Abstract:
A novel system for adaptively controlling an electric vehicle to maintain desired speed under variable driving conditions. The system includes a control circuit for producing a control signal to control an electric motor of the vehicle. The control signal is formed based on a control current required to achieve the desired speed. The control strategy selection circuit is configured in the system to determine a motor control scheme that provides an appropriate waveform profile of the control current.

Description:
RELATED APPLICATIONS 
   This application contains subject matter related to U.S. application Ser. No. 09/826,423 of Maslov et al., filed Apr. 5, 2001, now U.S. Pat. No. 6,492,756, U.S. application Ser. No. 09/826,422 of Maslov et al., filed Apr. 5, 2001, now U.S. Pat. No. 6,49,864, U.S. application Ser. No. 09/966,102, of Maslov et al., filed Oct. 1, 2001, now U.S. Pat. No. 6,787,951, U.S. application Ser. No. 09/993,596 of Pyntikov et al., filed Nov. 27, 2001, now U.S. Pat. No. 6,927,524, U.S. application Ser. No. 10/173,610, of Maslov et al., filed Jun. 19, 2002, now U.S. Pat. No. 6,727,668, U.S. application Ser. No. 10/290,537, of Maslov et al., filed Nov. 8, 2002, now U.S. Pat. No. 6,794,839, and U.S. application Ser. No. 10/353,075, of Maslov et al., filed Jan. 29, 2003, now U.S. Pat. No. 6,940,242. The disclosures of these applications are incorporated by reference herein. 
   FIELD OF THE INVENTION 
   The present invention relates to electric vehicles, and more specifically, to an electric vehicle equipped with a cruise control system adaptable in accordance with driving conditions. 
   BACKGROUND ART 
   A cruise control system in a vehicle provides automatic speed control to enable the vehicle to maintain constant speed under variable driving conditions without driver&#39;s intervention. A conventional cruise control system in an electric vehicle controls an electric motor of the vehicle to request a torque value required to achieve a desired speed. 
   For example, U.S. Pat. No. 5,615,933 describes an electric vehicle having an electric motor propulsion system, a brake system and a control unit, which includes a motor control controlling the propulsion system, and a brake control for controlling the brake system. The motor propulsion system includes a three-phase AC electric motor that provides traction force to vehicle drive wheels. The motor control provides control commands to the electric motor. In particular, the motor control derives a torque command that defines the torque to be applied to the wheels by the motor. The motor control comprises an electronic cruise control system that receives a vehicle speed signal from a vehicle speed sensor. In response to the vehicle speed signal, the motor control issues a command to set the motor either into an accelerating or a braking mode to achieve a desired speed. In the accelerating mode, an accelerating torque command is issued by the motor control to request the motor to increase the torque applied to the wheels so as to reach the desired speed. 
   Under typical driving conditions, torque values required to achieve a desired speed are subject to wide variability with little, if any, long term predictability. Moreover, driving conditions, such as steep uphill grade or heavy vehicle load or the like, may impose limitations on available speed and acceleration. 
   Higher acceleration or greater speed may be required than the system can accommodate at maximum torque restricted by available motor performance. In particular, the voltage signal that the motor control needs to produce in order to request the torque required to achieve the desired speed may be greater than the supply voltage. Therefore, the motor would not be able to develop the required torque. 
   Accordingly, there is a need for a cruise control system that would be able to control the electric motor of a vehicle in order to produce high torque required to maintain the vehicle at a desired speed. 
   In addition, in a vehicle drive environment, wherein power availability is limited to an on-board supply, it is highly desirable to attain a high torque output capability at minimum power consumption. Motor structural arrangements described in the copending applications contribute to these objectives. As described in those applications, electromagnet core segments may be configured as isolated magnetically permeable structures in an annular ring to provide increased flux concentration. Isolation of the electromagnet core segments permits individual concentration of flux in the magnetic cores, with a minimum of flux loss or deleterious transformer interference effects occurring from interaction with other electromagnet members. 
   The above-identified copending application Ser. No. 10/173,610 describes a control system for a multiphase motor that compensates for variations in individual phase circuit elements. A high degree of precision controllability is obtained with each phase control loop closely matched with its corresponding winding and structure. Successive switched energization of each phase winding is governed by a controller that generates signals in accordance with parameters associated with the respective stator phase components. The phase windings are energized with current of sinusoidal waveform for high efficiency operation. The control system varies the output current to respond to, and accurately track, the torque command input. 
