Sensor control apparatus

A sensor element drive circuit is a circuit including a plurality of semiconductor elements formed on a semiconductor substrate for realizing a current control function and a switching function. The current control function is a function of controlling the current flowing between electrodes such that the potential difference between electrodes becomes constant. The switching function is a function for switching between a connected state in which the electrodes are electrically connected to a sensor control apparatus and a cut-off state in which electrical continuity therebetween is broken. When one of an Ip+ terminal, a COM terminal, and a Vs+ terminal is determined to have an anomalous potential, the sensor control apparatus causes the sensor element drive circuit to perform switching from the connected state to the cut-off state, and connects the semiconductor substrate to a negative voltage lower than a ground potential applied to the sensor element drive circuit.

This application claims the benefit of Japanese Patent Application No. 2017-083786, filed Apr. 20, 2017, which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present disclosure relates to a sensor control apparatus for controlling a gas sensor.

BACKGROUND OF THE INVENTION

As disclosed in Japanese Patent Application Laid-Open (kokai) No. 2008-70194, there has been known a technique of electrically cutting off (breaking) the continuity between a gas sensor and a sensor control apparatus, thereby preventing flow of current into the gas sensor, when a wiring anomaly occurs and some of wiring lines electrically connected to the gas sensor are shorted to a power supply potential or a ground potential. The technique can reduce the possibility of an excessive current flowing into the gas sensor and breaking the gas sensor.

Problem to be Solved by the Invention

However, there has been a problem that even when the continuity between the gas sensor and the sensor control apparatus is cut off, in some cases, an excessive current flows into the gas sensor and the gas sensor breaks.

An object of the present disclosure is to prevent breakage of the gas sensor.

SUMMARY OF THE INVENTION

Means for Solving the Problem

One mode of the present disclosure is a control system comprising a gas sensor for detecting the concentration of a particular gas contained in a target gas and a sensor control apparatus which controls the gas sensor.

The gas sensor includes an electromotive force cell and a pump cell. The electromotive force cell has a first solid electrolyte member and paired first electrodes formed on the first solid electrolyte member, and is configured to generate electromotive force between the paired first electrodes in accordance with a difference in concentration of the particular gas therebetween. The pump cell has a second solid electrolyte member and paired second electrodes formed on the second solid electrolyte member, and is configured to pump the particular gas between the paired second electrodes.

The sensor control apparatus of the present disclosure comprises a control circuit, an anomaly determination section, a state switching section, and a potential connection section.

The control circuit has a plurality of semiconductor elements formed on a semiconductor substrate, the semiconductor elements being provided for a current control function and a switching function.

The current control function is a function of controlling current flowing between the paired second electrodes such that a constant potential difference is produced between the paired first electrodes.

The switching function is a function for switching between a connected state in which the paired first electrodes and the paired second electrodes are electrically connected to the sensor control apparatus and a cut-off state in which electrical continuity between the sensor control apparatus and the paired first electrodes and electrical continuity between the sensor control apparatus and the paired second electrodes are cut off.

The anomaly determination section determines, on the basis of an anomaly determination condition set in advance, whether or not at least one of a plurality of connection points for connecting the control circuit to the paired first electrodes and the paired second electrodes of the gas sensor has an anomalous potential.

The state switching section causes the control circuit to perform switching from the connected state to the cut-off state, when the anomaly determination section determines that the at least one connection point has an anomalous potential. The potential connection section connects the semiconductor substrate to an anomaly-time potential lower than a reference potential applied to the control circuit, when the anomaly determination section determines that the at least one connection point has an anomalous potential.

In the sensor control apparatus of the present disclosure configured as described above, parasitic diodes are formed between the semiconductor substrate and the semiconductor elements formed on the semiconductor substrate. The parasitic diodes have characteristics of limiting the current flowing from the semiconductor elements to the semiconductor substrate. The parasitic diodes also have characteristics of allowing the current flowing from the semiconductor substrate to the semiconductor elements to flow more easily as compared with the current flowing from the semiconductor elements to the semiconductor substrate.

In the sensor control apparatus of the present disclosure, leak currents flow from the semiconductor elements to the semiconductor substrate through the parasitic diodes. The magnitudes of the leak currents increase as the temperature of the control circuit increases. If the leak current from a certain semiconductor element reaches the semiconductor substrate through the corresponding parasitic diode, the leak current flows toward another semiconductor element whose potential is set to the reference potential among the plurality of semiconductor elements.

