Patent Publication Number: US-8970156-B2

Title: Path display apparatus considering correction data

Description:
RELATED APPLICATIONS 
     The present application claims priority from Japanese Application No. 2011-289045, filed Dec. 28, 2011, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a path display apparatus. More specifically, the present invention relates to a path display apparatus that displays the path of the tip point of a tool taking into account correction data such as backlash correction and pitch error correction. 
     2. Description of Related Arts 
     Numerical control apparatuses are commonly used to drive a plurality of motors, drive a table via a ball screw and position a tool in a desired position. Such a mechanism produces mechanical backlash and variations in the pitch of the ball screw (pitch error). 
     Consequently, to reduce the influence of such backlash and pitch error, correction data such as backlash correction and pitch error correction is generated. When controlling the tip portion of a tool, correction of the tool length and correction of the tool diameter are used as correction data depending on the machining conditions. 
       FIG. 8  is a block diagram relating to the position control of a motor, for explaining the handling of correction data. As illustrated in  FIG. 8 , correction data C that is generated is added to a position command P of the motor, to generate a corrected position command Pc. Then, a position feedback Pf that is detected by a detector D of the motor M is subtracted from the corrected position command Pc and the subtraction result is output to a motor controller, and by this means, the motor M is controlled. 
     As described with reference to  FIG. 8 , it is common that correction data is superimposed on a position command of a motor, which is controlled. For example, Japanese Patent Laid-Open Publication No. 3-58102 discloses improving the accuracy of positioning by changing the amount of backlash correction according to the feeding direction and feeding speed. 
       FIG. 9A  is a diagram illustrating a command path that is calculated from a position command of an end of the motor when backlash correction is added to a position command for a circular arc, and  FIG. 9B  is a diagram illustrating its feedback path. The circular arcs illustrated in these drawings correspond to the position command P of the motor. Furthermore, the command path indicated by the broken lines in  FIG. 9A  corresponds to the corrected position command Pc, and the feedback path indicated by the broken lines in  FIG. 9B  corresponds to the position feedback Pf. In these drawings, since the correction data C (backlash correction) is added to the position command P, the command path ( FIG. 9A ) and the feedback path ( FIG. 9B ) have shapes that deviate from the circular arcs corresponding to the position command P. 
     Furthermore,  FIG. 9C  is a drawing that is similar to  FIG. 9A  and that illustrates a command path calculated from a position command of an end of the motor when pitch error correction is added to the position command for the circular arc, and  FIG. 9D  is a drawing that is similar to  FIG. 9B  and that illustrates its feedback path. Furthermore, the command path illustrated in  FIG. 9C  corresponds to the corrected position command Pc, and the feedback path illustrated in  FIG. 9D  corresponds to the position feedback Pf. In these drawings, since the correction data C (pitch error correction) is added to the position command P, the command path ( FIG. 9C ) and the feedback path ( FIG. 9D ) deviate from the circular arcs corresponding to the position command P. 
     In this way, when the correction data C is superimposed on the position command P of the motor, the tool path to be calculated based on the corrected position command Pc does not match the path corresponding to the position command P. In other words, these paths differ from each other based on the correction data. 
     Compared to a mechanical error, a transient error due to the delay of the servo is significant. Consequently, when checking the response delay of the motor in the actual machine tool to which correction data is applied, or when adjusting the servo, it is necessary to temporarily invalidate the correction data. In other words, the path is checked by making the path calculated from the corrected position command Pc match the path calculated from the position command P of the motor. However, such a technique is very complex and time-consuming for an operator. 
     The present invention has been made in view of the above backgrounds, and it is therefore an object of the present invention to provide a path display apparatus that is capable of checking the response delay of the motor and performing servo adjustment using even a simpler method even when using an actual machine tool to which correction data such as backlash correction and pitch error correction is applied. 
