Patent Publication Number: US-5838123-A

Title: Automatic phase adjuster for a brushless motor driving signal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to motor drivers, and more particularly, to an automatic phase adjuster for the driver of a brushless motor which can automatically adjust the phase of the driving signal used to drive the motor so as to allow the motor to be operated with optimal output characteristics. 
     2. Description of Related Art 
     It is a well-known principle in electrical motor designs that the torque-versus-speed characteristic (the T-N curve) of a brushless motor changes as the phase of the driving signal used to drive the motor is varied. For details on this principle, readers can refer to Section 7.6 of the textbook &#34;Electrical Motion Devices&#34; authored by Paul C. Krause. 
     FIG. 1 shows, for example, a graph of the torque-versus-speed characteristic of a conventional brushless motor, in which the horizontal axis represents the speed of the motor and the vertical axis represents the output torque of the same. This graph shows that the starting torque in the first quadrant decreases as the phase-lead angle, represented by Φ.sub.ν, is increased from 0° to 90°. In the high-speed region, the torque increases as Φ.sub.ν  is increased. The T eM  curve in FIG. 1 represents an envelope curve of the T-N curve obtained from variation of Φ.sub.ν ; and the Φ.sub.νMT curve represents the phase-lead angle corresponding to the T-N curve. Similarly, from the efficiency-versus-speed characteristic (E-N curve) of the motor, an envelope curve E M  of the E-N curve and the corresponding phase-lead angle Φ.sub.νME can be obtained; and from the power-versus-speed characteristic (P-N curve), an envelope curve P M  of the P-N curve and the corresponding phase-lead angle Φ.sub.νMP can be obtained. 
     The ideal phase-lead angles are those following the Φ.sub.νMT, Φ.sub.νME, and Φ.sub.νMP curves. If the motor driver can adjust the phase-lead angle of the driving signal used to drive the motor in accordance with these Φ.sub.νMT, Φ.sub.νME, and Φ.sub.νMP curves, the motor will output the optimal torque-versus-speed, efficiency-versus-speed, and power-versus-speed characteristics. In other words, if the motor driver can generate a driving signal with the optimal phase angle at all speeds, the motor will achieve the optimal output characteristics at all speeds. 
     In typical single-phase brushless motors, such as fan motors or pump motors, the optimal phase-lead angle is within the range from 15° to 40° which allows the motor to be operated with the optimal output characteristics. This scheme, however, has several drawbacks. First, the starting torque of the motor will be low. Second, due to the low starting torque, the starting voltage required to start the motor will be high. Third, when the cogging torque and frictional torque of the motor are high, dead points easily occur in the motor. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide an automatic phase adjuster for the driver of a brushless motor which is capable of adjusting the phase-lead angle of the driving signal in response to the motor speed so as to allow the motor to be operated with the optimal output characteristics, including output torque, output efficiency, and output power. 
     In accordance with the foregoing and other objectives of the present invention, a new automatic phase adjuster for the driver of a brushless motor is provided. This automatic phase adjuster allows the driver to output a driving signal with a small phase-lead angle when the motor speed is low, and with a large phase-lead angle when the motor speed is high, so as to allow the motor to produce the optimal output characteristics in torque, efficiency, and power at all speeds. 
     The automatic phase adjuster of the invention are configured in various embodiments in accordance with the particular type of motor on which the automatic phase adjuster is to be used. Three embodiments are disclosed, respectively for a first type of motors which are operated only in one direction at a fixed speed, a second type of motors which are operated only in one direction and can be operated at various speeds, and a third type of motors which can be operated in both directions and at various speeds. 
     For motors which are operated only in one direction at a fixed speed, such as fan motors or pump motors, the automatic phase adjuster includes a phase-lead filter coupled to a Hall sensor which detects the motor speed. The output of the phase-lead filter is a driving signal with a phase-lead angle that allows the motor to be operated at the maximum speed. 
     For motors which are operated only in one direction and can be operated at various speeds, such as hard disk driving motors, the automatic phase adjuster includes a phase-lead filter, a phase-lag filter, a multiplexer, and a Hall sensor. The multiplexer operates in such a manner that when the motor is under acceleration, the output of the phase-lead filter is selected as the output of the multiplexer; and when the motor is under deceleration, the output of the phase-lag filter is selected as the output of the same. This allows the optimal driving signal to be output to the motor regardless of whether the motor is under acceleration or under deceleration. 
