Abstract:
An method for driving a motor is provided. A plurality of pulse width modulation (PWM) signals are generated from a commanded voltage signal and a commanded angle signal, and these PWM signal are used to drive a motor (which has a plurality of phases). Currents through the phases of the motor are measured, and a Park transformation is performed on the measured currents to determine a projection current measurement. Based at least in part on the projection current measurement, the adjusting the commanded voltage signal and the commanded angle signal can be adjusted.

Description:
TECHNICAL FIELD 
     The invention relates generally to motor control and, more particularly, to sensorless control of a permanent magnet synchronous motor (PMSM) or brushless direct current motor (BLDC). 
     BACKGROUND 
     Turning to  FIG. 1 , a conventional system  100  can be seen. This system  100  generally comprises a motor controller  102 , a power supply  104 , an inverter  106 , and a motor  108  (which is typically a PMSM or BLDC). In operation, the motor controller  102  provides generally continuous pulse width modulation (PWM) signals (i.e., 6 PWM signals if the motor  108  is a three-phase motor). These PWM signals are used to control the inverter  106 , so that the inverter  106  can provide the commanded voltage to each phase of motor  108  from power supply  104 . 
     The motor controller  102  provides control of motor  108  (through the application of the PWM signals) based on a field-oriented control (FOC) algorithm. For conventional FOC control, there are typically three control loops (one speed loop and two current loops) that are employed to provide adjustments. Typically, the observer  120  forms a portion of the speed loop and determines a feedback speed or feedback signal ω from the PWM signals (provided to the inverter  106 ) and from the motor  108 . A difference between this feedback signal ω and a reference speed or reference signal ω* (which is determined by assert  110 - 1 ) is adjusted by the proportional-integral (PI) controller  112 - 1  to generate the reference torque current i q * for the quadrature axis or q-axis. Additionally, a field weakener  114  provides the reference field current i d * for the direct axis d-axis (in normal operation, i d *=0). The observer  120  also determined the rotor position or angle and provides the angle signal θ to the Park converter  118  and PWM controller  116 . The current loops generally includes the Park converter  118 , which determines currents i d  and i q  from phase current measurements and the angle signal θ. These currents i d  and i q  are then compared to or subtracted from the reference current i d * and i q * by adders  110 - 2  and  110 - 3 , respectively, to generate errors ΔI d  and ΔI q . These errors ΔI d  and ΔI q  can then be further adjusted by PI controllers  112 - 2  and  112 - 3 , and the commanded voltages V d  and V q , along with the angle signal θ (which form a voltage command vector {right arrow over (V)}), can be used to generate the PWM signals, and generation of the PWM signals is usually accomplished by an inverse Park transformation (performed by an inverse Park converter within PWM controller  116 ) and a space vector PWM generator (within the PWM controller  116 ) so as to generate three phase voltages. 
     There are some drawbacks, however, to using conventional, sensorless FOC controls for PMSMs. Namely, the observer  120  is usually the limiting feature because of computationally intensive processes performed by the observer  120  and because of the complexity of the system  100  with simultaneous current and/or voltage measurements. Usually, there are multiple observers employed (i.e., one for speed/position and one for online parameter estimation), and these observers will oftentimes compete with one another, creating performance degradation, largely because decoupling the observers is difficult. Therefore, it is desirable to have a sensorless FOC-type system with robust performance and a low cost. 
     Some examples of conventional systems are: U.S. Pat. No. 5,886,498; U.S. Pat. No. 7,202,629; U.S. Pat. No. 7,208,908; U.S. Pat. No. 7,339,344; U.S. Pat. No. 7,646,164; U.S. Pat. No. 7,808,201; U.S. Patent Pre-Grant Publ. No. 2011/0012544; Ancuti et al., “Sensorless V/f control of high-speed surface permanent magnet synchronous motor drives with two novel stabilizing loops for fast dynamics and robustness,”  IET Electr. Power Appl ., Vol. 4, Iss. 3, 2010, pp. 149-157; Itoh et al., “A comparison between V/f control and position-sensorless vector control for the permanent magnet synchronous motor,”  Proc. of the Power Conversion Conf.,  2002. PCC Osaka 2002, pg. 1310-1315; and Perera et al., “A Sensorless, Stable V=f Control Method for Permanent-Magnet Synchronous Motor Drives”,  IEEE Trans. on Ind. Appl ., Vol. 39, No. 3, May/June 2003. 
