Abstrict A gas flow meter having a gas flow detection circuit for detecting
a current flowing through a resistor installed in a gas passage
and a voltage generated across the resistor and outputting a voltage
signal representing a gas flow passing through the gas passage;
a noise reduction circuit for reducing external noise; and a digital
adjusting circuit for digitally adjusting a signal representing
the detected gas flow and outputting the adjusted signal; wherein
a voltage signal based on the signal adjusted by the digital adjusting
circuit is output.
Claims What is claimed is:
1. A gas flow meter comprising: a gas flow detection circuit for
detecting a current flowing through a resistor installed in a gas
passage and a voltage generated across the resistor and outputting
a voltage signal representing a gas flow passing through the gas
passage; a noise reduction circuit for reducing external noise;
and a digital adjusting circuit for digitally adjusting a signal
representing the detected gas flow and outputting the adjusted signal;
wherein said gas flow meter outputs a voltage signal based on the
signal adjusted by the digital adjusting circuit.
2. The gas flow meter according to claim 1 wherein said digital
adjusting circuit includes: a digital conversion circuit for converting
an output from the gas flow detection circuit into a digital signal;
adjusting means for adjusting the digital signal to produce a desired
output characteristic; and a regulator circuit for supplying a reference
voltage to the digital conversion circuit and/or the adjusting means.
3. A gas flow meter comprising: a gas flow detection circuit for
detecting a gas flow passing through a gas passage; an adjusting
circuit for adjusting an output characteristic of said gas flow
meter to a desired output characteristic and outputting a gas flow
signal; and a noise reduction circuit including an overvoltage protection
circuit and supplying to the gas flow detection circuit and the
adjusting circuit a voltage whose surges and overvoltages applied
to a power supply terminal are reduced; wherein there are two or
more voltage supply paths for supplying different voltages to the
gas flow detection circuit and the adjusting circuit through the
overvoltage protection circuit.
4. The gas flow meter according to claim 3 wherein in one of the
voltage supply paths for supplying a voltage having reduced surges
and overvoltages to various circuits, a voltage limiter circuit
that turns on when applied with a voltage in excess of a predetermined
voltage is connected between a voltage supply terminals and a ground
terminal and a current limiting resistor is connected between the
power supply terminal and the voltage supply terminals; in the other
voltage supply path, another current limiting resistor is connected
between the power supply terminal and the voltage supply terminals;
and an overvoltage protection circuit is provided in which a diode
is connected between each of the voltage supply terminals.
5. The gas flow meter according to claim 3 wherein in all of said
voltage supply paths for supplying a voltage having reduced surges
and overvoltages to various circuits, a voltage limiter circuit
that turns on when applied with a voltage in excess of a predetermined
voltage is connected between voltage supply terminals and a ground
terminal and a current limiting resistor is connected between the
power supply terminal and the voltage supply terminals; and an overvoltage
protection circuit is provided in which the current limiting resistors
connected to the respective voltage supply terminals have different
resistance values.
6. The gas flow meter according to claim 4 further including an
overvoltage protection circuit having an additional diode connected
between the voltage supply terminals and the ground terminal.
7. The gas flow meter according to claim 5 further including an
overvoltage protection circuit having an additional diode connected
between the voltage supply terminals and the ground terminal.
8. The gas flow meter according to claim 3 wherein a part or all
of devices included in said overvoltage protection circuit, said
gas flow detection circuit and said adjusting circuit are formed
in a common integrated circuit.
9. The gas flow meter according to claim 3 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
10. The gas flow meter according to claim 4 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
11. The gas flow meter according to claim 5 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
12. The gas flow meter according to claim 6 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
13. The gas flow meter according to claim 7 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
14. The gas flow meter according to claim 8 wherein two voltage
supply paths are provided; and a circuit connected to a higher supply
voltage is an operational amplifier in the gas flow detection circuit
and a circuit connected to a lower supply voltage is a regulator
that supplies a voltage to the digital adjusting circuit.
15. A gas flow meter comprising: a gas flow detection circuit for
outputting a voltage signal representing a gas flow passing through
a gas passage; and an adjusting circuit for adjusting the voltage
output from the gas flow detection circuit; wherein an input range
of the voltage signal entered into the adjusting circuit is divided
in two or more and, in each divided range, a different adjustment
calculation formula is set in advance; wherein said gas flow meter
further comprises means for selecting the adjustment calculation
formula according to an input value of the voltage signal entered
into the adjusting circuit and performing adjustment calculation
to produce an output value.
16. The gas flow meter according to claim 15 wherein the adjusting
circuit is a digital adjusting circuit which digitally adjusts the
signal representing the detected gas flow and outputs the adjusted
signal.
17. The gas flow meter according to claim 15 wherein the adjusting
circuit has input/output characteristics represented by each of
the adjustment calculation formulas expressed as a first-degree
function of y=a.multidot.x+b where x is an output value of the gas
flow detection circuit, i.e., input value for the adjustment calculation,
y is an output of the adjustment calculation, and a and b are adjustment
coefficients.
18. The gas flow meter according to claim 16 wherein the adjusting
circuit has input/output characteristics represented by each of
the adjustment calculation formulas expressed as a first-degree
function of y=a.multidot.x+b where x is an output value of the gas
flow detection circuit, i.e., input value for the adjustment calculation,
y is an output of the adjustment calculation, and a and b are adjustment
coefficients.
19. The gas flow meter according to claim 15 further including:
a temperature sensor; and a digital conversion circuit for converting
an output of the temperature sensor into a digital value; wherein
said adjusting circuit also uses the output of the temperature sensor
in performing the adjustment calculation.
20. The gas flow meter according to claim 16 further including:
a temperature sensor; and a digital conversion circuit for converting
an output of the temperature sensor into a digital value; wherein
said adjusting circuit also uses the output of the temperature sensor
in performing the adjustment calculation.
21. The gas flow meter according to claim 17 further including:
a temperature sensor; and a digital conversion circuit for converting
an output of the temperature sensor into a digital value; wherein
said adjusting circuit also uses the output of the temperature sensor
in performing the adjustment calculation.
22. The gas flow meter according to claim 18 further including:
a temperature sensor; and a digital conversion circuit for converting
an output of the temperature sensor into a digital value; wherein
said adjusting circuit also uses the output of the temperature sensor
in performing the adjustment calculation.
23. The gas flow meter according to claim 19 wherein said adjusting
circuit has an input/output characteristic expressed byy=(a1.multidot.t+a2).multidot.x+(b1.multidot.t+b2)where
x is an output value of the gas flow detection circuit, t is an
output value of the temperature sensor, and a1 a2 b1 and b2 are
adjustment coefficients.
24. The gas flow meter according to claim 16 wherein said adjusting
circuit has an input/output characteristic expressed byy=(a1.multidot.t+a2).multidot.x+(b1.multidot.t+b2)where
x is an output value of the gas flow detection circuit, t is an
output value of the temperature sensor, and a1 a2 b1 and b2 are
adjustment coefficients.
25. The gas flow meter according to claim 17 wherein said adjusting
circuit has an input/output characteristic expressed byy=(a1.multidot.t+a2).multidot.x+(b1.multidot.t+b2)where
x is an output value of the gas flow detection circuit, t is an
output value of the temperature sensor, and a1 a2 b1 and b2 are
adjustment coefficients.
26. The gas flow meter according to claim 18 wherein said adjusting
circuit has an input/output characteristic expressed byy=(a1.multidot.t+a2).multidot.x+(b1.multidot.t+b2)where
x is an output value of the gas flow detection circuit, t is an
output value of the temperature sensor, and a1 a2 b1 and b2 are
adjustment coefficients.
27. The gas flow meter according to claim 17 wherein said adjusting
circuit includes a programmable storage device wherein the adjustment
coefficients a, a1 a2 b, b1 and b2 are written into.
28. The gas flow meter according to claim 17 or 13 wherein the
adjusting circuit includes an erasable and programmable storage
device wherein the adjustment coefficients a, a1 a2 b, b1 and
b2 are written into.
29. The gas flow meter according to claim 18 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
30. The gas flow meter according to claim 19 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
31. The gas flow meter according to claim 20 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
32. The gas flow meter according to claim 21 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
33. The gas flow meter according to claim 22 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
34. The gas flow meter according to claim 23 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
35. The gas flow meter according to claim 24 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
36. The gas flow meter according to claim 25 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
37. The gas flow meter according to claim 26 wherein said adjusting
circuit includes an erasable and programmable storage device wherein
the adjustment coefficients a, a1 a2 b, b1 and b2 are written
into.
38. A gas flow meter comprising: an adjusting circuit for adjusting
a voltage output of a gas flow detection circuit which outputs a
voltage signal representing a gas flow passing through a gas passage;
a storage device for storing data for adjustment; and a data input/output
circuit; wherein said data input/output circuit has two external
data communication terminals for writing adjust data from outside
into said storage device and for reading data from said storage
device to the outside.
