Closed-loop gain control system for limiting the ramp rate of oxygen sensors in exhaust systems

Abstract

System (100), comprising: a driver circuit (106) configured to drive a heater (104) connected to a sensor (102) in an exhaust system of a vehicle with a duty cycle; a feedback circuit (110) configured to generate a feedback signal indicative of a temperature of the sensor (102); a ramp circuit (112) configured to output: a first ramp setpoint indicating a first rate at which the temperature of the sensor (102) is to be changed over a first period of time after an engine of the vehicle is turned on; and a second ramp setpoint indicating a second rate at which the temperature of the sensor (102) is to be changed after the first period of time until the temperature of the sensor (102) reaches a predetermined temperature; an error circuit (116) configured to: generate a first error signal based on the feedback signal and the first ramp setpoint during the first period; and generate a second error signal based on the feedback signal and the second ramp setpoint after the first period of time until the temperature of the sensor (102) reaches the predetermined temperature; and a controller (108) configured to: controlling the duty cycle of the driver circuit (106) to drive the heater (104) based on one or more gains; adjust the one or more gains based on the first error signal during the first period; and adjusting the one or more gains based on the second error signal after the first time period until the temperature of the sensor (102) reaches the predetermined temperature.

Description

INITIATIONThe present invention relates generally to vehicle exhaust systems and, more particularly, to a closed loop boost control system for limiting a ramp rate of oxygen sensors in the vehicle exhaust systems.For background information, reference is made at this point beforehand to the publications US 2005 / 0 189 343 A1, DE 10 2008 005 110 A1 and U.S. Pat. No. 9,874,549 B2.An oxygen (O 2-) sensor is mounted in an exhaust manifold to detect unburned oxygen in exhaust gases discharged from the engine of a vehicle. The number of oxygen sensors in an exhaust system varies. Many vehicles have two oxygen sensors: a first oxygen sensor located upstream of a catalytic converter and a second oxygen sensor located downstream of the catalytic converter. The first (upstream) oxygen sensor measures the air-fuel ratio of the exhaust gas coming from the exhaust manifold and sends high and low voltage signals to a powertrain control module to regulate an air-fuel mixture. When the powertrain control module receives a low voltage signal indicative of a lean air-fuel mixture, the powertrain control module compensates for this by increasing the amount of fuel in the air-fuel mixture. When the powertrain control module receives a high voltage signal indicative of a rich air-fuel mixture, the powertrain control module makes the air-fuel mixture lean by reducing the amount of fuel in the air-fuel mixture. The use of the oxygen sensor input signal by the powertrain control module to regulate the air-fuel mixture is referred to as closed loop control. This closed loop operation results in a steady rich-to-lean transition that allows the catalytic converter to minimize emissions by maintaining an average total air-fuel mixture ratio in proper balance.However, when a cold engine is started or an oxygen sensor fails, the powertrain control module enters open-loop operation. During open loop operation, the powertrain control module receives no signal from the oxygen sensor and commands a solid rich fuel mixture. Open loop operation results in increased fuel consumption and emissions. Many more recent oxygen sensors have heaters to help them reach operating temperature quickly to minimize the amount of time it operates in open loop.The second (downstream) oxygen sensor measures the air-fuel ratio coming from the catalytic converter to ensure that the catalytic converter is functioning properly. The catalytic converter operates to maintain a desired stoichiometric air-fuel ratio while the powertrain control module switches between rich and lean air-fuel mixtures due to input from the upstream oxygen sensor (sensor 1). Therefore, the downstream oxygen sensor (sensor 2) needs to generate a constant voltage.Wide-range air fuel (also called wide-range air fuel or WRAF) sensors and air / fuel (A / F) sensors replace conventional oxygen sensors in recent vehicles. A WRAF sensor is essentially a more intelligent oxygen sensor with some additional internal circuitry that allows it to accurately determine the exact air / fuel ratio of the engine. Like an ordinary oxygen sensor, it responds to changing oxygen levels in the exhaust gas. However, unlike an ordinary oxygen sensor, its output does not change abruptly when the air-fuel mixture becomes rich or lean. While an ordinary oxygen sensor is a rich / lean indicator, the WRAF sensor provides a gradually varying signal corresponding to the exact air / fuel ratio. The output voltage of the WRAF sensor is converted by its internal circuit into a variable bi-directional current signal that may gradually increase in a positive direction as the air / fuel mixture becomes leaner. At the "stoichiometric" point, when the air / fuel mixture is perfectly balanced (14.7 to 1), the current flow from the WRAF sensor stops. As the air / fuel ratio becomes progressively richer, the current flows in a negative direction. Like ordinary oxygen sensors, WRAF sensors also have an internal circuit of a heater to assist them in rapidly reaching operating temperature. To operate properly, WRAF sensors typically require a higher operating