JP7786238B2 - AC motor monitoring device - Google Patents

Description

The present invention relates to a monitoring device for AC motors.

In a servo system, servo control of a servo motor is generally performed by a servo driver in accordance with commands from a controller such as a PLC. Techniques for detecting abnormalities in such servo motors have been proposed (see, for example, Patent Documents 1 and 2).

Japanese Patent Application Laid-Open No. 2019-204155 Japanese Patent Application Laid-Open No. 2020-035187

Servomotors switch their rotation direction, rotation speed, and torque (hereinafter, the combination of rotation direction, rotation speed, and torque is also referred to as the "operating mode") according to servo control. In such servomotors, the fluctuation in current value caused by switching operating modes can be greater than the fluctuation in current value caused by an abnormality in the servomotor. Therefore, even if an attempt is made to detect an abnormality in the servomotor by fluctuations in the servomotor current value, the fluctuation in current value caused by the abnormality in the servomotor may be obscured by the fluctuation in current value caused by switching operating modes, which could reduce the detection accuracy of the servomotor abnormality.

In order to prevent a decrease in detection accuracy due to switching of the operation mode, it is preferable to perform abnormality detection in a specific operation mode. Therefore, for example, it is conceivable to input an external trigger to the servo driver in a specific operation mode to start abnormality detection. However, in order to adopt such a configuration, it is necessary to provide an input terminal for the external trigger to the servo driver and to configure the programmable controller to execute processing in response to the external trigger.
This has been a major obstacle to adopting a configuration using an external trigger, as it requires modifying the program of a logic controller (PLC) or the like. This problem is not limited to servo motors, but can also occur in AC motors including servo motors and induction motors.

One aspect of the disclosed technology is to provide an AC motor monitoring device that can start measurements to detect abnormalities in an AC motor while the AC motor is operating in a desired operating mode, without providing an external trigger.

One aspect of the disclosed technology is exemplified by the following AC motor monitoring device. This AC motor monitoring device includes a memory unit that stores a trigger range, a calculation unit that calculates at least one of the current amplitude and frequency of the AC current that drives the AC motor, and a measurement unit that begins measuring parameters for detecting an abnormality in the AC motor when at least one of the current amplitude and frequency calculated by the calculation unit falls within the trigger range.

The current amplitude of the AC current that drives the AC motor is related to the torque of the AC motor, and the frequency of the AC current that drives the AC motor is related to the rotation speed of the AC motor. Therefore, by starting measurement by the measurement unit when the current amplitude and the frequency fall within the trigger range, measurement by the measurement unit can be started when the AC motor is operating in a desired operating mode. In other words, with this AC motor monitoring device, measurement for detecting an abnormality in the AC motor can be started when the AC motor is operating in a desired operating mode, without providing an external trigger.

The AC motor monitoring device may further include the following feature: The calculation unit calculates both the current amplitude and the frequency, generates a histogram indicating the frequency of occurrence of pairs of the current amplitude and the frequency calculated at the same time, and stores the generated histogram in the memory unit. The AC motor monitoring device further includes a setting unit that sets the trigger range based on the histogram. By including these features, the AC motor monitoring device can reduce the storage capacity of the memory unit required to store the frequency and current amplitude.

The AC motor monitoring device may further include the following feature: The setting unit outputs the histogram to a display unit, and sets a specified range of the output histogram as the trigger range. By including this feature, the AC motor monitoring device can set a user-desired range of the AC motor as the trigger range.

The AC motor monitoring device may further include the following feature: The calculation unit generates the histogram by excluding pairs in which at least one of the current amplitude and the frequency is below a threshold value indicating a stop of the AC motor. By including this feature, the AC motor monitoring device can prevent the measurement unit from starting measurement when the AC motor is stopped.

The AC motor monitoring device may further include the following feature: The calculation unit further calculates the rate of change of the frequency of the AC current, and the setting unit sets, in the histogram, an area where the rate of change is within a predetermined range indicating steady-state operation of the AC motor as the trigger range. By including this feature, the AC motor monitoring device can more accurately measure parameters that are preferably measured when the AC motor is operating steadily.

