Abstract:
To achieve peak acoustic and power performance, the coil or applied current should be in phase or substantially aligned with the back electromotive force (back-EMF) voltage. However, there are generally phase differences between the applied current and back-EMF voltage that are induced by the impedance of the brushless DC motor (which can vary based on conditions, such as temperature and motor speed). Traditionally, compensation for these phase differences was provided manually and on an as-needed basis. Here, however, a system and method are provided that automatically perform a commutation advance by incrementally adjusting a drive signal over successive commutation cycles when the applied current and back-EMF voltage are misaligned.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates generally to control of a brushless direct current (DC) motor and, more particularly, to automatically applying a commutation advance. 
       BACKGROUND 
       [0002]    There are a number of applications that employ brushless DC motors, and, in many of these applications (i.e., hard disc drives or HDDs), acoustic performance and power consumption of the brushless DC motor can be important. To achieve peak acoustic and power performance, the coil or applied current should be in phase with the back electromotive force (back-EMF) voltage. However, there are generally phase differences between the applied current and back-EMF voltage that are induced by the impedance of the brushless DC motor (which can vary based on conditions, such as temperature and motor speed). Traditionally, compensation for these phase differences was provided manually and on an as-needed basis. Conventional motor controllers would often provide a “commutation advance.” This commutation advance would usually be a digital value (i.e., 3 to 4 bits) that could be stored in a register. However, a limited number of bits generally does not provide much control over the system, and usually the end user would adjust the controller to “fit” the motor, which is a tedious and time consuming practice that drives up cost. Therefore, there is a need for an improved motor controller. 
         [0003]    Some other conventional systems are: U.S. Pat. No. 6,498,446; U.S. Pat. No. 7,0344,78; and U.S. Patent Pre-Grant Publ. No. 2002/0027423. 
       SUMMARY 
       [0004]    A preferred embodiment of the present invention, accordingly, provides a method for reducing acoustic noise in a brushless direct current (DC) motor. The method comprises applying a drive signal to a drive circuit; determining a zero-crossing for a back electromotive force (back-EMF) voltage for the brushless DC motor based at least in part on the drive signal; measuring an applied current applied to the brushless DC motor by the driver in response to the drive signal; comparing the zero-crossing for the back-EMF voltage to a zero-crossing for the applied current; and incrementally adjusting the drive signal so as to substantially align the zero-crossing for the back-EMF voltage and the zero-crossing for the applied current. 
         [0005]    In accordance with an embodiment of the present invention, the step of determining further comprises determining a position for the zero-crossing for the back-EMF voltage within a window. 
         [0006]    In accordance with an embodiment of the present invention, the step of comparing further comprises: determining a plurality of signs of a magnitude of the applied current within the window, wherein each sign is determined at a predetermined interval; incrementing a counter for each sign that is one of positive and negative to generate a position for the zero-crossing for applied current; and comparing the position for the zero-crossing for the back-EMF voltage to the position for the zero-crossing for the applied current to generate a position difference. 
         [0007]    In accordance with an embodiment of the present invention, the step of incrementally adjusting further comprises iteratively adjusting the drive signal until the position difference is less than the predetermined interval. 
         [0008]    In accordance with an embodiment of the present invention, the drive signal further comprises a plurality of pulse width modulation (PWM) signals. 
         [0009]    In accordance with an embodiment of the present invention, the step of measuring further comprises sampling the applied current for each PWM cycle. 
         [0010]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a drive circuit that drives a brushless DC motor; a sense circuit that is coupled to the drive circuit so as to measure an applied current for the brushless DC motor; and a control circuit that is coupled to the sense circuit and the drive circuit, wherein the control circuit: applies a drive signal to the drive circuit; determines a zero-crossing for a back-EMF voltage for the brushless DC motor based at least in part on the drive signal; compares the zero-crossing for the back-EMF voltage to a zero-crossing for the applied current; and incrementally adjusts the drive signal so as to substantially align the zero-crossing for the back-EMF voltage and the zero-crossing for the applied current. 
         [0011]    In accordance with an embodiment of the present invention, the drive circuit further comprises a plurality of half-H-bridges. 
