Electric cylinder system and method of detecting abnormality of electric cylinder

An electric cylinder system includes: a rod; a ball screw mechanism; a nut connected to the rod; a cylindrical body including a contact portion, the contact portion configured to be abutting the nut making the linear motion, the cylindrical body configured to support the ball screw mechanism in such a manner that the ball screw mechanism is displaceable in the axis direction of the ball screw; a strain detector fixed to the cylindrical body and configured to detect a value corresponding to a displacement of the ball screw mechanism; and a control section configured to make the ball screw mechanism be displaced by making the nut abut against the contact portion, and configured to detect an abnormality of the strain detector based on the value detected by the strain detector according to the displacement of the ball screw mechanism.

TECHNICAL FIELD

The present disclosure relates to an electric cylinder system and a method of detecting an abnormality of an electric cylinder.

BACKGROUND ART

Patent Document 1 discloses an electric press that presses a workpiece by moving a ram up and down. The electric press includes a load cell that detects a load applied on the ram. The electric press stores an output of the load cell when the load is zero, as a calibration quantity. Where the calibration quantity is equal to or exceeds a predetermined value, the electric press provides notification to urge replacement of the load cell. The predetermined value is a calibration quantity that may cause the load cell output to exceed an upper limit value or fall below a lower limit value during displacement of a position of pressing operation of the ram from a zero point at which there is no load to a maximum load position. The predetermined value is set on an assumption that the relationship between the load cell output and the position of the ram is linear.

CITATION LIST

Patent Document

SUMMARY OF INVENTION

Technical Problem

Since the electric press described in Patent Document 1 determines replacement of the load cell based only on the output of the load cell when there is no load, the electric press may fail to correctly make a determination on replacement of the load cell. For example, even though the output of the load cell when there is no load falls within an allowable range, the output of the load cell when there is a load may fail to fall within the allowable range.

The present disclosure provides an electric cylinder system that enables enhancing the accuracy of detection of an abnormality of a strain detector.

Solution to Problem

An electric cylinder system according to an aspect of the present disclosure includes: a rod; a ball screw mechanism including a ball screw making a rotary motion via a drive force of an electric motor; a nut connected to the rod, the nut making a linear motion in an axis direction of the ball screw together with the rod via the rotary motion of the ball screw; a cylindrical body including a contact portion, the nut making the linear motion abutting against the contact portion, the cylindrical body supporting the ball screw mechanism in such a manner that the ball screw mechanism is displaceable in the axis direction of the ball screw; a strain detector fixed to the cylindrical body and detecting a value corresponding to the displacement of the ball screw mechanism; and a control section making the ball screw mechanism be displaced by making the nut abut against the contact portion, and detecting an abnormality of the strain detector based on the value detected by the strain detector according to the displacement of the ball screw mechanism.

In this electric cylinder system, the nut makes a linear motion in the axis direction of the ball screw via a rotary motion of the ball screw. The nut making the linear motion abuts against the contact portion of the cylindrical body, causing a load to be generated on the ball screw mechanism with the contact portion of the cylindrical body as a fulcrum. The ball screw mechanism is displaced in the axis direction of the ball screw by the generated load. The strain detector detects a value corresponding to the displacement of the ball screw mechanism. In this way, this electric cylinder system can apply a load to the strain detector without using a workpiece. Therefore, the control section of this electric cylinder can detect an abnormality of the strain detector based on the value detected by the strain detector when there is a load. Therefore, this electric cylinder system enables enhancing the accuracy of detection of an abnormality of the strain detector in comparison with the case of detecting an abnormality of the strain detector based only on an output of the strain detector when there is no load.

In an embodiment, the strain detector may include an outer edge portion fixed to the cylindrical body and a movable portion being provided on an inner side of the outer edge portion and being displaceable in the axis direction of the ball screw, the ball screw mechanism may include a plurality of bearings rotatably supporting the ball screw, and the plurality of bearings may include a first bearing located between the contact portion of the cylindrical body and the strain detector and a second bearing holding the movable portion of the strain detector jointly with the first bearing. In this case, no matter which side in the axis direction of the ball screw the ball screw mechanism is displaced to, the strain detector can detect the displacement.

