JP7599398B2 - Sensor Control Device - Google Patents

Description

The technology disclosed in this specification relates to a sensor control device.

A gas sensor that detects the concentration of specific components (oxygen, nitrogen oxides, ammonia, etc.) in the exhaust gas of an internal combustion engine is known (see Patent Document 1). The sensor element of the gas sensor includes a solid electrolyte body, an electrode provided on the surface of the solid electrolyte body, and a porous protective layer that covers the electrode. The porous protective layer prevents moisture and poisonous substances (e.g., P, Si, S, Mg, etc.) in the exhaust gas from adhering to the electrode and causing deterioration of the electrode. The sensor element is housed and protected inside a cylindrical protective cover (protector) with a bottom.

JP 2020-165693 A

The protector may be made of a material that contains chromium, such as stainless steel. In such cases, when the temperature around the sensor element rises, chromium may become liberated from the protector and adhere to the surface of the porous protective layer. When chromium adheres to the surface of the porous protective layer, the ammonia in the exhaust gas may be oxidized (burned) on the surface of the porous protective layer by the action of the chromium, causing a decrease in measurement accuracy.

The sensor control device disclosed in this specification is a sensor control device that is connected to a gas sensor including a sensor element having a detection unit that detects a specific gas in a measurement target gas containing ammonia and a heater that heats the detection unit, and a protector that is made of a metal containing chromium and is arranged surrounding the sensor element, and controls the gas sensor. The sensor control device is provided with a heater control unit that selectively executes normal control to control the heater so that the detection unit reaches a first temperature at which the detection unit is activated when it is determined that the flow rate of the measurement target gas around the gas sensor is equal to or greater than a predetermined value, and suppression control to control the heater so that the detection unit reaches a second temperature that is lower than the first temperature when it is determined that the flow rate of the measurement target gas around the gas sensor is less than a predetermined value.

The sensor control device disclosed in this specification can accurately measure specific components in the measurement target gas.

FIG. 1 is a diagram showing a schematic diagram of an overall configuration of a gas measurement device connected to an exhaust pipe of a vehicle in an embodiment. FIG. 2 is a cross-sectional view of the gas sensor according to the embodiment. FIG. 3 is a diagram showing a cross section of a sensor element according to the embodiment and an electrical configuration of a sensor control device. FIG. 4 is a partially enlarged cross-sectional view of an ammonia sensor unit according to an embodiment. FIG. 5 is a flowchart showing a flow of processing by the gas meter device of the embodiment. FIG. 6 is a timing chart showing the operation of the heater control in this embodiment. FIG. 7 is a timing chart showing the operation of heater control in the modified example. FIG. 8 is a graph showing the relationship between the continuous measurement time and the output ratio of the sensor element when the temperature of the sensor element is changed in a test example.

[Overview of the embodiment]
(1) A sensor control device disclosed in this specification is a sensor control device that is connected to a gas sensor including a sensor element having a detection unit that detects a specific gas in a measurement target gas including ammonia and a heater that heats the detection unit, and a protector that is made of a metal containing chromium and is arranged to surround the sensor element, and controls the gas sensor. The sensor control device is equipped with a heater control unit that selectively executes normal control to control the heater to a first temperature at which the detection unit is activated when it is determined that a first state is reached in which the flow rate of the measurement target gas around the gas sensor is equal to or greater than a predetermined value, and suppression control to control the heater to a second temperature at which the detection unit is activated when it is determined that a second state is reached in which the flow rate of the measurement target gas around the gas sensor is less than a predetermined value.

The decrease in measurement accuracy due to ammonia combustion is thought to occur when the flow rate of the measurement target gas around the gas sensor becomes slow, and chromium liberated from the protector adheres to the sensor element. On the other hand, it has been found that the decrease in measurement accuracy due to ammonia combustion can be significantly suppressed if the sensor element is controlled at a low temperature. When the flow rate of the measurement target gas around the gas sensor is equal to or higher than a predetermined value, the heater is controlled to bring the temperature to a first temperature at which the detection unit is activated, and when the flow rate of the measurement target gas around the gas sensor is less than the predetermined value, the heater is controlled to bring the detection unit to a second temperature lower than the first temperature, suppressing the combustion of ammonia and allowing the specific component in the measurement target gas to be measured with high accuracy.

(2) In the sensor control device described in (1) above, the specific gas may be ammonia.

The sensor control device described in (1) above is preferably applicable when the specific gas is ammonia.

(3) In the sensor control device described in (1) or (2) above, the heater control unit may switch from the normal control to the suppression control a predetermined time after determining that the ambient environment of the gas sensor has changed from the first state to the second state.

If the time during which the second state is reached is extremely short, it is believed that there will be almost no impact on the measurement accuracy, and performing suppression control even in such a case would be rather troublesome. For this reason, it is preferable to switch from normal control to suppression control after a predetermined time has elapsed since the environment surrounding the gas sensor reached the second state, and not perform suppression control if the time during which the second state is reached is extremely short.

(4) In the sensor control device described in (1), (2) or (3) above, the heater control unit may switch from the suppression control to the normal control within 100 ms after determining that the ambient environment of the gas sensor has changed from the second state to the first state.

This is to allow measurement of specific gases in exhaust gas to begin immediately when suppression control is no longer necessary.

(5) In the sensor control device described in (1), (2), (3) or (4) above, the second temperature may be equal to or higher than 300°C and lower than 740°C. Alternatively, the second temperature may be equal to or higher than 300°C and lower than 670°C.

