Abstract:
A commutation circuit for a sensorless three-phase BLDC motor enables soft switching in the commutation process without the need for a complex delay circuit and generates signals separate from the back EMF signals of the stator to start the rotation of the motor rotor. The commutation circuit includes a starting circuit adapted to generate a starting clock signal to forcibly start the motor without information associated with the position of the motor rotor and generates first, second and third reference clock signals that are used to drive the motor in constant velocity operation. The commutation circuit also includes a constant velocity controlling circuit that compares the voltages from each of the phases to the neutral voltage of the stator and a stair/common voltage generating circuit that provides a first group of stair voltages and a common voltage to forcibly start rotation of the rotor or, alternatively, a second group of stair voltages with a second common voltage to optimally commutate the phases when the rotor is accelerating is turning at a constant operational speed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a three-phase brushless direct current motor. More particularly, the invention relates to a commutation circuit for a sensorless three-phase brushless direct current motor. 
     2. Description of Related Technology 
     In controlling currents through the windings of a three-phase brushless direct current (BLDC) motor, commutation is conventionally accomplished using Hall-effect sensors. The Hall-effect sensors are used to detect the position of the permanent magnets in the motor rotor to provide signals associated with the absolute position of the rotor to the commutation circuitry. Conventional commutation circuits use the phase and amplitude of the Hall-effect sensor signals to soft-switch the current in the motor windings during constant velocity operation of the motor. Additionally, the Hall-effect sensor signals may be used to provide the desired commutation during initial starting of the motor. 
     FIG. 1 illustrates a typical input stage of an output driving circuit in a conventional three-phase BLDC motor commutation circuit. The input stage includes three differential amplifiers  10 ,  20 ,  30  that receive signal pairs U_Hall+/U_Hall−, V_Hall+/V_Hall−, W_Hall+/W_Hall− from three respective Hall-effect sensors (not shown). The phase difference between the signals of each signal pair is 180°. 
     Where the difference between the individual signals of the signal pairs is less than 100 mV (i.e., approximately four times the thermal voltage V T ), the output currents I c   1 -I c   6  of the differential amplifiers  10 ,  20 ,  30  are linearly increased or decreased. Where the difference between the individual signals of the signal pairs is greater than 100 mV, the output currents Ic 1 -Ic 6  of the differential amplifiers  10 ,  20 ,  30  are limited by the current source I EE . Because the output currents I c   1 -I c   6  of the differential amplifiers  10 ,  20 ,  30  are linearly increased or decreased for signal pair differences in an interval surrounding zero volts, the output stage of the output driving circuit can be soft switched in the interval surrounding zero volts. Soft switching prevents the excitation current of the stator coil of the motor from changing rapidly, thereby allowing the motor to rotate freely without sparks. 
     In contrast to the above-described conventional commutation circuit, which uses Hall-effect sensors, a sensorless three-phase BLDC motor commutation circuit uses back electromotive force EMF, which is generated in the unexcited phase of a stator coil, instead of Hall-effect signals. In particular, a zero crossing point at which the back EMF and neutral point voltage intersect is detected, and commutation is accomplished by using the zero crossing point to determine the absolute position of the motor rotor. 
     FIG. 2 illustrates a timing diagram that shows the time domain relationship between the conventional commutation signals of a three-phase BLDC motor having three Hall-effect sensors and the zero crossing point of a sensorless three-phase BLDC motor. Detail (a) in FIG. 2 highlights a region in which the voltage difference between the signals of the U_Hall signal pair is less than 100 mV, thereby allowing conventional commutation to accomplish soft switching within the highlighted region. Detail (b) highlights a region surrounding the point at which the back EMF U_BEMF and the neutral point N intersect for a sensorless three-phase BLDC motor. As shown by detail (b) of FIG. 2, when the motor is driving with a constant velocity, the zero crossing point electrically leads the conventional commutation soft switching point by 30°. Thus, to optimize commutation in a sensorless three-phase BLDC motor using the back EMF signal, the phase of the back EMF signal must be electrically delayed by 30° before it is used in conjunction with the zero crossing point to perform commutation. 
     Generally, a complex delay circuit having a plurality of delay elements is needed to delay the back EMF signal. Furthermore, the combination of the current flowing through the motor windings and the resistance of the windings introduces substantial non-ideal voltages that make detection of the back EMF signal difficult and prone to error. Still further, a commutation mode using back EMF signals is typically a hard switching mode, which may produce undesirable sparks together with electromagnetic interference (EMI) in the switching process. While a snubber circuit may used to suppress the sparking and the EMI, this further increases the complexity of the delay circuit. 
