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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor pressure converter which uses a
semiconductor, such as silicon, as a diaphragm for pressure detection and
converts pressure into electrical signal by utilizing the semiconductor
piezo-resistance effect; and, more particularly, to an improvement in the
semiconductor pressure converter which is capable of adjusting various
characteristics of the semiconductor pressure converter utilizing a
shearing type gage without mutual interferences.
2. Discussion of the Prior Art
An existing circuit for obtaining an output voltage in response to pressure
applied to semiconductor pressure detector comprises an ordinary type gage
formed on the surface of a semiconductor diaphragm, wherein resistance
value changes corresponding to applied pressure. The strain gage is formed
in a bridge structure. A constant voltage is applied to a power supply end
of the strain gage and an output voltage is obtained from an output end
corresponding to the applied pressure.
Since the ordinary type gage is formed by a semiconductor material, the
zero point changes depending on slight differences of temperature of each
ordinary type gage forming the bridge circuit.
It is known to connect resistance material having a resistance temperature
coefficient which is different from the resistance temperature coefficient
of the ordinary type gage in series or parallel to the ordinary type gage
to compensate for temperature point shift.
Some ordinary type gages do not change in a linear manner the resistance
change for increase of applied pressure and tend to show reduction of
sensitivity of resistance change in accordance with increase of pressured
applied. In this case, an output signal of a bridge circuit for applied
pressure is detected by a first amplifier and is then fed back in positive
to a second amplifier which supplies a voltage to the power supply end of
the bridge circuit. In this manner, a voltage applied to the bridge is
boosted with increase of applied pressure and reduction of sensitivity can
be prevented. Moreover, relation of output voltage to applied pressure is
linearized.
However, the circuit which compensates for temperature zero point shift, as
explained previously, further requires a resistance for zero adjustment of
unbalance of the bridge circuit which is generated when the resistance
material for compensation is connected in series or parallel to the
ordinary type gage. These resistances generate temperature variations if
these are not equivalent to the temperature coefficient of the gages. In
addition, since the temperature coefficients of different gages are
different, it is difficult to adjust the temperature coefficient.
Moreover, series and parallel connections of resistances to gages result
in the disadvantage that span is influenced and adjustment is complicated.
Moreover, the existing linear compensation circuit, wherein an output
voltage of the first amplifier is additionally applied to the second
amplifier for addition to the power supply of the bridge circuit still has
the following deficiencies. If the first amplifier contains an offset
voltage, for example, the voltage including this value is fed back and the
characteristic of the second amplifier effects direct influence on the
amount of compensation for linearization. For example, the amount of
feedback itself changes when the degree of amplification of the first
amplifier changes. Adjustment of linearity requires complicated procedures
in case the amplifier is given zero adjustment, span adjustment and
temperature compensation function, which are usually required. As
described above, if the respective adjustments are improperly done,
repeated adjustments may be required by changing many times the ambient
temperature during temperature compensation. Accordingly, adjustments
become expensive.
Thus, the prior art arrangements still leave something to be desired.
SUMMARY OF THE INVENTION
Accordingly, an object of the invention is to overcome the aforementioned
and other deficiencies and disadvantages of the prior art.
Another object is to provide a semiconductor pressure converter which
realizes high precision and high stability temperature compensation, does
not generate mutual interference even in setting of zero span adjustment
and is capable of executing linear compensation with a simplified
arrangement.
The foregoing and other objects are attained by the invention which
encompasses a shearing type gage which is formed at the strain generating
part of a semiconductor diaphragm and which outputs a voltage in
accordance with the shearing stress generated at the strain generating
part according to a measured pressure. The invention further comprises a
drive circuit which applies a drive voltage to the power supply end of the
shearing type gage, an amplifying means which amplifies an output voltage
generated at the output end of the shearing type gage, a zero adjusting
means which adjusts deviation of zero point by zero point shift of the
shearing type gage by adding a bias voltage in relation to the drive
voltage of the drive circuit to such amplifying means, a span adjusting
means which adjusts span by changing a feedback voltage of the amplifying
means and functions which can adjust various characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1(a), 1(b) and 1(c) are diagrams depicting an illustrative embodiment
of the invention.
