Abstract:
Systems and methods are disclosed to provide torque linearity in the field-weakening region for an electric (e.g., IPM) machine. The systems and methods implement a field weakening and a torque linearity control loop for linearizing torque generated by an electric machine. As a result, torque linearity is maintained when the electric machine operates in the field weakening region.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/829,298, filed Jul. 27, 2007. 
    
    
     TECHNICAL FIELD 
     Embodiments of the present invention relate generally to electric machine control, and more particularly relate to techniques that affect torque linearity in a field weakening region of an electric machine. 
     BACKGROUND 
     An electric machine converts electrical power into mechanical force and motion. Electric machines are found in numerous applications including household appliances such as fans, refrigerators, and washing machines. Electric drives are also increasingly used in electric and hybrid-electric vehicles. 
     A rotary electric machine generally has an internal rotating magnet, called the rotor, which revolves inside a stationary stator. The interaction between the rotor electromagnetic field with the field created by the stator winding creates the machine torque. The rotor may be a permanent magnet or it may be made of coils. However, if the rotor has permanent magnets embedded therein (i.e., the permanent magnets are not in the rotor surface), the electric machine may be referred to as an interior permanent magnet (IPM) machine. The part of the machine across which the input voltage is supplied is called the “armature”. Depending upon the design of the machine, either the rotor or the stator can serve as the armature. In an IPM machine, the armature is the stator, and is a set of winding coils powered by input voltage to drive the electric machine. 
     The reverse task of converting mechanical energy into electrical energy is accomplished by a generator or dynamo. An electrical machine as mentioned above may also function as a generator since the components are the same. When the machine/generator is driven by mechanical torque, electricity is output. Traction machines used on hybrid and electric vehicles or locomotives often perform both tasks. 
     Typically as an electric machine accelerates, the armature (and hence field) current reduces in order to keep stator voltage within its limits. The reduction in field which reduces magnetic flux inside the machine is also called flux or field weakening. Field weakening control techniques can be used to increase performance in the torque-speed characteristic of the machine. To retain control of stator current, the machine field may be reduced by a field weakening control loop. The field or flux weakening in an IPM machine can be accomplished by adjusting the stator excitation. Stator excitation in an IPM machine may be controlled by voltage pulse width modulation (PWM) of a voltage source inverter. 
     Flux weakening techniques have been used in the past where IPM flux is purposely made weak to reduce the problems associated with high flux, such as over voltage due to high Back-EMF. For example, during a constant torque region of operation of an electric machine, closed loop current regulator control has been used to control the applied PWM voltage excitation so that the instantaneous phase currents follow their commanded values. However, saturation of the current regulators may occur at higher speeds when the machine terminal voltage approaches the maximum voltage of the PWM inverter. Beyond this point, the flux should be weakened to maintain proper current regulation up to the maximum available machine speed. Reducing the magnetic flux inside the machine provides improved power characteristics of the IPM machine at high speeds. However, torque may decrease in direct proportion to the flux. 
     Accordingly, it is desirable to keep torque linearity in the field-weakening region for an IPM machine within the voltage and current system constraints. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     BRIEF SUMMARY 
     Systems and methods are disclosed to provide torque linearity in the field-weakening region for an electric (e.g., IPM) machine. The systems and methods implement a field weakening and a torque linearity control loop for linearizing torque generated by an electric machine. First and second voltage commands are used at the field weakening loop to generate a first adjusting current command. The first adjusting current command is then provided to the torque linearity control loop, where it is multiplied by a gain to generate an output current command, which can then be limited to generate a second adjusting current command. The second adjusting current command can then be added to a current command to generate an adjusted current command. The first adjusting current command can also be added to another current command to generate another adjusted current command. The adjusted current commands and feedback currents are then used at a synchronous current regulator to generate new first and second voltage commands. As a result, torque linearity is maintained when the electric machine operates in the field weakening region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
         FIG. 1  is a functional block diagram that includes an existing control system without a torque linearity block for a vector controlled IPM machine; 
         FIG. 2  is a functional block diagram of a control system with a torque linearity control block for a vector controlled IPM machine; 
         FIG. 3  is a functional block diagram of a phase current limit module of the control system of  FIG. 2 ; 
         FIG. 4  illustrates current regulation performance for an IPM machine with and without the torque linearity control block; and 
         FIG. 5  is a flowchart illustrating a process for operating an electric machine with a torque linearity control block. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
     Embodiments of the invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various integrated circuit components, e.g., memory elements, controlled switches, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that embodiments of the present invention may be practiced in conjunction with any number of vehicle applications and that the system described herein is merely one example embodiment of the invention. 
