Abstract:
A method for determining the onset of positive torque through an automatic transmission (the power-on point), includes saving engine torque magnitudes within a specified tolerance of each other and within a adaptive limits, the highest torque value of the sampled torque magnitudes being stored in memory. The saved value plus a calibrated safety margin is used to compare to the calculated transmission input torque. If the input torque is higher, the powertrain is considered “power-on.” In addition to the safety margin, other calibratable adder torque values can be used to compensate for air conditioning, electrical loads, temperature, etc. If the samples fall below the lower adaptive limit, the lower limit is used plus any torque adders. If the adaptive samples are above the adaptive limit, the upper adaptive limit is used without any torque adders. A calibrated baseline torque value is used for power-on determination until enough valid samples are obtained.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates control of a powertrain in an motor vehicle. More particularly it pertains to determining incipient positive torque in a powertrain, i.e., the threshold of a power-on condition. 
     2. Description of the Prior Art 
     The powertrain of a motor vehicle is tested according to a procedure defined by the federal government for compliance with governmental standards including conformance with onboard diagnostic capability (the OBD II test standard) and for other system diagnostics purposes. During execution of the federal test procedure, it is necessary to determine the torque produced by the engine of the powertrain. Conventionally, this torque magnitude is determined from a calibratable scalar value inferred by mapping engine torque magnitudes conforming to a range of engine parameters including throttle position, engine speed, MAP, temperature, etc. Preferably the engine torque determined in this way should be sufficiently high in order for the OBD II test to be conducted. However, variations in the vehicle weight, the gear ratio of the axle and performance variations associated with the service life of the powertrain affect the power-on threshold point, and in that way influence whether the calibrated scalar torque estimate is sufficiently high to permit federal testing to occur. 
     It is preferable to know precisely the threshold of the power-on condition rather than to arbitrarily estimate or guess a conservatively high torque magnitude. For example, when a fairly highly conservative torque magnitude is assumed for the threshold of the power-on condition, a large portion of the operating range of the federal test procedure can fail to run. 
     It is preferable that a control of a powertrain be capable of determining precisely the onset of a power-on condition, and that the control adapt to variations in vehicle gross weight, axle ratio, and the effects of service life and other vehicle-to-vehicle variations that influence the threshold of the power-on condition. 
     The federal test procedure requires that the onboard diagnostic system demonstrate its ability to detect a failure and to produce an accurate indication thereof to a vehicle operator. If the diagnostic system fails to detect and indicate the failure, the vehicle is determined to be non-compliant with a federal onboard diagnostic standard. 
     Typically the calculation of inferred torque into and out of the automatic transmission is not accurate at low torque levels due to vehicle variations and mapping inaccuracies. Since many of the functional tests used for OBDII diagnostics require a power-on indication to run, proper power-on indication is crucial, especially when running the federal test procedure drive cycle for OBDII compliance, which tends to run at a fairly low torque level. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide a torque-based method for determining the threshold of positive torque flow through an automatic transmission. The method adapts to the particular powertrain characteristics and compensates for vehicle-to-vehicle variations and changes in the powertrain of a motor vehicle during its service life. As a result, this invention provides a better power-on indication than conventional techniques. It is another object to provide an accurate, consistent power-on indication during the performance of functional and OBDII diagnostic tests. 
     In realizing these objects, the method of this application for determining, with the aid of an electronic computer system, the power-on torque magnitude in a powertrain of a motor vehicle having an engine, and an automatic transmission having a torque converter that includes a bypass clutch for mechanically connecting and disconnecting the impeller and turbine of the torque converter, the impeller connected to the engine, the turbine connected to a transmission input shaft, includes the steps of repetitively determining that the torque converter clutch is disengaged; repetitively determining that the speed ratio across the torque converter is within a predetermined speed range; repetitively determining that the rate of vehicle deceleration is lower than a predetermined deceleration rate; repetitively determining the magnitude of torque produced by the engine; repetitively storing successive engine torque magnitudes in retrievable electronic memory; deleting from memory the stored torque magnitudes, if any of the stored torque magnitudes is greater than a predetermined magnitude from the other stored torque magnitudes; setting the power-on torque magnitude equal to the maximum stored torque magnitude, if all stored torque magnitudes are within a predetermined range of torque magnitudes; adding to the maximum stored torque magnitude the corresponding magnitude of engine torque currently driving accessory equipment; and using the sum of the torque magnitudes as the power-on torque magnitude. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a powertrain that includes an engine and transmission and an electronic control for controlling the powertrain. 