     SUMMARY 
     An embodiment of the present invention, accordingly, provides a method. The method comprises generating a plurality of pulse width modulation (PWM) signals from a commanded voltage signal and a commanded angle signal; driving a motor with the plurality of PWM signals, wherein the motor has a plurality of phases; measuring currents through the phases of the motor; performing a Park transformation on the measured currents to determine projection current measurements; and adjusting the commanded voltage signal and the commanded angle signal based at least in part on the projection current measurement. 
     In accordance with an embodiment of the invention, the method further comprises generating the commanded voltage signal and the commanded angle signal from a reference signal, and wherein the projection current measurement further comprises a field current measurement. 
     In accordance with an embodiment of the invention, the step of generating the commanded voltage signal and the commanded angle signal from the reference signal further comprises: generating the commanded voltage signal from a frequency of the reference signal; and integrating the reference signal to determine the commanded angle signal. 
     In accordance with an embodiment of the invention, the step of adjusting further comprises: controlling the field current measurement with a proportional-integral (PI) controller to generate a control signal; adding the control signal to the commanded voltage signal; and subtracting the control signal from the commanded angle signal. 
     In accordance with an embodiment of the invention, the step of driving further comprises applying the plurality of PWM signals to an inverter. 
     In accordance with an embodiment of the invention, the projection current measurement further comprises a field current measurement, and wherein the step of generating further comprises performing an inverse Park transformation on the voltage and commanded angle signals. 
     In accordance with an embodiment of the invention, the step of adjusting further comprises: reducing a command voltage if the field current measurement is greater than zero, wherein the command voltage corresponds to the PWM signals; weakening the magnetic field of the motor if the field current measurement is greater than zero; and increasing the command voltage if the field current measurement is greater than zero. 
     In accordance with an embodiment of the invention, the method further comprises generating the commanded voltage signal and the commanded angle signal from a reference signal, and wherein the projection current measurement further comprises a torque current measurement. 
     In accordance with an embodiment of the invention, an apparatus is provided. The apparatus comprises a feedback loop that determines a projection current measurement by performing a Park transformation on measured currents and that generates a control signal; a voltage generator that generates a commanded voltage signal from a reference signal; an integrator that generates a commanded angle signal from the reference signal; a first adder that adds the commanded voltage signal to the control signal; a second adder that subtracts the control signal from the commanded angle signal; and a PWM controller that generates a plurality of PWM signals in response to outputs from the first and second adders. 
     In accordance with an embodiment of the invention, the projection current measurement further comprises a field current measurement, and wherein the feedback loop further comprises: a Park converter that determines the field current measurement from the measured currents; a field weakener that weakens a magnetic field if the field current measurement is less than zero; and a PI controller that generates the control signal based at least in part on the field current measurement. 
     In accordance with an embodiment of the invention, the PWM controller further comprises: an inverse Park converter that performs an inverse Park transformation on the commanded voltage signal and the commanded angle signal; and a space vector PWM (SVPWM) generator that generates the plurality of PWM signals based at least in part on outputs from the inverse Park converter. 
     In accordance with an embodiment of the invention, the voltage generator, the integrator, the first adder, the second adder, the Park converter, the PI controller, and the inverse Park converter are implemented in software that is embodied on a processor and memory. 
     In accordance with an embodiment of the invention, the apparatus further comprises: an inverter that is coupled to the SVPWM so as to receive the plurality of PWM signals; and a motor that is coupled to the inverter. 
     In accordance with an embodiment of the invention, the motor further comprises a permanent magnet synchronous motor (PMSM). 