The AC motor monitoring device may further include the following feature: It may further include a notification unit that notifies of an abnormality in the AC motor when the parameter exceeds a threshold value indicating an abnormality in the AC motor. By including this feature, the AC motor monitoring device can notify of an abnormality in the AC motor.

In the above-mentioned AC motor monitoring device, the parameters may include the harmonic content of the AC current.

The disclosed technology makes it possible to start measurements to detect abnormalities in an AC motor when the AC motor is operating in a desired operating mode, without providing an external trigger.

FIG. 1 is a diagram schematically illustrating an example of the configuration of a servo system according to an embodiment. FIG. 2 is a diagram showing a schematic configuration of the motor. FIG. 3 is a diagram showing a schematic configuration of the functional units of the servo driver. FIG. 4 is a diagram showing a schematic diagram of the current value supplied to the motor. FIG. 5 is a diagram showing a typical harmonic content measured by the monitoring device when the motor is operating normally. FIG. 6 is a diagram showing a schematic diagram of the harmonic content measured by the monitoring device when an abnormality occurs in the motor. FIG. 7 is a diagram showing a schematic configuration of the functional units of the monitoring device. FIG. 8 is a diagram schematically illustrating the current amplitude calculated by the calculation unit. FIG. 9 is a diagram schematically illustrating the frequency calculated by the calculation unit. FIG. 10 is a diagram schematically showing the current amplitude and frequency calculated by the calculation unit. FIG. 11 is a diagram illustrating time-series data of current amplitude calculated by the calculation unit. FIG. 12 is a diagram illustrating time-series data of frequencies calculated by the calculation unit. FIG. 13 is a diagram illustrating an example of a histogram generated by the calculation unit. FIG. 14 is a diagram showing a schematic view of an area set as a trigger on a histogram. FIG. 15 is a flowchart illustrating an example of a process flow for setting the ranges of current amplitude and frequency by the setting unit. FIG. 16 is a diagram illustrating an example of a processing flow of a process for detecting an abnormality in a motor by a monitoring device. FIG. 17 is a diagram showing another example of the processing flow of the process of detecting an abnormality in a motor by the monitoring device. FIG. 18 is a diagram schematically showing an area set as a trigger on a three-dimensional histogram in the first modified example.

<Embodiment>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, identical or corresponding parts are designated by the same reference numerals, and their description will not be repeated. In this disclosure, an industrial system is shown as one exemplary embodiment of a servo system. However, the use of the servo system according to the present invention is not particularly limited.

<Embodiment>
1 is a diagram schematically illustrating an example configuration of a servo system 100 according to an embodiment. The servo system 100 includes a PLC 1, a servo driver 2, and a monitoring device 9. The servo driver 2 is arranged to drive and control a servo motor 3. An output shaft 32 of the servo motor 3 is connected to a screw shaft 52 by a coupling 51. A precision stage 53 is arranged on the screw shaft 52. The precision stage 53 is displaced on the screw shaft 52 by being driven by the servo motor (hereinafter referred to as "motor") 3.

Stoppers (not shown) are provided at both ends of the driving range of the precision stage 53 along the screw shaft 52. The impact when the precision stage 53 comes into contact with the stoppers is reduced as much as possible by torque control of the motor 3. A workpiece 8 is placed on the precision stage 53. While the servo system 100 illustrated in Figure 1 has one drive shaft driven by the motor 3, two or more drive shafts may be provided.

PLC1 outputs a command signal to servo driver 2. PLC1 functions, for example, as a monitoring device for servo driver 2 by executing processing according to a pre-prepared program.

The servo driver 2 receives a command signal from the PLC 1. The servo driver 2 also receives a feedback signal from the motor 3. The servo driver 2 is formed with a servo system that performs feedback control using a position controller, a speed controller, a current controller, etc., and these signals are used to servo-control and drive the motor 3.

