Abstract:
A position control system includes a PID controller for generating a current target value of a linear motor from a difference between a positional detected value and a positional command value, and a disturbance observer. The disturbance observer includes a signal processing unit comprising a filter for filtering a torque command value for a motor drive and an input torque estimating filter carrying out estimation for obtaining an estimated input load torque from the detected positional value. An inverse model of a motor torque constant calculates an estimated disturbance load torque from the difference between the filtered torque command value and the estimated input load torque and generates a correction value for the current target value so as to cancel a disturbance torque on the basis of the estimated disturbance load torque being calculated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a position control system and a velocity control system for controlling movement and positioning of a mechanism having a motor and a load element, and in particular relates to a position control system and a velocity control system suitable for controlling a stage driving mechanism which is driven at least in one axial direction.  
           [0003]    2. Description of the Related Art  
           [0004]    As this kind of position control system, control systems using a PID control method or a PID and FF control method are known, for example.  
           [0005]    As will be described in detail later, in a stage driving mechanism employing a control system of the PID control method or the PID and FF control method, the performance such as a constant velocity and a positioning time may be degraded by the disturbance due to the effect of external unnecessary vibration, motor cogging, and tension of a cable system. In order to suppress the disturbance, a measure to increase a gain of a servo system has been taken. However, when increasing the gain of the servo system, an instability problem of the control system is arisen.  
           [0006]    In such situations, the practical point for suppressing the disturbance is measures such as high-grade vibration proof, a high-performance motor with small disturbance and improvement in cable mounting, and improvements in a mechanical system with high accuracy. Such measures, however, increase manufacturing cost.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, it is an object of the present invention to provide a position control system for a stage driving mechanism, which is capable of improving performance in a constant velocity and a positioning time by estimating and compensating the disturbance to the system without changing a hardware structure of the system.  
           [0008]    It is another object of the present invention to provide a velocity control system for a stage driving mechanism having the performance mentioned above.  
           [0009]    The present invention provides a position control system of a stage driving mechanism that drives a stage at least in one-axial direction, and the position control system comprises a position detector for detecting a position of the stage and a feedback control system for controlling a motor, which is a driving source of the stage, on the basis of a detected positional value obtained from the position detector and a positional command value. In accordance with a first aspect of the present invention, the position control system comprises a PID controller for generating a current target value for the motor from the difference between the detected positional value and the positional command value and a disturbance observer. The disturbance observer comprises a signal processing unit for carrying out high-speed calculation according to a predetermined program. The signal processing unit has a filtering function to filter a torque command value for the motor, an input torque estimating and filtering function carrying out estimation for obtaining an estimated input load torque from the detected positional value, and a function to calculate an estimated disturbance load torque from the difference between the filtered torque command value and the estimated input load torque and to generate a correction value for the current target value so as to cancel a disturbance torque on the basis of the estimated disturbance load torque.  
           [0010]    A position control system according to a second aspect of the present invention is applied to a stage driving mechanism having a linear motor as a driving source. In this case, the feed back control system comprises the position detector, a first calculator for calculating a first difference between the detected positional value and the positional command value, a PID controller connected to the first calculator for generating a current target value for the linear motor from the first difference, and a motor drive connected to the PID controller for generating a torque command value for the linear motor. The position control system further comprises a second calculator inserted and connected between the PID controller and the motor drive, and a disturbance observer connected to the output side of the second calculator, the output side of the position detector, and one input side of the second calculator. The disturbance observer comprises a signal processing unit for carrying out high-speed calculation according to a predetermined program. The signal processing means comprises a low-pass filter for filtering the torque command value for the linear motor, an input torque estimating filter carrying out estimation for obtaining an estimated input load torque from the detected positional value, a third calculator for calculating a third difference between the filtered torque command value and the estimated input load torque, and an inverse model of a motor torque constant for calculating an estimated disturbance load torque from the third difference and for generating a correction value for the current target value so as to cancel a disturbance torque on the basis of the estimated disturbance load torque calculated above. The second calculator calculates a second difference by subtracting the output value of the inverse model of the motor torque constant from the output value of the PID controller and produces the second difference to the motor drive as a current command value.  
