CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International Application No. PCT/JP2018/010161 filed Mar. 15, 2018 which designated the U.S. and claims the benefit of priority from Japanese Patent Application No. 2017-086979, filed Apr. 26, 2017, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a control apparatus for a gas sensor that measures a gas concentration.

BACKGROUND ART

A gas sensor is provided in the exhaust gas passage of a vehicle having an internal combustion engine, for measuring the concentration of a specific gas (for example, oxygen) that is contained in the exhaust gas. In the same manner as for an O2sensor or an A/F sensor, a sensor for measuring the oxygen concentration has a solid oxide layer through which oxygen ions can pass, and is configured such as to vary the resulting electromagnetic force in accordance with the oxygen concentration within a detection space.

In order to accurately measure the gas concentration by the gas sensor, it is necessary to maintain the temperature of the gas sensor within an activation temperature range. Since this activation temperature range is relatively narrow, it is difficult to maintain the temperature of the gas sensor within that range through heating the gas sensor only by the exhaust gas. For that reason, the gas sensor is generally provided with a heater, and the temperature of the gas sensor is adjusted to be held within the activation temperature range by energizing the heater.

It would be possible to provide a separate temperature sensor for measuring the temperature of the gas sensor, to perform temperature adjustment as described above. However, such a configuration is undesirable, in view of increased parts cost. The temperature of the gas sensor is therefore estimated by measuring its impedance, and using a correlation relationship between the temperature of the gas sensor and its impedance.

For example, with an oxygen sensor element impedance detection apparatus (that is, a control apparatus for a gas sensor) described in PTL 1 below, a sweep voltage is applied to the gas sensor, with the gas sensor in a condition in which measurement of gas concentration by the gas sensor is temporarily interrupted. The impedance of the gas sensor is then calculated by dividing the amount of increase in voltage across the gas sensor by the amount of increase in current. After calculating the impedance of the gas sensor, measurement of the gas concentration by the gas sensor is resumed.

CITATION LIST

Patent Literature

[PTL 1] JP-A-2004-177178

SUMMARY OF INVENTION

With the control apparatus described in PTL 1, after the sweep voltage is applied to the gas sensor for calculating the impedance, the sweep voltage is returned to zero and the measurement of the gas concentration is restarted. However, charge becomes accumulated in the gas sensor due to the application of the sweep voltage, and this charge is discharged from the gas sensor after the sweep voltage is returned to zero. As a result, the electromotive force which expresses the measured value of the gas sensor may be changed by the effects of such a discharge. That is, a value of electromotive force that differs from the electromotive force corresponding to the gas concentration may be produced by the gas sensor.

In order to prevent this phenomenon, a voltage which is in the reverse direction to the above sweep voltage (also referred to as a “reverse sweep voltage” in the following) is applied to the gas sensor before returning the sweep voltage to zero and resuming the measurement of the gas concentration. By applying the voltage in the reverse direction, discharge of the charge accumulated in the gas sensor can be promoted, and the above-described effects of the stored charge on the electromotive force can be suppressed.

Preferably, the absolute value of the reverse sweep voltage should be made equal to the absolute value of the sweep voltage, in order to keep the amount of charge accumulated in the gas sensor as low as possible when measurement of the gas concentration is resumed. Furthermore it is preferable to make the length of the period for which the reverse sweep voltage is applied coincide with the length of the period for which the sweep voltage is applied.

However, with a circuit for applying a sweep voltage or the like to a gas sensor, the absolute values of the sweep voltage and the reverse sweep voltage may vary, due to the parts tolerances of circuit components, and temperature variations. Thus, it is difficult to make the absolute value of the reverse sweep voltage and the absolute value of the sweep voltage exactly match each other. It is not practical, from the aspect of parts cost, to strictly control the parts tolerances or to provide some separate means for suppressing temperature variations, in order to make the absolute values of the reverse sweep voltage and the sweep voltage exactly match each other.

It is an objective of the present disclosure to provide a control apparatus which suppresses the accumulation of charge in a gas sensor that is caused by impedance measurement, and which enables a gas concentration to be accurately measured by the gas sensor.

A control apparatus according to the present disclosure serves to control a gas sensor that measures a gas concentration, and includes a voltage application section which applies a voltage to the gas sensor for measuring the impedance of the gas sensor, a control section which controls the operation of the voltage application section, and a sweep measurement section which measures the current that flows in the gas sensor, or voltage applied to the gas sensor, or both of these. The control section executes a first control, in a first period, for operating the voltage application section such that a current flows in the gas sensor in a first direction, and executes a second control, in a second period that follows the first period, for operating the voltage application section such that a current flows in the gas sensor in a second direction that is opposite the first direction, and changes the length of at least one of the first period and the second period based on a comparison between a first measurement value and a second measurement value, where the first measurement value is the absolute value of a value measured by the sweep measurement section during execution of the first control and the second measurement value is the absolute value of a value measured by the sweep measurement section during execution of the second control.

With a control apparatus having such a configuration, after a current is passed through the gas sensor in the first direction in the first period, a current is passed through the gas sensor in the second direction in the second period. Impedance measurement is performed in the first period, and the charge accumulated in the gas sensor is removed in the second period. Furthermore with the control apparatus, the length of at least one of the first period and the second period is changed based on a first measurement value and a second measurement value that are measured by the sweep measurement section.

For that reason, even if the first measurement value and the second measurement value are different from each other due to parts variations, etc., since the length of at least one of the first period and the second period is changed, the charge accumulated in the gas sensor can be held to approximately zero, so that the electromotive force is prevented from fluctuating due to accumulated charge after the impedance measurement is performed, and thus, the gas concentration can be accurately measured by the gas sensor.

The present disclosure provides a control apparatus which suppresses an accumulation of charge in a gas sensor, caused by executing impedance measurement, and enables measurement of gas concentration by the gas sensor to be accurately performed.