In an embodiment, the cylindrical body may include a first cylindrical body supporting the first bearing, the contact portion being provided in the first cylindrical body and a second cylindrical body supporting the second bearing. In this case, processing of the contact portion of the cylindrical body is easy, and thus, it is possible to reduce time necessary for manufacture of the cylindrical body in comparison with the case of manufacturing the cylindrical body from a single member.

In an embodiment, a gap may be provided between the contact portion of the cylindrical body and the first bearing, in an axis direction of the ball screw. Because of the gap being provided between the contact portion of the cylindrical body and the first bearing, the ball screw mechanism including the first bearing can move in the direction approaching the contact portion by the amount corresponding to the gap. Therefore, until the first bearing comes into contact with the contact portion, the strain detector can properly measure an amount of displacement of the ball screw mechanism for a load.

In an embodiment, the control section may detect the abnormality of the strain detector based on a comparison between the value corresponding to the displacement of the ball screw mechanism, the value being detected by the strain detector at a predetermined load value, and a reference value acquired in advance at the predetermined load value. In this case, this electric cylinder system can detect an abnormality of the strain detector based on a reference value acquired in advance.

A method of detecting an abnormality of an electric cylinder according to another aspect of the present disclosure includes: a step of making a ball screw of the electric cylinder make a rotary motion via a drive force of an electric motor, making a nut attached to the ball screw make a linear motion in an axis direction of the ball screw and making the nut making the linear motion abut against a contact portion provided in a cylindrical body of the electric cylinder; a step of making a ball screw mechanism including the ball screw be displaced in the axis direction of the ball screw with the contact portion provided in the cylindrical body as a fulcrum, by the abutted nut; a step of detecting a value corresponding to the displacement via a strain detector fixed to the cylindrical body; and a step of detecting an abnormality of the strain detector based on the value corresponding to the displacement, the value being detected in the step of detecting.

In the method of detecting an abnormality of an electric cylinder, the nut making a linear motion abuts against the contact portion provided in the cylindrical body of the electric cylinder. The abutted nut causes a load to be generated with the contact portion provided in the cylindrical body as a fulcrum, and the ball screw mechanism including the ball screw is displaced in the axis direction of the ball screw by the generated load. A value corresponding to the displacement of the ball screw is detected by the strain detector fixed to the cylindrical body. Then, an abnormality of the strain detector is detected based on the valve corresponding to the displacement of the ball screw. In this way, this method of detecting an abnormality of an electric cylinder enables applying a load to the strain detector without using a workpiece. Therefore, the control section of this electric cylinder can detect an abnormality of the strain detector based on the value detected by the strain detector when there is a load. Therefore, this method of detecting an abnormality of an electric cylinder enables enhancing the accuracy of detection of an abnormality of the strain detector in comparison with the case of detecting an abnormality of the strain detector based only on an output of the strain detector when there is no load.

Advantageous Effects of Invention

With an electric cylinder system according to the present disclosure, it is possible to enhance the accuracy of detection of an abnormality of a strain detector.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present disclosure will be described below with reference to the drawings. In the below description, elements that are identical or correspond to each other are provided with a same reference sign and overlapping description thereof is not repeated. Dimension ratios in the drawings do not necessarily agree with those in the description. The terms “upper (up)”, “lower (down)”, “left” and “right” are those based on the illustration and used for the sake of expediency.

[Configuration of Electric Cylinder System]

FIG.1is a schematic diagram illustrating an example of an electric cylinder system according to an embodiment. The X-direction and the Y-direction in the drawings are horizontal directions and the Z-direction in the drawings is a vertical direction. The X-direction, the Y-direction and the Z-direction are axis directions orthogonal to one another in a three-dimensional orthogonal coordinate system. The electric cylinder system100illustrated inFIG.1is a system that presses a workpiece (not illustrated) to perform, e.g., shaping or press-fitting. The electric cylinder system100includes an electric cylinder1and a control device50. The electric cylinder1is fixed to a frame2. The workpiece is placed on a workpiece table2aof the frame2and is pressed between the workpiece table2aand a distal end of a rod of the electric cylinder1by the rod being extended.