           [0011]    The present invention also provides a velocity control system of a stage driving mechanism that drives a stage at least in one-axial direction, and the velocity control system comprises a velocity detector for detecting a velocity of the stage and a feedback control system for controlling a motor, which is a driving source of the stage, on the basis of a detected velocity value obtained from the velocity detector and a velocity command value. In accordance with a third aspect of the present invention, the velocity control system comprises a PID controller for generating a current target value for the motor from the difference between the detected velocity value and the velocity command value and a disturbance observer. The disturbance observer comprises a signal processing unit for carrying out high-speed calculation according to a predetermined program. The signal processing unit has a filtering function to filter a torque command value for the motor, an input torque estimating and filtering function carrying out estimation for obtaining an estimated input load torque from the detected velocity value, and a function to calculate an estimated disturbance load torque from the difference between the filtered torque command value and the estimated input load torque and to generate a correction value for the current target value so as to cancel a disturbance torque on the basis of the estimated disturbance load torque calculated above.  
           [0012]    A velocity control system according to a fourth aspect of the present invention is applied to a stage driving mechanism having a linear motor as a driving source. In this case, the feedback control system comprises the velocity detector, a first calculator for calculating a first difference between the detected velocity value and the velocity command value, a PID controller connected to the first calculator for generating a current target value for the linear motor from the first difference, and a motor drive connected to the PID controller for generating a torque command value for the linear motor. The velocity control system further comprises a second calculator inserted and connected between the PID controller and the motor drive and a disturbance observer connected to the output side of the second calculator, the output side of the position detector, and one input side of the second calculator. The disturbance observer comprises a signal processing unit for carrying out high-speed calculation according to a predetermined program. The signal processing unit comprises a filter for filtering the torque command value for the linear motor, an input torque estimating filter carrying out estimation for obtaining an estimated input load torque from the detected velocity value, a third calculator for calculating a third difference between the filtered torque command value and the estimated input load torque, and an inverse model of a motor torque constant for calculating an estimated disturbance load torque from the third difference and for generating a correction value for the current target value so as to cancel a disturbance torque on the basis of the estimated disturbance load torque calculated above. The second calculator calculates a second difference by subtracting the output value of the inverse model of the motor torque constant from the output value of the PID controller and produces the second difference to the motor drive as a current command value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a position control system employing a conventional PID control method;  
         [0014]    [0014]FIG. 2 is a block diagram of a position control system employing a conventional PID and FF control method;  
         [0015]    [0015]FIG. 3 is a drawing showing a high-precision stage driving mechanism for one-axis;  
         [0016]    [0016]FIG. 4 is a block diagram of a position control system employing a PID control method according to a first embodiment of the present invention;  
         [0017]    [0017]FIG. 5 is a block diagram for explaining transfer functions of components illustrated in FIG. 4;  
         [0018]    [0018]FIG. 6 is a block diagram of a position control system employing a PID and FF control method according to a second embodiment of the present invention;  
         [0019]    [0019]FIG. 7 is a graph showing measured results of effects on a constant velocity when the present invention is applied to a precision stage mechanism using static air bearings and a linear motor;  
         [0020]    [0020]FIG. 8 is a graph showing measured results of effects on positioning characteristics when the present invention is applied to the precision stage mechanism using the static air bearings and the linear motor;  
         [0021]    [0021]FIG. 9 is a block diagram of a velocity control system employing a PID control method according to a third embodiment of the present invention;  
         [0022]    [0022]FIG. 10 is a block diagram of a velocity control system employing a PID and FF control method according to a fourth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    In order to facilitate to understand the present invention, a conventional position control system for a stage driving mechanism will be described. As a conventional position control system for a stage driving mechanism, control systems employing a PID control method shown in FIG. 1 or a PID and FF control method shown in FIG. 2 are known.  
