Abstract:
The present invention includes an induction motor and a system for controlling the induction motor including a battery providing a DC voltage and an inverter coupled to the induction motor and the battery, wherein the inverter is adapted to drive the induction motor from the battery. A motor controller is adapted to calculate a flux current and a slip frequency in response to an induction motor speed. The motor controller is further adapted to operate in one of a current control mode or a slip control mode. The motor controller is adapted to cause the slip frequency to increase by a slip decremental in a transition phase from the current control mode to the slip control mode, and further adapted to cause the slip frequency to increase by a slip incremental in a transition phase from the slip control mode to the current control mode.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to an induction motor control system, and in particular, the present invention includes a hybrid control system for an induction motor adapted for high speed generation and idle speed smoothing.  
         BACKGROUND OF THE INVENTION  
         [0002]    Conventional field oriented (FO) induction machine drives are being actively pursued in the automotive field as a high-power generation means. Specifically, it is of great interest to replace the common DC starter and claw-pole alternator of an internal combustion engine with an integrated starter/generator. The integrated starter/generator induction machine is electronically controlled and optimized to increase fuel economy and reduce vehicle emissions in both conventional and hybrid vehicles.  
           [0003]    An induction machine for automotive applications is usually required to generate at least 2-4 kW DC electric power with an internal combustion engine speed variation from 800 to 6000 rpm. Although induction machines are capable of variable-speed operation, the challenge presented is to meet all of the torque and power requirements for such a wide speed range, while simultaneously combining the motoring function of the induction machine with a starter/generator function.  
           [0004]    In order to minimize the power train modification, the induction machine is located in the space formerly occupied by the claw-pole alternator. In a typical configuration, the induction machine is coupled to the internal combustion engine through a belt. In order to amplify the torque output of the induction machine, the gear ratio between the induction machine and the internal combustion engine is preferably between 2 and 2.5.  
           [0005]    Given the aforementioned torque output and gear ratio, the induction machine will be required to operate at speeds up to 15,000 rpm. The fundamental frequency of the induction machine current is generally about 1 kHz. In order to save the costs inherent in an already complex machine, most induction machines use relatively low-cost current sensors. However, at very high speeds, the traditional current sensors may not be sufficient to maintain control over the induction machine. Similarly, because the pulse width modulation (PWM) frequency is generally on the order of 10 kHz, the inverter loss reduction can become sluggish. The foregoing design parameters common to a conventional current feedback control are not suitable for high-speed operation of an induction machine.  
           [0006]    There are other features of an induction motor that have not been successfully applied to the automotive field. For example, during idle speed operation, the speed of the internal combustion engine crank is subject to a great deal of fluctuation and vibration. Although it is desirable to smooth-out the operation of the vehicle in an idle state, conventionally-controlled induction machines have failed to replace the commonly-used flywheel for this purpose. A flywheel, however, possesses the dual limitations of increased weight and delayed dynamic response to torque and speed demands. The application of induction machines in this capacity has been delayed due to the power-requirements necessary for its control  
           [0007]    Due to limited BUS voltage, a typical induction machine is designed to have limited power capability in the high-speed operating region. The voltage shortage is compounded by the fact that the battery that supplies current to the induction machine does not always accept full charge. It is understood that the conventional current-loop control system for an induction machine requires up to 20% of the BUS voltage for current regulation, further hampering the controller&#39;s ability to utilize the available BUS voltage for high-speed operation.  
           [0008]    Even an induction machine that overcomes the foregoing limitations will likely suffer from an asymmetric instability. It has been generally assumed that, based upon steady-state models, the motoring and generating modes of an induction machine were dynamically symmetrical. Accordingly, it was assumed that in the generating mode an induction machine should be able to achieve comparable performance to that in a motoring mode.  
