GB2061505A

GB2061505A - Fluid Flow Meter

Info

Publication number: GB2061505A
Application number: GB8029830A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1979-09-17
Filing date: 1980-09-16
Publication date: 1981-05-13
Also published as: JPS5921485B2; DE3034936A1; JPS5642110A

Abstract

A fluid flow meter includes a vortex generating element 12 with hot wires 14, 16 supported on the exterior of the element. The meter has a circuit for generating a pulse signal indicative of the frequency of occurrence of vortices in the fluid and another circuit for generating an analog signal having a voltage corresponding to an average velocity of the fluid. The pulse signal and the analog signal are selectively outputted corresponding to the flow condition of fluid. For determining whether the pulse signal or the analog signal is to be outputted, the fluid flow meter is further provided with a fluid condition detecting means which senses whether or not the fluid is pulsable. <IMAGE>

Description

SPECIFICATION Fluid Flow Meter The present invention relates generally to a fluid flow meter for determining a velocity and amount of a fluid flowing through a fluid passage.

More specifically, the invention relates to a fluid flow meter applicable for determining the velocity and the amount of a pulsatile fluid and the like which the flowing condition of the fluid is frequently varied, and therefore applicable for determining, for example, the velocity and amount of an intake air to be delivered to an internal combustion engine.

Conventionally, there have been know two kinds of fluid flow meters. One is Karman voltex fluid flow meter for determining the velocity and amount of the fluid by measuring frequency of occurrence of the voltex in the fluid. The other is a hot wire fluid flow meter for determining the fluid velocity of measuring varying of impedance of a heated wire.

In general, the Karman voltex fluid flow meter has a bar-like element to be inserted in the fluid in lateral direction with respect to fluid flow for producing or generating voltex in the fluid. Since the frequency of occurrence of Karman voltex produced in the fluid is proportional to the velocity and amount of fluid flowing therethrough, the frequency of the occurrence of the Karman voltex is determined to determine the velocity and amount of fluid. In this purpose, the Karman voltex fluid flow meter is provided with a means for detecting occurrence of the Karman voltex and for generating a pulse signal in response to occurrence of the Karman voltex.

The Karman voltex fluid flow meter is advantageous in that, since the proportion coefficient between the frequency of occurrence of the voltex and the velocity and amount of the fluid is determined by a relationship of diameter of the fluid passage and size of the voltex generating element, the coefficient indicative of the relationship between the frequency of occurrence of Karman voltex and the velocity and amount of fluid flow is fixed value and not varied.

Further, the Karman voltex fluid flow meter is advantageous in that, since the means for detecting presence of the Karman voltex generates the pulse signal corresponding to frequency of occurrence of voltex, it is easy to incorporate with a digital processing means such as digital computer for processing the signal.

Contrary to this, the Karman voltex fluid flow meter contains a problem caused in the nature thereof, which problem is that it is difficult to determine velocity and amount of fluid in case of the fluid being frequently varied the flowing condition such as pulsatile fluid.

For example, in considering measurement of velocity and amount of an intake air for an internal combustion engine of an automotive vehicle, the Karman voltex fluid flow meter effects accurate measurement within a range of relatively low engine load condition, since, in such range, pulse hight of pulsatile fluid is substantially small to be disregarded even though the fluid introduced in the engine is pulsatile. However, in high engine load condition such as accelerating by fully opening a throttle valve, the pulse hight of the pulsatile fluid becomes considerably higher to interfere generating of Karman voltex or to generate the voltex in synchronism with the pulse of the fluid and therefore the voltex being generated without corresponding to fluid velocity.

In the meanwhile, the hot wire fluid flow meter comprises a electrically conductive and heat resistantive wire such as tungsten wire or platinum wire. The wire is heated to keep constant temperature thereof and is inserted into the fluid. By exposing the heated wire within the fluid flow, the temperature of the wire drops.

Corresponding to dropping of the wire temperature, impedance of the wire drops to reduce potential flowing therethrough. Therefore, in the hot wire flow meter, the fluid velocity is determined by measuring varying of potential flowing through the wire.

