WO1999060345A1

WO1999060345A1 - Fluid flow sensor

Info

Publication number: WO1999060345A1
Application number: PCT/AU1999/000383
Authority: WO
Inventors: Mark Payne
Original assignee: Automotive Computers Australia Pty. Ltd.
Priority date: 1998-05-20
Filing date: 1999-05-20
Publication date: 1999-11-25
Also published as: AUPP361198A0

Abstract

A fluid flow meter has a temperature dependant resistor (R2) located in the fluid flow. Control means (24, 52) maintains the temperature dependant resistor at a constant resistance as fluid flow varies by varying the power supplied from variable power supply means (30, 50). The variable power supply means inlcudes a variable voltage source (50) and a variable resistor (30) in series with the temperature dependant resistor, both of which are varied with fluid flow.

Description

Fluid Flow Sensor

This invention relates to air or other fluid meters, and more particularly to hot wire type airflow meters, such as those used for measuring the air intake of an internal combustion engine.

A hot wire airflow sensor utilises a resistor whose resistance is temperature dependant located in the fluid flow. Current is passed through the resistor and as fluid flow varies, so does the heat loss from the resistor, which changes the temperature and hence resistance of the resistor. The temperature dependant resistor is utilised as one arm of a wheatstone bridge and another temperature dependent resistor it utilised as another arm of the bridge, to compensate for differences in fluid temperature.

The differential voltage across the bridge is measured by a circuit which controls the input voltage to the bridge in response to the sensed differential voltage. As fluid flow increases the temperature of the temperature dependent resistor falls, inducing a voltage change. By increasing the supply voltage the current flow in the temperature dependant resistor increases, causing its temperature to rise and reducing the differential voltage across the bridge to zero. By measuring the supply voltage either directly or indirectly, this gives a measure of the fluid flow.

Conventional fluid flow meters utilise a fixed input voltage supply and a transistor between the supply and the wheatstone bridge. The control circuit merely opens and closes the transistor progressively to alter the voltage supplied to the wheatstone bridge.

Most spark ignition engines operate at low engine speeds and very small throttle openings with consequently low fluid flows, in comparison to maximum engine speed at full throttle opening. Accordingly there is a requirement for high power dissipation of the power transistor which reduces the energy efficiency and reliability of the air mass sensor.

The present invention aims to alleviate at least some of the disadvantages of the prior art by utilising pulse width modulation techniques. In a preferred form a second feedback loop is provided which varies the supply voltage by pulse width modulation so that the power transistor may be run at near maximum capacity, so reducing the voltage drop across the transistor and hence the power loss. As fluid flow increases the supply voltage may be increased to provide for the increased current demand.

In another form the transistor may be dispensed with and a pulse width modulated is provided to directly supply the wheatstone bridge. In another form a pulse width modulator is utilised to reduce the normal 12V supply of most vehicles to a stabilised 6.5V, thereby reducing power consumption.

In one broad form the invention provides a hot wire type fluid flow meter for measuring the flow of fluid in a passageway, the meter including:

a first temperature dependent resistor located in the passageway;

voltage supply means for supplying voltage and current to the first temperature dependent resistor and for varying the current in the first temperature dependent resistor so as to maintain said first temperature dependent resistor at a substantially constant temperature as the fluid flow varies, said voltage supply means utilising pulse width modulation to

i) supply a fixed reduced voltage compared to the supply voltage;

and/or

ii) supply a variable voltage to the first temperature dependent resistor.

Where a variable voltage is supplied by the voltage supply means, this may be achieved by utilising pulse width modulation only or by using pulse width modulation in conjunction with a transistor to further regulate the voltage.

In another broad form, the invention provides a fluid flow meter for measuring the flow of fluid in a passageway, the meter including

a first temperature dependant resistor located in the passageway; a variable voltage supply; voltage reduction means for reducing the voltage supplied from the variable voltage supply to the temperature dependant resistor; first sensing means for determining the temperature of the temperature dependant resistor and for controlling the voltage reduction means to maintain the temperature dependant resistor at substantially one temperature; and; second sensing means for controlling the variable voltage supply.

Preferably the voltage reduction means includes one or more valves or transistors located in series between the variable voltage supply and the temperature dependant resistor. Alternatively a variable resistor may be utilised.

Preferably the variable voltage supply is a switched mode power supply.