temperature than oxygen sensors.SUMMARYAccording to the invention, a system is presented which is distinguished by the features of claim 1.The system includes a driver circuit, a feedback circuit, a ramp circuit, an error circuit, and a controller. The driver circuit is configured to drive a heater connected to a sensor in an exhaust system of a vehicle at a duty cycle. The feedback circuit is configured to generate a feedback signal indicative of a temperature of the sensor. The ramp circuit is configured to output a first ramp setpoint indicative of a first rate at which the temperature of the sensor is to be changed over a first period of time after turning on an engine of the vehicle. The ramp circuit is configured to output a second ramp set point indicative of a second rate at which the temperature of the sensor is to be changed after the first period of time until the temperature of the sensor reaches a predetermined temperature. The error circuit is configured to generate a first error signal based on the feedback signal and the first ramp set point during the first period. The error circuit is configured to generate a second error signal based on the feedback signal and the second ramp set point after the first time period until the temperature of the sensor reaches the predetermined temperature. The controller is configured to control the duty cycle of the driver circuit to drive the heater based on one or more gains. The controller is configured to adjust the one or more gains based on the first error signal during the first period. The controller is configured to adjust the one or more gains based on the second error signal after the first time period until the temperature of the sensor reaches the predetermined temperature.In another feature, the controller adjusts the duty cycle of the driver circuit to drive the heater based on the adjusted one or more gains.In another feature, the second rate is faster than the first rate.In another feature, the predetermined temperature is a turn-off temperature of the sensor.In another feature, the temperature of the sensor remains within the limits specified by a manufacturer of the sensor due to the adjusted one or more gains.In another feature, the sensor heats to a first temperature that is less than the predetermined temperature at the end of the first time period.In another feature, the sensor heats up in the presence of water vapor during the first period.In another feature, the sensor heats up during the first period of time without being damaged.In another feature, the sensor reaches the predetermined temperature without being damaged.In further features, the driver circuit includes a pulse width modulation circuit and the controller includes a proportional-integral-differential controller.Further described is a method comprising driving a heater connected to a sensor in an exhaust system of a vehicle with a duty cycle, and receiving a feedback signal indicative of a temperature of the sensor. The method further includes outputting a first ramp setpoint indicative of a first rate at which to change the temperature of the sensor over a first period of time after turning on an engine of the vehicle. The method further includes outputting a second ramp setpoint indicative of a second rate at which the temperature of the sensor is to be changed after the first period of time until the temperature of the sensor reaches a predetermined temperature. The method further includes generating a first error signal based on the feedback signal and the first ramp set point during the first period. The method further includes generating a second error signal based on the feedback signal and the second ramp set point after the first time period until the temperature of the sensor reaches the predetermined temperature. The method further includes controlling the duty cycle to drive the heater based on one or more gains, adjusting the one or more gains based on the first error signal during the first period of time, and adjusting the one or more gains based on the second error signal after the first period of time until the temperature of the sensor reaches the predetermined temperature.In another feature, the method further includes adjusting the duty cycle to drive the heater based on the adjusted one or more gains.In another feature, the second rate is faster than the first rate.In another feature, the predetermined temperature is a turn-off temperature of the sensor.In another feature, the method further comprises maintaining the temperature of the sensor within limits specified by a manufacturer of the sensor based on the adjusted one or more gains.In another feature, the method further includes heating the sensor to a first temperature that is less than the predetermined temperature at the end of the first period.In another feature, the method further comprises heating the sensor in the presence of water vapor during the first period.In another feature, the method further includes heating the sensor during the first period of time without damaging the sensor.In another feature, the method further includes heating the sensor to the predetermined temperature without damaging the sensor.In another feature, the method further includes controlling the duty cycle to drive the heater using pulse width modulation and using a proportional-integral-differential controller.Further areas of applicability of the present invention will become apparent from the detailed description, claims and drawings. The detailed description and specific examples are intended for purposes of illustration only.