39. The gas flow meter according to claim 38 further comprising
means which, after a predetermined number, two or more, of pulses
have been supplied to one of the external data communication terminals
of the data input/output circuit, allows the adjust circuit to enter
into a data communication mode where it transfers data between the
storage device and external circuits.
40. A gas flow meter comprising: an adjusting circuit for adjusting
a voltage output of a gas flow detection circuit which outputs a
voltage signal representing a gas flow passing through a gas passage;
and a storage device for storing data for adjustment; wherein said
adjusting circuit retrieves as the output signal of the detected
gas flow a ratiometric analog output, a non-ratiometric analog output
and a digital output and selects one of these output signals by
an output selection means provided in the adjusting circuit.
41. The gas flow meter according to claim 40 wherein said circuits
for producing the ratiometric analog output, the non-ratiometric
analog output and the digital output are formed on a same integrated
circuit.
42. The gas flow meter according to claim 38 wherein the external
data communication terminals serve as a detected flow output terminal.
43. The gas flow meter according to claim 40 wherein the external
data communication terminals serve as a detected flow output terminal.
44. A gas flow meter comprising: a gas flow detection circuit for
detecting a current flowing through a resistor installed in a gas
passage and a generated voltage and outputting a voltage signal
representing a gas flow passing through the gas passage; a digital
conversion circuit for converting the detected gas flow into a digital
signal; and a digital adjusting circuit for digitally adjusting
the digital signal and outputting the adjusted digital signal; wherein
a voltage signal based on the digital signal adjusted by said digital
adjusting circuit is output, and the digital conversion circuit
has means for selecting either a single-phase input or a differential
input.
45. A gas flow meter comprising: a gas flow detection circuit for
detecting a current flowing through a resistor installed in a gas
passage and a voltage generated across the resistor and outputting
a voltage signal representing a gas flow passing through the gas
passage; a digital conversion circuit for converting the detected
gas flow into a digital signal; a digital adjusting circuit for
digitally adjusting the digital signal and outputting the adjusted
digital signal; and an analog conversion circuit for receiving the
adjusted digital signal and converting it into an analog signal;
wherein said analog conversion circuit is driven by a voltage based
on an external reference voltage and a voltage follower circuit
is arranged between a reference voltage terminal and a power supply
terminal which drives said analog conversion circuit.
Description BACKGROUND OF THE INVENTION
[0001] The present invention relates to a gas flow meter for automotive
control and more particularly to a noise reduction circuit, to an
adjustment circuit, to a reduction in the number of adjustment terminals
and output terminals, and to an output circuit.
[0002] A gas flow meter for detecting an air flow in internal combustion
engines has been in use. An example of the gas flow meter is a constant
temperature control hot wire type gas flow meter described in the
Journal of Fluid Mechanics, vol. 47 (1971), pp577-599. FIG. 25 shows
an outline configuration of a gas flow detection circuit DECT1 applying
the constant temperature control heat wire type gas flow meter.
This gas flow detection circuit mainly comprises an operational
amplifier OP1 a power transistor Tr1 a heating resistor (also
called a hot wire) Rh, a gas temperature measuring resistor (also
called a cold wire) Rc and resistors R1 R2 and keeps the temperature
of the heating resistor Rh constant at all times, i.e., keeps its
resistance constant by maintaining a bridge balance using the operational
amplifier OP1. As the gas flow increases, heat taken from the heating
resistor Rh increases resulting in an increased heating current.
Because this heating current is proportional to a voltage between
terminals of the resistor R1 the measurement of this voltage can
determine the gas flow. The voltage output produced by the current
detection resistor R1 is processed by an adjust circuit having a
predetermined input/output characteristic so that the voltage output
provides a predetermined signal characteristic required of the gas
flow meter.
[0003] There is another gas flow detection circuit DECT2 as shown
in FIG. 26 in which heat sensing resistors Ru, Rd for measuring
gas flow temperatures are arranged upstream and downstream of the
heating resistor Rh of the constant temperature control hot wire
type gas flow meter so that they are influenced by heat from the
heating resistor Rh. The resistor Ru on the upstream side is cooled
by the gas flow to lower its resistance and the resistor Rd on the
downstream side receives a gas flow heated by the heating resistor
Rh to raise its temperature and therefore its resistance. This changes
the potential at a connecting point between Ru and Rd and thus measuring
this voltage can determine the gas flow.
[0004] Still another gas flow detection circuit DECT3 as shown
in FIG. 27 is available, in which a total of four heat sensing resistors
for measuring gas flow temperatures are arranged two upstream and
two downstream of the heating resistor Rh of the constant temperature
control hot wire type gas flow meter so that they are influenced
by heat from the heating resistor Rh, and in which one pair of resistors
Ru1 Rd1 are serially connected in an upstream-downstream order
and another pair of resistors Rd2 Ru2 are serially connected in
a downstream-upstream order to form a bridge and measure a potential
difference between two connecting points. The resistors Ru1 Ru2
on the upstream side are cooled by the gas flow to lower their resistances
and the resistors Rd1 Rd2 on the downstream side receive a gas
flow heated by the heating resistor Rh, raising their temperatures
and therefore their resistances. This changes the potential difference
in the bridge and thus measuring this voltage difference can determine
the gas flow.
[0005] The electronic circuits that adjust the output characteristic
of a gas flow meter mounted on motor vehicles are subject to various
surges and overvoltages, as specified in the International Standard
Organization (ISO) 7637-1 7637-3 standard and Japan automotive
standard (JASO) D001-94. These standards are intended to prevent
undesired operations or failures of electronic circuits due to surge
voltages caused by ignition of engine, overvoltages caused by batteries
stacked in two tiers at time of starting engine in cold environment,
and high frequency noise caused by other electronic devices. On
the other hand, the electronic circuits are constructed in the form
of IC circuits for reducing the manufacturing cost and, in recent
years, to meet the emission control requirements the gas flow meter
is increasingly required to raise its precision in line with the
sophistication of engine control functions. Further, because the
service temperature range is as wide as -40.degree. C. to 130.degree.
C., measures should be taken to prevent a possible change in output
due to temperature variations.
[0006] For surges and overvoltages, a variety of overvoltage protection
circuits have been in use. One such example is a protection circuit
using a Zener diode ZD and a current limiting resistor R as shown
in FIG. 28.
[0007] The circuit of FIG. 28 is one type of a commonly used constant
voltage circuit in which a voltage applied to a connection terminal
VBB for the battery causes a current to flow through the current
limiting resistor R to the Zener diode ZE. When an overvoltage is
applied, the voltage of the power supply terminal Vcc to various
circuits is clamped by a Zener voltage of the Zener diode ZD to
put an overvoltage protection into action.
[0008] Further, JP-A-9-307361 proposes as a conventional technology
an overvoltage protection circuit that uses an overvoltage detection
circuit made up of a resistor and a Zener diode and a switching
circuit made up of bipolar transistors.
[0009] The overvoltage protection circuit described in this official
gazette is intended for protecting microwave FETs (field-effect
transistors). When an overvoltage higher than a voltage sum of the
Zener voltage of the Zener diode and the base-emitter voltage of
the switching transistor is applied to the power supply terminal,
the switching circuit is operated to cut off the load from the power
supply line and thereby prevent the overvoltage from being impressed
on the load.
[0010] The voltage outputs of the flow detection circuits DECT1-3
in FIG. 25 to FIG. 27 need adjustments in zero point and span (output
range) to produce the required sensor output characteristics. This
adjust circuit is mainly an analog circuit at present but a higher
precision adjustment is considered possible by using a digital circuit.
[0011] Table 1 shows comparison between an analog circuit and a
digital circuit ("CMOS Analog Circuit Design Technique"
published by Triceps (1998), compiled under the supervision of Iwata).
1 TABLE 1 Analog circuit Digital circuit No. of Few (about 20 pcs
in Many (2000 pcs in transistors multiplier) 8-bit multiplier) Chip
area Small (few devices) Large (many devices) Power Low power Large
(many gates consumption consumption because are switched) of fewer
devices Clock Low (determined by Higher (1/2 of cut- frequency settling
of off frequency of amplifier) device) Signal High (about 1/2 of
Low (1/10 of clock frequency cut-off frequency of frequency) device)
Precision Low (device High (depending on deviation, noise) bit number)
Stability Low (oscillation, High characteristic variation) Noise
Low (S/N) Strong (large noise resistance margin) Source: "CMOS
Analog Circuit Design Technique" published by Triceps (1998),
compiled under the supervision of Iwata
[0012] The analog circuit has a small size and a small power consumption
compared with the digital circuit. But the use of such devices as
resistors causes manufacturing variations and other variations due
to aged deterioration, and thus the analog circuit has less precision
and stability than the digital circuit. The digital circuit, while
it is superior to the analog circuit in terms of precision and stability,
has a larger circuit size and a larger power consumption. The rapid
advance in the integrated circuit manufacturing technology in recent
years, however, has enabled micro-fabrication and therefore reduced
the circuit size and power consumption. The digital circuit is now
finding many applications in various industrial fields. Example
applications of a digital adjust circuit to the gas flow meter are
found in Japanese Patent No. 3073089 and JP-A-8-62010 and JP-A-11-118552.
