Abstrict In a heating resistor type air flow meter, a temperature characteristic
of an air flow rate signal whose output value varies with temperature
of the air flow meter circuitry is compensated by the temperature
characteristic compensation circuit. The compensation circuit produces
a temperature characteristic signal for compensation which changes
in response to changes in an ambient temperature, and converts this
signal into a predetermined proportional signal. The predetermined
proportional signal is added to the air flow rate signal and such
an addition result signal is input to the output control circuit
and the temperature characteristic of the air flow rate signal is
compensated. The resistors for proportional adjustment are external
to and connected through a terminal to the IC of the air flow meter.
Claims What is claimed is:
1. A heating resistor type air flow meter comprising a heating
resistor placed in an air flow passage, a heating current control
circuit for controlling a heating current flowing through said heating
resistor to maintain said heating resistor at a constant temperature,
a flow rate sensing circuit for producing an air flow rate signal
through the use of heating current control of said heating resistor,
an output control circuit for amplifying and outputting said air
flow rate signal, a temperature characteristic compensation circuit
for compensating a temperature characteristic of said air flow rate
signal whose output value varies with changes of temperature of
the air flow meter circuitry, wherein said temperature characteristic
compensation circuit comprises a circuit for producing a temperature
characteristic signal for compensation which changes in response
to changes in an ambient temperature, and a circuit for converting
said temperature characteristic signal into a predetermined proportional
signal, and wherein said predetermined proportional signal is added
to said air flow rate signal and such an addition result signal
is input to said output control circuit.
2. A heating resistor type air flow meter comprising: a heating
resistor placed in a air flow passage; a heating current control
circuit for controlling a heating current flowing through said heating
resistor to maintain the temperature of said heating resistor at
a constant temperature; a flow rate sensing circuit for producing
an air flow rate signal through the use of heating current control
of said heating resistor, an operational amplifier for taking in
said air flow rate signal and amplifies the signal; and a temperature
characteristic compensation circuit for, in order to compensate
the temperature characteristic of said air flow rate signal, producing
a current having a temperature characteristic, namely a current
which changes depending on changes of an ambient temperature, dividing
this current into currents at a predetermined ratio, producing predetermined
proportional signals to the divided currents by converting the divided
currents into voltages, and inputs the proportional signals to positive
(+) and negative (-) input terminals of said operational amplifier.
3. The heating resistor type air flow meter according to claim
2 wherein said heating current control circuit, said output control
circuit, and said temperature characteristic compensation circuit
are integrally formed in a semiconductor integrated circuit except
for some of resistors, resistors for adjusting the ratio between
said divided currents is installed external to and connected through
a terminal to said semiconductor integrated circuit, and resistors
for converting said divided currents into voltages are formed in
the monolithic semiconductor integrated circuit.
4. The heating resistor type air flow meter according to claim
3 wherein said resistors for adjusting the ratio between the currents
have resistance temperature coefficients different from that of
said resistors in the monolithic circuit.
5. The heating resistor type air flow meter according to claim
3 wherein said temperature characteristic compensation circuit
divides said current having the temperature characteristic into
two currents by using current mirror circuits, based on voltage
values of said resistors for adjusting the ratio.
6. The heating resistor type air flow meter according to claim
2 wherein one of the predetermined proportional signals produced
by said temperature characteristic compensation circuit is added
to said air flow rate signal and such an addition result signal
is input to the positive (+) input terminal of said operational
amplifier, and the other predetermined proportional signal is added
to a zero offset signal to the air flow rate signal and such an
addition result signal is input to the negative (-) input terminal
of said operational amplifier.
7. The heating resistor type air flow meter according to claim
2 wherein one of the predetermined proportional signals generated
by said temperature characteristic compensation circuit is added
to said air flow rate signal and such an addition result signal
is input to the positive (+) input terminal of said operational
amplifier, and the other predetermined proportional signal is added
to a zero offset signal and a linearity offset signal to the air
flow rate signal and such an addition result signal is input to
the negative (-) input terminal of said operational amplifier.
Description CLAIM OF PRIORITTY
[0001] The present application claims priority from Japanese application
serial no. 2004-164145 filed on Jun. 2 2004 the content of which
is hereby incorporated by reference into this application.
