Patent Publication Number: US-2013238101-A1

Title: Load inertia estimation method and control parameter adjustment method

Description:
TECHNICAL FIELD 
     The present invention relates to a load inertia estimation method and a control parameter adjustment method applicable to industrial machines such as machine tools. 
     BACKGROUND ART 
     Feedback control which is a classical control theory is generally used for load position control of a feed system in an industrial machine such as a machine tool. 
       FIG. 4  shows an example of a machine tool. The machine tool of the illustrated example is a double column type machining center which includes a bed  1 , a table  2 , a gate-shaped column  3 , a crossrail  4 , a saddle  5 , a ram  6 , and a main spindle  7 . 
     The table  2  is disposed on the bed  1  and the column  33  is disposed in such a manner as to straddle the table  2 . A workpiece W is mounted on the table  2  at the time of machining, and the table  2  moves linearly in an X-axis direction along guiderails  1   a  on the bed  1  with the assistance of a feed system (not shown in  FIG. 4 , see  FIG. 5 ). The crossrail  4  moves linearly in a Z-axis direction along guiderails  3   b  on a column front face  3   a  with the assistance of a feed system (not shown). The saddle  5  moves linearly in a Y-axis direction along guiderails  4   b  on a crossrail front face  4   a  with the assistance of a feed system (not shown). The ram  6  is provided on the saddle  5  and moves linearly in the Z-axis direction with the assistance of a feed system (not shown). The main spindle  7  is supported rotatably inside the ram  6 , and a tool  9  is fitted onto a tip of the main spindle  7  via an attachment  8 . 
     Accordingly, when the workpiece W is machined with the tool  9 , the tool  9  is driven to rotate by the main spindle  7 . The main spindle  7  and the tool  9  move linearly in the Z-axis direction together with the crossrail  4  or the ram  6  and move linearly in the Y-axis direction together with the saddle  5 , and the table  2  and the workpiece W move linearly in the X-axis direction. In order to achieve high-precision machining of the workpiece W at this time, positions to which the main spindle  7  (the tool  9 ) and the table  2  (the workpiece W) are moved are required to be precisely controlled by the feedback control. 
       FIG. 5  shows a general configuration example of a feedback control system and a feed system. Although detailed description is omitted herein, a feed system  11  for the table  2  shown in  FIG. 5  includes a servo motor  12 , a reduction gear unit  13 , brackets  14 , a ball screw  15  (a screw portion  15   c  and a nut portion  15   b ), and so forth. The feed system  11  moves the table  2  and the workpiece W linearly in the X-axis direction. A feedback control system  16  controls this feed system  11  as follows. Specifically, the feedback control system  16  controls rotation of the servo motor  12  in such a way that a load position θ L , which is a position of the table  2  (the workpiece W) detected with a position detector  6 , follows a position command θ issued from a numerical control (NC) device  17 . 
     However, it is difficult to achieve a sufficient following performance with the feedback control system  16  as in the illustrated example, and a delay of the load position θ L  in following the position command θ (namely, a delay in the load position) occurs as a consequence. In order to deal with the follow delay (the delay in the load position), it is a common practice to add, to the feedback control system  16 , a feed-forward control function, which is not illustrated, to differentiate the position command θ and compensate for a position delay. 
     However, addition of the feed-forward control function to the feedback control system cannot compensate for a position delay or vibration caused by dynamic deformation such as deflection or torsion that occurs in a mechanical element in a controlled object. For example, in the case of the feed system  11  in  FIG. 5 , rigidity of the screw portion  15   c  of the ball screw  15  has a limitation and thus torsion or deflection corresponding to load inertia (the weight of a workpiece) or the load position θ L  occurs in the screw portion  15   c  at the time of moving the table  2 . The feed-forward control function cannot compensate for the follow delay of the load position θ L  thus caused. 
     In this context, Patent Document 1 listed below discloses a technique for compensating for a delay in a load position or a delay in a velocity caused by torsion or deflection of a ball screw in a feed system by finding a characteristic model (a transfer function) that approximates a characteristic of the feed system, then finding an inverse characteristic model (an inverse transfer function) of the characteristic model, and adding the inverse characteristic model to a feedback control system (see  FIG. 1  and  FIG. 2 : to be described later in detail). Meanwhile, such techniques for adding an inverse characteristic model of a controlled object to a control system are also disclosed in Patent Documents 2 and 3 listed below, for instance. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Publication No. 2009-201169 
         Patent Document 2: Japanese Patent No. 3351990 
         Patent Document 3: Japanese Patent No. 3739746 
         Patent Document 4: Japanese Patent No. 4137673 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, in  FIG. 5 , the weight of the table  2  remains constant whereas the weight of the workpiece W varies depending on the type of a machined product and the like. Accordingly, the load inertia to be determined by the weight of the table  2  and the weight of the workpiece W also varies with a change in the weight of the workpiece W. 
     As a consequence, if the load inertia included in the inverse characteristic model (the inverse transfer function) of the feed system is always set to a constant value, then the load inertia included in the inverse characteristic model of the feed system differs from actual load inertia of the feed system when the workpiece W having a different weight from the constant value is mounted on the table  2  for machining. Accordingly, even when the inverse characteristic model of the feed system is added to the feedback control system, the inverse characteristic model cannot sufficiently compensate for the follow delay of the load position θ L  caused by torsion, deflection or the like of the ball screw  15  when the workpiece W having a different weight from the constant value is machined. Hence, a position deviation between the position command P and the load position θ L  is increased. As a consequence, the workpiece W cannot be machined at high precision. 
     For this reason, in order to enable the feedback control system, to which the inverse characteristic model of the feed system is added, to perform high-precision machining on the workpiece W having any weight, it is necessary to estimate the load inertia corresponding to the weight of the workpiece W and to adjust the load inertia included in the inverse characteristic model of the feed system based on the estimated load inertia. 
     In view of the aforementioned circumstances, it is an object of the present invention to provide a load inertia estimation method of estimating load inertia corresponding to the weight of a workpiece, and a control parameter adjustment method of adjusting load inertia included in an inverse characteristic model of a feed system based on the estimated load inertia. 
     Incidentally, the above-mentioned Patent Document 4 discloses a method of calculating the weight of a load by using a difference between a motor torque when no load is applied and a motor torque when a load is applied. In contrast, the method of the present invention estimates the load inertia based on a position deviation and so forth. 
     Means for Solving the Problems 
     A load inertia estimation method according to a first aspect of the invention for solving the above problems is a load inertia estimation method of estimating load inertia of a feed system for a load position control system configured to cause a feedback control system, to which an inverse characteristic model of the feed system is added, to control a load position of the feed system on the basis of an amount of compensation outputted from the inverse characteristic model and used for compensating for a dynamic error factor of the feed system. The method is characterized in that the method comprises: in the load position control system, conducting a load position control test using the feedback control system by issuing a position command to the feedback control system, and measuring a position deviation between the position command and the load position arising at a prescribed load position at this time; and in a load inertia estimation model being a model of the load position control system, conducting load position control simulation on a model of the feed system using a model of the feedback control system by issuing the position command to the model of the feedback control system, repeating the load position control simulation while the load inertia included in the model of the feed system is adjusted until a position deviation between the position command and the load position arising at the prescribed load position in the load position control simulation becomes equal to the position deviation measured in the load position control test, and as a consequence, if the position deviation arising at the prescribed load position in the load position control simulation becomes equal to the position deviation measured in the load position control test, estimating the load inertia included in the model of the feed system at this time as the load inertia of the feed system. 