FIG. 2 is a circuit diagram depicting the converting part, including the
sensor of FIG. 1.
FIG. 3 is a graph depicting the relationship between output voltage and
pressure applied to the shearing type gage.
FIG. 4 is a circuit diagram depicting a 2-wire type pressure converter
formed using the converting part of FIG. 2.
FIG. 5 is a circuit diagram depicting another illustrative embodiment of
the invention.
FIG. 6 is a diagram depicting the linear compensation circuit of FIG. 2.
FIG. 7 is a circuit diagram depicting a circuit for eliminating offset
voltage.
FIG. 8 is a circuit diagram depicting another embodiment for eliminating
offset voltage.
FIG. 9 is a circuit diagram depicting a circuit for generating a voltage to
adjust offset voltage from current flowing into the shearing type gage.
FIG. 10 is a circuit diagram depicting a circuit for positively or
negatively adjusting the offset voltage.
FIG. 11 is a circuit diagram depicting a circuit having a high output
impedance of the shearing type gage.
FIG. 12 is a circuit diagram depicting another impedance converting circuit
used in FIG. 11.
FIGS. 13(a), 13(b), 13(c), 13(d) and 13(e) are circuit diagrams depicting
other temperature signal generating circuits used in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to FIGS. 1(a), 1(b) and 1(c), a diaphragm 10 is formed by an
n-type silicon single crystal which includes a recessed part 11. A strain
generating part 12 is formed by a thin single crystal due to the formation
of recessed part 11 and peripheral fixing part 13 and is fixed to a
substrate 15 by an anode junction through a glass thin film 16. Strain
generating part 12 is formed as a plane (100) and shearing gage 17 is
formed thereon by diffusion of a p-type impurity in the vicinity of the
boundary of strain generating part 12 and fixing part 13 in the direction
of crystal axis <001> passing the center of plane (100). Moreover, an
ordinary type gage 18, which changes resistance when a pressure P is
applied, is also formed by diffusion of a p-type impurity in the strain
generating part in the axial direction <011> which forms an angle of
45.degree. to the axis <001>.
In FIG. 1(c), which is an enlarged view of the shearing type gage 17, gage
17 has length l and width W. The power supply ends 19,20 are formed in the
longitudinal direction of the shearing gage and a voltage is applied to
power supply ends 19,20 from a source not shown. When a pressure P is
applied to diaphragm 10, a voltage corresponding to a shearing stress
.tau..sub.s can be obtained at the output ends 22,23 formed substantially
at the center of gage length l. However, the resistance of power supply
ends 19,20 does not change due to the applied pressure P.
The ordinary type gage 24 formed in the direction along the circumference
in the periphery of strain generating part 12 (see FIG. 1(a)) reduces its
resistance value against increase of applied pressure P, contrary to the
ordinary type gage 18, and is used for linear compensation in the forward,
that is positive, direction.
FIG. 2 depicts a converting part 25 which converts pressure into a voltage
using the pressure sensor of FIG. 1. In this diagram, the resistor
R.sub.1, ordinary type gage 18, resistor R.sub.2, shearing type gage 17,
are serially connected in this sequence between the output end of
amplifier Q.sub.1 and common potential point COM. These resistors and
gages form a linear compensating circuit 26. Resistor R.sub.3 is connected
between the output end of amplifier Q.sub.1 and the inverting input end
(-) of amplifier Q.sub.1. A negative voltage -V is also applied to
inverting input end (-) of amplifier Q.sub.1 through resistor R.sub.4. The
non-inverting input end (+) of amplifier Q.sub.1 is connected to common
potential point COM. These component comprise drive circuit 27. A drive
voltage E.sub.s is obtained at the output end of amplifier Q.sub.1.
