WO2005015190A1

WO2005015190A1 - Method and arrangement for detecting substances by means of regulation of an electrical resistance of a sensor

Info

Publication number: WO2005015190A1
Application number: PCT/CH2004/000486
Authority: WO
Inventors: Albert Schiess
Original assignee: Saia-Burgess Murten Ag
Priority date: 2003-08-06
Filing date: 2004-08-03
Publication date: 2005-02-17
Also published as: CH696777A5; DE112004001342D2; WO2005015190A8; WO2005015190A9

Abstract

According to the invention, the detection of materials with at least one sensor (10), the electrical resistance (11) of which changes due to the effect of a material for detection, whereby a regulation circuit (30) is used to regulate the electrical resistance to a given set value. Disturbances in the sensor measured signal can thus be reduced.

Description

CONTROLLABLE HEATING TO CONTROL THE ELECTRICAL RESISTANCE OF A GAS SENSOR

The invention relates to an arrangement with a sensor and a method for detecting substances according to the preamble of claim 1 and the first method claim.

Such arrangements and methods are used, for example, to detect air pollution in order to be able to control the ventilation system of an automobile accordingly. If the sensor detects increased air pollution in the sucked-in outside air, for example due to a short distance to the front automobile or when driving through a tunnel, the ventilation is switched to circulating air operation by means of a control signal, so that no further odors or pollutants get into the interior of the automobile can.

For example, a semiconductor with a heated active layer made of metal oxide is used as the sensor, the electrical resistance of which changes when the gases to be detected act upon it. 1 shows typical characteristics for such semiconductor sensors, the horizontal axis 4 corresponding to the gas concentration C and the vertical axis 5 corresponding to the resistance 5 R. When exposed to a reducing gas (e.g. CO, C _x H _y , C _x H _y OH), the active layer of the sensor becomes low-resistance (dash-dotted curve 6), when exposed to an oxidizing gas (e.g. NO _x , 0 ₃ ) they have a higher impedance (dashed curve 7). The horizontal 0 line 8 indicates the value for the resistance in normal air.

However, reliable detection of the substances is made difficult by various influencing factors: - The physical properties of the air such as temperature and humidity vary in time and location, so that the electrical resistance of the sensor changes even in the absence of a substance to be detected. - The basic pollution of the air with pollutants differs locally. For example, the air in the city is generally more polluted than in the country, and the levels of pollutant peaks also vary accordingly. - Each sensor has an individual response behavior due to tolerances in manufacture, aging, operating time, etc.

Various methods are known for taking these influencing factors into account when detecting it:

The respective current measured value for the electrical resistance is compared with the mean value formed over a certain time. In the case of pollutant peaks, the current measured value is significantly greater than the mean value (see, for example, the patents DE-C3-37 31 745 and US-A-4 930 407.)

- The differential is formed from the measured values by means of an electrical high pass. Pollutant peaks lead to a rapid change in the pollutant concentration and thus to a large differential value (see e.g. the patent specification DE-C3-33 04 324.)

The known arrangements and methods have in common that the sensor is operated at a constant temperature and that the changes in the electrical resistance are always evaluated. However, this has the disadvantage that relatively complex measures have to be taken to keep the temperature constant. Another disadvantage is that an interference signal originating from the influencing factors described above is superimposed on the actual measurement signal, which is generated by the action of the substances to be detected, and this has to be filtered out by a suitable downstream evaluation device. However, this can occasionally lead to incorrect measurements and, when using sensors of the same type, to a different response behavior of the arrangement.

Starting from this prior art, it is an object of the invention to largely eliminate these disadvantages and to provide an arrangement and a method such that the measurement signals generated by the sensor have a reduced proportion of interference signals.

An arrangement according to the invention and a method according to the invention which or which achieves this object are specified in claim 1 and independent method claim.

The invention is described below using a

Exemplary embodiment explained with reference to figures.

