used to determine the velocity. Hot wire anemometers are generally classed as being a constant temperature or constant current system.

With continued reference to Figure 1, it should be understood that the control of current through the hot wire anemometer 18 and the measurement of the relevant parameters are performed by circuitry contained in device 20. Since the device 20 shown in Figure 1 is commonly available in commercial quantities, a detailed explanation of the methodology used to operate this device will not be provided herein.

Figure 2 shows a sectional view of the device of Figure 1. A main fluid stream, indicated by arrows A, passes into the first diameter conduit 10. The diameter of the conduit is reduced, as illustrated, to provide a second diameter conduit 14. A bypass conduit 16 is provided in the face 12. Arrows B represent a diverted fluid stream that passes through the bypass conduit 16. The diverted flow of fluid, represented by arrows B, passes in thermal communication with the hot wire anemometer 18 and returns to the main stream through conduit 22. In this way, the main stream of fluid is sampled to determine the mass flow or velocity passing though main the conduit. The velocity measurement of fluid passing through the bypass conduit 16 can be calibrated to represent an analog value that is related to the total flow through the main conduit according to a predefined relationship. As discussed above, severe disadvantages are incumbent in the type of flow sensor illustrated in Figures 1 and 2. One disadvantage is the existence of the face 12 which operates as a flow disruption that adversely affects the overall efficiency of the system through which the fluid is flowing. Furthermore, the sample of diverted flow is taken from a wall portion of the flow stream and the flow proximate the walls of the main conduit may differ in characteristic and velocity from the flow in the central portion of the main conduit. Another severe disadvantage of the device shown in Figures 1 and 2 is the fact that calibration of the device requires a full flow of fluid through the main conduit. In other words, it is not likely that a calibration using only the reduced flow through bypass conduit 16 will result in the same information as would be determined if the same calibration is performed with a full flow of fluid through the main conduit. Because of the complex flow interactions between the flow through the main conduit, the flow through the by-pass conduit 16, the flow disruption cause by the face 12 and the relationship between the wall effects in the main conduit and the location of the hot wire an anemometer, the device illustrated in Figures 1 and 2 must be calibrated with the full main flow passing through the main conduit so that all of the actual conditions are duplicated during the calibration procedure. This characteristic causes the calibration procedure to be much more expensive and cumbersome than if the flow sensor could be calibrated with only the much reduced flow that passes over the flow sensor itself. Figure 3 illustrates a schematic representation of a typical example of the way in which a semiconductor device can be used to measure the flow of a fluid through a main conduit. A main flow of fluid, identified by arrows C, is caused to flow through a main conduit 30. A bypass conduit 32 is provided with connections through the walls of the main conduit 30 to permit a diverted flow of fluid to pass. The diverted fluid flow is identified by arrows D. The different diameters illustrated within the main conduit 30 created pressure differentials which induce fluid to flow through the bypass conduit 32. A semiconductor fluid sensor is schematically illustrated in the bypass conduit 32 and is identified by reference numeral 34. A disadvantage of the device shown in Figure 3 relates to the fact that the fluid passing through the bypass conduit 32 is taken from the wall portion of the main conduit 30 and may possess different characteristics than the remaining portion of the fluid passing through the central portion of the main conduit.

Figure 4 illustrates an exemplary semiconductor flow sensor. The flow sensor comprises a semiconductor base 40 having an insulating layer 42, such as silicon nitride or silicon dioxide, attached to the base. A portion of the base 40 is removed, either by etching or some other suitable process. This forms a cavity 44 in the base 40. Some forms of this type of device are referred to as microbridge sensors. The primary purpose for removing the semiconductor material in the region identified by reference numeral 44 in Figure 4 is to remove material that would otherwise conduct heat away from devices disposed in the oxide layer 42. Those devices generally comprise a heater 46 and two or more temperature sensitive resistors, 48 and 49. One resistor 48 is disposed upstream from the heater 46 and the other resistor 49 is disposed downstream from the heater 46. Since each of the two resistors, 48 and 49, is temperature sensitive, their resistivity varies as a function of their temperature. Therefore, related circuitry can monitor the resistances of the two temperature sensitive resistors to determine the mass flow passing across them. Several specific techniques are available to those skilled in the art to utilize this type of micro bridge circuit and, therefore, the specific algorithms utilized in these types of mass flow sensor circuits will not be described in detail. However, it should also be clearly understood that alternative sensors could employ one heater and one temperature sensitive resistor with the resistor being disposed downstream from the heater. Since the present invention is not directly related to the specific technology used in the semiconductor sensor itself, the structure and operation of the semiconductor device is not specifically related to the present invention. In a typical application, the sensor 41 is disposed on a circuit board chip 50. It should be understood that in many applications of the present invention, the sensor 41 is attached to a hybrid board which comprises both discrete components and deposited components. However, it should be understood that the particular type of support mechanism, whether it is a hybrid or a circuit board, does not directly relate to the operation of the present invention. Throughout the following description of the present invention, the device identified by reference numeral 50 will be referred to as a circuit board for the purposes of consistency. The sensor die is approximately 0.070 inches by 0.070 inches with the bridge thickness being less than 200 micrometers and the overall height of the sensor die being approximately 0.012 inches.

