Failure diagnosis device and failure diagnosis method

A failure diagnosis device for a mechanical device provided with a motor as a source to drive a motion axis, and configured to acquire a moving position of the motion axis and a disturbance torque value applied to the motion axis every predetermined period, and to diagnose that a failure is occurring when the disturbance torque value is larger than a failure determination threshold, includes a maintenance effect determination unit configured to calculate a change in the disturbance torque value before and after conducting of a maintenance task when the maintenance task is conducted on the motion axis, and a failure diagnosis unit configured to re-set the failure determination threshold only when the change in the disturbance torque value is larger than a predetermined threshold.

BACKGROUND

Technical Field

The present invention relates to a failure diagnosis device applicable to a mechanical device provided with a motor as a source to drive a motion axis, and to a method thereof.

Related Art

Patent Literature 1 has been disclosed as a conventional failure diagnosis method applicable to an articulated industrial robot. In the failure diagnosis method disclosed in Patent Literature 1, a moving position of a joint axis of a robot and disturbance torque applied to the joint axis are detected every predetermined period while the robot is in operation, and an average value of the disturbance torque is obtained for each detected moving position. Then, the average value is compared with a set threshold and the robot is diagnosed as having an abnormality or a failure when the average value exceeds the set threshold. As described above, the conventional technique has been designed to diagnose a failure based on determination as to whether or not the disturbance torque exceeds the certain set threshold. Thus, an abnormality in a robot drive system used to be detected irrespective of a motion posture of the robot or a weight of a workpiece or the like to be gripped with a robot hand.

Patent Literature 1: Japanese Patent Application Publication No. H 9-174482

SUMMARY OF INVENTION

However, if a maintenance task is conducted in such a way as to alter a grease viscosity by changing a grease in each motion axis, there may be a case where a disturbance torque value varies due to an effect of the maintenance task. In this case, continued operation of failure diagnoses by using the certain set threshold without eliminating the effect of the maintenance task may lead to frequent occurrence of diagnoses as being abnormal in spite of being normal as a matter of fact, thus causing deterioration in failure diagnosis accuracy.

In the meantime, it is also true that the disturbance toque is not always affected by every maintenance task. In this respect, a mistake may occur in detecting a failure when the set threshold is changed every time the maintenance task is conducted.

One or more embodiments of the present invention provides a failure diagnosis device and a method thereof, which are capable of improving failure diagnosis accuracy by eliminating an effect of a maintenance task only when the conducted maintenance task has the effect on disturbance torque.

In a failure diagnosis device and a method thereof according to one or more embodiments of the present invention, a change in disturbance torque value before and after conduct of a maintenance task is calculated when the maintenance task is conducted on a motion axis. Then, a failure determination threshold is re-set only when the change in disturbance torque value is larger than a predetermined threshold.

DETAILED DESCRIPTION

[Configuration of Failure Diagnosis System]

FIG. 1is a block diagram showing a configuration of a failure diagnosis system including a failure diagnosis device according to one or more embodiments of the present invention. As shown inFIG. 1, a failure diagnosis system100of one or more embodiments of the present invention is formed from a robot1, a robot control device2, a failure diagnosis device3, and a production management device4. As an example of a mechanical device, the robot1is a robot of a multi-axis-machine teaching-play back type and also of an articulated type. However, the robot1may be a single-axis machine instead of being the multi-axis machine.

Although the robot1includes multiple motor drive systems serving as joint axes that are motion axes,FIG. 1illustrates a motor drive system just for one axis. A robot arm5is driven by a servo motor (hereinafter simply referred to as a motor)6through a decelerator8. A pulse coder (a pulse generator or an encoder)7being a detector for a rotation angle position and a velocity is attached to the motor6.

The robot control device2includes an operation integrated control unit9, a communication unit10, a servo control unit11, and a servo amplifier unit14. The servo control unit11includes a disturbance torque calculation unit12and a status data acquisition unit13, and drives the motor6through the servo amplifier unit14by receiving an instruction from the host operation integrated control unit9. The pulse coder7attached to the motor6forms a feedback loop in conjunction with the servo control unit11in order for control processing of the rotation angle position and the velocity of the motor6.

