Abstract:
Some embodiments provide a system that generates a coil switching signal for a brushless DC motor. During operation, the system determines a magnetic field of the brushless DC motor at a first time and a magnetic field of the brushless DC motor at a second time. Then, the coil switching signal is generated based on a relationship between the magnetic field determined at the first time and a first predetermined threshold, and the magnetic field determined at the second time and a second predetermined threshold.

Description:
RELATED APPLICATION 
     This application hereby claims priority under 35 U.S.C. §119 to U.S. Provisional Application No. 61/428,666, entitled “Tail End Current Reduction Circuits for a Motor Driver,” by Ching-Yuh Tsay, and Chuan Hung Chi, filed 30 Dec. 2010, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present embodiments relate to techniques for controlling a brushless DC motor. More specifically, the present embodiments relate to techniques for generating a coil switching signal for a brushless DC motor. 
     2. Related Art 
     Typically, in order to drive the stater coils of a brushless DC motor, the magnitude and polarity of the magnetic field of the rotor is detected with a sensor. As the rotor rotates, the magnetic field of the rotor at a stator coil switches polarity. The sensor detects this polarity switch and a motor controller then generates a signal to reverse the polarity of the magnetic field generated by the stator coil. However, for some motor controllers, the threshold magnetic field at which the motor controller generates the signal to reverse the direction of the stator coil current is set so that the coil current switches only after the polarity of the magnetic field of the rotor has switched. Then, for the portion of the cycle in which the polarity of the magnetic field of the coil is the same as the polarity of the magnetic field of the rotor, the motor will act like a generator and back emf may cause the tail end of the coil driver current to peak. This peaking of the driver current may cause undesirable noise, and may decrease the efficiency of the motor. 
     Hence, use of brushless DC motors may be facilitated by improved techniques for generating a coil switching signal. 
     SUMMARY 
     Some embodiments provide a system that generates a coil switching signal for a brushless DC motor. During operation, the system determines a magnetic field of the brushless DC motor at a first time and a magnetic field of the brushless DC motor at a second time. Then, the coil switching signal is generated based on a relationship between the magnetic field determined at the first time and a first predetermined threshold, and the magnetic field determined at the second time and a second predetermined threshold. 
     In some embodiments, a magnitude of the first predetermined threshold is larger than a magnitude of the second predetermined threshold, and a polarity of the first predetermined threshold is the same as a polarity of the second predetermined threshold. 
     In some embodiments, determining the magnetic field of the brushless DC motor includes determining the magnetic field at the first time using a Hall effect sensor, and determining the magnetic field at the second time using the Hall effect sensor. 
     In some embodiments, the relationship between the magnetic field determined at the first time and the first predetermined threshold, and the magnetic field determined at the second time and the second predetermined threshold includes a magnitude of the magnetic field determined at the first time being larger than or equal to a magnitude of the first predetermined threshold and a magnitude of the magnetic field determined at the second time being less than or equal to a magnitude of the second predetermined threshold. 
     In some embodiments, the coil switching signal is generated based on a reduction in a magnitude of the sensed magnetic field. 
     Some embodiments further include switching a polarity of a magnetic field of a stator coil of the brushless DC motor from a first polarity to a second polarity based on the coil switching signal, wherein the first predetermined threshold and the second predetermined threshold are selected such that when the polarity of the magnetic field of the stater coil is switched from the first polarity to the second polarity, a torque is generated by the magnetic field of the stater coil on a rotor of the brushless DC motor in a direction opposite to a direction of rotation of the rotor. 
     In some embodiments, a net torque generated by the magnetic field of the stator coil on the rotor is in the same direction as the direction of rotation of the rotor during a period of time when the magnetic field of the stator coil of the brushless DC motor is in the second polarity. 
     In some embodiments, the brushless DC motor is a single coil brushless DC motor. 
     In some embodiments, the sensor and the coil switching mechanism are packaged on a single chip. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1A  shows a system for controlling a coil of a brushless DC motor in accordance with an embodiment. 
