US3662239A - Stitching machine with inertial (dc) motor for pull rolls and electronic control thereof - Google Patents

Abstract

An extremely simple, high speed stitching machine is disclosed which includes pull rollers for moving a workpiece to be stitched along a path adjacent to a stitching head, a DC motor for directly driving the pull rollers and control circuitry for coordinating the operation of the DC motor and stitching head. Additionally current limiting circuitry is provided for electronically controlling the acceleration and deceleration of the motor thereby limiting the torque which will be applied by the rollers to the workpiece. In a preferred embodiment the extent of acceleration and deceleration (and the torque control) can be selectively varied in accordance with the strength of the media comprising the workpiece.

Description

United States Patent Jefferts 51 May 9,1972

[22] Filed:

[72] Inventor: Keith Jefferts, Summit, NJ.

[73] Assignee: Flynn & Emrich Company, Baltimore,

Feb. 1, 1971 [21] Appl.No.: 111,198

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[56] References Cited UNITED STATES PATENTS 3/1970 Hermann ...227/7 3,582,746 6/1971 Nye ..3l8/332 Primary Examiner-Harold Broome Assistant Examiner-Thomas Langer Attorney-Lemer', David & Littenberg [57] ABSTRACT An extremely simple, high speed stitching machine is disclosed which includes pull rollers for moving a workpiece to be stitched along a path adjacent to a stitching head, a DC motor for directly driving the pull rollers and control circuitry for coordinating the operation of the DC motor and stitching head. Additionally current limiting circuitry is provided for electronically controlling the acceleration and deceleration of the motor thereby limiting the torque which will be applied by the rollers to the workpiece. in a preferred embodiment the extent of acceleration and deceleration (and the torque control) can be selectively varied in accordance with the strength of the media comprising the workpiece.

12 Claims, 1 Drawing Figure STITCIIING MACHINE WITH INERTIAL (DC) MOTOR FOR PULL ROLLS AND ELECTRONIC CONTROL THEREOF BACKGROUND OF THE INVENTION box blank passing therebeneath, and conveying means for incrementally advancing the box blank passed the stitching head. Conventionally, such conveying means has taken the form of cooperating pull rollers which sandwich the box blank therebetween and advance 'same through the machine in response to the driving rotation thereof.

In the control of such machinery, at start up it is necessary to have the pull rollers operating at a continuous high rate of speed to initially convey the box blank from the manual control of the operator) toward the stitching station defined by the stitching head. When the box blank has reached a preselected position with respect to the stitching head, appropriate sensing means (in the form of a mechanical sensing finger or a photocell arrangement) detects the leading edge of the box blank to energize a timer which after timed out, generates a command signal to start the operation of the stitching head which then begins to apply the first stitch to the box blank now positioned therebeneath.

Simultaneously, a signal must be generated to stop the aforementioned pull rollers such that the box blank will be stationary beneath the head when the first stitch is applied thereto. After the completion of the first stitch, and after the stitch driving means associated with the head has been lifted from the box blank, another signal must be generated to start the pull rollers moving again to advance the blank a preselected distance whereupon the pull rollers must again be stopped to halt the movement of the box blank to await the application of the next stitch being applied by the'now continuously operating stitching head. This cycle of stop and start movement of the box blank, and the application of a stitch at the stop or dwell period of the box blank motion, continues until a preselected number of stitches have been applied. At that point, the stitching'head must be de-energized, and the pull rollers returned to their earlier mode of continuous operation to rapidly eject the box blank out of the rear of the stitching machine.

lnmost prior art stitching machines, all of the above control signals are generated by purely mechanical means. For example, prior art apparatus of the type in question employ cams, cam followers, mechanical gear trains, etc., to generate, synchronize, and coordinate the various operations discussed above. Typical of this type of operation is the box stitching apparatus disclosed for example in US. Pat; No. 2,785,403. It will be appreciated however, that in addition to being relatively complex and expensive to manufacture, such mechanically controlled mechanisms are extremely slow. In fact, the slow stitching speed which necessarily results from this type of mechanical operation is one of the prime reasons that box stitching apparatus of this type incorporating wire stitching heads are not readily accepted in the box forming industry.

