Abstract:
A system and method of advancing the commutation sequence of a brushless DC motor is provided. The motor having a plurality of coils, each of the coils coupled together at one end to a common center tap and coupled at an opposite end, through a respective coil tap, to both a source voltage and ground via selectively actuateable switches having diodes coupled in parallel therewith. The motor operates in a pulse width modulation (PWM) mode having PWM-on states and PWM-off states. During PWM-off states, a coil tap voltage from the coil tap of a floating phase is provided to a preconditioning circuit. The preconditioning circuit adjusts the floating phase coil tap voltage to compensate for an amount of voltage substantially equal to an amount of voltage by which a voltage at the center tap deviates from zero. The preconditioning circuit further includes sharpening circuitry for amplifying the adjusted floating phase coil tap voltage. A signal output from the preconditioning circuit is provided to a zero-crossing detection circuit for detecting zero-crossing and determining when to advance in the commutation sequence.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to motor driving and control circuitry, and is more specifically related to an improved circuit and method for back electromotive force (BEMF) detection in a brushless motor.  
           [0003]    2. Discussion of Related Art  
           [0004]    Three-phase brushless DC motors have many uses, among which include both high speed and low speed applications. Conventional high speed applications include spindle motors for computer hard disk drivers, digital video disk (DVD) drivers, CD players, tape-drives for video recorders, and blowers for vacuum cleaners. Motor for high speed applications typically operate in a range from a few thousand rotations per minute (rpm&#39;s) to 20,000 rpm&#39;s, for example. Low speed applications include motors for farm and construction equipment, HVAC compressors, fuel pumps and the like. Motor for low speed applications typically operate in a range from less than a few hundred rpm&#39;s to a few thousand rpm&#39;s, for example. Compared to DC motors employing brushes, brushless DC motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.  
           [0005]    [0005]FIG. 1 shows a cross-section of a typical brushless, DC motor  10 . The motor  10  includes a permanent magnet rotor  12  and a stator  14  having a number of windings (A, B, C shown in FIG. 2). The windings are each formed in a plurality of slots  18 . The motor  10  illustrated has the rotor  12  housed within the stator  14 . The stator  14  may also be housed within the rotor  12 . The rotor  12  is permanently magnetized, and turns to align its own magnetic flux with one generated by the windings.  
           [0006]    Power to the motor  10  is often provided in a pulse width modulation (PWM) mode. The PWM mode is a nonlinear mode of power supply in which the power is switched on and off at a very high frequency in comparison to the angular velocity of the rotor. For example, typical switching frequencies may be in the range of 20 kHz. In a typical on-off cycle lasting about 50 μs, there may be 40 μs of “on” time followed by 10 μs of “off” time. Given the short duration of off times, current still flows through the motor windings so there is virtually no measurable slow down in the angular velocity of the rotor  12  during these periods. Accordingly, PWM mode provides a significant power savings advantage over modes in which power is continuously supplied.  
           [0007]    In order to operate the motor  10 , the flux existing in the stator  14  is controlled to be slightly in advance of the rotor  12  thereby continually pulling the rotor forward. Alternatively, the flux in the stator  14  may be controlled to be just behind the rotor  12 , in which case the polarity is set such as to repel the rotor  12 , thereby aiding rotation. Therefore, to optimize the efficiency of the motor  10 , it is advantageous to monitor the position of the rotor  12  so that the flux in the stator  14  may be appropriately controlled and switched from one stage to the next. If the rotor  12  movement and the flux rotation should ever get out of synchronization, the rotor  12  may become less efficient, start to jitter or stop turning.  
           [0008]    A conventional motor can be represented in circuit form as having three coils A, B, and C connected in a “Wye” or “Y” configuration, as shown by reference numeral  20  in FIG. 2, although a larger number of stator coils are often employed with multiple rotor poles. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three “Y” connected coils, connected in three sets of four coils, each physically separated by 90 degrees.  
