Patent Publication Number: US-10760994-B2

Title: Abnormality diagnostic method for feed axis and abnormality diagnostic device for the same

Description:
BACKGROUND OF THE INVENTION 
     This application is the National Stage of International Application No. PCT/JP2017/026040, filed Jul. 19, 2017, which claims the benefit of Japanese Application No. 2016-154748 filed Aug. 5, 2016, the entirety of which is incorporated herein by reference. 
     Field of the Invention 
     The disclosure relates to a method and a device that diagnose an abnormality of a feed axis, in a machine such as a machine tool including the feed axis by a ball screw. 
     Background Art 
     In a feed axis of a machine tool, a system that transmits rotational motion of a motor to a ball screw for a linear drive is often used. However, in a machine operated for several years, an accuracy failure, an abnormal noise, and the like possibly occur due to a preload loss by abrasion and a damage by entrance of foreign matter, a lubrication failure, or the like. In such a state, a trouble, such as a shape defect, a failure in a pick feed direction, and the like of a workpiece, occurs. Accordingly, it is preferable that machine components such as the ball screw, a bearing, and a linear guide that constitute the feed axis are replaced before deterioration or a damage occurs to generate the trouble. 
     To know a state of the machine component, it has been proposed that various diagnostic methods such as a method that detects and diagnoses vibration of the ball screw, the bearing, and the linear guide by a vibration sensor, and a method that measures positioning accuracy with a displacement sensor internally disposed. However, in these methods, it is necessary to additionally add a sensor near a site desired to be diagnosed, thus causing a cost increase. Since parts that possibly break down increase, there is also a problem to lead to increase of a breakdown risk. 
     Therefore, a method that performs the diagnosis using servo information used for control without an additional sensor for diagnosis has been proposed. As a method that determines an abnormality of a reducer, Japanese Patent No. 4112594 proposes a method that performs a frequency analysis on an estimated disturbance value and a torque command in the control to compare spectra in the frequency corresponding to an integral multiple of a rotation frequency of an axis. Japanese Laid-Open Patent Publication No. 2009-68950 proposes a method that, in a machine driven by a motor, performs Fourier transformation on a torque command value, obtains and indicates a spectrum, and focuses on the number of rotations when the motor is rotating and a spectrum in a higher mode caused by the number of rotations to confirm a damage of the machine. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In the feed axis by the ball screw, there are a plurality of driving parts such as the ball screw, the bearing, and the linear guide, and there are also a wide range of states of the breakdown such as the abrasion and the damage. The aforementioned diagnostic method focuses on the motor rotation or the spectrum in the higher mode of the motor rotation to detect the degradation in deflection in the rotation. That is, the degradation in the deflection in the rotation occurs caused by the abrasion of the ball screw and the bearing, or a severe damage roughly equivalent to an abrasion state where most of a transferring surface is damaged, in the machine in a normal state where there is no assembly failure and no accuracy failure of the parts. However, in consideration of processing accuracy required for a machine tool, to perform preventive maintenance, it is preferred to ensure detection of a mild damage before the degradation in the deflection in the rotation. 
     Therefore, the disclosure has been made in view of such problems, and provides a method and a device that perform an abnormality diagnosis of a feed axis such as a bearing, a ball screw, and a linear guide, without additionally adding a sensor or the like. 
     Solutions to the Problems 
     In order to achieve the above-described object, one aspect of the disclosure is an abnormality diagnostic method for feed axis, in a machine including a feed axis that moves a moving body via a ball screw that rotates by a servo motor. The method performs a frequency obtaining step of obtaining a frequency characteristic of the feed axis and a damage frequency that occurs when the feed axis that has been damaged performs an axis operation, a feed velocity calculating step of calculating a feed velocity configured to detect a peak of the damage frequency, from the obtained frequency characteristic, an axis operation step of performing the axis operation on the feed axis with the calculated feed velocity, a frequency analysis step of performing a frequency analysis on servo information regarding a control of the servo motor during the axis operation, and a determining step of confirming the presence and absence of the peak of the damage frequency from a result of the frequency analysis to determine the abnormality when the peak is present. 