[0013] FIG. 29 shows comparison between an analog adjustment and
a digital adjustment in the adjust circuit of the gas flow meter.
[0014] An outline circuit configuration for analog adjustment shown
in FIG. 29 comprises an operational amplifier OP2 trimming resistors
Rs1 Rz1 and resistors Rs2 Rz2. This circuit trims the trimming
resistors Rz1 Rs1 to adjust the voltage output from the flow detection
circuit DECT and thereby adjust the zero point and span to produce
an output for a desired gas flow. As the trimming resistors Rs,
Rz, thin-film resistors printed on a hybrid IC or thin-film resistors
on IC may be used. In trimming the resistors, a laser trimmer may
be used. The laser trimmer has a disadvantage that trimming with
high precision takes time and re-trimming cannot be done. Further,
because only a two-point adjustment is made, it is difficult for
the laser trimmer to make a complicated adjustment on the output
characteristic, such as a non-linear adjustment. In the analog circuit,
when the output specification for the gas flow is changed, the resistance
value needs to be redesigned and, in some cases, it is necessary
to redesign the hybrid IC substrate pattern, which in turn increases
the man-hour of designing works.
[0015] In the case of the digital adjust circuit of FIG. 29 since
the output specification can be changed by simply changing an adjust
coefficient while leaving the circuit pattern intact, the number
of design steps can be reduced. As an example digital adjust circuit,
a method described in Japanese Patent No. 3073089 has been proposed.
A rough circuit configuration for the digital adjustment is as follows.
The voltage output from the flow detection circuit DECT is converted
into a digital value by an analog-digital converter AD. Based on
the digital value, a digital processor CALC calculates the zero
point and span adjustments, which are then converted by a digital-analog
converter DA into an analog signal which is an analog output for
a desired gas flow. The adjust coefficient used in this calculation
is stored in a storage device MEM such as PROM. Further, the digital
processor CALC, because of its ability to easily perform non-linear
calculations, can make non-linear adjustments as well as zero point
and span adjustments during the output adjustment. With this non-linear
adjustment, the adjustment accuracy is within .+-.2%.
[0016] Another example configuration for the digital adjustment
is found in JP-A-11-118552. While its configuration is similar to
that of the digital adjust circuit of FIG. 29 this circuit reduces
its circuit size by using an oversampling type analog-digital converter
including a delta-sigma modulator as an analog-digital converter
AD.
[0017] Still another example configuration for the digital adjustment
is found in JP-A-2000-338193. The adjust coefficient used by the
processor in executing the adjustment calculation is written into
a storage device such as PROM through a terminal of a digital input/output
circuit that communicates with external circuits of the sensor.
This official gazette describes that a third-degree polynomial is
used for the adjustment calculation.
[0018] A further example configuration for the digital adjustment
is found in JP-A-11-94620. This circuit converts a flow signal from
the gas flow detection circuit into a rectangular wave signal and
counts up a counter at a certain rate only while the rectangular
wave is "1". To this count value is added the adjust coefficient
to produce an output.
[0019] Because the heating current flowing through the heating
resistor Rh is not affected by voltage variations in the power supply
(for example, battery), the voltage output of the gas flow detection
circuit DECT1 has a non-ratiometric characteristic. As output specifications
of the gas flow meter, there are ratiometric analog and digital
output specifications in addition to the non-ratiometric analog
output specification. A circuit configuration that realizes the
ratiometric analog output circuit is described in JP-A-2-85724.
This circuit divides an external ratiometric output reference voltage
into smaller voltages by two resistors and inputs the divided voltages
to an operational amplifier to realize a ratiometric output. With
a sum of the two resistors set to about 10 kilo-ohm, the current
to be supplied from the reference voltage is relatively small at
about 0.5 mA. An example of the digital output circuit is disclosed
in JP-A-8-247815. This circuit configuration comprises at least
a constant temperature control circuit, a zero point/span adjust
circuit and a voltage control oscillator, all integrated into one
chip.
[0020] Another configuration is described in JP-A-5-203475 in which
an analog output and a digital output are produced by a single circuit
board. In this configuration, a single circuit board is provided
with both an analog output terminal and a digital output terminal,
and both analog and digital outputs are supplied to an output connector
which selects and uses one of the two output signals or only one
of the outputs is connected through wire to the output connector.
SUMMARY OF THE INVENTION
[0021] When the gas flow meter circuit is integrated into a digital
circuit to reduce the cost and size of the gas flow meter and enhance
the accuracy of the output by adjustment, the conventional technique
described above is not optimized and thus has some problems that
cannot be solved by conventional technology.
[0022] C-MOSs are used for enhancing the level of circuit integration
and for building the gas flow meter circuit with a digital circuit.
The C-MOSs, however, are easily affected by surges and overvoltages
compared with bipolar transistors used in analog circuits and thus
need countermeasures.
[0023] In an overvoltage protection circuit shown in FIG. 28 when
a current through a connected circuit is large, a resistance of
a current limiting resistor must be reduced to prevent a voltage
drop. In this case, the electric withstandability of a Zener diode
ZD is increased large enough so that it can withstand an overcurrent.
This results in an increase in size and cost of components, which
is not desirable.
[0024] A circuit described in JP-A-2000-338193 performs a nonlinear
adjustment using a cubic equation. When the nonlinear adjustment
needs to be done with a quatic equation or higher order function,
the calculation time naturally increases. In addition, if individual
output characteristics have steep characteristic changes with respect
to an ideal characteristic, the output characteristics may often
not be able to be adjusted with such a polynomial.
[0025] Next, in integrating the electronic circuit of the gas flow
meter into a digital circuit, because adjustment coefficients need
to be written into a programmable storage device during the adjustment
process, terminals must be added. Further, there are different specifications
on the sensor output, i.e., a ratiometric analog output, a non-ratiometric
analog output and a digital output. For reduced manufacturing cost,
it is necessary during the integration process to make provisions
for coping with all these specifications. Simply adding terminals
to meet this requirement, as described in Japanese JP-A-11-94620
results in an increase in the chip area, which should be avoided.
[0026] Next, when the adjustment calculation is digitized, a digital-analog
converter may be required at the output stage. The digital-analog
converter includes an amplifier circuit for a signal output to external
circuits and thus its current consumption reaches several mA. When
the digital-analog converter is to be driven by using an external
reference voltage to produce a ratiometric output, if the maximum
current supplied from its power supply is small, the digital-analog
converter cannot be operated. This raises a problem that the reference
voltage cannot be connected directly to the power supply terminal
of the digital-analog converter.
[0027] It is therefore an object of the present invention to provide
a means which solves various problems encountered when reducing
the cost and size of the gas flow meter, enhancing the integration-level
of electronic circuits, making the output characteristics more accurate
and adjustable, and transforming the circuits into digital circuits.
[0028] To achieve the above objective, the present invention discloses
the following configuration:
[0029] (1) A gas flow meter comprising:
[0030] a gas flow detection circuit for detecting a current flowing
through a resistor installed in a gas passage and a voltage generated
across the resistor and outputting a voltage signal representing
a gas flow passing through the gas passage;
[0031] a noise reduction circuit for reducing external noise; and
[0032] a digital adjust circuit for digitally adjusting a signal
representing the detected gas flow and outputting the adjusted signal;
[0033] wherein a voltage signal based on the signal adjusted by
the digital adjust circuit is output.
[0034] (2) Preferably, the gas flow meter according to item (1),
wherein the digital adjust circuit includes:
[0035] a digital conversion circuit for converting an output from
the gas flow detection circuit into a digital signal;
[0036] an adjust means for adjusting the digital signal to produce
a desired output characteristic; and
[0037] a regulator circuit for supplying a reference voltage to
the digital conversion circuit and/or the adjust means.
[0038] With the above arrangement, the digital adjustment type
gas flow meter has a more appropriate circuit configuration.
[0039] (3) Preferably, a gas flow meter comprising: a gas flow
detection circuit for detecting a gas flow passing through a gas
passage;
[0040] an adjust circuit for adjusting an output characteristic
to a desired output characteristic and outputting it; and
[0041] a noise reduction circuit including an overvoltage protection
circuit and supplying to the gas flow detection circuit and the
adjust circuit a voltage whose surges and overvoltages applied to
a power supply terminal are reduced;
[0042] wherein there are two or more voltage supply paths for supplying
different voltages to the gas flow detection circuit and the adjust
circuit through the overvoltage protection circuit.
