JPH11174012A - Odor measurement device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent odor measurement accuracy from being deteriorated due to drift of resistance or the like of an odor sensor. SOLUTION: A sensor 26 is connected in series to a variable resistance 41 so as to form a half bridge. Before odor measurement, a calibration part 46 instructs to switch over the resistance value of the variable resistance 41 sequentially, searches a resistance value Rs at which a voltage Vo at a connection point 42 is nearest 1/2 Vd, and fixes the variable resistance 41. When odor is measured, a calculation processing part 45 calculates the resistance value R's of the sensor 26 from the resistance value Rs, voltage Vd, and output voltage Vo. When a balance is kept between Rv and Rs, S/N in resistance measurement becomes best and, because the adjustment of the balance is made for each measurement, a highly accurate odor detection can always be made without being affected by the drift.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an odor measuring device for measuring an odor component contained in a sample gas by using one or more odor sensors as one kind of gas sensor. The odor measuring device of the present invention is widely used for quality inspection of foods and fragrances, quantitative detection of odor pollution, fire alarm by detection of burnt odor, and further, criminal investigation such as tracking, identification, authentication and drug test of persons. Can be used in the field.

[0002]

2. Description of the Related Art An odor sensor electrically (or optically) converts a physical change of an odor component contained in air (or a supplied sample gas) caused by adhering to a sensitive surface of the sensor. It is to be measured. As an odor sensor, a sensor using an oxide semiconductor and a sensor using a conductive polymer are known.

For example, in an odor sensor provided with a sensitive film made of a conductive polymer, when molecules of various components contained in the odor substance adhere to the sensitive film, the conductivity of the molecule is increased by direct or indirect involvement of the molecules. The conductivity of the molecule changes. Therefore, the detection of the odor substance is performed by measuring the change in the resistance value between the electrodes provided with the sensitive film interposed therebetween.

[0004]

This type of odor sensor has a large initial resistance value variation even when manufactured by the same process. Therefore, in the conventional odor measuring device, by using a generally known bridge circuit to measure the resistance value of the sensor, by mounting the sensor to the device, by performing a balance adjustment of the resistance of the bridge circuit, The variation in the initial resistance of the sensor is absorbed.

However, the resistance value of this type of odor sensor varies depending on measurement conditions such as ambient temperature, heat generated by a voltage applied to read the resistance value, and the flow rate of a sample gas. In addition, a large drift occurs due to factors such as aging and use history. Therefore, even if the balance of the bridge circuit is adjusted initially, the balance is not always in the best state at the time of subsequent measurement. In addition, since the drift is considerably large, the balance of the bridge circuit is surely lost in the long term. If the balance of the bridge circuit is lost in this way, the S / N ratio of the resistance measurement deteriorates, which hinders accurate odor detection.

For this reason, in the odor measuring device, it is essential to periodically calibrate the bridge circuit. However, in the conventional device, fine balance adjustment has to be performed manually, and the user himself / herself must perform accurate calibration. Was difficult. As a result, it is necessary to request calibration from an external (for example, a device maker) maintenance staff, which requires a large cost. In addition, since such a periodic calibration work is required, the apparatus cannot be used continuously for a long period of time, and a great limitation has been placed on the measurement.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem, and has as its object to always provide a high S signal without being affected by fluctuation or drift due to the measurement conditions of the resistance value of the odor sensor. An object of the present invention is to provide an odor measuring device capable of performing measurement with a / N ratio.

[0008]