     In addition, a load inertia estimation method according to a second aspect of the invention is a load inertia estimation method of estimating load inertia of a feed system for a load position control system configured to cause a feedback control system, to which an inverse characteristic model of the feed system is added, to control a load position of the feed system on the basis of an amount of compensation outputted from the inverse characteristic model and used for compensating for a dynamic error factor of the feed system. The method is characterized in that the method comprises: in the load position control system, conducting a load position control test using the feedback control system by issuing a position command to the feedback control system and measuring a position deviation between the position command and the load position arising at a prescribed load position at this time, or in a model of the load position control system, conducting load position control simulation on a model of the feed system using a model of the feedback control system by issuing the position command to the model of the feedback control system and measuring the position deviation between the position command and the load position arising at the prescribed load position at this time; and finding load inertia corresponding to the position deviation measured in the load position control test or the load position control simulation on the basis of position deviation characteristic data which is preset based on the position deviation between the position command and the load position being measured in advance and arising at the prescribed load position when no load is applied and on the position deviation between the position command and the load position being measured in advance and arising at the prescribed load position when a certain load is applied and which increases linearly in proportion to an increase in the load inertia, and estimating the load inertia thus found as the load inertia of the feed system. 
     Further, a control parameter adjustment method according to a third aspect of the invention is a control parameter adjustment method of adjusting load inertia included in an inverse characteristic model for a load position control system configured to cause a feedback control system, to which the inverse characteristic model of a feed system is added, to control a load position of the feed system on the basis of an amount of compensation outputted from the inverse characteristic model and used for compensating for a dynamic error factor of the feed system. The method is characterized in that the method comprises adjusting the load inertia included in the inverse characteristic model on the basis of the load inertia estimated by the load inertia estimation method according to the first or second aspect. 
     Effect of the Invention 
     The load inertia estimation method of the first aspect of the invention provides the method of estimating the load inertia of the feed system for the load position control system configured to cause the feedback control system, to which the inverse characteristic model of the feed system is added, to control the load position of the feed system on the basis of the amount of compensation outputted from the inverse characteristic model and used for compensating for the dynamic error factor of the feed system. Here, the method is characterized in that the method includes, in the load position control system, conducting a load position control test using the feedback control system by issuing a position command to the feedback control system, and measuring a position deviation between the position command and the load position arising at a prescribed load position at this time; and in a load inertia estimation model being a model of the load position control system, conducting load position control simulation on a model of the feed system using a model of the feedback control system by issuing the position command to the model of the feedback control system, repeating the load position control simulation while the load inertia included in the model of the feed system is adjusted until a position deviation between the position command and the load position arising at the prescribed load position in the load position control simulation becomes equal to the position deviation measured in the load position control test, and as a consequence, if the position deviation arising at the prescribed load position in the load position control simulation becomes equal to the position deviation measured in the load position control test, estimating the load inertia included in the model of the feed system at this time as the load inertia of the feed system. For this reason, even when the weight of a load on the feed system (such as the weight of a workpiece mounted on a table of a machine tool) varies, the load inertia corresponding to the load weight can easily be estimated. 
     The load inertia estimation method of the second aspect of the invention provides the method of estimating the load inertia of the feed system for the load position control system configured to cause the feedback control system, to which the inverse characteristic model of the feed system is added, to control the load position of the feed system on the basis of the amount of compensation outputted from the inverse characteristic model and used for compensating for the dynamic error factor of the feed system. Here, the method is characterized in that the method includes, in the load position control system, conducting a load position control test using the feedback control system by issuing a position command to the feedback control system and measuring a position deviation between the position command and the load position arising at a prescribed load position at this time, or in a model of the load position control system, conducting load position control simulation on a model of the feed system using a model of the feedback control system by issuing the position command to the model of the feedback control system and measuring the position deviation between the position command and the load position arising at the prescribed load position at this time; and finding load inertia corresponding to the position deviation measured in the load position control test or the load position control simulation on the basis of position deviation characteristic data which is preset based on the position deviation between the position command and the load position being measured in advance and arising at the prescribed load position when no load is applied and on the position deviation between the position command and the load position being measured in advance and arising at the prescribed load position when a certain load is applied and which increases linearly in proportion to an increase in the load inertia, and estimating the load inertia thus found as the load inertia of the feed system. For this reason, even when the load weight on the feed system (such as the weight of the workpiece mounted on the table of the machine tool) varies, the load inertia corresponding to the load weight can easily be estimated. 
     The control parameter adjustment method according to the third aspect of the invention provides the control parameter adjustment method of adjusting the load inertia included in the inverse characteristic model for the load position control system configured to cause the feedback control system, to which the inverse characteristic model of the feed system is added, to control the load position of the feed system on the basis of the amount of compensation outputted from the inverse characteristic model and used for compensating for the dynamic error factor of the feed system. Here, the method is characterized in that the method includes adjusting the load inertia included in the inverse characteristic model on the basis of the load inertia estimated by the load inertia estimation method according to the first or second aspect of the invention. Therefore, even when the load weight on the feed system (such as the weight of the workpiece mounted on the table of the machine tool) varies, it is possible to cause parameters of the feed system to match parameters of the inverse characteristic model (such as coefficients (to be described later in detail) in differential terms of third and higher orders including the term of the load inertia). For this reason, it is possible to perform precise control over the load position such that the load position follows the position command, and thereby to cause, for example, a machine tool to perform high-precision machining. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a configuration of a load position control system which embodies a load inertia estimation method and a control parameter adjustment method according to a first embodiment of the present invention. 
         FIG. 2  is a view showing a configuration of a load inertia estimation model. 
         FIG. 3  is a view showing a configuration of a load position control system which embodies a load inertia estimation method and a control parameter adjustment method according to a second embodiment of the present invention. 
         FIG. 4  is a view showing a configuration of a conventional machine tool. 
         FIG. 5  is a view showing a configuration of a conventional load position control system (a feedback control system and a table feed system). 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described below in detail based on the drawings. 
     First Embodiment 
     (Description of Feedback Control System and Feed System) 
     A configuration of a load position control system (a feedback control system  21  and a feed system  22 ) of a machine tool (see  FIG. 4 ) which embodies a load inertia estimation method and a control parameter adjustment method according to an embodiment of the present invention will be described based on  FIG. 1 . 
     As shown in  FIG. 1 , the table feed system  22  includes a servo motor  23  being a drive source, a reduction gear unit  24  having a motor end gear  24   a  and a load end gear  24   b , brackets  26  each incorporating a bearing  25 , a ball screw  27  having a screw portion  27   a  and a nut portion  27   b , a position detector  28 , and a pulse encoder  29 . 
     The brackets  26  on two sides are fixed to a bed  1  and rotatably support the screw portion  27   a  of the ball screw  27  via the bearings  25 . The nut portion  27   b  of the ball screw  27  is attached to the table  2  and screwed to the screw portion  27   a . The servo motor  23  is connected to the screw portion  27   a  of the ball screw  27  via the reduction gear unit  24 . A workpiece W is placed on the table  2 . In addition, the position detector (which is an Inductosyn linear scale in the illustrated example)  28  is attached to the table  2 , and the pulse encoder  29  is attached to the servo motor  23 . 