To reliably suppress the combustion of ammonia, the second temperature is preferably less than 740°C, and more preferably 670°C or less. Furthermore, if the second temperature is 300°C or more, the detection unit can be quickly activated and measurement can be resumed when the ambient environment of the gas sensor changes from the second state to the first state.

(6) In the sensor control device described in (1), (2), (3), (4) or (5) above, the gas to be measured may be exhaust gas discharged from an internal combustion engine mounted on a vehicle, the first state may be a state in which the internal combustion engine is driven, and the second state may be a state in which the internal combustion engine is stopped.

The above sensor control device can be suitably applied when measuring specific components in exhaust gas emitted from an internal combustion engine mounted on a vehicle.

[Details of the embodiment]
Specific examples of the technology disclosed in this specification will be described below with reference to the drawings. Note that the present invention is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

<Embodiment>
The embodiment will be described with reference to Figs. 1 to 6. The gas metering device 1 of the present embodiment is used in a urea SCR (Selective Catalytic Reduction) system that purifies nitrogen oxides (NOx) contained in exhaust gas (an example of a gas to be measured) discharged from a diesel engine 401 (an example of an internal combustion engine) of an automobile. The urea SCR system is a system that purifies nitrogen oxides contained in exhaust gas by chemically reacting ammonia with nitrogen oxides to reduce the nitrogen oxides to nitrogen. In the urea SCR system, if the amount of ammonia supplied to the nitrogen oxides becomes excessive, there is a risk that unreacted ammonia will be released to the outside while remaining contained in the exhaust gas. The gas metering device 1 measures the concentrations of nitrogen oxides and ammonia (an example of a specific gas) contained in the exhaust gas.

The gas measurement device 1 includes a gas sensor 10 and a sensor control device 300 connected to the gas sensor 10. As shown in FIG. 1, an oxidation catalyst 403, a DPF (Diesel Particulate Filter) 404, and an SCR catalyst 405 are provided in this order from the upstream side in an exhaust pipe 402 connected to a diesel engine 401. The gas sensor 10 is installed in the exhaust pipe 402 on the downstream side adjacent to the SCR catalyst 405.

[Overall configuration of gas sensor 10]
As shown in Fig. 2, the gas sensor 10 includes a sensor element 100, a metal shell 11, an outer protector 12, an inner protector 13, an outer cylinder 14, and a holding member 18. The sensor element 100 has a narrow plate shape extending in the direction of the axis AX (the up-down direction in Fig. 2) and is held inside the metal shell 11. In the following description, the lower side in Fig. 2 will be referred to as the leading end side, and the upper side in Fig. 2 will be referred to as the rear end side.

The metal shell 11 is a cylindrical member having a through hole 11A that penetrates in the direction of the axis AX. The metal shell 11 holds the sensor element 100 in the through hole 11A with the leading end of the sensor element 100 protruding outward from its leading end side and the rear end of the sensor element 100 protruding outward from its rear end side.

Inside the through hole 11A of the metal shell 11, there are arranged an annular ceramic holder 15, two talc rings 16A and 16B filled with talc powder in an annular shape, and a ceramic sleeve 17. In detail, the ceramic holder 15, the talc rings 16A and 16B, and the ceramic sleeve 17 are arranged in this order, overlapping from the front end to the rear end of the metal shell 11, surrounding the periphery of the sensor element 100.

An external protector 12 (one example of a protector) and an internal protector 13 (one example of a protector) having multiple holes are attached by welding to the tip of the metal shell 11 so as to cover the tip of the sensor element 100. The external protector 12 and the internal protector 13 are made of a metal containing chromium, for example stainless steel. Meanwhile, an outer cylinder 14 is attached by welding to the rear end of the metal shell 11. The outer cylinder 14 is cylindrical and extends in the direction of the axis AX, and surrounds the rear end of the sensor element 100.

A retaining member 18 is disposed inside the outer tube 14. The retaining member 18 is a cylindrical member made of an insulating material (specifically, alumina) and has an insertion hole 18A penetrating in the direction of the axis AX. The rear end of the sensor element 100 and multiple terminal members 19 are disposed inside the insertion hole 18A. The multiple terminal members 19 are electrically connected by elastic contact with multiple electrode terminal portions provided on the surface of the sensor element 100. The multiple terminal members 19 are each connected to a multiple lead wire 20.

An elastic seal member 21 made of fluororubber is disposed at the opening at the rear end of the outer tube 14. A number of lead wires 20 pass through the elastic seal member 21 and are led out to the outside.

[Sensor element 100]
As shown in FIG. 3, the sensor element 100 includes a NOx sensor portion 101 (an example of a detection portion), a heater 170, and an ammonia sensor portion 201 (an example of a detection portion).

The NOx sensor section 101 includes a first pump cell 110, a Vs cell 120, a second pump cell 130, a first measurement chamber R1, a second measurement chamber R2, and a reference oxygen chamber R3.

The NOx sensor section 101 has a structure in which a first insulating layer 141, a first solid electrolyte body 111, a second insulating layer 142, a second solid electrolyte body 121, a third insulating layer 143, and a third solid electrolyte body 131 are stacked in this order.

The first measuring chamber R1 is a small space provided between the first solid electrolyte body 111 and the second solid electrolyte body 121, and penetrating the second insulating layer 142. The first measuring chamber R1 is separated from the external space by a first diffusion resistor 151 through which gas can pass. The first diffusion resistor 151 limits the amount of exhaust gas flowing from the outside to the first measuring chamber R1 per unit time.