Specifically, when given a choice, box manufacturers would rather employ tape dispensing and/or gluing machinery to fabricate cardboard boxes. This is so since such machinery can be operated in a continuous mode (that is the tape or glue being applied to the moving box blank) as compared to the incremental advancement (stop and start motion) utilized in wire stitching apparatus. However, for boxes designed for extremely heavy contents, tape and/or glue is not a sufficient joining means, and the fabricators must turn to wire stitches to provide the necessary strength.

However, and for the reasons discussed above, such wire stitching machines operate at such a slow rate of speed that fabricators have been faced with a choice involving the best of two evils: either they can refrain from manufacturing wire joined cartons (and lose the potential business income derived therefrom), or they incur great operating expenses by either shutting down, slowing down, or losing time on their highspeed tape and/or glue production lines in favor of manufacturing the wire joined boxes on the much slower wire stitching apparatus. This problems'becomes especially severe in situations, for example, where the customer desires a wire stitched carton (for strength), but also requires tape and/or glue on the box seam, for example, to make it impervious to water. In that situation, the boxes which have been taped and/or glued on the main high-speed production line must be slowed down to receive the wire stitches or physically removed from the production line to another location where the wire stitching apparatus is employed.

Equally as important, if not more so, is the problem relating to poor stitching accuracy which results from the above described mechanically controlled box stitching apparatus. Specifically, it is one of the basic requirements of the box stitching apparatus that the stitches be applied at precisely desired locations (for example with respect to the leading and trailing folds which define the intermediate body portion of the planar box blank). Similarly, it is essential that the spacing between adjacent stitches be maintained uniform throughout a given stitching sequence. However, with box stitching apparatus employing mechanical means to synchronize and control the operation thereof, extended usage over long periods of time necessarily results in mechanical wear, looseness or play, etc., all of which has the unstoppable effect of completely destroying the close tolerances which are essential in the industry.

In the copending patent application of Stephen Stanton and Edward Varholy, Ser. No. 72,764, filed Sept. l6, I970, and entitled Electronically Controlled Stitching Machine (and assigned to the same assignee as the instant patent application) an electronically controlled stitching machine is disclosed. In that electronically controlled stitching machine, a clutch-brake combination is interposed between a constantly running AC motor and a set of pull rollers of the stitching machine. The pull rollers are actuated by engaging the clutch and deenergizing the brake while the pull rollers are stopped by engaging the brake and disengaging the clutch. This system is a vast improvement over the prior art mechanically driven stitching machines but still requires mechanical clutch-brake combination disposed between the motor and the pull rollers. Obviously, the cost and maintenance involved with such clutch-brake mechanism is a serious drawback to the stitching machine; and more importantly, the speed of such machine is severely limited by the nature of the mechanical parts involved.

BRIEF DESCRIPTION OF THE INVENTION In accordance with this invention, a stitching machine is provided which includes means for engaging a workpiece for moving the workpiece along the path, a stitching head located adjacent to the path, a DC motor for directly driving the engaging means and control circuitry for coordinating the operation of the DC motor and the stitching head. By employing a DC motor to directly power the stop and start motion of the pull rollers, the conventional mechanical clutch-brake mechanism of the prior art stitching machines is completely eliminated, along with all the drawbacks and slowness associated therewith.

As a particularly advantageous feature of the invention, current limiting circuitry is provided for electronically controlling the acceleration and deceleration of the motor thereby limiting the torque (thus the stress) which will be applied by the rollers to the workpiece. In this manner tearing of the box blanks is prevented.

As a further feature, the current limiting circuitry is provided with adjustment means for selectively varying the acceleration and deceleration which the motor will experience. Thus for stronger box blanks, which can take greater shear stress by the rollers, the speed of the stitching cycle can be increased by simply setting the adjusting means for higher acceleration.