     Accordingly, when torque of the servo motor  23  is transferred to the screw portion  27   a  of the ball screw  27  via the reduction gear unit  24  and the screw portion  27   a  is rotated as indicated with an arrow A, the table  2  moves linearly in an X-axis direction together with the nut portion  27   b  of the ball screw  27 . At this time, the position detector  28  detects a load position θ L , which is a position to which the table  2  (the workpiece W) is moved, and sends a detection signal of the load position θ L  to the feedback control system  21  (position feedback). The pulse encoder  29  detects a motor position θ M  which is a rotational position of the servo motor  23 . A detection signal of the motor position θ M  is sent to the feedback control system  21 , then subjected to temporal differentiation by a differential operation unit  36 , and thereby converted into a motor velocity V M  which is a rotational velocity of the servo motor  23  (velocity feedback). 
     The feedback control system  21  is constructed by software to be executed by a personal computer, for example, and includes a position deviation operating unit  31 , a multiplication unit  32 , a velocity deviation operating unit  33 , a proportional integral operating unit  34 , a current control unit  35 , and a differential operating unit  36 . 
     Moreover, an inverse characteristic model  50  of the feed system  22  of the table  2  is added to the feedback control system  21 . Although the details will be described later, the inverse characteristic model  50  is an inverse characteristic model (an inverse transfer function) of a characteristic model (a transfer function) that approximates a characteristic of the feed system  22 , and is designed to compensate for a delay in the load position θ L  or a delay in a velocity caused, for instance, by torsion or deflection of the ball screw  27  (the screw portion  27   a ) of the feed system  22  (see  FIG. 2 : to be described further in detail). Here, s in  FIG. 1  denotes a Laplace operator, namely, is a first-order differential, s 2  is a second-order differential, s 3  is a third-order differential, s 4  is a fourth-order differential, s 5  is a fifth-order differential, and 1/s is an integral thereof (the similar applies to  FIG. 2  and  FIG. 3 ). 
     The position deviation operating unit  31  of the feedback control  21  finds a position deviation θΔ by calculating a deviation (θ−θ L ) between a position command θ, which is issued from a numerical control (NC) device  41  in order to control the load position θ L , and the load position θ L . The multiplication unit  32  finds a motor velocity command V for controlling the rotational velocity of the servo motor  23  by multiplying the position deviation Δθ by a position loop gain Kp. Meanwhile, the velocity deviation operating unit  33  finds a velocity deviation ΔV by calculating a deviation (V+V H −V M ) between a value (V+V H ), which is obtained by adding the amount V H  of velocity compensation outputted from the inverse characteristic model  5  to the motor velocity command V, and the motor velocity V M . 
     The proportional integral operating unit  34  finds a motor torque command τ to the servo motor  23  by performing a proportional integral operation of τ−ΔV×(K V (1+1/(T V s))) using a velocity loop gain K V  and an integration time constant T V . The current control unit  35  controls a current to be supplied to the servo motor  23  in such a way that the torque generated by the servo motor  23  follows the motor torque command τ. Although illustration is omitted, the current control unit  35  performs feedback control on the current such that the supply current to the motor  23  becomes a current that corresponds to the motor torque command τ. 
     As described above, the feedback control system  21  performs the feedback control using the triple loops of the position loop serving as a main loop, and the velocity loop as well as the current loop serving as minor loops, thereby performing control such that the load position θ L  follows the position command θ. 
     (Description of Load Inertia Estimation Model) 
     Furthermore, in the first embodiment, a model  60  for estimating load inertia J L  that corresponds to the weight of the workpiece W is added to the feedback control system  21 . The load inertia estimation model  60  will be described based on  FIG. 2 . Note that portions in  FIG. 2  similar to those in  FIG. 1  will be denoted by the same reference numerals and overlapping detailed description thereof will be omitted herein. 
     In the example shown in  FIG. 2 , the characteristic model (the transfer function) approximating the characteristic of the feed system  22  is specified as a two-mass-point mechanical system model defining the servo motor  23  as one mass point, and the table  2  and the workpiece W collectively serving as the load on the motor as another mass point. Further, the load inertia estimation model  60  includes the characteristic model (the transfer function) of the feed system  22 , the inverse characteristic model (the inverse transfer function)  50  of the characteristic model, and a model (a transfer function) of the feedback control system  21 . 
     As shown in  FIG. 2 , when a characteristic model of the servo motor  23  is expressed by transfer functions, the characteristic model is expressed by a transfer function (1/(J M s+D M )) in a block  62  and a transfer function (1/s) in a block  63 . Here, J M  is motor inertia and D M  is motor viscosity. The motor velocity V M  is outputted from the block  62  while the motor position θ M  is outputted from the block  63 . 
     When a characteristic model of the table  2  inclusive of the ball screw  27  is expressed by transfer functions, the characteristic model is expressed by a transfer function (C L s+K L ) in a block  64 , a transfer function (1/(J L s+D L )) in a block  65 , and a transfer function (1/s) in a block  66 . Here, J L  is load inertia, which is the inertia determined by the weight (a constant value) of the table  2  and the weight of the workpiece W mounted on the table  2 . Therefore, when the weight of the workpiece W mounted on the table  2  varies, the load inertia J L  also changes accordingly. Here, D L  is viscosity of the load (the table), C L  is spring viscosity of the ball screw  27  unit (the screw portion  27   a , the nut portion  27   b , and the brackets  26 ) in an axial direction, and K L  is spring rigidity of the ball screw  27  unit (the screw portion  27   a , the nut portion  27   b , and the brackets  26 ) in the axial direction. 
     A position deviation operating unit  67  finds a position deviation Δθ ML  by calculating a deviation (θ M −θ L ) between the motor position θ M  and the load position θ L . When the position deviation Δθ ML  is inputted, the block  64  finds reactive torque τ L  by performing calculation of τ L =Δθ ML ×(C L s+K L ) and outputs the reactive torque τ L . When the reactive torque τ L  is inputted to the block  65 , the load position θ L  is found by performing calculation of θ L =τ L (1/(J L s+D L ))×(1/s) in the block  65  and the block  66 , and the load position θ L  is outputted from the block  66 . 
     A torque deviation operating unit  61  finds a torque deviation Δτ by calculating a deviation (τ−τ L ) between the torque command τ and the reactive torque τ L . The block  62  finds the motor velocity V M  by performing calculation of V M =Δτ×(1/(J M s+D M )). The motor velocity V M  is outputted to the block  63  and fed back to the velocity deviation operating unit  33  of the feedback control system  21 . The block  63  finds the motor position θ M  by performing calculation of θ M =V M ×(1/s). The motor position θ M  is outputted to the position deviation operating unit  67 . The load position θ L  is fed back to the position deviation operating unit  31  of the feedback control system  21 . 
     The inverse characteristic model  50  includes a first-order differential term operating unit  51 , a second-order differential term operating unit  52 , a third-order differential term operating unit  53 , a fourth-order differential term operating unit  54 , a fifth-order differential term operating unit  55 , an addition unit  56 , and a proportional integral inverse transfer function unit  57 . 
     A transfer function for compensation control, which is provided for performing compensation control in such a manner as to compensate for dynamic error factors at the servo motor  23 , the ball screw  27 , and the table  2  of the feed system  22  and thereby to cause the load position θ L  to match (follow) the position command θ, is set to each of the differential term operating units  51  to  55  and the addition unit  56 . The transfer functions for compensation control are inverse transfer functions of the aforementioned transfer functions of the feed system  22  (a mechanical system including the servo motor  23 , the ball screw  27 , and the table  2 ). Note that the inverse transfer functions are formed as functions where operational elements are partially curtailed. 
     Specifically, the differential term operating units  51  to  55  of the inverse characteristic model  50  include operands a1s, a2s 2 , a3s 3 , a4s 4 , and a5s 5 , respectively. The differential term operating units  51  to  55  multiply the position command θ by the operands a1s, a2s 2 , a3s 3 , a4s 4 , and a5s 5 , respectively, and output multiplied values to the addition unit  56 . The addition unit  56  adds the multiplied values outputted from the differential term operating units  51  to  55 . 
     The coefficients a1, a2, a3, a4, and a5 in the operands a1s to a5s 5  are set as follows. Of the terms included in the formulae of the respective coefficients a1 to a5, K V  is the velocity loop gain, K L  is the spring rigidity of the ball screw  27  in the axial direction, τ V  is the integration time constant, D M  is the viscosity of the servomotor  23 , D L  is the load viscosity, J M  is the inertia of the servomotor  23 , and J L  is the load inertia as discussed previously. 
     A calculation method of setting (calculating) the coefficients a1 to a5 as below will be described later. 
     
