Method for controlling tenter clip gap spacing during simultaneous biaxial stretching using linear synchronous motors

A method is disclosed of temporarily speeding up delayed carriages on a return side of an endless loop of driven carriages in a linear motor web tenter.

BACKGROUND OF THE INVENTION 
The invention relates to control of clip carriages in simultaneous biaxial 
tenters for stretching webs of material wherein clip carriages are 
independently propelled in two opposed endless loops. The invention in 
particular relates to tenters using linear motors to propel the carriages 
throughout the endless loops. On the inner facing sides of the two loops, 
the web is grasped by clips on the carriages at the entrance of each loop 
and stretched longitudinally by progressively accelerating individual 
carriages thereby causing them to become spaced apart. At the same time 
the carriages can be guided laterally apart thereby stretching the web 
laterally. At the end of each loop, the web, now traveling at a higher 
speed than at the entrance, is released from the carriages and the 
carriages are returned to the beginning of the loop where the web is once 
again grasped and stretched by the diverging accelerating carriages. On 
the return side of each loop, the carriages must be slowed from the web 
release speed to the web grasping speed, at which speed the carriages are 
abutted with each other. Such a linear motor propelled, web tenter system 
with the above carriage control is described in U.S. Pat. No. 5,072,493 to 
Hommes et al. Additional details and improvements in such system are 
described in patent publications DE 4436676 to Steffl (clip carriages), DE 
19513301 to Briel et al (eddy current disc for carriages stacking), and DE 
19517339 to Ruehlemann (rail transitions at exit turn). 
Controlling the carriages on the return side of each loop has presented 
challenges in handling the carriages during start up of the tenter and 
process upsets, handling large gaps between carriages that may cause a 
shortage of carriages at the web grasping end of the loop, and handling 
the same number of carriages at different stretching ratios. At a low 
stretch ratio, a lot of carriages are on the web sides of the loops and 
fewer are on the return sides, while for high draw ratios, fewer carriages 
are on the web sides of the loops and more are on the return sides. There 
are also economic concerns about keeping the return side system simple and 
low cost since precise control of the carriages is not required on the 
return side where there is no interaction with the web. 
Nevertheless, one suggested method of operating the return side is to 
provide linear motors that interact synchronously with permanent magnets 
on the carriages to predictably control the speed and position of the 
carriages. These magnets are the same magnets that are required for 
precise control on the web stretching side of the loops. For precise 
return side control, the motor drives for control of adjacent motor 
windings can be frequency and phase synchronized or partially frequency 
and phase synchronized as the carriages pass from one motor winding to the 
next. For less precise control and lower cost, the motor drives may not be 
frequency and phase synchronized as the carriages pass from one motor 
winding to the next; some loss of control of speed and position will be 
experienced, but it may not affect overall operation. 
At the loop turns at the web entrance ends of the endless loops, an eddy 
current disc, as described in the '301 reference, may be employed for each 
loop. It is driven at a speed faster than the desired carriage speed at 
the web entrance of the loop. This removes all gaps between carriages and 
abutts them with a low collision force that does not damage the carriages, 
which are provided with bumpers for this purpose. The eddy current disc 
also develops sufficient cumulative force on the stack of carriages to 
insure that the carriages entering the film side of the machine are pushed 
tightly against the first few film side carriages with no gaps. This 
pushing force also contributes to the force required of the first few 
carriages on the film side to develop web tension prior to the tenter. 
Although this improved system achieves a simplified operation and reduced 
costs, there is a problem that this system does not handle large gaps that 
may occur between carriages unpredictably. For instance, on the web side a 
gap between carriages may occur when there is a web break during thread-up 
or continuous operation that causes the carriages to get out of 
synchronism with the electromagnetic wave developed by the motor winding. 
In this situation, one or several carriages may fall behind and bunch-up 
leaving a large upset gap ahead of the bunched-up carriages. For the 
bunched-up carriages to catch up to their steady state positions, the 
upset gap must be closed before it reaches the entrance end of the tenter. 
