1. Field of the Invention
The present invention relates to sensors for detecting particles, in particular sooty particles in an exhaust gas flow of an internal combustion engine.
2. Description of the Related Art
It is known from practice to measure a concentration of particles in an exhaust gas, for example, sooty particles or dust particles, with the aid of two electrodes which are situated on a ceramic. This may, for example, be carried out via a measurement of the electrical resistance of the ceramic material separating the two electrodes. Such sensors are, for example, used in an exhaust gas system of an internal combustion engine, for example, an internal combustion engine of the diesel version. They are generally situated downstream from the internal combustion engine or the diesel particulate filter. Due to growing environmental awareness and partly due to legal requirements, soot emissions must be monitored in the future during the driving operation of a motor vehicle, and the functionality of this monitoring must be ensured. This type of monitoring of the functionality is generally referred to as on-board diagnostics. In addition, it is necessary to predict the load of diesel particulate filters to achieve high system reliability with few efficient fuel-saving regeneration cycles, in order to be able to use more economical filter materials such as cordierite. A resistive soot sensor provides one option for this purpose, which uses the change in resistance of an interdigital electrode structure due to soot deposition to detect the soot. Based on its functionality, the resistive soot sensor is classified according to the collecting principles. Such soot sensors are, for example, known from published German patent application document DE 101 49 333 A1 or published international patent application document WO 2003/006976 A2.
In such resistive particle sensors for conductive particles, two or more metallic electrodes are formed on an electrically insulating substrate, the particles, in particular sooty particles which accumulate under the effect of a measuring voltage, short-circuiting the comb-like meshed electrodes, and a decreasing resistance or an increasing current being measurable between the sensor electrodes at a constant applied voltage. To regenerate the sensor element after soot has accumulated, the sensor element is burned clean with the aid of an integrated heating element. The evaluation of the sensor signal is carried out in the system by comparing the setpoint trigger time ascertained from a signal behavior model, taking into account the raw emission model and the actual sensor trigger time.
In order to monitor the functionality of the electrodes and thus of the sensor in the field, a measuring voltage is applied to the electrodes at the end of the regeneration. As a result, an ion stream is created, which is caused mostly by impurities in the form of sodium. If the ion stream exceeds a certain threshold value, the electrodes are to be considered to be intact.
Despite the numerous advantages of the method and device for detecting particles which are known from the related art, they still have potential for improvement. Thus, the above-described form of self-diagnosis is resistant to aging only to a limited extent. Up to now, the intrinsic conductivity of the electrode measuring cell has generally been measured once at a certain temperature. However, the aging of the sensor is generally associated with a drop in the self-diagnosis current. Thus, after the sensor has aged for a certain period of time, the self-diagnosis current may fall below the threshold value without this being attributed to a defect of the sensor. A sensor in which a current smaller than 2 μA is measured has, for example, previously been regarded in many cases as a reject, since currents up to 1.5 μA may be attributed to shunts. Thus, after a certain period of aging, it is no longer possible to differentiate whether the undershooting of the threshold value is caused by aging or by a defect.