Abstract:
Several control methods are presented for application in a hybrid electric vehicle powertrain including in various embodiments an engine, a motor/generator, a transmission coupled at an input thereof to receive torque from the engine and the motor generator coupled to augment torque provided by the engine, an energy storage device coupled to receive energy from and provide energy to the motor/generator, an engine controller (EEC) coupled to control the engine, a transmission controller (TCM) coupled to control the transmission and a vehicle system controller (VSC) adapted to control the powertrain.

Description:
This invention was made with Government support under Prime Contract No. DE-AC3683CH10093, Subcontract No. ZCB-4-13032-02, awarded by the Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to control systems and methods for hybrid electric vehicles. 
     2. Description of the Related Art 
     In a hybrid electric vehicle, a plurality of torque sources are available. Typically, such sources include an engine such as an internal combustion engine, and an electric machine. In one general topology of hybrid electric vehicles, the electric machine is a motor/generator interposed between the engine and the vehicle&#39;s transmission. The motor/generator can add torque to supplement the torque provided by engine. Further, the motor/generator can act as a generator in order to convert excess engine torque into electric energy for storage in a storage device such as a battery. This function as a generator can be in furtherance of the driver&#39;s request for the vehicle to decelerate (i.e., regenerative braking). The motor/generator can also act as a generator in a manner transparent to the driver of the vehicle, in order to assure that the battery maintains a reasonable state of charge. 
     Coordinated control of the engine, motor/generator and transmission is, of course, paramount for excellent vehicle performance. In one possible control partitioning, three controllers can be provided: an electronic engine controller (EEC), a transmission controller (TCM) and a vehicle system controller (VSC). In such a partitioning, the EEC would provide generally traditional engine control functions. The TCM would also provide generally traditional transmission control functions. The VSC would take accelerator position, vehicle speed, battery state of charge and other variables into consideration and partition a driver-commanded torque (as expressed primarily by accelerator position) into a desired motor/generator torque and a desired engine torque. 
     The driver-commanded torque would be provided from the VSC to both the EEC and the TCM. Because the TCM needs actual torque at its input for its control as well, the VSC provides a signal reflecting the sum of actual motor/generator torque (which the VSC knows because it performs control of the motor/generator) and actual measured engine torque or estimated engine torque (estimated within the VSC). When a transmission shift is impending or underway, the TCM provides a commanded transmission input torque to the VSC, which acts to cause the torque at the input to the transmission to conform to the command. This commanded torque allows the TCM to perform its shift as appropriate. 
     There are several concerns with the aforementioned control method. First, it requires the VSC to be able to anticipate dynamic effects, such as, for example, the effects of dynamic fueling strategy and manifold air flow, on actual engine torque, whereas in fact the EEC is the controller having the best knowledge of that information. This may be addressed in two ways, neither entirely satisfactory: 1) an engine torque sensor may be included in the system, or 2) the engine torque may be estimated within the EEC which has far better access to all engine control, sensor and calibration variables, as well as the control strategy itself. The first is an unusual and expensive solution, while the second will introduce a time delay during which the engine torque is computed in the EEC, passed to the VSC for combination with the starter/alternator torque and then passed to the TCM. This time delay will result in poor transmission shift quality. These specific issues derive from the genesis of this hierarchical control in a parallel HEV where the engine and electric machine each contribute substantial torque to propel the vehicle either separately or in combination. In such a case, it is absolutely essential that the VSC intervene completely in the connection between driver demand and the actual engine control inputs. In another type of HEV, the so-called “low storage requirement” or LSR hybrid having the topology described above, the torque contribution of the motor/generator is very small compared to that of the engine, and the vehicle has no electric-only propulsion mode at all. Thus the LSR system can take great advantage of the familiarity and proven base of control methodology of the more accustomed direct driver control of the engine with less severe intervention by the VSC. Such a strategy also allows use of non-hybrid powertrain components and controls with minimal modification. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above-described drawbacks of alternative control methods for hybrid electric vehicles. 
     The present invention provides several control methods adapted for application in a hybrid electric vehicle powertrain including in various embodiments an engine, a motor/generator, a transmission coupled at an input thereof to receive torque from the engine and the motor generator coupled to augment torque provided by the engine, an energy storage device coupled to receive energy from and provide energy to the motor/generator, an engine controller (EEC) coupled to control the engine, a transmission controller (TCM) coupled to control the transmission and a vehicle system controller (VSC) adapted to control the powertrain. 