DESCRIPTION OF THE DRAWING The sole FIGURE is a drawing partially in schematic form, partially in block diagram form and partially in isometric view shows the circuitry and mechanical apparatus comprising the stitching machine of this invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the sole FIGURE, we see stitching machine 8 of the instant invention. The stitching machine includes a pair of pull rollers 10 and 11 which cooperate with a pair of idler rollers 12 and 13 to advance a workpiece, such as a cardboard box blank 14, into operative relationship with a conventional stitching head 16. In the context of the instant invention the stitching head 16 takes the form of a conventional wire stitching head, but of course the invention is most limited to a wire stitching head. In accordance with a first aspect of the invention the pull rollers 10 and 11 are mounted on and are driven by a shaft 17 directly connected to and driven by a DC motor 18. (It is to be understood that the term directly is employed to denote the fact that the stop and start motion of the pull rollers is the result of the stop and start motion of the motor 18 with no intervening stop and start mechanism, such as the clutch-brake of the prior art being interposed between the motor and the pull rollers. However, it will be understood that if desired a conventional gear reduction box may be inserted between the motor and the pull rollers to mechanically interconnect the high speed, low torque motor with the relatively lower speed higher torque load of the pull rollers. Of course, such a gear box would have no influence on the stop-start characteristics of the system). Control circuitry 19 is provided to coordinate the operation of the stitching head and DC motor.

The particular choice of construction for the control circuitry 19 is not critical to the instant invention and will therefore not be described in detail. Its function is simply to permit the stitching head to apply a stitch only when the motor 18, (and rollers 10, 11) has the box blanks at a rest position. The circuitry 19 could either generate timing for controlling both the stitching head 16 and the DC motor 18 or signals can be derived from either the stitching head 16 to control the DC motor 18 or the DC motor 18 to control the stitching head 16 just so long as there is a coordination therebetween so that the stitching occurs when the DC motor has the cardboard box 14 in its rest position.

In operation, the cardboard box blank 14 is to be stapled along overlapping portions thereof and, therefore, is moved intermittently passed the stitching head 16 by the pull rollers 10, 11 under the direct stop and start action of the DC motor 18. With this simple arrangement, all of the mechanical difficulties and slowness associated with clutch-brake mechanisms of the prior art have been eliminated. It might be noted, that maximum stitching rates for conventional clutchbrake stitching machines are in the magnitude of 600-800 stitches per minute, whereas a stitching machine constructed in accordance with the teachings of the instant invention has been found capable of operating at stitching rates in excess of 1,500 stitches per minute.

Turning to a second aspect of the invention, it will be appreciated that for a given strength of the box blank, (frequently designated by the weight of the cardboard), the amount of force exerted by the pull rollers 10 and 11 on the cardboard box 14 must be maintained below a threshold level to prevent ripping or mutilation thereof. This problem has been found to be especially acute when the DC motor 18 is of the low inertia high acceleration type desirable because of the high operating speeds which can be attained therewith.

In accordance with this aspect of the invention, current limiting means is provided for limiting the maximum current that the DC motor 18 can draw thereby controlling the acceleration and deceleration of the motor 18. Such control thereby limits the torque and stress applied by the pull rollers 10 and l I to the cardboard l4 and thus prevents ripping.

To accomplish this end, the control circuitry 19 applies signals by lead 21 and 22 to a current limiting circuit 23. The current limiting circuit 23 controls the voltage and current supplied to the DC motor 18 by leads 24 and 26. A transistor bank 27 is in series with the leads 24 and 26 and a current sensing resistor 28 to apply power from a source of power +V" to the motor. A transistor bank 29 is in shunt with the leads 24, 26 and the current limiting resistor 28 to short them out. The remaining circuitry is employed to control the two transistor banks 27 and 29.

The control circuitry 19 is set up so that the transistor banks 27 and 29 cannot be on at the same time. Only one of the transistor banks 27 and 29 may be operative at a single time. They may, however, both be off at the same time.

