Abstract:
A method for estimating traction wheel torque in a hybrid electric vehicle powertrain that does not require a torque sensor. The method relies upon variables including speed, torque, moments of inertia and angular acceleration of powertrain components. Separate strategy routines are used for a parallel operating mode and for a non-parallel operating mode.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The invention relates to hybrid electric vehicles and a method for estimating vehicle wheel torque. 
   2. Background Art 
   Unlike pure electric vehicles that use a battery as a power source for a motor in a power flow path to traction wheels, a hybrid electric vehicle has an engine (typically an internal combustion engine) and a high voltage motor for powering the vehicle. A known powertrain configuration for a hybrid electric vehicle consists of two power sources that are connected to the vehicle traction wheels through a planetary gearset. A first power source in this powertrain configuration is a combination of an engine, a generator and a planetary gearset. A second power source comprises an electric drive system including a motor, a generator and a battery subsystem. The battery subsystem acts as an energy storing device for the generator and the motor. 
   In the case of the first power source, the engine speed can be decoupled from the vehicle speed since the generator acts as a torque reaction element for a reaction gear of the planetary gearset. This results in both a mechanical torque flow path and an electromechanical torque flow path, which function in tandem to deliver driving torque to the vehicle traction wheels. The generator reaction torque effects engine speed control as it provides a reaction torque in the torque flow path from the engine. This operating mode commonly is referred to as a non-parallel operating mode. If the generator is braked, the reaction element of the gearset also becomes braked, which establishes a fully mechanical power flow path from the engine to the traction wheels through the gearset. This is referred to as a parallel operating mode. An example of a powertrain configuration of this type can be seen by referring to co-pending U.S. patent application Ser. No. 10/248,886, filed Feb. 27, 2003, now U.S. Pat. No. 6,991,053, dated Jan. 31, 2006. This co-pending patent application is assigned to the assignee of the present invention. 
   In the powertrain configuration disclosed in the co-pending patent application, torque is delivered through the powertrain for forward motion only in the case of the first power source. In the case of the second power source, the electric motor draws electric power from the battery and provides driving torque independently of the engine in both forward and reverse drive. In this operating mode, the generator, using battery power, can drive against a one-way clutch on the engine output shaft to propel the vehicle forward. 
   A control system is used to effect integration of the two power sources so that they work together seamlessly to meet the driver&#39;s demand for power at the traction wheels without exceeding the limits of the battery subsystem. This is accomplished in the powertrain of the co-pending patent application by coordinating the control of the two power sources. Under normal powertrain operating conditions, a vehicle system controller interprets a driver demand for power, which may be an acceleration or deceleration demand, and then determines a wheel torque demand based on driver demand and powertrain limits. The vehicle system controller also will determine when and how much torque each power source must provide to meet the driver&#39;s demand and to achieve specified vehicle performance, such as fuel economy, emissions, driveability, etc. The vehicle system controller can control the engine operating speed for each torque demand so that an efficient operating point on the speed-torque engine characteristic curve will be established. 
   A control system of the type discussed in the preceding paragraphs requires a so-called drive-by-wire control system as the two power sources cooperate seamlessly to achieve optimal performance and efficiency. Such a drive-by-wire system requires a torque monitor strategy to ensure that the control system wheel torque demand and the actual powertrain torque output are within a predefined range so that unintended vehicle acceleration will be avoided. 
   U.S. Pat. No. 5,452,207, which is owned by the assignee of the present invention, discloses a torque estimation method based on a vehicle dynamics model, a torque converter model and an engine torque model. Estimates of torque are obtained from at least two of the models. The torque estimates are weighted according to a predefined strategy and then transferred to a controller for developing torque estimates based on the weighted individual torque estimates. 
   A wheel torque estimation strategy is disclosed also in U.S. Pat. No. 5,751,579, which also is owned by the assignee of the present invention. It provides an estimate of wheel torque based upon engine combustion torque. The estimated torque is proportional to engine acceleration and engine powertrain mass. 
   SUMMARY OF INVENTION 
   The control method of an embodiment of the invention will provide an estimate of the total output torque at the traction wheels for any given driving condition. The torque estimate is used to perform wheel torque monitoring. The method estimates total wheel torque for any given torque of the motor, the generator and the engine in various operating modes. These modes include a non-parallel mode and a parallel mode. 
   When the powertrain configuration is operating in a non-parallel mode, both the engine and the motor cooperate with the gearset to establish both a mechanical torque flow path and an electromechanical torque flow path. In a so-called parallel operating mode, the generator rotor is braked. 
   The method of the invention performs a torque monitoring function to ensure that the vehicle does not accelerate when acceleration is not intended. It eliminates the need for using a torque sensor for measuring total wheel torque. 
   The method uses multiple powertrain inputs, including motor speed, generator speed, engine speed, motor torque, generator torque, engine torque and generator brake status. After calculating engine and motor angular accelerations, the strategy will determine the operating mode. Separate subroutines are used for the non-parallel mode (both positive and negative power flow) and the parallel mode to calculate output torque of the gearset. 
   After the output torque of the gearset is computed in either of the separate subroutines, the strategy computes a total wheel torque estimate. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic representation of a hybrid electric vehicle powertrain for an automotive vehicle capable of embodying the present invention; 
       FIG. 2  is a flowchart illustrating the control software strategy for calculating an estimate of total wheel torque; and 
       FIG. 3  is a sub-routine used in carrying out the routine of  FIG. 2  wherein the operating mode for the powertrain is determined. 
   

