Abstract:
A circuit for controlling a brushless permanent magnet motor is provided. The circuit comprises windings, each of the windings having a first end connected at a common node and each of the windings having a second end connectable directly to supply voltages by switches, the second end connected to an upper supply voltage or connected to a lower supply voltage or disconnected from the supply voltages; blocking circuitry connectable with the second ends, the blocking circuitry producing a blocked voltage; a comparator receiving the blocked voltage on one input and a reference voltage on another input, the comparator result indicating polarity of a back emf voltage in the associated winding; and a latch providing control signals for the circuit, an input of the latch enabled by an enable signal, an output of the latch comprising a back emf voltage detection signal. The blocking circuitry and the comparator are duplicated for each of the windings.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to the control of a brushless DC (BLDC) motor. More particularly, the present invention relates to control a sensorless BLDC motor without Hall sensors. 
   2. Description of Related Art 
     FIG. 1  shows a schematic diagram for a BLDC motor drive. Such a motor usually comprises three phases A, B and C. The three phases A, B and C may be connected in a star configuration having a common node N as shown in  FIG. 1 , or in a delta configuration. For each phase, switch pairs XSA, XGA; XSB, XGB; XSC, XGC connect the free ends of the phases A, B, C to supply a power source Vs and a ground GND, respectively. The switches XSA, XGA, XSB, XGB, XSC, XGC are typically power transistors. Reverse biased diodes DSA, DGA; DSB, DGB; DSC, DGC are connected in parallel to the corresponding switches XSA, XGA, XSB, XGB, XSC, XGC. 
   The BLDC motor is controlled by detecting the position of the rotor, then a current is applied to the stator according to the detected position. Three Hall sensors are widely used as a position sensor for the BLDCM. However, the Hall sensors themselves increase the motor system size and manufacturing costs. Many position sensorless BLDCM drivers that detect the rotor position from the back electromotive force (BEMF) generated in the phases, have been introduced. 
     FIG. 2  shows a relationship between the BEMFs and armature currents of stator windings. The motor is controlled through six steps s 1 , s 2 , s 3 , s 4 , s 5  and s 6 . In each step, the current is conducted in only two phases, and the third phase is floating. The BEMF can be monitored from the terminal voltage of the floating phase. The commutation events occur at 30° delayed from the corresponding zero-crossing points (ZCP) of the BEMF waveforms. 
   U.S. Pat. No. 4,654,566 describes such a motor control system, which is based on sensing the difference between the virtual neutral potential and the voltage at the floating terminal. For the motor driven by a main power, the supply voltage Vs may be 300V respective to ground GND. The monitored BEMF must be attenuated into an allowable range for the sensing circuit. Therefore, the method tends to have a narrow speed range and a poor signal to noise ratio. 
   To achieve wide speed range, especially at a lower speed, U.S. Pat. No. 5,859,520 describes a direct BEMF detection circuit E referring to  FIG. 3 . With the clamping circuitry C, there is no attenuation of the monitored BEMF, which has good resolution even at low speed operation. However, the resistor R for limiting the injected current has a high value, such as 100 kΩ. The BEMF zero crossing detection is not correct sometimes when the PWM duty cycle is high. This is caused by the large time constant of the current limit resistor and the parasitic capacitance inside the microcontroller. 
   SUMMARY OF THE INVENTION 
   According to the foregoing description, an object of this invention is to provide a method and a circuit for controlling a brushless DC motor without using position sensors. Therefore, the motor system size and manufacturing costs can be reduced, and a wider speed range and a good signal to noise ratio can be provided. 
   According to the objects mentioned above, the present invention provides a circuit for controlling a brushless permanent magnet motor. The circuit comprises a plurality of windings, each of the windings having a first end connected at a common node and each of the windings having a second end connectable directly to supply voltages by switches, the second end connected to an upper supply voltage or connected to a lower supply voltage or disconnected from the supply voltages; blocking circuitry connectable with the second ends, the blocking circuitry producing a blocked voltage; a comparator receiving the blocked voltage on one input and a reference voltage on another input, the comparator result indicating polarity of a back emf voltage in the associated winding; and a latch providing control signals for the circuit, an input of the latch enabled by an enable signal, an output of the latch comprising a back emf voltage detection signal. The blocking circuitry and the comparator are duplicated for each of the windings. 
