Control device

Abstract

To provide a control device which can suppress damages on an air-fuel ratio sensor from thermal stress.SOLUTION: A controller includes: a control unit for controlling the conduction amount of a heater for heating a sensor element for detecting the air-fuel ratio of discharge gas in an internal combustion engine for driving a vehicle; and a detection unit for detecting the resistance value of the heater. The control unit reduces the conduction amount in a case where the amount of change of the resistance value with time is at least a threshold value.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a control device.

Background Art

[0002] Some of the air-fuel ratio sensors in the exhaust system of internal combustion engines are of a laminated type in which a sensor element for detecting the air-fuel ratio of exhaust gas and a heater for heating to activate the sensor element early are integrally provided. For example, Patent Document 1 describes that as the environmental temperature at the start of the engine is lower, by reducing the increase per unit time of the energization amount of the heater, damage to the sensor element due to heat shock caused by moisture in the exhaust gas at startup is suppressed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the warm-up process from a cold start of the engine, the sensor element may be damaged due to excessive thermal stress during temperature rise caused not only by moisture in the exhaust gas but also by other surrounding environments and individual differences in the sensor element itself.

[0005] Therefore, the present invention has been made in view of the above problems, and an object thereof is to provide a control device capable of suppressing damage to an air-fuel ratio sensor due to thermal stress.

Means for Solving the Problems

[0006] The control device of the present invention includes a control unit that controls the energization amount of a heater that heats a sensor element that detects the air-fuel ratio of the exhaust gas of an internal combustion engine that drives a vehicle, and a detection unit that detects the resistance value of the heater. When the amount of change over time of the resistance value is equal to or greater than a threshold value, the control unit reduces the energization amount.

Advantages of the Invention

[0007] According to the present invention, damage to the air-fuel ratio sensor due to thermal stress can be suppressed.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0009] (Configuration of Engine Exhaust System) FIG. 1 is a configuration diagram showing an example of an engine exhaust system 9. The engine exhaust system 9 is mounted on a vehicle such as a hybrid vehicle, and includes a sensor control device 1, an air-fuel ratio sensor (A / F sensor) 2, an engine (ENG) 3, an exhaust passage 30, a catalyst device 31, a muffler 32, and a water temperature sensor 33. The engine 3 is an example of an internal combustion engine that drives a vehicle. The water temperature sensor 33 detects the temperature of the cooling water of the engine 3 and outputs it to the sensor control device 1. The exhaust passage 30 is provided between the engine 3 and the muffler 32. The exhaust gas of the engine 3 flows through the exhaust passage 30 and is discharged outside the vehicle from the muffler 32. The catalyst device 31 is provided in the middle of the exhaust passage 30. The catalyst device 31 purifies the exhaust gas with, for example, a three-way catalyst.

[0010] The air-fuel ratio sensor 2 is provided between the engine 3 and the catalytic converter 31 in the exhaust passage 30. The air-fuel ratio sensor 2 includes a sensor element 20 and a heater 21. The sensor element 20 detects the air-fuel ratio of the exhaust gas. The heater 21 heats the sensor element 20 to activate it. The heater 21 and the sensor element 20 are integrated within a laminated structure.

[0011] The sensor element 20 is a solid electrolyte such as zirconia, for example, and is sandwiched from above and below by platinum electrodes. The sensor element 20 requires heating by the heater 21 because, by having a low resistance in a high-temperature region, a sensor current corresponding to the air-fuel ratio of the exhaust gas flows between the electrodes. The sensor element 20 outputs the sensor current to the sensor control device. Note that the air-fuel ratio sensor 2 may also be provided on the downstream side of the catalytic converter 31.

[0012] The sensor control device 1 includes a CPU (Central Processing Unit) 10, a ROM (Read Only Memory) 11, a RAM (Random Access Memory) 12, and an interface section (IF) 14. The CPU 10 is electrically connected to the ROM 11, the RAM 12, and the IF 14 via a bus 19 so that signals can be input and output to and from each other. Note that the sensor control device 1 is an example of a computer and is, for example, an ECU (Electronic Control Unit).

[0013] The ROM 11 stores a program for driving the CPU 10. The RAM 12 functions as a working memory for the CPU 10. The IF 14 is a circuit that performs communication processing between the CPU 10 and the water temperature sensor 33 and the air-fuel ratio sensor 2.

[0014] When the CPU 10 reads a program from the ROM 11, it forms, as software functions, an air-fuel ratio calculation section 100, a heater control section 101, and a resistance value detection section 102. The resistance value detection section 102 is an example of a detection section and detects the resistance value of the heater 21. For example, the resistance value detection section 102 acquires the current and voltage of the heater 21 from the heater 21 via the IF 14 and calculates the resistance value of the heater 21 from the current and voltage.