         [0024]    Referring to FIG. 1, in the PID control method, a motor M for driving a stage, which is a load L, has a position detector  11  so as to detect a stage position. A positional detected value x detected in the position detector  11  is fed back to a subtractor  12 . In the subtractor  12 , the positional detected value x and a positional command value XC are input. To the output of the subtractor  12 , a PID controller  13  is connected, and to the output of the PID controller  13 , a motor drive  14  is connected. The output of the motor drive  14  is supplied to the motor M.  
         [0025]    In the PID control method, the difference between the positional command value XC and the positional detected value x is calculated by the subtractor  12 , so that the difference signal is input in the PID controller  13 . The PID controller  13  provides a current target value iC to the motor drive  14  according to the difference signal. The motor drive  14  outputs a torque command value τ c .  
         [0026]    In FIG. 2, like references designates like elements common to FIG. 1. In the PID and FF control method, an FF controller  15  is connected to the PID controller  13  in parallel, and an adder  16  is connected to the output side of the PID controller  13 . The output of the FF controller  15  is added to the output of the PID controller  13  by the adder  16 . The FF controller  15  multiplies a predetermined control gain by the positional command value x c  so as to make the positional detected value x coincide with the positional command value x c .  
         [0027]    In the stage driving mechanism employing the PID control method or the PID and FF control method, the performance such as a constant velocity and a positioning time is degraded by the disturbance due to effects of external unnecessary vibration, motor cogging, and tension of a cable system. In order to suppress the disturbance, a measure to increase a gain of a servo system has been taken. However, when increasing the gain of the servo system, an instability problem of the control system is arisen.  
         [0028]    In such situations, the practical point for suppressing the disturbance is measures such as high-grade vibration proof, a high-performance motor with small disturbance and improvement in cable mounting, and improvements in a mechanical system with high accuracy. Such measures, however, increase manufacturing cost.  
         [0029]    Referring now to FIG. 3, a stage driving mechanism with high performance and accuracies for one-axis will be described. The high-precision stage driving mechanism may have various structures. In general, a stage driving mechanism for an X-axis and a stage driving mechanism for a Y-axis are stacked together perpendicularly to each other to form an X-Y stage driving mechanism. For the convenience of description, one-axis will be described with reference to the drawing.  
         [0030]    The high-precision stage driving mechanism has the following structure. A driving system including a linear motor  21  and a guide system including a slide guide  22  are equipped on a base member (not shown). A movable stage  23  is movably assembled to the slide guide  22 . A work (not shown) is placed on the movable stage  23 .  
         [0031]    The base member is made of a material such as aluminum, cast iron, and granite. In the driving system, a rotation-to-linear motion converter using a rotational motor and a ball screw may also be used instead of the linear motor for directly achieving linear motion. In the guide system, contact-type ball/roller bearings or static air bearings (air slider) are used. For the high-precision system, the static air bearing is advantageous.  
         [0032]    Such a high-precision stage driving mechanism may also be applied to the present invention. When considering the application of a disturbance observer according to the present invention which will be described later, because the disturbance observe requires high mechanical rigidity of each element, the high-precision stage driving mechanism of the direct driving structure utilizing the linear motor and the static air bearings may be most effective.  
         [0033]    The positional detection of the movable stage  23  is performed by a linear encoder  24 . In such a linear motor, a movable section has the linear encoder  24  and a guide section (fixed section) has a linear scale  25 , so that a relative position is calculated by counting the number of pulses corresponding to a travel. Other than the combination of the linear scale and the linear encoder, by a laser interferometer, for example, the same function may also be certainly achieved.  
         [0034]    In a control section  20 , a high-speed processor such as a digital signal processor (referred to as DSP for short below) is used. The control section  20  can be realized by a general-purpose servo-control board having an analogous output board for supplying a calculated result and an input board for receiving a detected signal from the linear encoder  24  while having the PID controller  13  or the PID controller  13  and the FF controller  15  as well, described in FIGS. 1 and 2.  
         [0035]    A position control system or a velocity control system according to the present invention is realized by using the general-purpose servo-control board described above and by further adding a disturbance observer control program to the servo-control board.  