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will be more fully understood from the detailed description and the accompanying drawings, wherein: FIG. 1 shows a system for controlling heating of a sensor (e.g., an oxygen or a wide-range air-fuel sensor) in an exhaust system of a vehicle according to the present invention; and FIG. 2 illustrates a method for controlling heating of a sensor (e.g., an oxygen or a wide range air and fuel sensor) in an exhaust system of a vehicle in accordance with the present invention.In the drawings, reference numerals may be reused to identify similar and / or identical elements.DETAILED DESCRIPTIONO 2- and WRAF sensors must have a limited rate of temperature rise to prevent sensor breakage. The proposed method alters the heater control gains to limit the temperature rise rate to a maximum rate recommended by the sensor manufacturer. The calibrated heater control gains of the O 2- sensor are automatically constrained during the warm-up period of the sensor using closed loop feedback. No additional gain calibration is required for the warm-up period, and the method uses sensor feedback to ensure that the constraint is not violated. In addition to avoiding breakage, the method reduces the off time of the O 2- and WRAF sensors for better emissions, the off time being the time taken by the sensor to reach an off temperature that is a temperature at which the sensor is sufficiently warm and usable.To reduce turn-off time, many newer sensors have a lower mass and use a higher heating power, which increases the thermal ramp rate of the sensors, which in turn may cause the sensor to crack. Currently, the controllers use a minimum open loop voltage level provided to the sensor during the warm-up period. Current controllers do not use feedback from the sensor and do not control the heater gains during the warm-up period of the sensor. The system and method of the present disclosure utilizes feedback and controls the heater gains to optimize the supplier's specified maximum ramp rate, resulting in more robust control of the heater that prevents sensor failure and also minimizes warm-up time and emissions. In particular, the system and method utilize sensor temperature measurement feedback to change the closed loop control gains to limit the temperature ramp rate to the maximum value specified by the supplier.For both sensors (i.e., O 2 or WRAF), the output of the sensor is a function of the temperature of the sensor. Heaters are used to heat these sensors to control the sensor temperature and thus control the sensor output to a desired value. The proposed system achieves this aim in a manner that also prevents the sensors from cracking due to heating during the warm-up period until shutdown is reached, as explained in detail below.When the engine is cold started, the engine exhaust gas often contains water vapor. This water vapor may result in breakage (e.g., crack) of the sensors if the temperature of the sensor is not properly controlled. In order to prevent breakage, some manufacturers specify to heat the sensor for a certain period of time after cold starting the engine. The assumption is that after this period of time the water vapor will no longer be present. After the period has elapsed, the sensor may be heated according to a predetermined heating or temperature profile until the sensor is heated to a desired temperature at which the sensor is ready for use (i.e., off temperature). However, this process may take a long time, which may adversely affect fuel efficiency and emissions.Some manufacturers allow the sensor to be heated in a controlled manner after cold starting the engine. These manufacturers typically offer different heating modes and corresponding heating profiles for the gradual heating of their sensors. For example, in a first mode, comprising heating the sensor for a first predetermined period of time immediately after cold start of the engine, the sensor is heated at a first rate according to a first profile. That is, after the engine cold start, the sensor temperature is increased at the first rate corresponding to the first profile for the first predetermined period of time. It is assumed (or verified) that at the end of the first predetermined period of time, the water vapor is no longer present.After the first predetermined period of time, the sensor is heated in a second mode at a second rate according to a second profile until the sensor temperature reaches the turn-off temperature. That is, after the first predetermined period of time, the sensor temperature is increased at the second rate according to the second profile until the sensor temperature reaches the turn-off temperature. Some manufacturers provide additional modes and specify different heating rates and corresponding profiles for each additional mode. Both the first and second rates need to be non-linear (i.e., the rates may be non-linear). The second rate may be faster than the first rate. Importantly, manufacturers limit (i.e., limit) these rates to avoid sensor breakage. That is, manufacturers specify maximum and minimum values for these rates in each mode. The system and method of the present invention controls the heater that heats the sensor so that the sensor temperature is varied according to the manufacturer's profile and within the manufacturer's constraints, regardless of the type and manufacturer of the sensor in each mode.FIG. 1 shows a system 100 for controlling the heater of a sensor according to the present invention. While only one sensor is shown and described for illustrative purposes, the system 100 may control a plurality of sensors in the manner described below. Although shown separately, one or more of elements 106, 108, 110, 112, and 116 may be integrated into a single