     Further, for motors which can be operated in both directions and at various speeds, such as servo-type brushless motors, the automatic phase adjuster includes a phase-lead filter, a phase-lag filter, a multiplexer, and a Hall sensor. The multiplexer is used to select either the output of the phase-lead filter or the output of the phase-lag filter as its output. In the case of the motor running in the clockwise direction and being under acceleration, the phase-lead filter is selected; and in the case of the motor running in the clockwise direction and being under deceleration, the phase-lag filter is selected. Further, in the case of the motor running in the counterclockwise direction and being under deceleration, the phase-lag filter is selected; and in the case of the motor running in the counterclockwise direction and being under acceleration, the phase-lead filter is selected. This allows the motor to be always running with optimal performance. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention can be more fully understood by reading the following detailed description of the preferred embodiments, with reference made to the accompanying drawings, wherein: 
     FIG. 1 is a graph showing the torque-versus-speed characteristic plot of a conventional brushless motor; 
     FIG. 2 is a graph showing a simulated torque-versus-speed characteristic plot (T-N curve) of a brushless motor by using a simulation software program; 
     FIG. 3 is a graph showing a simulated current-versus-speed characteristic plot (I-N curve); 
     FIG. 4 is a graph showing a simulated efficiency-versus-speed characteristic plot (E-N curve); 
     FIG. 5 is a graph showing a simulated power-versus-speed characteristic plot (P-N curve); 
     FIG. 6 is a graph showing the phase-lead angle versus speed curves for Φ.sub.νMT, Φ.sub.νME, φ.sub.νMP corresponding to the envelope curves of the plots of FIG. 2, FIG. 4, FIG. 5; 
     FIG. 7 is a schematic circuit diagram of a phase-lead automatic phase adjuster according to the present invention; 
     FIG. 8 is a schematic circuit diagram of a phase-lag automatic phase adjuster according to the present invention; 
     FIG. 9 is a graph showing the output characteristics of the phase-lead automatic phase adjuster shown in FIG. 7 and the phase-lag automatic phase adjuster shown in FIG. 8 in response to the motor speed signal; 
     FIG. 10 is a schematic block diagram of a first preferred embodiment of the automatic phase adjuster according to the present invention; 
     FIG. 11 is a schematic block diagram of a second preferred embodiment of the automatic phase adjuster according to the present invention; 
     FIG. 12 is a schematic block diagram of a third preferred embodiment of the automatic phase adjuster according to the present invention; 
     FIG. 13 is a graph showing the output characteristic of an optical disc driving motor on which the automatic phase adjuster of the invention is not provided; and 
     FIG. 14 is a graph showing the output characteristic of an optical disc driving motor on which the automatic phase adjuster of the invention is provided. 
     FIGS. 15a-15c are schematic block diagrams of the automatic phase adjuster of FIGS. 10-12, respectively, including an analog-to-digital converting means, and 
     FIGS. 16a-16c are schematic block diagrams of the automatic phase adjuster of FIGS. 10-12, respectively, including a plurality of filters. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
     The automatic phase adjuster of the invention was designed by first using a simulation program to simulate the output characteristics of a brushless motor in response to the phase-lead angle φ.sub.ν of the driving signal which is varied from -30° to 60°. Specifically, the output characteristics corresponding to the phase-lead angles -30°, -15°, 0°, 15°, 30°, 45°, and 60° are simulated. Through the simulation, the simulation program generates a number of T-N curves (i.e., the torque-versus-speed characteristics at the various phase-lead angles) as illustrated in FIG. 2, a number of I-N curves (i.e., the current-versus-speed characteristic plots at the various phase-lead angles) as illustrated in FIG. 3, a number of simulated E-N curves (i.e., the efficiency-versus-speed characteristic plots at the various phase-lead angles) as illustrated in FIG. 4, and a number of P-N curves (i.e., the power-versus-speed characteristic plots at the various phase-lead angles) as illustrated in FIG. 5. 
     To allow the motor to produce the optimal output torque, the envelope curve T eM  (not shown) for the T-N curves of FIG. 2 is obtained to determine the corresponding angle-versus-speed curve Φ.sub.νMT, as illustrated in FIG. 6, that will allow for the optimal output torque from the motor. 
     To allow the motor to produce the optimal output efficiency, the envelope curve E M  (not shown) for the E-N curves of FIG. 3 is obtained to determine the corresponding angle-versus-speed curve Φ.sub.νME, as illustrated in FIG. 6, that will allow for the optimal output efficiency from the motor. 
     To allow the motor to produce the optimal output power, the envelope curve P M  (not shown) for the P-N curves of FIG. 5 is obtained to determine the corresponding angle-versus-speed curve Φ.sub.νMP, as illustrated in FIG. 6, that will allow for the optimal output power from the motor. 
     FIG. 7 and FIG. 8 respectively show a phase-lead automatic phase adjuster and a phase-lag automatic phase adjuster which are devised in accordance with the invention to provide a driving signal with the optimal phase-lead or phase-lag angle that allows the motor to be operated with the optimal output characteristics. As shown in FIG. 7, the phase-lead automatic phase adjuster includes a phase-lead filter 10 and a Hall sensor 30; and as shown in FIG. 8, the phase-lag automatic phase adjuster is substantially similar in circuit configuration to the phase-lead automatic phase adjuster of FIG. 7, except for the incorporation of a phase-lag filter 20 instead of the phase-lead filter 10. 
     In the circuit of FIG. 7, the phase-lead filter 10 is an RC circuit composed of a resistor R 1 , a capacitor C 1  connected in parallel with the resistor R 1 , and a resistor R connected to R 1  and C 1 . In the circuit of FIG. 8, the phase-lag filter 20 is also an RC circuit but connected in a different manner in which the resistor R 1  and capacitor C 1  are connected in series. 
