Patent Publication Number: US-2013249465-A1

Title: Motor control apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present disclosure contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2012-066551 filed in the Japan Patent Office on Mar. 23, 2012, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosed embodiment relates to a motor control apparatus. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 2004-349494 discloses a technology related to a workpiece stage that positions a table holding a workpiece thereon by moving the table in any directions. The workpiece stage includes a laser interferometer that measures the position of the table using a laser beam, a position measuring device used to position the table, and a controller that determines whether or not the position data obtained by the laser interferometer is normal and that obtains an error in the positioning of the table on the basis of the position data obtained by the laser interferometer when the position data is determined as normal. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the disclosure, there is provided motor control apparatus including a position controller that generates a velocity command on the basis of a position difference between a position command and a position feedback signal, a switcher that switches the position feedback signal to be input to the position controller from one of a first position signal detected by a first position detector and a second position signal detected by a second position detector to the other, and a phase compensator that compensates for a phase delay of the first position signal or the second position signal switched by the switcher. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a motor control system including a motor control apparatus according to an embodiment. 
         FIG. 2  is a schematic block diagram of the motor control apparatus. 
         FIG. 3  is a block diagram of an example of the detailed structure of the motor control apparatus. 
         FIG. 4  is a block diagram of an example of the structure of a phase compensator. 
         FIG. 5A  shows a waveform graph of a command velocity of a motor control apparatus that does not include a phase compensator, and  FIG. 5B  shows a waveform graph of a command velocity of a motor control apparatus that includes a phase compensator. 
         FIG. 6  is a schematic block diagram of a motor control apparatus that corrects a position signal by using a correlation table. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an embodiment will be described with reference to the drawings. 
     Structure of Motor Control System 
     As illustrated in  FIG. 1 , a motor control system  1  includes a motor control apparatus  2 , a controlled object  9 , a laser interferometer  6  (first position detector), and a position sensor  8  (second position detector). The controlled object  9  includes a workpiece stage  3  and a linear guide  4  that supports the workpiece stage  3  so that the workpiece stage  3  can move in the front-back direction (the vertical direction in  FIG. 1 ). The laser interferometer  6  is disposed so as to face a reflection mirror  5  disposed on the workpiece stage  3 . A linear scale  7  is disposed, for example, on one side of the linear guide  4  in the width direction of the linear guide  4 , and the position sensor  8  is disposed so as to face the linear scale  7  with a predetermined gap therebetween. 
     The laser interferometer  6  emits a laser beam toward the reflection mirror  5  and receives a reflected laser beam reflected from the reflection mirror  5 , thereby detecting the position (movement amount) of the workpiece stage  3  in the movement direction, that is, the position of the controlled object  9 . Position data detected by the laser interferometer  6  (hereinafter referred to as a “first position signal Pfb 1 ”) is input to the motor control apparatus  2  as a position feedback signal and is used to control the position of the controlled object  9 . The position sensor  8  optically or magnetically reads position marks on the linear scale  7 , thereby detecting the position (movement amount) of the workpiece stage  3  in the movement direction, that is, the position of the controlled object  9 . Position data of the controlled object  9  detected by the position sensor  8  (hereinafter referred to as a “second position signal Pfb 2 ”) is input to the motor control apparatus  2  as a position feedback signal and is used to control the position of the controlled object  9 . 
     Structure of Motor Control Apparatus 
     As illustrated in  FIG. 2 , the motor control apparatus  2  includes a position controller  10 , a velocity controller  11 , a differentiator  12 , a determiner  13 , a switcher  14 , and a phase compensator  15 . The position controller  10  includes an integral position controller  16  that performs integral position control on the basis of the first position signal Pfb 1  and a proportional position controller  17  that performs proportional position control on the basis of the second position signal Pfb 2 . The position controller  10  generates a velocity command Vr on the basis of the position difference between a position command Pr input to the position controller  10  and the position feedback signals (the first position signal Pfb 1  and the second position signal Pfb 2 ). The velocity controller  11  generates a torque command Tr on the basis of the velocity difference between the velocity command Vr output from the position controller  10  and a velocity feedback signal Vfb generated by the differentiator  12  by differentiating the second position signal Pfb 2 . 
     The switcher  14  switches the position feedback signal to be input to the integral position controller  16  from one of the first position signal Pfb 1  detected by the laser interferometer  6  and the second position signal Pfb 2  detected by the position sensor  8  to the other. With the present embodiment, to perform high-accuracy positioning, the position controller  10  usually performs integral position control based on the first position signal Pfb 1  detected by the laser interferometer  6  and proportional position control based on the second position signal Pfb 2  detected by the position sensor  8 . However, the first position signal Pfb 1  may not be input normally if, for example, the axis of the laser beam of the laser interferometer  6  is blocked. In such a case, the switcher  14  switches the first position signal Pfb 1  to the second position signal Pfb 2 . Thus, the position controller  10  can continue integral position control based on the switched second position signal Pfb 2  and proportional position control based on the second position signal Pfb 2 , and thereby, for example, the workpiece stage  3  can be moved a predetermined stop position and stopped at the stop position. If the first position signal Pfb 1  of the laser interferometer  6  becomes normal again, position control using the second position signal Pfb 2  may be continued, or the second position signal Pfb 2  may be switched back to the first position signal Pfb 1  and machining of a workpiece on the workpiece stage  3  may be restarted. 