         FIG. 1B  shows an exemplary graph of the drive current direction for a coil in a brushless DC motor versus magnetic field sensed by a Hall sensor in accordance with an embodiment. 
         FIG. 2  shows a flowchart illustrating the process for generating a coil switching signal for a brushless DC motor in accordance with an embodiment. 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
       FIG. 1A  shows a system for controlling a coil of a brush less DC motor in accordance with an embodiment. Motor controller  102  includes coil switching mechanism  104 , and receives input from Hall sensor  106 . Motor controller  102  is coupled to stator coil  108  and can output current through stator coil  108  in positive direction  110  or negative direction  112 . 
     Motor controller  102  and coil switching mechanism  104  can be implemented in any technology including but not limited to any combination of one or more of the following: an integrated circuit (IC), discrete circuits, any other semiconductor device, software running on a computer, a special purpose processor, a microcontroller, an ASIC, or one or more other dedicated or special purpose processors. 
     Hall sensor  106  is a Hall effect sensor and can be implemented in any technology without departing from the invention. In some embodiments, Hall sensor  106  can be replaced any other sensor that can detect a magnitude and polarity of the magnetic field of a rotor of a brushless DC motor without departing from the invention. Note that Hall sensor  106  and motor controller  102  can be implemented on one or more ICs without departing from the invention. 
     During operation, Hall sensor  106  detects the magnitude and polarity of the magnetic field of the rotor at the position of the stator coil. A signal related to the magnitude and polarity of the magnetic field is transmitted by Hall sensor  106  to motor controller  102 . Coil switching mechanism  104  then controls motor controller  102  to output current through stator coil  108  in either positive direction  110  or negative direction  112  based on the magnitude and polarity of the magnetic field sensed by hall sensor  106  at two or more different times. This process is discussed in more detail below with respect to  FIG. 1B . 
       FIG. 1B  shows an exemplary graph of the drive current direction for a coil in a brushless DC motor versus magnetic field sensed by a Hall sensor in accordance with an embodiment. In coil drive current direction vs. magnetic field graph  120 , the horizontal axis represents magnetic field  124  detected by Hall sensor  106  with negative polarity (−G) to the left side of the graph and positive polarity (+G) to the right side of the graph. The vertical axis, current direction  122 , represents the output current direction from motor controller  102  through stator coil  108 . Horizontal line positive direction current  126  is an example of current output from motor controller  102  in positive direction  110 , and horizontal line negative direction current  128  is an example of current output from motor controller  102  in negative direction  112 . Additionally there are four magnetic field values marked on coil drive current direction vs. magnetic field graph  120 : SETN  130 , SETS  132 , BRP  134 , and BOP  136 . 
     We will now describe the operation of coil switching mechanism  104  with reference to coil drive current direction vs. magnetic field graph  120 . Note that for convenience the description starts from the state in which the initial magnetic field detected by Hall sensor  106  is below SETN  130  (i.e., larger in magnitude and the same polarity as SETN) and the current output by motor controller  102  is negative direction current  128 . Any start state could be used without departing from the invention. 
     Starting from the state in which Hall sensor  106  detects a magnetic field less than SETN  130 , when Hall sensor  106  then detects a magnetic field that is greater than BRP  134  (e.g., lower in magnitude with the same polarity as BRP  134 ), coil switching mechanism  104  controls motor controller  102  to switch the current through stator coil  108  from negative direction current  128  to positive direction current  126 . Then, after Hall sensor  106  detects a magnetic field greater than SETS  132 , coil switching mechanism  104  waits for a magnetic field less than BOP  136  to be detected. When a magnetic field less than BOP  136  is detected, coil switching mechanism  104  then controls motor controller  102  to switch the output current through stator coil  108  from positive direction current  126  to negative direction current  128 . Then, when a magnetic field less than SETN  130  is detected, the cycle is complete. This cycle continues at the rotor of the brushless DC motor rotates. 