   In addition, the present invention further provides a circuit for controlling a brushless permanent magnet motor. The circuit comprises a plurality of windings, each of the windings having a first end connected at a common node and each of the windings having a second end connectable directly to supply voltages by switches, the second end connected to an upper supply voltage or connected to a lower supply voltage or disconnected from the supply voltages; a single blocking circuitry periodically connected to the second ends for producing a blocked voltage; a comparator receiving the blocked voltage on one input and a reference voltage on another input, the comparator result indicating polarity of a back emf voltage in the associated winding; and a latch providing control signals for the circuit, an input of the latch enabled by an enable signal, an output of the latch comprising a back emf voltage detection signal. 
   The present invention further provides a circuit for driving a brushless DC motor. The circuit comprises a plurality of windings constructed to a star configuration, each of the windings having a first end connected at a common node and each of the windings having a second end connectable directly to supply voltages by switches, the second end connected to an upper supply voltage or connected to a lower supply voltage or disconnected from the supply voltages; a plurality of blocking circuits, each coupled to a free end of each winding to blocking a voltage at the free end to a blocked voltage indicative of the winding; a plurality of comparators, each receiving the blocked voltage generated by each blocking circuit on one input and a reference voltage on another input, the comparator result indicating polarity of a back emf voltage in the associated winding; and a plurality of latches, each providing control signals for the circuit, an input of the latch enabled by an enable signal, an output of the latch comprising a back emf voltage detection signal. 
   In the aforementioned circuits, the blocking circuit can further comprises a diode with its cathode end connected to the second end, and a resistor with one end connected to the blocked voltage and the other connected to a DC power source. 
   The present invention further provides a method for controlling a brushless DC motor. The brushless DC rotor comprises a rotor capable of generating a back electromotive force. The method comprising steps of: determining whether a time of a zero crossing point for the back electromotive force is detected; determining a rotor speed of the brushless DC motor is smaller than a lower speed if the time of the zero crossing point is detected; reducing a frequency of a control signal if the rotor speed is smaller than the lower speed; determining again whether there is a time of a zero crossing point; and resetting the frequency of the control signal if the time of the zero crossing point is detected again. 
   In the above method, whether a time of a zero crossing point is determined again when the rotor speed is larger than the lower speed. When the zero crossing point is not detected, the method is terminated. The control signal is a pulse width modulation (PWM) signal. In step of resetting the frequency, the frequency is reset to a normal operation frequency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings. 
       FIG. 1  shows a schematic diagram for a BLDC motor drive according to a prior art. 
       FIG. 2  shows relationship between the BEMFs and the armature current of stator windings of  FIG. 1 . 
       FIG. 3  shows the direct BEMF detection circuit described in U.S. Pat. No. 5,859,520 according to a prior art. 
       FIG. 4  and  FIG. 5  show a schematic diagram of a circuit for controlling a motor according to the invention during certain periods. 
       FIG. 6  shows voltages present in the circuits of  FIG. 4  and  FIG. 5 . 
       FIG. 7  shows a simplified flow chart diagram of a method of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will now be described more fully hereinafter with reference to the accompanying drawing, in which a preferred embodiment of the invention is shown. 
     FIGS. 4 and 5  show a circuit according to one embodiment of the present invention for measuring a BEMF in the windings of the motor. Elements with the same or similar functions as  FIG. 3  are labeled with the same numerals. 
   Referring to  FIG. 4  and  FIG. 1 , the circuits allow the measurement of the BEMF in winding B. The windings A, B, C are connected in star formation with a common node N. The terminal voltage of the windings A, B, C relative to the ground voltage GND will be referred to as Vag, Vbg, Vcg respectively. The voltage at the common node N relative to the ground GND will be referred to as Vng. For each phase, switch pairs XSA, XGA, XSB, XGB, XSC, XGC connect to the free ends of the phases A, B, C to supply power source Vs and ground GND, respectively. Reverse biased diodes DSA, DGA; DSB, DGB; DSC, DGC are connected in parallel with the corresponding switches XSA, XGA, XSB, XGB, XSC, XGC. 
   The features described above are also shown in  FIG. 2 , and represent a particular configuration of the step s 3  in  FIG. 2 . The switch XGC is kept ON during the whole step s 3 , and the switch XSA is controlled to turned on and off according to a pulsed signal. The forward voltage of the switches XSA, XGA, XSB, XGB, XSC, XGC and diodes DSA, DGA; DSB, DGB; DSC, DGC are neglected. 
