JP6350342B2 - Applied voltage controller - Google Patents

Description

  The present invention relates to an applied voltage control device that controls an applied voltage of an air-fuel ratio sensor.

  As shown in Patent Document 1, a gas sensor detection system including a gas sensor element and a gas sensor control circuit is known. The gas sensor control circuit applies a voltage to the gas sensor element to cause the pump current corresponding to the measured gas concentration to flow in the gas sensor element, and detects it.

  The gas sensor element and the gas sensor control circuit are electrically connected via a switch. The gas sensor control circuit outputs a battery voltage abnormality detection circuit that outputs an abnormality detection flag indicating whether or not the power supply voltage of the battery power supply is less than a threshold value, and a switch control circuit that performs ON / OFF control of the switch based on the abnormality detection flag; , Including. The battery voltage abnormality detection circuit sets the abnormality detection flag to 0 when the power supply voltage is higher than the threshold value, and sets the abnormality detection flag to 1 when the power supply voltage is lower than the threshold value. The switch control circuit controls the switch to an ON state when the abnormality detection flag is 0, and electrically connects the gas sensor element and the gas sensor control circuit. On the other hand, when the abnormality detection flag is 1, the switch control circuit controls the switch to the OFF state to cut off the electrical connection between the gas sensor element and the gas sensor control circuit. As a result, the gas sensor control circuit that has malfunctioned due to a drop in the power supply voltage (battery voltage) of the battery power supply can prevent abnormal current from flowing through the gas sensor element.

JP 2007-205805 A

  As described above, in the gas sensor detection system disclosed in Patent Document 1, when the battery voltage is less than the threshold value, the electrical connection between the gas sensor element and the gas sensor control circuit is interrupted, and the abnormal current flows to the gas sensor element. doing. However, with this configuration, the measured gas concentration cannot be detected at all when the battery voltage decreases.

  In addition, although the structure which applies a voltage to a gas sensor element (air-fuel ratio sensor) was not mentioned in particular, the structure shown below can be considered as the structure. That is, a differential amplifier circuit whose applied voltage changes according to the sensor current flowing through the air-fuel ratio sensor is connected to the upstream terminal of the air-fuel ratio sensor, and a voltage follower circuit for applying a constant voltage is connected to its downstream terminal. Possible configurations are possible. In this configuration, a current detection resistor for detecting the sensor current is provided between the downstream terminal and the output terminal of the voltage follower circuit, and the feedback wiring of the voltage follower circuit is between the downstream terminal and the current detection resistor. Is provided. Then, one end of the current detection resistor whose potential varies according to the sensor current is connected to the feedback wiring of the differential amplifier circuit. As a result, the differential voltage input to the differential amplifier circuit varies in accordance with the sensor current. However, in this configuration, if noise is added to the sensor current, the differential voltage input to the differential amplifier circuit becomes excessively large, which may cause an excessive voltage to be applied to the air-fuel ratio sensor. Therefore, it is conceivable to provide a filter for removing noise between one end of the current detection resistor and the feedback wiring. However, in this case, due to the delay due to the capacitor constituting the filter, when the battery voltage fluctuates, the difference in voltage input to the differential amplifier circuit temporarily becomes excessively large, thereby applying an excessive voltage to the air-fuel ratio sensor. There is a risk of being.

  Therefore, in view of the above-described problems, the present invention applies applied voltage control in which excessive voltage application to the air-fuel ratio sensor is suppressed while the air-fuel ratio sensor continues to detect current even if the battery voltage decreases. An object is to provide an apparatus.

One of the disclosed inventions for achieving the above object is an applied voltage control device for controlling the applied voltage of the air-fuel ratio sensor (200),
An upstream operational amplifier (81) having an output terminal connected to the upstream terminal (200a) of the air-fuel ratio sensor;
A constant voltage section (90) for keeping the potential of the downstream terminal (200b) of the air-fuel ratio sensor constant;
A filter (85) into which a sensor current flowing through the air-fuel ratio sensor flows;
A selector switch (87) provided between the upstream feedback wiring (83) connecting the inverting input terminal and the output terminal of the upstream operational amplifier and the filter;
A control unit (10) that controls opening and closing of the changeover switch based on the battery voltage,
The filter has a resistor (88) and a capacitor (89),
The upstream voltage obtained by stepping down the battery voltage at a constant rate is input to the non-inverting input terminal of the upstream operational amplifier.
The control unit
Storing a first level and a second level for determining a change in battery voltage;
By connecting the upstream feedback wiring and the filter by switching the changeover switch from the open state to the closed state after a lapse of a predetermined time from the time when the battery voltage becomes lower than the first level from the state lower than the first level, Apply a voltage that changes according to the battery voltage and sensor current to the upstream terminal,
When the battery voltage changes from the second level or more to a state below the second level, the upstream feedback wiring and the filter are disconnected by changing the changeover switch from the closed state to the open state, and the battery voltage is changed from the upstream operational amplifier to the battery voltage. Apply a voltage based on the upstream terminal.

