The invention relates to a circuit for determining the internal resistance of a linear lambda probe.
The transmission function of a linear lambda probe has a high degree of dependence on temperature, which has to be compensated by controlling the probe temperature. However, for reasons of cost the probe temperature is not measured by means of a separate sensor (for example Pt100) but instead the high temperature dependence of the probe impedance Ri is utilized. FIGS. 1a and 1b show the temperature dependence and equivalent circuit of the probe impedance. Here, R1/C1 represents the contact resistance between electrodes and ceramic material, R2/C2 represents the junction between the grain boundaries of the sintered ceramic grains, and R3 represents the intrinsic resistance of the sintered material.
R1 is highly subject to ageing and therefore cannot be used for measuring temperature. Given a suitable selection of the measurement frequency—for example 3 kHz—R1 is short-circuited in terms of AC voltage by means of C1; it therefore makes no contribution to the overall impedance any more. The series connection of R2/C2 and R1 yields an absolute value of 100 ohms with this measuring frequency at approximately 500° C. and can be used for determining the temperature.
The older patent application 2000 P 12334 DE (official file number not yet known) which was not published before the priority date describes a customary measurement method for determining the probe impedance Ri. According to said method, a square-wave AC current, for example 500 μAss (peak-to-peak), is applied to the probe impedance.
An AC voltage of 500 μAss*100 ohm=50 mVss is produced at Ri. This AC voltage is amplified and rectified and can then be fed to a microprocessor for controlling the temperature.
The AC current is generated according to FIG. 2, for example, by means of a 3 kHz square-wave oscillator which is supplied with 5 V. The signal is conducted to the probe impedance via a high impedance resistor Rv and a decoupling capacitor Cv.