     For the sake of brevity, conventional techniques and components related to vehicle electrical parts and other functional aspects of the system (and the individual operating components of the system) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the invention. 
     The following description may refer to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature is directly joined to (or directly communicates with) another element/node/feature, and not necessarily mechanically. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically. Thus, although the schematic shown in  FIG. 2  depicts an example arrangement of elements, additional intervening elements, devices, features, or components may be present in an embodiment of the invention (assuming that the functionality of the system is not adversely affected). 
     Embodiments of the invention are described herein in the context of one practical non-limiting application, namely, a control system for an IPM machine. In this context, the example technique is applicable to operation of a system suitable for a hybrid vehicle. Embodiments of the invention, however, are not limited to such vehicle applications, and the techniques described herein may also be utilized in other electric powered control applications. 
       FIG. 1  is a functional block diagram that depicts an existing control system  100  for a vector controlled IPM machine suitable for use with a hybrid vehicle. Such systems are well known and, therefore, the operation of system  100  will not be described in detail here. In summary, control system  100  adjusts the q-axis component of the stator current command I Q * (q-axis current command) of the IPM machine using a flux weakening control loop. Control system  100  includes: a current command 3-D table lookup module  102 , a synchronous current regulator module with dynamic over modulation  116 , a DC to AC transformation module  118 , a PWM inverter  120 , an AC to DC transformation module  122 , an IPM machine  124 , and a field weakening module  114 . Control system  100  operates as described below. 
     Based on a torque command T*, the rotor rotational speed ω R , and a DC-link voltage V DC , optimal current commands (I D * and I Q *) are generated using the current command 3-D table look-up module  102 . The inputs to the table look-up module  102  are provided by a voltage sensor from the V DC  input to the inverter  120 , and a position sensor (not shown in  FIG. 1 ) from the IPM machine  124 . The q-axis current command I Q * is adjusted to obtain an adjusted command (I Q **) as explained below. 
     The I D  and I Q  stationary currents (d-axis and q-axis components of the stator current) from the IPM machine  124  are fed to the synchronous current regulator module with dynamic over modulation  116 , which generates synchronous voltage commands (V D * and V Q *). The command voltages V D * and V Q * are vector rotated using the rotor angular position θ R , which is provided by IPM machine  124 . The outputs of the current regulator with dynamic over modulation  116  (namely, V D * and V Q *) are fed to the DC to AC transformation module  118  to generate stationary frame voltage commands (V AS * V BS *, and V CS *) based on V D * and V Q *. 
     The V AS *, V BS *, and V CS * stationary frame voltage commands are fed to the inverter  120  to generate I AS , I BS  and I CS , which are the respective stationary frame currents. The inverter  120  may be, for example, a PWM inverter which applies alternating three phase voltage to the stator winding of the IPM machine  124 . 
     The IPM machine  124  then operates at the rotational speed ω R  based on the stationary frame currents I AS , I BS  and I CS . 
     The AC to DC transformation module  122  generates I D  and I Q  (the d-axis and q-axis components of the stator feedback current) based on I AS , I BS , I CS , and θ R . Additional details of the control system  100  can be found in United States Patent Application Number 2005/0212471, the content of which is hereby incorporated by reference in its entirety. 
     The Back-EMF is proportional to the rotational speed, ω R . Moreover, the Back-EMF of the electric machine increases as the rotational speed ω R  of the electric machine is increased. Above a certain rotational speed, the voltage of the IPM machine may become higher than the voltage of the bus, resulting in reversal of current flow (regenerating instead of motoring). To control the I D  and I Q  components of the stator current, the machine flux is reduced by a field weakening control loop. The field weakening module  114  generates an adjusting current command ΔI Q  (ΔI Q  is the adjusting q-axis current, (which decreases the flux in the machine but also decreases the torque), based on V D * and V Q * to adjust the current command I Q *. ΔI Q  is then added to I Q * by an adder  112  to generate the adjusted current command I Q **. 