Means for Solving the Problems An odor measuring device of the present invention made to solve the above-mentioned problems has the following features: a) It has two or more electrodes and a sensitive film, and when a sample component adheres to the sensitive film, A sensor in which the resistance value between the electrodes changes, b) a variable resistor connected in series with the sensor, c) a voltage source for applying a predetermined voltage across the variable resistor and the sensor, d) E) a voltage detecting means for detecting a voltage value at a connection point between the sensor and the variable resistor; and And calibrating means for adjusting the resistance value of the variable resistor so that the predetermined ratio becomes substantially the maximum.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the odor measuring device according to the present invention, a half bridge circuit is constituted by a sensor and a variable resistor, and a detection signal is taken out from a connection point between the sensor and the variable resistor. In this configuration, when the sensor and the variable resistor have the same resistance value, the S / N ratio of the detection signal becomes the best. Therefore, the calibrating means adjusts the variable resistor so that the detection value of the voltage detecting means becomes closest to 1/2 of the predetermined voltage, for example, immediately before the start of the odor measurement. The variable resistor is, for example, a combination of a resistor array in which a plurality of resistors are arranged in parallel and an analog switch that switches the resistors, and a resistor whose value can be adjusted by setting the switching of the switch from the outside. I do. In this case, when the adjustment of the variable resistor is completed, the resistance value of the variable resistor can be known from the setting of the switch. Since the detection value of the voltage detection means changes at the time of odor measurement, the resistance value of the sensor can be calculated based on the detection value, the resistance value of the variable resistor, and the predetermined voltage.

[0010]

As described above, in the odor measuring device according to the present invention, the balance of the resistance is adjusted so that the measurement can be performed at the best S / N ratio with high frequency. Therefore, the resistance value of the sensor can always be measured with high accuracy, and the odor can be detected more accurately. In addition, since the balance adjustment is performed automatically without any need for human intervention, there is no need to ask maintenance personnel or the like for calibration work, so that measurement costs can be reduced. Continuous measurement of time is also possible.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the odor measuring device according to the present invention will be described below with reference to FIGS. FIG. 1 is a configuration diagram centering on the flow path of the odor measurement device of the present embodiment, and FIG. 2 is a configuration diagram of a resistance measurement unit that measures a resistance change between electrodes of the odor sensor.

First, the configuration of this odor measuring device will be described with reference to FIG. The flow paths on the outlet side of the constant pressure valve 11 provided at the gas outlet of the nitrogen gas container 10 filled with pure nitrogen gas are two first and second nitrogen gas flow paths having needle valves 13 and 15, respectively. It branches to 12 and 14. The sample gas flow path 16 and the first nitrogen gas flow path 12
6-way valve (6 port 2
Position valve) 18 while being connected to port a.
The second nitrogen gas flow path 14 is connected to the port d of the six-way valve 18. A collection tube 19 provided with a heater 20 for heating is provided between the port c and the port f of the six-way valve 18.
Is connected. The collection tube 19 is filled with, for example, a carbon-based adsorbent or another appropriate adsorbent according to the sample component to be measured.

A port b of the six-way valve 18 is connected to a first discharge passage 22 or a needle valve 24 by a three-way valve 21.
And a second discharge passage 23 provided with a pump 25. The port e of the six-way valve 18 is connected to a flow cell 27 having a plurality of odor sensors 26, and a downstream side thereof is a third discharge passage 29 provided with a valve 30 and a check valve 31. The detection signals of the plurality of odor sensors 26 are respectively input to the signal processing unit 34, and the processing described below is performed. The number of the odor sensors 26 can be any number of 2 or more, but here, c
Sixteen odor sensors numbered h1 to ch16 (each odor sensor is called a sensor channel) are installed. The temperature of the flow cell 27 can be freely set within a predetermined temperature range by the temperature adjusting unit 28.

An operation unit 33 is attached to the control unit 32, and according to a predetermined program, the three-way valves 17, 21, the six-way valve 18, the pump 25, the heater 2
0, and controls the temperature adjustment unit 28 and the like. In addition, as a piping material of each flow path, PTFE with little adsorption of sample components is used.
It is desirable to use (polytetrafluoroethylene) tubes.

Next, the operation of the odor measuring device will be described in detail. First, the control unit 32 switches the three-way valve 17 so that the sample gas flow path 16 is connected to the port a of the six-way valve 18 and the port b of the six-way valve 18
The three-way valve 21 is switched so as to be connected to the discharge channel 23. In addition, the six-way valve 18 is switched to the connection state shown by the broken line in FIG. Then, by the suction force of the pump 25, the sample gas introduced into the sample gas flow path 16 passes through the collection tube 19 via the three-way valve 17 and the six-way valve 18 (from left to right in FIG. The gas is discharged from the outlet of the second discharge channel 23 through the valve 18, the three-way valve 21, and the needle valve 24. This sample gas contains, for example, a sample component (odor component) to be measured in clean air. At this time, since the heater 20 is not energized, the collection tube 19 is maintained at approximately normal temperature, and when the sample gas passes through the collection tube 19, the sample components contained in the sample gas are adsorbed by the adsorbent.