           [0009]    However, it is now understood that at high-speed generation, the induction machine may exhibit instability, meaning that the controller is unable to regulate both the iq and id current loops. This instability is not present in the motoring mode, implying that for an identical rotor speed and slip speed, the performance of an induction machine will be asymmetrical in the motoring and generating modes. The dynamic asymmetry becomes more prominent at high speeds.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, the present invention provides a hybrid-control scheme for an induction machine that includes an induction motor, a battery providing a DC voltage, and a voltage inverter coupled to the induction motor and the battery for driving the induction motor from the battery. The present invention also includes a motor controller adapted to calculate a flux current and a slip frequency in response to the induction motor speed. The motor controller is also adapted to operate in one of a current control mode or a slip control mode.  
           [0011]    The motor controller of the present invention transitions between the current control mode and the slip control mode through a simple algorithm. Specifically, the motor controller causes the slip frequency to increase by a slip decremental in the transition from the current control mode to the slip control mode, while conversely causing the slip frequency to increase by a slip incremental in the transition from the slip control mode to the current control mode.  
           [0012]    The motor controller of the present invention is also adapted to receive and respond to control signals from a power train controller. In particular, the power train controller transmits signals indicative of a torque demand or a motor speed demand, to which the motor controller responds by operating in one of a torque control mode or a speed control mode. The torque control mode is particularly useful in engine starting, torque boost applications, and charging the battery coupled to the induction machine. The speed control mode is utilized when the internal combustion engine is idling, thus permitting the battery to be charged while smoothing the idle speed fluctuations. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram depicting the integrated starter/generator with an induction machine and associated control elements.  
         [0014]    [0014]FIG. 2 is a control scheme of the prior art means for indirect field-orientation control of an induction machine.  
         [0015]    [0015]FIG. 3 is a control scheme in accordance with the preferred embodiment of the present invention having torque control, speed control, and a hybrid slip-loop and current-loop control.  
         [0016]    [0016]FIG. 4 is a flowchart showing the transition from the current control mode to the slip control mode.  
         [0017]    [0017]FIG. 5 is a flowchart showing the transition from the slip control mode to the current control mode.  
         [0018]    [0018]FIG. 6 is a series of graphical representations of experimental data indicative of the operation of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    In accordance with its preferred embodiment, the induction machine system  10  of the present invention generally comprises the components depicted in FIG. 1. The induction machine system  10  includes an induction motor  20  coupled to an internal combustion engine  14  via a belt drive system  13 . The induction motor  20  is coupled to a voltage source inverter  18 , both of which receive electrical current from a battery bank  16 .  
         [0020]    The voltage source inverter  18  transmits current from the induction motor  20  to the battery bank  16  when the induction motor  20  is in the generating mode; and when the induction motor  20  is in the motoring mode, the voltage source inverter  18  transmits current from the battery bank  16  to the induction motor  20 .  
         [0021]    The voltage source inverter  18  and the induction motor  20  are controlled locally by the motor controller  12 , which in turn receives high-level commands from the power train controller  22 . In particular, the power train controller  22  generates and transmits signals indicative of a requested torque command or a requested speed command for operation in a torque control mode or a speed control mode, respectively. The particulars of the control scheme implemented by the motor controller  12  are discussed in more detail herein.  
         [0022]    A motor controller  11  typical of the prior art is depicted schematically in FIG. 2. This motor controller  11  is characterized in that it detects two input currents, ia  25  and ib  26 , and transforms these two input currents into regulated currents Id and Iq via a Park Transformation  28 . Using a flux model of induction machines, the flux and slip are calculated from Id and Iq at  30 . Additionally, the motor controller  11  is adapted to receive inputs indicative of the motor speed  24  and a reference rotor speed  23 .  
         [0023]    The motor controller  11  is generally comprised of two channels, a first channel for flux control and a second channel for speed control. The flux control channel includes a flux scheduler  44  for generating a reference flux. The reference flux is calculated based upon the motor speed, which is commonly calculated from the rotor position signal in order to take full advantage of the induction machine under the DC-BUS voltage limitations. The reference flux is compared to the calculated flux, and the difference is passed through the flux controller  42  to generate the d-axis current. The Id controller  40  regulates the difference between the reference and feedback d-axis current. The final d-axis voltage command  46  is the summation of the output from the Id controller  40  and the voltage feedforward  38 .  