  Thus, in the present invention, when the battery voltage falls below the second level, the changeover switch (87) is opened, and the upstream feedback wiring (83) and the filter (85) are disconnected. According to this, the voltage input to the two input terminals of the upstream operational amplifier (81) does not depend on the sensor current. Therefore, it is suppressed that the difference between the voltages input to the two input terminals of the upstream operational amplifier (81) is excessively different due to the fluctuation of the battery voltage. As a result, application of an excessive voltage to the air-fuel ratio sensor (200) is suppressed. Even in this state, since the voltage is applied to the air-fuel ratio sensor (200), the sensor current can be detected. Therefore, detection of the air-fuel ratio can be continued.

  In addition, as described above, when the battery voltage falls below the second level due to a drop in the battery voltage, the charge amount of the capacitor (89) of the filter (85) may be lowered. Even if the battery voltage rises to become the first level or higher after this, charging of the capacitor (89) may not be completed. Similarly, even if the battery voltage becomes equal to or higher than the first level when the battery voltage is supplied, there is a possibility that the charging of the capacitor (89) is not completed. Therefore, if the changeover switch (87) is closed immediately after the battery voltage becomes equal to or higher than the first level, the difference between the voltages input to the inverting input terminal and the non-inverting input terminal of the upstream operational amplifier (81) is excessively large. Thus, an excessive voltage may be applied to the air-fuel ratio sensor (200).

  Therefore, the changeover switch (87) is closed after a predetermined time has elapsed since the battery voltage has become equal to or higher than the first level as in the above invention. As a result, the difference in voltage input to each of the two input terminals of the upstream operational amplifier (81) due to the filter (85) is suppressed from being excessively large, and an excessive voltage is applied to the air-fuel ratio sensor (200). Is suppressed.

  In addition, the code | symbol with the parenthesis is attached | subjected to the element as described in the claim as described in a claim, and each means for solving a subject. The reference numerals in parentheses are for simply indicating the correspondence with each component described in the embodiment, and do not necessarily indicate the element itself described in the embodiment. The description of the reference numerals with parentheses does not unnecessarily narrow the scope of the claims.

It is a circuit diagram which shows schematic structure of the applied voltage control apparatus which concerns on 1st Embodiment. It is a graph for demonstrating the applied voltage at the time of fixed control. It is a graph for demonstrating the applied voltage at the time of variable control. It is a timing chart for explaining fixed control and variable control.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(First embodiment)
In the following, after mainly describing the air-fuel ratio sensor 200, each component of the applied voltage control apparatus 100 will be described in detail with reference to FIGS. FIG. 1 also shows an air-fuel ratio sensor 200 in addition to the applied voltage control device 100. In FIG. 4, the control state is also illustrated in addition to the electric signal in order to clearly indicate the control state. In FIG. 4, a step-down voltage Vsd and a battery voltage VB, which will be described later, are shown as the same electrical signal because they have the same behavior except for the voltage level. FIG. 4 shows an electrical signal when the air-fuel ratio is constant.

  The air-fuel ratio sensor 200 according to this embodiment is a limiting current oxygen sensor. The air-fuel ratio sensor 200 is provided, for example, in an exhaust pipe through which an exhaust gas of an internal combustion engine passes, and flows an electric current (hereinafter referred to as a sensor current) corresponding to the component concentration of the exhaust gas. The applied voltage control device 100 controls the applied voltage to the air-fuel ratio sensor 200 and detects the sensor current. The applied voltage control apparatus 100 includes an engine control circuit (not shown) in addition to the control circuit for the air-fuel ratio sensor 200, and the sensor current of the air-fuel ratio sensor 200, and the internal combustion engine such as the rotational speed and intake air amount of the internal combustion engine. The fuel injection amount of the fuel injection device is controlled based on the engine information.

