Document Edging And Stack Advance System

Abstract

The present disclosure describes an edging and stack advance system that is capable of automatically preparing documents such as letters and the like, that have been loaded onto a conveyor for subsequent feeding, sorting and/or processing. Significant features of the system include the use of separate zones for conveyance and edging, and a plurality of interconnected stack sensing switches. The latter switches cooperate to establish and maintain a stack geometry having a degree of looseness which permits reliable edging and subsequent feeding of the documents. The entire system has been designed to be relatively uncomplicated and may be manufactured at low cost.

Description

BACKGROUND OF THE INVENTION

The edging and stack advance system of the present invention has performed reliably in an actual operative mail sorting machine. In present day systems, stack advancement is accomplished by utilizing a single front sensing switch located in the forward wall toward which the documents to be processed are urged by the conveyor mechanism. In general, the front switch is usually custom designed to have a low differential travel, for example, in the order of 0.003 inches. Differential travel may be defined as the switch actuator movement required from the stop point to the start point of conveyor movement. The front mounted switch controls a high performance stepping motor which can advance the entire stack of documents in response to a single document of minimum thickness (about 0.008 inches) being fed out of the stack.

The foregoing systems require that the documents in the stack be maintained in a vertical edge position at all times. The stack is advanced to correspond with the time allowed for the feed rate required. If a front stack sensing switch is employed to move the documents out of the stack, the looseness of the documents, that is, the stack looseness, is a function of document thickness. In this system, the consistent looseness control needed for reliable edging is unpredictable with variable thickness documents.

In contrast to the above mentioned system employing a single front sensing switch, the present invention contemplates the use of a plurality of sensing switches. For example, in an actual operative embodiment of the present invention, a pair of sensing switches displaced from each other and mounted in a vertical plane in a side registration wall of the edging zone are employed. Additionally, a front mounted switch is incorporated.

As will become apparent in the description of the invention hereinafter, the stack advancement is accomplished primarily by the two side mounted switches, rather than the front mounted switch, once feeding is initiated. The system is characterized by slow response in that the stack is not advanced for every document that is fed out. Stack thicknesses of approximately 0.5 inches may be required to be fed out before the conveyor again advances the stack. The stack geometry is controlled by the side sensing switches and is substantially vertical in the forward portion of the edging zone after an initial stack advance. Subsequently, however, as documents are fed out of the edging zone an angular separation increases incrementally between the document adjacent the feeder face and the succeeding documents in the stack. When this separation angle has attained a predetermined value as sensed by the side sensing switches, the conveyor is again actuated to advance the stack and reestablish the aforementioned substantially vertical condition in the vicinity of the forward wall. Throughout this range of operation, looseness of the documents is maintained to a degree which permits reliable edging. The magnitude of the total travel of the side switches determines the maximum angular separation, and the travel is chosen to be considerably greater than that found in systems where only a single front mounted switch is used. This total travel is composed of a large differential travel together with an angular over-travel. Such switches need not be custom designed. In fact, standard low-cost microswitches may be utilized. Additionally, a readily available low response stepping motor may be used to drive the conveyor.

SUMMARY OF THE INVENTION

In the present system, the documents to be processed are loaded into the conveyor zone having conveyor belt transport means. The side edges of the documents are inclined at a predetermined angle from the vertical as determined by the face of a stack support block upon which they rest. The stack support block advances on the conveyor belts in unison with the stack of documents. The documents are advanced forward and after leaving the conveyor zone, they are pushed into the edging zone. A plurality of rotating edger bars in the latter zone perform both a vertical agitation and horizontal displacement of the documents to accomplish the edging function. The bars are driven by a timing belt and pulley system and continually rotate in a direction that causes the documents through friction contact between their bottom edges and the bars to be urged in a direction perpendicular to the stack movement and against a side registration wall. A retarder strip placed between two of the bars and having a saw-toothed shape serves to inhibit stack advancement to the edging zone by tripping the bottom edge of the mail pieces as they pass over the strip. This causes the documents on the edger bars to be tipped from an initial backward leaning position to a more vertical position, thereby enhancing the capability of the edger bars to perform their function.