     In one embodiment, the present invention provides a method for controlling the powertrain comprising: (a) providing a signal from the EEC to the TCM to reflect a sum of an actual or estimated electric machine torque and an actual or estimated engine output torque; (b) providing a signal from the TCM to the EEC to reflect a TCM-commanded transmission input torque; and (c) partitioning the TCM-commanded transmission input torque signal into a TCM-commanded engine torque signal and a TCM-commanded electric machine torque signal. 
     In a second embodiment, the present invention provides a method for controlling the powertrain comprising: (a) providing an accelerator position signal to the VSC; (b) in the VSC, calculating a first desired electric machine torque to reflect a driver-commanded boost or regenerative torque, the first desired electric machine torque being a function of the accelerator position signal; (c) providing a modified accelerator position signal from the VSC to the EEC, the modified accelerator position signal reaching 100% before the first accelerator position signal; and (d) controlling output torque of the engine at least partly as a function of the modified accelerator position signal. 
     In a third embodiment, the present invention provides a method for controlling the powertrain comprising: (a) providing an accelerator position signal to the VSC; (b) in the VSC, calculating a first desired electric machine torque to reflect a driver-commanded boost or regenerative torque, the first desired electric machine torque being a function of the accelerator position signal; (c) controlling output torque of the engine at least partly as a function of the accelerator position signal; (d) in the VSC, calculating a second desired electric machine torque to reflect a battery-charge-maintenance torque; (e) controlling the electric machine in view of the first desired electric machine torque and the second desired electric machine torque; (f) providing a first signal from the VSC to the EEC to reflect a driver-commanded boost or regenerative torque of the electric machine; and (g) providing a second signal from the VSC to the EEC to reflect a battery-charge maintenance torque of the electric machine. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the powertrain of a hybrid electric vehicle according to one embodiment of the present invention. 
     FIG. 2 is a block diagram illustrating in more detail the control functions performed by vehicle system controller (VSC)  50 , engine controller (EEC)  52  and transmission control module (TCM)  54  of FIG. 1, and the signals passed among those controllers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Refer first to FIG.  1 . Illustrated there is the “topology” of a hybrid electric vehicle powertrain according to one embodiment of the present invention. An engine  20 , such as an internal combustion engine, is coupled at its crankshaft  22  to a motor/generator such as starter/alternator  24 . Starter/alternator  24  is preferably, though not necessarily, a polyphase induction machine. Starter/alternator  24  is coupled to input  26  of a transmission  28 . In this embodiment, transmission  28  is an “automatic shift manual (ASM)” transmission/transaxle, though other transmission configurations can be readily substituted as well. Wheel shafts  30  and  32  couple torque from transmission  28  to drive wheels  34  and  36  of the vehicle. Depending upon the exact functionality required of the powertrain, clutches can be provided between engine  20  and starter/alternator  24 , and/or between starter/alternator  24  and transmission  28 . 
     A vehicle system controller (VSC)  50  receives inputs from the vehicle driver and regarding vehicle operation, such as (for example) accelerator pedal position (ACC), brake pedal force (BRK), vehicle speed (VSS) and battery state of charge (SOC). The VSC  50 &#39;s processing of those signals will be discussed in detail below. VSC  50  is preferably a microprocessor-based module having appropriate microcomputer resources (throughput, memory, inputs, outputs and the like) to perform the functions ascribed to it in this disclosure. In this embodiment of the present invention, VSC  50  also preferably includes the semiconductor switches to perform the inverter function of converting DC power stored in battery  51  to AC power for use when starter/alternator  24  acts as a motor, and for rectifying the AC power generated by starter/alternator  24  for storage in battery  51 . However, the inverter can also be a module separate from VSC  50 . 
     An electronic engine controller (EEC)  52  provides traditional control functions for engine  20 , including fuel injection control. If engine  20  is a spark ignition engine, EEC  52  can also provide control for an electronic throttle. A transmission control module (TCM)  54  provides control for transmission  28 . EEC  52  and TCM  54  are preferably microprocessor based devices. 
     VSC  50  is coupled to starter/alternator  24  by a plurality of circuits collectively referred to with reference numeral  60 . EEC  52  is similarly coupled to engine  20  by a plurality of circuits collectively referred to with reference numeral  62 . Also, TCM  54  is coupled to transmission  28  by a plurality of circuits collectively referred to with reference numeral  64 . Circuits  60 ,  62  and  64  are selected to be appropriate for control of the various powertrain components as known to those skilled in the art and as further described below. 
     VSC  50 , EEC  52  and TCM  54  are also coupled by a high speed data link  70 , such as a Controller Area Network (CAN) data link. 