The output ends 22,23 of the shearing type gage 17 are connected to the
input end of amplifier Q.sub.2 which forms a differential amplifier,
giving an output voltage V.sub.s to differential amplifier Q.sub.2. The
inverting input end (-) of amplifier Q.sub.2 is connected to the voltage
dividing point of resistors R.sub.5, R.sub.6 connected in series between
the output end 22 and common potential point COM. The non-inverting input
end (+) of amplifier Q.sub.2 is connected to the output end 23 through
resistor R.sub.7 and is also connected to the output end of amplifier
Q.sub.2 through resistor R.sub.8. The inverting input end (-) of amplifier
Q.sub.3 is connected to the output end of amplifier Q.sub.2 through
resistor R.sub.9 and is also connected to the output end of amplifier
Q.sub.3 through resistor R.sub.10. Non-inverting input end (+) of
amplifier Q.sub.3 is connected to common potential COM. Moroever,
inverting input end (-) of amplifier Q.sub.4 is connected to the output
end of amplifier Q.sub.3 through resistor R.sub.11 and is also connected
to the output end 28 of amplifier Q.sub.4 through variable resistor
R.sub.12. The span can be changed by changing the resistance value of
variable resistor R.sub.12.
Each inverting input end (-) of amplifiers Q.sub.3, Q.sub.4 is connected
with the series circuit of resistor R.sub.13 and variable resistor
R.sub.14 and the connecting point of resistor R.sub.13 and variable
resistor R.sub.14 is connected to the connecting point of ordinary type
gage 18 and resistor R.sub.2 and a drive voltage E'.sub.s is applied
thereto. The zero point can be adjusted by adjusting variable resistance
R.sub.14. Resistor R.sub.13, variable resistor R.sub.14 and variable
resistor R.sub.12 comprise a zero span setting circuit 29.
A temperature signal generating circuit 30 is provided comprising
temperature sensor T.sub.H, variable resistors R.sub.12, R.sub.16
connected in series between the power supply designated positive voltage
+V and negative voltage -V. A diode, not shown, can also be connected in
the series circuit. A temperature signal can be obtained between the
connecting point of the variable resistors R.sub.15 and R.sub.16 and
common potential COM.
A variable resistor R.sub.17, for adjusting temperature span, is connected
between the connecting point of variable resistors R.sub.15, and R.sub.16
and the inverting input end (-) of amplifier Q.sub.1. In addition,
resistor R.sub.18 and variable resistor R.sub.19 are connected in series
between respective inverting input ends (-) of amplifiers Q.sub.3 and
Q.sub.4 and the connecting point of resistor R.sub.18 and variable
resistor R.sub.19 is connected to the connecting point of variable
resistors R.sub.15 and R.sub.16. Variable resistor R.sub.19 is used for
adjusting temperature zero point. The respective power supply ends of
amplifiers Q.sub.1 -Q.sub.4 receive the positive and negative voltages
+V,-V. Each element is respectively integrated on a silicon single crystal
such as the fixing part 13 of diaphragm 10 formed by the integrated
circuit technique.
The capacitors respectively connected in parallel to resistors R.sub.3,
R.sub.6, R.sub.8, and R.sub.10 and variable resistor R.sub.12 are provided
for noise elimination. For example, it is formed by using silicon dioxide
insulation film covering the single crystal of the fixing part 13 of
diaphragm 10.
The converting part depicted in FIG. 2 may be formed with one chip on
diaphragm 10 or may be formed by two chips separate from the sensor part.
Moreover, the converting part may also be formed as a hybrid integrated
circuit. Also, the diaphragm 10, while shown to be circular in FIG. 1, may
be of other shapes, such as a square shape.
The circuit operates as follows. Zero point adjustment, linear compensation
and span adjustment at a reference temperature t.sub.o, and then
temperature zero point adjustment and temperature span adjustment at
another temperature are accomplished as follows.
ZERO POINT ADJUSTMENT
First, temperature signal V.sub.t is adjusted to zero by adjusting variable
resistors R.sub.15 and R.sub.16 at reference temperature t.sub.o.
