US3590969A

US3590969A - Fixed point stop mechanism for driven rotary machine

Info

Publication number: US3590969A
Application number: US828844A
Authority: US
Inventors: Sadayuki Kajitani; Masahiro Yokoyama; Yukio Kajino
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1968-06-03
Filing date: 1969-05-29
Publication date: 1971-07-06
Anticipated expiration: 1988-07-06
Also published as: BE733928A; GB1247264A; DE1928138A1

Abstract

An electric motor puts a sewing machine in its normal operation through a clutch. Upon stopping the machine, a transistor and a capacitor fire a thyristor to permit the energization of an electromagnet. This causes the clutch to be partly braked to put the machine in sustained rotation at a low speed. When a position sensor senses the machine being at a selected one of its predetermined positions the electromagnet is deenergized to stop the machine at that selected position through the operation of the brake.

Description

United States Patent Inventors Sadayuki Kajitani;

Masahiro Yokoyama, Yukio Kajino. all of Nagoya, Japan Appl. No. 828,844

Filed May 29, 1969 Patented July 6, 1971 Assignee Mitsubishi Denki Kabushiki Kaisha Tokyo, Japan Prlority June 3, I968 FIXED POINT STOP MECHANISM FOR DRIVEN ROTARY MACHINE 7 Claims, 5 Drawing Figs.

11.8. C1 192/146, 112/219,192/16,192/18 Int. Cl ..F16d 71/04, D05b 69/26 Field 01 Search..... 192/16, 18,

[56] References Cited UNlTED STATES PATENTS 2.481.028 9/1949 Lear. 192/18 .21 2,739,251 3/1956 lngalls ..192/18 (.2) (X) 2,920,730 1/1960 Shapiro 192/18 3,089,573 5/1963 Walker.... 192/18 (.2) 3.157.261 11/1964 Bono. 192/146 (X) 3,160,128 12/1964 Hcidt 112/219 (A) 3,268,047 8/1966 Grygera et a1. 112/219 (A) 3,352,396 11/1967 Moseley 112/219 (A) Primary Examiner-Allan D. Herrmann Artorneys- Robert E. Burns and Emmanuel J. Lobato ABSTRACT: An electric motor puts a sewing machine in its normal operation through a clutch. Upon stopping the machine, a transistor and a capacitor fire a thyristor to permit the energization of an electromagnet. This causes the clutch to be partly braked to put the machine in sustained rotation at a low speed. When a position sensor senses the machine being at a selected one of its predetermined positions the electromagnet is deenergized to stop the machine at that selected position through the operation of the brake.

FIXED POINT STOP MECHANISM FOR DRIVEN ROTARY MACHINE BACKGROUND OF THE INVENTION This invention relates in general to a fixed point stop mechanism for use with a driven rotary machine such as a sewing machine, and more particularly to improvements in such a mechanism for selectively stopping a driven rotary machine at its predetermined positions by controlling a clutch and a brake disposed between the machine and a driven port thereof without stopping the drive.

When driven rotary machines, for example, sewing machines are put in rotation by the respective electric motors it is commonly practiced to control the clutch and brake disposed between the machine and motor to selectively stop the machine at its predetermined positions while the motor is always put in operation. For sewing machines such predetermined positions may be, for example, the upper and lower ends of a stroke for the associated sewing needle. In the normal operation, a sewing machine is operatively coupled to its clutch to be driven with a predetermined number of rotation of from I000 to 5000 rpm. by the associated motor. Upon stopping the sewing machine, the clutch is caused to disengage from the motor and instead the associated brake is applied to the machine. Under these circumstances, if the sewing machine rotating with the number of rotation ranging from 1000 to 5000 rpm. is suddenly stopped it will undergo a considerable mechanical shock while it is very difficult to stop the machine at the particular predetermined position. To avoid those drawbacks, it has been previously proposed to prepare a pair of routes of transmission one of which is a primary route on which the sewing machine is driven in the normal mode of operation and the other of which is a secondary route on which the machine is driven with the number of rotation less than the normal number of rotation in unit time by the associated speed reduction gearing. Upon stopping the sewing machine, the primary route is changed over to the secondary route and then a time point when the machine has reached its predetermined position during its low speed operation through the secondary route is sensed whereupon a braking action is applied to the machine.