The semiconductor sensor 41 shown in Figure 4 is disposed on the circuit board 50 for purposes of providing electrical connections between the sensor and the external components. Reference numeral 41 is used to identify the flow sensor which comprises the semiconductor base 40, the oxide layer 42, the heater 46 and temperature sensitive resistors, 48 and 49. Reference numeral 50 is used to describe the larger supporting structure for the sensor 41 and associated electronic components.

The present invention utilizes first and second means for directing fluid flow along an appropriate path for proper sensing of the mass flow of the fluid. The first directing means will be described below in conjunction with Figures 9, 10, 11 and 12. However, before discussing the first directing means in greater detail, a second directing means will be described. The second directing means is illustrated in Figure 5 and identified generally by reference numeral 52. In Figure 5, arrows F are used to illustrate the passage of a fluid through the second directing means 52. The upstream end 54 of the second directing means 52 is provided with an opening or conduit 56 to permit a diverted flow of fluid F to pass into the second directing means 52. The downstream end 58 is also provided with an opening or conduit 59 through which the diverted fluid can exit from the second directing means 52. Also shown in Figure 5 is a dashed line 50 which represents a circuit board that will be associated with the second directing means. The circuit board 50 will be described in greater detail below in conjunction with Figure 8. The second directing means 52 is provided ith feet 53 which align it with a predetermined position on the circuit board 50.

Figure 6 illustrates a sectional view of Figure 5 that more clearly illustrates the fluid passage of the diverted fluid flow passing through the second directing means 52. As the fluid F passes through the conduit 56 at the upstream end of the device, it is directed toward a region 62 where it experiences a reduction in cross sectional area caused by the tapered upper portion 64 of the second directing means 52. This tapered portion 64 causes the diverted fluid to increase slightly in velocity because of the reduced cross section and, more importantly, causes the diverted fluid flow to pass more intimately in thermal communication with the flow sensor 41. The flow sensor 41 is represented by dashed lines in Figure 6 to illustrate its relative position.

After passing in thermal communication with the flow sensor 41, the fluid continues to flow through the second directing means 52 and eventually exits through conduit 59 at the downstream end 58 of the device. Several components are represented by dashed lines in Figure 6. The flow sensor 41, which has been discussed above, is attached to a circuit board 80. In addition, a flow aligning mechanism 70 is disposed upstream from the flow sensor 41. The relationship between the flow sensor 41, the fluid aligning means 70 and the circuit board 50 will be described in greater detail below in conjunction with Figure 7. Throughout this discussion, it should be understood that the device referred to as the flow sensor 41 is extremely small and it is not possible to clearly illustrate it in Figures 6 and 7. However, the semiconductor flow sensor 41 is supported on a circuit board 50 which acts as its carrier and provides a means for connecting the flow sensor 41 in electrical communication with other components.

Figure 7 illustrates the circuit board 50 which supports the flow sensor 41 on its upper surface. Also, the circuit board 80 and the fluid aligning means 70 are shown. As the fluid, represented by arrows F, passes through the second directing means 52, it could normally be expected to strike the upstream edge 72 of the fluid sensor 41. If the fluid is permitted to strike the upstream edge of the sensor '41 turbulent flow would likely result. Since turbulent flow in the region of the flow sensor would distort the magnitudes of the values being sensed, the present invention provides the fluid aligning plate 70 for the purpose of aligning the flow of fluid approximately level with or above the upper surface of the fluid sensing components. By comparing Figures 6 and 7, the location of the fluid aligning means 70 relative to the fluid sensor 41 components and the circuit board 50 can be seen.