           [0009]    In operation, coils A, B and C are energized with a PWM drive signal that causes electromagnetic fields to develop about the coils. The resulting attraction/repulsion between the electromagnetic fields of the coils A, B, and C and the magnetic fields created by the magnets in the motor causes the rotor assembly of the motor to rotate.  
           [0010]    The coils are energized in sequences to produce a current path through two coils of the “Y”, with the third coil left floating (or in tristate), hereinafter floating coil FC. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. The sequences are defined such that when the floating coil is switched into the current path, the direction of the current in the coil that was included in the prior current path is not changed. In this manner, six commutation sequences, or phases, are defined for each electrical cycle in a three phase motor, as shown in Table A.  
                                             TABLE A                       Phase   Current Flows From:   Current Flows To:   Floating Coil                                1   A   B   C       2   A   C   B       3   B   C   A       4   B   A   C       5   C   A   B       6   C   B   A                  
 
           [0011]    When the motor is on, the rotation of the rotor induces a BEMF voltage in each of the windings of the motor. Such BEMF is represented by the Bemf voltage sources in FIG. 2. With respect to whichever phase is currently floating, the BEMF in that phase is monitored to determine when to advance in the communication sequence. More particularly, the BEMF in the floating coil is monitored to determine when it crosses zero at which point the position of the rotor is assumed to be known. The point at which the BEMF crosses zero is referred to as the “zero-crossing”. Each time a zero-crossing is detected, the motor advances in its commutation sequence by 30 electrical degrees.  
           [0012]    A conventional technique to measure the BEMF is to measure, during a floating period, the voltage at a coil tap (nodes Va, Vb, and Vc. in FIG. 2) for the floating coil. The measured voltage at the coil tap is presumed to be the BEMF. Accordingly, the coil tap voltage for the floating coil is monitored to detect zero-crossings at which times the commutation sequence is advanced. However, unless the center tap voltage V CT  is zero, this BEMF calculation is not fully accurate.  
           [0013]    Known methods of detecting BEFM include comparing the floating phase coil tap voltage with the center tap voltage, or a virtual center tap voltage configured by a resistor network. During the PWM-on and PWM-off states, the center tap voltage V CT  is significantly deviated from zero. This generates high common mode noise. To offset the center tap voltage V CT  for zero-crossing detection, voltage divider and filter circuits have been used. However, such voltage divider and filter circuits reduce the sensitivity of the circuits and delay zero-crossing detection.  
         SUMMARY OF THE INVENTION  
         [0014]    A system and method of advancing the commutation sequence of a brushless DC motor is provided. The system and method advantageously monitors for zero crossing detections during PWM-off states. Because a PWM signal typically oscillates at a frequency significantly greater than the frequency at which the commutation sequence advances, zero-crossings which may happen to begin during a PWM-on state are still detectable during the PWM-off state with minimal delay. For example, the frequency of the PWM signal may be in the range of 20 kHz-100 kHz while the frequency at which the commutation sequence advances is typically on the order of 100 Hz. Accordingly, timely advancement of the commutation sequence is minimally impacted if the zero-crossing begins to occur during a PWM-on state. Further, as zero-crossing detection is accomplished during PWM-off states, the filter circuits previously used to offset the center tap voltage for zero-crossing detection are no longer needed, thereby avoiding reduced circuit sensitivity and delays in zero-crossing detection.  
           [0015]    It has been observed that, especially in low speed and/or low voltage applications, variations in the center tap voltage V CT  from zero during PWM-off states may have an adverse effect on zero-crossing detection. Variations in the center tap voltage V CT  from a zero often occur during PWM-off states due to voltage drops across diodes in each coil of the motor. The diodes are typically connected in parallel with the switches which couple the coils to Vdc and ground. Accordingly, the present invention further provides a zero-crossing precondition circuit which adjusts for variances in BEMF measurements which occur due to a non-zero center tap voltage during PWM-off states.  