When this gap gets to the return side, the only way proposed by known 
systems to close the gap is by increasing the speed of the eddy current 
disc, but this has been found to be insufficient for closing a gap greater 
than about 2 meters, and cannot handle multiple, or recurring, closely 
spaced gaps of 1.5-2.0 meters. The eddy current disc is torque controlled 
so its speed increases over a limited range when a gap is experienced by 
the disk. This is because the resisting force the carriages exert on the 
disk decreases with fewer carriages adjacent the disk. Since the disk is 
running at a constant torque, the lower resistance causes the disk to 
speed up. As the disk speeds up, the relative motion between the disk and 
carriages stacked adjacent the disk increases until the resisting force of 
the carriages and the driving torque of the disk is again in balance. The 
increased speed of the disk allows carriages that are up to two meters 
behind to catch up to the stack of carriages on the disk. With a gap 
greater than two meters, the bunched-up carriages arrive too late at the 
entrance end and a gap occurs on the web side of the loops which shuts 
down the tenter. Additionally, there may be so few carriages engaged by 
the eddy current disc that there is insufficient force developed to 
achieve the necessary web tension by the first few carriages on the web 
sides of the loops. Shut down of the tenter is highly undesirable for a 
tenter that stretches continuously cast polymer sheet that must be 
diverted to waste during resynchronization of the carriages. There is a 
need for a simple system for handling large gaps (greater than 2 meters) 
or frequently recurring gaps of 1.5-2.0 meters to maintain a continuous 
supply of abutted carriages to the eddy current disc and the web side of 
the loops. 
SUMMARY OF THE INVENTION 
The invention is directed to a method of temporarily speeding up delayed 
carriages on a return side of an endless loop of driven carriages in a 
linear motor web tenter comprising two such loops, each driven carriage 
provided with a linear motor secondary adapted for web stretching 
propulsion on a web engaging side of the endless loop, comprising: 
(a) propelling the driven carriages, at a constant speed in a transport 
portion on the return side, with a propulsion means interacting with the 
carriage secondary, the transport portion extending over a length of the 
return side between a carriage entrance end and a carriage exit end, said 
constant speed being such as to provide a steady state supply of carriages 
to a continuously abutted length of carriages in a moving stack at a 
turning portion adjacent the exit of the return side; 
(b) propelling the carriages, at a speed less than said constant speed, at 
said turning portion with a propulsion means interacting with the carriage 
secondary; 
(c) sensing the distances between carriages arriving in the transport 
portion on the return side and detecting the excess length of an 
undesirable gap between carriages arriving in the transport portion on the 
return side; 
(d) temporarily increasing the speed of all carriages in the transport 
portion on the return side in response to said undesirable gap; 
(e) continuing said temporarily increasing to increase the speed of a 
delayed plurality of carriages immediately following the undesirable gap 
and to temporarily increase the length of the stack before the gap reaches 
the end of the transport portion; 
(f) decreasing the speed of all carriages in the transport portion to said 
constant speed; 
wherein the undesirable gap is eliminated between carriages exiting the 
return side. 
