JP2571016Y2 - Gas detector - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas detector, and more particularly to a gas detector which converts a small signal from a gas sensor into an A / D signal with high resolution.

[0002]

2. Description of the Related Art A gas detection device in which a signal from a gas sensor is AD-converted and processed by a microcomputer is well known. Such a gas detection device is used in the field of detection of a trace gas, particularly in the field of air conditioning and odor detection, with the development of pattern recognition.

[0003] A low-end 4-bit one-chip microcomputer is mainly used as a microcomputer, and an AD converter having a resolution of 8 bits built in the microcomputer is mainly used.

However, when detecting a very small amount of gas,
The signal of the gas sensor is small, and for example, when AD conversion is performed with 8 bits, only a signal of 2 to 3 digits may be obtained. Here, 2 to 3 digits are the amplitude of the gas sensor signal by the gas to be detected, which is the maximum value of the signal.

A one-digit signal is meaningless and requires at least a two-digit signal for detection. Amplitude 2
If a gas is detected after a 2-digit signal is generated with respect to a signal of 検 出 3 digits, detection is performed after the gas sensor signal reaches a maximum value. This is unsuitable for air-conditioning control, which requires quick detection in real time. In addition, in applications other than air conditioning control, the lack of resolution of the AD converter makes accurate detection difficult.

The resolution problem can be solved by using, for example, a 10-bit AD converter instead of the 8-bit AD converter. A 10-bit AD converter is built into a 16-bit one-chip microcomputer used for controlling an automobile engine, for example, but such a one-chip microcomputer is extremely expensive. Even if the AD converter is provided separately from the microcomputer, a 10-bit or 12-bit AD converter is extremely expensive.

In order to compensate for the lack of resolution of the A / D conversion of the gas sensor signal, for example, in the case of a resistance type gas sensor, it has been proposed to sequentially switch the value of a load resistance connected to the gas sensor. For example, when the detection voltage is 5 V, the voltage applied to the load resistance is the largest change in the sensor resistance.
The load resistance is switched so as to be set at about 5V. However, the improvement in resolution obtained by this method is not large. For example, it is assumed that the ratio of the resistance value between the sensor resistance and the load resistance is 9: 1 and an output of 500 mV appears in the load resistance. Here, assuming that the ratio between the sensor resistance and the load resistance is 1: 1, the output is 2500 mV. In these cases, it is assumed that the sensor resistance has changed by 10%. Then, when the original sensor output is 500 mV, the change in the output to the load resistance is 41.3 mV. On the other hand, the original sensor output is 250
In the case of 0 mV, the change of the output is 119 mV. The improvement in resolution by switching the load resistance is only about 2.9 times.

[0008]

An object of the present invention is to convert a signal from a small gas sensor into an A / D signal at a high resolution without using an A / D conversion circuit with a high resolution so that gas can be detected accurately.

[0009]

The gas detecting device according to the present invention is a gas detecting device in which a gas sensor signal is AD-converted by an AD converter and processed by a microcomputer. A means for DA conversion and a differential amplifier circuit for amplifying a difference between the DA-converted reference signal and the gas sensor signal are provided, and the AD converter converts the output from the differential amplifier circuit to AD. and things, and depending on the gas sensor signal
Fixed reference signal Te, means to keep the difference between the gas sensor signal and the reference signal within a predetermined range, characterized in that a city. The correction of the reference signal may be performed, for example, when the value obtained by AD-converting the output of the differential amplifier deviates from a predetermined range.

[0010]

According to the present invention, a reference signal is generated based on the gas sensor signal obtained by the AD conversion, and the reference signal is DA-converted.
A differential amplifier differentially amplifies the difference between the gas sensor signal and the DA-converted reference signal. And the output of the differential amplifier circuit is A
A / D conversion is performed by a D converter and processed by a microcomputer.

In this way, the difference between the gas sensor signal and the reference signal is amplified and A / D converted.
The resolution of the D converter is substantially increased significantly. Next, the difference between the gas sensor signal and the reference signal is monitored by, for example, the output of an AD converter, and the difference between the gas sensor signal and the reference signal is monitored.
Gas sensor signal so that the difference between
The reference signal may be corrected according to

When performing processing such as first-order differentiation or second-order differentiation of a gas sensor signal inside a microcomputer,
The relative value of the gas sensor signal is important, and the AD-converted signal may be used as it is. On the other hand, when the absolute value of the gas sensor signal is important, the signal obtained by digitally adding the reference signal to the AD-converted signal becomes the AD-converted value of the gas sensor signal with high resolution. Therefore, the gas is detected using this signal.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment will be described with reference to FIGS. In FIG. 1, 1 is WO3
RL is a load resistance and RH is a heater. In addition to the above, a NOx sensor such as lead phthalocyanine or SnO
A flammable gas sensor such as 2 or an electromotive gas sensor such as a proton conductor may be used. In addition to the outside air introduction control of the automobile, it may be used for detection of smell, detection of ozone, control of an indoor air cleaner, detection of alcohol in the fermentation industry, and the like. The output VRL to the load resistor RL is used as a sensor signal.