Figure 8 shows the second directing means 52 attached to a circuit board 50. A portion of the central region of the second directing means 52 has been removed in Figure 8 to illustrate the internal components disposed on the circuit Dϋard 50, but under the second directing means 52. In Figures 5, 6, 7 and 8, like components have been identified by identical reference numerals to permit comparison to be made between these figures. The feet 53 of the second directing means 52 permit it to be accurately positioned on the circuit board 50. The semiconductor flow sensor 41 is positioned under the second directing means 52 and in thermal communication with the diverted fluid flow stream indicated by arrows F. As can be seen in Figure 8, the circuit board 50 permits the flow sensor 41 to be electrically connected to conductive runs on the circuit board 50. It should be understood that in a typical application of the present invention, many other components would be attached to the circuit board 50. However, for purposes of this discussion, only a few exemplary components have been illustrated on the circuit board 50. Those components are identified by reference numerals 81-84. It should be clearly understood that the particular circuit and circuit components on the circuit board 50 are not directly related to the present invention and, in addition, many different electrical circuits can be utilized within the scope of the present invention. For purposes of comparison between Figures 8 and 9, components 81-84 are identified on the circuit board 50 along with leads 85. These components, 81-85, and their positions are used solely for facilitating the comparison of Figures 8 and 9.

In Figure 9, the circuit board 50 is disposed within a housing structure 120 that comprises two halves. The halves are identified by reference numerals 90 and 92 in Figure 9. The exemplary components, 81-85, are shown on circuit board 50 as the circuit board is disposed in the housing half 90. For purposes of this description, the second directing means 52, which is shown in Figures 5, 6 and 8, is not shown in Figure 9. However, it should be clearly apparent that the second directing means 52 would be disposed in the cavity region 94 of the housing member half 90. The housing half 90 is provided with a depression 96 that permits the circuit board 50 to be disposed in it. The other half 92 of the housing structure is also provided with a cavity 95. The two cavities, 94 and 95, combined to form a chamber in which the second directing means 52 can be disposed and sealed from fluid communication with the outside of the housing structure other than as desired. For these purposes, a ridge 97 and a groove 98 are shaped to receive each other in sealing association around the chamber formed by the two cavities, 94 and 95. As can also be seen in Figure 9, the second half 92 of the housing structure

120 is provided with a depression 99 that is shaped to permit the components 81-84 of the circuit board 50 to extend into it. The provision of the two depressions, 96 and 99, allow the two halves, 90 and 92, of the housing structure 120 to be associated together with their mating surfaces, 100 and 102, in contact to be in sealed association with each other. In on particular embodiment of the present invention, the circuit board 50 is approximately 1 inch by 2 inches in dimension and approximately 0.03 inches thick. However, it should be very clearly understood that the particular dimensions of the circuit board 50 are not critical to the present invention and do not limit its scope. It should also be clearly understood that specific details relating to the shapes and characteristics of the depressions, chambers and grooves or ridges illustrated in Figure 9 can vary significantly within the scope of the present invention for adaptation to specific applications.

When assembled, the surfaces identified by reference numerals 100 and 102 in Figure 9 are disposed against each other with the depression identified by reference numeral 99 disposed over the components 81-84 of the circuit board 50. In addition, the edges identified by reference numerals 106 and 108 are associated proximate each other when the two halves are assembled and the edges identified by reference numerals 110 and 112 are associated together when the two halves, 90 and 92, are associated together to form a unitary housing structure 120. In addition, when the two halves are attached together, the second directing means 52 is disposed in the chamber formed by the two cavities, 94 and 95.

