JP2002365319A - Detection circuit for minute resistance change - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a detection circuit for minute resistance change, capable of obtaining similar performance, while minimizing precision and stable parts by bringing the detection of minute resistance changes closer to the measurement principle of resistance value. SOLUTION: This circuit comprises two resistance sensors 2a and 2b, having the electrically same characteristic and connected in series, an amplifier circuit 3 for adding the voltage Va, generated between the connecting point C of the resistance sensors 2a and 2b and one end A to a voltage Vb, generated between the connecting point C and the other end B followed by amplifying, and a current source 4 for making a prescribed constant current I flow to the two sensors 2a and 2b, in a state of insulation, to the amplifier circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a micro-flow sensor used in an intermittent optical analyzer represented by an infrared-based analyzer, for example, in which a change in a resistance value is a detection signal. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit for detecting a small resistance change of a resistance sensor and, more particularly, to a circuit for detecting a small resistance change which improves the detection performance.

[0002]

2. Description of the Related Art FIGS. 4 and 5 show a general configuration of a circuit for detecting a small resistance change used in an infrared gas analyzer. First, FIG. 4 is an overall view schematically showing a configuration of a conventional infrared gas analyzer 20 using a detection circuit for a small resistance change, which is of a so-called single beam (single cell) type.

[0003] The infrared gas analyzer 20 is configured as follows. That is, in FIG. 4, reference numeral 21 denotes a cylindrical measuring cell, both ends of which are sealed by cell windows 22 and 23 made of an infrared transmitting material, and an inlet 24 and an outlet 25 of the sample gas S. Are formed.
Reference numeral 26 denotes an infrared light source provided to face one of the cell windows 22. An infrared IR emitted from the infrared light source 26 is a light chopper 27 interposed between the infrared light source 26 and the cell window 22.
Is converted into intermittent light of a predetermined frequency.

Numeral 28 denotes a detector provided opposite to the cell window 23. Both ends of a main body block 29 made of a corrosion-resistant metal are sealed by windows 30 and 31 made of an infrared transmitting material. The inside is divided into two gas chambers 33 and 34 by a window 32 made of an infrared transmitting material, and these gas chambers 33 and 34 are connected to an infrared light path (arrow IR).
(Indicated by) are arranged with respect to the measurement cell 21 so as to be sequentially connected in series. Note that the window 31 does not need to be a transmission window, and may be formed of the same material as the main body block 29 as a sealing body.

The gas chambers 33 and 34 are filled with a gas (or a gas to be measured) G having the same absorption characteristics as the gas to be measured. 35 is a gas chamber 33, 34
And gas passages 35a and 35b are gas passages 35.
Are open to the gas chambers 33 and 34 of FIG. The gas passage 35 has a slightly expanded space 35c, and a pair of resistance sensors 36 as a flow sensor for detecting the flow rate of the gas G flowing through the gas passage 35 in the space 35c. Voltage is always applied.

That is, the resistance sensor pair 36 is composed of resistance sensors 36a and 36b, which are connected to each other at one end and to the bias resistance 3 at the other end.
7a and 37b, and the other ends of the bias resistors 37a and 37b are connected.
A bridge circuit 38 as shown in FIG. 5 is configured. Then, one end of the resistance sensor pair 36 (the resistance sensor 36
a, 36b are connected to a common potential.

Reference numeral 39 denotes a bridge power supply.
Is applied as a bridge voltage. That is, a bridge voltage is applied to the bridge circuit 38 including the resistance sensor pair 36, and the bridge circuit 38 becomes a measurement circuit. 40
Is a subtraction circuit for detecting the difference between the voltages at the other ends of the pair of resistance sensors 36a and 36b to obtain an output signal O from the bridge circuit 38.

