JPH0675049B2 - Sensor drive - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a sensor driving device that utilizes a change in electric resistance of a sensitive substance, and more specifically, a sensor applicable to a gas sensor, a temperature sensor, a humidity sensor, an alcohol sensor, or the like. The present invention relates to a drive device.

2. Description of the Related Art Conventionally, as a gas detection device, a heater coil also serving as an electrode is built in a gas sensitive material made of a metal oxide semiconductor, and the resistance value of the metal oxide semiconductor heated to a predetermined temperature by the heater coil is measured on the surface. There is one that utilizes the fact that it decreases due to the adsorption of gas. In this case, in order to achieve low power consumption suitable for driving dry batteries, normally, an intermittent pulse voltage is applied to the heater to generate heat, and the output from it is synchronized with the timing when the gas-sensitive substance is heated. It is configured to detect.

However, in the above-mentioned method, the current for heating the heater leaks to the gas sensitive material, and the signal output of the sensor is apparently reduced by the leak current. For example, if the resistance of the gas sensitive material is 1 MΩ in the normal state and it is sensitive to the gas and drops to 500 KΩ, the resistance is 2μ.
When the constant current for detection of A is passed, a signal whose output changes from 2V to 1V is obtained. However, if the above-mentioned leakage current exists,
Equivalent to the current equivalent to the case where a resistance of 500 KΩ is connected in parallel to the gas sensitive material, the total resistance is 333 KΩ in the normal time and the difference is 250 KΩ during the reaction, which is the same as the detection current of 2 μA. On the other hand, only a small signal changing from 0.67V to 0.5V can be obtained.

The present invention has been made in view of the above points, and an object of the present invention is to provide a sensor drive device capable of stably detecting a signal having a large SN ratio with low power consumption.

Configuration The present invention, in order to achieve the above object, in a sensor drive device that utilizes a change in the resistance value of the sensitive substance according to the adsorption and desorption phenomenon of the sensitive object by heating the sensitive substance to a predetermined temperature with an electric heater, The present invention is characterized in that the presence of the sensitive object is detected by detecting the resistance value of the sensitive substance at a timing when the electric heater is not energized.

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 in which a drive device according to an embodiment of the present invention is implemented. In FIG. 1, a sensor S is formed by providing a gas sensitive material portion 2 made of a metal oxide semiconductor in a bridge shape between a pair of lead portions 1a and 1b extending in parallel. A pair of leads 1
One of the lead portions 1a of a and 1b functions as one electrode of the gas sensitive material portion 2 and also as a heater for heating the gas sensitive material portion 2. Gas sensitive material part 2 of this example
Is made of SnO ₂ and is formed into a small thin film in order to reduce the heat capacity.
When heated to 00 ℃, it acts to adsorb and desorb gas. Therefore, when gas is present in the atmosphere, gas molecules are adsorbed to reduce the resistance value of the gas-sensitive substance itself, and when the gas is exhausted, it is released to increase the resistance value to the original value. The change in the resistance value is detected by applying a current from a detection circuit described later, and the presence of gas is notified.

Both ends of the lead portion (heater) 1a as a heater are connected to the heater drive circuit 3, and from this, for example, 1.5 to 3V.
Is applied to the heater 1a to heat the heater 1a. Also, one end of the other lead portion 1b and one end of the heater 1a are connected to the detection circuit 4, and a current is supplied to the gas sensitive material portion 2 from the detection pulse power source of 1.5 to 3V which is also provided in the detection circuit 4. Current is caused to flow, and a change in current due to a change in resistance value is 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.

In this case, as described above, the heating current from the heater drive circuit 3 leaks into the gas sensitive material portion 2, and the leak current causes a problem that the signal output is lowered when the gas of the sensor S is detected. Avoids the inconvenience as follows.

The inventors of the present application have found that the temperature of the gas sensitive material part 2 and the gas adsorption /
The following findings were obtained regarding the relationship of the withdrawal action. FIG. 3 shows the changes over time in the adsorption / desorption states of air molecules A and gas molecules G on the surface of the gas sensitive substance portion 2 when the pulse voltage is intermittently applied to the heater 1a in stages I to VII. It is divided and shown schematically. Air molecules A are adsorbed on the surface of the gas sensitive material portion 2 in the first stage when a pulse voltage is applied to the heater 1a in a state where no gas exists in the atmosphere and only air is present. Then, in the stage II when the heater is turned off,
Since the gas sensitive material portion 2 has a small heat capacity, the temperature thereof rapidly drops and reaches the equilibrium temperature within 1 ms. In this case, the state of the stage I is maintained even if the temperature of the gas sensitive material portion 2 becomes low. Next, even if the gas molecules G flow in, the surface state of the gas sensitive material portion 2 does not change because it is at a low temperature (step III). After that, when a voltage is applied and the heater 1a is turned on, the air molecules A on the surface of the gas sensitive substance portion 2 are desorbed and the gas molecules G are adsorbed instead (step IV). In this case, since the gas sensitive material portion 2 is formed in a minute and thin film, the action of adsorbing and desorbing the gas from the surface of the gas sensitive material portion 2 is performed extremely sensitively. Then, even if the heater 1a is turned off, the temperature of the gas sensitive material portion 2 is promptly lowered, so that the surface state of the IVth stage is maintained (the Vth stage). This state is maintained even when the atmospheric state changes (step VI), and the heater 1a is turned on next to cause desorption of gas molecules G and adsorption of air molecules A (step VII).
Stage).

