Abstract:
A method and system for controlling motor torque provides improved responsiveness and reliability in motor control applications. The motor control system includes a power converter, a voltage detector for detecting the converter input voltage and a control circuit for controlling the converter. The control circuit includes a torque command limit value generator and a torque command limiter for limiting a torque command value. The system may also include a torque command coefficient generator for generating a coefficient corresponding to an input current of the power converter, and a multiplier for calculating a final torque command value for the motor by multiplying together the torque command value and the torque command coefficient. The torque command limit and coefficient values may be determined from the converter input voltage, calculated or detected converter input current and calculated input impedance between a power supply and the converter.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally motor control systems, and more specifically to motor control system that controls positioning motor torque.  
         [0003]     2. Background of the Invention  
         [0004]     Motor control systems are used in many applications, including industrial and consumer applications. The control of a motor&#39;s position, velocity and/or acceleration is the object of a motor control system, which attempts to position or control the speed of the motor at a commanded rotation, location and/or velocity.  
         [0005]      FIG. 8  shows a motor drive control system as is known in the related art. The apparatus includes a power converter  25  such as an inverter for driving a 3-phase alternating-current (AC) motor  27 , having a direct-current (DC) power supply  21  as an input, and a control device  100  for controlling the system.  
         [0006]     Power converter  25  converts DC power from DC power supply  21  into AC power and supplies it to motor  27 . A circuit breaker  23  interrupts the supplied power when there is a failure such as a short-circuit of power converter  25 , interrupting the circuit so that excessive current does not flow from DC power supply  21  to power converter  25 . A voltage detector  24 , such as an instrument transformer detects the input voltage of power converter  25  and a current detector  26 , such as a current transformer, detects the output current of power converter  25 .  
         [0007]     Control device  100 , which supplies a drive signal to power converter  25  includes a speed calculating part  3  for determining a speed of motor  27  from a detected position value provided by an encoder  4  mechanically coupled to motor  27 . The control device  100  also includes a torque command calculating part  2  for calculating a torque command value for motor  27  from the difference between a speed command value and the speed value determined by speed calculating part  3 . An output calculating part  1  is provided for providing feedback control based on the difference between an output current command value for power converter  25  obtained from the torque command value and a detected output current value provided by the current detector  26 . Output calculating part  1  then generates a drive signal provided to power converter  25  using the detected input voltage value from voltage detector  24  and the detected position value.  
         [0008]     In the system depicted in  FIG. 8 , considering the input impedance  22  due to the resistance of the wiring between the DC power supply  21  and power converter  25  and other elements, the input voltage V 1  and the output power P of power converter  25  are given by the following equations: 
 
 V   1   =V   0   −R·I  and  P=I·V   1   =I ( V   0   −R·I ), 
 
 respectively, where V 0  is the output voltage of DC power supply  21 , I is the input current of power converter  25 ; and R is the value of the input impedance  22 . 
 
         [0010]     The maximum output power of the system P max  is then V 0   2 /4R and the values of the input voltage V 1  and the input current I at that time are V 1 =V 0 /2 and I=V 0 /2R, respectively. From the above expressions it is apparent that input impedance R cannot be ignored, since if the input current is increased, at a certain point a maximum output power is reached. Even if the input current is increased beyond the maximum power point, output power P decreases due to a voltage drop caused by input impedance  22 .  
         [0011]     The specification for maximum output power level is set in consideration of the above constraints, but when the required output power is greater than the maximum output value P max  at a given time, the required torque command value cannot be met by actual torque, and the difference between the output current command value obtained from the torque command value and the detected output current value widens. Consequently, if output-calculating part  1  integrates the difference, the integral term becomes large, and the current control output becomes saturated. As a result the responsiveness of the system falls.  
         [0012]     In the related art shown in  FIG. 8 , when the maximum output value of the power converter  25  is reached, when an output demand such as for further motor acceleration is made, an overcurrent condition typically arises on the input side of power converter  25  and the circuit is broken by circuit breaker  23 . The output of the power converter  25  ceases supplying power, and in an apparatus in which continuous operation of the motor  27  is required, the overload condition inevitably results in total system failure.  