           [0016]    According to an additional embodiment of the invention, the precondition circuitry additionally and/or alternatively includes sharpening circuitry for sharpening a coil tap voltage of the floating coil as the coil tap voltage approaches the zero-crossing. Again, particularly in low voltage and low speed motor applications, it has been observed that the coil tap voltage varies at a slow rate as it passes through the zero-crossing. Due to standard deviations/offsets in a comparator or other circuitry used to monitor for a zero-crossing (hereinafter “zero-crossing detection circuitry”), a zero-crossing detection may be triggered either earlier or later than the actual zero-crossing. Accordingly, by sharpening the coil tap voltage in comparison to the standard deviations/offset of the zero-crossing detection circuitry, a more accurate zero-crossing detection is made possible.  
           [0017]    Thus, according to one embodiment of the present invention, a driver circuit for a brushless motor is provided. The driver circuit includes a first coil coupled between a first coil tap and a center tap, a second coil coupled between a second coil tap and the center tap, and a third coil coupled between a third coil tap and the center tap. The driver circuit operating in a pulse width modulation (PWM) mode having a PWM-on state and a PWM-off state. The driver circuit includes a precondition circuit configured to receive, during a PWM-off period, a floating phase coil tap voltage from one of the first coil tap, the second coil tap and the third coil tap. The precondition circuit is configured to precondition the floating phase coil tap voltage for zero-crossing detection. The driver circuit further includes zero-crossing detection circuitry configured to receive the preconditioned floating phase coil tap voltage and determine when a zero-crossing event has occurred.  
           [0018]    According to another embodiment of the present invention, a method for determining when to advance in a commutation sequence of a brushless DC motor is provided. The motor including a first phase having a first coil coupled between a first coil tap and a center tap, a second phase having a second coil coupled between a second coil tap and the center tap, and a third phase having a third coil coupled between a third coil tap and the center tap. The method includes the steps of: providing to the motor a pulse width modulation (PWM) signal having a PWM-on state and a PWM-off state; during the PWM-off state, supplying a floating phase coil tap voltage from a floating one of the first, second and third phases of the motor to a preconditioning circuit; performing by the preconditioning circuit, preconditioning to the floating phase coil tap voltage; monitoring for a zero-crossing of the preconditioned floating phase coil tap voltage; and when a zero-crossing is detected, advancing a step in the commutation sequence of the motor.  
           [0019]    According to yet another embodiment of the present invention, a motor is provided. The motor includes a plurality of coils, coupled together in one of a delta or wye configuration, each of the coils coupled at one end, through a respective coil tap, to both a source voltage and ground via selectively actuateable switches. Each of the switches includes a diode coupled in parallel with the respective switch. The motor includes a preconditioning circuit, the preconditioning circuit configured to adjust a voltage received from an associated one of the coil taps by an amount of voltage substantially equal to an amount of voltage by which the diodes offset a voltage at the center tap from zero, and a zero-crossing detection circuit coupled to the precondition circuit for receiving a signal output from the preconditioning circuit and monitoring the signal to detect a zero-crossing.  
           [0020]    To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 shows a cross section of a known brushless permanent magnet motor;  
         [0022]    [0022]FIG. 2 shows a schematic diagram of a known circuit for controlling the motor of FIG. 1;  
         [0023]    [0023]FIG. 3 is a theoretical diagram depicting the voltages and zero-crossings detected in each of three phases of the circuit shown in FIG. 2 with the PWM signal removed;  
         [0024]    [0024]FIG. 4 is a schematic diagram of a circuit for controlling a motor in accordance with one embodiment of the present invention;  
         [0025]    [0025]FIG. 5 is a schematic diagram of one embodiment of a preconditioning circuit of the present invention; and  
         [0026]    [0026]FIG. 6 is a theoretical diagram depicting the voltages and zero-crossings detected in each of the three phases of the circuit shown in FIG. 4 with the PWM signal removed. 
     
    
     DETAILED DESCRIPTION  
       [0027]    The present invention will now be described with reference to the drawings, wherein like reference labels are used to refer to like elements throughout.  
