Abstract:
A method and apparatus for limiting current and voltage in a PM electric machine comprising: receiving a command indicative of a desired position control; obtaining a velocity value indicative of the rotational velocity of the electric machine; and computing a first voltage threshold for the electric machine based on a selected operating condition. If the voltage command exceeds the first voltage threshold, the method also includes establishing a modified voltage command as substantially equivalent to the first voltage threshold, otherwise establishing a modified voltage command as substantially equivalent to the voltage command.

Description:
BACKGROUND 
     Electric Steering applications such as Electric Power Steering (EPS), four wheel steering e.g., Quadrasteer™ (Qsteer) and Active Front Steer (AFS) are used in vehicles to improve performance fuel economy and stability of the vehicle. Commonly, in such systems an electronic controller is configured to drive an electric motor to provide torque, velocity or positioning control. 
     Steering applications such as AFS and Quadrasteer™ utilize motor position control. It is desirable to use a brushless permanent magnet (PM) motor in such applications for its higher efficiency and high torque density. Generally, the motor can be designed and controlled to exhibit a sinusoidal back EMF (electromotive force), which provides smoother torque feel or a trapezoidal back EMF, which while easier to control, can suffer from commutation ripple and noise. The sinusoidal back EMF motor can be controlled utilizing phase advance, thus further reducing the size of the motor. Therefore, it is often desirable to use brushless permanent magnet motors with sinusoidal back EMF for these applications. Brushless permanent magnet motors can be position controlled employing either current mode control or voltage mode control. Voltage mode control advantageously, provides damping when applied voltage is not compensated for back EMF. In voltage mode control, the voltage command to the motor is primarily a function of application control variables disregarding the motor characteristics. In addition, voltage mode control systems may be desirable in certain applications because the need for external sensors to provide feedback is minimized. Unfortunately, however, with voltage mode control the torque, and therefore, the current flowing through the motor is not measured or controlled. For position control applications using voltage command, the voltage is a direct function of the position error, therefore, a high voltage is applied across the motor winding at high position errors while a small voltage is applied for small position motor irrespective of motor velocity. By the principal of the operation of the motor, the voltage applied across the motor is function of motor torque and the back EMF of the motor. At very low velocity, even small voltage applied across the motor can result into high torques and therefore high current. At higher voltage and low velocity, the torque, and thereby the current of the motor can be several times a motors rating. Steering control systems employing voltage mode control algorithms, generally do not use the motor phase current for torque control. Moreover, it may be beneficial to limit motor torque and current to avoid exceeding motor or controller ratings. While the phase current is readily available for measurement, such measurement would require additional sensors and interfaces. Therefore, in a voltage control system, it may be desirable to constrain maximum voltages and thereby avoid exceeding rated torques and currents without relying upon current sensors or measurements. 
     BRIEF SUMMARY 
     Disclosed herein in an exemplary embodiment is a method for limiting current and voltage in a PM electric machine comprising: receiving a voltage command indicative of a desired position control; obtaining a velocity value indicative of the rotational velocity of the electric machine; and computing a first voltage threshold for the electric machine based on a selected operating condition. If the voltage command exceeds the first voltage threshold, the method also includes establishing a modified voltage command as substantially equivalent to the first voltage threshold, otherwise establishing a modified voltage command as substantially equivalent to the voltage command. 
     Also disclosed herein in yet another exemplary embodiment is a system for limiting current and voltage in a PM electric machine comprising: a PM electric machine; a position sensor configured to measure the rotor position of the electric machine and transmit a position signal; a controller, the controller in operable communication with a voltage source and the electric machine and the position sensor, the controller computing a voltage command responsive to a position control. 
     If the voltage command exceeds a first voltage threshold, the controller establishes a modified voltage command as substantially equivalent to the first voltage threshold, otherwise the controller establishes a modified voltage command as substantially equivalent to the voltage command. 
     Further disclosed herein in another exemplary embodiment is a system for limiting current and voltage in a PM electric machine comprising: a means for receiving a voltage command indicative of a desired position control; a means for obtaining a velocity value indicative of the rotational velocity of the electric machine; and a means for computing a first voltage threshold for the electric machine based on a selected operating condition. If the voltage command exceeds the first voltage threshold, the system also includes a means for establishing a modified voltage command as substantially equivalent to the first voltage threshold, otherwise a means for establishing a modified voltage command as substantially equivalent to the voltage command. 
     Also disclosed herein in yet another exemplary embodiment is a storage medium encoded with a machine-readable computer program code for limiting current and voltage in a PM electric machine, the storage medium including instructions for causing controller to implement the abovementioned methodology. 
     Disclosed herein in yet another exemplary embodiment is a computer data signal embodied in a carrier wave for limiting current and voltage in a PM electric machine, the data signal comprising code configured to cause a controller to implement the abovementioned methodology. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of an example, with references to the accompanying drawings, wherein like elements are numbered alike in the several figures in which: 
         FIG. 1  is a simplified block diagram depicting a motor control system with voltage-profiling in accordance with an exemplary embodiment; 
         FIG. 2  depicts an illustrative motor torque at different voltages if the torque is allowed to exceed the maximum current/voltage; 
         FIG. 3  is a simplified block diagram depicting a voltage-profiling algorithm in accordance with an exemplary embodiment; 
         FIG. 4  depicts an illustrative motor torque at different voltages if the torque is limited to the maximum current/voltage; 
         FIG. 5A  show an exemplary torque velocity profile for a motor in all four quadrants of operation; and 
