Method for fetching heads from stacks

In a method for manipulating paper roll heads by a multi-axis robot (8), heads are picked-up and transferred by the robot (8) from multiple stacks in accordance with stored data concerning the height and position of each stack as well as the dimension characteristics of the articles in the stack. At initial start-up, or immediately after a shut-down, previously stored information on the position or height of the stacks is considered not known by or otherwise unreliable by the robot (8) controller. That is, when the operation of the robot (8) is halted to accommodate replacement or replenishment of the head stacks, the stored parameter data for all stacks is set unknown. In accordance with its operating program, the system operation is restarted, the robot performs a slow-speed approach toward an uncalibrated stack, measures the stack data, that is, calibrates the stack height and position, and thus can perform the next stack approach through a normal transfer cycle on the basis of the measured data.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to article handling, and more 
particularly, to a method for picking up roll end header disks from a 
plurality of stacks using a multiple axis robot. 
2. Description of the Prior Art 
The finished product of the papermaking process is a continuously issued 
web approximately 6 meters wide. For shipment to customers and converters, 
the web is slit into a plurality of more manageable widths and wound into 
cylindrical reels or rolls of normally 1 to 2 meters in diameter. Shipment 
weights of such rolls may range from 1700 to 9200 pounds. To protect the 
finished rolls from damage during shipment and handling, they are 
typically wrapped with a heavy grade of paperboard having a thickness of 
0.23 mm or greater. 
The presently prevailing technique for such roll wrapping is to draw a 
strip of wrapping board from a supply reel of greater axial length than 
the roll to be protected. This web strip is wrapped tightly about the 
cylindrical surface of the protected roll. The axially overhanging portion 
is crimped radially inward toward the roll center and tightly creased 
against the circular roll end. To seal the roll end and hold the crimps 
down against the end faces, two circular header disks or "heads" of 
approximately the same diameter as the protected roll are used at each 
roll end-face. One head is inserted within the surface wrap overhang flush 
against the roll end-face. Adhesive, typically a hot-melt glue, is then 
applied to the outer face of this first or inner head. Next, the 
overhanging portion of the surface wrap is crimped and pleated into the 
inner head adhesive. Thereafter, a second header disk or "outer head" 
having adhesive applied to the inner face thereof is pressed against the 
outer face of the surface wrap pleats, thereby covering the crimped 
overlap of the wrapper and the inner head. 
The inner head is usually relatively thick and thus capable of protecting 
the roll end against mechanical damage. The outer head, which serves to 
bind the wrapper at the roll ends and protect the roll from humidity, can 
be substantially thinner than the inner head. Frequently the coloring and 
printed pattern of the outer head are designed to give the roll a neat 
appearance. 
Common to all prior art wrapping methods is the need for circular, usually 
die cut, heads of substantially the same diameter as the protected roll. 
If several different diameter sizes are prepared for shipment by the 
producing mill, it is necessary to make, or purchase and store, such 
respectively sized heads preparatory to use. Prior to wrapping, the width 
and diameter of the roll to be protected is measured, and heads of 
suitable diameter and width are selected from this group for placement on 
the roll ends. 
The end heads can be placed on the roll ends in a variety of ways. Manual 
placement of the heads is the oldest method, and it is still suited for 
relatively small capacity wrapping lines or applications not requiring a 
substantial degree of automation. In accordance with the manual technique, 
the operator positions the appropriate inner heads manually on the roll 
ends. The operator then places the corresponding outer heads onto heated 
press platens, which platens press the outer heads into adherence with the 
roll ends. The inner heads are kept against the roll ends by means of 
separate arms for the duration of the crimping of the wrapper overlaps 
against the roll ends. The adherence of the outer heads to the platens 
during the operation is achieved using vacuum. In order to overcome the 
processing speed limitations on the manual technique described above, 
various types of automatic heading machines have also been utilized. A 
common feature of almost all such conventional automated heading devices 
is that, for each end of the roll, a separate heading machine with an 
associated head clamp is provided. The head clamp transfers the head from 
the head stack or stacks to a corresponding roll end. 