     Adjusting I Q * in this manner results in a decrease in the torque, as will be explained in the context of  FIG. 3 . The aforementioned reduction in torque reduces the maximum torque available from the IPM machine, and it may reduce the machine efficiency. Additional details of the field weakening control loop module  114  can be found in U.S. patent application Ser. No. 11/552,580, filed Oct. 25, 2006, which is hereby incorporated by reference in its entirety. 
     To keep torque linearity in the field weakening region of an IPM machine, a torque linearity loop according to an embodiment of the invention is utilized as explained below. 
       FIG. 2  is a block diagram that illustrates a control system  200  for a vector controlled IPM machine, which is suitable for use in a hybrid vehicle. System  200  includes a torque linearity control loop that is suitably configured to perform a torque linearity control function according to an embodiment of the invention. System  200  is suitable for use with a vehicle having an electric traction machine (e.g., an electric vehicle or a hybrid vehicle). A practical control system  200  may include a number of electrical components, circuits and controller units other than those shown in  FIG. 2 . Conventional subsystems, features, and aspects of the control system  200  will not be described in detail herein. The control system  200  has components that are similar to control system  100  (common features, functions, and elements will not be redundantly described here). For this embodiment, as shown in  FIG. 2 , the control system  200  generally includes: a current command 3-D table lookup module  202 , a torque linearity module  204 , a phase current limit module  211 , a synchronous current regulator module with dynamic over modulation  216 , a DC to AC transformation module  218 , a PWM inverter  220 , an AC to DC transformation module  222 , an IPM machine  224 , and a field weakening control loop module  214 . System  200  operates with an IPM machine  224 . In particular, inverter  220  drives IPM machine  224 . 
     The torque linearity module  204  generates an adjusting current command ΔI D  (ΔI D  is the adjusting current in the d-axis, which decreases the flux in the machine while torque linearity is maintained), which is based on ΔI Q  as explained below. In practice, ΔI Q  is provided by the field weakening control loop module  214 . ΔI D  is added by an adder  210  to I D * to generate an adjusted current command I D **. The adjusted current command I D ** is fed to the synchronous current regulator module with dynamic over modulation  216 . 
     For this embodiment, the torque linearity module  204  includes a proportional gain module  206  and a limiter module  208  coupled to the proportional gain module  206 . The proportional gain module  206  applies a proportional gain, K, to ΔI Q . K may be a constant having a value that typically ranges from about one to about three, or it may be a variable that varies as a function of the torque command (T*) and the adjusted current command (I Q ***). For example, K may be calculated based on the following relationship: 
                 4     3   *   P       ·       T   *         (       L   Q     -     L   D       )     ⁢     I   Q     **     *   2               ,         
where P is the number of poles of the machine, L D  and L Q  are the d-axis and q-axis machine inductances, T* is the torque command, and I Q *** is a limited q-axis current command.
 
     ΔI Q  is multiplied by K to obtain an output current adjusting command (ΔI D ). ΔI D  is then fed to the limiter  208  to keep the current adjusting command ΔI D  within its range (about −30 to about 0 AMPS). 
     To keep the I D −I Q  vector within the maximum torque per flux boundaries, the phase current limit module  211  is used. The phase current limit module  211  is configured to set the maximum phase current at any DC-voltage V DC  and machine rotor speed ω R .  FIG. 3  is a functional block diagram that depicts the phase current limit module  211  (see  FIG. 2 ). The maximum available current block  230  provides the maximum phase current I S(max)  as a function of V DD  and ω R . The maximum phase current I S(max)  is constant in the constant torque region. However, in the field weakening region, I S(max)  is decreased accordingly to follow the maximum torque per flux machine curve. I Q ** is first limited by I S(max)  resulting in the limited q-axis current command I Q ***. The maximum d-axis current command is calculated as I D(max) =√{square root over (I S(max)   2 −I Q *** 2 )}. Then, I D ** is limited by I D(max)  resulting in the limited d-axis current command I D ***. 
