BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates generally to the operation of a hybrid vehicle, and more particularly to method and system controller for controlling torque matching in a hybrid electric vehicle to limit noise, vibration, and harshness (NVH). 
   2. Background Art 
   Hybrid electric vehicles (HEVs) having a primary power source, whether the primary power source is an internal combustion engine or a fuel cell arrangement, typically include a transfer brake and a motor/generator to controllably transfer energy between the primary power source and a secondary electric system. 
   The transfer brake can be engaged to prevent the motor/generator from producing or receiving torque from the primary power source. The transfer brake is then disengaged to permit the motor/generator to produce or receive torque. It is desirable for the motor/generator torque to match the torque provided by the primary power source when the transfer brake is disengaged. The failure or mismatch of the motor/generator to match the torque of the primary power source can produced noise, vibration and harshness (NVH). In some cases, the NVH can cause erratic acceleration or deceleration, excessive vibration, or other discomforting sensation to be felt by the passengers. 
   The severity of the NVH is generally related to the degree of torque mismatching. As such, it is desirable to accurately determine the actual torque produced by the primary power source so that the motor/generator can be controlled to produce closely matching torque. 
   In the past, a torque estimate derived from operating parameters of the primary power source would be used by a vehicle system controller to match the motor/generator torque. One problem with this approach is that the operating parameters used to estimate the torque can and often do include inaccuracies which limit the accuracy of the estimated torque to the actual torque. This inaccuracy can make it difficult to match the motor/generator torque. 
   Accordingly, there exists a need to provide a more accurate estimate of the torque actually produced by the primary power source that would permit better torque matching during transfer brake disengagement. 
   SUMMARY OF INVENTION 
   The present invention overcomes the above-identified torque mismatching problem with a novel controller. According to one aspect of the present invention, the controller utilizes a torque correction factor to improve upon the accuracy of estimating torque actually produced by a primary power source. In this manner, torque produced by an motor/generator can be more accurately controlled to match torque produced by the primary power source during transfer brake disengagement. Consequently, noise, vibration and harshness (NVH) is limited. 
   One aspect of the present invention relates to a controller for use in a hybrid electric vehicle (HEV). The controller can be used with any HEV configuration which includes a transfer brake for braking a motor/generator. The controller, for example, can be used with a series hybrid electric vehicle (SHEV), a parallel hybrid electric vehicle (PHEV), or a parallel-series hybrid electric vehicle (PSHEV). 
   The controller can be used to control noise, vibration, and harshness (NVH) by matching torque produced by a primary power source, which is generally an engine or a fuel cell arrangement, with torque produced by the motor/generator during disengagement of the transfer brake. 
   With respect to the primary power source being an internal combustion engine, the controller is configured to determine a first torque estimate and a second torque estimate for the internal combustion engine. The first torque estimate is based on operating parameters for the internal combustion engine and the second torque estimate is based on operating parameters for the motor/generator. Preferably, the second torque estimate is a more accurate estimate of actual torque being produced by the internal combustion engine. In this manner, the second torque estimate can be used to increase the accuracy of the first torque estimate. 
   According to one aspect of the present invention, the controller compares the first torque estimate to the second torque estimate during steady-state vehicle operation. The comparison permits the controller to calculate a difference between the first and second torque estimates under steady-state operation. The difference is incorporated into a correction factor calculation. The correction factor can then be used to adjust the first torque estimate when the second torque estimate is unavailable, such as when the transfer brake is engaged. 
   The adjusted first torque estimate is a more accurate estimate of the torque actually being produced by the internal combustion engine. As such, when the transfer brake is disengaged, the controller can rely on the adjusted first torque estimate to match the torque provided by the motor/generator. In this manner, NVH and other discontinuities in demanded torque are limited. 