Next, pressure P is applied to diaphragm 10 and set to zero condition
P.sub.o. Under this condition, since offset voltage E.sub.off is generally
generated at output ends 22,23 of shearing type gage 17, the zero point
adjustment is carried out so that an output becomes 0% by flowing a
current to cancel the current produced by the offset voltage E.sub.off to
the adding point of amplifier Q.sub.4 through adjustment of variable
resistor R.sub.14 to one end of which the drive voltage E'.sub.s is
applied.
LINEAR COMPENSATION
Thereafter, a pressure P is applied to diaphragm 10 for linear compensation
and the relation between pressure P and corresponding output voltage
V.sub.s is obtained by changing pressure P.
The shearing type gage 17 does not change linearly its output voltage
V.sub.s but non-linearly for increase of applied pressure P. The output
sometimes increases or decreases more than that in the case of linear
change for the increase of applied pressure P. Ordinary type gage 18 tends
to increase its resistance value for increase of pressure P, while
ordinary type gage 24 shows an inverse trend. Thus, ordinary type gage 18
or 24 is selectively used in accordance with the pressure characteristic
of gage 17. In this case, gage 18 is selected under the supposition that
gage 17 has the characteristic in such a trend that output voltage V.sub.s
increases more than that in the linear change for increase of pressure P.
As will be explained later, the resistance of gage 18 can be changed. Thus,
the linear compensation of gage 17 can be realized by adjusting the
resistance value. Namely, an output voltage V.sub.s of gage 17 tends to
increase more than that in linear change of output for increase of
pressure P. In this case, since the resistance value of gage 18 also
increase simultaneously, voltage applied to power supply ends 19 and 20 of
gage 17 and output voltage V.sub.s of gage 17 suppresses the trend of
increase and can be linearized. Resistor R.sub.2 is used for priorly
adjusting the series resistance of resistor R.sub.2 and the resistance
between the power supply ends 19 and 20 of gage 17 to a constant value in
spite of fluctuation of gage 17 under the condition that the temperature
is the reference temperature t.sub.o and applied pressure P is zero.
The drive voltage E'.sub.s is changed by changing the resistance value of
gage 18, but this change does not influence the zero point because it
gives effect at the same rate to the current flowing into the adding point
of amplifier Q.sub.4 and zero adjusting current flowing thrugh variable
resistor R.sub.14 and resistor R.sub.13 due to the offset voltage
E.sub.off of gage 17.
Linear compensation is explained in further connection with FIG. 3. If the
characteristic curve of FIG. 3, which is the pressure and output voltage
of gage 17 before compensation, is expressed as a parabola passing three
points (P.sub.A, V.sub.A), (P.sub.A +P.sub.B, V.sub.A +V.sub.B), (P.sub.A
+P.sub.C, V.sub.A +V.sub.C), the output voltage V.sub.s can be expressed
as follows.
V.sub.s =[(P-P.sub.A)$.sub.1 /(P.sub.C P.sub.B)]+V.sub.A (1)
wherein
$.sub.1 =[(V.sub.B P.sub.C /P.sub.B)-(V.sub.C P.sub.B /P.sub.C)]-(V.sub.B
/P.sub.B -V.sub.C /P.sub.C) (P-P.sub.A).
For simplification, when P.sub.A =V.sub.A =0, equation (1) can be
transformed as
V.sub.s =[P$.sub.2 /(P.sub.C -P.sub.B)] (2)
wherein $.sub.2 =(V.sub.B P.sub.C /P.sub.B)-(V.sub.C P.sub.B
/P.sub.C)]-(V.sub.B /P.sub.B -V.sub.C /P.sub.C)P.
Equation (1) can also be transformed as
V.sub.s =P(P.sub.C V.sub.B /P.sub.B -P.sub.B V.sub.C /P.sub.C)/(P.sub.C
-P.sub.B)$.sub.3 (3)
wherein
$.sub.3 =1-[P(V.sub.B /P.sub.B -V.sub.C /P.sub.C)/(P.sub.C V.sub.B /P.sub.B
-P.sub.B V.sub.C /P.sub.C)].