[0015] The heater control unit 101 is an example of a control unit and controls the energization amount of the heater. The heater control unit 101 outputs, for example, a pulse signal for turning the heater 21 on and off to the heater 21 via the IF14. The heater control unit 101 controls the energization amount of the heater 21 by adjusting the duty ratio of the pulse signal according to the impedance of the sensor element 20 acquired via the IF14. The heater control unit 101 activates the sensor element 20 by controlling the energization amount.

[0016] After the sensor element 20 is activated, the air-fuel ratio calculation unit 100 acquires the sensor current output from the sensor element 20 via the IF14 and calculates the air-fuel ratio from the current value. When the air-fuel ratio calculation unit 100 determines that the temperature of the sensor element 20 has reached the activation reference value based on the notification from the heater control unit 101, the air-fuel ratio calculation unit 100 calculates the air-fuel ratio.

[0017] In the warm-up process from the cold start of the engine 3, the sensor element 20 may be damaged due to excessive thermal stress generated during temperature rise caused not only by moisture in the exhaust gas but also by other surrounding environments and individual differences of the sensor element 20 itself.

[0018] Therefore, when the amount of change over time of the resistance value detected by the resistance value detection unit 102 is equal to or greater than the threshold value, the heater control unit 101 reduces the energization amount. For example, the heater control unit 101 calculates the temperature based on the correlation between the resistance value and the temperature indicated by the symbol G.

[0019] T = a×r + b ···(1)

[0020] As an example, the correlation between the temperature T and the resistance value r is represented by the above formula (1). The constant a is the temperature coefficient according to the material of the heater 21. The constant b is the temperature T when the resistance value r = 0, and as shown by the dotted line, it varies due to individual differences of the heater 21. For this reason, as an example, the heater control unit 101 regards the temperature of the water temperature sensor 33 at the cold start of the engine 3 as the temperature T of the heater 21, and calculates the constant b from the resistance value r of the heater 21 at that time. Here, as the temperature T, instead of the temperature of the water temperature sensor 33, the intake air temperature of the engine 3 may be used, or further, the lower temperature of the water temperature sensor 33 and the intake air temperature may be used.

[0021] In this way, the heater control unit 101 can calculate the temperature T from the resistance value r of the heater 21 with high accuracy. Here, when the heater control unit 101 calculates the constant b when sufficient time (for example, 120 minutes) has elapsed after the engine 3 last stopped operating and the engine 3, the air-fuel ratio sensor 2, and its surroundings are in a cooled state, the constant b can be calculated with higher accuracy, which is preferable. Note that the heater control unit 101 may store the calculated constant b in a non-volatile memory such as the ROM 11 and use it at the next start of the engine 3.

[0022] The heater control unit 101 regards the temperature T of the heater 21 as the temperature of the sensor element 20, and monitors the temperature change amount per unit time (for example, 100 ms). Here, since the heater 21 and the sensor element 20 are integrally laminated as described above, the temperatures of both can be regarded as substantially the same. Also, the temperature change amount per unit time is a measure of the magnitude of the thermal stress generated in the sensor element 20. In this way, the heater control unit 101 monitors the temperature change of the sensor element 20 without providing a temperature sensor in the sensor element 20, so that the air-fuel ratio sensor 2 can be miniaturized.

[0023] The heater control unit 101 compares the amount of temperature change per unit time with a predetermined threshold value. When the amount of temperature change ≥ the threshold value, it determines that the risk of damage to the sensor element 20 is higher than normal, and reduces the energization amount of the heater 21 by, for example, setting the duty ratio lower than the current set value. For this reason, when the temperature of the sensor element 20 rises rapidly, the sensor control device 1 can suppress damage to the sensor element 20 by suppressing the heating from the heater 21. Here, the threshold value is preset to include a safety margin with respect to the temperature at which damage occurs, taking into account the variation in the durability of the sensor element 20 against thermal stress.

[0024] As described above, the heater control unit 101 determines the risk of damage to the sensor element 20 according to the amount of temperature change per unit time. Since the temperature is calculated from the resistance value r of the heater 21 by Equation (1), it can be said that the determination is made according to the amount of time change of the resistance value r per unit time. That is, when the amount of time change of the resistance value r per unit time is equal to or greater than a predetermined threshold value, the heater control unit 101 reduces the energization amount of the heater 21, and when the amount of time change of the resistance value r per unit time is less than the predetermined threshold value, the heater control unit 101 maintains or increases the energization amount of the heater 21.

[0025] When reducing the energization amount of the heater 21, the heater control unit 101 may set the duty ratio to half of the current set value so that the energization amount becomes half of the current value, or may set the duty ratio to 0 (%) so that the energization amount becomes 0. Alternatively, the heater control unit 101 may determine the reduction amount of the energization amount according to the difference between the amount of temperature change and the predetermined threshold value.