Therefore, in the case where there exists a path of leak current which extends from one of the connection points, passes through the corresponding semiconductor element, reaches the semiconductor substrate, and extends from the semiconductor substrate toward another semiconductor element whose potential is the reference potential, a current corresponding to the magnitude of the leak current flows to the gas sensor even in the above-described cut-off state.

In view of this, in the case where one of the connection points has an anomalous potential, the sensor control apparatus of the present disclosure connects the semiconductor substrate to an anomaly-time potential which is lower than the reference potential applied to the control circuit.

Therefore, even when a leak current from a certain semiconductor element reaches the semiconductor substrate through the corresponding parasitic diode, the sensor control apparatus of the present disclosure can prevent the leak current from further flowing from the semiconductor substrate toward another semiconductor element whose potential is the reference potential.

Thus, the sensor control apparatus of the present disclosure can prevent a current corresponding to the magnitude of the leak current from flowing into the gas sensor, which flow would otherwise occur even in the above-described cut-off state. As a result, the sensor control apparatus can prevent breakage of the gas sensor.

In the one mode of the present disclosure, the potential connection section may include a negative charge pump for generating a negative voltage, and when the anomaly determination section determines that the at least one connection point has an anomalous potential, the potential connection section may activate the negative charge pump and use the negative voltage generated by the negative charge pump as the anomaly-time potential.

Since the sensor control apparatus of the present disclosure configured as described above applies a negative voltage to the semiconductor substrate as the anomaly-time potential, the reference potential applied to the control circuit can be made equal to or higher than 0 V. In general, the reference potential of the control circuit is 0 V. Therefore, the sensor control apparatus of the present disclosure can prevent breakage of the gas sensor without changing the reference potential applied to the control circuit.

In the one mode of the present disclosure, the gas sensor may be configured such that when constant current flows between the paired first electrodes of the electromotive force cell, the concentration of oxygen around one of the paired first electrodes becomes constant.

The sensor control apparatus of the present disclosure configured as described above can prevent a current corresponding to the magnitude of the leak current from flowing into the electromotive force cell, which flow would otherwise occur even in the above-described cut-off state. As a result, the sensor control apparatus can prevent blackening of the electromotive force cell. Blackening is a phenomenon in which the external color of the solid electrolyte member changes and metallic oxide constituting the solid electrolyte member is reduced and oxygen is removed, whereby the crystalline structure is disordered.

DETAILED DESCRIPTION OF THE INVENTION

A control system1of the present embodiment executes various control processes for controlling the operation state of an internal combustion engine, and executes, as one of them, a process of detecting the concentration of a particular gas contained in exhaust gas. In the present embodiment, the particular gas is oxygen.

As shown inFIG. 1, the control system1includes an electronic control unit2and a gas sensor3. The electronic control unit2includes a sensor control apparatus4, an engine control apparatus5, and a heater control circuit6.

The gas sensor3includes a sensor element10for detecting the oxygen concentration in exhaust gas for a wide range and a heater30for maintaining the sensor element10at an operating temperature.

The sensor element10includes an oxygen pump cell11, porous diffusion layers12, an oxygen concentration detection cell13, and a reinforcing plate14.

The oxygen pump cell11has an oxygen-ion conductive solid electrolyte member21formed of partially stabilized zirconia and having a plate-like shape, and pump electrodes22and23provided on the front and back surfaces of the oxygen-ion conductive solid electrolyte member21and formed mainly of platinum. The pump electrode22is electrically connected to a connection terminal7of the electronic control unit2through a wiring line17. The pump electrode23is electrically connected to a connection terminal8of the electronic control unit2through a wiring line18. The pump electrode22is covered with a porous protection layer24which protects the pump electrode22from poisoning substances or the like.

The oxygen concentration detection cell13has an oxygen-ion conductive solid electrolyte member25formed of partially stabilized zirconia and having a plate-like shape, and detection electrodes26and27provided on the front and back surfaces of the oxygen-ion conductive solid electrolyte member25and formed mainly of platinum. The detection electrode26is electrically connected to the connection terminal8of the electronic control unit2through the wiring line18and is electrically connected to the pump electrode23. The detection electrode27is electrically connected to a connection terminal9of the electronic control unit2through a wiring line19.

An unillustrated insulating layer formed mainly of an insulating material (e.g., alumina) is provided between the oxygen pump cell11and the oxygen concentration detection cell13so as to electrically insulate the two cells11and13from each other. The porous diffusion layers12are provided in portions of the insulating layer. Notably, the porous diffusion layers12are formed mainly of an insulating material (e.g., alumina) to be porous for limiting the diffusion rate of a gas under measurement introduced into the sensor element10.

