Abstract:
A method is presented for controlling powertrain torque by minimizing the error between the actual powertrain torque (as read by the torque sensor) and the desired powertrain torque (as requested by the vehicle driver). Since torque sensors are known to drift under certain conditions, such as high ambient temperature, the output of the torque sensor is adjusted by an offset value. This offset value is determined by reading the torque sensor output when the speed ratio (engine speed/turbine speed) is substantially unity, and the net torque at the torque converter is substantially zero. This adjusted output is then filtered to avoid abrupt fluctuations in the powertrain torque, and used to improve powertrain control so that better drive feel and increased fuel economy can be achieved.

Description:
FIELD OF THE INVENTION 
     The field of the invention relates to a method for controlling a vehicle powertrain having a torque sensor by adjusting torque in response to the information provided by the torque sensor. 
     BACKGROUND OF THE INVENTION 
     Vehicles driven by an internal combustion engine having a torque converter and an automatic transmission have used a torque sensor. The output of the torque sensor can be used to control engine or transmission performance. A number of approaches have been proposed for utilizing the engine torque sensor signal to achieve improved powertrain control. One such method is described in U.S. Pat. No. 5,319,555. Using the engine torque sensor output signal, vehicle driving resistance can be calculated with high precision. This information is then used to determine the most appropriate transmission gear ratio for various driving conditions such as hill climbing. In other words, transmission performance can be improved by using the output signal of the engine torque sensor. 
     The inventors herein have recognized a disadvantage with this approach. A typical torque sensor is usually a piezoelectric or a magnetostrictive device, which has a tendency to drift under certain operating conditions. For example, changes in ambient temperature may cause errors in the output signal values. Such errors are especially significant near very low engine torque levels such as those experienced in idle conditions. These errors lead to degraded engine control and cause reduced fuel economy and degradation in drive feel and vehicle performance. In other words, when the output signal of the engine torque sensor is not representative of the actual engine torque, engine performance optimization is degraded. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for controlling engine torque by using the output signal of a torque sensor. 
     The object of the invention is achieved and disadvantages of prior approaches overcome by a method for controlling torque of a powertrain coupled to a torque sensor. The method comprises indicating when the powertrain torque is substantially zero, in response to said indication, correcting an output signal of the torque sensor, and controlling the powertrain torque in response to said corrected torque sensor output signal. 
     An advantage of the above aspect of the invention is that a more precise value of torque can be determined from the torque sensor and, therefore, better engine and transmission control can be achieved. These improvements will contribute to improved drive feel, vehicle performance, and fuel economy. 
     In another aspect of the present invention, the object is achieved and disadvantages of prior approaches overcome by a method for controlling torque of a powertrain coupled to a torque sensor where the powertrain has a torque converter. The method comprises determining desired powertrain torque based on an operator command, indicating when speed ratio across the torque converter is substantially unity, in response to said indication, correcting an output signal of the torque sensor, filtering said corrected powertrain torque sensor output signal, and controlling the powertrain in response to said filtered corrected powertrain torque sensor output signal and said desired powertrain torque. 
     An advantage of the above aspect of the invention is that it is possible to correct offsets in the torque sensor. Further, by filtering these corrections, it is possible to prevent abrupt changes in measured actual torque. Thus, is it possible to provide smooth powertrain control even when correcting for offsets. These improvements will further contribute to improved drive feel, vehicle performance, and fuel economy. 
     Other objects, features and advantages of the present invention will be readily appreciated by the reader of this specification. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The object and advantages of the invention claimed herein will be more readily understood by reading an example of an embodiment in which the following invention is used to advantage with reference to the following drawings herein: 
     FIG. 1 is a block diagram of a vehicle powertrain illustrating various components related to the present invention; 
     FIG. 2 is a block diagram of an engine in which the invention is used to advantage; 
     FIGS. 3,  4 , and  5  are block diagrams of embodiments in which the invention is used to advantage. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, internal combustion engine  10 , further described herein with particular reference to FIG. 2, is shown coupled to torque converter  11  via crankshaft  13 . Torque converter  11  is also coupled to transmission  15  via turbine shaft  17  which is also known as a transmission input shaft. Torque converter  11  has a bypass clutch (not shown) which can be engaged, disengaged, or partially engaged. When bypass clutch is either disengaged or partially engaged, torque converter  11  is said to be in an unlocked state. Transmission  15  is an electronically controlled transmission with a plurality of selectable discrete gear ratios. Transmission  15  also includes various other gears such as, for example, a final drive ratio (not shown). Transmission  15  is also coupled to tire  19  via axle  21 . Tire  19  interfaces the vehicle (not shown) to the road  23 . In an alternative embodiment for use with manually shifted vehicles, transmission  15  can be replaced with a manual transmission and torque converter  11  can be deleted. 
