JPH0665984B2 - Detector - Google Patents

Description

TECHNICAL FIELD The present invention relates to a detection device that utilizes a change in electric resistance value of a sensitive substance, and more particularly to a detection device that can be applied to a gas sensor, a temperature sensor, a humidity sensor, an alcohol sensor, or the like. It is a thing.

2. Description of the Related Art Generally, in a detection device that utilizes a change in electric resistance value depending on the adsorption / desorption action of a sensitive object in a sensitive material composed of, for example, a metal oxide semiconductor, the temperature of the sensitive material largely determines its performance. It becomes a factor. That is, if the temperature distribution of the sensitive substance is not uniform, the high temperature region is selectively subjected to strong stress, and the performance thereof deteriorates or burns out over time. Therefore, in order to increase the reliability and life of the detection device, it is required to efficiently heat the sensitive substance and keep its temperature uniformly within the proper range.

The present invention has been made in view of the above points, and provides a detection device capable of efficiently heating a sensitive substance to an appropriate temperature uniformly with low power consumption and stably obtaining an accurate signal. The purpose is to do.

In order to achieve the above-mentioned object, the present invention provides a detection device for heating a sensitive substance to a predetermined temperature with an electric heater to detect a change in resistance value of the sensitive substance according to adsorption / desorption phenomenon of a sensitive object, A small thin film detection part made of the sensitive substance and at least a pair of detection lead parts connected to both ends of the detection part are provided in a bridge shape to generate heat by energizing a drive pulse to the pair of detection lead parts. A heat driving circuit that functions as a heater that heats the detection unit to a predetermined temperature, a detection circuit that applies a detection pulse to the pair of detection lead units to detect the resistance value of the detection unit, and the pair of detection lead units. When the timing control is performed so that the detection circuit energizes the detection pulse during the time when the heater driving circuit and the heater driving circuit do not energize the driving pulse. A control means for controlling the switch means to be closed when the heater drive circuit applies a drive pulse, and to open the switch means when the detection circuit applies a detection pulse. It is what

Hereinafter, a specific description will be given based on one embodiment of the present invention. FIG. 1 is a block diagram showing the configuration of a gas detection device as one embodiment of the present invention. In FIG. 1, a sensor S is formed by connecting a detection section 2 made of a gas sensitive material between detection leads 1a and 1b that also serve as a pair of heaters extending in parallel in a bridge shape in order to reduce the heat capacity. There is. The detection unit 2 of this example is made of SnO ₂ which is a metal oxide semiconductor and is formed in a thin film in order to reduce the heat capacity. When heated to 350 to 400 ° C. by the detection lead portions 1a and 1b also serving as heaters, Atmosphere gas molecules are adsorbed and released on the surface. Therefore,
When there is a gas to be detected in the atmosphere, the gas molecules are adsorbed to reduce the resistance value of the gas-sensitive substance itself, and when the gas disappears, the gas is released and the resistance value returns to the original value.
A change in the resistance value is detected by applying a current from a detection circuit described later, and the presence of the target gas is notified.

Thus, the two ends of the pair of detection lead portions 1a and 1b are connected via the switches 3a and 3b, respectively. This switch 3
Each of a and 3b is composed of an electronic switch, a reed relay, or the like having a high responsiveness, and is normally in an open state. And both ends of one detection lead 1a and each switch
3a and 3b are respectively connected to a heater drive circuit 4 that causes the detection lead portion to function as a heater. Further, one end of each of the pair of detection lead portions 1a and 1b is connected to the detection circuit.

Therefore, when heating the detection unit 2, the switches 3a and 3b are closed to connect the detection lead unit 1a and the detection lead unit 1b in parallel as shown in FIG.
A pulse voltage of ˜3 V is applied to both the detection lead portion 1a and the detection lead portion 1b to generate heat so that the two heaters can efficiently heat the detection portion 2 and keep the entire temperature thereof uniformly within an appropriate range. it can. Further, when detecting the resistance value of the detection unit 2, if the switches 3a and 3b are opened and the respective connections are disconnected as shown in FIG. 3, the same 1.5-3V provided in the detection circuit 5 is provided. The detection current from the detection pulse power source is sent to the detection unit 2, and the current change due to the resistance value change can be detected as a voltage change. The power supply for detection is not limited to the pulse power supply, and may be a normal DC power supply.

By the way, normally, the resistance value is measured when the detection unit 2 is heated to an appropriate temperature, but in this case, the heating current from the heater drive circuit 4 leaks into the detection unit 2 and this leakage occurs. There is an inconvenience that the detection signal output of the sensor S is lowered by the current. Therefore, in the present invention, the disadvantage is avoided as follows.

