Reciprocating linear actuator

A digitally indexed reciprocating linear actuator includes a frame and a servomotor mounted to the frame. A crank is gear-coupled with integer gear ratio to the servomotor for unidirectional rotation. An actuator slide with a non-home index point is slideably-mounted to the frame for linear motion. A connecting rod couples the crank to the actuator slide. The actuator also includes means, including a sensor, for detecting the index point at two angular positions of the crank. A wrapping machine for wrapping a plurality of articles has repeated cycles of article-handling operations and a plurality of carriages, each carriage capable of moving at least one article for wrapping. The wrapping machine includes at least one digitally indexed reciprocating linear actuator having a crank, and an actuator slide with a non-home index point. The wrapping machine records first and second servomotor angular positions on detecting the index point at first and second angular positions of the crank. The wrapping machine identifies home position as an angular position midway between the first and second positions.

TECHNICAL FIELD

The invention relates generally to improvements in reciprocating linear actuators and methods for general use, including use in machines for handling a plurality of articles, particularly wrapping machines that have repeated cycles of article-handling operations.

BACKGROUND OF THE INVENTION

Machines, including wrapping machines, for handling a plurality of articles and having repeated cycles of article-handling operations require synchronization to be maintained between the multiple article-handling mechanisms cycle after cycle. Without synchronization, collisions would occur. Prior art wrapping machines use linked mechanical drives typically driven by a common motor. Synchronization was not perceived as a serious problem because all mechanisms were typically driven from the common motor. However, modern machines, mainly for cost and control flexibility reasons, are increasingly using servomotors. These servomotors usually drive mechanisms independently, often one per mechanism. This aggravates the synchronization problem. Important design considerations for wrapping machines that have repeated cycles of article-handling operations are synchronization, speed, durability and energy consumption.

SUMMARY OF THE INVENTION

A first embodiment of the present invention provides a reciprocating linear actuator comprising a frame, a servomotor mounted to the frame, a crank gear-coupled with integer gear ratio to the servomotor for unidirectional rotation, an actuator slide slideably-mounted to the frame for linear motion, and a connecting rod coupling the crank to the actuator slide. Preferably, the crank includes a crank arm with an outer end, and the connecting rod includes a driven end pivotally connected to the outer end, and a drive end pivotally connected to the actuator slide.

A first preferred embodiment of the present invention provides a digitally indexed reciprocating linear actuator, comprising a frame, a servomotor mounted to the frame, a crank gear-coupled with integer gear ratio to the servomotor for unidirectional rotation, an actuator slide mounted to the frame for linear motion, a connecting rod coupling the crank to the actuator slide, and means for detecting a selected non-home index position of the actuator slide. Preferably, the crank includes a crank arm with an outer end, and the connecting rod includes a driven end pivotally connected to the outer end, and a drive end. The actuator slide defines an index point. The means for detecting includes a sensor positioned to detect the index point. Preferably, the digitally indexed reciprocating linear actuator includes a toothed gear train having a gear ratio equal to a selected integer.

Another embodiment of the present invention provides a machine for handling a plurality of articles, the machine having repeated cycles of article-handling operations and a plurality of carriages, each carriage capable of carrying at least one article. This embodiment comprises a frame, a servomotor mounted to the frame, the servomotor having an angular position encoder, a crank gear-coupled with integer gear ratio to the servomotor for unidirectional rotation, an actuator slide slideably-mounted to the frame for moving a carriage in linear motion, a connecting rod coupling the crank to the actuator slide, and means for detecting a selected non-home index position of the actuator slide. Preferably, the machine includes means for interpolating encoder angular position values from a first and second detection of a non-home position of the slide to produce an angular position value corresponding to a home position of the slide. In one embodiment, the machine is a wrapping machine.