The hot wire fluid flow meter is advantageous, since it is applicable to various measurement of fluid velocity and has good response characteristics. Further, this hot wire fluid flow meter can be applied for measuring fluid velocity of pulsatile fluid. On the other hand, the hot wire fluid flow meter is affected by stain on the surface of the wire which will vary heat radiation characteristics to gradually increase abrasion in the result of measurement. For correcting this, the conventional hot wire fluid flow meter is accompanied with a reference fluid flow generating device. The reference fluid flow generating device generates a reference fluid flowing in constant velocity.By measuring fluid velocity of the reference fluid, a difference between the accurate velocity and measured velocity can be determined, and based on the determined difference, a correction coefficient is obtained. In this case, it is required to change fluid passage construction for adapting to application of the reference fluid flow generating device. If the reference fluid flow generating device is not provided, it is required to frequently take off the hot wire from the fluid passage to clean.

As explained above, even though the conventional fluid flow meters have respective advantages, they have not been applicable to all of fluids. Namely, the conventional fluid flow meter is limited the fluid or fluid passage construction to be applied.

Therefore, it will be advantageous to combine both of the Karman voltex flow meter and the hot wire flow meter in such manner that the flow meters are selectively used depending on flow condition of the fluid. For accomplishing this, the basic or fundamental problem to be solve is that each flow meter is provided standing substantially different logic of fluid velocity measurement. The problem can be solved by using an electric circuit in common, which processes the result of measurement and selectively operating either one of Karman flow meter or hot wire flow meter.

Summary of the Invention Therefore it is an object of the present invention to provide a fluid flow meter having both of the Karman voltex flow meter and the hot wire flow meter which are selectively operated corresponding to fluid flow condition.

Another object of the present invention is to provide a method for measuring fluid velocity, which fluid is introduced into an internal combustion engine, such as intake air introduced in the combustion chamber through an air intake passage.

According to the present invention, there is provided a fluid flow meter having both of advantages of the Karman voltex flow meter and the hot wire flow meter. The fluid flow meter includes a voltex generating element with hot wires supported and stretched on the exterior of the voltex generating element. The voltex generating element and the hot wire is inserted into fluid flow. The fluid flow meter has a circuit for generating a pulse signal indicative of frequency of occurrence of voltex in the fluid and another circuit for generating an analog signal having voltage corresponding to an average velocity of fluid. The pulse signal and the analog signal is selectively outputted corresponding to flow condition of fluid. For determining either the pulse signal or the analog signal is to be outputted, the fluid flow meter is further provided with a fluid condition detecting means.

Thus, the fluid flow meter according to the present invention is provided both of characteristics of the Karman voltex flow meter and the hot wire flow meter and by using either one of measuring system coupled in the flow meter corresponding to flow condition of fluid, the result of measurement becomes accurate and is capable of measuring any fluid.

According to the preferred embodiment of a fluid flow meter according to the present invention, the fluid flow meter is applied to an internal combustion engine to be mounted on an automotive vehicle and so on for determining an intake air flow rate thereof. In this case, the pulse signal and the analog signal is selectively outputted corresponding to the throttle valve position.

Brief Description of the Drawings The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings of the preferred embodiment of the invention, which, however, are not be taken limitative the present invention but for elucidation and explanation only.

In the drawings: Fig. 1 is a perspective view of a preferred embodiment of a fluid flow meter according to the present invention, which is to be inserted and exposed to the flowing fluid; Fig. 2 is a transversary sectional view of the fluid passage to which is inserted the fluid flow meter of Fig. 1 , taking along a longitudinal vertical plain along longitudinal central axis; Fig. 3 is a sectional view of the fluid flow meter of Fig. 1 taken along longitudinal horizontal plain; Fig. 4 is a graph showing a fundamental logic of measurement of fluid flow according to the present invention; Fig. 5 is a circuit diagram of a circuit for determining fluid velocity based on the result of measurement of the fluid flow meter of Fig. 1;; Fig. 6 is a graph showing output signals of the circuit of Fig. 5 respectively in analog and pulse signal form; and Fig. 7 is a circuit diagram of another embodiment of a circuit for determining fluid velocity based on the result of measurement of the fluid flow meter.