In another embodiment, the propulsion means for said transport portion 
comprises means for varying the speed of carriages in discrete subsections 
of said transport portion, and temporarily increasing the speed comprises 
temporarily increasing the speed of a plurality of delayed carriages that 
are bunched-up and trailing said undesirable gap to thereby move said 
plurality of carriages so the bunched-up carriages are closely spaced and 
leading said undesirable gap, before said plurality of carriages reach the 
end of the transport portion.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a schematic plan view of a typical tenter-type stretcher for a 
web, such as a film. The polymer is cast onto a chilled roll at 20 to form 
a cast film 21 that is fed to the entrance 22 of the stretcher 23 where it 
is grasped by clips on carriages, such as carriages 24 and 26, riding on 
tracks, such as tracks 28 and 30, in opposed recirculating endless loops, 
32 and 34. The carriages, in addition to containing clips, contain a 
linear motor secondary (not shown), such as a permanent magnet. These 
secondaries are closely spaced from linear motor primaries 31 and 33 
adjacent the tracks 28 and 30, respectively, that develop moving 
electromagnetic fields that propel the carriages along the loops. There 
may also be film clips on idler carriages (not shown) nested in between 
the powered carriages containing secondaries. The idler carriages would 
further support the film and be propelled by the film, which is propelled 
by the powered carriages. At the entrance end 22, there are cam surfaces 
or magnetic means 36 and 38 that act on a clip lever on each carriage to 
close the clips to grip the edge of the film. As the carriages proceed 
along the tracks in the direction of arrows 40 and 42, the film is 
stretched laterally in direction 44 and, in the case of a simultaneous 
stretcher, the film is stretched longitudinally in direction 46. At the 
entrance 22, the carriages 24 are abutted in a stack 45 in loop 32 and the 
carriages 26 are abutted in a stack 47 in loop 34. At the position of 
arrows 40 and 42 in both loops, the carriages are spaced apart, such as 
carriages 24a and 24b, and carriages 26a and 26b in loops 32 and 34 
respectively. Such a simultaneous-type stretcher is described in U.S. Pat. 
No. 5,072,493 to Hommes et al. 
The stretched film 48 then proceeds to the exit end 50 of the stretcher 
where there are cam surfaces or magnetic means 52 and 54 that act on the 
clip lever on each carriage to open the clips to release the edge of the 
film. The film is released before it reaches the end 50 of the stretcher. 
The stretched film 48 leaving the stretcher has tension applied by winding 
devices (not shown) that wind the film into rolls. This tension insures 
the film edges are pulled from the spaced apart carriages as the carriages 
24 and 26 move away from the film at end turns 56 and 58 to return to the 
entrance end 22 of the stretcher where the carriages are again abutted in 
a stack. Near the exit end 50 of the stretcher, and located along a 
distance that spans the cam surfaces 52 and 54, are jet means 60 and 62 
directed at the clips on the carriages that act, during a film tear, to 
remove film edges and debris from the clips so no edge or debris is 
carried to the return side of the stretcher. 
Frequently, there is an oven enclosure 61 (shown in dashed lines) 
surrounding a major portion of the stretcher 23 for controlling the 
thermal condition (heating and cooling) of the film as it is being 
stretched, and for keeping the clips heated. There are a plurality of 
sensors, such as sensor 63, located in the oven and directed at the film 
to monitor film temperature for control purposes. In the case of film 
tears, the temperature sensed by the sensor 63 changes dramatically as the 
film moves away from the sensor, and this can be used to detect film tears 
in the stretcher. In this case, the sensor 63 also serves as a position 
detector to sense the presence of the planar film in the stretcher. A 
controller 65 is used to operate the stretcher and monitor the output from 
sensors, such as sensor 63. 
After leaving the end turns 56 and 58 of each loop, the carriages must 
begin slowing down and closing up the spaces 66 and 68 between carriages 
in each loop so the carriages are abutted when they return to the entrance 
end 22. FIG. 2 shows a typical speed versus position plot (solid line) of 
the carriages in each loop as they travel along the return side from the 
exit end 50 to the entrance end 22. The carriages are moving from right to 
left in the FIG. 2 plot as they would be in FIG. 1. The return side is the 
same for each loop and comprises several portions. In portion 70, the 
carriages decelerate gradually so any idler carriages present gently pile 
up against the leading powered carriage without damaging collision forces; 
elastomeric bumpers on the idler and powered carriages are provided to 
minimize these forces. In portion 72, steeper deceleration may be applied 
to slow the carriages quickly before arriving in portion 74, a constant 
speed, transport control, portion. The transport portion for each loop may 
be positioned as at 74' in FIG. 1. The horizontal heavy line 76 in FIG. 2 
indicates the constant speed versus position of the carriages in the 
transport portion 74. Some variations may occur for individual carriages 
but the average speed is indicated by the solid line. Portion 78 is a 
space closing portion where the carriages are brought closer together just 
before abutting. Portion 80 is the abutting portion where the carriages at 
the entrance end 22 are traveling abutted at a speed indicated at 
horizontal line 82 which is fixed by the speed of the film entering the 
stretcher. The carriages in portion 80 are urged to go faster than the 
abutted speed as indicated by line 84 so they form a moving abutted stack 
45 (FIG. 1) at position 87 (FIG. 2) before reaching the entrance end 22. 