     In accordance with an embodiment of the invention, an apparatus is provided. The apparatus comprises a processor having a memory with a computer program embodied thereon, the computer program including: computer code for generating a commanded voltage signal from a reference signal; computer code for integrating the reference signal to generate a commanded angle signal; computer code performing a Park transformation on measured currents to determine a projection current measurement; computer code for generating a control signal from the projection current measurement; computer code for adjusting the commanded voltage signal and the commanded angle signal based on the projection current measurement; and computer code for performing an inverse Park transformation on the adjusted voltage and commanded angle signals to generate drive signals; and a PWM generator that is coupled to the processor so as to receive the drive signals and generate a plurality of PWM signals from the drive signals. 
     In accordance with an embodiment of the invention, the PWM generator further comprises an SVPWM generator. 
     In accordance with an embodiment of the invention, the projection current measurement further comprises a field current measurement, and wherein the computer code for adjusting further comprises: computer code for controlling the field current measurement with a PI controller to generate the control signal; computer code for adding the control signal to the commanded voltage signal; and computer code for subtracting the control signal from the commanded angle signal. 
     In accordance with an embodiment of the invention, the apparatus further comprises: an inverter that is coupled to the SVPWM so as to receive the plurality of PWM signals; and a motor that is coupled to the inverter. 
     In accordance with an embodiment of the invention, the motor further comprises a PSMS. 
     In accordance with an embodiment of the invention, the apparatus further comprises: an inverter that is coupled to the SVPWM so as to receive the plurality of PWM signals; and a motor that is coupled to the inverter. 
     In accordance with an embodiment of the invention, the computer code for adjusting further comprises: computer code for reducing a command voltage if the field current measurement is greater than zero, wherein the command voltage corresponds to the PWM signals; computer code for weakening the magnetic field of the motor if the field current measurement is greater than zero; and computer code for increasing the command voltage if the field current measurement is greater than zero. 
     In accordance with an embodiment of the invention, the projection current measurement further comprises a torque current measurement. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an example of a conventional system; 
         FIG. 2  is an example of a system in accordance with an embodiment of the present invention; and 
         FIG. 3-5  are illustrations of stabilization control of the motor of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Turning to  FIG. 2 , an example of a system  200  in accordance with an embodiment of the present invention is provided. Contrasting motor controller  102  to motor controller  202 , the organization and structure is completely different in that motor controller  202  utilizes a hybrid volt-per-hertz (V/f) and FOC control, thereby eliminating the need for observer  120 . With motor controller  202 , there is one control loop that includes Park converter  118 . Generally, in operation, a voltage generator  204  and integrator  206  are employed to generate the commanded voltage signal V q * and commanded angle signal θ*, respectively, from reference speed or reference signal ω*. The control loop can then provide adjustments to the commanded voltage signal V q * and commanded angle signal θ* by way of adders  208 - 1  and  208 - 2 , which adds and subtracts a control signal to and from the commanded voltage signal V q * and commanded angle signal θ* (respectively). 
     Typically, the control signal is determined from phase current measurements. Namely, Park converter performs a Park transformation on these measured phase currents to generate projection currents I d  and I q . (which correspond to the d-axis and q-axis, respectively). For the system  200 , field current I d  is used, while torque current I q  can be ignored. Alternatively, current torque I q  can be used instead of field current I d . Typically, direct current (DC) motors have a totally independent field and torque control, and a FOC algorithm generally controls a PMSM or AC Induction Motor (ACIM). However, currents I d  and I q  are actually the projections of the resultant current on d-axis and q-axis, so there are not any true independencies. Hence, a single control for projection currents I d  and I q  is generally sufficient. So, in this example, the field current I d  can be adjusted by field weakener  114  (normally, I d =0) and adder  208 - 3  (i.e., based on the speed of the motor), and a PI control  210  can be applied to generate the control signal. 