   The sinusoidal current waveform profile obtained with this commutation strategy can extend battery life through efficient operation. However, in vehicle driving operation there may be a need for torque capability in excess of that available from the most efficient control scheme. Typically, the power supply is rated for a maximum current discharge rate, for example, 10.0 amps. If the cruise control system requests a torque command that correlates to this maximum current draw, then the motor torque output for a sinusoidal current waveform profile is limited, for example, to approximately 54.0 Nm in a motor with a configuration such as described above. 
   The need thus exists for an electric vehicle cruise control system that is capable of adaptively controlling the electric motor to enable its operation with high efficiency yet can deliver increased torque output when required to maintain a desired speed. 
   DISCLOSURE OF THE INVENTION 
   The present invention fulfills this need by providing a system for adaptively controlling an electric vehicle to maintain desired speed under available driving conditions. The system includes a control circuit for producing a control signal to control an electric motor of the vehicle. The control signal is formed based on a control current required to achieve the desired speed. The control strategy selection circuit is configured in the system to determine a motor control scheme that provides an appropriate waveform profile of the control current. 
   For example, the control strategy selection circuit may select a high-efficiency control scheme that provides a waveform profile for achieving operating efficiency of the motor. Alternatively, a high-torque control scheme may be selected to provide a waveform profile for achieving high torque, when the high-efficiency control scheme is not sufficient to achieve the desired speed. While the operating efficiency waveform profile may have a substantially sinusoidal waveshape, the high-torque waveform profile may have a substantially rectangular waveshape. 
   The motor control scheme may be selected in response to actual speed of the vehicle and torque required to achieve the desired speed. The high-efficiency control scheme is chosen when the required torque at a given speed does not exceed a threshold level. The high-torque control scheme is selected when the required torque exceeds the threshold level. 
   In particular, when the control signal to achieve the required torque does not exceed a power supply signal for energizing the motor, the control strategy selection circuit may select the high-efficiency control scheme. However, when the control signal needed to achieve the required torque exceeds the power supply signal, the control strategy selection circuit selects the high-torque control scheme. 
   The control strategy selection circuit may comprise a look-up table responsive to actual speed of the vehicle and torque required to achieve the desired speed for outputting the control current representing the selected motor control scheme. 
   The present invention may be manifested in a control system for a multiphase motor having a plurality of stator phase components, each stator phase component comprising a phase winding formed on a core element, and a permanent magnet rotor. Preferably, each of the stator core elements comprises ferromagnetic material separated from direct contact with the other core elements, each stator phase component thereby forming an autonomous electromagnet unit. Stator energization current is provided by a direct current power supply through circuitry coupled to a controller. The controller can access any of a plurality of stored motor control schemes to implement stator energization current having a corresponding waveform profile. The stored motor control schemes are determinative of the current waveform profiles and, when accessed, are incorporated into controller operation. 
   Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is an exemplary view showing rotor and stator elements in a configuration that may be employed in the present invention. 
       FIG. 2  is a block diagram of an adaptive cruise control system in accordance with the present invention. 
       FIG. 3  is a block diagram that illustrates controller methodology for use in the cruise control system of  FIG. 2 . 
       FIG. 4  is a flow chart for operation of the adaptive cruise control system in accordance with the present invention. 
       FIG. 5  is a curve representing motor control scheme selection for ranges of torque and speed in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is applicable to a vehicle driven by an electric motor such as disclosed in the copending application Ser. No. 09/826,422, although the invention can be used with various other permanent magnet motors.  FIG. 1  thus is an exemplary view showing rotor and stator elements of a motor  10  as described in that application, the disclosure of which has been incorporated herein. Rotor member  20  is an annular ring structure having permanent magnets  21  substantially evenly distributed along cylindrical back plate  25 . The permanent magnets are rotor poles that alternate in magnetic polarity along the inner periphery of the annular ring. The rotor surrounds a stator member  30 , the rotor and stator members being separated by an annular radial air gap. Stator  30  comprises a plurality of electromagnet core segments of uniform construction that are evenly distributed along the air gap. Each core segment comprises a generally U-shaped magnetic structure  36  that forms two poles having surfaces  32  facing the air gap. The legs of the pole pairs are wound with windings  38 , although the core segment may be constructed to accommodate a single winding formed on a portion linking the pole pair. Each stator electromagnet core structure is separate, and magnetically isolated, from adjacent stator core elements. The stator elements  36  are secured to a non-magnetically permeable support structure, thereby forming an annular ring configuration. This configuration eliminates emanation of stray transformer flux effects from adjacent stator pole groups. The stator electromagnets are thus autonomous units comprising respective stator phases. The concepts of the invention, more fully described below, are also applicable to other permanent magnet motor structures, including a unitary stator core that supports all of the phase windings. 