         FIG. 5B  show the operation voltage boundaries for this motor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Electric steering applications such as electric power steering, Quadrasteer and Active Front Steering systems (AFS) often use voltage mode controlled sinusoidal Brushless permanent magnet drives for position control actuators. The voltage mode controlled sinusoidal PM drive used may not include active current measurement. Without an active current measurement, it may be difficult to limit or clamp the current, especially at low velocities. Addition of active phase current sensing is possible but can result into an increase in cost of the drive. Disclosed herein in an exemplary embodiment is a system and methodology for implementing a current limiting clamp without active motor current sensing. An algorithm is presented which estimates the motor voltage as a function of maximum rated motor torque and velocity at each operating point and limits the motor voltage at each operating point. 
     The disclosed embodiments may be utilized in various types of vehicles employing motor control systems such as may be employed in electronic steering, four wheel steer, active front steer or steer by wire systems. A preferred embodiment, by way of illustration is described herein as it may be applied to an automobile employing a steering system for position control of a steerable wheel. While a preferred embodiment is shown and described by illustration and reference to a motor control system as may be employed in a automobile steering system, it will be appreciated by those skilled in the art that the invention is not limited to the automobiles alone by may be applied to other motor control systems employing electronic motor controls. 
     Referring now to the drawings in detail,  FIG. 1  depicts a PM motor system  10  where numeral  10  generally indicates a system for controlling the torque, velocity or position of a sinusoidally excited PM electric machine  12  (e.g. a motor, hereinafter referred to as a motor). The system includes, but is not limited to, a motor rotor position encoder  14 , an optional velocity measuring circuit  16 , an optional velocity sensor  17 , a controller  18 , power circuit or inverter  20  and power source  22 . Controller  18  is configured to develop the necessary voltage(s) out of inverter  20  such that, when applied to the motor  12 , the desired response is generated. Because these voltages are related to the position and velocity of the motor  12 , the position and velocity of the rotor are determined. A rotor position encoder  14  is connected to the motor  12  to detect the angular position of the rotor denoted θ. The encoder  14  may sense the rotary position based on optical detection, magnetic field variations, or other methodologies. Typical position sensors include potentiometers, resolvers, synchros, encoders, and the like, as well as combinations comprising at least one of the forgoing. The position encoder  14  outputs a position signal  24  indicating the angular position of the rotor. 
     The motor velocity denoted ω m  may be measured, calculated or a combination thereof. Typically, the motor velocity ωm is calculated as the change of the motor position θ as measured by a rotor position encoder  14  over a prescribed time interval. For example, motor velocity ω m  may be determined as the derivative of the motor position θ from the equation ω m =Δθ/Δt where Δt is the sampling time and Δθ is the change in position during the sampling interval. Another method of determining velocity depending upon the type of encoder employed for the motor position θ, may be to count the position signal pulses for a predetermined duration. The count value is proportional to the velocity of the motor. In the figure, a velocity measuring circuit  16  determines the velocity of the rotor and outputs a velocity signal  26 . In yet another option, a velocity sensor  17  may be employed to directly measure the velocity of the motor  12  and provide a velocity signal  26 . 
     The temperature of the motor  12  may be measured utilizing one or more optional temperature sensors located at the motor windings (not shown). The temperature sensor transmits a temperature signal  27  to the controller  18  to facilitate the processing prescribed herein. Typical temperature sensors include thermocouples, thermistors, thermostats, and the like, as well as combinations comprising at least one of the foregoing sensors, which when appropriately placed provide a calibratable signal proportional to the particular temperature. 
     The position signal  24 , velocity signal  26 , (optional temperature signal  27 ), and position control input signals  28  are applied to the controller  18 . The position control input signals  28  is representative of the desired position command and feedback input for position control applications. For the position control the controller  18  processes all input signals to generate values corresponding to each of the signals resulting in a rotor position value, a motor velocity value, (an optional temperature value) being available for the processing in the algorithms as prescribed herein. Measurement signals, such as the above mentioned are also commonly linearized, compensated, and filtered as desired or necessary to enhance the characteristics or eliminate undesirable characteristics of the acquired signal. For example, the signals may be linearized to improve processing velocity, or to address a large dynamic range of the signal. In addition, frequency or time based compensation and filtering may be employed to eliminate noise or avoid undesirable spectral characteristics. 
     In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the execution of position control algorithm(s), the voltage clamping process as prescribed herein, and the like), controller  18  may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interfaces, and input/output signal interfaces, as well as combinations comprising at least one of the foregoing. For example, controller  18  may include signal input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. Moreover, controller  18  may include or be implemented with various processors, controllers, microcontrollers, logic/gate arrays, programmable logic arrays (PLA), programmable logic devices, and the like, as well as combinations including any of the foregoing. Additional features of controller  18  and certain processes therein are thoroughly discussed at a later point herein. 
     The controller  18  determines the voltage command V ref    30  required to develop the desired position control by using the position control input signals  28  and may also be responsive to the position signal  24 , velocity signal  26 , temperature signal  27  and the like. For a three-phase motor, three sinusoidal reference signals that are synchronized with the motor back EMF {right arrow over (E)} are required to generate the required motor input voltages. 
     In an exemplary embodiment an additional voltage profiling process  100  is interjected in the voltage control loop for a sinusoidally controlled permanent magnet motor  12 . A motor voltage command denoted as V ref    30  for a desired position control is commanded from the Position Control Algorithm as depicted at  50 . The voltage profiling process  100  receives the voltage command V ref    30  and motor velocity signal  16  and generates a modified voltage command V ref     —     mod    31 . 
     The modified voltage command V ref     —     mod    31  and motor position θ are fed into the motor control algorithm as depicted at block  60 , which generates the PWM switching signal to control the sinusoidal voltage across each phase of the motor. The motor control of process block  60  transforms the modified voltage command signal V ref     —     mod    31  into phase commands by determining phase voltage command signals V a , V b , and V c  from the modified voltage command signal V ref     —     mod    31  and the position signal  24  according to the following equations:
 