For a steady state operating condition, each carriage is controlled to be 
at a particular position along the return side at a particular time. The 
steady state position of any given carriage along the return side is 
predictable. 
FIG. 3A is a schematic plan view of the carriages 26 on a return side 86 of 
loop 34 of the stretcher of FIG. 1. The carriages along the film 
stretching side 88 of the loop are not shown for clarity. The return side 
carriages are generally following the carriage speed versus position 
illustrated in FIG. 2, and along transport line 76. The carriages are 
regularly spaced along most of the return side and the spacing closes up 
and the carriages are abutted in a moving stack 45 at position 87. In 
normal operation the trailing end carriage 26e in the stack 86 is moving 
forward and is being rapidly replaced by the next carriage 26f so the end 
of the stack stays at about the same position at 87. In practice, the 
abutting portion 80 or the return side (FIG. 2) is represented by an eddy 
current disc, such as a metal surfaced wheel 91, operating in a carriage 
supply turn 92 that interacts with the carriage secondary magnets to 
asynchronously propel them. The speed imparted by the interacting surface 
of the wheel is greater than the speed the carriages are being consumed at 
the entrance end 22 of the stretcher. 
FIG. 3B shows what happens to the carriages when there is a disturbance on 
the film side 88 that causes some of the powered carriages to loose 
synchronism with the electromagnetic wave, become delayed, and fall back 
and bunch up with following carriages. This creates an upset gap 90 
between the first bunched-up, delayed, carriage at 26c and the leading 
undelayed carriage at 26d. If a small upset gap occurs, when the carriages 
26c and 26d arrive at abutting portion 80 (FIG. 2), the overspeed in 
abutting portion 80 may be sufficient to permit the delayed carriage 26c 
to catch up with the undelayed end carriage 26d at the end of the stack 
before the stack gets too short. Such a system is described in patent 
publication DE 19513301 assigned to Bruckner-Machinenbau by Briel et al. 
However, when a large upset gap is present, the undelayed end carriage 26d 
may proceed a significant distance around the carriage supply turn 92 
before being replaced by the next carriage, delayed carriage 26c. This can 
cause problems with the operation of the stretcher. The force applied to 
the entering film by the first few carriages is dependent on the number of 
abutted carriages being propelled together in the carriage supply turn 92. 
If too few carriages are being propelled, the total combined force exerted 
by the stacked carriages may be too low to reliably pull the film into the 
stretcher. In the worst case, the next stack end replacement carriage does 
not catch up with the moving stack end carriage before the film must be 
grasped, and the replacement carriage will be out of synchronism with the 
magnetic wave propelling the film stretching carriages at the entrance end 
22 of the stretcher. This will result in shutdown of the stretcher and 
film will be wasted as the stretcher is restarted. 
It has been discovered that the upset gap can be handled without upset to 
the stretcher if: 1) an excess of carriages are accumulated in the stack 
and the delayed carriages following the upset gap are caught up to their 
steady state position before the upset gap reaches the abutting portion, 
or 2) the delayed carriages are caught up to their steady state position 
and the upset gap is repositioned behind the bunched-up carriages instead 
of in front of them before the upset gap reaches the abutting portion. In 
both cases, preferably the upset gap is accommodated before it reaches the 
end of the transport portion. Strategy 1) can be accomplished by sensing 
the upset gap and speeding up all the carriages in the transport portion 
to cause a number of carriages ahead of the gap to get to the stack early 
and the delayed carriages immediately behind the gap to get to the stack 
at least on time. Strategy 2) can be accomplished by selectively speeding 
up the bunched-up, delayed, carriages so they move ahead of the upset gap 
to thereby arrive at the abutting portion at least on time before the 
upset gap reaches the abutting portion. To accommodate a large upset gap, 
the stack end may temporarily move beyond abutting portion 80 (FIG. 2) and 
into portion 78. The process must be controlled so the end of the stack 
does not move significantly beyond portion 80 and into a section of 
portion 78 where the carriages are traveling at an excessive speed 
relative to the stack speed so damaging collisions would occur. 