FIELD OF THE INVENTION
[0002] The present invention relates to an air flow meter using
a heating resistor, for example, a heating resistor type air flow
meter that is used for a device for measuring an intake air flow
rate of an engine to control an internal combustion engine for vehicle
or the like.
BACKGROUND OF THE INVENTION
[0003] A heating resistor type air flow meter of prior art related
to the present invention will be described with FIG. 2. This prior
art example is disclosed in Japanese Patent Laid-Open No. H9 (1997)-203650.
[0004] A heating resistor 1 is formed by a temperature-sensitive
resistor having a temperature-dependent resistance characteristic
and placed in an passage of airflow to be measured. Actually, the
heating resistor 1 is provided in a Wheatstone bridge circuit which
is not shown, together with a flow rate signal producing element
(resistor 2) and a temperature-sensitive resistor for air-temperature
compensation, which is not shown. A heating current flowing through
the heating resistor 1 is controlled by a heating current control
circuit 10 to maintain the heating resistor 1 at a predetermined
heating temperature (resistance value) with regard to the air-temperature.
[0005] Since this heating current varies depending on air flow
rate, the heating current is converted into a voltage as an air
flow rate signal V2.
[0006] The heating current flows through the heating resistor 1
even when the flow rate of the air is zero. Therefore, the sensed
flow rate signal V2 is subjected to zero adjustment as well as amplification
using an output control circuit 20 and a flow rate characteristic
signal Vout is output. A reference voltage V0 required for zero
adjustment is supplied by an internal reference voltage generator
circuit 30.
[0007] In the resistor elements and the internal reference voltage
generator circuit which constitute the air flow meter, characteristics
of resistance or the like thereof vary with changes in circuit temperature,
resulting in variation in the flow rate characteristic signal Vout.
Therefore, it is needed to compensate its temperature characteristic
(output variation).
[0008] Such a characteristic compensation is performed using a
plurality of voltage dividing resistors 31 32 33 34 and one
of which is a resistor for adjustment. Variations (gradient) in
outputs of the flow meter at known two-point temperatures are observed
beforehand, and the resistance value of the resistor for adjustment
is adjusted to produce an offset signal to offset the variations
in the output. Moreover, Variations (non-linear line) in outputs
of the flow meter at known three-point temperatures is observed
beforehand, and the resistance value of a resistor 35 is adjusted
to output a signal for compensating for the linearity. The adjustments
as above are easily implemented by a laser trimming method.
[0009] The internal reference voltage generator circuit 30 is configured
by a bandgap reference voltage generator circuit utilizing the bandgap
of silicon. This voltage generator circuit 30 the heating current
control circuit 10 and the output control circuit 20 are integrally
formed in a semiconductor integrated circuit (IC) 3 except for some
of the resistors.
[0010] In the above circuit configuration, if the temperature characteristic
compensation circuit is constructed by using transistors integrated
into the IC 3 the number of components of the flow meter can be
reduced and the circuit board can be made smaller.
[0011] The adjusting resistor for temperature characteristic compensation
is external to the IC 3 because of its adjustment function. In the
bandgap reference voltage generator circuit, it is unsuitable that
only the adjusting resistor is external to the IC and other resistors
are integrated into the monolithic IC for the following reason.
[0012] The resistors integrated into the monolithic IC are affected
by variance in diffusion, which results in variation in the temperature
characteristic of the reference voltage. Consequently, it is difficult
to adjust the characteristic of the flow meter. Therefore, if the
adjusting resistor in the bandgap reference voltage generator circuit
is external to the IC, all resistors in the voltage generator circuit
need to be external to the IC to ensure the precision of the flow
meter.
[0013] For this reason, in the prior art embodiment of FIG. 2
the adjusting resistors 31 32 33 34 for adjusting the flow meter
characteristic at two-point temperatures are external to the IC.
Besides, the adjusting resistor 35 for linearity compensation for
the bandgap reference voltage generator circuit (with a temperature
coefficient different from that of the above adjusting resistors)
is also incorporated into an external circuit with the adjusting
resistors. In consequence, it is necessary to equip the IC with
three or more terminals. Increase in the number of connection terminals
of the IC becomes a bottleneck in reducing the dimensions of the
air flow meter and reducing its cost.