         [0036]    Referring to FIG. 4, a first embodiment in which a position control system employing the PID control method is applied to the present invention will be described. As described above, in the PID control method, the motor M for driving the stage, which is the load L, has the position detector  11  so as to detect a stage position. A positional detected value x detected in the position detector  11  is fed back to the subtractor  12  (a first calculator). In the subtractor  12 , the positional detected value x and the positional command value x c  are input, so that the difference (a first difference) between these values is calculated. To the output side of the subtractor  12 , the PID controller  13  is connected, and to the output side of the PID controller  13 , the motor drive  14  is connected. The output of the motor drive  14  is supplied to the motor M.  
         [0037]    The control section  20  shown in FIG. 3 comprises the PID controller  13 , a disturbance observer  40 , and a subtractor  45  (a second calculator). To the output side of the PID controller  13 , the subtractor  45  is connected. The disturbance observer  40  is connected to one of inputs and the output side of the subtractor  45  and to the output side of the position detector  11 . The PID controller  13  performs a proportional action (P), integral action (I), and differential action (D) on the deviation (x c -x) between the positional command value x c  and the positional detected value x so that they coincide with each other, thereby calculating the current target value τ c  for the motor drive  14 .  
         [0038]    The disturbance observer  40  comprises a low-pass filter  41 , an input torque estimating filter  42 , a subtractor  43  (a third calculator), and an inverse model of a motor torque constant  44 . In the disturbance observer  40 , the difference (a third difference) between a torque command value τ c  filtered by the low-pas filter  41  for the motor drive  14  and an estimated input load torque estimated by the input torque estimating filter  42  from the positional detected value x is calculated by the subtractor  43 . Also, in the disturbance observer  40 , a current equivalent to an estimated disturbance load torque is also calculated by the inverse model of a motor torque constant  44  from the difference obtained in the subtractor  43 . By subtracting the current calculated by the subtractor  45  from the current target value i c , the current target value i c  is corrected so as to cancel the disturbance torque, so that a current command value i r  for the motor drive  14  is calculated. In addition, while the current command value i r  isgiven to the motor drive  14 , the low-pass filter  41  obtains a torque command value x c  by multiplying the current command value i r  by a constant K t .  
         [0039]    [0039]FIG. 5 is a block diagram of the position control system shown in FIG. 4. Referring to FIG. 5, the operation of the disturbance observer  40  will be described. In symbols written in boxes in FIG. 5, numerals  13 ,  41 , and  42  indicate transfer functions of the respective corresponding constituent elements shown in FIG. 4. That is, the PID controller  13  is defined as the transfer function [G p (s)], the low-pass filter  41  as the transfer function [(Kιω 2 )/(s 2 +2ζωs+ω 2 )], and the input torque estimating filter  42  as the transfer function [(J nom s 2 ω 2 )/(s 2 +2ζωs 2 +ω 2 )]. In these transfer functions, ω represents a cut-off frequency, ζ represents a damping coefficient of the filter, s represents a Laplace operator, and J nom  represents an inertia term of a discipline model in a movable portion. Furthermore, numeral  46  denotes the torque constant (Kι) of the motor M, numeral  47  denotes the transfer function (1/Js 2 ) of a controlled object, namely, the mechanism including the motor M and the load L, and numeral  48  denotes the inverse number (1/Kι) of the torque constant of the motor M.  
         [0040]    The low-pass filter  41  filters the torque command value eτ c  in the disturbance suppression frequency bandwidth and calculates an estimated torque command value eτ c . The input torque estimating filter  42  calculates an estimated value (eτ c +eτ d ) of an input torque (τ c +τ d ) from the detected positional value x on the basis of an inverse model of a transfer function  46  from the input torque of the mechanism including the motor M and the load L to the position. By giving the same filtering function as that of the low-pass filter  41  also to the input torque-estimating filter  42 , the estimated input torque value (eτ c  +eτ d ) is calculated only in the disturbance suppression frequency bandwidth.  