A plurality of document sensing switches and associated actuator fingers that control the stack feeding and orientation of the mail pieces over the edging mechanism, play an important role in the edging process. In the present system, three stack sensing switches are employed -- a front switch and a pair of side sensing switches. The front sensing switch is positioned to sense the lower front face of the mail stack as it arrives at the forward wall of the edging zone, in proximity to the feeder mechanism. A pair of side switches displaced a predetermined distance from each other and mounted one above the other in the side registration wall, control the stack advancement after feed out of the documents from the edging zone has begun. The side switches monitor respectively the upper and lower edges of the mail stack.

In the edging zone, the stack in its initial tilted back position, is operated upon by the rotating edger bars and the retarder strip, causing it to assume a more vertical position as it approaches and contacts the forward wall of the edging zone. In approaching the forward wall, the stack actuates the lower side switch first. Then, the front switch is actuated by the initial document in the stack and finally the actuation of the upper side switch takes place. With the front switch actuated, the electrical circuit conditions are such that the motor driving the stack conveyor is halted. The additional actuation of both side switches does not affect this condition.

An important requirement for successful edging of documents which may be nonuniform in size and weight is that they be kept as loose as possible to reduce the interaction of adjacent documents. Such interaction could inhibit an individual document having its lower edge above the edger bars from moving to the side registration wall when the documents on either side of the misregistered document have already been edged.

A significant advantage of the present invention is that edging is independent of the stack length. A small portion of the stack in the edging zone is effectively separated from the main body of documents in the conveyor zone, and the bulk of the documents do not interfere with the edging process. Stated another way, the pressure of the stack is transmitted to the support block, rather than to the forward part of the edging zone. Thus, the documents to be edged and situated at the front of the stack are located at a point of minimum compression, and are therefore in the best condition to be edged. The present system therefore relieves most of the stack pressure required to maintain a feeding geometry in order to enhance edging.

After the initial advancement of the stack into the edging zone and the halting of conveyor movement, feedout of the documents in the forward portion of the edging zone continues by virtue of the attraction of succeeding documents toward the face of a vacuum feed chamber. As documents leave the area, a separation angle develops until the actuator finger of a side switch, for example the upper switch, is no longer pushed forward and the switch is no longer actuated. During this time, the stack documents outside the edging zone, that is, those in the conveyor zone have remained in a static, tilted-back condition, leaning against the angled support block face. When either of the side switches is no longer actuated, the conveyor motor is again energized and more documents from the main body of the stack are moved into the edging zone, thereby again halting the conveyor movement. This action of the side switches provides an adequate range of looseness for proper edging both in the initial condition where the documents are substantially vertical in the forward portion of the edging zone, and subsequently in an angled condition as the documents leave the zone.

Other features and advantages of the present invention will become apparent in the detailed description appearing hereinafter.

DESCRIPTION OF THE DRAWING

FIG. 1 is a plan view of the major components of the edging and stack advance system.

FIG. 2 is a section view taken along the lines 2--2 of FIG. 1 and providing a detailed view of the drive means for the edger bars.

FIG. 3 is a section view taken along lines 3--3 of FIG. 1 to better illustrate the placement of the stack sensing switches with respect to the document stack.

FIG. 4 is a detailed view of the side stack sensing switches as they appear mounted on the opposite side of the registration wall.

FIG. 5 is an electrical schematic depicting the interrelationship of the sensing switches in their control of the stack conveyor motor.

FIG. 6 illustrates diagrammatically the initial stack geometry after the documents have been loaded into the conveyor zone.

FIG. 7 illustrates particularly the geometry of the forward portion of the stack in the edging zone at the time of halting of conveyor movement and prior to the commencement of feedout.

FIG. 8 illustrates the angular displacement .theta. in a tilt-back mode for the forward documents of the edging zone as documents are fed out therefrom.

FIG. 9 depicts the maximum angle .theta.' (equivalent to its converse in a forward-tilt mode) permitted in the leading documents and corresponding to the point of resumption of conveyor movement.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The basic elements used in the edging and stack advance system of the present invention are depicted in FIG. 1 and the section views of FIGS. 2 and 3, derived therefrom. With general reference to FIG. 1, and particular reference to the other Figures where indicated, the details of the system may be described as follows.

A stack of documents 10 such as mail pieces, which stack may vary from a few inches to several feet in overall length, is loaded into the conveyor zone shown in FIG. 3. This zone comprises conveyor belts 12 disposed on pulleys 13, driven by a motor 14 via shaft 15. As seen in FIG. 3, the documents are supported by a support block 16 and the angular disposition of the documents is a function of the sloped upper-face of the block as seen in FIG. 3. In an actual operative embodiment of the invention, the sloped face of the block was displaced approximately 30.degree. from the vertical. The surface of the block contiguous with the conveyor belt is of a resilient high-friction material 18.