[0043] With this arrangement, the minimum required voltage can
be properly supplied to the gas flow detection circuit and the adjust
circuit of the gas flow meter.
[0044] (4) Preferably, the gas flow meter according to item (3),
wherein in one of the voltage supply paths for supplying a voltage
having reduced surges and overvoltages to various circuits, a voltage
limiter circuit that turns on when applied with a voltage in excess
of a predetermined voltage is connected between a voltage supply
terminals and a ground terminal and a current limiting resistor
is connected between the power supply terminal and the voltage supply
terminals; in the other voltage supply path, another current limiting
resistor is connected between the power supply terminal and the
voltage supply terminals; and an overvoltage protection circuit
is provided in which a diode is connected between each of the voltage
supply terminals.
[0045] With this arrangement, the overvoltage protection circuit
has a more appropriate configuration.
[0046] (5) Preferably, the gas flow meter according to item (3),
wherein in all of the voltage supply paths for supplying a voltage
having reduced surges and overvoltages to various circuits, a voltage
limiter circuit that turns on when applied with a voltage in excess
of a predetermined voltage is connected between voltage supply terminals
and a ground terminal and a current limiting resistor is connected
between the power supply terminal and the voltage supply terminals;
and
[0047] an overvoltage protection circuit is provided in which the
current limiting resistors connected to the respective voltage supply
terminals have different resistances.
[0048] With this arrangement, the noise reduction circuit has a
more appropriate configuration.
[0049] (6) Preferably, the gas flow meter according to item (4)
or (5), further including an overvoltage protection circuit having
an additional diode connected between the voltage supply terminals
and the ground terminal.
[0050] With this arrangement, the overvoltage protection circuit
has a more appropriate configuration.
[0051] (7) Preferably, the gas flow meter according to any one
of items (3) through (6), wherein a part or all of devices included
in the overvoltage protection circuit, the gas flow detection circuit
and the adjust circuit are formed in the same integrated circuit.
[0052] With this arrangement, the circuit can be reduced in size.
[0053] (8) Preferably, the gas flow meter according to any one
of items (3) through (7), wherein the number of the voltage supply
paths are two; and
[0054] a circuit connected to a higher supply voltage is an operational
amplifier in the gas flow detection circuit and a circuit connected
to a lower supply voltage is a regulator that supplies a voltage
to the digital adjust circuit.
[0055] With this arrangement, the digital adjustment type gas flow
meter has a more appropriate configuration.
[0056] (9) Preferably, a gas flow meter preferably comprising:
[0057] a gas flow detection circuit for outputting a voltage signal
representing a gas flow passing through a gas passage; and
[0058] an adjust circuit for adjusting the voltage output from
the gas flow detection circuit;
[0059] wherein an input range of the voltage signal entered into
the adjust circuit is divided in two or more and, in each divided
range, a different adjustment calculation formula is determined
in advance;
[0060] wherein a means is provided which selects the adjustment
calculation formula according to an input value of the voltage signal
entered into the adjust circuit and performs adjustment calculation
to produce an output value.
[0061] With this arrangement, the gas flow meter can perform a
more precise adjustment during the output characteristic adjustment.
[0062] (10) Preferably, the gas flow meter according to item (9),
wherein the adjust circuit is a digital adjust circuit which digitally
adjusts the signal representing the detected gas flow and outputs
the adjusted signal.
[0063] With this arrangement, the adjustment as described in item
(9) can be realized.
[0064] (11) Preferably, the gas flow meter according to item (9)
or (10), wherein the adjust circuit has input/output characteristics
represented by each of the adjustment calculation formulas expressed
as a first-degree function of y=a.multidot.x+b where x is an output
value of the gas flow detection circuit, i.e., input value for the
adjustment calculation, y is an output of the adjustment calculation,
and a and b are adjustment coefficients.
[0065] With this arrangement, the calculation time can be shortened.
[0066] (12) Preferably, the gas flow meter according to any one
of items (9) through (11), further including:
[0067] a temperature sensor; and
[0068] a digital conversion circuit for converting an output of
the temperature sensor into a digital value;
[0069] wherein the adjust circuit also uses the output of the temperature
sensor in performing the adjustment calculation.
[0070] With this arrangement, the temperature adjustment can be
made.
[0071] (13) Preferably, the gas flow meter according to item (12),
wherein the adjust circuit has an input/output characteristic expressed
by
y=(a1.multidot.t+a2).multidot.x+(b1.multidot.t+b2)
[0072] where x is an output value of the gas flow detection circuit,
t is an output value of the temperature sensor, and a1 a2 b1 and
b2 are adjustment coefficients.
[0073] With this arrangement, the digital adjust circuit can perform
an appropriate adjustment. (14) Preferably, the gas flow meter according
to item (11) or (13), wherein the adjust circuit writes the adjustment
coefficients a, a1 a2 b, b1 and b2 into a programmable storage
device.
[0074] With this arrangement, the digital adjust circuit has an
appropriate circuit configuration.
[0075] (15) Preferably, the gas flow meter according to item (11)
or (13), wherein the adjust circuit writes the adjustment coefficients
a, a1 a2 b, b1 and b2 into an erasable and programmable storage
device.
[0076] With this arrangement, the digital adjust circuit has an
appropriate circuit configuration.
[0077] (16) Preferably, a gas flow meter comprising:
[0078] a gas flow detection circuit for outputting a voltage signal
representing a gas flow passing through a gas passage;
[0079] an adjust circuit for adjusting the voltage output of the
gas flow detection circuit;
[0080] a storage device for storing data for adjustment; and
[0081] a data input/output circuit;
[0082] wherein the data input/output circuit has two external data
communication terminals for writing adjust data from outside into
the storage device and for reading data from the storage device
to the outside.
[0083] With this arrangement, the gas flow meter can be made small
in size.
[0084] (17) Preferably, the gas flow meter according to item (16),
wherein the adjust circuit has a means which, after a predetermined
number, two or more, of pulses have been supplied to one of the
external data communication terminals of the data input/output circuit,
allows the adjust circuit to enter into a data communication mode
where it transfers data between the storage device and external
circuits. With this arrangement, the adjust circuit can be prevented
from undesirably entering into the data communication mode even
when pulse noise is impressed during normal operation.
[0085] (18) Preferably, a gas flow meter comprising:
[0086] a gas flow detection circuit for outputting a voltage signal
representing a gas flow passing through a gas passage;
[0087] an adjust circuit for adjusting the voltage output of the
gas flow detection circuit; and
[0088] a storage device for storing data for adjustment;
[0089] wherein the adjust circuit retrieves as the output signal
of the detected gas flow a ratiometric analog output, a non-ratiometric
analog output and a digital output and selects one of these output
signals by an output selection means provided in the adjust circuit.
[0090] With this arrangement, a single gas flow meter can cope
with a variety of output specifications and contribute to standardization
and reduction in manufacturing cost.
[0091] (19) Preferably, the gas flow meter according to item (18),
wherein circuits for producing the ratiometric analog output, the
non-ratiometric analog output and the digital output are formed
on the same integrated circuit.
[0092] With this arrangement, the circuit can be reduced in size.
[0093] (20) Preferably, the gas flow meter according to item (16)
or (18), wherein the external data communication terminals serve
as a detected flow output terminal.
[0094] With this arrangement, the gas flow meter can be reduced
in size.
[0095] (21) Preferably, a gas flow meter comprising:
[0096] a gas flow detection circuit for detecting a current flowing
through a resistor installed in a gas passage and a generated voltage
and outputting a voltage signal representing a gas flow passing
through the gas passage;
[0097] a digital conversion circuit for converting the detected
gas flow into a digital signal; and
[0098] a digital adjust circuit for digitally adjusting the digital
signal and outputting the adjusted digital signal;
[0099] wherein a voltage signal based on the digital signal adjusted
by the digital adjust circuit is output, and
[0100] the digital conversion circuit has a means for selecting
either a single-phase input or a differential input.
[0101] With this arrangement, the adjust circuit can deal with
either a gas flow detection circuit with a single-phase output or
a gas flow detection circuit with a differential output
[0102] (22) Preferably, a gas flow meter comprising:
[0103] a gas flow detection circuit for detecting a current flowing
through a resistor installed in a gas passage and a voltage generated
across the resistor and outputting a voltage signal representing
a gas flow passing through the gas passage;
[0104] a digital conversion circuit for converting the detected
gas flow into a digital signal;
[0105] a digital adjust circuit for digitally adjusting the digital
signal and outputting the adjusted digital signal; and
[0106] an analog conversion circuit for receiving the adjusted
digital signal and converting it into an analog signal;
[0107] wherein the analog conversion circuit is driven by a voltage
based on an external reference voltage and a voltage follower circuit
is arranged between a reference voltage terminal and a power supply
terminal which drives the analog conversion circuit.
[0108] With this arrangement, the digital-analog converter can
be operated even when the current supplied from the external reference
voltage is small.