The measure just described has been necessary to meet two requirements for the number of rotation in unit time derived from the secondary route. First, the number of rotation should be low enough to instantaneously stop the sewing machine upon applying the brake to the machine being driven at a low speed due to the secondary route. Secondly, the number of rotation should be sizably high in order to decrease an interval of time required to stop the machine after the brake has been applied to the machine because the machine is stopped within one complete rotation thereof after the application of the brake. For example, if the number of rotation reduced is set to be equal to 150 to 200 rpm. the sewing machine is allowed to be stopped at a selected one of its predetermined positions as rapidly as within 0.3 to 0.4 seconds.

As above described, the conventional sewing machines have included the secondary route composed of a train of speed reduction gears. Thus if the normal number of rotation is controlled in accordance with the type of sewing operation such as the type of cloth to be sewn, it is accompanied by a change in the number of rotation provided by the secondary route. For example, if the normal number of rotation is set to be too low then a time required for effecting one complete rotation becomes longer with the result that the machine can not be expected to be rapidly stopped. On the contrary, if the normal number of rotation is set to be too high then a considerable amount of slip occurs in the machine after the brake has been applied to the latter until the machine is stopped. Therefore it is difficult to stop the machine at the particular predetermined position.

SUMMARY or THE INVENTION Accordingly, it is an object of the invention to provide a new and improved fixed point stop mechanism for a driven rotary machine capable of very accurately and simply stopping the driven machine at a selected one of its predetermined positions.

It is another object of the invention to provide a new and improved fixed point stop mechanism for a driven rotary machine which, upon stopping the driven machine, first puts the machine in its low speed operation and then stops it for the purpose of accurately stopping the machine at a selected one of its predetermined position.

It is still another object of the invention to provide a new and improved fixed point stop mechanism for a driven rotary machine ensuring that the driven machine is accurately stopped at a selected one of its predetermined positions such that the associated clutch device is controlled in coupling degree in accordance with the rotational speed of the driven machine to stabilize the low speed operation thereof.

Briefly the invention accomplishes these objects by the provision of a mechanism for selectively stopping a driven rotary machine, comprising a drive for driving the rotary machine, clutch means disposed between the driven and the machine to transmit a power from the drive to the driven machine therethrough in the normal mode of operation of the machine, and brake means for applying a braking action to the driven machine, characterized by control means responsive to a decrease in the number of rotation in unit time of the driven machine occurring upon stopping the latter for driving the clutch means into its partly clutching state thereby to effect the sustained rotation of the driven machine at a reduced speed, and position sensing means responsive to the driven machine being at its predetermined position to permit the release of the clutch means and the application of the brake means to stop the machine at the predetermined position.

In a preferred embodiment of the invention, the mechanisms for selectively stopping the driven rotary machine at its predetermined positions may comprise a flywheel disposed on the drive, a clutch plate opposing the flywheel and capable of engaging the flywheel to transmit power from the drive to the driven machine therethrough, brake means for applying a braking force to the driven machine, electromagnet means for controlling a coupling degree between the clutch plate and the flywheel to put the driven machine in its low speed operation, an electric valve for controlling an electric current for exciting the electromagnet means, control means for controlling the conduction of the electrical valve in accordance with the number of rotation in unit time of the driven machine, and position sensing means operative during the low speed operation of the machine to permit the clutch plate to disengage from the flywheel when the driven machine is located at its predetermined position while at the same time applying a braking action to the driven machine to stop the latter at that predetermined position.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic front elevational view of a sewing machine embodying the principles of the invention;

FIG. 2 is a front elevational view, partly in section of the essential parts of the device shown in FIG. 1;

FIG. 3 is a wiring diagram of a control circuit constructed in accordance with the principles of the invention;

FIGS. 4a and b are curves useful in explaining the operation of the invention.

Throughout several FIGURES like reference numerals designate the identical components.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is applicable to a variety of driven rotary machines it is particularly suitable for use with sewing machines and the description will now be made in conjunction with a domestic sewing machine.

Referring now to FIG. 1 of the drawings, it is seen that a sewing machine generally designated by the reference numeral comprises a bed 11, an arm 12 including a root portion 13 vertically uprising from the upper surface of the bed 11, and a trunk portion 14 horizontally projecting beyond the upper end of the root portion 13 and a head portion 15 disposed at the end of the trunk portion 14. An arm shaft 16 is rotatably disposed within the trunk portion 14 and serves to vertically move a sewing needle 17 through a cam mechanism (not shown) disposed within the head portion 15. That end portion of the arm shaft 16 extending beyond the trunk portion 12 has a driven pulley 18 rigidly secured thereon and a position sensing device mounted thereon and generally designated by the reference numeral 20.

