Abstract:
The present invention controls torque of a hybrid vehicle that calculates power and torque of each motor when the hybrid vehicle provided with two motors operates at a transient state are used. More specifically, target power of a battery is determined. Then calculations are performed to determine target torque of the first motor, target torque of the second motor, target torque of an engine, and target speed of the engine at a steady state. The torque of the first motor at a transient state is calculated from the target torque of the second motor at the steady state and speeds of the first and second motors. Finally, torque of the second motor at the transient state is calculated from the torque of the first motor at the transient state and the speeds of the first and second motors.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0123450 filed in the Korean Intellectual Property Office on Dec. 6, 2010, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a method and a system for controlling torque of a hybrid vehicle. More particularly, the present invention relates to a method and a system for controlling torque of a hybrid vehicle that calculates power and torque of each motor when the hybrid vehicle provided with two motors operates at a transient state. 
     (b) Description of the Related Art 
     Generally, a hybrid vehicle is a vehicle which uses an engine and a motor as power source. The hybrid vehicle may be provided with one motor and one engine, but the hybrid vehicle provided with two motors and one engine is increasingly more popular in today&#39;s market. In this case, a first motor is used for controlling engine speed and a second motor is used for compensating engine torque according to the engine speed controlled by the first motor and generating demand torque. 
     A control portion of the hybrid vehicle determines target driving points of the engine and the first and second motors by using vehicle&#39;s speed, the demand torque, and the state of charge (SOC). However, such target driving points of the engine and each motor are determined under the assumption that the hybrid vehicle operates at a steady state. Therefore, when a hybrid vehicle operates at a transient state, actual driving points of the engine and each motor differs from the target driving points and logic for determining driving points of the engine and each motor at the transient state may be necessary. 
     In order to determine the driving points of each motor at the transient state, a priority order is necessary. That is, if the driving points of two motors at the transient state are simultaneously determined, a large error may occur. Therefore, after the driving point of one motor is determined, the driving point of the other motor is determined by using the driving point of just the one motor. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide a method and a system for controlling torque of a hybrid vehicle having advantages of determining power and torque of each motor at a transient state. 
     Particularly, a method and a system for controlling torque of a hybrid vehicle in which power and torque of a first motor for controlling engine speed is determined, and power and torque of a second motor for compensating engine torque and generating demand torque is determined by using the power and the torque of the first motor. 
     A method for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention may determine torque and power of each motor in the hybrid vehicle provided by a first motor controlling engine speed and a second motor for compensating engine torque and generating demand torque. The method may include: determining target power of a battery based on vehicle speed, the demand torque, and SOC; calculating target torque of the first motor, target torque of the second motor, target torque of an engine, and target speed of the engine at a steady state based on the vehicle speed, the demand torque, and the target power of the battery; calculating torque of the first motor at a transient state from the target torque of the second motor at to the steady state and speeds of the first and second motors; and calculating torque of the second motor at the transient state from the torque of the first motor at the transient state and the speeds of the first and second motors. 
     Calculation of the torque of the first motor at the transient state may include: calculating the target power of the second motor at the steady state from the target torque of the second motor at the steady state and the speed of the second motor; calculating maximum power of the first motor at the transient state from the target power of the second motor at the steady state and power limit of a power source; and calculating maximum torque of the first motor at the transient state from the maximum power of the first motor at the transient state and the speed of the first motor. 
     The calculation of the torque of the first motor at the transient state may further include: calculating the target torque of the first motor at the transient state based on the target speed of the engine at the steady state and the speeds of the first and second motors; and determining the torque of the first motor at the transient state by comparing the maximum torque of the first motor and the target torque of the first motor at the transient state. 
     Calculation of the torque of the second motor at the transient state may include: calculating power of the first motor at the transient state from the torque of the first motor at the transient state and the speed of the first motor; calculating maximum power of the second motor at the transient state from the power of the first motor at the transient state and the power limit of the power source; and calculating maximum torque of the second motor at the transient state from the maximum power of the second motor at the transient state and the speed of the second motor. 
     The calculation of the torque of the second motor at the transient state may further include: calculating target torque of the second motor at the transient state based on the demand torque and the torque of the first motor at the transient state; and determining the torque of the second motor at the transient state by comparing the maximum torque of the second motor and the target torque of the second motor at the transient state. The power of the first motor at the transient state may be filtered so as to calculate the maximum power of the second motor at the transient state. 
     A system for controlling torque of a hybrid vehicle according to the exemplary embodiment of the present invention may include: an engine; a first motor for controlling engine speed; a second motor compensating engine torque and generating demand torque; and a control portion controlling the engine, the first motor, and the second motor. In particular, the control portion calculates torque of the first motor at a transient state by using target torque of the second motor at a steady state, and calculates torque of the second motor at the transient state by using the torque of the first motor at the transient state. 