     SUMMARY OF THE INVENTION 
     To achieve the above object, according to a first mode, a path display apparatus that displays the path of the tip point of a tool of a machine tool that controls the position and posture of the tool by a plurality of drive axes is provided, the path display apparatus including: a first position command acquiring unit that acquires first position command, generated by a numerical control apparatus, for a plurality of motors that drive the plurality of drive axes, respectively; a first position feedback acquiring unit that acquires first position feedback of each of the plurality of motors, from a plurality of position detectors that detect the respective positions of the plurality of motors every predetermined control cycle; a correction data acquiring unit that acquires correction data generated for each of the plurality of motors; a second position command calculating unit that subtracts the correction data from the first position command to calculate a second position command; a second position feedback calculating unit that subtracts the correction data from the first position feedback to calculate second position feedback; a command path display unit that displays a command path of the tip point of the tool, based on the second position command calculated by the second position command calculating unit; and a feedback path display unit that displays a feedback path of the tip point of the tool, based on the second position feedback calculated by the second position feedback calculating unit. 
     According to a second mode, based on the first mode, the path display apparatus further includes a second correction data calculating unit that calculates second correction data including control delay, using the correction data acquired by the correction data acquiring unit and a transfer function indicating responses of respective position loops of the plurality of motors, wherein the second position feedback calculating unit calculates second position feedback by subtracting the second correction data from the first position feedback. 
     According to a third mode, based on the second mode, the second correction data calculating unit uses a first-order lag filter corresponding to a position gain, as the transfer function. 
     According to a fourth mode, based on the first mode, the first position command is generated by adding the correction data to a position command read from an operation program of the machine tool. 
     The objects, features, advantages and other objects features, and advantages will become apparent from the detailed description of typical embodiments of the present invention given herein below illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a machine tool connected to a path display apparatus based on the present invention; 
         FIG. 2  is a function block diagram of a path display apparatus based on a first embodiment of the present invention; 
         FIG. 3  is a flowchart illustrating the operation of the path display apparatus according to the first embodiment of the present invention; 
         FIG. 4A  is a diagram illustrating a command path generated based on a second position command; 
         FIG. 4B  is a diagram illustrating a position feedback path generated based on second position feedback; 
         FIG. 5  is a function block diagram of a path display apparatus based on a second embodiment of the present invention; 
         FIG. 6A  is a block diagram relating to position control of a motor; 
         FIG. 6B  is another block diagram relating to position control of a motor; 
         FIG. 7  is a flowchart illustrating the operation of the path display apparatus according to the second embodiment of the present invention; 
         FIG. 8  is a block diagram relating to position control of a motor, for explaining the handling of correction data; 
         FIG. 9A  is a diagram illustrating a command path calculated from a position command of an end of the motor when backlash correction is added to a position command for a circular arc; 
         FIG. 9B  is a diagram illustrating the feedback path in  FIG. 9A ; 
         FIG. 9C  is a diagram illustrating a command path calculated from a position command of an end of the motor when pitch error correction is added to a position command for a circular arc; and 
         FIG. 9D  is a diagram illustrating the feedback path in  FIG. 9C . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following drawings, the same members are assigned the same reference codes. The scale in these drawings has been changed for ease of explanation. 