       
         
           
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     A term (T V /K V (T V s+1)) in an inverse transfer function (T V /K V (T V s+1))×s of the transfer function K V (1+1/(T V s)) of the proportional integral operating unit  34  is set to the proportional integral inverse transfer function unit  57 . The differential operators in (T V /K V (T V s+1))×s is assigned to each of the operands a1s to a5s 5  in the differential term operating units  51  to  55 . 
     Then, load position control of the feed system  22  is conducted while the amount V H  of velocity compensation outputted from the inverse characteristic model  50  including the set coefficients a1 to a5 is applied to the feedback control system  21 . Thus, it is possible to compensate for error factors such as distortion, deflection, and viscosity which may occur in the servo motor  23 , the ball screw  27 , the table  2 , and so forth of the feed system  22 , and thereby to perform precise control over the load position θ L  such that the load position θ L  follows the position command θ. As a consequence, high-precision machining is enabled. 
     (Description of Load Inertia Estimation Method and Control Parameter Adjustment Method) 
     However, if the weight of the workpiece W mounted on the table  2  varies (when a workpiece W having a different weight is mounted on the table  2 ), the load inertia J L  also changes in response to the variation in the weight of the workpiece W. Hence, parameters of the feed system  22  no longer match parameters of the inverse characteristic model  50 . Specifically, the coefficients a3 to a5 of the differential terms of the third and higher orders (i.e., the terms a1s 3  to a5s 5 ) including the term of the load inertia J L  do not match the corresponding parameters of the feed system  22 . At this rate, the position deviation Δθ is increased whereby the load position θ L  causes a delay in following the position command θ. 
     Therefore, the load inertia J L  corresponding to the weight of the workpiece W is estimated in accordance with the following method prior to the machining of the workpiece W. 
     First, in the actual load position control system (the feedback control system  21  and the feed system  22 ) shown in  FIG. 1 , a load position control test on the feed system  22  is conducted using the feedback control system  21  by issuing the position command θ (a motion command in the X-axis direction) from the NC device  41  to the feedback control system  21  while mounting the workpiece W on the table  2 . Then, the position deviation Δθ arising at this time is measured. Here, since the spring rigidity K L  varies depending on the load position θ L , the position deviation Δθ arising at a point of time when the table  2  reaches a prescribed (predetermined) load position θ L  (i.e., a point of time when the table  2  reaches the load position θ L  where the spring rigidity becomes the prescribed spring rigidity K L ) is measured. 
     Next, in the load inertia estimation model  60  shown in  FIG. 1  and  FIG. 2 , which is the model of the load position control system, load position control simulation on a model of the feed system  22  is conducted using a model of the feedback control system  21  by issuing the position command A (the motion command in the X-axis direction) from the NC device  41  to the model of the feedback control system  21  while mounting the workpiece W on the table  2 . 
     Here, the load position control simulation is repeated while the load inertia J L  of the table  2  as well as the workpiece W included in the model of the feed system  22  are adjusted until position deviation Δθ arising in the load position control simulation becomes equal to the position deviation Δθ measured in the load position control test conducted by the actual system. 
     However, as described previously, the spring rigidity K L  varies depending on the load position θ L . Accordingly, the position deviation Δθ arising at the point of time when the table  2  reaches the prescribed load position θ L  (i.e., the point of time when the table  2  reaches the load position θ L  where the spring rigidity becomes the prescribed spring rigidity K L ) is compared with the position deviation Δθmeasured in the load position control test conducted by the actual system to estimate whether or not both of the position deviations Δθ are mutually equal. Meanwhile, the load inertia J L  in the inverse characteristic model  50  at the time when the load position control test is conducted by the actual system is set to the same value as the load inertia J L  in the inverse characteristic model  50  at the time when the load position control simulation is conducted. For example, these values are set equal to load inertia J L0  when no load is applied, i.e., no workpiece W is mounted on the table  2 . 
     If the position deviation Δθ arising in the load position control simulation becomes equal to the position deviation Δθ measured in the load position control test conducted by the actual system as a consequence of repeating the load position control simulation while adjusting the load inertia J L  included in the model of the feed system  22 , then the load inertia J L  included in the model of the feed system  22  at this time is estimated as the actual load inertia J L  corresponding to the weight of the workpiece W mounted on the table  2 . 
     Next, the load inertia J L  thus estimated is outputted from the load inertia estimation model  60  to the inverse characteristic model  50  of the actual system as shown in  FIG. 1 . In the inverse characteristic model  50  of the actual system, the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L  are adjusted (set) on the basis of the load inertia J L  outputted from the load inertia estimation model  60 . In this way, the parameters of the feed system  22  match the parameters (the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L ) of the inverse characteristic model  50 . For this reason, when the workpiece W is machined, it is possible to perform precise control over the load position θ L  such that the load position A L  follows the position command θ, and thereby to achieve high-precision machining. 
     Operation and Effect 
     As described above, the load inertia estimation method of the first embodiment provides the method of estimating the load inertia J L  of the feed system  22  for the load position control system configured to cause the feedback control system  21 , to which the inverse characteristic model  50  of the feed system  22  is added, to control the load position θ L  of the feed system  22  on the basis of the amount V H  of compensation outputted from the inverse characteristic model  50  and used for compensating for the dynamic error factor of the feed system  22 . Here, the method is characterized in that the method includes: in the load position control system, conducting the load position control test using the feedback control system  21  by issuing the position command θ to the feedback control system  21 , and measuring the position deviation Δθ arising at the prescribed load position θ L  at this time; and in the load inertia estimation model  60  being the model of the load position control system, conducting the load position control simulation on the model of the feed system  22  using the model of the feedback control system  21  by issuing the position command θ to the model of the feedback control system  21 , repeating the load position control simulation while the load inertia J L  included in the model of the feed system  22  is adjusted until the position deviation Δθ arising at the prescribed load position θ L  in the load position control simulation becomes equal to the position deviation Δθ measured in the load position control test, and as a consequence, if the position deviation Δθ arising at the prescribed load position θ L  in the load position control simulation becomes equal to the position deviation Δθ measured in the load position control test, estimating the load inertia J L  included in the model of the feed system  22  at this time as the load inertia J L  of the feed system  22  of the actual system. For this reason, even when the weight of a load on the feed system  22  (the weight of the workpiece W mounted on the table  2 ) varies, the load inertia J L  corresponding to the load weight can easily be estimated. 
