Pressure detecting apparatus having linear output characteristic

A pressure detecting apparatus which comprises a bridge circuit with semiconductor pressure elements; a constant voltage circuit for applying voltage to the bridge circuit; an amplifier circuit for amplifying the output of the bridge circuit thereby to provide an electrical signal proportional to the pressure variation; and a voltage correcting circuit connected between the input of the amplifier circuit and the input of the bridge circuit for amplifying the output of the bridge circuit to feed it back to the bridge circuit, and in which the output of the voltage correcting circuit and the output of the constant voltage circuit are applied to the bridge circuit to improve the linearity of the response characteristic or the pressure-electrical signal output characteristic.

BACKGROUND OF THE INVENTION 
This invention relates to pressure detecting apparatuses, and more 
particularly to an improved pressure detecting apparatus in which the 
linearity of the response characteristic is improved and temperature 
compensation is effected for the output voltage variation. 
A pressure detecting apparatus is known in the art for measuring a fluid 
pressure comprising a bridge circuit having semiconductor pressure 
sensitive elements adapted to convert a pressure variation into an 
electrical signal on the basis of the piezo resistance effect and a 
reference resistor providing a constant resistance independent of 
temperature variation. A constant voltage is applied to the bridge circuit 
and the output of the bridge circuit is amplified and converted into a 
current or voltage signal, whereby an electrical signal corresponding to 
the pressure variation is obtained. 
The pressure-resistance variation characteristic of the semiconductor 
pressure sensitive element itself is non-linear. Therefore, even if such a 
bridge circuit is formed to detect the pressure, the pressure-voltage 
characteristic, or the response characteristic, of the bridge circuit is 
non-linear. 
This nonlinear response characteristic is due to the error of the pressure 
sensitive element. For instance, in the case where a measuring instrument 
such as a recorder having a linear type meter is connected at the rear 
stage, measurement errors are caused. 
Accordingly in order that the pressure detection should not be affected by 
the error of the pressure sensitive element, only a part of the output 
characteristic of the pressure sensitive element, which is satisfactory in 
linearity, is employed, or the output of the bridge circuit is 
electrically corrected after being amplified. This is the conventional 
method of minimizing the effect of the error of the pressure sensitive 
element. 
However, in the first means of the conventional method, only a linear part 
of the output characteristic is utilized as was described, and therefore 
the first means is disadvantageous in that the pressure detection range is 
reduced. On the other hand, in the second means the nonlinearity can be 
corrected by signal process only in a predetermined pressure detection 
range; however, the second means is also disadvantageous in that in a 
diffrent pressure detection range the output variation width for the 
pressure detection range fluctuates thereby causing error even with the 
one and same pressure width. For this reason, in detecting a pressure in a 
different pressure range it is necessary to change the aforementioned 
electrically correcting circuit constant employed after amplification, 
which results in inconvenience in pressure detection. 
Furthermore, adjustment of the linearity of the pressure-electricity 
characteristic is made at several points in a predetermined pressure 
detection range. Therefore, it is difficult to maintain the linearity over 
the entire predetermined pressure detection range. In addition, 
maintaining the linearity is more difficult when the pressure detection 
range is changed. 
The pressure widths corresponding to certain output variation widths are 
not always coincident with one another. As a result, adjustment of the 
input to a measuring instrument connected at the rear stage becomes rather 
intricate, which leads to errors in measurement. 
The output characteristic of the bridge circuit causes a pressure detection 
error depending on temperature variations because the semiconductor 
pressure sensitive element has a resistance-temperature coefficient. In 
order to overcome this difficulty, a thermistor having a negative 
resistance-temperature coefficient is inserted between the bridge circuit 
and the constant voltage source, so that the input voltage to the bridge 
circuit is changed in association with the temperature variation, to carry 
out thereby the temperature compensation. 
However, in this conventional method, it is difficult to vary the input 
voltage in coincidence with the temperature-resistance characteristic of 
the bridge circuit, and the temperature compensation by the thermistor is 
applicable only to temperature variation in a very small range. 
It may be considered to drive the bridge circuit by a constant current 
source instead of the constant voltage source. However, in this case a 
rather intricate circuit is required for the compensation of the 
nonlinearity. Accordingly, it is difficult to put such a circuit in 
practical use in view of stability and reliability. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a pressure detecting 
apparatus which has a substantially linear output characteristic in any 
pressure detection range. 
Another object of the invention is to provide a pressure detecting 
apparatus which is capable of carrying out temperature compensation over a 
wide temperature variation range. 