In one conventional heading device, a rotatable arm is mounted on a 
vertical guide, the arm having at its distal end a rotatable vacuum clamp 
for grasping the heads. This type of heading machine is typically employed 
in conjunction with different kinds of head storage shelves situated 
beside the heading machine. Using this machine, the heads are placed on 
the roll ends so that the support arm of the clamp is transferred along a 
vertical guide to the level of the shelf containing the correctly sized 
heads. Next, the support arm of the clamp and the heading machine itself 
are rotated until the clamp is aligned parallel with the shelf. 
Thereafter, the head is gripped and transferred from the shelf to the roll 
end by rotating both the support arm of the clamp and the heading machine 
and moving the heading machine along the guide. 
In another type of known automated heading machine, the heads are placed in 
stacks on the factory floor and then transferred therefrom to the roll 
ends by means of an overhead gripping device, typically a carriage mounted 
manipulator. The gantry transfer carriage of this device is constructed 
above the stacks of heads and the heading manipulators are generally 
movable along a single, crosswise movable rail. Thus, a separate stack of 
heads of a predetermined size must be provided for each heading 
manipulator. 
A major disadvantage of the prior art automated heading systems discussed 
above is that a separate heading manipulator and a dedicated head storage 
shelf or supply stack is required for each roll end. The heading machines 
employed are of a special-purpose configuration in that they are designed 
for manipulating heads only and their associated control software must be 
tailored particularly for a particular operating environment. Accordingly, 
modifications to the operation of the system are cumbersome and require 
specialized design capabilities. 
In addition to the known techniques discussed above, it would also be 
possible to manipulate the heads using a standard multiple axis robot. 
Such a robot, for example, could be integrated with the layout of a 
wrapping line in a manner permitting the robot to pick up and place a head 
on each end of the roll or onto a press platen. To obviate the need for 
two separate head transferring cycles, it would be particularly desirable 
to configure such a robot with a two sided clamp capable of a flipping 
movement by which both roll ends could be sequentially gripped and both 
heads simultaneously transferred to the roll during a single transfer 
cycle. 
When retrieving the outer heads from respective stacks utilizing a robot, 
certain information must be known in order to optimize the processing 
speed of the transfer operation. The location of the stacks must be known 
so as to allow direct picking up of the heads without requiring that a 
stack location determination be made during each head transfer cycle. 
Another reason why stack position must be known is that the clamp carried 
by the arm of the robot must grasp the head by a predetermined point to 
ensure correct positioning of the head upon release. Another item of 
information which must be utilized by the robot control software to 
achieve an efficient transfer operation is the height of each stack. 
Specifically, if the height of a stack is known, the clamp can be 
immediately moved to a pick-up position relative to the uppermost head of 
the stack so that incremental clamp position adjustments or "inching 
steps" for finding the top of the stack are unnecessary. 
Accurate placement of head stacks relative to a robotic gripping device can 
not be achieved without unwieldy special arrangements, principally because 
the stacks are brought into place by fork-lift or other transfer means 
unsuitable for such placement. Consequently, the actual position of the 
stack subsequent to replenishment or replacement must be reported in some 
manner to the control software. The reporting can be manually performed at 
start-up or after stack replenishment, for example, by requiring the 
wrapping station operator to update information concerning the heads of 
the replenished stacks as well as the height and position of the stacks 
using a data terminal keyboard. Such a manual procedure, however, is 
awkward, inefficient, and possibly dangerous. To enter the stack height 
and location accurately requires a sufficiently accurate method of 
measuring such dimensions. The measurement cycle increases the system 
downtime consumed by the stack replenishment operation and introduces 
several error sources. Moreover, the robot control software is incapable 
of distinguishing between accurate and inaccurate information. Thus, if 
the wrapping station operator, due to stress or other reasons, were to 
enter incorrect measurement data to the control system or completely 
neglect to enter the altered measurements of a stack, the robot would 
perform the head transfer cycle on the basis of the incorrect or outdated 
information and possibly invoke extremely serious hazards. In the least 
severe cases, the head transfer cycle fails or the head is incorrectly 
positioned relative to the roll, whereby the error may be immediately 
detected and corrected. In the most serious case, the robot can topple or 
move the stack, or even cause damage to itself or other structures. In any 
event, operation of the wrapping station must be halted until a fault 
situation can be rectified. 
