Abstract:
Torque demand is coordinated in a vehicle. Information defining at least one torque production limitation for a first torque producing device is received. A request for torque is compared with the first torque producing device torque production limitation. If the comparison does not result in the request for torque exceeding a limitation, a first coordinated torque request is determined as the request for torque and a null torque is determined as a second coordinated torque request. Otherwise, a first excess requested torque is determined as the difference between the request for torque and the exceeded limitation, the first coordinated torque request is determined as the exceeded limitation, and the second coordinated torque request is determined as the first excess requested torque.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to the control of torque in a vehicle. More particularly, the present invention relates to coordinating torque demands amongst a plurality of vehicular torque producing devices. 
   2. Background Art 
   Vehicle control systems accept requests from the vehicle driver and various vehicle components as well as output from vehicle parameter sensors. Vehicle controllers use these inputs to generate control signals for vehicle equipment. Conventional control systems applied to automotive vehicle applications were used to improve engine operation in order to reduce vehicle emissions. Since these early attempts, engine controls have continued to grow in complexity as opportunities are identified to make further improvements in performance, emissions, fuel economy, and the like. Since the engine controller is still typically the most complex control system on the vehicle, it remains the primary repository for most new vehicle control algorithms as they are developed. This has resulted in two problems with conventional engine controllers. 
   First, several control features that reside in the engine controller are not engine specific. For example driver demand algorithms, which determine the desired traction torque or force required by the driver, are often resident in the engine controller. These algorithms are required for any vehicle, regardless of the type and number of torque generators, and are not therefore engine specific. Another example of algorithms routinely integrated into the engine controller is passive anti-theft algorithms. By not purposely distinguishing these algorithms from the base engine control algorithms, modular design, testing and implementation of the control system becomes much more difficult. 
   A second problem with conventional engine controllers is that many of the algorithms in the engine controller are engine system centric. Since the engine controller has historically been the predominant controller in the vehicle, many algorithms have been written assuming that the engine specific information is always available. For example, the interface between the transmission and engine control functions used for torque reduction during shifting is written in terms of spark angle rather than torque. This type of architecture is not conducive to adding other torque producing devices to the drive line such as, for example, an electric motor. 
   At the same time that engine control systems have been growing in complexity, control systems have been added to other subsystems on the vehicle with the intention of improving various aspects such as safety, durability, performance, emission control and the like. Typically, these control systems are implemented as stand alone systems that provide little or no interaction with the other control systems on the vehicle. 
   New vehicle technologies such as hybrid electric power trains, advanced engines, active suspensions, telematics, and the like are increasingly incorporated into the vehicle. As these technologies emerge and are targeted towards production vehicles, the interaction between subsystems grows ever more complex. To achieve increasingly more stringent requirements on vehicle objectives for emissions, safety, performance, and the like, the interactions between major subsystems in the vehicle need to be coordinated at the vehicle level. 
   Further, conventional controllers are easily adaptable to a variety of drive train configurations. Each hardware configuration requires a unique control solution. Arbitration among requests and coordination among actuators is often ad-hoc and device specific. Control subsystems need to know information buried within other subsystems. The possibility even arises for different subsystems to issue conflicting control commands. 
   Conventional torque coordinating schemes require different algorithms for different hybrid vehicle events such as charging, power assist, bleed, regenerative braking, and the like. This results in discontinuous torque control due to state switching while one or more torque generators are running. 
   What is needed is a functional structure that allows several torque producing devices to be coordinated at the vehicle level. This structure should be flexible, permitting application in a wide variety of vehicle configurations. In addition, this structure should be readily implemented in current and future vehicle control systems. 
   SUMMARY OF INVENTION 
   The present invention coordinates torque requests amongst a plurality of torque producing devices. 
   Torque coordination under the present invention is more robust and less prone to failure than conventional systems which use different algorithms for hybrid functions. The present invention also results in improved driveability, fuel economy and exhaust emissions. 
   A method for coordinating torque demand in an automotive vehicle is provided. Information defining at least one torque production limitation for a first torque producing device is received. A request for torque is determined. The request for torque is compared with the at least one first torque producing device torque production limitation. If the comparison does not result in the request for torque exceeding a first torque producing device torque production limitation, a first coordinated torque request is determined as the request for torque and a null torque is determined as a second coordinated torque request. If the comparison results in the request for torque exceeding a first torque producing device torque production limitation, a first excess requested torque is determined as the difference between the request for torque and the exceeded first torque producing device torque production limitation, the first coordinated torque request is determined as the exceeded first torque producing device torque production limitation, and the second coordinated torque request is determined as the first excess requested torque. The first coordinated torque request is sent to the first torque producing device and the second coordinated torque request is sent to at least one second torque producing device. 
   In an embodiment of the present invention, information defining at least one torque production limitation for the at least one second torque producing device is received. The first excess requested torque is compared with the second torque producing device torque production limitations. If the first excess requested torque exceeds any second torque producing device torque production limitation, an exceeded second torque producing device torque production limitation is sent as the second coordinated torque. A second excess requested torque may be determined as the difference between the first excess requested torque and the exceeded second torque producing device torque production limitation. The first coordinated torque request is then determined as the sum of the exceeded first torque producing device torque production limitation and the second excess requested torque. The sum of the exceeded first torque producing device torque production limitation and the second excess requested torque may be compared with first torque producing device torque production limitations. If the sum of the exceeded first torque producing device torque production limitation and the second excess requested torque is greater than an exceeded first torque producing device torque production limitation, the exceeded first torque producing device torque production limitation is determined as the first coordinated torque request. 
   In another embodiment of the present invention, the first torque producing device includes an engine and the at least one second torque producing device includes a motor. 
   In still another embodiment of the present invention, the comparison is performed at a wheel level and the first torque producing device generates torque at a transmission input level. Information defining at least one torque production limitation for the first torque producing device is translated through any transmission effects between the transmission input level and the wheel level. 
   In yet another embodiment of the present invention, the comparison is performed at a transmission input level and the first torque producing device generates torque at a wheel level. At least one of the first coordinated torque request and the second coordinated torque request is translated through any transmission effects between the wheel level and the transmission input level. 
   In a further embodiment of the present invention, the request for torque is determined by summing a plurality of torque requests. 