circuit.The system 100 includes a sensor 102 (e.g., an O 2- or WRAF sensor) with a heater 104 connected, a driver circuit (e.g., a PWM circuit) 106 that drives the heater 104 at a duty cycle, and a controller (e.g., a PID controller) 108 that controls the duty cycle of the heater 104 according to one or more gains. A battery 109 of the vehicle provides power to the sensor 102.The system 100 further includes a feedback circuit 110 that detects a resistance of the sensor 102 that varies as a function of the temperature of the sensor 102. The feedback circuit 110 converts the resistor into a temperature measurement using a look-up table and generates a feedback signal indicative of the temperature of the sensor 102.The system 100 further includes a ramp circuit 112. The ramp circuit 112 generates a ramped (i.e., ramp-varying over time) setpoint for the sensor 102. For example, the ramp circuit 112 may generate a first ramp set point that varies at a first rate during a first time period after the engine is on (i.e., a cold start). After the first period of time, the ramp circuit 112 may generate a second ramp set point that varies at a second rate until the sensor temperature reaches a predetermined temperature (e.g., a turn-off temperature). The second rate may be faster than the first rate. The units of ramp setpoints may be degrees Celsius per second.An engine control module 114 controls operation of the engine (e.g., an air-fuel ratio) based on the output of the sensor 102. The engine control module 114 may store heating profiles for various sensors. The engine control module 114 may detect the type of sensor 102 and provide the heating profile or profiles (depending on the number of heating modes) to the ramp circuit 112 for the sensor 102. The ramp circuit 112 generates the ramp setpoints corresponding to the heating profile or profiles. The ramp circuit 112 generates the ramp setpoints that are within the constraints specified by the heating profile or profiles.The system 100 further includes an error circuit 116 that compares the feedback signal to the ramp set points output by the ramp circuit 112 and generates error signals based on the comparisons. For example, during the first period after engine power-up (i.e., cold start), the error circuit 116 generates a first error signal based on the feedback signal and the first ramp set point. The controller 108 adjusts its gains based on the first error signal and adjusts the duty cycle of the driver circuit 106 based on the adjusted gains. As the first ramp set point varies during the first period, the first error signal, the gains of the controller 108, and the duty cycle of the driver circuit 106 also vary during the first period.The driving circuit 106 drives the heater 104 with the set duty until the first period elapses. Because the ramp circuit 112 generates the ramp set points that are within the constraints specified by the heating profile or profiles, the adjusted duty cycle at which the driver circuit 106 drives the heater 104 ensures that the sensor temperature remains within the constraints specified by the heating profile or profiles so that the sensor does not crack.The engine control module 114 may monitor the first period using a counter. The engine control module 114 may determine the value of the counter based on the amount of time it takes the water vapor in the exhaust system to disappear after the engine cold starts. The engine control module 114 may estimate the amount of time that water vapor needs to disappear based on factors such as ambient temperature, humidity, engine temperature, the amount of time elapsed since the engine was last turned off, etc.Thus, at the end of the first period, the sensor 102 heats to a temperature that is less than the turn-off temperature without cracking or otherwise being damaged due to the above-described controlled heating in the presence of water vapor. The system 100 may use additional ramp set points during the first period.Upon elapse of the first period of time, the ramp circuit 112 generates the second ramp set point that varies at the second rate until the sensor temperature reaches a predetermined temperature (e.g., a turn-off temperature). The error circuit 116 generates a second error signal based on the feedback signal and the second ramp set point. The controller 108 adjusts its gains based on the second error signal and adjusts the duty cycle of the driver circuit 106 based on the adjusted gains. Since the second ramp set point also varies with time, the second error signal, the gains of the controller 108, and the duty cycle of the driver circuit 106 also vary with time. The driver circuit 106 drives the heater 104 at the adjusted duty cycle until the sensor temperature reaches a predetermined temperature (e.g., a turn-off temperature of the sensor 102).Thus, the sensor 102 heats up to the off temperature without breaking or otherwise being damaged. Because the sensor 102 is already warmed up at the end of the first period, the second rate at which the second ramp set point changes may be faster than the first rate at which the first ramp set point changes during the first period. As a result, the sensor 102 reaches the turn-off temperature quickly and without cracking or other damage. The system 100 may use additional ramp setpoints after the first time period.The system 100 may determine whether the sensor 102 has reached a predetermined temperature (e.g., a turn-off temperature) in many ways. For example, the engine control module 114 may make the determination based on the output of the sensor 102. Alternatively, the error circuit 116 may make the determination based on the feedback signal.FIG. 