It is therefore an object of the present invention to provide a method in 
which the robot is controlled in accordance with a program which 
autonomously calibrates the position and height of the stacks, thus 
avoiding the problems and disadvantages discussed above. 
SUMMARY OF THE INVENTION 
It is therefore object of the present invention to provide a method through 
which the robot software can autonomously calibrate the position and 
height of the stacks, thus avoiding the fault situations described above. 
The invention is based on the principle of resetting all stack parameter 
data if the robot operations is halted during, e.g., the replenishment of 
the head stacks. 
At the restart of the operation, the robot performs a slow-speed approach 
including measurement steps to any uncalibrated stack thus measuring the 
stack height and position, while during the next head pick-up cycle the 
approach is made at normal speed on the basis of the measured information. 
The principal benefit of the method is the inherently safe operation of the 
heading system. As the number of human errors is minimized and the system 
operation is highly systematic, the possibilities of error are minimal. 
The measurements related to the calibration of stack heights and positions 
cause almost no impediment of system operating speed. The method is safe, 
because its operation is essentially based on the principle that when the 
operator poses a request to enter the robot's danger zone to replenish the 
stack, the robot is controlled to the home position and is allowed to 
continue its operation only after receiving a clearing message of the 
danger zone. Due to the position calibrating steps, good accuracy in the 
placing of the heads is attained, and the position and height of the 
stacks can be periodically verified based on, e.g., the number of head 
removals performed from a certain stack. Such verification is necessary as 
the stacks are not generally entirely straight. The method can be applied 
without performing major structural or software changes thus making it 
suitable for all standard types of industrial robots. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages, and specific object attained by its use, reference 
should be had to the drawings and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
When a multi-axis robot 8 is used for transferring the heads, it is 
preferably positioned so that the rotational center of the robot is 
coincident with the symmetry axis of the wrapping station/press platen 
unit. From this position, the heads are easy to move to the roll ends or 
press platens 11, 12. With reference to FIG. 1, the robot 8 is positioned 
in the abovementioned manner on the symmetry axis of the press platens 11, 
12. Then, the head stacks 1-7 are grouped to form a semi-circular or "C" 
arrangement as shown in the diagram, or alternatively, a "U"-shaped 
pattern about the robot. The entire piling area shown in the diagram is 
surrounded by a protective fence which prevents access to the working 
envelope of the robot 8. During the operation of the robot 8, the heads 
are picked up from a stack in accordance with the diameter of the roll to 
be wrapped, by means of a clamp 10 mounted to the end of the arm 9 of the 
robot 8, and are subsequently transferred to respective press platens 11, 
12. Although any suitable clamping structure may be utilized, a clamp 
capable of gripping and simultaneously transferring two heads is preferred 
to avoid the need for separate transfer cycles for each end of the roll 
being wrapped. The clamp 10 employed can be configured, for example, as a 
two-sided clamp, such as the one shown in FIGS. 2-4. As best shown in FIG. 
2, each side of the clamp includes a base plate 10 having mounted thereon 
a plurality of resilient suction cups 13. The outer rims of the suction 
cups 13 collectively define a suction plane 14. Reference numeral 16 in 
the diagram indicates a sensing means utilized for position calibration, 
and reference numeral 15 indicates a head stack. In the illustrative 
embodiment, sensing means 16 is a single photocell. At startup of the 
system, the robot control operates in accordance with a control program 
routine in which the exact height and position information for all stacks 
1-7 is assumed to be unknown. However, if the system operation is 
interrupted in a situation in which the software of the robot 8 possesses 
valid information of such stack height and position data, the operation is 
continued from the situation in which the system was stopped. In a 
situation of having the stack heights set as unknown, the robot 8 measures 
the height and position of the stack as it for the first time removes a 
head from each unknown stack. In this context the term unknown stack 
refers to a stack whose height and position are not yet stored in memory 
in accordance with the control. The stack data is always reset to an 
unknown condition when the robot operation is halted for any reason. A 
halt in this context refers to all system states causing the robot to 
stop, with the exception of a controlled stop, such as a work shift not 
involving a replenishment of any stack. In the case of a controlled stop, 
the latest updated information on the stacks is retained in memory for 
continued use by the control program. In practice the stack data are 
preferably always reset unknown for reasons of safety, irrespective of the 
reason for interrupting robot operation. 