     Additional complications arise in starting a stopped sensorless three-phase BLDC motor because the back EMF signal exhibits a poor signal-to-noise ratio and is an unreliable indicator of the rotor position until the motor has a reached a sufficient rotor speed. As a result, if commutation is started by using information from the back EMF signal that is detected during the forcible start of the motor an unstable commutation could result, which could stop rotation of the motor or rotate the rotor in an undesirable direction. 
     SUMMARY OF THE INVENTION 
     Generally, the invention provides a commutation circuit for a sensorless three-phase BLDC motor that enables soft switching in the commutation process without the need for a complex delay circuit and which provides signals separate from the back EMF signals to start the rotation of the motor rotor. 
     The commutation circuit includes a starting circuit adapted to generate a selection signal that indicates whether a motor is in a starting mode or a constant velocity mode by detecting the size of each phase voltage of the motor stator coil. The starting circuit also generates a starting clock signal to forcibly start the motor without information associated with the position of the motor rotor and generates first, second and third reference clock signals having respective phase differences of 120° and that are synchronized with the starting clock signal. 
     The commutation circuit further includes a constant velocity controlling circuit that is adapted to compare each phase voltage and a neutral point voltage of the stator to generate first, second and third comparison signals and a constant velocity clock signal having a frequency three times the frequency of the comparison signals. 
     The commutation circuit still further includes a stair/common voltage generating circuit, that is responsive to the selection signal generated by the starting circuit in the starting mode such that the reference clock signals simultaneously generate a first group of three stair voltages having respective phase difference of 120° when iterating three-phase with a first voltage equal to a reference voltage during a first period of the starting clock signal, iterating three-phase with a second voltage less than the reference voltage during subsequent second and third periods of the starting clock signal, iterating three-phase with the reference voltage during a subsequent fourth period of the starting clock signal, and iterating three-phase with a third voltage greater than the reference voltage during subsequent fifth and sixth periods of the starting clock signal, and, in the constant velocity mode of the motor, uses the reference clock signals to simultaneously generate a second group of three stair voltages having respective phase difference of 120°, when iterating three-phase with a fourth voltage equal to a second reference voltage during a first period of the constant velocity clock signal, iterating three-phase a fifth voltage less than the second reference voltage during subsequent second and third periods of the constant velocity clock signal, iterating three-phase with the second reference voltage during a subsequent fourth period of constant velocity clock signal, and iterating three-phase with a sixth voltage greater than the second reference voltage during subsequent fifth and sixth periods of the constant velocity clock signal. 
     The commutation circuit still further includes an output driving circuit, controlling the direction of the current in the respective phases of the stator coil using the voltage differences between the first group of stair voltages and a first common voltage in the starting mode of the motor and using the voltage differences between the second group of stair voltages and a second common voltage in the constant velocity mode of the motor. 
     The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a typical input stage of an output driving circuit in a conventional three-phase BLDC motor commutation circuit; 
     FIG. 2 illustrates a timing diagram showing the time domain relationship between the conventional commutation signals of a three-phase BLDC motor using three Hall-effect sensors and the zero crossing point of a sensorless three-phase BLDC motor; 
     FIG. 3 illustrates a block diagram of a commutation circuit for a sensorless three-phase BLDC motor according to the invention; 
     FIG. 4 illustrates an exemplary schematic diagram of the constant velocity controlling circuit of FIG. 3; 
     FIG. 5 illustrates an exemplary schematic diagram of an input stage of the output driving circuit of FIG. 3; 
     FIGS. 6A-6F illustrate the principles of generating stair voltages according to the invention; 
     FIG. 7 illustrates an exemplary schematic diagram of a circuit capable of generating common voltages according to the invention; 
     FIG. 8 illustrates a more detailed block diagram of the starting circuit of FIG. 3; 
     FIG. 9 illustrates a more detailed schematic diagram of the amplifying and mode selection circuits of FIG. 8; 
     FIG. 10 illustrates a timing diagram of signals associated with the sensorless three-phase BLDC motor starting circuit of FIG. 8; 
     FIG. 11 illustrates a timing diagram of signals associated with the starting mode of the motor according to the invention; and 
     FIG. 12 illustrates a timing diagram of the signals associated with the constant or accelerated velocity mode of the motor according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 illustrates a block diagram of a commutation circuit  50  for a sensorless three-phase BLDC motor according to the invention. The commutation circuit  50  includes a starting circuit  100 , a constant velocity controlling circuit  200 , a stair/common voltage generating circuit  300 , and an output driving circuit  400 . 