Figure 10 illustrates the housing structure 120 of the present invention when the two halves, 90 and 92, of the housing structure are assembled. Only the bottom portion of the housing structure 120 is shown for the purposes of this description. Figure 10 illustrates several important characteristics of the present invention. First, the housing structure is shaped to provide a means for directing particulates away from the measuring means which is contained therein. The first means for directing particulates away from the measuring means comprises the upstream portion of the housing structure 120. This upstream portion of the housing structure 120 includes the leading edges, 106 and 108, and the generally conically cross sectional area that is identified by reference numeral 122 in Figure 10. It tapers backward and outward from the leading edge and is generally conical in cross section as it extends back from the upstream edge. Immediately behind the conical particulate directing means is disposed a first port 124. A portion of the fluid is diverted from the main stream and flows into the first port 124 which is located immediately downstream from the particulate directing means 122. A second port 126 is disposed downstream from the first port 124. The second port 126 permits a diverted fluid flow to exit from the housing structure 120 after passing in thermal communication with the flow sensor 41 in the manner that has been described above. The diverted flow of fluid enters the housing structure 120 by passing through the first port 124. As will be described in greater detail below, fluid passing over the first means for directing particulate away from the first port 124 is caused to move radially away from the housing structure as it passes over the expanded portion of the conical cross section. As the fluid passes over the lip 128 of the broadest portion of the conical cross sectional shape, the particulate-free fluid can rapidly change directions and enter the first port 124, but particulate matter that may be entrained in the fluid stream is not able to make the transition into a higher pressure region proximate the first port 124. This characteristic will be described in greater detail below in conjunction with Figure 11. Figure 11 shows a cross sectional view of the housing structure 120 of Figure 10 along with its first means for directing particulates away from the measuring means of the present invention. In the cross sectional view of Figure 11, streamlines G show the paths through which the fluid passes proximate the housing structure. As the fluid reaches the vicinity of the first directing means 122, the streamlines diverge to pass around the housing structure. As they diverge, the velocity of the fluid increases and its pressure decreases. This results from the well known relationship between pressure and velocity that is defined by Bernoulli's equation. Particulate matter is directed to flow along the streamlines and away from the housing structure as the fluid passes the lips 128 of the first directing means 122. Because of the lower velocity of fluid proximate the first port 124, particulate matter would have to move from a region of lower pressure to a region of higher pressure if this particulate matter is to enter the first port 124. Since the particulate matter will not generally move from a region of lower pressure to a region of higher pressure, the particles pass the first port and continue downstream while the particulate-free fluid is permitted to enter the first port 124. The generally conical shape of the upstream portion of the housing, which is identified by reference numeral 122, therefore provides a means for directing particulates away from the sensing means of the present invention and prevents damage from occurring to the sensing means as would otherwise be possible if a particle in the main flow stream was permitted to strike the semiconductor sensor at a high velocity.

In Figure 11, the fluid passing through the first port 124 is directed into the upstream conduit 56 of the second directing means 52. As can also be seen in Figure 11, all of the fluid entering the housing structure through the first port 124 is forced to pass through the second directing means 52.

Figure 12 illustrates the present invention disposed within a conduit. The simplified illustration of Figure 12 shows the first means 122 for directing particulate matter away from a sensing means which is disposed within the housing structure 120. The first port 124 and the second port 126 are shown at their relative positions in the housing structure 120. The housing structure is disposed in the main conduit with the first and second ports disposed proximate the center line 130 of the conduit. The housing structure is attached to a wall 132 of the main conduit for support. As a flow of fluid, represented by arrows G, passes through the main conduit a portion of the fluid is diverted into the first port 124. Because of the characteristic shape of the first directing means 122, particulate matter is prevented from entering the first port 124. The diverted flow, after passing through port 124, continues to pass through a second directing means (not shown in Figure 12) disposed with the housing structure 120, as explained in greater detail above. As the diverted flow passes through the second directing means, it flows in thermal communication with a flow sensor. Eventually, the diverted flow exits the housing structure through port 126 and returns to the main stream of fluid. Entry into the first port 124 and exit from the second port 126 is induced by the different diameters that are illustrated in Figure 12. The pressure differential created by these different diameters induces flow into the first port and out of the second port to divert a portion of the flow over the flow sensor. Dimension D and R illustrate the fact that the first and second ports are disposed at a generally central region of the main conduit. This provides a significant advantage of the present invention in comparison with flow sensors known to those skilled in the art. The flow sensor of the present invention samples the mass flow at a region proximate the central portion of the main conduit and is therefore not affected by skin effects or other disturbances that can be caused by the walls of the main conduit. In addition, the semiconductor flow sensor of the present invention can be disposed in the middle portion of the main conduit without fear of damage by particulates striking it at high velocity. The first directing means 122 of the present invention directs particulates away from the first port 124 and, therefore, does significantly reduces the particulate matter passing over the flow sensor. In addition, the present invention provides^* the second directing means to cause the diverted flow to pass in intimate thermal communication with the flow sensor after it enters the housing structure 120 through the first port 124.