     The control portion may calculate target power of the second motor at the steady state by using the target torque of the second motor at the steady state and speed of the second motor. The maximum power of the first motor is then calculated at the transient state by using the target power of the second motor at the steady state and power limit of a power source. Further, the maximum torque of the first motor is calculated at the transient state by using the maximum power of the first motor at the transient state and speed of the first motor. 
     The control portion may calculate target torque of the first motor at the transient state based on target speed of the engine at the steady state and the speeds of the first and second motors, and may determine the torque of the first motor at the transient state to by comparing the maximum torque of the first motor at the transient state with the target torque of the first motor. 
     The control portion may also calculate power of the first motor at the transient state by using the torque of the first motor at the transient state and the speed of the first motor. Then the maximum power of the second motor is calculated at the transient state by using the power of the first motor at the transient state and the power limit of the power source. The maximum torque of the second motor is calculated at the transient state by using the maximum power of the second motor at the transient state and the speed of the second motor. 
     The control portion may calculate target torque of the second motor at the transient state based on the demand torque and the torque of the first motor at the transient state. Then the control portion may determine the torque of the second motor at the transient state by comparing the maximum torque of the second motor at the transient state and the target torque of the second motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic diagram illustrating a transmission of a hybrid vehicle to which a method for controlling torque according to an exemplary embodiment of the present invention can be applied. 
         FIG. 2  is a block diagram of a system for controlling torque of a hybrid vehicle to according to an exemplary embodiment of the present invention. 
         FIG. 3  is a flowchart of a method for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating calculation of power and torque of the first motor according to an exemplary embodiment of the present invention. 
         FIG. 5  is a flowchart illustrating calculation of power and torque of the second motor according to an exemplary embodiment of the present invention. 
     
    
    
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     An exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram illustrating a transmission of a hybrid vehicle to which a method for controlling torque according to an exemplary embodiment of the present invention can be applied. As shown in  FIG. 1 , a transmission of a hybrid vehicle to which a method for controlling torque according to an exemplary embodiment of the present invention can be applied uses an engine  10  and first and second motors  30  and  40  as power sources. The engine  10  generates power by burning e.g., fuel. Various types of engines such as a gasoline engine, a diesel engine, an LPI engine may be used as the engine  10 . 
     Power of the engine  10  is input to first and second planetary gear sets PG 1  and PG 2  through an input shaft  12 . The first planetary gear set PG 1  includes a first sun gear S 1 , a first planet carrier C 1 , and a first ring gear R 1  as rotation elements thereof. The first sun gear S 1  is may be always connected to the first motor  30 , and the first planet carrier C 1  may be always connected to the engine  10 . In addition, a first brake BK 1  is interposedly connected between the first sun gear S 1  and the first motor  30  so as to selectively stop the first motor  30  upon demand. 
     The second planetary gear set PG 2  includes a second sun gear S 1 , a second planet carrier C 2 , and a second ring gear R 2  as rotation elements thereof. The first planet carrier C 1  is selectively connected to the second ring gear R 2  through a first clutch CL 1 , and the first ring gear R 1  is directly connected to the second planet carrier C 2 . In addition, the second sun gear S 2  is selectively connected to the engine  10  through a second clutch CL 2  while at the same time being continuously connected to the second motor  40 . The second ring gear R 2  may be selectively stopped by a second brake BK 2 , and the second planet carrier C 2  may be continuously connected to an output gear  20 . 
     Operationally, the first motor  30  controls engine speed input to the first planet carrier C 1  through the first sun gear S 1 . The first planet carrier C 1  delivers the engine speed to the output gear  20  through the first ring gear R 1  and the second planet carrier C 2 . [The speed of the first motor is combined with the speed of the engine, and the target speed (i.e., controlled by the speed of the first motor and the speed of the engine) to that is output to the output gear. Thus, if the speed of the first motor is determined, the engine speed is controlled according to the speed of the first motor and the target speed. 
     The second motor  40  compensates engine torque input through the first planet carrier C 1  and engine torque selectively input through the second sun gear S 2  so that demand torque is output through the output gear  20 . That is, the second motor  40  compensates the engine torque so as to generate the demand torque accordingly. 
     A battery  50  supplies electricity/power to the first and second motors  30  and  40  while operating in a first mode and is charged by electricity generated at the first and second motors  30  and  40  under a predetermined driving condition in a second mode to thereby keep the battery charge above a certain level. 
       FIG. 2  is a block diagram of a system for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention. As shown in  FIG. 2 , a system for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention includes a vehicle speed detector  62 , a first motor speed detector  64 , a second motor speed detector  66 , an SOC detector  68 , a control portion  60 , and the first and second motors  30  and  40 . In addition, a plurality of sensors for detecting operations of the engine  10 , the transmission, the first and second motors  30  and  40 , and the battery  50  may be further included. 