  The output voltage of the air-fuel ratio sensor 200 varies according to the ratio of air and fuel (air-fuel ratio) contained in the exhaust gas. More specifically, the output voltage of the air-fuel ratio sensor 200 is such that the air-fuel ratio of the exhaust gas is lower than the ideal air-fuel ratio (ideal air-fuel ratio) at which air and fuel react without excess or deficiency in the internal combustion engine (oxygen). When the concentration is low), it rises higher than the ideal air-fuel ratio. On the other hand, when the air-fuel ratio is higher than the ideal air-fuel ratio (oxygen concentration is high), the output voltage is lower than that at the ideal air-fuel ratio. Therefore, when the air-fuel ratio is maintained at the ideal air-fuel ratio, the applied voltage control device 100 controls the fuel injection amount of the fuel injection device so that the oxygen concentration becomes higher when the output voltage of the air-fuel ratio sensor 200 increases. On the contrary, when the output voltage of the air-fuel ratio sensor 200 decreases, the applied voltage control device 100 controls the fuel injection amount of the fuel injection device so that the oxygen concentration becomes thin. The output voltage of the air-fuel ratio sensor 200 described above corresponds to a second voltage V2 described later.

  Although not shown, the air-fuel ratio sensor 200 is formed by sequentially stacking an exhaust side electrode, a solid electrolyte, and an atmosphere side electrode on a diffusion resistance layer. The diffusion resistance layer is made of porous alumina or the like having small holes, and the exhaust side electrode and the atmosphere side electrode are made of platinum or the like. The solid electrolyte is a zirconia solid electrolyte. Exhaust gas flows into the exhaust side electrode through the diffusion resistance layer, and the atmosphere side electrode is released to the atmosphere. The atmosphere side electrode is connected to the upstream terminal 200a of the air-fuel ratio sensor 200, and the exhaust side electrode is connected to the downstream terminal 200b.

  When the air-fuel ratio of the exhaust gas is higher than the ideal air-fuel ratio (when the air-fuel ratio is lean), oxygen molecules contained in the exhaust gas are sucked into the exhaust-side electrode. The inhaled oxygen molecules are ionized and move to the solid electrolyte, and then move to the atmosphere side electrode through the solid electrolyte. Then, oxygen ionized at the atmosphere side electrode is returned to oxygen molecules and released to the atmosphere. Thus, when the air-fuel ratio of the exhaust gas is lean, ionized oxygen flows from the exhaust side electrode to the atmosphere side electrode. In other words, when the air-fuel ratio is lean, current flows from the atmosphere-side electrode to the exhaust-side electrode as shown by the solid line arrow in FIG.

  In contrast, when the air-fuel ratio of the exhaust gas is lower than the ideal air-fuel ratio (when the air-fuel ratio is rich), oxygen molecules contained in the atmosphere are sucked into the atmosphere-side electrode. The inhaled oxygen molecules are ionized and move to the solid electrolyte, and move to the exhaust side electrode through the solid electrolyte. Then, oxygen ionized in the exhaust side electrode is returned to oxygen molecules and released into the exhaust gas. The oxygen molecules released from the exhaust side electrode react with unburned gas (carbon monoxide, hydrogen chloride, hydrogen, etc.) contained in the exhaust gas. In this way, when the air-fuel ratio of the exhaust gas is rich, ionized oxygen flows from the atmosphere side electrode to the exhaust side electrode. In other words, when the air-fuel ratio is rich, current flows from the exhaust side electrode to the atmosphere side electrode.

  When the applied voltage is low, the current flowing through the above-described air-fuel ratio sensor 200 (hereinafter referred to as sensor current) flows according to the applied voltage and the resistance value of the air-fuel ratio sensor 200. However, when the applied voltage exceeds a predetermined value, the sensor current is saturated as shown in FIGS. When the air-fuel ratio of the exhaust gas is lean, the sensor current is saturated because the inhalation of oxygen molecules contained in the exhaust gas is limited by the diffusion resistance layer. On the other hand, when the air-fuel ratio of the exhaust gas is rich, the reaction between the unburned gas and oxygen molecules is limited by the diffusion resistance layer, so that the sensor current is saturated. Thus, the sensor current is saturated, and a limit current flows through the air-fuel ratio sensor 200.

  This limit current has a property of increasing according to the oxygen concentration (air-fuel ratio) contained in the exhaust gas. As shown in FIGS. 2 and 3, when the air-fuel ratio (AF) is equal to the atmosphere, the current value of the limit current is higher than when the air-fuel ratio is 18. Therefore, the oxygen concentration can be detected based on the current value of the limit current. In the present embodiment, as shown in FIG. 2, the voltage level of the applied voltage is fixed in the fixed control described later. In addition, as shown in FIG. 3, the applied voltage characteristics are determined so as to pass through the limit current region of each air-fuel ratio in the variable control. In FIGS. 2 and 3, sensor currents having different behaviors are indicated by a solid line and a broken line, but the difference in behavior is caused by the temperature of the air-fuel ratio sensor 200. Hereinafter, the components of the applied voltage control apparatus 100 will be described in detail.