Description of the Preferred Embodiment Referring now to the drawings, particularly to Figs. 1 to 3, there is illustrated a preferred embodiment of a fluid flow sensitive element 10 according to the present invention. In Figs. 2 and 3, the fluid flow sensitive element 10 is positioned in a fluid passage 1 8 and therefore showing a condition being actually used. The fluid flow sensitive element 10 generally comprises a voltex generating element 12 and hot wires 14 and 1 6 vertically extending along vertical axis of the voltex generating element 12. The voltex generating element 12 and the hot wires 14 and 1 6 are inserted into the fluid passage 1 8 for measuring velocity and/or amount of fluid flowing therethrough.

The voltex generating element 12 consists a part of the sensitive elements which comprises the voltex generating element 12 and a mounting plate 22 provided on one end of the voltex generating element 12. In the preferred embodiment, the voltex generating element 12 is of sectionally rectangular shaped configuration and the mounting plate 22 is of stepped disc shaped configuration. Though there are illustrated the voltex generating element 12 and the mounting plate 22 in the specific form, the shapes of these element are unnecessarily specified thereto. For example, with respect to the voltex generating element is merely required the form symmetric with respect to flow direction of the fluid, and therefore, it can be formed to cylindrical shape, trigonal shape or other multi-angled bar shape.

The voltex generating element 12 is inserted into the fluid flow through an opening 24 formed in the periphery 26 of the fluid passage 1 8.

Preferably, the size of the opening 24 is adapted that of the stepped down portion of the mounting plate 22 so that the stepped down portion can engage therewith, as shown in Fig. 2. The mounting plate 22 is secured on the periphery 26 of the fluid passage with tightening of a plurality of screws 28 extending to the periphery 26 through a plurality of openings 30 formed in the mounting plate 22. At this time, the voltex generating element 12 is placed within the fluid flow directing the vertical plain 32 having the hot wires 14 and 1 6 thereon downstream with respect to flow direction of the fluid.For supporting and stretching the hot wires 14 and 1 6 on the vertical plain 32, a pair of substantially L-shaped projections 34 and 36 and a substantially T-shaped projection 38 are protruded from the vertical plain 32 adjacent upper and lower ends of the voltex generating element. These projections are made from electrically conductive materials. The projections are respectively connected with connecting terminals 40, 42 and 44 protruding upwardly from the top of the mounting plate 22. Thus, the hot wires 14 and 1 6 are stretched along the vertical plain of the voltex generating element 12 through leads 46, 48 and 50.

It is advisable that, in case of the voltex generating element 12 being made from an insulating materials, the leads 46, 48 and 50 are molded therein in apart from one another and in case of the element 12 being made from electrically conductive material, each lead should be covered or coated by insulating member.

The hot wires 14 and 1 6 are stretched in parallel relationship with respect to one another and are symmetrically placed with respect to a central axis of the voltex generating element 1 2. Further, the hot wires 14 and 1 6 are placed on substantially the same vertical plain in lateral with respect to the fluid flow direction.

It should be noted that the range in which the voltex effect to the fluid is 10 to 20 times of width of the voltex generating element with respect to the fluid flow. Therefore, the hot wires 14 and 1 5 are to be placed within this range. On the other hand, the shapes of the electrically conductive projections 34, 36 and 38 are not necessarily specified to the disclosed forms. For these projections, it is only required to stretch the hot wires 14 and 1 6 in parallel relationship and in symmetrical relationship with respect to the central axis of the voltex generating element.

Therefore, for example, the T-shaped projection can be replaced by two L-shaped projections and further for example, the two L-shaped projections provided upper portion of the voltex generating element can be the T-shaped projections. The hot wire is made from tungsten or platinum wire and is secured to the free ends of the projections in know suitable manner such as welding and the like. In considering response characteristics of the hot wire, it is preferable that the thickness or diameter of the wire is less than 100,u0.

However, the thickness or diameter may not always require to be less than 100 , and it can be determined corresponding to range to measure the voltex, wire length, wire strength and so on.

Before moving into description of the circuit structure for processing signal generated by the fluid flow meter of the present invention, the theory or logic of detecting of velocity and/or amount of fluid according to the present invention is explained hereafter for helping understanding of the concept of the present invention.