In addition to the disturbance torque calculation unit12and the status data acquisition unit13, the servo control unit11includes a processor which performs processing for controlling the rotation angle position, the velocity, and a current of the motor6, a ROM which stores a control program, and a non-volatile storage unit which stores set values and various parameters. Moreover, the servo control unit11includes a RAM which temporarily stores data in the course of calculation processing, a register for detecting an absolute rotation angle position of the motor6by counting position feedback pulses from the pulse coder7, and the like.

Incidentally, the robot1includes multiple joints and therefore requires as many motor drive systems as illustrated inFIG. 1as the number of joints. Nonetheless,FIG. 1illustrates the motor drive system just for one axis and illustration of the rest of the motor drive systems is omitted therein. In the meantime, a speed change gear train may be interposed between the motor6and the decelerator8inFIG. 1as appropriate.

The operation integrated control unit9is ranked higher than the servo control unit11and governs direct control of operations of the robot1. The communication unit10transfers necessary data to and from a communication unit15in the failure diagnosis device3to be described later through a LAN, for example. Meanwhile, the status data acquisition unit13has a function to regularly collect various types of data concerning operating statuses of the respective joint axes of the robot1. The collected data include data indicating a collection period. The disturbance torque calculation unit12has a function to calculate a disturbance torque value based on the data acquired by the status data acquisition unit13. Since the servo control unit11is designed to include the disturbance torque calculation unit12and the status data acquisition unit13, the disturbance torque value obtained by the calculation of the disturbance torque calculation unit12is outputted to the failure diagnosis device3through the communication unit10. According to this configuration, the servo control unit11takes the form of so-called software servo.

The failure diagnosis device3includes the communication unit15, a disturbance torque selection unit16, a disturbance torque database17, a failure diagnosis unit18, and a maintenance record database19. Here, the failure diagnosis device3is formed of a general-purpose electronic circuit inclusive of a microcomputer, a microprocessor, and a CPU, and of a peripheral device such as a memory. Accordingly, the failure diagnosis device3operates as the communication unit15, the disturbance torque selection unit16, and the failure diagnosis unit18by executing specific programs.

The communication unit15has a function to transfer the necessary data to and from the communication unit10and20in the aforementioned robot control device2and the production management device4through the LAN, for example. The disturbance torque selection unit16has functions to acquire necessary production information from the production management device4and to select a value to be stored out of the disturbance torque values collected depending on the operational status of the robot1. Meanwhile, the disturbance torque database17has a function to sequentially store the disturbance torque values selected by the disturbance torque selection unit16. As a consequence, the disturbance torque database17accumulates previous disturbance torque values.

The maintenance record database19has a function to store maintenance records on the respective joint axes when maintenance tasks are conducted on the robot1. As a consequence, the maintenance record database19accumulates previous maintenance record data.

The failure diagnosis unit18has a function to execute a failure diagnosis actively based on the disturbance torque values. The failure diagnosis unit18is equipped with a memory function. Hence, the failure diagnosis unit18temporarily stores data acquired by accessing the disturbance torque database17and the maintenance record database19, respectively, and executes the failure diagnosis based on those data. In particular, the failure diagnosis unit18acquires a moving position of each motion axis and a disturbance torque value applied to each motion axis at each moving position every predetermined period, and diagnoses that a failure is occurring if the acquired disturbance torque value is larger than a failure determination threshold. Furthermore, the failure diagnosis unit18includes a maintenance effect determination unit25, which determines an effect of a maintenance task and re-sets the failure determination threshold when the maintenance task is conducted.

The maintenance effect determination unit25calculates a change in disturbance torque value before and after conduct of a maintenance task when the maintenance task is conducted on the motion axis, and re-sets the failure determination threshold only when the change in disturbance torque value thus calculated is larger than a predetermined threshold. Here, the maintenance effect determination unit25calculates a rate of change in disturbance torque value as the change in disturbance torque value.