The pick-up cycle of the heads and their transfer to the roll ends or onto 
the press platens occurs so that the sensor station of the wrapping line 
instructs the robot 8 as to the size and type of head required. Then, the 
robot 8 picks up the proper heads by means of two-sided clamp 10 from the 
correct stack and transfers them onto press platens 11, 12. If the heights 
and positions of the stacks are known, the robot 8 transfers the clamp 10 
by a high-speed movement to the uppermost position above the stack 15. 
Only the vertical picking up movements of the head are performed using the 
slow inching speed. Thus, the pick-up cycle of the heads can be 
implemented in an extremely short time. 
When the stacks 1-7 are desired to be replenished, moved or otherwise 
changed, the operation is as follows. The station operator informs the 
robot controller of a need to enter the robot's danger zone or work 
envelope. Then, the robot finishes a possible uncompleted transfer cycle, 
drives itself to predetermined position and assumes a safety state, after 
which the interlocks of the danger zone are deactivated and switched off. 
Next, the station operator can open the safety gate to the danger zone and 
enter the danger zone to change and/or replenish the head stacks. After 
the operator has left the danger zone, closed the safety gate and 
reinstated the integrity of the danger zone, the height and position data 
for all stacks are reset unknown. The robot then continues the 
manipulation of the heads in a programmed manner though performing its 
initial approach to any unknown stack in accordance with an incremental 
approach and measurement cycle. 
The given unknown approach to an unknown stack takes place so that the 
presumed pick-up height at a stack 15 is set equal to the maximum height 
associated with the stack plus a preset constant. The clamp 10 is thus 
transferred by a high-speed movement above the stack 15 to a level equal 
to the maximum height of the stack plus the preset constant wherefrom the 
aforementioned incremental slow-speed approach is commenced. The maximum 
travel of the clamp 10 from the starting height is limited to a constant 
length. When the upper surface of the stack 15 is detected by means of 
photocell 16, the system either chooses to verify the position of the 
stack 15, or alternatively, sets the linear speed of the clamp to a still 
slower inching speed of the picking step, and the remaining travel is set 
equal to the computed travel to the surface of the stack 15 or even 
slightly below the upper surface. The movement is stopped when a contact 
with the surface of the stack 15 is indicated by a mechanical sensor such 
as a limit switch (not shown), and this measured position is stored as the 
stack height. In the event that photocell 16 cannot detect the surface of 
the stack 15 within the constant maximum approach travel distance, that 
is, the stack height is less than the stack maximum height, a new approach 
length is allocated for the travel of the clamp 10 and the approach cycle 
is repeated. This cycle can be repeated until the upper surface of the 
stack 15 is detected, or alternatively, a preset minimum stack height is 
reached, whereby a message indicating a depleted stack is issued and a 
stack replenishment is requested. 
After the height of the stack 15 has been measured, the deviation of the 
stack position relative to a predetermined location stored in memory is 
measured. Each stack has a preset position where the stack should be 
placed. During the above-described incremental approach cycle, the clamp 
10 is controlled above the stack to the point where head pick-up should 
occur for a correctly placed stack. Consequently, after the stack height 
is measured, the clamp remains in the position relative to the stack where 
the head could be picked up from a correctly placed stack. Next, the 
position of the stack 15 is calibrated via a sequence in which the clamp 
10 is elevated slightly above the head surface and transferred toward the 
stack edge; in practice, this is performed as soon as the photocell 
detects the upper surface of the stack 15. The transfer takes place along 
the symmetry axis of the head stack. By definition, the symmetry axis of 
the head stack is the line passing through the center point of the stack 
and being aligned equidistant from the stacks immediately adjacent to the 
stack being measured. Each head diameter is assigned a preset transfer 
distance X extending from the initial position of the clamp 10 to the edge 
of the head stack 15. As is evident from FIG. 2, the transfer distance X 
is equal to the distance of the photocell 16 of the clamp 10 from the edge 
of a correctly placed head stack 15. While the mounting position of the 
photocell 16, and thus the distance X, can be selected freely, the 
mounting position is advantageously selected, such transfer movement 
occurs over the symmetry axis K of the head stack, whereby the position of 
the clamp 10 relative to the head stack need not be altered for 
small-diameter stacks. The transfer movement is preferably always 
performed as accurately as possible radially over the head. 