A hollow measurement chamber28surrounded by the porous diffusion layers12and the unillustrated insulating layer is formed between the oxygen pump cell11and the oxygen concentration detection cell13. Namely, the measurement chamber28communicates with a measurement gas atmosphere through the porous diffusion layers12. The pump electrode23and the detection electrode26are disposed in the measurement chamber28.

The reinforcing plate14is disposed on a surface of the oxygen concentration detection cell13on the side opposite a surface thereof facing the measurement chamber28such that the reinforcing plate14is in close contact with the former surface while sandwiching the detection electrode27. As result, the reinforcing plate14increases the overall strength of the sensor element10. Notably, the reinforcing plate14has a size approximately the same as those of the solid electrolyte members21and25of the oxygen pump cell11and the oxygen concentration detection cell13and is formed of a material whose main component is ceramic.

The detection electrode27of the oxygen concentration detection cell13is isolated from the outside by the reinforcing plate14, and a reference oxygen chamber29which is a closed space is formed between the oxygen concentration detection cell13and the reinforcing plate14around the detection electrode27.

In the sensor element10configured as described above, a small constant current Icp is caused to flow from the detection electrode27of the oxygen concentration detection cell13toward the detection electrode26so as to pump oxygen from the measurement chamber28toward the detection electrode27. As a result, oxygen of an approximately constant concentration is accumulated in the reference oxygen chamber29formed around the detection electrode27. The oxygen of an approximately constant concentration accumulated in the reference oxygen chamber29serves as a reference oxygen concentration when the oxygen concentration in the gas under measurement is detected by the sensor element10. Therefore, the detection electrode27is also called a self-generating reference electrode.

The heater30is formed to have a flat-plate-like shape and is disposed to face the oxygen pump cell11of the sensor element10. The heater30is formed of a material whose main component is alumina, and includes a heater wire31formed of a material whose main component is platinum. The heater30is controlled by electric power supplied from the heater control circuit6such that the temperature of the sensor element10becomes an activation temperature (e.g., 550 to 900° C.). Opposite ends of the heater wire31are electrically connected to the heater control circuit6.

Notably, when the sensor element10becomes active as a result of heating by the heater30, the gas sensor3enters a gas detectable state.

The sensor control apparatus4has an Ip+ terminal, a COM terminal, and a Vs+ terminal. The Ip+ terminal, the COM terminal, and the Vs+ terminal are electrically connected to the connection terminal7, the connection terminal8, and the connection terminal9, respectively, of the electronic control unit2. Therefore, the pump electrode22of the sensor element10is electrically connected to the Ip+ terminal of the sensor control apparatus4through the connection terminal7. The pump electrode23and the detection electrode26of the sensor element10is electrically connected to the COM terminal of the sensor control apparatus4through the connection terminal8. The detection electrode27of the sensor element10is electrically connected to the Vs+ terminal of the sensor control apparatus4through the connection terminal9.

In the sensor element10, oxygen contained in the gas under measurement diffuses into the measurement chamber28through the porous diffusion layers12. The sensor element10has characteristics as follows. In a state in which an air-fuel mixture supplied to an internal combustion engine is maintained at a stoichiometric air-fuel ratio, an electromotive force of 450 mV is produced in the oxygen concentration detection cell13due to the difference in oxygen concentration between the measurement chamber28and the reference oxygen chamber29. Namely, a potential difference of 450 mV is produced between the detection electrode26and the detection electrode27.

Notably, the oxygen concentration detection cell13has characteristics that it generates voltage corresponding to the difference in oxygen concentration between the detection electrode26and the detection electrode27. The oxygen within the reference oxygen chamber29which the detection electrode27faces has an approximately constant concentration. Therefore, the oxygen concentration detection cell13generates, between the detection electrode26and the detection electrode27, voltage corresponding to the oxygen concentration within the measurement chamber28.

Incidentally, when the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine changes, the concentration of oxygen contained in exhaust gas changes, whereby the concentration of oxygen contained in the measurement chamber28of the sensor element10changes. In view of this, in the control system1of the present embodiment, the electronic control unit2controls the Ip current (pumping current) flowing through the oxygen pump cell11such that the potential difference between the detection electrode26and the detection electrode27is maintained at 450 mV. Namely, as a result of control of the Ip current such that the atmosphere within the measurement chamber28becomes the same as that in the case where the air-fuel ratio is the stoichiometric air-fuel ratio, pumping of oxygen is performed by the oxygen pump cell11. Therefore, the oxygen concentration in the exhaust gas can be computed on the basis of the flow state of the Ip current (for example, flow direction, current cumulative value, etc.).