The electric cylinder1includes an electric motor10, a rotation transmission mechanism20and a cylinder portion30. InFIG.1, the rotation transmission mechanism20and the cylinder portion30are each illustrated in section. The electric motor10is connected to the cylinder portion30via the rotation transmission mechanism20. A drive force generated by the electric motor10is transmitted to the cylinder portion30via the rotation transmission mechanism20. A control device50is connected to the electric cylinder1and controls operation of the electric cylinder1. More specifically, the control device50are communicably connected to the electric motor10and the cylinder portion30and controls the electric motor10.

The electric motor10generates a drive force via electric power supplied from the control device50. The electric motor10is, for example, a servomotor. The electric motor10includes a motor body11and an encoder12. The motor body11includes a motor shaft111. The motor body11is supplied with electric power from the control device50and rotates the motor shaft111around an axis direction (here, the Z-direction) of the motor shaft111. The encoder12detects a rotational angle of the motor shaft111and feeds the rotational angle back to the control device50. The drive force generated by the electric motor10is transmitted to the rotation transmission mechanism20through the motor shaft111.

The rotation transmission mechanism20transmits the drive force of the electric motor10to the cylinder portion30. The rotation transmission mechanism20includes a casing21, two timing pulleys22,24, a timing belt23, a rotary shaft25and two bearings26,27. The casing21receives the two timing pulleys22,24, the timing belt23, the rotary shaft25and the two bearings26,27inside. The electric motor10is fixed to the outer side of the casing21and the motor shaft111of the electric motor10is connected to the timing pulley22. The cylinder portion30is fixed to the outer side of the casing21in such a manner as to be provided side by side with the electric motor10. The rotary shaft25rotatably supported by the two bearings26,27is connected to the timing pulley24. The two timing pulleys22,24are linked by the timing belt23. The drive force generated by the electric motor10is transmitted from the timing pulley22to the timing pulley24via the timing belt23and rotates the rotary shaft25around an axis direction (here, the Z-direction) of the rotary shaft25.

The cylinder portion30operates based on the drive force transmitted from the rotation transmission mechanism20. The cylinder portion30includes a cylindrical body30a, a strain detector33, a ball screw mechanism35, a nut356, a rod357, a reducer36and a sliding key37. The cylindrical body30areceives or holds the strain detector33, the ball screw mechanism35, the nut356, the rod357and the reducer36. An opening that enables access of the sliding key37to a groove of the rod, which will be described later, is provided in an outer circumferential surface of the cylindrical body30a. The reducer36is connected to the ball screw mechanism35via a spline portion35eof a ball screw353. The rotary shaft25is connected to the reducer36. The drive force of the rotary shaft25is transmitted to the ball screw mechanism35via the reducer36and the spline portion35eof the ball screw353and then transformed to a drive force for linear motion of the rod by the ball screw mechanism35and the sliding key37.

FIG.2is a partial enlargement of the section of the ball screw mechanism inFIG.1. As illustrated inFIGS.1and2, the ball screw mechanism35includes two bearings351,352(an example of a plurality of bearings), the ball screw353, a collar354and a bearing nut355. The ball screw353extends in the axis direction of the rotary shaft25and includes a thread portion35a, a columnar portion35band a spline portion35e. The thread portion35ais a rod-like member including a thread at an outer circumferential surface, the nut356being threadably engaged with the rod-like member. The columnar portion35bis connected to an end of the thread portion35aand has a shape with a diameter reduced relative to the thread portion35a. At the end of the thread portion35a, a step surface35dis formed because of a difference in diameter from the columnar portion35b. The spline portion35eis formed at an end of the columnar portion35b, has, for example, a spline shape that is based on an involute curve at an outer circumferential surface, and is fitted in a spline shape provided at an inner circumferential surface of the reducer36. The columnar portion35bis rotatably supported by the two bearings351,352. The bearing351(an example of a first bearing) is provided in such a manner as to be in contact with the step surface35dof the thread portion35a. The bearing352(an example of a second bearing) is located on the end of the columnar portion35b. The two bearings351,352jointly hold the collar354. The collar354is held between the two bearings351,352. Furthermore, the bearing nut355is provided on the end of the columnar portion35b. The bearing nut355holds the bearings351,352holding the collar354, jointly with the step surface35dof the thread portion35a.

