JP4044700B2 - Slip detection method and slip detection device in drive transmission unit by friction - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a slip detection method and a slip detection device for a drive transmission unit in a transport facility having a drive transmission unit by friction such as a belt conveyor.
[0002]
[Prior art]
Conventionally, in a transport facility having a drive transmission unit by friction, for example, a belt conveyor, a belt loaded with a transport object is circulated by a drive pulley serving as a drive transmission unit, and the transport object is transported. If slip occurs between the belts, the conveyance accuracy is hindered, and the frictional heat caused by the slip may cause the belt and drive pulley to burn out, resulting in a fire accident. Efforts have been made to take early measures for slip suppression such as recovery processing of the friction coefficient of the drive pulley and load adjustment.
[0003]
For example, as a slip detection method in this belt conveyor,
(1) A method in which the belt speed is detected by the peripheral speed based on the rotational speed of the driving pulley and the touch roll brought into contact with the back surface of the belt, and the slip between the driving pulley and the belt is detected by the speed difference.
(2) As disclosed in Japanese Patent Application Laid-Open No. 5-201519, the load current of the drive motor is detected, and the value of this load current is a predetermined time or longer than a no-load current range given as a preset value. A slip detection method for determining that a slip has occurred when the voltage drops.
Etc. are known.
[0004]
However, in the former slip detection method based on the belt speed detection by the touch roll, it is necessary to install the rotation speed detection device on the drive pulley and the touch roll on the back side of the belt. In addition, these detectors are installed directly on belts and pulleys, so the object to be transported is in contact with these detectors and wiring, and is easily affected by the environment of the installation location, resulting in functional deterioration and abnormal signals. Therefore, it is difficult to ensure stable detection accuracy.
[0005]
In the former slip detection method based on a decrease in load current with respect to a preset no-load current, the former problem can be solved, but only the last continuous slip can be detected. It is impossible to detect a steady minute slip that occurs at a high level and a sporadic transient slip, and an early countermeasure cannot be taken. For this reason, efforts have been made to take measures to detect slips at an early stage and suppress slips.
[0006]
[Problems to be solved by the invention]
The present invention is a belt conveyor having a drive transmission unit by friction described above, which does not require various detectors that are susceptible to environmental influences on pulleys and belts, and is capable of slipping stably and accurately by simple and inexpensive means. A slip detection method and a slip detection device capable of detecting the above are provided.
[0007]
[Means for Solving the Problems]
The present invention comprises the following inventions (1) to (3).
(1). In a transport facility where a transport medium is interposed between the drive transmission unit and the object to be transported due to friction, the current that changes every moment from the start of operation with the motor that is the drive source is measured, and this measured current is passed through a preset filter The current change amount (ΔI) with the average current in the preset average processing time immediately before is calculated by absolute value processing , and this current change amount (ΔI), the immediately preceding average current (Ia), and the conveyance medium intervene. Based on the no-load average current (Ib) corrected with the drive loss correction coefficient (K), the current change rate (A) is obtained by the following equation (1), and the obtained current change rate (A) is A slip detection method in a frictional drive transmission unit, characterized in that a slip occurrence is determined when a current change rate threshold value equal to or greater than a set current change rate threshold value is compared with a set current change rate threshold value. .
(2). (1) The slip detection method in the friction drive unit of the transport facility, characterized in that the transport facility is a belt conveyor and detects slip between the drive pulley as the drive transmission unit and the belt as the transport medium. .