         [0024]    The speed control channel includes a speed controller  32 , an integrator  34 , and an Iq controller  36 . Analogous in structure to the flux control channel, the speed controller  32  generates the q-axis current by processing the difference between the reference speed  23  and the measured speed. The Iq controller  36  regulates the difference between the reference and feedback q-axis current. The final q-axis voltage command  46  is the summation of the output from the Iq controller  36  and the voltage feedforward  38 .  
         [0025]    In the final phase, the q-axis and d-axis voltage commands  46  are converted into pulse width modulated (PWM) three-phase variables  46 . The resultant signals, PMW_a  48 , PMW_b  50 , and PMW_c  52  actuate the voltage source inverter  18 . This form of motor controller  11  has shown problems as described above.  
         [0026]    [0026]FIG. 3 is illustrative of a preferred embodiment of the motor controller  12  of the present invention. The present motor controller  12  is characterized in that it detects two input currents, ia  58  and ib  60 , and transforms these two input currents into regulated currents Id and Iq via a Park Transformation  62 . Using the flux model of induction machines, the flux and slip are calculated from Id and Iq  64 . Additionally, the motor controller  12  is adapted to receive inputs indicative of the motor speed  56 , a reference torque command  53 , and a reference rotor speed  23 .  
         [0027]    The present motor controller  12  also comprises in part a flux control channel and a speed control channel. The flux control channel includes a flux scheduler  78  for generating a reference flux. The reference flux is compared to the calculated flux, and the difference is passed through the flux controller  76  to generate the d-axis current. The Id controller  74  regulates the difference between the reference and feedback d-axis current. The final d-axis voltage command  88  is the summation of the output from the Id controller  74  and the voltage feedforward  72 .  
         [0028]    The speed control channel includes a speed controller  84 , an integrator  68 , and an Iq controller  70 . Analogous in structure to the flux control channel, the speed controller  84  generates the q-axis current by processing the difference between the reference speed  54  and the measured speed. The Iq controller  70  regulates the difference between the reference and feedback q-axis current. The final q-axis voltage command  88  is the summation of the output from the Iq controller  70  and the voltage feedforward  72 .  
         [0029]    The motor controller  12  of the present invention is also characterized in that it is operable in a torque control mode. The torque reference command  53  is transmitted to an Iq torque mapping control  80 . The Iq torque mapping control  80  may operate through a look up table method, such that for each torque reference command  53 , there is a corresponding Iq command calculated by the Iq torque mapping control  80 .  
         [0030]    The torque control mode is employed for engine starting, torque boost, and battery charging. The torque reference command  53  is determined by the power train controller  22  and transmitted to the motor controller  12 . The Iq torque mapping  80  is selected at the reference select  82  terminal, which selects between the torque control mode and the speed control mode via a switch  86 . The speed control mode is employed during engine idling, in particular for induction machine systems having a diesel engine.  
         [0031]    A second feature of the present motor controller  12  is the mode transition terminal  88 . As noted, the induction motor  20  of the present invention operates in a hybrid control including a slip control mode and a current control mode. The mode transition terminal  88  functions to determine, select, and transition between each of the foregoing modes. In doing so, the mode transition terminal  88  receives inputs indicative of the motor speed  56 , Vd_pi, Vd_ff, Slip, Vq_ff, and Vq_pi, which are transformed into the known outputs Vd_cmd, Slip_cmd, and Vq_cmd.  
         [0032]    In the final control phase, the q-axis and d-axis voltage commands determined at the mode transition terminal  88  are converted into pulse width modulated (PWM) three-phase variables  90 . The resultant signals, PMW_a  92 , PMW_b  94 , and PMW_c  96  actuate the voltage source inverter  18 . The details of the mode transition between the slip control mode and the current control mode are discussed below.  