The inventors of the present application have obtained the following knowledge about the relationship between the temperature of the detection unit 2 and the gas adsorption / desorption action. FIG. 4 shows changes with time in adsorption / desorption states of air molecules A and gas molecules G on the surface of the detection portion 2 when a pulse voltage is intermittently applied to the detection lead portions 1a and 1b as heaters. VI
It is schematically shown in stages I. Detection lead 1a, 1b only in the air without gas in the atmosphere
At the I-th stage in which the pulse voltage is applied to, the air molecules A are adsorbed on the surface of the detection unit 2. Then the heater is turned off
In stage II, the temperature of the detection unit 2 decreases rapidly because of its small heat capacity, and reaches the equilibrium temperature within 1 ms. In this case, the state of stage I is maintained even if the temperature of the detection unit 2 becomes low.
Next, even if the gas molecules G flow in, the surface state of the detector 2 does not change because the temperature is low (step III). After that, when a voltage is applied and the detection lead portion 1a is turned on, air molecules A on the surface of the detection portion 2 are desorbed and gas molecules G are adsorbed instead (step IV). In this case, since the detection unit 2 is minute and formed in a thin film, the action of adsorbing and desorbing the gas from the surface of the detection unit 2 is extremely sensitive. Then the detection lead section
Even if 1a is turned off, the temperature of the detection unit 2 quickly becomes low.
The surface state of stage IV is maintained (stage V). This state is maintained even if the atmospheric state changes (step VI), and the detection lead portion 1a is next turned on to cause desorption of gas molecules G and adsorption of air molecules A (step VII).

As described above, the adsorbed state of each molecule on the surface of the gas sensitive material obtained in the heated high temperature state is maintained until the temperature of the surface of the gas sensitive material portion decreases, and then it is heated and rises to a predetermined temperature. It In this case, the pulse voltage application interval T ₂ ,
That is, even when the temperature of the detection unit 2 is low for a long period of time, for example, about 100 minutes, it is similarly held. Further, according to the sensor S that uses the detection unit 2 that is made of a metal oxide semiconductor and has a small heat capacity in the form of a small thin film, the response of the gas adsorption / desorption action to the on / off of the detection lead unit 1a is good, so that the pulse Voltage on time T ₁ is 1ms to 1s, off time T ₂ is 0.1s to 100
It is possible to set various application timing patterns according to the application by combining the times within the preferable range of min.

Based on the above findings, in this example, as shown in FIG. 5, a detection pulse voltage is applied to the detection unit 2 in which the gas adsorption state is held after the application of the pulse application to the heater drive circuit 4 is completed. The driving method is to obtain a detection signal. As a result, it is possible to obtain the signal r corresponding to the leak current due to the heater driving pulse voltage and the distinct signal t indicating the adsorbed state of the gas, which is different from the signal r.

In order to realize the above-described driving method, in this example, as shown in FIG. 1, a timing circuit 6 is connected to the heater driving circuit 4 and the detection circuit 5 to send a signal instructing the application timing of the pulse voltage in each of them. ing. Timing circuit 6
In FIG. 6, as shown in FIG. 6, a pulse generator circuit P is formed from a suitable number of inverters 6a, resistors 6b, diodes 6c and capacitors 6d, and is connected to the heater drive circuit 4 via the iterverter 6a. In this example, the detection circuit 6 is connected via two multivibrators 6e. As a result, the pulse drive circuit P causes the heater drive circuit 4
It is possible to generate a pulse signal having a small duty ratio to be sent to the detecting circuit 5 and generate a pulse shifted by a desired timing by the multivibrator 6e and send it to the detection circuit 5.

Next, a driving method and its operation carried out in the gas detecting apparatus configured as described above will be described based on the timing chart of FIG. Now, it is assumed that the gas to be detected has flowed into the atmosphere of the detection unit 2. Under this condition, the relay is turned on by the signal from the heater drive circuit 4 and the switches 3a and 3b are closed, and at the same time, the heater drive circuit 4 is closed.
When a pulse voltage is applied to the detection leads 1a and 1b from
The detection part 2 having a very small heat capacity is efficiently heated by the detection lead parts 1a and 1b as two heaters, and quickly becomes a state where the whole is uniformly maintained at an appropriate temperature. When the detection unit 2 is heated to an appropriate temperature, gas molecules are adsorbed on the surface thereof and the resistance value is reduced. At this time point, the signal output r corresponding to the leak current of the heater drive current to the detection unit 2 appears in the detection circuit 6. However, since the detection pulse voltage is not applied, this signal is below the signal level at which the alarm should be issued and the alarm cannot be issued.

After the application time of the predetermined heater drive pulse voltage is over,
Heater drive circuit 4 turns off the relay and switches 3a, 3b
And a signal is sent from the multivibrator 6e to the detection circuit 5 at a timing shifted by a predetermined time, and in response thereto, the detection pulse voltage is applied from the detection circuit 5 to the detection unit 2. In this case, since the surface state of the detection unit 2 is maintained at the surface state at the time when the heater driving pulse is applied, that is, the state of low resistance value in which gas is adsorbed, a high-level signal according to the detection pulse voltage. An output t ₁ is obtained and an alarm is issued to signal the presence of gas.