The leading group of letters are the first to leave the belt surface as the belts start downward in their return excursion to the opposite end of the conveyor. These letters come to rest within the edging zone (FIG. 3), and although they are no longer on the belt they are pushed toward the edger bars 20 by the stack of mail and the support block 16 still on the conveyor. Three edger bars 20 are depicted in FIGS. 1 and 2. Depending upon the application, a greater or lesser number of edger bars may be required. Each of the bars 20 is square in cross section and tapered on the extremity which first contacts the oncoming documents. This tapered portion provides a transition ramp from the conveyor to the main body of the edger bars. Increased edging capability results from this portion of the bars due to the greater force exerted on the lower document edge as it is lifted and pushed up the tapered incline. FIG. 2 indicates the configuration of belts 22a - 22c and pulleys 24a - 24f to continually rotate the edger bars in a direction to cause the documents to move up and down in a vertical plane and also horizontally toward the registration wall 26. The main drive pulley 24f is driven by belt 22c coupled to a source of power, not shown.

The tapered ends of the edger bars 20 begin the edging process as soon as the bottom edge of the mail pieces come into contact with its surface. As a result, some of the documents are driven to the side registration wall 26 while they also continue advancing onto the edger bars. Since the mail stack is still leaning against the support block 16, a portion of the letters will not have been edged due to the resisting frictional forces that are evident because of the higher normal forces present between adjacent letters.

The documents continue to advance on the rotating edger bars 20 so that their bottom edges contact the saw-tooth shaped teeth of the retarder strip 28 as seen clearly in FIG. 3. Sloped or leaning letters generate higher normal forces between adjacent mail pieces and this prevents a letter situated between two edged letters from effectively being registered due to the resisting frictional forces between the letters. The edges of the documents are tripped by the retarder strip 28 such that the vertical agitation provided by the edger bars 20 permits those letters 10 temporarily restrained by the retarder strip to assume a more vertical geometry than those in the remainder of the stack. As the letters continue to advance toward the forward wall 30 of the edging zone, their bottom edges contact the surface of belt 22a which drives the edger bars 20. The high frictional surface of the latter belt helps to maintain the mail pieces in their edged condition and urges them toward the side registration wall 26 in preparation for their subsequent feed out from the edging zone.

A brief explanation of feeder operation is pertinent at this time. It should be understood that a variety of feeder mechanisms are well known and that the following description of a particular feeder is presented merely for purposes of example. It should not be construed as limiting the present invention. The leading document in the forward end of the edging zone is drawn against the feeder control fingers 32 by virtue of the high air flow entering the feeder vacuum chamber 34 through lines 33. The control fingers 32 intermesh the moving feeder belts 36 and extend above the surface thereof. The latter belts are driven by drive pulley 37 through the action of belt 38 coupled to a drive source, not shown. The document is thereby prevented from contacting the feeder belts 36 until feed out of the document is desired. The latter is accomplished by retracting the control fingers 32 and allowing the documents to contact the moving belts 36.

As the stack of documents 10 is advanced by the conveyor system, the leading document contacts the forward wall 30 of the edging zone, thereby depressing the actuator finger 40 and actuating the front switch 42. A return spring 39 assures more positive switch operation. Concurrently, the leading documents are driven against the side registration wall 26 by the edger bars 20 and push both actuator fingers 44 and 46 as seen in FIG. 3 toward the forward wall 30, thereby actuating respectively both side switches 48 and 50. The mounting of the side switches 48 and 50 on a bracket 41 behind registration wall 26, and the projection of the actuator fingers 44 and 46 through respective slots 43 and 43' in the wall, are clearly shown in FIG. 4. Springs 45 and 45' are coupled respectively between the fingers 44, 46 and posts 47, 47'. The movement of fingers 44, 46 forward toward the forward wall 30 (FIG. 3) in an arc about pivot points 49, 49' extends the respective springs, which are designed to become fully extended with a small force of about one ounce applied thereto. The purpose of springs 45, 45' is to provide a more reliable return motion of the actuator when it is being pushed by the oncoming documents. Moreover, the springs permit the fingers to rotate in the opposite direction as documents exit the edging zone, in minimum time. It an automatic mode, this permits more consistent earlier initiation of conveyor motion.