         [0028]    Referring initially to FIG. 2, there is shown an electrical schematic diagram of a conventional motor  20  having three coils A, B, and C connected in a “Y” configuration. As will be described in more detail below, the present invention provides an improved method and apparatus for advancing the commutation sequences of the motor  20  by monitoring for zero-crossings during PWM-off states. During such PWM-off states, a precondition circuit  50 , (FIG. 4) is used to offset variances in BEMF detection which occur due to a non-zero center tap voltage V CT . Further, the precondition circuit  50  may be used to sharpen a coil tap voltage near the zero-crossing. While the precondition circuit  50  is useful in any brushless DC motor application, it finds particular benefit in low speed and/or low voltage motor applications as discussed in more detail below.  
         [0029]    The motor  20  comprises three phases or coils A, B. C. Each phase has a respective inductor La, Lb, Lc and line resistance Ra, Rb, Rc. The three phases may be connected in a star (“Y”) configuration having a center tap CT, or in a delta configuration. The invention may be applied to either. For each coil, a pair of switches Xsa, Xga, Xsb, Xgb, Xsc, Xbc (collectively “switch(s) X”) connect a free end of a coil (also referred to as a coil tap) at Va, Vb, Vc, to supply Vs and GND voltages respectively. The switches are typically power transistors such as Mosfets or the like. A reverse biased diode Dsa, Dga, Dsb, Dgb, Dsc, Dgc (collectively “diode(s) D”) is placed in parallel with each of these switches. The diodes are power rectifiers, and typically serve to protect the switches and windings against induced voltages exceeding the supply or ground voltage. As described in more detail below, during PWM-off states, the voltage drop across the diodes D has been found to cause the center tap voltage V CT  to deviate from zero which, in turn, creates undesirable variances in the measurement of the BEMF.  
         [0030]    Continuing to refer to FIG. 2, it will be described below, by way of example, how the diodes D deviate the center tap voltage CT from zero during a PWM-off state. For this example, is presumed that the motor  20  is in its first phase of a six phase commutation sequences, wherein current flows from phase A to phase B, while phase C is left floating. Further, it is presumed preferably that during the PWM-off state, the PWM signal does not turn on the switch Xga coupling phase A to ground. In this manner, during the PWM-off state, all of the current freewheeling from phase A to phase B passes through diode Vdga. By not turning on, during the PWM-off state, the switch that couples the high phase (e.g. the phase “from” which current is flowing in a given commutation phase) to ground, there is reduced switching loss and noise introduced into the motor  20 . It will be appreciated, however, that the present invention may be applied to motors which turn on the switch (e.g. Xga) coupling the high phase to ground during PWM-off periods, except that in such circumstances the precondition circuit  50  is appropriately adjusted to take into account the fact that all of the current during the freewheeling period is not passing through the diode (e.g. Dga) alone.  
         [0031]    In view of the above assumptions, and by way of example, the following equations can be derived from FIG. 2:  
         [0032]    From Phase A:  
           V   CT =0− V   Dga   −R   a   i−L   a   di/dt−Bemf   a   (1)  
         [0033]    Where:  
         [0034]    V CT  is the voltage at the center tap CT;  
         [0035]    V Dga  is the voltage across the diode Dga  
         [0036]    Ra is the resistance of resistor Ra  
         [0037]    i is the current  
         [0038]    La is the inductance of the inductor La  
         [0039]    Bemf a  is the BEMF induced in phase A.  