   A vehicle is also provided. The vehicle includes at least one source of torque requests. An engine receives commands for generating a first torque. At least one motor receives commands for generating a second torque. Control logic determines a torque request. An initial coordinated torque request is determined as the determined torque request limited by at least one engine torque limit. A first excess requested torque is determined as a difference between the received torque request and the initial coordinated torque request. A second coordinated torque request is determined as the first excess requested torque limited by at least one motor torque limit. A second excess requested torque is determined as a difference between the first excess requested torque and the second coordinated torque request. A first coordinated torque request is determined as a sum of the initial coordinated torque request and the second excess requested torque. 
   In an embodiment of the present invention, the vehicle further comprises a transmission for converting the first torque from a transmission input level to a wheel level driving a first axle and wherein the at least one motor comprises at least one motor mechanically connected to a second axle. The system may also include a traction controller determining a balancing torque request to reduce a difference in speed between the first axle and the second axle. The control logic determines the initial coordinated torque request as a difference between the determined torque request and the balancing torque request as limited by at least one engine torque limit. 
   In another embodiment of the present invention, the determined torque request includes an arbitrated driver request exceeding the ability for the engine to generate as the first torque. In response, the control logic determines the second coordinated torque request as a power assist request. 
   In still another embodiment of the present invention, the determined torque request is a negative torque request. In response, the control logic determines the second coordinated torque request as a regenerative braking request. 
   In yet another embodiment of the present invention, the vehicle includes at least one battery controller determining a charging torque request to change a state of charge of at least one battery using at least one motor mechanically connected to at least one of the first axle and the second axle. In response, the control logic determines the initial coordinated torque request as a sum of the determined torque request and the charging torque request as limited by at least one engine torque limit. 
   The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the preferred embodiments for carrying out the invention when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram illustrating torque producing devices according to an embodiment of the present invention; 
       FIG. 2  is a block diagram illustrating multilevel torque resolution according to an embodiment of the present invention; 
       FIGS. 3   a  and  3   b  are a block diagram illustrating motion control functions for an integrated starter-generator hybrid vehicle according to an embodiment of the present invention; 
       FIGS. 4   a - 4   c  are block diagrams illustrating a generalized architecture for vehicle motion control according to an embodiment of the present invention; 
       FIG. 5  is a schematic diagram illustrating a vehicle with electric four-wheel drive according to an embodiment of the present invention; 
       FIGS. 6   a  and  6   b  is a block diagram illustrating a vehicle motion controller for electric four-wheel drive according to an embodiment of the present invention; 
       FIG. 7  is a block diagram illustrating wheel level torque coordination according to an embodiment of the present invention; 
       FIG. 8  is a block diagram illustrating transmission input level base torque coordination according to an embodiment of the present invention; 
       FIG. 9  is a block diagram illustrating fast torque coordination at the transmission input level according to an embodiment of the present invention; 
       FIG. 10  is a block diagram illustrating arbitration among base requests at the wheel level according to an embodiment of the present invention; 
       FIG. 11  is a block diagram illustrating arbitration at the transmission input level according to an embodiment of the present invention; and 
       FIG. 12  is a block diagram illustrating multilevel torque resolution according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a schematic diagram illustrating torque producing devices according to an embodiment of the present invention is shown. Vehicle  20  may include a plurality of torque producing devices. Torque producing devices include any of a wide variety of internal combustion engines (ICE). Various types of motors may also be employed, including those powered by energy storage devices such as batteries, accumulators and the like; powered by power generating devices, such as engines, fuel cell systems, solar cell systems, and the like; or powered by any combination of these. 
   For example, engine  22  transmits torque through engine transmission  24  to front axle  26  thereby driving wheels  28 . Engine transmission  24  is controlled to convert torque from engine  22  to axle  26  using various mechanisms such as torque converters, gears, and the like. Transmission  24  may be manual, automatic, continuously variable, composed of one or more planetary gear sets, or of any other suitable construction or operation. Vehicle  20  may also include electric motor  30  mechanically connected to engine transmission  24 . Motor  30  may be, for example, an integrated starter-generator (ISG). Engine  22  may be connected to motor  30  through clutch  31 . Disengaging clutch  31  allows motor  30  to drive axle  26  without driving engine  22 . Various torque producing devices may be interconnected by one or more of a variety of mechanisms, including mechanical coupling, electromagnetic coupling, hydraulic coupling, and the like. Vehicle  20  may also include motor  32  connected through an intermediate stage of engine transmission  24  to axle  26 . 
   Many alternative drive configurations are possible. For example, internal combustion engine  33  transmits torque through transmission  34  to rear axle  36  propelling wheels  28 . Electric motor  38  transmits torque through separate transmission  40  to rear axle  36 . Transmission  40  may also transmit torque from rear axle  36  to motor  38  when motor  38  is generating electric power. One or more motor/generators  42  may also be directly connected to axle  36 . Motor/generators  42  may be electric or hydraulic, the latter storing energy in accumulators during deceleration for later delivery to wheels  28  for acceleration. Various combinations of front drive and/or rear drive sources can be implemented. In addition, any number of axles or other output shafts may be driven. The present invention is not limited to a specific configuration of drive or torque generating devices. 
   Vehicle  20  typically includes at least one mechanism for decelerating. Each wheel  28  may include one or more friction brake  44 . Engine  22 ,  33  may implement compression braking. Motor  30 ,  32 ,  38 ,  42  may implement regenerative braking. 
   Vehicle  20  with a multitude of torque producing devices is more efficiently controlled through a coordinated effort to receive torque requests and generate torque commands. A multilevel consideration is appropriate since torque producing devices and torque requesting sources operate at different levels. For example, some torque producing devices operate at a transmission input level whereas other torque producing devices operate at a transmission output or wheel level. Similarly, torque requests may be received at either the transmission input or wheel levels. It should be noted that the term transmission generally refers to any means for converting torque such as gears, belts, torque converters, clutches, shafts, pulleys, and the like, as well as traditional engine transmissions. 
   Referring now to  FIG. 2 , a block diagram illustrating multilevel torque resolution according to an embodiment of the present is shown. A level may be any point in a drive train where torque is requested or generated. Possible levels include at a wheel, axle, transmission input, transmission output, intermediate transmission stage, power take-off point, and the like. 
