Device for Detecting Different Pollutants in Gas Streams

The invention relates to a device for the detection of different pollutants in gas streams, preferably for feeding to the cabins of motor vehicles or to protection cabins for operators, involving the use of one or more pollutant-detecting semiconductor gas sensors.

It is known to use pollutant-detecting gas sensors for the control of fresh air/ambient air valves. These sensors additionally may be employed for switching on- and off filter devices in motor vehicle cabins and protection cabins for equipment operators. Furthermore, it is known that these semiconductor gas sensors, due to their measuring mode and manufacture, have tolerances too high for this type of application.

A great number of proposals have been made to improve the sensitivity and stability of these sensors. These have, however, not been successful. For instance, when the pollutant-detecting gas sensors are used in motor vehicles, the transverse sensitivity towards various travel speeds, temperatures, degrees of humidity and so on, is still unacceptably high.

In order to eliminate these disadvantages, the invention proposes that a gas sensor should be exposed alternately to the gas streams to be evaluated. The sensor is mounted at the separation point between the outside air and the cabin air. Thereby through switching, effected within seconds, practically a constant comparison between the quality of the inner air and the outer air is established. For this purpose, the gas sensor cooperates with a switching device. The gas sensor and the switching device can be arranged on a fresh air/ambient air valve. The output signals of the gas sensor serve for controlling the fresh air/ambient air valve.

It is known that semiconductor sensors, particularly of the stannic dioxide variety, have a very high transverse sensitivity towards temperature- and humidity variations such as in the degree of humidity. Sometimes the variations are so high that they can lead to deviations up to 70% in the measured value, namely due to various humidity degrees. The influence of temperature variations can have similar effects. It is known to create a temperature compensation by means of temperature sensors. Humidity can be technically measured only as relative humidity. This measured value, however, leads to very different absolute humidities at various air temperatures. Measurement of the realtive humidity thus cannot be directly used for the correction of the humidity flow. As a result, the correction becomes very complicated and can be generally figured out only by a computer.

In order to be capable to perform especially a correction of temperature- and humidity variations in a gas stream, the invention proposes that the measured value established by a semiconductor sensor be corrected by a microprocessor-originated correction factor, determined by an average value of temperature and humidity. The temperature- and humidity values are set in a two-dimensional matrix, so that in a microprocessor of only 4 K capacity a field with 200 values can be stored, which is sufficiently adequate to carry out a quite accurate compensation.

Specific embodiments of the invention are now detailed with the aid of the drawing, which shows:

FIG. 1 is a schematic representation of a motor vehicle which can be equipped with a device for detecting various pollutants in a gas stream according to the invention;

FIG. 1A is a detail of the region in which the gas sensor is placed in the vehicle of FIG. 1;

FIG. 1B is a view of a portion of FIG. 1A drawn to a larger scale showing the location of the gas sensor with its switching device on the air valve flap; and

FIG. 2 is a circuit diagram in block form of a switching arrangement for correcting the temperature and humidity oscillations in a gas stream.

FIG. 1 illustrates a motor vehicle 1 and the air supply area 2 for the motor vehicle. The pollutant-detecting gas sensor 4 is placed at the separation point between the outer air and the cabin air, being provided with a switching device. Gas sensor 4 with its switching device can be arranged on the ambient-air valve 3 as will be apparent from FIGS. 1A and 1B.

Due to this configuration, it is insured that the sensor mounted at a stationary measuring point for detecting the absolute values of pollutant, no longer requires an enormous checking and setting effort. Moreover the possibility now exists to use simple sensors, which can safely evaluate the difference between better or more polluted air and, in the end, give the output signal for the control of the fresh air/ambient air valve.

The basic concept of this embodiment consists thus in bringing about the continuously running comparison between the quality of two air streams, by means of a pollutant-detecting gas sensor with the assistance of a mechanical switching device.

In the embodiment of FIG. 2 there is schematically illustrated a gas sensor 11, temperature sensor 12, and humidity sensor 13. The three sensors 11, 12, 13 transmit their values in common to the microprocessor 14.

The microprocessor 14 seeks for the temperature- and humidity values each time the corresponding correction value, which corrects the gas sensor value.

This way, a heat- and humidity-independent output signal is obtained.