         [0040]    From Phase B:  
           V   CT   =V   Xgb   +R   b   i+L   b   di/dt−Bemf   b   (2)  
         [0041]    Where:  
         [0042]    V Xgb  is the voltage across switch Xgb  
         [0043]    Rb is the resistance of resistor Rb  
         [0044]    i is the current  
         [0045]    Lb is the inductance of inductor Lb;  
         [0046]    Bemf b  is the BEMF induced in phase B  
         [0047]    Adding (1) and (2):  
         2 V   CT   =V   xgb   −V   Dga −( Bemf   a   +Bemf   b )  (3)  
         [0048]    Solving for V CT :  
               V   CT     =           V   Xgb     -     V   Dga       2     -         Bemf   a     +     Bemf   b       2               (   4   )                               
 
         [0049]    Also, from a three phase balanced system:  
           Bemf   a   +Bemf   b   +Bemf   c =0  (5)  
         [0050]    Combining equations (4) and (5), V CT  equals:  
         [0051]    [0051]               V   CT     =           V   Xgb     -     V   Dga       2     +       Bemf   c     2               (   6   )                                 
         [0052]    From FIG. 2, the phase C coil tap voltage Vc equals:  
           Vc=Bemf   c   +V   CT   (7)  
         [0053]    As can be seen from equation (7), in this example, the coil tap voltage Vc is equal to the BEMF for phase C only when V CT  is zero. Combining equations (6) and (7):  
             Vc   =         3   2          Bemf   c       +         V   Xgb     -     X   Dga       2               (   8   )                               
 
         [0054]    If the last term of equation (8) is ignored, the coil tap voltage Vc is directly proportional to the BEMF for that phase. However, especially at low speeds and low voltages, the BEMF itself is very small. Accordingly, the last term of equation (8) plays a significant role. For low voltage switches X such as those typically used in brushless motor  20 , the internal resistance of the switch is also very small, often in the order of 10 milli ohms. In such cases, V Xgb  can be ignored, so equation (8) can be re-written as follows:  
             Vc   =         Bemf   c     +     V   CT       =         3   2          Bemf   c       -       V   Dga     2                 (   9   )                               
 
         [0055]    As can be seen from equation (9) the coil tap voltage Vc is proportional to the BEMF in phase C with the exception of one-half the voltage across diode V Dgn . Accordingly, as will be described in more detail below, the precondition circuitry  50  of the present invention provides circuitry for adjusting or offsetting the effect of the diode D.  
         [0056]    Referring now to FIG. 3, a theoretical graphical representation of the zero-crossing detection is depicted in a motor  20  which does not include the precondition circuitry  50 . The graph of FIG. 3 shows theoretical data presuming, for sake of simplicity, the high frequency PWM signal has been removed. Ideally, zero-crossings of each phase A, B, C of the motor  20  would be distributed evenly in 60 degree intervals. However, without the precondition circuitry  50 , the detection of the zero-crossing for each phase A, B, C is shown to be unsymmetrical due to the effect of the diodes D during the PWM-off states. More particularly, as shown in FIG. 3, each time the coil tap voltage Va, Vb, Vc of phases A, B, C cross the zero voltage line, a zero-crossing signal  30  is shown to transition from high-to-low or low-to-high. Due to the effect of the diodes D as indicated by equation (9) above, the zero-crossing signal  30  does not transition in equal 60 degree intervals.  
         [0057]    Referring briefly back to FIG. 2, in systems not having the precondition circuitry  50 , the zero-crossing signal  30  was typically obtained by comparing the floating phase coil tap voltage, e.g. voltage Vc, with a reference voltage Rref by way of a comparator  35 . While for sake of example only phase C is shown to be coupled to a comparator  35  for detecting zero-crossings, it will be appreciated that each phase A, B, C is coupled to a comparator for this purpose. Especially in low voltage and/or low frequency applications, it has been determined that because the slope of change of the coil tap voltage Va, Vb, Vc as it approaches zero-crossing may be very gradual, accurately detecting the time a zero-crossing actually occurs can be difficult. In particular, with a gradual change in coil tap voltage around zero-crossing the actual timing of the zero-crossing is often difficult to determine in view of the inherent standard deviation/offset of the comparator  35 . Accordingly, as a further feature of the present invention, the precondition circuitry  50  discussed below optionally includes a sharpening feature for sharpening the signal output from the coil tap prior to performing zero-crossing detection. In this manner, the standard deviation/offset of the comparator  35  will less significantly effect zero-crossing detection especially for low speed and/or low voltage applications.  