The motor 3 includes a motor main body 30 and an encoder 31. The motor 3 is, for example, an AC servo motor. The motor 3 is supplied with drive current from the servo driver 2 via a power line 40. The encoder 31 detects the displacement of the output shaft 32 of the motor main body 30. Examples of the displacement of the output shaft 32 detected by the encoder 31 include the direction of rotation, amount of rotation, and rotation speed of the output shaft 32. The encoder 31 outputs a feedback signal indicating the detected displacement to the servo driver 2 via an encoder cable 41. A monitoring device 9 that monitors the motor 3 is provided on the power line 40. The "drive current" is an example of an AC current.

Next, the functional configuration of the encoder 31 will be described. Figure 2 is a diagram showing the general configuration of the motor 3. The encoder 31 provided in the motor 3 includes a signal generation unit 311, a communication unit 312, and a memory unit 313.

The signal generation unit 311 detects the operation of the motor main body 30 of the motor 3 driven by the servo driver 2, and generates a feedback signal indicating the detected operation. The feedback signal is output to the communication unit 312. The feedback signal includes, for example, information about the rotational position (angle) of the rotating shaft of the motor main body 30, information about the rotational speed of the rotating shaft, and information about the rotation direction of the rotating shaft. The signal generation unit 311 can be configured, for example, as a known incremental or absolute type.

The communication unit 312 is an interface for communicating with the servo driver 2. In this embodiment, the communication unit 312 sends feedback signals to the servo driver 2 via the encoder cable 41. In this embodiment, serial communication is used to transmit feedback signals and detection signals from the communication unit 312. This reduces the number of signal lines included in the cable. Serial communication via the encoder cable 41 can use known communication standards such as Recommended Standards 232 (RS-232C), RS-422, or RS-485.

The memory unit 313 stores data related to servo control of the motor 3. The memory unit 313 is, for example, an electrically erasable programmable read-only memory (EEPROM). The motor 3 is an example of an "AC motor."

The motor 3 is driven by servo control performed by the servo driver 2. Figure 3 is a diagram showing the general configuration of the functional units of the servo driver 2. The servo driver 2 can be considered a computer equipped with an arithmetic unit, a storage device, etc. The functional units shown in Figure 6 are realized by the execution of predetermined programs, etc. in the servo driver 2. The servo driver 2 has a communication unit 21 and a servo control unit 22, but may have other functional units as well.

The communication unit 21 is a functional unit that manages communication with the outside world via the communication cable 11. For example, the communication unit 21 functions as an interface for communication with the PLC 1. Furthermore, the communication unit 21 also functions as an interface for communication with the encoder 31 via the encoder cable 41.

The servo control unit 22 is a functional unit that servo-controls the motor 3 based on commands from the PLC 1. Specifically, it is a functional unit that performs feedback control using a position controller, speed controller, current controller, etc. Note that for the position controller, speed controller, current controller, etc., control parameters such as speed gain are appropriately set so that servo control of the motor 3, which is the control target, is performed optimally. The servo control unit 22 switches the operating mode of the motor 3, for example, depending on the workpiece 8 placed on the precision stage 53.

The monitoring device 9 detects abnormalities in the motor 3 by monitoring the motor 3, which is driven by servo control using the servo driver 2. Parameters used for abnormality detection include the harmonic content of the current supplied to the motor 3, the average current value, and the effective current value. In this embodiment, the monitoring device 9 detects abnormalities in the motor 3 using the harmonic content of the current supplied to the motor 3.

Figure 4 is a diagram showing a schematic diagram of the current value supplied to motor 3. Figure 4 illustrates an example in which the servo driver 2 switches between the operation modes "Operation A," "Operation B," "Operation C," and "Stop" through servo control. Referring to Figure 4, it can be seen that the current value supplied to motor 3 fluctuates greatly each time the operation mode is switched. Here, it is assumed that during "Operation B," workpiece 8 is placed on precision stage 53, and workpiece 8 is transported by driving motor 3. The user of servo system 100 would like to detect any abnormalities in motor 3 when transporting workpiece 8.