The second measurement chamber R2 is a small space provided between the first solid electrolyte body 111 and the third solid electrolyte body 131, and penetrates the second insulating layer 142, the second solid electrolyte body 121, and the third insulating layer 143. The first measurement chamber R1 and the second measurement chamber R2 are separated by a second diffusion resistor 152 that allows gas to pass through. The second diffusion resistor 152 limits the amount of exhaust gas flowing per unit time from the first measurement chamber R1 to the second measurement chamber R2.

The reference oxygen chamber R3 is a small space provided between the second solid electrolyte body 121 and the third solid electrolyte body 131, and penetrates the third insulating layer 143. The inside of the reference oxygen chamber R3 is filled with a porous body 161 that allows gas to pass through.

The first pump cell 110 includes a first solid electrolyte body 111, a first electrode 112 arranged on the surface of the first solid electrolyte body 111 (the lower surface in FIG. 3), and a second electrode 113 arranged on the other surface of the first solid electrolyte body 111 (the upper surface in FIG. 3) and serving as a counter electrode to the first electrode 112. The first electrode 112 is arranged inside the first measurement chamber R1, and the second electrode 113 is arranged outside the first measurement chamber R1. The surface of the first electrode 112 is covered with a porous body 162 through which gas can pass. The second electrode 113 is arranged in an opening provided in the first insulating layer 141. This opening is connected to the outside of the sensor element 100, and the opening is filled with a porous body 163 through which gas can pass.

The Vs cell 120 includes a second solid electrolyte body 121, a third electrode 122 arranged on a surface of the second solid electrolyte body 121 (upper surface in FIG. 3), and a fourth electrode 123 arranged on another surface of the second solid electrolyte body 121 (lower surface in FIG. 3). The third electrode 122 is arranged inside the first measurement chamber R1. The fourth electrode 123 is arranged inside the reference oxygen chamber R3.

The second pump cell 130 includes a third solid electrolyte body 131, a fifth electrode 132 arranged on the surface of the third solid electrolyte body 131 (upper surface in FIG. 3), and a sixth electrode 133 arranged on the surface of the third solid electrolyte body 131 (upper surface in FIG. 3) and serving as a counter electrode to the fifth electrode 132. The fifth electrode 132 is arranged inside the second measurement chamber R2. The sixth electrode 133 is arranged inside the reference oxygen chamber R3, facing the fourth electrode 123.

The first electrode 112, the third electrode 122, and the fifth electrode 132 are each connected to a reference potential.

The heater 170 includes a fourth insulating layer 171 laminated on the third solid electrolyte body 131, a fifth insulating layer 172 laminated on the fourth insulating layer 171, and a resistive heating element 173 embedded between the fourth insulating layer 171 and the fifth insulating layer 172 and generating heat when electricity is passed through it.

The solid electrolyte bodies 111, 121, and 131 are mainly composed of zirconia, which has oxygen ion conductivity. The insulating layers 141, 142, 143, 171, and 172, the porous bodies 161, 162, and 163, and the diffusion resistors 151 and 152 are mainly composed of alumina. The electrodes 112, 113, 122, 123, 132, and 133, and the resistive heating element 173 are mainly composed of platinum. "Mainly composed" means that the content is 50% by mass or more.

As shown in FIG. 3, the ammonia sensor section 201 is formed on the surface of the fifth insulating layer 172. As shown in FIG. 4, the ammonia sensor section 201 includes a first ammonia sensor section 201A and a second ammonia sensor section 201B. The first ammonia sensor section 201A and the second ammonia sensor section 201B function as a mixed potential type sensing section.

As shown in FIG. 4, the first ammonia sensor unit 201A includes a first reference electrode 202A formed on the surface of the fifth insulating layer 172, a fourth solid electrolyte body 203A formed to cover the first reference electrode 202A, and a first detection electrode 204A formed on the surface of the fourth solid electrolyte body 203A. Similarly, the second ammonia sensor unit 201B includes a second reference electrode 202B formed on the fifth insulating layer 172, a fifth solid electrolyte body 203B formed to cover the second reference electrode 202B, and a second detection electrode 204B formed on the surface of the fifth solid electrolyte body 203B.

The first ammonia sensor part 201A and the second ammonia sensor part 201B are integrally covered by a protective layer 205 made of a porous material. The protective layer 205 prevents the adhesion of poisonous substances to the first detection electrode 204A and the second detection electrode 204B, and adjusts the diffusion speed of exhaust gas flowing from the outside into the first ammonia sensor part 201A and the second ammonia sensor part 201B.

The first detection electrode 204A and the second detection electrode 204B are mainly made of Au, and the first reference electrode 202A and the second reference electrode 202B are made of Pt alone or mainly made of Pt. The electrode materials constituting the first detection electrode 204A and the second detection electrode 204B are selected so that the ratio of the sensitivity to ammonia to the sensitivity to nitrogen oxides is different in the first ammonia sensor part 201A and the second ammonia sensor part 201B. The material forming the protective layer 205 can be at least one material selected from the group consisting of alumina (aluminum oxide), spinel (MgAl ₂ O ₄ ), silica alumina, and mullite. The diffusion speed of the exhaust gas through the protective layer 205 is adjusted by appropriately setting various conditions such as the thickness, particle size, particle size distribution, porosity, and compounding ratio of the protective layer 205.

The heater 170 is used to heat the NOx sensor section 101 and the ammonia sensor section 201 to a temperature at which the solid electrolyte bodies 111, 121, 131, 203A, and 203B are activated, thereby increasing the oxygen ion conductivity of the solid electrolyte bodies 111, 121, 131, 203A, and 203B and stabilizing their operation.

In the sensor element 100, the NOx sensor portion 101 and a portion of the heater 170 protruding from the metal shell 11, and the ammonia sensor portion 201 are covered with a protective coat C made of a porous material that allows gas to pass through, as shown in FIG. 2. The protective coat C is mainly composed of alumina, for example.