     Note that when coil switching mechanism  104  controls motor controller  102  to switch the direction of the current in stator coil  108  from negative direction current  128  to positive direction current  126  at magnetic field BRP  134 , the polarity of the magnetic field from stator coil  108  is the same as the polarity of the magnetic field of the rotor as detected by Hall sensor  104 . Therefore, stator coil  108  and the rotor will tend to repel each other, generating a torque on the rotor opposing the direction of rotation of the rotor. As the rotor continues rotating, the polarity of the magnetic field of the rotor at the position of stator coil  108  will eventually switch to a polarity opposite to that of stator coil  108 . This will tend to generate a torque on the rotor in the direction of rotation of the rotor. Note that the net torque generated by the magnetic field of stator coil  108  on the rotor during the time that the current in stator coil  108  is positive direction current  126  is in the direction of rotation of the rotor. 
     A similar process occurs when the current direction changes from positive current direction  126  to negative current direction  128  at BOP  136 . The polarity of the magnetic field generated by stator coil  108  is the same as that of the rotor and will therefore tend to exert a torque on the rotor in the direction opposite to its rotation. As the rotor continues rotating it will eventually rotate enough so that the polarity of the magnetic field from the rotor at stator coil  108  is opposite to the polarity of the magnetic field generated by stator coil  108 . The torque generated by stator coil  108  on the rotor is then in the direction of rotation. Note that the net torque generated by the magnetic field of stator coil  108  on the rotor during the time that the current in stator coil  108  is negative direction current  128  is in the direction of rotation of the rotor. 
     Note that in some embodiments, the absolute value of BOP  136  and BRP  134  can be any value including but not limited to 10 gauss, 30 gauss, 50 gauss, or any value higher or lower without departing from the invention. Additionally, in some embodiments the magnitude of the difference in magnetic field between SETN  130  and BRP  134  is not equal to the magnitude of the difference in magnetic field between SETS  132  and BOP  136 , as long as SETN  130  is less than BRP  134  and SETS  132  is larger than BOP  136 . Furthermore, in some embodiments the values of parameters SETN  130 , SETS  132 , BRP  134 , and BOP  136  are determined based on one or more operating characteristic of the brushless DC motor being driven by motor controller  102 . For example, the values of one or more of these parameters may be selected to reduce audible noise generated by back emf current in stator coil  108 , or to increase motor efficiency. 
     Additionally, note that the change in output current from positive direction current  126  to negative direction current  128  and vice-versa can be along any curve without departing from the present invention. 
     Note that some embodiments can be used with brushless DC motors that include any number of rotor poles and/or stator poles. Additionally, in brushless DC motors with multiple rotor poles, Hall sensor  106  can be located to detect the magnetic field of any position that is in phase with stator coil  108 , or any position that is antiphased with stator coil  108  if the polarity of stator coil  108  or of the output current of motor controller  102  is reversed. 
       FIG. 2  shows a flowchart illustrating the process for generating a coil switching signal for a brushless DC motor in accordance with an embodiment. First the magnetic field, A, of the rotor of the brushless DC motor is measured using a Hall effect sensor (step  202 ). If A is not greater than SETS (step  204 ), then the process proceeds to step  212 . At step  212  if A is not less than SETN then the process returns to step  202 . This loop will continue until either A is greater than SETS (steps  204 ) or A is less than SETN (step  212 ). 
     At step  204 , if A is greater than SETS, then the magnetic field, A, is measured again (step  206 ). Then, if A is not less than BOP (step  208 ), the process returns to step  206  and continues in this loop until A is less than BOP (step  208 ) and the process proceeds to step  210 . At step  210 , the direction of the current in the stator coil is toggled (e.g., from positive current direction  126  to negative current direction  128 ). The process then returns to step  202 . 
     The process will then loop from step  202  to step  204  and step  212  and back to step  202  until the rotor has rotated so that the magnetic field measured at step  202  is less than SETN. When A is less than SETN (step  212 ), the process continues to step  214  where A is measured again. At step  216 , if A is not greater than BRP, then the process returns to step  214  and continues to loop until A is greater than BRP (step  216 ) and the process continues to step  210 . In step  210 , the direction of the current in the stator coil is toggled (e.g., from negative current direction  128  to positive current direction  126 ). The process then loops back to step  202 . 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.