  The applied voltage control apparatus 100 includes a control unit 10 and a driver 60. The control unit 10 performs fixed control or variable control of the driver 60, and the driver 60 applies voltage to the air-fuel ratio sensor 200. The control unit 10 of the present embodiment also functions to detect the air-fuel ratio based on the sensor current.

  As shown in FIG. 1, the control unit 10 includes a power supply circuit 20, a reset control unit 30, a step-down unit 40, and a microcomputer 50 (hereinafter referred to as a microcomputer 50). The power supply circuit 20 converts the input battery voltage VB into the drive voltage Vo, and the reset control unit 30 varies the voltage level of the reset signal Init according to the voltage level of the drive voltage Vo. The step-down unit 40 steps down the input battery voltage VB to the step-down voltage Vsd, and the microcomputer 50 controls the driver 60 based on the input drive voltage Vo, reset signal Init, and step-down voltage Vsd. is there. The reset control unit 30 corresponds to the monitoring unit described in the claims, and the reset control unit 30 and the microcomputer 50 correspond to the opening / closing control unit described in the claims.

  The power supply circuit 20 converts the battery voltage VB into a voltage level suitable for the microcomputer 50 to operate, and outputs it to the microcomputer 50 as the drive voltage Vo. In the present embodiment, the battery voltage VB is 14V in a stable state, and the drive voltage Vo corresponds to 5V. As shown in FIG. 4, the drive voltage Vo does not fluctuate even if the voltage level of the battery voltage VB drops to some extent (for example, 9V). However, when the battery voltage VB drops to about 5V, the drive voltage Vo has a property of dropping accordingly.

  The reset control unit 30 monitors the voltage level of the drive voltage Vo, and changes the voltage level of the reset signal Init output to the microcomputer 50 based on the monitoring result. As shown in FIG. 4, when the voltage level of the drive voltage Vo is lower than the first threshold value Vth1, the reset control unit 30 sets the voltage level of the reset signal Init to Lo level. By doing so, the microcomputer 50 is brought into a non-driven state. However, as shown in FIG. 4, when the voltage level of the battery voltage VB rises and the drive voltage Vo exceeds the first threshold value Vth1 at time t1, the reset control unit 30 performs the first operation for confirming the stability of the battery voltage VB. Wait for a predetermined time T1. After that, the reset control unit 30 sets the voltage level of the reset signal to Hi level. By doing so, the microcomputer 50 is changed from the stopped state to the activated state. As shown at time t5 in FIG. 4, when the drive voltage Vo falls below the first threshold value Vth1 due to a decrease in the voltage level of the battery voltage VB, the reset control unit 30 changes the voltage level of the reset signal Init from the Hi level to the Lo level. To level. By doing so, the microcomputer 50 is changed from the activated state to the deactivated state.

  As described above, the example in which the reset control unit 30 changes the voltage level of the reset signal Init according to whether the drive voltage Vo is higher or lower than the first threshold value Vth1 is shown. However, when the drive voltage Vo falls below another threshold Vth different from the first threshold Vth1, the reset control unit 30 may change the voltage level of the reset signal Init from the Hi level to the Lo level. The other threshold value Vth is lower in voltage level than the first threshold value Vth1. The Hi level reset signal Init corresponds to the activation signal described in the claims.

  The step-down unit 40 divides the battery voltage VB to generate a step-down voltage Vsd and outputs it to the microcomputer 50. The step-down unit 40 has step-down resistors 41 and 42 connected in series from the battery power source to the reference potential (ground), and outputs the midpoint voltage to the microcomputer 50 as the step-down voltage Vsd. In the present embodiment, the step-down unit 40 steps down the battery voltage VB by a factor of 4, and outputs it to the microcomputer 50 as the stepped-down voltage Vsd.