Here, when a drive current flowing into gage 17 is I, resistivity of unit
thickness namely sheet resistance of gage 17 is R.sub.s, shearing
piezo-resistance coefficient is .pi..sub.1 and shearing stress working on
gage 17 is .pi..tau..sub.1, the following equation can be obtained.
V.sub.s =IR.sub.s .tau..sub.1 .pi..sub.1 (4)
Here, .tau..sub.1 is changed proportional to pressure P, if
.tau..sub.1 =K.sub.o P (wherein K.sub.o is a constant)
V.sub.s =If(P) (5)
wherein
f(P)=R.sub.s .pi..sub.1 K.sub.o P (6)
Then, refering to FIG. 3,
V.sub.B =If(P.sub.B) (7)
V.sub.C =If(P.sub.C) (8)
Therefore, equations (7) and (8) are substituted into equation (3)
V.sub.s =PI$.sub.4 [(P.sub.C f(P.sub.B)/P.sub.B -P.sub.B
f(P.sub.C)/P.sub.C)]/(P.sub.C -P.sub.B) (9)
wherein
$.sub.4 =1-[P(f(P.sub.B)/P.sub.B -f(P.sub.C)/P.sub.C)/(P.sub.C
f(P.sub.B)/P.sub.B -P.sub.B f(P.sub.C)/P.sub.C)].
In equation (9), if
##EQU1##
then equation (9) can be transformed as
V.sub.s =PIK.sub.1 (1+K.sub.2 P) (12)
and output voltage V.sub.s changes in proportion to drive current I as the
quadratic equation of pressure P.
Next, when resistance value between power supply ends 19,20 of gage 17 is
R.sub.P, resistance value of ordinary gage 17 is R.sub.n (1+K.sub.3 P)
(wherein K.sub.3 is rate of change of resistance for the pressure) and
resistance value of diffusion resistor R.sub.2 is R.sub.2 in FIG. 1, a
drive current I is expressed as follows if R.sub.1 =0 for simplification.
##EQU2##
wherein $.sub.5 =(R.sub.P +R.sub.n +R.sub.2)[1+R.sub.n K.sub.3 P/(R.sub.P
+R.sub.n +R.sub.2)].
When equation (13) is substituted in equation (12)
V.sub.s =PE.sub.s K.sub.1 (1+K.sub.2 P)/$.sub.5 (14)
In equation (14), when resistance value R.sub.n of gage 18 is selected so
that
K.sub.2 =K.sub.3 R.sub.n /(R.sub.P +R.sub.n +R.sub.2) (15)
the following equation is obtained
V.sub.s =K.sub.1 E.sub.s P/(R.sub.P +R.sub.n +R.sub.2) (16)
That is to say, the output voltage V.sub.s is proportional to the applied
pressure.
SPAN ADJUSTMENT
After linear compensation, span adjustment is carried out by applying the
specified pressure P.sub.1 to the diaphragm 10. Variable resistor R.sub.12
is adjusted so that voltage V.sub.o at output end 28 indicates 100% for
the specified pressure P.sub.1. Thereafter, an output voltage fof
amplifier Q.sub.2 is zero even when specified pressure P.sub.1 is set to
zero, giving no influence on the zero point. Moreover, no effect is
applied to the linearity adjusting result.
TEMPERATURE ZERO POINT ADJUSTMENT AT CHANGED TEMPERATURE
Next, adjustment of the zero point at a changed or specified temperature is
explained. First, temperature is changed to the specified value t.sub.1,
for example, 80.degree. C., from reference temperature t.sub.o, under the
condition that the applied pressure P is zero. In this case, an output
voltage V.sub.s changes because gage 17 has a temperature coefficient.