[Sensor control device 300]
Next, a description will be given of an example of the configuration of the sensor control device 300. The sensor control device 300 includes an electric circuit section 310 and a microcomputer 320, as shown in FIG.

The electric circuit section 310 is an analog circuit arranged on a circuit board. The electric circuit section 310 includes a reference voltage comparison circuit 311, an Ip1 drive circuit 312, a Vs detection circuit 313, an Icp supply circuit 314, an Ip2 detection circuit 315, a Vp2 application circuit 316, a heater drive circuit 317, a resistance detection circuit 318, a first electromotive force detection circuit 319A, and a second electromotive force detection circuit 319B.

The Ip1 drive circuit 312 supplies a current Ip1 between the first electrode 112 and the second electrode 113, and outputs the value of the supplied current Ip1 to the microcomputer 320.

The Vs detection circuit 313 detects the voltage Vs between the third electrode 122 and the fourth electrode 123 and outputs it to the reference voltage comparison circuit 311.

The reference voltage comparison circuit 311 compares a reference voltage (e.g., 425 mV) with the output (voltage Vs) of the Vs detection circuit 313, and outputs the comparison result to the Ip1 drive circuit 312. The Ip1 drive circuit 312 then controls the direction and magnitude of the Ip1 current so that the voltage Vs is equal to the reference voltage, and adjusts the oxygen concentration in the first measurement chamber R1 to a predetermined value that does not affect the nitrogen oxide measurement in the second measurement chamber R2, and does not decompose the nitrogen oxides.

The Icp supply circuit 314 passes a weak current Icp between the third electrode 122 and the fourth electrode 123, pumping oxygen from the first measurement chamber R1 to the reference oxygen chamber R3 and exposing the fourth electrode 123 to a predetermined reference oxygen concentration.

The Vp2 application circuit 316 applies a constant voltage Vp2 (e.g., 450 mV) between the fifth electrode 132 and the sixth electrode 133, sufficient to decompose the nitrogen oxides in the exhaust gas into nitrogen and oxygen, thereby decomposing the nitrogen oxides into nitrogen and oxygen.

The Ip2 detection circuit 315 detects the current Ip2 that flows between the fifth electrode 132 and the sixth electrode 133 when oxygen generated by the decomposition of nitrogen oxides on the fifth electrode 132 is pumped to the sixth electrode 133 side through the third solid electrolyte body 131, and outputs the detection result to the microcomputer 320.

The heater driving circuit 317 applies a heater voltage Vh to the resistive heating element 173 of the heater 170, causing the resistive heating element 173 to generate heat, thereby heating the NOx sensor section 101 and the ammonia sensor section 201. The resistive heating element 173 is a single electrode pattern, one end of which is grounded and the other end of which is connected to the heater driving circuit 317.

The resistance detection circuit 318 periodically passes a pulse of a current having a predetermined value through the Vs cell 120, detects the amount of voltage change (amount of change in voltage Vs) obtained in response to the current, and outputs the result to the microcomputer 320.

The first electromotive force detection circuit 319A detects the electromotive force between the first reference electrode 202A and the first detection electrode 204A (hereinafter, the first ammonia electromotive force EMF1) and outputs it to the microcomputer 320. The second electromotive force detection circuit 319B detects the electromotive force between the second reference electrode 202B and the second detection electrode 204B (hereinafter, the second ammonia electromotive force EMF2) and outputs it to the microcomputer 320.

The microcomputer 320 controls the entire sensor control device 300, and includes a CPU (Central Processing Unit) 321, a RAM 322, a ROM 323, a signal input/output unit 324, an A/D converter 325, etc. The CPU 321 executes processing for controlling the sensor element 100 based on a program stored in the ROM 323. As shown in FIG. 3, the CPU 321 is connected to an ECU (Engine Control Unit) 400 via the signal input/output unit 324. The ECU 400 is a device for electronically controlling the drive of the vehicle's diesel engine 401, etc., and outputs information such as the drive state of the diesel engine 401 to the CPU 321.

The A/D converter 325 digitally converts the output values from the Ip1 drive circuit 312, the Vs detection circuit 313, the Ip2 detection circuit 315, the resistance detection circuit 318, the first electromotive force detection circuit 319A, and the second electromotive force detection circuit 319B, and outputs the digital values to the CPU 321 via the signal input/output unit 324. The CPU 321 reads various data from the ROM 323, and performs various calculations from the values of the current Ip1, the current Ip2, the first ammonia electromotive force EMF1, and the second ammonia electromotive force EMF2.

The CPU 321 also controls the power supplied to the heater 170 by pulse width modulation by outputting a drive signal to the heater drive circuit 317 via the signal input/output unit 324 and the A/D converter 325, so that the NOx sensor unit 101 and the ammonia sensor unit 201 (more specifically, the solid electrolyte bodies 111, 121, 131, 203A, and 203B) reach the target temperature. The control of the power supply to the heater 170 can be realized by a known method of calculating the internal resistance (impedance) of the Vs cell 120 based on a value indicating the change in the voltage Vs detected by the resistance detection circuit 318 and a table in which the change in the voltage Vs and the internal resistance value of the Vs cell 120 stored in the ROM 323 are previously associated, and controlling the amount of power supply so that the impedance becomes the target value. The CPU 321 and the heater drive circuit 317 are an example of a heater control unit.