Specifically, the rate of change can be obtained by the following formula:
Rate of change in disturbance torque value=(average value before conduct of maintenance task−average value after conduct of maintenance task)/(average value before conduct of maintenance task).

However, the change in disturbance torque value may be derived not only from the rate of change but also from calculation of a difference between the average value before the conduct of the maintenance task and the average value after the conduct of the maintenance task. In addition, a different numerical value may be calculated when that numerical value represents the change in disturbance torque value before and after the conduct of the maintenance task.

Meanwhile, in the case of calculating the average value of the disturbance torque values after the conduct of the maintenance task, an average value of the disturbance torque values after a date on which a predetermined period has elapsed from a date of conduct of the maintenance task is calculated. For example, when the date of conductor of the maintenance task is day N as shown inFIG. 3, the average value of the disturbance torque values is calculated by using data after an (N+2)-th day, namely, data after the date on which two days have elapsed from the date of conduct of the maintenance task. This is due to the following reason. Specifically, the disturbance torque values significantly vary right after the conduct of the maintenance as shown inFIG. 3, so that the average value can be calculated more accurately by calculating the average value after the change is calmed. Here, the case of using the data after two days from the date of conduct of the maintenance task is explained inFIG. 3. However, the period required for stabilization of the disturbance torque values after the conduct of the maintenance task varies depending on the contents of the maintenance task and/or the motion axes subjected to the maintenance task. For this reason, a given period after the conduct of the maintenance task may be set on a case-by-case basis.

The production management device4is a device to manage production information including an operating status of a production line in a plant, for example. The production management device4includes a communication unit20and a production information database21. The communication unit20transfers the necessary data to and from the communication unit15in the failure diagnosis device3through the LAN, for example. The production information database21has a function to store a variety of collected production information. As a consequence, the production information database21accumulates a variety of previous production information. Here, the production information includes emergency stop information on the robot1and its incidental equipment as well as information on maintenance records and the like.

Here, in one or more embodiments of the present invention, disturbance torque (disturbance load torque) applied to the motor6that drives each joint axis of the robot1is detected and an abnormality of the corresponding motor drive system is diagnosed as a failure of the robot based on this disturbance torque value. Procedures to obtain the disturbance torque are as follows.

As shown in a block diagram inFIG. 2, an acceleration rate is obtained by differentiating actual velocities Vr of the motor6derived from velocity feedback signals from the pulse coder7, and then acceleration torque Ta is obtained by multiplying the acceleration rate by all inertia J to be applied to the motor6. Next, the obtained acceleration torque Ta is subtracted from a torque command Tc to the motor6obtained by velocity loop processing by the servo control unit11, and a moment M is further subtracted therefrom to obtain disturbance torque Tb. Thereafter, irregular components of the disturbance are removed by conducting given filtering processing, and disturbance torque TG is thus obtained. The servo control unit11executes the above-described processing every predetermined sampling period, thereby obtaining the disturbance torque TG.

To be more precise, the servo control unit11includes a register, and the register obtains an absolute position of the motor6by counting the position feedback pulses from the pulse coder7every predetermined sampling period. Accordingly, the servo control unit11detects the absolute position of the motor6from the register, and obtains the rotation angle position (the moving position) of the joint axis driven by the motor6from the absolute position of the motor6. Moreover, the servo control unit11obtains the disturbance torque TG by performing the processing ofFIG. 2as described previously.

Next, disturbance torque selection processing by the disturbance torque selection unit16of the failure diagnosis device3according to one or more embodiments of the present invention will be described with reference toFIG. 4.FIG. 4is a flowchart showing procedures of the disturbance torque selection processing by the disturbance torque selection unit16.

As shown inFIG. 4, in step S1, the disturbance torque selection unit16acquires the disturbance torque values calculated by the robot control device2. Each disturbance torque value represents a value at each moving position of each motion axis. Moreover, information indicating a data collection period for the disturbance torque values is also acquired at the same time.