     FIG. 2 shows a torque converter, a bypass or lockup clutch, and a portion of a hydraulic system for controlling the clutch. 
     FIG. 3 is a block diagram showing a microprocessor in a control system for an automatic transmission that includes multiple speed ratio gearing and a hydrokinetic torque converter having a lockup clutch. 
     FIGS. 4A and 4B comprise a logic flow diagram showing the adaptive torque based power-on method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 3, air and fuel are inducted by an internal combustion engine  10 , which drives a shaft connecting the engine output and transmission  11  input. That shaft rotates at the speed of the engine and carries the magnitude of torque produced by the engine minus torque that drives vehicle accessories such as an air conditioning compressor, or power steering pump, electrical power production loads, etc. Both the engine  10  and transmission  11  are coupled to an electronic engine control module  12 , which includes a microprocessor  50 , input conditioning circuit  56 , an electronic memory  62  containing various control algorithms for processing spark timing input, exhaust gas recirculation rate input, percent methanol fuel input, air/fuel ratio input, engine RPM input, engine air charge input, engine coolant temperature, firing cylinder indication input, engine operating hours, power steering pressure, timer input, air conditioning head pressure or air change temperature input, and a flag indicating whether the air conditioning compressor is on or off. These engine operating parameters and other such parameters are described in U.S. Pat. No. 5,241,855, which is owned by the assignee of the present invention. A method for determining engine torque is described in U.S. Pat. No. 5,241,855, the entire disclosure of which is incorporated herein by reference. 
     The microprocessor  50  shown in FIG. 3 is an integrated processor supplied with signals representing engine throttle position, engine speed, engine coolant temperature, torque converter speed, vehicle speed, a selected range of a gear selector, throttle valve pressure, the state of the selected transmission operating modes  52 , the state of a brake switch  54 , and signals representing the state of other operating parameters. Information conveyed by these input signals is conditioned by input conditioning circuitry  56  and transmitted on data bus  58  to a central processing unit  60  accessible to electronic memory  62 . The electronic memory contains control transmission algorithms for controlling gear shift scheduling, electronic pressure control EPC, and engagement and disengagement of the torque converter bypass clutch  82 . The processing unit recalls information and control algorithms from electronic memory  62 , executes the algorithms, and produces output signals carried on data bus  64  to output driver circuits  66 , which produce electronic signals from the signals produced by the microprocessor. The output signals drive electrical solenoid-operated valves  70 ,  72 ,  74 ,  76 ,  78  located in an hydraulic valve body  68  adapted to respond to the output signals. 
     The results of logical and arithmetic computations executed by the processor are stored in RAM, which is addressed, erased, rewritten, or changed in accordance with logic of the control algorithms. Certain values are stored in keep alive memory KAM, whose contents are maintained despite opening the engine ignition switch, provided the battery remains connected to the power supply. 
     The algorithms that control operation of the transmission are divided into several control modules executed sequentially in a known fashion during a background pass. The algorithms of each module are executed sequentially just as the modules themselves are executed sequentially. Information that results from the sensor input data and information stored in memory and learned from previous executions of the algorithms is used during execution of the control algorithms to produce electronic signals present at the output ports of the microprocessor. 
     Referring now to FIG. 2, the lock-up clutch  82  of a torque converter  84  is alternately hard-locked or soft-locked (modulated) by directing hydraulic fluid through converter bypass-clutch control valve  86 , which is supplied with regulated line pressure in line  88 . A variable pressure valve  92  is supplied with constant pressure through line  94  from a solenoid-pressure regulator valve and is controlled by a pulse-width modulated (PWM) signal applied to solenoid  78  from the microprocessor output. Valve  86  produces a pressure difference across bypass clutch  82 . When clutch  82  is hard-locked, a direct mechanical connection between impeller  96  and turbine  98  is produced. The impeller of torque converter  84  is driven from the crankshaft  100  of an engine, and turbine  98  drives a transmission input shaft  102 . When clutch  82  is disengaged, the turbine is driven hydrodynamically by the impeller. 
     The method of the present invention for determining the engine torque corresponding to the onset of positive torque at the transmission input, i.e., the threshold of the power-on condition, is described next with reference to FIG.  4 . At  104 , the transmission oil temperature is compared to a predetermined reference temperature to determine whether the transmission is at a sufficiently high temperature. Alternatively the elapsed run time can be compared to the length of a predetermined reference period for the same purpose. If the transmission temperature is below approximately 20° F., control passes to  106  where a calibrated scalar torque value is used as the power-on value, and the adaptive torque base routine is exited at  108 . 