Another embodiment of the present invention provides a wrapping machine for wrapping a plurality of articles, the machine having repeated cycles of article-handling operations. The wrapping machine includes a plurality of carriages, each carriage capable of moving at least one article for wrapping, and at least one digitally indexed reciprocating linear actuator, coupled to move a carriage, the linear actuator having a crank and an actuator slide, the actuator slide having a non-home index point. The wrapping machine further includes means for recording first and second servomotor angular position values on detecting the index point at first and second angular positions of the crank, and means for identifying home position as an angular position value midway between the first and second position values.

The invention further provides a method for identifying a home position of the slide of a digitally indexed reciprocating linear actuator. The method requires a linear actuator including an actuator slide with an index point and a servomotor with an angular encoder. The method requires that the servomotor be configured for unidirectional rotation drive of the actuator slider via a crank, and that the crank be gear-coupled with integer gear ratio. The method comprises a) determining a first servomotor angular position value at a first detection of the index point, b) determining a second servomotor angular position value at a second detection of the index point, c) calculating a third servomotor angular position value midway between the first servomotor angular position and the second servomotor angular position; and d) identifying the third servomotor angular position value as a home position.

DETAILED DESCRIPTION OF THE INVENTION

Prior art servomotor-based drivers of reciprocating linear actuators typically use direct drive. In the direct-drive mode of operation, the servomotor rotates in one direction to advance the linear actuator and then rotates in the opposite direction to retract the linear actuator. Because of the need to reverse direction with every cycle of the linear actuator, such drivers suffer significant energy losses and significant limitations on machine speed. The present invention uses a servomotor running continuously in unidirectional mode to reduce energy consumption and to increase maximum machine speed.

In machines, such as wrapping machines, that handle a plurality of articles and have repeated cycles of article-handling operations, it is important to maintain synchronization between the multiple article-handling mechanisms cycle after cycle. Otherwise the articles or the mechanisms would collide with each other. Prior art wrapping machines use linked mechanical drives that automatically achieve synchronization because all mechanisms are driven by a common motor. However, these machines suffer from wear in cams, backlash in gears, and stretching in chains and belts. The use of independent servomotors for driving all mechanisms largely eliminates the wear, backlash and stretching problems. However, mechanisms driven by independent servomotors must be individually synchronized, preferably to a home position.

Synchronization to home (end of stroke) position of the slide of a reciprocating linear actuator presents a special problem in that it is difficult to detect home position directly using a motion detector or a proximity sensor with a high degree of precision. This is because linear motion of the actuator slide at the end of its stroke is zero, and linear motion of the actuator slide close to the end of stroke is very small. In the present invention this difficulty is overcome by using the high resolution of the output of a servomotor encoder output and interpolating two angular position values measured at points in the cycle of the reciprocating linear actuator where the linear motion of the slide is large. In a preferred embodiment, the method includes detecting the slide at one selected position, well removed from an end-of-stroke position, during an outward stroke and during an inward stroke, recording a servomotor encoder output value at each detection, and interpolating the two encoder output values to calculate encoder output value at home position.

It is, of course, necessary to establish an initial known position for every mechanism of the machine. This is accomplished during initialization of the machine by setting each servomotor angular position output to zero at home position.

To maintain synchrony through multiple cycles of the machine, the present invention uses a gear train that has an overall gear ratio of servomotor revolutions to crank arm revolutions that is a selected integer. This ensures that one full rotation of the crank arm corresponds to an integral number of full rotations of the servomotor, thereby allowing synchrony to be maintained from one cycle of the machine to the next through an unlimited number of cycles.

In a machine involving handling a plurality of articles and having repeated cycles of article-handling operations, the present invention provides a digitally indexed reciprocating linear actuator for synchronizing home position of a servomotor-driven actuator slide with angular position of the servomotor. The present invention provides an improved apparatus and method for synchronizing the multiple mechanisms of wrapping machines. In particular, the present invention provides an improved apparatus and method for precisely identifying the home position of a digitally indexed reciprocating linear actuator used to drive a carriage in a wrapping machine, the carriage carrying an item to be wrapped. The term “carriage” is used herein to include a “pusher” for moving an article horizontally, an “elevator” for moving an article vertically and a “linear translator” or like term for moving an article in any other direction. The apparatus and method provided by the present invention benefits machines having at least one carriage and having repeated cycles of operation. Benefits result from more precise synchronization of multiple mechanisms that handle a given article as it moves through the machine. Direct benefits include improvements in speed, durability or reliability of the machine. Indirect benefits include improvements in quality of the output product. In a wrapping machine indirect benefits include improvements in the quality of the package.