As shown in FIG. 3, the position sensing device 20 comprises a circular annulus 21 including an annular segment 22 of any suitable electrically conductive material such as copper and an annular segment 23 of any suitable electrically insulating material such as mica plate disposed around the arm shaft 16. The annulus 21 is fixed in electrically insulated relationship on the arm shaft 16. A pair of diametrically opposite brushes 24 and 25 of any suitable electrically conductive material such as carbon is disposed in contact with the annulus 21. One of the brushes in this case the upper brush 24 cor responds in position to a predetermined position on a stroke for the needle 17, for example, the upper end thereof while the other brush 25 corresponds to another predetermined position on the stroke or the lower end thereof. Further a third brush 26 is continuously put in contact with the conductive annular segment 22 by having its circumferential dimension greater than that of the insulating annular segment 23. These three brushes 24, 25 and 26 are suitably supported in a casing 27 (see FIG 1). Referring back to FIG. 1, the casing 27 is supported to the bed 11 through a supporting and connecting plate 28 and a pilot generator 29 such as a tachometer generator is secured on the arm shaft 16 at the end. The generator 29 includes a stator carried by the casing and a rotator mounted on the shaft 16 although they are not illustrated.

In FIG. I, the reference numeral 30 generally designates a combined driving and controlling device including a housing 31 provided on the upper portion with a mounting block 32 serving to connect the housing to the lower side of the bed 11.

Referring now to FIG. 2, there is illustrated the combined driving and controlling device 30 just mentioned. As shown, the device 30 comprises an electric motor generally designated by the reference numeral 310 and serving as a drive for rotating the sewing machine as shown in FIG. 1. The motor 310 includes a rotator core 311 rigidly secured to the inner peripheral surface of the housing 31 and having a stator winding 312 inductively disposed on the core 311. The housing 31 is provided on that end remote from the associated load, in this example the left-hand end as viewed in FIG. 2 with a bracket 313 and on the intermediate portion with an

intermediate brackets

313 and 314 through a pair of

ball bearings

316 and 317 and has a rotor core 318 rigidly secured thereon so as to oppose to the stator core 311 with a small annular gap formed therebetween. The rotor core 318 has a rotor winding 319 inductively disposed thereon.

As shown in FIG. 2, the electric motor 310 is axially aligned with a clutch device generally designated by the reference numeral 320. The clutch device 320 comprises a driving rotary block or a flywheel 321 in the form of flanged hollow cone rigidly mounted on the motor shaft 315 at the right-hand end as viewed in FIG. 2 by a nut 322 and a rotation preventing washer 323.

The housing 31 is closed at the other end or the right-hand end as viewed in FIG. 2 with a bracket 324. The bracket 324 has integrally extending through the central portion a sleevetype bearing 325 whose longitudinal axis is aligned with that of the motor shaft 315. Fitted into the axial bore in the bearing 325 is a hollow cylindrical bearing member 326 for both rotational and limited axial movements. Then the movable bearing member 326 has an output shaft 327 rotatably supported on the inner peripheral wall surface through a pair of ball bearings 326 and 329. Therefore it will be appreciated that the output shaft 327 has the longitudinal axis aligned with that of the motor shaft 315. The end portion of the output shaft 327 remote from the motor 310 and projecting beyond the movable bearing member 326 has a driving pulley 330 keyed at 331 thereon for limited axial movement along the output shaft. A locking nut 331' is screw-threaded onto that end of the output shaft 327 to prevent the driving pulley 330 from disengaging from the output shaft.

This driving pulley 330 is operatively coupled to the driven pulley 18 on the sewing machine 10 through an endless belt as designated at dot-and-dash line 33 in FIG. 1.

The output shaft 327 is provided on the other end or the left-hand end as viewed in H6. 2 with a driven rotary block or a clutch plate 322 in the form ofa disc rigidly secured thereon by a locking nut 333 and a rotation preventing washer 334. The driven block 332 is provided on both sides of the outer peripheral edge portion with a pair of annular friction discs 335 and 336 one of which 335 opposes to the flywheel 321 to normally form a small gap therebetween.