1 shows the resistance R of a semiconductor sensor as a function of the concentration C of an oxidizing or reducing gas;

2 schematically shows an arrangement according to the invention for the detection of substances;

Fig. 3 shows the resistance R of a semiconductor sensor as

Function of sensor temperature T;

Fig. 4 shows the time course of the heating power when switching on; FIG. 5 shows a possible embodiment of the arrangement according to FIG. 2, in particular the evaluation device;

Fig. 6 shows schematically a housing with the sensor according to Fig. 2, - and

FIG. 7 shows a supplemented variant of the arrangement according to FIG. 2.

As shown schematically in FIG. 2, the arrangement comprises a sensor 10 with an active layer 11 and a heater 12 for heating the active layer 11, a heating controller 16 for regulating the heater 12, a central evaluation and control unit 17, and a measuring unit 18 for recording the current value of the active layer resistance R and an evaluation device 19 for generating switching pulses. The active layer 11 and the heater 12 are electrically insulated from one another, but are thermally coupled to one another.

The sensor 10 is, for example, a semiconductor gas sensor with a heated active layer 11 made of metal oxide such as tin dioxide, tungsten trioxide, gallium oxide, zinc oxide, etc. or a mixture of such metal oxides. Platinum and / or palladium can also be added as a catalyst in order to accelerate chemical reactions between metal oxide and the substances to be detected, in particular gases. 1 shows, the electrical resistance R of the active layer 11 changes strongly non-linearly with the gassing. The sensitivity of the sensor 10 corresponds approximately to the concentration range typically found in air contaminated by road traffic (for reducing gases in the Range from approx. 1 ppm to approx. 100 ppm, for oxidizing gases in the range from approx. 100 ppb to approx. 2000 ppb).

As a semiconductor, the active layer 11 has a strongly temperature-dependent resistance R. Fig. 3 shows R as a function of the sensor temperature T with normal air (no pollutants, ambient temperature 20 ° C, relative humidity 65%). At normal operating temperature 26 (in the example according to FIG. 3 about 350 ° C.), a certain value 27 is set for the resistance R (e.g. 10 kOhm). As curve 28 in Figure 3 shows, R decreases as T increases, while R increases as T decreases. The resistance R changes more in the temperature range from 250 ° C to 450 ° C than it would due to gas concentration fluctuations, as typically occur in road traffic.

Depending on the application, a correspondingly designed sensor, in particular a semiconductor sensor, can of course be used. For example, it is conceivable to use a sensor that is suitable for the detection of air pollutants such as CO, C _x H _y , C _x H _y 0H, NO _x , 0 ₃ and other harmful or odorous substances, vaporous substances, certain liquids and / or gases.

With the arrangement shown in FIG. 2, the sensor 10 is operated in such a way that the electrical sensor resistance R is brought to a predetermined setpoint value by regulating the heating temperature or is kept there. (This is in contrast to the common methods in which the

Temperature T of sensor 10 is kept constant and is therefore not R, but T is the controlled variable.) The electrical resistance of the active layer 11, the measuring unit 18, the evaluation and control unit 17, the heating controller 16 and the heater 12 form a (closed) control circuit 30. During the control, the current value for R (“actual value”) is continuously determined by the measuring unit 18 and compared in the central evaluation and control unit 17 with the predetermined target value. This controls the heating controller 16 in accordance with the control deviation (difference between the actual value and the target value), which then influences the heating power of the heater 12 so as to act on the resistance of the active layer 11. The heating power is set, for example, by changing the voltage at the connections of the heater 12.

To determine the target value, the sensor 10 is gassed with normal air (no pollutants, ambient temperature 20 ° C., relative air humidity 65%) and the resistance of the active layer 11 is measured. This measured value specifies the setpoint at which the active layer resistance is regulated.

The heating controller 16 is designed, for example, as a PI controller (proportional integral controller), wherein the proportional coefficient K _p and the integration time (readjustment time) T _N can be freely specified. The integration time T _N is chosen such that it is greater than the time in which the changes in the substance concentration to be detected occur. Typically, T _{κ is} in the range of minutes, for example in the range of 5 to 10 minutes or more. As a result of this measure, the control circuit 30 acts in such a way that the sensor resistance is only updated slowly and with a time delay.