     If the transmission temperature is sufficiently high, upper and lower limits for engine torque corresponding to the power-on threshold are set at  110 , the range being approximately between 10-60 ft-lb. 
     Keep alive memory KAM is used to store pertinent values determined by the control algorithm so that the values are obtained and not lost when the ignition switch is turned to the off position. An inquiry is made at  112  to determine whether KAM contains a power-on torque magnitude from a previous execution of the control algorithm. If so, at  114  that value is recalled from KAM and is used for the power-on determination. If, however, the power-on torque magnitude is absent, either because the battery was disconnected from the electrical power supply since the last execution of the algorithm or because there has been no prior execution of the control algorithm, at  116  a calibrated based torque magnitude for the power-on condition is determined and used during the current execution of the control algorithm. 
     In order to prevent potential inaccuracy in the power-on torque magnitude determined by this algorithm, at  118  the vehicle&#39;s deceleration rate is compared to a calibrated or predetermined reference deceleration. If the vehicle decelerates at a rate that exceeds the calibrated value, control passes to  120  where the control algorithm is exited. If the test at statement  118  is positive, control passes to  122 . 
     The state of the torque converter bypass clutch must be open or unlocked, and the speed ratio across the torque converter must be within predetermined limits, preferably between 0.97 and 1.02. It has been determined that the powertrain torque does not change rapidly when this speed ratio is within the specified limits. However, when the speed ratio is below 0.97, the transmission loads the engine and the torque carried by the engine output shaft  13  can change rapidly. Therefore if the tests at statement  122  are passed there is a high level of confidence that the torque produced by the engine is not changing rapidly and that the power-on condition can be sensed with a high level of precision. If the test at statement  122  is failed, control again passes to  120  where execution of this algorithm is terminated. 
     If the tests of statement  122  are passed, the present engine torque magnitude, as determined by mapping its value with reference to engine and vehicle parameters, or using a torque sensor on the engine shaft, is saved in KAM and a sample counter is incremented at  124 . 
     If the number of saved torque magnitudes, the count stored in a sample counter, equals or exceed a predetermined reference count, statement  126  directs control to statement  128  where it is determined whether all of the saved torque magnitudes are within a predetermined range of each other, preferably about 5.0 ft-lb. If the saved values are not within that tolerance range, the sample counter and saved values are cleared from memory at  130  and control exits the algorithm at  132 . If the test result at  128  is negative, at  130  the counter is zeroed and the saved torque values are deleted from memory. 
     If the saved torque values are within the tolerance range, at  134  the saved torque magnitudes are compared to an acceptable range of torque magnitudes, preferably 10-50 ft-lb. If any saved torque magnitude is outside those limits, control passes to statement  136  where it is determined whether any saved torque magnitude is above the upper limit. If so, the power-on torque magnitude is set equal to the high limit at  138 , control passes to statement  140 . If no saved torque magnitude fails the high limit test at  136 , at  142  the power-on torque magnitude is set equal to the highest magnitude above the low limit, if any. Otherwise the power-on torque magnitude is set equal to low limit plus adder torque that accounts for accessory drive and electric power loads currently applied to the engine. Then control passes to statement  140 . 
     The adder torque values are calibrated torque magnitudes used to provide a margin of safety and to compensate for loads associated with the air conditioning compressor and other accessory power requirements. The magnitude of these adder torque loads is determined by mapping operating parameters of the accessories and vehicle operating conditions. Separate adaptive values could be learned based on current accessory loads. 
     If the inquiry at statement  134  is positive, the power-on torque magnitude is set, at statement  144 , equal to the maximum of the saved torque magnitudes increased by the adder torque values. 
     Statement  140  clears the sample counter and the saved torque magnitudes. Execution of the control algorithm ends at statement  146 . 
     In this way, the onset of the power-on condition discounts the torque values present when the transmission operating temperature is too low, when the vehicle is decelerating too quickly, the torque converter is locked or partially locked, and the speed ratio is outside of a range of tolerance close to unity. 
     The method of this invention relies on the speed ratio across an open torque converter being at or very near 1.0. Therefore the power flow through the powertrain is at or near zero. The optimum time for those sampling conditions to occur in the vehicle is at very low driver demand decelerations with the torque converter unlocked, especially in the upper gear ranges. As long as the speed ratio is approximately 1.0, a reliable power-on torque magnitude can be learned. 
     Although the form of the invention shown and described here constitutes the preferred embodiment of the invention, it is not intended to illustrate all possible forms of the invention. Words used here are words of description rather than of limitation. Various changes in the form of the invention may be made without departing from the spirit and scope of the invention as disclosed.