  The microcomputer 50 controls the driver 60 in a fixed or variable manner by controlling the voltage level of the control signal Sc output to the selector switch 87 described later. As described above, the driving voltage Vo, the reset signal Init, and the step-down voltage Vsd are input to the microcomputer 50. Although not shown, the applied voltage control apparatus 100 has a plurality of step-down power supplies that step down the battery voltage VB, and the microcomputer 50 drives a voltage supplied from a microcomputer core power supply (not shown) as a power supply voltage. The microcomputer 50 determines whether to start or stop by the reset signal Init. As described above, when the voltage level of the reset signal Init is Lo level, the microcomputer 50 is stopped. In this case, the selector switch 87 is not controlled by the microcomputer 50, and the driver 60 enters a fixed control state. As shown in FIG. 1, the microcomputer 50 and the changeover switch 87 are electrically connected via a control wiring 51. The control wiring 51 connects the changeover switch 87 and the ground, and a shunt resistor 52 is provided between the connection point of the microcomputer 50 and the ground. Therefore, when the control signal Sc is not output to the control wiring 51 because the microcomputer 50 is in a stopped state, the voltage level of the control wiring 51 is fixed to the Lo level. As a result, the changeover switch 87 is fixed in the open state, and the driver 60 is fixed in the fixed control state. Unlike this, for example, as shown at time t2 in FIG. 4, when the voltage level of the reset signal Init changes from the Lo level to the Hi level, the microcomputer 50 is activated. After the microcomputer 50 enters the activated state, the microcomputer 50 waits for a second predetermined time T2 that is equal to or longer than the time (time constant) to finish charging of the filter 85 described later, and then sets the voltage level of the control signal Sc to the Hi level. By doing so, the microcomputer 50 changes the changeover switch 87 from the open state to the closed state, and switches the driver 60 from the fixed control state to the variable control state. As shown at time t4 in FIG. 4, when the step-down voltage Vsd falls below the second threshold value Vth2, the microcomputer 50 changes the voltage level of the control signal Sc from the Hi level to the Lo level. Thus, the changeover switch 87 is changed from the closed state to the open state, and the driver 60 is changed from the variable control state to the fixed control state. In FIG. 4, the second predetermined time T2 is shown to be shorter than the time constant. However, as described above, the second predetermined time T2 is actually a time longer than the time constant. In FIG. 4, the time constant corresponds to a time during which a midpoint voltage Vα described later reaches the upper limit value from the lower limit value.

  As described above, when the voltage level of the battery voltage VB decreases, the changeover switch 87 is opened, so that the driver 60 enters the fixed control state. However, when the voltage level of the battery voltage VB is stable, the changeover switch 87 is closed so that the driver 60 is in a variable control state. Switching of the changeover switch 87 from the open state to the closed state is performed after a time longer than the time constant has elapsed since the stability of the voltage level of the battery voltage VB has been confirmed. The changeover switch 87 is switched from the closed state to the open state immediately after it is confirmed that the voltage level of the battery voltage VB is lowered.

  As shown in FIG. 1, the driver 60 includes a voltage dividing unit 70, a voltage applying unit 80, and a constant voltage unit 90. The voltage dividing unit 70 divides the drive voltage Vo, and the voltage applying unit 80 determines a voltage (upstream voltage) to be applied to the upstream terminal 200a. The constant voltage unit 90 applies a constant voltage to the downstream terminal 200b.

  The voltage dividing unit 70 includes voltage dividing resistors 71 to 73 connected in series in order from the output terminal of the power supply circuit 20 toward the reference potential (ground). A midpoint voltage (hereinafter referred to as upstream voltage) of the voltage dividing resistors 71 and 72 is input to the voltage application unit 80, and a midpoint voltage (hereinafter referred to as downstream voltage) of the voltage dividing resistors 72 and 73 is input to the constant voltage unit 90. Is input. The upstream voltage and the downstream voltage are obtained by stepping down the drive voltage Vo at a ratio determined by the resistance values of the voltage dividing resistors 71 to 73, and the upstream voltage has a higher voltage level than the downstream voltage.