     FIG. 2  is a block diagram of an adaptive cruise control system in accordance with the present invention. A plurality of stator phase windings  38  ( FIG. 1 ) of the multiphase motor  10  are switchably energized by driving current supplied from d-c power source  40  via hybrid power block  42 . The power block  42  may comprise electronic switch sets that are coupled to controller  44  via a pulse width modulation converter and gate drivers. Each phase winding is connected to a switching bridge having control terminals connected to receive pulse modulated output voltages from the controller. Alternatively, the switching bridges and gate driver components may be replaced by amplifiers linked to the controller output voltages. 
   Rotor position and speed sensor  46  provides rotor position and speed feedback signals to the controller  44 . The sensor  46  may comprise a well-known resolver, encoder or their equivalents and a speed approximator that converts the position signals to speed signals in a well-known manner. 
   The controller  44  may comprise a microprocessor or equivalent microcontroller, such as Texas Instrument digital signal processor TMS320LF2407APG. Coupled to the controller may be RAM and ROM memories for storing programs and data used in the controller&#39;s operation. Profile memory  48  is shown separately in the drawing for purposes of illustration of the inventive concepts. The profile memory  48  may comprise a look-up table for storing motor control scheme data that determine motor current waveform profiles selectable in accordance with driving conditions. 
   As discussed in more detail below, the motor control schemes stored in the profile memory  48  are selected based on a torque command τ d , actual speed ω of the vehicle and rotor position θ that may be determined by the position/speed sensor  46 . The torque command τ d  determines torque required to achieve the desired speed ω d  maintained by the cruise control system. 
   In a well known manner, the desired speed is defined by set/resume switch  50  that sets the desired speed in the cruise control system, or commands the system to resume the desired speed set previously. The desired speed value is supplied to latch  52  that monitors cruise control switch  54  to determine whether a cruise control mode is set, and monitors brake pedal  56  to determine whether the cruise control mode is released. Subtracting unit  58  determines the difference Δω between the actual speed and the desired speed. Based on this difference, acceleration/deceleration characteristics unit  60  determines the torque command τ d  required to achieve the desired speed. The acceleration/deceleration characteristics unit  60  calculates torque using a well-known algorithm for determining torque for particular acceleration and deceleration characteristics. 
     FIG. 3  is a block diagram that illustrates operation of the controller  44  in the cruise control system of the present invention. In order to develop the desired phase currents the following per-phase voltage control expression is applied to the driver for the phase windings:
   V   i ( t )= L   i   dI   di   /dt+R   i   I   i   +E   i   +k   si   e   i   
where
     V i (t) is the voltage across the phase winding;   I di (t) is the desired phase current to be produced to obtain torque required to achieve the desired speed ω d ;   I i (t) is the phase current;   R i  is the winding resistance;   E i (t) is the back-EMF;   L i  is the winding self-inductance;   k si  is the current loop feedback gain; and   e i  is the phase current error.   
     FIG. 3  represents the methodology, generally indicated by reference numeral  60 , by which the controller  44  derives the components of this voltage control expression in real time. As described in more detail below, the desired phase current I di (t) is provided by the profile memory  48  in accordance with a motor control scheme selected by the cruise control system. 
   The desired phase current I di (t), dependent upon the particular motor control scheme implemented, defines the manner in which the cruise control system responds to the torque command requested by the system to achieve the desired speed. A significant aspect of the present invention is provision of a plurality of available motor control schemes adaptively selected by the cruise control system to obtain a desired response. Each control scheme effects a particular motor current waveform profile having unique characteristics with respect to efficiency, torque capacity, response capability, power losses, etc., in comparison to other control schemes. 
   As shown in  FIG. 3 , rotor position θ(t) is input to controller function block  64  that produces an output representing excitation angle θ i (t) based on the rotor position, the number of permanent magnet pole pairs (N r ) the number of stator phases (N s ) and the phase delay of the particular phase. The phase current I i (t) in each phase winding is sensed by a respective one of a plurality of current sensors whose outputs are provided to controller  44 . 