The nut356is threadably connected to the ball screw353. A rotative force of the ball screw353is transmitted to the nut356. The nut356is connected to the rod357. A keyway provided in the rod357fits in the sliding key37provided at the cylindrical body30a. The rotative force transmitted to the nut356is restricted from rotary motion, by the keyway of the rod357and the sliding key37and becomes a drive force in an axis direction (here, the Z-direction) of the ball screw353. In this way, the nut356makes a linear motion together with the rod357in the axis direction of the ball screw353via a rotary motion of the ball screw353.

The cylindrical body30asupports the ball screw mechanism35in such a manner that the ball screw mechanism35is displaceable in the axis direction of the ball screw353. The cylindrical body30aincludes a cylindrical body31(an example of a first cylindrical body), a cylindrical body32(an example of a second cylindrical body) and a cylindrical body34and is configured by the cylindrical body31, the cylindrical body32and the cylindrical body34being joined. The cylindrical body31receives the bearing351and supports the bearing351in such a manner that the bearing351is displaceable in the axis direction of the ball screw353. The inner diameter of the cylindrical body31is substantially equal to the outer diameter of the bearing351. The cylindrical body31includes a contact portion31bthat the nut356in a linear motion abuts against. The contact portion31bis a part (flange) that projects to the inner side of the cylindrical body31at a distal end of the cylindrical body31. The inner diameter of the contact portion31bis smaller than the outer diameter of the nut356. Therefore, the nut356abuts against the contact portion31bwhen moving to an end of the ball screw mechanism35.

A gap31ais provided between the contact portion31band the bearing351. In other words, the contact portion31band the bearing351are not in contact with each other, allowing displacement, in the axis direction of the ball screw353, of the ball screw mechanism35by the amount corresponding to the gap31a. The gap31ais, as an example, 0.5 to 2.0 mm, but is not limited to such size range, in other words, may be a gap of any size as long as such gap enables the later-described strain detector33to detect displacement of the ball screw mechanism35.

The cylindrical body32receives the bearing352and supports the bearing352in such a manner that the bearing352is displaceable in the axis direction of the ball screw353. In other words, the bearing351and the bearing352are displaceably supported by the cylindrical body31and the cylindrical body32, respectively. The ball screw mechanism35is supported by the cylindrical body31and the cylindrical body32in such a manner as to be displaceable in the axis direction of the ball screw353.

The strain detector33detects a value corresponding to a displacement of the ball screw mechanism35. The strain detector33is, as an example, a load cell. The strain detector33includes an outer edge portion33aand a movable portion33b. The strain detector33is, as an example, a plate-like member. The outer edge portion33aforms an edge of the plate-like member and is fixed to the cylindrical body30a. As an example, the outer edge portion33ais held between the cylindrical body31and the cylindrical body32. The movable portion33bis provided on the inner side of the outer edge portion33aand is displaceable in the axis direction of the ball screw353. For example, the movable portion33bis connected to the outer edge portion33avia an elastic portion33con the inner side of the outer edge portion33aof the plate-like member. A strain gauge is provided in the elastic portion33c. The strain gauge outputs a value corresponding to a displacement of the movable portion33bbased on strain of the elastic portion33caccording to the displacement of the movable portion33b.

The bearing351is located between the contact portion31bof the cylindrical body30aand the movable portion33bof the strain detector33. The bearing352holds the movable portion33bof the strain detector33jointly with the bearing351. Consequently, the movable portion33bis held between the bearing351and the bearing352and is displaced together with the bearing351and the bearing352. A displacement of the ball screw mechanism35is transmitted to the movable portion33bvia the bearing351and the bearing352.