The basic operation of the detector 28 having the above configuration will be described. It is assumed that the same gas (for example, CO ₂ ) as the object to be measured is sealed in the gas chambers 34 and 35.
In the above state, while supplying the sample gas S containing CO ₂ to the measurement cell 21,
When R is irradiated on the measurement cell 21, the infrared rays IR are absorbed by CO ₂ in the sample gas S, and then enter the gas chamber 33 of the detector 28 on the measurement cell 2 side. Gas G sealed in this gas chamber 33 (in this case, CO ₂ )
Absorbs a part of the infrared IR and expands in temperature. The expanded gas G enters the gas passage 35 through the opening 35a, passes through the space 35c, and passes through the opening 35b into the gas chamber 3.
Flow into 4.

At this time, heat is taken from the resistance sensor 36a on the upstream side of the flow in the resistance sensor pair 36 in accordance with the flow rate (flow rate) of the gas G flowing in the gas passage 35. For this reason, the resistance sensor 36a is cooled according to the flow rate of the gas G, and its temperature is lower than when there is no wind. On the other hand, the gas G is heated by the heat taken, and the heated gas G warms the resistance sensor 36b on the downstream side of the flow. Since the electric resistance of the resistance sensor 36a changes at a constant rate due to a change in temperature, the change in the resistance causes a difference in the voltage at the other end of the pair of resistance sensors 36a and 36b. The output signal O is output.

As described above, the gas chamber 33 of the detector 28,
The degree of expansion of the gas G sealed in 34 differs, and the gas flow rate when the flow of the gas G occurs in the gas passage 35 is measured. Since the measured value has a certain relationship with the concentration of the measurement target component (in this case, CO ₂ ) contained in the sample gas S supplied to the measurement cell 21 of the infrared gas analyzer 1, the measurement target The concentration of the component will be measured.

Here, in the circuit shown in FIG. 5, the voltage of the bridge power supply 39 is E [V], and the resistance sensors 36a, 36
b is Rs ₁ , Rs ₂ [Ω], and the bias resistor 37
Assuming that the resistance values of a and 37b are Rb [Ω], the detection signal Δe [V] input to the arithmetic circuit 40 in this circuit is as shown in Expression (1).

The resistances Rs ₁ and Rs ₂ of the sensor resistors 36 a and 36 b decrease in temperature due to heat radiation due to a minute gas flow due to a pressure difference due to infrared light absorption in the detector, and the resistances Rs ₁ and Rs ₂ decrease. Since there is a slight change, the change in the resistance values Rs ₁ and Rs ₂ is detected and amplified, and is output as a measured value. The change in the resistance values Rs ₁ and Rs ₂ is caused by the infrared light IR
The change of the AC voltage difference Δe is output due to the intermittent operation.

[0013]

However, in the above-described measuring method, another bias resistor 37 is connected in series to the resistor 36 to be measured, and a predetermined voltage is applied to these combined resistors. To measure the voltage divided by. In other words, it is far from the principle of resistance measurement, which causes various problems.

For example, as can be seen from the above equation (1),
The voltage difference Δe, that is, the stability of the output signal O is obtained, and the resistance value R
It can be seen that in order to obtain sufficient sensitivity due to the change in s (Rs ₁ , Rs ₂ ), it is necessary to obtain a stable supply of the voltage E from each of the resistance values Rs, Rb and the voltage source 39 supplied by the power supply 39. However, the voltage supplied from the voltage source 39 to the bridge 38 is not limited to the voltage drop due to not only the internal resistance on the power supply side but also the resistance Re which causes an error such as a wire resistance and a contact resistance existing in the wiring. Without
This caused measurement errors.

Further, in order to cause the resistance sensor 36 to generate heat and thereby raise the temperature, it is necessary to supply a constant power W ₀ as shown in the following equation (2). Therefore, when adjusting the power supplied to the resistance sensor 36, it is necessary to adjust the resistance value Rb of the bias resistor 37 accordingly, and it has been difficult to uniquely determine each constant.

In other words, the above equation (2) indicates that in order to supply more power to the resistance sensor 36, it is necessary to reduce the resistance value Rb of the bias resistor 37, and the above equation (1) ) Indicates that the resistance value Rb of the bias resistor 37 needs to be increased to some extent in order to obtain a stable output. Since these are contradictory, in the prior art, the resistance value Rb of the bias resistor 37 needs to be examined from both aspects of the improvement of the detection sensitivity and the magnitude of the power for supplying power to the resistance sensor 36. In addition, it was difficult to respond to a change in the specification of the resistance sensor 36 (resistance value, heat generation amount, etc.).