     The determiner  13  determines whether or not the first position signal Pfb 1  detected by the laser interferometer  6  is input to the position controller  10  normally. The method of determination may be such that it is determined as abnormal when the intensity of light received by the laser interferometer  6 , which is an optical detector, becomes lower than a predetermined threshold. 
     The phase compensator  15  compensates for a phase delay of the feedback signal switched by the switcher  14  (here, a phase delay of the second position signal Pfb 2  relative to the first position signal Pfb 1 ) and inputs the feedback signal, for which the phase delay has been compensated, to the position controller  10 . The structure of the phase compensator  15  will be described below in detail. 
     Detailed Structure of Motor Control Apparatus 
       FIG. 3  is a block diagram of an example of the detailed structure of the motor control apparatus  2 . In  FIG. 3 , numerals  20 ,  22 ,  24 ,  26 , and  32  denote subtractors; a numeral  21  denotes a position integrator; a numeral  23  denotes a position loop gain; a numeral  25  denotes a velocity loop gain; a numeral  29  denotes a machine spring constant; numerals  27  and  28  denote linear motors; and numerals  30  and  31  denote loads. The position controller  10 , the integral position controller  16 , the proportional position controller  17 , the velocity controller  11 , and the controlled object  9  in  FIG. 3  respectively correspond to those in  FIG. 2 . 
     The first position signal Pfb 1  detected by the laser interferometer  6  is input to the phase compensator  15  through the switcher  14  as a feedback signal and changed into a position feedback signal Po (estimated position) for which a phase delay is compensated by the phase compensator  15 . Then, the position feed back signal Po is input to the subtractor  20  of the position controller  10 . The second position signal Pfb 2  detected by the position sensor  8  is input to the subtractor  22  of the position controller  10  as a position feedback signal. The second position signal Pfb 2  is also changed into the velocity feedback signal Vfb by the differentiator  12  and input to the subtractor  24  of the velocity controller  11 . In addition, the first position signal Pfb 1  and the second position signal Pfb 2  are input to the subtractor  32  to obtain a position difference, and the position difference is input to the subtractor  26  through the machine spring constant  29 . 
     In the motor control apparatus  2 , the subtractor  20  of the integral position controller  16  subtracts a position feedback signal Po from the phase compensator  15  from the position command Pr to obtain a position difference, and the position integrator  21  integrates the position difference. The subtractor  22  of the proportional position controller  17  subtracts the second position signal Pfb 2  from the integrated position command to obtain a position difference, and the position difference is multiplied by a gain Kp at the position loop gain  23  to generate the velocity command Vr. The subtractor  24  of the velocity controller  11  subtracts the feedback velocity Vfb from the velocity command Vr to obtain a velocity difference. The velocity difference is multiplied by a gain Kv at the velocity loop gain  25  to generate a torque command Tr, and the torque command Tr is output to the controlled object  9 . 
     In the controlled object  9 , the subtractor  32  subtracts the first position signal Pfb 1  from the second position signal Pfb 2  to obtain a position difference. The position difference is multiplied by the machine spring constant  29  to obtain a torque To, and the subtractor  26  subtracts the torque To from the torque command Tr to obtain a torque difference. The torque difference is integrated by the velocity integrator  27  and is integrated by the integrator  28 . In  FIG. 3 , Jm denotes the mass of a slider of a linear motor. The torque To from the machine spring constant  29  is integrated by the velocity integrator  30  and integrated by the integrator  31 . The subtractor  32  represents the difference between the first position signal Pfb 1  output from the integrator  31  and the second position signal Pfb 2  output from the integrator  28 . With a force generated by multiplying the output of the subtractor  32  by the machine spring constant  29 , the first position signal Pfb 1  and the second position signal Pfb 2  are made to coincide with each other. 
     Detailed Structure of Phase Compensator 
       FIG. 4  illustrates an example of the detailed structure of the phase compensator  15 . In  FIG. 4 , the phase compensator  15  includes a position control system model  33  and a phase delay element model  34 , and is configured as a so-called phase-control position observer. In  FIG. 4 , a numeral  35  denotes a position integration gain; numerals  36 ,  39 ,  40 ,  46 ,  47 , and  51  denote subtractors; numerals  37 ,  41 , and  48  denote integrators; numerals  38  and  42  denote position loop gains; numerals  43 ,  44 , and  50  denote observer stabilization gains; and numerals  45  and  49  denote phase delay gains. 