FigureFIGS.3A and3Binclude conceptual diagrams of a case where the ball screw mechanism35is displaced by the nut356abutting against the contact portion31b.FIG.3Aindicates a position of the ball screw mechanism35before the nut356abuts against the contact portion31b. There is a space between the nut356and the contact portion31band the nut356and the contact portion31bare not in contact with each other. The gap31ais provided between the bearing351and the contact portion31b. A gap32athat is substantially equal in width to the gap31ais also provided between the bearing352and the cylindrical body32. The movable portion33bis not displaced and positioned in plane with the outer edge portion33a.

FIG.3Billustrates the position after the ball screw mechanism35is displaced. Upon the nut356being brought into a linear motion in a contraction direction of the rod357, the nut356abuts against the contact portion31b. The nut356displaces the ball screw mechanism35in an extension direction of the rod357with the contact portion31bas a fulcrum. As a result of the displacement of the ball screw mechanism35in the extension direction of the rod357, the gap31abecomes smaller and the gap32abecomes wider. The movable portion33bis displaced together with the ball screw mechanism35and becomes closer to the nut356than the outer edge portion33ais. In this way, the electric cylinder1can provide a load to the strain detector33without using a workpiece. Note that since the ball screw353and the reducer36are connected via the spline portion35eof the ball screw353, transmission of a rotative force is not affected even if the ball screw mechanism35is displaced in the axis direction of the ball screw353.

[Configuration of Control Device]

FIG.4is a schematic diagram illustrating the relationship among the control device50, the cylinder portion30and the electric motor10. The control device50includes a control section51and a motor driver52. The control section51bidirectionally communicably connected to the motor driver52. The control section51is, for example, a servo controller or a programmable logic controller. The control section51may be configured by, for example, a general-purpose computer including, e.g., an arithmetic device such as a CPU (central processing unit), storage devices such a ROM (read-only memory), a RAM (random access memory) and an HDD (hard disk drive) and a communication device.

A signal corresponding to a displacement of the movable portion33bof the strain detector33is input to the control section51. The motor driver52is, for example, a servo amplifier. A signal corresponding to a rotational angle of the motor shaft111is input from the encoder12to the motor driver52. The motor driver52controls the electric motor10by making an electric current flow in the motor body11based on the signal input from the encoder12. A torque load factor of the electric motor10is calculated based on a value of the current flowing at this time and a rated current value of the electric motor10.

FIG.5is a block diagram illustrating the control section51of the control device50. The control section51includes a load cell amplifier section511, an A/D conversion section512, a gain setting section514, an offset setting section513, a load value conversion section515, an arithmetic section516, a storage section517and a motor driver communication section518.

The load cell amplifier section511converts a displacement of the movable portion33binto a voltage value signal. The A/D conversion section512converts the voltage value signal input from the load cell amplifier section511into a digital electric signal. The gain setting section514adjusts a gain by multiplying the digital electric signal input from the A/D conversion section512by a multiplier. The offset setting section513adjusts an output of the strain detector33when there is no load, by adding a correction value to the digital electric signal input from the gain setting section514. The load value conversion section515converts the digital electric signal input from the offset setting section513from the digital electric signal corresponding to the displacement of the movable portion33bto a digital electric signal corresponding to a load applied to the movable portion33b.

The motor driver communication section518bidirectionally communicates with the motor driver52and the arithmetic section516. For example, the value of the current flowing in the motor body11, the torque load factor based on the current value or the rotational angle of the motor shaft111is input from the motor driver52to the motor driver communication section518. The motor driver communication section518outputs a signal for controlling the electric motor10to the motor driver52based on an instruction input from the arithmetic section516.

The arithmetic section516outputs the load received by the movable portion33bto the storage section517based on the digital electric signal input from the load value conversion section515. The arithmetic section516outputs, e.g., the value of the current applied to the motor body11, the torque load factor based on the current value or the rotational angle of the motor shaft111(an example of a value corresponding to the displacement of the ball screw mechanism), which has been input from the motor driver communication section518, to the storage section517. Also, the arithmetic section516outputs an instruction for controlling the electric motor10to the motor driver52by referring to the storage section517.