 V   a   =V   ref     —     mod  sin (θ)
 
 V   b   =V   ref     —     mod  sin (θ−120°)
 
 V   c   =V   ref     —     mod  sin (θ−240°)
 
     In a motor drive system employing phase advancing, the phase advancing angle denoted as δ may also be calculated as a function of the input signal. The phase voltage signals V a , V b , V c  are then phase shifted by this phase advancing angle δ. Phase voltage command signals V a , V b  and V c  are used to generate the motor duty cycle signals D a , D b , and D c    32  using an appropriate pulse width modulation (PWM) technique. Motor duty cycle signals  32  [SHOW  32  ON FIGURE] of the controller  18  are applied to a power circuit or inverter  20 , which is coupled with a power source  22  to apply phase voltages  34  to the stator windings of the motor in response to the motor voltage command signals. 
     Turning now to  FIG. 2 , the voltage at each operating point of the motor  12  is a function of velocity and torque at that operating point. A maximum rated profile may readily be established for the maximum rated torque as the function of motor velocity. It will be appreciated then that the torque of the motor  12  may be limited to the maximum rated torque independent of the motor voltage.  FIG. 2  depicts an illustrative motor torque at different voltages if the torque is allowed to exceed the maximum current/voltage. In this illustration it may readily be observed that the torque of the motor attains almost four times the maximum rated torque at low velocities with an excitation of 12V. 
     In an exemplary embodiment, the torque of the motor  12  may be limited by limiting the maximum voltage available to be applied to the motor  12  as a function of motor velocity, ω. This may be accomplished by calculating the motor voltage for the maximum torque at a given velocity with the angle δ between the back EMF {right arrow over (E)} and the voltage {right arrow over (V)} vectors is fixed to a selected value. The maximum voltage across the motor  12  may be computed as: 
     
       
         
           
             
               
                 
                   
                     V 
                     max 
                   
                   = 
                   
                     
                       1 
                       
                         
                           R 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           δ 
                         
                         + 
                         
                           
                             P 
                             2 
                           
                           ⁢ 
                           
                             ω 
                             r 
                           
                           ⁢ 
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           sin 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           δ 
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             
                               
                                 R 
                                 2 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       P 
                                       2 
                                     
                                     ⁢ 
                                     
                                       ω 
                                       r 
                                     
                                     ⁢ 
                                     L 
                                   
                                   ) 
                                 
                                 2 
                               
                             
                             
                               3 
                               ⁢ 
                               
                                 K 
                                 e 
                               
                             
                           
                           ⁢ 
                           
                             T 
                             
                               cmd 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               max 
                             
                           
                         