Eventually, the gas disappears from the atmosphere of the detection unit 2, but the state in which the gas is adsorbed on the surface of the detection unit 2 is maintained until the heater driving pulse voltage is applied. When the heater driving pulse voltage is applied at the same time when the switches 3a and 3b are closed at a predetermined interval in the same manner as the previous time, the detection unit 2 is immediately heated to a predetermined temperature, the gas is released from the surface, and the air is removed. By adsorption, the resistance value of the detection unit 2 rises to the original normal value (atmosphere is air). In this case as well, the output signal r due to the leak current appears. Similarly, when the heater drive pulse voltage is turned off and the detection pulse voltage is applied at a timing shifted by a predetermined time after the switches 3a and 3b are opened, the detection signal t ₂ is obtained accordingly. In this case, since the resistance value of the detection unit 2 is high, the signal level obtained is correspondingly low and the alarm cannot be issued.

Next, another embodiment of the present invention will be described with reference to FIG. The same components as those in the above embodiment are designated by the same reference numerals, and the description thereof will be omitted. In the detection device of this example, when the detection lead portions 1a and 1b are driven as heaters, they are connected in series. Therefore, only one end side of the pair of detection lead portions 1a and 1b is connected via the switch 7, and the other end portions are connected to the heater drive circuit 4, respectively. One end of each of the detection lead portions 1a and 1b is connected to the detection circuit 5 as in the above embodiment. in this case,
Diodes 8 and 9 are provided in each circuit in order to prevent each energization current from flowing around. Other configurations are the same as those in the above embodiment.

Therefore, during the heating operation of the detection part 2, a circuit as shown in FIG. 8 is formed and the detection lead parts 1a, 1 connected in series are connected.
By b, the detection unit 2 is efficiently heated and the whole is uniformly maintained at an appropriate temperature. When a signal is detected, a circuit similar to that of the above embodiment is formed as shown in FIG. 9 to stabilize a highly reliable detection signal. You can get it.

Although the resistance value is measured when the detection unit 2 is not heated in the above-mentioned embodiments, the resistance value is measured when the detection unit 2 is heated as in the conventional case. Good. In this case, the drive circuit may be connected to the detection circuit 5 so that the switch 3a, 3b or 7 can be opened / closed according to the operation of the detection circuit 5, and the drive pattern is opened at the time of detection.

Effect As described above in detail, according to the present invention, by connecting a pair of detection lead portions that also serve as heaters via a switch, all the detection lead portions can be used as heaters and the entire detection portion can be made uniform. It can be heated to a proper temperature. Therefore, a highly reliable detection signal can be stably obtained with low power consumption. The present invention is not limited to the above-mentioned specific embodiments, and it goes without saying that various modifications can be made within the technical scope of the present invention. For example, the object to be detected is not limited to gas, and the present invention can be applied to the detection of various substances such as liquid particles suspended in the gas phase that adsorb and desorb gas sensitive substances.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of a gas detector as one embodiment of the present invention, FIGS. 2 and 3 are explanatory views showing the operation of one embodiment of the present invention, and FIG. FIG. 6 is an explanatory view showing the operation principle of the first embodiment of the present invention, FIG. 5 is a timing chart illustrating the operation of the first embodiment of the present invention, and FIG.
FIG. 7 is a circuit diagram showing a timing circuit in one embodiment of the present invention, FIG. 7 is a block diagram showing another embodiment of the present invention, and FIGS. 8 and 9 are other embodiments of the present invention. FIG. 8 is an explanatory diagram showing the operation in FIG. (Description of symbols) 1a: Detection lead part 2: Detection part 3a, 3b, 7: Switch 4: Heater drive circuit 5: Detection circuit

Claims (1)

[Claims] 1. A detection device for detecting a change in the resistance value of a sensitive substance according to a phenomenon of adsorption and desorption of a sensitive object by heating the sensitive substance to a predetermined temperature with an electric heater. The detection part and at least a pair of detection lead parts connected to both ends of the detection part are provided in a bridge shape,
A heater drive circuit that functions as a heater that heats the pair of detection lead portions by applying a drive pulse to heat the detection portion to a predetermined temperature, and the pair of detection lead portions for detecting the resistance value of the detection portion. A detection circuit for energizing the detection pulse, a switch means interposed between the pair of detection leads, and the detection circuit energizes the detection pulse within a time when the heater drive circuit is not energizing the drive pulse. With such timing control, when the heater drive circuit energizes the drive pulse, the switch means is closed, and when the detection circuit energizes the detection pulse, the control means is controlled to open. A detection device having.