The present application is related to, and claims priority in, Japanese Patent Application No. Hei. 10-197174, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to impedance detecting apparatuses, and particularly to an apparatus for detecting the element impedance of an oxygen concentration sensor.
2. Discussion
In conventional vehicle air/fuel ratio control systems, catalyst exhaust gas purifying performance is enhanced by detecting the oxygen concentration in the exhaust gas with an oxygen concentration sensor. Based on the detected value, the air/fuel ratio of the mixture to be sucked into the engine may be feedback-controlled. Since the oxygen concentration sensor output voltage generally is highly dependent on temperature, the element temperature has to be kept at a proper level to maintain the detection accuracy of the sensor.
Therefore, some oxygen concentration sensors are equipped with a heater, the energization of which is feedback-controlled to keep the element temperature at or above the activation temperature (e.g., about 600xc2x0 C. or higher). In this system, the element temperature has to be detected for feedback control of the heater energization. However, the temperature sensor increases the size of the oxygen concentration sensor and increases the associated system cost.
Due to the fact that the sensor element impedance changes with the element temperature, it has been proposed to compute the element temperature from an element impedance by detecting the latter. Unexamined Published Japanese Patent Application No. Hei. 9-292364 discloses a method for detecting element impedance. In this method, an element application voltage Vo is switched from a reference voltage at the oxygen concentration detecting time to a sweeping voltage for detecting the element impedance so that the element impedance is detected from a voltage change xcex94Vo at that time and a voltage change xcex94Vi according to a current change in response to the voltage change xcex94Vo.
In the oxygen concentration sensor system, an element current detecting voltage Vi according to the element current (or oxygen concentration) and the element application voltage Vo are fetched for a constant sampling period (e.g., period of 4 ms) by a microcomputer through the A/D converter to detect the oxygen concentration and accordingly the element impedance. Immediately-before the element application voltage Vo is to be swept to a sweeping voltage, as shown in FIG. 18, the element application voltage Vo and the element current detecting voltage Vi are sequentially A/D-converted twice during the sweep. Since one A/D conversion takes a time period of 50 xcexcs, for example, an A/D conversion time period of at least 100 xcexcs is required during the sweep if two A/D conversions are made during the sweep.
In the practical system, channel signals other than the voltages Vo and Vi may be A/D converted during the sweep, and the A/D conversions of the voltages Vo and Vi may be delayed during the sweep by the A/D conversions of the other channel signals. It is therefore necessary to retain the time period for the A/D conversion to some extent. On the other hand, the period for detecting the oxygen concentration is preferably set to a short value (e.g., a period of 4 ms) to improve the detection responsiveness to the change in the oxygen concentration.
After the sweeping time period lapses, to converge the element current quickly into an ordinary state, the element application voltage Vo is switched to a return voltage deviated to the opposite side from the reference voltage by the difference between the reference and sweeping voltages. This deviation promotes the release of the charge stored in the sensor element during the sweep and latches the return voltage for the same time period as the sweeping time period. The element application voltage Vo is returned to the reference voltage at the instant when the charge stored by the capacity component of the element is released. If the sweeping time period is set long considering that the A/D conversions of the two voltages Vo and Vi during the sweep are delayed by the A/D conversions of the other channel signals, the return time period has to be accordingly increased to delay the more the element application voltage Vo to the reference voltage after the sweep. As a result, before the element application voltage Vo returns from the return voltage to the reference voltage, the timing of the A/D conversions of the next voltages Vo and Vi may arrive, and the voltage Vi according to the element current (or oxygen concentration) may not be accurately detected. Such inaccurate detection thereby compromises the detection accuracy of the oxygen concentration. If the sweeping time period is shortened to avoid this problem, the A/D conversions of the other channel signals cannot be made during the sweep.
Therefore, it is an object of the present invention to provide an oxygen concentration sensor element impedance detecting apparatus that shortens the sweeping time period for detecting the element impedance of the oxygen concentration sensor to detect the oxygen concentration with a high degree of accuracy without being influence by the sweep, and that -performs the A/D conversion of the signal to be used in subsequent control during the sweep.
More particularly, with an oxygen concentration sensor element impedance detecting apparatus according to the present invention, at least two sweeping operations are performed to switch the element application voltage from the reference voltage to the sweeping voltage when the element impedance is detected. Thus, the detection (or A/D conversion) of the element application voltage and the detection (or A/D conversion) of the element current detecting voltage can be performed during the different sweeping time periods.
Therefore, the element application voltage is controlled so as not to be changed by the element current (or oxygen concentration), so that detection results substantially identical to those when the detection of the element application and element current detecting voltages are made for one sweep can be obtained even if the detection of the two voltages are made for different sweeps. On the basis of these detection results, the element impedance is detected.
As a result, only one of the element application voltage and the element current detecting voltage need be detected for one sweep, so that the sweeping time period can be accordingly made shorter than that of conventional impedance detecting apparatuses. Thus, oxygen concentration can be accurately detected without any influence from the sweep, and the signals to be used for another control during the sweep can be A/D converted.