When the CPU 321 determines that the gas sensor 10 is in a first state where the flow rate of the gas to be measured around the gas sensor 10 is equal to or greater than a predetermined value, the CPU 321 selectively executes normal control to control the heater 170 to a first temperature at which the NOx sensor section 101 and the ammonia sensor section 201 (more specifically, the solid electrolyte bodies 111, 121, 131, 203A, 203B) are activated, and when the CPU 321 determines that the gas sensor 10 is in a second state where the flow rate of the gas to be measured around the gas sensor 10 is less than a predetermined value, the CPU 321 selectively executes suppression control to control the heater 170 to a second temperature at which the NOx sensor section 101 and the ammonia sensor section 201 are activated, which is lower than the first temperature.

[Operational Mode of Gas Measuring Device 1]
A process for measuring the concentrations of nitrogen oxides and ammonia in the exhaust gas of a vehicle using the above-mentioned gas measurement device 1 will now be described.

When the diesel engine 401 is started and receives power from an external power source, the heater drive circuit 317 passes a drive current through the resistance heating element 173 (step S11). The resistance heating element 173 rises in temperature, and the solid electrolyte bodies 111, 121, 131, 203A, and 203B are heated and activated.

The exhaust gas is introduced into the first measurement chamber R1 while the flow rate is restricted by the first diffusion resistor 151.

Next, the Icp supply circuit 314 passes a weak current Icp from the fourth electrode 123 to the third electrode 122 in the Vs cell 120 (step S12). This allows the oxygen in the exhaust gas to receive electrons from the third electrode 122 in the first measurement chamber R1, which is the negative electrode, and becomes oxygen ions that flow through the second solid electrolyte body 121 and move into the reference oxygen chamber R3. In other words, by passing the current Icp between the third electrode 122 and the fourth electrode 123, the oxygen in the first measurement chamber R1 is sent into the reference oxygen chamber R3.

Next, the CPU 321 determines whether the voltage Vs detected by the Vs detection circuit 313 is equal to or lower than a predetermined value (1.5 V) (step S13). If the CPU 321 determines that the voltage Vs is not equal to or lower than the predetermined value, it waits until the voltage Vs becomes equal to or lower than the predetermined value.

When the CPU 321 determines that the voltage Vs is equal to or lower than the predetermined value, it then determines whether the surrounding environment of the gas sensor 10 is in a first state in which the flow rate of the exhaust gas is equal to or higher than a predetermined value, or in a second state in which the flow rate of the exhaust gas is lower than a predetermined value (step S14). This determination is made, for example, using as an indicator whether the diesel engine 401 is in a driven state or in a stopped (idling-stop) state. If the diesel engine 401 is driven, it is considered that the exhaust gas is flowing around the gas sensor 10 at a certain flow rate, and if the diesel engine 401 is stopped, it is considered that the flow rate of the exhaust gas around the gas sensor 10 is small or almost zero. Therefore, by using as an indicator whether the diesel engine 401 is driven or in an idling-stop state, it is possible to easily determine whether the surrounding environment of the gas sensor 10 is in the first state or the second state.

Whether the diesel engine 401 is in an idling stop state is determined based on an idling stop flag (ISF) that is updated by, for example, an instruction to automatically stop the operation of the diesel engine 401 and an instruction to automatically resume the operation of the diesel engine 401 after the automatic operation is stopped. Specifically, for example, in an automatic vehicle, when the vehicle decelerates with the brake pedal depressed and comes to a stop (for example, when the vehicle speed is zero), it is determined that the state is suitable for idling stop and the idling stop flag is set (on), and when any condition that is not suitable for idling stop (for example, when the accelerator pedal is depressed) is established, the idling stop flag is turned off. The CPU 321 detects whether the idling stop flag is on or off based on an output signal from the ECU 400.

When the CPU 321 detects in step S14 that the idling stop flag is off, i.e., that the diesel engine 401 is being driven, it determines that the state is a first state in which the flow rate of the measurement target gas around the gas sensor 10 is equal to or greater than a predetermined value, and outputs a drive signal to the heater drive circuit 317 to control the heater 170 so that the NOx sensor section 101 and the ammonia sensor section 201 are at a first temperature (normal control: step S15). The first temperature is, for example, 750°C or higher.

The CPU 321 measures the nitrogen oxide concentration and ammonia concentration in the exhaust gas while the heater 170 is controlled so that the NOx sensor section 101 and the ammonia sensor section 201 are at the first temperature (step S17). The measurements can be performed by a known procedure. A specific procedure is, for example, as follows.

The Vs detection circuit 313 detects the voltage Vs between the third electrode 122 and the fourth electrode 123. The voltage Vs corresponds to the difference in oxygen concentration between the first measurement chamber R1 and the reference oxygen chamber R3. The Vs detection circuit 313 compares the detected voltage Vs with a reference voltage (425 mV) using the reference voltage comparison circuit 311, and outputs the comparison result to the Ip1 drive circuit 312. If the oxygen concentration in the first measurement chamber R1 is adjusted so that the voltage Vs is constant at around 425 mV, the oxygen concentration in the exhaust gas in the first measurement chamber R1 will approach a predetermined value (for example, 10 ⁻⁸ to 10 ⁻⁹ atm).

When the oxygen concentration of the exhaust gas introduced into the first measurement chamber R1 is lower than a predetermined value, the Ip1 drive circuit 312 passes a current Ip1 through the first pump cell 110 so that the second electrode 113 side becomes negative, and pumps oxygen from the outside of the sensor element 100 into the first measurement chamber R1. On the other hand, when the oxygen concentration of the exhaust gas introduced into the first measurement chamber R1 is higher than a predetermined value, the Ip1 drive circuit 312 passes a current Ip1 through the first pump cell 110 so that the first electrode 112 side becomes negative, and pumps oxygen from the inside of the first measurement chamber R1 to the outside of the sensor element 100. In other words, the first pump cell 110 pumps or pumps oxygen between the inside and outside of the first measurement chamber R1 so that the oxygen concentration in the first measurement chamber R1 is constant. In addition, the oxygen concentration in the exhaust gas that flows into the first measurement chamber R1 can be determined based on the current Ip1 detected by the Ip1 drive circuit 312. The oxygen concentration is used to detect the ammonia concentration.