   DETAILED DESCRIPTION 
   The powertrain of  FIG. 1  includes an internal combustion engine as shown at  10 . A planetary gear unit  12  includes a ring gear  14 , which is connected driveably to a torque input countershaft gear element  16 . The engine torque output shaft is connected driveably to carrier  18  for the planetary gear unit  12 . Sun gear  20  of the planetary gear unit  12  is connected driveably to generator  22 . The generator is electrically coupled, as shown at  24 , to a high voltage electric motor  26 , which may be an induction motor. The output rotor of the motor is connected to gear element  28  of torque output countershaft gearing  30 . A countershaft gear  32  engages gear  16 . A countershaft gear of larger pitch diameter, shown at  34 , driveably engages motor output drive gear element  28 . A smaller pitch diameter countershaft gear element  36  driveably engages torque output gear  38 , which distributes torque to a differential-and-axle assembly  40  to deliver driving torque to vehicle traction wheels  42 . 
   A generator brake  44 , when applied, anchors the rotor of generator  22 , which also anchors sun gear  20 . When the generator brake is applied, a mechanical torque flow path from the engine to the differential-and-axle assembly  40  is established. This is referred to as a parallel driving mode. When the brake  44  is released, reaction torque of the generator establishes torque reaction for the sun gear  20  because of the direct mechanical coupling between the sun gear and the generator rotor. Engine speed thus can be controlled by controlling generator. 
   The generator torque is under the control of transmission control module  46 , which communicates with vehicle system controller  48 . Input variables for the vehicle system controller  48  include a driver-controlled drive range selection at  50  and a signal from an accelerator pedal position sensor  52 . Another driver input for the vehicle system controller is a brake pedal position sensor signal  56 . 
   Battery  58  is connected to the generator  22  and the motor  26  through a high voltage bus  60 . The battery is under the control of the vehicle system controller by means of a contactor control signal at  62 . 
   The transmission control module receives from the vehicle system controller a desired wheel torque signal, a desired engine speed signal and a generator brake command as shown at  64 . The transmission control module  46  distributes a generator control signal through signal flow path  68  that extends from the module  46  to the brake  44 . 
   For the purpose of describing the output torque estimation method, reference will be made to the strategy flow charts of  FIGS. 2 and 3 . The various method steps involved in the strategy of  FIGS. 2 and 3  make use of moment of inertia terms, torque ratio terms, torque terms and angular acceleration terms for elements of the powertrain. Some of these terms are as follows: 
   J eng  is the combined inertia values for engine and carrier; 
   J gen     —     couple  is the combined moment inertia of the generator/sun gear; 
   J mot     —     eff  the sum of the combined motor/gear inertia and the generator inertia reflected at the motor; 
   T gen2mot  is the torque ratio from generator shaft to motor shaft; 
   T eng2mot  is the torque ratio from engine shaft to motor shaft; and 
   T mot2wheel  is the torque ratio from motor shaft to wheel. 
   In  FIG. 2 , the strategy routine begins at  70 , where the various inputs for the controller  48  are read and then stored in computer memory (RAM). The inputs are motor speed, ω mot , generator speed, ω gen , engine speed, ω eng , motor torque, τ mot , generator torque, τ gen , engine torque, τ eng , and generator brake status (the brake  44  is either “on” or “off”). The default operating mode is a non-parallel mode indicated by the statement “Parallel Mode=FALSE.” 
   The first entry in the initialization step sets the parallel mode (internal variable) to FALSE. This occurs in the first entry only. As the strategy routine proceeds, the operating mode will be determined for each control loop of the processor, as will be explained subsequently. 
   The routine then proceeds to action block  72 , where motor angular acceleration is calculated. This is done using the functional relationship: dotω mot =dω mot /dt, which is a derivative of the angular velocity of the motor rotor. The result of the calculation at action block  72  is stored in memory, and the routine then proceeds to action block  74  where the engine angular acceleration is calculated. This is done in accordance with the following relationship: dotω eng =dω eng /dt, which is the derivative of the angular engine velocity. 
   After the information obtained at action block  74  is stored in memory, the routine proceeds to action block  76 , where the operating mode is determined. The routine at action block  76  is a subroutine indicated in  FIG. 3 . That subroutine will determine whether the powertrain is in the parallel mode or in the non-parallel mode. As explained previously, the generator brake is applied when the powertrain is in the parallel mode and is released to establish plural power flow paths in the non-parallel mode. As previously explained also, the default operating mode is a non-parallel mode. 
   The routine then will proceed to decision block  78 , where the controller will determine whether the generator brake is on. If the inquiry at  78  is negative, the operating mode set during initialization is confirmed. If the inquiry at  78  is positive, the routine will proceed to decision block  80 , where it is determined whether the generator speed is less than a predetermined threshold generator speed C gen     —     spd . 
   If the result of the inquiry at  80  is negative, the non-parallel mode determination is confirmed. If the result of the inquiry at  80  is positive, the routine then will proceed to decision block  82 , where it is determined whether the generator torque is less than a predetermined threshold C gen     —     tq . 
   If the result of the inquiry at  82  is negative, the non-parallel mode is confirmed. If the result of the inquiry at  82  is positive, the parallel mode is set to “TRUE”, which is a change in mode from non-parallel operation to parallel operation. This occurs at action block  84 . 
   If the powertrain is in a parallel operating mode, as determined at decision block  86 ′, the control routine will proceed to decision block  88 , where it is determined whether the generator speed is less than a predetermined threshold generator speed C gen     —     spd . If the result of the inquiry at  88  is positive, the operating mode is changed at action block  90  from the parallel mode to the non-parallel mode (the setting is “FALSE”). 
   If the result of the inquiry at  88  is negative, the routine proceeds to action block  92 , where it is determined whether the generator torque is less than a predetermined threshold C gen     —     tq . If the result of the inquiry at  92  is positive, again the operating mode is changed at action block  90  from the parallel mode to the non-parallel mode. If the result of the inquiry at  92  is negative, the parallel mode at decision block  88  is confirmed. Likewise, a negative result of the inquiry at decision block  88  is a confirmation of the parallel mode indicated at  86 ′. 
   Based upon the operating mode that is determined in the subroutine of  FIG. 3 , the method uses one of two different ways to calculate planetary output torque at the motor shaft. If the powertrain is in a non-parallel operating mode as confirmed at  86  in  FIG. 2 , the routine of  FIG. 2  will proceed to action block  94 , where static planetary output torque is calculated. This is done using the relationship:
 