   An exemplary torque resolution system, shown generally by  50 , operates on both wheel level  51  and transmission input level  52 . Wheel level resolver  53  receives a plurality of wheel level torque requests  54  and generates at least one of wheel level base requests  55  and wheel level fast requests  56 . Wheel level resolver  53  may also coordinate wheel level requests  55 ,  56  between wheel level torque producing devices. 
   Operation at wheel level  54  may be expressed in one or more of a variety of reference domains. These domains apply to both vehicle acceleration and deceleration. The wheel torque domain expresses variables in terms of the torque requested at, or delivered to, one or more wheels  28 . The drive shaft domain is related to the wheel torque domain through differential gear ratios. The tractive force domain is related to the wheel torque domain through the wheel radius. The vehicle acceleration domain is related to the tractive force domain through vehicle mass. The present invention applies regardless of which domain is considered. Without loss of generality, operation at the wheel level will be described in terms of wheel torque. 
   Translator  57  accepts wheel level base requests  55  and wheel level fast requests  56  and translates requests  55 ,  56  to compensate for the effect of any torque conversion between transmission input level  52  and wheel level  51 . Translator  57  generates translated base requests  58  and translated fast requests  59  by translating wheel level base requests  55  and wheel level fast requests  56 , respectively. 
   Transmission input level resolver  60  accepts translated base requests  58 , translated fast requests  59  and transmission input level requests  61 . Transmission input level resolver  60  arbitrates requests  58 ,  59 ,  61  to produce transmission input level base requests  62  and transmission input level fast requests  63 . Transmission input level resolver  60  may also coordinate torque requests  62 ,  63  between multiple transmission input level torque producing devices. 
   One aspect of the present invention is that torque may be arbitrated at two or more levels. For example, wheel torque and transmission input torque are arbitrated separately by torque resolution system  50 . The first arbitration compares all wheel torques that are requested at wheel level  51 . After drive line disturbance control, the desired value of wheel torque is translated or converted to a desired crankshaft torque by adjusting for transmission torque ratio and losses. Since this is the point in vehicle  20  at which torque is summed on the drive line, it is an appropriate place for the second arbitration to occur. Here, all requests for crankshaft (transmission input) torque, including the arbitrated and translated wheel torque, are arbitrated to determine a final desired crankshaft torque. 
   A second aspect of the present invention propagates arbitrated desired torque requests into two signals: a base value and a fast value. As will be recognized by one of ordinary skill in the art, there are several ways to affect the torque in vehicle  20 . Thus, an effort is made to distinguish between base requested values, associated primarily with meeting driver demand and other relatively slow requests within the system, from fast values related to vehicle subsystem protection, safety, and other high speed requests for torque. This dichotomy also conveniently reflects the variation and abilities to produce torque within an engine. An internal combustion engine has methods for modifying torque that can cover the entire range of operation such as, for example, air flow modification, that typically have a low response time. These methods are best used for achieving base torque response. The internal combustion engine can also modify torque rapidly but often within only limited authority such as, for example, in spark modification. Similarly, an ISG is another device that can produce fast torque response within only limited torque capability. These types of torque production are best matched with fast torque demands. 
   Translator  57  may implement a fixed algorithm or a variable algorithm depending on the operation and type of transmission represented by translator  57 . For example, engine transmission  24  may be represented by translator  57  implementing, for each fast and/or slow torque, the following formula: τ c =rFm+y, where τ c  is a transmission input torque as represented by translated wheel level base requests  58  or translated wheel level fast requests  59 , r is an effective wheel rolling radius, F is a traction force representing wheel level base requests  55  or wheel level fast requests  56 , m is a torque ratio, and y is a torque offset. In addition, while only one translator  57  is shown in  FIG. 2 , a plurality of translators  64  may be used if multiple transmissions convert torque within vehicle  20 . Examples of other levels between which translation may occur include differential input, planetary gear stages, and the like. 
   Referring now to  FIGS. 3   a  and  3   b , a block diagram illustrating motion control-functions for an integrated starter-generator (ISG) hybrid vehicle according to an embodiment of the present invention is shown. A vehicle system controller, shown generally by  70 , contains the set of distinguishing characteristics for torque control in vehicle  20 . Vehicle system controller  70  also coordinates the interactions of various subsystems in vehicle  20  as represented by transmission controller  72 , battery controller  74 , ISG controller  76 , and engine controller  78 . Vehicle system controller  70  is preferably implemented on a microcontroller system within vehicle  20 . As will be recognized by one of ordinary skill in the art, functions performed by vehicle system controller  70  may be implemented in more than one special purpose controller, may be split amongst other vehicle controllers, and may implement functionality that may otherwise be assigned to various other vehicle controllers. Functionality in vehicle system controller  70  may be implemented as hardware, software, firmware, or any combination. 
   Vehicle system controller  70  may be divided into a plurality of functional elements, as illustrated here by way of example. Accelerator pedal interpreter  80 , vehicle speed limiting  82 , and cruise control  84  generate wheel level torque requests. Accelerator pedal interpreter  80  accepts accelerator pedal position  86  and vehicle speed  88  and determines driver&#39;s desired tractive force  90 . Cruise control  84  accepts desired vehicle speed  92  and vehicle speed  88  and determines cruise desired tractive force  94  needed to maintain a desired vehicle speed. Vehicle speed limiting  82  determines maximum tractive force  96  as a limit needed to avoid vehicle overspeed condition. Tractive force arbitration  98  accepts desired tractive forces  90 ,  94  and maximum tractive force  96 . Tractive force arbitration  98  arbitrates requests for tractive force from these various sources and generates desired tractive force base. Desired tractive force base  100  is a wheel level base request. 
   Tractive force arbitration  98  also generates tractive force source  104  propagated along with base desired tractive force  100 . Tractive force source  104  provides an indication of the requirements of the torque command and is used to help the torque and speed coordination function and torque producing subsystems to determine the appropriate method for achieving the desired torque values. For example, engine  22  can produce a fast torque reduction by either modifying spark advance or fuel cutoff to cylinders. The utility of these two methods varies, however, as spark is limited in the range of reduction that can be achieved whereas fuel is limited in the precision of the torque reduction produced. By encoding either the source of the torque request or the desired affect of the request in tractive force signal  104 , torque and speed coordination function and torque producing subsystems can make better decisions as to the appropriate course of action. 