The oxygen pump cell11is configured such that, in accordance with the flow direction of current flowing between the pump electrode22and the pump electrode23, the oxygen pump cell11can selectively perform the pumping out of oxygen from the measurement chamber28and the pumping of oxygen into the measurement chamber28. Also, the oxygen pump cell11is configured such that it can adjust the oxygen pumping rate in accordance with the magnitude of the current flowing between the pump electrode22and the pump electrode23.

As shown inFIG. 2, the sensor control apparatus4includes a sensor element drive circuit41, a differential amplifier circuit42, a terminal voltage output circuit43, an anomaly detection circuit44, a control section45, a logical operation circuit46, and a negative charge pump47. The sensor control apparatus4is realized by an application specific integrated circuit (i.e., ASIC). ASIC is an abbreviation of Application Specific IC.

The sensor element drive circuit41drives and controls the oxygen pump cell11and the oxygen concentration detection cell13of the sensor element10.

The sensor element drive circuit41includes an operation amplifier51for supplying the Ip current for driving the oxygen pump cell11, a PID control circuit52for improving the Ip current control characteristic, a constant current source53for supplying constant current Icp to the oxygen concentration detection cell13, a constant voltage source54for supplying a control target voltage used for control of the Ip current, and semiconductor switches55,56, and57.

The sensor element drive circuit41has the Ip+ terminal, the COM terminal, and the Vs+ terminal through which the sensor element drive circuit41is connected to the sensor element10. The sensor element drive circuit41has a P1terminal, a P2terminal, and a Pout terminal to which elements for determining the characteristics of the PID control circuit52are externally attached.

The Ip+ terminal, the COM terminal, and the Vs+ terminal are electrically connected to the connection terminal7, the connection terminal8, and the connection terminal9, respectively.

Of the two pump electrodes22and23of the oxygen pump cell11, the pump electrode22is connected to the Ip+ terminal through the wiring line17and the connection terminal7. The pump electrode23is connected, through the wiring line18and the connection terminal8, to the COM terminal which provides a common reference voltage of the sensor element10.

Of the two detection electrodes26and27of the oxygen concentration detection cell13, the detection electrode26is connected to the COM terminal through the wiring line18and the connection terminal8, and the detection electrode27is connected to the Vs+ terminal through the wiring line19and the connection terminal9.

The PID control circuit52is connected to the inverting input terminal of the operation amplifier51through the semiconductor switch56, the COM terminal, and a resistor element R2. A reference voltage of 3.6 V is applied to the non-inverting input terminal of the operation amplifier51. The output terminal of the operation amplifier51is connected to the Ip+ terminal through the semiconductor switch55. Namely, the operation amplifier51constitutes a portion of a negative feedback circuit which controls the current supplied to the sensor element10; specifically, to the oxygen pump cell11.

The PID control circuit52has a function of performing PID computation on the deviation ΔVs of the output voltage Vs of the oxygen concentration detection cell13from 450 mV (control target voltage), thereby improving the control characteristics of the above-mentioned negative feedback control. The PID control circuit52includes operation amplifiers61and62, resistors R3, R4, and R5, and capacitors C1, C2, and C3. The resistors R3to R5and the capacitors C1to C3are attached to the P1terminal and the P2terminal so as to determine the control characteristics of the PID control circuit52.

The input terminal of the PID control circuit52(namely, the inverting input terminal of the operation amplifier62) is connected to the Vs+ terminal through an operation amplifier63. As a result, the output voltage Vs of the oxygen concentration detection cell13is input to the PID control circuit52. The output end of the PID control circuit52is connected to the Pout terminal. The Pout terminal is connected to the COM terminal through the resistor element R2and is finally connected to the inverting input terminal of the operation amplifier51.

The output of the constant voltage source54is input to the inverting input terminal of the operation amplifier62through an operation amplifier64. The constant voltage source54is a circuit for supplying 450 mV, which is a voltage serving as a control target for control of the Ip current, to the PID control circuit52through the operation amplifier62.

The constant current source53is connected to the Vs+ terminal through the semiconductor switch57. The constant current source53is a circuit for supplying constant current Icp (for example, 17 μA) which is supplied to the oxygen concentration detection cell13so as to make the oxygen concentration around the detection electrode27of the oxygen concentration detection cell13(namely, the reference oxygen chamber29) constant. The non-inverting input terminal of the operation amplifier63is connected to the Vs+ terminal through the semiconductor switch57.