This sluggish control ensures that there is a rapid change in the sensor resistance, which is caused by a sudden increase in the one to be detected Concentration of substances ("pollutant peak") is caused, is not corrected and can therefore be subjected to an evaluation. Rapid changes in the substance concentration can thus be recorded. On the other hand, the influencing factors, which, as mentioned in the introduction, have a disruptive effect on the resistance measurement, can be regulated, since they change only slowly over time. The measurement signals supplied by the sensor 10 to the evaluation device 19 therefore have a greatly reduced proportion of interference signals, which are caused by the influencing factors.

So that the sensor 10 is operated neither in the area of the overload nor the ineffectiveness, the control circuit 30 is designed such that during the control the heating temperature of the heater 12 or the heating power does not exceed a freely defined maximum value and does not fall below a freely defined minimum value.

In addition to the proportional and integral part, the heating controller 16 preferably also includes one

Differential component, which is designed in such a way that a control deviation has a disproportionate effect, but nevertheless acts subcritically, so that tendencies to oscillate are suppressed. Supplementing the differential component therefore means that the controlled variable reaches its setpoint earlier and settles faster.

Optionally, the control circuit 30 can be designed asymmetrically in its control behavior, so that the case in which the sensor resistance tends to be based on the

Setpoint moved (positive control deviation), and the case in which it tends to move away from the setpoint (negative control deviation) are regulated differently. This results in an asymmetrical control behavior, for example achieved that the control circuit 30 has different integral parts by assigning different values for K _p and / or T _N to the positive and negative control deviations. An asymmetrical control is particularly advantageous if the behavior of the sensor 10 is different in the high-resistance range (R greater than the target value) and in the low-resistance range (R less than the target value).

If the active layer 11 of the sensor 10 is unheated, its surface is contaminated by foreign substances in the air. In order to obtain an active layer 11 with reproducible properties for operation, the foreign substances deposited on its surface must be removed in a suitable manner. So that this can be done in a controlled manner, the heater 12 or the heating controller 16 is first subjected to a forced operation after the sensor 10 is switched on, before the closed control circuit 30 is released for control. For this purpose, the evaluation and control unit 17 controls the heating controller 16 so that the

Voltage on the heater 12 and thus the heating power has a predetermined time course.

4 shows an example of the course of the heating power H (ordinate) as a function of the time t (abscissa). In the start phase, t ₀ <= t <= t_, the heating power H or voltage across the heater 12 is slowly increased in the form of a ramp function. In this phase, the foreign substances desorb serially from the active layer surface according to their volatility, as a result of which gentle cleaning is achieved without coking occurring. The gentle cleaning is continued by the heating 12 being operated with the final heating power for a specific time interval, ti <= t <= t ₂ or t ₃ <= t <= t ₄ . To remove resistant particles or foreign matter ("forced desorption") there is an overheating phase in the time interval t ₂ <t <t ₃ , in which the heater 12 is operated with an overload. The amplitude and duration of this overload are limited in such a way that damage to the heater 12 or the active layer 11 is avoided. At the point in time t, the forced control of the controller is ended and free regulation by the control circuit 30 follows.

Depending on the application, they will be different

Evaluation devices are used to evaluate the measurement signals supplied by the sensor 10. 5 shows a possible embodiment of the evaluation device 19 in the form of an analog circuit.

The regulating and control device 31 comprises the measuring unit 18, the heating controller 16 and the central evaluation and control unit 17. The output of the regulating and control device 31 is connected to the anode 12 of the heater 12.

The anode 36 of the active layer 11 is connected to a potential via a first resistor 37 and is connected to the input of the regulating and control device 31 or the inverting input 41 of a first comparator 40. The inverting input 41 and the non-inverting input 42 of the first comparator 40 are connected to the non-inverting input 52 and inverting input 51 of a second comparator 50, respectively. The output 43 of the first comparator 40 is via a second one

Resistor 44 is fed back with its non-inverting input 42, which is connected to an adjustable resistor 47, to which a voltage is applied. The output 43 of the second comparator 50 is with its non- inverting input 52 fed back via a third resistor 54. The two resistors 44 and 54 are designed such that a control signal is generated at the output 43, 53 of the first and second comparators 43 and 53, respectively, when the voltage at the anode 36 exceeds a freely defined upper threshold value or a freely defined lower value Falls below the threshold. The upper and the lower threshold value can be selected differently, for example to better take into account the fact that the sensor 10 can be more sensitive in the high-resistance area than in the low-resistance area.