The exhaust gas whose oxygen concentration has been adjusted in the first measurement chamber R1 is introduced into the second measurement chamber R2 while the flow rate is restricted by the second diffusion resistor 152. Nitrogen oxides in the exhaust gas that come into contact with the fifth electrode 132 in the second measurement chamber R2 are decomposed into nitrogen and oxygen on the fifth electrode 132 by applying a voltage Vp2 between the fifth electrode 132 and the sixth electrode 133 by the Vp2 application circuit 316. The oxygen generated by the decomposition becomes oxygen ions and flows through the third solid electrolyte body 131 and moves into the reference oxygen chamber R3. At this time, the current Ip2 flowing through the second pump cell 130 indicates a value corresponding to the nitrogen oxide concentration.

In addition, an electromotive force is generated between the first reference electrode 202A and the first detection electrode 204A of the first ammonia sensor part 201A according to the concentration of ammonia contained in the exhaust gas. The first electromotive force detection circuit 319A detects the electromotive force between the first reference electrode 202A and the first detection electrode 204A as a first ammonia electromotive force EMF1. Similarly, an electromotive force is generated between the second reference electrode 202B and the second detection electrode 204B of the second ammonia sensor part 201B according to the concentration of ammonia in the exhaust gas. The second electromotive force detection circuit 319B detects the electromotive force between the second reference electrode 202B and the second detection electrode 204B as a second ammonia electromotive force EMF2.

The CPU 321 calculates the nitrogen oxide concentration and ammonia concentration in the exhaust gas using various relational expressions stored in the ROM 323 based on the values of the current Ip1, the current Ip2, the first ammonia electromotive force EMF1, and the second ammonia electromotive force EMF2, and the oxygen concentration in the exhaust gas calculated based on the current Ip1.

The ammonia sensor unit 201 detects not only ammonia but also nitrogen oxides, so there is a risk that the accuracy of detecting ammonia may decrease if nitrogen oxides are contained in the exhaust gas. Therefore, the ammonia sensor unit 201 uses two ammonia sensor units 201A and 201B, each of which has a different ratio of sensitivity to ammonia and sensitivity to nitrogen oxides, to calculate the concentrations of ammonia and nitrogen oxides.

When the CPU 321 detects in step S14 that the idling stop flag has been turned on, i.e., that the diesel engine 401 has entered an idling stop state, it determines that the flow rate of the measurement target gas around the gas sensor 10 is less than a predetermined value, and outputs a drive signal to the heater drive circuit 317 to control the heater 170 so that the NOx sensor section 101 and the ammonia sensor section 201 reach a second temperature that is lower than the first temperature (suppression control: step S17). During the suppression control, the CPU 321 does not measure the ammonia concentration in the exhaust gas, and waits until the idling stop flag is turned off.

When the CPU 321 detects that the idling stop flag has been turned off again, i.e., that the operation of the diesel engine 401 has been resumed, it determines that the ambient environment of the gas sensor 10 has returned to the first state, and repeats the processing of steps S15 and S16. In this embodiment, as shown in FIG. 6, the suppression control is switched to normal control promptly (for example, within 100 ms) after it is determined that the ambient environment of the gas sensor 10 has changed from the second state to the first state. This makes it possible to promptly start measuring the ammonia concentration in the exhaust gas when the suppression control is no longer necessary. In FIG. 6, the darkly shaded area indicates the first state, and the lightly shaded area indicates the second state.

After step S16 is completed, the CPU 321 determines whether or not there is a request to end the measurement process (step S18). If it is determined that there is no request to end the measurement process, the process returns to step S14 and is repeated. If it is determined that there is a request to end the measurement process, the measurement process is terminated.

To reliably suppress the combustion of ammonia, the second temperature is preferably less than 740°C, and more preferably 670°C or less. Also, the second temperature is preferably 300°C or more. If the second temperature is 300°C or more, the sensor element 100 can be quickly activated and measurement can be resumed when the ambient environment of the gas sensor 10 changes from the second state to the first state.

Here, the protective layer 205 and the protective coat C allow oxygen, nitrogen oxides, ammonia, etc. in the exhaust gas to pass through, and trap moisture and poisonous substances (e.g., P, Si, S, Mg, etc.) that adversely affect various electrodes including the detection electrodes 202A and 202B. The ammonia contained in the exhaust gas would normally pass through the protective layer 205 and the protective coat C and reach the surfaces of various electrodes including the detection electrodes 202A and 202B. However, if the chromium liberated from the external protector 12 and the internal protector 13 due to the high temperature surrounding the sensor element 100 adheres to the surfaces of the protective layer 205 and the protective coat C, the ammonia in the exhaust gas will burn on the surfaces of the protective layer 205 and the protective coat C. This is thought to cause a decrease in the measurement accuracy due to a change in the amount of ammonia that reaches the various electrodes including the detection electrodes 202A and 202B.

When the gas to be measured flows around the gas sensor 10 at a certain flow rate, the area around the sensor element 100 is ventilated, and chromium released from the protectors 12 and 13 is carried by the exhaust gas flow and discharged from the environment surrounding the sensor element 100, so it is thought that it does not adhere to the protective layer 205 or protective coating C very much. However, when the flow rate of the gas to be measured around the gas sensor 10 decreases, the released chromium is not discharged from the environment surrounding the sensor element 100, but adheres to the protective layer 205 or protective coating C, which is thought to cause a decrease in measurement accuracy.