In the case where the gas under measurement is excessive in fuel supply (namely, rich), the oxygen concentration within the measurement chamber28becomes lower than that in the case of the stoichiometric air-fuel ratio, and the output voltage Vs of the oxygen concentration detection cell13becomes higher than 450 mV which is the control target voltage. Accordingly, the deviation ΔVs of the output voltage Vs from the control target voltage is produced, and the deviation ΔVs undergoes the PID computation performed by the PID control circuit52and is fed back by the operation amplifier51. Therefore, the Ip current required for the oxygen pump cell11to pump oxygen into the measurement chamber28so as to compensate a shortage of oxygen flows through the oxygen pump cell11.

Meanwhile, in the case where the gas under measurement is insufficient in fuel supply (namely, lean), the oxygen concentration within the measurement chamber28becomes higher than that in the case of the stoichiometric air-fuel ratio, and the output voltage Vs of the oxygen concentration detection cell13becomes lower than 450 mV which is the control target voltage. In the same manner as in the above-described case, the deviation ΔVs is fed back by the operation amplifier51. Therefore, the Ip current required for the oxygen pump cell11to pump out the excessive portion of oxygen from the measurement chamber28flows through the oxygen pump cell11.

As described above, the sensor element drive circuit41controls the Ip current supplied to the oxygen pump cell11such that the output voltage Vs of the oxygen concentration detection cell13becomes 450 mV.

The sensor element drive circuit41converts the Ip current flowing through the oxygen pump cell11to voltage through use of the resistor element R2whose one end is connected to the COM terminal and whose other end is connected to the Pout terminal.

The differential amplifier circuit42amplifies the difference between the voltage at the COM terminal and the voltage at the Pout terminal and outputs the amplified difference to the engine control apparatus5as a gas detection signal.

The engine control apparatus5computes the oxygen concentration on the basis of the gas detection signal. The engine control apparatus5controls the operation state of the internal combustion engine by executing air-fuel ratio feedback control, etc. for the internal combustion engine on the basis of the oxygen concentration obtained as a result of the computation.

The terminal voltage output circuit43is a circuit for outputting the terminal voltages of the Ip+ terminal, the COM terminal, and the Vs+ terminal to the control section45. Although connection lines are omitted inFIG. 2, the input terminals of the terminal voltage output circuit43are connected to the Ip+ terminal, the COM terminal, and the Vs+ terminal, respectively.

The anomaly detection circuit44is a circuit for determining (detecting) whether or not any one of the Ip+ terminal, the COM terminal, and the Vs+ terminal is shorted to a power supply potential or a ground potential and for outputting an anomaly detection signal showing the results of the determination (detection). As shown inFIG. 3, the terminal voltages of the Ip+ terminal, the COM terminal, and the Vs+ terminal are input to the anomaly detection circuit44. The anomaly detection circuit44is configured to output an anomaly detection signal of high level when any one of the input terminal voltages deviates from a normal voltage range. Also, the anomaly detection circuit44is configured to output an anomaly detection signal of low level when all the terminal voltages of the Ip+ terminal, the COM terminal, and the Vs+ terminal fall within the normal voltage range.

The control section45executes various control processes in the sensor control apparatus4and is mainly composed of a well known microcomputer which includes a CPU, a ROM, a RAM, an input port, an output port, and a bus line for connecting these components.

The various functions of the microcomputer are realized by a program which is stored in a non-transitory tangible recording medium and executed by the CPU. In this example, the ROM corresponds to the non-transitory tangible recording medium storing the program. Also, a method corresponding to the program is performed as a result of execution of this program. Notably, the control section45may include a single microcomputer or a plurality of microcomputers. Also, some or all of the functions of the microcomputer(s) may be realized by hardware; for example, by a single IC or a plurality of ICs.

The control section45includes a communication section71and a switch control section72. The communication section71performs data communication with the engine control apparatus5through a transmission cable73. When the communication section71receives a switch open/close instruction from the engine control apparatus5, on the basis of the received switch open/close instruction, the switch control section72outputs first, second, and third switch control signals for bringing the semiconductor switches55,56, and57into an ON state or an OFF state. Specifically, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch55into the ON state, the switch control section72outputs a high level signal as the first switch control signal. Meanwhile, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch55into the OFF state, the switch control section72outputs a low level signal as the first switch control signal.

Similarly, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch56into the ON state, the switch control section72outputs a high level signal as the second switch control signal. Meanwhile, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch56into the OFF state, the switch control section72outputs a low level signal as the second switch control signal.

Further, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch57into the ON state, the switch control section72outputs a high level signal as the third switch control signal. Meanwhile, in the case where the switch open/close instruction instructs the switch control section72to bring the semiconductor switch57into the OFF state, the switch control section72outputs a low level signal as the third switch control signal.