   Controller function block  66  calculates the difference between the desired phase current I di (t) received from the profile memory  48  and the phase current I i (t) to output a phase current track error signal e i (t). This error signal is multiplied by gain factor k s  in controller function block  68 . The effect of the current feedback gain is to increase overall system robustness via the rejection of system disturbances due to measurement noise and any model parameter inaccuracies. The output of block  68  is fed to controller function block  70 . Block  70  outputs time varying voltage signals V i (t) to the power block  42  for the selective controlled energization of the phase windings  38 . Voltage V i (t) has components that compensate for the effects of winding inductance, induced back-emf and winding resistance. 
   To compensate for the presence of inductance within phase windings, the term L i dI di /dt, wherein dI di /dt denotes the standard time derivative of the desired phase current I di (t), is input to the controller function block  70  to be added in the phase voltage calculation. The term L i dI di /dt shown in block  72  is provided by a look-up table in the profile memory  48  based on the received inputs of τ d (t), θ i (t) and ω (t). 
   To compensate for the induced back-emf voltage the term E i  is added in the phase voltage calculation as an input to function block  70  from controller function block  74 . The back-emf compensation value is derived from the excitation angle and speed, received as inputs to block  74  using back-emf coefficient K ei . To compensate for voltage drop attributed to phase winding resistance and parasitic resistance, the term R i I i (t) is added in the phase voltage calculation as an input to function block  70  from controller function block  76 . In operation, controller  44  successively outputs control signals V i (t) for individual energization of respective phase windings. 
     FIG. 4  is a flow chart illustrating operation of the adaptive cruise control system of the present invention. After an appropriate delay to wait until a cruise control loop begins (step  82 ), the cruise control system checks whether or not the switch  54  for engaging cruise control operation is in an ON state (step  84 ). If so, the cruise control system checks whether the brake pedal  56  was pressed after engaging the cruise control operation (step  86 ). If so, the cruise control operation is terminated. However, if the brake pedal was not pressed, the system determines the desired speed ω d  to be maintained (step  88 ). The desired speed is defined in a well know manner by set/resume switch  50  that sets the desired speed in the cruise control system, or commands the system to resume the desired speed set previously. 
   The actual speed ω measured in a well-known manner (step  90 ) is compared with the desired speed ω d  to calculate the speed error Δω as the difference between the actual speed and the desired speed (step  92 ). The torque command τ d  that defines torque required to achieve the desired speed ω d  is determined based on the speed error and a desired acceleration/deceleration characteristics (step  94 ). For example, the torque command may be determined for linear or S-curve acceleration/deceleration characteristics based on well-known algorithms. 
   The torque command, actual speed and rotor position are input to the look-up table of pre-computed motor control scheme data in the profile memory  48 . The look-up table may store motor control scheme data for supporting various modes of operation manifesting different operational aspects. For example, a high-efficiency motor control scheme may be utilized to energize the phase windings with current of sinusoidal waveform for high efficiency operation. The sinusoidal current waveform profile obtained with this motor control scheme can extend battery life. 
   The sinusoidal wave current trajectory I sin (t) of the desired phase current I di (t) supplied to the controller  44  is generated from the following equation
 
 I   sin   =I   m sin( N   r θ i ),
 
where I m =2τ d /N s K τi  denotes the phase current magnitude, N r  denotes the number of permanent magnet pairs, θ i  denotes the measured per phase rotor position signal, N s  represents the total number of phase windings, and K τi  denotes a per-phase torque transmission coefficient. This sinusoidal current waveform profile provides efficient motor operation.
 
   Different expressions can be used to obtain different current waveform profiles for manifesting other operational aspects, although sacrificing some of the efficiency achieved with the sinusoidal waveform profile. For example, for higher torque operation, a high-torque motor control scheme may be utilized to obtain a square wave current waveform trajectory I sq (t) of the desired phase current I di (t) supplied to the controller  44 . The square wave current waveform trajectory I sq (t) may be obtained using the following expression:
 
 I   sq   =I   m   sgn (sin( N   r θ i ))
 
where sgn (x) denotes the standard signum function and is defined as 1 if x&gt;0, 0 if x=0, and −1 if x&lt;0. Practically, the square wave current waveform I sq (t) has a trapezoidal shape with configurable rising and falling edges.
 
   Profile memory  48  stores data that are used by the controller  44  to obtain the current values that satisfy the expressions exemplified above. The data may be stored as a lookup table, each entry of which represents a value of the desired phase current I di (t) supplied to the controller  44 , for a particular combination of torque command value and rotor position for the corresponding motor control scheme. Another section of the lookup table may store corresponding values of the term L i dI di /dt for the selected desired phase current I di (t). 