(3). In a transport facility in which a transport medium is interposed between a drive transmission unit due to friction and an object to be transported, a current detector that is installed in a power supply circuit of an electric motor that is a drive source of the drive transmission unit and detects a current from the start of operation of the motor; A diagnostic condition setter that determines the diagnostic condition based on the operation signal relay and the signal from the operation signal relay, a filter that removes the low frequency component of the current signal from the current detector, and the absolute value of the current that has passed through this filter A current change amount (ΔI) is calculated from the average current at the immediately preceding average time after processing, and this current change amount (ΔI), the immediately preceding average current (Ia), and the driving loss correction coefficient (K The slip calculator for calculating the current change rate (A) by the following formula (1) based on the no-load average current (Ib) corrected in (1) , and the current change rate and the set slip occurrence. A slip judging device that compares and calculates a threshold value of a current change rate to be performed, a display device that displays the occurrence of a slip based on a slip occurrence signal from the slip judging device, and a monitoring device such as an alarm device. Slip detecting device in the friction drive unit of
A = ΔI / (Ia−K * Ib) (1)
here,
ΔI: Amount of current change due to slip (high frequency)
Ia: Last average current (low frequency)
Ib: Average current in the no-load state (setting or measurement)
K: Correction coefficient corresponding to drive drive unit independent operation
DETAILED DESCRIPTION OF THE INVENTION
The present invention is mainly applied to the case where slip is detected in a transport facility that does not have a speed control function for each drive unit, and an AC motor is used as a motor that drives a drive transmission unit of the transport facility. Applies to
Basically, a conveyance medium loaded with objects to be conveyed is supported by a drive transmission unit by friction driven by an electric motor, and a belt is wound around a conveyance facility that conveys an object to be conveyed through the conveyance medium, for example, a driving pulley. In a belt conveyor that loads the object to be transported on the belt, drives the drive pulley to move the belt, and conveys the object to be conveyed, the belt that moves by loading the object to be conveyed and the driving pulley, which is a drive transmission unit by friction It detects slips between them.
[0009]
In this case, the load with respect to the difference between the current value in the no-load state from the start of operation of the electric motor as the drive source and the value obtained by correcting the average current value of the electric motor after shifting to the load state in consideration of the drive loss due to the belt The occurrence of slip between the drive pulley and the belt is detected by the ratio of the steep current amount in the state.
[0010]
The above-described current change rate of the motor will be described with reference to FIGS. For example, the current value of the motor that drives the drive pulley of the belt conveyor changes from time to time according to changes in the load state from the start of operation to the end of operation, and slip occurs between the drive pulley and the belt. In this case, it is known that the current value is greatly reduced.
[0011]
FIG. 1 conceptually shows an example of current change over time from the start of operation. In general, the change in the current of the motor shifts from the start fluctuation range to the no-load stable range to the initial load fluctuation range to the load stable range, but slipping may occur in each process from the no-load stable range. Even when slip occurs in the above process, a significant current change occurs. This change in current is particularly noticeable when a slip occurs in a load state and has a great influence on wear. Therefore, the present invention selectively detects and manages a slip generated in the load state.
[0012]
Although it is conceivable to determine and manage the occurrence of slip at the lower limit of the current value, as described above, the current value due to steady minute slip that occurs at high load, that is, when the current value is high, That is, since the lower limit value set so as not to be misreported does not reach the current fluctuation due to the load fluctuation when the current value is small, an early countermeasure cannot be taken at the initial stage of the slip.
For example, FIG. 2 shows the current value over time from the start of the motor to no-load operation-load operation-stop, but the current changes greatly when a slip occurs, but the value is the no-load current. It has not dropped below the value.
[0013]
The inventors of the present invention have noticed that a significant current change that occurs with the occurrence of slip is a high frequency, and that a current change due to load fluctuations other than slip is a low frequency, and the low frequency is removed to indicate the occurrence of slip. We knew that steep high frequencies could be detected with high accuracy, and considered to determine and manage slip occurrence based on the ratio of steep current change to average current before removing low frequencies.
[0014]
Note that a high-frequency current change caused by turning on and off the current is determined so as not to cause erroneous detection of slip using an operation signal of the motor. In other words, the slip is diagnosed after the operation signal is applied when the current is turned on, and the detection unit that does not output an alarm when the operation signal is not applied immediately after detecting the high frequency is provided to prevent erroneous detection when the power is shut off. I will respond.
FIG. 3 shows an example of a current waveform obtained by passing the change in the current value of FIG. 2 through the high-pass filter and removing the low frequency, and then performing the absolute value processing of the current change value. FIG. 3 shows the current change in FIG. 3 as a change in current change rate.
[0015]
The present invention has been made on the basis of the above-mentioned knowledge. Basically, as shown in FIG. 5, a current that changes from start to start from the start of operation is filtered to remove low frequencies below the reference frequency, and the absolute The steep current change amount obtained by the value processing is calculated by calculating the average current immediately before the preset average processing time, the average current in the no-load state, and the correction coefficient corresponding to the no-load current in the single operation of the drive pulley. The current change rate obtained by dividing by the difference current from the current obtained from the above is compared with a set current change rate threshold, and if this current change rate exceeds the current change rate threshold, the occurrence of slip Judgment. Ideally, the current when the drive pulley is operated alone corresponding to the current at the time of a complete slip should be measured with the belt removed, but this is difficult to realize.