[0014] [Patent document 1] Japanese Patent Laid-Open No. H9 (1997)-203650
SUMMARY OF THE INVENTION
[0015] The present invention is to reduce the number of connection
terminals for external elements for adjustment in the temperature
characteristic compensation circuit, and to realize an air flow
meter that is practically immune to variance in diffusion even if
the resistors for temperature characteristic compensation are integrated
into the monolithic IC.
[0016] The present invention is principally configured as follows.
[0017] In a heating resistor type air flow meter, a temperature
characteristic of an air flow rate signal whose output value varies
with temperature of the air flow meter circuitry is compensated
by a temperature characteristic compensation circuit as follows.
[0018] This temperature characteristic compensation circuit comprises
a circuit for producing a temperature characteristic signal for
compensation which changes in response to changes in an ambient
temperature, and a circuit for converting the temperature characteristic
signal into a predetermined proportional signal. The predetermined
proportional signal is added to the air flow rate signal and such
an addition result signal is input to an output control circuit.
[0019] According to the above configuration, in an overall circuit
which produces the air flow rate signal, when an overall resistance
value of resistor elements and resistive component of circuits changes
depending on changes in ambient temperature (circuitry temperature),
the output value of the air flow rate signal varies. At that time,
the temperature characteristic compensation circuit produces a signal
(temperature characteristic signal) that changes following the same
gradient as the temperature characteristic of the output value of
the air flow rate signal. By inputting this signal (temperature
characteristic signal) to the output control circuit to offset the
temperature-varying output of the air flow rate signal, the temperature
characteristic of the air flow rate signal is compensated. In this
case, a proportion of the temperature characteristic signal is adjusted
and input to the output control circuit to properly compensate the
temperature characteristic of the air flow rate signal.
[0020] According to a preferred embodiment of the present invention,
for instance, the output control circuit is configured by an operational
amplifier that amplifies the air flow rate signal. The temperature
characteristic compensation circuit produces a current having a
temperature characteristic, namely a current which changes depending
on changes of an ambient temperature, divides this current into
two currents at a predetermined ratio, and produces predetermined
proportional signals to the divided by converting the divided currents
into voltages. Then, the compensation circuit inputs the predetermined
proportional signals to positive (+) and negative (-) input terminals
of the operational amplifier.
[0021] According to the above configuration, resistors for proportional
adjustment of the signal for use in the temperature characteristic
compensation circuit are formed by external resistors to a semiconductor
integrated circuit (IC) that forms a main circuit of the air flow
meter, and other related resistors are formed in the monolithic
IC. Even if the temperature coefficient of the external resistors
differs from that of the resistors in the monolithic IC, the precision
of temperature characteristic compensation can be maintained at
a favorable level. Further details will be described in the following
description of embodiments.
[0022] Because the present invention can configure a temperature
characteristic adjustment circuit with a single connection terminal
on the IC, a high-precision and smaller air flow meter and its IC
can be realized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is an equivalent circuit diagram of an embodiment
of the present invention.
[0024] FIG. 2 is a circuit schematic diagram of an embodiment of
prior art.
[0025] FIG. 3 is an equivalent circuit diagram of an embodiment
of the present invention.
[0026] FIG. 4 is a circuit schematic diagram of an embodiment of
the present invention.
[0027] FIG. 5 shows an example of an output control circuit that
is used in the above embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] FIG. 1 is an equivalent circuit diagram of a heating resistor
type air flowmeter according to a first embodiment of the present
invention.
[0029] In FIG. 1 a heating resistor 1 a heating current control
circuit 10 and a flow rate sensing element 2 are configured the
same as the corresponding ones in the air flow meter of the prior
art example described for FIG. 2 and their explanation is not repeated.
[0030] The air flow meter of the first embodiment is provided with
a temperature characteristic compensation circuit 40 that compensates
the characteristic of an air flow rate signal whose output value
varies depending on changes of the air flow meter circuitry-temperature,
that is, the temperature characteristic of the air flow rate signal.
[0031] The temperature characteristic compensation circuit 40 includes
a temperature characteristic voltage producing circuit (hereinafter
referred to as a "Vt circuit") 41 which produces a temperature
characteristic voltage (for example, a voltage proportional to temperature)
Vt and a proportional adjustment circuit 42 which converts the temperature
characteristic voltage (signal) Vt into a predetermined proportional
voltage.