     It is preferable that in the feed velocity calculating step, as the feed velocity, a feed velocity is calculated such that a frequency where a gain is maximized in the obtained frequency characteristic matches the damage frequency. 
     It is preferable that in the feed velocity calculating step, as the feed velocity, a feed velocity is calculated such that a maximum value of the damage frequency is included in a frequency band where a gain becomes a certain value or more in the obtained frequency characteristic. 
     It is preferable that in the feed velocity calculating step, a feed velocity that has a non-integral multiple relationship with the calculated feed velocity is additionally calculated, in the axis operation step, the axis operation is performed on the feed axis with a plurality of the feed velocities, in the frequency analysis step, the frequency analysis is performed on the servo information obtained for the respective feed velocities, and in the determining step, the presence and absence of the peak of the damage frequency is confirmed from a result of the frequency analysis for the respective feed velocities. 
     It is preferable that an indicating step that, based on the result of the frequency analysis obtained in the frequency analysis step, creates two kinds of graphs, one of which indicates a frequency and the other indicates a frequency ratio with respect to a rotation frequency of the feed axis, and collectively indicates the result of the frequency analysis for the respective feed velocities is additionally performed. 
     It is preferable that the servo information on which the frequency analysis is performed in the frequency analysis step is a torque waveform. 
     It is preferable that the servo information on which the frequency analysis is performed in the frequency analysis step is a position deviation between a position command to the feed axis and a current position of the servo motor when the damage frequency is less than a predetermined value, and is a torque waveform when the damage frequency is the predetermined value or more. 
     In order to achieve the above-described object, another aspect of the disclosure is an abnormality diagnostic device for feed axis, in a machine including a feed axis that moves a moving body via a ball screw that rotates by a servo motor. The device includes a storing means that stores a frequency characteristic of the feed axis and a damage frequency that occurs when the feed axis that has been damaged performs an axis operation, a feed velocity calculating means that calculates a feed velocity configured to detect a peak of the damage frequency from the stored frequency characteristic, an axis operation performing means that performs the axis operation on the feed axis with the calculated feed velocity, a frequency analysis means that performs a frequency analysis on servo information regarding a control of the servo motor during the axis operation, and a determining means that confirms the presence and absence of the peak of the damage frequency from a result of the frequency analysis to determine the abnormality when the peak is present. 
     Effects of the Invention 
     With the disclosure, without additionally adding the sensor or the like, the abnormality diagnosis for the feed axis such as the bearing, the ball screw, and the linear guide is allowed at low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a position control unit and an abnormality diagnostic device for a feed axis. 
         FIG. 2  is a flowchart of an abnormality diagnostic method in an embodiment 1. 
         FIG. 3  is a view illustrating an exemplary frequency response characteristic of the feed axis. 
         FIG. 4  is a flowchart of an abnormality diagnostic method in an embodiment 2. 
         FIG. 5  is a view illustrating a damage frequency of a bearing. 
         FIG. 6  is a view illustrating an FFT analysis result of a torque waveform when a feed velocity Fg is 7500 [mm/min]. 
         FIG. 7  is a view illustrating the FFT analysis result of a torque waveform when the feed velocity Fg is 20000 [mm/min]. 
         FIGS. 8A and 8B  are views illustrating an exemplary result indication when a diagnosis is performed with a plurality of feed velocities,  FIG. 8A  indicates a frequency, and  FIG. 8B  indicates a frequency ratio with respect to a rotation frequency. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following describes embodiments of the disclosure based on the drawings. 
     First, an embodiment 1 is described below. 
       FIG. 1  is a block diagram illustrating exemplary position control unit and abnormality diagnostic device for a feed axis in a machine tool to which the disclosure is applied. 
     A feed axis  1  in the machine tool includes a ball screw  2  and a moving body  5 . 