     In addition, the control parameter adjustment method of the first embodiment is characterized in that the method includes adjusting the load inertia J L  included in the inverse characteristic model  50  of the actual system on the basis of the load inertia J L  estimated by using the load inertia estimation method. Accordingly, even when the load weight on the feed system  22  (the weight of the workpiece W mounted on the table  2 ) varies, it is possible to cause the parameters of the feed system  22  to match the parameters of the inverse characteristic model  50  (the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L ). For this reason, it is possible to perform precise control over the load position θ L  such that the load position θ L  follows the position command θ, and thereby to achieve high-precision machining. 
     Second Embodiment 
     (Description of Load Inertia Estimation Method and Control Parameter Adjustment Method) 
     A load inertia estimation method and a control parameter adjustment method according to a second embodiment of the present invention will be described based on  FIG. 3 . Note that portions in  FIG. 3  similar to those in the first embodiment will be denoted by the same reference numerals and overlapping detailed description thereof will be omitted herein. 
     As shown in  FIG. 3 , a position deviation characteristic data unit  70  for estimating the load inertia J L  corresponding to the weight of the workpiece W is added to the feedback control system  21  in the second embodiment. 
     A relational expression F=ma=K L Δθ (F: force, m: weight of workpiece, K L : spring rigidity of ball screw, Δθ: position deviation) holds between the position deviation Δθ (i.e., deflection of the ball screw  27  and the like) and the weight of the workpiece W. When the force F and the spring rigidity K L  are made constant, the position deviation Δθ is thought to increase linearly in proportion to the increase in the weight of the workpiece W. 
     In the meantime, the amount of compensation in proportion to the load inertia J L  is determined for the differential terms of the third and higher orders (a3s 3  to a5s 5 ) in the inverse characteristic model  50 . Hence, the position deviation Δθ can be thought to increase linearly in proportion to the increase in the weight of the workpiece W mounted on the table  2 . 
     Therefore, if data on the position deviation Δθ under the load inertia J L0  when no load is applied, i.e., no workpiece W is mounted on the table  2  and on the position deviation Δθ under the load inertia J L  when a maximum load is applied, i.e., a workpiece W having a maximum probable weight is mounted on the table  2  are available, then it is possible to estimate load inertia J L1  at the time of mounting a workpiece W having an unknown weight on the table  2  by use of the data. 
     Accordingly, in the actual load position control system (the feedback control system  21  and the feed system  22 ) shown in  FIG. 3 , a load position control test is conducted using the feedback control system  21  on the feed system  22  in the cases where no load is applied and where the maximum load is applied, by issuing the position command θ (the motion command in the X-axis direction) from the NC device  41  to the feedback control system  21 . Then, a position deviation Hθ L0  arising when no load is applied as well as a position deviation Δθ LM  arising when the maximum load is applied are measured. 
     Alternatively, using the models of the load position control system as shown in  FIG. 2 , load position control simulation is conducted using the model of the feedback control system  21  on the model of the feed system  22  in the cases where no load is applied and where the maximum load is applied, by issuing the position command θ (the motion command in the X-axis direction) to the model of the feedback control system  21 . Then, the position deviation Δθ L0  arising when no load is applied as well as the position deviation Δθ LM  arising when the maximum load is applied are measured. 
     Here, as described previously, the spring rigidity K L  varies depending on the load position θ L . Accordingly, the position deviations Δθ L0  and Δθ LM  each arising at the point of time when the table  2  reaches the prescribed (predetermined) load position θ L  (i.e., the point of time when the table  2  reaches the load position θ L  where the spring rigidity becomes the prescribed spring rigidity K L ) are measured. 
     Moreover, in order to define the position deviation Δθ L0  when no load is applied as a reference, the load inertia J L  in the inverse characteristic model  50  is set at the load inertia J L0  when no load is applied. As a consequence, the position deviation Δθ L0  when no load is applied is substantially equal to 0. 
     Position deviation characteristic data ΔV D  which increases linearly in proportion to an increase in the load inertia J L  is set in the position deviation characteristic data unit  70  on the basis of the position deviation Δθ L0  when no load is applied and the position deviation Δθ LM  when the maximum load is applied, which are measured in advance. 
     Then, the load inertia J L  corresponding to the weight of the workpiece W is estimated prior to the machining of the workpiece W in accordance with the following method. 
     First, in the actual load position control system (the feedback control system  21  and the feed system  22 ) shown in  FIG. 3 , the load position control test on the feed system  22  is conducted using the feedback control system  21  by issuing the position command θ (the motion command in the X-axis direction) from the NC device  41  to the feedback control system  21  while mounting the workpiece W on the table  2 . 
     Then, the position deviation characteristic data unit  70  measures (inputs) the position deviation Δθ (which is Δθ 1  in the illustrated example) arising at this time. However, as described previously, the spring rigidity K L  varies depending on the load position θ L . Therefore, the position deviation characteristic data unit  70  measures (inputs) the position deviation Δθ (which is Δθ 1  in the illustrated example) arising at the point of time when the table  2  reaches the prescribed (predetermined) load position θ L  (i.e., the point of time when the table  2  reaches the load position θ L  where the spring rigidity becomes the prescribed spring rigidity K L ). 
     Next, the position deviation characteristic data unit  70  finds the load inertia J L  (which is J L1  in the illustrated example) corresponding to the position deviation Δθ (which is Δθ 1  in the illustrated example) measured (inputted) either in the load position control test conducted by the actual system or in the load position control simulation, on the basis of the preset position deviation characteristic data ΔV D , and estimates that the load inertia J L  (which is J L1  in the illustrated example) is the load inertia J L  corresponding actually to the weight of the workpiece W mounted on the table  2 . The estimated load inertia J L  is outputted from the position deviation characteristic data unit  70  to the inverse characteristic model  50  of the actual system. 