         [0012]    In accordance with an embodiment of the present invention, the sense circuit further comprises a plurality of current sensors, wherein each current sensor is associated with at least one of the transistors from at least one of the half-H-bridges. 
         [0013]    In accordance with an embodiment of the present invention, the control circuit further comprises: an analog-to-digital converter (ADC) that is coupled to the sense circuit so as to generate a digitized applied current; a processor having a memory with a computer program embodied thereon, wherein the processor is coupled to the ADC, and wherein the processor compares the zero-crossing for the back-EMF voltage to the zero-crossing for the applied current and incrementally adjusts the drive signal; and a controller that is coupled between the processor and drive circuit, wherein the controller applies a drive signal to the drive circuit and determines the zero-crossing for the back-EMF voltage. 
         [0014]    In accordance with an embodiment of the present invention, the drive circuit further comprises: a pre-driver that is coupled to the controller; and a driver that is coupled to the pre-driver and the sense circuit. 
         [0015]    In accordance with an embodiment of the present invention, the apparatus further comprises the DC brushless motor, which is coupled to the driver. 
         [0016]    In accordance with an embodiment of the present invention, the processor further comprises a digital signals processor (DSP). 
         [0017]    In accordance with an embodiment of the present invention, an apparatus is provided. The apparatus comprises a drive circuit; a controller that a drive signal to the drive circuit and that determines a zero-crossing for a back-EMF voltage for a brushless DC motor based at least in part on the drive signal; a sense circuit that measures an applied current applied to the brushless DC motor by the drive circuit in response to the drive signal; and a processor having a storage medium with a computer program embodied thereon, wherein the computer program includes: computer code for comparing the zero-crossing for the back-EMF voltage to a zero-crossing for the applied current; and computer incrementally adjusting the drive signal so as to substantially align the zero-crossing for the back-EMF voltage and the zero-crossing for the applied current. 
         [0018]    In accordance with an embodiment of the present invention, the controller determines a position for the zero-crossing for the back-EMF voltage within a window. 
         [0019]    In accordance with an embodiment of the present invention, the computer code for comparing further comprises: computer code for determining a plurality of signs of a magnitude of the applied current within the window, wherein each sign is determined at a predetermined interval; computer code for incrementing a counter for each sign that is one of positive and negative to generate a position for the zero-crossing for applied current; and computer code for comparing the position for the zero-crossing for the back-EMF voltage to the position for the zero-crossing for the applied current to generate a position difference. 
         [0020]    In accordance with an embodiment of the present invention, the computer code for incrementally adjusting further comprises computer code for iteratively adjusting the drive signal until the position difference is less than the predetermined interval. 
         [0021]    In accordance with an embodiment of the present invention, the drive signal further comprises a plurality of PWM signals. 
         [0022]    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 
         [0023]    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: 
           [0024]      FIG. 1  is a diagram of an example of a system in accordance with an embodiment of the present invention; 
           [0025]      FIG. 3  is a diagram depicting a general operation of a portion of the system of  FIG. 1 ; 
           [0026]      FIG. 2  is a diagram of an example of the driver, sense circuit, and motor of  FIG. 1 ; and 
           [0027]      FIG. 4  is a diagram depicting an example of an automatic commutation advance for the system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    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. 
         [0029]    Turning to  FIG. 1 , a system  100  in accordance with a preferred embodiment of the present invention can be seen. System  100  generally comprises an integrated circuit (IC)  102  and motor  104 . The IC  102  generally comprises a digital signals processor (DSP)  106  (which typical includes a memory with a computer program embodied thereon), a controller  108 , pre-driver  110 , phase comparator  112 , driver,  114 , sense circuit  116 , and an analog-to-digital converter (ADC)  118 . In operation, the IC  102  generates an applied voltage and applied (or coil) current for the motor  104  (which can be a sensorless, brushless DC motor with any number of phases) to drive the motor  104 . The motor  104  for this example (which is three-phase brushless DC motors) can generate sinusoidal back-EMF voltages for each of three phases, which are a function of velocity and rotor position and which are separated by  120  degrees. To control this motor  104 , IC  102  can use the back-EMF voltage generated by the motor to commutate the motor when the motor is operating at a generally constant rotational speed (also known as “run speed” for HDD applications), or it can be used in acceleration. For example, in a three-phase DC motor, two of the phases are engaged while the third phase is in a high impedance state. Commutation logic (i.e., controller  108  and DSP  106 ) within the IC  100  can use the third phase to measure the time for a zero-crossing of the back-EMF voltage during a window (as shown in  FIG. 2 ). Additionally, controller  108  issues a commutation clock signal that can be converted into the applied voltage (and applied or coil current) for the motor  104  by the pre-driver  110  and driver  114 . 