  The voltage application unit 80 includes an upstream operational amplifier 81, an upstream resistor 82, an upstream feedback wiring 83, an upstream feedback resistor 84, a filter 85, an adjustment resistor 86, and a changeover switch 87. The non-inverting input terminal of the upstream operational amplifier 81 is connected to the midpoint of the voltage dividing resistors 71 and 72, and the output terminal is connected to the upstream terminal 200 a via the upstream resistor 82. One end of the upstream feedback wiring 83 is connected to the inverting input terminal of the upstream operational amplifier 81, and the other end is connected between the upstream resistor 82 and the upstream terminal 200a. Further, the upstream feedback wiring 83 is provided with an upstream feedback resistance 84, and one end of the relay wiring 99 is connected between the upstream feedback resistance 84 in the upstream feedback wiring 83 and the inverting input terminal of the upstream operational amplifier 81. The other end of the relay wiring 99 is connected between a downstream operational amplifier 91 and a downstream resistor 92 which will be described later. The relay wiring 99 is provided with a filter 85, an adjustment resistor 86, and a changeover switch 87, and these are connected in series in order from the other end of the relay wiring 99 to one end. Therefore, when the changeover switch 87 is in the open state, the voltages input to the inverting input terminal and the non-inverting input terminal of the upstream operational amplifier 81 are constant, and the voltage level of the upstream terminal 200a is fixed. As will be described later, the voltage level of the downstream terminal 200b is also fixed at a constant level by the constant voltage unit 90. Therefore, when the changeover switch 87 is in the open state, the voltage applied to the air-fuel ratio sensor 200 is constant as shown in FIG. However, when the changeover switch 87 is closed, the inverting input terminal of the upstream operational amplifier 81 is connected between the downstream operational amplifier 91 and the downstream resistor 92 via the relay wiring 99. The voltage level varies between the downstream operational amplifier 91 and the downstream resistor 92 according to the sensor current. Therefore, the voltage input to the inverting input terminal of the upstream operational amplifier 81 varies according to the sensor current. Therefore, the voltage level of the upstream terminal 200a varies according to the sensor current, and the applied voltage of the air-fuel ratio sensor 200 varies according to the sensor current as shown in FIG. That is, the applied voltage increases as the sensor current increases.

  The filter 85 has a resistor 88 and a capacitor 89. The resistor 88 is connected to the relay wiring 99, and the capacitor 89 is connected between the relay wiring 99 and the ground. Therefore, as shown in FIG. 4, when the drive voltage Vo rises due to the supply of the battery voltage VB, charging of the capacitor 89 starts accordingly. The charging completion time of the capacitor 89 is determined by the time constant of the resistor 88 and the capacitor 89, and after the time corresponding to this time constant has elapsed, the midpoint voltage Vα between the filter 85 and the adjustment resistor 86 becomes constant. However, as shown in FIG. 4, when the voltage level of the drive voltage Vo decreases as the battery voltage VB decreases, the amount of power stored in the capacitor 89 decreases, and thereby the midpoint voltage Vα also decreases. Thereafter, when the voltage level of the drive voltage Vo rises with the rise of the battery voltage VB, charging of the capacitor 89 is completed, whereby the midpoint voltage Vα also becomes constant again.

  As shown in FIG. 4, the changeover switch 87 is switched from the open state to the closed state after the first predetermined time T1 + the second predetermined time T2 has elapsed since the drive voltage Vo exceeded the first threshold value Vth1. The first predetermined time T1 + second predetermined time T2 is longer than the time constant of the filter 85. Therefore, for example, as shown at times t3 and t8 in FIG. 4, at the timing when the changeover switch 87 is changed from the open state to the closed state (timing when the fixed control is switched to the variable control), the voltage level of the midpoint voltage Vα is changed. It is constant. As described above, in the variable control, the delay of the voltage input to the inverting input terminal of the upstream operational amplifier 81 due to the filter 85 is suppressed.

  The constant voltage unit 90 includes a downstream operational amplifier 91, a downstream resistor 92, a downstream feedback wiring 93, and a downstream feedback resistor 94. The non-inverting input terminal of the downstream operational amplifier 91 is connected to the middle point of the voltage dividing resistors 72 and 73, and the output terminal is connected to the downstream terminal 200 b via the downstream resistor 92. One end of the downstream feedback wiring 93 is connected to the inverting input terminal of the downstream operational amplifier 91, and the other end is connected between the downstream resistor 92 and the downstream terminal 200b. The downstream feedback wiring 93 is provided with a downstream feedback resistor 94. Therefore, the voltage input to each of the inverting input terminal and the non-inverting input terminal of the downstream operational amplifier 91 is constant, and the voltage level of the downstream terminal 200b is fixed.

  The downstream resistor 92 is used for detecting the sensor current. The microcomputer 50 detects the first voltage V1 on the downstream terminal 200b side and the second voltage V2 on the downstream operational amplifier 91 side in the downstream resistor 92, respectively. The first voltage V1 is constant because the downstream operational amplifier 91 is virtually short-circuited, but the second voltage V2 varies according to the sensor current. The microcomputer 50 detects the sensor current based on the stored resistance value of the downstream resistor 92 and the detected voltages V1 and V2, and calculates the air-fuel ratio based on the detected sensor current. The downstream resistor 92 corresponds to the current detection resistor described in the claims.