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, the time during which the gas sensitive material part 2 is at a low temperature is, for example, 100 m.
The same holds for a long period of time such as in. Further, according to the sensor S using the gas sensitive material portion 2 made of a metal oxide semiconductor and having a small thin film and having a small heat capacity, the response of the gas adsorption / desorption action to the on / off of the heater 1a is good, and therefore the pulse is generated. Voltage on time T ₁ is 1ms to 1s, off time T ₂
It is possible to set various application timing patterns according to the application by combining times within a suitable range of 0.1 s to 100 min.

Based on the above findings, in the present example, as shown in FIG. 4, the gas sensitive substance portion 2 in which the gas adsorption state is held after the application of the pulse voltage to the heater driving circuit 3 is finished is detected. A drive system is adopted in which a detection pulse voltage is applied 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 driving method as described above, in this example, as shown in FIG. 1, the heater driving circuit 3 and the detection circuit 4 are connected with a timing circuit 5 for sending a signal instructing the application timing of the pulse voltage in each of them. ing. Timing circuit 5
In FIG. 2, as shown in FIG. 2, a pulse generator circuit P is formed from an appropriate number of inverters 5a, resistors 5b, diodes 5c and capacitors 5d, and is connected to the heater drive circuit 3 via the inverter 5a. In this example, the detection circuit 4 is connected via two mono multivibrator 5e. As a result, the pulse generation circuit P generates a pulse signal with a small duty ratio to be sent to the heater drive circuit 3,
On the other hand, a pulse shifted by a desired timing can be generated by the mono-multivibrator 5e and sent to the detection circuit 4.

Next, a driving operation as one embodiment of the method of the present invention which is carried out in the gas detecting apparatus configured as described above will be explained based on the timing chart of FIG.
Now, it is assumed that the gas to be detected has flowed into the atmosphere of the gas sensitive material portion 2. In this state, when the pulse voltage of the heater drive circuit 3 is applied, the gas sensitive substance portion 2 is heated to a predetermined temperature, gas molecules are adsorbed on the surface thereof, and the resistance value is lowered. At this point, the signal output r corresponding to the leak current of the heater driving current to the gas sensitive material portion 2 appears in the detection circuit 4. 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.

When the application time of the predetermined heater drive pulse voltage is completed, a signal is sent from the multivibrator 5e to the detection circuit 4 at a timing shifted by a predetermined time, and in response thereto, the detection pulse voltage is sent from the detection circuit 4 to the gas sensitive substance portion 2. Is applied. In this case, the surface state of the gas sensitive material portion 2 is maintained at the surface state at the time when the heater driving pulse is applied, that is, the low resistance value state in which the gas is adsorbed. A signal output t _{1 of} is obtained and an alarm is issued to notify the presence of gas.

Eventually, the gas disappears from the atmosphere of the gas sensitive material portion 2, but the state in which the gas is adsorbed on the surface of the gas sensitive material portion 2 is maintained until the heater driving pulse voltage is applied. Then, when the heater driving pulse voltage is applied at a predetermined interval, the gas sensitive material portion 2 is immediately heated to a predetermined temperature, the gas is released from the surface thereof, and air is adsorbed, and the resistance of the gas sensitive material portion 2 is increased. The value rises to the original value. still,
In this case as well, the output signal r due to the leak current appears. Similarly, when the detection pulse voltage is applied at a timing shifted by a predetermined time after the heater driving pulse voltage is turned off, the detection signal t ₂ is obtained accordingly. In this case, since the resistance value of the gas sensitive material portion 2 is high, the signal level obtained is correspondingly low, and the alarm cannot be issued.

Effect As described above in detail, according to the present invention, by detecting the resistance value of the sensitive material at the timing when the heater is intermittently energized and not energized, the leak current from the heater current and the like with low power consumption can be obtained. It is possible to stably obtain an accurate signal with a large SN ratio that is not affected by. 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, but is adsorbed by a sensitive substance such as liquid particles suspended in the gas phase.
The present invention can be applied to the detection of various substances that have a withdrawal action.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a gas detection device in one embodiment of the present invention, FIG. 2 is a circuit diagram showing a timing circuit in one embodiment of the present invention, and FIG.
FIG. 4 is an explanatory view showing the operating principle of the embodiment, and FIG.
It is a timing chart figure explaining operation in an example. (Description of symbols) 1a: Heater (lead) 2: Gas sensitive material 3: Heater drive circuit 4: Detection circuit 5: Timing circuit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Miyoshi 1-9-17 Omorinishi, Ota-ku, Tokyo Within Ricoh Seiki Co., Ltd. (56) References JP-A-61-128149 (JP, A) Kaisho 59-185657 (JP, U)

Claims (1)

[Claims] 1. A sensor driving device for heating a sensitive substance to a predetermined temperature with an electric heater to detect a change in the resistance value of the sensitive substance in accordance with a phenomenon of adsorption and desorption of a sensitive object. A detection unit, a heater connected to the detection unit, and a detection lead unit connected to the detection unit in a bridge shape, and a heater drive circuit for driving the heater by energizing the heater with a drive pulse. A detection circuit for supplying a detection pulse to the detection lead part to detect the resistance value of the detection part; and the detection circuit for supplying a detection pulse within a time when the heater drive circuit is not supplying a drive pulse. And a control unit for performing timing control as described above.