   Max/min crankshaft torque  106  determines total minimum and maximum available crankshaft torque from all sources. In this example, inputs include ISG max/min torque available  108  from ISG controller  76  and engine max/min torque available  110  from engine controller  78 . Max/min crankshaft torque  106  generates max/min available crankshaft torque  112 . Shift scheduling  114  accepts accelerator pedal position  86 , vehicle speed  88 , and max/min available crankshaft torque  112 . Shift scheduling  114  determines transmission configuration as desired gear signal  116  to transmission controller  72 . Converter clutch scheduling  118  determines the desired lock up status of the torque converter bypass clutch based on accelerator pedal position  86  and vehicle speed  88 . Specifically, converter clutch scheduling  118  generates desired converter clutch state and desired converter clutch slip  119  for transmission controller  72 . Transmission controller  72  controls clutch and valve solenoids within engine transmission  24 . Transmission controller  72  also generates a variety of signals including torque ratio and torque loss offset signals, shown generally by  120 , used for translating torque requests. Signal  122  from transmission controller  72  indicates the maximum and minimum crankshaft fast torque and maximum crankshaft base torque. Signal  124  indicates transmission stop permission and signal  126  indicates desired crankshaft speed. 
   Block  128  performs translation down through engine transmission  24 . Actual crankshaft torque  130  is translated using torque ratio and torque loss offset signals  120  to produce actual tractive force  132 . Driveline disturbance control  134  accepts desired tractive force base  100  and actual tractive force  132  to smooth driveline responses to rapid changes in torque demand. The result is filtered desired tractive force base  136 . 
   Block  140  translates desired tractive force to desired crankshaft torque. Filtered desired tractive base force  136  is translated using torque ratio and torque loss offset signals  120  to produce translated desired tractive force base  142 . 
   Crankshaft torque arbitration  146  accepts translated desired tractive force base  142  and tractive force source  104  as well as requests of crankshaft torque from any other source. Crankshaft torque arbitration  146  arbitrates these requests to generate desired crankshaft torque base  148 , desired crankshaft torque fast  150 , and crankshaft torque source  152  reflecting tractive force source  104 . 
   Referring now to  FIG. 3   b , energy management block  154  represents energy management functions of vehicle system controller  70 . Energy management  154  generates desired generation power  156  and energy management stop okay flag  158 . Driveline idle speed coordination  160  accepts desired generation power  156  and desired crankshaft speed  126  to determine the desired operating speed for driveline during periods without driver demand. This desired operating speed is expressed as desired idle speed  162  used by engine controller  78 . 
   Torque and speed coordination function  174  splits requested torque between various torque producers. In this example, torque producers are internal combustion engine  22  and ISG motor  30  as controlled by engine controller  78  and ISG controller  76 , respectively. Torque and speed coordination  174  accepts desired crankshaft torque base  148 , desired crankshaft torque fast  150 , and crankshaft torque source  152  from crankshaft torque arbitration  146 . Inputs also include transmission stop okay flag  124 , energy management stop okay flag  158 , ISG stop okay flag  166  from ISG controller  76 , engine stop okay flag  168  from engine controller  78 , battery stop okay flag  170  from battery controller  74 , and desired generation power  156 . ISG controller  76  receives desired ISG torque, desired ISG speed, and ISG torque or speed control mode, represented by signals  184 , from torque and speed coordination  174 . Engine controller  78  receives desired engine torque base, desired engine torque fast, and engine torque source, represented by signals  186 , from torque and speed coordination  174 . Energy management  154  receives desired crankshaft torque base and desired crankshaft torque fast, represented by signals  188 , from torque and speed coordination  174 . 
   Referring now to  FIGS. 4   a - 4   c , block diagrams illustrating a generalized architecture for vehicle motion control according to an embodiment of the present invention are shown. In certain applications, there is a need to coordinate torque requests at the wheels. Examples of such applications include when electro-hydraulic brakes (EHB) are used to more efficiently capture braking energy, when a traction motor is introduced on an axle not driven by an internal combustion engine to provide four-wheel drive functionality, and the like. A generalized architecture covers the case where some propelling devices apply torque to the crankshaft/output shaft, with this torque passed through one or more typically variable transmissions before reaching the wheels, and other devices apply torque directly coupled to the wheels. An example of such an architecture is an electric four-wheel drive system with one or more electrical motors applying power directly to an axle or wheel. 
   Referring now to  FIG. 4   a , wheel level torque resolution is illustrated. Speed control arbitration function  240  accepts accelerator desired wheel force  242  from driver evaluator and wheel force limit signals  244  from vehicle speed control and produces desired wheel force  246 . Front torque translation  248  uses front transmission parameters  250  to convert front crankshaft torque  252  to front tractive force  254 . Rear torque translation  256  uses rear transmission parameters  258  to convert rear crankshaft torque  260  to rear tractive force  262 . 
   Anti-jerk control  264  filters desired wheel force  246 , front tractive force  254 , rear tractive force  262 , and other slowly changing tractive requests such as driver evaluator signals  266 , engine controller signals  268 , transmission controller signals  270 , and the like. Anti-jerk control  264  generates base tractive force requests  272  which are multiplied by one or more wheel constants  274  to produce acceleration torque requests  276 . Acceleration torque requests  276 , braking torque requests  278  from a braking controller, and vehicle speed signal  280  are combined in calculation block  282  to produce overall vehicle desired torque signal  284 . Wheel torque arbiter  286  accepts over-all vehicle desired torque signal  284  together with fast acting torque requests  288  from the brake controller. Fast brake signals  288  are generated by components including anti-lock brake systems (ABS), stability and traction control (STC), interactive vehicle dynamics (IVD), and the like. Torque vehicle speed limit  290  provides allowable torque limits. Wheel torque arbiter  286  generates wheel level base requests  292  and wheel level fast requests  294 . 
   Signals along the interface among functions can be either scalars or vectors. For example, fast brake signals  288  can be expressed individually for each wheel or for each axle. The respective signals can then be propagated as vectors and considered individually for torque coordination. 