On the other hand, since the gas pressure at the gas outlet of the nitrogen gas container 10 is high, the nitrogen gas supplied through the second nitrogen gas flow path 14 flows through the six-way valve 18 to the flow cell 27, It is discharged from the outlet of the discharge channel 29. The flow rate of the nitrogen gas is appropriately adjusted by the opening of the needle valve 15. Thereby, the odor sensor 26
Is always kept in a nitrogen gas atmosphere.

After flowing the sample gas through the collection tube 19 for a predetermined time, the control unit 32 switches the three-way valve 17 to connect the first nitrogen gas flow path 12 to the port a of the six-way valve 18 and By switching the valve 21, the port b of the six-way valve 18 is connected to the first discharge channel 22. Then, instead of the sample gas, the nitrogen gas supplied from the nitrogen gas container 10 is supplied to the first nitrogen gas flow path 12-the three-way valve 1.
The gas is discharged from the outlet of the first discharge channel 22 through the 7-way valve 18, the collecting pipe 19, the 6-way valve 18, and the 3-way valve 21. As a result, the sample gas remaining inside the flow path including the collection tube 19 is pushed out to the outside by the nitrogen gas. At this time, since the collection tube 19 is maintained at room temperature,
The sample component previously adsorbed by the adsorbent remains as it is. On the other hand, since nitrogen gas continues to flow through the flow cell 27, each odor sensor 26 is kept in a nitrogen gas atmosphere.

After flowing nitrogen gas through the collection tube 19 for a predetermined time, the control unit 32 switches the six-way valve 18 to a connection state shown by a solid line in FIG. Then, the second nitrogen gas flow path 14-the six-way valve 18-the collecting pipe 19-the six-way valve 18-
A flow path from the flow cell 27 to the third discharge flow path 29 is formed. In this state, energization of the heater 20 is started and the collection tube 1 is turned on.
Heat 9 rapidly. As a result, the sample component adsorbed on the adsorbent in the collection tube 19 is separated from the adsorbent, and rides on the nitrogen gas flowing in the opposite direction (from right to left in FIG. 1) to the flow cell. Carried to 27. At this time,
The temperature of the flow cell 27 is maintained at about 40 ° C. by the temperature controller 28. When the nitrogen gas containing the sample component passes through the flow cell 27, the sample component is adsorbed on the sensitive film of each odor sensor 26, and the electrical resistance between the electrodes of the odor sensor 26 changes according to its characteristics.

This change in electric resistance is measured by the resistance measuring section 40 shown in FIG. The sensor 26 and the variable resistor 41 are connected in series, and a DC voltage Vd is applied to both ends thereof. A connection point 42 between the sensor 26 and the variable resistor 41 is connected to an A / D conversion unit 44 via a buffer amplifier 43, and a digital value corresponding to the voltage value Vo at the connection point 42 is sent to an arithmetic processing unit 45 and a calibration unit 46. Is entered. Variable resistor 41
Are a resistor array 411 composed of a plurality of resistors, an analog switch 412 for switching the resistors, and an analog switch 4
And a decoder 413 for controlling the switching of Twelve. The resistance value changes according to the digital control value given to the decoder 413.

The resistance measuring section 40 configured as described above measures a resistance change in the following procedure. First, a calibration process is performed prior to detection of an odor substance. That is, the control unit 4
7 instructs the calibration unit 46 to start calibration processing. Calibration unit 4
6 changes the resistance value Rv by sequentially changing the digital control value given to the variable resistor 41. Variable resistance 4
When the resistance value Rv of 1 changes, the voltage value Vo of the connection point 42 changes. Therefore, the signal input from the A / D converter 44 causes the calibration unit 46 to make the voltage value Vo with respect to the voltage Vd closest to the predetermined ratio. Search for such a resistance value Rv. To find an appropriate resistance value Rv faster, an appropriate search algorithm such as a dichotomy may be used. If a resistance value Rv meeting the above condition is found, the digital control value at that time is maintained. This completes the calibration operation.
At this time, the resistance Rs of the sensor 26 and the resistance Rv of the variable resistor 41 are substantially the same, and the voltage Vo at the connection point 42 can be measured with the best S / N ratio.