     The transfer function G lead  (s) of the phase-lead filter 10 and the transfer function G lag  (s) of the phase-lag filter 20 are respectively shown below: ##EQU1## where 
     τ=R 1  ·C 1 , which is the time constant of the RC circuit; ##EQU2## and 
     s is the complex frequency variable. 
     FIG. 9 is a graph showing three waveform plots A, B, C. The plot A is the waveform of the output signal V i  of the Hall sensor 30; the plot B is the waveform of the output signal V o  of the phase-lead filter 10 of the circuit of FIG. 7; and the plot C is the waveform of the output signal V o  of the phase-lag filter 20 of the circuit of FIG. 8. The output signal V o  is the driving signal used to drive a brushless motor (not shown) connected to the phase-lead filter 10 or the phase-lag filter 20. 
     In practical applications, either one of the phase-lead filter 10 and the phase-lag filter 20, or both, are selected for use depending on the types of the motors being driven. 
     For motors which are operated only in one direction at a fixed speed (i.e., which are driven without the need of deceleration during operation), such as fan motors or pump motors, the automatic phase adjuster of the invention is configured as shown in FIG. 10, in which only one phase-lead filter (here labeled with the reference numeral 12) is coupled to the Hall sensor 30. The output V o  of the phase-lead filter 12 allows the motor (not shown) being driven thereby to be operated at maximum speed. 
     For motors which are operated only in one direction and can be operated at various speeds, such as hard disk driving motors, the automatic phase adjuster of the invention is configured as shown in FIG. 11, which includes a phase-lead filter (here labeled with the reference numeral 22), a phase-lag filter (here labeled with the reference numeral 24), a multiplexer 26, and a Hall sensor (here labeled with the reference numeral 28). The multiplexer 26 operates in such a manner that when the motor is under acceleration, the phase-lead filter 22 is selected by the multiplexer 26; and when the motor is under deceleration, the phase-lag filter 24 is selected by the same. The selection is controlled by a binary signal ACC. When ACC=1, the phase-lead filter 22 is selected; and when ACC=0, the phase-lag filter 24 is selected. This allows the optimal driving signal V o  to be output to the motor (not shown) regardless of whether the motor is under acceleration or under deceleration. 
     Further, for motors which can be operated in both directions and at various speeds, such as servo-type brushless motors, the automatic phase adjuster of the invention is configured as shown in FIG. 12, which includes a phase-lead filter (here labeled with the reference numeral 32), a phase-lag filter (here labeled with the reference numeral 34), a multiplexer 36, and a Hall sensor (here labeled with the reference numeral 38). The multiplexer 36 is used to select either the output lead of the phase-lead filter 32 or the output lag of the phase-lag filter 34 as its output V o . The selection is determined by two binary control signals ACC and CW/CCW. When the motor is under acceleration, ACC=1; and when the motor is under deceleration, ACC=0. Further, when the motor is operated in the clockwise direction, CW/CCW=1; and when the motor is operated in the counterclockwise direction, CW/CCW=0. Whether the phase-lead filter 32 or the phase-lag filter 34 is selected by the multiplexer 36 is determined by ACC and CW/CCW in accordance with the following table: 
     
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Condition              Selection                                          
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CW/CCW = 1, ACC = 1    Lead                                               
CW/CCW = 1, ACC = 0    Lag                                                
CW/CCW = 0, ACC = 0    Lead                                               
CW/CCW = 0, ACC = 1    Lag                                                
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     Alternatively, the circuits of FIG. 10, FIG. 11, and FIG. 12 can be implemented by software or firmware. The motor speed can be detected either by an encoder, a speed sensor, or a Hall sensor and subsequently converted to digital form by analog-to-digital converting means 40, as shown in FIGS. 15a-15c allowing the processing by the phase-lead filter and the phase-lag filter to be carried out by a digital signal processor (DSP). The multiplexer can also be implemented by software. The software implementation allows the designer to design digital filters of various purposes, as shown in FIGS. 16a-16c such as an optimal output-torque filter 42, an optimal output-power filter 44, an optimal output-efficiency filter 46, and so on, and then select a suitable one from these digital filters during operation of the motor. For instance, the optimal torque filter is selected when the motor starts, and the optimal efficiency filter is selected when the motor is running at a constant speed. 
     FIG. 13 is a graph showing the output characteristics, including the torque-versus-speed, the current-versus-speed, the efficiency-versus-speed, and the power-versus-speed characteristics, of an optical disc driving motor on which the automatic phase adjuster of the invention is not provided. For comparison, FIG. 14 is a graph showing the same of an optical disc driving motor on which the automatic phase adjuster of the invention is provided. It can be seen from FIG. 14 that the automatic phase adjuster of the invention allows the motor to be operated with about a 100% increase in maximum speed and about a 40% increase in maximum efficiency. 
     The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.