     In this embodiment, the vehicle speed detector  62  detects the current vehicle speed and delivers a signal corresponding thereto to the control portion  60 . The first motor speed detector  64  detects the current speed of the first motor  30  and delivers a signal corresponding thereto to the control portion  60 . The second motor speed detector  66  detects the current speed of the second motor  40  and delivers a signal corresponding thereto to the control portion  60 . The SOC detector  68  detects SOC of the battery  50  and delivers a signal corresponding thereto to the control portion  60 . 
     The control portion  60  determines driving points of the engine and each motor at a steady state based on the vehicle speed, the speeds of the first and second motors  30  and  40 , and the SOC of the battery  50 , and determines driving points of each motor at a transient state. The control portion  60  controls operations of the engine  10 , the first motor  30 , and the second motor  40  according to the determined driving points. 
     Hereinafter, a method for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention will be described in detail with reference to  FIG. 3  to  FIG. 5 . 
       FIG. 3  is a flowchart of a method for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention,  FIG. 4  is a flowchart illustrating calculation of power and torque of the first motor according to an exemplary embodiment of the present invention, and  FIG. 5  is a flowchart illustrating calculation of power and torque of the second motor according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 3 , a method for controlling torque of a hybrid vehicle according to an exemplary embodiment of the present invention includes determining target power of the battery at a step S 100 , calculating driving points of the engine  10  and each motor  30  and  40  at the steady state at a step S 200 , and calculating driving points of each motor  30  and  40  at the transient state at a step S 300 . 
     The control portion  60  calculates demand torque based on the current vehicle speed, a position of an accelerator pedal, and engine speed at a step S 112 , receives the current vehicle speed from the vehicle speed detector  62  at a step S 114 , and receives the SOC of the battery  50  from the SOC detector  68  at a step S 116 . In addition, the control portion  60  receives a charge/discharge limit at a step S 118 , and receives constraint conditions of the engine  10  and each motor  30  and  40  at step S 120 . The charge/discharge limit and the constraint conditions of the engine  10  and each motor  30  and  40  may be stored in the control portion  60  for later use in for example RAM memory. 
     The control portion  60  then determines power of the battery  50  from the vehicle speed, the demand torque, and the charge/discharge limit at a step S 121 , and determines target power of the battery  50  by comparing the power of the battery  50  with the charge/discharge limit at a step S 122 . 
     After that, the control portion  60  determines the target driving point at the steady state by using the demand torque, the vehicle speed, the target power of the battery  50 , and the constraint conditions of the engine  10  and each motor  30  and  40  at a step S 202 . That is, target torque of the first motor  30  at the steady state is calculated at a step S 204 , the target torque of the second motor  40  at the steady state is calculated at a step S 206 , the target torque of the engine  10  at the steady state is calculated at a step S 208 , and target speed of the engine  10  at the steady state is calculated at a step S 210 . In addition, the control portion  60  receives the speed of the second motor  40  from the second motor speed detector  66  at a step S 212 , and receives the speed of the first motor  30  from the first motor speed detector  64  at a step S 214 . 
     Next, the control portion  60  calculates target speed of the first motor  30  from the target speed of the engine  10  at the steady state and the speed of the second motor  40  at a step S 216 , and subtracts the speed of the first motor  30  from the target speed of the first motor  30  at a step S 302 . 
     The control portion  60  calculates the target torque of the first motor  30  at the transient state by using the difference between the target speed of the first motor  30  and the speed of the first motor  30  at a step S 304 , and limits torque of the first motor  30  by using the target torque of the first motor  30 , the charge/discharge limit of the first motor  30  (determined from the charge/discharge limit of a power source (the battery  50 )), the target power of the second motor  40  at the steady state, and the speed of the first motor  30  at a step S 310 . Accordingly, torque of the first motor  30  at the transient state is calculated at a step S 312 . 
     In addition, the control portion  60  calculates target torque of the second motor  40  at the transient state by subtracting the torque of the first motor  30  at the transient state from the demand torque at a step S 316 . In addition, the control portion  60  limits torque of the second motor  40  by using the target torque of the second motor  40  at the transient state, the charge/discharge limit of the second motor  40 , the power of the first motor at the transient state, and the speed of the second motor  40  at a step S 320 . Accordingly, the torque of the second motor  40  at the transient state is calculated at a step S 322 . 
     Referring to  FIG. 4 , processes for calculating the torque of the first motor  30  at the transient state will be described in detail. 
     The control portion  60  calculates target power of the second motor  40  at the steady state by multiplying the target torque of the second motor  40  at the steady state and the speed of the second motor  40  at a step S 330 , and multiplies a first gain to the target power of the second motor  40  at the steady state at a step S 332 . 