     The starting circuit  100  determines whether the motor (not shown) is in a starting mode or a constant velocity mode and outputs a selection signal SEL that corresponds to the determination. The starting circuit  100  also generates a constant frequency starting clock signal STR_CLK that is used to control the commutation of the motor during the initial start-up of the motor, thereby eliminating the unstable commutation that results from using the back EMF signal to control commutation during motor start-up (i.e., before the motor rotor has reached a nominal operating speed). Additionally, the starting circuit  100  generates reference clock signals D 0 -D 2  that each have a period six times that of the starting clock signal STR_CLK and respective phase differences of 120°. 
     The constant velocity controlling circuit  200  generates three comparison signals U_Comp, V_Comp, W_Comp by comparing each of the phase voltages U_Out, V_Out, W_Out of stator coils U, V, W to the neutral point voltage N of the stator coils to generate a constant velocity clock signal FG. The constant velocity clock signal FG has a frequency three times that of the comparison signals U_Comp, V_Comp, W_Comp and is used as a clocking signal for the commutation process during constant velocity operation of the motor (i.e., when the rotor of the motor has reached an operating speed). 
     FIG. 4 illustrates an exemplary schematic diagram of the constant velocity controlling circuit  200  of FIG.  3 . The constant velocity controlling circuit  200  includes first through third comparators  201 ,  202 ,  203  that receive the phase voltages U_Out, V_Out, W_Out of the stator coils at the respective noninverting inputs of the comparators  201 ,  202 ,  203 . The inverting inputs of the comparators  201 ,  202 ,  203  receive the neutral point voltage N. The comparators  201 ,  202 ,  203  provide the first through third comparison signals U_Comp, V_Comp, W_Comp at the respective output terminals of the comparators  201 ,  202 ,  203 . The comparison signals of the comparators  201 ,  202 ,  203  are coupled to first through third NAND gates  204 ,  205 ,  206 , and the outputs of the NAND gates  204 ,  205 ,  206  are coupled to the input terminals of an AND gate  207 . The first through third NAND gates  204 ,  205 ,  206  and the AND gate  207  logically combine the comparison signals U_Comp, V_Comp, W_Comp to generate the constant velocity clock signal FG. 
     FIG. 5 illustrates an exemplary schematic diagram of an input stage of the output driving circuit  400  of FIG.  3 . Generally, the output driving circuit  400  drives the motor by controlling the direction of the current that is flowing through each phase of the stator coil according to the voltage difference between a first group of stair voltages U_Stair 1 , V_Stair 1 , W_Stair 1  and a first common voltage Com 1  or, alternatively, according to a second group of stair voltages U_Stair 2 , V_Stair 2 , V_Stair 2  and a second common voltage Com 2 . The first group of stair voltages U_Stair 1 , V_Stair 1 , W_Stair 1  and first common voltage Com 1  are applied to the input terminals of first through third differential amplifiers  401 ,  402 ,  403  in the starting mode, and the second group of stair voltages U_Stair 2 , V_Stair 2 , V_Stair 2  and second common voltage Com 2  are applied to the input terminals of the first through third differential amplifiers  401 ,  402 ,  403  in the constant velocity mode of the motor. 
     In the constant velocity mode of the motor, the second group of stair voltages U_Stair 2 , V_Stair 2 , W_Stair 2  are generated by and synchronized with the constant velocity clock signal FG. The second group of stair voltages U_Stair 2 , V_Stair 2 , W_Stair 2  are applied to respective base terminals of the differential amplifiers  401 ,  402 ,  403 , and the common voltage Com 2 , which has a sawtooth waveform, is commonly applied to the other base terminals of the amplifiers  401 ,  402 ,  403 . 
     FIGS. 6A-6F illustrate the principles of generating the above-described stair voltages according to the invention. FIG. 6A illustrates a schematic diagram of a switched resistive network that may be used in the stair/common voltage generating circuit  300  to generate cyclical stair voltage waveforms having three discrete levels. The resistors R 1 -R 4  are of substantially the same value and, as shown, the switches S 1 , S 2  are controlled by control signals U 1 , U 2  and may be independently opened or closed to vary the effective voltage division provided by the resistor network to vary an output voltage Vout. 
     FIG. 6B shows a truth table associated with the operation of the circuit of FIG.  6 A. As shown in the table, the output voltage Vout is varied between three discrete levels depending on the combined states of the switches S 1 , S 2 . In particular, Vout is defined as equal to a reference voltage level when S 1  and S 2  are both open or both closed, Vout is greater than the defined reference voltage when S 1  is closed and S 2  is open, and Vout is less than the defined reference voltage when S 1  is open and S 2  is closed. 
