Abstract:
A method and apparatus for determining rotor position in a stationary rotor of a sensor-less permanent magnet synchronous machine that employs a rotating magnetic field to identify a magnetic axis of the stator without a magnetic direction and then determines magnetic direction by applying pulses along the magnet axis in two polarities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    — 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    — 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to electric motors and in particular to permanent magnet synchronous machines (PMSM) and more particularly to a method and apparatus for determining the starting position of the rotor of a PMSM without a position sensor such as a resolver. 
         [0004]    Permanent magnet synchronous machines are electric motors having a rotor holding a permanent magnet that may turn about an axis within a stator. The stator holds conductive coils that may be energized to create a rotating magnetic field. The rotating magnetic field is coordinated with the rotor position to draw the rotor along synchronously. 
         [0005]    Simple PMSM&#39;s employ power transistors to switch the electrical currents in the stator coils to create the necessary rotating magnetic field. “Sensor-less” PMSM&#39;s eliminate the rotor position sensors and deduce rotor position from its effect on the electrical signals used to drive the stator coils. 
         [0006]    In these sensor-less PMSM&#39;s, rotor position may be simply determined while the motor is operating (and the rotor is spinning) by means of the electrical voltages generated (induced) by the rotating magnetic rotor in the stator windings (so called “back-EMF”). Unfortunately, when the rotor is moving at a low speed or stationary, the back-EMF is low or nonexistent making it difficult to determine rotor position. Starting a PMSM motor without knowing the rotor position causes sudden accelerations of the rotor (possibly in the wrong direction) as the rotor attempts to align itself with the generated field. In many important motor control applications, such abrupt and unpredictable motion is undesirable. 
         [0007]    To overcome this problem, an approach has been developed for identifying rotor position that does not rely on back-EMF and thus that can work for a stationary rotor. This approach relies on variations in magnetic saliency of the rotor. Magnetic saliency refers to a change in the inductance of the stator windings as a function of the orientation of the rotor and results generally from the anisotropic magnetic properties of the rotor. 
         [0008]    A typical approach to identifying rotor position using magnetic saliency is described in U.S. Pat. No. 6,172,498 in which pulses are applied to each of the stator windings in sequence and variations in measured saliency is used to deduce the approximate location of the rotor. 
         [0009]    One limitation to this approach is that special hardware may be required. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention employs saliency techniques to identify the orientation of the rotor. The rotational speed of the field used to deduce saliency is set high enough so as to avoid rotor movement. After the orientation of this axis is determined, additional pulses can be applied along the magnetic axis also without promoting rotor movement. By applying positive and negative pulses along the magnetic axis, rotor magnetic direction is revealed by slight differences in induction caused by changes in saturation of surrounding magnetic components. In this way, absolute rotor position (magnetic axis and magnetic direction) may be determined. 
         [0011]    Specifically then, the present invention provides a method of determining rotor position in a PMSM having a permanent magnet rotor having magnetic direction, the permanent magnet rotor rotatable within stator coils receiving an electric signal to generate a stator field. In a first period, the method applies a first electrical signal to the stator coils to create a rotating stator field having a high rotational speed with a strength insufficient to cause a movement of the rotor. Current flow through the stator during this first period is used to detect a current fluctuation caused by anisotropic saliency of the permanent magnet rotor to deduce the magnetic axis. Next, in a second period after the first period, a second electrical signal is applied to the stator coils to produce a magnetic field aligned with the magnetic axis to prevent rotor motion, and current flow through this stator during the second period is used to deduce the magnetic direction and thus the orientation of the rotor. 
         [0012]    Thus it is an object of at least one embodiment of the invention to provide an unambiguous identification of rotor position without substantial rotor movement. 
         [0013]    The second electrical signal may include a first voltage pulse of a first polarity aligned with the axis of magnetic direction and a second voltage pulse of a second polarity aligned with the axis of magnetic direction and a magnitude of current flow through the stator during the first and second stator voltage pulses may be compared to deduce the magnet direction. 
         [0014]    It is thus an object of at least one embodiment of the invention to identify direction of magnetization along the stator axis by variations in electrical response of the stator to opposed magnetic fields caused by opposed voltage pulses. 
         [0015]    The first and second stator field pulses may each be followed with an equal energy current suppression pulse of opposite polarity. 
         [0016]    It is thus an object of at least one embodiment of the invention to provide extremely rapid assessment of rotor position with minimal rotor motion. 