In the above-described conventional method, the bias resistor 3 is provided on a circuit connected from the power source 39 to the resistance sensor 36.
7, it is necessary to supply a certain amount of current to the bias resistor 37 in order to generate heat by the resistance sensor 36. Therefore, the performance of the bias resistor 37 greatly affects the detection performance. That is, it is necessary to select a high-precision, high-stability resistor having a constant resistance value regardless of a temperature change due to its own heat generation, and the price of this resistor must be increased. Furthermore, since power is also consumed by the bias resistor 37, there is a problem that energy waste must be increased.

In addition, when there is a voltage drop due to wiring resistance or the like between the power supply 39 and the bridge circuit, a change in the resistance values Rs ₁ and Rs ₂ of one of the resistance sensors 36a and 36b causes the other resistance sensors 36b and 36a to change. This may affect the flowing current, which may adversely affect the measurement result.

Further, the measurement results are transmitted to the resistance sensors 36a and 36a.
6b, it is necessary to form a relatively complicated subtraction circuit in the operational amplifier. Since the subtraction circuit 40 has both input terminals of the operational amplifier connected to a measurement point far from the ground point, the subtraction circuit 40 is liable to be affected by disturbance and needs to use a special amplifier for instrumentation. there were.

The present invention has been made in consideration of the above-described circumstances, and has as its object to provide a highly accurate and highly stable component by making the detection of a small resistance change closer to the principle of measuring a resistance value. An object of the present invention is to provide a small resistance change detection circuit capable of obtaining the same performance at a minimum.

[0021]

In order to achieve the above object, a detection circuit for detecting a small resistance change according to the present invention comprises two resistance sensors having the same electrical characteristics and connected in series, and An amplifier circuit for adding a voltage generated between the connection point and one end to a voltage generated between the connection point and the other end to amplify the voltage; and a predetermined constant value for two resistance sensors insulated from the amplifier circuit. And a current source through which a current flows. (Claim 1)

That is, the detection circuit for detecting a small resistance change is a state in which a constant current is applied to the two resistance sensors.
Since the voltage generated at both ends is measured, the measurement principle of the resistance value is applied. Therefore, the resistance value of the resistance sensor can be obtained with extremely high accuracy irrespective of the magnitude of the contact resistance. In addition, since a bias current is not required as in the prior art, and a constant current is applied only to the resistance sensor to detect the resistance value, energy is saved, and generation of undesired heat for an electric circuit is suppressed as much as possible. be able to. And
It is necessary to select a bias resistor with high precision as before,
There is no need to carefully design the resistance value. In other words, the circuit can be designed to flexibly cope with a change in specifications such as the resistance value of the resistance sensor and the calorific value.

Further, since the current source is insulated from the amplifier circuit, the current of a constant flow rate flowing from the current source to one end of the first resistance sensor flows through the second resistance sensor. Spill from the other end. Therefore, even if a change occurs in the resistance value of the resistance sensor, the current value flowing through the resistance sensor does not change, and the change in the resistance value of the resistance sensor can be reliably detected without being affected by interference as in the related art. it can. Furthermore, since the voltage generated between the connection point of these resistance sensors and the other end is a voltage opposite to the voltage generated between the connection point of the resistance sensors and one end, simply adding both voltages, the both resistance sensors Can be determined very easily.

The amplifier circuit has the same electrical characteristics, a sufficiently large resistance value as compared with the resistance value of the resistance sensor, and connects between one end and the other end of the resistance sensor. In the case of an addition circuit that inputs two resistors connected in series and the connection point of the resistors to the inverting input terminal of the operational amplifier and negatively feedbacks the output, there is no need to use a special instrumentation amplifier. It is possible to use an operational amplifier generally configured to have high precision for forming the adder circuit, so that manufacturing costs can be reduced. Further, the non-inverting input terminal of the operational amplifier can be grounded together with the connection point of the resistance sensor, and the measured value can be further stabilized.