Figure 5 is a diagram that schematically shows the harmonic content measured by the monitoring device 9 when the motor 3 is operating normally. Also, Figure 6 is a diagram that schematically shows the harmonic content measured by the monitoring device 9 when an abnormality has occurred in the motor 3. Figure 6 illustrates an example in which an abnormality has occurred in the motor 3 during operation B. Also, Figure 6 illustrates a threshold value that has been set to detect an abnormality in the motor 3 during "operation B." Here, the threshold value is set lower than the harmonic content during "operation A."

In Figure 6, it can be seen that the occurrence of an abnormality has caused the harmonic content rate in operation B to exceed the threshold. However, in the example of Figure 6, the occurrence of an abnormality in operation B causes the difference between the harmonic content rate in operation A and the harmonic content rate in operation B to be greater than the fluctuation in the harmonic content rate.

In such a case, if an attempt is made to detect an abnormality by setting a uniform threshold value as shown in the example in Figure 6, an abnormality occurring in operation B may be detected, while an abnormality may be erroneously detected as occurring in operation A even though no abnormality has occurred. Therefore, in order to prevent such erroneous detections from occurring, this embodiment employs the following configuration of the monitoring device 9.

Figure 7 is a diagram showing the general configuration of the functional units of the monitoring device 9. The monitoring device 9 can be considered a computer having an arithmetic unit, a storage device, etc. The functional units shown in Figure 7 are realized by the monitoring device 9 executing predetermined programs, etc. The monitoring device 9 has a calculation unit 91, a setting unit 92, a measurement unit 93, a notification unit 94, and a storage unit 95, but may also have functional units other than these.

The calculation unit 91 is a functional unit that calculates the current amplitude and frequency of the drive current supplied to the motor 3. The frequency can be calculated, for example, from the current waveform. The calculation unit 91 can, for example, detect zero-crossing points in the current waveform and calculate the frequency based on the time until the next zero-crossing point. The current amplitude and frequency calculated by the calculation unit 91 are used, for example, as triggers for the measurement unit 93 to start measurement. The current amplitude of the drive current is related to the torque of the motor 3, and the frequency of the drive current is a parameter related to the rotation speed of the motor 3.

Furthermore, the calculation unit 91 calculates the frequency of occurrence of the current amplitude and frequency of the drive current supplied to the motor 3 at the same time. Figures 8 to 10 are diagrams that schematically explain how the calculation unit 91 calculates the frequency of occurrence of pairs of current amplitude and frequency. Figure 8 is a diagram that schematically shows the current amplitude calculated by the calculation unit 91. The vertical axis of Figure 8 represents the current amplitude, and the horizontal axis represents time. Figure 9 is a diagram that schematically shows the frequency calculated by the calculation unit 91. The vertical axis of Figure 9 represents the frequency, and the horizontal axis represents time. In Figures 8 and 9, the current amplitude and frequency calculated at the same time are associated with circled numbers. For example, the current amplitude illustrated by the circled "3" in Figure 8 and the frequency illustrated by the circled "3" in Figure 9 were calculated at the same time.

Figure 10 is a diagram that schematically shows the current amplitude and frequency calculated by the calculation unit 91. In Figure 10, the vertical axis represents the current amplitude, and the horizontal axis represents the frequency. In Figure 10, pairs of current amplitude and frequency calculated at the same time are plotted on a graph. By plotting pairs of current amplitude and frequency in a space where one axis represents the current amplitude and the other axis represents the frequency, it is possible to understand which pairs of current amplitude and frequency appear frequently.

Based on the above explanation, we will further explain how the calculation unit 91 calculates the frequency of occurrence of pairs of current amplitude and frequency calculated at the same time. The calculation unit 91 calculates the current amplitude and frequency of the drive current during a predetermined learning period. The calculation of the current amplitude and frequency by the calculation unit 91 is performed for each operating mode of the motor 3 switched by the servo driver 2.