   Referring to  FIGS. 4 and 5 , position detection circuits E are connected to each of the phases A, B, C, but only one position detection circuit E is depicted for simplicity. A voltage comparator P of the position detection circuit E has a non-inverting input connected via a diode d 4  to the free end of phase B, and also to an auxiliary +5V voltage supply by a resistor R. The comparator P also has an inverting input connected to a node M between a resistor R′ and a forward biased diode D 5  that are connected respectively in series between the auxiliary +5V supply and ground GND. The output CompOut of the comparator P is connected to an input of a latch D. The latch D is controlled by an enable signal ValidComp. The output of the latch D is Z B , i.e., a BEMF ZCP detection signal, BemfDet, that is supplied to a central processing unit CPU (not shown). These elements form circuitry E, which is repeated three times, once for each winding. Alternatively, a single circuitry E may be provided, connectable by controlled switches to each of the winding A, B, C. Also, the latch D can be replaced by the software. 
     FIG. 4  shows the motor control circuit at an instant during a PWM “on” period within step s 3 , ignoring a commutation interval. In  FIG. 4 , the PWM “on” period is referred to as t 2 . The switch XSA is controlled by a PWM signal, for example. In the instant as shown in  FIG. 4 , the power source Vs (300V) is supplied to the free end of the winding A. The ground voltage GND is supplied to the free end of the winding C through the switch XGC, while the winding B is disconnected to either 300V of the power source Vs, or the ground voltage GND. A current  11  flows in the windings A and C, but there is no current flowing in the winding B. 
   The node N voltage Vng is at approximately 150V due to a voltage divider effect of the windings A and C. As the winding B is disconnected to either the 300V power source Vs, or the ground voltage GND, the terminal voltage Vbg of winding B is offset by the 150V of the node N voltage Vng. Therefore, the diode d 4  is blocked off. 
   As shown in  FIG. 5 , when switch XSA is opened due to the operation of the PWM control, the inductance of the windings A and C maintains the current I 1  flowing in an arrow direction as shown. This current I 1  biases diode DGA forwards, holding a zero voltage between the terminal voltages of the windings A and C. This means that the node N voltage Vng will be about zero. This period is referred to as t 1  in  FIG. 6 . 
   As the motor is running, a BEMF is induced in the winding B by the rotor, and the BEMF appears at Vbg at this period. When the BEMF is negative, the voltage Vbg is clamped to zero by the diode DGB. 
   To detect the BEMF and in particular the zero crossing points of BEMF, a reference voltage needs to be defined for a voltage comparison. The inverting input node M of the voltage comparator P is held at about 0.7V by the forward voltage of diode D 5 . Although the free end voltage Vbg of the phase B varies between 300V and zero values, the input to the comparator P is limited to vary between 5V and +0.7V by the diode d 4  and the resistors R. By comparing the voltage Vbg, which contains the BEMF, with the +0.7V fixed by D 5 , the output CompOut of the comparator P gives a signal indicative of a polarity of the BEMF in the winding B. This signal is continually switched high and low as the power source Vs is applied to the winding A by turning on and off switch XSA. The output CompOut is connected to the input of the latch D, which is clocked by an enable signal ValidComp to record the polarity of the BEMF each time when the signal ValidComp is at a high value. As discussed below, this high value occurs near to the end of a PWM “off” period. 
   The circuit as shown in  FIGS. 4 and 5  may equally be used with delta-connected motors, but the derivation of the actual BEMF in a winding is slightly more complicated. 
     FIG. 6  shows a typical voltage-time signal for the terminal voltage of the floating phase B, referenced to ground voltage GND and ignoring the commutation interval. The time axis is magnified to enable a visibility of the PWM control. The signal ValidComp for enabling the latch D and the output BemfDet of the latch D are also shown. The time period represented covers the step s 3  and its next step s 4  in  FIG. 2 . 
   The BEMF is visible on the voltage Vbg as an amount bemf, either subtracted from the mid value of the power source Vs (300V), during each PWM “on” period t 2 , or added to the ground GND when the power source Vs is disconnected ( FIG. 5 ) during a bemf+ period. It can be seen that during each PWM “off” transition from t 2  to t 1 , the voltage Vbg goes fully to ground level. This is due to the discharging procedure of the parasitic capacitance inherent in the switches and the motor. Therefore, the value of the BEMF can only be meaningfully measured near to the end of the PWM off period t 1 . Since the control of the motor is performed by the CPU, the BEMF sampling instant can be easily timed, because the CPU knows when the end of each PWM off period will be. 