The driven rotary block 332 is operatively associated with a brake device generally designated by the reference numeral 340. The brake device 3411 include an annular stationary plate 341 fixed to the inner peripheral wall surface of the said righthand bracket 324 to oppose to the friction disc 336 serving as a stationary element in the brake device 3411 adapted to engage the annular friction disc 336 on the driven block 332.

An electromagnet device generally designated by the reference numeral 350 is disposed to control the movements of the clutch and brake devices 320 and 340 respectively. The electromagnet device 356 comprises a stationary unit 351 including a supporting annulus 352 rigidly secured to the annular stationary plate 341, an iron core 353 in the form of an annulus of U-shaped cross section, and an exciting winding 354 inductively disposed around the iron core 353. The electromagnet device 350 further comprises a movable unit 355 including a movable flanged annulus 356 loosely fitted onto that end portion near to the clutch disc 332 of the movable bearing member 326 by having a pair of opposite retaining elements 357 and 358 fixed to that end of the bearing member to sandwich the same therebetween, and a movable iron piece 359 in the form of an annulus fixed on that surface adjacent the iron core 353 of the flange of the annulus 356 to oppose to the latter with a small airgap normally formed therebetween.

Between the inner peripheral surface of the cylindrical portion of the movable annulus 356 and the opposite outer peripheral surface of the movable bearing member 325 a ringshaped guide 360 is fixed to the bearing member 325 to guide the movable annulus 356. A plurality of threaded rods 361 extend at substantially equal angular intervals through both the outer peripheral edge portion of the flange of the annulus 356 and the right-hand bracket 324 and fixed at one end to the latter by nuts 362. One helical spring 363 is disposed around each of the rods 361 between the flange of the movable annulus 356 and a flange disposed at the inner or other end of the rod. The springs 363 serve to impart to the movable annulus 356 a force opposing to an attraction exerted by the energized iron core 359 thereby to tend to move the driven rotary block 332 away from the driving rotary block 321 and toward the stationary plate 341.

In order to move the movable bearing member 326 toward and away from the driven rotary block 332. An operating L- shaped lever 370 is pivotally mounted on one arm thereof to the right-hand bracket 324 by a pivot pin 371 and the one arm has its end portion 372 formed into a fork serving to put the sleeve-type bearing 325therebetween. The forked portion is then provided on the opposite ends with a pair of opposite bolts 373 extending therethrough Each bolt 373 extends through an axially elongated slot (not shown) disposed on the sleeve-type bearing 325 until it is loosely fitted into an elongated recess 374 disposed on the movable bearing member 326. A partly threaded rod 375 extends through the one arm of the lever 370 at its position below the position of the pivot pin 371 and is supported at one end to the right-hand bracket 324. The threaded rod 375 has a butterfly nut 376 screw threaded onto the other end portion and a helical spring 377 disposed around the same between the nut 376 and the one arm of the lever 370. The helical spring 377 tends to bias the movable bearing member 326 away from the driven rotary block 332 or in the right-hand direction as viewed in FIG. 2 to separate the driven block 322 away from the driving rotary block 321.

In FIG. 2 a normally closed microswitch generally designated by the reference numeral 380 is shown as being attached to the outer surface of the right-hand bracket 324 on the upper portion as viewed in FIG. 2. The microswitch 380 includes its actuator 381 which, in turn, engages an operating element projecting from one of the legs of the forked end portion 372. If the lever 370 is rotated in the counterclockwise direction as viewed in FIG. 2 and against the action of the spring 377 about the axis of the pivot pin 371 to push the driven rotary block 321 against the driving rotary block 321 with a sufficiently high force then the microswitch 380 is open for the purpose as will be apparent hereinafter.

Referring back to FIG. 1, a pedal 40 rotatably mounted on a shaft 41 is articulated through an adjustable connecting rod 42 to the lever 370 at the other end 378. An electric control device generally designated by the reference numeral 50 is attached to the lower surface of the bed 11 and includes therein an electric circuit as illustrated in FIG. 3. The control device 50 includes an operating panel 501 provided with a pair of

pushbutton switches

502 and 503 for opening and closing a source switch 504 (see FIG. 3).

Referring now to FIG. 3, the source switch 504 comprising gauged switches 504A and 5048 is electrically connected through a lead 505 (also shown in FIG. 1) to a source 506 of alternating current which has, in turn, the electric motor 310 electrically connected across the same through the lead 505, the switch 504 and a cable 507 (also shown in FIG. 1). The motor 310 is electrically connected across pair of alternating current input terminals of a single phase full-wave rectifier 508 through a bidirectional three-terminal thyristor 509. The rectifier 508 has a pair of direct current output terminals electrically connected across the exciting winding 354 of the stationary iron core 353 through a resistor 511 and a lead 512.