         [0058]    Referring now to FIG. 4, a driver circuit for a blushless DC motor  100  of the present invention is depicted. The motor  100  is substantially similar to the motor  20  described above with reference to FIG. 2 and therefore common elements will not again be discussed. However, in addition to the elements described above, the motor  100  of the present invention includes precondition circuits  50  coupled to the coil tap voltage Va, Vb, Vc for each phase. As described in detail below, the precondition circuit  50  includes circuitry for offsetting or compensating the coil tap voltage Va, Vb, Vc from the effect of the diodes D and for sharpening the coil tap voltage Va, Vb, Vc prior to zero-crossing detection. An output of the precondition circuits  50  is coupled to a zero-crossing detection circuit  52 . The zero-crossing detection circuit  52  may, for example, take the form of the comparator  35  described above with reference to FIG. 2 or other known circuits for detecting zero-crossings.  
         [0059]    Referring now to FIG. 5, the precondition circuit  50  of one embodiment of the present invention is depicted. For sake of example, the precondition circuit  50  shown in FIG. 5 is that of phase C. However, it will be appreciated that similar precondition circuits  50  are coupled to phases A and B as shown in FIG. 4. The precondition circuit  50  of the present embodiment preferably includes offset circuitry  55  and sharpening circuitry  60 . The offset circuit  55  includes circuitry for offsetting from the coil tap voltage, the voltage effect from the diode D so that the coil tap voltage is substantially directly proportional to the BEFM. The sharpening circuit  60  includes circuitry for enhancing the coil tap voltage as the coil tap voltage approaches zero-crossing. Thus, the sharpening circuit  60  allows the comparator  35  which receives the output from the precondition circuit  50  to more clearly detect a zero-crossing. In this example, the coil tap voltage Vc is input to the precondition circuit  50  and coupled to node Z through current limiting resistor r.  
         [0060]    Continuing to refer to FIG. 5, in order to offset from the coil tap voltage Vc, the effect from the diode Dga, the offset circuitry  55  includes a voltage divide circuit  57 . In the present example, the voltage divide circuit  57  includes a first resistor RI coupled between common node n and constant supply voltage Vcon, and a second resistor R 2  coupled between the common node n and ground. For this example, R 1  and R 2  are chosen such that voltage at common node n=Vcon*R 2 /(R 1 +R 2 )=V Dga /2. The voltage at common node n is then coupled to node Z through current limiting resistor r, thereby serving as an additive component to the coil tap voltage Vc for purposes of offsetting the effect of the diode Dga. In this manner, the second term of equation (9) (discussed above with reference to FIG. 2) may be offset such that the phase C coil tap voltage Vc is made substantially directly proportional to the BEFM. For the present example, the voltage drop V Dga  across the diode Dga during a freewheeling period is referred to as V Dga-fw . Accordingly, in accordance with readily known techniques in the art, Vcon, R 1  and R 2  are selected such that the voltage divide circuit  57  provides a voltage substantially equal to ½ V Dga-fw . In this manner, the offset circuit  55  is able to add back to the coil tap voltage Vc at node Z, a constant voltage which substantially eliminates the effect of the diode Dga. In turn, the coil tap voltage Vc is adjusted prior to zero-crossing detection so as to be substantially directly proportional to the BEMF.  