     FIGS. 6C-6F illustrate equivalent circuits associated with the various combinations of the states for the switches S 1  and S 2  shown in FIG.  6 A. Namely, FIG. 6C represents the equivalent circuit where S 1  is closed and S 2  is open such that the output voltage Vout is defined by Equation 1 below.              Vout   =       R4       1       1   R1     +     1   R2         +   R4          V                 cc             Equation 1                                
     FIG. 6D represents the equivalent circuit where S 1  and S 2  are both closed, and FIG. 6E represents the equivalent circuit where both S 1  and S 2  are open. For the equivalent circuits represented in FIGS. 6D and 6E, the output voltage Vout is defined by Equation 2 below.                Vout        (     reference                 voltage     )       =     Vcc   2             Equation 2                                
     FIG. 6F represents the equivalent circuit where S 1  is open and S 2  is closed, which results in an output voltage Vout according Equation 3 below.              Vout   =         1       1   R3     +     1   R4             1       1   R3     +     1   R4         +   R2          Vcc             Equation 3                                
     In operation, the first group of stair voltages U_Stair 1 , V_Stair 1 , W_Stair 1  are generated during the starting mode of the motor by using the reference clock signals D 0 -D 2  as the control signals U 1 , U 2  to switch the switches S 1  and S 2 . For example, reference clock signals D 0  and D 1  are used as the control signals U 1 , U 2  to generate the stair voltage U_Stair 1 , signals D 1  and D 2  are used to generate the stair voltage V_Stair 1 , and signals D 2  and D 0  are used to generate the stair voltage W_Stair 1 . 
     To generate the second group of stair voltages U_Stair 2 , V_Stair 2 , W_Stair 2  in the constant velocity mode of the motor, the comparison signals U_Comp, V_Comp, W_Comp are used as the control signals U 1 , U 2  to switch the switches S 1 , S 2 . For example, the comparison signals W_Comp and U_Comp are used as the control signals U 1 , U 2  to generate the stair voltage U_Stair 2 , the comparison signals U_Comp and V_Comp are used to generate the stair voltage V_Stair 2 , and the comparison signals V_Comp and W_Comp are used to generate the stair voltage W_Stair 2 . 
     FIG. 7 illustrates an exemplary schematic diagram of a circuit  450  capable of generating common voltages according to the invention. The circuit  450  includes a switch SW that is switched on/off (i.e., closed/open) by the edge signal FG_Edge of the constant velocity clock signal FG, first and second current sources  1 ,  2 *I, and a capacitor C. 
     In operation, the switch SW is repeatedly switched on/off by the edge signal FG_Edge such that the capacitor C integrates a current I or −I, thereby generating a sawtooth voltage waveform having substantially constant slopes across the capacitor C. In this manner, the circuit  450  may be used in the constant velocity mode of the motor to generate the second common voltage Com 2 . 
     FIG. 8 illustrates a more detailed block diagram of the starting circuit  100  of FIG.  3 . The starting circuit  100  includes a clock generating circuit  110 , an amplifying circuit  120 , and a starting/acceleration mode selecting circuit  130 . The clock generating circuit  110  generates the starting clock signal STR_CLK having a constant frequency and generates the three reference clock signals D 0 -D 2 . The three reference clock signals D 0 -D 2  have a period six times the period of the starting clock signal STR_CLK and respective phase differences of 120°. The starting circuit  100  additionally generates mask clock signals U_MSK, V_MSK, W_MSK to detect the phase voltage of the unexcited coil at the time of commutation. 
     The amplifying circuit  120  amplifies the respective phase voltages U_Out, V_Out, W_Out of the stator coil and generates amplified signals A 0 -A 2 . When the amplified signals A 0 -A 2  increase to a constant level while the mask clock signals U_MSK, V_MSK, W_MSK are enabled, the starting/acceleration mode selecting circuit  130  is enabled and generates the selection signal SEL to drive the motor from the starting mode to the acceleration mode. 
     FIG. 9 illustrates a more detailed schematic diagram of the amplifying and mode selection circuits  120 ,  130  of FIG.  8 . The mode selection circuit  130  includes first through third NAND gates  131 - 133 , which receive the amplified signals A 0 -A 2  of the amplifying circuit  120  as their respective inputs, and which receive the mask clock signals U_MSK, V_MSK, W_MSK as further respective inputs. The first through third NAND gates  131 - 133  perform logical operations with the U_MSK and A 0  signals, with the V_MSK and A 1  signals, and with the W_MSK and A 2  signals, respectively. A fourth NAND gate  134  receives the outputs of the first through third NAND gates  131 - 133  to generate the selection signal SEL. 