   Wheel torque coordinator  296  distributes torque requests between front torque request base  298 , front torque request fast  300 , rear torque request base  302  and rear torque request fast  304 . Front brake torque intent  306  and rear brake torque intent  308  are nonzero only during braking. Braking controlled torque distribution  310  accepts front brake torque intent  306 , rear brake torque intent  308 , wheel level fast requests  294  and internal brake subsystem controller signals and generates brake torque requests  312  for the brake controller, as well as front axle torque limits  314  and rear axle torque limits  316 . Wheel torque coordinator  296  accepts as input various torque requests including wheel level base requests  292 , wheel level fast requests  294 , front generator torque requests at the wheel level  318 , and rear generator torque requests at the wheel level  320 . Wheel torque coordinator  296  also accepts torque limits including front axle torque limit  314 , rear axle torque limit  316 , front motor torque availability limit  322 , front engine torque availability limit  324 , rear motor torque availability limit  326 , and rear engine torque availability limit  328 . Not all of these signals will be present in every application. 
   Referring now to  FIG. 4   b , front crankshaft input level torque resolution is illustrated. Front torque translator  340  uses front transmission parameters  342  such as gear ratios, torque ratios, transmission internal losses and the like, to translate front torque request base  298  and front torque request fast  300  to translated wheel level front torque request base  344  and translated wheel level front torque request fast  346 , respectively. Front crankshaft torque arbitration  348  arbitrates translated wheel level front torque request base  344  and fast  346  with limits such as torque limit during shift  350  from front transmission controller resulting in transmission input level front torque request base  352  and fast  354 , respectively. 
   Front axle torque coordinator  356  distributes torque requests among front axle torque producing devices. To this end, front axle torque coordinator  356  generates base and fast engine torque requests  358  for a front engine controller and motor torque requests  360  for a front motor. In addition front axle torque coordinator  356  generates front generator torque request at the wheel level  318  and actual front crankshaft torque  252 . Front axle torque coordinator accepts requests such as transmission input level front torque request base  352  and fast  354  and electrical power generation torque request  362  from generation torque requestor  364  based on energy management front generated power request  366  and engine speed idle target  368  from front engine controller. Front axle torque coordinator  356  distributes torque requests based on availabilities and capabilities of torque producing devices as represented, for example, by engine torque capability signal  370  and front motor torque availability signal  372 . 
   Front motor torque availability signal  372  is generated by motor availability logic  374  based on state of charge signal  376  from an energy storage management module and torque capacity signal  378  from a front motor control. Engine torque capability signal  370  and front motor torque availability signal  372  are translated by front down torque translator  380  based on front transmission parameters  342  to generate front engine torque availability limit  324  and front motor torque availability limit  322 , respectively. 
   Referring now to  FIG. 4   c , rear transmission level torque resolution is illustrated. In the general case, rear transmission level torque resolution operates fundamentally the same as front transmission level torque resolution. Rear torque translator  390  uses rear transmission parameters  391  such as gear ratios, torque ratios, transmission internal losses and the like, to translate rear torque request base  302  and rear torque request fast  304  to translated wheel level rear torque request base  392  and translated wheel level rear torque request fast  393 , respectively. Rear crankshaft torque arbitration  394  arbitrates translated wheel level rear torque request base  392  and fast  393  with limits such as torque limit during shift  350  from rear transmission controller resulting in transmission input level rear torque request base  396  and fast  398 , respectively. 
   Rear axle torque coordinator  400  accepts rear transmission input level torque request base  396  and fast  398 , rear electrical power generation torque request  402  based on rear generated power request  404 , as well as engine torque capability signal  405  and rear motor torque availability signal  406 . Rear axle torque coordinator  400  generates base and fast engine requests  408 , motor torque requests  410 , rear generator torque requests at the wheel level  320 , and rear crankshaft torque signals  260 . Rear motor torque availability signal  406  is generated by motor availability logic  412  based on torque capacity signal  414  from rear electric motor controller. Rear down torque translator  416  translates rear motor torque availability signal  406  and engine torque capability signal  405  into rear motor torque availability limit  326  and rear engine torque availability limit  328 . 
   Referring now to  FIG. 5 , a schematic diagram illustrating a vehicle with electric four-wheel drive according to an embodiment of the present invention is shown. Vehicle  430  includes front axle  432  and rear axle  434 . Internal combustion engine  436  and integrated starter-generator (ISG)  438  are coupled to rear axle  434  through automatic engine transmission  440 . Traction motor  442  is either directly coupled to front axle  432  or coupled to front axle  432  through a fixed transmission, the effects of which may be ignored without loss of generality. 
   Torque control within vehicle  430  is distributed amongst a plurality of modules. Engine controller (EC)  444  controls various engine functions including spark, air, fuel, cam timing, exhaust gas recirculation control, and the like. Engine controller  444  provides indications of the maximum and minimum engine torque available. Rear electric motor controller (REM)  446  provides control signals to ISG  438 . Transmission controller (TC)  448  provides clutch and valve solenoid control for transmission  440 . Front electric motor control (FEM)  450  provides control signals to traction motor  442 . Brake control  452  handles braking functions such as actuation for hydraulic brakes  454 , anti-lock brake control, and the like. Battery management module (BMM)  456  provides state of charge and state of health estimation and current and voltage limit calculations, as well as actual voltage and current measurements. Vehicle speed control (SC)  458  provides cruise control and maximum allowed vehicle speed-based torque limits. Driver evaluator (DE)  460  provides signals based on driver input. Vehicle system controller (VSC)  462  provides top level torque resolution for vehicle  430 . Sensors  464  on axles  432 ,  434  provide axle rotation information to wheel slip controller  466  for balancing wheel speeds. As will be recognized by one of ordinary skill in the art, one or more of the modules illustrated may be implemented with the same hardware. Further, functions attributed to each module may be divided amongst various hardware components. 
   Referring now to  FIGS. 6   a  and  6   b , a block diagram illustrating a vehicle motion controller for electric four-wheel drive according to an embodiment of the present invention is shown. Vehicle system controller  462  implements logic to arbitrate between torque requests and coordinate request distribution amongst torque producing devices. The logic illustrated in  FIG. 6  is similar to the generalized logic illustrated in  FIGS. 4   a - 4   c.    
   Various wheel level torque requests are filtered, combined, limited, and otherwise arbitrated to produce wheel level base requests  292  and wheel level fast requests  294 . Additional inputs include four-by-four request interpreter signal  470  for balancing axle or wheel speeds. Wheel torque coordinator  296  generates rear torque request base  302  and rear torque request fast  304  which are translated by rear torque translator  390 . No such translation may be required for traction motor  442  driving front axle  432 . If translation is required, the translation is fixed. Thus, wheel torque coordinator  296  generates wheel level torque request signal  472  for front electric motor controller  450 . 