Reference numeral 2 denotes a constant voltage power supply which generates two outputs, a detection power supply VC such as 5 V for driving the gas sensor 1 and a power supply VDD for driving an auxiliary circuit such as 7 V. 3 is a power supply.
Reference numeral 4 denotes a buffer operational amplifier having a high resistance (for example, 1 to 1).
It is used to extract the sensor signal VRL from the load resistance RL of 3 MΩ). 5 differential amplifier, 6 Ri <br/> Oh an operational amplifier, for example, further connect the amplifier in the subsequent stage of the differential amplifier 5
You may.

Reference numeral 7 denotes a microcomputer, 8 denotes a circuit for generating an 8-bit reference signal VREF, 9 denotes an AD converter having a resolution of 8 bits, 10 denotes a memory of 12 bits, and 11 denotes a digital second-order differential for performing a second-order differentiation. The circuit is an edge filter for detecting the edge of the gas sensor signal. 12 is a timer, and 13 is an arithmetic logic circuit. The output of the microcomputer 7 is connected to a drive circuit 14,
The output controls a load 15, for example, a damper of an outside air introduction control device of an automobile.

Reference numeral 16 denotes an 8-bit switch network, and 17 denotes an 8-bit R-2R resistance ladder circuit. The switch network 16 is driven by an 8-bit digital signal of the reference signal generation circuit 8, and an R-2R resistance ladder circuit 17 is provided. And the operational amplifier 6 converts the analog output into an 8-bit analog output. After all, this is equivalent to DA conversion of the reference signal VREF by 8 bits,
The accuracy of the DA conversion may be 6 bits, 4 bits, or the like in addition to 8 bits.

The operation of the embodiment will be described with reference to FIGS. FIG. 3 shows an operation waveform of a gas sensor signal when the vehicle runs. The region marked with a circle in the figure is the response waveform to NOx from the diesel vehicle. The characteristic of the gas sensor signal for NOx from a diesel vehicle is that an edge is generated. A second-order differentiation circuit 11 for detecting an edge is provided in the microcomputer 7 to detect NOx due to exhaust gas from the diesel vehicle. However, as is clear from FIG. 3, the sensor signal for NOx is small, and when AD conversion is simply performed with 8 bits, there are usually only 2 to 3 digits. Of the signals of 2 to 3 digits, 1 digit cannot be distinguished from noise or quantization error of A / D conversion.
x cannot be detected. Therefore, if the detection is performed at the second digit, the detection is performed when the pollution of the outside air by NOx becomes maximum. In this case, the detection is performed after the odor outside the vehicle is introduced into the vehicle compartment, which is practically meaningless.

In the embodiment, detection is performed by the algorithm shown in FIG. The gas sensor signal VRL appearing in the load resistance RL is extracted by the operational amplifier 4 for the buffer, and the differential amplifier 5 differentially amplifies the difference from the reference signal VREF by, for example, 16 times. Next, this is AD-converted by an AD converter 9 to, for example, 8 bits. The AD-converted signal is input to, for example, a 12-bit memory 10, and the output of the AD converter 9 is set to the lower 8 bits. Then, the reference signal VREF stored in the reference signal generation circuit 8 is digitally added as the upper 8 bits of the 12-bit memory 10. In this manner, the memory 10 can store a value obtained by AD-converting the gas sensor signal VRL with a resolution of 12 bits.

In the embodiment, NOx from a diesel vehicle or the like is used.
To detect the contamination of the outside air by the vehicle, and turn on and off the introduction of the outside air into the vehicle interior. The signal processing for this is performed by the second order differentiation circuit 11
And the absolute value of the gas sensor signal VRL is not important. Therefore, what is required in the second-order differentiating circuit 11 is the signal of the AD converter 9, and there is no need to restore the VRL by adding the upper 8 bits of data from the reference signal generating circuit 8. That is, the data obtained by AD conversion into 8 bits by the AD converter 9 is stored in the memory 10 as it is, and the second-order differentiator 11 performs second-order differentiation on the 8-bit signal. Further, in the embodiment, for example, 1
Second order differentiation is performed using sensor signals sampled every second over five points. For this purpose, the memory 10 has 5
The AD-converted sensor signal of the point is stored and cyclically replaced every second.