BRIEF DESCRIPTION OF THE DRAWINGS
[0109] FIG. 1 is an outline circuit configuration of a gas flow
meter according to the present invention.
[0110] FIG. 2 is an outline circuit configuration of a noise reduction
circuit applied to a gas flow meter according to the present invention.
[0111] FIG. 3 is an outline circuit configuration of an overvoltage
protection circuit used in a gas flow meter according to the present
invention.
[0112] FIG. 4 is an example of a voltage limiter circuit used in
the overvoltage protection circuit according to the present invention.
[0113] FIG. 5 is an outline circuit configuration of the overvoltage
protection circuit used in the gas flow meter according to the present
invention.
[0114] FIG. 6 is an outline circuit configuration of the overvoltage
protection circuit used in the gas flow meter according to the present
invention.
[0115] FIG. 7 is an example of a diode used in the overvoltage
protection circuit according to the present invention.
[0116] FIG. 8 is an outline circuit configuration of the overvoltage
protection circuit used in the gas flow meter according to the present
invention.
[0117] FIG. 9 is an outline circuit configuration of the overvoltage
protection circuit used in the gas flow meter according to the present
invention.
[0118] FIG. 10 is an example output characteristic of a gas detection
circuit of the gas flow meter.
[0119] FIG. 11 is an example input/output characteristic of an
adjust circuit of the gas flow meter according to the present invention.
[0120] FIG. 12 is an example of an adjusted output characteristic
of the gas flow meter according to the present invention.
[0121] FIG. 13 is an error of the adjusted output characteristic
of the gas flow meter according to the present invention.
[0122] FIG. 14 is a flow chart representing an adjustment calculation
used by the gas flow meter according to the present invention.
[0123] FIG. 15 is a diagram showing a principle of the adjustment
calculation used by the gas flow meter according to the present
invention.
[0124] FIG. 16 is an outline circuit configuration of the adjust
circuit used in the gas flow meter according to the present invention.
[0125] FIG. 17 is an example of a temperature sensor circuit.
[0126] FIG. 18 is an example of a regulator circuit.
[0127] FIG. 19A and FIG. 19B are examples of temperature-characteristic-ad-
justed output characteristics of the gas flow meter according to
the present invention.
[0128] FIG. 20 is an outline circuit configuration of the adjust
circuit used in the gas flow meter according to the present invention.
[0129] FIG. 21 is an outline circuit configuration of a data communication
input/output circuit and a sensor output circuit, both used in the
gas flow meter of the present invention.
[0130] FIG. 22 is an outline timing chart for terminals of the
data communication input/output circuit of FIG. 17.
[0131] FIG. 23 is an outline circuit configuration of a data communication
input/output circuit and a sensor output circuit, both used in the
gas flow meter of the present invention.
[0132] FIG. 24 is an outline circuit configuration of a power supply
circuit unit for a digital-analog converter circuit used in the
gas flow meter according to the present invention.
[0133] FIG. 25 is a schematic diagram showing an example of the
flow detection circuit.
[0134] FIG. 26 is a schematic diagram showing an example of the
flow detection circuit.
[0135] FIG. 27 is a schematic diagram showing an example of the
flow detection circuit.
[0136] FIG. 28 is an example outline circuit configuration of a
conventional overvoltage protection circuit.
[0137] FIG. 29 is a table showing comparison between an example
analog adjustment system and an example digital adjustment system
of the adjust circuit in the gas flow meter.
DESCRIPTION OF THE EMBODIMENTS
[0138] Now, the configurations of a gas flow meter, an integrated
circuit and an adjust circuit according to the present invention
will be described in detail by referring to the embodiments shown
in the accompanying drawings.
[0139] FIG. 1 shows a configuration of the gas flow meter as a
first embodiment of the invention. This configuration comprises
a gas flow detection circuit 10 a digital adjust circuit 20 a
regulator 30 and a noise reduction circuit 100.
[0140] The gas flow detection circuit 10 outputs a voltage signal
representing a gas flow passing through a gas passage. The gas flow
detection circuit 10 may be a gas flow detection circuit DECT1 shown
in FIG. 21 which detects a current flowing through a resistor arranged
in the gas passage or a voltage across the resistor and outputs
a voltage signal representing the gas flow passing through the gas
passage.
[0141] The voltage output is supplied to the digital adjust circuit
20. In an example configuration of the digital adjust circuit 20
as shown in FIG. 29B, the voltage output from the flow detection
circuit is converted by an analog-digital converter AD1 into a digital
value, which is processed by a digital processor CALC to adjust
a zero point and a span. The adjusted zero point and span are converted
into an analog signal by a digital-analog converter DA to produce
an analog output corresponding to a desired gas flow. The gas flow
meter also has the regulator 30 which drives these analog-digital
converter AD, digital processor CALC and digital-analog converter
DA, and produces a reference voltage for the analog-digital converter
AD and the digital-analog converter DA.
[0142] The noise reduction circuit 100 is a circuit to reduce surges,
overvoltages and high frequency noise and to supply a stable power
supply voltage. A part of the digital adjust circuit 20 uses C-MOSs
that may be damaged by surges or overvoltages or operate undesirably
due to high frequency electromagnetic noise generated by a variety
of electronic devices. Hence, the power supply terminal of the digital
adjust circuit is connected to the noise reduction circuit 100 through
the regulator. A part or all of the digital adjust circuit 20 may
use bipolar transistors, and the noise reduction circuit 100 may
be connected to various circuits in the gas flow meter.
[0143] As shown in FIG. 2 the noise reduction circuit 100 comprises
an overvoltage protection circuit 101 for protecting the gas flow
detection circuit 10 and the digital adjust circuit 20 from surges
and overvoltages impressed on the power supply terminal VBB, and
a high frequency noise reduction circuit 102 for reducing high frequency
noise. The noise reduction circuit 100 supplies a regulated power
supply voltage with reduced overvoltages, surges and high frequency
noise via two or more terminals to circuits Ld1 Ld2 such as gas
flow detection circuit and adjust circuit. When minimum voltages
required by the circuits Ld1 Ld2 differ, the voltages supplied
through the voltage supply terminals need only be differentiated.
[0144] Next, the overvoltage protection circuit, a part of the
noise reduction circuit 100 will be described by referring to FIG.
3.
[0145] The overvoltage protection circuit 103 comprises a voltage
limiter circuit 110 two current limiting resistors Ra, Rb, and
diodes D2 D3. The overvoltage protection circuit 103 receives a
DC power (VB) from a car battery not shown via the power supply
terminal VBB and the ground terminal GND and then supplies an overvoltage-protected
DC power to the circuits Ld1 Ld2 connected to the two voltage supply
terminals Vcc1 Vcc2.
[0146] The voltage limiter circuit 110 when the voltage exceeds
a predetermined value, turns on to pass a current. This circuit
has current limiting resistors connected in series to clip an overvoltage
such as surge voltage impressed between the power supply terminal
VBB and the ground terminal GND to absorb an overvoltage energy.
[0147] The voltage limiter circuit 110 may use a circuit which,
for example, has a certain number of Zener diodes ZD1-ZDn connected
in series, as shown in FIG. 4 and also an Nch D-MOS M added to
them. In this configuration, the group of Zener diodes ZD1-ZDn,
when applied with a voltage higher than a predetermined level, is
turned on to turn on the Nch D-MOS M to pass a surge current. This
arrangement can reduce the size of the Zener diodes through which
almost no current flows, thus contributing a reduction in the circuit
size. The Nch D-MOS of the voltage limiter circuit 110 may be replaced
with a bipolar transistor, or the voltage limiter circuit may be
formed by using only Zener diodes.
[0148] Returning to FIG. 3 the diode D3 prevents a current from
the resistor Rb from flowing into the circuit Ld1 or a current from
the resistor Ra from flowing into the circuit Ld2 and also has a
function of supplying different supply voltages from the voltage
supply terminals Vcc1 Vcc2.
[0149] The diode D3 uses a bipolar transistor with its base and
emitter connected, as shown in FIG. 5. This arrangement allows the
diode to be fabricated in the same step that the bipolar transistor
is made, thus reducing the number of manufacturing steps.
[0150] Here, suppose that resistance Ra is larger Ad than resistance
Rb. When a positive surge voltage, against which the circuits Ld1
Ld2 are to be protected, is impressed between the power supply terminal
VBB and the ground terminal GND, a surge current flows mainly through
the current limiting resistor Rb and the voltage limiter circuit
110 thus protecting the circuit Ld2 from the surge. Because of
the diode D3 the voltage at the voltage supply terminal Vcc1 is
almost equal to that of the voltage supply terminal Vcc2 thus protecting
the circuit LD1 too, from the surge.
[0151] When a negative surge voltage is applied, the surge current
flows through the diode D2 and the current limiting resistor Rb,
thus protecting the circuits Ld1 Ld2 from the surge.
