Design and implementation of origami inspired miniature parallel mechanisms

Abstract

Nowadays, robotic systems are not only used for heavy industries, but also they are used for medicine, agriculture, service sectors or such places that can interact with daily life of people. As it can be expected, each sector has its own unique and different requirements. These differences can be mechanical properties such as scale, material, weight, or it can be new working conditions, workspaces and even physical appearance. However, these new demands cannot be met with the existing traditional methods. Particularly, the design and fabrication of small-scale robotic systems is very difficult. The reasons for this can be said that reduced strength due to the small size of the dimensions, the need for very small fasteners and the difficulty in the assembly of their small structures, which results in time consuming operations and high cost. In this study, parallel mechanisms with embedded sensor are designed by making use of origami-inspired design methods and 2D monolithic layered production techniques, which is commonly used for small-scale robotic systems. In addition, it is aimed that there are no intermediate steps such as folding and bonding outside of basic steps of the chosen methods that disrupts the planar structure during assembly. The assembly process of the mechanisms to be produced consists only of the cut – bond – repeat cycle that comes from the 2D fabrication methods. The sensor to be embedded in the mechanisms must also be suitable for the manufacturing technique. The fabrication method used in the project has been selected as "Smart Composite Microstructures" (SCM). This fabrication method can basically be described as cutting and joining sheet materials of different properties with certain patterns on top of each other. As a result, a simple five-layered structure emerges. These layers are rigid – adhesive – flexible – adhesive – rigid layers, respectively. The used materials are American Bristol paper with 400 grams for rigid layer, while plastic sheets are called PET for flexible layer and single and two layered 3D printed TPU material. Origami-inspired design methods were used for the patterns to be created (more specifically the joints) for the mechanisms. There are different types of joints present in the designs, which are one, two and three degrees of freedom joints. The reason for choosing parallel mechanisms is that they are scalable in terms of their structures, that is, they do not lose their properties as their dimension decreases. Even in some cases, the performance increases even if the structure gets smaller. Another reason is the compatibility of parallel mechanisms with closed loop kinematic chains to 2D production techniques. The reason for this is that all the arms are interconnected, have no open ends, and therefore the layers can be designed completely integrated. There are three different parallel mechanisms to be designed in this project. These are called the Pantograph, Delta and Stewart mechanism. The design process started with the Pantograph mechanism with 2 degrees of freedom and continued with the three degrees of freedom Delta mechanism, and finally ended with the six degree of freedom Stewart mechanism. As it can be seen, the aim is to design mechanisms that have different degrees of freedom; from simple to complex. For all three mechanisms, initially the traditional counterparts are examined. The structure of the mechanisms and the joints types that are going to be used are determined. Then according to those analyses, the kinematic models are obtained. Using these information, 2D pattern designs for the fabrication are created. After that, 3D models of the mechanisms are drawn in order to compare with the kinematic models to see if the behavior of the designed joints are matching with mathematical models. Finally, after validating the mechanisms, the fabrications are realized using the 2D production technique "SCM". A sensor must be designed to measure the required angles when calculating the position of the end point of the mechanisms. With the help of this sensor, control of motors and the end effectors can be made using the right dynamic models in the future projects. The sensor designed in this project was made to measure the angle between two rigid arms. A strain gauge is designed for the flexible layer between the rigids to take advantage of the stresses and elongations happening on the surface of the flexible layer due to the rotations between the links. Silver nano-particle ink and inkjet printers have been used since the strain gauges currently on the market are not suitable for integration in between layers. When the printed PET sheet is placed in the joints in the rigid sheets, a connection is formed with the angle between the two rigid sheets and the elongation in the flexible sheet. Detailed studies have been made not only on the design part of the sensor, but also on its location, layouts, cuts and cable connections. Later, sensors were designed and integrated into the production method into the required parts of each mechanism. Test platforms were designed specifically for each of the mechanisms. These test platforms are produced with 3D printers and contains mechanism's connection location, motor locations, motor-leg connection equipment and the reference apparatus of the position sensor, which tracks the translational and rotational motions of the end effectors. Robot servo motors are used for the experiments. Carried out experiments can be listed as point tests, continuous motion tests with the circle profile, and rotational motion tests only for the Stewart mechanism. In addition, in order to see the effect of different flexible materials, the same experiments were repeated using the Delta mechanism with different flexible layers. The sensor data is collected using a voltage divider circuit and a DAQ signal processing card.