         [0017]    The lesser magnitude of current flow may indicate alignment between the polarity of the pulse and the magnetic direction. 
         [0018]    It is thus an object of at least one embodiment of the invention to make use of saturation effects in ferromagnetic components to deduce the direction of the magnetic field of the rotor. 
         [0019]    The invention may further include the step of, in a third period, controlling a startup of the motor based on the deduced orientation of the rotor. 
         [0020]    It is thus an object of at least one embodiment of the invention to provide an improved method of starting a PMSM. 
         [0021]    The invention may include the further step of applying a third electrical signal to the stator coils, during the third period, to cause movement of the rotor according to a command signal, and further applying the first electrical signal to the stator coils having a substantially higher frequency than the third electrical signal during the third period, and further monitoring the current flow through the stator caused by the first electrical signal to update the rotor position. 
         [0022]    It is thus an object of at least one embodiment of the invention to provide for sensor-less operation of the motor using the ambiguous saliency information as initialized by the present invention&#39;s determination of magnet direction. 
         [0023]    It is thus an object of at least one embodiment of the invention to provide a simple yet high-resolution method of determining rotor axis independent of the number of stator poles. 
         [0024]    These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a simplified cross-sectional view through a single pole pair permanent magnet synchronous machine of the type suitable for use with the present invention; 
           [0026]      FIG. 2  is an enlarged view of the rotor of  FIG. 1  showing various axes and a rotating magnetic field described in the specification; 
           [0027]      FIG. 3  is a block diagram of an electronic drive for the motor of  FIG. 1  such as may incorporate the present invention; 
           [0028]      FIG. 4  is a flow chart showing the steps implemented by a program in the electronic drive of  FIG. 3 ; 
           [0029]      FIG. 5  is a simplified plot of saliency as a function of stator field angle showing features indicating the rotor magnetic axis; 
           [0030]      FIG. 6  is a frequency domain plot showing isolation of a high-frequency saliency signal and its phase to deduce rotor axis; 
           [0031]      FIG. 7  is a block diagram of one method of extracting the position information of  FIG. 6  that may be implemented by the position detection system of the present invention; 
           [0032]      FIG. 8  is a pulse sequence used in the present invention for determination of magnetic direction once rotor axis has been determined; 
           [0033]      FIGS. 9 and 10  are schematic representations of the fields applied by the pulses of  FIG. 8  superimposed on a fragmentary view of the rotor and stator of  FIG. 1  showing the influence of magnetic saturation. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0034]    Referring now to  FIG. 1 , a PMSM motor  10  provides a rotor  12  attached to a rotatable shaft (not shown) for rotation about an axis through the center of the rotor  12  perpendicular to the plane of the figure. The rotor  12  includes a permanent magnet  14  and ferromagnetic flux directors  16 . For clarity, a rotor having a single pole pair (e.g. only a single north and south pole) is shown; however, it will be understood that the present invention is equally applicable to multi-pole rotors. 
         [0035]    The rotor  12  may be surrounded by a stator  18  having one or more coils  20   a ,  20   b , and  20   c  shown here positioned at regular angles of 120° about the rotor  12 . Again additional coils may be provided according to techniques well known in the art for motors having larger numbers of poles. 
         [0036]    Generally, appropriately phased signals may be applied to each of coils  20   a ,  20   b  and  20   c  to produce a rotating magnetic field vector  22  synchronously attracting the rotor  12  to drive the motor  10 . 
         [0037]    Referring now to  FIG. 2 , the permanent magnet  14  of the rotor  12  has a magnetic vector defined by the north pole and one south pole of the permanent magnet  14  and defining both a magnetic axis  34  and a magnet direction  36 , (the latter having a direction, from south to north). A d-axis  34 ′ of the rotor  12  is aligned with the magnetic axis  34  and a q-axis  37  of the rotor is perpendicular to the magnetic axis  34 . The magnetic axis  34  and magnet direction  36  also define a rotor angular position θ measured between an arbitrary stationary reference point and the d-axis. 
         [0038]    Referring now to  FIG. 3 , a motor drive  30  suitable for practicing the present invention may receive command signals at motor control logic  32 , for example position and velocity or acceleration commands, and may interpret these commands into a q and d current Iq and Id respectively. As is understood in the art, these currents define stator currents that will produce magnetic fields in the q-axis and d-axis directions respectively. In order to effect acceleration and speed control, the motor control logic  32  must receive an indication of rotor position θ. 