The thyristor 509 includes a gating circuit including a triggering element such as a bidirectional three-layer diode 513 and a gating capacitor 514. When the capacitor 514 has charged to a predetermined magnitude V0 of a voltage with the polarity as illustrated it fires the corresponding portion of the thyristor 509 when it has charged to the same magnitude V0 with the polarity reversed from that illustrated it fires the other portion of the thyristorv The capacitor 514 is electrically connected on one side to the above-mentioned brush 26 of the position sensing device through a reactor 516, a cable 517, the microswitch 380 and a cable 519, both cables being also shown in FIG. 1. The opposite brushes 24 and 25 are adapted to be selectively connected to the switch 504 through a selector or transfer switch 518 and leads 519A and 5198 respectively which are also shown in FIG. 1 as being the cable 519. If the selector switch 518 has its movable arm connected to the upper brush 24, the latter is selected to be effective for the operation of the system. On the other hand, if the arm is connected to the lower brush 25 the latter is effective for the operation of the system.

The cable 519 as shown in FIG. 1 includes in addition to the leads connected to the brushes 24, 25 and 26. leads C connecting the pilot generator 29 across a single phase full-wave rectifier 520. The output of the rectifier 520 is connected to a filtering network generally designated by the reference numeral 521 including a shunt capacitor 522, a series resistor 523 and a variable resistor or potentiometer 524. The network 521 serves to smooth the direct current output from the rectifier 520. The slide on the potentiometer 524 is connected to a base of an NPN-type transistor 525 including a collector and an emitter connected across a single phase full-wave rectifier 526. More specifically, the collector of the transistor 525 is directly connected to one of diametrically opposite terminals of the rectifier 526 and the emitter thereof connected to the other terminal of the rectifier through a resistor 527. Then the rectifier 526 includes another pair of diametrically opposite terminals connected across the gating capacitor 514.

The arrangement thus far described is operated as follows: In order to drive the sewing machine 10, one can depress the pushbutton switch 502 to close the source switch 504 thereby to drive the motor 310 to rotate the rotary shaft 315. Under these circumstances, the pedal 40 is depressed to pull the connecting rod 42 downwardly. This causes the operating lever 370 to be rotated in the counterclockwise direction as viewed in FIG. 2 about the axis of the pin 371 and against the action of the spring 377 until the bolts 373 on the lever 370 move the movable bearing member 326 leftwards as viewed in FIG. 2 or toward the driven rotary block or the clutch disc 332. This leftward movement of the movable bearing member 326 is accompanied by the movement of the output shaft 327 along with the ball bearing 328 and 329 toward the driven rotary block 332 until the friction disc 335 on the latter is strongly pushed against the driving rotary block 321. Therefore the driven block 332 is driven by the driving block 321 It is noted that hardly any slip occurs between the rotating blocks 321 and 332 ensuring that the output shaft 327 is driven with the substantially same number of rotation as the motor shaft 315.

The rotational movement of the output shaft 327 is transmitted to the arm shaft 16 through the

pulleys

330 and 18 and the endless belt 33. Therefore the sewing needle 17 vertically reciprocates to perform the particular sewing operation in the well-known manner. It is now assumed that the arm shaft 16 is rotating with a predetermined number of rotation in unit time for example 5000 rpm. Since the counterclockwise rotational movement of the lever 370 also causes the microswitch 380 to be brought into its open position the gating capacitor 514 (see FIG. 3) is prevented from charging by the source 506 while the thyristor 509 remains open. Also no current flows through the winding 354 for exciting the electromagnet device 350.

If it is desired to stop the machine 10, the pedal 40 is released. This permits the driven rotary block 332 to be moved away from the driving rotary block by the action of the spring 363 until the clutch device 320 is put in its disengaged state. Instead the friction disc 336 on the driven rotary block 332 is forced to push against the stationary plate 341 on the brake device 340. It is here noted that upon the disengagement of the driven block 332 from the driving block 321 the lever 370 is prevent from moving in the clockwise direction as viewed in FIG. 2. Therefore the lever 370 does not cause a further movement of the driven block 332 but the spring 377 permits the driven block to be moved away from the driving block to an extent determined by a clearance formed between each bolt 373 and the associated elongated groove 374. Thus the braking action is applied to both the output and

arm shafts

327 and 16 respectively tending to be rotated due to their inertias. This results in a sudden decrease in the number of rotation of the arm shaft 16 as shown at curve S in FIG. 40 wherein the axis of ordinates represents the number of rotation per minute and the axis of abscissas represents time in seconds.