Still another object of the invention is to provide a pressure detecting 
apparatus in which even if the pressure detection range is changed, the 
variation width of an output for a pressure is substantially constant if 
the width of the pressure is maintained unchanged. 
A further object of the invention is to provide a pressure detecting 
apparatus in which the pressure-electrical signal characteristic can be 
made linear entirely over a predetermined pressure detection range by 
adjusting an output electrical signal corresponding to a particular 
pressure. 
A still further object of the invention is to provide a pressure detecting 
apparatus in which the amount of temperature compensation can be varied. 
A yet further object of the invention is to provide a pressure detecting 
apparatus in which the effect to the output due to the fluctuation in 
resistance-temperature coefficient of a plurality of semiconductor 
pressure sensitive elements in a bridge circuit is eliminated. 
The foregoing object and other objects of the invention have been achieved 
by the provision of a pressure detecting apparatus which, according to the 
invention, comprises a bridge circuit having semiconductor pressure 
sensitive elements, a constant voltage circuit for applying a constant 
voltage to the bridge circuit, an amplifier circuit for amplifying the 
output of the bridge circuit, and a voltage correcting circuit connected 
between the input of the amplifier circuit and the input of the bridge 
circuit for feeding the output of the bridge circuit back to the input of 
the bridge circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, wherein like reference numerals designate 
identical or corresponding parts throughout the several views, and more 
particularly to FIG. 1 thereof, in FIG. 1, reference numerals 1 and 2 
designate semiconductor pressure-sensitive elements which utilize the 
piezo resistance effect of a semiconductor such as silicone or germanium 
as well known in the art. The semiconductor pressure sensitive elements 1 
and 2 have resistances R1 and R2. In general, in measuring the pressure of 
an examined fluid, the fluid is received in a diaphragm so that a pressure 
is exerted on a different transmitting fluid through the diaphragm, and 
the pressure of the transmitting fluid is transmitted to the semiconductor 
pressure sensitive elements to distort the latter, the resultant 
distortions being converted into electrical signals, thereby to detect the 
pressure of the examined fluid. 
These semiconductor pressure sensitive elements 1 and 2 are 
series-connected at the connection point a, while resistors 3 and 4 having 
resistances R3 and R4, respectively are series-connected at the connection 
point b. The series circuit of the pressure sensitive elements 1 and 2 and 
the series circuit of the resistors 3 and 4 are connected in parallel to 
each other at the connection points c and d to form a bridge circuit 5. An 
input voltage is applied between the points c and d of the bridge circuit 
5, while an output voltage is developed between the points a and b. More 
specifically , a constant voltage from a constant voltage circuit 6 is 
applied between the points c and d, and the output voltage provided 
between the points a and b is applied to a known differential amplifier 7 
which comprises a operational amplifier well known in the art. 
The output of the bridge circuit 5 is applied to the differential amplifier 
7 as was described above, and to a voltage correcting circuit 18, the 
output of which is fed back to the input terminal c of the bridge circuit 
5. The voltage correcting circuit 18 is made up of an operational 
amplifier and has an amplification factor G as described later. That is, 
the circuit 18 operates to amplify the output of the bridge circuit 5 
which is fed back to the input of the bridge circuit. 
The voltage signal amplified by the differential amplifier 7 is applied to 
a known operational amplifier 8, the output of which is applied to the 
base of a transistor 9 operating as an amplifier. A voltage is applied to 
the collector of the transistor 9 through a line 11 by a power supply 10. 
In general, the power supply 10 is provided at a position remote from 
measuring points fluid, while the bridge circuit 5 and so forth are 
provided in the vicinity of measuring points for measuring the pressure of 
the latter. 
The power supply 10 is connected through a conventional constant current 
circuit 12 to the constant voltage circuit 6, the differential amplifier 7 
and the operational amplifier 8. The constant current circuit 12 operates 
to apply the current from the power supply 10 to the concerned sections 
while maintaining the current to the concerned sections unchanged. That 
is, the constant current circuit 12 is provided to maintain the operating 
points of the sections unchanged. The emitter of the transistor 9 is 
connected to a bias resistor 14. A current proportional to the current 
applied to the base of the transistor 9 is allowed to flow from the 
collector of the transistor 9 to the emitter and furthermore flow through 
the bias resistor 14, a span adjusting resistor 15, an output confirming 
resistor 16 and the line 11 to the power supply 10. The span adjusting 
resistor 15 is a variable resistor, in which the resistances between 
terminals e and f and between f and g can be varied by shifting the 
terminal f which is connected to one input of the operational amplifier. 