The amplifying circuits are provided at one end and the other end of the resistance sensor, respectively, and are connected in series with two amplifiers having an amplification factor of 1 so as to electrically connect the outputs of the two amplifiers with the same characteristics. In the case of an adder circuit that inputs the two resistors and the connection point of the resistors to the inverting input terminal of the operational amplifier and negatively feedbacks the output, the current output from the current source does not shunt at all. Since it surely flows into both resistance sensors, it is possible to achieve further improvement in accuracy.

In the case where the current source is a converter that obtains power from the power supply of the amplifier circuit and outputs a predetermined constant current while maintaining insulation from the power supply of the amplifier circuit, a detection circuit for detecting a small resistance change Power supply as a whole. In recent years, I / O insulated DC-DC
Since the converter can be mounted relatively easily, by combining this input / output isolated DC-DC converter with a constant current circuit, a predetermined constant current is output in a state where insulation is maintained with respect to the power supply of the amplifier circuit. A converter can be formed.

[0027]

1 and 2 show a first embodiment of the present invention. FIG. 1 shows a detector of an infrared gas analyzer for implementing a detection circuit 1 for detecting a small resistance change according to the present invention. It is a figure showing composition of a formed part. FIG. 2 is a diagram illustrating only the circuit configuration in detail. In FIG. 1, portions denoted by the same reference numerals as those in FIGS. 4 and 5 are the same or equivalent portions, and thus detailed description thereof will be omitted.

In FIGS. 1 and 2, reference numerals 2a and 2b denote a pair of resistance sensors disposed on the upstream and downstream sides of a flow of a gas G, which is an example of a fluid, respectively, and A and B denote a pair of resistance sensors 2 (2a , 2b), C is a connection point between the two resistance sensors 2a, 2b, and 3 is an amplifier for adding and amplifying a voltage generated at one end A to a voltage generated at the other end B based on the potential of the connection point C as a reference. The circuit 4 is a current source for supplying a predetermined constant current to the two resistance sensors 2a and 2b while being insulated from the amplifier circuit 3. The DC source 5 supplies power to the electric circuits 3 and 4. Power supply.

The resistance sensors 2a and 2b have exactly the same characteristics and generate heat when a current flows, and a heating wire as an example of a resistor having resistance values Rsa and Rsb in a predetermined relation to heat. Element. This resistance sensor 2
Since a and 2b are directly exposed to the sealing gas G, they are made of, for example, a metal having excellent corrosion resistance and a large temperature coefficient, such as platinum and nickel. The connection point C is grounded so that the potential is stabilized.

The amplifying circuit 3 has, for example, the same electrical characteristics and the resistance values Rsa and Rsb of the resistance sensors 2a and 2b.
It has a sufficiently large resistance value R ₁ as compared with, and the two resistors 6a connected in series so as to connect between the end A and end B of the resistor sensor 2, 6b and this resistance 6a, 6
operational amplifier 7 for inputting the connection point D of b to the inverting input terminal
And a resistor 8 for negatively feeding back the output of the operational amplifier 7 to the inverting input terminal.

Since the voltage Va at one end A with respect to the connection point C of the resistance sensors 2a and 2b and the voltage Vb at the other end B with respect to the connection point C are voltages of opposite polarities, the addition circuit 3 converts both voltages Va and Vb. The subtraction of the voltage drops Va and -Vb caused by the resistance sensors 2a and 2b can be performed only by the addition. The type of the operational amplifier 7 used in the adder circuit 3 is not limited as long as it is generally a high-precision operational amplifier. On the other hand, the stability of the output is improved.

The current source 4 is a converter that obtains power from the power supply 5 of the amplifier circuit 3 and outputs a predetermined constant current while maintaining insulation from the power supply 5 of the amplifier circuit 3. Input / output isolated DC-DC converter (DC
(DC converter) 9 and a constant current generating circuit 10 for generating a predetermined constant current I.

That is, in the detection circuit 1 for detecting a small resistance change of the present invention, the constant current I is supplied to the resistance sensors 2a and 2b in a state where the current source 4 is insulated from the power supply 5,
Even if the connection point C is grounded, the current flowing out of the current source 4 does not pass through another loop and the resistance sensor 2
a, 2b and surely returns to the current source 4.