On the other hand, the resetting of the lever 370 causes the microswitch 380 to be put into its closed position. Assuming that the selector switch 518 has its movable arm connected to the upper brush 24 as viewed in FIG. 3, the contacting of the first brush 24 with the conductive segment 22 completes a circuit traced from the source 506 through the switch 504A, the transfer switch 518, the first brush 24, the conductive segment 22, the third brush 26, the microswitch 380, the reactor 516 and the capacitor 514 and thence back to the source through the switch 5048. If the arm shaft 16 is still fairly high in the number of rotation with the output voltage from the pilot generator 29 exceeding a predetermined magnitude then a base current flowing from the rectifier 520 through the baseto-emitter circuit of the transistor 525 is large enough to maintain the collector-to-emitter circuit thereof completely conductive.

Under these circumstances, a current due to a voltage e across the source 506 with the polarity designated by the solid arrow denoted adjacent the source will shunt from the capacitor 514 to flow through a circuit including the transistor 525, the resistor 527 and the rectifier 526 in the direction of the solid arrows alongside the circuit. On the other hand a current due to a voltage e across the source 506 with the polarity designated by the dotted arrow adjacent the source will also shunt from the capacitor 514 to flow through a circuit including the rectifier 526, transistor 525 and the resistor 527 in the direction of the dotted arrows alongside the circuit. Therefore the voltage across the source 506 with either polarity does not charge the gating capacitor 514 to a magnitude sufficient to effect the breakdown of the triggering element 513. Thus the thyristor 509 remains open to prevent an exciting current from flowing through the electromagnets winding 354 with the result that the friction plate 336 on the driven rotary block 332 continues still to push against the driving rotary block 321 while the arm shaft 16 is decreasing its number of rotation in unit time.

After the number of rotation of the arm shaft 16 has decreased to a predetermined magnitude, the output voltage from the pilot generator 29 also decreases to bring the transistor 525 into its conductivity region where the conductivity varies. That is, the transistor 525 increases in resistance between the collector and emitter thereof. This increase in resistance weakens the shunting action with respect to the gating capacitor 51d permitting the latter to be charged to a voltage sufficient to breakdown the triggering element 513. When the source 506 is generating a voltage thereacross in the direction of e the capacitor 514 is charged with the polarity illustrated in FIG. 3 until the capacitor 514 fires the corresponding portion of the thyristor 509 at such a phase angle of the voltage that the capacitor has charged to a predetermined magnitude V0. On the contrary, when the source 506 is generating the voltage thereacross in the direction of e the capacitor 514 charges with the polarity reversed from that illustrated until it fires the other portion of the thyristor 509 at such a phase angle of the voltage that the capacitor has charged to the same magnitude V but in the reversed direction. This follows that the full-wave rectifier 508 rectifies an output waveform of alternating current provided by the thyristor 509 and in accordance with the firing angle thereof to supply the rectified waveform to the winding 35% of the electromagnetic device 350.

As the output voltage from the pilot generator 29 decreases to increase the resistance of the transistor 525 the capacitor 514 increases in charging rate thereby to advance the firing angle of the thyristor 509 resulting in an increase in duration of the alternating current output waveform provide by the thyristor 509 and therefore, in current flowing through the exciting winding 354. Once the increase in current flowing through the winding 354 has caused a force to overcome the resilience of the spring 363 thereby to permit the annular core 353 to attract the movable iron piece 359 the driven rotary block 332 disengages from the stationary plate 341 to be again contacted by the driving rotary block 321. That is the clutch device 320 is again put in its engaging coupling state. However it is noted that a force with which the friction disc 355 on the driven block 332 is caused to push against the driving block 321 varies in proportion to the current flowing through the electromagnets winding 354. Therefore such a force is not w sufficiently high and the output shaft 327 is driven by the driving block 321 while the friction disc 335 on the driven block 332 is slipping with respect to the driving block 321, that is to say, it is maintained in partly clutched stated. Under these circumstances, the clutch device 320 is adapted to transmit a torque from one to other portions thereof increased with a decrease in the number of rotation of the arm shaft 16. Therefore, as shown in FIG. ta, the arm shaft 16 normally effects the sustained rotation whose number of rotation vibrates about a predetermined one, for example 200 r.p.m.