     The ball screw  2  is rotatably driven by a servo motor  3  by receiving a position command from a feed axis controller  21  of an abnormality diagnostic device  20  formed in an NC device. The moving body  5  is screwed with the ball screw  2  via a nut  4  to perform a screw feeding movement to an axial direction by the rotation of the ball screw  2 . The ball screw  2  has both ends pivotally supported by bearings (not illustrated). Linear movement of the moving body  5  is guided by a linear guide (not illustrated). A position detector  6  is mounted on the servo motor  3 . 
     A position control unit  10  includes an adder  11 , a position controller  12 , a differentiator  13 , a velocity controller  14 , and a current controller  15 . The adder  11  calculates a position deviation such that a position command from the feed axis controller  21  and a current position from the position detector  6  are input to the adder  11 . The position controller  12  generates a speed command value corresponding to the position deviation calculated in the adder  11 . The velocity controller  14  generates a torque command value corresponding to the speed command value generated in the position controller  12  and a speed detection value obtained such that the current position from the position detector  6  is calculated in the differentiator  13 . The current controller  15  controls current to the servo motor  3  based on the torque command value input from the velocity controller  14 . Information used in this position control unit  10 , such as the current position detected in the position detector  6 , is allowed to be recorded in a storage unit  22  in the abnormality diagnostic device  20 , and displayed on a display unit  23 . 
     The abnormality diagnostic device  20  includes a feed velocity calculator  24  which is a feed velocity calculating means that calculates a feed velocity for abnormality diagnosis based on a frequency characteristic and a damage frequency stored in the storage unit  22  as a storing means. The abnormality diagnostic device  20  also includes a frequency analyzer  25  which is a frequency analysis means that performs frequency analysis on the servo information regarding the control of the servo motor  3  obtained from the position control unit  10 , when the feed axis controller  21  as an axis operation performing means performs axis operation on the feed axis  1  with the feed velocity calculated in the feed velocity calculator  24 . Further, the abnormality diagnostic device  20  includes a determining unit  26  is a determining means that determines presence/absence of abnormality from an analysis result in the frequency analyzer  25 . 
     When the abnormality diagnostic device  20  receives a command of a diagnostic mode from an operator, the abnormality diagnostic device  20  calculates the feed velocity considering a frequency response characteristic of the feed axis  1 , and performs the frequency analysis on the servo information by performing the axis operation on the feed axis  1  with this feed velocity to perform the abnormality diagnosis. The following describes this abnormality diagnostic method based on a flowchart in  FIG. 2 . 
     First, the frequency characteristic of the feed axis  1  and the damage frequency of a diagnostic target are obtained to be stored in the storage unit  22  at S 1  (a frequency obtaining step). As a method to know the frequency characteristic of the feed axis  1 , for example, there is a sweep test that performs velocity input of a sine wave whose frequency continuously varies to confirm an input/output response. However, the frequency characteristic and the damage frequency may be preliminarily stored in the storage unit  22 , or may be obtained just before a diagnostic operation. 
       FIG. 3  illustrates an exemplary obtained frequency characteristic of the feed axis  1 . The horizontal axis indicates a frequency, and the vertical axis indicates a gain with respect to the input. 
     A frequency of vibration (the damage frequency) that occurs during damage of the machine component such as the bearing, the ball screw, and the linear guide is calculated by a geometrical formula. For example, in the damage of the bearing, the frequency of vibration (the damage frequency) is calculated by following expressions (1) to (3) for each of an inner race (the damage on a race face), an outer race (the damage on the race face), and a rolling element (the damage on a surface). The damage of the ball screw and the damage of the nut can be obtained from a formula similar to that of the bearing, and the damage of the linear guide can be obtained from a pitch of the rolling element. 
     
       
         
           
             
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         f in : frequency by the inner-race damage [Hz], f out : frequency by the outer-race damage [Hz], 
         f ball : frequency by the rolling-element damage [Hz], 
         f r : rotation frequency [Hz], D: bearing pitch diameter [mm], d: diameter of rolling element [mm], 
         Z: the number of rolling element, a: contact angle [deg.] 