[0026] (Example of Heater Control) Figure 2 is a time chart showing an example of the control of the heater 21. Figure 2 shows the changes in the temperature (°C) of the heater 21 and its amount of change over time (temperature change amount) (°C) with respect to time (seconds), and the changes in the magnitude (distinguished as large, medium, and small) of the duty ratio of the pulse signal for driving the heater 21. Since the correlation between the temperature and the resistance value of the heater 21 is based on Equation (1), the illustration thereof is omitted, but the resistance value also shows the same time change as the temperature.

[0027] The heater control unit 101 sets the duty ratio to "small" during the period from time 0 to Ta, and sets the duty ratio to "medium" during the period from time Ta to Tb. Therefore, the temperature rises gently during the period from time 0 to Ta, and the temperature rises more rapidly during the period from time Ta to Tb than during the period from time 0 to Ta.

[0028] Since the heater control unit 101 sets the duty ratio to "large" during the period from time Tb to Tc, the temperature rises most rapidly. When the amount of temperature change reaches the threshold value TH at time Tc, the heater control unit 101 determines that the risk of damage to the sensor element 20 due to thermal stress is high, and sets the duty ratio to "small". As a result, the energization amount of the heater 21 decreases and the temperature rise is alleviated, so that the sensor element 20 can avoid damage.

[0029] In this way, the heater control unit 101 can increase the energization amount of the heater 21 so that the sensor element 20 is activated earlier when the engine 3 is started, and can reduce the energization amount to reduce the risk when the risk of damage to the sensor element 20 is high.

[0030] (Operation of the sensor control device) FIG. 3 is a flowchart showing an example of the operation of the sensor control device 1. This operation is executed, for example, at a fixed time period (for example, 100 ms).

[0031] First, the heater control unit 101 determines whether or not the constant b in the above formula (1) has been calculated (step St1). If it has not been calculated (No in step St1), the heater control unit 101 determines whether or not the engine 3, the air-fuel ratio sensor 2, and its surroundings are in a cooled state (step St10). For the determination, various parameters related to the engine 3 and the air-fuel ratio sensor 2 are used, for example. If the engine 3 or the like is not in a cooled state (No in step St10), this operation ends.

[0032] Further, when the engine 3 or the like is in a cooled state (Yes in step St10), the heater control unit 101 detects the temperature of the cooling water of the engine 3 from the water temperature sensor 33 (step St11). Next, the heater control unit 101 energizes the heater 21 for a predetermined time sufficient to measure the resistance value of the heater 21 (step St12), and calculates the resistance value from the current value and the voltage value (step St13). Next, the heater control unit 101 regards the temperature of the cooling water as the temperature of the heater 21, and calculates the constant b from the formula (1) based on the temperature and the resistance value (step St14). The constant b is held in a non-volatile memory or the like.

[0033] After step St14, or when it is determined that the constant b has been calculated (Yes in step St1), the heater control unit 101 detects the impedance of the sensor element 20 (step St2). Next, the heater control unit 101 calculates the duty ratio of the pulse signal based on the impedance (step St3).

[0034] Next, the heater control unit 101 detects the resistance value from the current value and the voltage value of the heater 21 (step St4). Next, the heater control unit 101 calculates the temperature from the formula (1) based on the resistance value (step St5). Next, the heater control unit 101 calculates the amount of change in temperature per unit time from the difference from the previously calculated temperature (step St6).

[0035] Next, the heater control unit 101 compares the amount of change in temperature with the threshold value TH (step St7). When the amount of change in temperature ≥ TH is satisfied (Yes in step St7), the heater control unit 101 sets the duty ratio lower than the calculated value in step St3 (step St8). When the amount of change in temperature < TH is satisfied (No in step St7), the heater control unit 101 maintains the duty ratio at the calculated value in step St3 (step St15).

[0036] Next, the heater control unit 101 energizes the heater 21 by outputting a pulse signal (step St9). Thus, the sensor control device 1 operates.

[0037] The above-described embodiments are preferred examples of the present invention. However, the present invention is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

Explanation of Reference Numerals

[0038] 1 Sensor control device (control device), 2 Air-fuel ratio sensor, 3 Engine (internal combustion engine), 20 Sensor element, 21 Heater, 101 Heater control unit (control unit), 102 Resistance value detection unit (detection unit)

Claims

[Claim 1] a control unit that controls an amount of electricity supplied to a heater that heats a sensor element that detects an air-fuel ratio of exhaust gas from an internal combustion engine that drives a vehicle; a detection unit that detects a resistance value of the heater, The control unit reduces the amount of current flow when a time change in the resistance value is equal to or greater than a threshold value. Control device.

Images