During this sustained rotation of the arm shaft 16, the first or upper brush 24 can contact the insulating segment 23 of the position sensing device 20. At that time the insulating segment 23 provides a pause interval of time for a charging circuit with the capacitor 514 sufficient to permit the electromagnet's winding 354 to be fully deenergized. Thus the winding 354 is deenergized to cause the driven rotary block 332 to again push against the stationary plate 341 to be braked resulting in the stoppage of the arm shaft 16. At the same time the sewing needle 17 has stopped at the upper end of its stroke corresponding to that angular position of the insulating segment 23 where it contacts the first brush 24.

If the movable arm of the transfer switch 518 is in engagement with the second or lower brush 25, then the sewing needle 17 stops at the lower end of its stroke as will be readily understood from the foregoing description.

It is assumed that a time point at which the arm shaft 16 has reached the sustained rotation as above described corresponds to a time point a shown in FIG. 4a. Then a time between the time point a and a time point b (see FIG. 4a) where the arm shaft 16 has completely stopped is equal to at most an interval of time required for effecting substantially one complete rotation thereof on the bases of the particular number of rotation during the sustained rotation in the case at the time point a the insulating segment 23 hasjust passed over the first brush 24 (or in the case at the time point a it has just passed over the second brush 25 when the sewing needle 17 is to stop at the lower end of its stroke). If during the time between the time points a and b not exceeding the one complete rotation just described the insulating segment 23 has been just positioned below the first or second brush 24 or 25 respectively when the time point a was reached then the sewing needle 17 will stop immediately from the time point a.

FIG. 4b wherein like reference characters has the same meaning as in FIG. 4a shows an ideal curve along which the arm shaft 16 decreases in speed. In FIG. 4b it is essential that at the instant the number of rotation of the arm shaft 16 has reached a predetermined magnitude for example 200 r.p.m. a transmitted torque in the partly clutched state be equal to a torque for driving the arm shaft 16just with 200 r.p.m.

In order to decrease the interval between the time point a where the arm shaft 16 has reached the sustained rotation and the time point b where it has completely stopped as shown in FIG. 4a or b, it is required only to increase the number of rotation of the arm shaft during the sustained rotation. However if that number of rotation is too high then it is impossible that within the period of time for which the insulating segment 23 passes over either of the brushes 24 or 25 the electromagnets winding 354i is deenergized to apply the braking action to the driven rotary block 332 until the arm shaft 16 is caused to stop. The reason for this is that if the arm shaft 16 does not stop within the period of time for which the insulating segment 23 is passing over either of the brushes 241 or 25 that the conductive segment 22 again contacts those brushes to energize the winding 354. If the insulating segment 23 increases in circumferential dimension its number of rotation somewhat may increase during the sustained rotation thereof but an angle through which the insulating segment 23 is in contact with either of the brushes 241 or 25 is also increased with the result that the sewing needle 17 may stop at any one various positions other than either of both ends of the stroke.

Therefore it has been found that the arm shaft 16 in the sustained rotation has preferably the number of rotation in unit time predetermined to range, for example, from to 200 r.p.m. On the other hand, the number of rotation with which the motor 310 is driven is adapted to be adjusted so as to change the number of rotation in the normal operation of the sewing machine 10 in accordance with the type of sewing operation or the type and thickness of cloth to be sewn, the accuracy required for the sewing operation, the skillfulness of the operator etc.

As previously described, the clutch device 320 is put in its partly clutching state to maintain the sewing machine in sustained rotation. Under these circumstance, if the transmitted torque is appropriately adjusted as by changing the position of the slide on the potentiometer 524 to which output voltage from the pilot generator 329 is applied the the number of rotation of the arm shaft 16 in its sustained rotation can readily be controlled to any desired magnitude irrespective of the number of rotation of the electric motor 310. This is because a change in resistance of the potentiometer 524 causes a variation in the number of rotation with which the arm shaft 16 is rotating when the transistor 525 changes from its fully conductive state to its conductivity varying state. Also it will readily be understood that for the purpose of adjusting the number of rotation of the arm shaft 16 in its sustained change as the case may be.