       
    
     Next, in the feed velocity calculator  24 , a frequency fg where the gain is maximized is calculated from the result of the frequency characteristic at S 2 , and then, a feed velocity Fg where each damage frequency matches the frequency fg where this gain is maximized is calculated at S 3  (S 2  and S 3 : a feed velocity calculating step). This feed velocity Fg can be calculated by the rotation frequency fr of the feed axis used in the above-described expression×a lead of the ball screw×60. 
     Taking the frequency characteristic in  FIG. 3  as an example, the frequency fg where the gain is maximized is 142 Hz, and the feed velocity Fg that diagnoses the damage of the bearing is as follows. 
     Inner Race: 10650 [mm/min] 
     Outer Race: 12241 [mm/min] 
     Rolling Element: 11451 [mm/min] 
     When the frequency characteristic is preliminarily obtained, the feed velocity Fg may be also preliminarily calculated to be stored in the storage unit  22 . 
     Next, the feed axis controller  21  performs the operation of the feed axis  1  with each feed velocity Fg at S 4  (an axis operation step), and the frequency analyzer  25  performs the frequency analysis on the servo information (here, a torque waveform) during the axis operation at S 5  (a frequency analysis step). 
     Then, the determining unit  26  determines whether a peak of the damage frequency is present or not from the frequency analysis result at S 6  (a determining step). 
     For presence/absence of this peak, for example, when an absolute value of the damage frequency exceeds a threshold value preliminarily set, it is determined that the peak is present. It can be also determined that the peak is present when the frequency analysis result in a normal state without any damage in the bearing is preliminarily obtained to compare it with this frequency analysis result in the normal period and the difference from the frequency analysis result in the normal state exceeds the threshold value preliminarily set. Since there are different levels of influence in processing depending on damage parts, when these threshold values are set per damage part, damage detect can be more accurately performed. 
     Thus, when the peak of the damage frequency is confirmed at S 6 , it is determined that it is abnormal (the damage is present) at S 7 , and when the peak of the damage frequency is not confirmed, it is determined that it is normal (the damage is not present) at S 8 , thus displaying the determination result on the display unit  23 . 
     Thus, with the abnormality diagnostic method and device of the feed axis  1  in the above-described embodiment 1, the frequency characteristic and the damage frequency of the feed axis  1  are obtained, the feed velocity Fg that can detect the peak of the damage frequency from the obtained frequency characteristic is calculated, the axis operation is performed on the feed axis  1  with the calculated feed velocity Fg, the frequency analysis is performed on the servo information regarding the control of the servo motor  3  during the axis operation, the presence/absence of the peak of the damage frequency is confirmed from the frequency analysis result, and when the peak is present, it is determined that it is abnormal. Thus, the damage determination of the feed axis such as the bearing, the ball screw, and the linear guide is allowed at low cost without additionally adding the sensor or the like. 
     In particular, here, since the feed velocity where the frequency where the gain is maximized in the obtained frequency characteristic matches the damage frequency is used as the feed velocity Fg, the presence/absence of the peak of the damage frequency can be clearly detected. 
     In the above-described embodiment 1, the damage regarding the bearing is mainly described as the example. However, detectable damage is wide-ranging, for example, in the ball screw, the nut, and the linear guide. Since a kind of the feed velocity with which the diagnosis is performed increases, a diagnostic period may be relatively longer. The following is an embodiment 2 that decreases the relatively longer diagnostic period. However, since the configurations of the position control unit  10  and the abnormality diagnostic device  20  for the feed axis  1  themselves are similar to those in the embodiment 1, the overlapping description will be omitted. The abnormality diagnostic method by the abnormality diagnostic device  20  will be described based on a flowchart in  FIG. 4 . 
     First, the frequency characteristic of the feed axis  1  and the damage frequency of the diagnostic target are obtained to be stored in the storage unit  22  at S 11  (a frequency obtaining step). This is similar to that in the embodiment 1. 