Next, in step S2, the disturbance torque selection unit16acquires the emergency stop information on a facility from the production information database21in the production management device4. In step S3, the disturbance torque selection unit16determines whether or not the emergency stop of the facility occurred in the collection period for the acquired disturbance torque values. The processing proceeds to step S4in the case of determination that the emergency stop occurred. On the other hand, the processing proceeds to step S5in the case of determination that the emergency stop did not occur.

In step S4, the disturbance torque selection unit16deletes only the disturbance torque values at the time of occurrence of the emergency stop out of the acquired disturbance torque values, and then the processing proceeds to step S5. In step S5, the disturbance torque selection unit16records the acquired disturbance torque values into the disturbance torque database17, and terminates the disturbance torque selection processing according to one or more embodiments of the present invention.

By selecting the disturbance torque values through the above-described processing, the disturbance torque database17stores and accumulates only the disturbance torque values that do not include abnormal values attributed to the emergency stop of the facility.

Next, failure diagnosis processing by the failure diagnosis unit18of the failure diagnosis device3according to one or more embodiments of the present invention will be described with reference toFIG. 5.FIG. 5is a flowchart showing procedures of the failure diagnosis processing by the failure diagnosis unit18.

As shown inFIG. 5, in step S11, the failure diagnosis unit18acquires the recent disturbance torque values as well as disturbance torque values in the same month last year as the date the diagnosis takes place in a lump from the disturbance torque database17. In step S12, based on the disturbance torque values in the same month last year as the date the diagnosis takes place, the failure diagnosis unit18calculates at least one (or more) of an average value, a variance value, and a median value thereof, and then calculates and sets a failure determination threshold based on the calculated value. For example, any one of the average value, the variance value, and the median value may be set to the failure determination threshold or two or more of these values may be set to the failure determination thresholds.

In step S13, the failure diagnosis unit18calculates at least one (or more) of the average value, the variance value, and the median value of the recent disturbance torque values, and determines whether or not the calculated value is equal to or less than the failure determination threshold set in step S12. Then, if the calculated value out of the average value, the variance value, and the median value of the recent disturbance torque values is equal to or less than the failure determination threshold, then the failure diagnosis unit18determines that a failure is not occurring, and immediately terminates the failure diagnosis processing according to one or more embodiments of the present invention. On the other hand, if the calculated value out of the average value, the variance value, and the median value of the recent disturbance torque values is larger than the failure determination threshold, then the failure diagnosis unit18determines that there is a possibility of a failure, and the processing proceeds to step S14.

In step S14, the failure diagnosis unit18determines whether or not maintenance has been conducted within the last three months based on the data accumulated in the maintenance record database19. Then, if no maintenance has been conducted, the failure diagnosis unit18determines that the failure is occurring, and the processing proceeds to step S21. On the other hand, the processing proceeds to step S15for determining the effect of the maintenance task when the maintenance has been conducted within the last three months.

In step S15, the maintenance effect determination unit25calculates the rate of change in disturbance torque value before and after the conduct of the maintenance task for all the motion axes of the robot which underwent the maintenance task. The robot that underwent the maintenance task includes the multiple motion axes, and some of the motion axes underwent the maintenance task while other motion axes did not undergo the maintenance task. This is due to the reason that some motion axes have to undergo a maintenance task frequently while other motion axes do not have to undergo a maintenance task for a long period of time. Here, the rate of change in disturbance torque value is calculated for all these motion axes. Note that a difference in disturbance torque value before and after the conduct of the maintenance task may be calculated instead of the rate of change in disturbance torque value.