                         + 
                         
                           
                             K 
                             e 
                           
                           ⁢ 
                           
                             ω 
                             r 
                           
                           ⁢ 
                           R 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where δ between the back EMF {right arrow over (E)} and the terminal voltage {right arrow over (V)}, 
     T cmd max (ω r ) is the maximum motor torque at velocity ω r , both negative and positive maximum torque are used in the equation. 
     K e  is the back EMF constant of the motor, 
     P is the number of poles in the motor, 
     R is the motor phase resistance, and 
     L is the motor phase inductance. 
     Equation 4 shows the maximum voltage may be computed utilizing known motor parameters and motor operating information. Advantageously, only information regarding motor parameters, rotor velocity, ω and position angle, θ is required for the controller  18  to develop a signal to produce a desired motor response, no current feedback is needed. 
     In an alternative embodiment for implementation of these equations, a further simplification is made when phase advancing is not used. Under these conditions Equation 4 reduces to: 
     
       
         
           
             
               
                 
                   
                     V 
                     max 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             
                               R 
                               2 
                             
                             + 
                             
                               
                                 ( 
                                 
                                   
                                     P 
                                     2 
                                   
                                   ⁢ 
                                   
                                     ω 
                                     r 
                                   
                                   ⁢ 
                                   L 
                                 
                                 ) 
                               
                               2 
                             
                           
                           
                             3 
                             ⁢ 
                             
                               K 
                               e 
                             
                             ⁢ 
                             R 
                           
                         
                         ⁢ 
                         
                           T 
                           
                             cmd 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             max 
                           
                         
                       
                       + 
                       
                         
                           K 
                           e 
                         
                         ⁢ 
                         
                           ω 
                           r 
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
       FIG. 3  depicts a block diagram of the voltage profile algorithm  100  in accordance with an exemplary embodiment. In the voltage profile process block  106 , the maximum permissible voltage V max  and minimum permissible voltage V min  (voltage corresponding to maximum rated negative torque) are calculated as a function of motor velocity ω fed into the block  106  and positive and negative maximum torque at that velocity. The maximum rated torque profile is used from the machine design parameters, which gives the rated maximum torque (or current) permitted for operation of motor. Alternatively, these values may readily be calculated off line and stored as a look up table indexed as a function of operational parameters, motor velocity for example. Values for V max  and V min  may also readily be interpolated for instances when a lesser number of points are stored. It will be appreciated that in this instance, maximum and minimum correlate with direction of the torque commands and velocity. That is, maximum voltage corresponds with a maximum rated torque for one direction, while minimum voltage corresponds with a maximum rated torque for the opposite direction. 
     At decision block  104 , the voltage command, V ref    30  is compared to maximum voltage V max  fed from a voltage profile process block  106 . If the voltage command, V ref    30  is greater than the maximum voltage V max  for a particular operating point a modified voltage command denoted V ref     —     mod    31  is set equal to maximum voltage V max , as depicted at process block  108 . Otherwise, the voltage command V ref    30  is transmitted into second decision block  111 . At the decision block  111 , the voltage command V ref    30  is compared to minimum voltage V min  fed from the voltage profile process block  106 . If the voltage command V ref    30  is less than the minimum voltage V min  for a particular operating point, the modified voltage command denoted V ref     —     mod    31  is set equal to minimum voltage V min , as depicted at process block  112 . Otherwise the modified voltage command V ref     —     mod    31  is set equal to V ref    30  as depicted at process block  110 . It will be appreciated that the process and decision blocks and comparison of the voltages can be implemented by numerous other methods. It should be understood that the methodology of the disclosed embodiments provide a methodology for performing the comparison, it should now be apparent that other methods are conceivable. 
       FIG. 4  depicts an illustration of output torque of the motor  12  at different voltage commands including the voltage limiting of an exemplary embodiment. It may readily be appreciated that the torque of the motor  12  is always within the maximum velocity torque profile of the motor  12 . Moreover, it may be seen that the torque is limited to the maximum profile at the points where it hits that bound and is allowed follow it own trajectory when it is within the bound.  FIG. 5A  show the torque velocity profile for the motor considered in this application in all four quadrants of operation.  FIG. 5B  shows the rated operation range voltage boundaries calculated equation (5) for this motor. 
     The disclosed invention can be embodied in the form of computer or controller implemented processes and apparatuses for practicing those processes. The present invention can also be embodied in the form of computer program code containing instructions embodied in tangible media  13 , such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer or controller  18 , the computer becomes an apparatus for practicing the invention. The present invention can also be embodied in the form of computer program code embodied as a data signal, for example, whether stored in a storage medium, loaded into and/or executed by a computer or controller, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits. 
     It will be appreciated that the use of first and second or other similar nomenclature for denoting similar items is not intended to specify or imply any particular order unless otherwise stated. 
     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.