         [0013]     Known solutions exist to the above-described problem, as exemplified by a system as disclosed in Japanese Patent JP-A-2003-153575. In that system, a control device is included using current control to calculate an output voltage command value by integrating the difference between an output current command value and a detected output current value. The system stops the integral control when the output voltage command value has risen above a predetermined limit value, thereby preventing saturation of current control output accompanying output voltage limitation is disclosed.  
         [0014]     However, the above-described solution has disadvantages, as using a predetermined limit value on the output voltage command value, as the current control output can become saturated when the power supply voltage falls. In that condition, the maximum output value of the power converter falls below the maximum output specification, and because the output voltage command value then never reaches the predetermined limit, again causing saturation of the control function and failure of responsiveness of the system.  
         [0015]     A second solution provided in another electric motor drive control device wherein control is prevented from becoming unstable when the power supply voltage of a power converting device drops is set forth in Japanese Patent JP-A-2002-218799.  
         [0016]     In the system described in the above-referenced Japanese Patent, when the power supply voltage drops and the output voltage of the power converting device becomes saturated, the condition is recognized and the electric motor current is lowered. Thus, the drop in the power supply voltage is suppressed and simultaneously the voltage across output demand lines is suppressed, thereby preventing the current feedback value from failing to follow the current command value.  
         [0017]     The above-described second solution also has disadvantages. Although torque limitation during a drop in power supply voltage is possible, the effect of the voltage drop at the converted input that is caused by the input impedance is not considered. Again, as in the case of the related art shown in  FIG. 8 , continuous operation of the motor is not guaranteed.  
         [0018]     Therefore, it would be desirable to provide a motor control system in which a voltage drop due to input impedance or due to a fall in power supply voltage has reduced effect on the responsiveness of the system. If is further desirable to provide a motor control system in which torque control is maintained via an upper limit on power converter output and in which motor operation can be continued without stopping.  
       SUMMARY OF THE INVENTION  
       [0019]     The above objectives of providing for continued operation and responsiveness of a motor control system are achieved in a motor control system and method of operation therefor.  
         [0020]     The motor control system includes a power converter having a DC power supply input, a voltage detector for detecting the input voltage of the power converter and a control circuit for controlling the power converter. The control circuit includes a torque command limit value generator for generating a torque command limit value using the detected power converter input voltage and a torque command limiter for limiting a torque command value for the motor. The system may also include a torque command coefficient generator, for generating a torque command coefficient corresponding to an input current of the power converter, and a multiplier for calculating a final torque command value for the motor by multiplying together the torque command value and the torque command coefficient.  
         [0021]     As an alternative, the system may include a current detector for detecting the input current of the power converter. The control circuit may then include an impedance calculator for calculating the input impedance between the power supply and the converter. The torque command limit value generator generates the torque command limit value using the detected converter input voltage, the detected converter input current and the calculated input impedance. As a second alternative, the control circuit may include an input current calculator for calculating the input current of the power converter using an output power value and a detected input voltage value of the power converter, without requiring a current detector in the system. An input impedance calculator is provided for calculating an input impedance value between the power supply and the converter using the detected input voltage value and the calculated input current. The torque command limit value generator generates the torque command limit value using the detected converter input voltage, the calculated converter input current and the calculated input impedance and the torque command limiter limits the final torque command value in conformity with the torque command limit value.  
         [0022]     A torque command coefficient generator may be used with either of the alternative embodiments described above and the final torque limit value adjusted in accordance with the generated torque command coefficient. Further, in the alternative embodiments, the torque command coefficient may be calculated in conformity with the power converter input voltage, input current and input impedance between the power supply and the converter.  
         [0023]     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a block diagram depicting a motor control system in accordance with an embodiment of the present invention.  
         [0025]      FIG. 2  is a graph depicting operation of torque command limit generator in the motor control system of  FIG. 1 .  
         [0026]      FIG. 3  is a graph depicting operation of torque command coefficient generator in the motor control system of  FIG. 1 .  
         [0027]      FIG. 4  is a block diagram depicting a motor control system in accordance with another embodiment of the present invention.  