Accordingly, as the output voltage of the differential amplifier 7 is 
changed, the base current of the transistor 9 changes so that the feedback 
voltage from the terminal f is equal to the output voltage of the 
amplifier 7. As a result, the current flowing between the collector and 
emitter of the transistor 9, namely, the output current of the latter is 
changed. Thus, the amplifier 8 and the transistor 9 form a voltage-current 
converter. 
A negative bias generating circuit 17 is connected to the differential 
amplifier 7 and the operational amplifier 8. The negative bias generating 
circuit 17 is a conventional circuit, which operates to generate a 
negative bias voltage with the aid of a current applied thereto from the 
constant current circuit 12. The negative bias generating circuit 17 is 
connected through a constant voltage diode 13 to the constant current 
circuit 12. That is, the constant voltage diode 13 is employed as a base 
for generating a reference voltage. The negative bias generating circuit 
17 is provided for generating the reference voltage. However, this circuit 
17 may be omitted if the input line, on the low voltage side, of the 
bridge circuit 5, that is, the line connecting the input terminal d to the 
power supply 10 is employed. 
As the voltage correcting circuit 18 amplifies the input applied to the 
differential amplifier 7, it can correct the output characteristic of the 
bridge circuit 5, that is, the voltage-current characteristic, as shown in 
FIG. 2. In FIG. 2, the curve l indicates a conventional pressure vs. 
voltage characteristic, while the curve m indicates a pressure vs. voltage 
characteristic corrected. The arrow is intended to indicate the fact that 
the non-linearity of the curve has been corrected. 
Accordingly, if the amplification factor G of the voltage correcting 
circuit is obtained, and the output of this circuit is applied to the 
input terminal c of the bridge circuit 5, then the relation between the 
output voltage of the bridge circuit 5 and the detected voltage is 
substantially linear over the entire pressure detection range. In 
addition, even if the pressure detection range is changed, the range of 
variation of the output with respect to the pressure can be made 
substantially constant. That is, even if the pressure-resistance output 
characteristics of the semiconductor pressure sensitive elements 1 and 2 
suffer from nonlinearity, a satisfactory linear characteristic can be 
obtained for the output, because it is fed back to the bridge circuit 5 by 
means of the voltage correcting circuit 18 before it is applied to the 
differential amplifier 7. 
Furthermore, if in the predetermined pressure detection range the output 
characteristic linearity is checked at several points, the linearity can 
be maintained over the entire pressure detection range. In addition, the 
measurement of a different pressure detection range the linearity can be 
satisfactorily maintained. 
As conducive to a full understanding of this invention, an embodiment of 
the invention shown in FIG. 3 will be described. In FIG. 3, a section 
surrounded by a broken line 20 is the constant voltage circuit 6 indicated 
in FIG. 1, whose ground line is connected to the input terminal d of the 
bridge circuit 5. Since FIG. 1 is a block diagram for a description of the 
function of the circuitry, the relationship between the constant voltage 
circuit 6 and the voltage correcting circuit 18 is as follows: In FIG. 3, 
the bridge circuit is identical to that shown in FIG. 1, and therefore it 
is designated by the same reference number as that in FIG. 1. The constant 
voltage circuit 20 has an operational amplifier 21, in which two inputs 
applied thereto are subjected to comparison, and the difference between 
the two inputs is applied, as an output, to a transistor 22. The constant 
current circuit 12 is connected through a constant current circuit, such 
as a constant current diode 23, to one input terminal of the amplifier 21, 
which input terminal is connected through a resistance element 24 to the 
terminal d of the bridge circuit 5, that is, the negative polarity side of 
the power supply 10. The resistance element 24 is provided for generating 
the reference voltage, while the constant current diode 23 is provided for 
supplying a constant current to the resistance element 24. The collector 
of the transistor 22 is connected to the constant current circuit 12, and 
the output of the operational amplifier 21 is applied to the base of the 
transistor 22. The emitter of the transistor 22 is connected to the input 
terminal c of the bridge circuit 5 to apply a voltage V.sub.IN to the 
bridge circuit 5. The emitter of the transistor 22 and the terminal d of 
the bridge circuit 5 are connected through resistors 25 and 26 to each 
other. The connection point of the resistors 25 and 26 is connected to the 
other input terminal of the operational amplifier 21. 
The division voltage input of the resistors 25 and 26 is compared with the 
reference voltage provided by the resistance element 24 in the amplifier 
21, whereby the transistor 22 is driven and the voltage V.sub.IN is 
outputted through the emitter of the transistor 22. 
