Abstract:
A method of controlling a motor vehicle engine having an electronic throttle including positioning the electronic throttle in response to a requested throttle area, determining a fault in the vehicle engine, limiting the throttle area after a fault has been determined to control vehicle speed, limiting acceleration of the vehicle after the fault has been determined, and determining if the engine may be operated at full throttle after the fault has been determined to allow the vehicle to operate in a high power demand application.

Description:
TECHNICAL FIELD 
     This invention relates to a method of operation for a vehicle electronic throttle control (ETC) system, and more particularly to a method of using the throttle control system to govern vehicle acceleration, speed and power during a fault. 
     BACKGROUND OF THE INVENTION 
     In a vehicle ETC system, the engine throttle plate is mechanically de-coupled from the driver-operated accelerator pedal, and instead is positioned by an electric motor under the control of an electronic or powertrain control module (PCM). The motor is activated to position the throttle plate in response to accelerator pedal movement, but may also be controlled to achieve other functions such as idle speed control, engine speed governing, cruise control, torque reduction for traction control, and vehicle acceleration governing. In general, the PCM or another controller determines a desired effective throttle area to achieve a given function, and the PCM activates the motor to move the throttle plate to a position corresponding to the desired throttle area. In addition to electronic throttle control, the PCM can regulate vehicle acceleration and engine power with the fuel injectors or spark retard. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an improved method of vehicle acceleration governing an ETC system. The acceleration governing function is typically requested under certain fault conditions, such as a sensor fault, and operates under such fault conditions to limit the vehicle acceleration to a threshold value, which may be determined based on a change in vehicle speed. Sensor faults may include, but are not limited to, an airflow sensor fault, and pedal or throttle sensors generating an out-of-range value. The throttle limits the vehicle acceleration and engine power by limiting the throttle area to a predetermined calibrated value during such a fault condition. On flat driving conditions at sea level, this limited calibrated throttle area will correspond to a vehicle speed, but during operation on terrain inclinations, at altitude, or during high power or torque demand operating conditions such as towing, this limited throttle area may not be great enough to ensure a minimum speed. Thus, the limited speed produced by the fixed throttle limit is heavily dependent on operating environment and vehicle loading. This invention overcomes these limitations while maintaining a safe vehicle response during a sensor failure. When sufficient redundancy exists (such as having two brake indications and two vehicle speed readings), it would be desirable to allow full throttle operation if acceleration can be limited. The present invention will monitor vehicle speed and acceleration and allow the throttle plate to move to a position beyond its limited calibrated throttle area during a fault condition to allow a desired torque output during a high power torque demand operating condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a vehicle having an electronic throttle control system according to this invention, including an electronic control unit; and 
     FIG. 2 is a flow diagram representative of a computer program executed by the electronic control unit of FIG. 1 in carrying out the acceleration governing control of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, and particularly to FIG. 1, the reference numeral  10  generally designates a vehicle drive train including an engine  12  coupled to a multiple-speed ratio transmission  14 , which in turn is coupled via drive shaft  16  and differential  18  to a pair of driven wheels  20   a - 20   b.  The position of a throttle  22  disposed within an intake manifold  23  of engine  12  is controlled to produce power for driving the wheels  20   a - 20   b.  The throttle  22  is mechanically de-coupled from a driver-manipulated accelerator pedal (not shown) and instead is positioned by an electric motor  24  under the control of a engine or powertrain control module (PCM)  26 , which also controls the operation of engine  12  and transmission  14 . The PCM  26  is microprocessor based and operates in response to a number of inputs, including, but not limited to, an engine speed signal ES on line  28 , a vehicle speed signal VS on line  30 , an accelerator pedal position signal TPS on line  32 , an accessory loading signal ACC on line  34 , a throttle position feedback signal on line  36 , and a barometric or ambient air pressure signal BARO on line  38 . These inputs are provided by various conventional sensors such as the illustrated shaft speed sensors  40 ,  42  and throttle position sensor  44 . In general, the PCM  26  activates motor  24  to position the throttle  22  in accordance with a desired throttle area TAdes determined in response to accelerator pedal position and various control functions such as idle speed control, engine governor control, cruise control, and traction control. Additionally, the PCM  26  controls conventional spark and fuel control devices  50 ,  52  coupled to engine  12 . 
     According to this invention, the PCM  26  controls the motor  24  during periods of engine power limiting so as to limit the vehicle acceleration to a calibrated value based on vehicle speed. The control is described in U.S. Pat. No. 6,167,343 to Bauerle, which is incorporated in its entirety by reference herein. 
     During certain operating conditions such as a single point sensor failure, the speed of the vehicle will be limited by the position of the throttle  22 . The position of the throttle  22  is related to both the speed of the vehicle and the torque output of the engine  12 . As described previously, during certain driving conditions that involve high torque demand, the air flow through the throttle  22  at a limited position will not be great enough to produce a desired torque for applications such as towing. The limited torque produced by the fixed throttle limit is heavily dependent on operating environment and vehicle loading. This invention overcomes these limitations while maintaining a safe vehicle response during the sensor failure. 
