Thermal flow meter

The current flowing through a thermosensitive element is amplified and output as a signal indicating its magnitude. The signal is processed by being converted into a digital signal and is used to calculate, for example, the quantity of fuel that should be supplied for the thus measured quantity of intake air in an internal-combustion engine. The current is supplied to the thermosensitive element from the positive electrode of a power source, and returns to the negative electrode thereof via a return line. The return line and a ground potential line of the amplifier are separate lines. The magnitude of the current flowing through the thermosensitive element is varied by varying the potential of a point on the return line accordingly. Because the return line and the ground potential line are separate, the ground potential can be kept constantly stable.

BACKGROUND OF THE INVENTION 
This invention relates generally to a thermal flow meter, and more 
particularly to a thermal flow meter which is suitable for measuring the 
quantity of intake air in an internal combustion engine. 
As a thermal flow meter of the kind described above, a flow meter in which 
a temperature-dependent resistor made of a material such as platinum is 
disposed inside an air intake pipe has been known in the past (e.g. U.S. 
Pat. No. 3,747,577). Negative feedback control is effected so that the 
temperature-dependent resistor is kept at a constant temperature. The 
quantity of heat lost from the temperature-dependent resistor changes with 
changes in the intake air quantity, but since feedback control is effected 
so that its temperature remains constant, the intake air quantity can be 
measured by measuring the value of the current flowing through the 
resistor. However, the prior art device has the problem that its 
measurement accuracy is low. 
SUMMARY OF THE INVENTION 
The present invention is therefore designed to obviate the problem 
described above and is directed to providing a thermal flow meter with a 
high measurement accuracy. 
As a result of intensive studies on measurement accuracy, the inventor of 
the present invention has clarified the following fact. The heating 
current varies between about 10 mA and about 150 mA, this heating current 
is detected and amplified, but the variations in the heating current 
strongly affect the detected and amplified signal, which reduces the 
measurement accuracy. 
Accordingly, the present invention provides a separate return line for 
returning a current from a thermosensitive element to a power source, and 
a ground potential line for an amplifier connected to the thermosensitive 
element, and grounds this ground potential line within a signal processing 
section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, one embodiment of the present invention will be described with 
reference to the accompanying drawings. 
FIG. 1 shows the arrangement of a sensor. In the drawing, a bypass pipe 2 
is provided by the side of an intake pipe 1. The downstream side of the 
bypass pipe 2 communicates with an annular passage defined between it and 
an insertion pipe 6 encompassing the intake passage. The insertion pipe 6 
opens to the intake passage through an outlet 6A. A venturi section is 
defined by the insertion pipe 6 and a skirt portion 1B, and the outlet 6A 
opens into the most constricted portion of the venturi. Hence, the outlet 
6A is kept at a negative pressure, and part of the air flowing through the 
intake passage flows into the bypass pipe. Air is taken into the bypass 
pipe 2 by static pressure and its flow is made laminar by a bell mouth 2A. 
A thermosensitive element 3, made of platinum or a similar material, to 
measure the intake air quantity and a temperature compensation element 4, 
made similarly of platinum or the like, are provided inside the bypass 
pipe 2. A circuit for controlling the temperature of the thermosensitive 
element 3 to be at a constant level, etc., is disposed inside a circuit 
case 5. A detection signal is input to a CPU for signal processing through 
a cable 7. The air flowing in the direction shown by the arrow W is 
supplied to the combustion chambers of an internal combustion engine. 
The circuit configuration will now be explained with reference to FIG. 2. 
The section encircled by a dashed line on the left of the drawing is a 
sensor section 10. It includes the resistors 3, 4 and the circuit inside 
the circuit case 5. The section encircled by a dashed line on the right of 
the drawing is a signal processing section 30, which includes the CPU, 
etc., and is mounted on the vehicle. The current from a battery 40 is 
input to the sensor section 10 and the signal processing section 30. 
The construction of the sensor section 10 will be described next. As will 
be described elsewhere, a sensor driving circuit 12 includes the 
thermosensitive element 3, the temperature compensation element 4 and a 
control circuit which keeps the thermosensitive element at a predetermined 
temperature. A current is input to this driving circuit 12 from the 
battery 40 through a signal line 50 and a connector 20 provided at an 
intermediate portion on the signal line. The current returns to the 
battery 40 through a return line 52 and a connector 22 provided at an 
intermediate portion on the return line. An air flow rate signal detected 
by the sensor-driving circuit 12 is applied to the positive input terminal 
of a differential amplifier 16 through a resistor 14. A terminal of the 
sensor-driving circuit 12 on the return line 52 side is connected to the 
negative input terminal of the differential amplifier 16 through a 
resistor 15. The voltage across the two ends of a resistor detecting the 
heating current which flows through the thermosensitive element 3 is input 
to the differential amplifier 16, and a differential voltage is output. In 
the amplification of an air flow rate signal, a differential amplifier 
with the construction described above is used for the first time in the 
present invention. In other words, in accordance with the prior art a 
simple amplifier amplifies the output of the resistor 14. 
A feedback resistor 17 is provided in the differential amplifier 16, the 
resistor 17 is variable. A constant voltage circuit 18 whose output is 
variable applies a reference voltage V.sub.R to the negative input 
terminal of the differential amplifier 16. The feedback resistor 17 and 
the reference voltage V.sub.R make the adjustment between zero and span. 
