Design of Feedforward Controller to Reduce Force Ripple for Linear Motor using Halbach Magnet Array with T Shape Magnet

AbstractRecently, in micro/nano fabrication equipments, linear motors are widely used as an actuator to position workpiece, machining tool and measurement head. To control them faster and more precise, the motor should have high actuating force and small force ripple. High actuating force enable us to more workpiece with high acceleration. Eventually, it may provide higher throughput. Force ripple gives detrimental effect on the precision and tracking performance of the equipments. In order to accomplish more precise motion, it is important to make lower the force ripple. Force ripple is categorized into cogging and mutual ripple. First is dependent on the shape of magnets and/or core. The second is not dependent on them but dependent on current commutation. In this work, coreless mover i.e. coil winding is applied to the linear motor to avoid the cogging ripple. Therefore, the mutual ripple is only considered to be minimized. Ideal Halbach magnet array has continuously varying magnetization. The THMA (Halbach magnet array with T shape magnets) is proposed to approximate the ideal one. The THMA can not produce ideal sinusoidal flux, therefore, the linear motor with THMA and sinusoidal commutation of current generates the mutual force ripple. In this paper, in order to compensate mutual force ripple by feedforward(FF) controller, we calculate the optimized commutation of input current. The ripple is lower than 1.17% of actuating force if the commutation current agree with the magnetic flux from THMA. The performance of feedforward(FF) controller is verified by experiment

Experimental Investigation of Two-Phase Flow Distribution with Different Vertical Header Configurations

In this study, we investigated the behavior of two-phase flow distribution inside a vertical header of a microchannel heat exchanger (MCHX) that functions as an evaporator of a heat pump system. In general, the two-phase flow distribution behavior of the refrigerant differs depending on the target application, which ranges from small-scale automobile air-conditioners to large-scale building heat pump systems. Particularly, it is reported that the distribution characteristics in the vertical header of the MCHX vary extensively according to the inlet flow conditions of the refrigerant and the physical profile of the header. In this study, the physical configurations (header height, branch tube diameter) of four types of vertical headers were considered. Thereafter, the operating conditions in an experimental device that simulates an MCHX with a vertical header were selected. The experiment was performed under R410A as the working fluid, with a saturation temperature of 15 °C, inlet mass flow rate of 50–150 kg h−1 (mass flux of 908–2723 kg m−2 s−1), and an inlet vapor quality of 0.1–0.2. The liquid and vapor flow ratios and the relative standard deviation were adopted as metrics to characterize the uniformity of flow distribution. The distribution characteristics were subsequently described according to Reynolds and Froude numbers. The larger the Reynolds number and the smaller the Froude number, the more uniform the two-phase flow distribution becomes. A correlation was proposed as a function of the Reynolds and Froude numbers to predict the flow distribution characteristics for the considered vertical headers

Vulnerability Resilience in the Major Watersheds of the Korean Peninsula

Water resources management requires policy enforcement in a changing environment. Climate change must be considered in major watershed river restorations in Korea. The aim of river restorations is to provide better water resource control - now and in the future. To aid in policy making in the government sector, ¡§vulnerability-resilience indexes¡¨ (VRIs) with a Delphi survey method have been adopted to provide a possible reference. The Delphi survey offers prioritized vulnerability proxy variables based on expert opinions regarding the changing environment in terms of climate change and river restorations. The VRIs of watersheds were improved after river restorations, with the exception of some locations. However, when climate change was taken into consideration in the analysis of conditions after the restorations were completed, the results showed that governments need to provide better mitigation strategies to increase vulnerability resilience in the face of climate change