On the other hand, it was found that the decrease in measurement accuracy due to ammonia combustion can be significantly suppressed if the sensor element 100 is controlled at a low temperature.

The mechanism by which measurement accuracy decreases due to ammonia combustion is presumed to be as follows.
(A) Chromium volatilizes from the protectors 12 and 13 and is adsorbed onto the sensor element 100 (for example, the protective layer 205 or the protective coat C).
(B) The adsorbed chromium reacts with some substance (for example, alumina contained in the protective layer 205 or protective coat C, or oxygen) to produce a compound or complex.
(C) The compounds and complexes produced act as catalysts to burn ammonia.

When the flow rate of the measurement target gas around the gas sensor 10 decreases, the sensor element 100 is controlled at a low temperature, so that it is believed that the reaction (B) above can be suppressed even if chromium is adsorbed to the sensor element 100. Note that chromium that does not undergo the reaction (B) above is easily released from the sensor element 100 when the flow rate of the measurement target gas increases, so that when the flow rate of the measurement target gas recovers, raising the temperature of the sensor element 100 is thought to have little effect.

[Action and Effect]
As described above, according to this embodiment, the sensor control device 300 is connected to and controls the gas sensor 10, the gas sensor 10 including the sensor element 100 including the NOx sensor portion 101 that detects nitrogen oxides in a gas to be measured, the ammonia sensor portion 201 that detects ammonia in the gas to be measured, and the heater 170 that heats the NOx sensor portion 101 and the ammonia sensor portion 201, and the protectors 12, 13 that are made of a metal containing chromium and are arranged to surround the sensor element 100. The gas sensor 321 and the heater driving circuit 317 selectively execute a normal control for controlling the heater 170 to a first temperature at which the NOx sensor portion 101 and the ammonia sensor portion 201 are activated when it is determined that the flow rate of the measurement target gas around the gas sensor 10 is equal to or higher than a predetermined value, and a suppression control for controlling the heater 170 to a second temperature at which the NOx sensor portion 101 and the ammonia sensor portion 201 are activated when it is determined that the flow rate of the measurement target gas around the gas sensor 10 is lower than a predetermined value.

It is believed that the decrease in measurement accuracy due to the combustion of ammonia occurs when the flow rate of the measurement target gas around the gas sensor 10 decreases, for example, during idling stop, and chromium released from the protectors 12 and 13 adheres to the sensor element 100. On the other hand, it has been found that the decrease in measurement accuracy due to the combustion of ammonia can be significantly suppressed if the sensor element 100 is controlled at a low temperature. When the flow rate of the measurement target gas around the gas sensor 10 is equal to or higher than a predetermined value, the heater 170 is controlled to make the NOx sensor portion 101 and the ammonia sensor portion 201 reach a first temperature at which they are activated, and when the flow rate of the measurement target gas around the gas sensor 10 is less than the predetermined value, the heater 170 is controlled to make the NOx sensor portion 101 and the ammonia sensor portion 201 reach a second temperature lower than the first temperature, thereby suppressing the combustion of ammonia, thereby enabling accurate measurement of a specific component in the measurement target gas.

<Modification>
In the modified example, as shown in FIG. 7, the CPU 321 executes switching from normal control to suppression control after a predetermined time has elapsed since it was determined that the ambient environment of the gas sensor 10 has changed from the first state to the second state. For example, when the time during which the gas sensor 10 is in the second state is extremely short, such as in a short-term idle stop, it is considered that there is almost no effect on the measurement accuracy, and lowering the temperature of the sensor element 100 in such a case is rather troublesome. For this reason, it is preferable not to execute suppression control by executing switching from normal control to suppression control after a predetermined time has elapsed since the ambient environment of the gas sensor 10 is in the second state, in the case where the time during which the gas sensor 10 is in the second state is extremely short. The "predetermined time" is preferably, for example, 10 s or more. In FIG. 7, the darkly shaded area indicates the first state, and the lightly shaded area indicates the second state.

<Test Example>
A gas sensor having the same configuration as the above embodiment was prepared and placed in an exhaust gas atmosphere. The surrounding environment of the gas sensor was in a windless state (a state in which the flow rate of the exhaust gas was almost zero), and the output value from the gas sensor corresponding to the ammonia concentration in the exhaust gas was continuously obtained. The ratio of the output value from the gas sensor after a specific time to the output value from the gas sensor at the start of measurement was calculated to obtain the output ratio.

As shown in FIG. 8, when the temperature of the sensor element was controlled at 740°C, the output ratio dropped rapidly immediately after the start of the measurement, and the output ratio after 600 hours was 10% or less. When the temperature of the sensor element was controlled at 700°C, the output ratio after 600 hours was 20% or less. The drop in the output ratio was more gradual than in the case of 740°C. When the temperature of the sensor element was controlled at 680°C, the output ratio after 600 hours was 50% or less, and the output ratio after 1200 hours was 20% or less. The drop in the output ratio was even more gradual than in the cases of 740°C and 700°C. When the temperature of the sensor element was controlled at 662°C, the output ratio hardly dropped, and the output ratio after 600 hours was 90% or more, and even after 1200 hours, the output ratio was 80% or more. As described above, it was confirmed that the lower the temperature of the sensor element, the more the decrease in measurement accuracy due to the combustion of ammonia is suppressed.