The arithmetic section516performs control of the electric motor10based on the load on the strain detector33, the load being stored in the storage section517, and detects an abnormality of the strain detector33based on the torque load factor of the electric motor10at this time. For example, the arithmetic section516stores a torque load factor of the electric motor10and a load on the movable portion33bin the storage section517as reference values. The reference value is a torque load factor when a load is actually imposed. Next, the arithmetic section516outputs an instruction for pushing the nut356against the contact portion31bto the motor driver52, until the resulting load becomes equal to the load stored in the storage section517. The arithmetic section516compares the torque load factor of the electric motor10obtained as a result of the nut356being pushed against the contact portion31band the torque load factor (an example of a reference value) stored in the storage section517with each other and the arithmetic section516detects an abnormality of the strain detector33.

[Operation of Electric Cylinder System]

Next, an example of the process of the electric cylinder1detecting an abnormality of the strain detector33will be described.FIG.6is a flowchart illustrating an example of the process of detecting an abnormality of a strain detector. The flowchart illustrated inFIG.6is executed by the control device50.

As illustrated inFIG.6, first, as abutment processing (step S1), the control device50makes the ball screw353of the electric cylinder1make a rotary motion via a drive force of the electric motor10and makes the nut356attached to the ball screw353make a linear motion in the axis direction (contraction direction) of the ball screw353to bring the nut356in a linear motion into abutment with the contact portion31bprovided in the cylindrical body30aof the electric cylinder1.

Subsequently, as displacement processing (step S2), the control device50makes the ball screw mechanism35including the ball screw353be displaced in the axis direction of the ball screw353with the contact portion31bof the cylindrical body30aas a fulcrum, by the abutted nut356(FIG.3B).

Subsequently, as displacement detection processing (step S3), the control device50detects a value corresponding to the displacement, via the strain detector33fixed to the cylindrical body30a.

Subsequently, as abnormality determination processing (step S4), the control device50detects an abnormality of the strain detector33based on the value corresponding to the displacement, which has been detected in the displacement detection processing (step S3). The control device50calculates the torque load factor of the electric motor10based on, for example, a displacement of the ball screw mechanism35, the displacement being detected by the strain detector33at a predetermined load value. Then, the control device50detects an abnormality of the strain detector33based on a comparison between the torque load factor of the electric motor10and the reference value (torque load factor when a load is actually imposed) stored in the storage section517. For example, if the absolute value of the difference between the torque load factor of the electric motor10and the reference value is equal to or below a threshold value set in advance, the control device50determines that the strain detector33is normal. For example, if the absolute value of the difference between the torque load factor and the reference value of the electric motor10exceeds the threshold value set in advance, the control device50determines that the strain detector33is abnormal. Upon an end of the abnormality determination processing (step S4), the flowchart illustrated inFIG.6ends.

By the flowchart illustrated inFIG.6being executed, it is possible to compare a past torque load factor actually measured at a predetermined load value and a current torque load factor actually measured for the predetermined load value with each other and detect an abnormality of the strain detector33.

The control device50can also detect an abnormality of the strain detector33by varying the predetermined load value described with reference toFIG.6and performing comparison for a plurality of load values.FIG.7is a flowchart illustrating the process of torque load factors being stored as a reference value.FIG.8is a flowchart illustrating the process of detecting an abnormality of the strain detector33by comparing a torque load factor with a reference value, for each target load.FIG.9is a graph illustrating the relationship between time and the torque load factor for each target load. The processes illustrated inFIGS.7and8are executed by the control device50.

In the process inFIG.7, as an example, three target loads are set. This process is performed using the electric cylinder system100calibrated. A target load is a value of a load applied to the strain detector33by the control device50when detecting an abnormality of the strain detector33. In the process inFIG.7, with a rated thrust force of the electric cylinder1as a maximum load value, values obtained by dividing the maximum load value into three are set as a first target load, a second target load and a third target load, respectively. These target loads are equal to the target loads inFIG.8. Also, torque load factors for these target loads are indicated inFIG.9.