   Referring now to  FIG. 7 , a block diagram illustrating wheel level torque coordination according to an embodiment of the present invention is shown. In most hybrid configurations, there is a need for torque coordination function at wheel or axle level  52 . Inputs to such a coordination function include arbitrated at wheel level torque requests for the vehicle as a whole, torque requests for individual axles, torque requests for individual wheels, driver demand information, and limitations from various sources such as vehicle stability, and the like. In addition, inputs should include torque capabilities and limitations of devices applying torque to the wheels either directly or translated through a transmission. The coordination function prioritizes torque application sources based on driver requirements, efficiency considerations, performance considerations, and the like. Torque coordination effectively funnels torque requests through torque availability limits in a priority order. This results in the issuance of torque commands to torque producing devices within the capability of these devices. 
   The embodiment illustrated in  FIG. 7  implements torque coordination at the wheel level for an electric four wheel drive vehicle as depicted schematically in FIG.  5 . Electric motor  442  drives front axle  432  and internal combustion engine  436  provides torque through transmission  440  to rear axle  434 . 
   A wheel level torque coordinator, shown generally by  480 , accepts arbitrated torque request  482 . Wheel level torque coordinator  480  may accept additional torque requests as well. In the embodiment shown, requests include 4×4 torque request  484  for regulating axle speeds and generator torque request  486  from energy management controller  154 . Selector  488  passes inverted 4×4 torque request  484  as auxiliary torque request  490  if 4×4 torque request  484  is non-zero. Otherwise, selector  488  passes generator torque request  486  as auxiliary torque request  490 . 
   Auxiliary torque request  490  is added to arbitrated torque request  482  in summer  492  to produce summed torque  494 . Since auxiliary torque request  490  is either the negative of 4×4 torque request  484  or generator torque request  486 , which can be a negative requested torque, summed torque request  494  may be less than arbitrated torque request  482 . 
   Engine maximum torque limit  496  and engine minimum torque limit  498  provide inputs to engine torque limiter  500 . Engine torque limiter  500  outputs initial coordinated torque request  502  as summed torque request  494  limited by engine maximum torque limit  496  and engine minimum torque limit  498 . Differencer  504  subtracts initial coordinated torque request  502  from arbitrated torque request  482  to produce first excess requested torque  506 . First excess requested torque  506  represents requested torque in excess of the capability of engine  436 . 
   Motor torque limiter  508  accepts motor maximum torque limit  510  and motor minimum torque limit  512  representing torque limits for electric motor  442 . Motor torque limiter  508  outputs front axle torque request  514  as first excess requested torque  506  limited by motor maximum torque limit  510  and motor minimum torque limit  512 . Differencer  516  subtracts front axle torque request  514  from first excess requested torque  506  to produce second excess requested torque  518 . Second excess requested torque  518  indicates requested torque which cannot be handled by electric motor  442 . 
   Summer  520  adds initial coordinated torque request  502  and second excess requested torque  518  to produce coordinated torque request  522 . Rear torque limiter  524  generates rear axle torque request  526  by limiting coordinated torque request  522  with engine maximum torque limit  496  and engine minimum torque limit  498 . 
   Wheel level torque coordinator  480  may be used to implement a wide variety of torque coordinating functions. For example, power assist is provided whenever powertrain wheel torque requests, as represented by arbitrated torque request  482 , exceed the torque availability estimated for engine  436  at the wheels. The excess request will be directed to traction motor  442  through front axle torque request  514 . 
   Another function is 4×4 balancing. 4×4 torque request  484  represents the need to regulate to zero the difference in speeds between front axle  432  and rear axle  434 . In this situation, arbitrated torque request  482  is subtracted from the engine torque request and added to the motor torque request. Effectively, the request for engine torque is reduced by 4×4 torque request  484  and the request to front axle traction motor  442  is increased by 4×4 torque request  484 . This redistributes torque between the axles for better vehicle traction without the need for driver intervention. 
   Another function is charging through the road. In the absence of a 4×4 request and in the event of a low state of charge on the high voltage battery, traction motor  442  can be used to charge the battery. This is accomplished by increasing the torque request to engine  436  and subtracting this increase from the torque requested to traction motor  442 . This effectively requests motor  442  to apply negative torque. This negative torque converts traction motor  442  into a generator for charging the battery. 
   Yet another function is regenerative braking. During a braking maneuver, powertrain wheel torque request  482  will have a negative sign. After subtracting the effect of engine compression braking at the wheels, if any, the remainder of the powertrain request is sent to electric motor  442 . Electric motor  442  applies negative torque within its torque availability and within the state of battery charge. Remaining braking torque may be provided by foundation brakes. 
   Still another function is bleed through the road. In the event of a very high battery state of charge, battery energy may be depleted to create room for future regenerative events by using motive torque from motor  442  in parallel with engine  436 . The energy management function sends a negative torque request as generator torque request  486 . This negative request effectively reduces the torque command to engine  436  and increases the torque command to motor  442 , thus using excess battery energy. 
   Wheel level torque coordinator  480  may be used to calculate powertrain braking torque requests. Rear axle torque request  526  is multiplied by vehicle rolling direction  530  in multiplier  532 . Vehicle rolling direction  530  has a value of 1.0 if vehicle  430  is traveling in a forward direction and a value of −1.0 if vehicle  430  is traveling in a reverse direction. Rear powertrain brake torque request  534  is the output of multiplier  532  if this output is less than zero and is zero otherwise. Similarly, front axle torque request  514  is multiplied by vehicle rolling direction  530  in multiplier  536 . Front powertrain brake torque request  538  is the output of multiplier  536  if this output is less than zero and is zero otherwise. 
   Torque limits within wheel level torque coordinator  480  may each be based on one or, more torque limitation inputs. In the embodiment shown, engine maximum torque limit  496  is the minimum of wheel level maximum engine torque capability  540  and rear axle maximum torque  542 . Engine minimum torque limit  498  is the maximum of wheel level minimum engine torque capability  544  and rear axle minimum torque  546 . Motor maximum torque limit  510  is the minimum of wheel level maximum motor torque capability  548  and front axle maximum torque  550 . Motor minimum torque limit  512  is the maximum of wheel level minimum motor torque capability  552  and front axle minimum torque  554 . 