<Other embodiments>
(1) In the above embodiment, the gas sensor 10 is an ammonia sensor. However, the type of the gas sensor is not limited to the above embodiment and may be, for example, an oxygen sensor. This is because oxygen is also consumed in the ammonia oxidation reaction, which affects the measurement.
(2) In the above embodiment, the idling stop condition of the diesel engine 401 is used to determine whether the ambient environment of the gas sensor 10 is in the first state or the second state, but the conditions used to determine whether the ambient environment of the gas sensor 10 is in the first state or the second state are not limited to those in the above embodiment. For example, the engine speed may be determined to be the first state when it is equal to or greater than a predetermined value, and the engine speed may be determined to be the second state when it is less than the predetermined value. Alternatively, the vehicle speed may be determined to be the first state when it is equal to or greater than a predetermined value, and the vehicle speed may be determined to be the second state when it is less than the predetermined value. Alternatively, a flow velocity meter may be attached in the exhaust pipe, and the flow velocity measured by the flow velocity meter may be determined to be equal to or greater than a predetermined value, and the flow velocity measured by the flow velocity meter may be determined to be the first state when it is equal to or greater than a predetermined value, and the flow velocity measured by the flow velocity meter may be determined to be the second state when it is less than the predetermined value.

1: Gas measuring device 10: Gas sensor 11: Metal shell 11A: Through hole 12: External protector (protector)
13: Internal protector (protector)
14: Outer cylinder 15: Ceramic holder 16A, 16B: Talcum ring 17: Ceramic sleeve 18: Holding member 18A: Insertion hole 19: Terminal member 20: Lead wire 21: Elastic seal member 100: Sensor element 101: NOx sensor portion (detection portion)
110: first pump cell 111: first solid electrolyte body 112: first electrode 113: second electrode 120: Vs cell 121: second solid electrolyte body 122: third electrode 123: fourth electrode 130: second pump cell 131: third solid electrolyte body 132: fifth electrode 133: sixth electrode 141: first insulating layer 142: second insulating layer 143: third insulating layer 151: first diffusion resistor 152: second diffusion resistor 161: porous body 162: porous body 163: porous body 170: heater 171: fourth insulating layer 172: fifth insulating layer 173: resistance heating element 201: ammonia sensor section (detection section)
201A: First ammonia sensor section 201B: Second ammonia sensor section 202A: First reference electrode 202B: Second reference electrode 203A: Fourth solid electrolyte body 203B: Fifth solid electrolyte body 204A: First detection electrode 204B: Second detection electrode 205: Protective layer 300: Sensor control device 310: Electric circuit section 311: Reference voltage comparison circuit 312: Ip1 drive circuit 313: Vs detection circuit 314: Icp supply circuit 315: Ip2 detection circuit 316: Vp2 application circuit 317: Heater drive circuit (heater control section)
318: Resistance detection circuit 319A: First electromotive force detection circuit 319B: Second electromotive force detection circuit 320: Microcomputer 321: CPU (heater control unit)
323: RAM
323: ROM
324: Signal input/output unit 325: A/D converter 400: ECU
401: Diesel engine (internal combustion engine)
402: Exhaust pipe 403: Oxidation catalyst 404: DPF
405: SCR catalyst AX: axis C: protective coat R1: first measuring chamber R2: second measuring chamber R3: reference oxygen chamber

Claims (7)

A sensor control device is connected to a gas sensor including a sensor element having a detection unit that detects a specific gas in a measurement target gas containing ammonia and a heater that heats the detection unit, and a protector that is made of a metal containing chromium and is disposed to surround the sensor element, the sensor control device controlling the gas sensor,
a heater control unit selectively executing a normal control for controlling the heater to have a first temperature at which the detection unit is activated when it is determined that a first state is reached in which a flow rate of the measurement target gas around the gas sensor is equal to or higher than a predetermined value, and a suppression control for controlling the heater to have a second temperature at which the detection unit is activated when it is determined that a second state is reached in which a flow rate of the measurement target gas around the gas sensor is less than a predetermined value ,
The sensor control device is configured to switch from the normal control to the suppression control after a predetermined time has elapsed since the heater control unit determined that the ambient environment of the gas sensor has changed from the first state to the second state . The sensor control device according to claim 1 , wherein the second temperature is equal to or higher than 300° C. and lower than 740° C. A sensor control device is connected to a gas sensor including a sensor element having a detection unit that detects a specific gas in a measurement target gas containing ammonia and a heater that heats the detection unit, and a protector that is made of a metal containing chromium and is disposed to surround the sensor element, the sensor control device controlling the gas sensor,
a heater control unit selectively executing a normal control for controlling the heater to have a first temperature at which the detection unit is activated when it is determined that a first state is reached in which a flow rate of the measurement target gas around the gas sensor is equal to or higher than a predetermined value, and a suppression control for controlling the heater to have a second temperature at which the detection unit is activated when it is determined that a second state is reached in which a flow rate of the measurement target gas around the gas sensor is less than a predetermined value,
The second temperature is equal to or higher than 300°C and lower than 740°C. The sensor control device according to claim 1 , wherein the second temperature is equal to or higher than 300° C. and equal to or lower than 670° C. The sensor control device according to claim 1 , wherein the specific gas is ammonia. 6. The sensor control device according to claim 1, wherein the heater control unit executes switching from the suppression control to the normal control within 100 ms after determining that the ambient environment of the gas sensor has changed from the second state to the first state. the measurement target gas is exhaust gas discharged from an internal combustion engine mounted on a vehicle,
7. The sensor control device according to claim 1, wherein the first state is a state in which the internal combustion engine is driven, and the second state is a state in which the internal combustion engine is stopped.

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