Abstract:
A vehicle has multiple electrical propulsion systems. Torque is split among the various propulsion systems to meet a variety of requirements and limitations including operator demand, vehicular traction limits, electric machine torque capabilities, machine efficiency operating points and available electrical power.

Description:
TECHNICAL FIELD 
     The present invention is related to electric propulsion vehicles. More particularly, the invention is concerned with optimal torque distribution among multiple propulsion systems in accordance with various constraints. 
     BACKGROUND OF THE INVENTION 
     Vehicles having multiple propulsion systems are characterized by two or more independently operable sources of tractive torque. Most typically, one propulsion system is associated with driving a front set of tires with a first electric machine and another propulsion system is associated with driving a back set of tires with a second electric machine. However, systems are known having electric machines associated with each of the tires of a vehicle. Vehicle dynamics control has been an area of substantial focus in such multiple propulsion system vehicles. Efficiency improvements in such vehicles have mostly come from improved electric machine designs, power electronics improvements (including improved power controls), power sources and power distribution control in systems having multiple power sources. 
     SUMMARY OF THE INVENTION 
     The present invention approaches system efficiency optimization of a multiple propulsion system vehicle in real-time considering operator demands, vehicle dynamics and propulsion system capabilities. Torque is distributed among the various ones of the multiple propulsion systems to meet the operator demands, tractive capabilities, machine torque capabilities, minimal overall electric machine power loss and electrical power availability. 
     A vehicle includes a source of electrical power and multiple electrical propulsion systems. A control method for distributing tractive torque among the multiple propulsion systems includes providing a tractive torque request which may be based on an operator demand and may be limited by traction limits at the tire and road surface. Complementary tractive torques for the propulsion systems operative to effect the tractive torque request are determined. The complementary tractive torques may be determined in accordance with electrical machine torque limitations. The complementary tractive torques meeting predetermined efficiency criteria are selected for use in controlling the propulsion systems in accordance with said selected set of complementary tractive torques. Preferably, the efficiency criteria include the lowest combined machine power loss. The complementary selected tractive torques further may be scaled in accordance with available electrical power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an exemplary dual-propulsion system vehicle including front and rear propulsion systems in accordance with the present invention; and 
         FIG. 2  is a block diagram representing various exemplary control tasks for implementing torque distribution control in accordance with the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Beginning with reference to  FIG. 1 , an exemplary dual-propulsion system vehicle  10  includes a front electric propulsion system  13  and a rear electric propulsion system  11 . The front propulsion system includes multiphase AC electric machine MF  15  mechanically coupled to final drive train DF  17  which may include such common elements as reduction and differential gearing. Final drive train  17  is mechanically coupled to traction wheels  19 ,  21 . The front propulsion system is operative to provide propulsion torque at the traction wheels via electric machine MF  17  motor operation or to provide braking torque at the traction wheels via electric machine MF  17  generator operation. Similarly, the rear propulsion system includes multiphase AC electric machine MR  23  mechanically coupled to final drive train DR  25  which may include such common elements as reduction and differential gearing. Final drive train  25  is mechanically coupled to traction wheels  27 ,  29 . The rear propulsion system is operative to provide propulsion torque at the traction wheels via electric machine MR  23  motor operation or to provide braking torque at the traction wheels (i.e. regenerative braking) via electric machine MR  17  generator operation. Other electric machines, for example the brushless DC variety, may serve as the electric machines in the present system. 
     Each electric machine has associated therewith a respective machine controller  31 ,  33 . Machine controllers include power inverter electronics and respective motor controllers configured to receive motor control commands and control inverter states therefrom for providing motor or generator functionality. Motor controllers are microprocessor based controllers comprising such common elements as microprocessor, read only memory ROM, random access memory RAM, electrically programmable read only memory EPROM, high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O) and appropriate signal conditioning and buffer circuitry. In motoring control, the respective inverter receives current from the DC bus  39  and provides AC current to the respective motor over phase lines  35  or  37 . In generator control, the respective inverter receives AC current from the corresponding motor over phase lines  35  or  37  and provides current to the DC bus  39 . 
     DC bus  39  is coupled to at least one source of electrical potential including, for example, batteries and fuel cells. While as single common bus source for electrical potential is illustrated, independent electrical sources of electrical potential may be employed for sourcing the front and rear propulsion systems. 