     Referring to FIG. 2, the method of the present invention is illustrated in flow chart form. The method begins at block  50  as a periodic loop to allow continuous evaluation of the conditions allowing limited acceleration mode. This method repeats every 50 ms to monitor current conditions often enough to allow this mode to be disabled if a more serious system fault develops. Such serious faults may include the loss of all throttle or pedal sensors, loss of ETC motor control (actuator fault), or internal PCM processor faults. 
     At block  52 , if the vehicle speed is greater than or equal to the vehicle speed above which limited acceleration operating mode is not allowed to be enabled, if it has been previously disabled, and there has been no brake application (typically set to around 5 MPH, below which limited acceleration throttle area limit defaults to a fixed throttle area due to the inaccuracy of acceleration at this low speed), and limited_accel_mode_not_allowed=TRUE, as indicated by a flag from the last loop at block  68 , then disable the limited acceleration mode for the ignition cycle at block  54  and continue to block  62 . The flag, limited_accel_mode_not_allowed, indicates whether the system may enter the limited acceleration operating mode and if TRUE will not allow entry into the limited acceleration operating mode. The determination at block  52  is required in case the vehicle has a single sensor fault at startup before the driver applies the brake pedal at least once. Normally, the ETC system security requires the system to never transition from a more restrictive mode (limited power) to a less restrictive mode (limited acceleration) during the same ignition cycle to maintain system stability and security. This transition may be made if the vehicle speed is low since there will always be a limited throttle area clamp on the throttle. If this check was not present, in a situation where the vehicle starts up with a single sensor fault that causes the PCM to enter a limited power operation (because the brakes have not yet been applied), the system would stay in limited power and never transition into limited acceleration mode. 
     If the vehicle speed is not greater then the speed allowed under the limited acceleration operating mode or the limited acceleration mode is allowed, then the method will continue to block  56  to determine if both the primary brake and a secondary brake are applied. The brakes provide an important security function for reducing the throttle limit to idle whenever the brakes are applied when operating during an ETC sensor failure. If both brakes are applied, the brake_transition flag is set to true at block  58  and the method continues to block  60 . If both brakes are not applied, then the method will also continue to block  60  to determine if any single ETC sensor is faulted and the system is not in a more severe remedial action mode. If there is not a single ETC sensor fault or the system is in a more severe remedial action, then the method will continue to block  62  and disable the limited acceleration mode. If there is a single ETC sensor fault and the vehicle is not in a severe remedial action mode, then the method will continue to block  64 . Sensor failures may include at least the failure of one pedal or throttle sensor and severe remedial action will occur when there is a loss of all throttle or pedal sensors, a loss of ETC motor control (actuator fault), or internal PCM processor faults. The present invention includes redundant sensors for brake pedal and accelerator pedal position, throttle position feedback and vehicle speed such as a transmission output shaft speed sensor or individual wheel speed sensors. 
     At block  64 , if the redundant brakes sensors exist and are operating correctly, the brake_transition flag is true, and redundant vehicle speeds are valid and in agreement with each other, the method will enter limited acceleration mode at block  66  and continue to block  67 . The conditions tested in block  64  provide sufficient redundancy to allow the throttle to reach full throttle even during a single sensor fault. If these condition are not true, at block  62 , the method will disable limited acceleration mode and continue to block  67 . 
     At block  67 , the method will determine if any ETC failures exist and limited acceleration is disabled. If these conditions are met, the method will continue to block  68  and not allow entry into a limited acceleration mode. The variable/flag, limited_accel_mode_not_allowed, set at block  68  is used for the next iteration through this method in block  52 , by indicating the ETC failure exists but limited acceleration mode is currently not allowed. Normally, the ETC system security requires the system to never transition from a more restrictive mode (limited power) to a less restrictive mode (limited acceleration) during the same ignition cycle to maintain system stability and security. This transition may be made once if the vehicle speed is low since there will always be a limited throttle area clamp on the throttle, and the driver of the vehicle will not notice any change in operation. If the conditions of block  67  are not met, limited_accel_mode_not_allowed at block  70  will be set to false and the method will continue to block  72 . 
     At block  72 , the method will determine if any ETC failures are present and continue to block  74 . If no ETC failures are present, the method will continue to block  73  and not impose a limit to the engine  12  power or torque and acceleration. The driver has normal control through the accelerator pedal or cruise control system, since all system diagnostics are reporting normal operation. 
     With ETC failures present, at block  74  the method determines if the limited acceleration mode is currently disabled or disabled in the present ignition cycle. To address driveability concerns, the driver will not have a limited throttle area operation on one drive cycle and a limited acceleration (with full throttle) on the next drive cycle. A drive cycle here is defined as a tip-in of the accelerator pedal and the resulting vehicle speed increase, followed by a brake application and a return to a stop. If the limited acceleration mode is disabled, then engine power and vehicle acceleration at block  76  will be limited. If the limited acceleration mode is enabled, the method at block  78  will limit vehicle acceleration but allow full throttle for a high torque or load condition. With the above-described control, vehicle acceleration governing can be carried out with a high degree of stability and accuracy and still allow open throttle operating conditions. 
     While this invention has been described in reference to the illustrated embodiment, it is expected that various modifications in addition to those suggested above will occur to those skilled in the art. In this regard, it will be understood that the scope of this invention is not limited to the illustrated embodiment, and that controls incorporating such modifications may fall within the scope of this invention, which is defined by the appended claims.