     One reason for the simplicity of motor controller  202  is that some approximations can be made to simplify the system  200  (as compared to system  100 ). From currents I d  and I q , the commanded voltages V d  and V q  can be determined by the following equations: 
                       V   d     =         I   d     ⁢     R   s       +       L   d     ·       ⅆ     i   d         ⅆ   t         -     ω   ⁢           ⁢     Ψ   q           ,           (   1   )                   V   q     =         I   q     ⁢     R   s       +       L   q     ·       ⅆ     i   q         ⅆ   t         +     ω   ⁢           ⁢     Ψ   d           ,           (   2   )                   Ψ   d     =         I   d     ⁢     L   d       +     Ψ   m         ,     
     ⁢   and           (   3   )                   Ψ   q     =       I   q     ⁢     L   q         ,           (   4   )               
where Ψ d  and Ψ q  are flux linkages for the d-axis and q-axis, respectively, L d  and L q  are stator inductances for the d-axis and q-axis, respectively, Ψ m  is the flux linkage of the permanent magnet, and R s  is the stator resistance. These stator voltages V d  and V q  are typically used to generate the PWM signals for inverter  106 . However, it should be noted that magnetic fluxes generated by L d *I d  and L q *I q  are typically very small (i.e., ≈0), and voltage generated by i q *R s  (even under full load) is very small (i.e., ≈0), compared with the flux linkage of the permanent magnet Ψ m . Thus, equations (1) through (4) can be reduced as follows:
 
                     Ψ   d     =           I   d     ⁢     L   d       +     Ψ   m       ≈     Ψ   m               (   5   )                   Ψ   q     =         I   q     ⁢     L   q       ≈   0       ,           (   6   )                   V   d     =           I   d     ⁢     R   s       +       L   d     ·       ⅆ     i   d         ⅆ   t         -     ωΨ   q       ≈       I   d     ⁢     R   s       ≈   0       ,     
     ⁢   and           (   7   )                   V   q     =           I   q     ⁢     R   s       +       L   q     ·       ⅆ     i   q         ⅆ   t         -     ωΨ   d       ≈         I   q     ⁢     R   s       +     ωΨ   d       ≈     ωΨ   d         ,     
     ⁢   So   ,           (   8   )                 V   ≈     V   q     ≈     ωΨ   d       ,           (   9   )               
Equation (9), thus, implies that V/f control (as shown in  FIG. 2 ) would be appropriate. Moreover, equations (5) through (8) imply that the position (i.e., angle signal θ) of the voltage command vector {right arrow over (V)} (which is generated by the PWM signals) is generally aligned with the q-axis of the rotor  109 , as shown in  FIG. 3 .
 
     Knowing that the voltage command vector {right arrow over (V)} should be approximately aligned with the q-axis of the rotor  109 , to achieve a maximum theoretical torque, stabilization would also be relatively easy to achieve. Under these circumstances, a projection current I d  that is greater than zero would indicate a voltage command vector {right arrow over (V)} having a positive real synchronous angle (as shown in  FIG. 4 ), and a stator current I d  that is less than zero than would indicate a voltage command vector {right arrow over (V)} having a negative real synchronous angle (as shown in  FIG. 5 ). For the positive real synchronous angle, the projection current I d  contributes unexpected magnetic saturations, so the commanded voltage (which is generated by the PWM signals and applied to the motor  108  by the inverter  106 ) can be reduced to compensate. For the negative real synchronous angle, the magnetic field can be weakened by the field weakener  114  and the command voltage can be increased. 
     Implementing the motor controller  202  can also be accomplished in a number of ways. For example, each element of motor controller can be implemented software that is embodied on a processor (i.e., digital signals processor or DSP) and memory, implemented in hardware, or some combination thereof. Typically, the motor controller  202  includes a processor and memory having the voltage generator  204 , integrator  206 , adders  208 - 1 ,  208 - 2 , and  208 - 3 , PI control  210 , field weakener  114 , Park converter  118 , and inverse Park converter (part of the PWM controller  116 ) and SVPWM (part of the PWM controller  116 ) implemented in software. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.