The voltage correcting circuit 18 is made up of an operational amplifier 27 
and a variable resistor 28 whose one terminal is connected to the output 
of the operational amplifier 27. The other terminal is connected to the 
connection points of the resistors 25 and 26 which are connected in series 
to each other. 
It is assumed that the resistances R1 and R2 of the resistors 1 and 2 in 
the bridge circuit 5 are represented by the following equations, 
respectively: 
EQU R1=R0-.DELTA.R 
EQU R2=R0+.DELTA.R 
where R0 is the resistance obtained when no pressure is applied to the 
pressure element, and .DELTA.R is the resistance variation obtained when a 
pressure is applied thereto. 
Then, the output voltage Vout between the terminals a and b of the bridge 
circuit 5 is: 
##EQU1## 
This output voltage Vout is applied to the differential amplifier 7, and 
also to the operational amplifier 27. 
If R3=R4, then 
##EQU2## 
EQU Therefore, V.sub.IN =(2R0/.DELTA.R)Vout (1) 
EQU or, V.sub.IN =V.sub.REF +(I25)(R25) (2) 
where V.sub.REF is the reference voltage across the resistance element 24, 
I25 is the current flowing in the resistor 25, and R25 is the resistance 
of the resistor 25. 
The current I25 is the sum of a current I250 which flows in the resistor 25 
when no pressure is applied to the pressure sensitive elements 
(.DELTA.R=0), and a current I28 flowing in the resistor 28. Therefore, 
EQU I25=I250+I28 (3) 
If an input voltage to the bridge circuit 5 obtained when no pressure is 
applied to the pressure sensitive elements is represented by V.sub.INO, 
then 
EQU V.sub.INO =V.sub.REF +(I250)(R25) (4) 
From Equations (3) and (4), Equation (2) can be rewritten as follows: 
EQU V.sub.IN =V.sub.INO +(I28)(R25) (5) 
The current I28 flowing in the variable resistor 28 can be expressed as 
follows: 
##EQU3## 
where V27 is the output of the operational amplifier 27, and R28 is the 
resistance of the resistor 28. 
If in the case of no pressure application, adjustment is made so that 
V27=V.sub.REF, then V27=V.sub.REF -(1+G) Vout, where G is the 
amplification factor of the amplifier 27. 
##EQU4## 
Then, Equation (5) can be rewritten as follows: 
##EQU5## 
Based on Equation (7), Equation (2) can be expressed as follows: 
##EQU6## 
In this equation, (.DELTA.R/R).sup.2 is substantially zero (0), and 
##EQU7## 
According to experiment, .DELTA.R can be approximated in a quadratic 
function; that is, .DELTA.R=AP.sup.2 +BP+C (where A, B and C are the 
constants). A is close to 0 (zero). Therefore, the approximation is 
.DELTA.R.sup.2 =(BP+C).sup.2. 
##EQU8## 
Accordingly, if the amplification factor of the amplifier 27 is adjusted so 
that K=-(2R0/B.sup.2)A, a substantially linear pressure-voltage 
characteristic is obtained for the bridge circuit 5. 
Thus, in the case of the embodiment shown in FIG. 3, the output 
characteristic can be made linear over the entire pressure detection 
range. Therefore, even if a different pressure detection range is measured 
with the same pressure variation width, a coincident output characteristic 
is obtained. Since the output characteristic is linear as was described, 
linearity can be maintained over the entire pressure detection range by 
checking several points thereof. A constant pressure variation width is 
obtained for a constant pressure variation width, and therefore detection 
can be achieved with a measuring instrument with accuracy, which is 
connected at the rear stage. Linearity between the pressure to be measured 
and the output electrical signal provides the advantage that despite the 
use of indicating instruments having a single scale (or graduation), a 
change of full scale value thereof can be made. Thus the error in reading 
the instruments can be easily reduced by employing a multiple full scale 
arrangement. 
Shown in FIG. 4 is another embodiment of this invention, in which one input 
of the operational amplifier in the voltage correcting circuit is obtained 
from a different resistance type voltage divider circuit. In FIG. 4, those 
components which have been previously described with reference to FIG. 3 
have been similarly numbered. The resistance-type voltage divider circuit 
is made up of a resistor 30, a variable resistor 31 and a resistor 32 
which are connected in series, and the two end terminals of the voltage 
divider circuit are connected to the terminals c and d of the bridge 
circuit 5. One input of the amplifier 27 is obtained through the variable 
resistor 31. Reference numeral 33 designates a diode. 