The arithmetic processing unit 45 can know the resistance value Rv from the digital control value.
The resistance value Rs can be calculated from v, the voltage value Vd, and the output voltage value Vo by the following equation (1). Rs = Rv · Vo / (Vd−Vo) (1)

The predetermined ratio that gives the best S / N ratio is:
In the case of this embodiment,

(Equation 1) From the condition, Vo = (1/2) Vd.

Then, when the gas containing the odor substance starts flowing into the flow cell 27 following the above-mentioned calibration operation, the arithmetic processing unit 45 obtains the output voltage value Vo at predetermined time intervals, and obtains the resistance by the above equation (1). Calculate the value Rs. Thereby, the resistance change of the odor sensor 26 due to the odor substance can be obtained with high accuracy.

If the input voltage value for the full scale of the A / D converter 44 is set to Vd,
The initial output digital value of the A / D converter 44 is about 1/2 of the full scale. This prevents a voltage exceeding the full scale from being input to the A / D conversion unit 44 even if the resistance value Rs fluctuates in either of the decreasing and increasing directions due to the detection of the odor substance. Can be done.

It is preferable that the resistance adjustment range of the variable resistor 41 is appropriately selected in advance in consideration of, for example, a change in the resistance value of the sensor 26 with time. Normally, in the case of a variable resistor having a wide resistance adjustment range, the step of adjusting the resistance value becomes coarse, and the accuracy of calibration deteriorates. Therefore, in practice, a variable resistor having a moderately wide resistance adjustment range is selected, and when the calibration operation becomes impossible as described above, it is considered that the sensor 26 has reached the end of its life and the sensor 26 itself is replaced. It is advisable to take measures such as replacement.

The above-mentioned calibration operation is preferably performed every time a certain sample is measured. For example, when a plurality of samples are continuously measured, the calibration operation is performed at an appropriate time every time a predetermined time elapses. A calibration operation may be performed.

In the above embodiment, a DC current is applied to the sensor 26 to measure the resistance between the electrodes.
A configuration may also be adopted in which an alternating current such as a square wave or a sine wave flows through the sensor 26 to measure the impedance such as the inductance component and the capacitor component between the electrodes.

Further, instead of measuring the voltage value, a configuration may be adopted in which a variable resistor is connected in parallel with the sensor and the current flowing through the sensor is measured using a current / voltage conversion amplifier or the like.

Also, as shown in FIG.
Odor measurement devices that use a combination of different types of odor sensors with different detection principles, such as odor sensors with oxide semiconductor films or conductive polymer films, especially odor sensors with a large drift over time May be provided with a function of performing the above-mentioned calibration operation.

It should be noted that the above embodiment is merely an example, and it is apparent that modifications and modifications can be made as appropriate within the spirit of the present invention.

[Brief description of the drawings]

FIG. 1 is a configuration diagram centering on a flow channel of an odor measuring device according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a resistance measuring unit in the odor measuring device of the present embodiment.

[Explanation of symbols]

 26 odor sensor 40 resistance measuring unit 41 variable resistor 411 resistance array 412 analog switch 413 decoder 42 connection point 43 buffer amplifier 44 A / D converter 45 arithmetic processing unit 46 calibration unit

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroshi Nakano 1 Shiwazu Works Sanjo Plant, Nishinokyo Kuwabaracho, Nakagyo-ku, Kyoto

Claims (1)

[Claims] A) a sensor having two or more electrodes and a sensitive film, wherein a resistance value between the electrodes changes when a sample component adheres to the sensitive film; and b) a variable sensor connected in series with the sensor. A resistor, c) a voltage source for applying a predetermined voltage across the variable resistor and the sensor, d) voltage detecting means for detecting a voltage value at a connection point between the sensor and the variable resistor, and e) Calibration that adjusts the resistance value of the variable resistor so that the detection value of the voltage detection means with respect to the predetermined voltage becomes a predetermined ratio at which the resolution of the sensor resistance becomes substantially maximum every time measurement is performed or every predetermined time has elapsed. A odor measuring device comprising: