JP3424784B2 - Solid electrolyte type carbon dioxide sensor control device for carbon dioxide concentration measuring instrument - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon dioxide gas sensor control device for a carbon dioxide gas concentration measuring instrument having a solid electrolyte type carbon dioxide gas sensor for heating a sensor element by a heater.

[0002]

2. Description of the Related Art A solid electrolyte type carbon dioxide gas sensor is disclosed in JP-A-4-168356 or JP-A-4-2130.
Some are known from Japanese Patent Publication No. 49 and the like. Most of them have a pair of electrodes with a carbonate ion conductor sandwiched between them, and one electrode is contacted with a gas of unknown carbon dioxide concentration, and the other electrode is contacted with a gas of known concentration so that the difference in concentration is Is to measure the electromotive force generated between these electrodes.

Here, the one described in Japanese Patent Laid-Open No. 4-213049 will be described. This has a structure as shown in FIG. That is, the sensor element formed on the substrate 11 made of gas impermeable ceramic or the like comprises a reference electrode 12 made of platinum or the like, a solid electrolyte thin film layer made of a solid solution of an alkaline earth metal carbonate and an alkali metal carbonate. 13, a sensing electrode 14 made of palladium or silver, and a sealing material 15 for blocking the solid electrolyte thin film layer near the reference electrode from the external atmosphere.

In such a sensor element, in a carbon dioxide gas atmosphere, the carbon dioxide gas causes a difference in concentration of carbonate ions near the detection electrode and the reference electrode in the solid electrolyte thin film layer, and an electromotive force is generated between these electrodes. By measuring this electromotive force with the voltage measuring means, the concentration of carbon dioxide in the atmosphere can be measured.

In order to obtain sufficient sensor sensitivity, the sensor element needs to be heated to 400 ° C. to 450 ° C. at which the solid electrolyte has good conductivity due to carbonate ions. Therefore, this sensor has its ceramic substrate 1
1 has an electric heater 16 on the back surface.

The heater of the sensor which needs to be heated by such an electric heater is used by being connected to a voltage applying means and being applied with a voltage. It should be noted that such a sensor can be used in an environment without carbon dioxide, that is, even at the time of zero signal.
An electromotive force corresponding to the temperature is provided due to the difference in the materials of the reference electrode and the detection electrode and the temperature dependency of the conductivity of the solid electrolyte. Therefore, a constant voltage is applied to this heater and the sensor temperature is kept constant.

The solid electrolyte type carbon dioxide sensor element absorbs moisture when it is not used, that is, when it is not heated by the heater. Due to the influence of this moisture, the sensor output (at the zero signal) immediately after the power is turned on becomes low. After that, the temperature gradually rises with the lapse of time, and finally becomes stable and becomes a steady state. However, depending on the moisture absorption condition, this stabilization may take several days. Since the measured value cannot be relied upon during the period until the sensor reaches a steady state in this manner, it has been a serious problem when this carbon dioxide sensor is used for manufacturing process control and safety control.

[0008]

The problems in the above-mentioned prior art will be described with reference to FIG. This figure is a diagram showing the relationship between the time after the power is turned on and the sensor electromotive force (output) in an environment without carbon dioxide. In addition, what is shown as "reference level" in the figure is stable at zero signal,
That is, it is the sensor output in the steady state.

Reference numeral a in FIG . 6 shows the change over time in the output when a normal applied voltage (hereinafter referred to as "steady applied voltage") is supplied immediately after the power is turned on as the voltage applied to the heater of the sensor. As described above, the sensor output leveled off at a low value compared with the reference level, and it took several days until the sensor output finally reached the reference level and reached a steady state.

At the same time when the power is turned on, a voltage higher than the steady applied voltage is applied to the heater section to keep the sensor element at a high temperature (470 ° C. to 550 ° C.) at which the sensor element does not deteriorate, and the absorbed moisture is removed. Treatment to release quickly,
That is, the so-called heat-up process can shorten the time required to return the sensor output to the reference level.

However, since the optimum heat-up time differs depending on the moisture absorption state of the sensor when the power is turned on, it is necessary to perform heat-up according to the situation. Here, when the optimum heat-up time is selected, that is, when the high voltage is applied after the power is turned on and the voltage is switched to the low voltage, that is, the steady applied voltage at the time Tc, as indicated by reference sign c in FIG. After the heat-up process, the sensor output quickly enters a steady state and its output matches the reference level.

However, when the heat-up time is insufficient (switching time: Tb), the sensor output becomes lower than the reference level (reference numeral b in FIG. 6). When the heat-up process is excessive (switching time: Td), the output becomes high (reference numeral d in FIG. 6), and in these cases, it takes several hours for the sensor output to recover to the reference level. There was a drawback left.

When the heat-up termination conditions were examined, the following was found. That is, the sensor output increases due to heat-up, but the optimum end time is when the increase ends and reaches the peak (reference numeral Tc in FIG. 6). Therefore, when this sensor electromotive force peaks, T
It was found that the heat-up process was terminated at c and the voltage supplied to the heater was switched to the steady applied voltage.

However, the detection of the arrival of the peak is difficult due to the small output of the sensor and the influence of noise and the like, and the detection becomes slower than when the peak is actually reached. Here, this delay in the detection results in a significant delay in the steady state of the sensor output. As described above, the method of controlling the heater applied voltage by detecting the peak of the sensor electromotive force during the heat-up process cannot be said to be optimal,
The solution was desired.

The present invention provides the above-mentioned object, that is, a carbon dioxide concentration measuring instrument having a solid electrolyte type carbon dioxide sensor,
An object of the present invention is to provide a control device that eliminates instability of sensor output immediately after power-on due to moisture absorption when the sensor is not used in a short time.

[0016]

As shown in the basic configuration diagram of FIG. 1, a sensor control device for a carbon dioxide concentration measuring instrument according to the present invention has a high voltage and a low voltage with respect to a heater 1a for heating a sensor element 1b. Voltage applying means 2a capable of supplying any one of two voltage levels, voltage switching means 2b for switching the voltage, and voltage switching means 2 after the power is turned on.
initializing means 3a for instructing b to supply a high voltage
And a signal detecting means 3b for measuring the output from the sensor element 1b to detect that it has become equal to a reference level, and an output from the sensor element to the reference level after the initial setting means gives the instruction to supply a high voltage. The time elapsed before reaching
That is, the holding time calculation section 3c is configured to calculate the holding time based on the reference level reaching time and to instruct the voltage switching means 2b to switch to the low voltage when the holding time is reached.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Timer means for instructing the voltage switching means 2b by the initial setting means 3a to supply a low voltage immediately after power-on, and then, after a lapse of a certain time, to supply a high voltage. In this case, it is possible to prevent deterioration of the sensor element due to a sharp rise in temperature due to application of a high voltage immediately after the power is turned on, which is preferable. Further, it is preferable that the holding time for supplying the high voltage is shorter than the time when deterioration of the sensor element due to high temperature exposure due to the high voltage starts.

An example of the present invention will be described below with reference to the drawings. FIG. 2 (a) is a circuit diagram of a carbon dioxide concentration measuring instrument having a solid electrolyte type carbon dioxide sensor controller according to the present invention. In the figure, 1 is a sensor, 1a is a heater,
1b is a sensor element. Further, 2 is a voltage application circuit, and the heater 1 a is connected to the output of the voltage application circuit 2.

The output end of the sensor element 1b is connected to an input port 3i with an A / D converter of a microprocessing unit (hereinafter referred to as "MPU") 3, and the electromotive force output from the sensor element 1b is Input port 3
Quantified by i. The MPU 3 has an output port 3o in addition to the input port 3i, and stores a control program, a sensor constant, a reference level value, etc.
ro (see FIG. 2 (b-1)), time measurement data of elapsed time after power-on (hereinafter referred to as "timer I"), and time measurement data of elapsed time from arbitrary start time (hereinafter, referred to as "timer I")
RAM3ra (see FIG. 2 (b-2)) for storing various data such as "timer II"), central processing unit (hereinafter referred to as "CPU") 3cp for executing the above program Built in.

The voltage application circuit 2 applies a two-step voltage of a high voltage and a low voltage to the heater 1a of the sensor 1 according to the change of the output signal of the output port 3o of the MPU 3. That is, when Hi of the output port 3o is output as the output signal, a high voltage is output, and as the output signal, the output port 3o is output.
When Lo is output, a low voltage is applied as a voltage to the heater 1a. As the voltage applying circuit 2, specifically, a switch mechanism and a power supply circuit were used in combination.

The carbon dioxide gas concentration measuring device has the structure of the solid electrolyte type carbon dioxide gas sensor control device according to the present invention. Therefore, immediately after the power is turned on, the heater 1a is gradually heated by the low voltage applied by the voltage application circuit 2. To be done. For this reason,
The sensor element does not deteriorate due to a rapid temperature rise.

At this time, the CPU 3cp checks the elapsed time after the power is turned on by the timer I of the RAM 3ra, and switches the voltage applied to the heater to a high voltage after a fixed time (10 seconds in this case) after the power is turned on. At this time, the temperature of the sensor element 1b is raised, the absorbed moisture is promptly released, and the output thereof is increased. At the time of this switching, the timer II is started.

The high voltage causes the temperature of the sensor element 1b to rise, and it is detected when the output value of the sensor element 1b reaches the reference level stored in the ROM 3ro, and the value of the timer II at this time, that is, the reference level is detected. The time when the high voltage should be applied, that is, the holding time is calculated by calculation based on the arrival time.

Thereafter, the high voltage is continuously applied until the value of the timer II becomes equal to the value of the holding time, and then the voltage is switched to the low voltage. However, if the holding time becomes too long, the sensor element 1b is deteriorated because the sensor element 1b absorbs much moisture before the power is turned on. Therefore, in order to prevent this deterioration, the MPU 3 monitors this and the heat-up process for a certain time or longer is interrupted. In this example, the interruption is 120 seconds, but it is desirable that the interruption be 180 seconds or less at the maximum.

Here, the significance of the holding time and the calculation method thereof will be described. As described above, in the solid electrolyte carbon dioxide sensor that heats the sensor element with the heater, when heating up at high voltage, the sensor output rises even after exceeding the reference level, then reaches a peak and then changes with time. Disappears.

As a result of various studies, assuming that the reference level arrival time from when the applied voltage is a high voltage to reach this reference level is x and the subsequent peak arrival time (peak arrival time) Tc is shown in FIG. Tc = ax + b (a is the slope of this straight line, b is the intercept when the reference level arrival time is 0 seconds, and these are constants specific to the sensor (hereinafter referred to as "sensor constant"). ).) It was found that there is a primary relationship.

Here, if the values of these sensor constants are investigated in advance, the peak arrival time Tc can be predicted by measuring the time x until the sensor output reaches the reference level. , The time for holding the high voltage (holding time y) can be obtained.

Therefore, according to the configuration of the present invention, the time x from when the voltage supplied to the heater is set to a high voltage for heating up until the sensor output signal value reaches the reference level is measured. An appropriate holding time y is calculated from the values a and b of and the high voltage is applied for the holding time y, so that the sensor can be brought into a steady state quickly.

Next, details of the operation from when the power is turned on to when the sensor output is stabilized will be described with reference to the flowchart of FIG. The MPU 3 starts its operation by a power source or the like when a switch (not shown) is turned on, and the timer I is first started in step S1. In step S2, the signal Lo is output to the output port 3o, which results in the heater 1a.
A low-voltage normal applied voltage is applied to the sensor element by the voltage application circuit 2, and the sensor element is gradually heated so as not to deteriorate.

Then, in step S3, a low voltage is applied for a certain period of time (here, 10 seconds) after the low voltage is applied, and then the process proceeds to step S4, in which the signal Hi is output to the output port 3o. Application circuit 2
By this, a high voltage is applied to the heater 1a, and the sensor element is heated up.

Thereafter, step S6 is performed until the output of the sensor measured by the input port 3i reaches the reference level.
When the electromotive force reaches the reference level, the value of the timer II, that is, the elapsed time after the high voltage is applied to the heater 1a is immediately set to the reference level reaching time x in step 7. . Next, in step S8, the holding time y is calculated from this x and the sensor constants a and b.

In step S9, the elapsed time after applying the high voltage to the heater 1a, that is, the value of the timer II is
If the time is less than the certain time (120 seconds in this case), the process proceeds to step S10, and if the elapsed time is more than the certain time, the process proceeds to step S11 to prevent deterioration of the sensor element due to excessive heat-up.

In step S10, the elapsed time after the high voltage is applied to the heater 1a is compared with the holding time y. If this elapsed time reaches the holding time, the process proceeds to step S11, and if it is less than the holding time. Returns to step S9. In step S11, the signal Lo is sent to the output port 3o. As a result, the voltage applied to the heater 1a becomes a low voltage, that is, the steady applied voltage, and the heat-up ends, and the sensor output quickly becomes the steady state and becomes the measurable state.

As is clear from the above description with reference to the flow chart, the MPU 3 executes the flow chart to instruct the voltage switching means 2b to supply a low voltage immediately after the power is turned on, and then, for a certain period of time. Timer means for instructing to supply a high voltage after a lapse of time, initial setting means 3a for instructing to supply a high voltage, and signal detection for detecting that the output from the sensor element 1b is equal to the reference level. The holding time is calculated based on the time elapsed from the means 3b and the instruction for supplying the high voltage by the initialization means until the sensor element output reaches the reference level, that is, the reference level reaching time, and the holding time is calculated. When the voltage reaches, the voltage switching means functions as a holding time calculation unit 3c that gives an instruction to switch to a low voltage.

[0035]

As described above, according to the structure of the present invention, the output of the sensor element 1b is detected by the signal detecting means 3b, the reference level reaching time is measured, and based on this time, the moisture of the sensor element that has absorbed moisture is measured. The heat-up process for promptly releasing the heat, that is, the optimum value of the time for applying the high voltage is estimated and calculated as the holding time, the high voltage is applied to the heater 1a for this holding time, and then the low voltage is applied. By switching, the sensor output reaches a steady state in a very short time and measurement becomes possible. As described above, according to the present invention, the optimum heat-up process time can be set based on the reference level arrival time, so that the time required for stabilizing the sensor output can be shortened.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration block of a solid electrolyte type carbon dioxide sensor control device of a carbon dioxide concentration measuring instrument of the present invention.

FIG. 2 is a circuit diagram showing an example of a heater control circuit of the solid electrolyte type carbon dioxide gas sensor according to the present invention.

FIG. 3 is a diagram showing a relationship between a reference level arrival time and a peak arrival time.

4 is a flowchart showing a procedure of work performed by the MPU in FIG.

FIG. 5 is an example of a solid electrolyte type carbon dioxide gas sensor.

FIG. 6 is a diagram showing an influence of a heat-up condition on a sensor output.

[Explanation of symbols]

1 Solid electrolyte type carbon dioxide sensor 1a heater 1b Sensor element 2a voltage applying means 2b Voltage switching means 3a Initial setting means 3b signal detection means 3c Retention time calculator

─────────────────────────────────────────────────── --Continued from the front page (56) Reference JP-A-5-307018 (JP, A) JP-A-6-94674 (JP, A) JP-A-4-55751 (JP, A) JP-A-4- 22861 (JP, A) JP-A-4-22860 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 27/406 G01N 27/416

Claims (3)

(57) [Claims] 1. A sensor control device for a carbon dioxide concentration measuring instrument having a solid electrolyte type carbon dioxide gas sensor for heating a sensor element by a heater, wherein a high voltage or a low voltage is supplied to the heater. Voltage applying means, voltage switching means for switching the voltage, initial setting means for instructing the voltage switching means to supply a high voltage after the power is turned on, and measuring the output from the sensor element. The signal detection means for detecting that the reference level is equalized, and the time elapsed until the sensor element output reaches the reference level after the initial setting means gives the instruction to supply the high voltage, that is, the reference level arrival time Calculate the retention time based on
A carbon dioxide sensor control device, comprising: a holding time calculation unit for instructing the voltage switching means to switch to a low voltage when the holding time is reached. 2. The timer means for instructing the voltage switching means to supply a low voltage immediately after power-on, and then, after a lapse of a predetermined time, to the high voltage, the initialization means. The carbon dioxide sensor control device according to claim 1, which is characterized in that. 3. The solid electrolyte according to claim 1, wherein a holding time for supplying the high voltage is shorter than a time when deterioration of the sensor element due to high temperature exposure due to the high voltage starts. Type carbon dioxide sensor control device.