In this embodiment, the resistance sensors 2a,
Since the resistors 6a and 6b are connected to one end A and the other end B of the resistor 2b, strictly speaking, the currents Isa and Isb flowing through the resistance sensors 2a and 2b and the currents I ₁ and S1 divided by the resistors 6a and 6b, respectively. I ₂ is represented by the following formulas (3) to (6)
It becomes as shown in.

[0035] However, the resistor 6a, the resistance value R ₁ of 6b, resistive sensors 2a, 2b of the resistance Rsa, Rs
By making it sufficiently larger than b, the effect can be made negligibly small. For example the resistance value R ₁ of the resistance Rsa, by such a magnitude of more than 1000 times that Rsb, resistor 6a, the current I ₁ which shunted to 6b, I ₂ is about one thousandth of the total current I And Isa, Isb ≒ I,
The effect can be ignored.

Further, the power W ₀ applied to each of the resistance sensors 2a and 2b (this is related to the amount of generated heat) is given by the following equation (7).
It becomes what is shown in. That is, the heating value is one of the resistance sensor 2a, since it is proportional to the square of the current flowing through the 2b of the resistance value Rs (Rsa or Rsb) Is (Isa or Isb), the resistance value the resistance value R ₁ Rsa, Rsb If the resistance Rsa and Rsb are selected to be sufficiently large as compared with, the influence of the resistance values Rsa and Rsb on the heat generation can be reduced to a negligible level. W ₀ = Is ² × Rs Equation (7)

[0037] That is, resistor 6a, only a sufficiently large resistance value R ₁ of 6b, both resistive sensors 2a, 2b of the resistance Rsa, regardless of the size of the Rsb, the constant current I ho Isuzu the total amount both Since the current can flow through the resistance sensors 2a and 2b, interference between the resistance sensors 2 due to fluctuations in the resistance values Rsa and Rsb hardly occurs. In addition, even if the resistance Re, which is an error factor such as a contact resistance or a wire resistance, exists on a circuit for supplying current to the resistance sensors 2a and 2b, this does not adversely affect the measured value.

In the configuration of the micro-resistance change detecting circuit 1 according to the present invention, the resistors 6a and 6b are connected to the resistance sensors 2a and 2b.
Since it is not formed on a circuit for supplying power for heat generation due to the above, it is not necessary for a large current to flow through the resistors 6a and 6b, but rather it is desirable to minimize the current flowing therethrough. Therefore, the resistance 6a, the resistance value R ₁ of 6b resistance sensor Rsa, sufficiently large compared to Rsb, and resistor 6a, the current flowing to 6b as small as possible.

The smaller the current flowing through the resistors 6a and 6b, the more the heat generated by the resistors 6a and 6b can be suppressed. This contributes to energy saving and the suppression of unnecessary heat generation leads to a stable operation of the circuit. In other words, the resistors 6a and 6b do not require stability and accuracy against self-heating as in the conventional bias resistor Rb.
It is possible to form a highly accurate detection circuit for a small resistance change with less expensive parts.

Here, when the resistance value of the resistor 8 is R ₂ , the voltage E output as the output signal O is as shown in the equation (8). The resistance R ₁ is the resistance value Rsa, Rsb
I ₁ ≒ I ₂ ≒ I
When the currents I ₁ and I ₂ are represented by resistance values R ₁ , Rsa and Rsb, the result is as follows.

Further, in the above equation (8), assuming that a slight change in the resistance value δr occurs in one of the resistance values Rsa, the change amount δe of the voltage E output as the output signal O is expressed by the following equation (9). Can be represented.

Therefore, from the equation (9), the detection sensitivity can be increased by increasing the current I and by increasing the gain of the amplifier circuit 3 determined by R ₂ / R ₁ . Further, even when the amount of heat is increased by increasing the current I, the resistances 6a and 6
there is no need to adjust the resistance value R ₁ and other circuit constants b. In other words, it is possible to very easily cope with a change in the specifications such as the resistance values and the calorific values of the resistance sensors 2a and 2b.