Figure 11 is a diagram illustrating time series data of current amplitude calculated by the calculation unit 91. In Figure 11, the vertical axis represents current amplitude (A) and the horizontal axis represents time (seconds). Figure 12 is a diagram illustrating time series data of frequency calculated by the calculation unit 91. In Figure 12, the vertical axis represents frequency (Hz) and the horizontal axis represents time (seconds).

The calculation unit 91 calculates the occurrence frequency of each pair of current amplitude and frequency calculated at the same time based on the calculated time series data of current amplitude and time series data of frequency. The calculation unit 91 may store the occurrence frequencies for all calculated pairs of current amplitude and frequency in the memory unit 95. Note that if the occurrence frequencies for all calculated pairs of current amplitude and frequency are stored in the memory unit 95, the amount of data stored in the memory unit 95 will increase. For this reason, a memory unit 95 with a large storage capacity will be used, or the data will be transferred to a higher-level device. Therefore, in order to reduce the amount of data stored in the memory unit 95, the calculation unit 91 generates a two-dimensional histogram of the collected data indicating the occurrence frequencies of the pairs.

Figure 13 is a diagram illustrating a histogram 241 generated by the calculation unit 91. In Figure 13, the vertical axis represents frequency, and the horizontal axis represents current amplitude. In the histogram 241, the current amplitude and frequency are divided at predetermined intervals to define multiple regions. Each of the multiple defined regions includes a pair of current amplitude and frequency. The shade of color in each defined region indicates the frequency of occurrence of the pair of current amplitude and frequency associated with that region, with a darker color indicating a higher frequency of occurrence. By storing the frequency of occurrence of pairs of current amplitude and frequency in the memory unit 95 as such a histogram 241, the storage capacity used to store the frequency of occurrence of pairs of current amplitude and frequency can be reduced to a size obtained by multiplying the number of predetermined intervals for current amplitude by the number of predetermined intervals for frequency.

Here, the calculation unit 91 may, for example, exclude from the histogram 241 pairs of current amplitude and frequency where at least one of the calculated current amplitude and frequency is equal to or less than a predetermined value that indicates the stop of the motor 3. In other words, the calculation unit 91 may generate the histogram 241 by excluding pairs of current amplitude and frequency where at least one of the calculated current amplitude and frequency is equal to or less than a predetermined value that indicates the stop of the motor 3.

The setting unit 92 sets a trigger for starting measurement by the measurement unit 93. The setting unit 92 may, for example, set an area in the histogram 241 that satisfies certain trigger setting conditions as the trigger. Examples of trigger setting conditions include current amplitude and frequency that appear at a frequency of 5% or more of the total number of data points in the histogram 241. An example of an area in the histogram 241 that satisfies such trigger setting conditions is area R1 in Figure 13. By setting a trigger based on such trigger setting conditions, the operating state of the motor 3 that appears most frequently can be used as the trigger.

Also, for example, the trigger setting condition may be that the frequency is the highest in the region in histogram 241 where the occurrence frequency is equal to or greater than a predetermined value. In the example of Figure 13, an example of a region that satisfies such a trigger setting condition is region R1 in Figure 13. By setting a trigger with such trigger setting conditions, a high occurrence frequency and a high rotation frequency of motor 3 can be used as the trigger.

The trigger setting condition may also be the region in histogram 241 where the current amplitude is the largest among the regions where the frequency of occurrence is equal to or greater than a predetermined value. An example of a region in histogram 241 that satisfies such a trigger setting condition is region R2 in Figure 13. By setting a trigger with such trigger setting conditions, a high frequency of occurrence and a large torque of motor 3 can be used as the trigger.

The setting unit 92 may display the histogram 241 on a display or the like, and allow the user to specify the region to be set as a trigger. Since the frequency and current amplitude vary depending on the operating mode of the motor 3, for example, if it is desired to detect an abnormality in the motor 3 during "operation B" (see Figure 4), the region corresponding to "operation B" may be specified on the histogram 241.