     A position signal output from the position control system model  33  is input to the subtractor  20  as the position feedback signal Po of the position controller  10  and also input to the phase delay element model  34 . A position signal output from the phase delay element model  34  is input to the subtractor  51 . The subtractor  51  subtracts the position signal from the first position signal Pfb 1  from the laser interferometer  6  (after switching, the second position signal Pfb 2  from the position sensor  8 , the same applies hereinafter) to obtain a position difference. The position difference is input to the subtractors  36 ,  39 , and  46  respectively through the observer stabilization gains  43 ,  44 , and  50 . 
     In the position control system model  33  of the phase compensator  15 , the position difference between the position command Pr and the feedback position Po is multiplied by a gain l/Ti at the position integration gain  35 , and the subtractor  36  subtracts a value calculated by multiplying the position difference from the subtractor  51  by a gain K 1  at the observer stabilization gain  43 . The position difference obtained by the subtractor  36  is integrated by the integrator  37  and multiplied by a gain Kp at the position loop gain  38 , and the subtractor  39  subtracts a value calculated by multiplying the position difference from the subtractor  51  by a gain K 2  at the observer stabilization gain  44 . The subtractor  40  subtracts a value calculated by multiplying a position signal output from the position control system model  33  by a gain Kp at the position loop gain  42  from the position difference obtained by the subtractor  39 . The integrator  41  integrates the position difference obtained by the subtractor  40 , and the obtained value is output from the position control system model  33  as a position signal. 
     The position signal output from the position control system model  33  is input to the subtractor  20  as the position feedback signal Po of the position controller  10  and is also input to the phase delay element model  34 . 
     In the phase delay element model  34  of the phase compensator  15 , the position signal output from the position control system model  33  is multiplied by a gain l/T at the phase delay gain  45 , and the subtractor  46  subtracts a value obtained by multiplying the position difference from the subtractor  51  by a gain K 3  at the observer stabilization gain  50 . The subtractor  47  subtracts a value obtained by multiplying a position signal output from the phase delay element model  34  by a gain l/T at the phase delay gain  49  from the position difference obtained by the subtractor  46  to calculate a position difference. The integrator  48  integrates the position difference, and the obtained value is output from the phase delay element model  34  as a position signal. The subtractor  51  subtracts the position signal output from the phase delay element model  34  from the first position signal Pfb 1  from the laser interferometer  6 . With such a structure, the phase compensator  15  performs control so that the position signal output from the phase delay element model  34  coincides with the first position signal Pfb 1 . 
     The phase of the position signal output from the phase delay element model  34  is delayed relative that of the position signal output from the position control system model  33 . Thus, the phase of the position signal from the position control system model  33  is advanced relative to the first position signal Pfb 1  (after being switched, the second position signal Pfb 2  input from the position sensor  8 ) input from the laser interferometer  6 . By outputting the position signal with advanced phase to the position controller  10 , even if a phase delay occurs when switching from the first position signal Pfb 1  to the second position signal Pfb 2 , the position feedback signal Po input to the position controller  10  can be made to be a position signal without phase delay. 
     Advantage of Embodiment 
     With the motor control apparatus  2  according to the present embodiment, the switcher  14  switches a position feedback signal to be input to the integral position controller  16  of the position controller  10  from the first position signal Pfb 1  detected by the laser interferometer  6  to the second position signal Pfb 2  detected by the position sensor  8 . When switching between position detectors used for position control, a shock (sharp change in motor velocity) may occur due to the following reasons: an error in a position signal due to the difference between objects to be detected by the position detectors and time lag for switching; and a phase delay of a position signal due to a delay of a control cycle and a delay of communication time between the detectors. 
     In the present embodiment, the motor control apparatus  2  includes the phase compensator  15 . The phase compensator  15  can compensate for the phase delay of the second position signal Pfb 2  switched by the switcher  14  relative to the first position signal Pfb 1 , and can interpolate an error between the first position signal Pfb 1  and the second position signal Pfb 2 . Therefore, occurrence of a shock when switching between the position detectors can be reduced. Moreover, with the phase compensator  15 , there is an advantage in that the rising edge and the falling edge of the motor velocity can be made smooth and the response of the control system can be made close to ideal characteristics. 
     This advantage will be described with reference to  FIGS. 5A and 5B .  FIG. 5A  shows a waveform graph of a motor velocity of a motor control apparatus that does not include the phase compensator  15 , and  FIG. 5B  shows a waveform graph of a motor velocity of the motor control apparatus  2  that includes the phase compensator  15 . With a comparative example, which does not include the phase compensator  15 , a shock (sharp change in motor velocity) occurs as indicated by an arrow A in  FIG. 5A  when the position detector is switched from the laser interferometer  6  to the position sensor  8 . Moreover, sharp edges are generated at the rising edge and the falling edge of the waveform of motor velocity as indicated by arrows B and C in  FIG. 5A . 