As shown in Fig. 3, assuming the fluid flowing from left to right, the fluid colides to the upstream side surface of the voltex generating element 12 to generate voltex alternatively on both sides of the voltex generating element. Therefore, the hot wires 14 and 1 6 are cooled alternatively.^When the voltex is generated at the side where is placed the hot wire 14, the velocity of the fluid can be obtained from the following equation.

u^=u+Au sin a)t (1) where Ue is a velocity of the fluid flowing therethrough; u is an average velocity of fluid; Au sin ot is an amount of varying of velocity of the fluid in response to generating of the voltex.

Alternatively, at this time, reversal varying of fluid velocity is occurred at the opposite side of the voltex generating element where is placed the hot wire 1 6. Therefore, fluid velocity flowing through the hot wire 16 can be obtained from the following equation.

u7=u-Au sin wt (2) where u, is fluid velocity flowing through the wire 1 6.

From the above-recited two equations (1) and (2), difference between the fluid velocity at both sides with respect the voltex generating element can be obtained as follows: u0-u7=2Au sin wt (3) Therefore, as seen from the above-recited equation (3), by subtracting of the average fluid velocity, the amount of varying of fluid velocity in synchronism with alternation of sides of generating the voltex can be obtained. On the other hand, by addition of the fluid velocities at both sides of the voltex generating element 12, the average fluid velocity can be obtained as follows: u,+u,=2 (4) Here, radiation characteristic of the hot wire 14 and 1 6 is generally represented by following equation.

wnere i is amount of current supplied to the wire; R is resistance value of the wire; and u is fluid velocity Further here, assuming the resistance value of the hot wire 14 and 1 6 is constant, i.e. assuming the temperature of the wire being kept at constant, the relationship of the potential at the end of the wire and fluid velocity can be illustrated by the following equation.

Vmul (6) The relationship can be illustrated in Fig. 4. As seen from Fig. 4, varying of fluid velocities flowing both sides of the voitex u,, u, with respect to the average velocity can be represented by varying of potential Ve, and V, at the ends of the hot wires 14 and 16. The amount of varying of the potential is AVa < V2 and the potential of each hot wire 14 and 16 is alternatively varied within this range.

Therefore, it is desirable to correct the value of potential at the end of the hot wire by multiplying four times to provide lineality between the potential and fluid velocity. After correction, the potential determined is processed for determining fluid velocity and amount of flow.

The corrected potential obtained by multiplying four times is limited at the upper limit of varying of average voltage V, as shown in function line in Fig. 4, caused by multiplying error. Thus, accurate average fluid velocity can be determined. On the other hand, in case of determining the fluid velocity from varying of velocity in synchronism with frequency of occurrence of generation of voltex, since the frequency of occurrence of the voltex is mereby important, the multiplying error can be disregarded.

It should be noted that correction of the potential is not always necessary, since the varying of fluid velocity caused by generation of voltex is substantially so smaller than the average fluid velocity as to disregarded.

Referring now to Fig. 5, there is illustrated a preferred embodiment of å circuit for determining fluid velocity based on output potential of the hot wires 14 and 16 of Figs. 1 two 3.

A wire heating current control circuit 50 includes the hot wire 14. The control circuit 50 controls an electric current supplied to the hot wire 14 for heating the latter at constant temperature. In response to drop of wire temperature by radiation of heat when the wire subjects to the voltex, the control circuit 50 increases the current supplied thereto for recover the wire temperature.

The control circuit 50 comprises a Wheatstone bridge circuit 52 including the hot wire 14 and resistors, R, to R3, a differential amplifier 54 including operation amplifier OP, and resistors R4 to R, and a feedback circuit 56 including transistor Trr and resistors R8 and R9. The control circuit 50 functions to balance the unbalance voltages at points 58 and 60 in the Wheatstone bridge circuit 50. Namely, the control circuit 50 function to reduce a different of potential at the points 58 and 60.