[0152] Another configuration of the overvoltage protection circuit
104 is shown in FIG. 6. This configuration has the diode D2 of FIG.
3 reconnected at the position of the diode D1 of FIG. 6.
[0153] When a positive surge voltage, against which the circuits
Ld1 Ld2 are to be protected, is applied between the power supply
terminal VBB and the-ground terminal GND, the surge current flows
mainly through the current limiting resistor Rb and the voltage
limiter circuit 110 protecting the Ld2 from the surge. The diode
D3 renders the voltage at the voltage supply terminal Vcc1 almost
equal to that of the voltage supply terminal Vcc2 also protecting
the circuit Ld1 from the surge.
[0154] As for the negative surge, the surge current flows through
the diodes D1 D3 and the current limiting resistor Rb, protecting
the circuits Ld1 Ld2 from the surge.
[0155] Another configuration of the overvoltage protection circuit
105 is shown in FIG. 7. This configuration has a voltage limiter
circuit 111 and a diode D1 connected in series with a current limiting
resistor Ra, as opposed to FIG. 3 and has a Zener diode ZD connected
between the voltage supply terminals Vcc1 and Vcc2.
[0156] If a positive surge voltage, against which the circuits
Ld1 Ld2 are to be protected, is applied between the power supply
terminal VBB and the ground terminal GND, the surge current flows
mainly through the current limiting resistor Rb, the Zener diode
ZD and the voltage limiter circuit 111 thus protecting the circuits
Ld1 Ld2 from the surge.
[0157] As for a negative surge, the surge current flows through
the diode D1 the Zener diode ZD and the current limiting resistor
Rb, thus protecting the circuits Ld1 Ld2 from the surge.
[0158] Still another configuration of the overvoltage protection
circuit 106 is shown in FIG. 8. This overvoltage protection circuit
106 comprises voltage limiter circuits 110 111 two current limiting
resistors Ra, Rb, and diodes D1 D2. The overvoltage protection
circuit 106 receives a DC power from a car battery not shown through
the power supply terminal VBB and the ground terminal GND and supplies
an overvoltage-protected DC power to the circuits Ld1 Ld2 connected
to the voltage supply terminals Vcc1 Vcc2.
[0159] It is assumed that the minimum voltages and minimum currents
required for the circuits Ld1 Ld2 differ. The current limiting
resistors Ra, Rb normally cause voltage drops as the current flows
through the circuits Ld1 Ld2. By increasing the resistances of
the current limiting resistors Ra, Rb within the minimum required
supply voltage range for the circuit Ld1 and circuit Ld2 the voltage
limiter circuit can be reduced in size, which in turn contributes
to a reduction in the overall circuit size.
[0160] The present invention is suitably applied to a circuit of
a gas flow meter that has a digital adjust circuit in particular.
One such example is shown in FIG. 9. Here, the overvoltage protection
circuit has the configuration 103 of FIG. 3. As the circuit Ld1
of FIG. 3 an operational amplifier OP1 a part of the gas flow
detection circuit DECT1 of FIG. 25 is connected. As the circuit
of Ld2 a regulator REG is connected that supplies a reference voltage
to various parts of the circuit of the gas flow meter.
[0161] The operational amplifier OP1 controls a power transistor
Tr1 so the current to be supplied to the operational amplifier
OP1 may be small (suppose it is about 1.5 mA) but a relatively high
voltage is needed to drive the power transistor Tr1. For example,
even when the battery voltage is low and the voltage at the power
supply terminal VBB is 6 V as during the starting of a car engine,
an output of approximately 5.5 V must be able to be produced. As
for the regulator REG which supplies voltage to the analog-digital
converter AD1 of the digital adjust circuit type (b) shown in FIG.
29 digital processor CALC and digital-analog converter DA, although
the current to be supplied to the regulator REG is relatively large
(suppose it is about 15 mA), it needs only to produce an output
of 5 V at all times even when the voltage at the power supply terminal
VBB falls to 6 V.
[0162] Suppose the overvoltage protection circuit has a configuration
shown in FIG. 28. When an overvoltage-protected voltage is supplied
from one voltage supply terminal to both the operational amplifier
OP1 and the regulator REG, the resistance of the current limiting
resistor R needs to have 30 ohm from the requirement that the supply
voltage is 5.5 V and the supply current is 16.5 mA when the voltage
at the power supply terminal VBB is 6 V.
[0163] In the overvoltage protection circuit 103 of FIG. 9 the
resistance can be made larger than in the general overvoltage protection
circuit of FIG. 28. That is, under the condition that the current
limiting resistor Ra causes a voltage drop of 0.5 V or less when
a current of 1.5 mA flows and that the current limiting resistor
Rb causes a voltage drop of 1 V or less when a current of 15 mA
flows, the resistor Ra may be set to 250 ohm and the resistor Rb
to 50 ohm, for example. Because the current, or energy, flowing
through the voltage limiter circuit 110 is reduced, the electrical
withstandability required also decreases, making it possible to
reduce the voltage limiter circuit 110.
[0164] Further, a part or all of the devices contained in these
overvoltage protection circuit, flow detection circuit DECT and
digital adjust circuit may be integrated into the same IC circuit
by using the BCD (bipolar, C-MOS, D-MOS) process to reduce the size
and manufacturing cost.
[0165] The configuration of the overvoltage protection circuit
according to the present invention can be applied in the similar
manner if the number of voltage supply terminals Vcc increases to
three or more.
[0166] Next, the accuracy enhancement in adjusting the sensor output
characteristic will be explained by referring to FIG. 10 through
FIG. 13 for a case of an example adjustment calculation of the present
invention, as compared with a conventional adjustment that adjusts
only the zero point and the span.
[0167] FIG. 10 shows a gas flow versus output voltage characteristic
of the flow detection circuit DECT. The output voltage is adjusted
by the adjust circuit to become a narrow line A, an ideal flow-output
characteristic, in FIG. 12.
[0168] First, in a conventional example (2) represented by a dotted
line in which only the zero point and span are adjusted, the adjustment
calculation formula in FIG. 11 used by the adjust circuit is a linear
relationship irrespective of the voltage value entered. When, by
using the input/output characteristic of this adjust circuit, the
flow-output voltage characteristic of the flow detection circuit
in FIG. 10 is adjusted, the characteristic will be as shown by a
curve (2) in FIG. 12. An error from the ideal output characteristic
is shown at (2) in FIG. 13. In an example of adjustment calculation
according to the present invention (1), as shown in FIG. 11 the
input/output characteristic of the adjust circuit has its input
range of voltage signal divided in two and defines different adjustment
calculation formulas in different divided ranges A, B (in this example,
the simplest first-degree equations). Adjusting the flow-output
voltage characteristic of the flow detection circuit of FIG. 10
by using the input/output characteristic of this adjust circuit
results in a curve (1) of FIG. 12. When the error from the ideal
output (A) is shown superimposed on the conventional case (2) in
FIG. 13 it is seen that the adjustment error (1) is reduced.
[0169] While in this example the input range of voltage signal
entered is divided in two, the simplest division number, it is possible
to increase the division number and define an adjustment calculation
formula in each divided range for further reduction in the adjustment
error. For example, when it is divided into four, the characteristic
curve will be as shown at (3) in FIG. 3. The adjustment calculation
formula of second or higher degree may be used to reduce the adjustment
error. This, however, raises a problem of an increased circuit size
and, in digital calculation, a longer calculation time.
[0170] Further, although in this example the error is discussed
as a characteristic of a quadratic function, this invention can
be applied to those errors that are characteristics of a third-
(or higher) degree function by increasing the division number to
reduce the adjustment error.
[0171] Such an adjust circuit can easily be realized by constructing
the adjust circuit in a digital form. An example of the digital
adjust circuit is shown in (b) of FIG. 29.
[0172] This digital adjust circuit converts the voltage output
of the flow detection circuit DECT1 DECT2 of FIG. 25 and FIG. 26
into a digital value by the analog-digital converter AD1 adjusts
the output characteristic by the digital processor CALC, and produces
an analog output by the digital-analog converter DA. Programs for
controlling the digital processor CALC and for adjustment calculation,
adjustment coefficients for adjustment calculations, and data temporarily
stored during calculation are stored in a storage device MEM, such
as read only memory ROM, programmable read only memory PROM, electrically
erasable & programmable read only memory EEPROM, and random
access memory RAM.
[0173] For an arbitrary function y=f(x) that can be differentiated,
when a is a constant and .vertline.x-a.vertline. is very small,
the function can be expressed, from the theorem of average value,
as f(x)=f(a)+f'(a)(x-a). That is, in a very small range of x an
arbitrary function can be replaced with a first-degree function.