In step S16, the maintenance effect determination unit25determines whether or not each rate of change in disturbance torque value calculated in step S15is equal to or less than a predetermined threshold. When the rate of change is equal to or less than the predetermined threshold, the maintenance effect determination unit25determines that there is no effect of the maintenance task and that a failure is occurring, and the processing proceeds to step S21. On the other hand, when the rate of change in disturbance torque value is larger than the predetermined threshold, the maintenance effect determination unit25determines that there is an effect of the maintenance task, and the processing proceeds to step S17. In other words, for all the motion axes of the robot which underwent the maintenance task, this step determines whether or not the disturbance torque value is significantly changed by the effect of the maintenance task.

In step S17, the failure diagnosis unit18calculates at least one (or more) of an average value, a variance value, and a median value of disturbance torque values after the conduct of the maintenance task, and calculates and re-sets a failure determination threshold based on the value. At this time, in the case of calculating any of the average value, the variance value, and the median value of disturbance torque values after the conduct of the maintenance task, such a value is calculated by using the disturbance torque values after the date on which the predetermined period has elapsed from the date of conduct of the maintenance task, since the disturbance torque values may significantly vary and therefore be unstable right after the conduct of the maintenance as described by usingFIG. 3.

In step S18, the failure diagnosis unit18determines whether or not there is a seasonal variation in the disturbance torque values of any of the joint axes. The processing proceeds to step S20when there is not the seasonal variation or proceeds to step S19when there is the seasonal variation. Here, the determination as to whether or not there is the seasonal variation in the disturbance torque values is made by using a degree of correlation between fluctuations in external temperature in each season and the disturbance torque values, for example. Such determination can be made by checking separately accumulated data of the external temperatures with the data of the disturbance torque values.

In step S19, the failure diagnosis unit18re-sets a failure determination threshold once again by multiplying the failure determination threshold that is re-set in step S17by a constant (a coefficient) corresponding to the seasonal variation.

In step S20, the failure diagnosis unit18determines whether or not at least one (or more) of the average value, the variance value, and the median value of the recent disturbance torque values of the corresponding joint axis is equal to or less than either the failure determination threshold that is re-set once or the failure determination threshold that is re-set twice. Then, if the calculated value out of the average value, the variance value, and the median value of the recent disturbance torque values is equal to or less than any of these failure determination thresholds, then the failure diagnosis unit18determines that a failure is not occurring, and terminates the failure diagnosis processing according to one or more embodiments of the present invention. On the other hand, if the calculated value out of the average value, the variance value, and the median value of the recent disturbance torque values is larger than the corresponding failure determination threshold, then the failure diagnosis unit18determines that a failure is occurring, and the processing proceeds to step S21.

In step S21, the failure diagnosis unit18displays a failure alarm on the corresponding joint axis on a display screen of a not-illustrated monitor that is installed as an attachment to the failure diagnosis device3, and the failure diagnosis processing according to one or more embodiments of the present invention is terminated.

Next, effects of the failure diagnosis device3according to one or more embodiments of the present invention will be described. First of all, when the maintenance task is conducted on the motion axes of the multi-axis machine, the disturbance torque values may cause a significant change in waveform. In this case, since the failure determination threshold is fixed in the related art, there may be a case of erroneous determination of a failure even though the motion axes are normal even after the conduct of the maintenance task. As shown inFIG. 6(a), for example, failure determination thresholds L1and L2are set with respect to a reference value S1of the disturbance torque before the conduct of the maintenance task, respectively. In this case, if the failure determination thresholds L1and L2remain fixed even when the conduct of the maintenance task causes a significant change in disturbance torque value, an alarm is activated as a consequence of erroneous determination of a failure after the conduct of the maintenance task.

On the other hand, when the maintenance task is conducted, the failure diagnosis device3according to one or more embodiments of the present invention calculates the change in disturbance torque value before and after the conduct of the maintenance task, and re-sets the failure determination thresholds when the change in disturbance torque value is larger than the predetermined threshold. As shown inFIG. 6(b), for example, in the case where the failure determination thresholds L1and L2are set with respect to the reference value S1of the disturbance torque before the conduct of the maintenance task, respectively, failure determination thresholds L3and L4are re-set with respect to a new reference value S2when the maintenance task is conducted. This makes it possible to improve failure diagnosis accuracy while preventing erroneous determination even when the maintenance task is conducted.