     In the inverse characteristic model  50  of the actual system, the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L  are adjusted (set) on the basis of the load inertia J L  (which is J L1  in the illustrated example) outputted from the load inertia estimation model  60 . In this way, the parameters of the feed system  22  match the parameters (the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L ) of the inverse characteristic model  50 . For this reason, when the workpiece W is machined, it is possible to perform precise control over the load position θ L  such that the load position θ L  follows the position command θ, and thereby to achieve high-precision machining. 
     Although the position deviation characteristic data ΔV D  is set by using the position deviation Δθ LM  when the maximum load is applied in the above-described embodiment, the present invention is not limited only to this configuration. The position deviation characteristic data ΔV D  may be set by using a position deviation Δθ L  when a certain load other than the maximum load is applied. Specifically, in the state where a workpiece W having a certain weight other than the maximum weight on the table  2  (i.e., in the state where the certain load other than the maximum load is applied), the position deviation Δθ when the certain load is applied may be measured by causing the actual system to conduct the load position control test or conducting the load position control simulation as similar to the above description, and the position deviation characteristic data ΔV D  which increases linearly in proportion to the increase in the load inertia J L  may be set on the basis of the measured position deviation  48  when the certain load is applied as well as the position deviation Δθ 0  when no load is applied. 
     (Operation and Effect) 
     As described above, the load inertia estimation method of the second embodiment provides the method of estimating the load inertia J L  of the feed system  22  for the load position control system configured to cause the feedback control system  21 , to which the inverse characteristic model  50  of the feed system  22  is added, to control the load position θ L  of the feed system  22  on the basis of the amount V H  of compensation outputted from the inverse characteristic model  50  and used for compensating for the dynamic error factor of the feed system  22 . Here, the method is characterized in that the method includes: in the load position control system, conducting the load position control test using the feedback control system  21  by issuing the position command θ to the feedback control system  21 , and measuring the position deviation Δθ (Δθ 1 ) arising at the prescribed load position θ L  at this time, or in the model of the load position control system, conducting the load position control simulation on the model of the feed system  22  using the model of the feedback control system  21  by issuing the position command θ to the model of the feedback control system  21 , and measuring the position deviation Δθ (Δθ 1 ) arising at the prescribed load position θ L  at this time; and finding the load inertia J L  (J L1 ) corresponding to the position deviation Δθ (Δθ 1 ) measured either in the load position control test or the load position control simulation on the basis of the position deviation characteristic data ΔV D  which is preset based on the position deviation Δθ (Δθ 0 ) being measured in advance and arising at the prescribed load position θ L  when no load is applied and on the position deviation Δθ (Δθ M ) being measured in advance and arising at the prescribed load position θ L  when the certain load is applied and which increases linearly in proportion to the increase in the load inertia J L , and estimating the load inertia J L  (J L1 ) as the load inertia J L  of the feed system  22  of the actual system. For this reason, even when the load weight on the feed system  22  (the weight of the workpiece W mounted on the table  2 ) varies, the load inertia J L  corresponding to the load weight can easily be estimated. 
     In addition, the control parameter adjustment method of the second embodiment is characterized in that the method includes adjusting the load inertia J L  included in the inverse characteristic model  50  of the actual system on the basis of the load inertia J L  estimated by using the load inertia estimation method. Accordingly, even when the load weight on the feed system  22  (the weight of the workpiece W mounted on the table  2 ) varies, it is possible to cause the parameters of the feed system  22  to match the parameters of the inverse characteristic model  50  (the coefficients a3 to a5 of the differential terms of the third and higher orders including the term of the load inertia J L ). For this reason, it is possible to perform precise control over the load position θ L  such that the load position θ L  follows the position command θ r  and thereby to achieve high-precision machining. 
     In the above-described first and second embodiments, the load inertia J L  in the inverse characteristic model  50  is adjusted based on the estimated load inertia J L . However, the present invention is not limited only to this configuration, but control parameters other than the load inertia J L  in the inverse characteristic model  50 , such as control parameters concerning machining conditions, may also be adjusted based on the estimated load inertia J L . For example, the estimated load inertia J L  may be outputted from the position deviation characteristic data unit  70  or the load inertia estimation model  60  to the NC device  41  as well, and control parameters to be set by the NC device  41 , including acceleration and deceleration time, corner velocity and acceleration, and so forth may be adjusted based on the estimated load inertia J L . 
     Meanwhile, the first and second embodiments have described the case of applying the present invention to the feed system  22  for the table  2 . However, the present invention is not limited only to this configuration but is also applicable to feed systems provided for components other than the table  2  (such as a feed system for a saddle or a ram). For example, if the weight of the attachment  8  or the tool  9  in  FIG. 4  is variable, then it is effective to apply the present invention to a feed system for the saddle  5  or the ram  6 . 
     Moreover, the first and second embodiments have described the case of applying the present invention to the feed system  22  including the servo motor  23 , the ball screw  27 , and the like. However, the present invention is not limited only to this configuration but is also applicable to feed systems having other configurations (such as feed systems using a hydraulic pump, a hydraulic motor, a hydraulic cylinder, and the like). 
     Furthermore, the first and second embodiments have described the case of application to the feed system in a machine tool. However, the present invention is not necessarily limited only to this configuration but is also applicable to feed systems in industrial machines other than machine tools. 
     &lt;Description on Calculation Method of Coefficients in Inverse Characteristic Model&gt; 
     Now, the calculation method of setting (calculating) the coefficients a1 to a5 in the inverse characteristic model  50  will be described. 
     In the mechanical system model shown in  FIG. 2 , the transfer functions for the inverse characteristic model involving the torque and the velocity can be calculated as follows. First, Formula (1) and Formula (2) shown below are found from equations of motion. Here, Formula (1) is an equation of motion representing an input-output relation concerning a motor transfer function that models a characteristic of the servo motor  23 , and Formula (2) is an equation of motion representing an input-output relation concerning a load transfer function that models a characteristic of the table  2  and the workpiece W collectively serving as the load. 
       [Expression 2] 
       τ−(θ M −θ L )·( C   L   s+K   L )=( J   Ms   2   D   M   s )·θ M   (1)
 