The transistor bank 27 is turned on by a common admitter transistor 31 which is driven by an operational amplifier 32. The operational amplifier 32 compares a reference signal, supplied by a potentiometer 33 connected between a source of negative voltage -V" and ground, to a current signal, generated across the resistor 28. Since the resistor 28, which is a low valved resistor such as a tenth of an ohm, is in series with the DC motor 18, the voltage thereacross will indicate the current through the DC motor 18. The reference signal supplied by the potentiometer 33 is applied to the operational amplifier 32 by resistor 34 while the current signal across the resistor 28 is applied to the operational amplifier 32 by a resistor 36. The other input to the operational amplifier 32 is returned to ground by a resistor 37.

When the signals supplied by the resistors 34 and 36 to the operational amplifier 32 balance out to the ground supplied by the resistor 37, the output of the operational amplifier 32 will control the transistor 31 to maintain the current through the DC motor 18 at the value represented by the voltage across the resistor 28. This is the limit current which the DC motor 18 can draw during start up (acceleration).

The operational amplifier 32 will have a positive output holding the transistor 31 hard on keeping the transistor bank 27 supplying current to the motor 18 when the voltage across the resistor 28 is below the value set by the potentiometer 33. In this way, it is seen that the DC motor 18 is connected to run in a current limited mode (and therefore a torque limited mode) when the DC motor 18 tries to draw more than the preset current in a normal voltage mode otherwise.

A diode 38 is interposed between the lead 21 and the input of the operational amplifier 32, to which the resistor 37 is connected, to enable or disable operation of the transistor bank 27. When a large negative signal is applied to the cathode of the diode 38, the voltage across the resistor 37 is brought to a large negative value driving the output of the operational amplifier 32 negative, thereby turning off the transistor 31 and the transistor bank 27.

The circuitry which controls the transistor bank 29 is shunt with the DC motor 18 operates to limit the current in the DC motor 18 upon deceleration. The transistor bank 29 operates as a dynamic brake to the DC motor 18. However, the current drawn during deceleration is limited thereby limiting the deceleration torque developed by the pull rollers 10 and 11.

To this end an operational amplifier 39 is employed to compare the voltage across the resistor 28 to the voltage across a second potentiometer 40. The voltage across the resistor 28 is applied by a resistor 41 to the non-inverting side of the operational amplifier 39 rather than the inverting side as was done to the operational amplifier 32. The voltage across the potentiometer 40 is applied to the inverting input terminal of the operational amplifier 39 by a resistor 42.

The reason that the voltage across the resistor 28 is applied to the opposite input terminal of the operational amplifier 39 is because upon deceleration the current flowing through the motor 18 is reversed. The output of the operational amplifier 39 is applied to a common emitter transistor 43 which drives the transistor bank 29. When the voltage across the resistor 28 is less than the voltage across the potentiometer 40, the operational amplifier 39 tends to turn off the transistor 43 holding the current through the DC motor at the limit value.

Again, a diode 44 is interposed between the non-inverting input terminal of the operational amplifier 39 and the lead 22 to control the operational amplifier 39 so that when a large negative signal is applied thereto, the transistor bank 29 is disabled from the control by the remaining circuitry. Therefore, it is seen that a current limiting circuit is provided in which the current in the DC motor 18 is limited upon acceleration and deceleration thereby limiting the torque developable by the pull rolers l0 and 11. This limit is independently adjustable for acceleration and deceleration by the potentiometers 33 and 40 respectively.

As noted previously, and as is apparent, cardboard box blanks of different strength or weight can withstand a higher degree of stress. Accordingly, when employing stronger cardboard, it is of course desirable to increase the speed of the stitching cycle (by increase of acceleration and deceleration) to maximize the capacity of the stitching machine.

Thus it will be appreciated, and in accordance with the third aspect of the instant invention that the adjustable potentiometers 33 and 40 constitute adjustment means for varying the acceleration and deceleration which the motor will experience. Preferably, the potentiometers 33 and 40 are controlled by a single knob (not shown) which is set by the machine-operator in accordance with the weight of the cardboard being stitched. Hence for stronger cardboard, the output of the machine can be increased.