     Internal combustion engine  10 , having a plurality of cylinders, one cylinder of which is shown in FIG. 2, is controlled by electronic engine controller  12 . Engine  10  includes combustion chamber  30  and cylinder walls  32  with piston  36  positioned therein and connected to crankshaft  13 . Combustion chamber  30  communicates with intake manifold  44  and exhaust manifold  48  via respective intake valve  52  and exhaust valve  54 . Exhaust gas oxygen sensor  16  is coupled to exhaust manifold  48  of engine  10  upstream of catalytic converter  20 . In a preferred embodiment, sensor  16  is a HEGO sensor as is known to those skilled in the art. 
     Intake manifold  44  communicates with throttle body  64  via throttle plate  66 . Throttle plate  66  is controlled by electric motor  67 , which receives a signal from ETC driver  69 . ETC driver  69  receives control signal (DC) from controller  12 . Intake manifold  44  is also shown having fuel injector  68  coupled thereto for delivering fuel in proportion to the pulse width of signal (fpw) from controller  12 . Fuel is delivered to fuel injector  68  by a conventional fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown). 
     Engine  10  further includes conventional distributorless ignition system  88  to provide ignition spark to combustion chamber  30  via spark plug  92  in response to controller  12 . In the embodiment described herein, controller  12  is a conventional microcomputer including: microprocessor unit  102 , input/output ports  104 , electronic memory chip  106 , which is an electronically programmable memory in this particular example, random access memory  108 , and a conventional data bus. 
     Controller  12  receives various signals from sensors coupled to engine  10 , in addition to those signals previously discussed, including: measurements of inducted mass air flow (MAF) from mass air flow sensor  110  coupled to throttle body  64 ; engine coolant temperature (ECT) from temperature sensor  112  coupled to cooling jacket  114 ; a measurement of throttle position (TP) from throttle position sensor  117  coupled to throttle plate  66 ; a measurement of transmission shaft torque, or engine shaft torque from torque sensor  121 , a measurement of turbine speed (Wt) from turbine speed sensor  119 , where turbine speed measures the speed of shaft  17 , and a profile ignition pickup signal (PIP) from Hall effect sensor  118  coupled to crankshaft  13  indicating an engine speed (We). Alternatively, turbine speed may be determined from vehicle speed and gear ratio. 
     Continuing with FIG. 2, accelerator pedal  130  is shown communicating with the driver&#39;s foot  132 . Accelerator pedal position (PP) is measured by pedal position sensor  134  and sent to controller  12 . 
     In an alternative embodiment, where an electronically controlled throttle is not used, an air bypass valve (not shown) can be installed to allow a controlled amount of air to bypass throttle plate  62 . In this alternative embodiment, the air bypass valve (not shown) receives a control signal (not shown) from controller  12 . 
     Referring now to FIG. 3, a routine is described for controlling engine parameters to provide desired engine torque. First, in step  300 , pedal position is determined from the pedal position sensor. Next, in step  310 , desired powertrain torque is determined based on the pedal position. In step  320 , desired engine torque is calculated based on desired powertrain torque. For example, if a desired wheel torque is determined from the pedal position, then gear ratio and torque ratio across the torque converter are used to calculate desired engine torque. Then, in step  330 , a routine is performed wherein engine parameters such as throttle position, ignition timing and air/fuel ratio are controlled to provide desired engine torque. This control routine in step  330  is described in further detail in FIG.  4 . First, in step  400 , a routine for calculating actual engine torque, T corr     —     torque , from the torque sensor is performed. Next, in step  410 , the error between desired engine torque and actual engine torque is calculated. Then, in step  420 , desired engine parameters such as throttle position, air/fuel ratio, ignition timing are calculated based on desired engine torque and the error calculated in step  410 . In other words, a combined feed-back and feed-forward control architecture is used to control engine torque to a desired level. Those skilled in the art will recognize, in view of this disclosure, that such a torque control architecture can be modified to accommodate any placement of the torque sensor. For example, the torque sensor may be placed on the engine output as shown in FIG. 1, torque converter input or output, or transmission input or output. 
     Moving on to FIG. 5, a routine for calculating actual engine torque from the torque sensor is described in detail. First, in step  500 , a determination is made whether the speed ratio (turbine speed/engine speed) across the torque converter is substantially equal to one. This is done, for example, by determining whether the speed ratio is within a predetermined range such as 0.95 and 1.05 when the torque converter is unlocked. In an alternative embodiment, a determination is made whether the transmission is in a neutral state and engine speed is substantially constant. Stated another way, a determination is made in step  500  as to whether net engine torque, or net powertrain torque, is substantially zero. When speed ratio across the torque converter is substantially unity and the torque converter is unlocked, or when slip between input and output speeds is substantially zero, this is an indication that powertrain torque is substantially zero. Further, when the transmission is in neutral, i.e., no coupling between engine and transmission, and when engine speed is substantially constant, this also is an indication that powertrain torque is substantially zero. 
     In an alternative embodiment, a determination can be made whether a transmission overrunning clutch becomes engaged to prevent engine braking. Engine braking is prevented by an overrunning clutch that produces a one-way drive connection between a gear unit and a nonrotating powertrain member, such as transmission casing. The overrunning clutch is engaged when the wheel speed becomes greater than the engine speed by a small preselected tolerance amount. When the overrunning clutch becomes engaged, engine braking is prevented. The point at which the overrunning clutch becomes engaged, i.e., a connection is made between a gear unit and a nonrotating member, is the point at which net torque is transitioning from positive to negative. At that point, net torque across the torque converter is essentially zero. 
     Continuing with FIG. 5, if the answer to step  500  is YES, the routine proceeds to step  510  wherein the offset is set to be equal to the output of the torque sensor: 
     