       FIG. 1  is a perspective view of a machine tool connected to a path display apparatus based on the present invention. The machine tool  1  illustrated as an example in  FIG. 1  is a five-axis machine tool. The machine tool  1  includes a table  2  on which a workpiece (not illustrated) is placed, and a column  3  that moves, relatively, in three directions (X axis, Y axis and Z axis) that are perpendicular to each other, with respect to the table  2 . As illustrated in this drawing, a head  4  extends horizontally from the column  3 , and the head  4  rotates around the B axis, which is parallel to the surface of the table  2 . Furthermore, a tool  5 , which can rotate around an A axis that is perpendicular to both the B axis and the surface of the table  2 , is attached to the head  4 . 
     Consequently, the machine tool  1  controls the position and posture of the tool  5  by means of three linear-motion axes (X axis, Y axis and Z axis) and two rotary axes (A axis and B axis), and processes the workpiece on the table  2 . However, even when the tool  5  is fixed to the table  2  and the workpiece (not illustrated) is attached to the tip of the head  4 , this is within the scope of the present invention. Also, the X axis, Y axis, Z axis, A axis and B axis may also be referred to as “drive axes.” 
       FIG. 2  is a function block diagram of the path display apparatus based on the first embodiment of the present invention. As illustrated in  FIG. 2 , the path display apparatus  20  is connected to the machine tool  1  via a numerical control apparatus  16 . The machine tool  1  includes motors M 1  to M 5  that drive respective drive axes. These motors M 1  to M 5  are provided with position detectors D 1  to D 5 , respectively, to detect the positions of the drive axes every predetermined control cycle. 
     Furthermore, the numerical control apparatus  16  includes a command generation unit  17  that generates a corrected position command Pc with respect to each drive axis, per predetermined control cycle. Furthermore, the numerical control apparatus  16  includes a correction data generation unit  18  that generates correction data C. The correction data C includes backlash correction, pitch error correction, tool length correction or tool diameter correction. The method of calculating correction data C such as these is publicly known, and therefore a description thereof is omitted. 
     As described with reference to  FIG. 8 , the command generation unit  17  reads a position command P from the operation program of the machine tool  1 , and, by adding the correction data C to this, generates a corrected position command Pc. Position feedback Pf detected by the position detector D is subtracted from the corrected position command Pc and supplied to a motor controller. 
     As illustrated in  FIG. 2 , the path display apparatus  20  includes a first position command acquiring unit  21  that acquires the corrected position command Pc of each drive axis generated by the command generation unit  17  as the first position command, a correction data acquiring unit  22  that acquires the correction data C of each drive axis generated by the correction data generation unit  18 , and a first position feedback acquiring unit  23  that acquires a position feedback Pf for each drive axis detected by the position detectors D 1  to D 5  as the first position feedback. 
     Furthermore, as can be seen from  FIG. 2 , the path display apparatus  20  further includes a second position command calculating unit  24  that calculates a second position command Pc′ by subtracting the correction data C acquired by the correction data acquiring unit  22  from the first position command Pc acquired by the first position command acquiring unit  21 , and a second position feedback calculating unit  25  that calculates second position feedback Pf′ by subtracting the correction data C acquired by the correction data acquiring unit  22  from the first position feedback Pf acquired by the first position feedback acquiring unit  23 . 
     Furthermore, the path display apparatus  20  also includes a command path display unit  28  that calculates and displays a command path of the tip of the tool  5  based on the second position command Pc′, and a position feedback path display unit  29  that calculates and displays a position feedback path of the tip of the tool  5  based on the second position feedback Pf′. The command path display unit  28  and the position feedback path display unit  29  are CRTs or liquid crystal monitors or the like, or the same CRT or liquid crystal monitor may be used in common for these. 
     In this connection, the technique of calculating the command path and position feedback path will be described. Referring back to  FIG. 1 , assume that the coordinates of the five drive axes are x(t), y(t), z(t), a(t) and b(t). Assuming that the intersection between the A axis and B axis is M, the coordinates of the intersection M are represented by (x(t), y(t), z(t)). Given that the length from the intersection M to the tip of the tool  5  is L and the position where the tool  5  is pointed straight downward is the reference position (origin) of the A axis and the B axis, the coordinates of the tip of the tool  5  are represented as follows:
 
