Industrial robots are increasingly employed in clean room environment semiconductor industries to eliminate contamination from human workers. A robot for manipulating wafers or large glass panels for flat panel display (FPD) is one of industrial robots used in the semiconductor industries, and the robot with higher payload and stiffness is required to increase productivity as the size of wafers and glass panels increases. However, in order to increase the payload and stiffness of robot, its mass and size should be increased, and this can reduce positional accuracy of the robot and induce large stresses on mechanical components due to a large inertia force. Increasing the mass and size of robot can also lower the fundamental natural frequency, performance and system stability. Therefore, the robot structure should have both low mass and high stiffness, but because conventional metals would not satisfy these requirements, attempts to apply advanced composite materials to robot structures are recently growing to meet the requirements.
Because composite materials have high specific stiffness, high specific strength, high damping and low coefficient of thermal expansion, they have been widely used in aircraft and spacecraft structures as well as in sports and leisure goods. The composite material is a promising material for the robot structure that requires lightweight and high stiffness to improve productivity because of its superior material properties to conventional metals.
The objective of this work is to develop the structure of robot that has lower mass and higher performances than conventional aluminum robot for manipulating large glass panel, a part of FPD, using composite materials. In order to develop the composite robot structure, fundamentals necessary for designing composite structures should be fully investigated: characteristics of materials and mechanically fastened joint. First, in-plane and out-of-plane material properties of composite materials were investigated by mechanical tests, and macro-mechanical analysis was performed to investigate the effect of stacking angle on the specific damping capacity of composite materials. The in-plane material properties were measured according to the ASTM standards, and the interlaminar shear behaviors were investigated by the Iosipescu shear test using thick composite laminates. However, during the curing process of thick composite laminates, substantial amount of temperature lag and overshoot at the center of the laminates is usually experienced due to the large thickness and low thermal conductivity of the composites, which requires a long time for full and uniform consolidation. Therefore, based on the numerical model, an optimized cure cycle with the cooling and reheating steps was developed by minimizing the objective function to reduce the temperature overshoot inside thick composites, and specimens for Iosipescu test were manufactured using the optimized cure cycle. Specific damping capacities of composite materials were estimated using macro-mechanical analysis on the composite beam under bending load.
When composites are employed as structural materials, joining of composites to other materials is necessary because manufacturing whole structure with composites only is not generally feasible. Because robot structures usually require mechanically fastened joints for regular inspection, repair and assembly, the characteristics of mechanically fastened joints for composite materials should be investigated to apply composite materials to the robot structure. The optimum bolted joints for composite materials under tensile loading were investigated using several design parameters: stacking angle, stacking sequence, the ratio of glass/epoxy to carbon/epoxy, clamping pressure and the outer diameters of washers. Three dimensional stress analyses of the bolted joints were performed using in-plane and out-of-plane material properties and compared with the experimental results. Because it is preferable for the robot structure to have sandwich structure with relatively high bending stiffness, bonding strength between face and core was also investigated varying core material, bonding method and applied external pressure.
With the mechanical properties and bearing strength of bolted joint of composite materials investigated, the structures of robot for manipulating large glass panels were designed and manufactured using composite materials. Considering specific stiffness and manufacturing cost, hybrid composite composed of carbon/epoxy with high specific stiffness and glass/epoxy with low material cost were selected as the main structural material for robot structure. The end effector that has the lowest stiffness and the wrist blocks on which the end effector is mounted are the most important parts among structural components of robot. Therefore, in this work, the focus was placed on development of the end effector and wrist block using sandwich structures composed of composite face and honeycomb or foam core. Finite element analysis was performed along with an optimization routine to design the composite end effector and wrist blocks. A box type sandwich structure was employed to reduce the shear effect arising from the low modulus of core materials. The static and dynamic characteristics of the composite robot end effector and wrist block ? end effector systems were measured: static deflection, natural frequency and damping ratio, and they were compared with those of the conventional aluminum robot end effector. The static stiffness of the composite end effector was 95% higher than that of the aluminum end effector, and the fundamental natural frequency and damping ratio of the composite end effector were 1.5 and 3.6 times higher, respectively, compared with those of the aluminum end effector. The weights of the composite wrist blocks were only half of those of the conventional aluminum wrist blocks, and the composite wrist block - composite end effector system had the best dynamic characteristics among wrist block - end effector systems.
