When designing a new product the design specification must be set first. These are the maximum requirements that are expected from the end product. For the basic requirements, let us summarise the demands and the known problems with the following list [5]:
− The foam parts to be demoulded and handled have sculptured surface.
− The opening of the mould is often 20-30% smaller than the largest cross section of the part. During demoulding, the part has to be deformed in order to get through the opening.
− Free surface for grasping the foam is the top surface. (This is the surface of the foam that can be seen when the mould is open).
− The needles of the gripper can only be loaded axially. Deformation of the foam must be produced by the reaction force at the mould side walls when pulling the foam outwards.
− The needles bend easily, which must be minimised in order to prevent needles braking.
− Length of an acceptable tearing should be less than 20 mm (set by the factory). Maximum number of acceptable surface tearing is unknown.
− Needle grippers require near flat contact surface.
− Length of the needles proved to be of 10-70 mm. Too short needles will only dent the sur-face instead of penetrating into it (See: Chapter: 4.6.2). Too long needles are undesirable because of flexion and possible contact with inserts and the mould’s lower surface.
− The gripper must work fast, reliably and accurately, (especially during grasping and real-ising) and cost effectively.
− Easy and efficient operation, using the infrastructure that is already available in the fac-tory.
− Must be safe to be operated with factory workers present.
− Lightweight and modular design is necessary. Using as many parts as possible that are commercial available
It is inevitable that not all requirements can be satisfied completely. Some less important will be neglected while others will be treated with more care.
Taking these into considerations we can start to design the gripper from the informations ob-tained in the previous parts of the work. Some data are still unknown at this stage.
A.3.2 Needle pad design
The needle pad is the part in which the needles are situated so it is the first part to be deter-mined in the design. For the first prototype a single pad is designed. This means that the pad is moved by the robot and has only one other independent motion and that is the insertion and pull out motion. The size of the pad is determined by the maximum number of needles that is needed for the grasping. The used needles and spacing were determined in the previous Chap-ters so based on these experiments and the theory the selected needle was the 0.9 mm diame-ter and 70 mm length hypodermic needle (manufactured by Dispomedicor Co., HU) that proved to be the optimal for the prototype gripper. The heuristic approach considered maxi-mal length along with minimaxi-mal surface tearing and reasonable flexion. From the number of needles the size of the needle pad, on which the needles will be fixed, can be calculated. The spacing between the needles is 20 mms because for the first approach the 5 mm spacing were too close (See: later). The shape of the pad has to follow the graspable surface of the foam. In our case (for a normal car front seat) the best shape is a rectangle. With these values we can construct the pad. A design of the pad in Pro/E can be seen on Fig. 102. This is the moving part of the gripper. The moving unit was designed to accommodate maximum 200 needles.
Identical perforated plates are used for fixing the needle heads and for provision guiding for the needles during penetration. Spacing of the needles can be adjusted in 5 mm increments in both orthogonal directions. Penetration length of the needles can be set by the stroke of a pneumatic cylinder. The guide plate is used also as the foam ejector. A disassembled model is also shown on Fig. 102 (b). for clarity.
(a) The assembled view of the part (b) Exploded view of the moving part Fig. 102 The design of the moving part
The top parts of the needles are sandwiched together by two plates that are made from plastic that is usually used for printed circuit boards. The bottom part is drilled out according to the determined needle spacing. The diameter of the holes has tight fitting (H7/p6) so the needles cannot move in them. The top plate is not drilled. A large 20 mm thick sheet made of thermo-set plastic is fixed to the top of the construction. The piston of the cylinder that is responsible for moving the part is fixed to this sheet. It has to be this thick to prevent the flexing of the
whole part. All the parts are fixed together by screws and distance rings are used to set the precise dimensions.
A.3.3 The gripper base and the cylinder design
To operate the insertion and pull out motion of the plate a pneumatically operated actuator were chosen. The pneumatic cylinder is the cheapest solution and is best suited for the task.
Only two positions have to be set. The one when the needles are completely pulled back, this is the case when the gripper lets go of the foam. The other is when all the needles are fully extended. This is the case when the gripper grasps the foam and the largest force is mented. Other positions are not acceptable because in those cases not enough force is imple-mented, or if the needles are not in the foam they can be dangerous to the environment and break easily. The needles are guided by the guiding plate and the cylinder. Every needle has a guiding hole in the front plate. The needles can only be implemented by forces that are paral-lel to their axis, otherwise they break. That is why it is very important for the gripper to have a proper guidance. Care should be taken not to move the gripper sideways inside the foam to prevent needle breakage. To select an appropriate cylinder for the gripper the following method is advised. We use the gross push in force for this calculation.