The logical operation circuit46includes a clock circuit74and AND circuits75,76,77, and78.

In order to operate the negative charge pump47, the clock circuit74generates a clock signal at a frequency (for example, 8 MHz) set in advance, and outputs the clock signal to the AND circuit78.

The AND circuits75,76, and77obtain the logical products between a signal obtained by inverting the voltage level of the anomaly detection signal output from the anomaly detection circuit44and the first, second, and third switch control signals output from the switch control section72.

The AND circuit78obtains the logical product between the clock signal output from the clock circuit74and signals obtained by inverting the voltage levels of the signals output from the AND circuits75,76, and77.

The negative charge pump47is a circuit for generating a negative voltage when the clock signal is input to the negative charge pump47through the AND circuit78. The negative charge pump47includes capacitors C11and C12and diodes D11and D12.

One end of the capacitor C11is connected to the output terminal of the AND circuit78, and the other end of the capacitor C11is connected to the connection point between the diode D11and the diode D12. The anode of the diode D11is connected to the capacitor C11, and the cathode of the diode D11is grounded. The anode of the diode D12is connected to the output terminal of the negative charge pump47, and the cathode of the diode D12is connected to the connection point between the diode D11and the capacitor C11. One end of the capacitor C12is connected to the anode of the diode D12, and the other end of the capacitor C12is grounded.

In the present embodiment, the negative charge pump47generates a negative voltage of, for example, −3 V.

Each of the semiconductor switches55,56, and57is formed by connecting an N-channel MOS FET and a P-channel MOS FET in parallel.

The gates of the N-channel MOS FETs of the semiconductor switches55,56, and57are respectively connected to the output terminals of the AND circuits75,76, and77. The gates of the P-channel MOS FETs of the semiconductor switches55,56, and57are respectively connected to the output terminals of the AND circuits75,76, and77through respective logical inversion circuits. As a result, the semiconductor switch55(56,57) enters the ON state when a signal of high level is output from the AND circuit75(76,77) and enters the OFF state when a signal of low level is output from the AND circuit75(76,77).

Notably, parasitic diodes are formed on a semiconductor substrate on which the sensor control apparatus4realized by an ASIC is formed. As a results, parasitic diodes D21, D22, D23, D24, D25, and D26are formed in the sensor element drive circuit41.

The cathodes of the parasitic diodes D21, D23, and D25are connected to the Ip+ terminal, the COM terminal, and the Vs+ terminal, respectively, The anodes of the parasitic diodes D21to D26are connected to the output terminal of the negative charge pump47.

The cathodes of the parasitic diodes D22, D24, and D26are grounded. Therefore, in a state in which the clock signal is not input to the negative charge pump47, the potential of the above-mentioned semiconductor substrate becomes substantially equal to the ground potential due to presence of the parasitic diodes D22, D24, and D26.

As shown inFIG. 4, the sensor control apparatus4is constituted by forming a plurality of semiconductor elements82on a semiconductor substrate81. The semiconductor elements82are electrically insulated from one another by an oxide film83formed on the semiconductor substrate81.FIG. 4shows four semiconductor elements82a,82b,82c, and82d. The semiconductor element82ais connected to the ground potential. The ground potential applied to the semiconductor element82acorresponds to the reference potential set in the sensor control apparatus4. The semiconductor element82cis electrically connected to the Vs+ terminal.

The semiconductor substrate81is a p-type silicon substrate. An n+ layer84is formed on the semiconductor substrate81, and an n layer85is formed on the n+ layer84. The n+ layer84and the n layer85are layers into which ions of an n-type impurity are implanted. The n+ layer84is higher than the n layer85in terms of concentration of the n-type impurity.

Parasitic diodes are formed at the junction surface between the p-type semiconductor substrate81and the n+ layer84. The parasitic diodes86and87shown inFIG. 4respectively correspond to the parasitic diodes D25and D26shown inFIG. 3.

The sensor control apparatus4configured as described above controls the gas sensor3which detects the concentration of oxygen contained in exhaust gas.

The gas sensor3includes the oxygen concentration detection cell13and the oxygen pump cell11. The oxygen concentration detection cell13includes the oxygen-ion conductive solid electrolyte member25and the paired detection electrodes26and27formed on the oxygen-ion conductive solid electrolyte member25and generates electromotive force between the paired detection electrodes26and27in accordance with the oxygen concentration difference therebetween. The oxygen pump cell11includes the oxygen-ion conductive solid electrolyte member21and the paired pump electrodes22and23formed on the oxygen-ion conductive solid electrolyte member21and pumps oxygen between the paired pump electrodes22and23.