         [0061]    It will be appreciated that while values for Vcon, R 1  and R 2  are shown above for sake of example, other values could have been chosen so as to achieve a similar result. Further, it will be appreciated that while the offset circuit  55  above is shown to be formed of a voltage divide circuit, the present invention is intended to cover any circuit configuration which serves to offset the value of the diode D and is not limited to a voltage divide circuit. Additionally, as mentioned above, in the present example during a PWM-off state, the switch Xga in the high phase is not turned on in order to minimize switching loss and noise. Thus, in the example leading to equation (9) the effect of the diode D was shown to be V Dga /2. It will be appreciated, however, that the present invention is suitable for use in other motor configurations where, for example, the ground switch (e.g. Xga) for the high phase is turned on during a PWM-off state. In such cases, the effect of the diode on the coil tap voltage will differ from the V Dga /2 described in the above example. Accordingly, in such alternative embodiments, the offset circuit  55  is correspondingly adjusted to offset the effect of the diode D by an appropriate amount as can be readily determined by one in the art.  
         [0062]    Continuing to refer to FIG. 5, the sharpening circuit  60  will now be described in more detail. The sharpening circuit  60  is shown to include an operational amplifier  65  having a negative feedback loop  70 . A non-inverting input  72  to the amplifier  65  is coupled to node Z. An inverting input  74  to the amplifier  65  is coupled to an output  76  of the amplifier  65  through resistor Rf. The inverting input  74  is also coupled to ground through current limiting resistor r. As will be discussed in more detail below, the output of the amplifier  65  is coupled to an input of the zero-crossing detection circuit  52  through a current limiting resistor r.  
         [0063]    In the present example, a gain of the amplifier  65  is preferably set to provide a gain or sharpening to the offset coil tap voltage of any desired value greater than one. The negative feedback loop  70  controls the gain of the amplifier  65  to equal 1+Rf/r in the present example. Thus, once a desired gain is selected, say for example a gain of ten, the values of Rf and r are selected to provide the desired gain in a manner readily known in the art. As discussed above, the gain may be set to any desired value and the present invention is not limited to any particular value or range.  
         [0064]    In addition to the amplifier  65  and associated circuitry described above, the sharpening circuit  60  further includes a clamping diode  80  coupled between the non-inverting input  72  of amplifier  65  and ground. The purpose of the clamping diode  80  is to protect the amplifier  65  from saturating. More particularly, the clamping diode  80  limits the maximum voltage introduced to the amplifier  65  to approximately 0.7 volts, which is the voltage drop across the clamping diode  80 . With an amplifier gain in the range of 10-15, the output  76  of the amplifier  65  will not exceed, for example, 7 volts-10.5 volts. With a source voltage Vs for the amplifier  65  set at, for example, 15 volts the clamping diode  80  ensures that the amplifier  65  does not amplify a signal beyond its 15 volt saturation point. Further, as the output  76  of the amplifier  65  is primarily used for zero-crossing detection, truncating voltages introduced to the amplifier  65  which are outside of the range clamped by the clamping diode  80  does not effect motor performance.  
         [0065]    In operation, the present invention provides enhanced zero-crossing detection for brushless DC motors as will now be described with reference to FIGS. 4 and 5. The motor  100  is driven by way of a PWM signal  110  which is applied to the motor  100  in one of several conventional manners. For example, in one embodiment, during PWM-on states the high switch (e.g. Xsa, Xsb, Xsc) for the “from” phase of the commutation sequence and the ground switch (e.g. Xga, Xgb, Xgc) for the “to” phase of the commutation sequence are turned on. During the following PWM-off state, the high switch in the “from” phase is turned off and all of the freewheeling current is allowed to pass through the diode (e.g. Dga, Dgb, Dgc) in the “from” phase to ground through the ground switch in the “to” phase. Such a current path during the PWM-off state is representatively depicted in FIG. 4 by current path i off . By not turning on the ground switch in the “from” phase during the PWM-off state, it is possible to avoid switching delays and noise. However, it will be appreciated that the present invention is suitable for motors  100  which operate in any switching mode.  
         [0066]    The motor  100  of the present embodiment, advantageously monitors for zero crossing detections during PWM-off states. Because a PWM signal typically oscillates at a frequency significantly greater than the frequency at which the commutation sequence advances, zero-crossings which may happen to occur during a PWM-on state are still detectable during the PWM-off state with minimal delay. For example, the frequency of the PWM signal may be in the range of 20 kHz-100 kHz while the frequency at which the commutation sequence advances is typically on the order of 100 Hz. Further, by performing zero-crossing detection during PWM-off states, filters and delays associated with offsetting the center tap voltage CT during PWM-on states are avoided.  