   If a control scheme is selected for which the sinusoidal waveform is produced, the corresponding profile memory data is accessed. Square wave profile memory data would be accessed if the corresponding control scheme is selected. Alternatively, the profile memory may store data for each profile with which the desired current value I di  is repeatedly computed by the controller  44  in real time. While expressions for sinusoidal and square wave waveforms have been set forth above for purposes of illustration, other waveform profiles, such as sawtooth, etc., may be utilized for different operational purposes. 
   Hence, the controller  44  at step  96  interacts with the profile memory  48  to determine a control strategy appropriate for current driving conditions. In particular, the controller determines whether or not the cruise control system, in the high-efficiency profile motor control scheme, has the capability of meeting the torque command requirements to maintain the desired speed. Such determination can be made with reference to the value of the voltage V i (t) from the output of block  70  of  FIG. 3  that would be derived from values of the torque command and the motor speed. 
   The torque demands can be met if the derived control voltages do not exceed the voltage level of the power supply. If the derived level of this output does not exceed the power supply voltage as determined in step  96 , the controller can apply the voltage required by the high efficiency motor control scheme for torque tracking. The controller, after any appropriate delay, will then access the profile memory  48  to retrieve efficiency motor scheme data from the lookup table at step  98 . If, instead, the derived voltage level is higher than the power supply voltage, determination is made at step  96  that the power supply capacity is exceeded. The controller, after any appropriate delay, will then access the profile memory to retrieve high-torque motor control scheme data from the lookup table at step  100 . 
   With the sinusoidal waveform profile, if the cruise control system requests a torque command that correlates to the maximum current draw, then the motor output is limited to approximately 54 Nm. The square wave profile, however, can produce 68 Nm without exceeding the 10 amp maximum rating of the power supply. However, the increase in torque production of the square wave profile is gained at the expense of efficiency with respect to the sinusoidal profile. 
   While the motor control scheme selection represented by step  96  can be performed by repeated calculation of a torque capacity threshold on a real time basis, calculations of voltage for various combinations of torque request and motor speed can be made in advance and linked with the appropriate profile in a lookup table in the profile memory.  FIG. 5  is a curve that represents a boundary in such a lookup table between ranges for efficiency motor control scheme selection and high-torque motor control scheme selection for values of torque and speed in accordance with the illustrated example. The lookup table is formulated by making the above described controller voltage/supply voltage comparison for a multitude of speed/requested torque combinations. With the abscissa of the graph representing speed and the ordinate representing requested torque, the curve is asymptotic to both axes with speed/torque combinations above the curve being beyond the capacity of the system to track torque in the high efficiency profile operational mode. The controller  44  upon accessing this lookup table will select the high-efficiency motor control scheme for speed/torque combinations represented by points below the curve and select the high-torque motor control scheme for the remaining points. 
   Controller  44  successively outputs control signals V i (t) to the power block  42  for individual energization of respective phase windings in a sequence established in the controller. Each successive control signal V i (t) is related to the particular current sensed in the corresponding phase winding, the immediately sensed rotor position and speed, and also to model parameters, K ei  and K τi , that have been predetermined specifically for the respective phases. 
   Based on selected waveform profile of the desired phase current I di (t), the controller derives the value of the voltage V i (t) in real time, utilizing the torque command value and the signals received from phase current sensors, position sensor and speed detector (step  102 ). The computations illustrated in  FIG. 3  are performed successively in real time. In addition to the term L i dI di /dt, the lookup table in the profile memory  48  may store the back-EMF value E i  used in the calculation of the voltage V i (t). The value E i  may be selected from the lookup table based on a combination of speed and rotor position. 
   Then, the controller  44  successively outputs control signals V i (t) to the power block  42  for individual energization of respective phase windings in a sequence established in the controller (step  104 ). Each successive control signal V i (t) is related to the particular current sensed in the corresponding phase winding, the immediately sensed rotor position and speed, and also to model parameters, K ei  and K τi , that have been predetermined specifically for the respective phases. 
   In this disclosure there is shown and described only preferred embodiments of the invention and a few examples of its versatility. It is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein. For example, various other motor control schemes defining different current waveform profiles may be utilized. The profile memory thus may store a plurality of motor control scheme data sets accessible by the controller in response to receipt of specific motor control scheme selection commands. Various lookup tables of varying complexities can be formulated for appropriate profile mode selection by the controller.