The sensor control apparatus4includes the sensor element drive circuit41, the anomaly detection circuit44, the control section45, the logical operation circuit46, and the negative charge pump47.

The sensor element drive circuit41is a circuit which includes the plurality of semiconductor elements82formed on the semiconductor substrate81for realizing a current control function and a switching function.

The current control function is a function of controlling the current flowing between the paired pump electrodes22and23such that the potential difference between the paired detection electrodes26and27assumes a constant value.

The switching function is a function for switching between a connected state in which the paired detection electrodes26and27and the paired pump electrodes22and23are electrically connected to the sensor control apparatus4and a cut-off state in which the electrical continuity between the sensor control apparatus4and the paired detection electrodes26and27and the electrical continuity between the sensor control apparatus4and the paired pump electrodes22and23are cut off.

The anomaly detection circuit44determines, on the basis of an anomaly determination condition set in advance, whether or not one of the plurality of terminals (i.e., the Ip+ terminal, the COM terminal, and the Vs+ terminal) for connecting the sensor element drive circuit41to the paired detection electrodes26and27and the paired pump electrodes22and23of the gas sensor3has an anomalous potential. In the present embodiment, the anomaly determination condition is that the terminal voltage falls outside a normal voltage range.

In the case where the anomaly detection circuit44determines that one of the Ip+ terminal, the COM terminal, and the Vs+ terminal has an anomalous potential, the AND circuits75,76, and77of the logical operation circuit46cause the sensor element drive circuit41to perform switching from the connected state to the cut-off state.

In the case where the anomaly detection circuit44determines that one of the Ip+ terminal, the COM terminal, and the Vs+ terminal has an anomalous potential, the clock circuit74and the AND circuit78of the logical operation circuit46and the negative charge pump47connect the semiconductor substrate81to a negative voltage of −3 V which is lower than the ground potential applied to the sensor element drive circuit41.

In the sensor control apparatus4configured as described above, the parasitic diodes D21to D26are formed between the semiconductor substrate81and the semiconductor elements82formed on the semiconductor substrate81. The parasitic diodes D21to D26have characteristics of limiting the current flowing from the semiconductor elements82to the semiconductor substrate81. The parasitic diodes D21to D26also have characteristics of allowing the current flowing from the semiconductor substrate81to the semiconductor elements82to flow more easily as compared with the current flowing from the semiconductor elements82to the semiconductor substrate81.

In the sensor control apparatus4, leak currents flow from the semiconductor elements82to the semiconductor substrate81through the parasitic diodes D21, D23, and D25. The magnitudes of the leak currents increase as the temperature of the sensor element drive circuit41increases. If the leak current from one semiconductor element82reaches the semiconductor substrate81through the parasitic diode D21, D23, or D25, the leak current flows toward a semiconductor element82whose potential is set to the ground potential (i.e., the semiconductor element82a) among the plurality of semiconductor elements82.

Therefore, in the case where there exists a path of leak current which extends from one of the Vs+ terminal, the COM terminal, and the Ip+ terminal, passes through the corresponding semiconductor element82, reaches the semiconductor substrate81, and extends from the semiconductor substrate81toward another semiconductor element82whose potential is the reference potential, a current corresponding to the magnitude of the leak current flows to the gas sensor3even in the above-described cut-off state.

In views of this, in the case where one of the Ip+ terminal, the COM terminal, and the Vs+ terminal has an anomalous potential, the sensor control apparatus4connects the semiconductor substrate81to −3 V which is lower than the reference potential applied to the sensor element drive circuit41.

Therefore, even when a leak current from a certain semiconductor element82reaches the semiconductor substrate81through the parasitic diode D21, D23, or D25, the sensor control apparatus4can prevent the leak current from further flowing from the semiconductor substrate81toward another semiconductor element82whose potential is the reference potential.

Thus, the sensor control apparatus4can prevent a current corresponding to the magnitude of the leak current from flowing into the gas sensor3, which flow would otherwise occur even in the above-described cut-off state. As a result, the sensor control apparatus4can prevent breakage of the gas sensor3.

Further, even when a wiring line anomaly (in which at lest one of the wiring lines electrically connected to the gas sensor3is shorted to the power supply potential or the ground potential) has occurred, the sensor control apparatus4can prevent breakage of the gas sensor3by switching to the above-mentioned cut-off state, thereby increasing the possibility that the control system1can be restored by exchanging wiring lines.