The setting unit 92 then stores the set trigger in the memory unit 95. Figure 14 is a diagram schematically showing an area set as a trigger on the histogram 241. The vertical axis of Figure 14 represents frequency, and the horizontal axis represents current amplitude. Figure 14 illustrates an example of area R3 set as a trigger. Area R3 is a rectangle with a frequency range of A1 to A2 and a current amplitude range of B1 to B2. When area R3 is set as a trigger, the measurement unit 93, described below, starts measurement when the frequency calculated by the calculation unit 91 is between A1 and A2 and the current amplitude calculated by the calculation unit 91 is between B1 and B2.

The measurement unit 93 performs measurements to detect abnormalities in the motor 3 when the current amplitude and frequency calculated by the calculation unit 91 fall within the region set as a trigger by the setting unit 92. The measurement unit 93, for example, measures the harmonic content rate contained in the current waveform supplied to the motor 3. The measurement unit 93, for example, performs a fast Fourier transform (FFT) on the current waveform supplied to the motor 3 to determine its fundamental wave and nth-order harmonics (n is an integer greater than or equal to 2). The measurement unit 93 then measures the harmonic content rate by calculating the ratio of second-order and higher harmonics to the fundamental wave.

The notification unit 94 notifies of an abnormality when the harmonic content measured by the measurement unit 93 exceeds a threshold value stored in the memory unit 95. Methods for notifying of an abnormality include outputting an alarm sound, outputting a warning message on a display, and sending a notification by email.

The storage unit 95 is, for example, an EEPROM. The storage unit 95 stores triggers set by the setting unit 92 and thresholds used by the notification unit 94. The storage unit 95 may also store a measurement time during which the measurement unit 93 performs measurement. The monitoring device 9 is an example of a "monitoring device."

Figure 15 is a flowchart showing an example of the processing flow for setting the current amplitude and frequency ranges by the setting unit 92. Below, an example of the processing flow for setting the current amplitude and frequency ranges by the setting unit 92 will be described with reference to Figure 15.

At T1, PLC 1, servo driver 2, and motor 3 are connected to form servo system 100. Then, the servo control unit 22 of servo driver 2 begins driving motor 3 in a predetermined operating mode (e.g., operation A in Figure 4) in response to a command signal from PLC 1.

At T2, the calculation unit 91 of the monitoring device 9 calculates the current amplitude and frequency of the drive current supplied to the motor 3. The calculation unit 91 continues to calculate the current amplitude and frequency of the drive current supplied to the motor 3 for a predetermined time. Furthermore, the calculation unit 91 calculates the occurrence frequency of pairs of current amplitude and frequency calculated at the same time.

At T3, the servo control unit 22 of the servo driver 2 switches the operating mode of the motor 3. For example, if the servo control unit 22 is driving the motor 3 in operating mode "Operation A," it can switch to operating mode "Operation B" and drive the motor 3. If recording of the current amplitude and frequency of the drive current supplied to the motor 3 for all operating modes of the motor 3 has been completed ("YES" at T4), processing proceeds to T5. If recording of the current amplitude and frequency of the drive current supplied to the motor 3 for all operating modes of the motor 3 has not been completed ("NO" at T4), processing proceeds to T2.

At T5, the setting unit 92 of the monitoring device 9 sets a trigger. The setting unit 92 may calculate a histogram 241 based on the current amplitude and frequency calculated by the calculation unit 91 at T2, and set an area in the histogram 241 that satisfies predetermined trigger setting conditions as a trigger. The setting unit 92 may also display the histogram 241 on a display or the like, and allow the user to specify the area to be set as a trigger.

Figure 16 is a diagram showing an example of the processing flow of the process by the monitoring device 9 to detect an abnormality in the motor 3. Figure 16 illustrates a case in which measurement by the measurement unit 93 continues while the current amplitude and frequency calculated by the calculation unit 91 are within the region set as a trigger. Below, an example of the processing flow of the process by the monitoring device 9 to detect an abnormality in the motor 3 will be described with reference to Figure 16.