     Refer now additionally to FIG. 2, where the functions of VSC  50 , EEC  52  and TCM  54  and the signal flow among them are illustrated in more detail. VSC reads vehicle speed (VSS) and accelerator position (ACC) signals, and at functional block  100 , generates signals TQ_NET_DES and TP_PCT. TQ_NET_DES indicates the target torque to be provided by the combination of engine  20  and starter/alternator  24 . TQ_NET_DES is a function of ACC (i.e., the driver&#39;s “expressed” command for torque) and vehicle speed VSS. The accelerator position signal ACC, expressed as a percentage of full throttle, is passed on from block  100  as signal TP_PCT. TP_PCT is also passed on to TCM  54  via data link  70 . 
     At functional block  102 , VSC  50  partitions TQ_NET_DES. VSC  50  determines, based on state of charge (SOC) of battery  51 , whether starter/alternator  24  needs to either generate electrical power to charge battery  51  (if the SOC of battery  51  is below an acceptable threshold), or needs to bleed charge from battery  51  (if the SOC of battery  51  is above an acceptable threshold). The torque command for these “battery maintenance” functions is provided as signal TQ_SA_BC to EEC  52  via data link  70 . 
     At block  102 , VSC  50  also generates a boost torque signal TQ_SA_BB. This signal represents boost (or conversely, drag) torque to be applied by starter/alternator  24  as a result of the driver&#39;s command as expressed by accelerator position signal ACC or brake pedal force BRK. This signal is generated at block  102  through very general a priori knowledge of the torque generating characteristics of engine  20 . If the driver is demanding more torque than engine  20  can generate, boost torque via signal TQ_SA_BB will be indicated (assuming the SOC of battery  51  is appropriately high to provide energy for the boost torque, and subject to limiting in view of signal TQ_SA_SSM, as will be discussed below). 
     The sum of the two starter/alternator torque signals TQ_SA_BB (subject to limiting in view of signal TQ_SA_SSM) and TQ_SA_BC is provided by functional block  102  as a desired starter/alternator torque signal TQ_SA_DES to functional block  104 . Functional block  104  provides control for starter/alternator  24 , such that starter/alternator  24  provides the torque indicated by TQ_SA_DES. Such control can be in accordance of any of numerous conventional electric machine control methods known in the art. From functional block  104 , an actual starter/alternator torque TQ_SA_ACT is provided to functional block  102 . TQ_SA_ACT is provided to serve several purposes. First, temperature and/or voltage limitations in the motor or inverter may unexpectedly limit available torque. Second, there may be a small delay in generating the desired torque. Third, TQ_SA_ACT is used to compute TQ_SA_BB for use by VSC  50 . 
     A signal TP_PCT* is also provided from VSC  50  to EEC  52 . The value of TP_PCT* is preferably an augmented version of TP_PCT, increased to reach 100% before TP_PCT reaches 100%. Thus, the accelerator position signal to EEC  52  will reach its maximum value before the actual accelerator position signal reaches 100%. The remainder of the range for TP_PCT is available to indicate the driver&#39;s desire for starter/alternator boost. Alternatively, TP_PCT* can be equal to TP_PCT. Also alternatively, TP_PCT* can instead be a direct torque command for engine  20 , as opposed to a signal in units of accelerator pedal position. 
     Within EEC  52 , TP_PCT* is provided to functional block  110 , where traditional fueling control is performed based at least in part on throttle position, including any limits or filters which may typically be applied to assure reasonable driveability and emissions. The output of block  110  is provided to functional block  112 , where a fuel correction factor is added (or subtracted) based on the battery maintenance torque TQ_SA_BC. The fuel correction makes the battery maintenance torque transparent to the driver of the vehicle. The output of block  112  is provided to an arbiter block  116 , as will be discussed below. 
     The output of block  110  is also provided to block  114 , which contains a map of fuel to output torque of engine  20 . The output of block  114  is summed with TQ_SA_BB at summing block  118  in order to generate a signal TQ_NET_MBT. This signal, which represents a target torque to be provided at the input to transmission  28 , is provided to TCM  54  via data link  70 . 
     If a transmission shift is impending or in progress, TCM  54  requires control of torque at its input in order to facilitate that shift. Therefore, TCM  54  provides a signal TQ_DES_SSM, which represents input torque to transmission  28  commanded by TCM  54 , to EEC  52 . (TCM  54  may also send a flag, TQ_REQ, to VSC  50  and EEC  52  to indicate that TCM  54  desires to take control of the torque.) At functional block  120 , EEC partitions the commanded torque TQ_DES_SSM into an engine  20  torque, TQ_IC_SSM, and a starter/alternator  24  torque, TQ_SA_SSM. In order to allow engine  20  to run as much in a steady state condition as possible, functional block  120  will preferably partition the torques such that the torque of starter/alternator  24  is reduced first and added back in last. 