On the other hand, temperature voltage V.sub.t also changes to a negative
value from zero. Thus, the temperature zero shift amount is compensated so
that output becomes 0% at the specified temperature t.sub.1 by flowing a
current for cancelling a current changed by variation of output voltage
V.sub.s resulting from temperature change to the adding point of amplifier
Q.sub.4 through adjustment of variable resistor R.sub.19 to one end of
which a temperature voltage is applied. Since temperature voltage V.sub.t
is zero at the reference voltage even when temperature is lowered to the
reference temperature t.sub.o under the condition that the value of
variable resistance R.sub.1 is changed at the specified temperature, a
current is not applied or flows out from the adding point of the amplifier
Q.sub.4. Therefore, there is no influence of change in resistance value of
variable resistor R.sub.19 at the zero point at the reference temperature.
TEMPERATURE SPAN ADJUSTMENT AT CHANGED TEMPERATURE
Next, pressure P is set to the specified pressure P.sub.1 from zero at the
specified temperature t.sub.1. In this case, since gage 17 has a
temperature characteristic, an output voltage is different from that in
the reference temperature t.sub.o.
Temperature voltage V.sub.t is changed from a negative voltage to zero. The
temperature span shift amount is thus compensated so that an output
becomes 100% by flowing a compensating current to the adding point of
amplifier Q.sub.1 through adjustment of variable resistor R.sub.17 to one
end of which temperature voltage V.sub.t is applied and then changing the
drive voltage E.sub.s of the output end.
Even when pressure P is set to zero under the condition that drive voltage
E.sub.s is changed through temperature compensation of span, the offset
voltage of gage 17 and drive voltage E'.sub.s applied to variable resistor
R.sub.14 and resistor R.sub.13, change at the same rate as the drive
voltage E.sub.s. Thus, output is kept at 0% producing no interference of
the zero point.
As above discussed, the linear compensation, the zero point adjustment, the
span adjustment, the temperature zero point adjustment and the temperature
span adjustment can be realized without any interference. Moreover,
ambient temperature change is required only once to t.sub.o or t.sub.1 and
it is no longer necessary to know the value of temperature allowing
adjustment of temperature compensation. Thus, the converter can be easily
adjusted and with high precision.
FIG. 4 is a circuit diagram of a 2-wire system pressure converter formed
using converting part 25 indicated in FIG. 2. Power supply 31 is connected
to the 2-wire type transmission line l.sub.1, l.sub.2 through a load 32.
The other ends of transmission lines l.sub.1, l.sub.2 are connected in
series with diode D.sub.1, current regulating circuit CC, Zener diode
D.sub.z and diode D.sub.2 for temperature compensation and feedback
resistance R.sub.f. Moreover, the current regulating circuit CC is
connected between the collector and base of transistor Q.sub.5 providing a
constant voltage between the connecting point of diode D.sub.2 and
feedback resistance R.sub.f and the emitter of transistor Q.sub.5. This
constant voltage is divided by resistors R.sub.20 and R.sub.21 and the
voltage dividing point is connected to non-inverting input (+) of
amplifier Q.sub.6 formed as the voltage follower connected to common
potential COM at its output end, providing positive and negative voltages
+V,-V for common potential COM. The positive and negative voltages +V and
-V are used as the power supply of converting part 25 and amplifiers
Q.sub.6,Q.sub.7.
The non-inverting input end (+) of amplifier Q.sub.7 is respectively
connected to the output end 28 through resistor R.sub.22 and to one end of
feedback resistor R.sub.f through resistor R.sub.23. Voltages obtained by
dividing the voltages of output voltages V.sub.o of output 3nd 28, voltage
-V and voltage across the feedback resistor R.sub.f are applied thereto.
The inverting input end (-) of amplifier Q.sub.7 is connected to the common
potential COM through the series circuit of resistor R.sub.24 and variable
resistor R.sub.25. Both ends of resistor R.sub.24 are connected to voltage
-V through resistors R.sub.26 and R.sub.27. Voltage obtained by dividing
voltage -V with resistors R.sub.24 and R.sub.27 is applied to the
inverting input end (-) of amplifier Q.sub.7. The output of amplifier
Q.sub.7 is applied to the base of output transistor Q.sub.8 through
resistor R.sub.28. The collector of output transistor Q.sub.8 is connected
to the cathode of diode D.sub.1 and the emitter of transistor Q.sub.8 is
connected to the other end of feedback resistor R.sub.f through series
circuit of diode group D.sub.3 and resistor R.sub.29.