The upset gap can be detected by placing a sensor 94 (FIG. 3B) that senses 
each passing carriage near the exit end 50 and comparing the actual time 
between carriages to the expected time between carriages. In this way the 
position and size of the upset gap is detected and appropriate action can 
be taken to make sure the gap is accommodated before it reaches the end of 
the stack. If the gap is not detected well before reaching the end of the 
stack, there will not be enough time to make necessary corrections. The 
sensor may be located at the entrance to the transport section or ahead of 
the transport section for efficient operation of the system. 
Strategy 1) will be discussed further referring to FIG. 2. When the upset 
gap is detected and the first delayed carriage is in the transport 
portion, the speed of all carriages in the transport portion 74, 
represented normally by line 76, is momentarily increase slightly, 
represented by dashed line 76'. This causes some of the carriages affected 
by the temporary speed increase to get to the end of the stack slightly 
earlier than normal which will cause the moving stack to increase in 
length and the end position of the stack at 87 to move toward portion 78. 
The most delayed of the plurality of delayed carriages will get to the 
abutting portion on time. The time that the temporary speed increase is 
maintained is determined by the length of the transport portion, the 
magnitude of the speed increase, and the size of the upset gap. For 
instance, for a given upset gap, if the transport portion is long, the 
speed increase can be low; if the transport portion is short, the speed 
increase must be high. The size of the upset gap is only determined after 
the sensor detects the first bunched-up following the gap. Compensation 
for the gap must begin immediately since it may take a long time to 
determine a large gap is present and then the time and distance to correct 
the gap is already diminished. Ordinarily, the speed increase compensates 
for the delayed carriages being out of position during the time the gap 
takes to travel the length of the transport portion. In this way, by the 
time the gap reaches the critical abutting portion 80, there are already 
enough carriages accumulated to make up for the diminished number of 
carriages arriving due to the upset gap. After compensation by speeding up 
the delayed carriages and lengthening the stack, the speed in the 
transport portion is returned to normal (represented by line 76). 
Eventually the upset gap reaches the end of the stack lengthened by the 
accumulated carriages, and there is then a lag before the on-time delayed 
carriage reaches the end of the stack. During this lag time, the moving 
stack length decreases from its compensated state, and the end of the 
stack moves to the left in the figure away from portion 78 until it 
reaches the normal position represented by 87 in FIGS. 2 and 3A. Normal 
operation has then been restored. 
A variation of stategy 1), as just discussed, is illustrated in FIG. 4 
where the transport portion 74 is not operated as a single constant speed 
portion, but as two constant speed subsections 74a and 74b with the short 
section 74a being variable to provide catch up capability to momentarily 
speed up delayed carriages and increase the stack length to accommodate 
upset gaps. In this case, the speed increase in the short portion would 
ordinarily be higher than that required in the longer portion illustrated 
in FIG. 2 since it acts on fewer carriages for the same time. The speed 
increase of portion 74a is started when it is estimated that the delayed 
carriages that are behind their steady state position arrive in 74a and 
can continue until the delayed carriages exit zone 74a. 
Another variation of strategy 1), which was mentioned earlier, is to 
provide the catch up ability only in the abutting portion 80. Since the 
abutting portion has a relatively short length compared to the transport 
portion, it provides only limited repositioning of delayed carriages and 
accumulation of additional carriages. If the upset gap is large, say more 
than 2 meters, this variation is not expected to be effective. Since the 
abutting portion often only includes the turning wheel 91, there is 
limited ability to physically increase the length of the abutting portion. 