 Px ( t )= x ( t )+ L ×cos( a ( t ))×sin( b ( t ))
 
 Py ( t )= y ( t )+ L ×sin( a ( t ))
 
 Pz ( t )= z ( t )− L ×cos( a ( t ))×cos( b ( t ))
 
     In this way, the coordinates of the tip of the tool  5  can be calculated from the position information and mechanical structural conditions of the five drive axes. 
       FIG. 3  is a flowchart illustrating the operation of the path display apparatus according to the first embodiment of the present invention. The operation of the path display apparatus  20  according to the first embodiment of the present invention will be described below with reference to  FIG. 2  and  FIG. 3 . 
     First, in step S 10  in  FIG. 3 , time t is set to 0. In step S 11 , the first position command acquiring unit  21  acquires the corrected position commands Pc(t) for the respective motors M 1  to M 5  at time t as the first position commands. In step S 12 , the first position feedback acquiring unit  23  acquires position feedback Pf(t) for the respective motors M 1  to M 5  at time t as the first position feedback. Furthermore, in step S 13 , the correction data acquiring unit  22  acquires correction data C(t) at time t. 
     Then, in step S 14 , the second position command calculating unit  24  subtracts the correction data C(t) from the first position command Pc(t) to generate second position commands Pc′(t) for the respective motors M 1  to M 5  at time (t). Then, in step S 15 , the second position feedback calculating unit  25  subtracts the correction data C(t) from the first position feedback Pf(t) to calculate second position feedback Pf′(t) of the respective motors M 1  to M 5  at time (t). By this means, the second position commands Pc′(t) and the second position feedback Pf′(t) do not include the correction data C(t). 
     Then, in step S 16 , the command path display unit  28  displays the command path of the tip point of the tool  5  at time t based on the second position commands Pc′(t). Furthermore, in step S 17 , the position feedback path display unit  29  displays the position feedback path of the tip point of the tool  5  at time t based on the second position feedback Pf′(t). 
     In step S 18 , a comparison is made between the current time t and a predetermined end time, and, when time t is equal to or later than the end time, the process is finished. On the contrary, when time t is not equal to or later than the end time, in step S 19 , a predetermined minute time Δt corresponding to the control cycle is added to time t, and the step returns to step S 11 . Then, the processes of step S 11  to step S 19  are repeated until time t reaches or exceeds the end time. 
       FIG. 4A  is a diagram illustrating a command path that is generated based on the second position command Pc′, and  FIG. 4B  is a diagram illustrating a position feedback path that is generated based on the second position feedback Pf′. These drawings illustrate machining errors when cutting a workpiece along a path of a circular-arc and assume the center of the circular-arc path as the origin. Furthermore, the solid lines in these drawings represent the position commands P determined in accordance with the operation program, and the broken lines represent second position commands Pc′ and second position feedback Pf′, respectively. In  FIG. 4A , the position command P is substantially superimposed on the second position command Pc′. 
     In the present invention, correction data C is subtracted from the first position command Pc and from the first position feedback Pf, so that it is possible to eliminate the influence of the correction data C. Therefore, as can be seen from  FIG. 4A  and  FIG. 4B , the second position command Pc′ and second position feedback Pf′ can be easily compared with the position command P. 
     Therefore, the present invention allows the response delay of the motor to be checked very easily even in the case of the actual machine tool  1  to which the correction data C such as backlash correction and pitch error correction is applied. Moreover, since it is not necessary to invalidate the correction data C temporarily, it is possible to appropriately adjust the servo for the actual machine tool to which correction data is already applied, using a simpler method. 
       FIG. 5  is a function block diagram of a path display apparatus based on a second embodiment of the present invention. In  FIG. 5 , a first correction data acquiring unit  22  has a function similar to that of the correction data acquiring unit  22  illustrated in  FIG. 2 . 
     Furthermore, a second correction data calculating unit  27  illustrated in  FIG. 5  calculates second correction data C′ based on the correction data C acquired by the first correction data acquiring unit  22  and a transfer function  26 . Then, a second position command calculating unit  24  subtracts the first correction data C from a first position command Pc to generate a second position command Pc′, and a second position feedback calculating unit  25  subtracts the second correction data C′ from the first position feedback Pf to generate second position feedback Pf′. Furthermore, other members similar to those in  FIG. 2  have functions similar to those described above, and therefore their detailed descriptions will be omitted. 
     In this connection, an output in response to an input, in control, normally has a certain degree of delay. In other words, a response of the position feedback Pf(t) in response to the correction data C(t) also has a delay. In the first embodiment, when the correction data C(t) is directly subtracted from the position feedback Pf(t), this means that a delay in control is not taken into account. Therefore, in such a case, the position feedback Pf(t) decreases by an amount corresponding to the control delay. Consequently, it is preferable to calculate the transfer function of the position loop illustrated in  FIG. 8  in advance and calculate the second correction data C′(t) based on the transfer function. 
       FIG. 6A  is a block diagram relating to position control of the motor. In  FIG. 6A , G(s) represents a transfer function of a motor controller including position control, speed control and current control. M(s) represents the transfer function of the motor. 
     Then, assume that the output when the correction data C(t) is input to the position control loop is second correction data C′(t). Assume that the Laplace transforms of the correction data C(t) and second correction data C′(t) are C(s) and C′(s), respectively (see  FIG. 6A ). C′(s) is represented by following equation 1. Then, by applying the inverse Laplace transform to equation 1, second correction data C′(t) is obtained as represented in equation 2. 
     