[0032] The voltage from an internal reference voltage generator
30 that is formed by a bandgap reference voltage generator circuit
is divided by resistors 44 45. The divided voltage is input as
a proportional adjustment signal Vex to the proportional adjustment
circuit 42.
[0033] The proportional adjustment circuit 42 converts the temperature
characteristic voltage Vt into a predetermined proportional voltage,
based on the proportional adjustment signal Vex. The predetermined
proportional voltage Voff4 as a result of the conversion is added
to an air flow rate signal V2 and such an addition result signal
is input to the output control circuit 20.
[0034] A series resistance value of the resistors 44 and 45 is
set as a resistance value to make a zero offset signal Ves. The
zero offset signal Ves is input to the output control circuit 20
through a terminal Es.
[0035] The output control circuit 20 is constituted by, for example,
a differential amplifier circuit or the like. The air flow rate
signal V2 to which the proportional signal Voff4 for temperature
characteristic compensation was added and the zero offset signal
Ves are input to the output control circuit 20. FIG. 5 shows an
example of the output control circuit 20 which is formed, for example,
by an operational amplifier 201 and its associated resistors (e.g.,
voltage dividing resistors, input resistor, and feedback resistor)
202 to 205. V2 is input to a positive (+) terminal of the operational
amplifier 201 and Ves is input to a negative (-) terminal thereof
via the voltage dividing resistors 202 203 and the input resistor
204.
[0036] The output control circuit 20 outputs an air flow rate signal
Vout with temperature characteristic compensation and zero adjustment
based on these input signals.
[0037] A divided voltage Vex is used to determine the characteristic
(gradient) of the output Voff4 of the proportional adjustment circuit
42 at two-point temperatures. That is, Vex is determined by adjusting
the voltage dividing resistors 44 45 beforehand to set Voff4 suitable
for compensating (correcting) temperature-variation in the output
Vout of the air flow meter.
[0038] Here, the IC 3 comprises the heating current control circuit
10 the output control circuit 20 the internal reference voltage
generator circuit 30 the temperature characteristic voltage generating
circuit 41 and the proportional adjustment circuit 42. On the other
hand, the resistors 44 and 45 are external to the IC 3 as described
hereinbefore, and formed by thick-film resistors having a stable
temperature characteristic. The resistors 44 45 and the sensing
resistor 2 are placed together with the IC 3 on a printed circuit
board and connected to the IC 3 through their appropriate terminals.
A terminal ES is for zero adjustment voltage input and the terminal
T4 is for proportional adjustment voltage input.
[0039] The resistance values of the resistors 44 45 are determined
by, for example, laser trimming in the stage of manufacturing the
air flow meter. More specifically, prior to the laser trimming,
a known flow rate signal V2 (its dummy signal is acceptable) is
produced beforehand under an environment where the air flow meter
experiences two-point temperatures (e.g., 25.degree. C. and 85.degree.
C.). And the temperature characteristic of the output V2 is found
at the two-point temperatures. The resistance values to produce
the proportional output Voff4 and the zero adjustment voltage Ves
that can compensate the above temperature characteristic are determined
based on the above found characteristic, and the resistors 44 45
are laser trimmed to provide such resistance values.
[0040] As described above, the output (flow rate signal) Vout of
the air flow meter varies with changes in the circuit temperature.
This is due to the temperature characteristics of the resistor elements
and the internal reference voltage generator circuit which are components
of the air flow meter and, therefore, such characteristic variation
needs to be corrected (compensated).
[0041] Since the output Voff4 that is added to the air flow rate
signal V2 offsets the temperature-variation in the flow rate signal
V2 variation in the output Vout of the air flow meter due to the
temperature characteristics can be prevented.
[0042] The Vt circuit 41 generates the temperature characteristic
voltage that is expressed by Vt=a1+a2.times.T, where T is temperature
and a1 a2 are proportionality constants.
[0043] According to the first embodiment, the air flow rate signal
that was adjusted by temperature characteristic compensation and
zero compensation is output from the output control circuit 20.
[0044] Moreover, output precision of the flow rate signal of the
air flow meter can be enhanced over a wide range of temperature.
