Lightweight positioning : design and optimization of an actuator with two controlled degrees of freedom

Abstract

It is known that internal vibrations decrease the performance characteristics and life time of mechanisms, and in some cases they even may lead to mechanical failures. In motion systems used in precision technology (wafer scanners, scanners, pick-and-place machines for production of PCBs, wire-bonders etc.), internal vibrations limit the performance parameters. The vibrations are still a challenge for the generally accepted design approach at present time, which is heading towards higher system accuracy, speed and throughput. Currently, the design approach to precision positioning applications places the dominant vibration frequencies of the mechanical parts several times higher than the required control bandwidth. However, these high mechanical frequencies are reached by constructing the mechanical parts with high stiffness, often at the cost of relatively high mass. To eliminate the negative consequences of the classical methodology, another design philosophy is used in this thesis. A three-disciplinary lightweight positioning approach (control, mechanics and electromechanics) focuses on mass reduction of the moving parts of motion systems. For this purpose, a principle based on over-actuation is used, which allows designing a lighter overall kinematical structure (force-path). In order to evaluate this approach on a general level, benchmarks for classical and lightweight positioning systems are proposed, namely, a so-called stiff beam system and a flexible beam system. The main focus of the thesis is on the design and optimization of a novel Lorentz force actuator for a lightweight positioning system that can also be applied in other precision technology applications. The objective is to reach the maximum mass reduction of the flexible beam system. In order to evaluate and design the novel actuator, a comprehensive static electromagnetic analysis of the actuator is elaborated. The resulting analytical model is based on a magnetic equivalent circuit, which has been identified by means of preliminary finite element calculations. The analytical model plays an essential role in the complete design. It is later used for the optimal dimensioning of the actuator for required performance specifications. Then, a numerical finite element model is built and the results are used to evaluate the accuracy of the analytical model and to identify parasitic forces and torques of the actuator. Another important aspect that determines the operating conditions is the thermal behavior of the actuator. It is also described analytically by a thermal lumped parameter model. The suggested description of the heat transfer captures the static as well as the dynamic behavior. To determine the optimal dimensions of the actuator an optimization approach, which uses the magnetic equivalent circuit and the thermal analytical model, is proposed. In terms of nonlinear programming, the problem statement consists of finding the dimensions of the actuator with minimal mass, where given force and torque are used as constraints. Because of the nonlinear nature of the problem the optimal solution is found numerically. The resulting optimal actuator incorporating two degrees of freedom (DoF) has 22.2% less mass than two equivalent 1-DoF actuators. It may be concluded, based on simulation and measurement results, that the proposed actuator can be analyzed with sufficient accuracy by the presented methods. The invented short-stroke actuator uniquely combines two controlled degrees of freedom: translational and rotational. This combination ensures that the mass of the actuators used in the flexible beam system has been reduced compared to that in the stiff beam system. The actuators support the flexible beam system in a way that introduces less disturbances. Meanwhile, the controllability of higher order vibration modes and, consequently, the global performance are improved. Two lightweight positioning systems were built, one with three 1-DoF actuators and the other with two novel Lorentz force actuators. In both setups the flexible beam has its mass reduced to 38.6% of that of the stiff beam. The total mass of the actuators in both cases is almost the same, but the setup with the innovative actuators allows to control the beam with two forces and two torques, while the setup with three 1-DoF actuators produces only three controlled forces