τ p@mot   =T   gen2mot *τ gen 
 
   where: 
   τ p@mot =torque at motor shaft; 
   T gen2mot =torque ratio from generator to motor shaft; and 
   τ gen =generator torque. 
   On the other hand, if it is determined that the powertrain is in the parallel operating mode at  86 , the routine will proceed to action block  96 , where static planetary output torque is computed using the relationship:
 
τ p@mot   =−T   gen2mot *(τ eng   −J   eng *dotω eng )
 
   where: 
   τ p@mot =torque at motor shaft; 
   T gen2mot =torque ratio from engine to motor shaft; 
   τ eng =engine torque; 
   J eng =lumped moment of inertia of engine and the element of the gearing to which it is connection; and 
   dotω eng =engine angular acceleration. 
   Following either of the calculations at action blocks  94  and  96 , the routine will proceed to action block  98  where the total wheel torque is estimated. This is done using the following relationship:
 
τ total     —     wheel   =T   mot2wheel *(τ mot −τ p@mot   +J   gen     —     couple *dotω eng   −J   mot     —     eff *dotω eng )
 
   where: 
   τ total     —     wheel =total wheel torque estimate; 
   T mot2wheel =torque ratio from motor to wheels; 
   τ mot =torque @ motor shaft; 
   J gen     —     couple =coupled moment of inertia of generator and the element to which it is connected; 
   dotω eng =engine angular acceleration; and 
   J mot   eff =sum of the lumped motor and gearing inertia and the lumped generator inertia reflect at the motor. 
   Although an embodiment of the invention has been described, it will be apparent to a person skilled in the art that modifications may be made to the invention without departing from the scope of the invention. All such modifications and equivalents thereof are intended to be covered by the following claims.