         [0067]    During PWM-off states, zero crossing detection occurs by initially providing the coil tap voltage Va, Vb, Vc for the floating phase to the precondition circuit  50 . The precondition circuit  50  then offsets the coil tap voltage for the effect of the diode D so that the coil tap voltage is more closely proportional to the BEMF for that phase. For instance, in the present example, the precondition circuit  50  adjusts the floating phase coil tap voltage by an amount substantially equal to an amount by which the voltage at the center tap V CT  is deviated from zero as discussed above with reference to equations (8) &amp; (9). Further, the precondition circuit  50  sharpens or amplifies the offset coil tap voltage so as to enhance the signal as it approaches zero-crossing. In this particular example, the precondition circuit  50  passes the offset coil tap voltage through amplifier  65  having a gain of 11. The amplifier is preferably clamped by a claming diode  80  so to avoid saturation of the amplifier  65  as discussed in more detail above.  
         [0068]    Following the offset and signal enhancement stages of the precondition circuit  50 , the output of the precondition circuit is provided to the zero-crossing detection circuit  52 . The zero-crossing detection circuit  52  may, for example, include a comparator for comparing the output of the precondition circuitry with a reference voltage to determine when a zero-crossing has occurred. As the precondition circuit  50  of the present invention has adjusted the coil tap voltage for variations introduced by the diode D, the offset coil tap voltage is more closely proportional to the BEMF. Further, as the offset coil tap voltage is further sharpened prior to performing a zero-crossing detection, offsets inherent in, for example, the comparator used in the zero-crossing detection circuit  52 , less significantly effect proper zero-crossing detection. Thus, zero crossing detection is made more accurate especially for low voltage and/or low speed applications where the effect of the diode and offset in the zero-crossing detection circuit are more pervasive.  
         [0069]    Referring briefly to FIG. 6, there is shown a theoretical graph with the adjusted the coil tap voltages Va, Vb, Vc and resulting output from the zero-crossing detection circuit  52  in a motor utilizing the preconditioning circuit  50  discussed herein. For sake of simplicity, the theoretical data shown in FIG. 6 presumes the high frequency PWM signal is removed. As shown, with the preconditioning circuit  50 , the output of the zero-crossing detection circuit  52  which controls advancement of the commutation sequence of the motor is substantially close to having the desired 60 degree switching intervals. Accordingly, the present invention provides for smoother switching through the commutation sequence which in turn provides a more efficient motor which is less likely to jitter or stall.  
         [0070]    Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalents and modifications will occur to others skilled in the art upon the reading and understanding of the specification. For example, although the embodiment described above shows that the precondition circuitry  50  performs an offset to the coil tap voltage Va, Vb, Vc prior to amplifying the signal, the precondition circuit  50  may have alternatively been designed so as to perform an amplification first followed by an appropriately adjusting an offset. Alternatively, to the extent that only an offset or a sharpening of the coil tap voltage is desired, the precondition circuit  50  may include only one of these two features. Further, although a diode  80  was shown to clamp the voltage input to the amplifier  65 , alternative clamping means could have been used. Still further, while a voltage divide circuit  57  was shown to be utilized in the offset circuit  55  to compensate for the effect of the diode D, it will be appreciated that alternative offset voltage circuit could have been used, including for example a direct application of the desired offset voltage to the non-inverting input of the amplifier  65 . Yet still further, while the precondition circuits  50  and zero-crossing detection circuits  52  are depicted as separate components for each phase, it will be appreciated that such circuitry may be combined into fewer circuits and/or fully consolidated without departing from the spirit or scope of invention. The present invention includes all such equivalents and modifications, and is limited only by the scope of the following claims.