     At functional block  122 , a fueling map to convert torque command TQ_IC_SSM into a fueling command is applied. The fueling commands from blocks  112  and  122  are arbitrated at block  116 . If flag TQ_REQ indicates that TCM  54  has assumed torque control, arbiter block  116  will pass the fuel command from block  122 . If flag FQ_OFF indicates that engine  20  is to be stopped, arbitration block  116  commands no fuel. The output of arbitration block  116  is a fuel command FQ_COM, which is provided to a conventional fuel injection control system, generically shown at functional block  124 . 
     FQ_COM, the actual fuel command provided to engine  20  (and therefore a measure of torque being produced by engine  20 ), is provided to functional block  130 , which estimates the output torque of engine  20  based upon the actual fuel command FQ_COM and other engine parameters. This estimate is preferably based on a map stored a priori in the memory of EEC  52 . The output of block  130 , estimated engine torque, is combined with TQ_SA_BB at summing block  132 , in order to yield a signal TQ_BRAKE_S. TQ_BRAKE_S, a measure of the actual torque being provided to the input of transmission  28 , is provided via data link  70  to TCM  54  for use in TCM  54 &#39;s control of transmission  28 . The control by TCM  54  of transmission  28  is according to conventional control methods known to those skilled in the art. 
     Signal TQ_SA_SSM, the starter/alternator component of the torque command from TCM  54 , is provided from EEC  52  to VSC  50 . TQ_SA_SSM is used by torque partitioning block  102  in determining TQ_SA_BB, the driver-desired boost torque and, consequently, TQ_SA_DES, the total commanded torque to which starter/alternator  24  is controlled by functional block  104 . In general, if a torque is commanded via TQ_SA_SSM, this torque will override other considerations in determining TQ_SA_BB and, consequently, TQ_SA_DES. 
     Signal FQ_OFF is provided by VSC  50  to EEC  52  if it is desired for engine  20  to be stopped. This is advantageous for fuel savings in conditions where the vehicle would otherwise be idling. Signal FQ_OFF causes EEC  52  to cut off fuel from engine  20 . Using starter/alternator  24 , engine  20  can be restarted very quickly upon demand for power from ACC. 
     Control partitioning according to this embodiment of the present invention has the following advantages. First, VSC  50  is not required to know in detail the dynamic torque-producing characteristics of engine  20 . VSC  50  needs only very general knowledge of these characteristics in order to perform torque partitioning at functional block  102 . Any inaccuracies will have relatively limited effect, as the starter/alternator in a “low storage requirement” hybrid electric vehicle contributes much less torque than does the engine. Further, because the driver&#39;s command for vehicle torque is provided to EEC  52  by signal TP_PCT*, the driver can correct for any errors which occur in torque partitioning (that is, the driver remains “in the loop” of torque partitioning in the system). 
     A further advantage of this embodiment of the present invention is that the measure of actual transmission input torque, signal TQ_BRAKE_S, is provided to TCM  54  by EEC  52  rather than by VSC  50 . If provided by VSC  50 , as discussed in the Background section above, part of the data for actual transmission input torque (specifically, the component provided by the engine) is delayed in reaching TCM  54 . This can result in inadequate shift quality. In this embodiment of the present invention, there is a delay in VSC  50 &#39;s responding to TCM  54 &#39;s torque command TQ_SA_SSM, because that signal is routed through EEC  52 . However, because the contribution of starter/alternator torque to total powertrain torque is relatively small and starter/alternator  24  responds much more quickly to its torque command than does engine  20 , this delay will not have a large effect on shift quality. 
     A further advantage of this embodiment of the present invention is in aiding the vehicle development process. VSC  50  does not need detailed engine control information. Further, TCM  54  operates as would a transmission controller in a non-hybrid vehicle, with the same inputs and outputs. Therefore, the algorithm development for VSC  50 , TCM  54  and EEC  52  can proceed with relatively little interaction. This will lower costs and accelerate development by sharing both hardware and control with conventional (i.e., non-hybrid) powertrain systems. 
     Various other modifications and variations will no doubt occur to those skilled in the arts to which this invention pertains. Such variations which generally rely on the teachings through which this disclosure has advanced the art are properly considered within the scope of this invention. This disclosure should thus be considered illustrative, not limiting; the scope of the invention is instead defined by the following claims.