         [0028]      FIG. 5  is a graph depicting operation of torque command limit generator in the motor control system of  FIG. 4 .  
         [0029]      FIG. 6  is a block diagram depicting a motor control system in accordance with yet another embodiment of the present invention.  
         [0030]      FIG. 7  is a block diagram depicting a motor control system in accordance with still another embodiment of the present invention.  
         [0031]      FIG. 8  is a block diagram depicting a prior art motor control system. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     The present invention provides a system and method for controlling motor torque that avoids the above-described disadvantages of the prior art.  
         [0033]      FIG. 1  depicts a system in accordance with an embodiment of the present invention that maintains a high responsiveness of torque control even when a power source voltage drop due to input impedance or a variation in power supply voltage occurs at the input of the system. In all of the depicted embodiments, constituent elements that are the same as those in  FIG. 8  have been given the same reference numerals and the following description will center on the elements and functionality, which differ from those in  FIG. 8 .  
         [0034]     The input voltage at a power converter  25  that supplies power to motor  27  is detected via a voltage detector  24 . A control device  50  provides for control of motor in conformity with the detected voltage, permitting control device  50  to set a limit on the commanded torque that avoids control function saturation problems by providing a torque limit that varies with the detected input voltage. Further, control device  50  limits the torque command provided to converter  25  in correspondence with the input current of the power converter, preventing the input current from becoming excessive and avoiding current shutoff so that the motor can be operated continuously.  
         [0035]     Within control device  50 , a detected input voltage value provided by voltage detector  24  is sent to a torque command limit value generating part  5 . Torque command limiter  6  receives a torque command limit value from torque command limit value generating part  5  and limits a torque command value provided by the torque command calculating part  2  to the torque command limit value. The limited torque command value is sent to a multiplier  8  serving as a torque command protection calculating part.  
         [0036]     Torque command coefficient generating part  7  generates a torque command coefficient corresponding to the input current of power converter  25 , and the torque command coefficient generated by generating part  7  is multiplied with the torque command value the multiplier  8  and the result is sent to an output calculating part  1 . Output calculating part  1  uses the detected input voltage value provided by voltage detector  24 , a detected position value from an encoder  4  and the output of the multiplier  8  to generate an output signal. The above-mentioned output signal is then used to drive a semiconductor switching device of the power converter  25 , which is an inverter or the like.  
         [0037]     The input current of the power converter  25  sent to the torque command coefficient generating part  7  may be a detected value or may be obtained by calculation from the input power (or output power and efficiency) and input voltage of the power converter  25 .  
         [0038]     From the equations V 1 =V 0 −R·I and P=I·V 1 =I(V 0 −R·I), where V 0  is the output voltage of DC power supply  21 , I is the input current of power converter  25  and R is the value of the input impedance  22 , it can be seen that if the output power P of power converter  25  is constant, when a voltage drop caused by input impedance  22  makes input voltage V 1  of the power converter  25  lower than the output voltage V 0  of DC power supply  21 , the input current I of the power converter  25  increases.  
         [0039]     If the drop in the input voltage V 1  is marked, input current I increases and rises above the set value of the circuit breaker  23 , and the circuit is broken by circuit breaker  23  and motor  27  stops. When the input impedance  22  is large and the voltage drop is large, because the maximum power P max =V 0   2 /4R decreases, and even if the input current I increases the maximum output power specification cannot be met.  
         [0040]     The present invention avoids the above-described problem by the action of torque command limit value generating part  5  and torque command limiter  6 , which reduce the motor torque in correspondence with the drop in the DC power supply voltage V 0  and/or the input voltage V 1 . When motor  27  is being driven at a constant speed the output power P is proportional to the torque, and the output power P of the power converter  25  is limited as shown in  FIG. 2 . Torque command limit value generating part  5  generates a torque command limit value T Lim  on as T Lim =(T max /V S )V 1  using a preset reference voltage V S , a preset maximum torque command value T max  of times of maximum output, and the detected input voltage value V 1 . Here, the reference voltage V S  is set in consideration of the rated voltage of the motor  27 , and the maximum torque command value T max  is set for example as the torque value at the maximum output rating.  