At T11, the calculation unit 91 calculates the current amplitude and frequency of the drive current supplied to the motor 3. If the calculated current amplitude and frequency fall within the region set as the trigger at T5 in FIG. 15 ("YES" at T12), for example, the process proceeds to T13. If the calculated current amplitude and frequency do not fall within the region set as the trigger at T5 in FIG. 15 ("NO" at T12), for example, the process proceeds to T11.

At T13, the measurement unit 93 measures the harmonic content contained in the current waveform of the current supplied to the motor 3. If the measured harmonic content exceeds the threshold value stored in the memory unit 95 ("YES" at T14), processing proceeds to T15. If the measured harmonic content is equal to or less than the threshold value stored in the memory unit 95 ("NO" at T14), processing proceeds to T11.

At T15, the notification unit 94 notifies that an abnormality has occurred in motor 3.

Figure 17 is a diagram showing another example of the processing flow for detecting an abnormality in the motor 3 by the monitoring device 9. Figure 17 describes the processing flow when measurement by the measurement unit 93 continues for a certain period of time. Below, with reference to Figure 17, another example of the processing flow for detecting an abnormality in the motor 3 by the monitoring device 9 will be described. Note that the same processes as in Figure 16 are assigned the same reference numerals, and their description will be omitted.

At T14a, if the harmonic content measured at T13 exceeds the threshold value stored in the memory unit 95 ("YES" at T14a), processing proceeds to T15. If the harmonic content measured at T13 is equal to or less than the threshold value stored in the memory unit 95 ("NO" at T14a), processing proceeds to T16.

At T16, the measurement unit 93 determines whether the measurement time for measuring the harmonic content has elapsed the measurement time stored in the memory unit 95. If the measurement time has elapsed ("YES" at T16), the process proceeds to T11. If the measurement time has not elapsed ("NO" at T16), the process proceeds to T13.

<Effects of the embodiment>
In this embodiment, a trigger region is set based on the pre-calculated current amplitude and frequency of the drive current supplied to the motor 3. The current amplitude of the drive current is related to the torque of the motor 3, and the frequency of the drive current is related to the rotation speed of the motor 3. Therefore, in this embodiment, by setting a trigger based on such current amplitude and frequency, it is possible to perform measurement for abnormality detection on the motor 3 in a desired operating mode without inputting a trigger from outside to start measurement for abnormality detection.

In this embodiment, the frequency and current amplitude calculated by the calculation unit 91 are stored in the memory unit 95 as a histogram 241. Through this processing, in this embodiment, the storage capacity of the memory unit 95 required to store the frequency and current amplitude can be reduced.

In addition, in this embodiment, the setting unit 92 can display the histogram 241 on a display or the like, and allow the user to specify the area to be set as a trigger. By providing this feature, this embodiment allows the user to set the desired area as a trigger.

In this embodiment, the calculation unit 91 can generate the histogram 241 by excluding pairs of current amplitude and frequency where at least one of the calculated current amplitude and frequency is equal to or less than a predetermined value indicating that the motor 3 is stopped. By generating the histogram 241 in this manner, this embodiment can prevent the measurement unit 93 from performing measurements when the motor 3 is stopped.

<First Modification>
In the embodiment described above, a trigger for starting measurement by the measurement unit 93 is set based on a pair of current amplitude and frequency. In the first modified example, a trigger for starting measurement by the measurement unit 93 is set based on the rate of change of frequency in addition to the current amplitude and frequency.

In the first modified example, the calculation unit 91 may, for example, generate a three-dimensional histogram in which the current amplitude, frequency, and frequency change rate are arranged on three orthogonal axes, instead of the histogram 241. Figure 18 is a diagram schematically showing an area set as a trigger on the three-dimensional histogram in the first modified example. Even when a three-dimensional histogram including three parameters is generated as in the first modified example, the trigger area can be set based on the histogram.