Now, we move more detail with respect to functions of the wire heating current control circuit 50. When the hot wire 14 is subject to voltex generated in the fluid, the heat of the wire is radiated to drop the wire temperature. By this, the impedance of the wire 14 drops accordingly to balance of potentials in the wheatstone bridge circuit 52 is destroyed. The difference of potentials is amplified by the differential amplifier 54. Responsive to operation of the difference amplifier 54, the potential applied to the base electrode of the transistor Trl is increased to cut off the same. The transistor Tr, is kept in cut off position until the difference of the potentials at the points 58 and 60 reduced to zero. Through the period in which the transistor Tr, is kept in cut off position, current supplied to the wire 14 through the resistor R1 is increased.The control circuit 50 can respond to relatively small varying of fluid velocity such as varying caused byvoltex.

However, the control circuit 50 may not respond to relatively large varying of fluid velocity, for example caused by pulsatile fluid since it is difficult to maintain the wire temperature at constant temperature. In this case, varying of potential at the point 62 is in a wave form signal V6 as shown in Fig. 6. The potential of the wave signal V6 is in relation VBaSU1/4 with respect to fluid velocity u.

Likewise, a wire heating current control circuit 64 comprises a Wheatstone bridge circuit 66, a differential amplifier 68 and a feedback circuit 70.

The control circuit 64 includes the hot wire 1 6 in the Wheatstone bridge circuit 66. The control circuit 64 controls an electric current supplied to the hot wire in response to drop of impedance of the wire 1 6. Therefore, likewise to the control circuit 50 as explained above, varying of potential at the point 72 is in a wave form signal V, as shown in Fig. 7. The potential of the wave signal V, is in relation V7au114 with respect to fluid velocity u.

A first detecting circuit 74 determines a signal Q corresponding to difference of the outputs of the control circuits 50 and 64, (V7-V6), and generates a pulse signal Q' based on the signal Q and in synchronism with frequency of occurrence of generation of voltex. The first detecting circuit includes a differential amplifier 76 comprising an operational amplifier OP3 and resistors R19 to R22 and a shaping circuit 78 comprising a capaciter C,, resistors R23 to R25 and operational amplifier OP4.

A second detecting circuit 80 determines an average of outputs V6 and V, obtain an average fluid velocity. The second detecting circuit 80 determines an analog signal P as shown in Fig. 6.

The second detecting circuit 80 further corrects the determined analog signal P by way of subtracting error in one-fourth power of the outputs V6 and V, to generate a corrected analog signal P'. The second detecting circuit 80 comprises resistors R26 and R2, of the same resistance value. The resistors R26 and R2, function to reduce potential of outputs V6 and V7 to obtain the analog signal P of (V,+V,)/2. The analog signal P is outputted from a point 82 being intermediate between the resistors R26 and R27.

The second detecting circuit 80 further comprises a upper peak holding circuit 84 including a diode D, resistor R28 and capacitor C2. The upper peak holding circuit 84 corrects the analog signal P by way of subtracting error in one-fourth power of outputs V6 and V, to determine the corrected analog signal P'.

Both of the outputs of the first and second detecting circuits 74 and 80 are inputted to a switching circuit 86. The switching circuit 86 is connected with a pulsatile fluid detector 88. The pulsatile fluid detector 88 outputs a signal in response to detecting of pulsatile fluid.

Responsive to the signal fed from the pulsatile fluid detector 88, the switching circuit 86 outputs the analog signal P' of the second detecting circuit. On the other hand, the pulsatile fluid detector 88 is inoperative and thereby no signal is fed, the switching circuit 86 outputs the pulse signal Q' of the first detecting circuit 74.

In case of application of the fluid flow meter according to the present invention and as described hereabove to measurement of air flow rate of an internal combustion engine, the flow of intake air in an air intake passage becomes pulsatile when a throttle valve is fully opened. At this time, pulsatile air flow will be caused by transmission of varying of pressure in the combustion chamber. Therefore, the pulsatile fluid detector 88 of the circuit of Fig. 5 can be comprises either one of a throttle valve switch or a vacuum sensor. The throttle valve switch detects fully opened position of the throttle valve and generate a signal in response thereto. On the other hand, vacuum sensor is provided in an intake manifold of the engine and determines vacuum pressure flowing therethrough. The vacuum sensor generates a signal when the determined vacuum becomes less than a given value.In response to the signal, the switching circuit 86 of Fig. Soutputs the analog signal P' of the first detecting circuit 50. On the other hand, if the signal is not generated by the throttle valve switch or vacuum sensor and therefore the throttle valve is not fully opened, the switching circuit 86 outputs the pulse signal Q'.