         [0033]    [0033]FIG. 4 is a flow chart illustrating the transition from the current control mode to the slip control mode. The motor speed  56  is a measured quantity that is compared to a predetermined SPD_enter value in step S 102 . If the SPD_enter value is not less than the motor speed, or if the Iq_cmd value is non-negative, then the motor controller  12  maintains the current control mode as shown in step S 104 . If the SPD_enter value is less than the detected motor speed and the Iq_cmd value is less than zero, then the motor controller  12  progresses to step S  106 , in which the input values shown in the mode transition terminal  88  are recognized by the motor controller  12 . The input values include Vd_pi, Vd_ff, Slip, Vq_ff, and Vq_pi. In step S 108 , the motor controller  12  calculates the slip decremental Slip_dec, as given by the following equation:  
             Slip_dec   =       (     1   -     abs   (     Vd_pi   Vd_max     )       )     *   Slip             (   1   )                               
 
         [0034]    where Vd_max is a predetermined constant.  
         [0035]    In step S 110 , the motor controller  12  gradually transitions into the slip control mode by increasing the slip decremental from zero to Slip_dec. Correspondingly, the Vd_cmd is ramped from Vd_pi to Vd_max, and the Vq_cmd is ramped from Vq_pi to Vq_noload, a predetermined constant. The transition from the current control mode to the slip control mode is then completed in step S 112 .  
         [0036]    [0036]FIG. 5 is a flow chart illustrating the transition from the slip control mode to the current control mode. The motor speed  56  is a measured quantity that is compared to a predetermined SPD_exit value in step S 114 . If the SPD_exit value is less than the motor speed and the Iq_cmd value is not positive, then the motor controller  12  maintains the slip control mode as shown in step S 116 . If the SPD_exit value is greater than the detected motor speed or the Iq_cmd value becomes positive, then the motor controller  12  progresses to step S 118 , in which the input values shown in the mode transition terminal  88  are recognized by the motor controller  12 . As before, the input values include Vd_pi, Vd_ff, Slip, Vq_ff, and Vq_pi. In step S 120 , the motor controller  12  calculates the slip incremental Slip_inc, as given by the following equation:  
             Slip_inc   =       (     1   -     abs   (     Vd_ff   Vd_max     )       )     *   Slip             (   2   )                               
 
         [0037]    where Vd_max is a predetermined constant.  
         [0038]    In step S 122 , the motor controller  12  gradually transitions from the slip control mode by increasing the slip decremental from Slip_inc to zero. Correspondingly, the Vd_cmd is ramped from Vd_max to Vd_ff, and the Vq_cmd is ramped from Vq_noload to Vq_ff, a predetermined constant. In step S  124 , the Vd_cmd is ramped from Vd_ff to Vd_pi, and the Vq_cmd is ramped from Vq_ff to Vq_pi. The transition from the slip control mode to the current control mode is then completed in step S 126 .  
         [0039]    As evidenced by the data displayed in FIG. 6, the transition between the current control mode and the slip control mode are smooth, stable, and fast. The variation and stabilization of the BUS voltage, Vd and Vq, Slip, and Id and Iq are shown respectively over a short amount of time. Consequently, the induction machine of the present invention provides a simple and cost-effective solution to the problems associated with the prior art. Notably, utilization of a hybrid control scheme with a current control mode and a slip control mode makes efficient use of the BUS voltage limitations, while simultaneously regulating the aforementioned instabilities present at high speed operation.  
         [0040]    As described, the present invention consists of a system for controlling the high-speed usage of an induction machine for particular application in an automobile. Nevertheless, it should be apparent to those skilled in the art that the above-described embodiments are merely illustrative of but a few of the many possible specific embodiments of the present invention. Numerous and various other arrangements can be readily devised by those skilled in the art without departing from the spirit and scope of the invention as defined in the following claims.