Preferably the first sensing means includes a wheatstone bridge with one arm of the bridge consisting of the first temperature dependant resistor.

Preferably the meter includes a first feedback loop for controlling the voltage reduction means and for outputting an output signal representing fluid flow. A second feedback loop controls the variable voltage supply.

In another form the invention provides a fluid flow meter for measuring the flow of fluid in a passageway, including:

a temperature dependant resistor located in the passageway;

power adjusting means provided in series with the resistor and for adjusting the power supplied to the resistor;

control means for controlling said power adjusting means to maintain the resistor at a substantially constant resistance as the fluid flow vanes;

said power adjusting means being the equivalent of a variable voltage source with an output voltage, V_ps and a variable resistance R_v means and

said control means causes the values of V_ps and R_v to both vary as fluid flow varies.

The value of the variable resistor R_v is preferably the same value at different fluid flows.

The value of the voltage V_ps is preferably the same at different fluid flows.

The voltage V_ps most preferably follows a step function as fluid flow increases. The value of the resistance R_v preferably follows a saw tooth function as fluid flow increases.

Preferably the variable voltage source is a pulse width modulation circuit.

Preferably the variable resistor means includes one or more transistors.

The invention will be better understood from the following non-limiting, description of an embodiment of the invention and the drawings, in which:

Fig 1 schematically represents the prior art, and

Fig 2 schematically represents an embodiment of the invention;

Fig 3 schematically represents a second embodiment of the invention;

Fig 4 schematically represents a third embodiment of the invention.

Fig 5 is a graph of the voltage output of the variable voltage source of the Fig 2 embodiment plotted against fluid flow;

Fig 6 is a graph of the effective resistance of the transistor of the Fig 2 embodiment plotted against fluid flow;

Fig 7 is a graph of power dissipated by the thermally dependant resistor of the Fig 2 embodiment plotted against fluid flow.

Referring to Figure 1 , there is shown a prior art fluid flow measuring device.

A wheatstone bridge 10 is provided which comprises a hot wire R_1t fluid temperature measuring resistor R₂ and two fixed resistors R₃ and R₄. The hot wire resistor R₁ and the temperature measuring resistor R₂ are positioned in the fluid flow. Given that R^IR₃ = RJR*, the voltage at nodes 20,22 will be equal and the differential voltage sensed by feedback circuit 24 will be zero. Resistors R₂ and R₄ each have a high resistance so the power dissipated by R₂ will be low, so that changes in fluid flow will have little effect on the heat loss rate and hence temperature of R₂. Conversely R₁ and R₃ each have a low resistance so heat loss through R, is high and so its heat loss rate is highly dependant on fluid flow. As fluid flow decreases or increases, the rate at which heat is lost from R, changes and so its temperature and resistance changes. This causes an imbalance in the bridge and so the differential voltage sensed at 24 is non-zero.

Feedback circuit 2 takes this voltage and increases or decreases the input voltage V| to a power transistor 30 at 32. The input voltage V> is adjusted to open or close transistor 30. This in turn causes the voltage at node 34 to increase or decrease the current flow in R The input voltage to transistor 30 is adjusted to return the differential voltage across nodes 20,22 to zero. A signal V₀, which depends on V_|, is output as a measure of fluid flow.

Referring to Figure 2, there is shown an embodiment of the invention which includes the same structure as the prior art device of Figure 1. Accordingly like parts are numbered the same.

In the Figure 2 embodiment there is provided a switched mode power supply 50. A switched mode supply is a variable voltage power supply. Variation in voltage is achieved by rapidly turning the voltage on or off so that the average voltage is dependant on the ratio of on to off time. Since the output from the device is either fully on or fully off, the efficiency is relatively high.

The switched mode power supply 50 supplies its output V_ps to the power transistor 30.

A second feedback loop 52 is provided. This loop 52 takes its input from the input voltage V, input to transistor 30, the voltage at node 34 and the supply voltage V_ps and provides an output signal V_H as an input to the switched mode power supply 50.