     Next, the feed velocity calculator  24  calculates a frequency band where the gain becomes a certain value or more from the obtained frequency characteristic at S 12 . Here, in the frequency characteristic in  FIG. 3 , the gain is set to −5 dB or more, and 5 to 200 Hz is considered as a frequency band A having a good responsiveness. 
     Next, based on specifications of the bearing, the feed velocity calculator  24  calculates the feed velocity Fg where the maximum value of the damage frequencies at a plurality of positions desired to be diagnosed is included in the frequency band A at S 13  (S 12  and S 13 : a feed velocity calculating step). For example, it is considered that the feed velocity is calculated similarly to that in the embodiment 1 for each of the inner race, the outer race, and the rolling element, and the feed velocity Fg is selected from a range of the feed velocity specified by the three feed velocities. 
     For example,  FIG. 5  illustrates the damage frequencies of the bearing when the feed velocity Fg is 7500 [mm/min] and when the feed velocity Fg is 20000 [mm/min], and  FIG. 6  and  FIG. 7  illustrate graphs where the frequency analysis is performed on the torque waveform when the feed velocity Fg is 7500 [mm/min] and when the feed velocity Fg is 20000 [mm/min]. 
     In this feed axis, scratches are present on the inner race and the outer race of the bearing. When the feed velocity Fg is 7500 [mm/min], the peaks are present at 87 Hz and 100 Hz, and the damage on the inner and outer races of the incorporated bearing can be detected. However, when the feed velocity Fg is 20000 [mm/min], since the damage frequencies of the inner and outer races of the bearing are 231 Hz and 268 Hz, and exceed 200 Hz that is an upper limit of the frequency band having the good responsiveness, it is known that the peak is not confirmed. 
     Therefore, the feed velocity Fg calculated here is 7500 [mm/min]. 
     However, even in the normal feed axis, by influence of vibration in the operation of the machine, vibration of disturbance, and the like, as a result of the frequency analysis of the torque waveform, the peak is sometimes present. There is also a case where change of peak values in the normal period and the diagnostic period becomes hard to know when this peak overlaps the damage frequency. 
     To deal with such a case to enhance detection accuracy of the abnormality, it is only necessary that the feed velocity calculator  24  calculates a plurality of feed velocities Fg where a relationship between the respective feed velocities will be a non-integral multiple, in addition to the feed velocity Fg with 7500 [mm/min] calculated at S 13 . For example, in a case based on Fg with 7500 [mm/min], in addition to Fg with 7500 [mm/min], 4500 [mm/min] that will be 0.6 times of this, and 10500 [mm/min] that will be 1.4 times of this are set as the feed velocities Fg. Since the vibration by the machine operation and the vibration of the disturbance do not vary by the feed velocity, state grasping separated from the damage of the machine components that varies in a proportional relationship to the feed velocity can be easily performed by performing the diagnosis with the plurality of feed velocities that will be the non-integral multiple. 
     The subsequent process is similar to that in the embodiment 1, the feed axis controller  21  performs the operation of the feed axis  1  with the set plurality of feed velocities Fg at S 14  (an axis operation step), and the frequency analyzer  25  performs the frequency analysis on the servo information (the torque waveform) during the axis operation at S 15  (a frequency obtaining step). 
     Then, the determining unit  26  determines whether the peak of the damage frequency is present or not from the respective frequency analysis results at S 16  (a determining step). Here, when the peak of the damage frequency is confirmed, it is determined that it is abnormal (the damage is present) at S 17 , and when the peak of the damage frequency is not confirmed, it is determined that it is normal (the damage is not present) at S 18 , thus displaying the determination result on the display unit  23 . 
