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.
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   (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 . 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 .
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.
Pedro Serra, José Silva, Mauro Martins
Instituto Pedro Nunes
António Pina, Edilásio Carreira da Silva
António Baptista, Centimfe
Rui Cortesão, Departamento de Engenharia Eletrónica e Computadores da Universidade de Coimbra
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