In the digital second-order differentiation circuit 11, for example, V
0, V1, V2, V3, and V4 (where V0 to V4 represent a sequence of AD-converted sensor signals, the oldest data being V0 and the newest data being V4), for example, (V0-2).
V1 + 2V2-2−V3 + V4) and the like, and second order differentiation is performed. Here, the digital second-order differentiation is performed using the sensor signal sequence V0 to V4 for 4 seconds, but the time width of the sensor signal used for the digital second-order differentiation and the weighting coefficient for the signal at each point ((1, (1) −2, 2, −2, 1)) and the like are optional.

The digital second-order differential signal is 0 at the edge in FIG.
From which the NOx is detected, and the introduction of outside air is shut off for one minute. The reason why the duration is set to one minute is that the distance between the vehicle and the diesel vehicle is generally increased in about one minute, and the outside air returns to a clean state. The time for shutting off the introduction of the outside air per one time, the condition for ending the shutting off of the introduction of the outside air, and the like are arbitrary. For example, since the sensor signal (V0) at the time of detection of outside air pollution is stored, and the difference between the sensor signal and V0 has recovered to a predetermined range,
The introduction of outside air may be resumed.

The reference signal VREF is constantly in a predetermined range from the gas sensor signal VRL. This range determines the dynamic range when the gas sensor signal VRL is AD-converted. In the embodiment, the difference between VRL and VREF is kept between, for example, 78.12 mV and 234.38 mV as a value before amplification. This is a 64- to 192 digit signal obtained by performing 16-bit differential amplification by the differential amplifier circuit 5 and then performing 8-bit AD conversion by the AD converter 9. Therefore, when the output of the AD converter 9 is out of this range, the reference signal VREF stored in the reference signal generation circuit 8 is changed so that the output of the AD converter 9 falls within this range. Specifically, an appropriate reference signal VREF is generated at the time of reset, and when the reference signal VREF is out of a predetermined range, the reference signal VREF is corrected,
After that, the reference signal V follows the change of the gas sensor signal VRL.
Modify the REF. That is, in the embodiment, feedback control of the reference signal VREF is performed so that the difference between the gas sensor signal VRL and the reference signal VREF does not deviate from a predetermined range.

The reference signal generating circuit 8 stores and generates, for example, an 8-bit digital output VREF, and controls the switch network 16 with this signal. Then, the R-2R resistance ladder circuit 17 is driven by the signal of the switch network 16, and the signal obtained by DA-converting the reference signal VREF into 8 bits by the operational amplifier 6 is extracted. This is input to the differential amplifier 5 as an analog reference signal. This operation converts the reference signal VREF to DA
This is equivalent to conversion and input to the comparison potential side of the differential amplifier 5.

Although the reference signal VREF is generated in the microcomputer 7 in the embodiment, for example, the bottom of the gas sensor signal VRL is obtained by an integrating circuit and this is used as the reference signal VRE.
As F, the difference between VRL and VREF may be amplified and AD converted. In this case, the original VRL can be restored as a digital signal by separately AD-converting the output VREF of the integration circuit and digitally adding the result to the AD conversion value of the difference from VREF.

[0025]

According to the present invention, even a signal from a small gas sensor can be A / D-converted at a high resolution without using a high-resolution A / D conversion circuit, and gas can be detected accurately.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a gas detection device according to an embodiment.

FIG. 2 is a flowchart illustrating an operation algorithm of the gas detection device according to the embodiment.

FIG. 3 is a characteristic diagram showing an output of the gas sensor when the vehicle is running.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Gas sensor RH Heater RL Load resistance 2 Constant voltage power supply 3 Power supply 4 Operational amplifier for buffer 5 Differential amplifier 6 Operational amplifier 7 Microcomputer 8 Reference signal generation circuit 9 AD conversion unit 10 12-bit sensor signal memory 11 Digital differentiation circuit 12 Timer 13 arithmetic logic circuit 14 drive circuit 15 load 16 8-bit switch network 17 8-bit R-2R resistance ladder circuit VRL gas sensor output VC detection power supply VDD circuit power supply

Claims (2)

(57) [Scope of request for utility model registration] An AD converter converts a gas sensor signal into an AD signal.
In a gas detection device which is converted and processed by a microcomputer, a means for generating a reference signal, a means for DA-converting the reference signal, and a difference between the DA-converted reference signal and the gas sensor signal In addition to providing a differential amplifier circuit for amplifying, the A / D converter outputs an output from the differential amplifier circuit to A
D conversion, and means for correcting the reference signal according to the gas sensor signal, so as to keep the difference between the gas sensor signal and the reference signal within a predetermined range, Gas detector. 2. The method according to claim 1, wherein the means for correcting the reference signal corrects the reference signal when an output of the differential amplifier deviates from a predetermined range.
The gas detection device according to claim 1.