FIG. 1shows a first preferred embodiment of a digitally indexed reciprocating linear actuator20having actuator slide21. Slide21is driven by servomotor22operating in unidirectional rotation via gear train24, crank arm25and connecting rod26. Proximity sensor30is positioned to detect a selected position of the actuator slide. The selected position is shown as index point31inFIGS. 2A and 2B.FIGS. 2A and 2Bshow the proximity sensor detecting the selected position during both the outward stroke and the inward stroke of the actuator slide. Note that because what is being detected is the location of the selected point on its line of motion as the point moves with the actuator slide. The preferred embodiment ofFIG. 1uses a Siemens type 3R640-12-3AG01 proximity sensor. However, any type of proximity sensor or any other type of sensor capable of determining the location of the index point along its line of motion could be used, including a position sensor, a motion sensor, or even an optical alignment sensor located at a distance from the linear actuator and configured to detect a tiny index hole in the actuator slide.

In the preferred embodiment ofFIG. 1, gear train24includes gearbox27and toothed belt drive28. Belt drive28includes toothed drive wheel34, toothed driven wheel35, and toothed belt36. Preferably, gearbox27and toothed belt drive28each have a gear ratio that is an integer. Driven wheel35turns crank arm25about its axis end52. Outer end53of crank arm25is pivotally attached to driven end32of connecting rod26. Actuator slide21is shown mounted for sliding motion on guide rail39. Guide rail39is supported by frame38. Drive end33of connecting rod26is pivotally attached to actuator slide21. Actuator slide21is attached at its operative end to elevator mounting post43for reciprocating (raising and lowering) elevator plate45.

Servomotor22includes position encoder23that generates angular position data corresponding to the current angular position of the servomotor.

Gear train24provides an overall gear ratio of motor revolutions to crank arm revolutions that is a selected integer. Because the gear ratio is an integer, a number of full rotations of the servomotor when the number equals the selected integral number, produces exactly one full rotation of the crank arm, so in subsequent rotations synchrony is maintained.

Suitably sized servomotors are preferably selected from the Allen Bradley® Ultra Series Servo drive family of servomotors. Control equipment is preferably selected from the AllenBradley® Control Logix PLC family.

FIGS. 2A and 2Bare schematic elevation views of the embodiment ofFIG. 1, showing the crank arm at equal and opposite angular distances “A” away from the angular position of the crank arm corresponding to the linear actuator being at its home position.

A standard (prior art) package produced by a wrapping machine is shown in FIG.6. This standard package has a bottom long seam, glued or heat-sealed, and double point end-folds on both ends of the package.

A reverse double point folds (prior art) package produced by a wrapping machine is shown in FIG.7.

FIG. 3is a schematic front elevation view of a novel wrapping machine that uses a digitally indexed linear actuator in accordance with the present invention. Preferably, the machine is 9-16 feet long, depending on the size range of packages to be wrapped. A machine according to the present invention can easily be sized to accommodate packages at least as large as 13.5×8×7 inches or at least as small as 2×1.125×0.375 inches. In a preferred embodiment, the machine will wrap up to 150 packages per minute in either paper or plastic, using trays or boxes.

FIG. 3shows the general location of several parts of the machine and the location of the wrapper sheet with which the article is to be wrapped. Articles (not shown) are introduced to the machine via in-feed conveyor40and dead plate41.