The actuation of all the sensing switches is indicative of the proper positioning of the documents for feeding purposes. The corresponding electrical condition is such that power is removed from the stack conveyor motor 14, thereby halting the advance of the stack. The electrical schematic of FIG. 5 indicates the circuit interconnections of the front switch 42, the upper side switch 48, lower side switch 50, and the conveyor drive motor 14, for the condition that none of the switches is actuated. That is, no documents are in contact with switch actuator 40, 44 and 46. Consideration of the various switch combinations possible during the actuation of one or more of the switches, reveals that the motor 14 will be energized by an electrical power source applied to terminals 51--51', and conveyor motion initiated under all switch conditions except where the front switch 42 alone is actuated, or where all three switches 42, 48 and 50 are actuated concurrently. This latter condition is the one mentioned hereinbefore. The former condition, wherein the front switch 42 is actuated and both side switches 48 and 50 remain unactuated can occur under conditions of imperfect edging where a vertical gap appears along the registration wall 26 which encompasses the actuator fingers 44 and 46 of both side switches 48 and 50. The side switches therefore sense this abnormal condition which would create excessive stack tightness and halt the stack advance. The function of the front switch 42 is to permit advancement of the stack after the stack conveyor is first loaded, even though the side switches are not actuated.

Assuming that the documents have arrived in the edging zone, that all three switches are actuated and that conveyor motion has been halted, the subsequent motion of the conveyor is dependent upon either switch 48 or switch 50 reverting to the unactuated condition. This further assumes that the front switch remains generally in an actuated condition as a result of the sequential drawing of the documents from the stack and their being held against the forward wall 30 and feeder fingers 32, as a result of the vacuum force exerted by the feeder mechanism. The two side switches 48 and 50 therefore perform an exclusive-or logic function, that is, the conveyor motor will be energized, if and only if, one of the switches is actuated but not if both are in an actuated or unactuated condition. As documents are fed out of the edging zone while the conveyor motion is halted, a condition is reached in the angular displacement of documents in this zone which causes either one of the side switches 48 or 50 to revert to its unactuated condition. Conveyor motion is resumed and more documents in the main body of the stack are moved from the conveyor zone into the edging zone.

The actual operation of the system is best explained in connection with the diagrammatic illustration of FIGS. 6-9 inclusive which nevertheless should be considered in connection with the structural details of FIGS. 1-5 inclusive described hereinbefore.

In FIG. 6, the documents 10 are shown loaded onto the conveyor belts 12 at an angle .alpha. which is determined by the face of the support block 16. No support means at the front of the leading document is needed because the belts have an adequate frictional coefficient to prevent the stack from collapsing or even increasing the initial angular condition. The support block 16 absorbs the stack compression force.

As the documents are moved forward by the conveyor belts 12, they are forced off the belt, leaving the conveyor zone for the edging zone. At this point, the documents contact the edger bars 20 and are agitated in a substantially vertical plane perpendicular to the stack advance direction. The coefficient of friction of the edger bars is lower than that of the conveyor belts and would normally not permit the stack to advance without an increase in angle .alpha. or indeed a collapse of the stack. However, the retarder strip 28, positioned between the edger bars permits the stack to continue toward the forward wall 30 without collapse. The vertical teeth of the retarder 28, extending above the edger bars, interweave between documents to prevent forward movement of the bottom edges of the documents, and initially to permit the stack to maintain the angle .alpha. in the edging zone. Since the leading edges of the retarder teeth are sloped, sufficient force from the conveyor belts during stack advance drives the document in the edging zone over the retarder teeth to permit them to arrive at the forward wall 30. At this point the documents assume a substantially vertical configuration as seen in FIG. 7.

As the stack advances and reaches the feeder, all three stack sensing switches 42, 48 and 50 (FIG. 3) will have been actuated and advancement is halted. This condition is illustrated in FIG. 7. The leading document 10a is attracted to the feeder by the air flow associated therewith and the remaining documents in the edging zone each has an increasing component of angularity proceeding from the initial document 10a whose angularity is zero toward the last document in the edging zone, designated 10b, which has an angularity substantially the same as that possessed by the documents now stored in the conveyor zone and awaiting the edging function.