       FIG. 4  illustrates current regulation performance with and without the torque linearity control block. The field weakening control loop module  214  keeps the current regulator stable at the available voltage by adjusting the I Q  current by an amount ΔI Q  as explained above. ΔI Q , however, moves the current vector  310  from point  304  on the T1 constant torque curve to point  308  on the T2 constant torque curve, thereby decreasing the torque in direct proportion to the flux. It is desirable to keep the current vector on the T1 constant torque curve in the field weakening region of the IPM machine. To this end, the control loop of the torque linearity module  204  generates ΔI D , which moves the current vector  310  from point  308  on the T2 constant torque curve to point  306  on the T1 constant torque curve, thereby keeping the torque constant and maintaining torque linearity in a field weakening region of the IPM machine. The techniques described herein adjust both I D  and I Q  to decrease flux in the field weakening region, while keeping torque linearity. 
       FIG. 5  is a flowchart illustrating a torque linearity operating process  400  for an electric, hybrid electric, or fuel cell vehicle. Process  400  may be performed by control system  200  as described above. The various tasks performed in connection with process  400  may be performed by software, hardware, firmware, or any combination thereof. It should be appreciated that process  400  may include any number of additional or alternative tasks, the tasks shown in  FIG. 5  need not be performed in the illustrated order, and process  400  may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein. For illustrative purposes, the following description of process  400  may refer to elements mentioned above in connection with  FIGS. 1-3 . 
     Process  400  adjusts the q-axis and the d-axis components of the stator current commands (I D * and I Q *) of the IPM machine, so torque remains linear during the field weakening region of the IPM machine, which would otherwise fall proportional to the reduction in the flux. In practical embodiments, portions of process  400  may be performed by different elements of control system  200 , e.g., the current command 3-D table lookup module  202 , the torque linearity module  204 , the phase current limit module  211 , the synchronous current regulator module with dynamic over modulation  216 , the DC to AC transformation module  218 , the PWM inverter  220 , the AC to DC transformation module  222 , the IPM machine  224 , and the field weakening control loop module  214 . 
     Torque linearity operating process  400  begins by generating first and second current commands (I Q * and I D *) based on a torque command T*, a rotor angular velocity ω R , and a DC-link voltage V dc  (task  402 ). 
     Process  400  also generates an adjusting current command ΔI Q  based on the V D * and V Q * voltage commands (task  406 ), and adds ΔI Q  to I Q * to obtain the I Q ** adjusted current command (task  408 ). However the torque is reduced as explained in the context of  FIG. 3  above. To keep the torque linearity, process  400  generates a ΔI D  adjusting current command as a function of the ΔI Q  adjusting current command (task  410 ). Then, a torque linearity loop applies a current adjusting gain K (task  412 ), multiplies K by the ΔI Q  to obtain an output current command ΔI D  (task  414 ), and limits the output current command to obtain the ΔI D  adjusting current command within a desired range (about −30 to about 0 Amps) (task  416 ). Values of K, and the lower and upper limits of the limiter are explained above. 
     Process  400  then adds ΔI D  to I D * and the second current command to obtain the I Q ** adjusted current command (task  418 ). ΔI D  adjusts the I D * current command such that the torque linearity remains constant as shown in  FIG. 4  above while the IPM field is weakened. Process  400  then limits the q-axis and the d-axis currents I Q ** and I D ** to generate the limited q-axis and d-axis current commands I Q ***, and I D *** (task  419 ). In turn, I Q *** and I D *** are used as inputs to module  216  for the generation of the voltage commands (V D *, and V Q *). Thereby, the current is suitably regulated to weaken the field in the IPM machine. 
     Process  400  then generates V D * and V Q * voltage commands based on I D ***, I Q ***, I D  and I Q  (task  420 ). 
     Process  400  also rotates the IPM machine stator at ω R  by delivering load-driving currents to the motor (task  422 ). To do this, the V AS *, V BS *, and V CS * stationary frame voltage commands are generated based on the V D * and V Q * synchronous voltage commands, to produce the I AS , I BS  and I CS  stationary frame currents. The load is then delivered via the stationary frame currents to the IPM machine. 
     With this approach, the torque linearity is maintained in a field weakening region of the IPM machine. 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.