Design of a Parallel Hybrid Micro-scara Robot for High Precision Assembly

The trend to miniaturization in the field of consumer and investment goods is leading to increasing interest in the field of precision assembly of small components. Until now most machines for precision assembly have been many orders of magnitude larger than the workpieces to be handled or the necessary workspace. The Institute of Machine Tools and Production Technology of the Technical University of Braunschweig and Micromotion GmbH, a manufacturer of miniaturized zero-backlash gears and actuators are now working together to develop a small-scale SCARA robot featuring a parallel hybrid kinematic structure. This new robot, with a base area of less than 150 x 150 mm2 will position small workpieces with a mass of up to 50 g with a repeatability of better than 1 μm. This paper describes the basic design process and design decisions made in the development of this parallel hybrid micro SCARA robot, which will be exhibited at the HANNOVER FAIR 2005 as a technology demonstrator. I. THE IMPORTANCE OF MICROASSEMBLY In a wide range of consumer goods and investment goods there is a clear trend to miniaturization. This trend is leading to a yearly growth of the world market for products based on micro system technology of 20%, and therefore, increased interest in microproduction technologies [1],[6]. The findings of the “Mikropro” study into the international state of the art in the field of microproduction technology can be summarized as follows for the area of automated microassembly [2]: • The trend to multifunctional, hybrid subassemblies in the field of microsystem technology is leading to increasing demand for devices that can precisely position and assemble micro-components with an accuracy of less than 1 μm • Up to 80% of production costs are incurred in assembly, which is increasing the need for high speed and high process stability during the assembly process • Most existing precision assembly machines and robots are very expensive, so there is a clear need for simple, cost-effective assembly automation • There is a trend to place the complete assembly system inside a clean room environment, leading to a requirement for very compact, modular assembly automation Until now there have been a variety of different technical solutions for dealing with the above mentioned trends. II. PREVIOUS APPROACHES The state of the art for precision robots can be summarized as shown in Fig. 1. The simplest classification is into serial, parallel and hybrid structures, which in turn can be sub-divided into further categories. Fig. 1 State of the art for precision assembly robots The first category covers cartesian robots. These are typically very large in comparison to the components to be handled and are often as a result, very expensive. However, they do provide a repeatability between 1 and 3 μm, as demonstrated by, for example, the “Sysmelec Autoplace 411”. The second category covers SCARA robots, which have a large workspace in relation to their physical size, but only achieve a repeatability of +/5μm, even in the case of the most accurate designs. In the field of parallel robots, there are few examples in industrial use. The Mitsubishi RPX is an exception, achieving a repeatability of +/5μm. Most other developments in this area are limited to university research projects, in particular at the Technical University of Braunschweig in Germany, where extensive experience with parallel structures has been gathered, for example with the Triglide robot, which has achieved a repeatability of better than 1 μm [5],[6],[7]. As the “MikroPro” study has shown, these existing solutions have the common feature of being very expensive and very large and there is now growing market demand for smaller, cheaper robotic devices for positioning and assembly. The development of such robots is now being made possible by new enabling technologies, in particular zero-backlash micro-gears and highly dynamic micro-motors with integrated incremental encoders, which are allowing proven robot arm structures to be miniaturised. Furthermore they allow the use of proven control technology and avoid the complexity of “alternative” actuator technologies such as Piezo actuators [5],[7]. III. BASIC DESIGN REQUIREMENTS Based on the findings of the “MikroPro” study and also direct contact to a number of potential users the following basic specifications could be established. Furthermore quantitative specifications were also fixed, as shown in table 1. • Positioning repeatability better than 1μm • Simple and modular structure • Small envelope, to allow easy integration into a “table factory” system • Lower production costs than existing systems • Easy access to the working area of the robot to allow automatic feeding of components to be assembled IV. ROBOT ARM DESIGN APPROACH Before commencing with the detailed design of a robot in device to fulfill the above mentioned requirements, a number of basic design decisions needed to be made. • Required number of degrees of freedom? • Parallel, serial or hybrid structure? • Appropriate degree of miniaturisation? (At which point does further miniaturisation have a negative effect on accuracy?) • Use of conventional joints or compliant mechanisms? From the practical requirements of potential users it became quickly clear that 4 degrees of freedom are necessary. 3 translational axes are required to allow the workpiece to be positioned in three orthogonal linear coordinates. In most practical micro assembly applications the workpiece must also be orientated, so a fourth rotational axes is also required. Table 1: Basic requirements Criterion Value Unit Workspace 54 x 85 x 20 mm Footprint (area of robot base) < 150 x 150 mm Repeatability < 1 μm Linear speed (X,Y,Z directions) > 100 mm/s Rotational speed (θ axis) > 160 /s Angular resolution (θ axis) < 0.005 0