   At an instant tz, a BEMF zero crossing occurs. After the instant tz, the BEMF is positive. The instant tz is thus the zero crossing point (ZCP) that needs to be detected. 
   In a system controlled according to the invention, a minimum PWM “off” period, T OFFMIN , as shown in  FIG. 6 , must be provided to allow sampling the BEMF. The signal ValidComp is generated synchronously by the CPU according to the timer that is used to generate the PWM signal. 
   When the motor speed is very low, the signal of the BEMF is very small. The effect of the parasitic capacitance becomes worse, the BEMF sampling period tends to be narrower than T OFFMIN . 
     FIG. 7  shows a simplified flow chart diagram of a method according to the present invention to enlarge the BEMF sampling period. At step  100 , whether it is time to ZCP detecting is determined, i.e., whether the expected commutation interval elapses. If not, the process moves to end. If it is time to ZCP detecting, whether the rotor speed is less than the lower speed is detected at step  110 . The rotor speed is computed from the interval of the BEMF ZCPs. The lower speed may be 80 rpm for a BLDCM 90ZW(Ke=0.084V/rpm). If the rotor speed is less than the lower speed, the frequency of PWM, which is synchronized to the enable signal CP, is reduced to enlarge the BEMF sampling period. Then, the process moves to step  130 . On the other hand, if the rotor speed is not less than the lower speed, the process moves to step  130  directly. In step  130 , whether a BEMF ZCP is detected is determined. If there is no BEMF ZCP, the process is terminated. If the BEMF ZCP is detected, the frequency of the PWM is reset to a normal operation frequency. The process may be performed periodically interrupt. 
   The BEMF has crossed the zero value around the instant tz as shown in  FIG. 6 , while the positive BEMF value is detected as a change in signal BemfDet when the next signal ValidComp comes at time td. The detection of the BEMF ZCP is thus performed to the next cycle of the PWM control. For a 5 kHz PWM, this will introduce a maximum error of only 200 μs. 
   Timing of the period T/x begins at this instant, where T is the interval between the two consecutive ZCP, and x is a constant. The interval T used to calculate T/x is of the last two consecutive BEMF ZCP or the previous two consecutive BEMF ZCP. The previous BEMF ZCP may be intended for compensation in case of dissymmetrical motors. The constant x may be adjusted in real time to take account to variations in the mechanical loading of the motor, the required acceleration, required stability and efficiency. When the period T/x elapses, commutation is performed to begin step s 4 , connecting the winding B to the power source Vs and disconnecting the winding A from the power source Vs. During this step s 4 , the BEMF measurement is performed on the winding A. 
   The magnitude of the BEMF depends on the speed of the rotor. As the circuit E does not divide the BEMF, no matter what its magnitude is, the ZCP is detected far more reliably because the rate of change of the BEMF seen by the comparator at zero crossing is the actual rate of change of the BEMF. It is not divided, nor filtered, either of which would reduce the speed or accuracy of zero crossing detecting. In addition, the diode d 4  not only blocks the high voltage, but also eliminates the wrong BEMF detection caused by the large time constant due to the large current limit resistor described in U.S. Pat. No. 5,859,520. 
   The power supply Vs of 300V described is often used for mains controlled motors operating from 220–250 Vrms mains. The invention applies equally to use with motors at very different supply voltages, varying at least between 12V and 1200V. Although described in relation to a motor having three windings, the method and the circuit of the invention is easily adapted to motors with fewer or more windings. Different step configurations than that illustrated herein are possible, along with the use of unipolar biasing to the windings, where each winding has its free end connected to one power source by a reverse biased diode (DGA, DGB, DGC), and to the other supply voltage by a switch (XSA, XSB, XSC). The PWM control may be effected by periodically disconnecting only one or other of the supply and ground voltage Vs, GND. Such applications remain within the scope of the invention. 
   Although described with reference to pulse width modulation control, the invention also applies to motors not controlled in such a way, provided always that periods T OFFMIN  are provided to allow the required measurements to be made. 
   Although described with the zero crossing detection carried out during every step, it is possible to reduce the number of detections. For example, it is possible to perform a single detection corresponding to one of the step s 1 –s 6 , during each sequence of steps and then use memorized information for the other steps in the sequence. The memorized information is periodically refreshed. 
   While the present invention has been described with a preferred embodiment, this description is not intended to limit our invention. Various modifications of the embodiment will be apparent to those skilled in the art. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.