     In contrast, with the present embodiment, which includes the phase compensator  15 , occurrence of a shock when switching the position detector from the laser interferometer  6  to the position sensor  8  can be reduced as illustrated  FIG. 5B . Moreover, there are no sharp edges at the rising edge and the falling edge of the waveform of motor velocity, and the motor velocity can be changed smoothly. 
     In addition, the following advantage can be obtained with the present embodiment. That is, if the position feedback signal is not input normally while position controller  10  is performing position control on the basis of the position difference between the position command Pr and the position feedback signal, positioning operation may be disabled and malfunction or the like of a device that is a driving object, such as the workpiece stage  3 , may occur. With the present embodiment, the determiner  13  determines whether or not the first position signal Pfb 1  from the laser interferometer  6  is input to the position controller  10  normally. If the determiner  13  determines that the first position signal Pfb 1  is not normal, the switcher  14  switches the first position signal Pfb 1  to the second position signal Pfb 2 . Thus, the position controller  10  can position the workpiece stage  3  at a predetermined stop position and stop the workpiece stage  3  at the stop position by using the switched second position signal Pfb 2 , and thereby malfunction or the like of a device that is a driving object can be prevented. 
     In particular, with the present embodiment, the position controller  10  performs integral position control using the laser interferometer  6  and proportional position control using the position sensor  8 , and thereby a smooth response can be obtained and the number of peaks of torque is reduced and therefore a load applied to a device that is the driving object, such as the workpiece stage  3 , can be reduced. Moreover, after the switcher  14  has performed switching, the position controller  10  can continue integral position control using the position sensor  8  and the proportional position control using the position sensor  8 , and thereby a good response and the like can be maintained. 
     Modifications 
     Hereinafter, modifications of the embodiment will be sequentially described. 
     (1) Modification with which Position Signal is Corrected using Correlation Table 
     In the embodiment described above, it is assumed that the workpiece stage  3  is stopped and machining of a workpiece is stopped when switching from the first position signal Pfb 1  detected by the laser interferometer  6  to the second position signal Pfb 2  detected by the position sensor  8  is performed. However, there may be a need to continue machining of a workpiece. If machining of a workpiece is continued with the embodiment described above, the accuracy of machining may decrease after the position signals have been switched and a defect of the workpiece may occur, because the laser interferometer  6  generally has detection accuracy higher than that of the position sensor  8 . Therefore, a corrector using a correlation table may be provided to make the position signal after switching coincide with the position signal before switching. Referring to  FIG. 6 , an example of the present modification will be described. 
     As illustrated in  FIG. 6 , a motor control apparatus  2  according to the present modification includes a corrector  52  and a storage  53 . The storage  53  stores a correlation table used by the corrector  52 . The correlation table contains the correlation (the difference and the like) between the first position signal Pfb 1  and the second position signal Pfb 2 . The correlation table is made by, for example, making the controlled object  9  perform uniform linear motion and simultaneously recording detection data of the laser interferometer  6  and detection data of the position sensor  8  for one stroke of the motion. 
     When the switcher  14  switches from the first position signal Pfb 1  to the second position signal Pfb 2 , the corrector  52  performs correction so that the second position signal Pfb 2  after switching coincides with the first position signal Pfb 1  before switching on basis of the correction table stored in the storage  53 . The corrected first position signal Pfb 1  is input to the phase compensator  15 . Thus, decrease in the accuracy of detection when the position detectors are switched can be prevented. Therefore, machining of a workpiece can be continued and thereby the yield can be increased. 
     (2) Other Modifications 
     Heretofore, examples in which the position detector is switched from the laser interferometer  6  to the position sensor  8  (linear scale  7 ) have been described. Switching of position detector may be performed in various other ways. For example, conversely, switching may be performed from the position sensor  8  to the laser interferometer  6 . In this case, the determiner  13  may determine whether or not the second position signal Pfb 2  is normal. If a linear encoder, an external encoder, or the like is used as a position detector, switching may be performed from the laser interferometer  6  to the linear encoder, from the external encoder to the position sensor  8  (linear scale  7 ), and in various other ways. In any of these cases, advantages the same as those of the embodiment described above can be obtained. 
     Heretofore, a linear motor is used as an example. However, a rotary motor may be used. Also in this case, the position detector may be switched from the position sensor  8  (linear scale  7 ) to the rotary encoder, and in various other ways. When a rotary motor is used, advantages the same as those of the embodiment described above can be obtained. 
     In addition, methods used in the embodiment and modifications described above may be appropriately used in combination. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.