Abstract:
A fluid flow meter has a semi-conducting substrate and a number of integral flexible tines orientated across an aperture in the substrate to deflect in the direction of the fluid flow by an amount dependent on that fluid flow. Piezo-resistive regions are included on the respective tines to provide them with electrical characteristics which vary dependent on the degree of deflection. A monitor, operably coupled to the regions of the tines, is provided to monitor the changes in their electrical characteristics and to calculate the flow therefrom.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fluid flow meter and in particular to a flow meter of the type that operates by measuring changes in electrical properties of an obstruction in the fluid flow path in order to determine the fluid flow. 
     2. Description of the Prior Art 
     A known flow meter of this type has an obstruction, in the form of a rigid paddle, to which is attached a piezo-resistive rod. The paddle is arranged to be deflected by a force exerted by a flowing fluid so that it tends to twist the piezo-resistive rod. This twisting results in changes in the electrical resistance of the rod, Changes in the electrical properties (such as resistance) of electrical circuitry that includes the rod are then measured and are used to calculate the fluid flow. 
     One disadvantage of the known device is that, particularly when used to measure small flows, the paddle must be made relatively large in order to construct a sufficiently sensitive flow meter. This adds an appreciable and undesirable resistance to the flowing fluid. Additionally, as the mass of the paddle increases its resonant frequency decreases. This may lead to a reduction in the usefulness of the sensor in measuring oscillatory flows, since oscillatory flows with frequencies close to and above the resonant frequency cannot be easily or accurately measured using such a paddle. 
     Moreover, the paddle produces an“average” flow measurement from across the entire surface that faces upstream of the flow. This can lead to inaccurate measurements, especially when part of this surface lies in a region of turbulent flow. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a fluid flow meter wherein at least some of the problems associated with known flow meters are alleviated. 
     The above object is achieved in accordance with the principles of the present invention in a fluid flow meter having a fluid flow path therein with an obstruction disposed in the fluid flow path which is deflectable in response to fluid flow, the obstruction having electrical characteristics which vary dependent on the degree of deflection, and wherein the obstruction is formed by a number of flexible tines which are oriented so as to deflect in the direction of fluid flow. A monitor is operably coupled to the obstruction so as to monitor changes in electrical characteristics caused by deflection of the tines, and to calculate the fluid flow therefrom. 
     By employing number of tines instead of a single, large paddle, the mass of each deflectable element is considerably reduced which enables a meter to be constructed having a faster response time and which is better suited to the measurement of oscillatory flows. Moreover, since the tines deflect in the direction of the fluid flow the resistance the sensor offers to flowing fluid decreases as the flow increases, reducing turbulence effects which may occur at higher flows. 
     The degree of flexion of each tine in the flowing fluid can be separately selected and can be used to provide an“average” flow value using all of the tines. Alternatively, a selective flow value can be obtained using one or more of the tines chosen, for example, because the variations in determined flexes indicates that they lie outside regions of turbulent flow, or because the tines are of differing degrees of flexibility and those most responsive at the extant flow rate are selected. 
     Preferably, the tines are integral with a semi-conducting substrate, such a silicon, on to which other sensors and circuitry is integrated to form a flow meter that can be accurately and reproducibly fabricated using standard etching and doping techniques well established in the field of integrated chip technology. 
     Most usefully, the tines may be disposed to form a variable area obstruction across an aperture in the substrate and a differential pressure sensor can be integrated with the substrate to calculate the flow rate from pressure differences measured upstream and downstream of the tines in a manner common in the art, for example in the ways described in U. S. Pat. No. 4,006,634, the disclosure of which is incorporated herein by reference. With this arrangement two flow meters, both dependent on the flexibility of the tines, are incorporated in the same device, with practically no additional space. In such an arrangement one meter can be used as a safety backup device for the other. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a flow meter according to the present invention. 
     FIG. 2 shows an example of a tine of the flow meter of FIG. 1 configured with a piezo-resistive portion. 
     FIG. 3 is an illustration of a Wheatstone bridge  15  circuit useable with the tines shown in FIGS. 1 and 2 for the determination of the flow. 
     FIG. 4 shows a further embodiment of a flow meter according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a fluid flow sensor  1  has a silicon  20  substrate  2  that is etched to provide an aperture  3  disposed in a fluid flow path through the sensor  1 , across which extends a number of flexible tines  4   a . . . n  to form a variable area obstruction to a fluid flow. The substrate  2  can be etched to leave bars of silicon extending between opposite sides of the same face of the aperture  3 . The bars are then etched so that they form flexible tines  4   a . . . n  connected to the substrate at one end only. For example the tines  4   a . . . n  can be of different lengths as shown in FIG.  1 . The lengths of the individual tines  4   a . . . n  are selected to influence their degree of flexibility which will vary their individual responses to differing flow rates and hence increase the overall sensitivity and dynamic range of the flow meter. The tines  4   a . . . n  are doped to produce piezo-resistive areas  5   a . . . n , so that their electrical properties will vary as they flex. Associated conducting paths  6   a . . . n  connect the respective piezo-resistive areas  5   a . . . n  to the main body of the substrate  2 . 