FIG. 3 shows a detection circuit 1 for detecting a small resistance change according to the present invention.
It is a figure which shows the modification of. In FIG. 3, members denoted by the same reference numerals as those in FIG. 2 are the same or equivalent members, and therefore, detailed description thereof and redundant description of effects obtained by common portions will be avoided.

In FIG. 3, reference numeral 4 'denotes a current source having a completely independent voltage source 11 insulated from the power supply 5, and 3' denotes an amplification degree for monitoring voltages at one end A and the other end B of the resistance sensor 2. Are two amplifiers 12a and 12b
Is an amplifying circuit having:

Since the voltage source 11 'is a power source completely independent of the power source 5, the constant current I output from the constant current generating circuit 10 which converts this power into a stable current and outputs the current is two resistance sensors. It is constituted so that it may return through 2a and 2b reliably.

If an amplifier having a very high input impedance and a low bias current is selected for the amplifiers 12a and 12b, the provision of the amplifiers 12a and 12b causes a shunt or a merge at the one end A and the other end B. Things can be virtually non-existent. That is, since the entire amount of the constant current I output from the current source 4 'flows through the two resistance sensors 2a and 2b, a small resistance change can be detected by a method closer to the basic principle of resistance measurement. Stable measurement can be performed.

[0047]

As described above, by using the circuit for detecting a small resistance change of the present invention, it is possible to accurately detect a small change in the resistance value of the resistance sensor by a method closer to the principle of measuring the resistance value. As a result, it is possible to perform very stable detection of a minute resistance change while contributing to energy saving.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a detector using a detection circuit for detecting a small resistance change according to the present invention.

FIG. 2 is a diagram showing a circuit configuration of the detection circuit for detecting a small resistance change.

FIG. 3 is a diagram showing a modified example of the detection circuit for detecting a small resistance change.

FIG. 4 is a diagram showing an example of a conventional infrared gas analyzer using a detection circuit for a small resistance change.

FIG. 5 is a diagram showing a circuit configuration of a conventional small resistance change detection circuit.

[Explanation of symbols]

1: detection circuit for minute resistance change, 2 (2a, 2b): resistance sensor, 3, 3 '... amplification circuit (addition circuit), 4, 4' ...
Current source (converter), 5: power supply, 6a, 6b: resistor,
7 ... operational amplifier, 9 ... DC-DC converter, 12a,
12b: amplifier, A: one end, B: other end, C: connection point, D
... connection point, I ... constant current, Rsa, Rsb ... resistance value of the resistance sensor, R ₁ ... resistance, Va, Vb ... voltage.

Claims (4)

[Claims] 1. Two resistance sensors having the same electrical characteristics and connected in series, and a voltage generated between a connection point and one end of the resistance sensor is changed to a voltage generated between the connection point and the other end. A small resistance change detection circuit, comprising: an amplification circuit that adds and amplifies the current; and a current source that supplies a predetermined constant current to two resistance sensors while being insulated from the amplification circuit. 2. The amplifying circuit according to claim 1, wherein said amplifying circuit has electrically the same characteristics,
It has a resistance value sufficiently larger than the resistance value of the resistance sensor, and two resistances connected in series so as to connect between one end and the other end of the resistance sensor, and a connection point of this resistance. 2. The detecting circuit according to claim 1, wherein the adding circuit inputs the inverted input terminal of the operational amplifier and negatively feedbacks the output. 3. The amplifier circuit is provided at one end and the other end of the resistance sensor, respectively, so as to connect two amplifiers having an amplification factor of 1 with the same electrical characteristics and the outputs of both amplifiers. 2. The detection circuit according to claim 1, wherein the addition circuit inputs two resistors connected in series and a connection point of the resistors to an inverting input terminal of the operational amplifier and negatively feedbacks the output. 4. The converter according to claim 1, wherein the current source obtains power from a power supply of the amplifier circuit, and outputs a predetermined constant current in a state where the power supply is insulated from the power supply of the amplifier circuit.
3. The detection circuit for detecting a small resistance change according to any one of 3.