[0174] If we let
Dout=f(Din) (1)
[0175] then, for each small range of Din, the equation (1) can
be rewritten as
Dout=A.multidot.Din+B (2)
[0176] However, because it is not realistic to give a linear equation
for all Din and because almost the same linear equation adjustment
calculation formula can be used in some Din ranges, the axis of
Din is divided into n segments at dividing points Din(1), Din(2),
. . . , Din(n). For each divided segment, the following linear equation
adjustment calculation formula is given: 1 Dout = { A ( 1 ) D i
n + B ( 1 ) ( D i n ( 1 ) D i n < D i n ( 2 ) ) A ( 2 ) D i n
+ B ( 2 ) ( D i n ( 2 ) D i n < D i n ( 3 ) ) M M A ( n ) D i
n + B ( n ) ( D i n ( n ) D i n ) } ( 3 )
[0177] A calculation flow chart based on the above formula is shown
in FIG. 14. First, from Din entered into the digital processor CALC,
a search is made for k that satisfies Din(k)<Din.ltoreq.Din(k+1).
Next, coefficients A(k) and B(k) are retrieved from the storage
device MEM and the calculation is performed by the digital processor
CALC according to the adjustment calculation formula (3) to adjust
the output.
[0178] Here, it is assumed that the number of dividing points n
is 2 raised to the ith power. The digital value Din is expressed
in m-bit binary number (m=n+1 or more). As for the dividing point,
high order i bits are arbitrary values and the remaining low order
(m-i) bits are all 0. That is, Din(k) is
i bits (m-i) bits
Din(1)=000 00 . . . 00
Din(1)=001 00 . . . 00
. . .
Din(n)=111 00 . . . 00 (4)
[0179] That is, the division intervals are equal. To adjust the
sensor output, if the high order i bits of Din are k (binary notation),
then the Din will always be
Din(k).ltoreq.Din.ltoreq.Din (k+1)
[0180] Hence, the coefficients of the adjustment calculation formula
are A(k) and B(k). That is, what is required is to retrieve A(k)
and B(k) having high order i bits of Din as label from the storage
device and to perform the adjustment calculation by the digital
processor CALC.
[0181] With this retrieval method, the search time does not change
even when the division number increases. This method therefore is
particularly effective where the number of divisions is large.
[0182] As for the measurement during the adjustment of the gas
flow meter before the adjustment value is written, there is no need
to take measurements at all n measuring points and it is possible
to take measurements at arbitrary points, determine n adjustment
coefficients A(k) and B(k) by interpolation and write them.
[0183] A straight line for linear approximation using this uniform
division has a relationship as shown in a graph of FIG. 15. That
is, by using the remaining low order (m-i) bits excluding the high
order i bits used for retrieval in some segments of Din, the following
adjustment calculation formula (5) can be used to reduce the possibility
of overflow.
Dout=A.multidot.(lower order (m-i) bits of Din)+B (5)
[0184] There is a fourth-degree relationship as shown in FIG. 10
between the flow Q and the voltage output V produced by the current
detection resistor Rc of the gas flow detection circuit DECT. If
a necessity arises to produce a linear output characteristic as
defined by V.varies.Q, it is possible to increase the number of
divisions and represent the fourth-degree equation by a linear approximation.
In the case of this gas flow meter, the error between the fourth-degree
equation and the linear approximation is about 3% for 16 divisions,
about 0.8% for 32 divisions, about 0.2% for 64 divisions, and about
0.05% for 128 divisions. Increasing the division number. naturally
reduces the error produced by the linear approximation of the quartic
equation and approaches a linear output characteristic for the flow
Q. Considering the error tolerated for the output characteristic
of the gas flow meter, the division number is preferably 32 or more.
[0185] Next, the adjustment of a temperature characteristic will
be explained.
[0186] A temperature characteristic of the gas flow meter, i.e.,
changes in output characteristic due to temperature variations,
may be classified largely into two types: an intake air temperature
characteristic in which a circuit board temperature remains constant
while the gas temperature changes; and a circuit board temperature
characteristic in which the gas temperature remains constant while
the circuit board temperature changes (also called a module temperature
characteristic). Here let us explain about the adjustment of the
circuit board temperature characteristic. As for the intake air
temperature characteristic, the output variations can be minimized
by appropriately setting resistances of the heating resistor Rh
and gas temperature measuring resistor Rc in the gas flow detection
circuit DECT1 a resistor temperature coefficient (TCR), and resistances
of resistors R1 R2. The intake air temperature characteristic has
a flow dependency and is difficult to eliminate completely. So,
by deliberately providing the circuit board temperature characteristic
with a reverse characteristic for the intake air temperature characteristic,
the overall temperature characteristic for the gas flow meter can
be set to zero.
[0187] The circuit board temperature characteristic is mainly determined
by a temperature characteristic of the output voltage of the regulator
that supplies a reference voltage to the analog-digital converter
AD and the digital-analog converter DA.
[0188] An outline configuration of the circuit for adjusting the
temperature characteristic is shown in FIG. 16. Added to the digital
adjust circuit shown in (b) of FIG. 29 in order to adjust the temperature
characteristic are a temperature sensor TS and an analog-digital
converter AD2 for converting the output of the temperature sensor
into a digital value. The converted digital value is entered into
the digital processor CALC.
[0189] The temperature sensor TS necessary for the adjustment of
a temperature characteristic is arranged close to the regulator
that has a temperature characteristic. An example configuration
of the temperature sensor is shown in FIG. 17 in which a constant
current source IS and one to several diodes D are used. When three
diodes are connected in series, for example, the output changes
with temperature variations at a rate of about -6 to -5 mV/.degree.
C. exhibiting a good linearity.
[0190] Further, if the supply voltage of the regulator is set to
change linearly with respect to temperature variations, the temperature
adjustment needs only to have a linear expression.
[0191] Such a regulator can be realized by using a band gap reference
power supply circuit (band gap voltage source circuit). The outline
configuration of this circuit is shown in FIG. 18. The regulator
has two diode-connected transistors Q1 Q2 an operational amplifier
OP3 and resistors R7 R8 R9. By using the operational amplifier
OP3 the currents flowing through the transistors Q1 Q2 can be
made to have a constant ratio determined by the resistances of resistors
R8 R9. At this time, the output voltage of the operational amplifier
OP3 stabilizes in such a way that the sum of the base-emitter voltage
of the transistor Q2 and the voltage drop of the resistor R7 is
equal to the base-emitter voltage of the transistor Q1. The voltage
drop of the resistor R7 is equal to the difference in the base-emitter
voltage between the transistor Q2 and the transistor Q1 and is proportional
to a thermal voltage V.sub.T=kT/q. So, the currents flowing through
the resistors R8 R9 and the transistors Q2 Q1 have a linear positive
temperature characteristic. Generally the base-emitter voltage has
a negative temperature characteristic. Hence, the reference voltage,
which is an output of the band gap reference power supply circuit
and equal to the sum of the base-emitter voltages of the transistors
Q2 Q1 and the voltage drop of the resistor R7 proportional to the
thermal voltage V.sub.T, can set a linear temperature coefficient
by changing the resistances of the resistors R7 R8 R9. In practice,
elements in the band gap reference power supply circuit have slightly
non-linear temperature coefficients, so the output voltage of this
reference power supply circuit has a characteristic slightly non-linear
for temperature variations on the high temperature side.
[0192] Since the output of the temperature sensor TS and the supply
voltage of the regulator that supplies a reference voltage have
a linear temperature characteristic for the temperature variations,
the temperature characteristic adjustment needs only to add a linear
temperature adjustment term to the adjustment terms of A and B in
the adjustment calculation formula (2) for Din. That is, the temperature
characteristic adjustment can be given by the following equation
(6) with a, b, c and d as coefficients.
Dout=(a.multidot.Dtepm+b).multidot.Din+(c.multidot.Dtemp+d) (6)
[0193] The adjustment calculation formula therefore is given as
follows by combining and rewriting the equations (3) and (4) with
a(k), b(k), c(k) and d(k) as coefficients. 2 Dout = { ( a ( 1 )
D t e m p + b ( 1 ) ) D i n + ( c ( 1 ) D t e m p + d ( 1 ) ) (
D i n ( 1 ) D i n < D i n ( 2 ) ) ( a ( 2 ) D t e m p + b ( 2
) ) D i n + ( c ( 2 ) D t e m p + d ( 2 ) ) ( D i n ( 2 ) D i n
< D i n ( 3 ) ) ( a ( n ) D t e m p + b ( n ) ) D i n + ( c (
n ) D t e m p + d ( n ) ) ( D i n ( n ) D i n ) } ( 7 )
[0194] To adjust the output of the sensor, a search is made for
k that satisfies Din(k).ltoreq.Din <Din(k+1) as in the flow chart
of FIG. 14 coefficients in the adjustment calculation formula a(k),
b(k), c(k) and d(k) are retrieved from the storage device, and the
calculation is performed by the digital processor CALC according
to the adjustment calculation formula (7) to adjust the output.