   Referring now to  FIG. 8 , a block diagram illustrating transmission input level base torque coordination according to an embodiment of the present invention is shown. A transmission input level torque coordinator, shown generally by  560 , accepts crankshaft desired base torque  562  and generator requested torque  564 . Crankshaft desired base torque  562  and generator requested torque  564  are added in summer  566  to produce combined requested torque  568 . Limiter  570  produces initial coordinated torque request  572  by limiting combined requested torque  568  with engine maximum torque limit  574  and engine minimum torque limit  576 . 
   Initial coordinated torque request  572  is subtracted from crankshaft desired base torque  562  by differencer  578  to produce first excess requested torque  580 . Limiter  582  generates coordinated motor request  584  by limiting first excess requested torque  580  with motor maximum torque limit  586  and motor minimum torque limit  588 . Coordinated torque request  590  is generated in summer  592  by subtracting coordinated motor request  584  from the sum of initial coordinated torque request  572  and first excess requested torque  580 . Limiter  594  generates coordinated engine base request  596  by limiting coordinated torque request  590  with engine maximum torque limit  574  and engine minimum torque limit  576 . 
   Torque coordination may also include a variety of functions such as power assist, regenerative braking, charging, bleed, and the like. 
   Referring now to  FIG. 9 , a block diagram illustrating fast torque coordination at the transmission input level according to an embodiment of the present invention is shown. In this embodiment, fast torque coordination is selected only for certain types of fast requests. A fast torque coordinator, shown generally by  610 , receives arbitration winner  612  from one or both of wheel level arbitration and transmission input level arbitration. If arbitration winner  612  equals either traction control torque request  614  or transmission torque modulation request  616 , then binary match flag  618  is set. As will be described in greater detail below, binary match flag  618  is a control signal selecting outputs for fast torque coordinator  610 . 
   Actual engine base torque  620  is subtracted from desired fast torque  622  in differencer  624  to produce initial fast torque request  626 . Limiter  628  generates limited fast torque request  630  by limiting initial fast torque request  626  with maximum available motor torque  632  and minimum available motor torque  634 . If binary match flag  618  is not asserted, base intended motor torque  636  is output as motor torque request  638 . If binary match flag  618  is asserted, limited fast torque request  630  is output as motor torque request  638 . 
   Limiter  640  uses motor slew rate  642  to represent the dynamic response of the electric motor for estimating transient motor torque output. This value is subtracted from desired fast torque  622  to produce desired engine fast torque  644 . If binary match flag  618  is asserted, engine torque request  646  is the minimum of desired engine fast torque  644  and engine requested base torque  648 . If binary match flag  618  is not asserted, engine torque request  646  is simply engine requested base torque  648 . 
   Conventional, non-hybrid vehicles with automatic or automated shift manual transmissions have a large degree of interaction between the engine and transmission control systems. One of these interactions is torque modification requested of the engine by the transmission controller prior to and during a shift event. This modulation, typically a torque reduction, improves the quality or feel of the shift and protects the internal transmission components. 
   Typically, the engine controller has several options to achieve the requested torque modulation. Spark timing modification has generally been preferred over air or fuel modulation for a number of reasons. Although air modulation has the benefit of a wide range of authority with respect to torque command, the response time of the engine due to changes in air command are too slow to effectively modify the torque in the time required for the shift. Torque changes due to spark timing modification, on the other hand, are nearly instantaneous due to the direct impact of spark on combustion. Spark control is also preferable to fuel cut out due to the granularity of control associated with the fuel command. This is particularly true for individual cylinder fuel injection where the amount of fuel injected must be kept in proportion to the amount of air in the cylinder. Thus, torque can only be reduced by cutting out individual cylinders completely. This results in limited, discrete levels of torque production that are not sufficient to adequately control torque during shifting. Spark control has the advantage that continuous change in spark angle results in continuous change in the torque produced by the engine. 
   The use of spark angle modification for torque modulation does, however, have several disadvantages. First, the range of torque authority from spark control is limited to only about 30% of the current level of torque being produced. This directly limits the level of reduction that the transmission can request during a shift. Also, since the spark angle is normally commanded as closely as possible to that angle which would produce maximum level of brake torque (MBT) production from the engine, there is no opportunity to provide a torque increase using spark angle. Another problem related to moving the spark angle away from MBT timing for the purpose of torque modulation is that the efficiency of combustion is lower as more fuel is converted to heat rather than used to produce torque. This results in a slight fuel economy degradation for the vehicle. Finally, by moving away from MBT timing, there is an increase in the emissions produced by the engine as less of the fuel is burned in the cylinder. 
   The addition of electric motor  442  to the drive line provides an additional option for achieving torque modulation during shifting. Electric motor  442  provides several advantages over spark timing when used for torque modulation. Given that the normal request from transmission controller  448  is for torque reduction, electric motor  442  may achieve the torque reduction by providing a positive charging current to the battery. Whereas spark modification results in a net energy loss in the system, use of motor  442  results in an energy gain, thereby increasing fuel economy. In addition, motor  442  can be used to provide positive torque increases if requested by the transmission  440 . Such a torque increase is not readily available from typical spark timing control due to the use of MBT spark timing. The availability of positive torque modification potentially results in smoother shifts. Because motor  442  has a response time similar to that of spark timing control, no adverse delay is introduced. 
   For these reasons, it is desirable to use motor  442  for torque modulation whenever possible. There are a few limitations related to motor  442  that must be taken into account. For example, the available torque from motor  442  can be limited by several factors including motor temperature, battery state of charge, motor speed, and the like. In cases where motor torque is limited to less than the requested torque, a combination of spark and motor torque may be used. The torque command for motor  442  is expressed in Equation 1 as follows:
 
τ mot     —     req =min(τ mot     —     avail     —     max , max(τ mot     —     avail     —     min ,(τ desired     —     fast −τ eng     —     base ))),  (1) 
 
where □ mot     —     req  is requested motor torque  638 , □ mot     —     avail     —     min  is the minimum available motor torque  634 , □ desired     —     fast  is the arbitrated, desired torque from all fast requesters  622 , and □ eng     —     base  is a feedback signal from engine controller  444 , represented by estimated base engine torque  620 . To cover the event when motor  442  is used to temporarily increase torque, requested motor torque  638  is also limited by the maximum availability of motor  442 , expressed as □ mot     —     avail     —     max    632 . The corresponding command for engine  436  is expressed in equation 2 as follows:
 
τ eng     —     req     —     fast =τ desired     —     fast −τ mot     —     req ,  (2) 
 
where □ eng     —     req     —     fast  is requested fast engine torque  246  achieved with spark timing control. Under this definition, engine controller  444  commands fast actuators such as spark timing and fuel to meet requested fast engine torque  246 .