[0016]
Coefficients for correcting the current increment at the time of no load due to the equipment closer to the conveyed object than the drive transmission part due to friction such as bending loss of the belt and rotational friction loss of many driven pulleys are necessary. In a normal belt conveyor, this correction coefficient is 0.1 to 0.5.
[0017]
The current change rate A in the present invention can be obtained based on the following equation.
A = ΔI / (Ia−K * Ib) (1)
here,
ΔI: Amount of current change due to slip (high frequency)
Ia: Last average current (low frequency)
Ib: Average current in the no-load state (setting or measurement)
K: Correction coefficient corresponding to drive pulley single operation In the present invention, a preset current change rate threshold value AD is compared with a current change rate A obtained by equation (1), and
Current change rate A> Current change rate threshold value AD (2)
In this case, it is determined that slip has occurred.
[0018]
The threshold value AD of the current change rate is determined by the load movement of the object to be transported, the structure of the transport equipment, the type of the motor that drives the transport equipment, the capacity, the current characteristics, etc., the current change during no-load operation, load operation, The current change rate corresponding to the slip subject to management is set on the basis of the actual change in current when a slip actually occurs. For example, in FIG. 4, the current change rate threshold value AD is 15%, and it is determined that a slip has occurred when the current change rate A obtained from the measured current exceeds the current change rate threshold value AD. It is.
[0019]
The average processing time Ta means the time for averaging the current. This average processing time is preferably set in the range of 1 to 5 seconds. If it is less than 1 second, it is affected by a high-frequency disturbance, and it becomes impossible to detect an average current that affects a relatively gradual load fluctuation that does not change sharply. Further, if it exceeds 5 seconds, it is impossible to follow the current change caused by the variation of the conveyed object, and the reliability of the slip determination decreases.
[0020]
In the present invention, the current from the current detector is filtered to remove low frequencies below the reference frequency. As a filter for that purpose, a filter that passes a high frequency of 0.1 Hz or more is used.
[0021]
The occurrence of slip is a high relative to the difference between the average current immediately before the preset average processing time and the current obtained by multiplying the average current in the no-load state by the correction coefficient corresponding to the no-load current when the drive pulley is operated independently. The high frequency that has passed through the band pass filter is determined based on the ratio of values obtained by performing absolute value processing. This high-pass frequency is set in advance according to the rating of the motor, the configuration of the transport facility, the scale, and the like.
[0022]
In the present invention, the occurrence of slip is detected by comparing the current change rate A with a set current change rate threshold value AD. Therefore, relatively large current fluctuations that are not necessarily related to the occurrence of slips and the load caused by the conveyed object. The slip can be determined quantitatively without being affected by the current fluctuation caused by the fluctuation, and sufficient detection accuracy can be ensured.
[0023]
【Example】
Example 1
Embodiment 1 of the present invention will be described below with reference to FIGS. In this embodiment, in the belt conveyor 1, slip detection is detected between a driving pulley 2 serving as a drive transmission unit due to friction and a belt 5 on which a transported object 4 wound around the driving pulley and the driven pulley 3 is loaded. Is the case.
[0024]
(A). [When measuring the no-load current Ib in advance and setting it manually before starting operation]
In this embodiment, the no-load average current Ib used for calculating the current change rate A is measured in advance and is manually set before starting operation. 6 and 7 conceptually show a configuration example of the slip detection device used in this embodiment.
[0025]
As shown in FIG. 6, this slip detection device is provided with a current detector 7 and an operation signal relay 8 in a current circuit of an electric motor 6 that drives a drive pulley 2 that drives a belt 5, and a current from the current detector 7. The signal and the operation signal from the operation signal relay 8 are input to the slip detection processing device 9, the current signal from the current detector 7 is processed by the high-pass filter 10, and then the absolute value processor 11 performs absolute value processing. Thus, the current change amount ΔI is calculated, and the current signal from the current detector 7 is converted by the low-pass filter (or average processor) 12 into a current that changes every moment in advance. The average current Ia averaged in step S5 is calculated, and the current change rate A is calculated by the equation (1) from the no-load current Ib set in advance by adding the power loss correction coefficient K due to the belt intervention in the slip calculator 13. Calculated, in which to detect the occurrence of the slip by the difference compared with a threshold value AD of the current change rate set.