Elevator42lifts the article into a wrapper sheet (not shown) for wrapping. Elevator plate45is driven by servomotor22, via gear train24and digitally indexed reciprocating linear actuator20(elevator actuator) in accordance with the present invention. Push bar47of pusher46removes articles one at a time from elevator plate45of elevator42. Push bar47is driven by servomotor22′, via gear train24′ and digitally indexed reciprocating linear actuator20′ (pusher actuator) in accordance with the present invention.

Folding of the wrapper sheet takes place at folding station54(tuckers), and at first, second, and third folding stations (folders)55-57, respectively. Wrapped articles are carried out of the machine by out-feed conveyor59. The steps of the folding process are substantially the same as the steps of the wrapping process of the prior art wrapping machine made by Package Machinery Company illustrated in FIG.8. Such machines are widely used and the folding process is well known to those skilled in the art.

FIG. 4is a perspective view of a preferred embodiment of the in-feed region of the wrapping machine of FIG.3.FIG. 4shows in-feed conveyor40that frictionally advances a line of abutting articles50, across a dead plate41, to a wrapping machine10. (Machine10is not explicitly shown but is represented inFIG. 4by folding channel48). Elevator plate45receives the leading article from conveyor40when it is on its lowest position. A first digitally indexed reciprocating linear actuator20raises elevator plate45to an upper discharge position. As this occurs, a pre-positioned wrapper sheet (indicated by44inFIG. 3) is partially folded about the article. (Digitally indexed reciprocating linear actuator20is shown in FIG.1. It is represented inFIG. 4only by crank arm25and connecting rod26). The article is then advanced from elevator plate45through folding channel48to perform the operation of folding the wrapper sheet about the article. Elevator plate45is then lowered for a subsequent cycle of operation. Digitally indexed reciprocating linear actuator20reciprocates (raises and lowers) elevator plate45as indicated by arrows AA.

FIG. 4also shows pusher46that includes push bar47. Push bar47is driven by a second digitally indexed reciprocating linear actuator20′. Pusher46moves articles from the upper discharge position through folding channel48. Second digitally indexed reciprocating linear actuator20′ is similar to the first digitally indexed linear actuator shown inFIG. 1, except that it drives pusher46and push bar47horizontally. Digitally indexed reciprocating linear actuator20′ reciprocates (advances and retracts) push bar47horizontally as indicated by arrows AB. Digitally indexed reciprocating linear actuator20′ is represented inFIG. 4only by connecting rod26′ and actuator slide21′.

Both digitally indexed reciprocating linear actuators,20and20′, are synchronized, as discussed above, with respective servomotors, and all servomotors are synchronized with each other, thereby synchronizing actuators20and20′ with each other.

Except for the servomotors and the digitally indexed reciprocating linear actuators, what is illustrated inFIG. 4is well known to those skilled in the art concerning wrapping machines. For example, see U.S. Pat. No. 3,243,033, to Merchant, et al, issued Mar. 29, 1966.

In the embodiment ofFIG. 4, the common motor of the Merchant machine is replaced by a plurality of servomotors. More particularly, the elevator reciprocating mechanism and the pusher reciprocating mechanism of the Merchant machine are replaced by novel digitally indexed reciprocating linear actuators20and20′, respectively.

FIG. 5is a perspective view of a preferred embodiment of a stacking elevator of a wrapping machine in accordance with the present invention.

FIG. 5shows in-feed conveyor70advancing articles71across a dead plate72to elevator plate73, which is reciprocated by first digitally indexed linear actuator74′ (similar to20′ shown in FIG.1). Elevator plate73receives the leading article from conveyor70in its lower or receiving position and then raises that article to latches75in its upper or discharge position. Elevator plate73is then lowered to receive a next article from conveyor70. Elevator plate73is then raised again to latches75to assemble a stack of articles thereon. Elevator plate73is raised and lowered by second digitally indexed linear actuator74(similar to20shown in FIG.1).

After a predetermined number of cycles of elevator plate73(which determines the number of layers in the stack) the assembled stack is advanced by a pusher76to intermediate conveyor77which then advances the stack of articles to wrapping machine11.