The operation of the arrangement shown in Fig. 5 is as follows:

To set the target value for the electrical resistance of the active layer 11, the adjustable resistor 47 is set such that the desired voltage is established at the anode 36 of the active layer 11 (for example U / 2 in the event that a potential U is applied to the first resistor 37) and its value is equal to the setpoint). The sensor resistance is regulated to the predefined setpoint value by means of the heating control circuit 30. For this purpose, the current voltage is measured at the anode 36, the reciprocal of which is proportional to the respective actual value of the sensor resistance, and the heating 12 is regulated by means of the regulating and control device 31 in such a way that the voltage at the anode 36 and thus the sensor resistance tends to tend is kept constant. As described above, a large integration time T _{N is} selected for the control circuit 30, so that brief voltage changes are not corrected. If, for example, an increase in the concentration of a reducing gas occurs in a pulsed manner, the sensor resistance decreases briefly, as a result of which the voltage increases the anode 36 increases. If this voltage is above the threshold value of the first comparator 40, a switching signal is generated at its output 43. In a corresponding manner, a switching signal is generated at the output 53 of the second comparator 50 if its threshold value is not reached when the voltage at the anode 36 drops due to a pulse-like increase in the concentration of an oxidizing gas. The switching signals generated by the comparator 40 or 50 can be used, for example, to control a ventilation system.

Of course, it is also possible to use a digital circuit instead of an analog circuit according to FIG. 4. For this purpose, the analog voltage signal supplied by the sensor 10 is converted into a digital signal by means of an A / D converter and evaluated using a suitable logic circuit or a microcomputer.

In addition to the advantages already mentioned, regulation of the

Sensor resistance to a setpoint by means of the controlled sensor heating also advantageous for the following reason:

The maximum sensitivity of the semiconductor sensor for an oxidizing gas is at temperatures that are rather below the normal temperature. For a reducing gas, on the other hand, the sensitivity becomes maximum at temperatures that are above normal temperature. If, for example, the sensor 10 is gassed with carbon monoxide CO, the sensor resistance drops, so that the sensor temperature is also lowered due to the regulation. This negative feedback makes the sensor 10 less sensitive to carbon monoxide. As can be seen from Fig. 1, has a Semiconductor sensor typically has a strongly curved, exponentially extending characteristic curve 6 or 7. The negative feedback just mentioned leads to the characteristic curve 6 or 7 tending to be linearized. The same changes in the gas concentrations thus produce approximately the same

Amounts of change in the electrical resistance, irrespective of the basic pollutant load currently acting on the sensor 10.

Another advantage of the method and the arrangement according to the invention is particularly evident when there are large fluctuations in ambient temperature. In technical implementations, the sensors 10 are located in housings. 6 schematically shows a housing 60 in which the sensor active layer 11, which can be connected to the measuring unit 18 via the connections 11a and 11b, and the heater 12, which can be connected to the heating controller 16 via the connections 12a and 12b, are arranged , T _aItlb denotes the ambient temperature, T the temperature of the active layer 11. The sensitivity of the sensors 10, in particular semiconductor gas sensors, depends on the temperature T. An optimal working temperature is, for example, T = 350 ° C. The ambient temperature T _amb is generally much lower and fluctuates in practice in large areas, for example in automobiles in the range from -40 ° C to + 150 ° C. Thus, as shown in FIG. 6 by the double arrow 61, there is a temperature gradient between the active layer 11 and the surroundings or the wall of the housing 60.

If the heater 12 were now operated with constant heating power in accordance with the conventional methods, there would be considerable deviations in the temperature T der due to the temperature gradient 61 and radiation and convection in the housing 60 Active layer 11. This could lead to complete loss of sensitivity when certain temperature limits are exceeded or fallen below. This effect is prevented here by regulating the active layer resistance R to a constant setpoint.