   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates an exemplary hybrid electric vehicle system that is controlled for torque matching in accordance with one aspect of the present invention; 
       FIG. 2  illustrates power and torque flow in the hybrid electric vehicle system; 
       FIG. 3  illustrates a positive split mode of operation for the hybrid electric vehicle system; 
       FIG. 4  illustrates a negative split mode of operation for the hybrid electric vehicle system; 
       FIG. 5  illustrates a parallel mode of operation of the hybrid electric vehicle system; 
       FIG. 6  illustrates an electric mode of operation of the hybrid electric vehicle system; and 
       FIG. 7  illustrates a process schematic of a controller in accordance with one aspect of the present invention. 
   


   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary hybrid vehicle that is commonly referred to as a parallel-series or “split power” hybrid vehicle (PSHEV) system  10 . In accordance with the present invention, the HEV  10  is controlled to limit noise, vibration and harshness (NVH). The present invention, however, also is not limited to a particular type of HEV powertrain configuration. 
   The system  10  includes an engine  14 , a transmission  16 , and a battery  20  which operate with a planetary gear set  24 , a generator  26 , a motor  28 , and meshing gears  32  to provide the torque. The torque is received by a torque shaft  36  for transfer to a differential axle  38  mechanism for final delivery to wheels  40 . 
   The system  10  provides torque for driving the hybrid vehicle. The manner in which torque is provided is variable and controllable by a vehicle system controller  44 .  FIG. 2  illustrates the variable and controllable means by which the vehicle system controller  44  can control power distribution in the system  10  for providing torque to the wheels  40 . 
   In general, fuel is delivered to the engine such that the engine  14  can produce and deliver torque to the planetary gear set  24 . The power provided from the engine  14  is expressed as T e ω e , where T e  is engine torque and ω e  is engine speed. Power delivered from the planetary gear set  24  to the meshing gears  32  is expressed as T r ω r , where T r  is ring gear torque and ω r  is ring gear speed. Power out from the meshing gears  32  is expressed as T s ω s , where T s  is the torque of shaft and ω s  is the speed of the torque shaft, respectively. 
   The generator  26  can provide or receive power from the planetary gear set  24 . This is shown with the double arrows and expressed as T g ω g , wherein T g  is the generator torque and is ω g  the generator speed. As shown with path  48 , the generator  26  can then supply power to or receive power from the battery  20  or the motor  28  during regenerative braking. As shown with path  50 , the battery  20  can store energy received from the generator  26  and the motor  28  and it can release energy to the generator  26  and the motor  28 . As shown with path  52 , the motor  28  provides power to and receives power from the generator  26  and the battery  20 . In addition, the motor  28  provides power to and receives power from the meshing gears  32 . This is shown with the double arrows and expresses as T m ω m , where T m  is motor torque and ω m  is motor speed. 
     FIGS. 3-6  provide further illustration of the flow of power and the production of torque in the system  10 . 
     FIG. 3  illustrates a positive split mode of operation of the HEV powertrain of FIG.  1 . In this mode, the engine power is split between the meshing gears  32  and the generator  26 , respectively. The splitting of power is controlled by the planetary gear set  24 . The meshing gears  32  use the power received from the planetary gear set  24  to provide torque to the wheels  40 . The battery  20  and the motor  28  can be controlled to receive power from generator  26 . The motor  28  can provide torque to the meshing gears  32  based on power receive from one or both of the generator  26  and the battery  20 . 
     FIG. 4  illustrates a negative split mode of operation. In this mode, the generator  26  inputs power to the planetary gear unit  24  to drive the vehicle while the motor  28  acts as a generator and the battery  20  is charging. It is possible, however, that under some conditions the motor  28  may distribute power to the meshing gearing  32 , in which case the battery  20  would power both the generator  26  and the motor  28 . 
     FIG. 5  illustrates a parallel mode of operation. In this mode, a generator brake  60  is activated and the battery powers the motor  28 . The motor  28  then powers the meshing gearing  32  simultaneously with delivery of power from the engine  14  delivered to the meshing gearing  32  by way of the planetary gear set  24 . Alternatively, the motor  28  can act as a generator to charge the battery  20  while the engine  14  provides power to the wheels  40  or during regenerative braking. 