         [0041]    Furthermore, an estimated disturbance torque value eτ d  is calculated from the finite difference eτ d  between the estimated disturbance torque value eτ c  and the estimated input torque value (eτ c +eτ d ) in the inverse model of a motor torque constant  44 . The estimated disturbance torque value eτ d  calculated in such a manner is multiplied by an inverse number (1/Kι) of the torque constant of the motor M so as to calculate a current correction value ei d . In the subtractor  45 , the current correction value ei d  is subtracted from the current target value i c  to have the current command value i r  for the motor drive  14 .  
         [0042]    [0042]FIG. 6 shows a second embodiment in which the present invention is applied to a position control system employing PID and FF control methods, and the FF controller  15  and the adder  16  are added to the structure shown in FIG. 4. That is, one of inputs of the adder  16  is connected to the output side of the PID controller  13 , and the FF controller  15  is connected to the input side of the positional command value x c  in the subtractor  12  and to the other input side of the adder  16 .  
         [0043]    The FF controller  15  multiplies the positional command value x c  by a control gain so that the positional command value x c  and the positional detected value x coincide with each other. The multiplied result is added to the output of the PID controller  13  to calculate the current target value i c  for the motor drive  14 . Operation of other constituent components is the same as that of the first embodiment described by referring to FIGS. 4 and 5.  
         [0044]    Specifically, when the present invention is applied to a precision stage driving mechanism employing the static air bearings (air slider) and a linear motor, an example of measured data of constant velocity effects is shown in FIG. 7. FIG. 7 shows changes in a following error when moving at a constant velocity. An addition of the disturbance observer  40  according to the present invention enables the changes in a following error to be reduced. A primary factor of the disturbance is considered to be changes in a thrust ripple and in tension of cables (electrical and pneumatic) due to inequality of a magnetic circuit of the linear motor (identical to general rotary motors). Generally, the disturbance certainly exists in common products and troublesome cases increase with trends toward high accuracies of recent apparatuses. The present invention is advantageous for these problems.  
         [0045]    [0045]FIG. 8 shows measured effects on positioning characteristics when the present invention is applied to a precision stage driving mechanism employing the static air bearings (air slider) and a linear motor. FIG. 8 shows deviation of positioning when step moving by 150 mm. In controlling without the disturbance observer  40 , residual vibration due to base member vibration exists during positioning; however, addition of the disturbance observer  40  according to the present invention enables the residual vibration to be effectively suppressed. Generally, an X-Y stage driving mechanism is fixed on a vibration-proof base member for elimination of floor vibration, and the residual vibration during positioning generally exists for sure, so that this becomes a problem when increasing the throughput of the apparatus. The present invention is also advantageous for this problem.  
         [0046]    The present invention has been described above by applying it to a precision stage driving mechanism; however, the present invention also is widely applicable to position and velocity control methods of apparatuses for moving and positioning objects. When the present invention is applied to a velocity control method, as illustrated in FIGS. 9 and 10, the positional command value is to be a velocity command value vc and a velocity detector  51  for detecting a velocity v of the motor M is used instead of the position detector  11 . FIG. 9 illustrates a velocity control system which is a third embodiment of the present invention and which corresponds to the position control system of FIG. 4. FIG. 10 illustrates a velocity control system which is a fourth embodiment of the present invention and which corresponds to the position control system of FIG. 6. An input load torque is estimated from the velocity v detected by the velocity detector  51  on the basis of an inverse model of a transfer function from an input torque of a mechanism including the motor M and the load L to a velocity. Operation of other constituent components is the same as that of the first and second embodiments described by referring to FIGS. 4 and 6.  
         [0047]    The first through fourth embodiments have been described above by applying them to the one-axial stage driving mechanism shown in FIG. 3. However, the present invention is also applicable to a two-axial driving mechanism or further to three-axial or more driving mechanisms.  
         [0048]    At any rate, the present invention provides a position control system and a velocity control system capable of improving the performance in constant velocity and a positioning time by estimating and compensating the disturbance in the control system without changing the hardware structure of the control system.