       (θ M −θ L )·( C   L   s+K   L )=( J   Ls   2   D   L   s )·θ L   (2)
 
     The following Formula (3) and Formula (4) are derived from Formula (1) and Formula (2) shown above. 
     
       
         
           
             
               
                 
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     In order to move the load (the table  2  and the workpiece W) with no error, compensation control should be performed such that the load position θ L  matches the position command θ, i.e., such that θ=θ L  is satisfied. In order to satisfy θ=θ L , the torque command τ should be subjected to feed-forward compensation control in accordance with a formula in braces { } (a first transfer function formula) on the right side of Formula (3), and the velocity command V should be subjected to feed-forward compensation control in accordance with a formula in parentheses ( ) (a second transfer function formula) on the right side of Formula (4). Note that θ M s in Formula (4) is equivalent to the motor velocity V. 
     In Formula (3), θ L  is replaced with 9 and then the formula is translated into a command velocity Vi. Thus, Formula (3) is converted into Formula (5). Formula (5) is equivalent to Formula (3) multiplied by an inverse operation expression of a proportional integral operation expression set in the proportional integral operating unit  34 . In other words, Formula (5) is equivalent to Formula (3) divided by the proportional integral operation expression set in the proportional integral operating unit  34 . A portion on the right side of Formula (5) excluding θ constitutes a third transfer function. Meanwhile, Formula (6) shown below is obtained by replacing θ L  with θ in Formula (4) and then transforming Formula (4). In order to perform the compensation control such that the load position θ L  matches the position command θ, the compensation velocity V H  for achieving no error between θ and θ L  should be set equal to a sum of Formula (5) and Formula (6). Such a sum is expressed by Formula (7) below. A portion on the right side of Formula (7) excluding θ constitutes a fourth transfer function. 
     
       
         
           
             
               
                 
                   
                       
                   
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                                            
                                           
                                             s 
                                             4 
                                           
                                         
                                         + 
                                       
                                     
                                   
                                   
                                     
                                       
                                         
                                           
                                             ( 
                                             
                                               
                                                 
                                                   D 
                                                   M 
                                                 
                                                  
                                                 
                                                   D 
                                                   L 
                                                 
                                               
                                               + 
                                               
                                                 
                                                   K 
                                                   V 
                                                 
                                                  
                                                 
                                                   D 
                                                   L 
                                                 
                                               
                                               + 
                                               
                                                 
                                                   
                                                     K 
                                                     V 
                                                   
                                                    
                                                   
                                                     J 
                                                     L 
                                                   
                                                 
                                                 
                                                   T 
                                                   V 
                                                 
                                               
                                             
                                             ) 
                                           
                                            
                                           
                                             s 
                                             3 
                                           
                                         
                                         + 
                                         
                                           
                                             
                                               
                                                 K 
                                                 V 
                                               
                                                
                                               
                                                 D 
                                                 L 
                                               
                                             
                                             
                                               T 
                                               V 
                                             
                                           
                                            
                                           
                                             s 
                                             2 
                                           
                                         
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       C 
                                       L 
                                     
                                      
                                     s 
                                   
                                   + 
                                   
                                     K 
                                     L 
                                   
                                 
                               
                               + 
                             
                           
                         
                         
                           
                             
                               
                                 
                                   ( 
                                   
                                     
                                       J 
                                       M 
                                     
                                     + 
                                     
                                       J 
                                       L 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   s 
                                   3 
                                 