2 shows a method 200 for controlling heating of a sensor according to the present invention. The method 200 is performed, for example, by one or more elements (e.g., one or more of the elements 106, 108, 110, 112, and 116) of the system 100. While only one sensor is described for illustrative purposes, method 200 may control a plurality of sensors in the manner described below.At 202, method 200 determines whether the engine is on (i.e., cold started). Method 200 waits until the engine is powered on. Method 200 proceeds to 204 when the engine is on. At 204, method 200 starts a counter. When the counter expires, a first time period has elapsed. For example, method 200 selects the first time period (i.e., the value of the counter) such that no water vapor is present in the exhaust gas at the end of the first time (i.e., when the counter expires).At 206, method 200 outputs a first ramp set point for controlling a heater connected to the sensor. At 208, method 200 generates feedback regarding a temperature of the sensor based on a resistance of the sensor. At 210, the method 200 generates a first error signal based on the feedback and the first ramp set point. At 212, method 200 varies one or more gains of a controller controlling the heater based on the first error signal. The adjusted gains ensure that the sensor temperature remains within the specified limits. At 214, method 200 controls a heater duty cycle based on the adjusted gains of the controller. At 216, method 200 controls the heater based on the duty cycle generated based on the adjusted gains.At 218, method 200 determines whether the counter has expired. Method 200 returns to 206 if the counter has not yet expired. Method 200 continues to 220 when the counter has expired.At 220, the method 200 outputs a second ramp set point for controlling the heater connected to the sensor. At 222, method 200 further generates feedback regarding the temperature of the sensor based on the resistance of the sensor. At 224, method 200 generates a second error signal based on the feedback and the second ramp set point. At 226, method 200 varies one or more gains of the controller controlling the heater based on the second error signal. The adjusted gains ensure that the sensor temperature remains within the specified limits. At 228, method 200 controls the heater duty cycle based on the controller adjusted gains. At 230, method 200 controls the heater based on the duty cycle generated based on the adjusted gains.At 232, method 200 determines whether the sensor is heated to a predetermined temperature (e.g., the off temperature of the sensor). Method 200 returns to 220 if the sensor is not yet heated to the predetermined temperature (e.g., the off temperature of the sensor). The method 200 ends if the sensor is heated to the predetermined temperature (e.g., the off temperature of the sensor).

Claims

A system (100) comprising: a driver circuit (106) configured to drive a heater (104) connected to a sensor (102) in an exhaust system of a vehicle with a duty cycle; a feedback circuit (110) configured to generate a feedback signal indicative of a temperature of the sensor (102); a ramp circuit (112) configured to output: a first ramp set point indicative of a first rate at which the temperature of the sensor (102) is to be changed over a first period of time after an engine of the vehicle is turned on; and a second ramp set point indicative of a second rate at which the temperature of the sensor (102) is to be changed after the first period of time until the temperature of the sensor (102) reaches a predetermined temperature; an error circuit (116) configured to: generate a first error signal based on the feedback signal and the first ramp set point during the first time period; and generate a second error signal based on the feedback signal and the second ramp set point after the first time period until the temperature of the sensor (102) reaches the predetermined temperature; and a controller (108) configured to: control the duty cycle of the driver circuit (106) to drive the heater (104) based on one or more gains; adjust the one or more gains based on the first error signal during the first time period; and adjust the one or more gains based on the second error signal after the first time period until the temperature of the sensor (102) reaches the predetermined temperature.The system (100) of claim 1, wherein the controller (108) adjusts the duty cycle of the driver circuit (106) to drive the heater (104) based on the adjusted one or more gains.The system (100) of claim 1, wherein the second rate is faster than the first rate.The system (100) of claim 1, wherein the predetermined temperature is a turn-off temperature of the sensor (102).The system (100) of claim 1, wherein the temperature of the sensor (102) remains within the limits specified by a manufacturer of the sensor (102) due to the adjusted one or more gains.The system (100) of claim 1, wherein the sensor (102) heats up to a first temperature that is less than the predetermined temperature at the end of the first time period.The system (100) of claim 1, wherein the sensor (102) heats up during the first time period in the presence of water vapor.The system (100) of claim 1, wherein the sensor (102) heats up without being damaged during the first period of time.The system (100) of claim 1, wherein the sensor (102) reaches the predetermined temperature without being damaged.The system (100) of claim 1, wherein the driver circuit (106) includes a pulse width modulation circuit, and wherein the controller (108) includes a proportional-integral-derivative controller.

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