Oxygen sensor control apparatus

An oxygen sensor control apparatus for controlling an oxygen sensor which includes a detection element composed of a solid electrolyte body and a heater for heating the detection element, which is sensible to the concentration of oxygen contained in exhaust gas discharged from an internal combustion engine, and which has a characteristic such that its sensor output corresponding to the air-fuel ratio sharply changes around the stoichiometric air-fuel ratio in the course of changing between a rich state and a lean state, the oxygen sensor control apparatus comprising:

• internal resistance detection means for temporarily changing a voltage between electrodes of the detection element or a current flowing between the electrodes and detecting the internal resistance of the detection element on the basis of the amount of a change in voltage or current which occurs in response to the temporary change;
• controlled internal resistance obtaining means for obtaining a controlled internal resistance on the basis of the internal resistance;
• and
• heater energization control means for feedback-controlling the supply of electricity to the heater such that the controlled internal resistance becomes equal to a target resistance, wherein
• when a timing for detecting the internal resistance comes during a lean period during which the sensor output indicates that the air-fuel ratio is in the lean state, the controlled internal resistance obtaining means uses the detected internal resistance as the controlled internal resistance, and
• when a timing for detecting the internal resistance comes during a rich period during which the sensor output indicates that the air-fuel ratio is in the rich state, the controlled internal resistance obtaining means uses, as the controlled internal resistance, a corrected value obtained by correcting the detected internal resistance on the basis of a latest lean resistance which is the last one of the internal resistances detected during lean periods before the detection timing, such that a variation of the internal resistance which stems from a difference between the lean state and the rich state and which is contained in the detected internal resistance is removed.