Further the temperature characteristic of the air flow rate signal
is compensated by proportional adjustment of the temperature characteristic
signal for compensation. According to the proportional adjustment,
even if resistors for compensation are integrated into the monolithic
IC 3 except for the resistors 44 45 for proportional adjustment,
current mirrors can be adopted so that the characteristic variations
of the resistors in the monolithic IC can offset each other (details
will be described for circuitry shown in FIG. 4). Therefore, adjustment
for temperature characteristic compensation in the flow meter can
be performed by providing only one additional terminal for the elements
external to the IC 3 for compensation.
[0045] A second embodiment of the present invention will be described
with an equivalent circuit diagram shown in FIG. 3.
[0046] In FIG. 3 the same reference numbers as in the above FIG.
1 denote the same or common components as found in FIG. 1. The air
flow meter configuration of the second embodiment is essentially
the same as that of the first embodiment, but as a point of difference
between both, the second embodiment is characterized in that a linearity
compensation function is added to correct nonlinearity of the air
flow rate signal V2.
[0047] For this purpose, a linearity compensation circuit 46 a
linearity offset adjustment circuit 47 and an adjusting resistor
48 for use in the adjustment circuit are added.
[0048] The linearity compensation circuit 46 is configured by a
kind of multiplier. The output (temperature characteristic voltage)
of the Vt circuit 41 is input to one input terminal of the circuit
46 and a signal for linearity offset adjustment produced by the
linearity offset adjustment circuit 47 is input to the other input
terminal thereof.
[0049] The linearity offset adjustment circuit 47 generates a signal
(Vt/R.sub.48) as a result of dividing the output Vt from the Vt
circuit 41 by the value R.sub.48 of the resistor 48. The linearity
compensation circuit 46 multiplies Vt and Vt/R.sub.48 together,
thus outputting Vt.sup.2/R.sub.48 which is a quadratic function
of temperature T as VL.sub.4.
[0050] The output Voff4 (signal for temperature characteristic
compensation) from the proportional adjustment circuit 42 the zero
offset signal V.sub.0 and the output VL.sub.4 from the linearity
offset adjustment circuit 47 are added to the flow rate signal V2
from the flow rate sensing resistor 2. Such an addition result signal
is input to the output control circuit 20. The nonlinear component
depending to temperatures T is included in the output V2. VL.sub.4
is added in a direction which offsets the nonlinear component.
[0051] An output Vout subjected to linearity compensation in addition
to temperature characteristic compensation and zero compensation
is amplified and output from the output control circuit 20.
[0052] The linearity compensation circuit 46 and the linearity
offset adjustment circuit 47 are formed within the IC 3 and its
adjusting resistor 48 is external and connected to the IC 3 through
a terminal L4.
[0053] The resistance value R48 of the resistor 48 is adjusted
by laser trimming in the process of manufacturing the air flow meter.
Prior to the laser trimming, in an environment where the air flow
meter experiences three-point temperatures (e.g., -30.degree. C.,
+25.degree. C., and 70.degree. C.), a known flow rate signal V2
(its dummy signal is acceptable) is generated beforehand. Nonlinearity
of the output V2 is observed and a linearity offset Vt/R.sub.48
is determined based on the nonlinearity. The resistor 48 is laser
trimmed to provide the resistance value R.sub.48 determined from
the linearity offset Vt/R.sub.48. The resistor 48 is formed by a
thick-film resistor having a stable temperature characteristic.
[0054] After linearity adjustment is performed, laser trimming
of the resistors 44 45 for adjustments of temperature characteristic
compensation and zero compensation of the flow rate signals at two-point
temperatures is performed, as described for FIG. 1.
[0055] According to the second embodiment, output precision of
the flow rate signal of the air flow meter can be enhanced over
a wide range of temperature. And the adjustments of temperature
characteristic compensation at two-point temperatures and the linearity
adjustment (compensation) of the flow meter are performed through
two connection terminals T4 and L4 of the IC.
[0056] FIG. 4 shows a circuitry implementation of the air flow
meter of the second embodiment (FIG. 3) of the present invention,
using transistors.
[0057] In the embodiment of FIG. 4 series resistors RM24 RM26
for signal input are connected to a positive (+) input terminal
of an operational amplifier 23 as the output control circuit 20
and series resistors RM23 RM25 are connected to a negative (-)
input terminal thereof, where RM denotes a resistor in the monolithic
IC.