FIG. 8 represents the condition wherein several of the documents in the edging zone have been fed out. An angular separation, represented by the angle .theta., increases incrementally as the documents leave the zone. Actually, an exiting document creates a void between the lower edge of the succeeding document and the forward wall equal to the document thickness. The lower edge of the succeeding document translates forward and the upper edge then rotates to contact the feeder control fingers 32 (FIG. 1). That is, the document waiting to be fed out of the edging area, is pulled to a vertical position by the vacuum forces exerted by the feeder.

Each succeeding document is subject to the same motion. It should be noted that only the documents lying in the space between the last tooth 28a of the retarder strip 28 and the front wall 30 are not inhibited by the retarder teeth and are therefore attracted by the feeder vacuum.

As feeding continues and the angle .theta. increases, the number of documents in the edging zone decreases. This condition causes increased looseness, and the relaxation of pressure between adjacent documents promotes even more effective edging.

FIG. 9 illustrates the condition existing just prior to the commencement of the next cycle of conveyor motion. The exiting of a number of documents from the edging zone has caused a separation angle .theta.' to develop, which has a maximum predetermined value. Concurrent with such development, the actuator finger 44 associated with switch 48 which had been moved forward by the advancing documents, has moved incrementally backward toward its initial position as the documents are fed out. The angle .theta.' which is somewhat smaller than the initial bias angle .alpha. allows switch 48 to achieve its original unactuated condition, and as will be apparent from the electrical schematic of FIG. 5, an electrical circuit path is established to supply power to motor 14 and reestablish conveyor motion.

It should be noted that as documents are fed out of the edging zone and angle .theta. increases, the time to pull succeeding documents to the feeder face increases. A momentary angular void exists between the feeder and the document about to be pulled forward by the feeder vacuum. With small angles of .theta. and documents of minimal thickness, actuator 40 of the front switch 42 may extend somewhat during the transient period immediately following feed out, but this extension is insufficient to cause conveyor movement. On the other hand, as .theta. becomes larger and with thicker documents, it is possible that a momentary return of switch 42 to its unactuated condition can occur, with some stack advancement. It must be emphasized, however, that the latter advancement is minimal compared to the total stack advancement effected by the side sensing switches.

FIGS. 6 through 9 assume that the stack in the edging zone is in a normal tilt-back mode. It has been observed in an actual operating system that under certain circumstances, related to the initial loading of documents onto the conveyor, or as a result of a high feed-out rate, the documents in the edging area will assume a forward-tilt geometry. Thus, compared with FIG. 9, an illustration of the latter would depict the top edges of the documents leaning toward the forward wall 30, and the formation of a displacement angle, equal but converse to .theta.', that is, measured between the forward wall 30 and the bottom edge of the next document waiting to be attracted to the feeder. In this case, the actuation finger 46 associated with lower side wall switch 50, FIG. 3, will have moved to its original position and switch 50 is no longer actuated. Upper side wall switch 48 remains actuated. Under these conditions, switch 50 effects the resumption of conveyor motion.

The function of the stack zones may be summarized as follows. The documents in the edging zone are edged in preparation for feeding, whereas those in the conveyor zone are stored in an angular geometry determined by the face of the support block 16. In the edging zone, which accommodates a very small portion of the stack (only about 3 inches in an actual operative embodiment) the edger bars 20 move the documents in a plane perpendicular to the direction of stack advance. The bars do not advance the stack, but permit the documents in the edging zone to be advanced forward when required by the pushing action of the documents in the conveyor zone. The documents in the edging zone are supported in varying stages of angularity by the documents in the conveyor zone as well as the retarder strip 28. The documents in the conveyor zone however, are stored in a mode of angularity which remains unchanged while they are present in this zone. These documents are supported by the support block 16 as well as by the friction of the conveyor belts 12 themselves.

It should be apparent from the foregoing description and mode of operation that there is taught an improved system wherein optimum edging is maintained at all times through a zone stack arrangement that effectively relieves stack pressure against the documents being edged, and a plurality of stack sensing switches adapted to sense the geometry of the documents being edged and to advance the entire stack upon the attainment of predetermined angular conditions. These concepts have proved highly satisfactory in an actual operative mail handling system, resulting in reliable feeding with minimal numbers of double-fed documents. It should be understood, however, that changes and modifications of the mechanisms described herein may be necessary to suit particular requirements. Such changes and modifications, insofar as they do not depart from the spirit of the invention, are intended to be covered by the following claims.