[0026]
More specifically, the slip detection processing device 9 performs overall management of the slip detection processing device 9, and diagnoses based on a diagnosis setting device 14 for setting various diagnostic items and an operation signal from the operation signal relay 8. Diagnostic condition determiner 16 for determining the condition, high-pass filter 10 that removes the low frequency of the current signal from the current detector 7, and high frequency that has passed through the high-pass filter are subjected to absolute value processing and the amount of current change An absolute value processor 11 for calculating ΔI and a low-pass filter (or an average) for calculating an average current Ia obtained by averaging the current signal from the current detector 7 which changes every moment with a preset average processing time Ta (Processor) 12, current change amount ΔI, average current Ia, preset no-load current Ib, and belt-mediated power loss correction coefficient K (1) Formula A = ΔI / (Ia The slip calculator 13 that calculates the current change rate A using -K * Ib) and the current change rate A and the preset threshold value AD of the current change rate are compared with each other, and equation (2) A> AD A slip discriminator 15 for discriminating the occurrence of a slip, and a slip discriminating method in which a slip occurrence signal erroneously detected when the electric power of the motor 6 is cut off by a slip occurrence signal from the slip discriminator 15 and a signal from the diagnostic condition determiner 16 is preset. It comprises a false detection prevention device 17 that is excluded based on the time Twd, and a monitoring device 18 such as a display or alarm device that displays the occurrence of slip.
[0027]
FIG. 7 shows an example of a slip diagnosis procedure by the slip detection device configured as described above.
(1). The diagnosis setting unit 14 includes a delay time Td, an average processing time Ta, a threshold value AD of a current change rate, an average current Ib at no load, a slip determination time Twd, A = ΔI / (Ia−K * Ib), and a belt intervention. Set conditions such as the power loss correction coefficient K, and start the diagnostic process by the operation signal from the operation signal relay 8. (2). The delay time Td from the start of operation is determined by the diagnostic condition determination unit 16, and the current signal from the current detector 7 is averaged with the average processing time Ta, and after the high-pass filter 10. Processing by the absolute value processor 11 (3). The absolute value processor 11 calculates the current change amount ΔI by performing absolute value processing on the average current Ia within the average processing time Ta immediately before passing through the low-pass filter 12 and the current passing through the high-pass filter 10, Current change rate A = ΔI / (Ia−K * Ib) is calculated from the previous average current Ia and the no-load average current Ib corrected by the power loss correction coefficient K set by the belt intervention set in (1). Do (4). The current change rate A is compared with a preset current change rate threshold value (5). If the current change rate A> the threshold value AD of the current change rate and the operation signal is ON even after the slip determination time Twd has elapsed, it is determined that a slip has occurred, and a monitoring device such as a display or an alarm device An alarm is output to, and the electric motor 6 is stopped as necessary.
[0028]
(B). [When the no-load current Ib is measured and automatically set after the start of operation]
In this embodiment, the no-load average current Ib used for calculating the current change rate A is measured after the start of operation and automatically set. 8 and 9 conceptually show a configuration example of the strip detection apparatus used in this embodiment.
[0029]
As shown in FIG. 8, the strip detection device includes a transported object detector 19 on the supply side of the transported object 4 of the belt conveyor 1 and a load signal relay that is linked to load state determination in the transported object detector. 22, a current detector 7 and an operation signal relay 8 are provided in the current circuit of the electric motor 6 that drives the drive pulley 2 that drives the belt 5, and the current signal from the current detector 7 and the operation signal relay 8 The operation signal and the load signal from the load signal relay 22 are input to the slip detection processing device 21, and the no-load current setting unit 20 averages the current during no-load operation for a preset average processing time Ta. An average current Ib at the time of load is set, and the current passing through the high-pass filter 10 is subjected to absolute value processing by the absolute value processor 11 to calculate the current change amount ΔI. Current signal The low-pass filter (or the average processor) 12 calculates an average current Ia obtained by averaging the current that changes every moment with a preset average processing time Ta, and the slip calculator 13 corrects the power loss due to the belt intervention. In consideration of K, the current change rate A is calculated from the no-load current Ib by the equation (1), and compared with the set current change rate threshold value AD, the occurrence of slip is detected by the difference. It is a thing.