[0058] A transistor Q7 and a resistor RM7 in the monolithic IC
are components of the Vt circuit (temperature characteristic voltage
generating circuit) 41 in FIG. 3. A fixed voltage Vei as a result
of dividing the reference voltage Ves from the internal reference
voltage generator circuit 30 by resistors RM1 RM2 and RM3 is input
as VBB to the base of the transistor Q7. The emitter potential VE
of the transistor Q7 has a temperature characteristic which VE varies
in an increase direction depending on increase of the temperature
in the transistor (increase of the temperature of the IC 3).
[0059] The emitter potential VE is expresses as the product of
the resistance value of the resistor RM7 and the current I7. The
VE varying depending on temperature means that the current I7 also
varies depending on temperature. By taking advantage of variation
in the current I7 that is, variation in the VE (corresponds to
the temperature characteristic voltage Vt in FIG. 3; I7=VE/RM7),
temperature characteristic compensation is performed as follows.
[0060] The current I7 flowing through the resistor RM7 is expressed,
using the voltage Vei as a result of dividing the reference voltage
Ves, as in the equation below:
I7=VE/RM7=(Vei-VBE)/RM7 (Equation 1)
[0061] The current I7 is distributed into I3 and I4 via transistors
Q3 Q4 and a current distribute circuit (current mirror circuits)
60A, 60B. A distribution ratio between the currents I3 and I4 is
determined by voltage dividing resistors 44 45 transistors Q1
Q2 constituting a current mirror circuit 61 input transistors Q11
Q12 Q13 Q14 for distribution adjustment, and resistors RM81 RM82
in the monolithic IC, and the like.
[0062] The divided voltage Vex produced by dividing the internal
reference voltage Ves by the resistors 44 45 is input to the base
of the transistor Q13 via the transistor Q11 in a constant current
circuit. On the other hand, the divided voltage Vei produced by
dividing the internal reference voltage Ves by the resistors RM1
RM2 and RM3 in the monolithic IC is input to the base of the transistor
Q14 via the transistor Q12 in the constant current circuit.
[0063] With these voltages Vei, Vex, currents I1 I2 flowing through
the transistors Q1 Q2 are expressed by equations 2 and 3.
I1=Vex/RM81 (Equation 2)
I2=Vei/RM82 (Equation 3)
[0064] When RM81 and RM82 are set to have equal resistance values,
the following is expressed:
I1/I2=Vei/Vex (Equation 4)
[0065] Difference dV.sub.BE in base-emitter voltage VBE between
the transistors Q1 and Q2 is expressed by equation 5.
dVBE=(kT/q).times.{ln/I2} (Equation 5)
[0066] where, k is a Boltzmann constant, T is temperature, Q is
charge, and ln is a logarithm.
[0067] As shown in FIG. 4 by connecting the emitter of the transistor
Q1 to the base of the transistor Q3 and connecting the emitter of
the transistor Q2 to the base of the transistor Q4 equation 6 is
fulfilled.
(kT/q)ln}I1/I2}=(kT/q).times.ln{I4/I3} (Equation 6)
Hence,
I1/I2=I4/I3 (Equation 7)
[0068] By equations 4 7 the following is obtained.
I4/I3=Vei/Vex (Equation 8)
[0069] That is, a current distribution ratio between the transistors
Q3 and Q4 is determined by adjusting the voltage Vex that is applied
through an external connection terminal T4 to the transistor Q11
in the IC 3.
[0070] One distributed current I4 is converted into a voltage through
the resistors RM24 RM26 in the monolithic IC. This voltage signal
as a result of the conversion represents a portion of VE (the emitter
potential of the transistor Q7) that corresponds to the temperature
characteristic voltage Vt and is proportionally adjusted (divided).
This proportionally adjusted signal V4 is added to the detected
flow rate signal V2 from the transistor Q16 and such an addition
result signal is input to the positive (+) terminal of the operational
amplifier 23.