[0195] To further simplify the adjustment for temperature variations,
the adjustment calculation formula (6) may be replaced with the
following formula with C and D as coefficients.
Dout=(C.multidot.Dtemp+D).multidot.(A.multidot.Din+B) (8)
[0196] This formula first performs the adjustment calculation related
to the flow before performing the adjustment calculation for the
temperature.
[0197] Further, because the regulator has a slightly non-linear
temperature characteristic, the calculation formula may be changed
according to the output value of the temperature sensor as in the
flow adjustment, in order to improve the adjustment accuracy for
the temperature characteristic. An example of this method is shown
in FIGS. 19A and 19B. A case (a) where the calculation formula is
changed according to the output value of the temperature sensor
(here, the temperature range is divided in two) is compared with
another case (b) where one adjustment formula is applied over the
entire output range. Here it is assumed that the input value to
the adjust circuit is constant, i.e., the intake air temperature
characteristic is zero and the flow is constant. The adjustment
calculation formulas are shown in FIG. 19A for the temperature characteristic
of the regulator shown in FIG. 18. From FIG. 19B, which shows the
output characteristics after being adjusted based on the temperature
characteristic, it is seen that the error is reduced.
[0198] The calculation formula for the temperature characteristic
adjustment may be of second-degree or higher. It is also possible
to add a gas temperature sensor and an analog-digital converter
and perform the adjustment calculation similar to the above to adjust
the intake air temperature characteristic.
[0199] If rewritable storage devices such as EEPROM are used for
storing the adjustment coefficients, gas flow meters can be taken
out from unused motor vehicles and the output specifications changed
to enable their use on various types of motor vehicles. In the present
manufacturing process, the adjustment procedure involves first supplying
a gas before adjustment, determining the amount of adjustment on
the output characteristic, performing the adjustment, and then verifying
the characteristic. If an EEPROM is used, it is written with an
adjustment coefficient in advance and only those gas flow meters
that failed the characteristic verification test need to be adjusted
again. That is, the use of the EEPROM enhances the level of reuse
and offers the advantage of reducing the manufacturing cost.
[0200] A digital adjust circuit as shown in FIG. 20 is also possible.
This circuit is almost similar to that of FIG. 16 except that the
differential output type flow detection circuit DECT3 of FIG. 27
is connected to the analog-digital converter AD1. Further, this
circuit has a group of switches SWS to switch between a single-phase
input and a differential input so that the flow detection circuits
DECT1 DECT2 shown in FIG. 25 and FIG. 26 can also be connected.
Further, a frequency output circuit FC is also added as an output
circuit. The adjustment calculation can be performed in the similar
manner.
[0201] Next, FIG. 21 shows one embodiment of a circuitry according
to this invention which is capable of reducing the number of flow
signal output terminals used in the gas flow meter and the number
of input/output terminals used for data communication with the storage
device in which adjustment data is written; of outputting as a flow
signal output of the gas flow meter one of a ratiometric analog
output, a non-ratiometric analog output and a digital output; and
of reducing the number of terminals by commonly using an electric
path as a data communication input/output path and a flow signal
output path.
[0202] An output circuit 201 comprises mainly a digital-analog
converter DA, a frequency output circuit FC, and switches SW1 SW2.
[0203] A digital value produced by the digital processor CALC performing
the adjustment calculation is entered into the digital-analog converter
DA and the frequency output circuit FC. The digital-analog converter
DA converts the received digital value into an analog voltage output.
A reference used in generating this analog voltage output is a voltage
supplied to the digital-analog converter DA. This voltage is switched
by the switch SW2 between a voltage generated by an electronic circuit
in the gas flow meter and a voltage supplied from outside to a ratiometric
reference voltage terminal 232 (e.g., a reference voltage of an
analog-digital converter in an automotive engine control unit) to
enable selection between the non-ratiometric voltage output and
the ratio metric output. The frequency output circuit FC outputs
the received digital value as a desired digital output. An analog
voltage output and a digital output are selected by the switch SW1.
[0204] The switching operation of these switches SW1 SW2 is carried
out according to data in the storage device MEM which can be written
during sensor adjustment.
[0205] A data input/output circuit 202 for transferring data between
the outside of the gas flow meter and the storage device MEM into
which the adjustment coefficients and the switching setting are
written during the sensor adjustment, mainly comprises: a data conversion
circuit I/O for converting the number of bits (8 or 16 bits) of
data in the integrated circuit and one-bit data used during data
transfer to and from the external circuits; a direction signal output
circuit DIR for outputting a DIRECTION signal indicating whether
the data conversion circuit I/O inputs or outputs data; a clock
detection circuit CDECT for detecting a clock signal supplied to
a CLOCK terminal; and a switch SW4 for selecting whether a data
signal is to be entered into or output from the data conversion
circuit I/O according to the signal from the direction signal output
circuit DIR.
[0206] The detection signal from the clock detection circuit CDECT
is entered into the data conversion circuit I/O which is activated
by the signal. If a switch SW3 is added which is operated according
to the detection signa1 it is possible to combine the flow signal
output path and the data input/output path into one path in the
integrated circuit. To ensure that pulse noise to the CLOCK terminal
will not undesirably operate the switch SW3 the detection signal
is output only after a predetermined number of pulses are entered
to the clock detection circuit CDECT.
[0207] FIG. 22 shows an example of data timing chart when data
is input and output. When a CLOCK signal 251 is supplied to the
CLOCK terminal, the clock detection circuit CDECT is operated to
generate a START signal 252. The switch SW3 is operated by the START
signal 252. The DIRECTION signal 253 is turned on or off by a predetermined
number of clock pulses. The DIRECTION signal 253 operates the switch
SW4 to change the direction of data flow, i.e., to select between
a DATA IN signal 254 and a DATA OUT signal 255.
[0208] FIG. 23 shows another embodiment of a circuitry according
to this invention which is capable of reducing the number of flow
signal output terminals used in the gas flow meter and the number
of input/output terminals used for communication with the storage
device in which adjustment data is written; of outputting as a sensor
output one of a ratiometric analog output, a non-ratiometric analog
output and a pulse output; and of reducing the number of terminals
by commonly using the terminals as communication input/output terminals
and sensor output terminals.
[0209] What is different from FIG. 21 is that a VF conversion circuit
VF for converting an analog voltage into a digital output is inserted
at a stage downstream of the digital-analog converter DA and that
the switch SW1 selects between the analog voltage output and the
digital output. The operation of this configuration is the same
as that of FIG. 21 and thus its explanation is omitted.
[0210] With this configuration, therefore, the connection terminals
with the outside of the gas flow meter can be constructed of at
least four terminals: a power supply terminal, a ground terminal,
a common terminal for flow signal output and data input/output,
and a data input/output terminal.
[0211] If the maximum current supplied from the external reference
voltage of the engine control unit is small, there is a possibility
that simply connecting this external reference voltage directly
to the digital-analog converter DA, which includes an amplifier
circuit at an output stage and has a large current consumption,
may fail to drive the digital-analog converter DA. To deal with
this problem, a buffer circuit is inserted, as shown in FIG. 24
which has a power supply terminal of an operational amplifier OP4
connected to a battery voltage not shown. With the input of the
buffer circuit as a resistor, the buffer output is connected to
the power supply terminal of the digital-analog converter DA to
supply a current from the operational amplifier OP4 to the digital-analog
converter DA for operation. The load resistance Ri of the buffer
circuit is set to about 10 kilo-ohm.
[0212] The gas flow meter with a digital adjust circuit according
to the present invention has an advantage that even if C-MOS devices
not resistant to surges and overvoltages are used in the internal
circuit, a high level of circuit integration and the use of digital
circuitry can prevent failure or undesired operation.
[0213] Further, the overvoltage protection circuit included in
the electronic circuit noise reduction circuit in the gas flow meter
has an advantage of being able to minimize a drop in the supply
voltage from the voltage supply terminal due to a voltage drop caused
by the current limiting resistor used in the overvoltage protection
circuit. The voltage limiter circuit can also be reduced in size.
[0214] In the gas flow meter output characteristic adjustment calculation,
because a predetermined first-degree adjustment calculation formula
is selected for calculation according to an input value, the calculation
time is short and a non-linear adjustment can be made. Further,
the circuit board temperature adjustment can also be performed simultaneously.
[0215] Further, because one electric path is commonly used both
for the flow signal output and for the data input/output, it is
possible to cope with different flow signal output specifications
calling for a ratiometric analog output, a non-ratiometric analog
output or a digital output, without increasing the number of terminals.
A further feature of this invention is that the digital-analog converter
at the output stage can be driven even when the maximum current
supplied from the external reference voltage is small.
[0216] It is therefore possible to provide an optimum integrated
circuit and configuration when the gas flow meter circuit is integrated
into a digital circuit to reduce the cost of the gas flow meter
and enhance the accuracy of the output. |