While this invention has been described with respect to a particular embodiment thereof, it should be clear that numerous other embodiments will become obvious therefrom to to others of ordinary skill in the art.

I claim:

1. A stitching machine comprising:

means for engaging a workpiece for moving the workpiece along a path;

a stitching head located adjacent to the path;

motor means for directly driving the engaging means; and

control circuit means for coordinating the repetitive intermittent operation of the motor and the stitching head so that the stitching head can operate when said motor is deenergized leaving said workpiece at rest.

2. The stitching head of claim 1 wherein the motor is a DC motor.

3. The stitching machine of claim 1 and further including current limiting means for electronically controlling the ac celeration and deceleration of the motor thereby limiting the torque and the stress which will be applied by the engaging means to the workpiece.

4. The stitching machine of claim 3 wherein said current limiting means includes adjustment means for selectively varying the acceleration and deceleration of the motor;

whereby the stress which will be applied by the work engaging means to the workpiece can be varied in accordance with the strength of the workpiece.

5. The stitching machine as defined in claim 3 in which said current limiting means limits the maximum current which said DC motor can draw.

6. The stitching machine as defined in claim 3 in which said current limiting means includes:

means for limiting said current to a first value when said DC motor is accelerating; and

means for limiting said current to a second value when said DC motor is decelerating. 7. The stitching machine as defined in claim 3 in which said current limitin means includes:

means for rivmg said DC motor (a) in a constant speed mode when said current is below said maximum current and (b) in a constant torque mode when said current is above said limit current. 8. The stitching machine as defined in claim 7 also including:

means for adjusting said maximum current. 9. The stitching machine as defined in claim 8 in which said current limiting means includes:

means for limiting said current to a second value when said DC motor is decelerating. 10. The stitching machine as defined in claim 3 in which said current limiting means includes:

means for measuring said current to provide a current signal; means for generating a reference signal; and means for comparing said current signal to said reference signal to provide a DC motor drive signal. 11. The stitching machine as defined in claim 10 in which said reference signal generation means is adjustable.

12. The stitching machine as defined in claim 11 in which said comparing means is an operational amplifier.

* l 1i l

Claims (12)

1. A stitching machine comprising: means for engaging a workpiece for moving the workpiece along a path; a stitching head located adjacent to the path; motor means for directly driving the engaging means; and control circuit means for coordinating the repetitive intermittent operation of the motor and the stitching head so that the stitching head can operate when said motor is deenergized leaving said workpiece at rest.

2. The stitching head of claim 1 wherein the motor is a DC motor.

3. The stitching machine of claim 1 and further including current limiting means for electronically controlling the acceleration and deceleration of the motor thereby limiting the torque and the stress which will be applied by the engaging means to the workpiece.

4. The stitching machine of claim 3 wherein said current limiting means includes adjustment means for selectively varying the acceleration and deceleration of the motor; whereby the stress which will be applied by the work engaging means to the workpiece can be varied in accordance with the strength of the workpiece.

5. The stitching machine as defined in claim 3 in which said current limiting means limits the maximum current which said DC motor can draw.

6. The stitching machine as defined in claim 3 in which said current limiting means includes: means for limiting said current to a first value when said DC motor is accelerating; and means for limiting said current to a second value when said DC motor is decelerating.

7. The stitching machine as defined in claim 3 in which said current limiting means includes: means for driving said DC motor (a) in a constant speed mode when said current is below said maximum current and (b) in a constant torque mode when said current is above said limit current.

8. The stitching machine as defined in claim 7 also including: means for adjusting said maximum current.

9. The stitching machine as defined in claim 8 in which said current limiting means includes: means for limiting said current to a second value when said DC motor is decelerating.

10. The stitching machine as defined in claim 3 in which said current limiting means includes: means for measuring said current to provide a current signal; means for generating a reference signal; and means for comparing said current signal to said reference signal to provide a DC motor drive signal.

11. The stitching machine as defined in claim 10 in which said reference signal generation means is adjustable.

12. The stitching machine as defined in claim 11 in which said comparing means is an operational amplifier.

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