Figures 13, 14, 15 and 16 illustrate alternative shapes that could be used in association with the present invention. By comparing the cross sectional shape of the first directing means 122 that is illustrated in Figure 11 to the shapes illustrated in Figures 13-16, it can be seen that many modifications of the cross sectional shape can be made within the scope of the present invention. Although the alternative shapes shown in Figures 13- 16 are illustrated three-dimensionally, it should be understood that they represent shapes that would be used to provide the cross sectional configuration of the upstream edge of the housing structure.

Throughout the description of the present invention, the device identified by reference numeral 50 has been referred to as a circuit board. However, it should be understood that any different substrates, such as hybrid devices, can alternatively be used to support the sensor 41 within the scope of the present invention. In addition, the sensor 41 has been described with particular detail as a semiconductor device comprising a heater and one or two temperature sensitive resistors. It should be understood that alternative sensors can be used in association with the present invention and, when semiconductor devices are used, they need not be similar to micro bridge structures.

Although the present invention has been described with significant specificity and many particular details of the present invention have been illustrated in the drawings, it should be clearly understood that many alternative embodiments of the present invention are within its scope.

Claims

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. A fluid flow sensor apparatus, comprising: means for measuring the mass flow rate of a fluid flowing through a conduit; and means for disposing said measuring means in a central portion of a said conduit, said disposing means being attached to said conduct and extending from a wall of said conduit.

2. The apparatus of claim 1, further comprising: first means for directing particulates away from said measuring means, said first directing means being configured to permit said fluid to pass in thermal communication with said measuring means.

3. The apparatus of claim 2, wherein: said disposing means comprises a housing member shaped to receive said measuring means, said housing member being shaped to form said directing means at an upstream portion of said disposing means.

4. The apparatus of claim 3, further comprising: second means for directing said fluid to flow in thermal communication with said measuring means, said second directing means being disposed within said housing member, said measuring means being disposed within said second directing means.

5. The apparatus of claim 4, wherein: said second directing means comprises means for reducing the cross section area of said fluid stream passing through said second directing means proximate said measuring means.

6. The apparatus of claim 5, further comprising: means for preventing said fluid from flowing against an edge of said measuring means, said preventing means being disposed upstream of said measuring means and within said second directing means.

7. The apparatus of claim 6, wherein: said preventing means comprises a thin metallic plate.

8. The apparatus of claim 7, wherein: said housing comprises first and second ports, said first port being associated in fluid communication with an upstream end of said second directing means, said second port being associated in fluid communication with a downstream end of said second directing means.

9. The apparatus of claim 8, wherein: said measuring means comprises an upstream resistor, a downstream resistor and a heating element disposed between said upstream and downstream resistors.

10. The apparatus of claim 9, wherein: said first directing means comprises a generally conical cross sectional shape.

11. A fluid flow sensor apparatus, comprising: a support structure attached to a fluid conduit, said support structure comprising a housing member, said housing member having a cavity disposed therein, said cavity being disposed proximate a central portion of a cross sectional area of said fluid conduit; means for directing a fluid to flow through a region, said directing means being disposed within said cavity; and a flow measuring device disposed in said region.

12. The apparatus of claim 11, wherein: said directing means comprises a flow restriction proximate said region, said flow restriction comprising a tapered passage.

13. The apparatus of claim 12, further comprising: a flow deflector disposed within said directing means upstream of said flow measuring device, said flow deflector being configured to prevent said fluid from flowing against an upstream edge of said flow measuring device.

14. The apparatus of claim 13, wherein: said housing member has an upstream port and a downstream port, said upstream port being associated in fluid communication with an upstream end of said directing means, said downstream port being associated in fluid communication with a downstream end of said directing means.

15. The apparatus of claim 14, wherein: an upstream portion of said housing member is shaped to direct particulates away from said upstream port while permitting said fluid to flow into said upstream part and through said directing means.

16. The apparatus of claim 15, wherein: said upstream portion of said housing member is generally conical in cross sectional shape.

17. A method for measuring fluid flow, comprising: disposing a fluid flow sensor proximate a central portion of a fluid conduit; directing a flow of fluid in thermal communication with said fluid flow sensor; constricting said flow of fluid in a region proximate said fluid flow sensor; directing particulates away from said fluid flow sensor; and deflecting said flow of fluid away from an upstream edge of said fluid flow sensor.

18. The method of claim 17, further comprising: disposing said fluid flow sensor in a housing member; directing said flow of fluid into an upstream port of said housing member; directing said flow of fluid out of a downstream part of said housing member; and directing said flow of fluid between said upstream and said downstream ports to flow in thermal communication with said fluid flow sensor.