  Next, the effect of the applied voltage control apparatus 100 according to the present embodiment will be described. As described above, when the drive voltage Vo falls below the second threshold value Vth2, the changeover switch 87 is opened. As a result, the upstream return wiring 83 and the filter 85 are disconnected. According to this, it is possible to suppress the difference between the voltages input to the two input terminals of the upstream operational amplifier 81 from being excessively different due to the fluctuation of the battery voltage VB. Accordingly, application of an excessive voltage to the air-fuel ratio sensor 200 is suppressed. Even in this state, since the voltage is applied to the air-fuel ratio sensor 200, the sensor current can be detected. Therefore, detection of the air-fuel ratio can be continued.

  Further, as described above, when the step-down voltage Vsd falls below the second threshold value Vth2 due to the drop in the battery voltage VB, the charge amount of the capacitor 89 of the filter 85 may be lowered. Thereafter, even if the drive voltage Vo rises together with the battery voltage VB and the drive voltage Vo becomes equal to or higher than the first threshold value Vth1, there is a possibility that the charging of the capacitor 89 is not completed. Similarly, even when the drive voltage Vo becomes equal to or higher than the first threshold value Vth1 when the battery voltage VB is supplied, there is a possibility that the charging of the capacitor 89 is not completed. Therefore, if the changeover switch 87 is closed immediately after the drive voltage Vo becomes equal to or higher than the first threshold value Vth1, the difference between the voltages input to the inverting input terminal and the non-inverting input terminal of the upstream operational amplifier 81 is temporarily excessive. There is a concern that an excessive voltage may be applied to the air-fuel ratio sensor 200.

  Therefore, as described above, the changeover switch 87 is closed after the first predetermined time T1 + the second predetermined time T2 has elapsed since the drive voltage Vo becomes equal to or higher than the first threshold value Vth1. As a result, the difference in voltage input to each of the two input terminals of the upstream operational amplifier 81 due to the filter 85 is suppressed from being excessively increased, and application of excessive voltage to the air-fuel ratio sensor 200 is suppressed. .

  The time for which charge is accumulated in the capacitor 89 of the filter 85 depends on the time constant of the filter 85. Therefore, when the changeover switch 87 is closed after a time shorter than the time constant from the supply of the battery voltage VB, the difference between the voltages input to the two input terminals of the upstream operational amplifier 81 becomes excessively large, and the excessive voltage is increased. There is a risk of being applied to the air-fuel ratio sensor 200. Similarly, if the changeover switch 87 is closed after a time shorter than the time constant after the drive voltage Vo becomes equal to or higher than the first threshold value Vth1, the difference between the voltages input to the two input terminals of the upstream operational amplifier 81 is excessive. And an excessive voltage may be applied to the air-fuel ratio sensor 200. On the other hand, in the present embodiment, the first predetermined time T1 + the second predetermined time T2 is set longer than the time constant of the filter 85. As a result, the difference in voltage input to each of the two input terminals of the upstream operational amplifier 81 due to the filter 85 is suppressed from being excessively increased, and application of excessive voltage to the air-fuel ratio sensor 200 is suppressed. .

  FIG. 4 shows an example in which the battery voltage VB decreases as time advances from time t4 to t5, and thereby the drive voltage Vo also decreases. However, it is also conceivable that the battery voltage VB rises and exceeds the second threshold value Vth2 without dropping as much as the voltage level of the drive voltage Vo changes after the battery voltage VB falls below the second threshold value Vth2. In this case, since the capacitor 89 is not discharged, the midpoint voltage Vα does not change. Therefore, the microcomputer 50 sets the voltage level of the control signal Sc to the Hi level again when the drive voltage Vo exceeds the second threshold Vth2 without falling below the first threshold Vth1 after the battery voltage VB falls below the second threshold Vth2. . In this way, even if the variable control is switched to the fixed control due to a temporary decrease in the battery voltage VB, the control can be switched to the variable control again.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

  In the present embodiment, an example in which the second predetermined time T2 is greater than or equal to the time constant is shown. However, when the relationship that the sum of the first predetermined time T1 and the second predetermined time T2 is longer than the time constant holds, the second predetermined time T2 may be shorter than the time constant.

  In the present embodiment, an example in which the fixed control and the variable control are switched based on the comparison between the drive voltage and the first threshold and the comparison between the step-down voltage and the second threshold is shown. However, the control switching determination is not limited to the above example. For example, the microcomputer 50 has a first level and a second level for detecting a change in the voltage level of the battery voltage VB, and compares these two levels with the drive voltage or these two levels with the step-down voltage. Thus, the control switching determination may be performed.

