Sheet handling apparatus

In order to move a piece of fabric (1) over a work table (2) using a gantry robot (4), a gripper (7) comprising a flat foam-covered plate is lowered on to the fabric by the robot shaft (6). It is found that the best control over the fabric is obtained by allowing the gripper to rest on the fabric solely under its own weight, i.e. it should not be pressed down by the robot shaft. A flexible coupling (8) allows the gripper to float, while preventing relative rotation between the robot shaft and the gripper. The coupling is mounted to be movable up and down relative to the shaft, and a coupling part (13) to which the gripper is attached can tilt along two horizontal axes. When the gripper is raised, compressed air is introduced into the coupling, causing parts (11,15) of the two coupling members (10,13) to make contact, so that the gripper is held rigidly in a horizontal position. It is held in that attitude while it is lowered on to the fabric, and the pressure is then released.

This invention relates to apparatus for handling flexible sheets, and 
particularly to apparatus for moving pieces of fabric over a work area, 
such as in a garment manufacturing system. 
Garments, such as underclothes and blouses, have previously been made by 
passing suitably-shaped pieces of fabric ("cut parts") to a machinist, who 
then overlays and/or folds them as required, and passes them through a 
sewing machine. The machinist binds the edges and adds lace and 
elasticated waistbands, where necessary. The accuracy of that process 
relies on the machinist's skill, and nominally-identical garments can 
finish up with quite large differences in sizes and shapes. 
Systems have been proposed for automating at least a part of the 
manufacturing process by using a robot to move a cut part to a work 
station, such as a sewing machine. The movement of the cut part is 
effected by a "gripper" which is mounted on the output shaft of the robot. 
In the known systems the gripper has engaged with the cut part by virtue 
of suction applied to the gripper, which causes the cut part to adhere to 
the gripper, or by virtue of downward pressure applied to the gripper by 
the robot so that horizontal movement of the gripper by the robot then 
causes the cut part to slide over a smooth work table on which it is 
positioned. 
It is an object of the present invention to provide an improved arrangement 
for effecting engagement between a gripper and a flexible sheet. 
According to the invention there is provided a coupling device for coupling 
a gripper to an output shaft of a robot in a sheet handling system in 
which in use the lower surface of the gripper engages with the upper 
surface of a flexible sheet, whereby movement of the gripper by the robot 
shaft causes the sheet to slide over a surface therebelow, the coupling 
device comprising means to allow the gripper to bear against the sheet 
solely by virtue of the dead weight of the gripper, while substantially 
preventing relative rotation between the shaft and the gripper.