However, there may be a case of practically relaxing the failure determination thresholds if the failure determination thresholds are re-set when the maintenance task not having any effect on the disturbance torque is conducted. As a consequence, there is a risk of overlooking a failure since no alarm is activated. For example, as shown inFIG. 7(a), in the case where the failure determination thresholds L1and L2are set with respect to the reference value S1of the disturbance torque before the conduct of the maintenance task, respectively, the new reference value S2and the failure determination thresholds L3and L4are set after the conduct of the maintenance task. However, as the failure progresses slowly, the disturbance torque values will also rise gradually. Therefore, if the failure determination thresholds are re-set by using the disturbance torque values before and after the conduct of the maintenance task, the failure determination threshold L1is changed to the failure determination threshold L3even though the maintenance task does not have any effect on the disturbance torque, and the threshold is relaxed. As a consequence, if the operation is continued as it is, the failure occurs without activating the alarm before the disturbance torque reaches the failure determination threshold L3.

On the other hand, when the maintenance task is conducted, the failure diagnosis device3according to one or more embodiments of the present invention calculates the change in disturbance torque value before and after the conduct of the maintenance task, and re-sets the failure determination thresholds only when the change in disturbance torque value is larger than the predetermined threshold. As shown inFIG. 7(b), for example, in the case where the failure determination thresholds L1and L2are set with respect to the reference value S1of the disturbance torque before the conduct of the maintenance task, respectively, the reference value S1and the failure determination thresholds L1and L2are not re-set when the conducted maintenance task does not have any effect on the disturbance torque. As a consequence, the alarm is activated at the point when the disturbance torque value reaches the failure determination threshold L1, so that the failure can be prevented in advance. This makes it possible to eliminate the effect of the maintenance task by re-setting the failure determination thresholds only when the conducted maintenance task has the effect on the disturbance torque, and thus to improve the failure diagnosis accuracy.

Moreover, accordingly to the failure diagnosis device3of one or more embodiments of the present invention, the rate of change in disturbance torque value is calculated as the change in distance torque value, so that the effect of the conducted maintenance task on the disturbance torque can be detected accurately. This makes it possible to eliminate the effect of the maintenance task only when the conducted maintenance task has the effect on the disturbance torque, and thus to improve the failure diagnosis accuracy.

Furthermore, the failure diagnosis device3according to one or more embodiments of the present invention calculates the change in disturbance torque value before and after the conduct of the maintenance task for each motion axis that did not undergo the maintenance task as well, and re-sets the failure determination thresholds only when the change in disturbance torque value is larger than the predetermined threshold. In this way, regarding the motion axes not subjected to the maintenance task as well, it is possible to eliminate the effect of the maintenance by re-setting the failure determination thresholds when there is the effect of the maintenance task, and thus to improve the failure diagnosis accuracy.

Meanwhile, the failure diagnosis device3according to one or more embodiments of the present invention calculates the change in disturbance torque value by using the disturbance torque values before the conduct of the maintenance task and the disturbance torque values after the date on which the predetermined period has elapsed from the date of conduct of the maintenance task. This makes it possible to calculate the change in disturbance torque value except the period in which the disturbance torque significantly varies right after the maintenance task, and thus to improve the failure diagnosis accuracy.

Furthermore, the failure diagnosis device3according to one or more embodiments of the present invention re-sets the failure determination threshold by using at least one of the average value, the variance value, and the median value of the disturbance torque values after the conduct of the maintenance task. This makes it possible to re-set the failure determination threshold reflecting the effect of the maintenance task, and thus to improve the failure diagnosis accuracy while preventing erroneous determination.

Although embodiments of the present invention are described above, it should be understood that the descriptions and the drawings constituting part of this disclosure are not intended to limit this invention. Various alternative embodiments, examples, and application techniques will be obvious to those skilled in the art from this disclosure. While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

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