DESCRIPTION OF SYMBOLS 20 ... Power supply circuit, 30 ... Reset control part, 40 ... Step-down part, 50 ... Microcomputer (microcomputer), 81 ... Upstream operational amplifier, 83 ... Upstream feedback wiring, 85 ... Filter, 87 ... Changeover switch, 88 ... Resistance, 89 ... Capacitor, 90 ... constant voltage unit, 100 ... applied voltage control device, 200 ... air-fuel ratio sensor, 200a ... upstream terminal, 200b ... downstream terminal

Claims (7)

An applied voltage control device for controlling an applied voltage of an air-fuel ratio sensor (200),
An upstream operational amplifier (81) having an output terminal connected to the upstream terminal (200a) of the air-fuel ratio sensor;
A constant voltage section (90) for keeping the potential of the downstream terminal (200b) of the air-fuel ratio sensor constant;
A filter (85) into which a sensor current flowing through the air-fuel ratio sensor flows;
A selector switch (87) provided between the upstream feedback wiring (83) connecting the inverting input terminal and the output terminal of the upstream operational amplifier and the filter;
A control unit (10) for controlling opening and closing of the changeover switch based on a battery voltage,
The filter has a resistor (88) and a capacitor (89);
The upstream voltage obtained by stepping down the battery voltage at a constant ratio is input to the non-inverting input terminal of the upstream operational amplifier,
The controller is
Storing a first level and a second level for determining a change in the battery voltage;
The upstream feedback wiring and the filter are connected by switching the changeover switch from the open state to the closed state after a lapse of a predetermined time from the time when the battery voltage becomes lower than the first level from the state lower than the first level. Then, a voltage that changes according to the battery voltage and the sensor current from the upstream operational amplifier is applied to the upstream terminal,
When the battery voltage is changed from the state where the battery voltage is equal to or higher than the second level to a state where the battery voltage is lower than the second level, the upstream feedback wiring and the filter are disconnected by changing the switch from a closed state to an open state. An applied voltage control device that applies a voltage based on the battery voltage from the upstream operational amplifier to the upstream terminal. The controller is
A power supply circuit (20) for converting the battery voltage into a drive voltage;
A step-down unit (40) for generating a step-down voltage obtained by stepping down the battery voltage;
An open / close control unit (30, 50) for controlling open / close of the changeover switch based on the drive voltage and the step-down voltage;
The non-inverting input terminal of the upstream operational amplifier receives, as the upstream voltage, a voltage obtained by stepping down the drive voltage converted from the battery voltage by the power supply circuit at a certain ratio,
The controller is
Storing a first threshold for comparing with the drive voltage corresponding to the first level, and a second threshold for comparing with the step-down voltage corresponding to the second level;
The changeover switch is changed from an open state to a closed state after the predetermined time has elapsed from the time when the drive voltage is lower than the first threshold value to the first threshold value or more,
2. The applied voltage control device according to claim 1, wherein when the step-down voltage is changed from a state where the step-down voltage is equal to or greater than the second threshold value to a state where the step-down voltage is lower than the second threshold value, the changeover switch is changed from a closed state to an open state.   The applied voltage control apparatus according to claim 2, wherein the predetermined time is longer than a time constant of the filter. The open / close control unit includes a monitoring unit (30) that monitors a voltage level of the drive voltage, and a microcomputer (50) that is activated based on a monitoring result of the monitoring unit,
The monitoring unit outputs a start signal instructing the microcomputer to start after a first predetermined time after the drive voltage becomes equal to or higher than the first threshold,
The microcomputer has changed the switch from the open state to the closed state after a second predetermined time after the start signal is input,
The applied voltage control apparatus according to claim 2 or 3, wherein the predetermined time is equal to a sum of the first predetermined time and the second predetermined time.   The applied voltage control apparatus according to claim 4, wherein the change-over switch is in an open state when the microcomputer is in a stopped state. The constant voltage unit has a downstream operational amplifier (91) whose own output terminal is connected to the downstream terminal of the air-fuel ratio sensor,
The inverting input terminal and the output terminal of the downstream operational amplifier are electrically connected via a downstream feedback wiring (93),
The application according to any one of claims 2 to 5, wherein a downstream voltage having a voltage level different from that of the upstream voltage and stepped down at a constant ratio is input to a non-inverting input terminal of the downstream operational amplifier. Voltage control device. A current detection resistor (92) is provided between the downstream terminal and the output terminal of the downstream operational amplifier,
The filter and the changeover switch are provided in a relay line (99) connecting a midpoint between the current detection resistor and the output terminal of the downstream operational amplifier and the upstream feedback line. Applied voltage control device.

Images