[0030]
More specifically, the slip detection processing device 21 performs overall management of the slip detection processing device 21, and sets a diagnostic setting unit 14 for setting various diagnostic items, and an operation signal from the operation signal from the operation signal relay 8. Based on the diagnosis condition determination unit 16 that determines the slip diagnosis condition, the load signal from the load signal relay 22 and the operation signal from the operation signal relay 8, the no-load operation state is detected and the current signal from the current detector 7 is preliminarily detected. A no-load current setting device 20 for setting an average current Ib at the time of no load averaged with the set average processing time Ta, a high-pass filter 10 for removing a low frequency of a current signal from the current detector 7, and a high An absolute value processor 11 for calculating a current change amount ΔI by performing absolute value processing on the high frequency wave that has passed through the band pass filter 10 and a current signal that changes from time to time from the current detector 7 are set in advance. A low-pass filter (or an average processor) 12 for calculating an average current Ia averaged for the average processing time Ta, a current change amount ΔI, an average current Ib, and a power loss correction coefficient K due to a preset belt intervention. And the average current Ib, the slip calculator 13 for calculating the current change rate A using the equation (1) A = ΔI / (Ia−K * Ib), and the current change rate A and the preset current change rate A slip discriminator 15 that discriminates the occurrence of a slip satisfying the formula (2) A> AD as compared with the threshold value AD, and a motor 6 based on a slip occurrence signal from the slip discriminator 15 and a signal from the diagnostic condition determiner 16. A false detection prevention device 17 that excludes a slip occurrence signal that is erroneously detected when the power is turned off based on a preset slip determination time Twd, and a display or alarm device that displays the occurrence of slip And a viewing device 18.
[0031]
FIG. 9 shows an example of a strip diagnosis processing procedure performed by the strip detection apparatus configured as described above.
(1). Conditions such as delay time Td, average processing time Ta, threshold value Ad of current change rate, slip determination time wd, A = ΔI / (Ia−K * Ib), power loss correction coefficient K due to belt intervention, etc. And start the diagnostic process by the operation signal from the operation signal relay 8. (2). The delay condition Td has elapsed from the start of operation by the diagnostic condition determiner 16, and based on the load signal from the load signal relay 22, the no-load current setter 20 detects the no-load state by the no-load signal. The current signal from the current detector 7 is processed with an average processing time Ta, and this is set as the average current Ib at the time of no load (3). A low-pass filter 12 that detects the transition of the load state as the delay time Td elapses from the start of operation and averages the current signal from the current detector 7 with an average processing time Ta, and an absolute value process after the high-pass filter 10 Processing with the vessel 11 (4). The absolute value processing unit 11 calculates the current change amount ΔI by performing absolute value processing on the average current Ia within the average processing time Ta immediately before passing through the low-pass filter 12 and the current passing through the high-pass filter 10 by the absolute value processor 11. Current change rate A = ΔI / (Ia−K * Ib) from the average current Ia of the motor and the power loss correction coefficient K due to the belt intervention set in (1) and the average current Ib in no load obtained in (2). (5). The current change rate A is compared with a preset current change rate threshold AD (6). If the current change rate A> the threshold value AD of the current change rate and the operation signal is ON even after the slip determination time Twd has elapsed, it is determined that a slip has occurred, and the monitoring device 18 such as a display or an alarm device is used. An alarm is output to, and the electric motor 6 is stopped as necessary.
[0032]
The setting of the correction coefficient K in (A) and (B) of the above embodiment is generally determined by the number of pulleys that receive belt tension, but for example, two pulleys with 180 ° brazing and 90 ° When there are two brazing pulleys, it is preferable to set a correction coefficient K = about 0.2.
[0033]
It should be noted that the apparatus configuration for slip detection processing and the processing procedure for slip detection in the present invention are not limited to the contents of the above-described embodiments, but the type, structure, scale, and transferred object of the transfer equipment to be applied. Depending on the type of object, maximum load, coefficient of friction of the drive transmission unit, motor type, capacity, required level of slip management accuracy, etc., there are changes within a range that can satisfy the claims of the present invention. .
[0034]
【The invention's effect】
In the present invention, the current that changes every moment after the start of operation in the electric motor that is the drive source of the drive transmission unit is measured, and a high-frequency current that is directly related to the occurrence of slip is calculated as the current change amount. The occurrence of slip is detected by the current change rate, which is the ratio to the difference current obtained from the difference between the average current and the no-load average current. Slip management accuracy can be ensured stably.
Further, the strip detection process can be simplified and the equipment cost can be reduced without the necessity of installing many detectors (rotation detector, belt speedometer, etc.) as in the prior art.