The operation of the circuitry shown in FIG. 4 is similar to that of the 
circuitry shown in FIG. 3. However, it should be noted that since the 
input of the operational amplifier 27 is obtained through the 
resistance-type voltage divider circuit, the non-coincidence between the 
resistances of the semiconductor pressure sensitive elements can be 
corrected, that is, the fluctuation in value of the semiconductor pressure 
sensitive element can be corrected. 
FIG. 5 shows one example of a constant voltage circuit which is obtained by 
replacing the resistance element 24 shown in FIGS. 3 and 4 by a resistance 
element having a resistance-temperature coefficient equal to or higher 
than the resistance-temperature coefficient of the semiconductor pressure 
sensitive element. In the circuitry shown in FIG. 5, the voltage 
correcting circuit has been omitted from the drawing for the purpose of 
simplicity, and those components which have been previously described with 
reference to FIG. 3 are designated by similar reference characters. 
The resistances R1 and R2 of the semiconductor pressure sensitive elements 
1 and 2 can be expressed as follows when the temperature variation 
components thereof are taken into consideration: 
EQU R1=R10 (1+.alpha.1)-.DELTA.R10(1+.beta..sub.1 t) 
EQU R2=R20(1+.alpha..sub.2)+.DELTA.R20(1+.beta..sub.2 t) 
where R10 and R20 are the resistances at a temperature of 0.degree. C. and 
at a pressure of zero, R10 and R20 are the resistances which vary upon 
application of a pressure at a temperature of 0.degree. C., and .alpha.1, 
.alpha.2, .beta.1 and .beta.2 are the resistance-temperature coefficients 
thereof. 
If .alpha.1=.alpha.2=.alpha., .beta.1=.beta.2=.beta., and 
.DELTA.R10=.DELTA.R20=.DELTA.R, 
##EQU9## 
And if the pressure is changed from 0 (zero) to .DELTA.R, that is, from 
.DELTA.R=0 to .DELTA.R=.DELTA.R, 
##EQU10## 
This equation indicates a temperature effect in an output voltage 
variation corresponding to a pressure variation. 
In general, .beta. is a negative temperature coefficient, but it can be 
made to be substantially zero by suitably manufacturing the pressure 
sensitive element. However, .alpha. is the value which is determined by 
the material of the semiconductor pressure sensitive element. Therefore, 
it can be understood that if the temperature becomes higher than that in 
this equation, the output voltage is decreased. 
Accordingly, in this embodiment, the input voltage to the bridge circuit is 
varied in accordance with the temperature so that the output voltage is 
not affected by the temperature. For this purpose, in the embodiment, the 
temperature coefficient of the resistance element 30 providing the 
reference voltage is set to (1+.alpha.t)/(1+.beta.t). 
If it is assumed that the reference voltage provided by the resistance 
element 30 is represented by V.sub.REF and the resistance of the resistor 
26 is represented by R26, then the following relation can be obtained: 
##EQU11## 
Thus, the temperature compensation can be effected if the resistance 
element 30 has the temperature coefficient (1+.alpha.t)/(1+.beta.t). 
The resistance temperature coefficients .alpha. and .beta. may be not 
always equal to those of the semiconductor pressure sensitive elements. 
Since .beta. is, in general, close to zero (0), all that is necessary is 
that .alpha. is substantially equal to that of the pressure sensitive 
element, and especially in the case when .beta.&lt;0 is significant, the 
resistance temperature coefficient can be made higher than that of the 
pressure sensitive element. 
Silicon single crystal is employed as the material of the semiconductor 
pressure sensitive element, the resistance-temperature coefficient of 
which is of the order of 2000-2500 ppm. Therefore, metallic materials such 
as Nl and Cu having a relatively linear temperature-resistance 
characteristic may be employed as the material of the pressure sensitive 
element. In this case, even if .beta.&lt;0, a satisfactory temperature 
compensation can be obtained over a wide temperature variation range. 
Furthermore, the output voltage variation is not affected by the 
temperature. 
In addition, if a variable resistor (shown in dotted line in FIG. 5.) is 
connected in parallel to the resistance element 30 to vary the amount of 
temperature compensation, the temperature compensation can be considerably 
improved. In this case, the variable resistor may be one whose 
resistance-temperature coefficient is low. 
Obviously, numerous additional modifications and variations of the present 
invention are possible in light of the above teachings. It is therefore to 
be understood that within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically described herein.