As illustrated inFIG.7, in step S10, the control device50makes the rod357and the nut356make a linear motion in the contraction direction of the rod357in the axis direction of the ball screw353and abut against the contact portion31b. The abutted nut356displaces the ball screw mechanism35in the extension direction of the rod357with the contact portion31bas a fulcrum. The movable portion33bis displaced together with the ball screw mechanism35and becomes closer to the nut356than the outer edge portion33ais. The control device50measures a load on the strain detector33based on a signal corresponding to the displacement of the movable portion33b. When the load reaches the first target load, the control device50stops the rod357and the nut356with the rod357and the nut356maintained in abutment with the contact portion31b. In other words, the control device50controls the electric motor10in such a manner that the load applied to the strain detector33from the ball screw mechanism35reaches the first target load.

In step S12, the control device50calculates a torque load factor of the electric motor10when the first target load is applied to the strain detector33and stores the torque load factor in the storage section517as a reference value. As illustrated inFIG.9, a torque load factor largely varies immediately after reaching a target load and then stabilizes. If a largely varying torque load factor is stored as a reference value, detection of an abnormality of the strain detector33is not correctly performed, and thus, a stabilized torque load factor is stored as a reference value. As an example, the control device50can calculate a stable torque load factor by a torque load factor stable range being set.

The procedure of the control device50with a torque load factor stable range set calculating a torque load factor will be described with reference toFIG.9. Even if a load value of the strain detector33reaches the first target load (l1), the control device50with a torque load factor stable range set maintains such state for a certain period (t1). After a lapse of the certain period (t1), the control device50goes back in time and calculates an average value of the torque load factor and excludes a time (t1a) during which the torque load factor falls out of the torque load factor stable range from the calculation of the average value. The control device50stores the average value of the torque load factor that have been calculated through the above process in the storage section517as a reference value of the torque load factor. In the below, likewise, for the second target load (l2), the control device50performs processing for excluding a time (t2a) during which the torque load factor falls out from the torque load factor stable range from calculation of an average value and performs processing similar to the above also for the third target load (l3).

In steps S14to S16, operations that are the same as those in steps S10to S12above are performed with the target load changed to the second target load. Also, in steps S18to S20, operations that are the same as those in steps S10to S12above are performed with the target load changed to the third target load. In steps S10to S20, the torque load factors of the electric motor10for the three target loads are stored in the storage section517as reference values. In step S22, the control device50makes the rod357and the nut356abutting against the contact portion31bmove to a predetermined original position at which no load is applied to the strain detector33in the axis direction of the ball screw353. Upon an end of step S22, the flowchart illustrated inFIG.7ends.

Next, the process of detecting an abnormality of the strain detector33will be described. Detection of an abnormality of the strain detector33is performed during regular work, and for example, is performed during an inspection before a start of work. As illustrated inFIG.8, in step S30, the control device50performs an operation that is the same as that in step S10and controls the electric motor10in such a manner that a load applied from the ball screw mechanism35to the strain detector33reaches the first target load.

In step S32, the control device50calculates a torque load factor of the electric motor10when the first target load is applied to the strain detector33and performs a comparison with the torque load factor in step S12. More specifically, the arithmetic section516compares the torque load factor in step S12stored in the storage section517as a reference value and the actual torque load factor in step S32with each other. The comparison refers to the work of setting a threshold value for the torque load factor in step S12stored in the storage section517as a reference value and determining whether or not an actual torque load factor falls within the threshold value.

In steps S34to S36, operations that are the same as those in steps S30to S32above are performed with the target load changed to the second target load. Also, in steps S38to S40, operations that are the same as those in steps S30to S32above are performed with the target load changed to the third target load. In step S42, as in step S22, the control device50makes the rod357and the nut356move to the predetermined original position.

In step S44, an abnormality of the strain detector33is detected based on respective results of the comparisons in steps S32, S36and S40. As an example, if the results of the torque load factor comparisons for the respective target loads all fall within the threshold value, the arithmetic section516determines that the strain detector33is normal and outputs a normality determination in step S46. In a case not falling under the above, the arithmetic section516determines that there is an abnormality in the strain detector33and outputs the abnormality determination in step S48.

The process in steps S10to S22is stored in the storage section517as an operation program for master data acquisition. Also, the process in steps S30to S42is stored in the storage section517as an operation program for detection of an abnormality of the strain detector33. The control device50executes the above programs via an operator's operation.