During the transfer movement X, the photocell 16 detects the edge of the 
head stack. In FIG. 3 the edge of the stack 15 is shown correctly placed, 
whereby the stack position can be stored as the preset position into the 
memory of the robot 8. By contrast, the position of the stack 15 in FIG. 4 
is shown to be farther from the starting point of the measurement than the 
dimension X, whereby by stack position is set equal to (preset value(X) 
+.DELTA.X). Correspondingly, if the stack edge is detected before the 
clamp 10 has been transferred by the distance X, the stack position is set 
equal to (preset value(X)-.DELTA.X). When the heads are being picked-up 
during the next transfer cycle from the stack, the clamp can thus be 
directly controlled to the corrected pick-up position. 
The stack position measurement is performed in the radial direction alone, 
that is, along the axis aligned essentially radial relative to the 
rotational center of the robot 8. Lateral measurement of the stacks is not 
necessary in practice, since the stacks are so close in this direction 
that a major displacement hardly can occur. If desired, however, an 
additional sensor can be used to implement stack position calibration in 
the lateral direction, as well. 
After both the height and position data of the stack are stored into the 
memory of the robot 8, the height and position of the stack are now known 
and the next head can be picked-up directly from the calibrated position 
of the measured stack without resorting to the slower incremental approach 
step. All other stacks are calibrated in a similar manner when heads are 
picked-up from them for the first time. Thus, no separate measurement 
cycle for all stacks is needed prior to the start of the actual heading 
operation. 
The position and height of the stacks 1-7 is calibrated at preset 
intervals. Each head stack and type can be assigned a proper number of 
pick-up cycles after which the height and position of the stack are again 
measured in accordance with the slow, incremental approach. The essential 
purpose of the calibration measurement is to update the stack position 
information, since the stacks are often sideways skewed, whereby the 
coordinates of the stack top change as the stack height is lowered after 
successive head transfer cycles. The calibration cycle is also performed 
after fault situations. As the software executed by the controller of 
robot 8 can obtain accurate stack position and height information where 
needed, the number of operating and heading errors is significantly 
reduced. 
Besides those described above, the present invention can have alternative 
embodiments. 
In the above-described embodiment the sensor instrumentation of the clamp 
was implemented by means of mechanical and optical sensors. For the method 
according to the invention, however, the arrangement and types of sensors 
employed has no relevance, thus permitting the sensor instrumentation to 
be solved in any desired manner. The measurement order in the calibration 
of stack position and height data can be interchanged, while such change 
may complicate the detection of the stack edge if the stack is appreciably 
lower than the preset maximum height of the stack. In the measurement of 
the stack position, the direction of the transfer movement X can be 
opposite to that described above, and when necessary, also the lateral 
position of the head stack can be calibrated. The above-described 
embodiment performs calibration on two stack coordinates, namely the stack 
height plus the stack position in one direction, and both stack parameters 
are reset to unknown at the occurrence of a system halt. Besides these, 
the stack data update can include stack position calibration in some other 
direction, possibly also identification of the head type contained in the 
stack, whereby different types of heads can be placed in the stack without 
entering prior information to the robot software. To achieve 
identification of head type or grade, however, a head ID code and 
compatible code reader are required. 
The invention is not limited by the embodiments described above which are 
presented as examples only but can be modified in various ways within the 
scope of protection defined by the appended claims.