     In operation, the mask clock signals U_MSK, V_MSK, W_MSK and the amplified signals A 0 -A 2  are enabled at the unexcited point of each phase of the motor coil in order to detect the pure back electromotive force that is generated on the unexcited phase from among the phases of the stator coil (FIG.  10 ). The fact that the amplitude of the amplified signals A 0 -A 2  is greater than a predetermined fixed level during the above-noted detection interval indicates that the amplitude of the back EMF is more than a predetermined fixed value, which indicates that the motor is either accelerating or running at a constant operational speed. 
     When the selection signal SEL is disabled, the stair/common voltage generating circuit  300  is synchronized with the starting clock signal STR_CLK and the reference clock signals D 0 -D 2  to generate the first group of stair voltages U_Stair 1 , V_Stair 1 , W_Stair 1  and the first common voltage Com 1 . When the selection signal SEL is enabled, the stair/common voltage generating circuit  300  is synchronized with the constant velocity clock signal FG and the comparison signals U_Comp, V_Comp, W_Comp to generate the second group of stair voltages U_Stair 2 , V_Stair 2 , W_Stair 2  and the second common voltage Com 2 . 
     FIG. 11 illustrates a timing diagram of the above-described signals associated with the starting mode of the motor according to the invention. The stair/common voltage generating circuit  300 , according to the states of the selection signal SEL, generates the first common voltage Com 1  as a square wave having a period twice the period of the starting clock signal STR_CLK. The stair/common voltage generating circuit  300  uses the clock signals D 0 -D 2  to simultaneously generate the first group of three stair voltages U_Stair 1 , V_Stair 1 , W_Stair 1  having respective phase difference of 120° when iterating three-phase with a voltage equal to the reference voltage during a period of the starting clock signal, iterating three-phase with a voltage equal to and below the reference voltage during next two periods of the starting clock signal, iterating three-phase with a voltage equal to the reference voltage during next one period of the starting clock signal, and iterating three-phase with a voltage equal to and over the reference voltage during next two periods of the starting clock signal. 
     FIG. 12 illustrates a timing diagram of the signals associated with the constant or accelerated velocity mode of the motor according to the invention. The stair/common voltage generating circuit  300  uses the leading edge of the constant velocity clock signal FG to generate the edge signal FG_Edge. The edge signal FG_Edge is used to generate the second common voltage Com 2 . As shown, the second common voltage Com 2  has a sawtooth waveform that repeatedly ramps between a maximum value and minimum value at a constant slope in response to the rising and falling edges of the edge signal FG_Edge. 
     The stair/common voltage generating circuit  300  uses the comparison signals U_Comp, V_Comp, W_Comp to simultaneously generate the second group of three stair voltages U_Stair 2 , V_Stair 2 , W_Stair 2  having respective phase differences of 120° when iterating three-phase with a voltage equal to the reference voltage during a period of the edge signal, iterating three-phase with a voltage equal to and below the reference voltage during next two periods of the edge signal, iterating three-phase with a voltage equal to the reference voltage during next one period of the edge signal, and iterating three-phase with a voltage equal to and over the reference voltage during next two periods of the edge signal. 
     In an interval highlighted by detail (a) in FIG. 12, the difference voltage DFR 1  between the second common voltage Com 2 , which is generated based on the edge signal FG_Edge of the constant velocity clock signal and the stair voltage U_Stair 2 , is less than 100 mV. Because the input stage (FIG. 5) of the output driving circuit  400  is operating in a linear mode in this interval, it is possible to perform soft switching within the interval. For example, as can be seen in FIG. 12, the point at which the stair voltage U_Stair 2  intersects the second common voltage Com 2  is electrically delayed 30° with respect to the zero crossing point highlighted in detail (b). Thus, when the difference voltage between the second group of three stair voltages generated during the constant velocity mode of the motor and the sawtooth waveform of the second common voltage Com 2  is used, precise commutation can be executed for each phase at the point electrically delayed by 30° from the zero crossing point of the back EMF without additional delay circuitry. 
     A range of changes and modifications can be made to the preferred embodiment described above. The foregoing detailed description should be regarded as illustrative rather than limiting and the following claims, including all equivalents, are intended to define the scope of the invention.