[0071] The other distributed current I3 is converted into a voltage
through the resistors RM23 RM25 in the monolithic IC. This voltage
signal V3 as a result of the conversion is input to the negative
(-) terminal of the operational amplifier 23. The negative (-) terminal
of the operational amplifier 23 also takes in a voltage Vsn produced
by dividing the internal reference voltage Ves by external resistors
R18 r19 as a zero offset voltage. Moreover, the negative (-) terminal
of the operational amplifier 23 takes in a linearity offset signal
component VioL corresponding to VL4 of the flow rate signal V2
produced by converting current I6 which will be described later,
into a voltage through the resistors RM23 RM25.
[0072] The above voltages V3 V4 correspond to the output Voff4
(the signal for temperature characteristic compensation) from the
proportional adjustment circuit 42 in the embodiments of FIGS. 1
and 3.
[0073] Before explaining the linearity offset signal VioL, here,
influence of the temperature of the resistors RMn (e.g., RM7 RM23
to RM 26 etc.) in the monolithic IC on the current I7 produced
by the temperature characteristic voltage Vt will be described.
[0074] The base-emitter voltage VBE of the transistor Q7 for generating
the temperature characteristic voltage has a gradient with temperature
of about -2 mV/.degree. C. This can be approximated as in equation
9.
VBE=VBE20-0.002(T-20) (Equation 9)
[0075] where VBE20 is the base-emitter voltage of the transistor
at a normal temperature of 20.degree. C.
[0076] When the temperature coefficient of a resistor RMn in the
monolithic IC is .alpha. and the resistance value of the resistor
at 20.degree. C. is represented as RMn_20 the temperature characteristic
of the resistor is expressed as follows:
RMn=RMn_20(1+.alpha.(T-20)) (Equation 10)
[0080] Because the resistance values in the IC vary at a same ratio,
the resistance variations offset each other, as is obvious from
equation 17 and a temperature characteristic adjustment circuit
not depending on the variations in the IC is obtained.
[0081] Next, the current I6 that produces a linearity offset signal
component is explained.
[0082] A resistor 48 (with a resistance value of R48) connected
to a terminal L4 is a resistor element independent of the IC 3.
Thus, a resistor element in which resistance variation with temperature
is sufficiently smaller than that of a resistor in the monolithic
IC can be used as this resistor.
[0083] A voltage VBL produced by dividing the internal reference
voltage Ves by the resistors RM1 RM2 and RM3 in the monolithic
IC is applied to the base of a transistor Q6 connected to a current
mirror circuit 62. The current I6 flowing through the transistor
Q2 is expressed as follows:
I6=(VBL-VBE)/R48 (Equation 18)
[0084] The current I6 is caused to flow through a resistor RM23
in the monolithic IC by the current mirror 62 and a current mirror
63. Since the resistor RM23 in the monolithic IC is connected to
the negative (-) terminal of the operational amplifier 23 in addition
to the signal (voltage) corresponding to the foregoing current I3
the following signal is input to that terminal. 2 VioL = - RM23
.times. I6 ( Equation 19 ) = - RM23_ 20 ( 1 + ( T - 20 ) ) .times.
( VBL - VBE ) / R4 ( Equation 20 ) = - RM23_ 20 ( 1 + ( T - 20 )
) .times. { VBL - ( VBE_ 20 - 0.002 ( T - 20 ) ) } / R48 ( Equation
21 )
[0085] The equation 21 is a quadratic function of temperature T.
Therefore, by adjusting the resistance value of the resistor R48
connected to the terminal L4 beforehand and inputting the VioL signal
component to the negative (-) terminal of the operational amplifier
23 the linearity of the temperature characteristic can be compensated
besides the flow meter output characteristic at two-point temperatures.
However, the linearity offset is somewhat affected by resistance
variations of the resistors in the monolithic IC.
[0086] According to the present embodiment, the characteristic
of the flow meter at two-point temperatures and the linearity of
the temperature characteristic can be compensated by providing the
IC only two connection terminals T4 L4.
[0087] The present invention can be applied to various types of
sensors having a wide range of operating temperature and requiring
reduced size and high performance.
[0088] In the foregoing embodiments, current flowing through the
heating resistor is converted into a voltage, thus generating an
air flow rate signal; however, the invention is not so limited.
For instance, temperature-sensitive resistors are installed upstream
and downstream of the heating resistor and an air flow rate is sensed,
based on a temperature difference between both temperature-sensitive
resistors. Any method that can utilize the heating current control
of the heating resistor falls in the scope of application of the
present invention. |