         [0039]    The Iq and Id currents are received by PID controllers (proportional, integral, derivative controllers) or other similar feedback control circuits  40  and  38  respectively, which provide voltage commands Vq and Vd to null the error signals for the Iq and Id currents respectively. 
         [0040]    The error signals may be received by one pole ( 3 ) of a single pole, triple throw switch pair  42 , which during normal operation, is connected to a vector rotator  44  which, using knowledge of θ (during normal operation), converts the Vd and Vq voltage commands to A, B and C voltages corresponding to coils  20   a ,  20   b  and  20   c  described above and more generally to three phases of power provided to the motor  10 . The present invention, in which θ is initially unknown, substitutes a generated value of γ to produce the desired test waveforms to be described. 
         [0041]    These A, B and C voltage commands are received by a pulse width modulator  46  which provides pulse width modulated signals to an inverter  48  which in turn provides high power current to the coils  20  and thus to the motor  10  regulating the A, B and C currents. 
         [0042]    The current on coils  20  is monitored by current sensors  50  and provided to a vector de-rotator  52  performing the opposite function of vector rotator  44  in converting signals A, B and C into feedback currents I′d and I′q respectively. These feedback currents I′d and I′q are in turn provided to the feedback control circuits  38  and  40  to complete a feedback control loop as is understood in the art. 
         [0043]    The present invention augments this motor drive  30  through the addition of five blocks that form a position detection system  54 . The first block is a high-frequency injector  56  which may add a signal on top of the power provided to the motor  10  producing a high-frequency, low-power rotating magnetic field as will be described. In one embodiment, as shown, the signal from the high-frequency injector  56  is added to the A, B and C voltages after the vector rotator  44 ; however, it could also be added, with the appropriate transform, to the voltage commands Vα and Vβ, to similar effect. 
         [0044]    The second block of the position detection system  54  is a pulse injector  58  which is connected one pole ( 1 ) of pair switch  42  to be connected to the input of vector rotator  44  during a second stage of motor control before the rotor  12  is rotating. 
         [0045]    A third block of the position detection system  54  is a rotor position estimator  60  receiving the signals taken at taps  50  during application of the high-frequency from high-frequency injector  56  to determine a magnetic axis signal γ indicating the orientation of the magnetic axis  34  (without direction) as deduced from measurements of the power signals. Generally γ could either equal θ or θ+180 degrees as a result of the fundamental symmetry in saliency of the rotor  12 . 
         [0046]    A fourth block of the position detection system  54  is a magnetic direction analyzer  63  monitoring the feedback current Id during application of the pulses from the pulse injector  58  to determine magnetic direction. 
         [0047]    Finally, the fifth block  62  is a rotor position extractor using the magnetic axis signal γ and the magnetic direction to deduce θ as will be described. 
         [0048]    Generally the elements of the motor drive  30  will include discrete electrical components, including power semiconductors and the like as well as one or more computer processors executing stored programs to implement functional blocks described. 
         [0049]    Referring now to  FIG. 4 , the position detection system  54  may operate according to a stored program having initial process block  64  during which a high-speed rotational vector from injector  56  is applied to the power signals received by coils  20  of motor  10  through the pulse width modulator  46  and inverter  48 . At this time, the rotor  12  is stationary and switch pair  42  is connected to pole ( 2 ) disconnecting the feedback control circuits  38  and  40 . 
         [0050]    Referring momentarily to  FIG. 2 , this high-speed rotational vector  66  has a low-strength and high angular speed such as to not induce rotation in the rotor  12 . As a practical matter, the rotor  12  experiences a slight torque from the projection of the rotational vector  66  on the q-axis, but the direction of torque changes rapidly so that the rotational inertia of the rotor  12  prevents substantial motion. Generally the speed of rotation of the vector  66  will be substantially greater than the normal rotational speed of the motor but at a frequency low enough to prevent substantial inductive attenuation. 