An output voltage V.sub.o of converting part 25 is converted to a current
output by the 2-wire system transmission lines l.sub.1 and l.sub.2 and is
then supplied to the load 32. A current output (4 mA) of the converting
part 25 when the output voltage V.sub.o is zero can be adjusted by varying
the resistance value of variable resistor R.sub.25.
Resistors R.sub.3, R.sub.4, R.sub.9, R.sub.10, R.sub.13 and R.sub.18 among
various resistors shown in FIG. 2 should be selected to eliminate
fluctuations by temperature because the amplification factor for input and
output is determined in accordance with the ratio of resistance values.
For this purpose, it is recommended to use resistors having the same
distribution of impurity concentration as that of the shearing type gage,
for example, resistors formed on the semiconductor chips with thin films,
or resistors formed on ceramic substrate with thick films.
Moroever, resistors formed of tantalum nitride (Ta.sub.2 N) having small
temperature coefficient may be used. The resistors may be fabricated , for
example, by forming respective elements by diffusion on a silicon crystal
diaphragm, covering the surface with insulation film, such as silicon
oxide (SiO.sub.2), and then sputtering tantalum nitride on the surface
thereof.
Temperature voltage, zero point, span, temperature zero and temperature
span may be adjusted with resistors R.sub.16, R.sub.15, R.sub.14,
R.sub.12, R.sub.19 and R.sub.17. These resistors allow use of variable
resistors to be provided externally and semi-fixed resistor where aluminum
wiring between taps is cut by laser or Zener diodes may be
short-circuitted among those used in the process of producing monolithic
integrated circuits. Moreover, a semi-fixed resistor by laser like a thin
film resistor for hybrid integrated circuit may also be used.
FIG. 5 depicts converting part 25 of FIG. 2 as partly changed. Power supply
ends 19 and 20 of gage 17 are connected between the output end of
amplifier Q.sub.1 and common point COM. Resistor R.sub.3 is connected
between the inverting input (-) of amplifier Q.sub.1 and the output end.
Voltage -V is applied to the inverting input end (-) of amplifier Q.sub.1
though the series circuit comprising resistor R.sub.4 and ordinary type
gate 18. A voltage proportional to the ratio of the combined resistance
value of resistor R.sub.4 and ordinary type gage 18 and the resistance
value of resistor R.sub.3 is applied to both ends of shearing type gage
17. Since ordinary type gage 18 shows the trend where resistance value
increases for increase of applied pressure P, linearization can be made by
suppressing the trend where an output voltage of amplifier Q.sub.1 is
reduced and output voltage V.sub.s of shearing type gage 17 is increased.
In this case, in converting part 25 of FIG. 2, non-linearity of gage 17 is
compensated by the rate of resistance change to applied pressure P of the
resistance value of gage 18 for the resistance of series circuit
comprising gate 18, diffusion resistance R.sub.2 and gage 17. On the other
hand, in the converting part 25A of FIG. 5, non-linearity of shearing type
gage 17 is compensated by the rate of resistance change for applied
pressure P of the resistance value of ordinary type gage 18 for the
resistance of series circuit comprising resistor R.sub.4 and gage 18.
Therefore, the converter of FIG. 5 realizes compensation which is greater
than obtained by the converter of FIG. 2.
FIG. 6 illustrates apparatus for effecting linear compensation as in FIG.
2, wherein two ordinary type gages 18, 24 are connected to aluminum wiring
33. A plurality of aluminum wirings 34, 35 are guided from each point of
the ordinary type gages 18,24. These aluminum wires are previously
connected at the other end of the guiding point and the necessary points
are cut by laser for adjusting the degree of linearity compensation. Since
ordinary type gages 18, and 24, respectively, show changes of resistance
in the reverse directions for increases of applied pressure P, these can
compensate for non-linearity in both positive and negative directions.