As shown in Fig. 6, assuming within periods of time T, and T3 flow normal fluid and within period of time T2 flows pulsatile fluid, the pulse signal Q' which is generated in synchronism with frequency of occurrence of the voltex in the fluid, is outputted within the periods of time T, and T3. In this case, the velocity of fluid can be determined by counting the pulse of the pulse signal Q'.

Contrary, the analog signal P' which has a potential corresponding to average velocity of fluid, is outputted within the term T2. Therefore, according to the fluid flow meter of the present invention, the fluid velocity or amount of fluid can be accurately determined in spite of fluid flow condition.

Fig. 7 shows another embodiment of a detecting circuit of Fig. 5 according to the present invention. A wire heating current control circuit 102 includes a Wheatstone bridge circuit 104. In the Wheatstone bridge circuit 104, the hot wires 14 and 1 6 of Fig. 1 are connected with each other in series to consist a of the bridge which further comprises resistors R29 to R31. The control circuit 102 controls an electric current supplied to the hot wires so that average temperature of the hot wires 14 and 1 6 becomes constant, namely, so that total impedance of the hot wires 14 and 16 is constant.

It should be noted that the control circuit 102 further comprises a differential amplifier 106 having an operational amplifier OP4 and resistors R32to R3s, and a feedback circuit 108 including a transistor Tr3 and resistors R30 and R3,. The structures of the differential amplifier 106 and the feedback circuit 108 are substantially the same as foregoing embodiment of Fig. 5.

From a point 110 between the hot wires 14 and 16, an output V, having a potential corresponding to impedance of the hot wire 1 6 is outputted. On the other hand, from a point 112, another output V6+V7 is outputted, which output V8+V, has a potential corresponding to total impedance of hot wires 14 and 1 6. The outputs V7 and Vs+V, are varied the potentials thereof corresponding to varying of impedances of the hot wires 14 and 16. The output VB+V7 fed from the point 112 is divided the potential thereof by resistors R36 and R39.Therefore, the potential at a point 114 between the resistors R36 and R39 is (Ve+V,)/2. The outputs V, fed from the point 110 and output (V0+V7)/2 fed from the point 114 is inputted to an operational amplifier OP5 of a first detecting circuit 11 6. The first detecting circuit 11 6 is a differential amplifier comprising the operational amplifier OP5 and resistors R, to R43.

The first detecting circuit 11 6 has amplifying ratio of 2 to generate output signal Q whose potential can be obtained from following equation.

Ve+V7 Q=(--------- --V,) x2=V,--V, 2 Therefore, the output signal 0 generated by the first detecting circuit 11 6 is substantially the same as output signal of the first detecting circuit of the preceding embodiment. The output signal O is shaped and thus converted to a pulse signal Q' synchronized with occurrence of the voltex.

On the other hand, the output VB+V7 through the resistor R31 is also fed to a second detecting circuit 11 8. The second detecting circuit 11 8 is consisted by buffer circuit having an operational amplifier OP6. The second detecting circuit 118 converts the output VB+V7 into analog signal P which corresponds to varying of impedance of the hot wires 14 and 1 6 and thereby corresponds to an average fluid velocity.

It will be advisable that, in the shown embodiment of Fig. 7, the peak holding circuit employed in the first embodiment explained with reference to Fig. 5 is not provided. Further, though there is omitted, the switching circuit and the pulsatile fluid detecting circuit of the first embodiment are connected with the first and second detecting circuit in substantially the same manner as shown in Fig. 5.

In the second embodiment, the circuit structure of the fluid velocity determining circuit can be more simplified.

Thus, the present invention fulfills all of the objects and advantages sought therefor.