 
   Another scenario that can benefit from the present invention is traction control torque reduction. Similar to shift quality function, traction control requires a fast torque response. However, this response can be a more prolonged event depending upon the road surface. The present invention applies for limiting both base and fast torque requests while traction is compromised. In this case, motor torque provides the transient difference between the actual and the requested base engine torque. 
   Referring now to  FIG. 10 , a block diagram illustrating arbitration among base requests at the wheel level according to an embodiment of the present invention is shown. A wheel level arbiter, shown generally by  660 , generates arbitrated, desired wheel force  662  and arbitrated force winner  664  indicating which base request was selected by wheel level arbiter  660 . Wheel level arbiter  660  accepts driver desired wheel force  666  and cruise control desired wheel force  668 . Driver desired wheel force  666  is based on position of the accelerator pedal. Cruise control desired wheel force  668  is requested to maintain vehicle  430  at a constant speed or other set-point. Intermediate desired wheel force  670  is the maximum of driver desired wheel force  666  and cruise control desired wheel force  668 . Arbitrated desired wheel force  662  is the minimum of intermediate desired wheel force  670  and vehicle speed wheel force limit  672 , which is based on vehicle speed limitation. 
   In addition to generating arbitrated demand  662 , wheel level arbiter  660  outputs arbitrated force winner  664  providing an indication as to the source of arbitrated desired wheel force  662 . Torque limit speed control indicator  674  and driver force indicator  676  are integer values indicating speed limiting and driver force, respectively. Arbitrated force winner  664  is set to torque limit speed control indicator  674  either if driver desired wheel force  666  is not greater than cruise control desired wheel force  668  or if vehicle speed wheel force limit  672  is not greater than driver desired wheel force  666 . 
   Referring now to  FIG. 11 , a block diagram illustrating arbitration at the transmission input level according to an embodiment of the present invention is shown. A transmission input level arbiter, shown generally by  690 , generates arbitrated desired transmission input base torque  692 , arbitrated desired transmission input fast torque  694  and arbitrated torque winner  696 . Transmission input level arbiter  690  accepts a variety of inputs. Arbitrated desired transmission input base torque  692  and arbitrated desired transmission input fast torque  694  are translated based on the operation of transmission  440 . Arbitrated force winner  664  indicates the winner of arbitrated requests occurring at wheel level  56 . Fast torque shift limit  702  from transmission controller  448  requests torque limit during shift for better shift quality. Maximum base torque limit  704  and maximum fast torque limit  706  from transmission controller  448  are provided to protect against mechanical damage to transmission  440 . 
   Transmission input level arbitrator  690  generates torque limit signal  708  as a binary control signal asserted when transmission input desired base torque  698  is greater than maximum base torque limit  704 . Intermediate base torque request  710  is the minimum of transmission input desired base torque  698  and maximum base torque limit  704 . Arbitrated desired transmission input base torque  692  is the maximum of intermediate base torque request  710  and fast torque shift limit  702 . First intermediate fast torque request  712  is the minimum of transmission input desired fast torque  700  and maximum fast torque limit  706 . Second intermediate fast torque request  714  is the maximum of first intermediate fast torque request  712  and fast torque shift limit  702 . Arbitrated desired transmission input fast torque  694  is the minimum of second intermediate fast torque request  714  and arbitrated desired transmission input base torque  692 . 
   Arbitrated torque winner  696  provides an integer indicating the source winning arbitration within transmission input level arbiter  690 . Mechanical limit indicator  716  indicates limiting to protect transmission  440  from excessive base torque. Shift torque reduction indicator  718  indicates limiting due to modulation requested by transmission controller  448  during a shift event. Arbitrated torque winner  696  is set to mechanical limit indicator  716  when torque limit signal  708  is asserted. If this is not the case, arbitrated torque winner  696  is set to shift torque reduction indicator  718  if transmission input desired fast torque  700  is greater than maximum fast torque limit  706 . Otherwise, arbitrated force winner  664  is sent as arbitrated torque winner  696 . 
   Referring now to  FIG. 12 , a block diagram illustrating multilevel torque resolution according to an embodiment of the present invention is shown. Torque resolution may be performed at any number of levels. In the generalized representation shown, first level resolver  730  arbitrates and/or coordinates first level torque input requests  732  to produce first level resolved torque requests  734 . First level resolved torque requests  734  are translated by first level translator  736  to produce translated first level torque requests  738 . Second level resolver  740  arbitrates and/or coordinates translated first level torque requests  738  and any second level torque input requests  742  to produce second level resolved torque requests  744 . Second level resolved torque requests  744  are translated by second level translator  746  to produce translated second level torque requests  748 . 
   This process may be repeated to match the architecture of any drive train. Resolved (n−1) st  level torque requests  750  are translated by (n−1) st  translator  752  to produce translated (n−1) level torque requests  754 . An n th  level resolver  756  accepts translated (n−1) st  level torque requests and any n th  level torque input requests  758  to produce n th  level resolved torque requests. At any level, torque input requests  732 ,  742 ,  758  may be generated by torque requestors operating on that level and/or from torque requests translated from another level. 
   Various multilevel systems are possible. For example, a planetary gear set can have a different level for each of the sun gear, the planet gear carrier and the annulus rotations. 
   Another example is a three level system including a transmission input level, a differential input level and a wheel level. An engine and/or motor operates at the transmission input level. An electric motor is coupled to the drive shaft at the differential input. One or more additional motors or other torque producing devices operate at the wheel level. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. For example, the present invention may be applied to nonautomotive systems. It should be understood that the words used in the specification are words of description rather than limitation and that various changes may be made without departing from the spirit and scope of the invention.