The methods and arrangements described here can be used in a variety of ways. Among other things, you can used for the detection of substances in the air, in particular pollutants and / or odors, and / or for the control of air conditioning and / or ventilation systems, such as those found in motor vehicles, buildings, rooms, etc. In particular, 19 switching pulses can be generated with the evaluation device, which serve to control air recirculation flaps in air conditioning and / or ventilation systems of motor vehicles.

Several sensors 10 can also be used, which are coupled to one another, e.g. To be able to compare measurement signals from several locations.

Numerous modifications are accessible to those skilled in the art from the foregoing description without departing from the scope of the invention, which is defined by the claims.

As explained above, the sensor 10 comprises an active layer 11, the electrical resistance R of which is a measure of the substance concentration C, and a controllable heater 12, the heating power of which can be changed as a function of the resistance R to regulate the resistance R to a predetermined target value. The signal supplied by the sensor 10 has a greatly reduced proportion of interfering signals, for example, compared to the conventional methods Changes in ambient temperature or humidity are due. In order to further reduce the interference signal component, it is also conceivable to use a temperature sensor. FIG. 7 shows an arrangement with sensor 10 and control circuit 30 as well as a temperature sensor 65 which is in operative connection with evaluation device 17 and by means of which the ambient temperature is measured repeatedly. The influence of this disturbance variable is represented by a variable resistor 66, which acts on the heating controller 16. The ones from

Temperature sensor 65 and sensor 11 are processed by the evaluation and control unit 17, so that a measurement signal with a reduced interference component is available.

Glossary:

C: Concentration of a substance to be detected

K _p : proportional coefficient of the PI controller R: electrical resistance of the sensor

T: temperature of the sensor

Ta _mb : ambient temperature

Tu: integration time of the control circuit ppb: parts per billion (billionth, 10 ^"9 ) ppm: parts per million (millionth, 10 ^{" 6} )

Claims

claims

1. Arrangement for the detection of substances with at least one sensor (10), the electrical resistance of which can be changed by the action of a substance to be detected, characterized by a control circuit (30) for regulating the electrical resistance (11) to a predetermined target value.

2. Arrangement according to claim 1, characterized in that the control circuit (30) comprises a controllable heater (12).

3. Arrangement according to one of claims 1 to 2, characterized in that the control circuit (30) has a freely defined integration time (T _N ).

4. Arrangement according to one of claims 1 to 3, characterized by an evaluation device (19) connected to the sensor (10) for generating control signals.

5. Arrangement according to one of claims 1 to 4, characterized in that the sensor is a semiconducting gas sensor (10) and preferably has an active layer (11) made of metal oxide.

6. Arrangement according to one of claims 1 to 5, characterized in that the control circuit (30) with the active layer (11) of the sensor (10) connected measuring unit (18) and one with the heater (12)

Heater controller (16) includes.

7. Method for the detection of substances by means of at least one sensor (10), the electrical resistance (11) of which can be changed by the action of a substance to be detected, characterized in that the electrical resistance is regulated to a predetermined target value.

8. The method according to claim 7, characterized in that the regulation takes place more slowly than the changes in the substance concentration to be detected.

9. The method according to any one of claims 7 to 8, characterized in that the regulation of the electrical

Resistance (11) by acting on the temperature of the sensor (10). .

10. The method according to any one of claims 7 to 9, characterized in that one with the active layer (11) of

Sensor (10) thermally coupled heater (12) is controlled to act on the temperature of the active layer.

11. The method according to any one of claims 7 to 10, characterized in that the control is asymmetrical, so that positive and negative deviations of the actual value from the target value are compensated differently.

12. The method according to any one of claims 7 to 11, characterized in that a control signal is generated when the

Value of the electrical resistance exceeds an upper threshold and / or falls below a lower threshold.

13. Use of the arrangement according to one of claims 1 to 6 or the method according to one of claims 7 to 12 for controlling an air conditioning and / or ventilation system, in particular a motor vehicle, and / or for the detection of substances in the air, in particular harmful substances and / or odorous substances.