     The control portion  60  calculates maximum discharge power of the first motor  30  at the transient state from discharge power limit of the first motor  30 , the target power of the second motor  40  at the steady state, and the first gain at a step S 334 . In addition, the control portion  60  calculates maximum discharge torque of the first motor  30  at the transient state by using the maximum discharge power of the first motor  30  at the transient state and the speed of the first motor  30  at a step S 336 . The maximum discharge torque in this case may be calculated from a predetermined discharge efficiency map. 
     Similar to the calculation of the maximum discharge torque, the control portion  60  calculates maximum charge torque of the first motor  30  at the transient state at a step S 344 . That is, the control portion  60  calculates the maximum charge power of the first motor  30  at a transient state from the charge power limit of the first motor  30 , the target power of the second motor  40  at the steady state, and the first gain at a step S 342 , and calculates the maximum charge torque of the first motor  30  at the transient state by using the maximum charge power of the first motor  30  at the transient state and the speed of the first motor  30  at a step S 344 . 
     The control portion  60  determines whether charge or discharge occurs by multiplying the target torque of the first motor  30  at the transient state and the speed of the first motor  30  at a step S 338 , and determines whether the maximum charge torque or the maximum discharge torque of the first motor  30  at the transient state is used at a step S 340  based on whether the sign is positive (+) or negative (−), respectively. 
     The control portion  60  calculates absolute value of the target torque of the first motor  30  at the transient state at a step S 346 , and compares the maximum charge torque or the maximum discharge torque determined at the step S 340  with the absolute value of the target torque at a step S 348 . After that, the control portion  60  calculates the torque of the first motor  30  at the transient state at the step S 312 . At S 348 , a minimum value is chosen. Since the step S 348  is performed without sign, the sign is added at the step S 312  as shown in  FIG. 4 . 
     Referring to  FIG. 5 , processes for calculating the torque of the second motor  40  at the transient state will be described in detail. 
     The control portion  60  calculates power of the first motor  30  at the transient state from the torque of the first motor  30  at the transient state and the speed of the first motor  30  at a step S 350 , and filters the power of the first motor  30  at the transient state at a step S 352  such that the torque of the second motor  40  at the transient state is not affected by fluctuation of the power of the first motor  30  at the transient state. 
     The control portion  60  subtracts the filtered power of the first motor  30  at the transient state from discharge power limit of the second motor  40  at a step S 354 , and calculates maximum discharge power of the second motor  40  at the transient state from the discharge power limit of the second motor  40  (calculated from the charge/discharge limit of the power source (the battery  50 )), the filtered power of the first motor  20  at the transient state, and a second gain at a step S 356 . In addition, the control portion  60  calculates maximum discharge torque of the second motor  40  at the transient state by using the maximum discharge power of the second motor  40  at the transient state and the speed of the second motor  40  at a step S 358 . The maximum discharge torque is calculated from the predetermined discharge efficiency map. 
     Similar to calculation of the maximum discharge torque, the control portion  60  calculates maximum charge torque of the second motor  40  at the transient state at a step S 366 . That is, the control portion  60  calculates maximum charge power of the second motor  40  at the transient state from charge power limit of the second motor  40 , the filtered power of the first motor  30  at the transient state, and the second gain at a step S 364 , and calculates the maximum charge torque of the second motor  40  at the transient state by using the maximum charge power of the second motor  40  at the transient state and the speed of the second motor  40  at the step S 366 . 
     After that, the control portion  60  determines whether charge or discharge occurs by multiplying the target torque of the second motor  40  at the transient state and the speed of the second motor  40  at a step S 360 , and determines whether the maximum charge torque or the maximum discharge torque of the second motor  40  at the transient state is used at a step S 362  based on whether the sign is positive (+) or negative (−), respectively. 
     The control portion  60  calculates absolute value of the target torque of the second motor  40  at the transient state at a step S 368 , and compares the maximum charge torque or the maximum discharge torque determined at the step S 362  with the absolute value of the target torque at a step S 370  based on whether the sign is positive (+) or negative (−), respectively. After that, the control portion  60  calculates the torque of the second motor  40  at the transient state at a step S 322 . 
     Furthermore, the control mechanisms/portions of the present invention may be embodied as computer readable media on a computer readable medium containing executable program instructions executed by a processor. Examples of the computer readable mediums include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, and optical data storage devices. The computer readable recording medium can also be distributed in network coupled computer systems so that the computer readable media is stored and executed in a distributed fashion, e.g., wirelessly to a remote server. 
     As described above, since power and torque of a second motor generating final demand torque is determined after power and torque of a first motor controlling engine to speed is determined, optimal torque of each motor at a transient state may be calculated according to an exemplary embodiment of the present invention. 
     In addition, since each motor and an engine are controlled by using optimal torque of each motor, fuel economy may be improved and an SOC may be managed stably. 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.