     FIG. 6  illustrates an electric only mode. In this mode, a one way clutch  62  brakes the engine. The motor  28  draws power from the battery  20  and effects propulsion independently of the engine  14 , with either forward or reverse motion. The generator  26  may draw power from the battery  20  and drive against a reaction of the one-way coupling  62 . The generator  26  in this mode operates as a motor. 
   The vehicle system controller  44  (VSC) selects the power and torque delivery mode based on the vehicle operating conditions and a predefined strategy. To this end, the vehicle system controller  44  receives a signal from a transmission range selector  66  (PRND), a desired engine torque request  68 , as shown at, which is dependent on accelerator pedal position sensor output (APPS), and a brake pedal position sensor  70  (BPPS). 
   In response to the received signals, the vehicle system controller  44  generates signals to the engine  14 , a transmission control module  74  (TCM), and a battery control module  76  (BCM). Theses signals include a desired engine torque  80 , a desired wheel torque  82 , a desired engine speed  84 , a generator brake command  86 , a signal  88  indicating battery contactor or switch is closed after vehicle “key-on” startup. The modules then provide further signal to control the hybrid vehicle, such as a generator brake control  90 , a generator control  92 , and a motor control  94 . Rather, the present invention can be used with any number of different motor driven vehicles, whether electric driven, mechanically driven, or driven by a combination of electrical and mechanical means. 
   Torque produced by the internal combustion engine  14  is estimated by the vehicle system controller  44  according to operating conditions of the internal combustion engine  14 . The operating conditions typically include spark timing, air flow rate, speed, and load, but other engine operating parameters could be used. 
   Torque received or produced by the motor/generator  26  has a known relationship with torque produced by the internal combustion engine  14 . This permits estimating engine torque production based on operating parameters for the motor/generator  26 . Typically, current in the motor/generator  26  is used to estimate torque produced by the internal combustion engine  14 . 
   In general, the torque estimates derived from the motor/generator  26  are more accurate of the actual torque produced by the engine  14  in comparison to the accuracy of the torque estimates based on the operating conditions of the engine  14 . The motor/generator  26 , however, cannot be used for such estimations when the motor/generator  26  is not producing or receiving torque. Rather, the controller  44  must rely on an engine torque estimate derived from the engine operating parameters. 
   The motor/generator is unavailable for estimating torque if the transfer brake  60  is engaged with connector  95 . Connector  95  connects the motor/generator to the planetary gear set. The transfer brake  60  physically presses against the connector  95  to prevent the connector  95  from rotating. Typical operation requires engaging and disengaged the transfer brake  60 . 
   During disengagement, torque produced by the motor/generator  26  should match torque produced by the internal combustion engine  14 . NVH can arise depending on the degree to which the torque provided by the motor/generator  26  fails to match the torque provided by the internal combustion engine  14 . Torque matching does not require equal levels of torque production, although it can. Rather, torque matching refers to a predefined range of torque production of the internal combustion engine  14  relative to the motor/generator  26 . The relationship can change under different driving conditions and control methodologies. 
   The controller  44  compares the torque estimate derived from the internal combustion engine  14  operating parameters to the torque estimate derived from the motor/generator  26  operating parameters. Preferably, the controller  44  determines the correction factor during steady-state operation. 
   Steady-state operation occurs when the transfer brake  60  is released and the vehicle is operating under predefined conditions. Steady-state operation preferably eliminates transient torque requests. For example, steady-state can occur when an engine load is within a predefined range for a predefined period of time or when an engine speed is within a predefined range for a predefined period of time. The controller includes a number of vehicle operating inputs that can be used to determine whether the vehicle is in steady-state operation. 
   The comparison permits the controller to calculate a difference between the two torque estimates under steady-state operation. The difference is incorporated into a correction factor calculation. The correction factor can then be used to adjust the torque estimates derived from operating conditions of the internal combustion engine  14 . 
   The adjusted torque estimate is a more accurate estimate of the torque actually being produced by the internal combustion engine  14 . As such, when the transfer brake  60  is disengaged, the TCM controller  74  can rely on the adjusted torque estimate to to control the motor/generator  26  to match the torque provided by the engine  14 . 