The abutting portion can be virtually extended into the space closing 
portion 78 by suitable control of the carriage drive system, so after an 
upset gap is detected, a modest capability to reposition delayed carriages 
and increase the stack length may be possible to accommodate larger upset 
gaps. 
Obviously, some combination of the variations just discussed may be 
employed with the transport portion to accommodate the upset gap using 
strategy 1). That is, the speed increase may apply to the transport 
portion in whole or part and, if necessary, to the space closing portion 
and the abutting portion to achieve a capability that matches the upset 
gaps to be expected in operation. To summarize, strategy 1) is to detect 
the upset gap as it enters the return side and increase the speed of a 
plurality of carriages in the transport portion, and additionally in other 
following portions, as soon as the first delayed carriage is in the 
transport portion to catchup the delayed carriages and lengthen the stack 
before the gap reaches the stack. Preferably the delayed carriages are 
caught up before the gap leaves the transport portion and reaches the 
abutting portion. 
Strategy 2) will be discussed further referring to FIGS. 3B, 3C and 5. In 
FIG. 5, the transport portion 74 is normally operated as a single constant 
speed portion as indicated by line 76, but it has subsections 74c, 74d, 
74e, and 74f that can be controlled to vary the carriage speed in each 
subsection independently of other subsections, as indicated by line 76c 
for subsection 74c; and by line 76d for subsection 74d; and by line 76e 
for subsection 74e; and by line 76f for subsection 74f. In this way, the 
speed of a selected subsection can be increased as the bunched-up 
carriages are present to speed up only those carriages, and some 
immediately adjacent the bunched-up ones that happen to be in the same 
subsection at the same time. When the upset gap 90 is first detected by 
sensor 94, it is in the position shown in FIG. 3B with the gap 90 ahead of 
the bunched-up carriages 96 relative to the direction of carriage motion 
indicated by arrow 98. When the end of the gap is detected by detecting 
the first bunched-up carriage 26c and the first delayed carriage is in 
subsection 74c (FIG. 5), the subsection 74c can change the propulsion 
speed of the carriages from line 76 to line 76c. This would continue until 
it is calculated that the bunch of carriages 96 is passing into subsection 
76d. At this point subsection 76d would change the propulsion speed of the 
carriages from line 76 to line 76d and join subsection 76c in propelling 
the carriages at the increased speed. When it is calculated that the bunch 
of carriages 96 have left subsection 74c, the propulsion speed in this 
subsection is changed from line 76c to line 76. This process continues and 
the speed of subsection 74e is increased to that indicated by line 76e as 
the calculated position of the bunched-up carriages reaches subsection 
74e, and the speed of subsection 74d is decreased to that indicated by 
line 76; and the speed of subsection 74f is increased to that indicated by 
76f and the speed of subsection 74e is reduced to that indicated by line 
76. 
At about this point (if not sooner) the bunched-up carriages 96 being 
propelled by subsection 74f have caught up to an expected spacing with the 
leading undelayed carriage 26d as shown in FIG. 3C, and the upset gap 90 
is now trailing the bunched-up carriages 96. The bunched-up carriages may 
also have become spaced apart slightly due to the process. In this 
situation, the first and most delayed of the bunched-up carriages has now 
caught up to its steady state, expected, position and the bunched-up 
carriages provide a momentary increase in the length of the stack to 
accommodate the upset gap that now follows the bunched-up carriages. This 
strategy 2) is preferred to strategy 1) if there is a need to close very 
large gaps. Since the control zones are shorter, it provides the ability 
to more controllably increase the speed of only the carriages which are 
delayed. This permits a higher gap closing speed to be used without 
getting the undelayed carriages to arrive at abutting portion 80 too 
early. For very large gaps, adjacent zones (such as 76c and 76d) may be at 
the higher velocity simultaneously. For very large gaps, sones 76f, 76e, 
76d, and 76c may all be at the high velocity simultaneously for a short 
time and then one by one return to their normal velocity as the delayed 
carriages leave each zone.