Fpush= n FN N. (A. 105)
where: Fpush is the gross push-in force, FN the force of one needle, N is the number of needles, n is the safety coefficient
This large force is used to be on the safe side of the values in case inhomogeneities occur in the friction in the foam, so a large force is needed to push in the needles.
When this value is obtained the cylinder that has to be selected from a catalogue should have to have a force capacity of at least this calculated force. If it has the equal capacity then the next larger one has to be chosen for the design. The cylinder body is fixed through a dis-tance ring to the upper part of the gripper and the piston is attached to the needle pad by a nut and washers. The fixing of the cylinder on the gripper is seen on Fig. 103 and the assembled gripper is shown on Fig. 104. The side shafts of the base of the gripper are made from com-mercially available extrude aluminium parts. The gripper top is made from textile bakelite, which is fixed to the side shafts with screws. In order to give a proper guidance of the needles that is to keep the needle pad (moving part) moving parallel to the needles, four tempered guiding shafts were employed with linear bearings. The shafts are attached to the top and bot-tom part of the gripper and the bearings to the bakelite plate of the moving part. Much care should be taken during the manufacturing, because if the guide shafts are not parallel the moving part can stuck. If the adjustments (the holes in the needle pad and the holes in the bot-tom part must be aligned properly and the motions have to be collinear with the axis of the needles, of course the needle axis must also be collinear with each other) are not properly made then the motion can break the needles easily.
Fig. 103 Exploded view of the base part Fig. 104 The Pro/E model of the needle gripper Table 13: The part list of the prototype gripper
No. Description Pieces No. Description Pieces
1 Pneumatic cylinder 1 13. Screw 3 8
2 Screw 1 4 14. Bracket 8
3 Distance ring 1 15. Nut 2 1
4 Base plate 1 16. Plate 2 1
5 Adjustable bumper 4 17. Plate 3 1
6 Nut 1 8 18. Needle 200
7 Strut profiles 4 19. Nut 3 6
8 Moving plate 1 20. Spacer 6
9 Bearing 4 21. Screw 4 6
10 Plate 1 1 22. Screw 5 4
11 Shaft 4 23. Screw 6 8
12 Screw 2 8 24. Screw 7 4
The assembly structure of the gripper is shown on Fig. 105. The prototype gripper that was manufactured can be seen on Fig. 106.
Base plate Distance ring Pneumatic cylinder
Screw 1 BRACKET
Screw 6 Shaft
SCREW 4 Strut profiles NEEDLE PAD
Nut 2 MOVING PLATE
PROTOTYPE GRIPPER
Adjustable bumper Screw 5 BRACKET
Bracket Screw 2
PUSHING PLATE Plate 2
BRACKET
Screw 3
MOVING PLATE Moving plate
Bearing
Nut 1 Nut 3
PUSHING PLATE Plate 3
NEEDLE PAD Spacer
Needle
Plate 1
SCREW 4 Nut 3
Screw 4
Screw 7
Fig. 105 The assembly structure of the gripper
Fig. 106 The needle gripper prototype
A.3.4 The operation of the gripper
The prototype gripper is made for factory tests and is not meant to be fixed on a robot in this phase. However, it is a gripper, so it must work for robotic demoulding. The gripper is at-tached to a robot via a surface, which is fixed to the base plate. The attachment surface is per-pendicular to the pneumatic cylinder.
In the demoulding process the gripper approaches the foam (the mould is open and the nee-dles are retracted, this must be checked by the robot control). The robot places the underside of the gripper (plate 2) slowly on the chosen surface of the foam. It pushes the foam down few centimetres in order to adjust its position. The robot controller’s PLC sends the signal to the first electromagnetic valve that lets air into the push side of the cylinder. This pushes the mov-ing part downwards this way performmov-ing the insertion of the needles. The motion lasts until the full length of the needles are inside the foam. This is adjusted to be the bottom part of the cylinder; a mechanical stop is when the moving part hits the bottom part of the gripper (again it is checked by a sensor). The air is turned off by the first valve from the controller’s PLC.
The gripper is gripping the foam from now on. The robot lifts the foam upwards on a path more or less collinear to the axes of the needles. If the path differs much then there is a danger of breaking the needles. This motion is the demoulding motion. The motion lasts until the foam is entirely out of the mould. When the foam is out of the vicinity of the mould the robot switches to a faster transferring motion, when the robot moves the foam to the release posi-tion, (it can be above a conveyor belt). When the foam reaches the desired position and orien-tation the controller’s PLC switches the second electromagnetic valve on. This releases air into the second chamber of the pneumatic cylinder, which moves the piston upwards that lifts the moving part. This motion pulls out the needles from the foam. The needles are retracted by the pulling motion and the foam is held back by the reaction force from the base plate (plate 2). The needles are held in the retracted position when the gripper is not in use for safety reasons, (for more details of the robot motion See: Chapter 5).