This adjustment is carried out in the following manner. A reference output 
value V.sub.1 for the adjustment when the air flow rate is 20 kg/H and the 
reference output value V.sub.2 when the air flow rate is between 200 and 
300 kg/H, for example, are determined in advance. When making the 
adjustment, the output values when the air flow rate is 20 kg/H and 300 
kg/H are obtained, respectively, and are compared with the reference 
output values V.sub.1 and V.sub.2. The value of the feedback resistor 17 
is changed as required in accordance with each difference from the 
reference values to change the gain of the amplifier 16. The outputs with 
respect to the two flow rates are again determined, but this time the 
reference value V.sub.R is changed as required. These two adjustments are 
repeated two or three times so that the output values can be easily 
brought into conformity with the reference output values, and the 
adjustment can thus be carried out. It is another characterizing features 
of the present invention that this zero-span adjustment circuit is 
provided in the sensor section 10. 
The output of the differential amplifier 16 is applied to an A/D converter 
32 within the signal processing section 30 through a flow rate signal line 
54 and is converted into a digital signal. This digital signal is input to 
the CPU 36 and is used as data for determining the quantity of fuel 
injected. 
A ground potential line 56 for the differential amplifier 16 and of the 
constant voltage circuit 18 is provided separate from the return line 52. 
The ground potential line 56 leads from the sensor section 10 to the 
signal processing section 30 and is grounded at a point 34 close to the 
input terminal of the A/D converter 32. It is a further one of the 
characterizing features of the present invention that the ground potential 
line 56 is provided separate from the return line 52, and is grounded 
within the signal processing section 30. 
The input ends of each the lines 50, 54 and 56 to the input terminals of 
the sensor section 10 are passed through chip capacitors 24, 26, 28 to 
deal with noise. 
The sensor-driving circuit 12 will now be described in detail with 
reference to FIG. 3. A resistor 60 is connected in series with the 
thermosensitive resistor 3. A series circuit of resistors 62 and 64 is 
connected parallel to the thermosensitive resistor 3 and divides the 
voltage across the ends of the thermosensitve resistor 3. The temperature 
compensation resistor 4 is connected in series with a resistor 66. The 
node between the thermosensitive resistor 3 and the resistor 60 is 
connected to the non-inverting input of an amplifier 68, and the node 
between the resistors 4 and 66 is connected to the inverting input. The 
output of the amplifier 68 is connected to the non-inverting input of an 
amplifier 70, and the inverting input is connected to the node between 
resistors 62 and 64. The output of the amplifier 70 is applied to the base 
of a transistor 72. The collector of the transistor 72 is connected to the 
terminal 20 from the battery 40 via a resistor 74. The emitter of the 
transistor 72 is connected to the node between the thermosensitive 
resistor 3 and the resistor 62, and the node between the resistors 60 and 
66 is connected to the terminal 22. The node between the thermosensitive 
resistor 3 and the resistor 60 is connected to the amplifier 16 via the 
resistor 14. 
When the transistor 72 is conductive, the emitter current of the transistor 
72 flows through the series circuit of the resistors 3 and 60, and also 
through the voltage-dividing circuit of the resistors 62 and 64. The 
amplifier 68 compares the potential between its inverting input and 
non-inverting input, that is, the potential between the node of the 
resistors 3, 60 and the node of the resistors 4, 66, and controls the 
output voltage so that they are equal to each other. The amplifier 70 
controls the base voltage of the transistor 72 so that the potential at 
the node between the resistors 62, 64 and the output potential of the 
amplifier 68 is equal. In other words, the amplifier 70 controls so that 
the voltage of the thermosensitive element 3 divided between its terminals 
and the voltage between the ends of the resistor 4 is equal. Accordingly, 
the voltage between the ends of the resistor 4 can be made much smaller 
than that of the resistor 3. In this case, the current flowing through the 
thermosensitive resistor 3 is a function of the flow rate when in 
equilibrium, and this current is equal to the current flowing through the 
resistor 60 so that the flow rate of the fluid can be measured by 
measuring the voltage between the ends of the resistor 60. 
In the embodiment described above, the ground potential line 56 and the 
return line 52 are provided separately, and the grounding point of the 
ground potential line is within the signal processing section 30. This 
arrangement provides the following advantage. For the sake of comparison, 
it is assumed that the two lines are common, and the grounding point is 
within the signal processing section. In this case, a region whose 
resistance changes markedly with the passage of time is formed in the 
return line 52 from the grounding side of the circuit 12 to the grounding 
point. The heating current flowing through the heating resistor 3 from the 
circuit 12 flows through the return line. Since this heating current 
changes with the air flow rate, the voltage between the grounding point 
and the junction between the return line and the ground potential line 
changes. This change causes a voltage change in the ground terminal of the 
amplifier 16 so that the accuracy of the amplification of the amplifier 16 
drops. In contrast, the present invention eliminates such a problem. 
Incidentally, if the grounding point were disposed on the sensor side, 
grounding could be effected by grounding it to the car body; hence, 
another problem would occur in that it would be easily affected by high 
voltages such as that of the ignition signal. 
Since the amplifier 16 is a differential amplifier, only the voltage 
between the two ends of the resistor 60 detecting the heating current can 
be measured with a high level of accuracy. The influence of changes in the 
ground potential of the amplifier 16 is reduced. 
As described in detail in the foregoing, the present invention improves the 
measurement accuracy of a flow meter.