While the invention has been illustrated and described in terms of sewing machine rotated by an electric motor, it is to be understood that the invention is equally applicable to the case any driven rotary machine other than the sewing machines is required to be selectively stopped at its predetermined positions. Example of such machines involve riveting machines including the hammer adapted to be vertically moved through the driven rotation of the machine, and sawing machines including the fret saw adapted to be vertically moved through the driven rotation of the machine. If desired, the driving rotary source may be a prime mover other than an electric motor.

The invention has several advantages. For example, the driven rotary machine can readily be adjusted in the number of rotation in unit time in the sustained rotation by controlling a torque transmitted through the clutch device operated in its partly clutching state in which the driven rotary machine effects the sustained rotation at a reduced rate. This ensures that the driven rotary machine always effects the sustained rota tion with the proper number of rotation thereby to permit the machine to rapidly stop at a selected one of its predetermined positions, regardless of any variation in the number of rotation of the driving rotary source and/or any change in the normal number of rotation of the driven machine. In addition, the clutch device serving to transmit a torque from the driving source to the driven machine therethrough during the normal operation of the latter machine is also used to effect the sustained rotation of the driven machine upon its stoppage, This leads to a simple and compact transmission mechanism as compared to the prior art practice including separately the secondary driving route.

We claim:

1. A fixed point stop mechanism for a driven rotary machine comprising in combination, a drive for driving the rotary machine, a flywheel disposed on said drive, a clutch plate opposing to said flywheel and capable of engaging said flywheel to transmit power from said drive to said driven machine therethrough, brake means for applying a braking action to said driven machine, electromagnet means for controlling a selected degree of coupling between said clutch plate and said flywheel to put said driven machine in low speed operation, a thyristor for controlling an electric current for exciting said electromagnet means, control means for controlling the conduction of said thyristor in accordance with the speed of rotation of the driven machine, and position sensing means operative during the low speed operation of said driven machine to permit said clutch plate to disengage from said flywheel when the driven machine is located at its predetermined position while at the same time applying a braking action to said driven machine to stop the latter at said predetermined position, said control means including a capacitor for applying a firing signal to said thyristor when said capacitor has charged to a predetermined magnitude, a transistor having a conductivity variable in accordance with the speed of rotation of the driven machine to control a charging of said capacitor, a pilot generator for detecting the rotational speed of said driven machine and controlling the conductivity of said transistor, said position sensing means being operative to respond to said driven machine being at its predetermined position to open the charging circuit for the capacitor thereby to deenergize said electromagnet means to cause the disengagement of said clutch plate from said flywheel and the application of said brake means to said driven machine.

2. A fixed point stop mechanism as claimed in claim 1 wherein, in order to stop the driven machine at a selected one of its predetermined positions, said position sensing means includes a selector switch for selecting said predetermined positions of the driven machine.

3. A fixed point stop mechanism as claimed in claim 1, comprising manually operable means for moving said clutch plate into engagement with said flywheel to drive said driven machine at full speed and means connecting said manually operable means with said clutch plate, said connecting means including lost motion mechanism to permit movement of said clutch plate by said electromagnet means without movement of said manually operable means.

4. A fixed point stop mechanism as claimed in claim 1, wherein said pilot generator is an alternating current generator and wherein said control means further includes a fullwave rectifier connected with said generator to rectify the output of said generator and circuit means for applying the rectified output of said generator to the base-emitter circuit of said transistor.

5. A fixed point stop mechanism as claimed in claim 4, wherein said circuit means includes a variable resistor for varying the effect of said pilot generator on the conduction of said transistor.

6. A fixed point stop mechanism as claimed in claim 1, wherein said thyristor is bidirectional and is connected in circuit with an alternating current power supply and alternating current terminals of a full-wave rectifier, said electromagnet means being connected across the direct current terminals of said rectifier so as to be energized when said thyristor is conducting.

7. A fixed point stop mechanism as claimed in claim 1, wherein said control means includes a triggering element for said thyristor, said triggering element comprising a bidirectional three-layer diode, the triggering of which is controlled by the voltage to which said capacitor is charged.

Claims

4. A fixed point stop mechanism as claimed in claim 1, wherein said pilot generator is an alternating current generator and wherein said control means further includes a full-wave rectifier connected with said generator to rectify the output of said generator and circuit means for applying the rectified output of said generator to the base-emitter circuit of said transistor.