     FIG. 7  illustrates schematically the operation of controller  44  to limit NVH. The controller  44  receives a number of inputs from a number of vehicle sensors. The inputs are generally referred to with reference numeral  96 . The inputs  96  include an internal combustion engine torque estimate (CET) input, an motor/generator torque estimate (EMT), an engine speed input, an engine load input, an motor/generator current, an engine coolant temperature, a EGR rate input, a brake status flag input, an fuel level input, and an air conditioning input. 
   The CET input and the EMT input are torque estimates for the engine  14 . The EMT input is based on operating conditions for the engine  14  and the CET input is based on operating conditions for the motor/generator  26 . As described above, the CET input is more accurate of the actual torque produced by the engine  14 . The CET and EMT inputs are typically from other controllers in the HEV  10 , such as an engine controller and an motor/generator controller. However, the controller  44  could be configured to calculate these values. 
   The controller  44  determines at block  100  whether the vehicle  10  is operating under steady-state conditions. This is part of the process because it is more desirable to calculate the correction factor from data collected during steady-state operation. The controller  44  monitors the input sensors  96  to make this determination. A comparison of CET to EMT occurs at block  102  if steady-state operation is determined. The comparison typically comprises calculating a difference between the CET value and the EMT value. The comparison, however, can include an algorithm or other manipulation of the CET and EMT values. 
   A correction factor is then calculated at block  104  based on the comparison block  102 . The correction factor generally corresponds with the difference between the CET and the EMT values. With respect to the split power system shown in  FIG. 1 , nominally the CET value can be different from the EMT value up to a range of 10-15%. The correction would correspond with this percentage to adjust the CET value accordingly. Of course, as one of ordinary skill in the art will appreciate, the correction value calculation can vary depending on various factors, including the type and configuration of the engine  14  and the generator  24 . As describe below, the correction factor is preferably constantly updated from repeated correction factor computations. 
   At block  108 , the operating conditions proximate the time of determining the correction factor are determined and stored. The correction factor is then stored at block  110  with the corresponding operating conditions from block  108 . 
   An memory  114  stores the correction factor and the corresponding operating conditions. The memory  114  preferably stores at least one correction factor for each of a number of operating conditions corresponding with low engine speed in high engine load, high engine speed in low engine load, high engine speed in high engine load, and low engine speed in low engine load. 
   The stored correction factors are retrieved at block  116  according to the operating conditions determined at block  108  for each correction factor. The correction factor corresponding with the current vehicle operating condition is retrieved. For example, if the current operating condition is low speed and low load, then the correction factor corresponding with low speed and low load is retrieved. 
   The retrieved correction factor is then used to adjust the CET value at block  118 . The forgoing process continuously calculates and stores and updates the correction factors during steady-state operation. This allows the controller  44  to continuously update the stored correction factors. 
   The transfer brake input is monitor throughout the process so that the controller can determine when the transfer brake  60  engages the connector  95 . Typically, the EMT input is used unless the transfer brake is engaged because the EMT input is more accurate than the CET value of engine torque. However, once the transfer brake  60  is engaged, the controller  44  at block  118  adjusts the CET value according to the correction retrieved at block  116 . The adjusted CET values continue to be adjusted until the transfer brake  60  is disengaged. 
   The TCM controller  74  uses the CET values adjusted at block  118  to control the motor/generator  26  at block  120  The motor/generator  26  is controlled to provide a torque based on the adjusted CET value. Accordingly, the torque provided by the motor/generator should match the torque provided by the engine  14  within a predefined range to limit NVH. 
   While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.
A controller and a method for use in a vehicle to limit noise, vibration, and harshness (NVH) by controlling torque matching between a first power source and a second power source. A first and a second torque estimate are made for the first power source. The torque matching comprises calculating a correction factor based upon a comparison of the torque estimates made during steady-state operating conditions of the vehicle. The first torque estimate is based on operating parameters for the first power source and the second torque estimate is based on operating parameters for the second power source.