     As shown in FIG. 2 for one tine  4   a , the piezo-resistive area can be formed as elements  5   a ,  5   a ′ opposing one another along opposite faces of the respective tine  4   a . Thus, as the tine  4   a  flexes one piezo-resistive area, for example  5   a , expands and the other, for example  5   a ′ , contracts. Since the piezo-resistive elements  5   a ,  5   a ′ have piezo-resistive properties that vary substantially equally with temperature the sensor  1  is also provided with a degree of temperature stability. The areas  5   a ,  5   a ′ is connected to the main body of the silicon substrate  2  via respective electrical conductors  6   a ,  6   a ′ , from where they are connected electrically to a suitably configured personal computer  7  (or alternatively a dedicated microprocessor), represented by the connector S in FIG. 1 in a manner common in the field of integrated circuit technology. 
     The elements  5   a ,  5   a ′ are configured in an electrical circuit to act as variable resistors R 1 , R 2  as shown in FIG.  3 . The circuit may be constructed on a suitable circuit board (not shown) which may be conveniently mounted in a conventional“card slot” (such as an ISA or PCI slot)on the motherboard of the computer  7 . The variable resistors R 1 , R 2  together with the resistors R 3 , R 4  of known resistance and the voltage source V, form a conventional Wheatstone bridge arrangement  9  as shown schematically in FIG.  3 . As is known in the art, the resistors R 3 , R 4  are chosen to provide a balanced bridge circuit  9  when the tine  4   a  is undeflected. The computer  7  initiates measurements of an electrical current that will flow between the points P 1 , P 2  in the Wheatstone bridge  9  when the tine  4   a  flexes and the bridge circuit  9  becomes unbalanced. Alternatively, the Wheatstone bridge arrangement  9  may be housed separately from the computer  7  and a value representative of a measured current transferred to the computer  7 . 
     This electrical current is representative of the amount of flexing of the tine  4   a  as a fluid flows and is used in the calculation of the flow rate by the computer  7 , which can be programmed, for example, to carry out the calculation using an empirically determined mathematical relationship between the measured current and the extant fluid flow. 
     It will be appreciated that each of the remaining tines  4   b . . . n  are similarly configured with corresponding piezo-resistive areas  5   b . . . n ,  5   b′ . . . n ′ and conductors  6   b . . . n ,  6   b′ . . . n ′ and operate as described above for the tine  4   a . Thus the circuit board can be provided with further Wheatstone bridge circuits, one for each tine  4   b . . . n , identical to that bridge circuit  9  described above. The computer  7  can then be programmed to selectively utilize the measurements from individual tines  4   a . . . n  to provide a sensible output indicative of the extant fluid flow. It will be clear to those skilled in the art that such a selection will depend largely on the proposed use of the sensor  1  and the physical conditions it is expected to experience. For example a selection to use readings from just some of the tines  4   a . . . n  can be based on the responsiveness of individual tines  4   a . . . n  to a particular flow or on the flow conditions as indicated by the readings of the individual tines  4   a . . . n  made so as to exclude readings from those tines in regions of turbulent flow. Alternatively a selection to use the readings from all of the tines  4   a . . . n  may be made. 
     An advantage of using a semi-conducting material as the substrate  2  of the sensor  1  is also illustrated in FIG. 1 which shows other environmental sensors, such as a moisture sensor  10 , a temperature sensor  11 , and pressure sensors  12 ,  12 ′ on opposite faces of the substrate  2 , integrated onto the same substrate  2  using well established technology. Particularly useful are the two pressure sensors  12 , 12 ′ which can be used in a differential flow meter arrangement, similar to that described in U.S. Pat. No. 4,006,634. 
     A further embodiment of the flow meter according to the present invention is shown in FIG.  4 . The sensor  1  is as shown in FIG. 1, having tines  4   a . . . n  as shown in FIG.  2  and therefore only the differences in the two embodiments of FIGS. 1 and 4 will be described in detail. 
     FIG. 4 shows a controllable power supply  13  connected between the sensor  1  and the computer  7 . Again a Wheatstone bridge circuit  9  is employed similar to that shown in FIG. 3 except that one of the variable resistors, for example PS, is replaced with a resistor of known value. The piezo-resistive element  5   a  (being associated with R 1 ) is, in the embodiment of FIG. 4, connected via the conductors  6   a  to the controllable power supply  13 . In use the current that flows between the points P 1  and P 2  in the bridge circuit  9  as the tine flexes from its zero flow position is again monitored by the computer  7 . A control signal is passed from the computer  7  to the supply  13  to vary the power output to the connected piezo-resistive element  5   a  in such a way that the element  5   a  responds to provide a force on the tine  4   a  which tends to move it back to its zero flow position. At this position the current flow between the points P 1 , P 2  will return to a value which exists when no flow is present (for example zero current) and the computer  7  instructs the supply  13  to maintain its output constant. The value of the power supplied to the element  5   a  that is required to hold the tine  4   a  in its zero flow position is a measure of the fluid flow and a signal representative of this value can be supplied to the computer  7  which is programmed to calculate the fluid flow therefrom. 
     Although the above embodiments describe flow meters in which piezo-resistive materials are utilized it will be appreciated that piezo-electric materials may alternatively be used without departing from the inventive concept. For example, the silicon substrate  2  and integral tines  4   a . . . n  of the sensor  1  may be formed of a piezo-electric material such as quartz or suitable ceramics or plastics. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.