[Brief description of the drawings]
FIG. 1 is a conceptual explanatory diagram of a current change in a drive motor when a slip occurs between a drive transmission unit and a material to be transported in a conveyance facility having a drive transmission unit due to friction.
FIG. 2 is an explanatory diagram showing an example of current change over time in a drive motor when a slip occurs between a drive transmission unit and a material to be conveyed in a conveyance facility having a drive transmission unit due to friction.
FIG. 3 is an explanatory diagram showing an example of current change when the filter is passed in the example of current change of FIG. 2;
FIG. 4 is an explanatory diagram showing current change as a change in current change rate.
FIG. 5 is an explanatory diagram of an example of current change after passing through a filter that uses slip detection for detection in the present invention and a current change rate.
FIG. 6 is a conceptual explanatory view showing another embodiment in which the present invention is applied to slip detection between a driving pulley and a belt on a belt conveyor.
7 is an explanatory diagram showing an example of a slip detection processing procedure in the embodiment of FIG. 6. FIG.
FIG. 8 is a conceptual explanatory view showing another embodiment in which the present invention is applied to detection of slip between a driving pulley and a belt on a belt conveyor.
FIG. 9 is an explanatory diagram showing an example of a slip detection processing procedure in the embodiment of FIG. 8;
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Belt conveyor 2 Drive pulley 3 Driven pulley 4 Conveyed object 5 Belt 6 Electric motor 7 Current detector 8 Operation signal relay 9 Slip detection processing device 10 High-pass filter 11 Absolute value processor 12 Low-pass filter (or average processor) )
DESCRIPTION OF SYMBOLS 13 Slip calculator 14 Diagnosis setting device 15 Slip discrimination device 16 Diagnosis condition determination device 17 Error detection prevention device 18 Slip monitoring device 19 Conveyed object detector 20 No-load current setting device 21 Slip detection processing device 22 Load signal relay

Claims (3)

In a transport facility where a transport medium is interposed between the drive transmission unit and the object to be transported due to friction, the current that changes every moment from the start of operation with the motor that is the drive source is measured, and this measured current is passed through a preset filter The current change amount (ΔI) with the average current in the preset average processing time immediately before is calculated by absolute value processing , and this current change amount (ΔI), the immediately preceding average current (Ia), and the conveyance medium intervene. Based on the no-load average current (Ib) corrected by the drive loss correction coefficient (K), the current change rate (A) is obtained by the following equation (1), and the obtained current change rate (A) is set. A slip detection method in a drive transmission unit by friction, characterized in that, when compared with a threshold value of a current change rate corresponding to the occurrence of slip, a slip occurrence is determined when the current change rate threshold value is exceeded.
A = ΔI / (Ia−K * Ib) (1)
here,
ΔI: Amount of current change due to slip (high frequency)
Ia: Last average current (low frequency)
Ib: Average current in the no-load state (setting or measurement)
K: Correction coefficient equivalent to drive drive unit independent operation   The slip detection in the friction drive unit of the transport facility according to claim 1, wherein the transport facility is a belt conveyor, and detects slip between a drive pulley that is a drive transmission unit and a belt that is a transport medium. Method. In a transport facility in which a transport medium is interposed between a drive transmission unit due to friction and an object to be transported, a current detector that is installed in a power supply circuit of an electric motor that is a drive source of the drive transmission unit and detects a current from the start of operation of the motor; A diagnostic condition setter that determines the diagnostic condition based on the operation signal relay and the signal from the operation signal relay, a filter that removes the low frequency component of the current signal from the current detector, and the absolute value of the current that has passed through this filter A current change amount (ΔI) is calculated from the average current at the immediately preceding average time after processing, and this current change amount (ΔI), the immediately preceding average current (Ia), and the driving loss correction coefficient (K The slip calculator for calculating the current change rate (A) by the following formula (1) based on the no-load average current (Ib) corrected in (1) , and the current change rate and the set slip occurrence. A slip judging device that compares and calculates a threshold value of a current change rate to be performed, a display device that displays the occurrence of a slip based on a slip occurrence signal from the slip judging device, and a monitoring device such as an alarm device. Slip detecting device in the friction drive unit of
A = ΔI / (Ia−K * Ib) (1)
here,
ΔI: Amount of current change due to slip (high frequency)
Ia: Last average current (low frequency)
Ib: Average current in the no-load state (setting or measurement)
K: Correction coefficient equivalent to drive drive unit independent operation

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