Conclusion of Embodiment

According to the electric cylinder system100and the method of detecting an abnormality of the electric cylinder1, the nut356makes a linear motion in the axis direction of the ball screw353via a rotary motion of the ball screw353. The nut356in the linear motion abuts against the contact portion31bof the cylindrical body30a, causing a load to be generated on the ball screw mechanism35with the contact portion31bof the cylindrical body30aas a fulcrum. The ball screw mechanism35is displaced in the axis direction of the ball screw353by the generated load. The strain detector33detects a value corresponding to the displacement of the ball screw mechanism35. In this way, this electric cylinder system100can apply a load to the strain detector33without using a workpiece. Therefore, the control section51of this electric cylinder1can detect an abnormality of the strain detector33based on the value detected by the strain detector33when there is a load. In other words, the control section51can detect an abnormality of the strain detector33in consideration of variation in relationship between the output value of the strain detector33and the load due to age-related deterioration (variation due to curing and deterioration of an adhesive material for the strain gauge). Therefore, this electric cylinder system100enables enhancing the accuracy of detection of an abnormality of the strain detector33in comparison with the case of detecting an abnormality of the strain detector33based only on an output of the strain detector33when there is no load.

Because of the cylindrical body30aincluding the cylindrical body31supporting the bearing351, the contact portion31bbeing provided in the cylindrical body31, and the cylindrical body32supporting the bearing352, processing of the contact portion31bis easy, and thus, it is possible to reduce time necessary for manufacture of the cylindrical body30ain comparison with the case of manufacturing the cylindrical body30afrom a single member.

Because of the gap31abeing provided between the contact portion31bof the cylindrical body30aand the bearing351in the axis direction of the ball screw353, the ball screw mechanism35can move in the direction approaching the contact portion31bby the amount corresponding to the gap31a. Therefore, until the bearing351comes into contact with the contact portion31b, the strain detector33can properly measure an amount of displacement of the ball screw mechanism35for a load.

Although various exemplary embodiments have been described above, the present disclosure is not limited to the above exemplary embodiments, but various omissions, replacements and changes may be made. For example, detection of an abnormality of the strain detector33may be performed with the torque load factor replaced with the current value. Also, with target torque load factors set instead of the target loads, the process of detecting an abnormality of the strain detector33by performing comparison of a load at each target torque load factor may be provided. An abnormality of the strain detector33may be detected by calculating torque load factors for a plurality of target loads and evaluating the linearity of the output of the strain detector33.

The electric cylinder system100may detect an abnormality of the strain detector33when pressing a workpiece. In this case, a workpiece pressing portion35cof the rod357in a linear motion in the extension direction abuts against a workpiece (not illustrated). The rod357displaces the ball screw mechanism35in the contraction direction of the rod357with the workpiece as a fulcrum. As a result of the displacement in the contraction direction of the ball screw mechanism35, the gap32abecomes smaller and the gap31abecomes wider. The movable portion33bis displaced together with the ball screw mechanism35and becomes more away from the nut356than the outer edge portion33ais. The control device50measures a load on the strain detector33based on a signal corresponding to the displacement of the movable portion33b. When the load reaches the first target load, the control device50makes the rod357stop with the rod357maintained in abutment with the workpiece. By operating in this way, the electric cylinder system100can detect an abnormality of the strain detector33when pressing a workpiece. Such operation is implemented by holding the movable portion33bof the strain detector33via the two bearings351,352. With such configuration, no matter which side in the axis direction of the ball screw353the ball screw mechanism35is displaced to, the strain detector33can detect the displacement.

In the electric cylinder system100, a contact portion that a distal end of the nut356abuts against may be provided at an inner portion of the cylindrical body34. In this case, the electric cylinder system100can detect an abnormality of the strain detector33through a procedure that is the same as that of the above-described case of pressing a workpiece.

The electric cylinder1may include no bearings351,352. The cylindrical body30amay be configured by a single member. The cylindrical body31and the cylindrical body34may be formed integrally. The strain detector33only needs to be fixed to the cylindrical body30aand is not limited to being held between separated cylindrical bodies.

REFERENCE SIGNS LIST