       
         
           
             
               
                 
                   
                     
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     In addition,  FIG. 6B  is another block diagram relating to position control of the motor. Assuming that the transfer function to represent the response of the speed loop is 1, the block diagram of the position loop using a position gain PG is as illustrated in  FIG. 6B . The transfer function in  FIG. 6B  is a first-order lag system as represented in following equation 3. 
     
       
         
           
             
               
                 
                   
                     
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     For the transfer function  26  illustrated in  FIG. 5 , assume that the transfer function represented by equation 1 or equation 3 is employed. A first-order lag filter corresponding to the position gain may also be employed as the transfer function  26 . In this case, it is apparent that the control delay can be handled with a relatively simple configuration. 
       FIG. 7  is a flowchart illustrating the operation of the path display apparatus according to the second embodiment of the present invention. Hereinafter, the operation of a path display apparatus  20  according to the second embodiment of the present invention will be described with reference to  FIG. 5  to  FIG. 7 . 
     Since step S 20  to step S 23  illustrated in  FIG. 7  mostly correspond to step S 10  to step S 13  illustrated in  FIG. 3 , their descriptions will be omitted. However, in step S 23 , assume that the first correction data acquiring unit  22  acquires correction data C from the correction data generation unit  18 . 
     Then, in step S 24 , the second correction data calculating unit  27  multiplies the correction data C(t) of the motors M 1  to M 5  at time t by the transfer function  26  to calculate second correction data C′(t). As described with reference to equation 1 to equation 3, the calculated second correction data C′(t) includes control delay. 
     Then, in step S 25 , the second position command calculating unit  24  subtracts the first correction data C(t) from the first position command Pc(t) to generate second position command Pc′(t) of the respective motors M 1  to M 5  at time (t). Then, in step S 26 , the second position feedback calculating unit  25  subtracts the second correction data C′(t) from the first position feedback Pf(t) to calculate second position feedback Pf′(t) of the respective motors M 1  to M 5  at time (t). By this means, the second position command Pc′(t) does not include the first correction data C(t) and the second position feedback Pf′(t) does not include the second correction data C′(t). 
     After that, in step S 27 , the command path display unit  28  displays the command path of the tip point of the tool  5  at time t based on the second position command Pc′(t). Furthermore, in step S 28 , the position feedback path display unit  29  displays the position feedback path of the tip point of the tool  5  at time t based on the second position feedback Pf′(t). 
     Step S 29  and step S 30  are similar to aforementioned step S 18  and step S 19 , and therefore descriptions thereof are omitted. In short, with the second embodiment, assume that the processes of step S 21  to step S 30  are also repeated until time t reaches or exceeds the end time. Since the path display of the command path display unit  28  and the position feedback path display unit  29  with the second embodiment is similar to that illustrated in  FIG. 4 , descriptions will be omitted. 
     Thus, with the second embodiment, the second correction data C′(t) taking into account the control delay is subtracted from the second position feedback Pf′(t). In other words, since the control delay is taken into account in the second embodiment, it is apparent that the second embodiment can check a response delay of the motor more accurately or perform servo adjustment more accurately compared to the first embodiment. 
     Effects of the Invention 
     According to the first mode, since correction data is subtracted from the first position command (corrected position command) and the first position feedback, it is possible to remove the influence of backlash correction or pitch error correction, and calculate and display path data that can be directly compared with a path that is defined from the initial position command. 
     Therefore, even with an actual machine tool to which correction data such as backlash correction or pitch error correction is applied, it is possible to check the response delay of the motor or perform servo adjustment using a simpler method. 
     According to the second mode, since control delay is taken into account, it is possible to check the response delay of the motor or perform servo adjustment more accurately than the first mode of the invention. 
     With the third mode, it is possible to handle control delay with a relatively simple configuration. 
     With the fourth mode, the first position command is clarified. 
     Although the present invention has been described using typical embodiments, a person skilled in the art should understand that the above-described changes, and various other changes, omissions, and additions are possible without departing from the scope of the present invention.