Pedro Serra1, José Silva1,Mauro Martins1, António Pina2, António Baptista3, Rui Cortesão4 1 Instituto Pedro Nunes 2 Edilásio Carreira da Silva 3 Centimfe 4 Departamento de Engenharia Eletrónica e Computadores da Universidade de Coimbra
Abstract – This paper presents the development of a micro part handling solution for the plastic injection industry. The experimental setup has an industrial cell, which includes micro parts molded by micro injection, inspection systems and assembly lines. A XYZ Cartesian robot is used to assemble the micro parts with two other compatible test parts. The XYZ robot is also responsible of removing the defective micro parts previously identified by the visual inspection system. Index Terms – microgrippers, micromanipulation, vacuum gripper, micro parts.
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MicroHandling: assembling and manipulation of micro parts
I. INTRODUCTION Microinjection and micro part handling brought new challenges to the molding fabrication but also to manipulation and control. The literature is not clear and assertive to what defines a micro part, but the general community accepts that a micro part is a part that has reduced dimensions or details in the order of micrometer. Manipulation of micro parts presents serious challenges both in the catch and release process. The catch process requires enough force to pick the micro part without damaging it, and in the release process there should be enough force to overcome the strong adhesive force that makes the micro part adhere to the end-effector [1] [2] (gravity forces are rather weak and can be neglected). In this scenario, high precision is required to accomplish micro assembly tasks, creating challenges to the manipulation procedure. Two types of grippers are used for pick and place operations: micro grippers and vacuum systems. The micro grippers can be split into two categories: active and passive. Passive techniques rely on the contact between micro part and subtract so they depend on subtract surface properties and generally have poor repeatability [2]. Active parts use one or more actuators. These actuators can be piezoelectric which guarantee a great resolution and good response times, but are very expensive. Pneumatic actuators can also be used but their precision is not optimal. Vacuum grippers are typically used in pick and place machines to create pressure differences for catching and releasing [3]. The end effector is not only important for the catch and release task, but also for the course the micro part is going to execute during the operation. Conventional robot arms have a lot of degrees of freedom, so they can be used in systems that need very specific axis travel. However, these robots are expensive. The Delta robots are very used for pick and placing operations due to high repeatability and speed but they are also expensive. Another solution is the XYZ
Cartesian table that allows a high repeatability at a good price, being only capable of linear movements. XYZ Cartesian tables can only execute linear movements and but they are cheaper compared to other robots, having good repeatability and speed. Currently there are “Pick&Place” machines in the market that are used for SMD (Surface Mount Device) component placement in Printed Circuit Boards (PCB). This type of solution enables (via linear movements and a vacuum system) collecting electronic component coils which are supplied and installed in the correct positions on the PCB. There are many types of “Pick and Place” machines, differing primarily in accuracy, speed, and volume of production. The more economical machines that are typically used for prototyping or little productions do not have the necessary accuracy to handle the micro parts of the production cell. On the other hand, industrial and more expensive pick and place machines offer the necessary resolutions but are hard to integrate in the production cell, due to their high degree of specialization. This paper presents a manipulation system with high precision, easy to operate, that can be easily installed in industrial cells with a large production capacity.
II. SYSTEM DESCRIPTION The MicroHandling project aims to develop a new production cell targeted for the molds and injection industry. The production cell must effectively integrate the production of microinjected parts, assembling and palletizing. The envisioned production cell is divided into four main parts. The production zone where the micro parts are injected, the palletizing zone where a robotic arm is going to take the parts from the production zone, removing the excess of material and placing the micro parts in trays, the visual control system where the trays are inspected to detect damaged parts and finally the selection and assembly zone where the damaged parts are removed and the different types of micro parts are assembled together. This article is going to explore the final zone of the production cell where the different parts are assembled. A. Parts Geometry To validate the production cell and to test both the creation of molds and the injection system, three parts were designed. These parts are: t a micro part (Figure 1); t a cogwheel (Figure 2); t a pin (Figure 3).
the parts by the nozzle a spring is used. This spring acts as an opposite force when the nozzle is forcing the parts. D. Vacuum system In order to catch and release the micro parts a vacuum system is used. The vacuum system is designed to use the typical 6 bar pressure. Two vacuum generators were installed. One is used to control the vacuum in the nozzle to catch and release micro parts and pins and the other one is used to generate vacuum in the other two nozzles, working together with the other vacuum generator to catch and release cogwheels. The vacuum generator for the micro parts and pins is also used to generate a blow. This blow is fundamental since gravity is not enough to release micro parts and pins from the nozzle.
Figure 7. Photograph of the assembled part.
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B. Removal The removal of defective parts starts with an indication from the robot. The PLC waits from a file of the vision system with the report about the tray. The PLC then processes this file and sends back the position of the defective parts to the XYZ table. The XYZ table requests the tray with defective parts and a tray containing only good parts from the robot arm. The manipulation system starts to remove the defective parts from the tray, replacing them with good ones. The process can now be repeated until the tray of good parts is empty. At this point, the XYZ table should request a new tray of good parts from the robot.
Figure 6. Gripper transporting a micro part and a cogwheel.
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parts are assembled, the table informs the robot that it can pick up the tray.
A vacustat is used to detect if a part is being picked up. A threshold value is defined and if the value of the negative pressure after picking up a piece is below that threshold it means that the part has not been picked up by the gripper.
III. CONTROL SYSTEM The control system is responsible for the implementation of the two types of operations defined in our project, assembling and removal of defective parts. The controller of the table allows to control the motor of each axis individually, having also digital inputs/outputs to control the vacuum system. The control system has a technical cabinet connected to the vacuum system and PLC to communicate with the robot and the visual inspection system.
IV. CONCLUSION This paper presented a micromanipulation system that can be easily included in industrial cells. The system is fairly simple and can be used in different types and geometries of micro parts. During the tests some problems with the repeatability of the system were detected due to misalignments between trays and the industrial cell. Micro parts and specially pin small dimensions make any little misalignment a potential problem. Tray misalignments have been corrected but we are also studying new solutions to solve the problem based on the visual system that can help the XYZ table to correct the final gripper position. Other improvements are currently being implemented like a pressure regulator to limit the blow in the release of micro parts and pins.
REFERENCES [1] D. A. T. F. F. Arai, “Micro manipulation based on micro physics-strategy based
A. Assembly When the assembly process starts, the XYZ table asks for trays of micro parts, cogwheels and pins. After receiving the trails, the table starts placing the pins in the four holes of each cogwheel. The number of pins in one tray is not enough to assemble them in every cogwheel so the table must ask for new pin trays until all cogwheels have four pins mounted in. The micro parts are then placed on the top of the pins. The number of micro parts in one tray is also not enough to complete a tray of cogwheels so the table also needs to ask for new trays. When all cogwheel
on attractive force reduction and stress measurement,” in Intelligent Robots and Systems 95. ‘Human Robot Interaction and Cooperative Robots’, Proceedings. 1995 IEEE/RSJ International Conference on (Volume:2), Pittsburgh, PA, 1995; [2] B. K. C. X. L. Y. S. Yong Zhang, “Autonomous Robotic Pick-and-Place of Microobjects,” IEEE Transactions on Robotics, vol. 26, no. 1, pp. 200-207, 2010; [3]
B. M. ,. W. A. Zesch.W, “Vacuum tool for handling microobjects with a NanoRobot,” in Robotics and Automation, 1997. Proceedings., 1997 IEEE International Conference on (Volume:2), Albuquerque, NM, 1997.