FIG. 7 is another embodiment which eliminates offset voltage appearing at
the output end of gage 17. Drive voltage E.sub.s of amplifier Q.sub.1 is
divided by resistor R.sub.30 and R.sub.31, and variable resistor R.sub.32.
The voltage at both ends of variable resistor R.sub.32 is applied to the
power supply end of gage 17. Moreover, a voltage at the variable
intermediate point of variable resistor R.sub.32 is applied to the
inverting input (-) of amplifier Q.sub.2 through resistor R.sub.33. In
this case, the combined resistance of resistors R.sub.33 and R.sub.8 is
set equal to resistor R.sub.6. The offset voltage contained in output
voltage V.sub.s can be eliminated by varying the variable intermediate
point of variable resistor R.sub.32.
FIG. 8 is still another embodiment which eliminates the offset voltage
appearing at the output end of gage 17. Drive voltage E.sub.s of amplifier
Q.sub.1 is applied to the bridge circuit formed by resistors R.sub.35, and
R.sub.36, and R.sub.37, and R.sub.38, and variable resistor R.sub.32
through resistor R.sub.34. The variable intermediate point of variable
resistor R.sub.32 is connected to the inverting input end (-) of amplifier
Q.sub.2 through resistor R.sub.33. The connecting point of resistors
R.sub.35 and R.sub.36 is connected to the non-inverting input end (+) of
amplifier Q.sub.2 through resistor R.sub.39. The offset voltage contained
in output voltage V.sub.s can be eliminated by varying the variable
intermediate point of variable resistor R.sub.32.
FIG. 9 shows an embodiment for generating power supply to adjust offset
voltage from current flowing into the gage 17. The power supply for
adjusting offset voltage is generated from the current. Drive voltage
E.sub.s of amplifier Q.sub.1 is applied to the series circuit comprising
resistor R.sub.40, shearing type gage 17 and resistor R.sub.41. The
voltage at both ends of resistor R.sub.4 is respectively applied to the
inverting input end (-) and non-inverting input end (+) of amplifier
Q.sub.2 through resistors R.sub.42 and R.sub.43. In this case, if the same
temperature characteristic is given to resistors R.sub.s and R.sub.41
between the power supply ends of the gage 17, fluctuation of offset
voltage for temperature can be eliminated.
FIG. 10 depicts a circuit for positively and negatively adjusting the
offset voltage. A voltage, which makes negative the value of output end of
amplifier Q.sub.2 by resistor R.sub.41, is applied to the input of
amplifier Q.sub.2 and drive voltage E.sub.s is applied to the inverting
input end (-) of amplifier Q.sub.4 through variable resistor R.sub.14 as
the offset voltage adjusting voltage. The offset voltage can be adjusted
positively or negatively by adjusting variable resistor R.sub.14.
Therefore, in this case, amplifier Q.sub.3 in FIG. 2, can be omitted.
FIG. 11 depicts a circuit wherein output impedance of the shearing type
gage is high. When output impedance of gage 17 is high, the output voltage
V.sub.s is applied to amplifier Q.sub.2 through the impedance converting
circuit 36 which has the amplifiers Q.sub.9 and Q.sub.10 operating
respectively as voltage follower at the output ends 22 and 23 of gage 17.
The offset voltage adjusting voltage is supplied from drive voltage
E.sub.s through resistor R.sub.44.
FIG. 12 is similar to the impedance converting circuit of FIG . 9 but
changed as follows. The non-inverting input (+) of amplifiers Q.sub.11 and
Q.sub.12 are respectively connected to the output ends 22, 23 of gage 17.
The inverting input ends (-) are respectively connected to the output end
resistors R.sub.45 and R.sub.46. Variable resistor R.sub.47 is connected
between inverting input ends (-) of amplifiers Q.sub.11 and Q.sub.12 and
is used to adjust the amplification.
It is also possible to adjust the offset voltage in one direction by
variable resistor R.sub.14 with resistor R.sub.13 in F | | |