While the present invention has been shown and described in detail in terms of preferred embodiments, it should not be considered as limited to these, or mere and simple generalizations thereof, or other detailed modifications. Further variations to any particular embodiments may be made without departing from the scope of the present invention, which it is therefore desired should be delimited and defined not by any way of the perhaps purely fortuitous details of the shown embodiments, or of the drawings, but solely by the appending

Claims

claims. Claims

1. A fluid flow meter comprising: a voltex generating element with a pair of hot wires being inserted within fluid flowing through a fluid passage the longitudinal axis of said voltex generating element being directed in lateral direction with respect to the direction of fluid flow, and said hot wires being stretched along said longitudinal axis of said voltex generating element and in symmetric relationship with respect to said longitudinal axis; said hot wires being supplied electric current for heating at a given constant temperature; a first means for controlling supply of electric current to said hot wires, which first means increase current value to be supplied to the hot wires in response to drop of impedances of the hot wire corresponding to drop of wire temperature; a second means for determining frequency of occurrence of voltex in the fluid by obtaining difference of impedance of said hot wires and for generating a pulse signal which has a frequency corresponding to frequency of occurrence of voltex; a third means for determining an average fluid velocity by obtaining average impedance of the hot wires and for generating an analog signal value of which corresponds to determined average velocity of the fluid;; a fourth means for detecting a condition of fluid and generating a signal if the fluid is a pulsatile fluid; and a fifth means for selectively outputting said pulse signal and analog signal, which fifth means outputting said analog signal in response to said signal of said fourth means.

2. A fluid flow meter as set forth in claim 1, wherein said hot wires are placed apart from said voltex generating element within a range where said voltex generated in the fluid effects thereto.

3. A fluid flow meter as set forth in claim 1, wherein said first means comprised a circuit for outputting a signal corresponding to the impedance of the hot wires and a circuit for detecting drop of impedance and increasing current supplied to said hot wire until the temperature thereof becomes at a given temperature.

4. A fluid flow meter as set forth in claim 1, wherein said third means includes a means for holding an upper limit of said average potential corresponding to average velocity of the fluid.

5. A fluid flow meter as set forth in claim 1 or 2, wherein said third means corrects the output signal of said first means so that the voltage of the signal is linear to the fluid velocity.

6. A fluid flow meter for determining an intake air flow rate in an air intake passage of an internal combustion engine, comprising: a voltex generating element with a pair of hot wires being inserted within air flow flowing through an air intake passage the longitudinal axis of said voltex generating element being directed in lateral direction with respect to the flow direction of intake air, and said hot wires being stretched along said longitudinal axis of said voltex generating element and in symmetric relationship with respect to said longitudinal axis; said hot wires being supplied electric current for heating at a given constant temperature; a first means for controlling supply of electric current to said hot wires, which first means increase current value to be supplied to the hot wires in response to drop of impedances of the hot wire corresponding to drop of wire temperature; a second means for determining frequency of occurrence of voltex in the intake air by obtaining difference of impedance of said hot wires and for generating a pulse signal which has a frequency corresponding to frequency of occurrence of voltex; a third means for determining an average air flow rate by obtaining average impedance of the hot wires and for generating an analog signal value of which corresponds to determined average air flow rate;; a fourth means for detecting a flow condition of intake air and generating a signal if the intake air is a pulsatile; and a fifth means for selectively outputting said pulse signal and analog signal, which fifth means outputting said analog signal in response to said signal of said fourth means.

7. A fluid flow meter as set forth in claim 6, wherein said hot wires are placed apart from said voltex generating element within a range where said voltex generated in the intake air effects thereto.

8,A fluid flow meter as set forth in claim 6, wherein said first means comprised a circuit for outputting a signal corresponding to the impedance of the hot wires and a circuit for detecting drop of impedance and increasing current supplied to said hot wire until the temperature thereof becomes at a given temperature.

9. A fluid flow meter as set forth in claim 6, wherein said third means includes a means for holding an upper limit of said average potential corresponding to average air flow rate.

10. A fluid flow meter as set forth in claim 6 or 7, wherein said third means corrects the output signal of said first means so that the voltage of the signal is linear to the air flow rate.

11. A fluid flow meter as set forth in claim 1, wherein said fourth means is a throttle valve switch which is operative and generates a signal in response to fully opening of a throttle valve provided in the air intake passage.

12. A fluid flow meter as set forth in claim 1, wherein said fourth means is a vacuum sensor provided in an intake manifold of the internal combustion engine, which detects drop of vacuum pressure responsive to opening of the throttle valve and generates a signal when the vacuum drops lower than a give value.