Referring to FIG. 1, a cut part 1 is to be moved over a flat smooth table 2 
to a sewing station 3 by a robot 4. The robot is supported by an overhead 
gantry 5 for movement longitudinally and transversely of the table. The 
robot has a substantially vertical output shaft 6 which can rotate about 
its axis and can move longitudinally upwards and downwards. A gripper 7 is 
attached to the shaft 6 by means of a flexible coupling 8. 
We have found that the best gripper structure appears to comprise a foam 
pad mounted on a rigid support plate of whatever shape is required for the 
particular operation (e.g. it might be circular, rectangular, U-shaped for 
getting under the body of the sewing machine, etc.). The gripper may be 
quite small for holding a cut part of lace or it may be up to, say, 1 
meter long and 1/2 meter wide for holding cut parts for some types of 
garment. The optimum arrangement would give an even distribution of 
downward force on the cut part over the whole of the area of the gripper. 
However, various problems arise. If the robot 4 is made to bear down on the 
gripper 7 at its attachment point, which may not be anywhere near the 
centre of area of the gripper, the foam will be compressed against the cut 
part under the connection point, but may become completely separated from 
the cut part at more remote points and so provide no control. It would be 
possible to control the pressure exerted by the robot along a number of 
axes to prevent tipping of the gripper, but this would require a 
complicated arrangement of sensors and force controllers. 
We have found that, in order to obtain the desired substantially even force 
distribution in a simple manner, the gripper should not be pressed down by 
the robot but, in accordance with the present invention, should be allowed 
to act downwards on the cut part solely under its own weight. One possible 
arrangement would be to allow the gripper to float on a ball joint on the 
end of the robot shaft. However, this is not the best arrangement, as it 
has some drawbacks. Firstly, due to friction between the gripper and the 
cut part and/or the table, the leading edge of the gripper may tend to 
"dig-in" during movement of the cut part, so that the gripper tips up, 
leaving the cut part uncontrolled. Secondly, when the gripper is raised 
from the table, unless it is absolutely balanced (which it very probably 
will not be) it will tip. Then, when it is lowered on to a cut part, the 
edge of the heavier end of the gripper will contact the cut part first 
and, while the gripper is flattening out, the cut part will be moved away 
from its starting position, which may already have been accurately sensed. 
In a preferred embodiment of the invention the coupling 8 is so constructed 
as to alleviate the above-mentioned drawbacks. The coupling is shown in 
plan in FIG. 2 and in section in FIG. 3. It should be noted that the two 
halves of the sectional view are mutually rotated by 90.degree.. It 
comprises an upper hollow cylindrical member 9 which is attached to the 
robot shaft 6 by a ball spline device (not shown), so that the member 9 
can move up and down relative to the robot shaft but cannot twist relative 
thereto (as the robot is required to rotate the gripper accurately in 
order to achieve the necessary orientation of the cut part, there must be 
substantially no relative rotation between the robot shaft and the 
gripper). An outer collar 10 having an outwardly-pointing flange 11 is 
screwed on the outside of the member 9, and is locked by a nut 12. A lower 
hollow cylindrical cap member 13 has an outwardly-extending flange 14 to 
which a cover plate 15 is bolted. The cover plate 15 overlaps the flange 
11 when the components are assembled. The member 9 has an 
outwardly-pointing flange 16 to which a generally square multi-layer 
flexible coupling 17 is attached at two opposite corners. The other 
corners of the coupling are attached to the member 13. Hence, the members 
9 and 13 can tilt relative to each other along two orthogonal lines, but 
substantially no rotation between them is allowed by the coupling 17. 
To provide further control over the alignment of the axes of the members 9 
and 13, a ball 18 is seated in a recess 19 in the base of the member 13 
and in a recess 20 in the end of an axial, externally threaded member 21. 
The distance of the member 21 from the member 13 is adjustable by means of 
a screwed connection to a perforated circular support 22 which is attached 
to the inner surface of the cylindrical wall of the member 9. A resilient 
diaphragm 23 seals against the cover plate 15 so that a sealed chamber is 
formed within the members 9 and 13, the collar 10 and the plate 15. An 
inlet 24 for receiving compressed air is provided near the top of the 
member 9. The gripper 7 is rigidly attached to the member 13 so that the 
centre of the ball 18 is approximately half-way down the depth of the 
gripper. 
In use of the device, the mass of the ball spline and the coupling is 
approximately balanced by tension springs (not shown) coupled between the 
ball spline and to the robot shaft, so that the effective mass acting on 
the cut part is substantially only that of the gripper itself. When the 
robot shaft is raised to separate the gripper from the cut part, 
compressed air is introduced into the chamber through the inlet 24, and 
the member 13 moves downward under the air pressure, so that the cover 
plate 15 bears firmly against the flange 11. This causes contact to be 
made between the cover plate and an 0-ring 25, which effects further 
sealing of the chamber. This now rigid connection between the members 9 
and 13 holds the gripper horizontal, irrespective of its degree of 
unbalance. The pressure is maintained until after the gripper has been 
subsequently lowered on to the cut part, and then it is released. The 
gripper can now adopt its required attitude on the cut part, and the 
downward force is dependent only on the dead weight of the gripper. The 
flexible coupling 17 now allows slight tilting of the gripper, but has 
sufficient rigidity to prevent the excessive tilting due to friction 
mentioned above and, since the robot does not press down on the gripper, 
the above-mentioned tilting due to offsetting of the robot shaft 
connection position does not occur.