Further, the sensor control apparatus4includes the negative charge pump47, and when the anomaly detection circuit44determines that one of the Ip+ terminal, the COM terminal, and the Vs+ terminal has an anomalous potential, the sensor control apparatus4activates the negative charge pump47so as to connect the semiconductor substrate81to a negative voltage.

Since the sensor control apparatus4applies a negative voltage to the semiconductor substrate81as described above, the reference potential applied to the sensor element drive circuit41can be made equal to or higher than 0 V. In general, the reference potential of the sensor element drive circuit41is 0 V. Therefore, the sensor control apparatus4can prevent breakage of the gas sensor3without changing the reference potential applied to the sensor element drive circuit41.

Notably, since the negative charge pump47is activated in the above-descried cut-off state, noise generated as a result of switching of the negative charge pump47does not affect the concentration detection accuracy of the sensor control apparatus4.

The gas sensor3is configured such that when the constant current Icp flows between the paired detection electrodes26and27of the oxygen concentration detection cell13, the oxygen concentration around the detection electrode27becomes constant.

The sensor control apparatus4configured as described above can prevent a current corresponding to the magnitude of the leak current from flowing into the oxygen concentration detection cell13, which flow would otherwise occur even in the above-described cut-off state. As a result, the sensor control apparatus4can prevent blackening of the oxygen concentration detection cell13.

Notably, the oxygen concentration detection cell13corresponds to the electromotive force cell appearing in CLAIMS; the oxygen pump cell11corresponds to the pump cell appearing in CLAIMS; the oxygen-ion conductive solid electrolyte member25corresponds to the first solid electrolyte member appearing in CLAIMS; the detection electrodes26and27correspond to the first electrodes appearing in CLAIMS; the oxygen-ion conductive solid electrolyte member21corresponds to the second solid electrolyte member appearing in CLAIMS; and the pump electrodes22and23correspond to the second electrodes appearing in CLAIMS.

The sensor element drive circuit41corresponds to the control circuit appearing in CLAIMS; the Vs+ terminal, the COM terminal, and the Ip+ terminal correspond to the connection points appearing in CLAIMS; the anomaly detection circuit44corresponds to the anomaly determination section appearing in CLAIMS; the AND circuits75,76, and77of the logical operation circuit46correspond to the state switching section appearing in CLAIMS; and the clock circuit74and the AND circuit78of the logical operation circuit46and the negative charge pump47correspond to the potential connection section appearing in CLAIMS.

Exhaust gas corresponds to the gas under measurement appearing in CLAIMS; oxygen corresponds to the particular gas appearing in CLAIMS; and the negative voltage generated by the negative charge pump47corresponds to the anomaly-time potential appearing in CLAIMS.

One embodiment of the present disclosure has been described above, but the present disclosure is not limited to the above embodiment and can be embodied in various other forms.

For example, in the above-described embodiment, a full-range air-fuel ratio sensor for detecting the concentration of oxygen in exhaust gas has been described as a gas sensor for detecting the concentration of a particular gas contained in a gas under measurement. However, the gas sensor which is controlled by the sensor control apparatus of the present disclosure is not limited to the full-range air-fuel ratio sensor. No limitation is imposed on the gas sensor controlled by the sensor control apparatus of the present disclosure so long as the gas sensor includes an electromotive force cell and a pump cell. Examples of such a gas sensor include an NOx sensor, etc.

In the above-described embodiment, the reference potential applied to the sensor element drive circuit41is the ground potential, and the negative charge pump47generates a negative voltage. However, the reference potential applied to the sensor element drive circuit41is not limited to the ground potential, and may be a positive voltage or a negative voltage. However, the voltage applied to the semiconductor substrate81when the anomaly detection circuit44determines that one of the Ip+ terminal, the COM terminal, and the Vs+ terminal has an anomalous potential must be set to be lower than the reference potential.

The function of one component in the above embodiment may be distributed to a plurality of components, or the functions of a plurality of components may be realized by one component. Part of the configurations of the above embodiments may be omitted. At least part of the configuration of each of the above embodiments may be added to or partially replace the configurations of other embodiments. All modes included in the technical idea specified by the wording of the claims are embodiments of the present disclosure.

The present disclosure may be realized in various forms other than the above-described sensor control apparatus4. For example, the present disclosure may be realized as a system including the sensor control apparatus4as a constituent element, a program for causing a computer to function as the sensor control apparatus4, a non-transitory tangible recording medium, e.g., a semiconductor memory, in which the program is recorded, and a sensor control method.

DESCRIPTION OF REFERENCE NUMERALS