     Here, a display method on the display unit  23  will be described.  FIGS. 8A and 8B  illustrate a display example of the result when the diagnosis has been performed with the plurality of feed velocities. In  FIG. 8A , the horizontal axis is indicated as the frequency, and in  FIG. 8B , the horizontal axis is indicated as a frequency ratio with respect to the rotation frequency of the feed axis. The rotation frequency is calculated by the feed velocity/the lead of the ball screw/ 60 , the frequency ratio is calculated by the frequency/the rotation frequency. Both vertical axes show strengths after the frequency analysis, and are offset in a vertical direction so that the plurality of feed velocities are indicated in one graph for convenience. 
     A marker M 1  is an influence by the inner-race damage of the bearing, and while a peak position varies depending on the feed velocity in the graph indicating the frequency in  FIG. 8A , there is no variation in the peak position depending on the feed velocity in the graph indicating the frequency ratio with respect to the rotation frequency in  FIG. 8B . 
     A marker M 2  is an influence of the vibration by the axis operation, and while there is no variation in the peak position depending on the feed velocity in the graph indicating the frequency in  FIG. 8A , the peak position varies depending on the feed velocity in the graph indicating the frequency ratio with respect to the rotation frequency in  FIG. 8B . 
     Thus, after the determining step is performed, the graph indicating the frequency in the horizontal axis and the graph indicating the frequency ratio with respect to the rotation frequency are arranged to be collectively indicated (an indicating step). Then, it can be discriminated whether it is an influence of the damage by the machine component or not, thus facilitating the grasping of the state. Markers corresponding to the damages of the machine components such as the bearing, the ball screw, and the linear guide may be indicated. 
     Thus, also in the abnormality diagnostic method and device for the feed axis  1  in the above-described embodiment 2, the frequency characteristic of the feed axis  1  and the damage frequency are obtained, the feed velocity Fg that can detect the peak of the damage frequency is calculated from the obtained frequency characteristic, the axis operation is performed on the feed axis  1  with the calculated feed velocity Fg, the frequency analysis is performed on the servo information regarding the control of the servo motor  3  during the axis operation, the presence/absence of the peak of the damage frequency is confirmed from the frequency analysis result, and when the peak is present, it is determined that it is abnormal. Thus, the damage determination of the feed axis such as the bearing, the ball screw, and the linear guide is allowed at low cost without additionally adding the sensor or the like. 
     In particular, here, since the feed velocity where the maximum value of the damage frequency is included in the frequency band A where the gain becomes the certain value or more in the obtained frequency characteristic is used as the feed velocity Fg, the presence/absence of the peak of the damage frequency can be detected in a short time even when there are a plurality of positions required to be diagnosed. 
     In the above-described embodiments 1 and 2, the frequency analysis is performed on the torque waveform as the servo information. The similar method can be performed also on the position deviation and the velocity calculated based on displacement information of the position detector. However, the frequency easily detected in the displacement information becomes a frequency lower than that detected in the torque waveform. Therefore, the detection from the torque waveform and the detection from the position deviation are differently used in accordance with the damage frequency required to be detected, for example, such that the frequency is detected from the position deviation when the damage frequency is less than 20 Hz, and the frequency is detected from the torque waveform when the damage frequency is 20 Hz or more. In this way, accuracy in the damage detection can be increased. The predetermined values of these differently used frequencies can be changed as necessary. 
     In the above-described embodiments 1 and 2, the abnormality diagnostic device is formed in the machine tool to perform the abnormality diagnosis. However, the abnormality diagnosis may be performed such that the frequency characteristic of the feed axis and the damage frequency are stored in an external device such as an outside PC, the servo information during the axis operation is transmitted to the external device by wire or without wires, and the frequency analysis is performed in the external device. That is, the abnormality diagnostic device in the disclosure can be constituted including the machine tool having the feed axis as the diagnostic target and the external device. Thus, when the abnormality diagnostic device is constituted using the external device, there are advantages that the abnormality diagnoses for a plurality of machine tools can be simultaneously performed, and diagnosis data can be also centrally controlled. 
     Then, the disclosure is applicable to machines insofar as they include the feed axis, not limited to the machine tool. 
     It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate entity for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular as limits of value ranges.