At low fluid flow in the prior art, the heat dissipated by the hot wire resistor R, is low and normally transistor 30 is almost fully closed, so the voltage drop across the transistor 30 is large. In the Figure 2 device the switched mode power supply 50 is controlled by loop 52 so as to reduce its output voltage V_ps. This causes the voltage across the hot wire to reduce, reducing its heat output and hence temperature. This causes first feedback loop to increase the voltage V, to the transistor 30, causing it to open. This process continues until the transistor 30 is at a predetermined point on its input voltage/output voltage curves. Ideally the transistor 30 is only operated along the linear part of those curves with an arbitrary point considered to be 100% open. The system may be set so that in the steady state of the transistor 30 is 80% open, as opposed to 5% in the prior art.

As fluid flow increases the hot wire resistor 12 requires a higher voltage to maintain its temperature.

Initially loop 1 increases the input voltage V| to transistor 30 until transistor 30 is 95% open. At this point loop 2 causes the switched power supply 50 to increase its output voltage V_ps. This higher voltage allows loop 1 to reduce the voltage V, to close down the transistor 30 to about 75% open. The change in V_ps and the effective resistance of the transistor R_v is shown in Figs 5 and 6. The voltage V_ps is raised in steps as the fluid flow increases but the power increases steadily, as shown in Fig 7.

As fluid flow decreases, the hot wire resistor 12 requires a lower voltage to maintains its temperature. Initially loop 1 decreases the input voltage V, to transistor until it is, 75% open. A this point loop 2 causes the switched power supply 50 to reduce its output voltage V_ps. This lower voltage allows loop 1 to increase the voltage V| to open up the transistor 30 to 95% open.

Thus as fluid flow increases from negligible to full flow the switched mode power supply 50 outputs a voltage which increases in steps, so allowing the servo transistor 30 to operate substantially open all of the time, thereby reducing power consumption and enhancing durability. It will be appreciated that the range of the effective resistance of the transistor 30 shown in Fig 6 is significantly less than without the variable power supply 50.

Fig 3 shows a second embodiment of the invention and again like parts are numbered the same.

In this embodiment the switched mode power supply is used merely to "step down" the voltage from the normal 12V supply (or 24V in heavy duty vehicles) to about 6.5V. The circuitry then operates as per the Fig 1 device, in that feedback loop 24 controls the transistor to vary the voltage at node 34 to maintain the hot wire resistor R, at a constant temperature.

By utilising a switched mode supply the power consumption of the transistor can be reduced from about 10W to about 2W or 3W. When dissipating 10W the power transistor may be operating at about 100 °C which is near its maximum permissible operating temperature, with consequential reduced reliability. Figure 4 shows a third embodiment of the invention. In this embodiment a pulse width modulation circuit 60 is provided which is connected directly to node 34 - the transistor of the prior art and the embodiments of Figs 2 and 3 is not utilised. A feedback loop 62 is provided which takes as one of its inputs the differential voltage across nodes 20 and 22. The feedback loop outputs a feedback signal Vπ into the pulse width modulation circuit to control the pulse width and hence average voltage output Vps supplied to the wheatstone bridge. By varying Vps the temperature of the hot wire resistor Ri is maintained at a substantially constant temperature. The feedback loop also outputs a voltage signal Vo representing a measure of the fluid flow.

It will be apparent to those skilled in the art that many modifications and variations may be made to the embodiment described herein without departing from the spirit or scope of the invention.

Claims

The claims defining the invention are as follows:

1. A fluid flow meter for measuring the flow of fluid in a passageway, including:

a temperature dependant resistor located in the passageway;

said power adjusting means being the equivalent of a variable voltage source having an output, V_ps and a variable resistance R_v means and

2. The meter of claim 1 wherein resistance R_v has the same value at different fluid flows.

3. The meter of claim 1 or claim 2 wherein V_ps is the same at different fluid flows.

4. The meter of any one of claims 1 to 3 wherein V_ps increases as a step function with fluid flow.

5. The meter of any one of claims 1 to 4 wherein R_v varies as a saw tooth with fluid flow.

6. The meter of any one of claims 1 to 5 wherein the variable resistance means includes a first transistor which receives a first current from said control means at its base.

7. The meter of claim 6 wherein the transistor is operated so that the ratio of base current l_B to effective resistance is substantially constant.

8. The meter of claim 6 or claim 7 wherein the base current l_B is based on the values of V_ps , R_v and l_B.

9. The meter of any one of claims 1 to 8 wherein the variable voltage source includes a pulse width modulation circuit which receives a second signal from the control means.

lO.The meter of claim 9 wherein the second signal is based on the values of V_ps , R_v and l_B.