         [0041]     Torque command limiter  6  performs limit processing on the torque command value on the basis of the torque command limit value T Lim . In this manner, even when the input voltage V 1  of the power converter  25  drops, a torque command value is applied within the range of power which can be provided at any given time. Also, as motor  27  is driven with a limited torque command value, the detected output current value of the power converter  25  follows the output current command value and the power control system does not become saturated. Therefore a responsiveness of current control comparable to that available when input voltage V 1  has not fallen can be maintained.  
         [0042]     Further, the torque command limited by torque command limiter  6  is multiplied by the torque command coefficient generated by torque command coefficient generating part  7  by multiplier  8 . If the input voltage V 1  of power converter  25  falls while and motor  27  is accelerating, if input current I is close to the cutoff current value of circuit breaker  23 , the likelihood that the input current I will rise above the cutoff current value is high. Thus, the probability of the circuit being broken also increases.  
         [0043]     To avoid current cut-off, in the present embodiment a torque command coefficient C I  corresponding to the input current I is calculated by torque command coefficient generating part  7 . The coefficient C I  is then multiplied by the limited torque command value to adjust the limited torque command value.  
         [0044]     The operation of torque command coefficient generating part  7  is depicted graphically in  FIG. 3 . Torque command coefficient generating part  7  calculates a torque command coefficient C I  using an input cutoff current value I I break . I I break  is set in correspondence with: the trip level of circuit breaker  23 , an input limit current value I I Lim  is set at a certain margin lower than I I break , and the input current I.  
         [0045]     When input current I is below the input limit current value I I Lim , torque command coefficient C I  is set to 1 and when the input current I has risen above the input limit current value I I Lim , in conformity with the input current I a torque command coefficient C I  that assumes a set value C I b  at the input cutoff current value I I break  is generated. Accordingly, a value not greater than the limited torque command value is always generated. By applying a limit to the torque command value in conformity with the input current I, it is possible to prevent the input current I from rising above the cutoff current value and prevent the circuit from being broken during motor acceleration, thereby providing continuous operation of motor  27 .  
         [0046]     Referring now to  FIG. 4 , a block diagram showing another embodiment of the invention is shown. In the depicted embodiment, a current detector  9  for detecting the input current of power converter  25  is provided in the main power circuit. Also, control device  60  further includes an input impedance calculating part  10 , for obtaining an input impedance value from detected values of the input current and the input voltage of the power converter  25 . Control device  60  also includes a maximum torque command limit value generating part  11 , for generating a maximum torque command limit value from the detected values of the input current and the input voltage and from the input impedance value. A maximum torque command limit value generated by maximum torque command limit value generating part  11  is applied to torque command limiter  6 .  
         [0047]     Since V 1 =V 0 −R·I as pointed out above, when the voltage value of DC power supply  21  is constant, its output voltage V 0  can be expressed by V 0 =V 1H +I 1H ·R=V 1L +I 1L ·R. V 1H  is a detected input voltage value and I 1H  is a detected input current value measured when the output P of the power converter  25  is high. V 1L  is a detected input voltage value and I 1L  is a detected input current value measured when the output of the power converter  25  is low. The input impedance value R can therefore be expressed as R=(V 1L −V 1H )/(I 1H −I 1L ).  
         [0048]     Further, the maximum output value P max  of power converter  25  is P max =(V 1 +I·R) 2 /4R. Therefore, by using the input impedance value R, the detected input voltage value V 1  and the detected input current value I, it is possible to calculate the maximum output value P max  for any condition. In advance of actual operation, the motor  27  is tested driven in a high-load state and in a no-load state, and using the detected input voltage values and the detected input current values of those states, input impedance calculating part  10  calculates the input impedance value R from the expression above and stores it.  
         [0049]     Maximum torque command limit value generating part  11  uses P max =(V 1 +I·R) 2 /4R to calculate the maximum output value P max  at that time from the input impedance value R and the detected input voltage value V 1  and the detected input current value I and outputs a maximum torque command limit value T′ Lim  on the basis of the following conditional expressions. 