         [0051]    Referring again to  FIGS. 3 and 4 , at process block  68 , the currents at the coils  20  are monitored by the estimators  60  to determine the angle of the rotor magnetic axis  34 . As shown in  FIG. 5 , the magnetic axis  34  can be discovered by monitoring the currents  70  of the stator  12  with respect to the vector  66  over a full motor phase cycle. A motor phase cycle will be 360° for a single pole pair motor, or 360°/N for an N-pole pair motor where generally N is the number of duplications of coils  20   a - 20   c  around the stator. The currents are analyzed to identify saliency features  72  caused by anisotropic qualities of the rotor that indicate the angle θ of the rotor. The current, for example, may be highest when vector  66  is aligned with axis  34  and so the current  72  may indicate peaks in saliency. Note generally that there will be two saliency features  72  for every motor phase cycle and so this determination of process block  68  identifies only the magnetic axis  34  (γ) and not the magnetic direction  36  (θ). 
         [0052]    Referring to  FIG. 6 , generally, the angle of the magnetic axis  34  may be determined from the saliency by extracting a frequency component  74  rotating in the opposite direction of the frequency  76  of the rotating vector  66  (reflecting the symmetrical property of saliency) and identifying a phase  78  of that frequency component  74  such as indicates the angle γ. 
         [0053]    Referring to  FIG. 7 , this extraction may, in theory, be done through a series of filter blocks including a bandpass filter  79  receiving the stator currents iabc from the coils  20 , followed by a vector de-rotator  51 ′ producing quadrature signals iα and iβ. A first vector rotator  81  receives the quadrature signals and is followed by a high pass filter  82  which may be used to extract frequency component  74  which may be derotated by vector rotator  83  and smoothed by low pass filter  84  to be processed by an arc tangent circuit  85  and halving block  86  to deduce angle γ. 
         [0054]    Referring again to  FIG. 4 , once the magnetic axis  34  is determined at process block  86 , the magnet direction  36  is deduced by applying first and second polarity pulses along the d-axis (determined to be at angle θ). These pulses may be produced by pulse injector  58  as shown in  FIG. 3  when switch pair  42  is switched to poles ( 1 ). 
         [0055]    Referring to  FIG. 8 , a first polarity voltage pulse  90  produces a current pulse  92  in the stator windings reaching an amplitude of A 1  during the time of pulse  90 . The pulse  90  is quickly followed by a current suppression pulse  94  of equal and opposite area to stop the current flow through the stator windings. Because the pulse  90  and current suppression pulse  94  are applied along the d-axis, little or no torque is exerted on the rotor  12 . As indicated by process block  96  of  FIG. 4 , the first polarity pulse  90  and current suppression pulse  94  are followed by a second polarity voltage pulse  90 ′ having opposite polarity as pulse  90  to produce a negative polarity current pulse  92 ′ having amplitude A 2 . Pulse  90 ′ is followed by current suppression pulse  94 ′ having opposite polarity as pulse  90 ′ to suppress the current flow through the stator winding. Generally the amplitude A 1  will differ from amplitude A 2  and this difference will determine the magnetic direction  36 . 
         [0056]    Referring to  FIG. 9 , with the rotor  12  in a first orientation along the rotor axis  34 , the permanent magnet  14  will cause magnetization  36 ′ in ferromagnetic elements  97  associated with the rotor  12  or stator  18 . The first polarity pulse  90  may produce a magnetic vector  95  generally counter to the magnetic direction  36  and magnetization  36 ′, depending on the position of the rotor  12 . In this case, the opposite directions of the induced magnetization  36 ′ and magnetic vector  95  will reduce saturation of these ferromagnetic elements  97  increasing the peak inductances seen by the stator coils  20  thus decreasing the height of pulse amplitude A 1 . In contrast, as seen in  FIG. 10 , the opposite polarity pulse  90 ′ will produce a magnetic vector  95 ′ aligned with the magnetization  36 ′ causing deeper saturation of the ferromagnetic elements  97  decreasing the inductance and increasing the height of pulse  92 ′. Thus the actual magnet direction  36  may be deduced by comparing these two amplitudes A 1  and A 2  as indicated by process block  100 . 
         [0057]    Referring to  FIG. 3 , a comparison of the amplitudes A 1  and A 2  is performed by magnetic direction analyzer  63  to determine magnetic direction which is used by rotor position extractor  62  using the magnetic axis signal γ and the magnetic direction to provide a running determination of θ which in turn may be used by the vector rotator  44  and vector de-rotator  52  in lieu of γ during starting of the motor  10  as indicated by process block  102 . Just before the motor  10  has started, as indicated by process block  104 , the high-frequency signal of process block  64  is reapplied to the stator coils  20 , per process block  102 , and used to extract rotor magnetic axis  34  in the manner described with respect to process block  68  for ongoing sensor-less operation. 
         [0058]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.