                               
                               + 
                               
                                 
                                   ( 
                                   
                                     
                                       D 
                                       M 
                                     
                                     + 
                                     
                                       D 
                                       L 
                                     
                                     + 
                                     
                                       K 
                                       V 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   s 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   
                                     K 
                                     V 
                                   
                                   
                                     T 
                                     V 
                                   
                                 
                                  
                                 s 
                               
                             
                           
                         
                       
                       } 
                     
                     · 
                     
                       ( 
                       
                         
                           T 
                           V 
                         
                         
                           
                             
                               K 
                               V 
                             
                              
                             
                               T 
                               V 
                             
                              
                             s 
                           
                           + 
                           
                             K 
                             V 
                           
                         
                       
                       ) 
                     
                     · 
                     θ 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     It is not possible to organize the original Formula (7) in terms of the differential orders. However, the following Formula (8) is obtained by deleting the term C L , which has little effect on accuracy, from Formula (7). A portion on the right side of Formula (8) excluding θ constitutes a transfer function for compensation control. The following Formula (9) is obtained by replacing Formula (8) with the coefficients a1 to a5. In this way, the coefficients a1 to a5 are obtained from Formula (8) and Formula (9). 
     
       
         
           
             
               
                 
                   
                       
                   
                    
                   
                     [ 
                     
                       Expression 
                        
                       
                           
                       
                        
                       5 
                     
                     ] 
                   
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     V 
                     H 
                   
                   = 
                   
                     
                       { 
                       
                         
                           
                             
                               J 
                               M 
                             
                              
                             
                               J 
                               L 
                             
                              
                             
                               s 
                               5 
                             
                           
                           
                             K 
                             L 
                           
                         
                         + 
                         
                           
                             
                               ( 
                               
                                 
                                   
                                     J 
                                     M 
                                   
                                    
                                   
                                     D 
                                     L 
                                   
                                 
                                 + 
                                 
                                   
                                     J 
                                     L 
                                   
                                    
                                   
                                     D 
                                     M 
                                   
                                 
                                 + 
                                 
                                   
                                     K 
                                     V 
                                   
                                    
                                   
                                     J 
                                     L 
                                   
                                 
                               
                               ) 
                             
                              
                             
                               s 
                               4 
                             
                           
                           
                             K 
                             L 
                           
                         
                         + 
                         
                           
                             ( 
                             
                               
                                 J 
                                 M 
                               
                               + 
                               
                                 J 
                                 L 
                               
                               + 
                               
                                 
                                   
                                     
                                       D 
                                       M 
                                     
                                      
                                     
                                       D 
                                       L 
                                     
                                   
                                   + 
                                   
                                     
                                       K 
                                       V 
                                     
                                      
                                     
                                       D 
                                       L 
                                     
                                   
                                 
                                 
                                   K 
                                   L 
                                 
                               
                               + 
                               
                                 
                                   
                                     K 
                                     V 
                                   
                                    
                                   
                                     J 
                                     L 
                                   
                                 
                                 
                                   
                                     T 
                                     V 
                                   
                                    
                                   
                                     K 
                                     L 
                                   
                                 
                               
                             
                             ) 
                           
                            
                           
                             s 
                             3 
                           
                         
                         + 
                         
                           
                             ( 
                             
                               
                                 D 
                                 M 
                               
                               + 
                               
                                 D 
                                 L 
                               
                               + 
                               
                                 K 
                                 V 
                               
                               + 
                               
                                 
                                   
                                     K 
                                     V 
                                   
                                    
                                   
                                     D 
                                     L 
                                   
                                 
                                 
                                   
                                     T 
                                     V 
                                   
                                    
                                   
                                     K 
                                     L 
                                   
                                 
                               
                             
                             ) 
                           
                            
                           
                             s 
                             2 
                           
                         
                         + 
                         
                           
                             
                               K 
                               V 
                             
                             
                               T 
                               V 
                             
                           
                            
                           s 
                         
                       
                       } 
                     
                     · 
                     
                       ( 
                       
                         
                           T 
                           V 
                         
                         
                           
                             
                               K 
                               V 
                             
                              
                             
                               T 
                               V 
                             
                              
                             s 
                           
                           + 
                           
                             K 
                             V 
                           
                         
                       
                       ) 
                     
                     · 
                     θ 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   
                     V 
                     H 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           a 
                            
                           
                               
                           
                            
                           1 
                            
                           s 
                         
                         + 
                         
                           a 
                            
                           
                               
                           
                            
                           2 
                            
                           
                             s 
                             2 
                           
                         
                         + 
                         
                           a 
                            
                           
                               
                           
                            
                           3 
                            
                           
                             s 
                             3 
                           
                         
                         + 
                         
                           a 
                            
                           
                               
                           
                            
                           4 
                            
                           
                             s 
                             4 
                           
                         
                         + 
                         
                           a 
                            
                           
                               
                           
                            
                           5 
                            
                           
                             s 
                             5 
                           
                         
                       
                       ) 
                     
                     · 
                     
                       ( 
                       
                         
                           T 
                           V 
                         
                         
                           
                             
                               K 
                               V 
                             
                              
                             
                               T 
                               V 
                             
                              
                             s 
                           
                           + 
                           
                             K 
                             V 
                           
                         
                       
                       ) 
                     
                     · 
                     θ 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a load inertia estimation method and a control parameter adjustment method, which is useful for application to the case of adjusting load inertia included in an inverse characteristic model of a feed system that is added to a feedback control system of a machine tool and the like. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
           1  bed 
           2  table 
           21  feedback control system 
           22  feed system 
           23  servo motor 
           24  reduction gear unit 
           24   a  motor end gear 
           24   b  load end gear 
           25  bearing 
           26  bracket 
           27  ball screw 
           27   a  screw portion 
           27   b  nut portion 
           28  position detector 
           29  pulse encoder 
           31  position deviation operating unit 
           32  multiplication unit 
           33  velocity deviation operating unit 
           34  proportional integral operating unit 
           35  current control unit 
           36  differential operating unit 
           41  NC device 
           50  inverse characteristic model 
           51  first-order differential term operating unit 
           52  second-order differential term operating unit 
           53  third-order differential term operating unit 
           54  fourth-order differential term operating unit 
           55  fifth-order differential term operating unit 
           56  addition unit 
           57  proportional integral inverse transfer function unit 
           60  load inertia estimation model 
           64  torque deviation operating unit 
           62 ,  63  blocks of transfer functions concerning servo motor 
           64 ,  65 ,  66  blocks of transfer functions concerning table and ball screw 
           67  position deviation operating unit 
           70  position deviation characteristic data unit