 
 T′   Lim =( T′   max   /P   Lim ) P   max  (P max &lt;P Lim ) 
 
T′ Lim =T′ max  (P max ≧P Lim ) 
 
         [0050]      FIG. 5  is a graph illustrating operation of maximum torque command limit value generating part  11 , in which the maximum torque command value T′ max  is the torque value at, for example, the maximum output specification P Lim . Torque command limiter  6  limits the torque command value on the basis of this maximum torque command limit value T′ Lim  and outputs a post-limiting torque command value to output calculating part  1 .  
         [0051]     In the embodiment of  FIG. 4 , by calibrating high-output operation and low-output operation of power converter  25  to obtain an input impedance value R in advance, torque control can be performed on the basis of the maximum output value P max  at that time. The above calibration ensures that torque can be controlled even when the input voltage of power converter  25  falls. Also, because motor  27  is driven by a limited torque command value, the detected output current value follows the output current value, the current control system does not become saturated, and the same current responsiveness as when the input voltage has not fallen can be maintained.  
         [0052]     Referring now to  FIG. 6 , a block diagram showing a yet another embodiment of the invention is shown. The present embodiment differs from the previous embodiment in that the input current of power converter  25  is calculated from the input voltage and output power of power converter  25 . A control device  70  has an input current calculating part  14 , the input voltage of power converter  25  is detected by a voltage detector  24  and the output of a multiplier  13  serving as an output power calculating part are supplied to input power calculating part  14 . Multiplier  13  multiplies the output voltage of power converter  25  detected by a voltage detector  12  and an output current detected by a current detector  26 , and thereby calculates an output power value P out  of power converter  25 .  
         [0053]     The relationship between the input power P in  and the output power P out  of power converter  25 , using the efficiency η of the power converter  25 , is given by P out =η·P in , and therefore the input current I of power converter can be obtained from 
 
 I=P   out /(η· V   1 ). 
 
         [0054]     Input current calculating part  14  uses the output power P out  from multiplier  13 , the detected input voltage value from the voltage detector  24  and the known efficiency η to obtain a calculated input current value (estimated value) on the basis of the above expression. The efficiency η is obtained for example by referring to a table characterizing P out /V 1 .  
         [0055]     Input impedance calculating part  10  calculates the input impedance value R from the above-mentioned calculated input current value and detected input voltage value as in the previous, and maximum torque command limit value generating part  11  generates a maximum torque command limit value T′ Lim . Torque command limiter  6  then limits the torque command value on the basis of the maximum output value P max .  
         [0056]     In the above-described manner, by estimating the input current from the output power and input voltage value of power converter  25 , the torque can be limited to the maximum output value P max  without using an input current detector  9  of the kind used in the embodiment of  FIG. 4 .  
         [0057]     Next, referring to  FIG. 7 , a block diagram showing still another embodiment of the invention is depicted. In the illustrated embodiment, in contrast to the embodiment of  FIG. 6 , a control device  80  is made by adding generating part  7  and multiplier  8  as present in the embodiment of  FIG. 1 .  
         [0058]     As the operation of the system of  FIG. 7  can be easily understood from descriptions of the systems of  FIG. 1  and  FIG. 6 , a detailed description will not be given here. However, a brief description of the system of  FIG. 7  follows.  
         [0059]     A maximum torque command limit value T′ Lim  is generated by a maximum torque command limit value generating part  11  and then the torque command is limited on the basis of the maximum output value P max  at that time. Torque command limiter  6  performs the limiting according to the maximum torque command limit value T′ Lim , in correspondence with a calculated input current value. A generating part  7  generates a predetermined torque command coefficient C I  as described above with reference to  FIG. 3 , and the torque command coefficient C I  is multiplied by the output of torque command limiter  6  to adjust the torque command value.  
         [0060]     In the embodiment of  FIG. 7 , by adjusting (limiting) the torque command value in correspondence with a calculated input current value, the input current is prevented from becoming excessive. Circuit breaker  23  is also prevented from opening at times such as during motor acceleration, thereby making continuous operation of the motor  27  possible.  
         [0061]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.  
         [0062]     This application incorporates by reference the entire disclosure of Applicant&#39;s corresponding Japanese priority application no. 2003-299792, filed Aug. 25, 2003