       
         offset=T sensor   
       
     
     Then, in step  520 , a filtered value of the offset, f_offset, is calculated according to the following equation: 
     
       
           f_offset=(   fk )*offset+(1 −fk )*f_offset, 
       
     
     where fk is a filtering coefficient. The filtering coefficient can be selected so that smooth engine control is provided when a change in offset is detected. The routine then proceeds to step  530  wherein corrected torque, T corr     —     torque , is calculated based on the following equation: 
     
       
         
           T 
           corr 
           
             — 
           
           torque 
           =T 
           sensor 
           −f_offset. 
         
       
     
     If the answer to step  500  is NO, the routine proceeds to step  530  described above. 
     Thus, it is possible to eliminate the effects of the torque sensor drift by re-zeroing the torque sensor every time the speed ratio (engine speed/turbine speed) is substantially unity, and to use the corrected result to achieve better powertrain control. Those skilled in the art will recognize, in view of this disclosure, that torque sensor  121  can be placed on several different areas of the powertrain, such as the engine output as shown in FIG. 1, torque converter input or output, or transmission input or output. Irrespective of torque sensor location, and according to the present invention, it is possible to correct the torque sensor output to compensate for zero drifts. 
     This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. For example, the present invention may be used with both port fuel injected engine and direct injected engine, stoichiometric engines or lean-burn engines, or gasoline engines or diesel engines. Accordingly, it is intended that the scope of the invention is defined by the following claims.