Claims

What is claimed is:

1. A system for stack advance and document edging in preparation for the feedout of documents by a vacuum feeder mechanism comprising:

a conveyor zone and an edging zone contiguous to each other and having a common side registration wall, said edging zone having a forward wall in proximity to said feeder mechanism,

said conveyor zone having transport means for conveying a stack of said documents toward said edging zone, said edging zone being characterized by the absence of said transport means and having edger bar means operatively connected to agitate said documents in a vertical plane and to move them in a direction perpendicular to the motion of said documents in said conveyor zone toward said side registration wall, the documents in said edging zone being concurrently pushed forward toward said forward wall by the documents in the conveyor zone under the influence of said transport means,

said edging zone further including a plurality of stack sensing means, at least a pair of said sensing means being positioned respectively in said forward wall and said side registration wall, said sensing means being electrically interconnected for controlling the motion of said transport means and being initially in an unactuated condition,

the actuation of said forward wall sensing means in response to the physical contact made therewith by the initial document in said stack being indicative of the attainment of a proper feedout position by said stack and causing the termination of motion of said stack by said transport means, said side registration wall sensing means being actuated concurrently with said forward wall sensing means by physical contact of the documents in said edging zone upon the attainment of said last mentioned position,

the subsequent attraction of each of said documents in turn to said feeder mechanism as a result of the vacuum forces associated therewith, and the feedout of documents from said edging zone creating an incrementally increasing angular separation between the document in position for feedout and the next succeeding document, said side registration wall sensing means being responsive to said increasing angular separation such that upon the attainment of a predetermined angle, said last mentioned sensing means resumes its unactuated condition, thereby permitting said transport means to resume stack motion.

2. A system as defined in claim 1 wherein said conveyor zone includes a block-like member adapted to be carried by said transport means for supporting said stack of documents, said block-like member having its face portion inclined at a predetermined angle with respect to the vertical such that the documents standing on edge and leaning thereagainst assume a substantially like angle.

3. A system as defined in claim 2 further characterized in that said edging zone includes a plurality of said edger bar means having their longitudinal axes parallel to the direction of motion of the documents in the conveyor zone, a retarder strip positioned between a pair of said edger bar means, said retarder strip being comprised of a plurality of saw-tooth shaped teeth oriented with respect to the motion of the stack such that the documents contact consecutive teeth of the strip as they are pushed toward said forward wall by the documents in said conveyor zone.

4. A system as defined in claim 3 wherein said edger bar means each include an elongated portion which is square in cross section, and a tapered end which first makes contact with the documents advancing toward said forward wall, said edger bar means being continuously rotated by drive means in a direction to move said documents toward said side registration wall.

5. A system as defined in claim 4 further characterized in that the portion of each of said edger bar means opposite said tapered end and situated adjacent the front surface of said forward wall is a pulley, said means for driving said edger bar means including a friction belt disposed on said pulley, the edged documents approaching said forward wall contacting said friction belt and being maintained in, or further edged into, a registered condition thereby.

6. A system as defined in claim 5 wherein said transport means comprises a plurality of friction belts mounted on pulleys and driven by motor means.

7. A system as defined in claim 3 wherein each of said stack sensing means comprises an actuator finger operatively connected to an electrical micro-switch, physical contact with said actuator finger by said documents resulting in the actuation of said switch.

8. A system as defined in claim 7 further characterized in that the electrical switch portions of a pair of sensing means are mounted behind said side registration wall in a vertical plane and in spaced apart relation, said registration wall having a pair of slots therein to receive the actuator fingers of said sensing means, said fingers protruding therethrough to contact the documents in said stack, and being pivotally mounted to rotate in the direction of stack movement, thereby actuating the switches associated respectively therewith.

9. A system as defined in claim 8 wherein each of said pair of side registration wall sensing means includes a spring member coupled between its actuator finger and a fixed support to provide more consistent switch operational cycles.

10. A system as defined in claim 9 wherein the electrical switch portion of the front wall sensing means is connected in parallel with a circuit path containing the electrical switch portions of the pair of side wall sensing means, the series/parallel arrangement of all the switches being interposed between a source of electrical power and the motor driving said transport means, the front wall switch when unactuated being in a closed circuit condition to permit said motor to be energized, the pair of side wall switches being connected in an exclusive-or configuration whereby an electrical circuit path from said source of power to said motor is established therethrough if, and only if, one of said switches is actuated, but not if both switches are in an actuated or unactuated condition.