         [0030]    To be able to drive the motor  104 , the driver  114  (as shown in  FIG. 3 ) generally comprises three half-H-bridges (i.e., transistors Q 1  through Q 6 ) that generally function as discrete driving components for each of the phases PHA, PHB, and PHC of motor  104 . When motor  104  (in this example) is being driven one of phases PHA, PHB, and PHC sources current, one of phases PHA, PHB, and PHC sinks current, and the remaining phase PHA, PHB, or PHC can either sink or source current. For example, if current I AB  is being supplied, transistors Q 1  and Q 4  would be “on,” while phase PHC and its transistors Q 5  and Q 6  can be ignored. These transistors Q 1  through Q 6  are driven by the application of pulse width modulation (PWM) signals PWM 1  to PWM 6  from pre-driver  110  (which can collectively function as a drive signal for motor  104 ) to transistors Q 1  through Q 6  of driver  114 . 
         [0031]    Additionally, to provide automatic commutation advance, additional information (namely, applied or coil current) should be determined. This applied or coil current measurement is generally performed by sense circuit  116 . As shown in  FIG. 3 , this sense circuit  116  is generally comprised of current sensors  202 - 1  to  202 - 6  that are associated with transistors Q 1  to Q 6 , respectively. Generally, for each PWM cycle, appropriate current sensors (i.e., current sensors  202 - 1  and  202 - 4  for the example above) sample the applied or coil current; generally, currents sensors for the sourcing and sinking transistors are used. These sensed currents are then digitized by the ADC  118  so as to provide a zero-crossing measurement for the applied current to the DSP  106 . Controller  108  (which generates the commutation clock signal) also determines when there is a zero-crossing for back-EMF voltage, which is also provided to the DSP  106 . 
         [0032]    Based on the zero-crossings for the back-EMF voltage and the applied current, the DSP  106  is able to calculate an error between these zero-crossings. This is generally accomplished through a relative comparison between the zero-crossing positions within a window (which can be seen in  FIG. 4 ). As shown, a counter (which can be hardware or can be software within DSP  106 ) can be incremented to count the number of predetermined intervals for which the magnitude of is greater than (or, alternatively, less than) zero. A comparison (using, for example, an up-down counter) of the count values for the zero-crossings of back-EMF voltage and the applied current determines the error between these zero-crossings. In the example shown in  FIG. 4 , the error is three periods (which, if each period is 10 ns, would be 30 ns). Based on this error, the DSP  106  can provide an adjustment to the PWM signals PWM 1  to PWM 6  (through controller  108  and the commutation clock signal) by a fixed or predetermined amount iteratively over successive commutation cycles until the difference between the zero-crossings is, for example, less than one predetermined interval (i.e., less than 50 ns). A reason for incrementally adjusting this drive signal is that system  100  is nonlinear, and one large adjustment may result in instability in system  100 . Once adjusted, this automatic commutation advance can also then be performed periodically to generally ensure that the back-EMF voltage and the applied current are substantially aligned or within a predetermined error range (i.e., less than 50 ns apart). As an alternative, both the magnitude and the sign can be used to determine the zero-crossing for the applied current. Under these circumstances, the position of the zero-crossing for the applied current can be determined with much higher accuracy because interpolation of the zero-crossing can be employed, but this is at the cost of increased computing resources (i.e., within DSP  106 ) being consumed. 
         [0033]    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.