When the frequency change rate is included as a parameter for setting a trigger, it is preferable to select an area where the frequency change rate is close to 0 as the trigger area. An example of a frequency change rate being close to 0 is when the frequency change rate is within a predetermined range from 0 to indicating no fluctuations in the rotational speed of motor 3. By specifying an area in the three-dimensional histogram where the frequency change rate is close to 0 as the trigger, it is possible to set the trigger condition as being when motor 3 is operating steadily (operating at a substantially constant speed).

By starting measurement by the measurement unit 93 based on a trigger set in this way, parameters that are preferably measured when the motor 3 is operating steadily can be measured more accurately. An example of a parameter that is preferably measured when the motor 3 is operating steadily is the harmonic content calculated by FFT. In other words, by using the steady state operation of the motor 3 as a trigger, the harmonic content can be measured more accurately.

<Other Modifications>
In the above-described embodiment, the motor 3 is a servo motor, but the motor 3 is not limited to a servo motor. The motor 3 may be, for example, an induction motor.

In the embodiment described above, a trigger was set to cause the measurement unit 93 to start measurement based on both the current amplitude and frequency of the drive current supplied to the motor 3, but a trigger may also be set to cause the measurement unit 93 to start measurement based on at least one of the current amplitude and frequency of the drive current.

In the embodiment described above, a monitoring device 9 is provided separately from the servo driver 2, but the monitoring device 9 may also be built into the servo driver 2.

The embodiments and variations disclosed above can be combined with each other.

<Appendix 1>
a storage unit (95) for storing the trigger range;
a calculation unit (91) that calculates at least one of the current amplitude and frequency of an AC current that drives an AC motor (3);
a measurement unit (93) that starts measuring a parameter for detecting an abnormality in the AC motor (3) when at least one of the current amplitude and frequency calculated by the calculation unit (91) falls within the trigger range.
AC motor monitoring device (9).

1. PLC
2. Servo driver 3. Motor 8. Workpiece 9. Monitoring device 21. Communication unit 22. Servo control unit 91. Calculation unit 92. Setting unit 93. Measurement unit 94. Notification unit 95. Storage unit 30. Motor body 31. Encoder 241. Histogram 311. Signal generation unit 312. Communication unit 313. Storage unit 32. Output shaft 41. Encoder cable 51. Coupling 52. Screw shaft 53. Precision stage 100. Servo system

Claims (6)

a storage unit that stores the trigger range;
a calculation unit that calculates at least one of a current amplitude and a frequency of an AC current that drives an AC motor;
a measurement unit that starts measuring a parameter for detecting an abnormality in the AC motor when at least one of the current amplitude and the frequency calculated by the calculation unit falls within the trigger range,
the calculation unit calculates both the current amplitude and the frequency, generates a histogram indicating an appearance frequency of pairs of the current amplitude and the frequency calculated at the same time, and stores the generated histogram in the storage unit;
The AC motor monitoring device further includes a setting unit that sets the trigger range based on the histogram.
AC motor monitoring device. the setting unit outputs the histogram to a display unit, and sets a specified range of the output histogram as the trigger range.
2. The AC motor monitoring device according to claim 1 . the calculation unit generates the histogram by excluding the pairs in which at least one of the current amplitude and the frequency is equal to or less than a threshold value indicating a stop of the AC motor.
3. The monitoring device for an AC motor according to claim 1 or 2 . The calculation unit further calculates a rate of change of the frequency of the AC current,
the setting unit sets, as the trigger range, a region in the histogram where the rate of change is within a predetermined range that indicates steady operation of the AC motor.
The monitoring device for an AC motor according to any one of claims 1 to 3 . the parameters include a harmonic content of the AC current;
The monitoring device for an AC motor according to any one of claims 1 to 4 . a notification unit that notifies the abnormality of the AC motor when the parameter exceeds a threshold value that indicates an abnormality of the AC motor;
The monitoring device for an AC motor according to any one of claims 1 to 5 .