     Each motor controller  31 ,  33  communicates with system controller  43 , for example via controller area network (CAN) bus  41 . The CAN bus  41  allows for communication of control parameters and commands between the system controller, motor controllers and other controllers (not shown) such as antilock brake and traction controllers. System controller is microprocessor based comprising such common elements as microprocessor, read only memory ROM, random access memory RAM, electrically programmable read only memory EPROM, high speed clock, analog to digital (A/D) and digital to analog (D/A) circuitry, and input/output circuitry and devices (I/O) and appropriate signal conditioning and buffer circuitry. System controller  43  monitors various parameters for use in the control functions, including operator inputs such as brake effort BR and throttle request TH. Other system parameters including bus voltage V and front and rear electric machine speeds NF and NR, respectively, are also monitored by controller  43 . 
     Having thus described an exemplary environment for implementing the control of the present invention, attention is now turned to  FIG. 2  wherein a block diagram illustrates exemplary steps  100  used in the determination of optimum torque distribution between front and rear electrical propulsion systems. A variety of inputs are monitored from which a measure of desired longitudinal force  50  is derived. For example, vehicle operator inputs including throttle TH and brake BR operation provide key indicia of the desired longitudinal force and the propulsion or braking characteristic thereof. Vehicle speed and gear selection for effecting speed ratio, range and directional settings of the vehicle (e.g. P, R, N, OD, D, 2, 1) are also monitored and provide additional indicia of the desired longitudinal force. Vehicle speed may be derived, for example, from one or more conventional wheel rotation sensors (not shown) or rotation sensing of other driveline members including electric machine rotation. Electric machine rotation may be derived via conventional hardware sensing techniques or through inverter phase information in the case of multi-phase AC machines. 
     At block  101 , front and rear wheel sets tractive torque limits  51 ,  53  are independently determined, preferably in accordance with dynamic vehicle and road surface conditions such as vehicle speed, net vehicular acceleration (accounting for speed and directional changes), and road condition limits. 
     At block  103 , front and rear limitations are combined to provide overall tractive torque limits. Wheel slip at the road surface occurs at the tractive torque limits of the wheels and corresponds substantially to limitations on the longitudinal forces effectively applicable to the vehicle at the wheels. Front and rear tractive torque limits therefore may limit the actual longitudinal forces that may effectively be applied at the wheel to road interface. Therefore, the requested tractive torque  55  output from this block is the tractive torque corresponding to the desired longitudinal force as may be attenuated by tractive torque limits. 
     Achieving the requested tractive torque  55  can be with a variety of torque splits between front and rear wheels within the tractive limits established. Another set of limits related to the front and rear electric propulsion systems can affect the tractive torque that may effectively be applied to the vehicle wheels. Therefore, at block  105 , electric machine torque limits  57 ,  59  are determined in accordance with various known propulsion system parameters. In the illustrated embodiment, electric machine speeds NF, NF and bus voltage V are used to reference characteristic minimum and maximum machine torque data for the front and rear propulsion system torque limits  57 ,  59 . Additional reference variables, for example machine temperature, may provide further refinement to the characteristic machine torque data. Such characteristic machine torque data may be empirically derived from conventional dynamometer testing of the combined electric machine and power electronics and reduced to pre-stored table format. 
     At block  107 , a plurality of torque vectors for the front and rear propulsion systems that are within the front and rear machine torque limits  57 ,  59  and requested tractive torque  55  are synthesized for evaluation in accordance with efficiency considerations of the propulsion system. From the propulsion system torque vectors and the requested tractive torque  55  are derived a plurality of complementary propulsion torque vectors for the front and rear machines. Block  107  outputs complementary pairs of front and rear torque vectors  60 ,  61  effective to establish the requested tractive torque  55 . 
     In the illustrated embodiment, electric machine speeds NF, NR and bus voltage V are used to reference characteristic power loss data (Ploss) for the front and rear propulsion systems for complementary pairs of torque vectors at block  109 . Such characteristic loss data may be empirically derived from conventional dynamometer testing of the combined electric machine and power electronics and reduced to pre-stored table format. The loss data for the front and rear complementary torque vector pairs  63 ,  65  are aggregated and the torque vector pair associated with the smallest aggregate loss is selected as the preferred operational torque vector complement  67 ,  69  for the front and rear propulsion systems of the vehicle. 
     The electrical power limits  71  of the vehicle are next compared with the torque vector pairs  67 ,  69  to determine whether the preferred torque vector complement can be effected within the electrical constraints. If necessary, the preferred torque vector complement is scaled to comply with the available electric power and a scaled torque vector pair  73 ,  75  is provided for use in commanding the front and rear electric machines. 
     The present invention has been described with respect to certain preferred embodiments. However, these embodiments are intended as non-limiting examples of the invention, it being recognized that alternative implementations are within the scope of the invention. For example, while front and rear propulsion systems have been described, each wheel of a vehicle may have its own associated electric machine for practicing the present invention. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the fill scope permitted by the language of the following claims.